USER'S GUIDE TO

1IRECT Lo IGITAL ,_ ONTROL00

OF HVAC EQUIPMENT

74:

~by R.E Kirts

DTICSE LECTEDEC 2 6 1989

Sponsored by the Naval Facilities Engineering CoiQand

Naval Civil Engineering LaboratoryPort Hueneme, California 93043

pm for OL48C 2 C4itt c ilmltd 0 6 0

CONTENTS

f Page

I. INTRODUCTION ...... ............ 1

II. BACKGROUND ............. .. 1

III. FUNDAMENTALS ...... ............ 2

IV. APPLICATIONS ............ .. 35

V. SUMMARY . . ............ 65

REFhRENCES .... ................ 67

APPENDIXES

A - Proportional-Integral-DerivativeControl ............... .A-1

B - Control System Tuning .... .... B-I

i Accesion ForNTIS CRA&IDTIC TAB 0

Utanootnced 0;"' .J11stificat.oil

~BY:': Oistribtotion I

: Avaitabfitty Codes

: Avail awldjorDist Special

I NTRODUCTION

Heating, ventilating, and air conditioning(HVAC) systems are now under going the most signi-ficant change in control technology in the past100 years. This change is the introduction ofdirect digital control (DDC) to HVAC systems.Computer control has been introduced to manythings over the past few years, from automobileengines to home appliances to most industrialprocesses, but only recently have small computercontrol systems been introduced into the spaceconditioning industry. To understand the impli-cations of this new HVAC control technology, itis necessary to consider several questions:

<:4at is DDC and what are its capabilities?QUnder what conditions is DDC a costeffective alernative to conventionalcontrol? &44-What constitutes good system design andinstallation practice?

This report addresses these questions bypresenting information which describes DDC, pre-sents its advantages and disadvantages, and givesapplications guidance.__-

II. BACKGROUND

Direct digital control is becoming an impor-tant factor in the new construction and retrofitmarket for HVAC controls for several reasons:energy savings through improved control, improvedcontrol system reliability, and easier maintenance.

I I Ii ,1

As applied to HVAC systems, direct digitalcontrol is direct computer control of a devicesuch as a valve or damper actuator. In a DDCsystem, the computer accepts data from sensorsmeasuring variables such as temperature, humidity,and pressure, performs calculations on thesedata to determine the correct output response,then sends a control signal directly to the con-trolled device.

DDC has been expanding into the chemical,food, and metals process industries steadily forabout the past 10 years until today it has vir-tually supplanted conventional pneumatic control-lers. In the process, DDC has proven itself tobe reliable, flexible, easy to use and maintain,and cost effective. Where and when a DDC controlsystem should be installed in an HVAC system is,at the present time, a function of the complexityof the system to be controlled. At the presenttime, DDC is most cost effective in larger,complex HVAC systems, but the HVAC system sizefor cost effective DDC application is rapidlydecreasing.

III. FUNDAMENTALS

What is DDC?

Direct digital control uses a programmabledigital computer to process information for thepurpose of determining the correct control action.The input information for the computer comesfrom analog sensors (such as temperature sensors)and digital sensors (such as switches). Thisinformation is used as variable data in a pre-determined sst of instructions called the con-trol progran. -roe control program calculatesdifferent val. for control parameters and takesdifferent paths in the logic of the programdepending upon the values of the input variables.

2

j?

After the computer completes its calculations,the appropriate value of control signal is outputeither in analog form (such as a voltage between0 and 10 volts) or in digital form (such as aswitch closure). The complete input-calculation-output sequence is usually repeated several timesevery minute. Because of this repetitive ratherthan continuous processing of input data, a DDCcontrol system is called a "sampled data system."These concepts are illustrated in Figure 1.

The word "direct" in direct digital controlimplies that the controller has immediate controlover the final control element. This is in con-trast to a conventional energy monitoring andcontrol system (EHCS) in which a computer actsonly through a conventional pneumatic or analogelectronic control system to actuate the finalcontrol element.

The word "digital" in direct digital controlmeans that input data are converted into digitalform, i.e., a discrete number, so that they maybe operated on by a sequence of instructionscalled a computer program. The output from theprogram is also a number, which may be displayedon an output device for information purposes orconverted to a continuous (or analog) outputsuch as a voltage.

Direct digital control should not be con-fused with analog electronic control (Figure 2).An analog electronic controller is the electronicequivalent of a conventional pneumatic controller.As a consequence, each analog electronic control-ler is designed to perform only one specificfunction. Thus, an UVAC control system which

- - uses analog devices must be built up from a numberof interconnected special purpose devices toobtain the desired system performance. Analogelectronic devices use operational amplifiers,voltage dividers, and other analog components to

3

produce an output signal which hat. A fixed rela-tionship to the input signal. For example, theoutput might be the difference between two inputsignals multiplied by a constant. The output isalso a continuous function of the input. Thisis to be contrasted with a DDC controller inwhich a computer "takes a Jook" at the inputs,follows one of several different logical pathsdepending upon the values of the inputs, an±dgenerates an output (or outputs) accordingly.Figures 1 and 2 illustrate these differences.

What's the difference between DDCand an EMCS System?

A conventional energy monitoring and controlsystem is used when centralized monitoring and asupervisory level of remote control are desiredin an HVAC control system. A central computercommunicates with a remotely located electronicdevice (usually called FID, for field interfacedevice) which provides the computer with data onlocal conditions, such as temperature or motorstatus, and which can relay signals from thecentral computer to the local HVAC control systemto do things such as change the set point of apneumatic two-input controller. An EMCS systemis usually designed to provide detailed statusand alarm reports and log system data for trendanalyses and energy savings calculations. Thekey distinction to be made is that in an EMCSsystem, control of the individual devices (suchas valves, dampers, or fan speed) remains with alocal, more or less independent, control system.These local control systems are often called"local loop controllers." The local loop con-

* trollers may be either pneumatic, electric, analogor digital electronic. Thus, a DDC localloop controller may or may not be connected toan E CS system. The relationship between DDCand EMCS is illustrated in Figure 3.

t 4

001 Sensor0000

actuator

Special circuits convertanalog signals to digital DIA

form for use by computer Digital computer(and vice versa) Iperiodically

samples sensor,performs controlcalculation, and

control outputs thecalculation

Figure 1. A digital control system.

I ° +I ,..-m . ,5

000 sensor

000000

actuator

, -U :-Input andoutput signalsare continuousrather thanpeiodc

Analog electronictemperaturecontroller

Figure 2. An analog control system.

( BASE -WIDEEMCS

COORDINATINGCOMMANDS ARESENT DOWN THECONTROL

SINGLE HIERARCHYBUILDING

EMCS

SYSTEM STATUSAND DATA ARE ISENT UP THE IHIERARCHY

FIELDINTERFACE

S DEVICE,(,OPTIONAL)

DIRECT DIGITALLOCAL LOOP

CONTROL

IF FUNCTIONING OF LOCAL CONTROL IS INDEPENDENTOF OTHER DEVICES, CONTROL IS SAID TO BE

DISTRIBUTED CONTROL

Figure 3. Relationship between DDC and EMCS.

l Inl l~lll lll ll Iu nn n ,7

What are the Advantages andDisadvantages of DDC?

The major advantage of direct digital controlis that one programmable unit can be made toperform almost an infinite variety of controlfunctions. If, at some later date, the controlsystem needs to be modified, changing a DDC systemis mostly a matter of changing the computer prog-ram, whereas changing a pneumatic or analog elec-tronic control system could require majorreplacement and rewiring of components. This isillustrated in Figures 4 and 5 where pneumaticand DDC control systems are compared in the sameapplication.

Another point illustrated by the informationpresented in Figures 6 and 7 is that of accessto control system documentation. In actual prac-tice, it is often difficult to find current andaccurate drawings for a conventional, built-upcontrol system such as that illustrated in Fig-ure 4. Data on setpoints, reset schedules, andevent timing, such as that presented in Figure 6,are often even more difficult to find. In con-

trast, the "as-built" drawings for a DDC controlsystem are always available in the form of thecomputer program code (Figure 7). In additionto the control logic, setpoint and other dataare readily available. Program logic, setpointand schedule data, and other information storedin a DDC unit can be displayed on a video ter-minal or printed when the control documentationneeds to be examined or recorded.

8

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10

FAN CONTROL

The fan control circuit shall include safety controls,

a hand-off-auto switch, timer energized controls, and a

manual override switch.

Safety controls shall consist of an emergency disconnect

switch mounted in the mechanical room, a manual reset freeze-

stat, a supply air smoke detector, and a return air smoke

detector.

