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~ ;g Computer BasedndustrialControl3 5 5 Timer Counter ModuleT im er /c ou nt er mo du le b asic al ly c on si st s o f a n um be r o f t im es/c ou nt er s whi ch m ay b e c asca de dor us ed i nd ep end en tl y. Eac h time r/ cou nt er ma y be pr og rammed in di ff er en t mod es i n w hichc as e th e t imer /co un te r ou tp ut wi ll be di ffer en t. The mod es may b e i nt er ru pt on zer o co unt, ra teg en er ato r, mon os hot etc . We s hal l no t di scus s th e mo des o f ti mer/ co un ter h er e. Th e s tru ctu re ofa ti mer /co un te r is s ho wn in Fi g. 3. 50.

The processor loads the count in the form of data byte/words. The clock may be derivedf rom p ro ces so r cl ock o r may be p rov id ed ex te rn al ly. The ga te s ig na l i s u sed t o ena bl e/d is ab lethe t6unter operation. rhe processor may read the current counter value at any instant bys top pi ng t he cou nt er u sin g g at e s ign al or r ead it on th e F ly , i. e. , w it ho ut s top ping th e co un ter .T he o ut pu t ma y b e u sed to i nte rr upt t he p roce ss or or in any o the r way a s pr ogr ammed .

Clock OutputTimer/Counter

Gate

Data'bytelWord from processor

Figure 3.50 Timer/Counter module.

'iiIIIii11

ill.ilIIIi..ili

~In..1IIil:a.1'1H'IIII'j

5 6 Display Control ModuleDisplay control module consists of following independent sub-modules.

- Manual entry sub-moduleCRT controller sub-moduleLEDILCD control sub-moduleAlarm annunciator sub-module

- Printer controller sub-moduleThe manual entry to the system may be via thumb wheel switches, various ON-OFF

command switches and/or keyboard. The keyboard may be a full ASCII keyboard or aspe ci al iz ed n um er ic a nd t ask- or ie nt ed k ey bo ar d. T he re a re b ot h a dv an ta ge s a nd d isad va nt ag esin u sin g an y manu al e nt ry s ub m od ul es an d w e w il l n ot d is cus s t hes e her e. B as ica ll y, manu alen tr y s ub- mo du les hav e bu il t in b uf fer to s tor e c ur re nt s ett in g o f th umb w he el s wi tch es an d las tco mma nd /d ata en te re d th rou gh k ey bo ar d. The pr oces sor may r ea d th es e va lue s on its ow n o rm ay b e i nt el TUpt ed whe ne ve r a n ew d at a/ co mm an d i s e nt er ed .

C RT c on tr ol le r sub -m od ul e i nt er fa ce s m ai n p ro ce ssor t o Visua l Displ ay Uni t, whi ch i s u se dt o sho w t he sta tu s o f p ro ce ss b y d ispl ay in g t ra nsdu ce rs v al ue s, p re se nt set p oi nt s e nt er ed t hr ou ghmanual entrysub-module,his toricaltrend of various parameters,mimic diagram of process, alarmsta tu s e tc . All t he a bo ve d ispl ay t asks a re p ro vi de d i n t he m ai n p ro ce ssor t hr ou gh sof twar e.LEDILCD control sub-module interfaces array of LEDILCD to main processor. This sub-mod ule accep ts d ata b ytes/word fro m main p ro cessor and d isplay s it o n LEDILCD. Alarm

t

BuildingBlocksof utomationSystemannuciator controller sub-module generates ON-OFF signal for each type of alarm.p ro ce ssor m ay sen d a n a la rm b yt e/ wo rd t o t he a la rm a nn un ci at or c on tr ol le r sub -m od ul ed ec od es b yt e/ wo rd a nd sen ds O~- OFF sig na ls f or v ar io us t yp es o f a la rm s. T he se a la rm m oa re sep ar at e a nd r eq ui re o nl y d ig it al sig na l t o l ig ht t he i nc an de sc en t b ul bs a nd /o r a ud io a

Printer controller sub-module is printer interface to main processor. Generally it hasi nt el li ge nc e f or p ri nt er c on tr ol , a d at a b uf fe r t o sto re t he d at a f or p ri nt in g e tc . I t a cc ep ts t hb yt es /w or ds f ro m mai n p ro ces so r an d pr in ts it f or t he ben ef it of o per at or . Th e i nt er acti onmai n pr oces sor may b e t hr ou gh p er io di c d ir ect memo ry a cces s o r u nder t he d ir ect co ntmain processor programmed 110 transfer).

