Microprocessor-Based Vacuum-Pan Controller

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MICROPROCESSOR-BASED VACUUM-PAN CONTROLLER J.

M. Barrios and

J.

C. Remesar

Computn S.ntems Departmf1lt. Institute for Flll1rimllf1lta/ Tfrhllira/ Rf.lt'(l rrh. Aradem.,· of Srie1lrfs of Cuba

Abstract. In sugar industry, one of the most complex processes from the operational point of view is the masacuite cooking in vacuum-pan, where is realized the crystallization of sucrose and the growine of the crystals from the sucrose dissolved in mother-liquid. The sugar recovery achieved hardly depends on the operation of this process. In this paper is described a microprocessor system developed in our institute to control vacuum-pans. The system consists of an eight bit microcomputer with data acquisition, operation and control proerams running in a real-time environment. Some results are presented about the experimental version which worked durine the last cane harvest in a sugar factory. Keywords. Controllers; r.licroprocessors; Computer control; Real time systems; BOilers; Vacuum-pan control. Seeding and graining.

INTRODUCTIOl1 The process which must be controlled consists of of a set of vacuum-boiling pans, whose number may be from four to even more than ten. The main function of the pans consists in the production and growth of uniform crystals with high efficiency. The most cO);lplex type of proce~ure realized in the sugar vacuum-boiling pan is known as crystallization, which, in a few Vlords, consists in preparing the primary product for further procedures, upon the basis of seeding sugar crystals. The crystallization by itself presents, in prinCiple, all the steps done in the other procedures. Then, they might be analized as a subset of crystallization procedure, and the solution of the problem for it, could be taken as a general solution if some aspects of modularity are fulfilled. GENERAL RELlARKS ABOUT CRYSTALLIZATION ~OCEDu?'E 7his procedure involves a sequence of operation, which should be generally achieved in the \7ay explained belo';; : Vacuum settin!,:. All the valves must be closed in the initial stage. The vacuun is obtained in t v:o steps. First is obtained an initial value, and then should begin the first control loop Vlhich allows the adjustment of this variable to the set-point. enurging and concentration. Syrup valves nust be opened until is reached one third of tl:e total volmne of the pan. Then, the steac pressure is set and the level profits due to evaporn tior. should be replB ced I'/i th t he c0rrespor:.ding syrup supply. I, second and third control loops are present in the systerJ: the stes:;] pressure and the constant level in the pan. This process is dor.e until t h e conductivi ty of the masacui te reaches a ::Jagni tude I':hich alloVls seeding.

The sugar crystal seeds are introduced during a time interval (seeding time). Then, the constant level loop should be changed by the constant conductivity loop during another time interval (grainine time). In this step and later should be given the option of feedind the pan with hot water in addition to syrup if the concentration grows too fast. Dilution. At this step is only changed the set-point of the conductivity value. It is done during another ti me interval (dilution time). BoilinG' The first part of this step does not differ in sense from the dilution. The pan could be feeded using syrup, water or both duri ng the "boiling 1" interval. Then the constant conducti vi to' loop Dust be changed by a ne'.'! one in wl:ich the set-point l'Iill chant;e along a nen trajectory, selected fro n a fa mily of possible curves. The selection of the curve is based on the technical experience. A chnnge from one curve to another could also be b,ken into e ecount wh enever beca n e ir.1possible to conti nue the initially selected curve because of tiDe or energy losses. This situation is caused by sharp changes in syrup quality, which frecuently occurs in sugar cans factories. During this process the level in the pan must grow until its r.18ximur.1 value. In this situation t he conductivity should also reae!: t he final (predicted) value. If it is not obtained, a final part of the bOiline step is optionally done by feeding the pan only \,Ii t21 Y!ater. Final seguencing steps. At last, is executed the deliver process fUlfilling an establis~ed seque nce of operations.

3337

J. M. Barrios and J. C. Remesar

3338

Vacuum

brea~

______________________

Vacuum

~

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Washing WaI!!r inyecfion

Seeding

Syrup (cont) Syrup'fn','I/rj'R-JL-___ .......L...I----' ......

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-,_Viscosity

Hot Water ~l-+_Conducfivity

Cut- Over Steam preas'su.cJL________~ Steam preassure

'--_____ L eve!

Fig. 1 General diac:!"am of control and mea suring signals in a vacuum-boilinG pa!-. .

DSSCRJPC'I ON OF THE SYSTr,;):! HARDrlA.t."1~..

The microco,"puter system presents the folloyJinc :r.odule: clicrocomputer on a board. -SBC 00/20 J2 Kbyte of RfJ,! CiCr.;Ory. -SBC C32 Cor.,bina tion of EPEO:.i-RA1.T -SE C 428 T !:1emo ry.

