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Laboratory Pilot Plant for Modeling and Control Education
Copl'right © I FAC Trends in Control and Measurement Education. S,,'allsea, l ' K. l~IHH
LABORATORY PILOT PLANT FOR MODELING AND CONTROL EDUCATION B. ZupanCic, S. Strmcnik*, M. Sega*, R. Karba, D. Matko and M. Atanasijevic Faf/l/ly of EI('(/ri((ll ElIgillfl'flllf{. L"1I1l'I'rl liV of Ljllbljllllll. Tr:a,{ka 25. 61000 Ljllhljalla. hlf{oslm'ia *j. Slljilll [mlillllf. jalllOl '1I 39. 6J()()() Lj 11 blja 1111 , rllf{oslm'ill
Abstract. The paper deals with educational aspects of a hydraulic-thermal pilot plant connected to the PC workstation. First the hystorical background of the plant use for student exercises is given. The plant is described through hydraulic and thermal part as well as control elements and control cabinet. Then the software packages used for education as well as research work are described. The software consists of RTAC, ANA and SIMCOS packages. RTAC is program package for real time acquisition and control, ANA is a very complex package for computer aided analysis and design of control systems and SIMCOS is a die;ital simulation language. Linking all packages with a common data base a powerful PC workstahon was obtained. Such an equipment enables that students in relative short time perform various experiments on the plant. Some elementary experiments from the students exercises are presented at the end of the paper. Keywords. Computer aided design; Computer control; Control equipment; Control system synthesis; Education aids; Hydraulic systems; Identification; Level control.
INTRODUCTION The main topics of the course on automatic control at the Faculty of Electrical Engineerine; University of Ljubljana are simulation of systems, contmuous and discrete time control systems, computer control of processes and computer aided design of control systems.
to work with the program packages but when they become familiar with the software support, they can in relatively short time make very interesting and very different exercises from the various parts of the field of modeling and control. Fig.l represents the overall scheme that enables real time data acquisition, real time control, CACSD support and simulation support.
Many years experiences in the fields of modeling and control indicate that computer simulation is very usable tool lookin~ education as well as research aspects. On the other hand It comes out that modern teaching demands also exercises on laboratory devices which more realistically show the problems of modeling and control. Using the pilot plant in the process of education the interest of students significantly increase and it help them very much later in solving real industrial control problems. Some years ago we began exercises on hydraulic-thermal pilot plant without process computer. Students measured analog signals, which were displayed on analog plotter, on oscilloscope etc. The control was realized by our analog computer EAI 580. So students got a good insight into continuous modeling and control. Later we connected process computer PDP 11/34 with process interface with analog computer EAI 580. So students got an opportunity to make exercises on the field of discrete modeling and discrete real time control. The analog computer was the simulator of a real continuous process and the discrete controller was realized on the process computer. Finally we also connected the pilot plant with the process computer, at first with PDP 11/34 and later with IBM PC. The students had to write his own programs so their practical work was focused on programming (FORTRAN, PASCAL). Parallely two complex program packages were in development, the program package for analysis and design of control systems (ANA) and the program package for simulation of continuous dynamical systems (SIMCOS). So we got an idea to develop also a smaller program package for real time modeling and control requirements(RTAC) and to link all three packages with common data base. Realizing such a powerful PC workstation connected to the pilot plant or to the analog computer a completely new medium for teaching was established. The main problem became the complexity of the software. Students often spend much time in learning
PILOT PLANT INPUTS
OUTPUTS IH YDRAUllC PARTI ITHERMAL PART
I
ELECTRONIC PART INDICATION MANUAL CONTROL AUTOMATIC CONTROL
Fig. I.
