CONTROL VALVE CAVITATION: AN...

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30 August 1999 0:54 MHOO9-09.tex MH009-09.SGM MH009v3(1999/07/12)

VALVES, SERVOS, MOTORS, AND ROBOTS 9.63

4. The torque can be applied manually or with the aid of a hydraulic torque wrench. If the powerwrench is used, be extra careful to avoid slippage due to the high torques and the safety risk topersonnel if something slips or breaks. If the valve has been pulled from the line, a lathe or boringmill may be the easiest way to apply the torque to back the ring out.

5. The bonnet bolting can be used as a reaction point for the torque and to hold the puller down intothe body.

6. On particularly stubborn rings, using an impact wrench can help to break them loose.

7. As the ring starts to come out, the bolts holding the puller in the body must also be loosened topermit the ring to move up.

8. Once the ring is out, thoroughly clean and chase all threads.

9. Apply a heavy coat of lubricant or pipe compound to all threads and reinstall and torque to specifiedlevels. The ring may be tackwelded in place, as necessary.

10. On double-ported valves, the port the farthest distance from the actuator is the smallest and needsto be installed first.

REFERENCES

1. Preckwinkle, S. E., Maintenance Guide for Air Operated Valves, Pneumatic Actuators & Accessories, ElectricPower Research Institute, Palo Alto, Calif., 1991.

2. Ozol, J., “Experiences with Control Valve Cavitation Problems and Their Solutions,” Proceedings of EPRIPower Plant Valves Symposium EPRI, Palo Alto, Calif., 1987.

3. McElroy, J. W., Light Water Reactor Valve Performance Surveys Utilizing Acoustic Techniques, PhiladelphiaElectric Co., Philadelphia, Pa. 1987.

4. Fitzgerald, W. V., “Automated Control Valve Troubleshooting: The Key to Optimum Valve Performance,” ISA,Proceedings, ISA, Research Triangle Park, N.C., 1991.

5. Ferguson, Brian, “Air-Operated Valve—Preventive Maintenance Program,” Proceedings of the 2d NRC/ASMESymposium on Pump & Valve Testing, Washington, D.C., 1992.

6. Hutchison, J. W., ISA Handbook of Control Valves, ISA, Research Triangle Park, N.C., 1971.

7. Control Valve Handbook, 1st ed., Fisher Controls, Marshalltown, Iowa, 1977.

8. Instruction, Manual, EHD, EHS, & EHT, Form 5163, Fisher Controls, Marshalltown, Iowa, 1985.

CONTROL VALVE CAVITATION:AN OVERVIEW

by Marc L. Riveland∗

INTRODUCTION

Cavitation is of significant concern to the process control industry. It can be the source of unac-ceptable noise, vibration, material damage, and a decrease in the efficiency of hydraulic devices. Left

* Sr. Engineering Specialist, Applied Research, Fisher Controls International, Inc., Marshalltown, Iowa 50158.

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9.64 PROCESS/INDUSTRIAL INSTRUMENTS AND CONTROLS HANDBOOK

unaddressed, cavitation can shorten the operating life of critical and expensive hardware, upset processcontrol, and create hazardous or unsuitable work environments of plant personnel.

Theoretically, cavitation can occur in any process element or fitting that induces pressure changes.However, control valves, by virtue of their throttling function, are inherently problematic in thisregard. Understanding the basic nature of cavitation and becoming familiar with available cavitationcontrol products and techniques are the most effective means of avoiding these negative consequencesin practice.

CAVITATION FUNDAMENTALS

Valve Hydrodynamics

Cavitation is a specific fluid behavior whereby pressure dynamics induce explosive growth and col-lapse of cavities within a liquid. Weaknesses in the fluid continuum allow rapid growth and vaporizationof the liquid when the local fluid pressure decreases to near the liquid vapor pressure. Conversely, ifthe local pressure subsequently rises to a value above the vapor pressure, the reverse process occursand the cavity collapses. Localized pressures and velocities associated with the collapse phase are thesource of most of the aforementioned problems. This behavior can occur in cases in which the liquidis static (as in a propeller spinning in a large body of water) or in motion (as in the case of liquid flowthrough pipes and fittings).

Pressure dynamics conducive to cavitation within a control valve are related to the flow of the liquidthrough the restriction. The mean pressure profile associated with flow through a simple restriction,such as a control valve, is shown in Fig. 1.

The shape of this general curve is a consequence of fluid continuity and conservation of energy.The mean kinetic energy increases as the fluid accelerates through the restricted flow area of the valvethroat. Correspondingly, the mean fluid pressure decreases to maintain the fluid energy balance. Thefluid decelerates as it moves into the increased flow area of the valve outlet and downstream pipe,the mean kinetic energy decreases, and the pressure again increases. However, a small amount offluid energy is irreversibly dissipated in this recovery process so that the mean fluid pressure is notcompletely restored to its original value. This results in a pressure differential across the device, oftenreferred to as the observed pressure differential.

The point of minimum flow area is known as the vena contracta and, for all practical intents andpurposes, corresponds to the minimum pressure condition. The relationship between these two keypressure differentials is embodied in the liquid pressure recovery factor, FL, defined by the followingequation:

F2L = �Pobs

P1 − Pvc(1)

Proper use of this factor is discussed later in the section on valve selection.Incipient cavitation is the level of cavitation associated with discernible onset. This would the-

oretically occur when the vena contracta pressure is equal to the vapor pressure of the fluid andthe corresponding outlet pressure is above the vapor pressure. According to the simple model justdescribed, it would initially appear reasonable to utilize Eq. (1) to predict this condition. While thissimple model is suitable for conceptualizing the cavitation process, and it does provide a first-orderunderstanding of the forces that give rise to cavitation, it is not sufficiently complete enough to explainall observed behaviors. In fact, it is a particularly poor model for predicting the incipient cavitationcondition.

Recall that this model depicts mean fluid pressure. Flow through the complex geometry of controlvalves gives rise to significant deviations from this mean pressure. These include instantaneous pres-sure fluctuations associated with fluid turbulence and low pressures in the cores of vortices and eddiesassociated with boundary-layer separation, free shear zones, stagnation regions, and re-entrant zones.These phenomena can produce local pressures significantly higher or lower than the mean pressure,sufficient to initiate cavitation in very localized regions. Typically, cavitation begins well before the

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FIGURE 1 Idealized mean fluid pressure profile for a control valve. This modelrepresents mean pressure changes associated with mean velocity changes. It doesnot account for local pressure fluctuations.

minimum mean pressure is reduced to the vapor pressure. Additional empirically based parametersare required to predict different levels of cavitation. These are discussed later.

Cavity Mechanics

Understanding cavity behavior is the key to developing both systemic and valve-based solutions tocavitation–related problems. Cavitation has been the focus of both academic and industrial research fordecades, and much has been learned about individual and collective cavity behavior. A comprehensivereview of this subject is outside the scope of the immediate article and will be discussed only in thecontext of cavitation control. By necessity, many of the details are omitted. The interested reader isreferred to a very thorough coverage of the subject by Hammitt [1].

Cavitation is generally recognized to consist of four distinct events, that is, nucleation, growth,collapse, and rebound. All four events contribute to the overall extent of cavitation-related problems.However, the latter two events are the primary source of noise, vibration, and material damage.

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When pressure conditions are right, cavities begin to grow from weak spots in the fluid continuumknown as nuclei, undergoing an initial period of stable growth. Upon attaining a critical diameter,growth proceeds explosively with substantial vaporization of the liquid. Cavity growth ceases and thecollapse process begins when the fluid pressure increases. The ultimate degree and extent of cavitygrowth will be determined by the number and size of entrained nuclei, the properties of the liquid,and the extent of the low-pressure region.

Unlike the comparatively symmetrical growth of the cavity, the initial collapse is very rapidand highly asymmetrical. This results from the inertial forces acting on the cavity as it movesinto a lower-velocity, higher-pressure region. Experimental studies [2] have revealed the presenceof a very small, high-velocity jet (referred to as a microjet) formed during such asymmetricalcollapse.

Several additional growth–collapse cycles may follow the initial cycle in a phenomenon known asrebound. This occurs when the rate of cavity collapse exceeds the condensation rate, or if the cavitycontains substantial amounts of a noncondensable gas. The cavity contents are compressed by theliquid rather than condensed to the liquid state. Mechanical energy is stored in the compressed gasesand can be released to initiate another cycle. The total cavity volume decreases on each successivecycle until the process ceases. The collapse of a rebound cavity is generally more symmetrical thanthe initial collapse and is marked by the absence of the high-velocity microjet. However, the rapidmovement of the liquid surrounding the cavity induces a shock wave, which propagates away fromthe cavity.

The collapsing cavity serves as the primary source of hydrodynamic noise and vibration. In fact,hydrodynamic noise is usually attributed entirely to cavitation; noise levels at subcavitating flow arenot typically troublesome. The general subject of hydrodynamic noise and related prediction methodsis very involved and is discussed elsewhere in this handbook. In practice, treatment of cavitation (thesource) brings resultant hydrodynamic noise levels to within an acceptable level. A more detaileddiscussion is outside the immediate scope of this article.

Damage Mechanisms

Physical damage to the valve is probably the most frequent concern because of the associated cost,inconvenience, and unpredictable nature. Damage, as used in the context of this article, refers toany permanent deformation or loss of material. The collapsing cavity initiates an attack on adjacentmaterial surfaces. This attack on the material surface comprises a mechanical component and, moreselectively, a chemical component. The response or reaction of the material to the attack and the totaltime of exposure determine the extent of total damage to the material.

Mechanical attack may consist of high velocity, microjet impingement, shock-wave impingement,or most likely a combination of the two, on the material surface. Mechanical attack must alwaysoriginate from a cavity collapsing near the material surface in order to impart damage to that surface.If the microjet (established during the asymmetrical cavity collapse) is close to the surface andimpinges directly on the surface, a damaging attack will occur; otherwise no adverse material effectresults. Interestingly, these microjets exhibit a preferred orientation toward rigid surfaces. Presumablythe fluid resistance near the wall reduces the “supply” of fluid to that side of the cavity. Flow to theoutboard side of the cavity is relatively unimpeded, so that the jet orientation is in the direction of thewall. The coupling dynamics of a highly compliant surface (such as an elastomeric material) effectthe opposite behavior; that is, the orientation is away from the surface. Similarly, the intensity of theshock waves dissipates rapidly with the propagation distance so that shock waves originating fromcavities far removed from the surface (that is, more than one bubble diameter or so) have insufficientstrength to impart significant damage.

Corrosion can play a significant role in the damage process in that it interacts with the mechanicalforms of attack in a synergistic manner. Protective coatings can be removed by the mechanical attack,allowing chemical attack to occur. The chemical attack in turn deteriorates the material, making itmore susceptible to mechanical attack. The process continues, resulting in a more aggressive damageprocess than that associated with either of the constituent forms individually.

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Repeated attack by millions of microjets and shock waves results in the characteristic appearanceof cavitation—a very rough pitted surface.

Scale Effects

A number of factors can potentially affect the intensity of cavitation or, more importantly, the level ofthe associated negative effects of cavitation. Sometimes called scale effects, these factors can eitherintensify or diminish cavitation-related problems relative to equivalent installations under hydrody-namically similar operating conditions. The scale effects of most concern to control-valve applicationsare pressure- and velocity-related effects, size effects, and air-content effects. Other effects such asviscosity, surface tension, and various thermal property effects have also been investigated. However,in most industrial applications these are either of little significance or not sufficiently quantified to beable to adequately account for them.

In general, as the pressure, velocity, or valve size increase, the associated cavitation problemsget worse. Numerous investigations (as documented by Hammitt) have borne out the fact that thedegree of damage resulting from cavitation is very sensitive to the fluid velocity; that, in fact, the totaldamage imparted is an exponential function of the fluid velocity. The range of values reported for theexponent is very broad, usually between three and ten. However, there is some agreement that sixis a representative number. Tullis [3] and Mousson [4] both provide data showing an increase in thedamage rate as the upstream pressure increases. Investigations by Tullis [3] report an increase in theseverity of the negative side effects of cavitation associated with an increase in the nominal size ofthe device.

Likewise, there are effects associated with the change in backpressure applied to a valve. Fora monotonically decreasing outlet pressure at constant inlet conditions, two opposite trends can berationalized: an intensifying effect (due to increasing vapor volume) and a diminishing effect (due todecreasing collapse intensity). The issue becomes a matter of determining which effect dominates atany given outlet pressure. Mousson’s data [4] supply a partial answer to this by showing a maximumdamage level existing roughly midway between the two extremes. Field experience with controlvalves is consistent with this. In some cases unacceptable levels of noise and vibration existing at flowconditions well below choked flow have been observed to diminish to satisfactory levels at chokedflow conditions.

The presence of dissolved or entrained air (or any other noncondensable gas) has multiple effectson the cavitation process and associated problems. Increasing the amount of such gases has the effectof providing additional nucleation sites in the fluid. This contributes to an increase in the overallamount of cavitation and consequently the level of problems associated with cavitation. However,continued increase in the amount of air reduces the collapse velocities and disrupts the microjetand shock-wave attack mechanisms. This results in an overall attenuation of the negative effects ofcavitation, even though cavitation is still occurring. Mousson [4] reports a significant reduction in thedamage levels with only a few percent of air (by volume) entrained in water under otherwise constantconditions.

CAVITATION ABATEMENT STRATEGIES

There is no single best method for controlling the problems created by cavitation. A number oftechniques are available, each with inherent advantages and shortcomings. Familiarity with thesepractices provides greater opportunity to implement the most technically satisfactory and cost-effectivesolution for a given application.

All cavitation abatement techniques are based on effecting control over one or more of the basicelements of cavitation problems, that is, cavitation intensity and attack, material of construction(response to attack), or duty cycle (time of exposure). Cavitation control should be considered on twolevels—the system level and the control valve level. Whenever possible, it is desirable to contendwith cavitation at the system level.

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SYSTEM-LEVEL CAVITATION CONTROL

A primary and preferred strategy is to consider potentially problematic conditions at the time a processsystem is designed. Awareness and avoidance of conditions conducive to control-valve cavitation area highly effective means of reducing the risk of cavitation-related problems.

Valve Placement

Cavitation-related problems can be averted by simply locating the valve in a region of high overallpressure. The flow rate and pressure differential across the valve remain the same regardless oflocation, but overall fluid pressure within the valve body is increased proportionately, as depicted inFig. 2. This results in a greater margin between the minimum pressures throughout the valve and thevapor pressure, thus decreasing the likelihood that the fluid pressure will fall below the vapor pressureof the liquid.

This technique can be effected in practice by locating the valve as far upstream in the systemas possible. When the piping losses are minimized upstream and shifted downstream of the valve,backpressure to the valve is increased. This, as noted above, increases the overall pressures internallyin the valve.

Backpressure Devices

When placement of the valve within the system is not flexible, fluid pressures within the controlvalve may be increased by introducing additional resistance to flow downstream of the valve. When arestriction such as an orifice plate or a second valve is placed downstream of the valve, the backpressureis increased by the amount of the pressure differential across that restricting device.

Usually the pressure drop across the control valve is decreased by this amount, that is, the inletpressure remains constant but the backpressure increases. Consequently, the valve will realize lesspressure drop for the same flow rate and the required valve coefficient must increase accordingly. Inaddition to increasing the fluid pressure within the valve, the fluid velocities will generally be reducedsince the valve will operate at a larger opening. The combined effects of increased fluid pressure andreduced velocity can be very effective in controlling cavitation. A word of caution is needed, however.

FIGURE 2 The effect of locating a valve in a region of higher system pressure is to increase the overall mean pressurewithin the valve.

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Cavitation within the overall system may not always be controlled by this method, but rather merelydisplaced from the valve to another location within the system. It is important to account for thepossibility and consequence of any resulting cavitation at the downstream restriction.

The fixed-restriction alternative is best suited to on–off service since the device can only beoptimized for a single flow rate. If the restriction is sized for high load and the system is operating atlower load, the pressure drop across the restriction will be very low as a result of a decreased fluidvelocity through the restriction. Consequently, the valve will realize an increased pressure drop andagain be at risk of cavitation-related problems. At higher flow rates than designed for, the orifice maywell become the primary restriction and in turn limit or choke the flow at a lower flow rate than desired.The use of a second control valve in series affords a greater effective range, but this is usually a moreexpensive solution and requires a more sophisticated control scheme for optimum performance.

Gas Injection

Another method of controlling the damage, noise, and vibration resulting from cavitation (but nottotally eliminating cavitation) is through noncondensable gas injection. This method, while effective,is very selective since not all processes will tolerate the introduction of gases.

A gas that will not condense under the prevailing downstream conditions is injected (or aspirated)into the flowing fluid near the vena contracta. The continued presence of the gas phase during pressurerecovery disrupts the cavity collapse process and limits the negative effects associated with cavitycollapse. Caution must be exercised to introduce the gas at or downstream of the throat of the valvein order to avoid a reduction in flow from a two-phase mixture at the vena contracta. Individualvalve suppliers should be consulted as to the practicability of this method for the valve style beingconsidered, as well as to the minimum amount of gas required and the exact location of introductionto the flow stream.

CONTROL-VALVE SOLUTIONS

If it is not possible to avoid cavitating conditions in the process system, it is necessary to contendwith it at the control-valve level. Control-valve solutions are predicated on the concepts of cavitationreduction, resistance to cavitation attack, and control of the collapse region of cavities.

Material Selection

First consideration should be given to material selection. If physical damage is of primary concern, itis sometimes possible to create a cavitation-tolerant environment by selecting materials more resistantto cavitation attack. Standard trims constructed of materials suited to the process fluid, and serviceconditions often provide a cost-effective solution.

Proper material selection is not a black-and-white issue, nor is there one “best” material. Thecharacterization of a material’s resistance to cavitation lacks rigorous quantification. Currently, quali-tative force ranking of material resistance to cavitation attack in combination with empirical “rules ofthumb” governs selection. As a general rule, a material’s hardness and resistance to corrosion are theforemost properties considered. Other properties that have shown a correlation to cavitation damageresistance to varying degrees include the ultimate resilience and strain energy to failure. However, nosingle property offers a consistent numerical correlation.

It is important to base material selection on the total attack, that is, considering both the mechanicaland the chemical components. A notable exception to the “harder is better” rule is the widespreaduse of cobalt alloy 6 in cavitating service. Its combined hardness and corrosion resistance makes it apreferred choice to harder materials currently available. However, even this material is not universallysuperior. It provides very poor protection in applications of boiler feedwater treated with hydrazine.

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Even though the material is extremely hard, the amines attack the material and render it structurallyinferior. Other materials that are chemically more resistant to the amines, such as S44004 (440C), area preferred choice. Other popular materials frequently used in cavitating liquid service include otheralloy steels, tool steels, certain stainless steels (such as the 300 series), and precipitation-hardenedmaterials.

Ceramics is an emerging material category that shows promise of good cavitation damage re-sistance. Ceramics of practical interest to the control-valve industry consist of metals or metalloidscombined with oxygen, carbon, nitrogen, or boron. Examples include aluminum oxide, zirconiumdioxide, silicon carbide, and silicon nitride.

Other nonmetallic materials, such as elastomers and compliant materials in general, exhibit anability to withstand levels of cavitation attack greater than standard structural indicators would suggest.This paradox apparently results from a dynamic interaction between the surface and the cavity,which orients the microjet away from the surface, thus eliminating the mechanical attack. Whilesuch behavior is appealing from a damage-control standpoint, Sanderson [5] points out that bondingdifficulties, as well as potential pressure and temperature limitations, have curbed the widespread useof such materials in the industry.