-Disr. 83-01 -AD-DA 83-05 The systeL has the

Alphanumeric display board. Coubina tion analo[;-diC;i tal I/O board. follow~ng

perip!lerals:

-e,lphanun,eric keyboard. -paper tape reader. -paper tape puncher. -serial printer. In fig. 2 is sho·.m the arc"i tectnre of the systen. The 12 !':byte ,"emar:l , internally in the SBC 80/2Cl (nTTEL, 1976), is used to store all the programs necessary for the specific applica ti on • . The alphanlAr:: eric ke:iboard and the paper tape reador are coupled via the IC8255 #1 of the SBC 80/2':' . ':'<1is c hip is prograr..med i!1 node 1 input ar. d t1:e sienal IIlC'P.A, c;e~erated each ti tle a key is depressed, is connected to R6 input of tthe I88259 (interrup s~·ster.: co~troller). The keyboard is the principal ::Jeans of cor..r.:unicat icn of the operator with the system. Initial experinents vlere done usin£ a standard alphanuneric ke~'boardt but. :'or furt her reproductions of the c ontroller ~ill be used an specially deSigned industrial type keyboard.

The serial printer and the paper tape puncher are connected through the IC8255 #2 of the SBC 80/20. This C :lj.p is progranmed in mode 1 output and. the signal INTR3 , generated each time a character is printed, is connected to R5 input of the IC8259. The printer has been used to obtain periodical reports and operator's actions. The paper tape reader and the paper tape puncher are optional for the system. The output 0 0 of the IC8253 is used for generatill[; the real ti me clock necessary for the soft-clare to synchroni ze real tine processes. This output is connected to IU j.nput of the IC8259. Outputs 01 y 02 werf not used in this application. ':'he interrupt systec controller, IC8259, gene:'ates the INT sjenal to the 8080 chip. It is pro gra::1ned in the nested mode. The operator ca t! generate an interrupt of level 7 by depressing a push botto," in the front panel of the equipnent . As we have seen, the real tirJe clock is co nne cted to level I, the printer to level 5 and the kej'board to level 6 of this chip. The SBC 032 (I:rT:2L, 1977) consists of 32 !\byte of E:":: me:.cor;,· and the SBC 428 ':' consists of a cor::binaU.o !'! of EP2Ql.!-RAl'.! r.lemory. These boards vlet'e used durinc the developnent of the systerr: contt'ol softl':al'e. The cOLbination analoc-diGital I/O board, ADDA 83-05 (Harrios, 1983 ) , consists of eight double-ended channel inputs used for measure the analo[;ic variables of the process.

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Microprocessor-bas ed Vacuum-pan Controll er

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Architecture of the System_

These variables are co nne cted as follo ws: Variable Channel Ve cuum o DAC 0 1 Va pour 2 DAC 1 3 Conductivity 4 Leve l 5 Visco sity 6 DAC 2 7 This board als o has an ADC of t":elve bits used for measuring current in t h e f ange of 4-20 mA., four DAC's of twelve bits use d for the analog control of linear valves. Thp.ygive curr ent in in the ra nge of 4-20 mA. The DAC's are connected wi th the linear valVes a s fol), o·.:s: DACO Vacuum valve control DACl Vapour valve control DAC2 Feeding valve control And finally, the board (:o nstains 24 di gital I/O lines t hat peroit the system to measure or co ntrol on-off t ype variab les. The alpha numeric display board, Di sp 83- 01 , (Ro driguez, 1981) allows the in terfacing of a standard TV :10ni tor Set. The monitor set is

is t he main s ignalizatio n means for process behavior s upervisinG by the operator. PROGHAMI.IING STJPPORT OF TH3

COllTROLLER From t he de s cription of the process to be controlled i t is seen t ha t th e sof twa re suppor t must be organized s uch as in Fig. 3- In t '1i s diagram the "off -line " i niti alization i s not represente d. This "off-line" ta s k alloVls the operato r t o intro duce initial parameters a nd data. The tas){s '7hich r un in real-t:!. rr. e are: the followinG: !Jeasuring. In this task are handled the mUlt i plexer a nd analo G to di gital convert er. Thi s task must supply data of up to eight variables to t he tas ks of vapour, vacuum and feeding. Its resul t s are handled also by a diagrar dra~:inG task which pro vi des a representation on t he display screen of the tleasured values. Vacuu Vacuum, vapour a nd feedi nG. These tasks provide t '1e c ontro l of the three

J. M. Barrios and J. C. Remesar

3340

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I,lain tasks of the controller and its message flows.

continuous valves of the process. The simplest one is the vapour, v/hich works in only two modes Vacuum task presents an additional mode because of the two-step characteristic of vacuum setting. In both tasks the PID algorithms provide a good perfomance.

the operation stages \"lere reorganized. The actual version presents 18 stages. Due to thi s, practically all the messages received by the sequencer should request a new step setting. Internal procedures of the sequencer presents r~gher amount of its 10 Ci c in tables instead of in progrst"J2ing lOGic.

