A UNIFIED PID METHODOLOGY TO MEET PLANT OBJECTIVES. Introduction

Transcription

1 A UNIFIED PID MEHODOLOGY O MEE PLAN OBJECIVES Gregory. MMllan, CDI Proe & Indutral, Autn, X and Hétor H. orre, Eatman, laxala, Méxo Introduton Plant objetve for ba and advaned regulatory ontrol an be qute dvere. PID ontrol play an mportant role n mzng afety, envronmental and equpment proteton, proe effeny and proe apaty. here an be many dfferent oure of proe varablty that ontrol loop need to deal wth, uh a raw materal and reyle ompoton, weather, utlty temperature and preure, operator, nteraton, optmzaton, on-off aton, tartup, tranton, hutdown, meaurement and proe noe, and lmt yle. Proe nonlnearte, proe non-elf-regulaton, dead tme domnane, low meaurement and valve, analyzer yle tme, deadband, and threhold entvty or reoluton lmt an make the tak muh more dffult. he tehnque for applyng PID ontrol to aheve plant objetve are largely ndvdualt than ytemat. Eah applaton tend to be treated a unque requrng peal experte and tunng. he reult ha been over 100 tunng rule to meet dfferent pereved and atual need of the proe. Conultant are often requred and PID feature are not effetvely utlzed. In ome plant PID loop have been puhed nto model predtve ontrol (MPC) beaue PID funton applaton prate are largely heurt and undoumented. A unfed approah to tunng ha been found that enable a ommon and mplfed method for ettng PID tunng parameter. ey feature an be ued to elmnate the need for retunng to deal wth dfferent dynam and objetve. h paper how a methodology that ntegrate a unfed tunng approah and key feature that mnmze mplementaton and mantenane effort. A tep by tep method wll be ued that wll addre the myrad of dynam and objetve n PID applaton. ype of Proe Repone here are 4 major type of proe repone. Early tunng rule were developed wth one type of proe repone n mnd. unng rule developed n the 1990 have expanded the ope of proee addreed but few effetvely deal wth all type of repone. he tunng requrement are o dfferent for eah type of repone that uer an readly end up onfued. he frt tep to reolvng th defeny reognton of the dtntve haratert of eah type of proe repone. Repone Methodology and ermnology he effet of automaton ytem dynam wll be nluded and dtnguhed n the proe repone. A frt order plu dead tme (FOPD) approxmaton ued that ont of a dead tme, a tme ontant, and a gan. For a near-ntegratng proe the gan dvded by the tme ontant to gven an ntegratng proe gan ued for ntegratng proee. he dentfaton or approxmaton of a eondary tme ontant added to mprove the ettng of the rate tme for the dervatve mode.

2 In the lterature thee parameter to defne the dynam repone are referred to a proe dead tme, proe tme ontant, and proe gan. Sne n ndutral applaton thee parameter are determned by the automaton ytem bede proe dynam, more general term wll be ued. he total loop dead tme the um of all the pure tme delay plu the fraton of the mall tme ontant that equvalent dead tme. All tme ontant maller than the larget (prmary tme ontant) and next larget f a eondary tme ontant dentfed reate an effetve dead tme. he delay and tme ontant a hown n Fgure 1 n the fnal ontrol element (e.g. ontrol valve or varable peed drve), proe, meaurement, and ontroller mut be nluded. In the lterature prmary and eondary tme ontant are ondered to be n the proe a hown n Fgure 1. In ndutral applaton thee tme ontant an be n the automaton ytem. In th paper the larget tme ontant n Fgure 1 termed the open loop tme ontant. he open loop gan the produt of the fnal ontrol element, proe, and meaurement gan hown n Fgure 1. For elf-regulatng proee, the open loop gan dmenonle (%/%). For ntegratng proee, the reultng gan alled an ntegratng proe gan ha unt of 1/e (%/e/%). Proee wth a large proe tme ontant are termed near-ntegratng. he near-ntegratng proe gan attaned by dvdng a elf-regulatng proe gan by the prmary proe tme ontant. Fgure 1 Blok dagram reveal the myrad oure of dynam n the FOPD repone

3 Dead tme Domnant Self-Regulatng Repone A elf-regulatng proe wll reah a new teady tate f a manual hange n the PID output made provded there are no dturbane. he teady tate the reult of nternal elf-regulaton from a negatve feedbak proe tme ontant. he atual tme to teady tate oberved wll depend upon the open loop tme ontant (larget tme ontant n the loop), whh may be n the automaton ytem. If the total loop dead tme exeed the open loop tme ontant, the proe dead tme domnant. he proe dynam repone of onveyor, heet and pn lne eentally a pure dead tme from the tranportaton delay. However, f de bolt heater are ued ntead of hydraul atuator to ontrol heet thkne, the thermal lag ntrodued an be large enough that the proe no longer dead tme domnant. Blender, deuperheater, extruder, tat mxer, and plug flow reator have a neglgble proe tme ontant due to the lak of bak mxng. However, the temperature repone often not dead tme domnant due to the thermal lag n the equpment heat tranfer urfae and thermowell. he addton of a large wrele default update rate n a flow loop an make the flow loop dead tme domnant. A large wrele default update rate n a tat mxed ph loop or the ue of an at-lne or off-lne analyzer wth a large yle tme an make thee ompoton loop dead tme domnant. For thee ae where the dead tme domnane due to a dontnuou update of an automaton ytem omponent, we wll ee that a PID feature an be enabled to prevent ntablty and mplfy tunng. Lag Domnant Self-Regulatng Repone If the elf-regulatng proe repone ha an open loop tme ontant larger than the total loop dead tme, the proe termed lag domnant. Contnuou operaton wth mxng uh a olumn, rytallzer, evaporator, neutralzer, and reator have a lag domnant repone. Lqud flow ha a elf-regulatng repone but the proe tme ontant le than the total loop dead tme. A gnal flter ued to redue noe an make the proe lag domnant. A large automaton ytem lag an make a repone lag domnant that would have been dead tme domnant baed on the proe tme ontant. If the open loop tme ontant muh larger than the loop dead tme, the proe repone een by the PID a ramp n the ontrol regon of 4 dead tme. A PID tuned for dturbane rejeton wll have done nearly all of t orreton n 4 dead tme. So far a what the PID ee, the proe repone ntegratng. Suh proee are termed near-ntegratng and an ue ntegratng proe tunng rule. emperature and ompoton ontrol loop on ontnuou proee are near-ntegratng. Integratng Repone Proee that have no negatve feedbak proe tme ontant and hene no teady tate are termed ntegratng proee. he proe repone ntegrate any unbalane n the proe. Sne the unbalane nearly never exatly zero, the proe wll alway ramp when the PID n manual. For mot ntegratng proee the ramp rate extremely low (e.g %/e). he extreme exepton ga preure where the ramp rate an be a mllon to a bllon tme fater. An eletral