Overall scheme for real time data acquisition and control with CACSD support
50
B. Zupan{i{ 1'/ al.
GENERAL DESCRIPTION OF THE PILOT PLANT In designing the pilot plant the following guidelines were considered: it should represent a simple laboratory model of a hypothetic process plant, often used in industry, from the system theoretical viewpoint it should be a multivariable system consisting of a few subsystems with a moderate number of interrelated systems variables, as much as possible available standard process control components should be included to allow similarity with real process plants and on the other side to keep investment costs within reasonable limits, possibilities for either local or remote control should be provided, a convenient degree of flexibilitr of varIOUs configurations was sought with proVIsions for easy modifications or extensions, from the education point of view it should represent a plant providing basic exercises on the courses of modeling and control. Keeping these guidelines in mind, the plant was designed in the form of a laboratory process system conceptually consisting of a hydraulic and a thermal part. Fig. 2 represents the scheme of the pilot plant.
mounted in the plant were selected from cheaper standard production programs for process instrumentation. They include: two capacity-type liquid level sensors, five resistance-type (Pt-lOO) temperature probes, two diffused-!lilicon differential pressure transducers, eight electromagnetic control valves, two standard 2 kW electrical heaters. The pilot plant is complemented by a separate power and control cabinet containing: indication with displays of all measuring variables, potentiometers for manual control of valves and heaters, electronics for transducers and transmitters, electronics for controlling the electromagnetic valves, ventilator, pump and heaters (thyristor regulators), - galvanic isolation of all measurin~ signals, connectors with analog and digital signals for process computer connection, power supply , power switches, fuses, protection relays.
PC WORKSTATION WITH CACSD SUPPORT The hydraulic-thermal plant is connected to process IBM PC computer workstation through the control cabinet what is represented in Fig.I. The process hardware enables sampling of all measured signals using A/D converters and controlling of valves, heaters, pump and cooler using D / A converters and dis-ital outputs (DIG). Three program packages with unifIed data base are installed on the PC workstation: -RTAC real time acquisition and control, -ANA program package for computer aided analysis and design of control systems, -SIMCOS program package for simulation of dynamical systems. These packages are based on many years of experiences and research as well as education work in our control laboratory. Such hardware and software equipment represents a very powerful tool in the field of modeling and control. We shall give a brief description of these program modules.
Fig.2.
The scheme of the laboratory pilot plant
This program module enables the work in real time. The following procedures are provided: driving A/D, D/A converters and digital outputs, real time changing of parameters through the keyboard , automatic acquisition of all analog signals of the plant , digital filtering of sampled data signals, automatic measurements of the transient responses for various process configurations, automatic measurements of the static valve characteristics, automatic measurements of the frequency characteristics for various process configurations, real time control, graphic representation of results.
The symbols in the scheme have the following meaning:
C Gl,G2 Pl,P2
R
H VI, V2, V3, V4, VIO V5 V7,V8
circulation pump electric heaters level tanks reservoir for liquid storage cooler electromagnetic control valves manually operated valve electromagnetic on/off valves
The hydraulic part of the plant consists of two vertical cylindrical glass tanks in which the levels of the liquid can be measured by level transducers. The level tanks are connected with each other and with an auxiliary liquid-!llorage reservoir in the closed circulation circuit system which also contains a circulation pump, a number of piping cross connections and remotely controllable electromagnetic valves. The thermal part of the plant is superposed onto the hydraulic part simply by inclusion of two electrical heaters and cooler into the piping system. In this way the working liquid can be heated and also cooled to near normal room temperature. The sensors and actuators
It is possible to realize all standard and also advanced control schemes using this software. The following discrete controllers are included: PID controller, state controller, deadbeat controller, general dynamic controller, minimum variance controller. All control algorithms have different modifications with manual/automatic modes, with integral wind up protection and with continuous or digital (on/oft) output.
ANA is Qur package for analysis and design of control systems (Sega and others 1985a ,1985b). Operations are divided to input and output operations and to the following operations from control system theory: transformations among different system representations - analysis and verification of systems, - synthesis of control systems and simulation.
51
La\)oraton' Pilot Plallt for '.Iodclillg alld COlltrol Eelucatioll All operations in the packalje are based on the foll owing representations of multivanable systems and they are appropriately coded for the interactive work: in the time domain - sampled data series of inputs and outputs, - series of self and cross correlation functions in the frequency domain - transfer function matrix for continuous and discrete systems state space description - for continuous and discrete systems special representations used in simulation
Simulations of hybrid systems i.e. the combination of con t in uous and discret e st ruct ures (discrete transfer fun ctions) is very uncomfortable in most simulation languages. We introduced special discrete blocks which are called from the simulation plOgram as function subpro~rams included in SIMCOS library. Each block has arbItrary sampling interval. The following discrete blocks can be implemented : - DPID (discrete PID controller), - DTRAN (general discrete transfer function), - S&H (sample and hold element), - DELAY (delay element) .