With all safety controls closed and the hand-off-auto

switch in the auto position, the fan shall be controlled by

a seven day timer.With the fan off, should the space temperature fall

below 550F, night thermostat shall restart the fan.A manual override switch, located in the occupied space,

shall, when activated, restart the fan for a timed period of

1 hour.

FAN CONTROL AND TENPERATURE CONTROL INTERLOCK

AC power and DC power to each component in the systemshall be interrupted whenever the fan circuit is de-energized

or there is no air flow as sensed by air flow switch, AF.

MIXED AIR SECTION

Mixed air sensor, SM, through controller, TCM, shall

modulate the outside air and return air dampers to maintain

a mixed air temperature of 60*F 3*F.With the system in the occupied mode, the minimum

position switch, NP, shall insure the quantity of outsideair is not less than 10% of the total CFM. When the system

is in an operating but unoccupied mode, the minimum positionswitch shall be removed from the circuit.

High limit switch, HL, shall return the system to minimum

outside air (occupied) or zero outside air (unoccupied) when-

ever the outside air rises above 70*F.

Figure 6. Sequence of control for pneumatic system.

(See Figure 4 for nomenclature.)

11

k m. ..

Whenever the zone requires less cooling than the mixed

air section is providing, the discriminator, LSS, shall passcontrol of the OA/Rh dampers to the zone controller.

HEATING COIL

Zone sensor, SZ, through controller, TCZ, and

sequencer, SQl, shall modulate the hot water valve from fullheat to no heat over a zone temperature range of 66°F to

700F.

COOLING COIL

Zone sensor, SZ, through controller, TCZ, and

sequencer, SQ2, shall modulate the chilled water valve from

no cooling to full cooling over a zone temperature range of

76*F to 80°F.

Figure 6. Continued.

12

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Control system changes are much easier toimplement with DDC than with conventional controlsystems. Changes to a DDC control system aremade by changing the computer program and, per-haps, changing a sensor or actuator. Changes toa pneumatic or analog electronic control systemusually require extensive rework of the controlpanel, including component replacement and re-wiring or repiping. This is because the controllogic of a built-up control system, such as apneumatic system, is composed of many discrete,special purpose devices with the logic "hard-wiredin" by means of the interconnecting wire or pipes.This makes changing the system difficult andexpensive. Figure 8 presents a typical controlschematic for a small pneumatic system. Eventhis small system consists of more than 80 dis-crete components. It is often difficult to deter-mine the control logic and operating sequencefrom control diagrams such as Figure 8. Also,the original "as-built" control schematic may nolonger be very representative of the actual con-trol system installation.

A significant advantage of direct digitalcontrol systems is the energy savings achievablethrough improved control of RVAC systems.Improved control is achieved in two ways: betterfeedback control algorithms, and more sophisti-cated control strategies.

A major advantage of DDC is the ease withwhich proportional-integral-derivative (PID) andother advanced control algorithms can be imple-mented. Most pneumatic and analog electroniccontrollers offer only proportional control. Inproportional control, the output of the controlleris made proportional to the difference betweentwo inputs: one is the setpoint and the otheris the measured variable. The problem with pro-portional control is that corrective action (i.e.,an output from the controller) occurs only after

15

a measurable error occurs between the input andthe setpoint. This results in loose control anda steady state offset error, or control droop.See Figure 9. Addition of integral controleliminates the offset error and improves controlresponse. Integral control eliminates thesteady state error by generating an outputsignal proportional to the accumulated error.In some control applications, it is important to

consider not only the accumulated error, butalso the rate at which the error signal ischanging, or the derivative of the error. Theaddition of derivative control to the controlprogram causes corrections to be made based onthe rate to change of the error. Derivativecontrol gives a controller a kind of "lookahead" or "feedforward" capability that isespecially valuable when there are significanttime delays between when an action takes placeand when its effect is felt, for example when atemperature sensor controlling a damper islocated in the ductwork many feet downstreamfrom the damper. For most HVAC applications, PI

control is an adequate control technique. Amore detailed description of PID control ispresented in Appendix A.

Figure 10 illustrates the performance of a

system controller using PID control. Note thereduced settling time and the elimination of theoffset error. PID control saves energy over

proportional control because the offset errorrepresents an unnecessary energy consumption(e.g. the cooling coil discharge temperature is

lower than it would need to be with PIDcontrol). The faster settling time of PID

control also results in reduced energy consump-tion and may result in improved occupant comfort.

16

lis

.313 moo .

17

Figure 9. Proportional control.

I19

ofse

19

I tI I iI. ..l t . . . l i

less overshoot

noofe

Time

Figure 10. Proportional/integral/derivative control.

20j

Figures 11 and 12 present data on measuredcontrol system response for a pneumatic and adirect digital control system operating underthe same conditions. Note the improved perfor-mance achieved with DDC. Figure 13 presents the

. results of computer simulations which comparethe energy costs for two types of EVAC systems,each with conventional proportional control anda DDC controller with a PID algorithm. Notethat good control can save 10 to 15% of the energycost associated with an HVAC system.

A very important feature of DDC is the abil-ity to implement sophisticated energy managementstrategies, such as chiller or boiler optimiza-tion, enthalpy economizer cycles, load shedding,and optimum start-stop, with comparative ease.Many energy conservation strategies are difficultor impossible to implement with traditional con-trol components or require purchase of specializedcomponents.

Ease of operation is one of the major advan-tages of DDC control systems. Most control systemmanufacturers have tried to make their products"user friendly." Information on controller inputs,outputs, and setpoints is readily available bymeans of built-in or plug-in alphanumeric displayterminals. With the proper access codes, set-points and other controller parameters can bereadily changed by engineering or maintenancepersonnel. Other access codes permit changes tobe made in the computer program.

DDC control has been used in industry, espe-cially the chemical, food, and other processindustries for about the past 10 years. It hasbeen proven to be very reliable, even in veryharsh environments. As a consequence, it has

fvirtually supplanted pneumatic control in theseindustries. When there is a problem with amicrocomputer-based system such as DDC, the con-troller can often assist in problem diagnosis.

21

The operating system and support programs for aDDC controller can be written to perform manyself-test and self-diagnosis functions. Con-trollers are available which can alert the userto bad sensors, faulty peripheral functions, andinoperative output devices.

Direct digital control devices furthersimplify the task of system maintenance by thefact that the parts count of the system is much.ower than that of a comparable pneumatic oranalog electronic control system, so the problemsare easier to repair and the required inventoryof spare parts is small.

Finally, DDC control systems facilitate thedisplay of system data and alarm conditions, andare readily tied into EMCS systems if supervisorycontrol or enhanced data acquisition and analysiscapabilities are desired.

Direct digital control systems are not with-out disadvantages, however. One of the presentdisadvantages is the higher cost when comparedto pneumatic and analog electronic systems.This is especially true for smaller size systems.The relationship between system cost and systemsize is illustrated in Figure 14. Because tradi-tional control systems are assembled from a col-lection of discrete components, system cost tendsto rise linearly with increa.ing system size.With a DDC system, however, a substantial invest-ment is required for the components necessary tomake even a single loop system work, but after

that investment is made, the system can be inex-pensively expanded to control many loops andprovide other capabilities as well. Of course,

the addition of accessories such as a color CRToperator console, remote communications capabil-ity, or data logging and trend analysis capabil-ity will add significantly to the cost. It is afact, however, that the minimum competitive systemsize for DDC has been getting smaller as the

22

U

! I

prices for these systems decline. A recent (1984)study of a demonstration installation of a DDCunit on a small (20,000 CFM) single zone airhandler at the Pacific Missile Test Center, PointHugu, CA., concluded that the hardware costs ofa DDC system and a comparable pneumatic controlsystem were $4,300 and $1,200, respectively.These estimates do not include the costs of pneu-matic actuators and other components common toboth systems. The cost of installing and main-taining the DDC system is expected to be lowerthan that of a pneumatic system, but no data onthese costs are available at this time.

Another factor which has been acting as arestraint to the implementation of DDC is accep-tance by building owners and operators. Mostbuilding operators are familiar primarily withconventional controls, usually pneumatic controlas pneumatics comprise about 90Z of all installedcontrol systems. There is a natural reluctanceto abandon a technology that is basically simple,mechanical in nature, easy to understand, and(in theory at least) easy to operate and maintain.This is especially true if the replacement tech-nology appears to be complex or mysterious (ascomputer technology appears to be to many people)or if it seems that a great amount of trainingwill be required to understand and operate thesystem. Training in the operation of DDC systemsis offered primarily by the manufacturers of theequipment, as many features of a DDC system (in-cluding the programming language and problemdiagnostic features) are unique to the manufac-turers product. However, more general courseson DDC and on HVAC system operation and mainte-nance are offered by a variety of organizationsincluding trade organizations, vocational schools,and the extension services of universities. Ahigh school education is adequate for basic under-standing and operation of DDC systems for HVACapplications.

