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Design of a Realistic CACSD Course
Copyright~, IFAC Advances in Control Education. Tokyo. Japan. 1994
DESIGN OF A REALISTIC CACSD COURSE
DRAGO MATKO', REIMAR SCHUMANN" , BORUT ZUPANCIC' • Faculty of electrical and computer engineering University of Ljubljana, Slovenia •• Fachhochschule Hannover , Germany
Abstract. In the paper the layout of a realistic CACSD course and its implementation in a European Community TEMPUS project are described. Realistic means that the students are not only taught control system theory but also become familiar with the real problems encountered in practice. A laboratory air conditioning plant, designed and built in the frame of the project is described as well. A brief review of CACSD software and hardware necessary for the project's realisation is given . The status of the project is reviewed together with the first student 's experiment . Keywords. Educational aids, laboratory techniques, air conditioning.
1. INTRODUCTION
2. THE TEMPUS PROJECT The project was started in 1992 with the west European partners Fachhochschule Hannover (FRG), University of Glamorgan (Wales, UK), Fachhochschule Leipzig (FRG) and the eligible country partner University of Ljubljana (Slovenia) . The project is set up for three years with the final goal to run a realistic CACSD course not only in Ljubljana, but also in all other partner universities in 1995.
The theory of control systems has made great advances during the last thirty years. Adaptive systems, optimal systems , learning and intelligent control systems were invented and their practical realisation was made possible by digital computers . However their use in practice is still limited to only a few applications. The main reason for this gap between theory and practice is that many assumptions. required by theoretical theorems and lemmas which form the basis for such academic controller design approaches are simply not satisfied in practice. Leaving the "nice theoretical world" and entering into the" dirty real world" other problems such as unknown and unstationary working points , nonlinearities and/or disturbances of unknown character , real time implementation problems etc . arise and have to be taken into account by the control engineer in practice. Thus it is essential that students are not only taught control system theory but should become familiar during their education with the problems of controller design in the real world . So realistic courses on computer aided control systems design (CACSD) for realistic plants are due to become part of their education .
The course is set up on the basis of a computerised CACSD laboratory. This laboratory is to be equipped not only with CACSD workstations but especially with realistic laboratory plants with all problems encountered in practice as described above and with industrial control units. The CACSD workstations are used for the computer aided modelling of the plant and the computer aided design of the control system making intensive use of all required software tools for process identification, simulation , optimisation etc .. The implementation of the designed control system for the laboratory plants will be done at the end of the design process using industrial control devices which are to be programmable by block diagrams thus simplifying the (nontrivial) task of translating the results of the CACSD workstation into an industrial control system .
The aim of this paper is to describe the layout of such a realistic CACSD course which has been specified and is going to be implemented in a European Community TEMPUS project.
In the first year of the project the physical setup of the laboratory was primarily addressed ; besides the acquisition of computers, CACSD software and industrial controllers) this included es-
245
pecially the specification and realisation of an appropriate and realistic pilot plant.
3. THE LABORATORY AIR CONDITIONING PLANT
Fig. 2. Block scheme of the process
One of the major problems in this context was the fact that realistic plants occupy in general too much laboratory space, which all universities are short of, and even more troublesome, consequently have large time constants, what prevents them to operate in real time in student courses. Thus a laboratory scale air conditioning plant was designed by the TEMPUS group and built at the University of Ljubljana. An air conditioning plant was chosen because it is a multivariable, nonlinear plant quite often encountered in industry. The plant has two controlled outputs: air temperature and relative humidity which are influenced by two actuating elements for heating and humidification . Due to the crossconnections (increase of the temperature decreases the . relative humidity while humidifying decreases the temperature) the plant is a true multivariable system . After several experiments the following configuration shown in Fig. 1 proved to be acceptable : a ventilator conveying the air which serves as a transport medium , a heating coil for heating, a nebulizer for humidification of the air and a mixing chamber in which the outputs, i.e. temperature and relative humidity, are measured. With this setup the plant
(~_---,JLJ Mixing Nebulizer
Fig . 3. The air conditioning plant
I
-I Ohit':
Ventilator S - Temperature
Fig.!. Process scheme exhibits a lot of nice nonlinearties and other real world problems due to the underlying physics and the used actuating elements. The physical dimensions of the plant were designed to be very small to produce small time constants.