It should be emphasized that all materials are vulnerable to cavitation attack. The rate of damageis a complicated function of the intensity of the cavitation attack, the total time of exposure, and thematerial characteristics. Material selection by itself can prolong the life of a component, but it willnot completely eliminate the possibility of damage and therefore is best utilized in conjunction withother abatement strategies.

Special Trim Designs

If the protection offered by material selection is deemed inadequate by itself, or if noise and vibrationare also of concern, it may be necessary to use special trim designs. A number of proprietary productsare available from different valve manufacturers. These products and trims come in a wide variety ofconfigurations, but they are all based on one or more fundamental operational strategies with differenttradeoffs between cost and performance.

The most common design concepts embraced by different valve manufacturers parallel manyof the techniques used on a larger scale in the context of system strategies. The foremost objec-tive of good cavitation control product design is to control energy conversions within the valve.Pressure-recovery characteristics and trim velocities are favorably affected by strategically introduc-ing resistance into the flow path. In general, overall fluid pressure recovery is reduced in such trims.Reduced-pressure recovery effectively reduces the tendency of the valve to cavitate by raising over-all pressure in the valve compared to those in a high-recovery valve under the same conditions, asdepicted in Fig. 3. Further benefit is realized in that, if cavitation does occur, the pressure differentialdriving cavity collapse (P2 − Pv) is reduced, which in turn tends to reduce the negative effects ofcavitation.

This objective is commonly achieved in practice by forcing fluid flow through successive stages ortortuous paths. When the pressure drop across a valve is staged, a portion of the total pressure drop istaken across each of a series of restrictions, or stages. This creates a much less efficient hydrodynamicpath than an equivalent single restriction and results in a lower pressure recovery. Furthermore, thedecreased flow efficiency requires comparatively larger flow passages, hence lower velocities, undersimilar flow conditions. Tortuous-path treatment, in contrast, utilizes a labyrinthal flow path to induceirreversible energy conversions, which in turn have the same impact on pressure recovery and velocitythat staging does.

Another fundamental design strategy consists of dividing the flow stream into multiple parallelflow paths. Whereas many of the cavitation-related problems tend to scale with the physical size ofthe flow stream, the reduced size of individual flow paths helps to reduce overall cavitation-relatedproblems, particularly noise and vibration. To avoid potential plugging problems associated withrestrictive flow passages, a compromise between degree of cavitation control and passage clearancemust be reached.

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FIGURE 3 Successive contractions and expansions, and tortuous flow paths reducecaviation problems. Pressure is dissipated while maintaining higher overall fluid pressures.

Finally, it is possible to redirect the flow stream to avoid direct impingement of cavitating flowson critical control surfaces. As pointed out earlier, a cavity must collapse in close proximity to asurface in order to damage that surface. By separating collapsing cavities from the surface the threatof damage is minimized, even though the cavitation has not necessarily been eliminated or reduced.

The design of any control valve to minimize the effects of cavitation involves a tradeoff betweenhigher comparative costs, lower relative capacity, the degree of protection required by a particularapplication, and the ability of the valve to tolerate dirty fluids. Highly optimized designs actuallyprevent the formation of any significant cavitation, whereas standard trim would cavitate heavily.This degree of protection is not warranted by all process control applications. Therefore a variety oftrims generally designed to a reasonably specific set of conditions are available to meet the variety ofprocess needs.

CONTROL-VALVE SELECTION

Correct process operation and maximum valve life are predicated on proper valve selection forspecific application conditions. This is true not only for special cavitation control trims, but forso-called standard valves as well. The following discussion provides an overview of the generallyaccepted industry methods available at the time of this writing. This subject remains an active area ofdevelopment within the industry, and changes are likely forthcoming.

Background

Recognizing that cavitation is a complex phenomenon, it comes as no surprise that a single, simplemethod to properly select a valve for cavitation remains elusive. Numerous “manufacturer specific”methods have evolved over the years, with varying degrees of success. A fairly recent initiative by theInstrument Society of America (ISA) has resulted in the first meaningful step toward bringing thesediverse methods into a single, cohesive, and universally applicable framework. While the technologyis still evolving and many questions remain, this effort goes a long ways toward providing a commonvernacular and collection of parameters by which performance can potentially be quantified. This effort

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culminated with the release of a recommended practice [6] that provides background informationon the cavitation phenomenon, nominal testing and evaluation methods, and a basic applicationframework. The discussion that follows is based on this information. The interested reader is referredto this document for more information. (Note: At this juncture, the scope of discussion is narrowedto exclude noise prediction technology. This is an equally involved topic and is covered elsewhere inthis handbook.)

Cavitation Parameters and Coefficients

From the discussion presented earlier it is evident that pressure characteristics are relevant to cavitationbehavior. It comes as no surprise, then, that control-valve service and performance are generallycharacterized by variant forms of the pressure coefficient or pressure ratios. Several different formshave evolved over time, the exact choice of which varied between valve manufacturers. The ISAmethod [6] has adopted the following basic form:

σ = P1 − Pv

P1 − P2(2)

where P1 = absolute pressure upstream of control valve,P2 = absolute pressure downstream of control valve,Pv = absolute thermodynamic vapor pressure.

The application method is built on the notion of comparing an operating condition expressed interms of Eq. (2) to a meaningful limit expressed in the form of a valve cavitation coefficient, σ v.If σ is greater than σ v, then the valve satisfies the criterion for acceptable operation implied by thecoefficient.

Clearly, the critical step of this method is the establishment of a meaningful and appropriate valvecavitation coefficient. Numerous benchmarks are defined, some of which are readily measurable andothers that are desired but are more difficult to objectively quantify. Several principal values utilizedin the recommended practice [6] include the following:

� incipient cavitation coefficient, σ i: the value of σ at the onset of cavitation� constant cavitation coefficient, σ c: the value of σ at conditions of mild but steady cavitation� incipient damage coefficient, σ id: the value of σ at the onset of damage by cavitation� manufacturer’s recommend coefficient, σ mr: the minimum value of σ recommended by the valve

manufacturer for satisfactory operation of the control valve.

It is very difficult to observe and measure cavitation directly. No “scientifically pure,” or completelyobjective, laboratory method exists for evaluating the cavitation that occurs in a control valve or allcoefficients of interest. The usual approach is to monitor the effect of cavitation on characteristicssuch as noise levels, vibration levels, damage rate, or flow efficiency, and infer information about thebehavior of other cavitation effects under field conditions.

Means of evaluating σ i and σ c are available [6]–[8]. In essence, pipe wall vibration (or in somecases control-valve noise) are measured in a prescribed laboratory installation. The measured quantity(i.e., vibration or noise) is plotted over a range of σ . Figure 4 depicts a typical plot of this information.The incipient level of cavitation and the constant level of cavitation are associated with the inflectionpoints indicated on the plot.

The incipient level of cavitation is generally too conservative for most applications. Most controlvalves, especially those designed for cavitating service, can tolerate some degree of cavitation. Thislevel of cavitation is an appropriate limit only for those applications in which no cavitation whatsoevercan be tolerated by the application.

The constant level of cavitation is a limit defined primarily by the test. It is certainly a discerniblelevel of cavitation; however, it is difficult to say whether it is a tolerable level of cavitation or not with

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FIGURE 4 Typical pipe wall vibration curve,showing defined cavitation levels.

respect to any given application. In other words, constant cavi-tation is not necessarily an appropriate universal limit or thresh-old. Again, in many applications, it is likely to be too conser-vative, resulting in unnecessarily expensive solutions.

The incipient damage level of cavitation is a threshold ofsignificant practical interest. If the onset of damage could bepredicted, and damage subsequently avoided, many cavitationapplications would be solved. However, the problem again liesin the objective evaluation of this condition. From previousdiscussion it is evident that the onset and extent of materialdamage is a function not only of the extent and intensity ofcavitation, but also of the material of construction and the totalduration of attack (duty cycle). No test method currently existsthat consistently quantifies this process and accounts for all ofthese factors.

For the foreseeable future, the threshold described by σ mr

probably provides the greatest utility to the end user. The man-ufacturer’s knowledge of valve design parameters, tested performance, and accumulated experienceprovide a strong basis for establishing the value of meaningful operational limits.

It should be noted that none of these parameters by themselves are complete similarity parameters.They do not, without modification, account for the numerous scale effects previously identified (suchas pressure or size). Some scaling relationships have been established for certain parameters and areoffered in the recommended practice [6]. Others, such as σ mr, may or may not already account forone or more of the scale effects. They should not be subject to universal scaling laws without specificdirection to do so by the manufacturer.

Whereas the pressure-recovery characteristics of a control valve are quantified in the FL parameter,the question naturally arises as to the appropriate use of this parameter in the context of selecting avalve for cavitating service.

Universally, FL has only one quantitative use, and that is to determine the choked flow rate througha specific valve under a given set of conditions [9]. However, because it is a pressure recovery term,it is qualitatively related to a valve’s tendency to cavitate. Reflecting on the previous discussionsregarding pressure recovery, it is apparent that if the same pressure differential is applied to both ahigh-recovery device and a low-recovery device, the high-recovery device will have the lower venacontracta pressure (see Fig. 5).

FIGURE 5 Low-recovery valves tend to have higher overall meanfluid pressure than high-recovery valves under equivalent flow con-ditions. Local pressure fluctuations may offset this positive effect.

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Low-pressure recovery devices are characterized by large values of the pressure-recovery coeffi-cient. Therefore, a valve with a high value of FL is generally, but not universally, a better candidatefor cavitating service than one with a low value insofar as overall fluid pressures within the valve aregreater. Used in conjunction with other parameters and additional information, FL can play a role insizing valves for cavitation service. However, it should not be construed as the singular cavitation in-dex. Many cavitation control hardware features are not reflected in the value of the pressure-recoverycoefficient.

Example

Consider a cage-guided globe valve in the following service.Water

T1 = 26.7 ◦C,

P1 = 1034 kPa,

P2 = 470 kPa.

The associated valve coefficients are:

FL = 0.82,

σ i = 2.89,

σ mr = 1.57.

Water at the given temperature has a vapor pressure of 3.5 kPa; therefore by Eq. (2),

σ = 1034 − 3.5

10.34 − 470

= 1.83

Since σ < σ i, the valve will be cavitating. However, since σ > σ mr, the valve will operate satisfactorilyaccording to the manufacturer’s specification.

CLOSURE

Cavitation is a complex phenomenon that can have an adverse impact on process performance,equipment service life, and operational behavior. Useful application technology is emerging but is notfully mature and is still evolving. This technology is most effective when used in combination withengineering judgment that is based on an understanding of the basic behaviors and experience.

REFERENCES

1. Hammitt, F. G., Cavitation and Multiphase Flow Phenomena, McGraw-Hill, New York, 1980.

2. Knapp, R. T., and A. Hollander, “Laboratory Investigations of the Mechanism of Cavitation,” Trans. ASME,vol. 70, 1948.

3. Tullis, J. P., “Cavitation Scale Effects for Valves,” J. Hydraulics Div., ASCE, vol. 99, p. 1109, 1973.

4. Mousson, J. M., “Pitting Resistance of Metals under Cavitation Conditions,” Trans. ASME, vol. 59, pp. 399–408.1937.

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5. Sanderson, R. L., “Elastomers for Cavitation Damage Resistance,” paper C. I. 82–908, presented at the ISAInternational Conference And Exhibit, Philadelphia, Pennsylvania, Oct. 1982.

6. ISA RP75.23–1995, “Considerations for Evaluating Control Valve Cavitation,” Instrument Society of America,Research Triangle Park, North Carolina, 1985.

7. Riveland, M. L., “The Industrial Detection and Evaluation of Control Valve Cavitation,” ISA Trans., vol. 22,no. 3, 1983.

8. Ball, J. W., and J. P. Tullis, “Cavitation in Butterfly Valves,” J. Hydraulics Div., ASCE, vol. 99, p. 1303, 1973.

9. ISA S75.01–1985 (R 1995),“Flow Equations for Sizing Control Valves,” Instrument Society of America,Research Triangle Park, North Carolina, 1995.

CONTROL VALVE NOISE

by Allen C. Fagerlund∗

INTRODUCTION

Fluid transmission systems are major sources of industrial noise. Elements within the systems thatcontribute to the noise are control valves, abrupt expansions of high-velocity flow streams, compres-sors, and pumps. Control-valve noise is a result of the turbulence introduced into the flow stream inproducing the permanent head loss required to fulfill the basic function of the valve.

NOISE TERMINOLOGY

Noise is described or specified by the physical characteristics of sound. The definitive properties ofsound are the magnitude of sound pressure and the frequency of pressure fluctuation, as illustrated inFig. 1.

Sound pressure, PS, measurements are normally root-mean-square (rms) values of sound pressureexpressed in microbars. Because the range of the sound pressure of interest in noise measurements is∼108 to 1, it is customary to deal with the sound pressure level (LP) instead of sound pressure. LP isa logarithmic function of the amplitude of sound pressure and is expressed mathematically as

L P = 20 log10

Ps

0.0002 µbard B

The selected reference sound pressure of 0.0002 µbar is approximately the sound pressure requiredat 1,000 Hz to produce the faintest sound that the average young person with normal hearing can detect.

* Senior Research Specialist, R. A. Engel Technical Center, Fisher Controls International, Inc., Marshalltown, Iowa.

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FIGURE 1 Properties of sound.

The characteristic of the LP scale is such that each change of 6 dB in level represents a change in theamplitude of sound pressure by a factor of 2.

The apparent loudness of a sound varies not only with the amplitude of sound pressure but also asa function of frequency. The human ear responds to sounds in the frequency range between 20 and18,000 Hz. The normal ear is most sensitive to pressure fluctuations in the neighborhood of 3,000 to4,000 Hz. Therefore the degree of annoyance created by a specific sound is a function of both soundpressure and frequency.

L P measurements are often weighted to adjust the frequency response. Weighting that attenuatesvarious frequencies to approximate the response of the human ear is called A weighting. Figure 2shows L P correction as a function of frequency for A-weighted octave-band analysis.

Approximate overall sound levels of some familiar sound environments are shown in Table 1.Any study of valve noise will evaluate the following basic phenomena. Acoustic power, generated

by fluid flow through a control valve, propagates through the piping and creates a fluctuating pressurefield, which forces the pipe wall to vibrate. These vibrations in turn cause pressure disturbancesoutside the pipe that radiate as sound. The difference in sound pressure levels from inside to outsidethe pipe is called the transmission loss.

FIGURE 2 A-weighting curve.

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TABLE 1 Approximate Sound Levels of FamiliarSounds

Sound Sound Level (dB)

Pneumatic rock drill 130Jet takeoff [at 200 ft (61 m)] 120Boiler factory 110Electric furnace (area) 100Heavy street traffic 90Tabulation machine room 80Vacuum cleaner [at 10 ft (3 m)] 70Conversation 60Quiet residence 50Electric clock 20

SOURCES OF VALVE NOISE

The major sources of control valve noise are (1) mechanical vibration of valve components and (2)fluid-generated noise, namely hydrodynamic noise and aerodynamic noise.

Mechanical Noise

The vibration of valve components is a result of random pressure fluctuations within the valve bodyor fluid impingement on movable or flexible parts. The most prevalent source of noise resultingfrom mechanical vibration is the lateral movement of the valve plug relative to the guide surfaces.Sound produced by this type of vibration normally has a frequency of less than 1,500 Hz and is oftendescribed as a metallic rattling. The physical damage incurred by the valve plug or associated surfacesis generally of more concern than the noise emitted.

A second source of mechanical noise is a valve component resonating at its natural frequency.Resonant vibration of valve components produces a single pitched tone, normally having a frequencybetween 3,000 and 7,000 Hz. This type of vibration produces high levels of stress that may ultimatelyproduce fatigue failure of the vibrating part.

Noise resulting from mechanical vibration has, for the most part, been eliminated by improvedvalve design and is generally considered a structural problem rather than a noise problem.

Hydrodynamic Noise

Control valves handling liquid flow streams can be a substantial source of noise. The flow noiseproduced is referred to as hydrodynamic noise and may be categorized with respect to the specificflow classification or characteristic from which it is generated. Liquid flow can be divided into threegeneral classifications: (1) noncavitating, (2) cavitating, and (3) flashing.

Noncavitating liquid flow generally results in very low ambient noise levels. It is generally acceptedthat the mechanism by which the noise is generated is a function of the turbulent velocity fluctuationsof the fluid stream, which occur as a result of rapid deceleration of the fluid downstream of the venacontracta as the result of an abrupt area change.

The major source of hydrodynamic noise is cavitation. This noise is caused by the implosion ofvapor bubbles formed in the cavitation process. Cavitation occurs in valves controlling liquids whenthe following two service conditions are met: (1) the static pressure downstream of the valve isgreater than the vapor pressure and (2) at some point within the valve the local static pressure is lessthan or equal to the liquid vapor pressure because of either high velocity or intense turbulence.

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The cavitation phenomena is discussed in detail in another section of this handbook. Vapor bubblesare formed in the region of minimum static pressure and subsequently are collapsed or imploded asthey pass downstream into an area of higher static pressure. Noise produced by cavitation has abroad frequency range and is frequently described as a rattling sound similar to that which wouldbe anticipated if gravel were in the fluid stream. Since cavitation may produce severe damage to thesolid boundary surfaces that confine the cavitating fluid, noise produced by cavitation is generally ofsecondary concern.

Flashing is a phenomenon that occurs in liquid flow when the differential pressure across a restric-tion is greater than the differential between the absolute static and vapor pressures at the inlet to therestriction, that is, �P > P1 − Pv. The resulting flow stream is a mixture of the liquid and gas phasesof the fluid, which causes large variations in fluid density and acoustic wave speed. Noise resultingfrom a valve’s handling a flashing fluid is a result of the deceleration and expansion of the two-phaseflow stream.

Test results supported by field experience indicate that noise levels in noncavitating and flashingliquid applications are quite low and generally are not considered a noise problem.

Aerodynamic Noise

Aerodynamic noise is created by turbulence in a flow stream as a result of deceleration or impinge-ment. The principal area of noise generation in a control valve is the recovery region immediatelydownstream of the vena contracta, where the flow field is characterized by intense turbulence andmixing. Subcritical flow is similar to noncavitating liquid flow in that turbulence caused by shear isthe primary source of noise. Once the compressible flow is critical with shock waves present, thecharacter of the noise generation changes. The interaction of the turbulence with the shock wavesbecomes the predominant noise source.

NOISE PREDICTION

International standards now exist that provide methods for predicting valve noise under a wide varietyof conditions. A reader wishing detailed methods of calculation should refer to the most currentrevision of the appropriate standards mentioned below.

Hydrodynamic Noise: IEC 534-8-4 [1]

Methods in the standard are for both the noncavitating and cavitating conditions of liquid flow. Theyare based on the premise that the acoustic power generated in side the pipe is proportional to theavailable stream power. The proportionality factor or efficiency is developed empirically and differsgreatly between the two regimes. Transmission loss through the pipe wall is then calculated to yieldan estimate of the external sound pressure level at 1 m from the outside surface of the pipe. Flashingconditions are not covered in the standard. The large variation in the fluid density and the acousticwave speed must be accounted for in attempts to predict this noise, and to date a reliable method hasnot been developed.