23

I

74

G Zone 1-Zone 2

72

70

166

SPneunmatic controI64 * Building wArmip

startng at 5 on* Set point a72oF

for au CoUUnoler

62

Time, hr60 3 i e r 6 7 8

Figure 11. Measured system performance with pneumatic

control. (Ref 1)

24

74

a- Zone 2

72 0 - Zone 3 -

70

06

e Direct Digital Control* Building Wam-up

starting a 5 am6 Set point - 720 F

for all controfler

623 4 5 6 7

Time. hr

Figure 12. Measured system performance with Direct Digital

'ontrol. (Ref 1)

25

Reheat System

7.0 12.9%

TT VAV Systan6.0 15

Total 2.4

5.0 2.0Sav.39

~4.011.6

13.0 svn

2.0

0.0.2

0.0 ~ ~ .7 baee cl ek 0 0 .7with P1 1.6 oc.88 trol

Figure.5 130opaio.f1nrycosm tono0w

262

04 .baeieIWdc afre P

ConventionalBuilt-up Control

Syte

~Control System

System complexity

Figure 14. Relationships between control system size and cost.

What are the components of a DDC control

system?

The microcomputer

The heart (or perhaps brain is a betterterm) of a DDC control device is a small digitalcomputer, often referred to as the central proces-sing unit or CPU. The CPU is usually containedon a single micro-circuit chip. The CPU performsarithmetic and logical operations on data (numbers)it reads from memory or input devices, then writesthe result back to memory or to an output device.All computer-based devices must have memory, ordata storage, capability. This memory is usuallyin two forms: read only memory (or ROM), andrandom access memory (or RAM). ROM is calledinviolate memory because it not easily changedor lost, while RAM is the the part of the systemmemory which is constantly being modified. Datain RAM will be lost if power is removed from thecircuit (unless it is provided with a backupbattery power supply), while data in ROM will

27

not be affected. The type of ROM used in most

controllers is called an electrically erasableprogrammable read only memory (or EEPROM). Thistype of memory permits the read only memory tobe altered in the field by means of a controllerprogramming device. Program instructions and

data which will not be changed are stored inROM, while input data, the intermediate resultsof calculations, and other variables are storedin RAM. Two special purpose integrated circuitsare used to change analog data from sensors into

f a form the computer can use and to convert theresults of the computer operations into analog,

or continuous, form. An analog-to-digital con-vertor (ADC) converts an input voltage into adigital form (usually an 8 or 12 bit binary num-ber) that is compatible with the requirements ofthe CPU. A digital-to-analog converter performsjust the opposite function, i.e., it converts abinary number into a voltage level, which can beused to operate an actuator, transducer, or otherdevice.

In addition to the basic components listedabove, a DDC controller usually contains othercomponents such as an electronic clock to providetime-of-day and calendar functions, and a commu-nications device so that the controller can"talk" to other controllers, computers, or data

terminals.The components of a typical DDC system are

illustrated in Figure 15.

Sensors

The following types of sensors are commonlyused in DDC systems for HVAC applications: tem-perature, humidity, pressure, flow, status, andposition. All of the preferred types of sensorsused in DDC control systems have an electricaloutput. The temperature sensors are usually one

28

I

of three types: thermistor, resistance-temperaturedevices (RTDs), or integrated-circuit temperaturesensors. The preferred humidity sensor is themodern thin-film solid state device. Pressuresensors are usLally based on a strain gage loadedby a diaphragm. There are many types of flowmeasuring devices on the market. Some flow sen-sors have an analog output (such as those whichdepend on measuring a pressure drop through themeasuring device), while others have an outputthat is essentially digital in nature and consistsof a series of pulses. Examples of flow metershaving an analog output are an orifice and aventuri flow meter; flow meters having a digitaloutput are turbine and vortex shedding meters.Status sensors are basically switches, and areused to signal the controller whether or not adevice is on or off, open or closed, and so forth.It is sometimes valuable to have an independentmeasure of the position of final control elements,such as valves and dampers. This position feed-back information can be provided to the DDC com-puter by a number of methods, one of the morereliable being to connect the valve stem or dampershaft to a linearly variable differential trans-former (LVDT). An LVDT is a special purposetransformer which outputs a voltage proportionalto the distance a metal plunger is moved withinthe core windings.

A detailed description of sensor technologyfor DDC and EHCS is presented in Reference 3.

In general, it is worthwhile to pay extrafor accurate, calibrated, interchangeable sensors.The investment in an accurate control systemwill not pay off if it can not perform to itsdesigned capabilities, and it cannot provideaccurate, dependable control based on poor inputdata. Some specific recommendations on sensorselection are presented in Section IV - Applica-tions. The question of sensor accuracy (and

i ! 29

cost) versus controller performance (and energysavings) is the subject of several current re-search projects, and quantified answers to thisquestion should be available in the near future.

Actuators

Two types of actuator power are commonlyused with DDC systems: pneumatic and electric.Because of their relatively low cost, actuatingpower, and high reliability, pneumatic actuatorsappear to be the preferred technology at thepresent time. Since the output signal from aDDC device is either an analog voltage level ora switch closure, some method is required toconvert the electrical output to a branch lineair pressure signal to operate the pneumaticactuator. The device which converts an analogvoltage to a pressure level is called an elec-trical to pneumatic transducer of E/P trans-ducer. Some DDC units have built-in E/Ptransducers, but on most units it is a separatehardware item. A pair of digital outputs canalso be converted into a pressure level signalby using the output signals to activate anarrangement of solenoid valves. These inter-facing techniques are discussed in detail in alater section of this document.

Electronically controlled electric actu-ators, such as those which have a shaft thatrotates from rest to a certain position on theapplication of a signal from 3 to 9 volts (DC),can usually be connected directly to the analogoutput terminals (or ports) of the DDCcontroller.

30

ALA

COMM4UNICATION MMRINTER FACERO

LOCALPONDISPLAY &INEFC

ACCESS BNR

Figure 15. Digital controller hardware components.

31

Stepper motors are also used in some actu-

ator applications, especially those in which asmall motor can be used, such as control of avariable-air-volume (VAV) terminal. A steppermotor is a special purpose motor which rotates afixed amount (often 7.5 degrees), either clockwiseor counterclockwise, on each application of anelectrical pulse. As a consequence, the amountand direction of rotation of the motor, and hencethe position of the actuator, can be readilyachieved through a series of program commands.

Connecting the parts together

Usually the sensors are connected to thecontroller by means of a simple shielded, twisted2-wire pair. The input impedance of modern cir-cuits is usually so high that current loop trans-mitters are not required for long distance signaltransmission. Actuators are connected to theDDC unit by low voltage wiring or pneumatictubing. In some parts of the country where light-ning strikes are a problem, or in very noisyelectrical environments, fiber optic technologymay be used to connect sensors to the controlleror connect the controller to other controllersor a central computer. In fiber optic "wiring,"information is carried by light transmittedthrough a slender glass fiber rather than elec-tricity. A third method of connecting sensorsand actuators to the controller is by means ofpower line carrier (PLC) technology. PLC usesthe same wiring which distributes electricalpower through the building to carry coded mes-sages between components of the system.

All of these communication systems will bediscussed in detail later in this document.

32

IV. APPLICATIONS

Selection and Design Guidance

HVAC System Complexity and AppropriateTechnology. As a general rule, DDC control sys-tem technology is cost effective only for rela-tively large and/or complex HVAC systems. Thereason for this was discussed earlier and wasillustrated in Figure 14. Traditional controlsystems are composed of whatever number of dis-crete components are required to do the job.Thus, the small control systems have a only afew parts and relatively low cost, and largecontrol systems have many components and a highercost. Because the traditional control system isassembled from discrete components, the costtends to increase linearly with system size.

