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® CNT -APG0 02-EN Applications Guide PID Contr ol in T r acer Contr ollers.
Applications Guide PID Contr ol in T r acer Contr ollers CNT -APG0 02-EN Oct ober 200 1 ®.
PID Control in T racer Controll ers This manual and the information in it are the property of Am erican Standard Inc. and shall not be used or reproduc ed in whole o r in part, except as inte nded, without the written perm ission of American Standard Inc.
CNT -APG002-EN iii ® Cont ents C h a p t e r 1 Ov er view of PID cont r ol . . . . . . . . . . . . . . . . . . . . . . 1 What PID loops do . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 How PID loops work . . .
® Conte nts iv CNT -A PG002-EN C ha p t e r 4 A pplicat ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Disc harge-air temperature control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Building pres sure control .
CNT -APG002-EN 1 ® Chapt er 1 Ov erview of PID contr ol This guide will hel p you set up , tune , and troubleshoo t proportional, inte- gral, derivative (PID) control loops used in Tracer controllers . Thes e con- trollers in c lude the Tracer MP580 /581, AH540/541 , and MP501 controllers .
® Chapter 1 O v erview of PI D control 2 CNT -A PG002-EN Ho w PID loops w or k A PID l oop p erfo rms pr opor tio nal, inte gra l, an d deri vati ve cal cul ati ons to calculat e syst em output. Figure 2 i llustrat es how a typi cal PID l oop works .
PID calculations CNT -APG002-EN 3 ® PID calculatio ns A PID loop pe rforms three calculati ons: the proportion al ca lculatio n, the integral ca lculation, and the derivative calcu lation. Thes e calculations are indep endent of ea ch other but are combi ned to determine the response of the controller to the error .
® Chapter 1 O v erview of PI D control 4 CNT -A PG002-EN Figure 4: The ef fects of propor tional bias on system output Integr al calculation The int egral calculatio n responds t o the leng th of time the measured vari- able is not at setpo int.
PID calculations CNT -APG002-EN 5 ® Figure 5: Integral output adde d to propor tional output The value of the inte gral calculation can build up o ver time (becau se it is the sum of all past errors) , and this built -up value must b e overcome before the system can change direction.
® Chapter 1 O v erview of PI D control 6 CNT -A PG002-EN Because of these d isadvantages , derivati ve control is rarely used in HV AC applicatio ns (with the ex ception of ste am valve controlle rs and static pressure control).
V elocity model CNT -APG002-EN 7 ® V elocity model Trane controllers use a type of PID cont rol known as the velocity model. The velocity model minimizes the problem of integral windup , which occurs.
® Chapter 1 O v erview of PI D control 8 CNT -A PG002-EN.
CNT -APG002-EN 9 ® Chapt er 2 PID set tings This chapter describes some of the key variables used to set up and tune PID loop s . The variabl es discuss ed here are: • Throttling range • Gain •.
® Chapter 2 PID se t tings 10 CNT -A PG002-EN The throttling range determines the responsive ness of a contro l system to disturbances . The smaller the throttling range , the more respons ive the control.
Calculating the gains CNT -APG002-EN 11 ® Calculating th e g a ins T able 1 shows recommen ded initial values for the p roportional and inte- gral gai ns for several ap plications. Most applica tions do not re quire a derivative contribution, so the derivative gain is not shown.
® Chapter 2 PID se t tings 12 CNT -A PG002-EN Sampling f r equenc y The samp ling frequency is the rate at w hich the input s ignal is sample d and the PID calculations are performed. Using the right sampli ng fre- quency is v ital to achieving a res ponsive and stab le system.
S ampling frequency CNT -APG002-EN 13 ® Proble ms also a rise fr om samplin g too q uick ly . So me systems ha ve nat u- rally slow response times , such as when measuring room temperature . Slow res ponse time s can also be caused b y equipmen t lags .
® Chapter 2 PID se t tings 14 CNT -A PG002-EN Calculat ing the sampling fr eque nc y PID loops are carried out by programs , such as process cont rol language (PCL) progra ms and Tracer g raphical prog ramming (TGP) programs .
Calculating the sampling frequency CNT -APG002-EN 15 ® 6. Calcul ate two-thirds ( 66%) o f the change in meas ured variable d eter- mined in s tep 4. Add t his va lu e to the init ial temper at ure to deter - mine at what po int two-third s of the total change occurs.