4 furnae had an ntegratng proe gan of 1 %/e/%. A wate heat nnerator ga preure had an ntegratng proe gan of 1000 %/e/% (too fat for any dgtal ontroller). he mot ommon example of an ntegratng proe level. Ga preure ha an ntegratng repone when hange n operatng preure are a mall fraton of the preure drop aro nlet or outlet valve and onequently do not appreably affet the flow nto or out of the ytem. Bath operaton aue an ntegratng repone beaue there no dharge flow untl the operaton omplete. Bath temperature ntegratng unle the heat lo hange appreably wth temperature. Bath ompoton and bath ph ha an ntegratng repone unle the reatant or reagent rapdly onumed n a reaton. Runaway Repone Proee wth a potve feedbak tme ontant an aelerate. hee proee are termed open loop untable. If the PID put n manual, the proe an aelerate and run off-ale. Exotherm reaton where the reaton rate greatly nreae wth temperature an have runaway repone f the heat releae exeed the oolng apablty. A large eondary tme ontant (e.g. heat tranfer urfae or thermowell lag) or dead tme approahng the potve feedbak tme ontant an aue the proe to be untable regardle of PID tunng. Proe Objetve he man tak of the PID to determne the tranfer of varablty from a ontrolled varable uh a temperature to the manpulated varable uh a oolant flow. he proe nput mot often manpulated a proe or utlty flow. For mum dturbane rejeton, the tranfer of varablty mzed. h generally the ae for veel and olumn temperature, ompoton, ga preure, and ph ontrol. An nuffently aggreve tranfer of varablty an aue produt degradaton or an emon, unontrollable reaton, a relef valve or rupture d to blow, and a afety ntrumented ytem (SIS) to atvate. Safety, equpment and envronmental proteton, on-tream tme, and yeld depend upon dturbane rejeton. o meet thee objetve the ntegrated and peak error mut be mnmzed. he ultmate lmt to how mall thee error are depend upon the total loop dead tme ( o ) and the open loop tme ontant ( o ). Fgure 2 how that the ultmate lmt to the peak error proportonal to the rato of dead tme to the 63% repone tme whh the dead tme plu the open loop tme ontant. he peak error the alttude of the rght trangle whoe bae the dead tme ne no orreton an be enated and een tll one dead tme. A mrror mage of th rght trangle plaed bak to bak wth the frt trangle how the total reovery from the dturbane. he ultmate lmt to the ntegrated error the area of the two trangle whh end up beng the dead tme to 63% repone tme multpled by the dead tme. In ome applaton the tranfer mut be mnmzed beaue hange n the manpulated flow affet other loop and operaton. he mot ommon applaton urge tank level ontrol where the level PID manpulate the dharge flow to the degree only needed to keep the tank level wthn low and hgh lmt. he objetve to mnmze the drupton of feed hange to downtream operaton. h objetve alo effetve for a dtllate reever level PID manpulatng dtllate flow. h goal often termed mzaton of the aborpton of varablty.

5 Fgure 2 he ultmate lmt to the peak and ntegrated error depend upon the total loop dead tme and open loop tme ontant Aggreve ontrol aton an alo upet other loop. For nteraton that annot be redued uffently by deouplng tehnque, the tranfer of varablty for the leat mportant loop mnmzed to avod upettng the more mportant loop. When loop are manpulatng flow affetng a unt operaton, the tranfer of varablty to thee flow oordnated o the tohometr rato not momentarly upet from one flow beng fater or lower than another flow. In th ae the tranfer of varablty n all loop regulated to math the lowet loop. h oordnaton of manpulated flow partularly mportant for operaton wthout bak mxng to mooth out tranent. Coordnaton onequently rtal for blendng and plug flow lqud and ga reaton (e.g. atalyt bed ga reator). PID loop mut not nreae varablty. A PID loop an ollate from overly aggreve aton and nteraton and n ntegratng and runaway proee from nadequate mmedate aton due to a gan ettng that too low. A PID an reate varablty when there are automaton ytem dffulte. Automaton Sytem Dffulte he repone of the automaton ytem an ntrodue exeve dontnute, lag, noe, and dead tme. Here we look at the omponent of the ytem for the oure of thee dffulte. here only room n th paper to note ome of the more gnfant defene.