With DDC control systems, a certain minimumnumber of components must be included in eventhe controller regardless of the size or com-plexity of the HVAC system it controls. Forexample, a microcomputer, memory, A/D, D/A, powersupply, and other components are required in aDDC device whether the device controls one actu-ator or ten. In fact, it costs little more tobuild a DDC system to control many devices asto control one or two. Thus, DDC controllerscosts are characterized by a relatively highminimum cost and a comparatively small rate ofchange of cost with system size. The minimumsize of HVAC system for which DDC is the mostcost effective control technology is not a mat-ter of component costs alone; differences inoperating and maintenance costs, energy savingspotential, and other factors also need to beconsidered. The minimum size HVAC system forcost effective application of DDC has beendecreasing every year since the introduction of

33

DDC due to the decline in the cost ot the elec-tronic components, increased competition in themarket, and the recovery of investment by manu-facturers.

The cost effectiveness of a DDC system cansometime be improved by using DDC as part of a"hybrid" control system that mixes DDC and con-ventional control technology. For example,instead of applying DDC throughout a VAV or reheatHVAC system, it may be both technically accep-table and economically sound to use DDC to con-trol major HVAC functions such as the mixed airsections, hot and cold deck temperature, fans,and chiller, and use pneumatic controllers forlocal applications such as VAV terminal or reheatcoil control, where precise control has no sig-nificant benefits.

Direct digital control will probably neverentirely replace pneumatic or self-powered controldevices. For certain applications, such as fire-stats, freezestats, and other alarms, the inde-pendence and reliability of traditional controlcomponents will assure their continued use, atleast as a backup to the DDC system.

Features to Look for in DDC

There are several features of DDC systemswhich should be evaluated by those consideringinstalling DDC.

Programs. A DDC system should contain asstandard equipment a selection of prewrittencomputer programs for standard controller func-tions such as time-of-day, economizer cycles,reset, load shedding, chiller optimization, VAVfan synchronization, and optimum start/stop.The prospective buyer of a DDC system shouldinvestigate the issue of prewritten programscarefully to determine exactly what is available

34

I[| !mm m m m m m mm m m m m m mm -- . .

Iand how it works. It is helpful to obtain flow-charts or detailed narrative descriptions of theprograms to aid in assessing whether or not theycan do the anticipated job. Some HVAC controlprograms have been developed by the Governmentas standards against which vendor programs canbe compared for performance. If a vendor hasdemonstrated that the performance of their HVACcontrol programs meets or exceeds that of theGovernment bench mark, the code is assured to becapable of acceptable performance. The exist-ence of a large selection of prewritten controlalgorithms means that less custom programmingwill be required and, as a consequence, acquisi-tion cost will be reduced. Special purpose pro-grams can often be developed by connectingseparate prewritten programs (or parts of prog-rams, called subroutines) with custom programming.This technique substantially reduces programdevelopment costs.

Ease of Programming. It is always necessaryto do some custom program development becauseevery HVAC installation is unique. Most DDCvendors offer a programming service which willdo the program development work based on thecustomer's system schematics and a narrativedescription of the control system operation. Ifthe customer is interested in doing its own prog-ram development, it should investigate the pro-gramming language used by the vendor for easeand flexibility in programming. The class ofprogramming languages termed "high level" lan-guages, such as BASIC and FORTRAN, are theeasiest to learn and to use because the commandsare written in English language-like statements.At the other end of the language spectrum arethe assembly languages, machine codes, and otherdevice specific languages by which the electroniccomponents actually communicate with each other.

35

Programming in these fundamental languages re-quires substantially more training and is muchmore time consuming. Most controller programminglanguages offered by manufacturers of DDC equip-ment fall somewhere between a true "high level"language and a machine level language, althoughthe trend is toward use of a high level languagesuch as BASIC or PASCAL.

Diagnostics and Data Display. An importantfeature to look for in a DDC system is a largenumber of self diagnostic features. Self diag-nostics help take some of the mystery out of"computerized" control systems and aid in re-vealing and correcting problems with both thesystem hardware and software. For example, aDDC system can be easily programmed to detectopen circuit or short circuit sensor or actuatorwiring, sensors which are giving suspiciousreadings, or actuators that fail to respond tocontroller output. The self diagnostic featurecan also be used to identify possible problemsin the control program, or software. A datasearch procedure that fails to find the requiredvalue, an iterative calculation that does notconverge to a solution, calculated values whichare outside the range of expected values are allexamples of possible problems with the programcode that can be identified with the aid of ade-quate diagnostic messages. The ability to displaycurrent and historical values of temperature,humidity, pressure or other properties of theHVAC system will aid in understanding how theHVAC system is performing and can lead to in-creased energy conservation, lower operatingcosts, and improved maintenance.

Reliability. The reliability of DDC con-trollers should, in theory, exceed that of tradi-tional built-up control systems because of the

36

smaller number of components in the control sys-tem. If the circuit boards and other componentsof a DDC system are selected by the manufacturerwith care and assembled with a high level of

quality control, very high reliability rates canbe achieved. Also, the working parts of thecontroller cannot be made inoperative by dirt,oil, or water in the working medium as can pneu-

matic devices. Care must be taken, however, toprotect the DDC system from excessive voltages

and spurious signals. Adequate lightning protec-tion and shielding of signal lines are requiredin many DDC installations.

Overall system reliability is also improvedby constructing the control system from manyindependently operating controllers rather thanone central controller. This system design iscalled distributed control or distributed pro-cessing.

Data on the reliability of the DDC systemsoffered by different manufacturers are not avail-able at this time, so it is suggested that theprospective purchaser of a DDC control systemobtain the names of previous customers from thevendor and contact them to obtain first handinformation on the performance of the DDC unitbeing considered.

Maintenance and Service Support. The main-tenance requirements of a DDC control unit willbe less than those of conventional control sys-tems for several reasons. First, there arefewer parts to break. When something does need

*repair, the problem should be easier to isolate.(This is particularly true if the computer por-tion of the controller has remained operative,as the computer can often be used to help findthe problem.) Repair is generally by replacementof the defective circuit board, relay, sensor,or other component. Because of the comparatively

37

small number of parts in a DDC system, the inven-tory of spare parts will be much smaller thanthat required for a conventional control system.Also, because of the digital nature of controlleroperation, the control system will not "drift"over a period of time as do both pneumatic andanalog electronic systems. Once initial set-upand calibration have been performed on a DDCsystem, the output will remain consistent withthe input.

Digital devices such as a DDC controllertend either to work as designed or to not workat all. Digital circuits, if correctly designed,are more immune to noisy or unpredictable opera-tion than are most analog circuits. If controlaction appears to be changing in an unexplainablemanner, check the inputs - the problem is oftena bad sensor or wiring.

The quality c support you receive from thelocal DDC supplier and the manufacturer is animportant factor in determining the ease withwhich DDC can be implemented at your facilityand the degree of satisfaction you will havewith it. Check the history and qualificationsof the manufacturer in detail and visit localinstallations of their products. A great dealcan be learned by spending an hour talking withpast customers.

Interfacing with DDC. A DDC system mustinterface (or "talk") to at least thxee xterualentities: the input devices, the output devices,and the operator. Sometimes a fourth device,another computer, is added to the above list.

Interfacing sensors

Temperature sensors. The two types of tem-perature sensors most commonly used in DDC sys-tems are resistance temperature devices, or RTDs

38

and thermistors. The electrical resistance of ametal at some temperature T may be related toits resistance at some reference temperature Rby the equation:

RT a R0(1 + T),oh

If the coefficient of resistance (a) is high,this property can be used to accurately measuretemperature. One of the metals most widely usedin the manufacture of resistance thermometers isPlatinum, which is formed into a very thin wirearranged in a serpentine pattern similar to aresistance strain gage and housed in a protectivesheath. Balco alloy is another metal which isoften used in the manufacture of RTD temperaturesensors. RTDs have a positive temperature coef-ficient, which means that resistance increaseswith increasing temperature (see Figure 16).

6 1 1 i I I

High angeNickel

V T-4

ED3

C 0 100 200 300 400 500 600 700 800

F 32 212 392 572 732 932 1112 1292 1472

Temperature

Figure 16. Resistance/temperature relationship for severalmetals. (Ref 4)

39

Another commonly used kind of electricalresistance temperature measuring device is thethermistor. A thermistor is made by sinteringoxides of metals, such as manganese, nickel andcobalt, into a very small bead, then coating thebead with resin or glass for protection. Thesmall size of thermistors makes them very sensi-tive and rapid in response. The resistance ofthermistors decreases with increasing temperatureand is exponential in form as opposed to thealmost linear characteristic of RTDs. This charac-teristic of thermistors makes signal conditioningsomewhat more difficult than it is for a lineardevice. Another disadvantage of thermistors isthat they are not readily interchangeable andtend to drift with age. Thus, a thermistor tem-perature sensor needs to be periodically recali-brated to assure accurate measurement.

