Trane PID Control manual

® 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. Sinc e The Trane Company has a policy of cont inu- ous produc t improve ment, it reserves the [...]

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 PID calculations [...]

® 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Cascade control— firs[...]

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 . This chapter provides an overview o f PID control. What P[...]

® 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 . The si gma ( Σ ) symb ol indica tes that a sum is being perfor med. The plus (+) symbol indi cate[...]

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 . Pr oportional calculation The p[...]

® 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. The longer the mea sured variable is no t at set- poin t, the l arger the out put of th e integra l cal[...]

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. This prevents the contro ller from over -reacting to mi nor chan[...]

® 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). Derivative cont rol can affect the out put in two w ays: it slows the out put if the deri vative g ain is nega tive and [...]

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 when the sum of past errors in the integral calculation is too great to al low the co ntroll er to chan ge the output at one of the extre mes (see ?[...]

® 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 • Sampling freq uency • Action • Error deadband Thr ot tli ng r ang e The thro ttling range is the amount of error i t tak es to mov e the output o[...]

® 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. Y ou cannot directly program the throttling range in Tracer con- troll ers; rath er , the t hrottling ra nge is used t o calculat e the ga ins [...]

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. W e recom- mend usin g a ratio of 4:1 between th e proportion al and [...]

® 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. Pro blems can arise whe n the samplin g frequency is too slow or too f[...]

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 . Since PID loops respond to error and ch anges in error over time , if the measured variable c h[...]

® 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 . Since th e PID calcul ation occu rs when the program exec utes , the sa m- pling frequ ency and the prog ram execution fre[...]

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. In the exa mple , 0.66 × 50 ° F = 33 ° F , so two th irds of the total c hang[...]

® 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. Full y open t he outpu t. 4. The s tabilized tempe rature is 105 ° F (41 ° C). 5. The c hang e [...]

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 . A con- troller usi ng reverse a ction decrea ses the output when the meas ured vari- ab[...]

® 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. If so, then the [...]

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. F or exa[...]

® 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 0.1 ° F (0.06 ° C), so 0.5 ° F (0 .3 ° C) is an appropriate err or deadband. Th[...]

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 . Although it is not always possib le for a technician to do th[...]

® 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 . F or information on usi ng these editors, re fer to Univer sal Programmable Control Module (UP[...]

® 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 . 2. Run th e PID calculatio n and store[...]

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. In line 4, i f the fan is off , then the actu ator is closed . In line 6, if the inp ut sensor f a[...]

® 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. The setpo int, measured varia ble , p-g[...]

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 . Check for fa n-status a nd measured-v ariable inpu t failures . P rogr[...]

® 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. Disc har g e-air temper at ur e contro l When cont rolling hot/chilled-water valv es in discharge-air applica tions , a PID loop co ntrol s the po sition of a v[...]

® 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. Figure 20: TGP pro gram to control a hot-wate[...]

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 (5.6 ° C) band around setpoint . Slowing t he sampli ng frequency t o 30 second s stopped t he oscil- latio ns (see Chapt e r 5, “ Troubles hoo[...]

® 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. The spac e pressure in t he buil ding should remain slightly p ositive to [...]

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 1.0, which is e qual to 100 times t he minimum reso lution of the press ure sensor .[...]

® 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. This type o f control, called cascade control, result s in very tight cont r ol of space te m[...]

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 . The prop ortional and in[...]

® 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. The high limit [...]

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 . St aging appli- cations control a series of binary out[...]

® 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. Figure 28: TGP program for staging co oling tower fans T ab le 15: PCL program for staging c[...]

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 . 2. Based o n the error (the dif ference betw een the chilled- w ater setpoin t and the chilled-water temperature), the controller turn[...]

® 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 . The pro- portiona l bias should have the same value as th e point at whi ch the first stage tu rns on (see “ Dete rmining the stag ing points ” o n p[...]

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. Figure 30 il lus[...]

® 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. Turn that st [...]

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. The stag ing points are illustrated in Figure 31. Y ou sh ould not have t o adju st th[...]

® 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. 4. Calcul ate the third stage control po ints . The resu lts are summa rized in F[...]

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. Fo llo w these st eps t o[...]

® 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. If a 10% chang e in PID output r equires t wo minut es to affe ct the me asured v ari- able, it do[...]

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 . Alt hough the s pace temperatu re remains fairly stable, the d[...]

® 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). Ho wever , it might ope n in the follo wing cases: • The bui lding automatio n system has information that ch illed water is a[...]

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) Hot-w ater v alv e positio n (%) Disc harge-air tem perat ure and v alv e posi tion[...]

® 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 .6 ° C) . T he technici an graphs the disc harge-ai r temperature se tpoint[...]

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 . Change the set point to a re asonab le val ue manua lly o r add a li[...]

® 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 . Chang ing the ga ins may s lightly i mprove th e speed and[...]

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. Y ou may need to adjus t the samp ling fr equency (usua lly to slow it down). If the measure d [...]

® 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). V elocity model f or mu la The f ormula us ed to calc ulate th e output in the ve locity mo del[...]

® 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 . A controller using reve rse action decreases the outp ut when the mea- sured variable[...]

® 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. gai n The primary to ol for tuning PID loops , gain dete rmines how a particular part of the PID calculation contributes to the final output[...]

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. pr opor tional, int egr al, der iv ativ e (PID[...]

® 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 63.21% provides a good app roximation of the sy stem time constan t.[...]

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 dead band, 19 and PID outpu t, 2 cycling, troubleshootin g, 46 aliasing, 12 applicati ons, 29- 44 building pre ssure control , 18, 32-33 cascad e contro[...]

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 ng, 20-2 1, 37 calculat ing for staged outputs, 21, 40 for modulatin g outputs, 20 recom mended v alues, 19 executio n frequenc y , see sampling freq[...]

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 alues, 14 syste m time cons tant, 15 troubleshoo ting, 45, 46, 48 sensor resolu tion, 20, 32 setp oint high limit, 36 in PID calcula tion, 2 low l imit[...]

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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. Litera ture Order Numb er CNT -APG0 02-EN File Number PL -ES-C N T -APG0 02-EN- 1 0 0 1 Supe rsedes New Stocking Location La Crosse The T ra ne Comp an y An Americ an [...]