6 Fnal Control Element A ontrol valve an have gnfant tton from exeve frton often derbed a a reoluton lmt. Atuator and potoner an alo have a threhold entvty lmt. One or more ntegrator n the proe or PID n a loop wth a reoluton or threhold entvty lmt wll aue a lmt yle that annot be elmnated by tunng. A lmt yle a utaned equal ampltude ollaton. A ontrol valve or damper an have gnfant baklah from play n lnkage and onneton een a a dead band upon reveral of dreton of the gnal. wo or more ntegrator n the loop wll aue a lmt yle than annot be elmnated by tunng. hu, an ntegratng proe and a PID ontroller wth ntegral aton an aue a lmt yle. A elf-regulatng proe wth a aade loop where the prmary and eondary PID eah have ntegral aton wll exhbt a lmt yle. A varable peed drve (VSD) wth a reoluton lmt due to nuffent number of bt n the analog to dgtal gnal nput ard wll reult n a lmt yle for one or more ntegrator. If a dead band ntrodued n the VSD etup, lmt yle wll develop for two or more ntegrator. A loop wth a low fnal ontrol element that annot keep up wth the rate of hange of the ontroller output wll burt nto ollaton. he ollaton may not how up durng loop tet where typally mall hange are made. he burt of ollaton may only our for large fat dturbane and large etpont hange aung the rate of hange of the PID output to be fater than the valve lewng rate or VSD rate lmted repone. Large valve and damper and VSD wth margnally zed motor an be too low. Meaurement he enor degn, loaton, and operatng ondton an reult n exeve meaurement noe. Dfferental head meter and vortex meter are epeally entve to veloty profle and hene the uptream and downtream ppng onfguraton. Mot flow meter get noy a you approah the low flow lmt. ph eletrode are partularly vulnerable to noe from eletromagnet nterferene and onentraton flutuaton. he meaurement an alo ntrodue an exeve lag nto the ontrol loop. Fouled eletrode and thermowell an have an exeve lag from a dereae n the ma tranfer and heat tranfer oeffent, repetvely. Preure and dfferental preure tranmtter on ompreor ontrol an beome too low by mply nreang the tranmtter dampng ettng. If the meaurement lag beome by far the larget tme ontant n the loop an ndou tuaton develop where the trend hart may look moother beaue the meaurement howng an extremely attenuated (fltered) vew of the atual proe ollaton. he ontroller gan may even be able to be nreaed further mleadng the uer nto thnkng the meaurement lag benefal. he key the loop perod hould how an nreae due to the nreae n loop dead tme. Wrele meaurement an ntrodue exeve dead tme n fat loop by a low default update rate. At-lne analyzer and epeally off-lne analyzer an aue exeve dead tme n almot any loop from the ample tranport and proeng, analy yle tme, and multplex tme.

7 Controller he ontroller an ntrodue an exeve dead tme from too large of a module exeuton tme or gnal flter tme. Even wthout onderng the effet of the nreae n dead tme on the ultmate lmt to error and tunng, a gnal flter and module exeuton tme larger than 10% of the reet tme wll nreae the ntegrated error by more than 10% for an unmeaured dturbane. oo low of a eondary ontroller tunng an aue a aade loop to burt nto ollaton when the prmary ontroller output hangng fater the eondary loop an repond. Inuffent gan aton an aue low rollng ollaton n an ntegratng proe and ntablty n a runaway proe. Unfed unng he key breakthrough n thnkng to ue the lambda tunng rule for ntegratng proee for lag domnant elf-regulatng and runaway proee. he lambda tunng rule automatally prevent the produt of the PID gan and reet tme from beng le than the twe the nvere of the ntegratng proe gan that would reult n low rollng ollaton. he man deon whether to mze the tranfer of varablty or mze the aborpton of the varablty. ey feature are ued rather than tunng to addre the automaton ytem dffulte and to meet dfferent proe objetve. Maxmzaton of the ranfer of Varablty he mum dead tme for all poble operatng ondton ued a the arret tme, whh the tme to tart to turn bak an exuron from an unmeaured dturbane. he arret tme lambda. h mnmum lambda the dead tme a hown n Fgure 2. he mnmum lambda plu the mum ntegratng proe gan (fatet ramp rate) ued n the tunng equaton. he lambda tunng rule then gve the ame gan and reet tme a the Zegler Nhol Reaton Curve method. For temperature, ompoton, and ph loop, the rate tme hould be et equal to the eondary tme ontant. If th tme ontant annot be dentfed, ½ of the total loop dead tme an be ued a an approxmaton. h gve a rate tme that about 1/6 of the reet tme. Note that for the ISA Standard Form PID, the rate tme mut alway be le than the reet tme to avod ntablty (ee Appendx A). If a lower ontroller gan ued. he equvalent lambda need to bak alulated and ued n the alulaton of the reet tme to prevent low rollng ollaton. For runaway proee, the PID gan mut alway be greater than the nvere of the proe gan to prevent a runaway. Maxmzaton of the Aborpton of Varablty When the aborpton of varablty mut be mzed, lambda hoen to be a large a poble. he mum arret tme lambda depend upon the ntegratng proe gan, the allowable hange n the proe varable and the avalable hange n the manpulated flow. he followng equaton how how to alulate lambda for a gener applaton and then a urge tank level loop. he mum lambda ( ) the mum allowable % exuron ( %PV ) dvded by the mum poble PV ramp rate. he mum poble ramp rate the PV rate of hange per perent output hange multpled by the mum avalable perent output hange ( %CO ).