Aerodynamic Noise: IEC 534-8-3 [2]

Methods for predicting aerodynamic noise are built on an understanding of the structure of opencompressible jets over the full pressure ratio range. The acoustic power inside the pipe is definedby the stream power at the vena contracta multiplied by an efficiency term, which is dependent onvalve geometry and the pressure ratio. Pipe-wall transmission loss terms are well defined to yield aprediction of external noise levels.

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NOISE CONTROL

Either one or both of the following basic approaches can be applied for noise control:

1. Source treatment: prevention or attenuation of the acoustic power at the source (quiet valves andaccessories).

2. Path treatment: reduction of noise transmitted from a source to a receiver.

Quiet Valves

Based on the preceding discussion, the parameters that determine the level of noise generated bycompressible flow through a control valve are the geometry of the restrictions exposed to the flowstream, the total valve flow coefficient, the differential pressure across the valve, and the ratio of thedifferential pressure to the absolute inlet pressure.

It is conceivable that a valve could be designed that utilizes viscous losses to produce the permanenthead loss required. Such an approach would require valve trim with a very high equivalent length,which becomes impractical from the standpoint of both economics and physical size.

The noise characteristic or noise potential of a regulator increases as a function of the differentialpressure �P and the ratio of the differential pressure to the absolute static pressure at the inlet �P/P1.Thus for high-pressure-ratio applications (�P/P1 > 0.7), an appreciable reduction in noise can beeffected by staging the pressure loss through a series of restrictions to produce the total pressure headloss required.

Generally in control valves, noise generation is reduced by dividing the flow area into a multiplicityof smaller restrictions. This is readily accomplished with a cage-style trim, as shown in Fig. 3: (a)slotted multipath, (b) drilled-hole multipath, (c) drilled-hole multipath, multistage, and (d) plate-stylemultipath, multistage. Both acoustic and manufacturing technology have improved to where individualstages in a trim can be custom designed for the conditions they see, as well as take advantage of threedimensional flow passages, as shown in Fig. 3(e).

Proper hole size and spacing is critical to the total noise reduction that can be derived from theutilization of many small restrictions versus a single or a few large restrictions. It has been found thatoptimum size and spacing are very sensitive to the pressure ratio �P/P1.

For control-valve applications operating at high-pressure ratios (�P/P1 ≥ 0.7) the series restrictionapproach, splitting the total pressure drop between the control valve and a fixed restriction (diffuser)downstream of the valve, can also be very effective in minimizing the noise. In order to optimize theeffectiveness of a diffuser, it must be designed (special shape and sizing) for each given installationso that the noise levels generated by both the valve and diffuser are minimized. Figure 4 depicts atypical valve-plus-diffuser installation.

Pertaining to the design of quiet valves for liquid applications, the problem resolves itself intoone of designing to reduce cavitation. Service conditions that will produce cavitation can readily becalculated. The use of staged or series reductions provides a practicable solution to cavitation andhence hydrodynamic noise.

Path Treatment

A second approach to noise control is path treatment. Sound is transmitted through the medium thatseparates the source from the receiver. The speed and efficiency of sound transmission is dependenton the properties of the medium through which it is propagated. Path treatment consists of regulatingthe impedance of the transmission path to reduce the acoustic energy communicated to the receiver.

In any path-treatment approach to control valve noise abatement, consideration must be given tothe amplitude of noise radiated by both the upstream and the downstream piping. Since, when allelse is equal, an increase in static pressure reduces the noise transmitted through a pipe, the upstream

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FIGURE 3 Valve cage designs for noise attenuation.

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FIGURE 4 Two-stage pressure reduction with diffuser.

noise levels are always less than those downstream. Also, the fluid propagation path is less efficientmoving back through the valve.

Dissipation of acoustic energy by the use of acoustical absorbent materials is one of the mosteffective methods of path treatment. Whenever possible, the acoustical material should be located inthe flow stream either at or immediately downstream of the noise source. This approach to abatementof aerodynamic noise is accommodated by in-line silencers. In-line silencers effectively dissipate thenoise within the fluid stream and attenuate the noise level transmitted to the solid boundaries. Wherehigh mass-flow rates or high-pressure ratios across the valve exist, in-line silencers are often the mostrealistic and economical approach to noise control. The use of absorption-type in-line silencers canprovide high levels of attenuation; however, economic considerations generally limit the insertionloss to ∼25 dB.

Noise that cannot be eliminated within the boundaries of the flow stream must be eliminated byexternal treatment or isolation. This approach to the abatement of control-valve noise includes theuse of heavy-walled piping, acoustical insulation of the exposed solid boundaries of the fluid stream,and the use of insulated boxes, rooms, and buildings to isolate the noise source.

In closed systems (not vented to the atmosphere) any noise produced in the process becomesairborne only by transmission through the solid boundaries that contain the flow stream. The soundfield in the contained flow stream forces the solid boundaries to vibrate, which in turn causes pressuredisturbances in the ambient atmosphere that are propagated as sound to the receiver. Because of therelative mass of most valve bodies, the primary surface of noise radiation to the atmosphere is thepiping adjacent to the valve. An understanding of the relative noise transmission loss as a functionof the physical properties of the solid boundaries of the flow stream is necessary in noise control forfluid transmission systems.

A detailed analysis of noise transmission loss is beyond the scope of this article. However, it shouldbe recognized that the spectrum of the noise radiated by the pipe has been shaped by the transmissionloss characteristic of the pipe and is not that of the noise field within the confined flow stream. For acomprehensive analysis of pipe transmission loss, see [1].

Acoustic insulation of the exposed solid boundaries of a fluid stream is an effective means of noiseabatement for localized areas, if installed correctly. Specific applications should be discussed withinsulation suppliers, since results can vary widely.

Path treatment such as the use of heavy-walled pipe or external acoustical insulation can be avery economical and effective technique for localized noise abatement. However, it should be pointedout that noise is propagated for long distances by means of fluid stream and that the effectiveness ofheavy-walled pipe or external insulation terminates where the treatment is terminated. Path treatmenteffects are summarized in Fig. 5.

A simple sound survey of a given area will establish compliance or noncompliance with thegoverning noise criterion, but not necessarily either identify the primary source of noise or quantifythe contribution of individual sources. Frequently piping systems are installed in environments wherethe background noise due to highly reflective surfaces and other sources of noise in the area makeit impossible to use a sound survey to measure the contribution a single source makes to the overallambient noise level.

A study of sound transmission loss through the walls of commercial piping indicated the feasi-bility of converting pipe-wall vibrations to sound levels. Further study resulted in a valid conversiontechnique as developed in [2].

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FIGURE 5 Comparison of path-treatment methods.

The vibration levels may be measured on the piping downstream of a control valve or other potentialnoise source. Sound pressure levels expected are then calculated based on the characteristics of thepiping. Judgment can then be made as to the relative contribution of each source to the total soundfield as measured with a microphone. The use of vibration measurements effectively isolates a sourcefrom its environment.

REFERENCES

1. “Prediction of Noise Generated by Hydrodynamic Flow,” IEC-534-8-4-1994, 1994.

2. “Control Valve Aerodynamic Noise Prediction Method,” IEC-534-8-3-1995, 1995.

3. Fagerlund, A. C., “Sound Transmission through a Cylindrical Pipe Wall,” J. Eng. Ind. Trans. ASME, vol. 103,pp. 355–360, 1981.

4. Fagerlund, A. C., “Conversion of Vibration Measurements to Sound Pressure Levels,” Publ. TM-33, FisherControls International, Inc. Marshalltown, Iowa.

SERVOMOTOR TECHNOLOGY INMOTION CONTROL SYSTEMS

by David A. Kaiser∗

INTRODUCTION

Motion control systems that employ servo motors generally have the topology shown in Fig. 1. Theinnermost servo loop is usually a torque or force loop consisting of a servo motor and a servo drive.

* Staff Engineer, Compumotor Division, Parker Hannifin Corporation, Rohnert Park, California.

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FIGURE 1 Basic servo loop topology.

The input command to this loop is either a torque or force signal and is designated by T∗ in Fig. 1.Bandwidths of these loops are of the order of 500 Hz and greater. Servo drives apply a switchedvoltage waveform to the servo motor’s winding and sense the resultant current. Generally, controllinga servo motor’s current directly controls the servo motor’s shaft torque, or in the case of linear motors,carriage force.

The servo motor may have a tachometer connected to its shaft if a velocity loop is closed. Theinput command to this loop is a desired velocity and is designated in Fig. 1 by ω∗. Bandwidths ofthese loops are of the order of 50 Hz and higher; however, they must not be higher than the torqueloop or instability is almost always ensured. In general, motion systems use a position device andvelocity information is derived. Typical positioning devices are encoders and resolvers.

Servo systems might also employ a position loop controller and some type of motion trajectorymechanism commanding a desired position, θ∗. Bandwidths of these loops are of the order of 5 Hzand greater. Again, position loop bandwidths in this topology must be lower than the velocity loopfor stable and predictable control.

TYPES OF SERVO MOTORS

Electric motors have been used in servo systems for over a century. The improvement in powerelectronic devices in the past 20 years has greatly increased the popularity of servo motors. In general,the type of servo motor used is highly dependent on the application.

Before selecting a servo motor technology, the user must consider the following:

� the environment in which the motor be placed (i.e., temperature, vibration, fluids, air borne particles),� the required velocity, torque, and power profiles including the duty cycles� the acceptable torque and/or velocity ripple� the available operating voltages� the need to comply with local or worldwide regulatory standards (i.e., CE, UL, etc.)� the acceptable audible noise

The most popular servo motor technologies are summarized in Table 1, along with their respectivepower levels and typical advantages and disadvantages. This table is by no means a comprehensivelist. Table 2 [1] defines the different types of servo applications along with specific examples. It isvery typical that more than one type of servo motor will be adequate for a given application. Factorssuch as cost, availability, and user’s experience will ultimately play the central role in selecting amotor technology.

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TABLE 1 General Characterstics of Motors Used in Servo Applications

Major Major TypicalType Advantage Disadvantage Power Range (hp)

DC brushed low initial cost maintenance <20DC brushless low cost torque ripple <20AC synchronous high performance high initial cost <30Induction low cost, simple possible problems >5

construction with full torque atzero speed

Reluctance low cost torque ripple, <20audible noise

TABLE 2 Generic Motion Applications Using Servo Motors

Application Definition Examples

Feed to length Applications in which a continuous web, BBQ grill-making machinestrip, or strand of material is being indexed film advanceto length, most often with pinch rolls or on-the-fly weldersome sort of gripping arrangement. Theindex stops and some process occurs(cutting, stamping, punching, labeling, etc.).

X/Y Point to point Applications that deal with parts handling optical scannermechanisms that sort, route, or divert the circuit board scanningflow of parts.

Metering/dispensing Applications in which controlling displacement telescope driveand/or velocity are required to meter or engine test standdispense a precise amount of material. capsule filling machine

Indexing/conveyor Applications in which a conveyor is being indexing tabledriven in a repetitive fashion to index parts rotary indexerinto or out of an auxiliary process. conveyor

Contouring Applications in which multiple axes of motion engraving machineare used to create a controlled path, (e.g., fluted-bit cutting machinelinear or circular interpolation).

Tool feed Applications in which motion control is used to surface grinding machinefeed a cutting or grinding tool to the proper transfer machinedepth. flute grinder

disk burnisher

Winding Controlling the process of winding material monofilament winderaround a spindle or some other object. capacitor winder

Following Applications that require the coordination of labeling machinemotion to be in conjunction with an external window blind gluingspeed or position sensor. moving positioning systems

Injection molding Applications in which raw material is fed by plastic injection moldinggravity from a hopper into a pressurechamber (die or mold). The mold is filledrapidly and considerable pressure is appliedto produce a molded product.

Flying cutoff Applications in which a web of material is cut rotating tube cuttingwhile the material is moving. Typically, thecutting device travels at an angle to the weband with a speed proportional to the web.

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General Characteristics and Comparison of Servo Motors

Regardless of the motor technology, all servo motors consist of a motor housing or stack, some typeof position or velocity feedback device, and numerous options including connectorization, gearheads,brakes, flanges and shafts. Figure 2 shows the anatomy of a rotary servo motor with a typical list ofthe many options available.

In addition to mechanical options, servo motor manufacturers will often offer an assortmentof different motor windings for a given frame size. This allows additional flexibility especially inmatching a particular servo drive with the motor.

Since the mid-1990s, linear servo motors have increased in popularity in applications requiringdirect linear motion. All of the subsequent motor technologies discussed here apply to linear as wellas rotary motors.

dc Brushed Motor. Arguably the most common servo motor is the dc brushed motor. The basicstructure is shown in Fig. 3. The motor is made up of three main parts: the stator, containing thepermanent magnets; the rotor, made up of coils of wire wound in slots in an iron core; and thecommutator, which consists of a brush assembly that maintains the proper orientation of magneticfields to produce maximum shaft torque for a given motor current.

Shafts· Centered tapped· Double flats· Harder shaft materials· Hollow shafts· Pressed on gears· Rear shaft extensions· Shaft pinning· Special flats· Special keyways· Special lengths· Special shaft diameters

Feedback· Absolute encoders· Incremental encoders· Resolvers· Tachometers

Brakes· 24VDC activation· 90VDC activation

Gearheads· Custom ratios· Customer specified flanges· Customer specified output shafts

Flanges· Customer Specified flanges· Face mount· NEMA flanges· Tapped mounting holes

Connectorization· Back cover cable and/or connectors· Customer specified cables and connectors· Flying leads· High flex cables· MS connectors· Pipe threads (NPT)· Right angle connectors· Special cable lengths

FIGURE 2 Anatomy of a rotary servo motor.

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CommutatorBrushes

Rotor Winding

Stator Magnets

FIGURE 3 dc motor (courtesy of Robbinsand Myers/Electrocraft).

These motors are generally the lowest in initial cost for a given powerrating. Their long-term cost, however, might be higher because they re-quire routine brush maintenance. If an application requires the motor toturn in both directions, a see-sawing action occurs on the brushes, and thewear-out time is even shorter. It therefore has the potential to be the min-imal cost solution in cases in which the required motion is unidirectionaland/or limited in duty.

In terms of power density, the dc brush motor suffers from three fun-damental design constraints limiting its overall usage. First, because thecurrent-carrying conductors are on the rotor, it is difficult to keep thewinding temperatures cool because of the high thermal impedance be-tween the rotor and the motor case. Second, the commutator assemblycan take up significant space in the motor housing. Third, because me-

chanical contact is made by means of the brushes, there is a maximum speed limitation that is lowerthan typical brushless motors.

A key advantage for the this type of servo motor is the ease of driving and controlling it. Thesimplest of power amplifiers can be used keeping the overall system cost low.

dc brushed motors have enjoyed the advances made in permanent magnet materials, especiallyneodymium-iron-boron (NdFeB). Although these magnets are more expensive than the historicalAlnico (Aluminum, Nickel and Cobalt) types, the improvement in power density is substantial.

Brushless dc Motor. In the brushless dc motor, permanent magnets are mounted on the rotor andthe wound iron core field is contained in the stator. This is depicted in the cross-sectional view andthe equivalent three-phase winding model shown in Fig. 4. Since the current-carrying coils are nowlocated in the housing, there is a short, efficient thermal path from the windings to the outside air.Cooling can further be improved by finning the outer casing as this is typicaly done.

The term “brushless dc” can mean one of two types of motors. Using the term “dc” in its descriptionrelates to the nature of the currents going to and from the motor. The first type of brushless dc motorhas only two wires and the motor current is “dc.” This type of motor is generally not used in servoapplications because of the power requirements needed for the commutation mechanism mountedinside. This type of motor is shown in Fig. 5 and is commonly used in small fans to cool electronicenclosures.

The far more common type of dc brushless motor is a three-phase design with concentric windings.This type of motor operates with constant levels of dc current; however, the commutation of the currentsis done externally in the servo drive, with Hall effect devices mounted inside the motor providingshaft-position information. This topology is shown in Fig. 6.

S

S N

N

StatorLam Teeth

Magnets

Windings

N

B

A

C

(a) (b)

FIGURE 4 (a) Typical cutaway view of brushless dc motor; (b) three-phase winding represen-tation.

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DC Motor Current

BrushlessDC Motor

CommutationLogic and

power stage

+

-

FIGURE 5 One type of brushless dc motor.

The typical motor voltage and current waveforms for this type ofbrushless motor are depicted in Fig. 7. The dotted lines represent a Hallstate. These states are used to commutate the motor. These motors arealso referred to as “trapezoidal back EMF motors,” “trap” or “six-state”motors because of the nature of their phase voltages and currents.

The chief disadvantage of this type of motor is that it torque pulsationscan be sensed as the Hall position boundaries are crossed. This will affectthe overall velocity smoothness of the servo, especially in low-inertia orhigh-performance applications.

ac Synchronous Motor. Arguably the most popular motor used in generalpurpose motion control today is the ac synchronous motor. It is alsocommonly referred to as a permanent magnet ac (PMAC) motor. Thecutaway view of this motor is similar to the brushless dc motor in Fig.

4 with one major difference: the stator winding pattern and the rotor magnet placement are donein such a way as to minimize the torque ripple. Mechanically, this is done by spatially distributingthe stator windings in conjunction with either skewing the rotor magnets and/or the stator slots.Consequently, the servo drive must also be designed such that it can produce smooth sinusoidal acvoltages and currents to fully exploit this motor. This is shown in Fig. 8. Theoretically, the smootherthe resultant flux wave, the lower the shaft torque ripple.

Some manufacturers in an effort to further reduce the torque ripple, resort to a “slotless” statordesign in an attempt to reduce the electrical “cogging” torque to zero.

Depending on the mechanical constraints, these motors are designed in both a flat pancake and along rectangular shape. In the <10 hp range, these motors by and large dominate the market.

As with the brushed dc motors, the ac synchronous motors today also predominately use high-energy magnets composed of NdFeB.

Induction Motor. The induction motor has long been the work horse motor in the world. Figure 9shows the common elements that make up the induction motor. This motor is typically used in threeprimary ways: line-start applications, constant velocity or spindle applications, and finally, servoapplications.

The line start applications include general purpose machinery, pumps, fans, compresors, andconveyors. Line start refers to the method of control. Since induction motors tend to draw largecurrents when energized directly off line, line starters are typically circuit breakers with a high currentinrush capability. These motors operate at fixed rotational frequencies slightly less than the appliedelectrical frequency/pole pair combination. This limits their use.

Spindle applications, in contrast, employ some type of motor inverter. In this case, the input linefrequency no longer directly affects the motor speed. Spindle applications include more advance fancontrol, pumping, and general purpose turning machines. A classic example of this mode of controlis a machining station that has one large spindle drive controlling the main turning axis or mandrel.

Phase A

Servo Controller

Servo Drive

Bru

shle

ss D

C M

otor

Phase B

Phase C

Torque

Command Hall 1

Hall 2

Hall 3

Position Feedback

CommutationLogic

PowerStage

PowerStage

Control

FIGURE 6 Typical topology of a brushless dc motor used in servo applications.

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30A

ugust19990:54

MH

OO

9-09.texM

H009-09.SG

MM

H009v3(1999/07/12)

0

+

-

0

+

-

0

+

-

0

+

-

0

+

-

0

+

-

Phase ACurrent

Phase A-NVoltage

Phase B-NVoltage

Phase C-NVoltage

Phase BCurrent

Phase CCurrent

60electricaldegrees

FIGURE 7 Typical phase voltages and currents of a brushless dc motor used in servo applications.