The fee~ng task is the most complicated of this group. It should provide the different modes of feeding valve control, and the syrup and water on-off valves switching. Its cOlIDDunication with the sequencer is very wide. During a crystallization, the sequencer must change the feeding task mode up to ten times. In this loop were used PID algorithms and also a self-tunning regulator for the boiling stage (Aguado, 1983). Dialogue and keYboard. This task allows the operator to dynamically introduce or modify intej.'n.. :;' values such as regulator parameters, stage flow, valves status, etc. All commands are introduced fro::! t"e keyboard, and the operator has a helpinG menu to guide himself at the display screen. Seguencer. This task was built in such a way that; t llutOmatically provides the standard tion" and "commercial" procedures, allO\7ing any modification in the order of the steps introduced by the operator froD the keyboard through the dialogue task. This flexibility in the operation '/Ias an special user's request. In order to improve the sequencer modularity,

The sequencer and dialogue ~:ere ":ri tten in and t;ce rest of the prograr:s in assembler. GENl'RAL nfl!ARKS on D=VELO P!.!EII~

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OF TEE SYSTE.I

The initial approach taken to develop this cont::-oller was based on the use of t i~ e :.10 dula::- ::eal Tirr.e System, SRT1.~, (Felipe, 1983) developed in our departr.:ent for the cO!1trol of technological processes USi !1g microcomputers based on the 8080 8085 fanily. This system presents monitor call mechanism for co=unication between tasks. 1-: has a siople three priority level set Vii th FIFO queues rei thin each level. The communication betneen tasks is performed bay a monitor call mechan:'sm. Initial results in using this system vlere satisfactory, but t!1e progra=inc and debugGing of ::lore complicated tasles present up to noVl some difficul ties. Then, we prepared a ne~i version using the R!.!x-80, which strongly differs of S7R!.I_ The diagram showed in Fig. 3 corresponds closer to the nesssge-interchange mecr.anis;:. of the R:.!X80 than to the O!1e of STR;~.

Microprocess or-based Vacuum-pan Controller

334 1

CONCLUSIO!IS Initial results in measuri ng the free processor ti me showed the possibility to control more t~~ n one vacuum-pan vii th the t y pe of mic!'oco ",puter and t h e softl7are used here. Thi s is no t a definitory conclusion since sta t istics of overhead and queues le~gth are no t co c plete.

Aguado, A. ( 198) ; . Iden t ificati.o n and adapt iv e c ont _'01 of a sugar fact or:: '/ SCUUr:: pa n. l're-

Nevertheless, we can assert that: -microcomputer based controller in vacuum pans of sugar industry allows the automatic operation of the process I'd th a r..inimun operator intervention. - \,:i th an eight bit processor should be controlled at least t rio pans ','I i thout system degradation. It is an interesting result fro m the point of view of the policy for reaction against failures. -the flexibility given by this controller allol'/s the operator to qUickly actuate whenever arises any unpredictable situation.

repo!' t. Fel i pe, D.: l. ~198)). Ree pa::'a el control de procesos el; t i e: .. po real. Co :-: t rol, ~ i~) " 'T e ­ tica y Co cnu ~ ac i on • 17(1), pp. 7-12. I:1 t el (1976 ) . SB C 80/20 a nd SEC 80 /2C-4 si cgle board co nputers. P.ard'::a:'e reference :::anual. Intel (1977 ) . ISBC 032/048/064. ~ando m access menory boards. r.ardl'lare reference ma n'.Ull. Ro dribue~. J (1981 ) . Display a1fanur.erico. IN1}~EF internal report.

ACKNO'.1LEDG:.lENT This paper resunes the results of the \"Iork done by a group of collegues of Computer Systems and Theory of Control departments of our institute wi th the colaboration of the Automation department of the National Research Institute of 3ugar Industry, and the special participation of Dr. B. Dedina, fro m t he Czeehoslovakian Academy of SCiencies, in the fi nal version of the controller.

nrin t s of I FAC/IFIP S·r=noos i'..l.!':1 on TI eal-"'ir.:e

::ontrol Applica t io ns. Guadala ~ a r 3 y ';alisco :,:ex:.co. -:olu:.:e 1. pp. 114-120 Ear~~i c st

J.r,~# (19 8 3 ) .. 0o:;,b inac i on de Ent~a da J 3al i da Annlo c o-Di C-i. tal.. :!:!~1~ :::F i r.:';; e Y';.al