provided by a small compressor (20 W). A mixture of the heated and nebulized air enters the mixing chamber. The block diagram of the climate process is shown in Figure 2. The process is strongly nonlinear at first because of the quadratic relationship of heating power to the input voltage, second due to the nonlinear characteristics of the nebulizer and third due to the relation between temperature and humidity according to the Mollier diagram. Figure 2 therefore represents a Hammerstein type nonlinear model. The control paths can be modelled of four first order blocks with time delay. Their time constants and delays depend on the physical dimensions of the plant. The dynamic characteristics of the sensors are also represented as first order blocks. For pedagogical reasons and for more flexibility temperature and relative humidity are measured also at the air inlet and after the heating coil. It was decided to use two additional sensors for each variable, at the air inlet (19 1 , '1'd to the plant and at a point between the heater and nebulizer (19 2 , '1'2)' This enables more insight into the process and the implementation of cascade and feedforward control. The plant was build in a 19" rack as shown in Fig. 3. On the covering plate, which is closed down during transportation, the ventilator, the glass tube with built-in heater, the connections for the intermediate sensor and nebulizer and the mixing chamber are mounted. In the rack the compressor, the electronic interfaces and ac-
A suitable power for the heater for the process as it was realised, was found to be approximately 25 W. The heating coil is situated in a glass tube and the nebulizer is introduced downstream . The nebulizer is operated by compressed air which is 246
ffi li
u.
I
40 f
F--------cp
o
200
ffi j 40
cp
u.
L 0
ffiO
400
It
800
1000 Time [sI
\
~ Fig. 5. The working place
200
400
ffiO
800
1000 Time [sI
shown in Fig. 5. As basic CACSD software MATLAB with SIMULINK was chosen among others. A second laboratory scale plant for the control of water level and air pressure in a semi closed vessel will be installed in the laboratory in addition in the very near future . This second plant was designed in a preceding TEMPUS project by the TH Leipzig (Now FH Leipzig) .
Fig. 4. The step responses of the plant tuators are assembled . The front plate is divided into two parts: The left part represents the actuator 's part including manual- automatic switches , potentiometers for manual control and 0-10 V inputs . The heating and nebulizing inputs are equipped also with LCD displays, while the ventilator input used for changing the plant characteristics is not .
In spring 1994 the first student exercise will be performed with a smaller group of a students (46) . In this exercise the plant will be used as univariable plant where only the temperature part and relative humidity part will be used for two experiments separately. The objective of the exercise is to design and realise single variable PID controllers. First the step response will be evaluated and the PID controller parameters will be determined by Chien- Chrones-Reswick method and parameter optimization . First order process models with time delay are used in this case and the controllers will then be tested by simulations.
The right part of the front plate represents the sensors; for each measuring point the temperature and relative humidity is displayed normalized within 0-10 V outputs. Typical step responses of the plant are shown in Fig . 4. The upper part of the figure contains the responses to the step change due to heating while the lower part depicts step changes due to nebulizing. It can be seen that the nebulizing response is faster what is the consequence of the physical layout (nebulizer is mounted closer to the process output). The heating response is slower with the settling time about 300 s what is still acceptable for performing exercises in the frame of a student course.
In the next step the PID control scheme for the industrial controller will be designed by corresponding software package and downloaded . The controller configuration will then be tested in the hardware in the loop test where the supposed plant will be simulated on the computer and connected by AD and DA connecters to the standard industrial controller. In the last step the controller will be used on the real plant .
4. STATUS OF THE PROJECT 5. CONCLUSION In the first year of the project besides building the air conditioning plant also three CACSD stations joined in a local area network have been installed. All stations are equipped with analog to digital (AD) and digital to analog (DA) converters and combined with industrial standard process control units and programmable logic (PL) controllers as
The used combinations of laboratory scale plants , CACSD computers and industrial controllers enables a diversity of experiments such as process identification - controller design - validation of controllers with identified linear models by simulation - programming of industrial control units 247
- test of the programmed controllers as hardware in the simulated control loop with the identified linear models and finally the direct control of the real plant by the industrial control units. The detailed elaboration of CACSD courses remains to be done in the next two years of the project; a few ideas have been given in the paper . In the final stage of installation at least 9 CACSD stations will be installed with the above described equipment in CACSD laboratory enabling a realistic practice oriented teaching of control systems design to a group of twenty to thirty students simultaneously.
248
REFERENCES
TEMPUS (1992) . Financial support for cooperation and mobility In higher education between Central/Eastern Europe and the European Community. EC TEMPUS Office Brussels. Zupancic B., R. Karba, D. Matko, M. Atanasijevic - Kunc (1993) . Educational aspects of CAD supported real time control. IEEE Transaction on Education 36 No. 3, pp. 340-7.