9.8

8

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0

+

-

Phase ACurrent

Phase BCurrent

Phase CCurrent

0

+

-

Phase ANVoltage

Phase BNVoltage

Phase CNVoltage

60electricaldegrees

60electricaldegrees

FIGURE 8 Typical phase voltages and currents of a PMAC synchronous motor.

-16.53

29.17

FanBearing

Shaft

RotorBearing

StatorHousing

Iron CoreElectricalConnections

FIGURE 9 Induction motor (courtesy of Danfoss Drives) [2].

All cutting operations on the work piece are then controlled by smaller, higher-performance positioningservos that are synchronized with the main spindle axis position.

Finally, the third mode of induction motor control is servo position control. It is also sometimescalled “vector” or “field orientation” control. These drives either require position sensing or sometype of internal complex motor model to achieve field orientation. Torque production in inductionmotors requires a “slip” between the rotating stator field and the rotor. This results in rotor losses thatare unavoidable and limit the ability to produce full rated torque at zero speed. In the fractional andintegral horsepower range, these motors usually lose out to the ac synchronous type. In the high-powerrange, the cost of the induction motor is significantly less than the permanent magnet ac motor, andwith the performance demands not as high, these motors are the preferred choice.

Variable Reluctance. The final motor considered for use in servo applications is the variable re-luctance motor. It is also commonly called either a “switched reluctance,” “brushless reluctance,”

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Phase 1

Phase 2

Phase 4

Phase 3

FIGURE 10 Cutway view of a four-phase reluctancemotor.

0

+

0

+

0

+

0

+

15mech.

degreesPhase 1Current

Phase 2Current

Phase 3Current

Phase 4Current

FIGURE 11 Current waveforms of a reluctancemotor.

or “commutated reluctance” motor. It origins can be traced to the early 1800s [3]. The fact that nopermanent magnets are used not only keeps the manufacturing cost low, but also allows for high-temperature operation. Figure 10 shows a cut away view of a four-phase design. The stator teeth androtor geometry are similar to permanent magnet stepper motors, but with the absence of magnets, therotational losses are much smaller. One drawback of this motor is the relatively high audible noise itmakes under normal operation.

The motor currents for a variable reluctance servo motor are generally switched on and off in asimple pattern, making the control relatively simple. Figure 11 shows what the ideal current waveformslook like.

The chief drawback in this servo motor design is the large torque ripple associated with the currentpulses. This, in conjunction with the audible noise, ultimately limits the variable reluctance motor’suse in high-performance servo applications.

GENERAL CONSIDERATIONS

There are numerous considerations to take into account when selecting a servo motor. Before a motortechnology is chosen, the desired speed, torque, and duty cycle have to be approximated, the availablemounting space estimated, and finally a target cost set. Refer to Table 1 in the previous section fora list of the common motors used for a given power range. After the motor technology is selected,there is a plethora of both electrical and mechanical details to consider. Fortunately, there are severalregulatory agencies servo motor manufacturers conform to that ease in the selection of the propermotor configuration.

Motor Parameters, Definitions, and Terminology

Depending on the motor technology, motor parameter definitions can vary. Careful attention must bepaid to the implied assumptions and conditions for each parameter, as well as its tolerance, units, andmethod of measurement. One area that is consistently overlooked is a servo motor’s thermal rating.Motor parameters are typically given at ambient temperatures of 25◦C or 40◦C, with the assumptionthat the motor is mounted to a specified heatsink.

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Historically, dc motors were the first widely accepted, general purpose servo motor, resulting ina strong tendency to put as many ac servo motor parameters as possible in their equivalent dc motorterms. This is a source of great confusion. For example, a servo motor’s stall torque or its torque ratingat zero speed is given by Eq. (1) and is straightforward for a dc motor. The current is dc and the unitsfor the torque constant, KT, are Newton meters/Amp (or Nm/A).

Tstall = Istall KT (1)

To calculate this for an ac motor, the units of KT might be as follows: Nm/A, Nm/A rms, Nm/Apeak. In making this calculation, we now must understand what kind of current is assumed for the acservo motor.

Servo motors are compared to one another by either their current, voltage, power, and/or theirspeed ratings. For a given motor technology and frame size, there is a fixed amount of torque that canbe produced before either magnetic saturation sets in or the motor losses are so high that the motorbasically overheats. Equation (2) relates a servo motor’s power rating to its torque and speed rating.

Prated = Trated nrated (2)

A typical motor data sheet will include this information along with a host of other details. Table 3is an example of the kind of information manufacturers will provide.

In situations in which the servo motor has an integral gearbox or brake, the manufacturer willprovide additional derating information if necessary. Care must be taken to fully understand thethermal interface between the motor and its mounting to correctly apply any derating informationprovided.

Name-Plate Ratings

Name-plate information varies from manufacturer to manufacturer, although the basic information ofmake and model number is always provided. It is also typical that the dc winding resistance is givenso that a field technician can quickly determine if the motor has a shorted phase winding. Figure 12shows what a motor name plate might look like.

BRUSHLESS SERVO MOTORPARAMETERS AT 25OC AMB.

ICONT.(RMS AMPS)

RATEDVOLTAGE

RATED PWR.(KW)

RES L/L@25O C

RATED SPD(RPM)

MODEL NO.

TCONT.(Nm)

IPVOLTAGE

CLASS(O C)

KB(V/KRPM)

INERTIA(g cm2)

SERIAL NO.

Parker Hannifin Corporation5500 Business Park DriveRohnert Park, CA 94928Made in the U.S.A.

FIGURE 12 Typical servo motor name plate.

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TABLE 3 Typical Servo Motor Parameters Provided by the Manufacturer

Typical Motor Data Sheet Parameters

1. Acceleration at rated torque2. Bearing class, internal/external3. Bearing grease4. Constant(s): torque, voltage, motor, electrical time, mechanical time, thermal5. Current: rated, peak6. dc resistance terminal (line-line)7. Damping8. Dielectric strength (winding to frame)9. Inductance: terminal (line-line)

10. Insulation class11. IP classification12. Maximum winding temperature13. Mechanical dimensions with tolerances14. Number of motor poles15. Output power: rated16. Rotor inertia17. Shaft seal pressure18. Shaft: radial play (front to back), radial loading, material, magnet type19. Speed: rated, maximum20. Speed vs. torque curves21. Stator phase sequence22. Thermal impedance23. Thermostat reset temperature24. Thermostat trip temperature25. Torque(s): continuous stall at XX ◦C ambient, peak, static friction, % ripple, derating curves26. Vendor/supplier27. Voltage: rated, max28. Weight29. Winding capacitance to frame30. Wiring diagrams of the motor and any of its optional feedback and brake devices

OptionsBrakes: release time, holding torque, operating voltageGearbox: ratios, deratingsHall devicesIncremental encoder: manufacturer, supply voltage, resolution, accuracyKeywayResolver: manufacturer, electrical specs, accuracy, model no.Tachometer: manufacturer, electrical specs, model no.

NEMA (National Electrical Manufacturers Association) specifies (standard MG 7 [4]) that theminimum standard name-plate information for servo motors 3 in. in diameter or greater contain atleast the following information:

1. Manufacturer’s name

2. Manufacturer’s model number (includes motor type, i.e., ac, dc, and so forth)

3. Manufacturer’s serial number or date code

4. Maximum continuous stall torque at either 25◦C or 40◦C ambient

5. Maximum continuous rms current at either 25◦C or 40◦C ambient

6. Maximum continuous output power at either 25◦C or 40◦C ambient

7. Maximum allowable intermittent voltage (brush motors only)

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8. Maximum allowable speed

9. Phase-phase resistance at 25◦C

Note that the specifications are given at a certain operating temperature. This is important, as eventhe motor’s resistance can increase significantly at elevated operating temperatures.

ELECTRICAL CONSIDERATIONS

There are numerous electrical considerations to take into account when selecting a servo motor. Theavailable supply voltage is usually considered first. Invariably the servo motor will be connected to aservo drive, and therefore the supply voltage becomes more of a drive requirement than a motor. Inorder to minimize errors in the drive motor system interface, drive manufacturers either manufactureservo motors or provide guidelines on how to connect servo motors to them. This becomes extremelyimportant in the cases in which regulatory acceptance of the entire system is required by the customer.

Another area of equal importance is understanding the acceptable power loss in a servo motor.Electrical parameters such as motor inductance and resistance play an important role in determiningthese losses, as well as the servo drive’s current control methodology.

Regulatory Considerations

It is always important to determine where the servo motor will ultimately be used. This will affectthe need for regulatory approval. The common regulatory agencies that affect servo motors are ULand CE.

UL. The initials UL refer to Underwriters Laboratories, Inc., a nonprofit agency that has set fortha series of safety standards covering a wide range of products from consumer goods to industrialequipment. The primary goal of UL testing is to certify that a product will not be a source of asustaining fire under any condition of use or misuse.

The term UL Approved, however is a misnomer. The most common designation, UL Listed, can befound on virtually any consumer good. This designation means that a stand-alone product complieswith a particular UL safety standard appropriate for that product’s intended use. A second designationis UL Recognized Component. This designation covers components intended to be used as part of anend product or system. Since servo motors are used as part of an end product or system, UL RecognizedComponent is the appropriate designation. Depending on the motor and the application, the followinglist of UL standards might apply: UL519, UL547, UL674, and UL1004 (see [5]).

In addition, products that are certified by UL to meet the Canadian National Standards and Codesdisplay the UL Mark for Canada, known as cUR.

CE. Since the 1990s, there has been a strong push to buy and sell freely in the common Europeanmarkets. Although historically, the U.S. suppliers have met the regulatory requirements for a particularcountry they were doing business in, there is now a strong push to consolidate all the European countriesinto one set of “directives” mandated by the European Union laws. What makes this difficult is thatservo motors are always connected to something. The whole system consisting of the motor and drivenow must meet the regulatory requirements. This generally requires special attention paid to all wireconnections and particularly shielding.

There are two general directives spelled out in the CE mark. The first directive is commonly referredto as the low voltage directive (LVD). It refers to the product’s ability to withstand high-voltage surgeswithout degrading. Another part of the low-voltage directive (EC Directive 73/23/EEC) requires a“technical file” be kept on servo motors produced by a given manufacturer. This file contains a wealthof information about the safety testing of the motor as well as information about manufacturing qualitycontrol. The second directive is referred to as the electromagnetic compatibility, or EMC, directive.

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TABLE 4 Overall Required Test to claim CE Compliance

EC standard Description

EN 55022 (1987) radiated and conducted electromagnetic emissionsEN 61000-4-2 electrostatic discharge immunityEN 50140 (1994) radiated electromagnetic field immunityEN 61000-4-4 (1995) electrical fast transient burst immunityEN 61000-4-8 (1993-06) power frequency magnetic field immunityENV 50141 (1993-08) radio frequency common mode immunityTBD (Mains harmonic content) Power line disturbance

This directive is broken down into two additional classes: emissions and susceptibility. These classesrefer to the product’s ability to limit the amount of electrical noise it emits and to reject external noisefrom affecting its operation. Table 4 summarizes the required tests to become CE compliant.

Speed Versus Torque Curves

Servo motors operate with full torque at zero speed. This torque is sometimes referred to as stall orcontinuous torque (Tcontinuous). Associated with this value is the peak stall torque (Tmaximum). Dependingon the type of motor and the expected life, this value can be 2–5 times the continuous value. Otheruseful values from the speed versus torque curve include rated and no-load speeds and rated torque.These points are all illustrated in a typical speed versus torque curve shown in Fig. 13.

Depending on the servo motor technology, feedback limitations, and the assumed servo driveconnected to the motor, the actual speed torque curves from a servo motor manufacturer might lookvery different. It is therefore critical that both the drive and motor be considered together when thespeed versus torque curve is analyzed.

To determine if the amount of time in the intermittent region is acceptable, the rms, or root-mean-square, torque must be calculated for a motion profile and be within the continuous torque region atthe relevant speeds. To do that, first the required motion profile is used to determine the torque profile,and then that profile is used to determine the rms torque. Finally, a safety factor is included to provide

Torque

Tcontinuous Trated

Tmaximum

Speednrated

nno load

Continuous Region

Intermittent Region

FIGURE 13 Typical speed vs. torque curve for a servo motor.

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t1 t2 t3

.1 .2 .3 .4 .5 .6

t4

Velocity,RPS

20

Time, Seconds

.1 .2 .3 .4 .5 .6Torque,ft.lbs.

5

1

Time, Seconds

-3

FIGURE 14 Typical servo motion profile.

additional headroom in sizing. The equation for rms torque is given in Eq. (3).

torquerms =

√√√√√√√n∑

i=1torque2

i ti

n∑i=1

ti

(3)

An example of what a typical profile might look like is illustrated in Fig. 14.

Example. A required motion profile with its corresponding torque profile is given in Fig. 14. Cal-culate the rms torque.

The rms torque is calculated as follows:

torquerms =√

torque21 t1 + torque2

2 t2 + torque23 t3 + torque2

4 t4

t1 + t2 + t3 + t4

or

torquerms =√

52 × 0.1 + 12 × 0.1 + (−3)2 × 0.1 + 02 × 2

0.1 + 0.1 + 0.1 + 0.2= 2.6 ft. lbs.

It is also typical for servo motor vendors to provide a peak torque or current derating curve. Anexample of one is shown in Fig. 15.

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0.01 0.1 1 10 1000.001

% o

f con

tinuo

us o

utpu

t cur

rent

"on" time, minutes

duty cycle = 10%

duty cycle = 20%

duty cycle = 50%

duty cycle = 80%

100%

200%

300%

400%

FIGURE 15 Typical servo motor derating curve.

In this case, the percent over continuous torque is determined and the amount of time that is neededis used to find the allowable amount of “on” time. As the time gets longer, the amount of intermittenttorque approached the continuous value, as it should.

Torque ripple is another figure of merit used in comparing one servo type with another. Ideally,the exact shaft torque should not be affected by the shaft location. Motor characteristics, feedbackresolution, and servo drive current regulation all affect the motor’s torque ripple. In generally, Eq. (4)is used in comparing motors. Typical values are under 5%.

torque ripple = torque (peak − peak)

torque(continuous)(4)

Finally, it is not uncommon that a servo motor’s no-load speed is limited mechanically. Rotationalspeeds above 10,000 rpm invariably require special balancing, manufacturing, and, in some cases,special position feedback devices.

Thermal Ratings—Insulation Class

In general, the insulation class of the windings will dictate the maximum allowable winding tem-perature. In the case of permanent magnet machines, the magnets may limit the maximum operatingtemperature before the winding insulation does. Typical winding classes are shown in Fig. 16. Auseful rule of thumb is that motor insulation life is reduced by approximately a factor of 2 for every10◦ rise over the rated temperature. Altitude also plays a role in determining these numbers, as furtherderatings are needed for altitudes above 3300 ft (1000 m).

Servo motor manufacturers will generally put thermal cut-out switches inside the motor windingsto protect the motor from permanent thermal damage. These switches are sensed by the servo driveconnected to it and rely upon the drive to cease operation of the motor. The cut out or maximumallowable motor temperatures are summarized in Table 5. These maximum temperatures correspondto approximately 30,000 hs of operation. For the most part, servo motor windings are generallyclass H.

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TABLE 5 Insulation Class Rating

Max. Allow.Class Winding Temp. (◦C)

A 105B 130F 155H 180

100,000

70,000

50,000

30,000

20,000

10,000

7,000

5,000

3,000

2,000

1,000

700

500

300

200

100

70

50

30

20

10

7

5

3

280 100 120 140 160 180 200 220 240 260 280 300

CLASS C

AV

ER

AG

E L

IFE

- H

OU

RS

AGING TEMPERATURE - DEGREES C

CLASS A

CLASS B

CLASS F

CLASS H

CLASS 220C

CLASS 250C

FIGURE 16 Life temperature lines of varnished twist-test specimens as determined by AIEE57 test procedure. (Anaconda Wire and Cable Company.)

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MECHANICAL CONSIDERATIONS

Mounting

Unfortunately, there is no real dimensional consistency from servo motor manufacturers; however,some vendors do follow the NEMA dimensional specification (MG7-1993 [4]) for the larger framesize motors. As of 1998, all dimensions are metric, with the English dimensions under consideration.Figure 17 and Table 6 show NEMA’s servo motor dimensional specifications. Note that there aretwo standard NEMA mountings: the type “C” face mount with tapped mounting holes, and thetype “D” flange mount with free mounting holes. It is not uncommon to specify only the mountingface. This gives the servo motor manufacturer the ability to adjust the other motor dimensions whilemaintaining the mounting. The chief advantage of following the NEMA guidelines is the compatibilityof gearboxes and other transmission devices.

FIGURE 17 Dimensions for servo motors (NEMA MG7-1993 2.4).

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30A

ugust19990:54

MH

OO

9-09.texM

H009-09.SG

MM

H009v3(1999/07/12)

TABLE 6 Metric Mounting Dimensions for Servo Motors (NEMA MG7-1993 Section 2.4.1.3)

AJ AK AK TolcAM(mm) U Tolc (mm)

BC Pmax/BDmax BBmax BF BF Tolc XD (mm) S(mm)

Flangea Numberb (mm) (mm) (mm) (Pri) (Sec) (Pri) (Sec) (mm) (mm) (mm) (mm) Thread (Pri) (Sec) (Pri) (Sec)

55CM 55DM 55 40 +0.011 20 30 9 14 +0.007 +0.008 0 70 2.5 5.8 +0.300 M5 × 0.8 15 20 3×3 5× 5−0.005 −0.002 −0.003 −0

65CM 65DM 65 50 +0.011 20 30 9 14 +0.007 +0.008 0 80 2.5 5.8 +0.300 M5 × 0.8 15 20 3×3 5 × 5−0.005 −0.002 −0.003 −0

75CM 75DM 75 60 +0.012 23 40 11 16 +0.008 +0.008 0 91 2.5 5.8 +0.300 M5 × 0.8 18 30 4×4 5 × 5−0.007 −0.003 −0.003

85CM 85DM 85 70 +0.012 30 40 14 19 +0.008 +0.009 0 105 2.5 7 +0.360 M6 × 1.0 20 30 5×5 6 × 6−0.007 −0.003 −0.004 −0

100CM 100DM 100 80 +0.012 30 40 14 19 +0.008 +0.009 0 120 3 7 +0.360 M6×1.0 20 30 5×5 6 × 6−0.007 −0.003 −0.004 −0

115CM 115DM 115 95 +0.013 40 50 19 24 +0.009 +0.009 0 140 3 10 +0.360 M8×1.25 30 40 6×6 8 × 7−0.009 −0.004 −0.004 −0

130CM 130DM 130 110 +0.013 40 50 19 24 +0.009 +0.009 0 160 3.5 10 +0.3.60 M8×1.2 30 40 6×6 8 × 7−0.009 −0.004 −0.004 −0

145CM 145DM 145 110 +0.013 40 50 19 28 +0.009 +0.009 0 165 3.5 10 +0.360 M8×1.2 30 40 6×6 8 × 7−0.009 −0.004 −0.004 −0

165CM 165DM 165 130 +0.014 50 50 24 28 +0.009 +0.009 0 200 3.5 12 +0.430 M10×1.5 40 40 8×7 8 × 7−0.011 −0.004 −0

200CM 200DM 200 114.3 +0.0 79 – 35 – +0.01 − 0 235 4 13.5 +0.430 M12×1.75 70 10×8 –−0.25 −0.00 −0

215CM 215DM 215 180 +0.014 60 – 32 – +0.018 − 0 250 4 15 +0.430 M12×1.75 50 – 10×8 –−0.011 +0.002

265CM 265DM 265 230 +0.016 85 – 48 – +0.018 − 0 300 4 15 +0.430 M12×1.75 60 – 14×9 –−0.013 +0.002

300CM 300DM 300 250 +0.016 85 – 48 – +0.018 − 0 350 5 19 +0.520 M16×2.0 60 – 14×9 –−0.013 +0.002 −0

350CM 350DM 350 300 +0.016 110 – 55 – +0.030 − 0 400 5 19 +0.520 M16×2.0 90 – 16×10 –−0.016 +0.011 −0

U(mm)

(Pri) (Sec)

0.004−

(mm)

a The suffix CM denotes C face-mounting motors with tapped mounting holes.