® Chapter 2 PID se t tings 16 CNT -A PG002-EN Example In this scenario, we want to find the sa mpling frequency for a PID loo p control ling a heating application. 1. Full y cl ose the out put. 2. The s tabilized tempe rature is 60 ° F (1 6 ° C). 3.
Act ion CNT -APG002-EN 17 ® Ac t i on The acti on of a P ID loop det ermines how it reacts t o a cha nge in the mea - sured variable (such as a room temperature). A cont roller using direct actio n increases the out put when the me asured v ariable incre ases .
® Chapter 2 PID se t tings 18 CNT -A PG002-EN Deter mining the act ion T abl e 3 shows the action s ettings for s everal applicat ions . Thes e settings are a good starting place for most applicat ions. T o find the action for other appl ications, determine whe ther the actuato r and measured v ariable move in the same d irection.
Er ror d eadband CNT -APG002-EN 19 ® Er r or dead band Error deadband is typically used to minimize actuator acti v ity . It can also be used to allow for some slack in sys tem sensor s and actua tor mechanics. Error deadba nd preven ts the PID out put from c hanging whe n the abso- lute value of the error is less than the error deadband.
® Chapter 2 PID se t tings 20 CNT -A PG002-EN Adjusting er ror deadband f or modulating outpu ts In most a pplications, start with an erro r deadband o f five or ten times t he sensor res olution . F or example , thermis tors hav e a resolut ion of app roxi- mately 0.
Other PID set tings CNT -APG002-EN 21 ® With the precedin g guidelin es in min d, use the follow ing proc edure to determine error d eadband. T o adjust the er ror deadband f or stag ed outputs: 1. Run the syst em manually . If possible, do so under wo rst case conditions for the site .
® Chapter 2 PID se t tings 22 CNT -A PG002-EN.
CNT -APG002-EN 23 ® Chapt er 3 Pr ogr amming PID loops This chapter presents programs wr itten in process control language (PCL) and t he Trace r grap hical prog ramming (TGP ) editor . This cha pter does not dis cuss how to use the PCL or TGP e ditors .
® Chapt er 3 Progr amming PID loops 24 CNT -A PG002-EN Fol lo w these steps t o pr ogram PID loops in PCL: 1. Mak e su re th at the setpo int is wi thin r eas onab le li mits . Use the MIN an d MAX operators to set a ceilin g and floo r for the set- point, as shown in l ines 1 and 2 o f T abl e 7 on page 25 .
Pr ogramming in PCL CNT -APG002-EN 25 ® T abl e 8 sh ows a PCL prog ram with separate d enable/dis able and fail-sa fe logic . The l ogic is separate d becau se in this case t he enabl e/disab le and fail-saf e conditions h a ve dif f eren t results.
® Chapt er 3 Progr amming PID loops 26 CNT -A PG002-EN Pr ogr ammin g in TGP Figu re 17 shows the PID blo ck use d to program PID loops in T GP edit or . The PID block is more flex ible t han the DD C function in PCL. The e nabl e/ disable and failure inputs can accept any binary value, regardless of source.
Pr ogramming in TGP CNT -APG002-EN 27 ® Fol lo w these steps t o pr ogram PID loops in TGP: 1. Use th e Limit blo c k to m ake sure tha t the setp oint is within reason- able l imits . 2. Run the PID calc ulation. 3. Define fai lure and other op eration- dependent cond itions .
® Chapt er 3 Progr amming PID loops 28 CNT -A PG002-EN.
CNT -APG002-EN 29 ® Chapt er 4 Applications This chapter describes several HV AC applications that use PID control. It includes specific setting s and recommendations for each application.
® Chap t e r 4 App lic ati ons 30 CNT -A PG002-EN Figure 20 shows a TGP pro gram to control a hot-water valve. Output Sta- tus 1 (an a nalog outp ut) pro vides t he po sition of t he c hilled -w ater valve . If the chil led-water valv e position is greater than zero, the hot-water val ve will not open.
Disc harge - air tem perature co ntrol CNT -APG002-EN 31 ® After the i niti al in stal lat ion a nd test ing , the tech nici an n otice d that the discharge-air temperature w as oscillating in a 10 ° F (5.
