An Educational plant based on the Quadruple-tank process

AN EDUCATIONAL PLANT BASED ON THE QUADRUPLE-TANK PROCESS I. Al a ado1D. Limon1W. Ga c´ ıa-Gab´ ın2T. Alamo1 E.F. Camacho1 Abs ac : This pape p esen s an expe imen al ank sys em de eloped a he Uni e si y o Se ille o p ocess con ol educa ion. This plan is based on he well known quad uple- ank p ocess and some modi ica ions ha e been done in o de o ob ain a wide ange o applica ions. The quad uple ank p ocess is a mul i a iable labo a o y plan o in e - connec ed anks ha can be easily con igu ed o exhibi he e ec o mul i a iable ze o (minimum and non-minimum phase) on he sys em beha io , as well as he e ec o non linea dynamics, sa u a ion, cons ain s, e c. In he eal plan implemen a ion, he o iginal s uc u e o he p ocess has been modi ied o o e a wide a ie y o uses o bo h educa ional and esea ch pu poses.Thus, di e en plan s can be con igu ed such as one single ank, wo o h ee cascaded anks, a mix u e p ocess and hyb id dynamics. Mo eo e he dynamics pa ame e s o each ank can be se up by uning he c oss-sec ion o he ou le hole o he ank. Fu he mo e, he eal plan has been implemen ed using indus ial ins umen a ion and a PLC o he low le el con ol. Supe ision and con ol o he plan is ca ied ou in a compu e by means o OPC (Ole o P ocess Con ol) which allows one o connec he plan wi h a wide ange o con ol p og ams such as LabView, Ma lab o indus ial SCADA. Keywo ds: P ocess con ol educa ion, labo a o y plan design, mul i a iable ze os, OPC 1. INTRODUCTION One o he di icul ies encoun e ed in con ol edu- ca ion consis s o p o iding a heo e ical ounda ion main aining he p ac icali y. To his aim, expe imen al labs p o ide a powe ul ool o ill his gap. An expe i- men al lab should be designed o show in e es ing and indus ially ele an con ol p oblems which equi e no oo skilled con ol solu ions and eal ools, such as ins umen a ion, con ol p og ams, e c. The quad uple ank p ocess has p o ed o be a e y in e es ing sys em o con ol educa ion in ad- 1Dp o de Ingenie ´ ıa de Sis emas y Au om´ a ica, Escuela Supe io de Ingenie os, Uni e sidad de Se illa. A da/ Camino de los Descub imien os s/n. 41092, Se illa (Spain). {al a ado,limon,alamo,edua do}@ca uja.us.es 2Uni e si y o Los Andes, Venezuela. [email p o ec ed] 3The au ho s g ace ully acknowledges MCYT (Spain) con ac DPI 2004-07444 o unding his wo k. anced con ol cou ses as well as in esea ch cou ses (Johansson, 2000; Johansson e al., 1999; Rusli e al., 2004; Long e al., 2005). The main p ope y o his p ocess is ha i is app op ia e o illus a e he impo ance o mul i a iable ze os since hese can be loca ed a he igh and he le hal plane. Fu he - mo e, he e exis o he ins e es ing p ope ies, such as he coupled na u e o he plan , he measu able s a es, he nonlinea beha io , o ins ance. To his aim, a labo a o y plan based on he quad u- ple ank p ocess has been designed and de eloped a Uni e si y o Se ille. This plan is used o bo h educa ional and esea ch pu poses. The objec i e o he design has been o p o ide lexibili y o he plan . Thus, he plan can be easily con igu ed o ob ain di - e en p ocesses and he con ol sys em has been im- plemen ed o allow us o con ol om he PLC, om an ex e nal de ice o om a compu e by means o an open and s anda d p o ocol OPC (OLE o P ocess Fig. 1. The Quad uple Tank P ocess scheme. Con ol). Thus, any con ol so wa e wi h OPC con- nec i i y (such as MATLAB, LAbView o comme cial SCADAs) can be used o con ol he plan . The pape is o ganized as ollows: i s , he quad uple ank p ocess is p esen ed in sec ion 2 and he imple- men ed plan is desc ibed in he ollowing sec ion. In sec ion 4 he ins umen a ion used in he plan is p esen ed and in sec ion 5 he con ol s uc u e is demons a ed. The pape d aws o a close wi h some conclusions. 