8 % PV (1) % PV t % CO % CO 1 Realzng that the ntegratng proe gan the PV ramp rate per perent output hange: % PV (2) % CO An equvalent etpont rate lmt on the ontroller output (e.g. flow ontroller etpont): % CO t % CO % CO PV % 2 (3) For a PV lmt (% PVLmt ) and orrepondng CO lmt (% COLmt ) we have: 1 % PVLmt % SP (4) % CO CO Lmt % he above alulaton would be done for hgh and low operatng lmt and varou etpont. he mallet of the arret tme would be ued n tunng. We an obtan the more detaled requrement for urge level tank level ontrol by omputng the ntegratng proe gan for level. he ntegratng proe gan the produt of the valve, proe, and meaurement gan: (5) v p m he valve gan or varable peed drve gan for a lnear ntalled haratert or flow loop : v Fv F (6) % CO 100% he level proe gan for ma flow (omt denty term for volumetr flow): 1 p A (7)

9 he level meaurement gan : m % PV 100% (8) L L Subttutng n the valve, proe, and meaurement gan, the ntegratng proe gan : v p m F 1 100% F 1 % A L L A 100 (9) he onequental arret tme for a level loop : A L F (10) % PV % CO An equvalent etpont rate lmt on the ontroller output (e.g. flow ontroller etpont): % CO t % CO F % CO PV A L % 2 (11) Where: A ro etonal area (m 2 ) F mum hange n valve or varable peed drve flow (e.g. flow pan) (kg/e) L mum hange n level (e.g. level pan) (m) ntegratng proe gan (1/e) m p meaurement gan (%/m for level) proe gan (m/kg for level) v valve or varable peed drve gan (kg/e/%) arret tme (e) flud denty (kg/m 3 ) % CO operatng pont ontroller output (%) % ontroller output lmt (%) CO Lmt %CO mum avalable hange n ontroller output (%) % proe varable lmt (%) PV Lmt %PV mum allowable hange n proe varable (%) % SP operatng pont etpont (%)

10 ey PID Feature here are even key related PID ontrol feature; truture, external-reet feedbak, feedforward, etpont flter and lead-lag, etpont rate lmt, output trakng, and an enhanement for wrele. hee key feature ynergtally provde apablte that have not been fully realzed. See Appendx A for onveron of tunng ettng for dfferent PID Form and tunng ettng unt. PID Struture Mot modern DCS offer a hoe of truture hown n able 1. able 1 - Major PID Struture Choe (1) PID aton on error (= 1 and = 1) (2) PI aton on error, D aton on PV (= 1 and = 0) (3) I aton on error, PD aton on PV (= 0 and = 0) (4) PD aton on error, no I aton (= 1 and = 1) (5) P aton on error, D aton on PV, no I aton (= 1 and = 0) (6) ID aton on error, no P aton ( = 1) (7) I aton on error, D aton on PV, no P aton ( = 0) (8) wo degree of freedom ontroller (and adjutable 0 to 1) he truture hoe ue and et pont weghtng fator. A ontroller that ha both fator adjutable alled a two degree freedom ontroller. Other truture have the and fator et equal to 0 or 1. he uer an alo omt a mode entrely to get P-only, I-only, ID, and PD ontrol wth varou agned fator. PI ontrol aheved by mply ettng the dervatve (rate) tme to zero. In general, the uer mut not et the ontroller gan equal to zero n an attempt to get I-only or ID ontrol or et the ntegral (reet) tme to zero n an attempt to get P-only or PD ontrol. Note that the ue of P- only or PD ontrol requre addtonal hoe of how to et the ba and t ramp tme. In hoong the orret truture t mportant to know the relatve ontrbuton of the proportonal, ntegral, and dervatve mode. Fgure 3 how the ontrbuton of eah PID mode for a etpont hange. here no feedbak from the proe n th fgure (e.g. blok valve loed), o we an more learly ee the ontrbuton of eah mode. Struture 1 (PID on error) provde the fatet approah to a new etpont by vrtue of a tep and a kk n the ontroller output from the etpont hange a een n the eond repone n Fgure 3. For mall etpont hange and low ontroller gan, the tep and kk an help get through gnfant valve baklah and tk-lp to get the valve movng. he kk appear to be a pke on trend hart wth a large tme pan. he abrupt hange n output often een a druptve by operator when they make etpont hange. If the burt of flow through the ontrol valve doe not affet other uer of the proe or utlty flud, the tep and kk more of a pyhologal than a proe onern.

11 Fgure 3 Contrbuton of eah PID mode for a tep hange n etpont for a PID on error truture Struture 2 (PI on error, D on PV) the truture mot often ued. Struture 2 elmnate the kk from dervatve aton for a etpont hange by ettng gamma to zero (). he nreae n re tme gong from truture 1 to 2 neglgble for the more mportant loop, uh a olumn and veel temperature where dervatve aton ued. he tep n the output from the proportonal mode on a etpont hange large for a hgh ontroller gan. Inreae n proe gan or dead tme wll nreae the overhoot unle the ontroller gan dereaed aordngly. If the elmnaton of etpont overhoot muh more mportant than re tme (e.g. boreator temperature and ph hft), then truture 3 may be bet. Struture 3 (I on error, PD on PV) elmnate overhoot but wth qute a arfe n peed of approah to the etpont. For near and true ntegratng and runaway proee, truture 3 may have a re tme an order of magntude larger than the re tme for truture 1 for the ame tunng ettng and proe dynam. A etpont flter et equal to the reet tme provde the ame repone.