The characteristics of commonly used tempera-ture sensors are presented in Table 1. The pre-ferred temperature sensor is the 1000-ohm PlatinumRTD.

A Wheatstone bridge resistance network (Fig-ure 17) is usually used to measure the unknownresistance of the sensor and, hence, the tempera-ture. The main factor affecting the accuracy ofthe measurement is the the unknown changes thattake place in the resistance of the sensor leads.This is particularly true when the sensor islocated some distance from the bridge network.To compensate for lead wire resistance, the pre-ferred RTD design has two built-in compensatingresistors which are an integral part of the RTDunit. This compensating RTD is called a four-wireresistance temperature device (Figure 18). Whena four-wire RTD is used as the sensor, any changesin lead resistance also take place in the "dummy"leads, and, since each set of leads is in opposinglegs of the bridge network, these changes are

40

completely balanced out. The analog input ter-minals of most DDC controllers are connected toWheatstone bridge circuits built into the control-ler circuit.

An alternative approach is tu use a two-wireRTD bridge located very close to the sensor tominimize the influence of lead length, and thenconvert the resulting voltage difference into acurrent signal for transmission to the controller.The device which converts a potential difference(or voltage) into an electrical current is calleda current loop transmitter. The standard currentloop transmitter provides an output which rangesfrom 4 to 20 milliamperes, full scale. By con-necting a resistor across the output of the trans-mitter at the "receiver" end, the current signalis converted back into a voltage. A precision250-ohm resistor converts a 4-20 mA signal intoa 1 to 5 volt potential difference (see Fig-ure 19). The reason current loop transmittersare used is that although the voltage differencewill vary between a pair of wires along theirlength, the current flowing in the wires willnot; thus electrical current is an unambiguouscarrier of information.

A third way to transmit the sensed informa-tion to the DDC controller is digitally, i.e.,as a series of l's and O's or, equivalently, asa series of high and low voltage pulses. Totransmit information digitally, the analog-to-digital convertor must be located at the sensor(vice at the DDC unit) and digital informationconverted from parallel format into serial format.Additional circuitry controls the timing of thedata transmission from the sensor unit. Thedata wires are connected to a digital input portof the DDC controller rather than an analog port.The digital transmitter unit may also contain anadditional micro-circuit which functions as anaddressable send/receive unit. This permits all

41

of the sensors to be connected in a single loop

of field wiring as illustrated in Figure 20.This loop, or network, also carries the elec-trical current which powers the individualsensor/transmitter units. In operation, the DDCcontroller is programmed to send a series ofbinary digits (or bits) over the loop. Thisseries of bits contains information on whichsensor the controller wants to communicate with(i.e. a device address) and an instruction tothe sensor unit. For example, the DDC controllermight address a particular temperature sensorand request that the data from that sensor betransmitted back to the DDC controller. Thesensor would then respond by sending a digitallycoded value of temperature to the controller.In this manner, all of the sensing devices canbe "polled," or accessed. Because the data aretransmitted as digits, the data are less suscep-tible to electrical noise. Also, error checkcodes can be used in the DDC program to estimatethe accuracy of transmission of the data.

A control computer can also communicatewith remotely located sensors or actuators overthe same wires used to distribute electricalpower throughout the building by means of a methodcommonly called power line carrier or PLC. ThePLC method is much like the method described inthe preceding paragraph except that the 120 VACpower wiring is used to interconnect the systemcomponents. Instead of turning the current onand off to encode a message as the dedicatedline method does, the PLC method modulates aspecial carrier frequency placed on the powercircuit. The carrier frequency is selected suchthat it will not interfere with the operation ofother devices on the circuit, such as motors orclocks. Obviously, to use the PLC method, thecontroller sensors and actuators must be con-nected to the same electrical power circuit.

42

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46

Humidity sensors. There are many methodsin use to measure the moisture content of air(see Table 2). One of the most modern and accu-rate methods is based on the change in capacitancein a thin film polymer capacitor as it absorbswater vapor. This type of sensor is reported topermit measurement of high humidity over longperiods of time with high accuracy. The deviceis also claimed to be stable and linear through-out the range of 0 to 100 percent relative humid-ity. Changes in capacitance can be measured byan alternating current bridge circuit called aSchering bridge which is similar in principle tothe DC Wheatstone bridge, or by a single ratiotransformer bridge. The necessary detection andbridge circuitry for humidity sensors are usuallypackaged as part of a "humidity transmitter"which has as output a 4-20 mA signal proportionalto relative humidity.

A second popular type of humidity sensingdevice uses a sulfonated polystyrene ion exchangemembrane which changes electrical resistancewith changes in relative humidity. Changes inresistance can be measured with a conventionalDC bridge circuit.

Perhaps the most commonly used humidityscisor in HVAC applications is the lithium chlo-ride dew point indicator, commonly called a dew-cell. A dewcell works as follows: A solutionof lithium chloride is heated by a small electricelement until the water vapor pressure above thesurface of the solution is in equilibrium withthe water vapor pressure in the ambient air. Itcan be deterrxined when equilibrium conditionsare obtained because under equilibrium conditions,the current supplied to the heating element willbe constant or the electrical resistance of thesolution will not change with time. The equilib-rium vapor pressure above a saturated solution

47

L1

of lithium chloride is a known function of tem-perature. Thus, when the salt solution is inequilibrium with the air, the vapor pressure ofthe air becomes a known function of temperature.By measuring the temperature of the salt solution,the vapor pressure of the water in the air canbe determined. Dew point temperature can bedetermined once the partial pressure of the watervapor is known. If the temperature of the ambientair is also measured, the relative humidity canbe determined. Dew point sensors are often usedin economizer systems to control condensation ofwater vapor inside of a building.

In general, relative humidity and dew pointsensors consist of two parts: a replaceable sensorelement and a transmitter unit. The transmittersusually have an output of 0 to +5 VDC, 0 to +10VDC, or 4 to 20 mA.

The recommended humidity sensor is the thin-film capacitive type.

Pressure sensors. Pressure sensors areused in HVAC systems in several applications,the most common being the control of duct pressurein variable air volume (VAV) systems. The commondesign of pressure sensor is based on a straingage loaded by a diaphragm. The pressure to bemeasured is applied to one side of a diaphragm,and the reference pressure is applied to theother side. The deflection of the diaphragm ismeasured by a resistance strain gage. Solidstate piezoresistive devices are also used measurethe deflection of the diaphragm. The change inresistance of the strain gage is measured by abridge circuit similar to those used with RTDs.The output of the bridge circuit is often con-verted to a 0 to 10 VDC or a 4 to 20 mA signalfor compatibility with other sensors.

48

In general, it is desirable to select pressuresensors having a comparatively large diaphragm(for increased sensitivity) and built-in protectionagainst overloading the diaphragm and straingage. Filters should be placed in the fluidlines connected to the sensor input ports topLevent dirt and other contaminants from enteringthe sensor.

Flow sensors. Although flow sensors arewidely used in energy monitoring and controlsystems, they are not usually required for controlof an HVAC system and will not be addressed indetail. Detailed information can be found inReference 6.

The types of flow sensors usually found inan HVAC system seem to be divided into two basiccategories: those based on a change in pressurethrough the measuring device (such as an orificeor venturi) and those based on counting somequantity which varies with flow rate (such asrevolutions of a turbine wheel or the number ofvortices shed per unit of time). Flow sensorsbased on measuring pressure change usually usethe same pressure measurement techniques describedabove. If the flow sensor output consists of aseries of pulses, those pulses can be counted bythe computer and the flow rate determined.

Status sensors. A status sensor is simplya switch used to input on-off type informationinto the DDC computer. For example, a paddleswitch might be used to provide a positive indicationof flow, a smoke alarm to indicate the presenceof smoke in the ducts, and a freezestat to indicate

- low outdoor air temperature. Usually, the onlysignal conditioning required for switch closuresis that they be "debounced," which means filteringout the noise and other extraneous signals thatoccur at the instant of switch closure so as toprovide the computer with an unambiguous signal.

49

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Interfacing Actuators

Pneumatic actuators. Because of their rela-tively low cost, high power output, and proven

reliability, pneumatic actuators are often thepreferred controller device in an HVAC system.There are two basic methods of interfacing apneumatic actuator with a DDC system: an analogelectric to pneumatic transducer (or E/F forvoltage to pressure transducer), or modulationof the branch line air pressure by means of digitalsignals.

