The Computer Control System in Tanning Processes

International Carpathian Control Conference ICCC’ 2002 MALENOVICE, CZECH REPUBLIC May 27-30, 2002 THE COMPUTER CONTROL SYSTEM IN TANNING PROCESSES Thang PHAM MANH and Vladimír VAŠEK Department of Automatic Control, Faculty of Technology UTB, Zlín, Czech Republic Thang@seznam.cz, Vasek@ft.utb.cz Abstract: This paper brings up main information about the computer control of tanning processes. As an experimental laboratory model were used in the Department of Automatic Control Faculty of Technology Tomas Bata University in Zlín a washing vessel fully equipped with sensors and actuators. Monitoring and control system was tested on the real equipment with using computer with an industrial card Advantech PCL – 812/812PG. The software of the control system is built in the real time with idea of optimal technology. Key words: Computer control, real –time, process control, mathematical model, control algorithms. 1 Introduction Tanning industry is characteristic by lots of discontinuous operations by which unrefined leather is changed into hide. There is a great deal of hard work because of the fact that it is necessary to put among physics – chemical operations machine operations and at the same time there exists a lot of consumption of electrical energy, technological water and other tanning agents. Big problem seems to be working conditions and protection of the environment because of large amount of solid wastes and polluted water. For these reasons a large modernisation of tanning industry is necessary. Is consists at the introducing of automatic control of technological processes and computer technique in tanning industry, whose raise can be seen in last years. As a laboratory model of tanning process technological equipment (see fig.1) was created in our department in faculty of Technology UTB in Zlín. The main part is a washing vessel, in which a rotated cylinder is situated. The processed materials are put into it. This equipment is fitted out with necessary sensors and valves for reading physical quantity and control of action elements. It is necessary to control in washing process: 371 Figure 1. - - Filling technological water with defined temperature and amount. - Letting water out from washing process. - Scanning discreet signals defining the position or state individual procedure parts of technological system and taking analogy values prom the process (temperature). - Control of the time sequence of individual operation according to technological procedure with controlling of all parameters. Control of rotation of cylinder and control stirring and measuring out of prepared chemicals in assisting vessel and their releasing. Regulation of temperature in washing watering place. - Chemical stores I II III Granary for powder IV V Measure out of acid I II I Cool water Hot water WASHING VESSEL Motor Steam I/O Signals Actuators and action electronics PC with Industrial Card Comunication Unit Technological Panel Figure 2. The simplified schema of technological equipments 372 II Control consists of the commanding of equipments and measuring out of chemicals according planed program. Quantitatively higher-level automatic control is the using of backward connection, when action interference is made on the basis of real parameters of the process, which are just going. 2 Mathematical model of tanning process: One way how to reach it is adequately technical equipment with using of control algorithms, which are deduced from mathematical model given process. The goal of control is looking for such control algorithm to reach suitable quality of interproduct, protection of the environment and minimization of working cost, which depends on the amount of water, electrical energy used for this process. Mathematical model is base on salt dissolution and its rate is given by the equation: dc = k[cn − c] dt (1) Operational costs (N) are the sum of costs of electric energy (NE) and washing water consumption (NV): N = NE + NV = Ke.P.t + Kv.Vo (2) The final concentration c is determined from the mass balance: cp.V = c.Vo c = cp.V Vo (3) The time t is determined by the solution of equation (1): t= 1  cn  ln k  cn − c  (4) Substitution the values for c and t into (2) we get:     cn P.Ke  ln  N = Kv.Vo + k  cn − cp V   Vo   (5) The optimal technology is the one, operated at such parameter values at which the cost function has its minimum: P.Ke.cp.V dN = 0 = Kv − Vo(Vo.cn − cp.V ) dVo (6) The optimal capacity of washing water is given by solution of equation (6): Kv.cp.V + Vo =  2 2 2 4cn.Kv.cp.V.P.Ke   Kv .cp .V +  k   2.cn.Kv (7) The control with backward connection will work in case that concentration will be measured during filling, when the identification parameter k is determined. Dissolving of surface salt is given: 373 V' dc   = c + k  = k.cn dt  Vo + V '.t  (8) By solving of it we get:  V' k.cn.e −kt  V  + c = cn 1 −  − Vo    k (V ' t + Vo ) V '.t + Vo  k (9) For the short time (t ->0) the value k can be estimated from a much simpler relationship:  V '.t  + Vo  t.k.cn  2  c≈ V '.t.Vo Or c ≈ k.t.cn (10a,b) For the long time t:   V'  c ≈ cn1 − ( ) k V t + Vo '.   (11) For finding of identification parameter k we have to ensure filling of a optimal amount of water and its realizing in the optimum time. Other parameters in this case the amount of water and time of washing can get only values from certain technological interval, which is given by used equipment and work organization not to get to damage of leather during short time of watering place or to such a lengthening of time that it doesn’t suit working organization in the whole sequence of operations. According to the kind, character and state of entering raw material values of optimal parameters are changed. 3 Programming methods in applied in automation application Described technology is now realised in laboratory conditions in our department by computer control system with a programmable industrial card Advantech PCL – 812/812PG, which has own A/D and D/A converters. Each part of the technology process has its own control subsystem for the direct digital control of the physical values as a temperature, water level in vessel, value of turbidity etc. The software system is built in the C language. For the real time running of the program system there is used special preemptive real time operating system RTMON (Vašek, V., 1994), which was built for the using of monitoring and control system for technological processes. It allows multitasking of defined number of processes. User’s programs are structured on the basis of the priority hierarchically. The choosing of the program, which will be running on the processor, is carried out on the basis of its priority level. If there are more programs with the same priority, the processor is assign periodically for the time of defined account of time quantum. By the help of only one interrupt signal – timer interrupt / this simple priority system allows fill many requirements for the real time processes running. The structure of an application programs is shown in figure number 3. Program system includes the basic part of the real time operating system is created by process modules. Initialisation process INIC defines data structure of the real time operating system and fills the program variables by the initial values; another process SNI reads periodically the binary and analogue input values. The control algorithms processes 374 REG calculate optimal amount of washing water and time to filling it into the bath from the measured values of temperatures, value of turbidity. The visualisation process ZOB allows to write the information and important parameters in display and more. Design of control in the real time is a specific possibility how to use a long sampling period to master complicated mathematical equations in the real time. INITIALISATION BINARY & ANALOG INPUTS CONTROL ALGORITHMS R T M O N REAL TIME USER COMMANDS PROCESS START / STOP BINARY OUTPUTS PROCESS CONTROL SUPERVISOR VIZUALISATION Figure 3. Measured quantities are mostly concentrations of active substances in the bath and operational quantities are amounts of chemicals metered in the individual period of sampling. In the laboratory conditions for testing there were using PC equipped with card containing I/O of physical quantities from the process. Because of control algorithm and technological methods are universal and we can use the technical and programming tools of automation in several ways in the real industrial environment, including the chance of using microcomputer’s technique, PLC (Programmable Logic Controller), DCS (Distributed Control Systems). 4 Conclusion Described method of the computer automatic control in tanning processes realizing by laboratory washing vessel is still in the stage of research. It allows on the one hand testing new technological techniques and approach for transform of the natural hide, on the other hand testing control discreet algorithms for controlling analogue quantities. By the present experiences it can be told that the important factor affecting a lot of working results is right choice of kind and range sensor of turbidity. 375 List of symbols c t k cn cp NE NV NE Ke Kv P V V’ Vo - concentration of the salt in water time speed constant of dissolving concentration of salt in water initial concentration of salt in raw leather costs on consumption of electric energy costs on consumption of washing water total operational costs price for unit of electric energy price for unit of washing water input of electromotor capacity of washing raw leather capacity rate of water during filling hogshead capacity of water used for washing of surface salt kg m-3 s s-1 kg m-3 kg m-3 Kč Kč Kč Kč/KWh Kč m-3 KW m3 m3 s-1 m3 References BLAHA, A., KOLOMAZNÍK, K. (1989): J.Soc. Leather Techno. And Chem., 1989, 73, 136 VAŠEK, V., PHAM MANH, T. (2001): HC11 Control Unit Applications IFAC WORKSHOP on Programmable Devices and Systems PDS 2001, Gliwice, Poland. ROZEHNAL, Z. 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