® Chap t e r 4 App lic ati ons 32 CNT -A PG002-EN Building pr essur e contr ol Space pre ssure is typically co ntrolled by openin g and closing relie f damp- ers . A P ID loop con trols these da mpers ba sed on a s pace press ure setpoi nt and the mea sured space pressu re.
Building pressu re control CNT -APG002-EN 33 ® T abl e 12 lists t he settings for the PID loo p controlling building press ure . The sampli ng freq uency is slow b ecause b uilding pres sure cha nges slo wly . F or programs written in PCL, the er r or deadban d is 1.
® Chap t e r 4 App lic ati ons 34 CNT -A PG002-EN Cascade contr o l—first stag e A PID loop can be used to automatically d etermine a discharge-air tem- perature setpoint. Other programs or control syste ms can then make use of this calculated setpoint.
Cas cade c ontr ol —first st age CNT -APG002-EN 35 ® Figure 25: TGP program for disc ha rge -air temperat ure setpoint in cascade contr ol If you use t he settings shown in T abl e 14, you sho uld not hav e to tune t he loo p. These value s ca n be u sed in a lmo st an y casc ade con tro l app lic ation without cha nge .
® Chap t e r 4 App lic ati ons 36 CNT -A PG002-EN Figure 26: Space temperatur e and calculated disc harge-air setpo int The disc harge- air tempera ture setp oint cal culated b y the PI D loop ma y not control the di scharge -air temperature de pending on ot her conditions that have priority , such as high and low se tpoint limits.
Staging cooling-tow er fans CNT -APG002-EN 37 ® Staging co oling-t ow er f ans Staging appl ications org anize indivi dual pieces of equi pment into a group to accomplish a singl e task. F or example, seve ral fans might be use d to maintain t he supply water tempe rature in a cool ing tower .
® Chap t e r 4 App lic ati ons 38 CNT -A PG002-EN The PCL program in T able 15 s tages two cooling-tower fans . Figure 28 shows t he same prog ram in TGP . Th e beha vior of the sta ges prog rammed in this pro gram is illustrate d in Figure 30 on p age 41.
Staging cooling-tow er fans CNT -APG002-EN 39 ® The TG P program fol lows th is sequ ence of ope rati on: 1. Ch illed-water pump sta tus is ch ecked. If there is f low , the cooling towers are allowed to op erate .
® Chap t e r 4 App lic ati ons 40 CNT -A PG002-EN The challe nge in stagin g applications is to find the correct propor tional bias . This value de termin es the ou tput whe n the error is zero .
Staging cooling-tow er fans CNT -APG002-EN 41 ® F or staging applica tions , the re sult of the P ID calculation co ntrols binary output s rather than an analog outp ut. F or this kind of staging app lica- tion, it is typical t o use the de adband to ma ke sure th at the binar y output state is maintained for some s pecific range.
® Chap t e r 4 App lic ati ons 42 CNT -A PG002-EN Deter mining the sta ging points This section de scribes how to fi nd the points at wh ich sta ges are turned on and off. Start with thes e guideline s: • T o avoid hav ing a s tage t urn off at the lo west extreme , alw ay s ha ve at least o ne stage on at 10% of th e output range.
Staging cooling-tow er fans CNT -APG002-EN 43 ® Example 1: T wo-stag e f an system The stag ing points are calculated as follows: 1. Calc ulat e the ove rlap range . 2. Calc ulat e the first stag e control po ints . 3. Calcul ate the second stage control points.
® Chap t e r 4 App lic ati ons 44 CNT -A PG002-EN Example 2: Th r ee-stag e f an system The stag ing points are calculated as follows: 1. Calc ulat e the ove rlap range . 2. Calc ulat e the first stag e control po ints . 3. Calcul ate the second stage control points.
CNT -APG002-EN 45 ® Chapt er 5 T r oubleshooting This chapter offers a general troubleshooting proced ure and tips for spe- cific problems . T r o ubleshooting pr ocedur e When fo llowing this tr oubles hooting pro cedure , ch ange only o ne th ing at a time , the n wa it to see t he effect the cha nge has on the sys tem.
® Chapt er 5 T r oubleshooting 46 CNT -A PG002-EN Tips f or specifi c pr oblems T able 17 provid es tips fo r troubl eshooti ng specifi c problems . Changing the sampling frequency The major ca use of actua tor cyc lin g is time la gs in th e system.