12 Struture 4 (PD on error, no I) ued on proee adverely affeted by ntegral aton. he temperature ontrol of everely exotherm polymerzaton reator ue truture 4 beaue ntegratng aton n the ontroller nreae the rk of a runaway. If ntegral aton ued, the reet tme hould be nreaed by a fator of 10 for thee potve feedbak proee to be afe. Uer may not be aware of th requrement leadng to overhoot that an trgger a runaway. he ba for truture 4 et equal to the normal PD ontroller output when the PV at etpont. Struture 4 alo ued on bath proee that repond n only one dreton. For example, n brngng a bath ph up to a etpont by the addton of a bae where the bae not onumed n a reaton, the bath wll only repond n the dreton of nreang ph. he ph wll overhoot etpont f ntegral aton ued. If plt rangng added wth an ad reagent, there wll be ome wated reagent due to ro neutralzaton of reagent and lmt ylng aro the plt range pont from tton that greatet near the loed poton. For truture 4 and a ngle reagent, the ba et for zero reagent addton when the PV at etpont. Struture 5 (P on error, D on PV, no I) ued for many of the ame reaon a truture 4. Struture 6 (ID on error, no P) ued for valve poton ontroller (VPC) to elmnate the nteraton wth proe ontroller whoe valve poton beng optmzed. he VPC ould be optmzng the oare adjutment from a large ontrol valve n parallel wth a mall ontrol valve manpulated a fne adjutment by the proe ontroller. In Fgure 4 the proe ontroller FC215 adjut the mall valve nput to the proe. he output to the mall valve alo ued a a PV of the VPC ZC215. he mall valve provde preon and the large valve gve rangeablty. Fgure 4 - Valve poton ontrol nreae preon and rangeablty he VPC ould be optmzng utlty upply preure or temperature to mnmze energy ue or optmzng feed rate to mze produton rate. Normally, the rate tme zero. he tunng of th ntegral-only ontroller problemat. unng rule often ted ugget the ntegral tme hould be larger than 10 tme the produt of the gan and reet tme of the proe ontroller and 10 tme the redene tme of the proe to elmnate nteraton. However, the VPC repone for th tunng too low to prevent the proe ontroller from gettng nto trouble for large dturbane. Feedforward aton an be added to help. In addton external-reet feedbak, etpont dretonal rate lmt, and an enhaned PID developed for wrele an be ued a a flexble and eay to tune oluton to deal wth unknown. h oluton derbed n the Setpont Rate Lmt eton.

13 Struture 7 (I on error, D on PV, no P) ued for the ame reaon a truture 6. A wth truture 2, the pke from rate aton for etpont hange elmnated. Struture 8 (two degree of freedom va adjutable and ued to provde a balane between a fat re tme and overhoot. he ame effet an be aomplhed by the ue of a etpont lead-lag a dued n the etpont flter eton. h truture an be ued to get the truture wth proportonal or dervatve aton on PV by zerong the repetve and et pont fator. External-Reet Feedbak PID ontroller wth the potve feedbak mplementaton of ntegral aton a hown n Fgure 5 enable the ue of an external reet feedbak. When the wth n Fgure 5 n the poton to get an external reet feedbak to the ntegral mode from the proe varable (PV) of a etpont beng manpulated whether an Analog Output blok or a eondary PID, the PID output prevented from hangng fater than the PV of the ontrol valve, VSD, or eondary loop an repond. here an nreangly larger ope of potental beneft dovered a derbed n ubequent feature and the ummary of the unfed methodology. he only requrement that the external reet feedbak be properly onneted to a fat update of the PV for the valve, VSD, or loop beng manpulated. Update of atual valve poton, VSD peed, or eondary loop PV hould be at leat twe a fat a the the exeuton tme of the ontroller wth external reet feedbak. Fgure 5 ISA Standard Form PID wth External Reet Feedbak

14 Feedforward Feedforward ontrol partularly effetve when a dturbane large, fat, and preely meaured. Whle theoretally a feedforward multpler more effetve for hange n the lope of a plot of the manpulated flow veru the meaured dturbane, for many pratal reaon uh a alng and nonlnearty problem, a feedforward ummer ued for feedforward ontrol epeally on veel and olumn. he mot ommon feedforward gnal flow beaue the hange n feed flow are fater than hange n ompoton and temperature and the flow the mot ommon proe nput. A flow feedforward ummer mplemented va an external rato and ba taton o the operator an et the dered rato and ee the atual rato. On tartup, the operator may run on rato ontrol wth the proe ontroller n manual untl the equpment reahe operatng ondton. he ntal flow rato an be et baed on the proe flow dagram (PFD). he PFD an be put lve onlne on the operator graph to how the atual rato ued. he tehnque alo enable a plant wde feedforward ytem to rapdly nreae and dereae produton rate. Setpont Flter and Lead-Lag A etpont flter an provde the dered loed loop tme ontant mportant for the oordnaton of loop. he ue of eondary PID flow etpont enable tohometr rato to be enfored wthout a tranent unbalane for produton rate hange and etpont hange and dturbane to the prmary PID. External reet feedbak mut be turned on n the prmary PID to prevent t from tryng to hange the eondary PID et pont fater than flter wll allow the eondary loop to repond. urnng external reet on elmnate the need to retune the prmary PID when the eondary PID flter added or tuned. If oordnaton of loop not neeary, a etpont flter hould not be added to a eondary loop beaue t wll low down the rejeton of prmary loop dturbane. he re tme (tme to reah etpont) and overhoot for a etpont hange an be optmzed by the ue of a etpont lead-lag. In th ae the etpont lag tme et equal to the reet tme and the etpont lead tme et equal to about 20% of the lag tme. If preventng overhoot more mportant than redung re tme for a etpont hange, the lead tme an be dereaed. Setpont Rate Lmt he etpont rate lmt opton n analog output (AO) and PID funton blok n a modern DCS an provde move uppreon, a tunng parameter found to be very ueful n model predtve ontrol. Sne there often an up and down rate lmt, dretonal move uppreon an be ued provded the ablty needed to enable a low approah to an optmum and a fat getaway to prevent an abnormal ondton. h tehnque ued to provde a fat openng and low long ant-urge ontrol valve. here are many other applaton a we wll ee n the unfed methodology. If external reet feedbak turned on n the PID manpulatng the etpont, the PID doe not have to be retuned when the rate lmt are added or tuned n the manpulated AO or PID. hee rate lmt an be wrtten to onlne. Output rakng Dturbane an be too fat and the onequene too evere to rely jut on PID ontrol. Here output trakng ued to get the loop out of trouble for equpment and envronmental proteton. he mot ommon example ompreor urge preventon, where the PID output wll trak a urge valve