Transformations. Among described different system representations direct and indirect transformations exist . Analtsis. In the unit for analysis we have several possi ilit ies: in the state space this unit enables stability , controllability , observability and cyclicity tests of system as well as rank , norms, eigenvalues and eigenvectors computation for systems presented in state space. for continuous systems presented with transfer function matrix the usage of graphical methods for analysis in frequency domain is possible (Bode, Nyquist and root-locus plots). in the time domain we can get well known system erformance factors in the case of standard input signals overshoot , settling time, time delay , rise time , damping actor }. for verification of system model or controlled system the comparison of two or more I/O time series (sampled data series or computed time responses) can be realized giving also some statistic parameters.
r;
Synthesis. This unit enables: cont roller design (( opt imal) PID, generalized dead beat, optimal state controller}, Kalman filter calculation, two ways of feedback structure treatment: calculation of I/O state space equivalent simulation of general system structure includes precompensators, cascade, feedback and feed forward controllers as well as corresponding observers and estimators. The structure is realized through so called UNICUS (universal computer simulation pro~am) (Matko and others 1985}.Due to numerous ophmality problems the possibility of optimization and parameter estimation is included in UNI CUS. The optimization is based on some standardized criterion functions and some installed optimization algorithms. But both criterion function and optimization algorithm can be also written by user as FORTRAN subroutine.
EXAMPLE: LEVEL CONTROL OF THE PILOT PLANT The scheme of the laboratory pilot plant is represented in Fig. 2. A hydraulic subsystem with pump C, valve VI, level tank PI, valves V2, V8 and VlO and the storage reservoir R is considered for the purpose of control. The aim of the students exercise is to control the level in the tank PI with electromagnetic valve VI. The valves V2, V8 and VlO are constantly opened, so the output flow current from tank PI to reservoir R depends only on the level. Actually the output flow current is proportional to the square root of the lev el. But in the vicinity of working point the output current is approximately proportional to the level. If the fl ow /v oltage characteristic of the valve VI is linear, the system would be of the first order with appropriate dead time because the liquid has to come from valve VI to tank Pl. So it is important to measure the static characteristic of electromagnetic control valve which is controlled with voltage O-lOV. This characteristic can be measured automatically using a program from the RTAC module. This program measures times in which the tank PI is filled up at appropriate input valve VI voltage with shut output valves. Fig.3 represents this characteristic. A very strong effect of hysteresis and dead zone can be observed . Such valve is unusable for control. But the students can compensate the effect of hysteresis using: - inverse hysteresis function, - additive alternating signal to the control signal. The first method is rather complicated and very unrobust with respect to changes in hysteresis characteristic. The second method is very easy to implement and gives very good results. Such characteristic is shown in Fi&. 3 as the central curve. This curve shows the linear behaVIOur in the range O.8-2V . The additive signal is periodic square signal with frequency 1. 5 Hz and amplitude IV.
SIMCOS SIMCOS is CSSL like digital simulation language (Zupancic and others 1986, 1987). It works as compiler that compiles source program into FORTRAN modules and tables. Developing the simulation language SIMCOS we took into account the following basic principles: the syntax of the simulation language SIMCOS must be very similar to the syntax of CSSL language SIMCOS must possess some features that make it closer to hybrid computers than other languages. According to this principle controlling integrators were introduced in the simulation language, SIMCOS simulation language must possess some capabilities for simulation of discrete control systems. Besides all standard simulation language statements the following statements for signals and nonlinearities can be implemented: STEP (step function), RAMP (ramp function), PULSE (pulse function), HARM (harmonic function)( UNIF (random signal with uniform distribution) , GAUSS random signal with gaussian distribution}, PNBS (pseudo random binary signal), COMP AR (comparator) , FCNSW (function switch), SWIN (input switch), BOUND (bounding of signal), QNTZR (quantizator), DEAD (dead zone) , HSTRSS (hysteresis), BCKLSH (backlash).
Fig. 3.
