CNC SYSTEMS Electrical Component

7/22/2019 CNC SYSTEMS Electrical Component

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CNC SYSTEMS - ELECTRICAL COMPONENTS

(1) Power units

In machine tools, power is generally required for

For driving the main spindle For driving the saddles and carriages. For providing power for some ancillary units.

The motors used for CNC system are of two kinds

Electrical - AC , DC or Stepper motors Fluid - Hydraulic or Pneumatic

Electric motors are by far the most common component to supply mechanical input to a linear motionsystem. Stepper motors and servo motors are the popular choices in linear motion machinery due to

their accuracy and controllability. They exhibit favourable torque-speed characteristics and arerelatively inexpensive.

(1.1) Stepper motorsStepper motors convert digital pulse and direction signals into rotary motion and areeasily controlled. Although stepper motors can be used in combination with analog ordigital feedback signals, they are usually used without feedback (open loop). Steppermotors require motor driving voltage and control electronics. The rotor of a typicalhybrid stepper motor has two soft iron cups that surround a permanent magnetwhich is axially magnetized. The rotor cups have 50 teeth on their surfaces and

guide the flux through the rotor- stator air gap. In most cases, the teeth of one setare offset from the teeth of the other by one-half tooth pitch for a two phase steppermotor.


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Figure 24.1 Unipolar and Bipolar Stepper Motor

The stator generally has the same number of teeth as the rotor, but can have twofewer depending upon the motor's design. When the teeth on the stator pole areenergized with North polarity, the corresponding teeth on the rotor with South polarity

align with them. Similarly, teeth on the stator pole energized with South polarityattract corresponding teeth on the rotor that are energized with North polarity. Bychanging the polarity of neighbouring stator teeth one after the other in a rotatingsequence, the rotor begins to turn correspondingly as its teeth try to alignthemselves with the stator teeth. The strength of the magnetic fields can be preciselycontrolled by the amount of current through the windings, thus the position of therotor can be precisely controlled by these attractive and repulsive forces.

There are many advantages to using stepper motors. Since maximum dynamictorque occurs at low pulse rates (low speeds), stepper motors can easily acceleratea load. Stepper motors have large holding torque and stiffness, so there is usually noneed for clutches and brakes (unless a large external load is acting, such as gravity).Stepper motors are inherently digital. The number of pulses determines positionwhile the pulse frequency determines velocity. Additional advantages are that theyare inexpensive, easily and accurately controlled, and there are no brushes tomaintain. Also, they offer excellent heat dissipation, and they are very stiff motorswith high holding torques for their size. The digital nature of stepper motors alsoeliminates tuning parameters.

There are disadvantages associated with stepper motors. One of the largestdisadvantages is that the torque decreases as velocity is increased. Because most

stepper motors operate open loop with no position sensing devices, the motor canstall or lose position if the load torque exceeds the motor's available torque. Openloop stepper motor systems should not be used for high-performance or high-loadapplications, unless they are significantly derated. Another drawback is that dampingmay be required when load inertia is very high to prevent motor shaft oscillation atresonance points. Finally, stepper motors may perform poorly in high-speedapplications. The maximum steps/sec rate of the motor and drive system should beconsidered, carefully.

( 1.2) Servo Motors

Servo motors are more robust than stepper motors, but pose a more difficult controlproblem. They are primarily used in applications where speed, power, noise level aswell as velocity and positional accuracy are important. Servo motors are notfunctional without sensor feedback. They are designed and intended to be applied incombination with resolvers, tachometers, or encoders (closed loop). There areseveral types of servo motors, and three of the more common types are describedas follows. The DC brush type servo motors are most commonly found in low-end tomid-range CNC machinery. The "brush" refers to brushes that pass electric currentto the rotor of the rotating core of the motor. The construction consists of a magnetstator outside and a coil rotor inside. A brush DC motor has more than one coil. Each

coil is angularly displaced from one another so when the torque from one coil hasdropped off, current is automatically switched to another coil which is properly


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located to produce maximum torque. The switching is accomplished mechanically bythe brushes and a commutator as shown below.