DESIGN OF A REALISTIC CACSD COURSE
DRAGO MATKO', REIMAR SCHUMANN" , BORUT ZUPANCIC' • Faculty of electrical and computer engineering University of Ljubljana, Slovenia •• Fachhochschule Hannover , Germany
Abstract. In the paper the layout of a realistic CACSD course and its implementation in a European Community TEMPUS project are described. Realistic means that the students are not only taught control system theory but also become familiar with the real problems encountered in practice. A laboratory air conditioning plant, designed and built in the frame of the project is described as well. A brief review of CACSD software and hardware necessary for the project's realisation is given . The status of the project is reviewed together with the first student 's experiment . Keywords. Educational aids, laboratory techniques, air conditioning.
1. INTRODUCTION
2. THE TEMPUS PROJECT The project was started in 1992 with the west European partners Fachhochschule Hannover (FRG), University of Glamorgan (Wales, UK), Fachhochschule Leipzig (FRG) and the eligible country partner University of Ljubljana (Slovenia) . The project is set up for three years with the final goal to run a realistic CACSD course not only in Ljubljana, but also in all other partner universities in 1995.
The theory of control systems has made great advances during the last thirty years. Adaptive systems, optimal systems , learning and intelligent control systems were invented and their practical realisation was made possible by digital computers . However their use in practice is still limited to only a few applications. The main reason for this gap between theory and practice is that many assumptions. required by theoretical theorems and lemmas which form the basis for such academic controller design approaches are simply not satisfied in practice. Leaving the "nice theoretical world" and entering into the" dirty real world" other problems such as unknown and unstationary working points , nonlinearities and/or disturbances of unknown character , real time implementation problems etc . arise and have to be taken into account by the control engineer in practice. Thus it is essential that students are not only taught control system theory but should become familiar during their education with the problems of controller design in the real world . So realistic courses on computer aided control systems design (CACSD) for realistic plants are due to become part of their education .
The course is set up on the basis of a computerised CACSD laboratory. This laboratory is to be equipped not only with CACSD workstations but especially with realistic laboratory plants with all problems encountered in practice as described above and with industrial control units. The CACSD workstations are used for the computer aided modelling of the plant and the computer aided design of the control system making intensive use of all required software tools for process identification, simulation , optimisation etc .. The implementation of the designed control system for the laboratory plants will be done at the end of the design process using industrial control devices which are to be programmable by block diagrams thus simplifying the (nontrivial) task of translating the results of the CACSD workstation into an industrial control system .
The aim of this paper is to describe the layout of such a realistic CACSD course which has been specified and is going to be implemented in a European Community TEMPUS project.
In the first year of the project the physical setup of the laboratory was primarily addressed ; besides the acquisition of computers, CACSD software and industrial controllers) this included es-
245
pecially the specification and realisation of an appropriate and realistic pilot plant.
3. THE LABORATORY AIR CONDITIONING PLANT
Fig. 2. Block scheme of the process
One of the major problems in this context was the fact that realistic plants occupy in general too much laboratory space, which all universities are short of, and even more troublesome, consequently have large time constants, what prevents them to operate in real time in student courses. Thus a laboratory scale air conditioning plant was designed by the TEMPUS group and built at the University of Ljubljana. An air conditioning plant was chosen because it is a multivariable, nonlinear plant quite often encountered in industry. The plant has two controlled outputs: air temperature and relative humidity which are influenced by two actuating elements for heating and humidification . Due to the crossconnections (increase of the temperature decreases the . relative humidity while humidifying decreases the temperature) the plant is a true multivariable system . After several experiments the following configuration shown in Fig. 1 proved to be acceptable : a ventilator conveying the air which serves as a transport medium , a heating coil for heating, a nebulizer for humidification of the air and a mixing chamber in which the outputs, i.e. temperature and relative humidity, are measured. With this setup the plant
(~_---,JLJ Mixing Nebulizer
Fig . 3. The air conditioning plant
I
-I Ohit':
Ventilator S - Temperature
Fig.!. Process scheme exhibits a lot of nice nonlinearties and other real world problems due to the underlying physics and the used actuating elements. The physical dimensions of the plant were designed to be very small to produce small time constants.