Flow /voltage static lectromagnetic valve
characteristic
of
When students obtain this curve they choose appropriate working point in the middle of the linear region (approximately 1.5V). With the special program from RTAC module they record output signal at defined input signal. Fig. 4 represents input step signal and the system response. With input output data students identify system with program package ANA. They can choose parameter or structural-parameter black box identification method . The identification confirms our assumptions and the model has the transfer function
52
B. ZupanCic ('/ al. Students perform this experiments in relative short time. They give them a good feeling also to the problems that they meet later in practice. We describe only some simpler exercises. The other topics that can be realized using the pilot plant and the PC workstation are: control of higher order systems (two interconnected tanks), - cascade control , - state control, - multivariable control, - nonlinear contro!'
Fig.4.
Measured level signal and identified model output signal for step input signal
G(s) = s9a
+
1 e-4·5s.
The result of identification in the program package ANA is discrete state space description but students have the possibility to transform it to the continuous form, to the state space or to the transfer function form. To verify this model students use simulation capabilities of program package ANA or simulation language SIMCOS. SIMCOS is used especially in cases when nonlinearities must be included in the model. Fig. 4 represents also the identified model output signal at the same input signal. Another exercise is to obtain a discrete PID controller of the plant. Students use program package ANA that gives several possibilities of designing the PID controllers. One of the possibility is also to use the optimization method . By optimization the criterion function
I=
100 2 ~ le (k)
o
2
+ 0.1 u (k))
at the step reference change at sampling time 0.& the optimal discrete PID controller is
_ 1. 5 - 1.41z-1 + 0.072z 2 GR( z ) - 3 -1 1 - z Students realize discrete PID controller using RTAC software support. Fig. 5 represents the results of real time control of hquid level when the reference signal was changed.
Fig. 5.
The level signal on the pilot plant using discrete PID controller
CONCLUSION Our laboratory pilot plant built in the form of a hypothetical thermo--hydraulic technological system offers various possibilities for education as well as for research process for developing and testing classical and modern control algorithms. Its worth increases significantly when the plant was connected to software powerful PC workstation . Students can in relative short time make various experiments that are supported by three program packages, with graphics, etc. The most important is the fact that students have the possibility to model and control the real process with the similar problems they meet later in practice. Using this equipment for laboratory exercises the students interest increases significantly. It is urgent to improve some objects of the control equipment of the plant, especially the control valves and to install some new control equipement (flow measurement) . REFERENCES Agathoklis, P. , F .E. Cellier, M. Djordjevic, P.O. Grepper and F.J. Kraus (1979). Educational Aspects of Using Computer Aided Design in Automatic Control. Proc. _ IFAC Zurich Symp., pp. 441-446. . Cernetic J., S. Strmcnik, J. PetrovCic, D. Slebinger (1983). A Laboratory Set-up for the Investigation of ThermoHydraulic Control Systems. Proc. Simulators Brighton ~, pp.182-186. Freaena, D.K. (1982). Software Summaries on Computer Aided Control System Design Software Packages. Systems Magazine, Vo!. 2, No. 4, pp. 37-44. Matko, D., M. Sega, B . Zupancic (1985). UNICUS- A New Approach to the Simulation and Design of Control Systems. Proe. IMACS Oslo Symp. , Vol. 4, pp. 91-94. Schaufelberger W . (1987) .Teachware for Contro!' Prep. . IFAC Munchen Symp., VoI5. , pp. 267-272. Sega, M., S. Strmcnik, R. Karba and D. Matko (1985a). Interactive Program Package AN A for System Analysis and Control Design . Prep. CADCE'85 IFAC . Copenhagen Symp., pp. 145-150. Sega, M., S. Strmcnik, R. Karba and D. Matko (1985b). Control System Treatment by Program Package ANA. Proc. IMACS Oslo~, Vo!. 4, pp. 95-98. Walker, R ., C. Gregory and S. Shah (1982). MATRIX : A x Data Analysis, System Identification , Control Design and Simulation Package. Control Systems Magazine, _ Vo!. 2, No. 4, pp. 30-37. ZupanCic, B., D. Matko, M. Sega and P . Tramte (1986). Simulation in the Program Package ANA, Proc. ESC Antwerp Symp. pp.31-h318. . Zupancic, 8 ., D. Matko, R. Karba, M. Sega (1987) .SimcosDigital Simulation Language with Hybrid Capabilities. Proc. Simulationstechnik Zurich Symp.,pp 205-212.