2. THE QUADRUPLE TANK PROCESS Thisp ocess is alabo a o y plan p oposedin (Johansson, 2000) aimed o show he e ec o non-minimum ze os o a mul i a iable sys em. The o iginal plan consis s o ou in e connec ed anks as shown in Fig 1. The in- pu s a e he ol ages o he wo pumps and he ou pu s a e he wa e le els in he lowe wo anks. The model o he sys em (Johansson, 2000) is de i ed om i s p inciples as ollows dh1 d =−a1 A1p2gh1+a3 A1p2gh3+ γ 1 A1qa(1) dh2 d =−a2 A2p2gh2+a4 A2p2gh4+ γ 2 A2qb dh3 d =−a3 A3p2gh3+(1− γ 2) A3qb dh4 d =−a4 A4p2gh4+(1− γ 1) A4qa whe e he pa ame e s o he plan a e: S a e Va iables Uni Concep Aicm2C oss-sec ion o ank i aicm2C oss-sec ion o he ou le hole himWa e le el o he Tank i qa,qbm3/hFlow o e he pumps g m/s2The accele a ion o g a i y qim3/hFlow o e he each ank γ iPa ame e s o he h ee-way al es Linea izing he model in an ope a ing poin gi en by h0 iand de ining he a iables xi=hi−ho iand uj=qj− qo jwhe e j=a,band i=1,···,4 we ha e ha : dx d =             −1 T10A3 A1T30 0−1 T20A4 A2T4 0 0 −1 T30 0 0 0 −1 T4             x+             γ 1 A10 0 γ 2 A2 0(1− γ 2) A3 (1− γ 1) A40             u y=1 0 0 0 0 1 0 0 x (2) whe e Ti=Ai ai 2h0 i g≥0, i=1,···,4, a e he ime cons an s o each ank. The sys em is open loop s able wi h wo mul i a iable ze os. The na u e o hese ze os is de e mined by he pa ame e s γ 1and γ 2as ollows •I 0≤ γ 1+ γ 2<1 he sys em has Righ Hal Plane ansmission Ze os (RHPZ). •I 1 < γ 1+ γ 2≤2 has Le Hal Plane ansmis- sion Ze os (LHPZ) I is wo h ema king ha he sign o he eal pa o he ze os does no depend on he ope a ing poin . In addi ion o his ema kable p ope y, he plan pos- sesses ano he in e es ing ea u es ha make he plan app op ia e o be used o bo h educa ional and e- sea ch pu poses. These a e he ollowing: (1) The linea ized model o he quad uple- ank p o- cess has a mul i a iable ze o, which can be lo- ca ed in ei he he le o he igh hal -plane by simply changing a couple o al es. (2) All he s a es a e measu able. (3) The ou pu s a e s ongly coupled. (4) The sys em is nonlinea . (5) The s a es and inpu s o he plan a e cons ained. (6) The plan is easyly ha mlessly handled. Thus his plan can be used o show e y in e es ing con ol p oblems. Among hese p oblems, he ollow- ing ones can be highligh ed: •Con ol o mul i a iable sys ems •Con ol o sys ems wi h RHPZ and limi s o pe o mance. •Robus con ol. •S a e es ima ion. •T acking o cons an e e ences. •Con ol unde sa u a ing ac ions. •Con ol o sys ems subjec o cons ain s. The design and implemen a ion o he plan has been ca ied ou in such a way ha he po en ial educa ional in e es is maximized. This is de ailed in he ollowing sec ion. 3. IMPLEMENTATION OF THE LABORATORY PLANT One o he main objec i es in he implemen a ion o he quad uple ank p ocess has been o p o ide lexibili y o he plan in he ollowing aspec s: •Capabili y o se up di e en p ocesses in he same plan . •Capabili y o une some pa ame e s which allows us o con igu e he plan dynamics. •Wide ange o ope a ing poin s. Thus, he plan layou has been designed o mee hese speci ica ions (see igu e 8). The designed plan di e s om he quad uple ank p ocess p oposed in (Johansson, 2000) in he ollowing i ems: •The h ee-way al e has been eplaced by wo con ol al es con olling he low o he pipes. This allows us o ix a desi ed low a io be ween he wo pipes ( ha is, he pa ame e γ i) and hence ob ain an ideal h ee-way al