−0

−0

−0

–

b The suffix DM denotes D flange-mounting motors with free mounting holes.c The reference for the tolerances is ISO Recommendation 286, S.I. Metric System of Limits and Fits. If a stepper motor is designed to be a direct substitute for servo motors in servo motor applications, the stepper motor dimensions should comply with the dimensions specified for servo motors.

9.9

9

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FIGURE 18 Example of NEMA 100DM.

An example of an ac synchronous motor that conforms to the NEMA mounting standards is shownin Fig. 18. The length of the motor is generally not important provided that the user has space for it.

Standard Flat SquareKeyway

WoodruffKeyway

FIGURE 19 Examples of different shaft options.

Modifications to motor shafts are another mechanical considera-tion to consider. The default standard is not to make any shaft mod-ifications. At one time keyways were used, but with small, high-performance servo motors, they have two drawbacks. First, any sharpcuts made into a motor’s shaft will be a point of structural weakness;second, since keyways are generally made from a softer material, thispotentially causes a rotor imbalance. Figure 19 illustrates the commonshafts options.

Motor connections can vary from flying leads or pig tails to fullmilitary-style waterproof connectors. Figure 20 illustrates some ofthe more typical arrangements.

One of the more common connections made to servo motors is a gearbox. Planetary styles aregenerally preferred because of both high speed constraints and balancing considerations. Some servomotor manufacturers will offer the gearbox as an integral part of the servo motor. This is shown inFig. 21.

After all the options are selected for the servo motor, the general classification of the motor mustbe specified.

FIGURE 20 Examples of different connector options.

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FIGURE 21 Example of an in-line gearbox attached to a servo motor.

I P 6 5

Protection against Solid Objects

IP Protection Level0 = None1 = Objects greater than 50 mm. (hands)2 = Objects greater than 12 mm. (fingers)3 = Objects greater than 2.5 mm. (wires, basic tools)4 = Objects greater than 1 mm. (small wires, tools)5 = Limited dust protected machine6 = Totally protected against dust

Protection against Liquids

IP Protection Level0 = None1 = Protected from dripping water2 = Protected from dripping water with motor tiled at 15 degrees3 = Protected from spraying water4 = Protected from water splashing from all directions5 = Protected from water from low pressure jets from any direction6 = Protected from strong jets of water any direction7 = Protected against brief full immersion up to 1meter8 = Protected against full continuous immersion as specified by manufacture

FIGURE 22 IP classification.

IP Classification

Servo motors use a standard classification system to distinguish different working environments. TheIP classification comes from the IEC (International Electrotechnical Commission) standard (34-5)[14]. This rating system uses two numbers after the letters IP that relate to the degree of protectionagainst solids and liquids. Figure 22 illustrates the different classifications.

In general, servo motors used for industrial applications are rated at IP 65. For metals-cutting andother harsher environments, IP67 is typical.

Couplers

The ideal coupler joining a servo motor and load would be completely rigid, allowing the torqueproduced in the servo motor to be transmitted directly to the load. Rigid couplers require perfectalignment, however, which can be difficult or even impossible to achieve. In real systems, somemisalignment is inevitable, partly because of tolerance buildups of components.

The three misalignment conditions are:

� end float: a change in the relative distance between the ends of two shafts� angular misalignment: the center lines of two shafts intersect at an angle other than 0◦

� parallel misalignment: the offset of two mating shaft center lines, although the center linesremain parallel to each other

These conditions can exist in any combination. They are illustrated Fig. 23.

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TABLE 7 Coupler Comparison

Rel. Misalign.Type Rigidity Allowance Rel. Cost Rel. Inertia

Beam low high low highBellows medium medium medium lowMetal Disk high low high medium

Aligned

End Float

Angular misalignment

Parallel Misalignment

Combined Parallel & Angular Misalignment

FIGURE 23 Coupler misalignments.

There are several types of couplers to consider. In addition to coupler dynamics, the coupler’sinertia may also have to be considered. Table 7 summaries the characteristics of the three mostcommon types [8].

Beam or helical style couplers are low cost and they handle large misalignments, but they canbe extremely hard to tune for high-performance servo systems. Metal disk couplers offer the besttorsional rigidity, but they allow very little angular alignment. Bellows couplers offer very goodtorsional rigidity, relatively low inertia, and some tolerance to misalignment.

Bearings

The most common type of bearing used in servo motors is the radial ball bearing. One useful classi-fication for bearing tolerances is stated by the Annular Bearing Engineering Committee (ABEC) orAnti-Friction Bearing Manufacturers Association (AFBMA). Different classes or grades numberingodd from one to nine indicate increasing levels of tightness or tolerance. Grades 1 and 3 are the mostcommon for general purpose servo motors.

Servo motor manufacturers make an assumption on the life expectancy of the bearings used in themotors they produce. Factors such as temperature, bearing ball speed, and axial and radial loadingplay an important part in estimating the useful life of a bearing.

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TABLE 8 Relationship between Reliability (r) and a1

Reliability (r) Life Adjustment Factor(%) Ln for Reliability (a1)

90 L10(rated life) 195 L5 0.6296 L4 0.5397 L3 0.4498 L2 0.3399 L1 0.21

The most common method to determine the bearings reliability is to use a fatigue life equation(see [9]). An example of this is given in Eq. (5).

Ln = 16667a1a2asafety

N

(CB

P

)3

(5)

with the following definitions:

Ln = number of hours for a given reliability rate “r” (see Table 8)a1 = life adjustment factor based on reliabilitya2 = life adjustment factor for material (∼1–3, depending on the steel)

asafety = optional servo motor manufacturer’s safety factor (∼0.6–0.7)N = rotor speed (rpm)

CB = basic dynamic load rating from manufacturer (lb)P = equivalent radial load on bearings (lb)

To decide on the value for a1, a reliability rate must be determined. The most common one for servomotors is the L10 (n = 10) rating. This means that 90% of the bearings will meet the number of hourscalculated in Eq. (5). Table 8 illustrates the other values of a1 for their respective reliability rates.

Since there are generally two bearings in a servo motor, the equivalent radial load P must becalculated. P is a function of the actual radial load, FR, the distance the load is applied, x, and othergeometric dimensions of the motor. Figure 24 shows a cutaway view of a typical bearing arrangementin a servo motor.

B x

RearBearing

FrontBearingMotor

Shaft

RR

RF

FR

A

Motor Mounting Face

FIGURE 24 Bearing placement in servo motors.

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The reaction forces are found by summing the moments around each bearing. For the front bearing:

RF = FR

(A + x + B

B

)(6)

And for the rear bearing we have

RR = FR

(x + A

B

)(7)

The front bearings generally have significantly more loading than the rear ones and therefore are usedto determine the overall life of the motor.

P ≈ RF (8)

If axial loads are applied, they are compensated for by modifying the effective radial load. Thiseffective radial load is generally calculated by summing the real radial load with two times the axialload.

Equation (5) is usually shown graphically for a given reliability rate n. For a given reliability rate,the acceptable load as a function of rotor speed is given for radial load at a distance x. These curvesare generated for different values of distance. Figure 25 is an example of the nature of the curve.

For a fixed radial loading distance x, as the loading is increased, the corresponding speed must belowered to maintain the same number of reliable hours.

Lubrication

There are many different types of lubrication for servo motors. In selecting the lubrication, it isextremely important to understand the type of environment in which the servo motor will be placed.

1 10 100 1000 10000

0

500

1000

1500

2000

2500

Speed, rpm

Load

, lbs

.

FIGURE 25 Typical radial loading curve for a fixed radial load of x.

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TABLE 9 Lubricating Grease Components

Lubricating = Fluid+ Thickening Agents +Special Ingredients

mineral oils Soaps oxidation inhibitorsesters lithium rust inhibitorsorganic esters sodium VI improverglycols barium tackinesssilicones calcium perfumes

strontium dyesmetal deactivator

Nonsoaps (inorganic)microgel (clay)carbon blacksilica gel

Nonsoaps (organic)urea compoundsterepthlamateorganic dyes

Grease

In general, lubrication grease can be described by the three components that make it up: fluid, thickener,and special ingredients. Table 9 illustrates the common ingredients for these three components.

Seals

Servo motors cover such a wide range of speeds and operating environments that no one seal designis preferable in all cases. The two basic types of dynamic seals are the face seal and the lip seal.

Face seals or axial mechanical seals are mounted between the rotor and stator housing. A springmechanism provides the required axial pressure on the seal. These types of seals can be very lossy,and they are therefore limited in use.

FIGURE 26 Measuring points for a vibration test.

Lip seals create a radial barrier between the rotating shaft and the motor housing. This type ofseal looks like a U-shape O ring with a garter spring encompassing the entire shaft. Lip seals are self-adjusting but will wear out prematurely if operating at high speeds. Depending on the environment,many different types of materials are used. The most popular general purpose seals for servo motors

are made with either Teflon or Viton [11]. Lip sealsare by far the most common for the general purposeservo motor market.

Vibration

Vibrations in servo motors are caused by either anunbalance in the shaft, misalign bearings, or slightdeformations in the housing of the motor caused byan interaction between the rotor’s magnetic field andthe stator’s. Despite the fact that the rotors of servomotors can be mechanically balanced, it is possiblefor other secondary effects to dominate.

One standard that servo motor manufacturers ref-erence is ISO 2373 [10]. This standard provides aset of guidelines for maximum allowable vibrationvelocities as a function of shaft height. Figure 26 il-lustrates the points of vibration measurements, and

Table 10 summarizes the specifications for the different quality grades.

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TABLE 10 Recommended Limits of Vibration Severity

Maximum rms Valuesa of the Vibration Velocity

Speedfor the Shaft Height, H (in mm)

n 80 < H < 132 132 < H < 225 225 < H < 400

Quality Grade (rpm) mm/s in/s mm/s in/s mm/s in/s

N 600 < n < 3600 1.8 0.071 2.8 0.110 4.5 0.177(Normal)

R 600 < n < 1800 0.71 0.028 0.12 0.044 1.8 0.071(Reduce) 1800 < n < 3600 1.12 0.044 1.8 0.071 2.8 0.110

S 600 < n < 1800 0.45 0.018 0.71 0.028 1.12 0.044(Special) 1800 < n < 3600 0.71 0.028 1.12 0.044 1.8 0.071

a A single set of values, e.g., those applicable to the 132- to 225-mm shaft height, may be used if shownby experience to be required.

REFERENCES

1. Parker Hannifin Engineering Technical Reference, Parker Hannifin Corporation, Compumotor Division, 1997.

2. AC Technical Reference, Danfoss Drives.

3. Miller, T. J. E., Brushless Permanent-Magnet and Reluctance Motor Drives, Oxford University Press, NewYork, 1989.

4. NEMA Standards Publication MG 7-1993 Motion/Position Control Motors and Controls: Covers all ro-tational and linear electric servo and stepper motors and their power requirements, feedback devices, andcontrols intended for use in a motion/position control system that provides precise positioning, speed control,torque control, or in any combination. Adopted by the U.S. Department of Defense.

5. Underwriters Laboratories, Inc.: UL 508C Power Conversion Equipment, UL519 Impedance Protected Mo-tors, UL547 Thermal Protectors for Motors, UL674 Electric Motors and Generator sfor Use in Division 1Hazardous Locations; UL1004 Electric Motors.

6. IEC 60034-1 (1996), Rotating electrical machines, Parts 1–12, 1996.

7. IEC Standard Publication 34-5, 2nd ed., Bureau Central de la Commission Electrotechnique Interntionale,Geneva, Switzerland, 1981.

8. Kaiser, D. A., J. Morris, and C. Durkin, “Dynamic Modeling of Couplers for Drivescrew Applications,” PCIM,June 1997.

9. Torrington/FAFNIR/Kilian Engineering Reference Catalog 100-295-75M.

10. Teflon is a registered trademark of DuPont; Viton is a registered trademark of DuPont Dow Elastomers.

11. ISO 2373, 2nd ed., 1987-06-01 Mechanical Vibration of Certain Rotating Electrical Machinery with ShaftHeights between 80 and 400 mm. Measurement and Evaluation of the Vibration Severity, Switzerland, 1987.

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SOLID-STATE VARIABLESPEED DRIVES

by Richard H. Osman1

INTRODUCTION

The global power electronics industry continues the rapid pace of solid-state drive development. Overthe years, many drive circuits have become virtually obsolete, and new ones have been introduced.The user is still confronted with a wide variety of drive types that are suitable for virtually every kindof electrical machine, from the subfractional to the multimegawatt rating. The integral horsepowerstandard polyphase ac motor and dc motor are major consumers of electric power in industrial appli-cations, and they represent the opportunity for substantial improvement in the user’s process. Bothnew installations and the retrofit of existing machines are possible.

Despite the diversity of power circuits, there are two common properties of these drives:

1. All of them accept commonly available ac input power of fixed voltage and frequency and, throughswitching power conversion, create an output of suitable characteristics to operate a particular typeof electric machine; that is, they are machine specific.

2. All of them are based on solid-state switching devices. The development of new devices is themost important driver of the technology.

Figure 1 shows the basic structure of most common ac drives. There is an input conversion circuit,which converts the utility power into dc, and then an output inversion stage, which changes the dcback into variable ac. This type of drive is commonly called a “variable frequency drive,” or VFD.(For dc drives, the motor is in the dc link, or a chopper is used in place of the inverter.)

REASONS FOR USING A VARIABLE SPEED DRIVE

There are a number of reasons to use a variable speed drive:

1. Energy savings where variable flow control is required. In any situation in which flow is controlledby a throttling device (valve or damper), there is the potential for energy savings by removing thethrottle and slowing the fan or pump to regulate flow.

2. Optimizing the performance of rotating equipment; e.g., SAG mills, compressors, conveyors,pumps, and fans.

3. Elimination of belts and gears or other power transmission devices by matching the base speed ofthe motor to the driven load.

4. Automation of process control by using the VFD as the final control element, leading to moreefficient part-load operation.

5. Reduction of the rating and cost of the electrical distribution system by eliminating motor startinginrush.

6. Extending the life of motors, bearings, seals, liners, and belts.

1 Vice President of Technology, Robicon, 100 Sagamore Hill Road, Pittsburgh, Pennsylvania 15239.

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FIGURE 1 Basic circuit arrangement of modern commercial ac drives.

7. Reducing noise and environmental impact—electric drives are clean, nonpolluting, quiet, efficient,and easy to repair.

SEMICONDUCTOR SWITCHING DEVICES

Even though many of the basic power conversion principles were developed in the thirties, when thecircuits were constructed with mercury arc rectifiers, it was not until the invention of the thyristor in1957 that variable speed drives became truly practical. Figure 2 shows a comparison of the propertiesof devices commonly in use today.

The thyristor (SCR) is a four-layer semiconductor device that has many of the properties of an idealswitch. It has low leakage current in the off state, a small voltage drop in the on state, and takes only asmall signal to initiate conduction (power gains of over 106 are common). When applied properly, thethyristor will last indefinitely. After its introduction, the current and voltage ratings increased rapidly.Today it has substantially higher power capability than any other solid-state device but no longerdominates power conversion in the medium- and higher-power ranges. The major drawback of thethyristor is that it cannot be turned off by a gate signal, but the anode current must be interrupted inorder for it to regain the blocking state. The inconvenience of having to commutate the thyristor in itsanode circuit at a very high energy level has encouraged the development of other related devices aspower switches.

Transistors predate thyristors, but their use as high-power switches was relatively restricted (com-pared with thyristors) until the ratings reached 50 A and 1,000 V in the same device, during theearly 1980s. These devices are three-layer semiconductors that exhibit linear behavior but are usedonly in saturation or cutoff. In order to reduce the base drive requirements, most transistors used in

FIGURE 2 Comparison table of modern semiconductor switching devices.

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variable speed drives are Darlington types. Even so, they have higher conduction losses and greaterdrive power requirements than thyristors. Nevertheless, because they can be turned on or off quicklyvia base signals, transistors quickly displaced thyristors in lower drive ratings, and they were oncewidely used in pulse-width-modulated inverters. They in turn were displaced by insulated gate bipolartransistors (IGBTs) in the late 1980s. The IGBT is a combination of a power bipolar transistor and aMOSFET that combines the best properties of both devices. A most attractive feature is the very highinput impedance that permits them to be driven directly from lower-power logic sources. Their powerhandling capability has increased dramatically, and they are now practical alternatives to thyristors,GTOs, and IGCTs in the largest drive ratings.

It has long been possible to modify thyristors to permit them to be turned off by a negative gatesignal. These devices are four-layer types and are called gate-turn-off thyristors, or simply GTOs.These devices have been around since at least 1965, but only in the mid-1980s did their ratings increaseto high power levels. Present-day GTOs have about the same forward drop as a Darlington transistor(twice that of a conventional thyristor). GTOs require a much more powerful gate drive, particularlyfor turn off, but the lack of external commutation circuit requirements gives them an advantage overthyristors. GTOs are available at much higher voltage and current ratings than power transistors.Unlike transistors, once a GTO has been turned on or off with a gate pulse, it is not necessary tocontinue the gate signal because of the internal positive feedback mechanism inherent in four-layerdevices. Unfortunately, high cost and very large switching losses restricted the use of GTOs to onlythose applications in which space and weight were at a premium. In 1997 the IGCT (integrated gatecontrolled thyristor) was introduced. This is identical in construction to the GTO, but a new methodof turn off and special metallurgy has resulted in a device considerably better than the GTO in forwarddrop and switching losses.

Today the thyristor, IGBT, GTO, and IGCT form the technological base on which the solid-statevariable speed drive industry rests today. There are other device technologies and enhancements invarious stages of development that may or may not become significant, depending on their cost andavailability in large current (>50 A) and high voltage (1,000 V) ratings. These include: (1) trenchgate construction for IGBTs, (2) silicon carbide semiconductors, and (3) variants of the four-layerswitch, such as the MTO (MOS turn-off thyristor) and MCT (MOS controlled thyristor). We shouldexpect new switches to come along and significantly improve on the devices currently in use. Whilethe type of semiconductor device is not necessarily the most important issue to a user, in general thenewer devices provide a better drive performance.

DRIVE CONTROL TECHNOLOGY

Parallel to the development of power switching devices, there have been very significant advances inhardware and software for controlling variable speed drives. These controls are a mixture of analogand digital signal processing. The advent of integrated circuit operational amplifiers and integratedcircuit logic families made possible dramatic reductions in the size and cost of the drive control, whilepermitting more sophisticated and complex control algorithms without a reliability penalty. Thesedevelopments occurred during the 1965–1975 period. Further consolidation of the control circuitsoccurred after that as large-scale integrated circuits (LSI) became available. In fact, the pulse-widthmodulation control technique was not practical until the appearance of LSI circuits because of theimmense amount of combinatorial logic required.