15 poton large enough to prevent urge when a rong of the urge urve deteted or a preptou drop n ompreor uton flow our ndatng the tart of urge. he PID tay n output trakng holdng the protetve output for everal eond to enure the ytem ha tablzed. he fatet poble etpont repone obtaned from a full throttle approah (bang-bang ontrol method) where the PID put n output trakng at the output lmt to drve the PV to the etpont. he PID output tay at the lmt untl the PV one dead tme nto the future predted to reah etpont. A dead tme (D) blok wth t parameter et equal to the loop dead tme ued to reate an old PV that ubtrated from the new PV to reate a delta PV. he delta PV added to the new PV that the D blok nput gve the PV one dead tme nto the future. When the future PV projeted to reah etpont, the PID trak a fnal retng value (FRV) aptured from prevou bathe or tartup and hold th FRV for one dead tme before releang the PID bak to feedbak ontrol. Enhaned PID for Wrele he enhaned PID opton developed for wrele enable the followng feature: 1. Potve feedbak mplementaton of reet wth external-reet feedbak 2. Immedate repone to a etpont hange or feedforward gnal or mode hange 3. Supenon of ntegral aton untl there a hange n PV 4. Integral aton the exponental repone of the potve feedbak flter to the hange n ontroller output for the tme nterval ne lat update 5. Dervatve aton the PV or error hange dependng upon PID truture dvded by the tme nterval ne the lat update (ntead of PID exeuton tme) multpled by the gan and rate tme he enhaned PID wll go nto a holdng mode for a lo of ommunaton, tuk ontrol valve, or falure of an eletrode, analyzer, or ample ytem provdng a mooth ontnuaton upon retoraton of ommunaton and analyzer reult. For offlne analyzer, late lab reult do not aue a problem. he enhaned PID elmnate the need for retunng a dontnuou delay from wrele deve and analyzer are nreaed. For nteratng loop and valve poton ontrol and fnal ontrol element wth reoluton lmt and deadband, a threhold entvty ettng enable the enhaned PID to elmnate reaton to ngnfant hange that ould aue fghtng between loop or perpetuate a lmt yle. he threhold entvty et jut larger than the PV noe ampltude after flterng. A Unfed Methodology he followng a generalzed tep by tep proedure to addre a wde petrum of automaton ytem dffulte and applaton objetve. Note that PID feature vary onderably from one DCS manufaturer to another. Any ettng of opton or tunng mut be baed on nformaton from the uppler and hould be thoroughly teted before and arefully oberved after ommonng. 1. Set the output lmt to keep the manpulated etpont n the dered operatng range. For varable peed drve et the proe PID low output lmt o the peed annot aue the dharge head to approah the tat head n order to prevent exeve entvty to preure and to prevent revere flow. In general, et the ant-reet wndup lmt to math the output lmt. If the output ale engneerng unt, the output lmt and ant-reet wndup mut be baed on the