e. Mo eo e , gi en ha all he lows o he inle pipes o he anks a e con olled, di e en p o- cesses can be con igu ed. •Ex a pipes, manipulable al es and ank in e - connec ions ha e been added o se up he di e - en p ocesses. These will be shown in he ollow- ing sec ion. •A manipula ed al e wi h a posi ion display has been placed in he ou le s o he anks in o de o manipula e he c oss-sec ion o he ou le hole ai. This allows us o con igu e he dynamics o each ank o he p ocess. •The anks a e anspa en , wi h a ec angula c oss sec ion and can be easily emo ed. This allows one o pu in some ad hoc elemen o change he c oss sec ion, and hence change he dynamics o he ank. Ano he ele an aspec o he plan is ha i has been implemen ed using indus ial measu emen de ices and indus ial con ol al es, which p o ides a ealis- ic amewo k o es con olle s. These, oge he wi h he low-le el con ol sys em, will be shown la e on. A pho og aph o he implemen ed plan can be seen in igu e 2. As i can be seen, his plan is la ge ha he ypical scaled lab plan s (Rusli e al., 2004; Johansson e al., 1999) ( he o al heigh is 3.5 m and he anks a e 1.3 m all).  Fig. 2. The implemen ed labo a o y plan . 3.1 Se ing up he plan One o he main p os o he designed plan is he capa- bili y o con igu e di e en p ocesses and dynamics. In he appendix some o he possible con igu a ions a e shown (see igu e 9). These can be achie ed by opening and closing some o he manipulable al es. See ha he loca ion o con ol al es in he ank inle s allows us o ob ain a la ge numbe o possible con igu a ions. The mos immedia e p ocesses ha can be con igu ed a e wo, h ee and ou cascaded anks; i is also possi- ble o con igu e a mix u e p ocess, whe e he ese oi is spli in wo pa s, con aining he wo p oduc s o mix; hese a e mixed in one o he uppe anks and he mix u e is s o ed in he bo om anks, connec ed o enla ge he capaci y. Ano he in e es ing p ocess is a simple hyb id sys em. This can be ob ained by connec ing he wo lowe anks a a ce ain heigh . Thus, he dynamic changes when some o he wo le els a e abo e his heigh . Dynamics o he plan can be se up by adjus ing he c oss-sec ion o he ou le s o he anks. See ha his pa ame e de e mines he cons an ime o he anks and also he wa e le el a a gi en ope a ing poin . Thus hese pa ame e s can be ixed wi h wo objec i es: change he ime esponse o he sys em o change he ange o a ia ion o he le el. The dynamics can also be uned by changing he c oss sec ion o he ank, pu ing an elemen o change he a ia ion o he olume wi h he le el. This allows us o enla ge he deg ee o nonlinea i y o he plan o e en ob ain an hyb id sys em. In he ollowing sec ion, some de ails o he ins u- men a ion a e p esen ed. 4. INSTRUMENTATION OF THE PLANT. The designed plan equi es a leas some de ices o measu e he wa e le el in he anks and con ol al es a each inle o he ank. Mo eo e , in o de o ensu e he low o wa e in each inle , a low senso has been added. The layou o he ins umen s in he plan and hei wi ing diag am is shown in igu e 3. All he ins umen s used in he plan a e s anda d de- ices used in he p ocess indus y. The le el o he anks is measu ed by p essu e senso and he lows o he inle s by magne ic low-me e s. A pneuma ic con ol al e wi h posi ione has been chosen o ma- nipula e he lows o he inle s. The main eason why indus ial ins umen a ion has been used is o p o ide a ealis ic benchma k o es con olle s and each he s uden s o use, con igu e and wo k wi h such de ices. Box 1  Val e 3  Floa   Swi ch 3  Floa   Swi ch 4  Val e 4  Val e 1  Val e 2  Box 5  Box 4  Box 3  Box 2  Floa   Swi ch 1  Floa   Swi