Clearly, the most significant advance in drive control has been the introduction of microprocessorsinto drive control circuits. The introduction of cheap and powerful microprocessors continues toexpand the capability of drive controls. A modern drive should have most of these features. Theperformance enhancements include:

1. More elaborate and detailed diagnostics, resulting from the ability to store data relating to driveinternal variables, such as current, speed, firing angle, and so on; the ability to signal to the user ifa component has failed.

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2. The ability to communicate both ways over industry standard protocols with the user’s centralcomputers about drive status.

3. The ability to make drive tuning adjustments by means of keypads, with parameters such as loopgains, ramp rates, and current limits stored in memory rather than potentiometer settings.

4. Self-tuning and self-commissioning drive controls.

5. More adept techniques to overcome power circuit nonlinearities.

SOLID-STATE dc DRIVES

The introduction of the thyristor had the most immediate impact in the dc drive area. Ward Leonard(motor-generator) variable speed drives were quickly supplanted by thyristor dc drives of the typeshown in Fig. 3 (six-thyristor full converter), for reasons of lower cost, higher efficiency, and lowermaintenance cost. This circuit arrangement (the Graetz circuit) has become the workhorse of theelectrical variable speed drive industry, as will be seen by the number of other drives that use it. Byutilizing phase control of the thyristors, the converter behaves as a programmable voltage source.Therefore, speed variation is obtained by adjustment of the armature voltage of the dc machine.Because the phase control is fast and precise, critical features such as current limit are easily obtained.In fact, almost all thyristor dc drives today are configured as current regulators with a speed or voltageouter loop. The line-side characteristics are only fair in that input power factor is proportional to speed,and the input current has 31% harmonic content. The full converter offers low output ripple and theability to regenerate or return energy to the ac line. The system can be made into a four-quadrantdrive by the addition of a bidirectional field controller. Torque direction is determined by field currentdirection. Because of the large field inductance, torque reversals are fairly slow (100–500 ms) butadequate for many applications.

FIGURE 3 Thyristor converter dc drive.

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FIGURE 4 Dual thyristor converter dc drive.

For the best speed of response in four-quadrant thyristor dc drives, the dual armature converter ofFig. 4 is preferred. This is simply two converters (as shown in Fig. 3) connected back to back. Torquedirection is determined by the direction of armature current, and since this is a low inductance circuit,reversal can be accomplished in 10 ms (typically). Obviously, only one converter is conducting at onetime with the other group of six thyristors not being gated. This is called “bank selection.”

SUMMARY OF THYRISTOR dc DRIVES

The two types of thyristor dc drives just described all share a common property in that the thyristors areturned off by the natural polarity reversal of the input line. This is called natural or line commutation.Thus, the inability to turn off a thyristor from the gate is no practical drawback in these circuits.Consequently, they are simple and very efficient (typically 98.5%) because the device forward dropis very small compared with the operating voltage. These drives can be manufactured to match adc machine of any voltage (commonly 500 V) or horsepower (1/2 to 2500 HP, typically). The maindrawback of dc drives is the machine, not the power electronics. The dc machine, although easyto control, is larger, heavier, less robust, incompatible with corrosive or hazardous environments,generally not available above 750 V, and much more expensive than its ac counterpart of the samerating. Today, the only remaining reason for selecting a dc drive is if an inexpensive dc motor isavailable, or a retrofit situation exists. ac variable frequency drives now offer better response andgenerally better overall performance.

ac VARIABLE FREQUENCY DRIVES

The impact of new solid-state switching devices was even more significant on ac variable speed drives,but it occurred somewhat later in time as compared with dc drives, and it shows no sign of stopping.ac drives are machine specific and more complex than dc drives, mostly because of the simplicity of

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the ac machine. Solid-state variable speed drives have been developed and marketed for wound-rotorinduction motors (WRIMs), cage-type induction motors, and synchronous motors.

Historically, WRIM-based variable speed drives were in common use long before solid-stateelectronics. These drives operate on the principle of deliberately creating high-slip conditions inthe machine and then disposing of the large rotor power that results. This is done by varying theeffective resistance seen by the rotor windings. The WRIM is the most expensive ac machine. Thishas made WRIM-based variable speed drives noncompetitive as compared with cage induction motor(IM) drives or load commutated inverters using synchronous machines. The WRIM has become acasualty of the tremendous progress in ac variable speed drives as applied to cage induction motors.

INDUCTION MOTOR VARIABLE SPEED DRIVES

Because the squirrel cage induction motor is the least expensive, least complex, and most ruggedelectric machine, great effort has gone into drive development to exploit the machine’s superiorqualities. Because of its extreme simplicity, it is the least amenable to variable speed operation. Sinceit has only one electrical input port, the drive must control flux and torque simultaneously throughthis single input. As there is no access to the rotor, the power dissipation there raises its temperature;so very low-slip operation is essential. Induction motor variable speed drives in the past have hadthe greatest diversity of power circuits. Today, for drives rated 600 VAC or below (LV), there areessentially only two choices: (1) The IGBT pulse-width-modulated drive and (2) the autosequentiallycommutated current-fed inverter (ASCI; see Fig. 6).

1. The IGBT pulse-width modulated drive: In this type of voltage source inverter, both thefrequency and amplitude are controlled by the output switches alone. A representative circuit basedon IGBT’s is shown in Fig. 5. The input converter is a diode bridge so that the dc link operates

FIGURE 5 IGBT PWM variable frequency drive.

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at a fixed unregulated voltage. The diode front end gives virtually unity power factor, independentof load and speed. This type of drive is called pulse-width modulated (PWM), because the outputvoltage waveform is synthesized from constant amplitude, variable-width pulses at a high (1–3 kHz)frequency so that a sinusoidal output is simulated; the lower harmonics (5, 7, 11, 13, 17, 19, . . . ) arenot present in modern PWM drives. One advantage is smooth torque, low harmonic currents, and nocogging.

Although this approach eliminates the phase control requirements of the thyristor converter, itrequires fast output switches. Since every switching causes an energy loss in the output devices,fast switches are needed in order to support a high switching frequency without excessive losses inthe output switches. Occasionally, high-frequency switching may cause objectionable acoustic noisein the motor, but that has been overcome with special modulation techniques and higher switchingfrequencies (3 kHz and up).

There are IGBT PWM VFD’s on the market today in the range of 1 kW to 1 MW at 460, 600,and 690 VAC. As with all voltage source inverters fed by diode rectifiers, regeneration of power tothe line is not possible.

2. The autosequentially commutated current fed inverter (ASCI; Fig. 6): An entirely differentapproach to an IM drive is to generate a smooth dc current and feed that into different pair of windingsof the machine so as to create a discretely rotating magnetomotive force (MMF). This type of inverteris called the autosequentially commutated current-fed inverter. This circuit was invented later thanvoltage-fed inverters and is much more popular in Europe and Japan than in the United States. Theinput stage is a three-phase thyristor bridge, which is current regulated. A dc link choke smoothesthe current going to the output stage. There a thyristor bridge distributes the current into the motorwindings with the same switching function as the input bridge, except at variable frequency. (Notice

FIGURE 6 Thyristor Current-fed drive.

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the similarity to the LCI). The current waveform is a quasi-square wave whose frequency is set bythe output switching rate and whose amplitude is controlled by the current regulator. The capacitorsand rectifiers are used to store energy to commutate the thyristors, since the induction motor cannotprovide this energy and remain magnetized, in contrast to the synchronous motor. This type of drivehas simplicity, good efficiency (95%), excellent reliability, and four-quadrant operation up to ∼120Hz. Harmonics in the output current are greater than in the PWM drive, but reasonably low, givinga form factor of 1.05 (same as the LCI). The input power factor is equal to the product of the loadpower factor times the PU speed. Above 500 kW, the ASCIs are very cost effective. Because of thecontrolled current properties, this drive is virtually immune to damage from ground faults, load shorts,and commutation failures. Since MMF (current) is directly controlled and the drive is regenerative,ASCIs can readily be equipped with field oriented controls for the most demanding four-quadrantoperation.

CURRENT-FED VS. VOLTAGE-FED CIRCUITS:THE TWO BASIC TOPOLOGIES

Voltage-fed and current-fed refer to the two basic VFD strategies of applying power to the motor.In Europe, these are called voltage-impressed and current-impressed, which is a clearer description.In voltage-fed circuits, the output of the inverter is a voltage, usually the dc link voltage. The motorand its load determine the current that flows. The inverter doesn’t care what the current is (withinlimits). Usually, these drives have diode rectifiers on the input. The main dc link filter is a capacitor.In current-fed circuits, the output of the inverter is a current, usually the dc link current. The motorand its load determine the voltage. The inverter doesn’t care what the voltage is. Usually these VFD’shave a thyristor converter input stage, and the dc link element is an inductor. See Fig. 7 for a summarycomparison of the properties of the two types of VFDs in a six-pulse 600 VAC or smaller (LV)configuration.

MEDIUM-VOLTAGE VARIABLE FREQUENCY DRIVES

For drives rated 2300 VAC and above on the output, there are a number of choices of design of bothcurrent and voltage fed types.

1. The load commutated inverter (LCI; Fig. 8).

2. The filter commutated thyristor inverter (Fig. 9).

FIGURE 7 Comparison of low-voltage current-fed and voltage-fed drives.

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FIGURE 8 Load commutated inverter.

FIGURE 9 Filter-commutated thyristor drive.

3. The current-fed GTO inverter (Fig. 10.).

4. The neutral-point-clamped inverter (Fig. 11).

5. The multilevel series cell VFD (Figs. 12 and 13).

6. The cycloconverter (Fig. 14).

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FIGURE 10 Current-fed GTO inverter.

FIGURE 11 Neutral-point-clamped inverter.

THE LOAD COMMUTATED INVERTER

As shown in Fig. 8, the load-commutated inverter is based on a synchronous machine. All the thyristorsare naturally commutated, because the back EMF of the machine commutates the load side converter.The machine side converter operates exactly like the line side converter, except the phase back angle is∼150◦. The machine naturally applies reverse voltage to an off-going device before the next thyristoris gated. This imposes some special design criteria on the synchronous motor. It has to be able tooperate at a substantial leading power factor over the speed range, it must have enough leakageinductance to limit the thyristor di/dt, and it has to be able to withstand harmonic currents in the

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FIGURE 12 Multilevel series-cell drive.

FIGURE 13 Conversion cell of Multilevel VFD.

damper windings. The LCI uses two thyristor bridges—one on the line side and one on the machineside. The requirement for the machine to operate with a leading power factor requires substantiallymore field excitation and a special exciter compared with that normally applied to synchronous motor.This also results in a reduction in the torque for a given current. The machine side devices are firedin exact synchronism with the rotation of the machine, so as to maintain constant torque angle andconstant commutation margin. This is done either by rotor position feedback or by phase control

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FIGURE 14 Cycloconverter.

circuits driven by the machine terminal voltage. Only RC networks for voltage sharing are necessary.The output current is very similar in shape to the input current, which means a substantial harmoniccomponent. The harmonic currents cause extra losses in the damper bars, and they give rise to verysignificant torque pulsations. The drive is not self-starting because of the low machine voltage atlow speeds.

Therefore, the drive is started by interrupting the dc link current with the line-side converter inorder to commutate the inverter thyristors. The line-side converter is regulated to control torque. Achoke is used between converters to smooth the link current. LCIs came into commercial use about1980 and are used mainly on very large, medium-voltage drives (1–100 MW). At these power levels,multiple series devices are employed (typically four at 4 kV input), and conversion takes place directlyat 2.4 or 4 kV or higher. The efficiency is excellent, and reliability has been very good. Althoughthey are capable of regeneration, LCls are rarely used in four-quadrant applications because of thedifficulty in commutating at very low speeds where the machine voltage is negligible. Operation aboveline frequency is straightforward. Despite the need for special synchronous motors, the LCI drive hasbeen very successful, particularly in very large sizes, where only thyristors can provide the current andvoltage ratings necessary. Also, high-speed LCIs have been built. Now that self-commutated VFDsare available, the LCI is becoming less popular.

FILTER COMMUTATED THYRISTOR DRIVE

In the circuit shown in Fig. 9, the output capacitor filter is chosen to supply all the magnetizing currentof the motor at ∼50% speed. Above that point the load (machine and filter combined) power factorremains leading and the inverter thyristors are naturally commutated, that is, the voltage across the

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device is naturally reversed before the reapplication of forward voltage. In this mode of operation,the thyristor waveforms are similar to those in an LCI. The filter must supply (at a minimum) all ofthe reactive current requirements of the motor at full load, and is typically 1 PU of the drive kVArating. In addition to the large ac capacitors, the filter has to have some series inductive reactance tolimit the di/dt applied to the inverter thyristors. Since the filter is capable of self-exciting the motor,a contactor is required to isolate the filter from the motor when the drive is off line. The large filterhas the added advantage of providing a path for the harmonic currents in the inverter output (whichis a six-step current), so that the motor current waveform is good near rated frequency. As the outputfrequency decreases, the filter becomes less effective and the motor current waveform deteriorates.The fundamental current into the filter increases with the square of the frequency up to rated voltage,since the voltage is also increasing with the frequency. Maintaining control of the voltage requiresthat the inverter “drain off” more of the filter reactive current as the frequency increases, thereforemaking it difficult to achieve more than ∼1.1 PU of base frequency.

Since the filter cannot provide commutation down to zero frequency, it is necessary to providean auxiliary commutation means to get the drive started. This circuit acts on the dc link current andis commonly called the diverter. When it is time to switch inverter thyristors, the dc link current istemporarily interrupted (diverted), allowing the devices to recover. Then the next thyristor pair is gated,and dc link current is restored. The auxiliary circuit has to be able to withstand full link voltage, andinterrupt the rated dc link current for several hundred microseconds to permit the inverter thyristors torecover. (High-voltage thyristors require long turn-off times as a consequence of design compromisesin achieving a high blocking voltage). Thus, the auxiliary commutation circuit is quite significant inrating. It is not usually intended for continuous operation, but only to get the speed up to the pointwhere the filter commutation commences. The drive controller has to be able to manage two modesof operation.

This circuit has been implemented with four 3-kV thyristors in series per leg of the output bridge.(It is possible to add additional thyristors for redundancy.) Each leg of the bridge experiences the peakmotor line–line voltage of ∼6000 V in both polarities, so the devices must have symmetrical blockingvoltage. As in the input converter, the issue of voltage sharing during steady-state and switchingarises. Combinations of device matching and/or RC snubbers are needed. Gate circuits for thyristorsare simple and typically deliver 3–5 W of power, although they are designed for somewhat more.This approach has been most successful in those applications in which the drive operates more or lesscontinuously and in the range of 60–100% of rated speed.

CURRENT-FED GTO INVERTER

Another medium-voltage bridge inverter circuit is shown in Fig. 10. Here the output devices areGTOs (three 4-kV units per leg will be required) that can be turned off with the gate. This reducesthe size of the filter as compared to the filter-commutated inverter to perhaps 0.8 PU, but it doesnot eliminate it. Since the motor appears to be a voltage source behind the leakage reactance, it isnot possible to commutate the current between motor phases without a voltage to change the currentin the leakage inductance. When a GTO turns off, there must still be a path for the current trappedin the motor leakage inductance, which is provided by a capacitor bank. The capacitors resonate withthe motor leakage inductance during the transfer of current. The choice of capacitor is determinedby the permissible ring-up voltage during commutation. All current-fed VFDs have the need of a“buffering” capacitor between the impressed current of the inverter and the inductance of the motor.Furthermore, if the capacitor bank exceeds 0.2 PU, the possibility of self-excitation of the motorexists, necessitating a contactor between machine and drive. Voltage-fed circuits do not require theseelements, because the voltage can arbitrarily be changed across the leakage inductance.

Since the capacitor bank is smaller than in the filter-commutated VFD, it does not provide asmuch filtering of the output current. Motor current improvements are made by harmonic eliminationswitching patterns for the GTOs. At low frequencies, many pulses per cycle are possible and harmonicelimination is quite effective, but the GTO frequency limit of a few hundred hertz restricts harmonicelimination at rated frequency to the fifth and maybe the seventh. This frequency limit comes about

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because of the nature of the GTO turn-off (and to a lesser extent, turn-on) mechanism. The deviceis turned off by extracting charge from the gate over a period of a few tens of microseconds andinterrupting the regenerative turn-on mechanism. Near the end of the charge extraction period, thevoltage across the GTO rises and the current begins to fall. During this time the device experiencesextremely high internal power dissipation, which must be mitigated by the use of a large (1–5 µFcompared to 0.1 µF for thyristors) polarized snubber located very close to the GTO. In that snubber,the capacitor is connected through a diode (the diode requires the same voltage rating as the GTO) tothe GTO, so turn-off current can divert into the snubber, but the capacitor cannot discharge into thesnubber at turn on. The energy transferred to the snubber capacitor must be disposed of in some wayso that the capacitor is discharged before the next turn off. Thus GTOs typically have a minimum“on” time (say 10 µS) and a minimum “off” time (say 100 µS) to permit the internal switching heatto flow away from the junction and for the snubber to recover.

Violation of the minimum time limits, or an unsuccessful turn-off attempt can result in destructionof the GTO. This limits the the maximum switching rate with tolerable losses to a few hundred hertz.The GTO gate driver, in addition to a providing a turn-on pulse comparable to the thyristor driver,has to be able to deliver a peak negative current of 1/5 to 1/3 the anode current in order to turn-off thedevice. Thus, the GTO driver has a peak VA rating of 2–3 orders of magnitude higher than that fora thyristor, and perhaps ten times the average power requirement. This is an important factor in thatthe all the gate power must be delivered to a circuit floating at medium-voltage potential.

The snubber losses can have a noticeable effect on part-load efficiency for a GTO drive. Somecircuit implementations use patented energy recovery techniques to avoid efficiency deterioration, butthese add serious complexity. The snubber loss is proportional to the frequency and to the snubbercapacitance, but to the square of the voltage. Those circuits have to use devices with a comparablevoltage rating to the GTO.

The design compromises in the metallurgy of the GTO result in a noticeably higher forward drop(2.5–4 V) than that of the conventional thyristor. The device design is further complicated by therequirement for symmetrical voltage blocking in the current-fed topology.

NEUTRAL-POINT-CLAMPED INVERTER

Despite the obvious complications of series GTO designs, they have also been used successfullyin voltage fed drives. Figure 10 illustrates such a circuit, the neutral-point-clamped inverter. Therehave been many of this type applied at a 3,300-V output with 4.5-kV GTOs, but the circuit has onlyrecently been extended to 4-kV ac, probably because of the improved properties of the IGCT. Inthe new erversions of this drive, the GTOs are replaced with IGCTs. These devices are similar inconstruction to a GTO, but they are turned off quickly (1 µs) by drawing all the anode current outthrough the gate, so that the turn-off gain is unity. This requires a higher current gate driver, butlower average power requirements since the turn-off time is so short. The main claim of improvementis that the IGCT supposedly can operate with a very small or no snubber. In this 4- kV ac outputdesign, the total dc link voltage is 6 kV, with a midpoint established at the center of the capacitorfilter. Each leg of the bridge consists of two 6-kV IGCTs in series. There are diodes in reverse acrosseach GTO to permit motor current to flow back to the link, and still more diodes (same voltage ratingas the GTOs/IGCTs) connecting the midpoints of the inverter legs back to the midpoint of the dclink. The total device count is 12 GTOs and 18 diodes (plus 12 more diodes in the GTO snubbers, ifGTOs are used). The neutral-point-clamped inverter offers several advantages in those cases in whichseries devices would be necessary anyway. First, the clamping diodes permit another voltage level,the dc link midpoint, at the output. This cuts the voltage step seen by the motor in half, and moreimportantly, creates another degree of freedom in eliminating output harmonics. Also, the clampingdiode positively limits the voltage across any one device to half the link voltage, enforcing voltagesharing without additional RC networks.