6 SUPERVISORYCONTROL AND DATAACQUISITION SYSTEMSAfter having dealt with the basic hardwaremodulesof a real-time system,let uc on ce nt ra te o n Sup er vi so ry C on tr ol a nd Dat a Acq ui si ti on SCADA) syste m, sin ce i t i s t hs te pJ owa rd s au to mati on .. The ba si c f unct io ns car ried o ut b y an S CA DA s ys tem ar e:

- Channel scanning- Conversion into engineering units- Data processing.F ig ur es 3 .5 1 s ho ws th e b lo ck s ch emat ic of S CA DA . Bef or e co ns ider in g o th er f eat ur

SCADA, l et u s d iscu ss t he b asic f un ct io ns.

gc'(j) AnaloginputmodulePR0CES ,SC '' '' 'ij~'j;(j)

(;'c'(j)Digitalinputmodule

JExternalclock

Displaymodule

Micro-processor+Memory

Serialinterfacemodule

Alarmannunciatormodule

Timer/Countermodule

Communicationwithcentralcomputer

3 6 1

Figure 3.51 Supervisory control anddata acquisition system.

Channel ScanningTh er e are many wa ys i n w hic h mi cro pr oces so r can add re ss t he v ar io us ch ann el s an d redata.

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~1 2; Computer-BasedndustrialControl

I

PollingThe microprocessor scans the channels to read the data, and this process is called polling Inpolling, the action of selecting a channel and addressing.. :is the responsibility of processor.The channel selection may be sequential or in any particular order decided by the designer. It isalso possible to assign priority to somechannels over others, i.e. some channels can be scannedmore frequently than others. I t is also possible to offer this facil ity of selecting the order of

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r~n I . 4;. Computer-BasedndustrialControl

Channel number Timegap

1/II

91012593426897

FF HOF HFO H

nFigure 3.54 Scanarraywith time.

IIi:IIIII:Interrupt scanningAnother way of scanning the channels may be to provide some primitive facility aftert ran sducer t o ch eck fo r viol atio n of limi ts. It end s in terru t request si gnal to ro cesso r whenthe analog signal f rom transducer is not within High and Low limits ou~ry.J ieLby-Analog-HighanaA~n:ll.9g Low signals.}his ISalso called Scanning by EXCe IOIt WI'ien any parameter exceedsthe limit s then t he limit che cking ci rcui t \youJ.d.j e.nd in t ;ITUpt req~'(QPro

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~: Computer-Based Industrial Controlfor any channe l, appropr iat e a ct ion li ke a larm genera tion, p rin ting, e tc . i s in it ia ted. The limitarray shown in Fig. 3.55, s implifies the limit checking routine. Through this, the facility todynamical ly change the l im its for any chanue l may a lso be provided , on the l ines s im il ar to scanarray described in Section 3.6.1.

In addition to limit checking, the s ystem performance may als o be analysed and reportcould be generat ed for the manager level . Thi s r epor t wil l enab le the manager s to v isua li se t heproblems in the sys tem and to take decis ions regarding sys tem modification or alternateope rat ional s tr ategy to increase the sys tem per fo rmance. The analys is may include h is~ogra) Il

~ 1erat ion, standard devia tion calcula ti~n, plott ing one param~er with respect to ~ 1?therand soon. The sof tware can be wri tt en depend ing on the type of ana lysis r equ ir ed .The facil it ies l ike scanning, l imit checking, e tc . could be incorporated in a simple way using

arrays in s oftware. Let us first examine channel s canning, using s can array in memory whichcontains the channel numbers in one particular order of scan. For two applications using thesame hardware, only the scan array will have to be changed for the scanning. The samea rgument i s val id for l imi t check ing. The conve rs ion to eng iuee ri ng unit s can al so be achievedby s to ring the conve rs ion fac to rs for d if fe ren t channe ls in any a rray. The conver sion fac tor sstored in the array will change with the transducers i.e. it will be different for differentapplications. The simple print programme which takes the data in particular format frommemory and sends for printing may also be same for different applications. However, thedesigner will have to write specialized routine for arranging the data in memory in anypar ti cu lar format requ ir ed . The ana lysis and repor t gene ra ti on programs will be appJi ca ti ondependent and wil l have to he wri tten separately for different applica tions. Here also the routinesfor histogram etc . may be uscd by different applica tions i f they are wri tten in general ised manner.