The most widely used method of connecting aDDC output to a pneumatic device is the voltage-to-pressure transducer. In general, an E/P trans-ducer consists of a small, flapper type bleedvalve which is positioned by a solenoid (Figure 21).As the voltage across the solenoid is increased,the bleed port is closed and the pressure in thebranch line to the actuator is increased. Thus,for example, a 0 to 5 VDC output from the control-ler is transformed to a 3 to 13 psi branch linepressure. Features to look for in E/P transducersinclude linearity between input and output, repeat-ability (output consistently follows the input),low hysteresis (output is the same on decreasinginput signal as it is on an increasing signal),immunity to vibrations, and low power consumption.

The branch line pressure can also be control-led by using several digital methods. The branchline pressure can be controlled by means of apressure regulating valve driven by a bi-directioualmotor or a stepper motor. This method is compara-tively expensive, however, and offers no distinctadvantage over other methods.

53

main air branchsupply - line air

rbleed

airvalve-plungeramsmbly

solenoidc ioil

Figure 21. Schematic of electric to pneumatic transducer.

A digital control method which is mechanicallysimple and low in cost is illustrated in Figure 22.With this method, the branch line pressure isregulated by connecting it to the main air supplyor to the atmosphere via simple solenoid valvesas required to change the branch line pressureto the desired value. This method requires onlytwo digital output ports for implementation: oneto raise the branch line pressure and one tolower it. By varying the sequence and durationof supply and bleed valve actuation, the desiredbranch line pressure can be obtained. The restric-tion keeps the air flow through the control cir-cuit to a low value, while the small tank providesthe "capacitance" necessary for smooth operationand provides a way to accommodate small air leaksin the actuator circuit. The ratio relay is astandard pneumatic device used as a "volume ampli-fier" to increase the air flow to the pneumaticactuators. This interface technique is suscep-tible to errors from several sources, however,and the computer coding to make it work correctlycan be extensive. See Reference 7 for additionaldetails.

54

Vm

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55

Figure 23 illustrates the supply air tempera-ture coming off a chilled water coil controlledwith the digital-to-pneumatic method shown inFigure 22. The computer program, or algorithm,used for this experiment was the PI version ofthe incremental PID algorithm described in Appen-dix A. Figure 24 presents experimental resultsfor a different value of proportional gain.Figure 24 is presented to illustrate the sensi-tivity of system response to the choice of valuesfor controller gains, i.e., the importance ofgood control system tuning.

At the present time, there is no true digi-tal to pneumatic (D/P) convertor on the market,although several manufacturers are studying thedesign and market for such a device. The majorproblems are a lack of agreement on a standardfor digital transmission of data with the HVACcontrols industry, comparatively high cost, anda weak market for such a device.

The recommended approach is to use an analogoutput from the DDC device as input to an electric-to-pneumatic convertor.

Electric actuators. Most modern electricactuators used in HVAC systems lh.ve a controlsignal level that is compatible with the analogoutput of a DDC system. For example, a 24 voltmotor may have a control circuit which puts themotor shaft in full counterclockwise positionfor a control input of 3 VDC, and which puts theshaft in full clockwise position for an input of9 VDC. The actuator control terminals can usuallybe connected directly to an analog output terminalof the DDC unit. Actuator positioning is doneby simply setting the analog output voltage tothe desired value, e.g., 6 volts for a half openactuator. The major disadvantage of electricactuators is their comparatively higher cost.

56

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Note: The analog output device should becapable of sourcing at least 100 mA so that itcan drive distant actuators. Some digital-to-analog convertors may need to be followed by asmall power amplifier circuit to drive the actu-ators of choice.

Contact Closures

Certain low voltage, low power devices suchas annunciator lamps, can be powered from a digi-tal output of the DDC unit, but in most cases adigital output is used to activate an electro-magnetic or solid-state relay to permit the unitto control higher power devices such as a fanmotor. Digital outputs (and inputs) of the con-troller should be electrically isolated from therest of the control system. This is usuallyaccomplished with a solid-state device called an"opto-isolator" which consists of a light emit-ting diode located very close to a light sensi-tive transistor and packaged as a single device.Isolation helps protect the DDC unit from inputswhich would damage the controller circuitry.

Interfacing with Another Computer

Two decisions must be made before the DDCunit can communicate with another computer: whatlanguage will be spoken and in what form willthe information be transmitted. There are severalcommunication codes in use for information ex-change, but the most widely used is the AmericanStandard Code for Information Interchange (orASCII). In this code, every eight-bit sequenceof binary digits has been assigned a meaning,such as a letter of the alphabet, a decimal digit,or a punctuation mark. Thus, any message can betransmitted by sending the correct sequence of8-bit groupings. The second question is whether

59

to send all 8 bits at one time over 8 separatewires or to send the digits out serially, onebit at a time, over a single wire and reassemblethe word at the receiving end. The first methodis called parallel data transmission and thesecond method is called serial data transmission.Although parallel transmission is very fast, itrequires multiconductor cable and is not amenableto use over very long distances. Serial datatransmission is the method most commonly used tointerconnect computers. Specially designed circuitsmake the task of parallel-to-serial conversionand required conversion back to the parallelform easy to accomplish. These circuits arepart of the computer interface card if you purchasethat option. For communication of long distances,a device called a modem (for modulator-demodulator)is used. A modem converts the serial bit streamfrom the communications board of a DDC unit intoa series of audible tones for transmission overtelephone lines. A second modem at the receivingend converts the sequence of tones back into aserial bit stream for processing by the secondcomputer.

Installation Guidance

As with any other type of control system,the sensors should be located where they willprovide a true measure of the quantity beingsensed. Temperature sensors for liquids shouldbe mounted in an oil bath in a thermowell foraccurate readings and ease of replacement. Airtemperature sensors should also be located withcare. Often it is appropriate to use a longsensor probe or an array of sensors arranged ina serpentine pattern across a duct to measurethe average air temperature. Outdoor air tempera-ture sensors should be shielded from the sun andlocated away from exhaust air vents or otherobjects which might affect the temperature reading.

60

1 4The same precautions presented above also

apply to humidity sensors. Because humiditysensors require more maintenance than most othertypes of sensors, it is desirable to locate them

~where they can be readily accessed.All field wiring should be in accordance

with the applicable local or Navy codes. It isrecommended that the DDC unit, termination devices,and other components be housed in a suitablyventilated, lockable cabinet. Critical operatinginstructions should be posted on the inside ofthe cabinet door.

Commissioning

Commissioning is the process of putting anewly installed DDC system into proper workingorder. The commissioning process can be thoughtto consist of three parts: program verification,system tuning, and acceptance testing.

It is first necessary to determine if thecontrol program is correct as written. Two typesof programming errors are possible: errors incoding and errors in logic. Coding errors areerrors in the way the control logic has beentranslated into computer language. Coding errorscause the program to work incorrectly, which isusually manifest by the control system doingsomething unexpected. Severe coding errorsusually result in the program not working atall. Logic errors fall into two categories:those which are errors in the decision makingprocess embodied in the existing coding, andthose which are the result of omission of neces-sary logic. Checking a computer program forcoding errors is usually a straightforward pro-cess, but detecting logic errors can be verydifficult. Sometimes logic errors are foundonly after the DDC system has been in operationfor a period of time and encounters conditions

61

a m i m ~

not anticipated by the programmer. For thisreason, it is recommended that the control prog-ram be kept as simple as possible. Problem diag-nosis and correction is also facilitated if thecomputer program is written in modules, each ofwhich has a specific purpose such as reading allthe input data or controlling the chiller coil.

The control logic of a DDC system can beeasily bench-tested in most units by varying theinputs and recording the outputs. For example,variable resistors, current sources, and switchescan be used to simulate different input conditions.The sensor calibration curves are required totranslate the input resistance (for example)into the simulated temperature, humidity, orpressure. The outputs from the DDC unit arethen monitored to determine if the controllerresponse is correct for the specific inputconditions.

Tuning

Once the known problems (or bugs) in thecontrol program have been corrected, the nextstep in system installation is to optimize theperformance of the control system in relation tothe specific HVAC system being controlled. Thisprocess is known as tuning and it basically meansdetermining the values for the proportional gain,integral gain, throttling range and other factorsin the control algorithms such that smooth, stablesystem response will be achieved. System tuningis very important, for without correct tuning, aDDC system may perform no better than the poorestpneumatic control system. A method of tuning asimple (i.e., linear response) control loop ispresented in Appendix B. Often, fine adjustments

* to the control system parameters are made afterthe system has been in use for a period of timeand its performance using different values ofcontrol constants can be compared.

