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 .