3.6.4 Distributed SCADASystemIn any app li ca tion, i f t he number o f char~ ~s are q.uit e l a1] : t hen in order to i nte rf ace these toprocessor, one has to use multiplexers at different levels. Figure 3.57 shows the interfacing of

Building Blocks of Automation System

00.,;UO SJeAUOO

1PU3t:t:' >- c:l o()

I~I~'5N 1t3B-t: 0 00. 0

cl~l~iICIJ0oE OJ:;;:

Higher limit Lower limit10 0220 09I II II II II II II II II II I

Figure 3.56 Limit array.

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\JM Computer-BasedndustrialControl256 channels using 17multiplexer of 16channels each. The 8 address lines are usedto address256 channels. Out of the 8-address lines upper four are used to select a particular multiplexerand lower four lines are used to select a particular channel in the multiplexer.

The 8-bit channel address thus directly maps into channel number and can be manipulatedin any way. The other parts are sameasdescribed earlier. This approach will be suitable for thep ro ce sses w hich a re b asically slo w. Eve n if a cha nn el is sca nn ed o nly o nce in e ve ry sca n it isonly after 255 channels have been scanned limit checking and analysis have bcen performed aparticular channel will be addressed again. This is not acceptable in many processes.

For the process plants wherc the structure of Fig. 3.57 does not suit the only alternative isto us e mor e th an on e S CA DA s ys te m an d di st ri but e th e c ha nne ls among th em. B ut f orperformance analysis on the process plant it is mandatory that the data from various channelsshould reach a central location where it canbe consolidated andanalysed to generate thereportson plant performance. Figures 3 .5 8 a) an d b ) sh ow the interfacing of number of SCADAsystems with central computer in st r configuration and isy ch in configuration respectively.

The SCADA system directly connected to transducers are called nodes and are the sameasthe systems described earlier. They scan the channels using one of the techniques discussedearlier; convert the data into engineering units perfonn the limit checking generate alarm ifdata item crosses the limit and generatc print out.

In addition to these functions the data regarding the channels in the node are transfcrred tocentral computer which analyses the systemperfonnance and gcnerates print out. The print outsare generated by exception i.e. unnecessary data is not printed at any point. At the nodelevelthe print out is required for the operators to run the system. Depending on the nodeperformance operator may decide to monitor any channel more frequently change the limitse tc. The p rint o ut a t t h e cen tral n od e is req uire d f or t he m an ag ers t o t ake lon g t erm d ecisio n t oo pt im ise t he p erfo rm an ce . The d et ails o n t he cha nn el p erfo rm an ce l im it vio la tion a re n otreq uire d a t t his level. On t he o th er h an d hist og ra m o n t he inp ut a nd o ut pu t m at eria l f lo w a ndthe fuel consumption etc. may he more helpful.

The concept of local areanetworks or microprocessor interconnections can be usedin caseof distributed SCADA system very effectively. Thus we conclude that the distributed SCADAi s t he u lt imat e s ol ut io n for c ompl ex p ro ce ss p la nt mon it orin g.

3.7 REMOTE TERMINAL UNITThe Remote Terminal Units RTUs) are basically distributed SCADA based systems used inremot c l oc at io ns i n a pp li ca ti on s l ik e o il p ip el in in g, i rrig at io n c an al s, o il d ri ll in g p la tforms e tc .The y a re rugge d a nd shoul d be ablc to work una tt ende d for a l ong durat ion. The re a re t wom ode s i n whi ch Re mot e T er mi nal Uni ts w or k.