62

One area of current controls research isthe development of an adaptive or self-tuuingcontroller for HVAC systems. Because of thememory and computational capabilities of

microcomputer-based control devices, it istheoretically possible to design a controllerwith sufficient artificial intelligence to beable to determine and set the values of controlparameters which will produce optimum systemperformance. A well designed self-tuning con-troller could accommodate not only changes inload and setpoint, but also season changeover(heating to cooling, for example) and changes inequipment performance (e.g., coil fouling). Aprimary benefit would be the elimination of theneed to hire the skilled personnel necessary totune the controller for optimum performance. Anexcellent review of the status of adaptive con-trols research as applied to HVAC is presentedin Reference 8. Reference 9 presents the resultsof an analytical and experimental study of adap-tive control of an air handling unit.

The final task in the process of commis-sioning a DDC system is performing the acceptancetest. The acceptance test is a planned testprogram to demonstrate that the DDC system willperform as specified. It is recommended that atest plan be developed that will lead the controlprogram through all of the possible logic branchesof the program. It is sometimes difficult toimplement a full test program in the field envi-ronment in a brief period of time because of thelack of control the operator has over factorssuch as building load and outdoor air conditions.Usually a field acceptance test can be fullyaccomplished in a reasonable period of time onlyby substituting manually variable resistors forsensors and manually actuating switches as wasdescribed above in the discussion of bench testing.

63

Maintenance and Repair

Maintenance. A fundamental rule of anycontrol system, DDC or otherwise, is that controleffectiveness is limited by the performance ofthe equipment it is controlling. A DDC controlsystem cannot make poorly designed or poorlymaintained HVAC systems perform well. HVAC systemproblems, such as leaking pneumatic lines, dirtycoils, and sticking valves, must be correctedbefore the DDC system is commissioned.

The routine maintenance required on a DDCcontrol system is minimal. Routine calibrationof the control system is not required. Also,because of the digital nature of the controlsystem, it tends either to work or not to work.There is no degradation of performance over timeas is commonly the case with analog control sys-tems. Also, the self-diagnostic, data display,and alarm features of most DDC systems maketrouble shooting of both control and HVAC systemproblems much easier.

Repair. The control system should be sup-ported with an inventory of critical spare parts.A controls contractor can assist with generatinga spare parts list. Usually the required spareparts inventory is very small. Key spares usuallyinclude controller circuit boards, sensors, andtransmitters. System problems are usually cor-rected by replacement of the defective item.Repair of most components (circuit boards, sen-

sors, etc.) requires skills and equipment whichare generally not available to most Navy facil-ities managers.

64

Training

The maintenance people responsible for theDDC system will need a significant amount oftraining. Facilities managers should work withthe control system vendor to develop a trainingprogram to educate maintenance personnel on thefundamentals of digital control, system operation,and trouble shooting. Refresher courses should

also be scheduled on a regular basis. The impor-tance of proper training of maintenance personneland facilities managers cannot be overemphasized.Many aspects of DDC systems will be new to mostpersonnel, so training must be designed to take

some of the mystery out of "computer control,"and emphasize how DDC can be more reliable andeasier to maintain than conventional systems.Also, a review of the relevant HVAC system designsand operation and how a DDC system can be usedin trouble shooting system problems is also agood training investment.

Many of the topics discussed in this sectionwill be the subject of NAVFAC Guide Specificationsto be published in the future.

V. SUMMARY

Direct digital control is finding increased

application to heating, ventilation, and airconditioning systems for many reasons. A summarycomparison of DDC and pneumatic control systemsis presented in Table 3. Many different typesof sensors can be used with a DDC controller.It is recommended that the extra investment bemade in accurate, stable sensors as a DDC systemcannot perform to full capability given poorinput data.

65

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There are two basic methods of controllingproportional actuators: converting the digitalvalue of the control signal produced by the DDCunit into an analog (i.e., continuous) voltageor air line pressure or using the digital signaldirectly to modulate an actuator. Both methodshave proponents among manufacturers and controlsystem designers. More experience with DDC isrequired before a preferred DDC-actuator inter-facing method is identified. Thorough operatortraining prior to control system commissioningis required if the benefits of DDC are to berealized.

REFERENCES

1. "STORE CASE STUDY," by Mr. R. K. Rooney, ECCControls Corp., Milwaukee, WI. A presentationto DIRECT DIGITAL CONTROL, a University ofWisconsin Engineering Extension Course, 12 May1983.

2. "PERFORMANCE COMPARISONS," by Dr. D. C. Hittle,U.S. Army Construction Engineering Research Labo-ratory (CERL), Champaign, IL. A presentation toDirect Digital Control, a University of WisconsinEngineering Extension Course, 11 May 1983.

3. "User's Guide to Sensors for Energy Moni-toring and Control Systems," (draft) by G. A.Nowakowski, Naval Civil Engineering Laboratory,Port Hueneme, CA, Dec 1983.

4. Catalog, HY-CAL Engineering, Inc., Santa FeSprings, CA, 1980.

69

5. "General Guidelines for the On-Site Calibra-tion of Humidity and Moisture Control Systems inBuildings," by R. W. Hyland and C. W. Hurley,National Bureau of Standards (NBS) Science Seriesreport 157, NBS, Washington, DC, Sep 1983.

6. "On-Site Calibration of Flow Metering SystemsInstalled in Buildings," by D. W. Baker and C.W. 4urley, NBS Science Series Report 159, NBS,Washington, DC, Jan 1984.

7. "Direct Digital Control of a PneumaticallyActuated Air-Handling Unit," by W. B. May, B. A.Bonesen, Ph.D., and C. W. Hurley. American Societyof Heating, Refrigeration, and Air ConditioningEngineers (ASHRAE) paper TO-82-6, Aug 1982.

8. "Adaptive Control Strategies for ProcessControl: A Survey," by D. E. Seborg et al. Apaper presented to American Institute of ChemicalEngineers Annual Meeting, Washington, DC, Nov 1983.

9. "An Adaptive Controller for Heating and CoolingSystems: Modeling, Implementation, and Testing,"by C. Park, NBS, and A. J. David, Bell Labora-tories, Holmdel, NJ. American Society of Mechan-ical Engineers (ASME) paper 82-WA/DSC-22 or NBSreport 82-2591, Oct 1982.

10. "Microprocessors in Instrumentation andControl," by R. J. Bibbero, John Wiley and Sons,New York, 1977.

11. "Optimum Settings for Automatic Controllers,"by J. G. Ziegler and N. B. Nichols, ASME TRANSAC-TIONS, Nov 1942.

12. "Automatic Process Control," by D. P. Eckman,John Wiley and Sons, New York, 1958.

70

Appendix A

PROPORTIONAL-INTEGRAL-DERIVATIVE CONTROL

Let the desired value of the controlledvariable be called the setpoint, V . For example,it may be desired to control the teperature ina space to maintain a nominal value of 70*F. Inthis case, the setpoint equals 70. Let the actualmeasured value of the controlled variable bedesignated Vm , for measured variable. Any dif-ference between V and V is an error which thecontrol system mul try tm eliminate. The outputsignal from a controller is dependent on themagnitude and algebraic sign of this error.

In a proportional controller, the output ofthe controller is simply a constant multipliedby the error. If

E = V -Vsp m

then

G = K *Ep

where: G - controller output

K a proportional gain - a constantp

In an integral controller, the output of thecontroller is a constant multiplied times theaccumulated (or integral) error. Thus,

A-i

t2

G - K1 * f E dtti

where: K1 - integral gain - a constant

dt - differential of time [time]

In a derivative controller, the output is a con-stant multiplied by the rate of change (or deriv-ative) of the error signal.

G - KD dE/dt

where: KD derivative gain

dE/dt - rate of change of error

Usually, the integral and derivative gain factorsare redefined in terms of the proportional gain:

KI = KpIT

where T, = reset time (time]

and

K =K * TD p D

where TD rate constant [time]

Thus, a PID controller has the form

t 2

G K * E + KI f E *dt + dEldtP Iti

A-2

or

C K E + E *dt + K *Tp t p dt

In the form of a finite difference equation,

C K El + E• (E.Kp T ifi T N-

I s

+ constant

where: E N error signal at samplen = setpoint - measured valueat time n

T = sample period [time]

TI f reset time

TD = rate constant

constant - mid-point position ofactuator = Gmp

A PID controller is presented in block dia-gram form in Figure A-i. A schematic of how thePID control method might be implemented is pre-sented in Figure A-2.