16 output ale range and unt. 2. Chooe the bet truture for your applaton. Generally the bet hoe truture 2 wth PI on error and D on PV. For a ngle dreton repone (e.g. bath heatng or neutralzaton), ue truture 4 or truture 5 o that there no ntegral aton. For a hghly exotherm reaton, you mght want truture 5 to help prevent a runaway from ntegral aton. 3. Set the gnal flter noe jut large enough to keep the ontroller output flutuaton from exeedng the reoluton lmt or deadband of the fnal ontrol element. 4. For near-ntegratng, true ntegratng, and runway proee ue the lambda ntegratng proe tunng rule. o mze the tranfer of varablty from the proe varable to the manpulated varable, et the lambda (arret tme) equal to twe the mum poble dead tme and ue the larget ntegratng proe gan for all operatng ondton n the tunng. o mze the aborpton of varablty (e.g. urge tank level) ue the mnmum arret tme omputed from Equaton 1 through 10 for all poble operatng ondton. If you dereae the PID gan, proportonally nreae the PID reet tme to prevent low rollng ollaton. For runaway proee, the PID gan mut not be le than the nvere of the open loop gan. When hangng from the Sere Form to the ISA Standard Form n newer DCS, onvert the tunng ettng baed on the dfferene n Form. In the ISA Standard Form the rate tme hould not be greater than ¼ the reet tme. Be ure to realze and onvert any dfferene n Form and tunng ettng unt between dfferent PID vntage and uppler (ee Appendx A). You mut take nto aount the unt and nvere relatonhp between gan and proportonal band and reet tme n eond and mnute per repeat. Not addreng thee dfferene an reult n ettng off by order of magntude. 5. For elf-regulatng proee wth the open loop tme ontant le than 4 tme the dead tme, ue the lambda elf-regulatng tunng rule. o mze the tranfer of varablty from the proe varable to the manpulated varable et the lambda (loed loop tme ontant) equal to the twe the mum poble dead tme and ue the larget proe gan and mallet tme ontant for all operatng ondton n the tunng. Shedule tunng ettng preferably wth an adaptve tuner wth the pefed lambda to deal wth hange n the dynam. 6. urn on external reet feedbak. Make ure the external reet feedbak gnal orretly propagated bak to the PID (e.g. BCAL gnal) epeally f there are plt range, gnal haraterzer, or gnal eletor blok on the PID output. 7. For fnal ontrol element that are low or that have deadband or reoluton lmt, ue a fat readbak of the valve poton or varable frequeny drve peed a the external reet feedbak to prevent a burt of ollaton from the PID output hangng fater than the fnal ontrol element an repond. 8. For fnal ontrol element that reate lmt yle from reoluton lmt and deadband, ue a fat readbak of the valve poton or varable frequeny drve peed to top the lmt yle. 9. For aade ontrol, ue the PV of the eondary loop a the external reet feedbak to prevent a burt of ollaton from volaton of the aade rule where the eondary loop mut be gnfantly fater than the prmary loop. 10. For etpont flter of eondary loop for oordnaton of flow loop, ue the PV of the eondary loop a the external reet feedbak to prevent the need to retune the PID. 11. For etpont rate lmt ue the PV of the analog output blok or eondary loop a the external reet feedbak to prevent the need to retune the PID. Add etpont rate lmt to mnmze the nteraton between loop and n valve poton ontrol and to provde dretonal move uppreon to enable a fat getaway for abnormal ondton and a low approah to optmum. For valve poton ontrol, ue an enhaned PID developed for wrele wth a threhold entvty lmt to gnore ngnfant hange n the valve poton to be optmzed.

17 12. Add output trakng for equpment proteton and a full throttle (bang-bang ontrol) trategy for the fatet poble tme to reah etpont on tartup and for bath operaton. 13. Add output trakng log to momentarly trak an output that nure equpment and envronmental proteton. For ompreor urge proteton trak a uffently large openng of the urge valve. o prevent a RCRA ph volaton, trak a rapdly nrementng reagent valve poton to prevent an effluent exuron below 2 ph or above 12 ph. 14. Add feedforward ontrol for large and fat meaured dturbane. For flow feedforward, ue a rato and ba taton o the operator an enter a dered flow rato and ee the atual flow rato. Setup the PID to provde a ba orreton to the manpulated flow. Add dynam ompenaton (dead tme and lead-lag blok) to the feedforward o the manpulated flow arrve at the ame pont n the proe at the ame tme a the meaured dturbane. 15. For wrele deve or analyzer that ntrodue a large dontnuou PV update delay ue an enhaned PID to elmnate the need to retune the ontroller to prevent ollaton. If the delay muh larger than the 63% proe repone tme, the PID gan an be et a large a the nvere of the open loop gan for elf-regulatng proee. 16. For valve poton ontrol (VPC), make ure the key PID feature n able 3 are ued. For example of VPC for proe optmzaton, ee able 4. he ue of an enhaned PID enable the ue of ntegral aton n the VPC and mplfe the tunng of the VPC. Whle model predtve ontrol an provde a more ophtated optmzaton, the VPC an be mplemented by a mple onfguraton hange that nvolve ettng up a PID a the VPC. he mnmal mplementaton ot and tme attratve for mall optmzaton. able 3 ey Feature Funton and Advantage for Valve Poton Control

18 able 4 Example of Valve Poton Control for Proe Optmzaton Not mentoned the beneft of ung aade ontrol. Fat eondary loop an orret for many dturbane before they affet the prmary loop. Seondary loop alo redue nonlnearte een by the prmary loop. A eondary flow loop olate the nonlnearty of the ntalled haratert of the ontrol valve and enable flow feedforward. A eondary jaket or ol temperature loop olate a rytallzer, evaporator, or reator temperature loop from oolng or heatng ytem nonlnearte. Conluon he ue of key PID feature, a unfed PID tunng method, and an adaptve tuner enable an effetve applaton of PID ontrol for wde petrum of proe repone. Dratally dfferent oure of varablty, automaton ytem dffulte and hangng proe objetve do not requre PID retunng. PID feature an be ued to talor the PID repone to dfferent applaton requrement on a muh more undertandable ba by the uer. he role of the PID an be expanded to nlude proe optmzaton bede regulaton.