I . Undc r c omm and f rom cc ntr al c omput er2. Stand alone mode

Si nce t hc sc RT U s ha ve t o ope rat e f or a l ong dur ati on una ttc nded, t he ba si c r equi rcm entsw ould be t ha t t he y c ons ume mi ni mum power and have cons ide ra blc s el f- dia gnost ic f aci lit y.Following are the main parts of rcmotc terminal units. -

Building Blocks of Automation Syste

CPU

Serialinterfacemodule

Process

CH, CH2 CH-M,CH2

CH, CH-MnC H, C H2 C H- M2SensorsC H, CH 2 C H- Mn -

a)

,CPUSerialinterfacemodule

SerialinterfacemoduleDASn

From otherDASSerialinterfacemodule

DAS,~- \

CH, CH2 CH-M, CH2CH, CH-MnCH,CH2H-M2b)F ig ur e 3 .5 8 a ) D is tr ib ut ed S CA DA s tr uc tu re S ta r c on fi gu ra ti on ). a nd

b ) D is tr ib ut ed S CA DA s tr uc tu re D ai sy c ha in c on fi gu ra ti on ).I

,. j f.interface interface-s- ---1 I---:=h --:. I I DASn

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CHAPTER6~jr~CtPigital ontrol Structure a nct~ ~r~~ftware

6.1 INTRODUCTION:i fi ::

The advent of microprocessor has changed the field of proces s control completely. The taskswhi ch were performed by complex and cos tl y min icompute rs a re now eas il y p rogrammed usingmicrocomput er s. ,In the pas t comput er was not d ir ect ly connect ed to the proce ss but was usedfor s upervis ion of analog controllers. The analog controllers were interfaced to the processd ir ec tl y a s wel l as t hrough specia li sed cont ro l for ded ica ted functi ons (Fig. 6 .1 ). The analogcontrol lers and special ised control lers were cal led level 2 and level 1 control respect ively.~:h

)

::

ProcessFigure 6.1 Supervisory computer control.

The emcrgence of cconomical and fast microprocessor has made analog controllerscompletely out-dated, as the same functions can be performed by digital computers in moreefficient and cost effective way.

250

Direct Digital Control-Structure and Softwa6.2 DDC STRUCTUREThe DDC (Direct Digital Control) directly interfaccs to the proces s for data acquis iticontrol purpose . That is, i t has necessary hardware for direc tly interfacing (opto-isola tor,conditioner, ADC) and reading the data from proccss. It should also have memoar it hmeti c capabi li ty t o execu te requir ed P, P + I o r P + I + D cont ro l s tr at egy. At t he samthe interlace to control valve should also be part of DDC. Figure 6.2 shows thefunct ional blocks of a direc t digital control system. These funct ional blocks have been din number of books on microprocessor. The multiplexer acts l ike a switch under microprcon trol. It swit ches and presen ts a t it s output t he analog s ignal f rom a senso r/t ransmi tt eanalog to d ig it al conver te r convert s t he ana log s igna l t o d ig ita l value.

Fig. 6.2 Direct digital control.The microprocessor performs the fol lowing tasks.1. It eads the various process variables from different t ransmitters through multiand ADC.2. It determines the error for each control loop and executes control strategy for eac3. It outputs cor re cti on value to con trol valve th rough DAC.

6.3 DDC SOFTWAREThe main part, DDC software is program for control loops. There are two algorithprogramming a three-mode PID control loop:

- Posi tion algori thm- Veloci ty algori thm.

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252 Computer BasedndustrialControl6.3.1 The Position AlgorithmThe three-mode controller has been explained in Chapter 1. The PID correction has beenderived in Section 1.8.5. It can be represented by,

6.e 1 fY =KP. e + KD . - + - e. 6.1+ Y. 6.1 KI 0where

Y = valve posi tion a t time nYo = med ian val ve pos it ionKP = proport ional constant = lOOfPB (where, PB = proportional band in per cent),KI = integral constant = liT (where T = i nt eg ra l t ime const an t)KD = derivative cons tint =TD (where TD = der ivat ive constant)e = error at instant I = (S - V,,)\I = value o f cont rol led vari ab le a t ins tant IS = set-point

The PID control can be realised with a microprocessor based system, if only the aboveequat ion i s implemented in the sof tware . Apparent ly, i t i svery di ff icul t towr it e the so ftware forimplementing the above equation for a microprocessor based system. However, the aboveequation can be modified such that its soitware implementation becomes easy. Themodificat ions are discussed in the following section.