LABORATORY PILOT PLANT FOR MODELING AND CONTROL EDUCATION B. ZupanCic, S. Strmcnik*, M. Sega*, R. Karba, D. Matko and M. Atanasijevic Faf/l/ly of EI('(/ri((ll ElIgillfl'flllf{. L"1I1l'I'rl liV of Ljllbljllllll. Tr:a,{ka 25. 61000 Ljllhljalla. hlf{oslm'ia *j. Slljilll [mlillllf. jalllOl '1I 39. 6J()()() Lj 11 blja 1111 , rllf{oslm'ill
Abstract. The paper deals with educational aspects of a hydraulic-thermal pilot plant connected to the PC workstation. First the hystorical background of the plant use for student exercises is given. The plant is described through hydraulic and thermal part as well as control elements and control cabinet. Then the software packages used for education as well as research work are described. The software consists of RTAC, ANA and SIMCOS packages. RTAC is program package for real time acquisition and control, ANA is a very complex package for computer aided analysis and design of control systems and SIMCOS is a die;ital simulation language. Linking all packages with a common data base a powerful PC workstahon was obtained. Such an equipment enables that students in relative short time perform various experiments on the plant. Some elementary experiments from the students exercises are presented at the end of the paper. Keywords. Computer aided design; Computer control; Control equipment; Control system synthesis; Education aids; Hydraulic systems; Identification; Level control.
INTRODUCTION The main topics of the course on automatic control at the Faculty of Electrical Engineerine; University of Ljubljana are simulation of systems, contmuous and discrete time control systems, computer control of processes and computer aided design of control systems.
to work with the program packages but when they become familiar with the software support, they can in relatively short time make very interesting and very different exercises from the various parts of the field of modeling and control. Fig.l represents the overall scheme that enables real time data acquisition, real time control, CACSD support and simulation support.
Many years experiences in the fields of modeling and control indicate that computer simulation is very usable tool lookin~ education as well as research aspects. On the other hand It comes out that modern teaching demands also exercises on laboratory devices which more realistically show the problems of modeling and control. Using the pilot plant in the process of education the interest of students significantly increase and it help them very much later in solving real industrial control problems. Some years ago we began exercises on hydraulic-thermal pilot plant without process computer. Students measured analog signals, which were displayed on analog plotter, on oscilloscope etc. The control was realized by our analog computer EAI 580. So students got a good insight into continuous modeling and control. Later we connected process computer PDP 11/34 with process interface with analog computer EAI 580. So students got an opportunity to make exercises on the field of discrete modeling and discrete real time control. The analog computer was the simulator of a real continuous process and the discrete controller was realized on the process computer. Finally we also connected the pilot plant with the process computer, at first with PDP 11/34 and later with IBM PC. The students had to write his own programs so their practical work was focused on programming (FORTRAN, PASCAL). Parallely two complex program packages were in development, the program package for analysis and design of control systems (ANA) and the program package for simulation of continuous dynamical systems (SIMCOS). So we got an idea to develop also a smaller program package for real time modeling and control requirements(RTAC) and to link all three packages with common data base. Realizing such a powerful PC workstation connected to the pilot plant or to the analog computer a completely new medium for teaching was established. The main problem became the complexity of the software. Students often spend much time in learning
PILOT PLANT INPUTS
OUTPUTS IH YDRAUllC PARTI ITHERMAL PART
I
ELECTRONIC PART INDICATION MANUAL CONTROL AUTOMATIC CONTROL
Fig. I.
Overall scheme for real time data acquisition and control with CACSD support
50
B. Zupan{i{ 1'/ al.
GENERAL DESCRIPTION OF THE PILOT PLANT In designing the pilot plant the following guidelines were considered: it should represent a simple laboratory model of a hypothetic process plant, often used in industry, from the system theoretical viewpoint it should be a multivariable system consisting of a few subsystems with a moderate number of interrelated systems variables, as much as possible available standard process control components should be included to allow similarity with real process plants and on the other side to keep investment costs within reasonable limits, possibilities for either local or remote control should be provided, a convenient degree of flexibilitr of varIOUs configurations was sought with proVIsions for easy modifications or extensions, from the education point of view it should represent a plant providing basic exercises on the courses of modeling and control. Keeping these guidelines in mind, the plant was designed in the form of a laboratory process system conceptually consisting of a hydraulic and a thermal part. Fig. 2 represents the scheme of the pilot plant.