The PID controller equations presented aboveare called the position forms or whole valueforms of the control algorithm because they computethe absolute value of the position of the actuatorcontinuously. Thus, in the event of a temporaryloss of communication between the controller andthe actuator, or loss of computing capability,the correct signal to restore the actuator tothe proper position will be transmitted uponrestoration of control.

A-3

f

A-4

.3

C

E

0

A-5

IJ

0

u

A-74

Another form of the PID algorithm is the

incremental value form or the velocity form.The incremental form of the PID algorithm isobtained by subtracting two successive values ofposition. Because only the difference betweenthe currently computed whole value and the wholevalue from the previous calculation is trans-mitted to the actuator, only the change in posi-tion is transmitted to the actuator.

GN - GN-1 = AGN

M Kp * (ZN - E ) + KI * EN * Ts

+ (KD/Ts)(EN - 2ENi + EN 2)

The incremental form of the PID algorithm isbetter suited to many digital control systemswhere sampling is done at regular time intervals.An additional advantage is freedom from "windup,"

the condition in which the integral term of thecontroller reaches an upper limit value due tothe persistence of an error signal.

If the positional form of the PID controlleris used, the process will undergo a disturbance,or "bump," when control is switched between manualand automatic control unless the output of thecontroller is adjusted so as to coincide with

the present position of the actuator. The incre-mental form of the algorithm however is "bumpless"because the average value setting, G, disappearswhen successive values of absolute position aresubtracted.

If the final control element is a devicewhich moves at a constant velocity, such as anelectric motor which rotates at a constant speed,

the distance the actuator moves will be directlyproportional to the duration of the control signalapplied to the actuator terminals. If the incre-mental form of the PID control algorithm is used

A-8

I |1BiN• n iD and i aI

for control, then the change in actuator positionis directly related to the duration and polarityof the output "pulse" from the controller. Thismethod of control is widely used when gearmotorsare employed as actuator drives. The method iscalled pulse-width-modulation or PWM control.

The equations presented above are for anideal controller. In actual practice, the con-troller algorithm usually includes noise filters,error and limit detectors, and compensation forlead, lag and dead time. A good source of furtherinformation on control methods and digital controltechniques is Reference 10.

A-9

Appendix B

CONTROL SYSTEM TUNING

The operator of a new DDC system is oftenfaced with an immediate problem: for each localcontrol loop the DDC system controls, at leastone, and perhaps three or more numbers, must beentered into the controller as part of theinstructions on how to operate. These three

numbers are usually the proportional gain (K),the integral gain (K ), and the derivative gkin(KD). The effects oi these variables are illus-

trated in Figures B-I through B-8.Too large a value of K , however, results

in unstable, ox oscillatory, system behavior(Figures B-i and B-2). A small value of Kresults in stable operation but a large residual,or offset, error (Figure B-3). An optimum value

of K results in stable performance and an accep-tablB residual error (Figure B-4).

The residual error, however, can be elimi-nated by the addition of integral, or reset,control action. Integral control action is des-

cribed by the factor KT. The effect of integral

action is illustrated in Figures B-5 throughB-7. Too large a value for K results in anextended period of oscillation about the control

point (Figure B-6). The effect of derivative

control is to reduce peak excursions and dampenoscillations (Figure B-8).

B-I

*

New SP A

Koo largeUsiltea

Figure B-i

Settling time

Figure B-2

B-2

Kr too small, largeofse error, short

settling time

Figure B-3

Kp optimum, goodbalance betweenoffset and settlingtime

Figure B-4

B-3

Kp optimumK! - too smalllong time requiredto eliminate offset

Figure B-5

Kp - optimumKi - too largeoscillates

Figure B-6

B-4

Kp . Optimum

Kz = Optimum

Rapid eliminationof offset error

Figure B-7

Kp Optimum

Ki = Optimum

KD= Optimum

Figure B-8

B-5

How, then, does the operator determine what

values of K , KI , and Kn are correct for thecontrol loops in the HVXC system?

One method of determining the response ofan HVAC system to different control techniquesis to develop a detailed mathematical model ofthe system and use any of several classicalmethods of analysis, such as Nyquist, root-locus,or Bode analysis to estimate system performanceunder various conditions. Unfortunately,developing a realistic mathematical model ofeven the simplest of HVAC control loops is usuallya formidable task and performing the necessaryanalysis on the model requires specialized engi-neering training.

An experimental method, called the Ziegler-Nichols method after its developers (Refer-ence 11), offers a systematic way of determiningthe controller settings for optimum performanceof the controlled system and also provides asimple quantitative view of the behavior of thecontrol system. The Ziegler-Nichols method isbased on an approximation of a process composedof a single dead-time element and a single timeconstant element. The apparent dead time andthe apparent time constant are used to estimatesystem performance. To a first approximation,the process simulates that of a linear value-coilcombination (time constant element) and a temper-ature sensor located downstream of the coil (dead-time element).

An open-loop transient test is used to deter-mine the magnitudes of the dead time and timeconstant. Figure B-9 illustrates a generalclosed-loop control system. The system is madeopen loop by opening the connection between thecontroller and the actuator. A small step in-crease in controller output is then manuallyapplied to the controlled device (Figure B-10).

B-6

m I~a m m nI

The following precautions should be observedwhen conducting the open-loop transient responsetest:

(a) The input step change (M) should be assmall in magnitude as possible that yields resultsthat can be recorded and interpreted.

(b) If possible, the step change should beabout the nominal setpoint, i.e., it should beginwith the controlled variable just below the set-point and end with the controlled variable justabove the setpoint.

(c) No variable should be permitted toattain a maximum or minimum value, i.e., thetest should be performed near the actuator mid-position.

(d) Actuators should be in good workingcondition so there is no significant hysteresisor dead zone in any element of the system.

After some dead time, the process willrespond in a manner similar to that illustratedin Figure B-10. The process response is mea-sured by the feedback loop sensor, for example,the temperature sensor used to control a coil.The system response should be measured andrecorded. A strip chart recorder or similardevice is very useful for recording system per-formance.

The open-loop response is approximated fromthe recorded response using the measured valuesillustrated in Figure B-I. The measured valuesare:

K = magnitude of the change in thecontrolled variable in units of themeasured variable, e.g., *F

B-7

A

N = rate of change of controlled variablein units of measured variable perunits of time, e.g., *F/min.

L - apparent dead time in units of time,e.g., minutes

M - magnitude of step change in units ofcontroller output, e.g., volts or psi

As a check on applicability of the Zeigler-Nichols method, a horizontal line drawn at 63.2%of the total change, K, should give a value oftime constant T which is approximately equal tothe value of K/N. If the values of T and K/Nare not within about 15 percent of each other,the system is appreciably nonlinear and theZeigler-Nichols should be used only with caution.If it is suspected that the controlled system isnonlinear, the open-loop transient test shouldbe performed in the other direction, i.e., asmall step decrease in controller output is ap-plied to the controlled device. If the measuredvalues of T and L (taken in both directions)differ by more than 10 percent, the system isnonlinear.

The Cohen-Coon equations (Reference 12) areused to calculate the optimum controller variables

Proportional-Integral Control

K M ( 9 . units of K

p i 10 12 3R

S30+ 3R Time

where

R-NL LR - -- dimensionless

K T B

B-8

- m

a.

0

~0

E

0a.2 0

0

4,

4,

0'

4,

0

5-9

zn(t) - manual input to

controlled device

Output V (t)=-masured/ system response

0Time

Figure B-10). Open-loop transient input and response.

Line drawntangent to

calculations.

B-il

Proces

If KI is the controller adjustment ratherthan the reset rate, input

KK I = TP

TI

Propettional-Integral-Derivative Control

If the controller adjustments are expressedas Kp, Ti, and

TD' then

p NL 3 4

TKI 1 + 2R)

TD (L 2 + 6R(1 +8 R

D= 2"

R is defined above.If the controller adjustments are input as

Kn, Khe and K at, then

I =( 6 NL2R

MKD 2N

R is defined above.Af er the optimum values for the controller

gains have been calculated and input to the con-troller, a closed-loop transient response testcan be performed by Inputting a small change in

B-12

N ! !I

setpoint and measuring how the system responds.The equations presented above should result in asystem response similar to that depicted in Fig-ure B-8. The ratio of the amplitudes of succes-sive oscillations should be about 4:1 and the

* area enclosed by the oscillations will be approxi-mately a theoretical minimum. Settling timewill also be minimized.

The Zeigler-Nichols tuning process can beautomated with a microcomputer-based controlsystem.

'P.'U.S. GOVLRNMENT PRINTING OFFICE: 1985-679-102/40622

B-13

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