The integral term at any given instant I is equal to the algebraic sum of all the contr olforces genera ted by the integral cont rol action from the beginning to that in stant .

Thus integral t erm can be rep resented as

f

1KI I e,' 6.1,=0

and the differential term, KD . 6.el6.1 a t any in stant I i s propor tiona l to the rate of change of theerror.

Thus, di fferent ial t erm, can be represented asKD. e -c,,-l61

where c i s the curren t e rror and e ,,- l is the previous error calculated at instant t,,-l'Thus, with these modificat ions the three-mode controller equation will become:

e -c 1 IY =Kp.c +KD'~+- e6.1 +Y 6.1 KI 0i=O

(1)

(2)

DirectDigital Control Structureand Softwa.The in tegral and the di fferen ti al cont rol forces a re dependen t upon the in te rva l bl twe

two consecutive errors. This interval is the inverse of the rate at which the/waluecontrolled variable is measured i.e. the sampling rate. Hence the provision for definsampling rate should be made available in the software.

The f low-chart for calculating PID cont rol output based on above equa tion (Eq. 2) iin Fig. 6.3

YesNo

Reinitialise sampling interval counter (ST)Error Eo= Set point - Measure d val ue

Proportional P =KP x EoEo10=Ki

ST = Sampling interval counter~ Return)Figure 6 3 Flow chart of PID control

The sampl ing interval counter , the se t-point , the propor tiona l cons tant KP, theconstant KI and the derivative constant KD are defined by the user.

The two mod if icat ion s tha t can be performed on above equa tions are Trapezoida lintegral term and interpolation technique for derivative term.

-- ___Trapezoidal rule for integral termThe integral t erm can be represented by using t rapezoidal rule.

\ Ci + e,-lL. ~ 6.ti=O 2

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65 Computer-BasedndustrialControlThis will g ive be tt er a ccuracy than previous te rm ,

nL ei 1ti=Obased on rectangular rule.

Interpolation technique for derivative termThe first difference in the derivative term, (en - en-I) is affected by noise. and tJ1USdiff eren tia ti on is sensi ti ve t o da ta er ro r and noise. The noise can be reduced by us ing anal lg ordigital filters.

However t he t echn ique commonly used i s in terpo la ti on method with four-po in t cont ro ldifference technique (Fig. 6.4).

bf.' +,,,,,,,,,,,,,,Llt: -----r-'---- Llt:1:,~ '~: LltJi11'~,:):

Vo-3 VO-2 Vo Vo-F ig ur e 6 .4 I nt er po la ti on t ec hn iq ue f or d er iv at iv e term.

''',Let Vn>Vn-I, Vn-2 and Vn-3 be the values of controlled variable at current and their previous

consecutive sampling intervals.~~I Vn + Vn-l + Vn-2 + Vn-3V*= 4

111'= ~[Vn-1'* + V,,-I-V* + V*-Vn-2 + V*-Vn-3 ]11 4 1.51t 0.5 1t 0.5 1t 1.51tI= - [V -1'*+31' -31'*+31'*-31' +v*-v ]6111 n-l n-2 n-3

= 6~1 (Il + 3l'n-l - 3\1 -2-1~'-3]Since set-point is constant,

l1e1t

111' I~ t =_6 A [en + 3en-l - 3e 2 - e ]L1 L1t n- n-3

(4)

(5)

(6)

DirectDigital Control-Structureand SoftwareThus, with these two modificat ions the control ler equat ion becomes

JI Y KD I~ e+eln=KP.en +-[en +3en-1-3en-2 -en-3]+- L. ~+YO6I1t KI i=O 2In position algorithm computer recalculates the full value of the valve setting at

sampli ng in terval . As shown in Fig 6 .2 the analog s ignal i s sen t t o valve ac tua to r th roughThe position algorithm has distinct property that it maintains its own reference

However i thas two drawbacks namely lack of bumpless t ransfer from manual to auto switcand reset wind-up due to integral saturation in tes t mode. These drawbacks are not presevelocity algorithm.