mounted in the plant were selected from cheaper standard production programs for process instrumentation. They include: two capacity-type liquid level sensors, five resistance-type (Pt-lOO) temperature probes, two diffused-!lilicon differential pressure transducers, eight electromagnetic control valves, two standard 2 kW electrical heaters. The pilot plant is complemented by a separate power and control cabinet containing: indication with displays of all measuring variables, potentiometers for manual control of valves and heaters, electronics for transducers and transmitters, electronics for controlling the electromagnetic valves, ventilator, pump and heaters (thyristor regulators), - galvanic isolation of all measurin~ signals, connectors with analog and digital signals for process computer connection, power supply , power switches, fuses, protection relays.
PC WORKSTATION WITH CACSD SUPPORT The hydraulic-thermal plant is connected to process IBM PC computer workstation through the control cabinet what is represented in Fig.I. The process hardware enables sampling of all measured signals using A/D converters and controlling of valves, heaters, pump and cooler using D / A converters and dis-ital outputs (DIG). Three program packages with unifIed data base are installed on the PC workstation: -RTAC real time acquisition and control, -ANA program package for computer aided analysis and design of control systems, -SIMCOS program package for simulation of dynamical systems. These packages are based on many years of experiences and research as well as education work in our control laboratory. Such hardware and software equipment represents a very powerful tool in the field of modeling and control. We shall give a brief description of these program modules.
Fig.2.
The scheme of the laboratory pilot plant
This program module enables the work in real time. The following procedures are provided: driving A/D, D/A converters and digital outputs, real time changing of parameters through the keyboard , automatic acquisition of all analog signals of the plant , digital filtering of sampled data signals, automatic measurements of the transient responses for various process configurations, automatic measurements of the static valve characteristics, automatic measurements of the frequency characteristics for various process configurations, real time control, graphic representation of results.
The symbols in the scheme have the following meaning:
C Gl,G2 Pl,P2
R
H VI, V2, V3, V4, VIO V5 V7,V8
circulation pump electric heaters level tanks reservoir for liquid storage cooler electromagnetic control valves manually operated valve electromagnetic on/off valves
The hydraulic part of the plant consists of two vertical cylindrical glass tanks in which the levels of the liquid can be measured by level transducers. The level tanks are connected with each other and with an auxiliary liquid-!llorage reservoir in the closed circulation circuit system which also contains a circulation pump, a number of piping cross connections and remotely controllable electromagnetic valves. The thermal part of the plant is superposed onto the hydraulic part simply by inclusion of two electrical heaters and cooler into the piping system. In this way the working liquid can be heated and also cooled to near normal room temperature. The sensors and actuators
It is possible to realize all standard and also advanced control schemes using this software. The following discrete controllers are included: PID controller, state controller, deadbeat controller, general dynamic controller, minimum variance controller. All control algorithms have different modifications with manual/automatic modes, with integral wind up protection and with continuous or digital (on/oft) output.
ANA is Qur package for analysis and design of control systems (Sega and others 1985a ,1985b). Operations are divided to input and output operations and to the following operations from control system theory: transformations among different system representations - analysis and verification of systems, - synthesis of control systems and simulation.
51
La\)oraton' Pilot Plallt for '.Iodclillg alld COlltrol Eelucatioll All operations in the packalje are based on the foll owing representations of multivanable systems and they are appropriately coded for the interactive work: in the time domain - sampled data series of inputs and outputs, - series of self and cross correlation functions in the frequency domain - transfer function matrix for continuous and discrete systems state space description - for continuous and discrete systems special representations used in simulation
Simulations of hybrid systems i.e. the combination of con t in uous and discret e st ruct ures (discrete transfer fun ctions) is very uncomfortable in most simulation languages. We introduced special discrete blocks which are called from the simulation plOgram as function subpro~rams included in SIMCOS library. Each block has arbItrary sampling interval. The following discrete blocks can be implemented : - DPID (discrete PID controller), - DTRAN (general discrete transfer function), - S&H (sample and hold element), - DELAY (delay element) .