ch 2  Flow-me e 1  Flow-me e 3  Flow me e 4  Flow-ma e 2  P essu e  senso 3  P essu e  senso 1  P essu e  senso 4  P essu e  senso 2  Powe Supply  Con ol Boa d  Pump 2 Pump 1  Flow-me e   P essu e Senso   Pnema ic Val e  Floa Swi ch  Pump  220 V  S2 S3  S4 S5 E1  S1  24 V     Fig. 3. Ins umen a ion and wi ing o he plan The measu emen p o ided by he senso s as well as he ape u e o he al es a e elec ical signals (4-20 mA cu en loop). The wi ing o his signals allows one o connec hem o a P og ammable Logic Con olle (PLC) loca ed in he con ol boa d o o connec hem o an ex e nal de ice by means o plugs loca ed in he panel o he con ol boa d (see o ins ance he wi ing o he p essu e senso in he igu e 4, whe e his is illus a ed). In o de o a oid o e lows o he anks, a loa is ins alled a he op o each ank and he on/o signal is wi ed o he con ol boa d whe e an eme gency s op o he plan is ca ied ou closing he al es and s opping he pumps. The PLC used as da a acquisi ion de ice allows one o use di e en con ol s uc u es, which a e p esen ed in he ollowing sec ion. D+D- - RA A+A-RBB+B-RCC+C-RD M L+ EM 231 PLC Powe Supply + RCA Con ol Boa d 24 V Analog Inpu Module o he PLC P essu e Senso   Fig. 4. Wi ing o he p essu e senso . 5. THE CONTROL STRUCTURE OF THE PLANT. The con ol objec i e o he plan depends on he cho- sen con igu a ion al hough his is basically o egula e he le els o he ank. The plan is designed in such a way ha allows one o choose di e en con ol s uc- u es o ca y ou he con ol ask. This con ol ask is ypically ca ied ou in wo s uc u es: Cascaded Con ol: he con ol is di ided in wo le - els (o loops): a lowe le el (inne loop) aimed o con ol he low o each inle and a highe le el (ou e loop) whe e he he le els a e con olled. Di ec Con ol: he le els a e con olled manipula - ing di ec ly he ape u es o he al es. On he o he hand, he con ol law can be implemen ed in he PLC o by means o an ex e nal de ice gi en ha all he signals (measu emen s and con ol ac ions) a e accessible in he con ol boa d. The ex e nal con- ol is in e es ing om an educa ional and p ac ical poin o iew since his allows us o con ol he plan by means o s anda d low le el con olle s, such as indus ial PIDs, o using di e en PLCs o da a acqui- si ion sys ems. The PLC loca ed in he con ol boa d can be used o implemen (simple) con ol laws, such as he low le el con olle s o o be connec ed o a compu e , allowing us o con ol he plan by means o an applica ion unning on he lap op. In igu e 5 i is shown he diag am o he cascaded con ol s uc u e whe e he low le el PIDs a e implemen ed in he PLC and he high le el con olle is implemen ed in he compu e . PC Plan  h1 h2 h3 h4 q1 q3 q4 q2 a1 a2 a3 a4 q1 q3 q4 q2 h1 h2 h3 h4 a: Opening Ra e o he Val es. q: Flows. h: Le els. PID PID PID PID PLC S7 200 q1 q3 q4 q2  Fig. 5. The compu e -based cascaded con ol s uc u e All he p ocess a iables a e pe iodically sampled and s o ed in he PLC RAM in a eal ime da a base. These a iables can be accessed om he compu e hanks PLC Da a  base  Plan  Ma lab P o ool LabView Supe iso OPC OPC PC PC/PPI OPC Se e  Fig. 6. Compu e con olled sys em by means o OPC o he exis ing connec ion wi h he compu e . This connec ion is an s anda d se ial RS-232 unde an open and simple cha ac e -based p o ocol as enough o con ol he plan and ha can be easily implemen ed. Thus, he mos simple way o p og am an applica ion o con ol he plan is o code he p o ocol and sam- pling he s a e by eading he alues om he da a base in he PLC and manipula ing he plan by w i ing he con ol ac ions in his da a base. We ha e used his app oach o de