It is still necessary to equip each GTO with a close-coupled snubber and manage the snubber losses.Since the switching devices in this circuit are never subjected to reverse voltage, it is preferable to

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use asymmetrical devices in which the absence of reverse blocking is traded off for lower conductionand switching losses.

Device protection during a short circuit is a problem, as the GTO/IGCT can carry almost unlimitedfault current like a thyristor. Unlike the current-fed circuits in which fault current is limited, in thevoltage fed circuit, the dc link capacitor can source very large fault currents in the event of a short ora commutation failure.

Protection schemes generally attempt to detect the onset of fault current and turn off the devicesbefore it grows beyond the safe turn-off level. Another approach of turning on all the GTOs todistribute the fault current may protect the devices, but it applies a bolted fault to the motor, resultingin extremely large torques. It is possible to use the NPC topology with IGBTs as the switching devices.As IGBTs are currently limited to 3,300 V, the IGBT NPC cannot yet reach a 4-kV ac output, but aEuropean manufacturer has announced a 6-kV IGBT to be available in the first quarter of 1999. Theconcept of NPC can be extended to M-level inverters, although the number of diodes grows rapidly.Since each device is topologically unique, adding redundant devices would require twice as many,instead of just one more.

MULTILEVEL SERIES-CELL INVERTER

The patented series cell arrangement of Figs. 11 and 12, also known as the Perfect Harmony drive,addresses the previously mentioned design issues in a unique way. Since there are no devices in series,only series cells, the problem of voltage sharing does not exist. The rectifier diodes and the IGBTs areboth closely coupled to the dc link capacitor in the cell and thus cannot be exposed to more than thebus voltage, regardless of the load behavior. Since there is no dc link choke, a voltage transient on theac mains is converted into a current pulse by the relatively high leakage reactance of the transformersecondary, and does not add to the voltage seen by the diodes.

Each cell generates the same ac output. The fundamentals are equal in magnitude and in phase, butthe carrier frequency is staggered among the cells in a particular phase. Although an individual celloperates at 600 Hz, the effective switching frequency is 3 kHz, so the lowest harmonic is theoreticallythe 100th. This low switching frequency and the excellent high-frequency characteristics of the IGBThas the advantage that the IGBT switching losses are totally negligible. The devices can switch wellabove rated current without the need for snubbers which also helps in maintaining excellent efficiency.Waveform quality is unaffected by speed or load. For the 5 cell/phase VFD, there are ten 620-V stepsbetween the negative and positive peaks. With this technique, the concern for high dv/dt on the motorwindings is avoided entirely.

A major advantage of the IGBT over all other power switches is the extremely low gate powerrequired. The peak power is about ∼5 W, with an average of much less than 1 W. This dramaticallysimplifies the delivery of gate power compared to the GTO/IGCT. Although there are more activedevices in the Perfect Harmony (60 IGBTs and 60 diodes in the inverter sections) than in the other cir-cuits, the elimination of snubbers, voltage sharing networks, and high-power gate drivers compensatesfor the additional switching devices. The type of IGBTs employed are third and fourth-generationisolated base modules, generally the same mature product as those found in 460 VAC and 690 VACPWM drives, and that are also used in traction applications. The IGBTs are protected by an out-of-saturation detector circuit, which augments the built-in current limiting behavior. Since the cells areassembled into a nonconducting framework and are electrically floating, the mounting and cooling ofthe IGBTs is no more complex than in a low-voltage PWM drive. It is possible to put redundant cellsin the string and also to operate at reduced output with one cell inoperative.

CYCLOCONVERTER

Still another approach in an IM drive is to “synthesize” an ac voltage waveform from sections of theinput voltage. This can be done with three dual converters, and the circuit is called a cycloconverter

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FIGURE 15 Comparison of medium-voltage motor drives.

(see Fig. 13). The output voltage is rich in harmonics but of sufficient quality for IM drives as long asthe output frequency does not exceed 1/3 of the input frequency. The thyristors are line commutated,but there are 36 of them. The cycloconverter is capable of very heavy overloads and four-quadrantoperation, but it has a limited output frequency and poor input power factor. For special low-speedhigh-power (>10 MW) applications, such as cement-kiln drives, the cycloconverter has been usedsuccessfully.

COMPARISON OF MEDIUM-VOLTAGE MOTOR DRIVES

All the types of drives mentioned above are capable of providing highly reliable operation at ajustifiable cost, and they have been proven in service. They all have efficiencies above 95%. The mostsignificant differences among them have to do with power quality, that is, how close to a sinewaveis the input current, and how well does the output resemble the sinusoidal utility voltage. Figure 15compares them on a number of different factors. Note that the voltage-fed drives have an advantagein input harmonics and power factor, and the drives, which do not use thyristors, have a wider speedrange.

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BIBLIOGRAPHY

Bedford, B. D., and R. G. Hoft, Principles of Inverter Circuits, Wiley, New York, 1964.

Bose, B. K., Adjustable Speed ac Drive Systems, Wiley, New York, 1981.

Brichant, F., Force-Commutated Inverters, Macmillan, New York, 1984.

Ghandi, S. K., Semiconductor Power Devices, Wiley, New York, 1977.

Kosow, 1. L., Control of Electric Machines, Prentice-Hall, Englewood Cliffs, New Jersey, 1973.

Pelly, B. R., Thyristor Phase-Controlled Converters and Cycloconverters, Wiley, New York, 1971.

Schaefer, J., Rectifier Circuits: Theory and Design, Wiley, New York, 1965.

Scoles, G. J., Handbook of Rectifier Circuits, Wiley, New York, 1980.

Sen, P. C., Thyristor dc Drives, Wiley, New York, 1981.

ROBOTS

Early industrial robots date back several decades. Initially they were used mainly to assist or takeover dangerous or difficult handling operations, essentially to protect manual laborers from undueexposure to harmful substances, temperature, radiation, and so on. Research and development expensein such situations was relatively easy to justify, but all along the ultimate objective was to designrobots that could perform manual tasks better, more cheaply, and more quickly than people. Aswith numerous other specialized equipment technologies, the robot, in concept, was far ahead of thecomponents needed to enhance its performance, as have later emanated from advancements in solid-state electronics, computer controls, and communications. Very large strides in robot developmenthave been made since the mid-1970s, particularly as the result of some piece- and parts-handlingindustries (automotive being a major example) to improve their competitive position in terms ofincreased productivity and product quality.

BASIC FORMAT OF ROBOT

It is not always easy to distinguish a mechanized handling machine from what is generally consideredto be a robot. For example, a modern, complex conveyor system would meet some of the generaldescriptive criteria of a robot, but in professional parlance, a conveyor by itself would seldom beconsidered a robot. However, in terms of total robotic technology, one or many conveyors could beinvolved. A definition, coined several years ago by the Robot Institute of America, still providesa good definitive foundation, even though some of the words used are rather general and perhapssuperconclusive. The definition is:

A robot is a reprogrammable, multifunctional manipulator designed to move materials,parts, tools, or specialized devices through variable programmed motions for the performanceof a variety of tasks.

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In terms of their classification, robots may be considered from a number of viewpoints:

1. Axes of motion, including type of motion, number of axes, and the parameters of axis travel

2. Load capacity and power required, namely, weight of load, and electrically, pneumatically, orhydraulically operated

3. Dynamic properties

4. End-effectors or grippers used

5. Programming and control system

6. General-purpose or special task

AXES OF MOTION

A robot may be movable from one factory location to another, as may be required by factory layoutchanges or by major alterations in job assignment. However, for any given task that will be repeatedover and over for long periods, a robot will be firmly fastened to the operating floor (sometimes theceiling). The firm location establishes a fixed geometrical location of reference, an unchangeableposition that will geometrically relate precisely with an associated machine, or in the case of a workcell, involving several other machines and often other robots.

For relatively moderate changes in the robot’s working envelope, the average “stock” robot willincorporate considerable flexibility within its design so that changes can be made without alteringthe location of reference. Sometimes, in the case of a “smart” robot, final very small changes in thepositioning of an arm can be made by outputs from a machine vision or tactile system.

Less frequently, a robot will be intentionally designed for movability so that it can be transferredto the worksite, rather than grouping one or more robots about specific locations, as will be mentionedlater.

Degrees of Freedom

Designed or built-in axes of motion essentially define the robot’s ability to move parts and materials,sometimes referred to as degrees of freedom. The axis of motion refers to the separate motion a robothas in its manipulator, wrist, and base. The designer usually will select from one of four differentgeometric coordinates for any given robot.

Revolute (Jointed-Arm) Coordinates. In this system the robot arm is constructed of several rigidmembers, which are connected by rotary joints. Three independent motions are permitted (Fig. 1).These members are analogous to the human upper arm, forearm, and hand, while the joints areequivalent to the human shoulder, elbow, and wrist, respectively. The arm incorporates a wrist assemblyfor orienting the end-effector, in accordance with the demands of the workpiece (Fig. 2). These threearticulations are pitch (bend), yaw (swing), and roll (swivel). In some applications, fewer than sixarticulations may suffice, depending on the geometry of the workpiece and the machine which therobot is serving.

Cartesian Coordinates. In this system all robot motions travel in right-angle lines to each other.There are no radial motions. Consequently the profile of a Cartesian-based robot will have a rectan-gularly shaped work envelope (Fig. 3).

Some systems utilize rotary actuators to control end-effector orientation. Robots of this typegenerally are limited to special applications. A robot may incorporate rectilinear cartesian coordinatesas, for example, a continuous-path extended-reach robot gains much versatility through a bridge andtrolley construction, which enables the robot to have a relatively larger rectangular work envelope.When ceiling mounted, this system may service many stations with several functions, thus leaving

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FIGURE 1 Jointed-arm manipulator, incorporating revo-lute coordinates.

FIGURE 2 Wrist assembly on robot armfor orienting end-effector in accordance withrequirements of workpiece.

FIGURE 3 Manipulator incorporating Cartesian coordi-nates.

the floor clear. X and Y motions are performed by the bridge and trolley; the vertical motions areaccomplished by using telescoping tubes.

In a Cartesian coordinate system the location of the center of the coordinate system is the center ofthe junction of the first two joints. Except for literally moving the robot to another factory location, thecenter does not move. In effect, it is tied to the “world” as if anchored in concrete. If the X measurementline points toward a column in the work area where the robot is placed, the X line will always pointtoward that same column, no matter what way the robot turns while performing its programs. Theseare known as the world coordinates for a given robot installation (Fig. 4).

In the operation of a robot, having an origin for a measurement reference is not sufficient. One alsoneeds to know the point to which measurements will be made. This measurement is made from theorigin of the coordinate system to a point that is exactly in the center of the circle, on which the tool(end-effector) is to be mounted. This system moves with the tool and is aptly called the tool coordinatesystem. In the tool coordinate system the X and Y lines lie at right angles flat on the tool-mountingsurface. The Z line is the same as the axis of rotation for the point, that is, it points directly throughthe tool in one direction and through the wrist in the other direction. The system is not tied to theworld. Instead it stays in position on the tool-mounting surface and moves wherever the tool moves.

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FIGURE 4 World coordinate system of robot using Cartesian coordinates.

While the origin of the system is thus allowed to move around, the destination (where it measures to)is left to the discretion of the user. Sometimes the tool coordinate system is actually used to measurewhere the tip of the tool lies relative to where it is mounted; sometimes it is used to measure whereone position in space lies relative to some other point in space (Fig. 5).

Cylindrical Coordinates. Robots designed with this system have a horizontal shaft that goes in andout and rides up and down on a vertical shaft. The latter rotates about the base (Fig. 6). Additionalrotary axes are sometimes used to allow for end-effector orientation. Cylindrical-coordinate robots areoften well suited for tasks to be performed on machines to be serviced that are located radially from

FIGURE 5 Tool coordinate system of robot using Cartesiancoordinates.

FIGURE 6 Manipulator incorporatingcylindrical coordinates.

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FIGURE 7 Spherical-coordinate manipulator, the oper-ation of which is comparable to a tank turret.

FIGURE 8 Work envelope of a robot is that area in spacewhich the robot can touch with the mounting plate on the endof its arm.

the robot and where no obstructions are present. A robot that incorporates cylindrical coordinates hasa working area or envelope that is a portion of a cylinder.

Spherical (Polar) Coordinates. Robots using this system may be likened to a tank turret, that is,they comprise a rotary base, an elevation, and a telescoping extend-and-reach boom axis. Up to threerotary wrist axes (pitch, yaw, and roll) may be used to control the orientation of the end effector(Fig. 7).

Work Envelope

The area in space that a robot can touch with the mounting plate on the end of its arm is known as itswork envelope (Fig. 8).

LOAD CAPACITY AND POWER REQUIREMENTS

With need and proper design, robots can be designed to handle miniature (tiny) pieces that weigh afew ounces or grams, as found, for example, in electronics manufacture, up to heavy industrial loadsranging from 135 to 1045 kg (300 to 2300 lb) and even much greater loads where robotic equipment isused, for example, in earth-moving situations, as may be found in earthquake debris removal. A recentsurvey of user demand shows that robots lie within the range of 9 kg (20 lb) on the low side to 136 kg(300 lb) on the high side for the majority of applications. A majority of robots are electrically actuatedby servomotors, particularly stepping and permanent-magnet dc motors, as described previously indetail in this handbook section. Less frequently used are pneumatic and hydraulic actuators.

DYNAMIC PROPERTIES OF ROBOTS

Important dynamic properties of robots include (1) stability, (2) resolution, (3) repeatability, and (4)compliance. Considering these factors, the design of a robot is innately complex because of the manner

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FIGURE 9 Consider a robot arm that has a retracted handposition of 2 m and an extended hand position of 3.5 m. Con-sider also that this arm might carry a load of 150 kg, and thatthe arm should go from position to position, with or withoutload, at any extension and without overshoot. For the con-figuration shown, the variation in moment of inertia is from70 kg Msec2 when tucked in and unloaded to 230 kg Msec2

when fully extended and loaded. To achieve a critically dampedservo with position repeatability of 0.5 mm under all operat-ing conditions is difficult. Note that 0.5-mm resolution for anarm with 300◦ of rotation requires position encoding to an ac-curacy of 1 part in 33,000, or 215. The foregoing deals onlywith a major robot arm articulation. In a full arm the interac-tions among the various articulations complicate both dynamicperformance and accuracy. For example, a robot arm designedto achieve an individual-articulation natural frequency of 50Hz degenerates to an overall 17 Hz in a six-articulation arm.(Westinghouse.)

in which these properties interrelate. This also contributes to the difficulties of optimizing a design.Figures 9 through 12 illustrate specific examples of dynamic problems.

Stability

This characteristic is associated with oscillation in the motion of the tool. The fewer the oscillationsthat are present, obviously the more stable is the operation of the robot. Negative aspects of oscillationsinclude the following. (1) Extra wear is imposed on the mechanical, hydraulic, and other parts of therobot arm. (2) The tool will follow different paths in space during successive repetitions of the samemovement, thus requiring more distance between the intended trajectory and the surrounding parts.(3) The time required for the tool to stop at a precision position will be increased. (4) The tool mayovershoot the intended stopping position, possibly causing a collision with some object in the system.

Oscillations may be damped or undamped. Damped (transient) oscillations will degrade and ceasewith time; undamped oscillations may persist or may grow in magnitude (runaway oscillation) andare the most serious because of the potential damage they may cause to the surroundings.

Variations of internal and gravitational loads on the individual joint servos (as the arm’s posturechanges) make the operation of the robot prone to oscillation.

In one approach to solving oscillation problems, the joint servos operate continuously. Somesophisticated servo designs (the result of experience from numerical control of machine tools) preventoscillation from starting, regardless of the load carried. In another approach the robot controller lockseach joint independently the first time it reaches its set point. Special circuitry also decelerates thejoint after it comes within a prescribed distance of that position. When the joints are all locked (totalcoincidence), the arm is stationary, and it can then begin to move to the next position. If the positionis held for more than a few seconds, the tool slowly creeps away from its programmed position.

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FIGURE 10 Block diagram for functionally describing key el-ements of a single-articulation servo system, including velocityand acceleration feedback and interarticulation bias signals. In es-timating the time to complete a task (without actually simulatingthe entire process), the interface with the workplace complicatesthe process. Paths to avoid obstacles add program steps. Somesteps must be very precise, calling for closing out to zero errorbefore the program advances. Other steps may be the corners in amotion path, which can be passed through “on the fly” so to speak.The use of interlock switches may introduce transport lags. Sim-ple programs often permit using a rule of thumb. For example, ifone allows 0.8 second for each motion taught, short steps as wellas long, a time for program completion can be estimated quiteclosely. However, if a program is complex, as in spot welding acar body, there are too many variables to permit the use of suchmethods. Other factors that must be included are weld-gun inertia,weld-gun operating time, metal thickness, proximity of spots toone another, among other critical variables.

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FIGURE 11 It is common for robots to be offered with abbreviatedspecifications that list the slew rates and the repeatability of eacharticulation. What is really needed is the total amount of time requiredto go from position to position and net accuracy of all articulationsin consort. Shown here are two typical velocity traces for a short-arm motion and a large-arm motion. It is evident that the slew rate isno measure of elapsed time in making a motion, particularly a shortmotion in which the slew rate may not be attained at all.

Resolution and Repeatability

Repeatability is affected by resolution and component inaccuracy. Both short- and long-term repeata-bility exist in a robotic system. Long-term repeatability is of concern in robot systems that mustperform tasks over a several-month period. During long-term repetitive use, components wear andage to the extent that repeatability must be checked periodically. Short-term repeatability is influ-enced most by temperature changes within the control and the environment as well as by transientconditions between shutdown and start-up of the system. These factors frequently are grouped underthe umbrella term, drift. The accuracy of a robotic system can range from several hundredths of aninch for a simple robot to several thousandths of an inch for a robot doing precision assembly orhandling small parts. In the case of a robot used in testing printed-circuit boards and other electronicmanufacturing operations, the need for precision is paramount. Repeatability claims for standard orstock robots are listed in Table 1.

TABLE 1 Manufacturers’ Claimed Repeatabilityof Randomly Selected Contemporary Robots

Load Capacity Claimed Repeatability (±)

lb kg inches mm

5 2.2 0.004 0.114 6 0.004 0.122 10 0.008 0.235 16 0.001 0.0366 30 0.002 0.05

110 50 0.020 0.5132 60 0.020 0.5150 68 0.020 0.5176 80 0.020 0.5200 90 0.010 0.25264 120 0.04 1.0

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FIGURE 12 For some operations, program time is critical, such as when a robot is serving heavy, expensivecapital equipment. If the production rate is paced by the robot rather than the equipment, the project wouldnot seem viable because of loss of throughput. Optimizing such a program may involve a range of techniques.A typical application might be press-to-press transfer of sheet-metal parts. A line of presses runs at a grossproduction rate of up to 700 parts/h. At this rate a robot must make a complete transfer and return for the nextpickup in 5.16 s. With presses on center-to-center distances of 6 m, this is a demanding transfer speed. To meetthis rate, a robot was modified by increasing the capacity of both hydraulic supply and servo valves. Accelerationand deceleration times were reduced at some sacrifice in damping and accuracy. This was compensated byproviding the die nests with leads or strike bars. Finally, interlocks were refined so that the robot could makeapproaches and departures during the rise and fall of the moving platens of the press. The curves given hereshow how the time can be shortened by tight interlocks that do not wait for press-cycle operation. For safety,this approach cannot be used with human operators.