'2 6.3.2 The VelocityAlgorithmIn number o f control loops, t he fi nal cont ro l el emen t i s s tepper moto r o r s teppe r moto r dvalve. In such cases , the requi rement at t he comput er outpu t wil l be a pu lse tr ain spec ifyingchange in valve position. Thus output of position algorithm cannot be used, since it givenew posi tion of t he valve, in absolute t erm.

In velocit y algo ri thm , the comput er c al cu lat es the requir ed change in valve pos iti on.utput is digital pulse train which can bedirectly used in case valve is stepper motor dricase o f o ther valves , s tepper motor combined with s lide wire ar rangement as shown in Fican be used. The same funct ion can be performed by an integrati ng amplif ier .

Fixedpoint Voltage corresponding tonew positionChange invalve positionby velocity algorithm

~ Movable point drivenby stepper motor- -n_- - -- - -- - - n - ---FullscalevoltageorcontrolvalveW ~Figure 6.5 Slidewire arrangement.

The Eq. (2) o f posi tion algo rit hm der ived ea rli er i s,

Y =Kp.e +KD (Cn-C -I )+l... ~ e 1t+Y n 1t Kl L.' 0i=Owhere, Yn is valve position t t

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6 Computer-BasedndustrialControlAt In-I i .e. at previous instant, the valve posit ion was,

T (en-I - en-2 ) I ~} n-I = Kp. en-I + KD . + - L.. ej / :,1+ Yo/ :,1 KI ;=0The change in valve position / :,Ynat In wil l thus be,

/ :,Yn= Yn- Yn-Ij

KD 1= Kp. (en - en-I) + - (en - 2en-l + en-2) + - en, / :,1/ :,1 KIThe integral term and derivative term can be modified by using Trapezoidal rule andInterpolation technique similar to position algorithm

I 1[

n e. + e, n-I e. + e.]ntegra l term = K L -' ::L- L -' ::L / :,rI ;= 0 2 ; =0 2

= ~I [en +2en-1]MDerivat ive term = KD [(en + 3en-1- 3en-2- en-3) - (en-l + 3en-2- 3en-3- en-4)]MI .

KD- [en+ 2en-1- 6en-2+ 2en-3+ en-4]MIBy subst itut ing modif ied integral and dif ferent ia l terms in (10), we get

K KD 6 ] 1 [en + en-I ] (13:,Yn= p. (en - en-I) + - [en + 2en-1- en-2 + 2en-3 + en-4 + - - . / :,1 )6M ~ 2The relationship between position and velocity algorithm is,

/ :,Yn= Yn- Yn-II.e.

Yn = / :,1 + Yn-I= / :,Yn + [/ :,Yn-I + Yn-2]

= / :,Yn + / :,Yn-I + [/ :,Yn-2 + Yn-3] = ... = L / ': ,l ; + Yo;=0

(9)

(10)

(11)

(12)

f DirectDigitalControl-StructureandSoftwaorLogically also, the present valve posit ion is equal to origina l posi tion p lus su mochanges occun ed so far. ...The velocity algoritum at equation (13) exhibits two measure problems:- Controller drift- Integral overshoot.

We shall discuss these and modify the algorithm accurately,ontroller riftThe velocity algorithm should always include integral term otherwise it will givecontroller drift . To explain this let us substitute the following equation in velocity algo

en,= S - Vn,whereS = set-point and

Vn=value of cont rol led variable a t In-KD / :,1

[ Vn / :,Yn=KP Vn - Vn-I)+ 6/ :,1 Vn 2Vn-1 - 6Vn-2 2Vn-3 Vn-4) - KI S - 2From the above it is c lear tha t on ly in teg ra l t erm has set -po in t and thus thi s te rm wil

cont rol led vmiable to come to set -point . I f integral t erm is not p resent in ve loc ity a lgothen con trol le r dr if t may be caused . The propo rt iona l t erm of ve loc ity a lgo ri thm may gto oscil lations. Let us consider the velocity algorithm with only proportional and integral