provided by a small compressor (20 W). A mixture of the heated and nebulized air enters the mixing chamber. The block diagram of the climate process is shown in Figure 2. The process is strongly nonlinear at first because of the quadratic relationship of heating power to the input voltage, second due to the nonlinear characteristics of the nebulizer and third due to the relation between temperature and humidity according to the Mollier diagram. Figure 2 therefore represents a Hammerstein type nonlinear model. The control paths can be modelled of four first order blocks with time delay. Their time constants and delays depend on the physical dimensions of the plant. The dynamic characteristics of the sensors are also represented as first order blocks. For pedagogical reasons and for more flexibility temperature and relative humidity are measured also at the air inlet and after the heating coil. It was decided to use two additional sensors for each variable, at the air inlet (19 1 , '1'd to the plant and at a point between the heater and nebulizer (19 2 , '1'2)' This enables more insight into the process and the implementation of cascade and feedforward control. The plant was build in a 19" rack as shown in Fig. 3. On the covering plate, which is closed down during transportation, the ventilator, the glass tube with built-in heater, the connections for the intermediate sensor and nebulizer and the mixing chamber are mounted. In the rack the compressor, the electronic interfaces and ac-
A suitable power for the heater for the process as it was realised, was found to be approximately 25 W. The heating coil is situated in a glass tube and the nebulizer is introduced downstream . The nebulizer is operated by compressed air which is 246
ffi li
u.
I
40 f
F--------cp
o
200
ffi j 40
cp
u.
L 0
ffiO
400
It
800
1000 Time [sI
\
~ Fig. 5. The working place
200
400
ffiO
800
1000 Time [sI
shown in Fig. 5. As basic CACSD software MATLAB with SIMULINK was chosen among others. A second laboratory scale plant for the control of water level and air pressure in a semi closed vessel will be installed in the laboratory in addition in the very near future . This second plant was designed in a preceding TEMPUS project by the TH Leipzig (Now FH Leipzig) .
Fig. 4. The step responses of the plant tuators are assembled . The front plate is divided into two parts: The left part represents the actuator 's part including manual- automatic switches , potentiometers for manual control and 0-10 V inputs . The heating and nebulizing inputs are equipped also with LCD displays, while the ventilator input used for changing the plant characteristics is not .
In spring 1994 the first student exercise will be performed with a smaller group of a students (46) . In this exercise the plant will be used as univariable plant where only the temperature part and relative humidity part will be used for two experiments separately. The objective of the exercise is to design and realise single variable PID controllers. First the step response will be evaluated and the PID controller parameters will be determined by Chien- Chrones-Reswick method and parameter optimization . First order process models with time delay are used in this case and the controllers will then be tested by simulations.
The right part of the front plate represents the sensors; for each measuring point the temperature and relative humidity is displayed normalized within 0-10 V outputs. Typical step responses of the plant are shown in Fig . 4. The upper part of the figure contains the responses to the step change due to heating while the lower part depicts step changes due to nebulizing. It can be seen that the nebulizing response is faster what is the consequence of the physical layout (nebulizer is mounted closer to the process output). The heating response is slower with the settling time about 300 s what is still acceptable for performing exercises in the frame of a student course.
In the next step the PID control scheme for the industrial controller will be designed by corresponding software package and downloaded . The controller configuration will then be tested in the hardware in the loop test where the supposed plant will be simulated on the computer and connected by AD and DA connecters to the standard industrial controller. In the last step the controller will be used on the real plant .
4. STATUS OF THE PROJECT 5. CONCLUSION In the first year of the project besides building the air conditioning plant also three CACSD stations joined in a local area network have been installed. All stations are equipped with analog to digital (AD) and digital to analog (DA) converters and combined with industrial standard process control units and programmable logic (PL) controllers as
The used combinations of laboratory scale plants , CACSD computers and industrial controllers enables a diversity of experiments such as process identification - controller design - validation of controllers with identified linear models by simulation - programming of industrial control units 247
- test of the programmed controllers as hardware in the simulated control loop with the identified linear models and finally the direct control of the real plant by the industrial control units. The detailed elaboration of CACSD courses remains to be done in the next two years of the project; a few ideas have been given in the paper . In the final stage of installation at least 9 CACSD stations will be installed with the above described equipment in CACSD laboratory enabling a realistic practice oriented teaching of control systems design to a group of twenty to thirty students simultaneously.
248
REFERENCES
TEMPUS (1992) . Financial support for cooperation and mobility In higher education between Central/Eastern Europe and the European Community. EC TEMPUS Office Brussels. Zupancic B., R. Karba, D. Matko, M. Atanasijevic - Kunc (1993) . Educational aspects of CAD supported real time control. IEEE Transaction on Education 36 No. 3, pp. 340-7.