There are distinct advantages to using DC brush servo motors. They are veryinexpensive to apply. The motor commutates itself with the brushes and it appears

as a simple, two-terminal device that is easily controlled. Among the disadvantages itis the fact that they are thermally inefficient, because the heat must dissipate throughthe external magnets. This condition reduces the torque to volume ratio, and themotor performance may suffer inefficiencies. Also, the brushed motor will requiremaintenance, as the brushes will wear and need replacement. Brushed servo motorsare usually operated under 5000 rpm.

The DC brushless type offers a higher level of performance. They are often referredto as "inside out" DC motors because of their design. The windings of a brushlessmotor are located in the outer portion of the motor (stator), and the rotor isconstructed from permanent magnets as shown below. DC brushless motors aretypically applied to high-end CNC machinery, but the future may see midrangemachinery use brushless technology due to the narrowing cost gap.

AC servo motorsare another variety that offers high-end performance. Theirphysical construction is similar to that of the brushless DC motor; however, there areno magnets in the AC motor. Instead, both the rotor and stator are constructed fromcoils. Again, there are no brushes or contacts anywhere in the motor which meansthey are maintenance-free. They are capable of delivering very high torque at veryhigh speeds; they are very light and there is no possibility of demagnetization.

.However, due to the electronic commutation, they are extremely complex andexpensive to control. Perhaps the largest advantage of using servo motors is thatthey are used in closed loop form, which allows for very accurate position informationand also allows for high output torque to be realized at high speeds. The motor willdraw the required current to maintain the desired path, velocity, or torque, and iscontrolled according to the requirements of the application rather than by thelimitations of the motor. Servo motors put out enormous peak torque at or near stallconditions. They provide smooth, quiet operation, and depending upon the resolutionof the feedback mechanism, can have very small resolutions. Among thedisadvantages of servo motors are the increased cost, the added feedbackcomponent, and the increased control complexity. The closed loop feature can be a

disadvantage for the case when there is a physical obstacle blocking the path ofmotion. Rather than stalling, the servo motor will continue to draw current toovercome the obstacle. As a result, the system hardware, control electronics, signalamplifier and motor may become damaged unless safety precautions are taken.

( 2 ) Encoders

An encoder is a device used to change a signal or data into a code. These encodersare used in metrology instruments and high precision machining tools ranging from

digital calipers to CNC machine tools.


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( 2.1) Incremental encoders

With incremental linear encoders, the current position is determined by stating adatum and counting measuring steps. The output signals of incremental rotaryencoders are evaluated by an electronic counter in which the measured value isdetermined by counting "increments". These encoders form the majority of all rotaryencoders. Incremental rotary encoders with integral couplings used for lengthmeasurement are also in the market.

The resolution of these encoders can be increased by means of electronicinterpolation. There are, of course, the precision rotary encoders specificallydesigned for angle measurement. If finer resolution is required, standard rotaryencoders often utilize electronic signal interpolation. Rotary encoders for applicationsin dividing heads and rotary tables, with very small measuring steps (down to 0.36arc second) have in principle the same basic design features as standard rotary

encoders, but incorporate some overall varying construction.

Figure 24.2 Rotary encoders

( 2.2 ) Absolute encoders

Absolute linear encoders require no previous transfer to provide the current positionvalue. Absolute rotary encoders provide an angular position value which is derivedfrom the pattern of the coded disc. The code signal is processed within a computeror in a numerical control. After system switch-on, such as following a powerinterruption, the position value is immediately available. Since these encoder types

require more sophisticated optics and electronics than incremental versions, a higherprice is normally to be expected. Apart from these two codes, a range of other codeshave been employed, though they are losing their significance since moderncomputer programs usually are based on the binary system for reasons of highspeed. There are many versions of absolute encoders available today, such assingle-turn or multi-stage versions to name only two, and each must be evaluatedbased on its intended application.