Transformations. Among described different system representations direct and indirect transformations exist . Analtsis. In the unit for analysis we have several possi ilit ies: in the state space this unit enables stability , controllability , observability and cyclicity tests of system as well as rank , norms, eigenvalues and eigenvectors computation for systems presented in state space. for continuous systems presented with transfer function matrix the usage of graphical methods for analysis in frequency domain is possible (Bode, Nyquist and root-locus plots). in the time domain we can get well known system erformance factors in the case of standard input signals overshoot , settling time, time delay , rise time , damping actor }. for verification of system model or controlled system the comparison of two or more I/O time series (sampled data series or computed time responses) can be realized giving also some statistic parameters.
r;
Synthesis. This unit enables: cont roller design (( opt imal) PID, generalized dead beat, optimal state controller}, Kalman filter calculation, two ways of feedback structure treatment: calculation of I/O state space equivalent simulation of general system structure includes precompensators, cascade, feedback and feed forward controllers as well as corresponding observers and estimators. The structure is realized through so called UNICUS (universal computer simulation pro~am) (Matko and others 1985}.Due to numerous ophmality problems the possibility of optimization and parameter estimation is included in UNI CUS. The optimization is based on some standardized criterion functions and some installed optimization algorithms. But both criterion function and optimization algorithm can be also written by user as FORTRAN subroutine.
EXAMPLE: LEVEL CONTROL OF THE PILOT PLANT The scheme of the laboratory pilot plant is represented in Fig. 2. A hydraulic subsystem with pump C, valve VI, level tank PI, valves V2, V8 and VlO and the storage reservoir R is considered for the purpose of control. The aim of the students exercise is to control the level in the tank PI with electromagnetic valve VI. The valves V2, V8 and VlO are constantly opened, so the output flow current from tank PI to reservoir R depends only on the level. Actually the output flow current is proportional to the square root of the lev el. But in the vicinity of working point the output current is approximately proportional to the level. If the fl ow /v oltage characteristic of the valve VI is linear, the system would be of the first order with appropriate dead time because the liquid has to come from valve VI to tank Pl. So it is important to measure the static characteristic of electromagnetic control valve which is controlled with voltage O-lOV. This characteristic can be measured automatically using a program from the RTAC module. This program measures times in which the tank PI is filled up at appropriate input valve VI voltage with shut output valves. Fig.3 represents this characteristic. A very strong effect of hysteresis and dead zone can be observed . Such valve is unusable for control. But the students can compensate the effect of hysteresis using: - inverse hysteresis function, - additive alternating signal to the control signal. The first method is rather complicated and very unrobust with respect to changes in hysteresis characteristic. The second method is very easy to implement and gives very good results. Such characteristic is shown in Fi&. 3 as the central curve. This curve shows the linear behaVIOur in the range O.8-2V . The additive signal is periodic square signal with frequency 1. 5 Hz and amplitude IV.
SIMCOS SIMCOS is CSSL like digital simulation language (Zupancic and others 1986, 1987). It works as compiler that compiles source program into FORTRAN modules and tables. Developing the simulation language SIMCOS we took into account the following basic principles: the syntax of the simulation language SIMCOS must be very similar to the syntax of CSSL language SIMCOS must possess some features that make it closer to hybrid computers than other languages. According to this principle controlling integrators were introduced in the simulation language, SIMCOS simulation language must possess some capabilities for simulation of discrete control systems. Besides all standard simulation language statements the following statements for signals and nonlinearities can be implemented: STEP (step function), RAMP (ramp function), PULSE (pulse function), HARM (harmonic function)( UNIF (random signal with uniform distribution) , GAUSS random signal with gaussian distribution}, PNBS (pseudo random binary signal), COMP AR (comparator) , FCNSW (function switch), SWIN (input switch), BOUND (bounding of signal), QNTZR (quantizator), DEAD (dead zone) , HSTRSS (hysteresis), BCKLSH (backlash).
Fig. 3.