Examples CNT -APG002-EN 47 ® Exampl es This s ection p resents t rouble shooting scenarios from a h ot-water valv e applic ation. Th e three ex ample s have t he same sympt om but diff erent soluti ons t o the problem . Example 1 A hot-w ate r valve cyc les clo sed every few mi nutes .
® Chapt er 5 T r oubleshooting 48 CNT -A PG002-EN The applicati on is running in a cold cli mate during winte r , so the chille d- w ate r valv e shou ld not op en at a ll (becau se c hi lled wa ter is no t be ing used).
Examples CNT -APG002-EN 49 ® Figure 34: Hot-w ater valve position, sampling fr equency too sho rt Time (min utes) Discharge-ai r temper at ure s etpoint ( ° F) Disc h arge-air temperat ure ( ° F) H.
® Chapt er 5 T r oubleshooting 50 CNT -A PG002-EN Example 3 The tech nician ex periences t he same probl em as in the fi rst two exa m- ples: a hot-w ate r valve cyc les c lose d every few minutes , and the dis- charge-air tempe rature swings acros s a range of 10 ° F (5 .
CNT -APG002-EN 51 ® Chapt er 6 F r equently ask ed questions Wh y i s the output of m y PID loop alw a ys z er o? • Maximum PID output ma y be set to zero . • PID acti on setting ma y nee d to be cha nged. • Setpoi nt may be zero o r negative, dri ving the output to ze ro .
® Chapt er 6 Fr equently ask ed questions 52 CNT -A PG002-EN I tr ied the 4:1 ratio f or pr opor tional and integr al g ai ns, but this did not o ptimiz e m y syst em. Can I t ry anothe r r atio? W e recom men d maint ain ing a 4:1 r atio bet ween th e pr oport ion al an d integral gains .
F requently asked questions CNT -APG002-EN 53 ® What ’ s the best sampli ng fr equenc y ? The best sa mpling freque ncy depends on th e applicat ion. See “ Calculat- ing the sampling fr equency ” on page 14 for recommended sampling fre- quencie s.
® Chapt er 6 Fr equently ask ed questions 54 CNT -A PG002-EN.
CNT -APG002-EN 55 ® Appendix A The math behind PID loops This appendix presents the mathematical formulas used for PID con trol in Tracer MP580/581 controllers, the programmable control module (PCM), and the universal programmable control module (UPCM).
® Appendix A The math behind PID loops 56 CNT -A PG002-EN.
CNT -APG002-EN 57 ® Glossary action A PID se tting that de termines ho w the PID loo p reacts to a c hange in the measured varia ble (such as a room te mperature). A controll er using direct action i ncreas es the o utput when t he measured variab le incre ases .
® Glossa ry 58 CNT -A PG002-EN er r or deadband A PID setting that defines how much the error mus t change before the PID loop rea cts . Used to compens ate for bearings, linkages, and other mechanical items.
Glossa ry CNT -APG002-EN 59 ® pr ogr am fr equenc y The rate at which a prog ram executes or cycle s. pr opor tional contr ol Control action based on the error . T he most important determinant in how quic kly th e system re sponds t o the erro r . Some applicat ions us e pro- portional- only control.
® Glossa ry 60 CNT -A PG002-EN syst em time constant The time it takes t o reac h 63.2 1% of the di fference betwe en the sta rt point an d the end p oint when con trolling an out put over a k nown range. Used to calcu late the sampling freq uency . Using 2 / 3 (66%) r ather than 63.
CNT -APG002-EN 61 ® Index Numer ics 4 to 1 ra tio for g ain s, 11, 52 A acti on, 17 -18 determining, 18 direct, 17 , 52 examples , 18 recomm ended valu es, 18 reverse, 17 , 52 actu ator and er ror de.
Index 62 CNT -A PG002-EN ® E enable c onditions, 2 4, 25, 27 error and the ve locity model, 7 and thro tt ling range, 9-1 0 definition, 2 er ror dead band, 19 -21 and sensor res olution, 20 and stagi.
Ind ex CNT -APG002-EN 63 ® S sampling freque ncy , 1 2-16, 53 aliasing, 12 calcula ting, 14-15 causing outp ut to oscilla te, 13, 46, 48 example, 16 in di f feren t cont roller s, 14 recom mended v a.
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® Sinc e The T r ane Comp an y ha s a policy of cont inuous pr oduct a nd produc t data im provemen t, it reserves t he right to c h ange design and sp ecifications wit hout notice.
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