( 2.3 ) Rotary and Linear encoders

A linear encoder is a sensor, transducer paired with a scale that encodes position.The sensor reads the scale in order to convert the encoded position by a digital


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readout (DRO). Linear encoder technologies include capacitive, inductive, eddycurrent, magnetic and optical.

A rotary encoder, also called a shaft encoder, is an electro-mechanical device usedto convert the angular position of a shaft to a digital code, making it a sort of a

transducer.Rotary encoders serve as measuring sensors for rotary motion, and for linear motionwhen used in conjunction with mechanical measuring standards such as leadscrews. There are two main types: absolute and relative rotary encoders.Incremental rotary encoder uses a disc attached to a shaft. The disc has severalradial lines. An optical switch, such as a photodiode, generates an electric pulsewhenever one of the lines passes through its field of view. An electronic controlcircuit counts the pulses to determine the angle through which the shaft has turned.

As the present trend of machine tools evolves toward increasingly higher accuracyand resolution, increased reliability and speeds, and more efficient working ranges,so too must feedback systems. Currently, linear feedback systems are available thatwill achieve resolutions in the submicron range.

Figure 24.3: Exposed and sealed linear encoders

Submicron resolutions, for example, are required in the semiconductor industry andin ultra-precision machining. Achieving these resolutions is possible with the use oflinear scales which transmit displacement information directly to a digital readout. Asin rotary, linear scales operate on the same photoelectric scanning principle, but thelinear scales are comprised in an overall straight construction, and their outputsignals are interpolated or digitized differently in a direct manner. One of thesesignals is always used by the accompanying digital readout or numerical control todetermine and establish home position on the linear machine axis in case of a powerinterruption or for workpiece referencing. Overall, there are two physical versions ofa linear scale: exposed or enclosed as shown in the figure 24.3. With an enclosed or"sealed" scale, the scanning unit is mounted on a small carriage guided by ballbearings along the glass scale; the carriage is connected to the machine slide by abacklash-free coupling that compensates for alignment errors between the scale andthe machine tool guide ways.

A set of sealing lips protects the scale from contamination. The typical applicationsfor the enclosed linear encoders are primarily machine tools. Exposed linear

encoders also consist of a glass scale and scanning unit, but the two componentsare physically separated. The typical advantages of the non-contact system are


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easier mounting and higher traversing speeds since no contact or friction betweenthe scanning unit and scale exists. Exposed linear scales can be found in coordinatemeasuring machines, translation stages, and material handling equipment.

Another version of the scale and scanning unit arrangement is one that uses a metal

base rather than glass for the scale. With a metal scale, the line grating is a depositof highly reflective material such as gold that reflects light back to the scanning unitonto the photovoltaic cells. The advantage of this type of scale is that it can bemanufactured in extremely great lengths, up to 30 meters, for larger machines. Glassscales are limited in length, typically three meters. There are several mechanicalconsiderations that need to be understood when discussing linear encoders. It is nota simple matter to select an encoder based just on length or dimensional profile andinstall the encoder onto a machine. These characteristic considerations includepermissible traversing speeds, accuracy and resolution requirements, thermalbehaviour and mounting guidelines.

Figure 24.4: Principle of rotary and linear encoders

( 3 ) CNC Controller

There are two types of CNC controllers, namely closed loop and open loopcontrollers. These have been discussed in details in section 22.2.

( 3.1 ) Controller Architecture:

Most of the CNC machine tools were built around proprietary architecture and could

not be changed or updated without an expensive company upgrade. This method ofprotecting their market share worked well for many years when the controltechnology enjoyed a four-to-five year life cycle. Now a day the controller life cycle isonly eight-to-twelve months. So CNC manufacturers are forced to find better andless expensive ways of upgrading their controllers.

Open architecture is the less costly than the alternatives. GE Fanuc and othermanufacturers introduced control architecture with PC connectivity to allow users totake advantage of the new information technologies that were slowly gainingacceptance on the shop floor. They created an open platform that could easilycommunicate with other devices over commercially available MS Windows operating

system, while maintaining the performance and reliability of the CNC machine tool.


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CNC SYSTEMS Electrical Component

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