Flow /voltage static lectromagnetic valve
characteristic
of
When students obtain this curve they choose appropriate working point in the middle of the linear region (approximately 1.5V). With the special program from RTAC module they record output signal at defined input signal. Fig. 4 represents input step signal and the system response. With input output data students identify system with program package ANA. They can choose parameter or structural-parameter black box identification method . The identification confirms our assumptions and the model has the transfer function
52
B. ZupanCic ('/ al. Students perform this experiments in relative short time. They give them a good feeling also to the problems that they meet later in practice. We describe only some simpler exercises. The other topics that can be realized using the pilot plant and the PC workstation are: control of higher order systems (two interconnected tanks), - cascade control , - state control, - multivariable control, - nonlinear contro!'
Fig.4.
Measured level signal and identified model output signal for step input signal
G(s) = s9a
+
1 e-4·5s.
The result of identification in the program package ANA is discrete state space description but students have the possibility to transform it to the continuous form, to the state space or to the transfer function form. To verify this model students use simulation capabilities of program package ANA or simulation language SIMCOS. SIMCOS is used especially in cases when nonlinearities must be included in the model. Fig. 4 represents also the identified model output signal at the same input signal. Another exercise is to obtain a discrete PID controller of the plant. Students use program package ANA that gives several possibilities of designing the PID controllers. One of the possibility is also to use the optimization method . By optimization the criterion function
I=
100 2 ~ le (k)
o
2
+ 0.1 u (k))
at the step reference change at sampling time 0.& the optimal discrete PID controller is
_ 1. 5 - 1.41z-1 + 0.072z 2 GR( z ) - 3 -1 1 - z Students realize discrete PID controller using RTAC software support. Fig. 5 represents the results of real time control of hquid level when the reference signal was changed.
Fig. 5.
The level signal on the pilot plant using discrete PID controller
CONCLUSION Our laboratory pilot plant built in the form of a hypothetical thermo--hydraulic technological system offers various possibilities for education as well as for research process for developing and testing classical and modern control algorithms. Its worth increases significantly when the plant was connected to software powerful PC workstation . Students can in relative short time make various experiments that are supported by three program packages, with graphics, etc. The most important is the fact that students have the possibility to model and control the real process with the similar problems they meet later in practice. Using this equipment for laboratory exercises the students interest increases significantly. It is urgent to improve some objects of the control equipment of the plant, especially the control valves and to install some new control equipement (flow measurement) . REFERENCES Agathoklis, P. , F .E. Cellier, M. Djordjevic, P.O. Grepper and F.J. Kraus (1979). Educational Aspects of Using Computer Aided Design in Automatic Control. Proc. _ IFAC Zurich Symp., pp. 441-446. . Cernetic J., S. Strmcnik, J. PetrovCic, D. Slebinger (1983). A Laboratory Set-up for the Investigation of ThermoHydraulic Control Systems. Proc. Simulators Brighton ~, pp.182-186. Freaena, D.K. (1982). Software Summaries on Computer Aided Control System Design Software Packages. Systems Magazine, Vo!. 2, No. 4, pp. 37-44. Matko, D., M. Sega, B . Zupancic (1985). UNICUS- A New Approach to the Simulation and Design of Control Systems. Proe. IMACS Oslo Symp. , Vol. 4, pp. 91-94. Schaufelberger W . (1987) .Teachware for Contro!' Prep. . IFAC Munchen Symp., VoI5. , pp. 267-272. Sega, M., S. Strmcnik, R. Karba and D. Matko (1985a). Interactive Program Package AN A for System Analysis and Control Design . Prep. CADCE'85 IFAC . Copenhagen Symp., pp. 145-150. Sega, M., S. Strmcnik, R. Karba and D. Matko (1985b). Control System Treatment by Program Package ANA. Proc. IMACS Oslo~, Vo!. 4, pp. 95-98. Walker, R ., C. Gregory and S. Shah (1982). MATRIX : A x Data Analysis, System Identification , Control Design and Simulation Package. Control Systems Magazine, _ Vo!. 2, No. 4, pp. 30-37. ZupanCic, B., D. Matko, M. Sega and P . Tramte (1986). Simulation in the Program Package ANA, Proc. ESC Antwerp Symp. pp.31-h318. . Zupancic, 8 ., D. Matko, R. Karba, M. Sega (1987) .SimcosDigital Simulation Language with Hybrid Capabilities. Proc. Simulationstechnik Zurich Symp.,pp 205-212.