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Compliance

The compliance of a manipulator is indicated by its displacement relative to a fixed frame in responseto a force (torque) exerted on it. The force may be a reaction force (torque) that arises when themanipulator pushes (twists) the tool against an object, or it may be the result of the object pushing(twisting) the tool. High compliance means that the tool moves a lot in response to a small force, andthe manipulator is then said to be spongy or springy. If it moves very little, the compliance is low andthe manipulator is said to be stiff.

Compliance is a complex quantity to measure. In practice, a manipulator may be defined as anonlinear, anisostropic tensor quantity that varies with time and with the manipulator’s posture andmotion. It is a tensor because a force in one direction can result in displacements in other directionsand even rotation. A torque can result in rotation about any axis and displacement in any direction. A6 × 6 matrix is a convenient representation for a compliance tensor. Time can affect compliancethrough changes in temperature and hence the viscosity of hydraulic fluid. Compliance will often befound to be a function of the frequency of the applied force or torque. A manipulator, for example,may be very compliant at frequencies around 2 Hz, but very stiff in response to slower disturbances.Compliance may exhibit hysteresis. For example, the servos in one design of hydraulic manipulatorturn off when the arm stops moving. In this condition all the servo valves are closed, and the compliancehas a value that is determined by the volume of incompressible hydraulic fluid trapped in the hydraulichoses and the elasticity of the hoses. However, if an outside force on the tool should move any of thejoints more than a given distance from the position at which they are supposed to remain, then theservos on all joints will turn on again. The compliance then changes to a completely different value(presumably stiffer in some sense).

Both electric and hydraulic manipulators have complex compliance properties. In an electricmanipulator the motors generally connect to the joints through a mechanical coupling. The stickingand sliding friction in such a coupling and in the motor itself can cause strange effects on the compliancemeasured at the tool tip. In particular, some of these couplings are not very back-drivable. For example,if one pushes on the nut of a lead screw (backdrive), the lead screw will not turn unless the screw’spitch is very coarse and ball bearings are used between the threads to reduce friction. But one canturn the screw easily, and the nut will move.

For manipulators that (1) operate open-loop in the sense that (2) they go blindly to a given pointin space, (3) without any regard to the actual position in the environment, or (4) without regard to anyreaction forces (feedback) that those objects exert on the arm (or tool)—then less compliance thanthat of the surrounding objects would be an advantage. High-frequency oscillations can be filtered outwithout degrading the overall response. Such filtering actually requires no special effort inasmuch asthe combinations of servo valves and actuators commonly used have relatively low bandwidths.

Tactile sensors, which measure forces and moments exerted on the tool, can allow the manipulatorto track or locate objects. Even in such cases, however, oscillations may arise in the force-feedbackcontrol loop if the compliance at the point of sensing is too low (stiff). Examination of a particularservo design is needed to predict reliably whether it will provide the kind of compliance needed fora specific task. There is no substitute for an actual test with the real tool on the manipulator.

END-EFFECTORS (GRIPPERS)

The device that is fastened to the free end of a manipulator is known as an end-effector or gripper. Theusual function of the device is to grasp an object or a tool and then hold it while the manipulator moves,thereby moving the object, and finally releasing the object. Many end-effectors are widely used andavailable from stock. Custom-designed end-effectors, however, are not uncommon where standarddesigns do not satisfy specific application needs. Normally the end-effector is not included in the priceof a robot per se. Hence their cost often can create substantial additional expense. Mechanical clamps(grippers) are commonly used. Other forms include vacuum-operated holders and both permanentand electromagnetic holders (Fig. 13).

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FIGURE 13 Representative robot end-effectors. 9.133

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FIGURE 14 Die-casting installations to unload, quench, and dispose ofpart. In this installation, quite exemplary of earlier robot installations, thework is arranged around the robot.

WORKPLACE CONFIGURATIONS

There are four basic situations pertaining to the flow of work and the location of the robot:

1. Work may be arranged around the robot (Fig. 14).

2. Work is brought to the robot (Fig. 15).

3. Work may travel past the robot.

4. The robot travels to the work.

Work Cells

A robotic work cell may be defined as a cluster of two or more robots and several machine tools ortransfer lines that are interconnected in such a way that they work in unison. All of the necessaryaccessory equipment is embraced within the work cell and, together, establishes a particular workenvironment. The cell level is one step higher than the station level in the hierarchy of control andcommand. Keeping very close supervision over statistical quality control has become a paramountconsideration in instrumenting the cell-level concept (Fig. 16).

ROBOT PROGRAMMING AND CONTROL

The two general classifications of robots from a control standpoint are (1) nonservo robots and (2)servo-controlled robots. For noncritical, simple applications, nonservo robots may suffice, particularlywhere low cost is a major consideration. Most of the early designs were in this category. However, thewide acceptance of the servo-controlled robot, as technological advancements made them possible,are evident from the inspection of Table 2.

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FIGURE 15 Overhead robot system, where the robot travels to the work. In this system an overhead robot systemallows one robot to serve eight numerically controlled lathes.

Of historical interest, when robots and other automation techniques were largely associated withreplicating the skills of human operators, the detailed steps and operations of the human operator werecarefully studied and recorded. Initially this was the main source of robot programming information.Out of these studies the early “playback” concept was developed. In this method, the robot was“taught” by manually recording all of the movements that robot had to take to accomplish a giventask. Obviously, at that time this method represented a “shortcut” because the path of the robot did nothave to be measured or described in complex mathematical terms. Since those earlier years, of course,the techniques of mathematical simulation and appropriate motion algorithms have been developed.Much research along these lines has been conducted over the past decade or so by a combination effortmade by robot designers and manufacturers, by large robot-using firms, and by academic institutions.For example, a pioneer in the field has been the Robotics Institute, Carnegie Mellon University,Pittsburgh, Pennsylvania, which initially developed a program known as VAST (versatile robot-armdynamic simulation tool). As has become an accepted practice pertaining to instrumentation andcontrol in the process industries, many robots have developed a high dependence on manufacturersand consultants for robot programming and software systems. In cases where applications from oneuser to the next may differ only in minor detail, packaged computer controls and software are nowavailable and may be used with few, if any, alterations.

For example, large numbers of robot designs have been refined over several generations. Thusspecial controllers, visual operating displays, and software programs are available from robot sup-pliers. Standard applications that fit these criteria include those used in general materials handling,palletizing, arc welding, and, more recently, laser cutting, welding, etching, and surface-hardeningapplications.

Customized software for palletizing, for example, provides quick setup, easy modification ofexisting applications, and automatic calculation of all robot paths, eliminating the process of positionteaching. In connection with robot welding applications, touch-sensing systems adaptively locate weldjoints, and a through-arc seam-tracking system offers further enhancement by allowing the robot tocompensate for weld-joint deviations and to correct the robot’s path in real time.

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TABLE 2 Nonservo versus Servo-Controlled Robots

Characteristic Nonservo Robot Servo-Controlled Robot

Flexibility Limited in terms of program capacity Maximum flexibility provided by ability to program axesand positioning capability. Arms can of manipulator to any position within limits of travel.travel at only one speed and can stop Can vary speed at any point within envelope. Ability toonly at end points of their axes. move heavy loads in a controlled fashion.

Speed Relatively high. Relatively slow.

Repeatability Approximately ±0.5 mm. ±0.1 to 0.5 mm and better, depending on design andapplication.

Cost Comparatively low. Comparatively high.

Complexity Simple operation, programming, Permits storage and execution of more than oneand maintenance. program, with random selection of programs from memory

via externally generated signals. Subrouting andbranching capabilities may be available, permittingrobot to take alternative actions within a programwhen commanded.

FIGURE 16 Pseudopyramidal hierarchy where communications are predominantly vertical rather than hori-zontal.

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For parts and piece-dispensing operations, a software package minimizes the amount of codethat is needed. For more complex operations, vision systems equip robots with advanced gray-scalevision capabilities. This is covered in more detail elsewhere in this handbook. Software also has beendeveloped for painting applications and adds many teaching, editing, and programming capabilities.In these cases the robot can be controlled by a specially developed electronics package, or by usinga teach pendant. A representative grouping of contemporary industrial robots is given in Fig. 17.

FIGURE 17 Representative industrial robots. (Courtesy of GMFanuc Robotics Corporation.)(a) Spot welding, heavy part or tool handling, parts transfer, palletizing, material removal. Payload

120 kg (264 lb). Six axes of motion; floor- or wall-mounted; repeatability ±0.5 mm (0.02 in.); baserotation 300◦; vertical travel 2731 mm (107.5 in.); reach 2413 mm (95 in.). (S-420)

(b) Arc welding of large parts on conveyors and fixtures. Payload 5 kg (11 lb). Six axes of motion;overhead-mounted; repeatability ±0.1 mm (0.004 in.); base rotation 300; reach 1309 mm (51.5 in.).(ArcMate OH)

(c) Material handling, machine loading, palletizing, mechanical assembly in severe environments.Payload 50 kg (110 lb). Three to five axes of motion; floor-mounted; repeatability 0.5 mm (0.02 in.);base rotation 300◦; vertical travel 550 or 1300 mm (21.6 or 51.2 in.); horizontal travel 500 to 1100 mm(19.7 to 43.3 in.). (M-100)

(d) Palletizing and machine loading. Payload 50 kg (110 lb). Four to five axes of motion; floor-mounted; repeatability ±0.5 mm (0.02 in.); vertical travel 1850 mm (72.8 in.); radius reach 1930 mm(76 in.); access to two or more conveyors. (M-400) (Continues)

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FIGURE 17 (Continued)(e) Gantry robot for medium-to heavy-payload machine load and unload uses. Also palletizing, mechanical

assembly, parts transfer. Cartesian coordinates. Area (shown) or linear configurations. Payload 50 kg (110 lb).Repeatability ±0.5 mm (0.02 in.); very large work envelope; two to four axes of motion for linear design; threeto five for area design. (G-500)

(f) Multipurpose material handling; light-payload applications. Payload 10 kg (22 lb). Six axes of motion;floor-, ceiling-, or wall-mounted; repeatability 0.2 mm (0.008 in.); base rotation 300◦; front reach 1529 mm(60.2 in.). (S-10)

(g) Laser robot for integration with a laser generator. For precision-path laser processing-welding, cutting,heat treating, and cladding. Payload 5 kg (11 lb). Five axes of motion; floor-mounted; antibacklash drive;repeatability ±0.05 mm (0.002 in.); base rotation 200; vertical travel 1968.5 mm (77.4 in.); horizontal travel3964 mm (156 in.). Complete robotic laser cells available. (L-100)

(h) Industrial and automotive paint finishing of stationary or moving parts. Payload 7 kg (15.5 lb). Sixor seven axes of motion; floor-or rail-mounted; repeatability 0.5 mm (0.02 in.); maximum reach 2613 mm(103 in.), large work envelope. Robot also used for dispensing and applying antichip sealers and underbodydeadeners. (P-155)

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CURRENT-TO-PRESSURETRANSDUCERS FORCONTROL-VALVE ACTUATION

by Len Auer1

Current-to-pressure transducers (I/Ps) are used primarily in process control to change a 4-to 20-mAelectronic signal from a computer controller into a 3-to 15-psi (21-103 kPa) pneumatic signal. Theoutput signal from the I/P is then used to fill a diaphragm or piston actuator, which, in turn, modulatesa valve. An effective I/P must provide air to the receiver quickly, accurately, and in sufficient quantity.The I/P device also must be able to exhaust air quickly when the signal decreases, consume a minimumamount of supply air for operation, and be easy to repair. In most industrial applications the I/P alsomust be sufficiently rugged to withstand difficult environmental conditions, including vibration, dirtysupply air, temperature extremes, and corrosive conditions.

Since the mid-1960s I/Ps have used the traditional flapper-nozzle design concept with relativelyfew alterations. However, in the late 1980s several I/P manufacturers introduced new technologies in anattempt to overcome some of the difficulties encounted with the traditional design. These technologiesvary in approach, and application success often depends on the integration of several design featureswithin the same I/P.

TRADITIONAL FLAPPER-NOZZLE DESIGN

The traditional flapper-nozzle design is shown in Fig. 1. The input current (4 to 20 mA) is applied to acoil-armature arrangement that acts on a beam. The beam (“flapper”) positions itself against a nozzlethat has air flowing through it. The gap between flapper and nozzle determines the backpressure, alsocalled pilot pressure, that builds up in the nozzle. Other variations of the flapper-nozzle concept areshown in Fig. 2. A bellows sometimes is connected to the nozzle area to balance the forces on the

FIGURE 1 Traditional flapper-nozzle design. (Rosemount Inc.)

1 Product Marketing Manager, Rosemount Inc., Eden Prairie, Minnesota.

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FIGURE 2 Variations of flapper-nozzle concept. (Rosemount Inc.)

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armature-flapper. The pilot pressure usually is channeled to a pneumatic booster or relay. This boostertranslates the low pilot pressure into a higher output pressure and capacity, which typically is 3 to 15psi (20 to 100 kPa) and 4 ft3/min (0.11 m3/min), respectively.

I/Ps that use the flapper-nozzle principle alone may have difficulty with some of the environmentalfactors faced in an application. Such I/Ps essentially are mechanical in nature and do not use electronicfeedback sensors. The flapper is susceptible to vibration and traditionally has forced users to mountthe I/P separately on a pipe or rack. This requires additional tubing to carry the I/P output signal tothe valve. Output tubing installation costs offset any benefits from mounting the I/Ps together in acommon location. The dead time and lag time introduced into the loop by longer output signal tubingcan have a significant impact on loop performance. In addition to vibration, traditional I/Ps also can beadversely affected by fluctuations in air supply, downstream tubing leaks, temperature changes, andaging of the magnetic coil within the I/P. Periodic calibration checks are required in order to maintainthe output of the I/P within the desired range.

Dirty supply air can be a major cause of I/P downtime. While mechanical I/Ps do not have electronicfeedback to compensate for partial plugging, the nozzle opening traditionally has been designed tobe at least 0.015 in. (0.4 mm) in diameter. This has reduced the likelihood of the nozzles pluggingand is a strong point for those I/Ps that have maintained the larger nozzle diameters.

INTRODUCTION OF NEW I/P CONCEPTS

Since the late 1980s several new technologies have been introduced within the I/P. These new conceptshave changed the nature of the pilot stage and incorporated sensor-based electronics. There are someinherent tradeoffs when combining different versions of these new technologies, and they have metwith mixed results in the field. Two concepts are described here.

Piezoceramic Bender-Nozzle. This device, another variation of the flapper concept, is shown inFig. 3. The unit does not use the coil to move the flapper, but instead, the flapper itself is made oflayers of different materials which are laminated together. These different materials flex or bend whena voltage is applied across them. The 4- to 20-mA input signal to the I/P must be converted to a voltagein the range of 20 to 30 V dc.

FIGURE 3 Piezoceramic bender-nozzle. (Rosemount Inc.)

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FIGURE 4 Deflector bar design. (Rosemount Inc.)

FIGURE 5 Electronic feedback control as applied to I/P device. (Rosemount Inc.)

This design tends to be more stable in vibration than the typical flapper armature, particularly whencombined with an electronic feedback loop. Several drawbacks also have become evident from fieldapplications. The bender does not have a very good “memory” and will tend to locate in a differentposition for the same input signal. This creep can be cumulative and eventually will exceed theadjustment range of the calibration mechanism. An electronic feedback sensor can be combined withthe piezoceramic bender to compensate for the creep temporarily, but the feedback circuit typicallyuses much of the power available from the input signal. This leaves little power to energize the bender.The bender cannot balance against the force of the nozzle air, unless the nozzle is kept relatively small.Thus larger nozzles must be traded for improved bender control. Plugging of small nozzles or orificestypically is the leading cause of I/P field failure.

Deflector Bar Design. Shown in Fig. 4, this pilot stage concept also was introduced in the late1980s. This design uses an electromagnetic coil similar to the traditional flappers. The deflector bar,however, replaces the flapper as the main moving part in the assembly. The flapper no longer is usedto block the airflow coming out of the nozzle. The deflector bar design is based on the Coanda effect,which may be defined as the tendency of an airstream to attach itself to a surface with which it makesoblique contact.

The actual hardware consists of two opposed 0.015-in. (0.4-mm)-diameter nozzles, fixed on thesame centerline, spaced about 0.15 in. (0.4 mm) apart. One nozzle provides a high-velocity airstreamfrom the air supply, the second nozzle recaptures the airstream and converts its kinetic energy to apressure (potential energy).

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FIGURE 6 Possible variations in dynamic response of I/P devices. (Rose-mount Inc.)

To vary the pilot stage output pressure, a 0.019-in. (0.5-mm)-diameter solid deflector bar, whichis positioned crosswise and midway between the nozzles, is caused to move into the airstream. Thestream attaches to the surface of the bar and follows its curvature for some distance before separating.This deflection of the airstream away from the receiver nozzle results in a decrease in the pilot outputpressure. To increase the output pressure again, the bar is simply pulled out of the airstream.

The deflector bar is low mass, which adds vibration stability, and only travels about 0.003 in. (0.08mm) to produce a full-scale output change. This small travel requirement, coupled with the fact thatthe bar movement does not directly oppose the airflow, allows for a low-power magnetic actuatorcoil. The nozzle diameter is not limited by force-balance versus power tradeoffs, as is the case withflapper designs. The nozzle diameter is relatively large at 0.016 in. (0.15 mm), and is only limited bythe desirable range for air consumption.

When combined with an electronic feedback sensor, this type of pilot stage is virtually unaffectedby vibration and provides quick response to input changes. Lag time, the rate of change to reach anew output pressure, actually can be reduced by the quick response, as compared with other pilotstage designs.

ELECTRONIC FEEDBACK

Several I/Ps introduced since the late 1980s have incorporated pressure sensors and electronic feedbackcontrol. This feedback, shown in Fig. 5, detects the actual output pressure and is completely internal tothe I/P. There are several types of sensors used, the most common of which is a solid-state silicon strain-gage type. The electronics in the I/P contain an error-correction circuit that continuously comparesthe output sensor reading with the input signal. The electronics then adjust the current to the pilotstage to make any needed corrections to the output of the I/P.

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Electronic pressure sensor feedback is the foundation on which the installed advantages of recentI/Ps are based. These advantages include vibration immunity, calibration stability, repeatability, quickdynamic response, and reduced downtime. Performance of the electronic feedback circuits is relativelyconsistent across the I/Ps available as of the early 1990s. However, many of the advantages just givenare contingent on integration of the electronic feedback with the optimum pilot stage design. Iftradeoffs are made in order to incorporate electronics into the I/P, reliability can be adversely affected.In addition, dynamic response may vary considerably, as shown in Fig. 6. In particular this can beevident when the same I/P design is used to fill a wide range of output end volumes. Proper balanceof damping and responsiveness is critical to the operation of the I/P into the full range of typicaloutput volumes. The importance of the role played by the I/P in the performance of a loop cannot beoveremphasized.

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