19 Reoure 1. MMllan, G. PID unng Rule, Ineh, Jan.-Feb MMllan, G. Good unng: A Poket Gude, 3 rd Edton ISA, MMllan, G. Full hrottle Bath and Startup Repone Control, May Shnkey, G. he Power of External-Reet Feedbak Control, May MMllan, G. What I the ey PID Feature for Ba and Advaned Control? Control alk blog, Control, Apr. 26, MMllan, G. and orre, H. Effetve Ue of ey PID Feature Paper preented at ISA Automaton Week, Orlando, MMllan, G. Don t Overlook PID n APC Control, Nov Yu, C. and Luyben, W. Analy of Valve-Poton Control for a Dual Input Proe Ind. Eng. Chem. Fund., pp , Vol. 25, No. 3, Harrott, P. Proe Control MGraw-Hll, Bohl, A. and MAvoy,. Lnear Feedbak v. me Optmal Control. II. he Regulator Problem, Ind. Eng. Chem., Proe De. Dev., Vol. 15, No. 1, MMllan, G. Eental of Modern Meaurement and Fnal Element n the Proe Indutre ISA, MMllan, G. Advane n Flow and Level Meaurement Enhane Proe nowledge, Control Ineh, Jan.-Feb MMllan, G. Feedforward Control Enable Flexble, Sutanable Manufaturng Ineh, Mar.-Apr MMllan, G. ey Degn Component of Fnal Control Element Ineh, Mar.-Apr MMllan, G. What Your Flow Control Valve ellng You? Control Degn, May MMllan, G. Improve Control Loop Performane Chemal Proeng, Ot Shnkey, G. Flow and Preure Control Ung Varable-Speed Motor Proeedng of Control Conferene, pp , Chago, Al-halfah, M. and MMllan, G. Control Valve veru Varable Speed Drve for Flow Control Paper preented at ISA Automaton Week, Orlando, Smth, C. Wath Out wth Varable Speed Pumpng Chemal Proeng, May MMllan, G. Unlokng the Seret Profle of Bath Reator Control, Jul MMllan, G. I Wrele Proe Control Ready for Prme me Control, May MMllan, G. Wrele - Overomng Challenge of PID Control, Analyzer Applaton Ineh, Jul.-Aug

20 Appendx A PID Controller Form Converon of Settng to Corret Unt and Form he methodology n th paper baed on the ISA Standard Form wth pef tunng ettng unt where the ontroller gan dmenonle, the reet tme eond (eond per repeat), and the rate tme eond. PID tunng ettng mut frt be heked for unt and f neeary undergo a unt onveron before beng ued. (1) Convert proportonal band (%) to ontroller gan (%/%) (dmenonle) Gan = 100 % Proportonal Band (2) Convert reet ettng n repeat per mnute to reet tme n eond per repeat Seond per repeat = repeat per mnute (3) Convert rate tme n mnute to rate tme n eond Seond = 60mnute (4) After the tunng ettng unt are verfed, onvert the Sere or Parallel Form tunng ettng to the ISA Standard Form tunng ettng per equaton below. he prmed tunng ettng are for the Sere Form, the double prmed tunng ettng are for the Parallel Form, and the unprmed tunng ettng are for the ISA Standard Form. o onvert from Sere to ISA Standard Form ontroller gan: ' ' d ' (A.1) ' o onvert from Sere to ISA Standard Form reet (ntegral) tme: ' ' d ' ' ' d (A.2) ' o onvert from Sere to ISA Standard Form rate tme: d ' ' d (A.3) ' ' d Note that f the rate tme zero, the ISA Standard and Sere Form ettng are dental. When ung the ISA Standard Form, f the rate tme greater than ¼ the reet tme the repone an beome

21 ollatory. If the rate tme exeed the reet tme, the repone an beome untable from a reveral of aton form thee mode. he Sere Form nherently prevent th ntablty by nreang the effetve reet tme a the rate tme nreaed. We an onvert from ISA Standard Form to the Sere Form ung the followng equaton f the reet tme equal to or greater than 4 tme the rate tme ( 4 ). 0.5 ' d (A.4) ' d (A.5) ' d d (A.6) 2 d he Parallel form ued n ome older DCS and PLC to olate the proportonal mode tunng ettng from the other the mode. he gan ettng doe not affet the ontrbuton from the ntegral and dervatve mode and ometme alled non-nteratng. We an onvert from the Parallel to the ISA Standard Form ung the followng equaton. '' (A.7) (A.8) '' '' d (A.9) '' d '' We an onvert from the ISA Standard Form to the Parallel Form by the followng equaton: '' (A.10) '' (A.11) '' d d (A.12)

22 Where: ontroller gan for ISA Standard Form (%/%) (dmenonle) ' ontroller gan for Sere Form (%/%) (dmenonle) '' ontroller gan for Parallel Form (%/%) (dmenonle) ntegral tme (reet tme) for ISA Standard Form (eond) ' ntegral tme (reet tme) for Sere Form (eond) '' ntegral tme (reet tme) for Parallel Form (eond) d dervatve tme (rate tme) for ISA Standard Form (eond) ' d dervatve tme (rate tme) for Sere Form (eond) '' d dervatve tme (rate tme) for Parallel Form (eond) Potve Feedbak Implementaton of Integral Mode he potve feedbak mplementaton of the ntegral mode effetvely yeld the equaton above a een n lterature for a PI ontroller ung Laplae tranform. Intead of an ntegrator, a flter whoe nput the ontroller output and whoe output added to the ontrbuton from the proportonal mode n the potve feedbak mplementaton. he flter tme the ntegral tme ettng. When external reet feedbak enabled, the nput to the flter wthed from the ontroller output to the external reet feedbak gnal. ) ( 1 1 ) ( ) ( O E O (A.13) ) ( ) ( E O (A.14) ) ( 1 ) ( E O (A.15) E O 1 ) ( ) ( (A.16) Where: () E Laplae tranform of ontroller error (%) () O Laplae tranform of ontroller output (%)