elop a LabView based applica ion o moni o ize and con ol he plan . In o de o enhance he connec i i y o he p ocess we ha e chosen an open and ee p o ocol ha can be easily ound in con ol packe s and SCADAs: he OPC (OLE o P ocess Con ol). Thus, an OPC se e de eloped by Kepwa e Inc. has been ins alled. This so wa e eads he a iables om he da a base o he PLC and builds a eal ime da a base in he PC. These a iables can be used ( ead and/o w i e) by o he applica ions by means o he OPC p o ocol in a se e - clien a chi ec u e (see igu e 6. Con ol so wa e such as MATLAB , LabView o idus ial SCADAs as SIMATICi implemen an OPC clien , and hence can be connec ed o he plan . Based on he OPC p o ocol, connec i i y o he plan om he Wo ld- Wide-Web is also possible (Reyes e al., 2005) The connec i i y o he plan esul s o be e y in- e es ing o he objec i es o he plan : educa ion and esea ch. Fo ins ance o basic con ol cou ses, Simulink o LabView based con olle s can be easily implemen ed and applied o he plan by means o he OPC connec ion. Fo mo e ad ance cou ses, an SCADA can be used o design s anda d con olle s o comme cial packages such as DMC, o ins ance. Fo esea ch, he acili ies o he plan allows one o es he con olle s easily on he eal plan . Fo ins ance, in igu e 7 i is shown he e olu ion o he lowe le els o he plan in he quad uple ank con igu a ion. The non- minimum phase se ing o he plan has been chosen and he plan has been con olled using a mul i a ible GPC implemen ed in MATLAB wi h OPC connec- ion. 6. CONCLUSIONS This pape p esen s he design and implemen a ion o a labo a o y plan based on he quad uple ank p o- cess. The main objec i e o he design o he plan 0 1000 2000 3000 4000 5000 6000 −0.02 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 ime h1 h1 eal h1 e 0 1000 2000 3000 4000 5000 6000 −0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 ime h2 h2 eal h2 e Fig. 7. E olu ion o he le els o he non-minimum phase plan con olled by a GPC is i s lexibili y. Thus, he plan can be con igu ed o ob ain cascaded anks, a mix u e p ocess o a hyb id plan . Mo eo e he con ol s uc u e is also lexible allowing one o con ol in a cascade o in a di ec s uc u e by means o he PLC, by using an ex e nal de ice o by means o a compu e . In his case, OPC p o ocol has been chosen o p o ide connec i i y o con ol applica ions. In addi ion, i is in e es ing o highligh ha he plan has been buil wi h indus ial ins umen a ion. Finally i is wo h o ema k ha his plan has been success ully used o p ac ical wo ks o s uden s in basic con ol cou ses as well as ou esea ch g oup. REFERENCES Johansson, Ka l Hen ik (2000). The quad uple- ank p ocess. IEEE T ans. Au oma ic Con ol. Johansson, K.H., A. Ho ch, O. Wiljk and A. Hansson (1999). Teaching mul i a iable con ol using he quad uple ank p ocess. In: In p oceedings o he IEEE Con e ence on Decision and Con ol. Long, C. E., C. E. Holland, and E. P. Ga zke (2005). Expe imen al ai p essu e ank sys ems o p o- cess con ol educa ion. Chemical Enginee ing Educa ion. Reyes, C., a. Cepeda, B.Pon es, I. Al a ado and E.F. Camacho (2005). Con ol de la plan a de los cua o anques median e la ealizaci´ on de una pasa ela MATLAB-HTTP-OPC. In: Jo nadas de Au om´ a ica. Rusli, E., S. Ang and R.D. B aa z (2004). A quad uple ank p ocess con ol expe imen . Chemical Engi- nee ing Educa ion. APPENDIX: PLANT CONFIGURATIONS PLANS Fig. 8. Labo a o y plan layou . (a) Two cascaded anks (b) Two cascaded anks, wo inle s. (c) Th ee cascaded anks (d) Fou coupled anks. (e) Mix u e p ocess. ( ) Hyb id p ocess. Fig. 9. Some con igu a ions o he plan .