/ :,Yn= KP [en - en-I] + ~ [en + en-I ]I 2Case When error is increasing i.e. when value of controlled variable is moving away from set-

t\.. Proportional term =KP en- en-I) = Ve/ :,r[e + e ]ntegral term = - ~ =VeKI 2ThusOutput / :,Yn= VeCase II

W~n error is decreasing because of control action i.e. when value of controlled variamoving towards set-point.

since en < en-IProportional term =KP [en- en-d = - Ve

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58 Computer BasedndustrialControl/11

[e + e ]ntegralerm= - ~ =+VeKI 2 j

I f i nt eg ral te rm is not over bal ancing the proporti on t erm then , Output /1Yn = negative.The negative correction may increase the error thus, giving positive correction as per

Case I . The pos iti ve cor rec ti on may decrea se the er ro r and g ive nega tive correct ion.The con trol le r response may thus osci lla te . The osc il lat ion probl em expl ained above can be

solved by d is regarding the s ign of proporti onal t erm and assigning i t t he same s ign as int eg ra lterm. Thus,Proporti onal t ern = (si gn of integral te rm) , KP len - en -I I

Integral overshootThis modificat ion while solving one problem, creates another problem of integral overshoot andconsequent ly integral osc ill at ions . When proport iona l te rm is forced to have the same sign asintegral term, the va u e of controlled variable will reach the set-point at a faster rate andovershoot it.

The integral term,

. ~ [ en+en-I ]I 2; will regis ter the change in the direction of error and oppos e this along with proportional term

and g ive negat ive cor rec ti on . The value of con troll ed var iable may ove rshoot the set -point i n theopposite direction, giving rise to oscil la tion (Fig 6.6).::

IIISP e +----

Variablevalue

TimeF ig ur e 6 .6 C as e o f o sci ll at io n i n v el oc it y a lg or it hm .

The ideal solution problem will be to fix a band across the set-point. When value ofcontrolled variable is outside the band the proportiona1 term takes the sign of integral term.Thus controlled variable will reach the band steeply. \Vhereas when the valuc of controlledvariable is inside the band, the proportional term takes the sign as caleubted, i.e. en - en-I)This gives damping action ins ide the band whi1e controlled variable reaches s et-point, thuspreventing integral overshoot and oscillation (Fig. 6.7 .

Direct Digital Control-Structure and Software

SP~---~:o{Variablevalue

TimeFigure 6 .7 E ff ect o f s et -p oi nt b an d i n v e lo ci ty a lg or it hm.

I t has been found tha t 7% full scal e set -po in t band g ives good resul ts on s imulat ion ts ys tems w ith first and second order time cons tants . The above modification to the valgoritlllil will yield the following equation.

/11[

en+enl]

KD [ 2 6 2 ] P . IYn= - -- =--- + - en + en-I - en-2 + en-3 + en-4 + roportlOna termKI 2 661(se t-point Band) SB = 0 .07 of full scal eWhen len-II> SBProportional term= (sign of integral term) KP len - en-IiWhen len- II< SBProportional term =KP len - en-IIThe f low cha rt for velocit y a lgor ithm i s shown in Fig . 6 .8 . The ADC outpu t can be d

used for cal cul ati on i f set-po in t and SB are expressed in te rms of f ract ion of ADC outpu t vThe range of ADC output is taken as full scale value. This avoids time consuming portiConve rs ion to Eng ineer ing Unit in sof tware. I f p roport ional it y const an t KP, i ntegral coIlK/) an~rivat ive const an t KD can be expre ssed asf ract ion of ADC value, t hen in adi t wil l save computer t ime in calculation.

6.3.3 Position vs. Velocity AlgorithmReference positionThe major advan tage of posit ion algori thm i s the re ference pos it ion. In i ts equat ion it se lr eference pos it ion of con trol valve is main tai ned as Yo. This is however not t here in vealgorithm. Thus whenever there is disrupt ion due to shut down, communication eITorlfai luany.other reason, the median position of valve will be known in position algorithm7,ven tual ly valve wil l ca tch up withou t any synchron iza tion probl em. In veloci ty algorexternal device like stepper motor or integrating amplifier s hould s tore the last pos itiovalve.

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