Educational system for water level regulation: simulation and control.

Masnjak, Ivan ; Majetic, Dubravko ; Brezak, Danko 等

1. INTRODUCTION

The low cost laboratory setup for the purpose of education, analyzing and design of different control algorithms was developed and implemented in the Laboratory for automation and robotics of the Faculty of Mechanical Engineering and Naval Architecture in Zagreb (Petrlic et al., 2008). The concept of liquid level control in open water tank is selected due to its easy to understand dynamics properties and visibility for operators. The setup comprises from an open water container, a centrifugal pump, and various electronically controlled valves which provide desired water level disturbances. Water level control loop is closed via water pump and discrete-time PID control algorithm was applied. The paper outlines the design and ease usage of interactive graphical interface based on MatLab software (Masnjak, 2007).

2. SETUP DESCRIPTION

The experimental level control setup (ELCS) consists of eleven essential parts which are presented in the Figure 1.

These parts are as follows: 1. solenoid 2/2 valve, 2. gauge glass, 3. water pump which is activated by DC motor, 4. two-way valve with motor step drive, 5. level sensor, 6. control unit, 7. water tank, 8. - 11. connective hoses.

[FIGURE 1 OMITTED]

Among all these parts, control unit and two-way valve module were developed in the laboratory. All ELCS building parts are located into the casing which is made from transparent plastic i.e. Perspex, so the system properties are easy to note. Construction of casing is realized as the union of three chambers. In first chamber the electronics elements of control unit are located. The control unit enables the connection and control of main system parts (water pump, two-way valve, solenoid valves and sensor). By RS232 protocol the control unit communicates with PC. All drivers are written as an M-functions using MatLab software. Function of second chamber is electronics parts water isolation. Finally, all water is stored in third chamber i.e. the water tank. From water tank the water pump pumps the water and distributes it through the system. After passing throughout the gauge glass, two-way valve and 2/2 valves, the water returns to the water tank (recirculation process). DC motor is used to drive the water pump. The pulse-width modulation (PWM) of voltage signal is used for the pump power control. The two-way valve module is made from polished transparent plastic, so the separation of water flow is visible and easy to note. Its main characteristic is that it has one input and two outputs (Petrlic, 2008). By shifting the piston which is located inside the valve, the total volume of liquid that flows into the valve distributes in different rates at two outputs. The stepping motor is used for shifting the piston. The gauge glass (GG) is water tank in which the water level height is controlled. It is calibrated (from 100 mm to 300 mm with spacing of 5 mm) and it has four holes. The first hole is located at the front of the GG (water input). The second hole is at its bottom. It is drain that is always active, independently of the system state. The other two holes on the left and right side are responsible to solenoid valves connection to GG. As solenoid valves operate in simple on/off modes, they can be used to simulate disturbances. Water level height is measured by sensor which is implemented into the gauge glass.

3. GRAPHICAL INTERFACE OPERATING MODES

Two interactive graphical interface operating modes, named simulation and control are developed (Masnjak, 2007), based on Matlab software (Matlab, 2006). In both of them, simply by few mouse clicks, operator is able to monitor all variables in the control process and to manipulate with all elements of the level control processes. It is obvious that both modes must have the same dynamical behavior in time domain. Modes allow the operator to control the water level height in two possible ways, automatic by using discrete-time PID controller, or manual via water pump power adjustments or changing the openness of two-way valve. After stopping the operating modes, operator can plot graphs of level height h (mm) and water pump power P (%) with all monitored data from start to the end of each mode. Finally, some safety features are implemented to protect level sensor and the water leakage on the top of gauge glass.

[FIGURE 2 OMITTED]

3.1 Simulation operating mode

The first, simulation mode is given in Figure 2. In this mode simply by few mouse clicks operator can put the controller on/off, to choose the controller parameters, define height reference as desired water level, and switch on/off disturbance valves (PVL, PVD) at any instant of time. He can change the water pump power, or change the openness of two-way valve. By pressing the controller on/off button, the automatic or manual operating mode is chosen. If the controller is set to 'off, operator can control the water level manually. He can do that in three possible ways. The first way of controlling the water level height includes changing only the water pump power. During that, the position of two-way valve is fixed to an in advance preset value. The second way of controlling the water level height includes changing the openness of two-way valve, while the pump power is fixed to desired value. And finally, the third way implies combining of actions with water pump and two-way valve at the same time.

Existence of an accurate model of object dynamics is of crucial importance for the object control-related purposes (Isermann, 1996). In order to define the mathematical model of ELCS system, static and dynamic characteristics of system parts are defined (Lenart, 1995). On the basis of extensive measurements the mathematical equations for all elements are defined and mathematical model for ELCS is derived (Petrlic et al., 2008). For the purpose of control operating mode, the ELCC mathematical model given in Figure 3., is extended with discrete-time PID controller (Kuo at al, 2003) as follows:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (1)

where [K.sub.P] presents the proportional gain, [K.sub.i] integral gain, [K.sub.d] derivative gain and [T.sub.s] sampling time ([T.sub.s]=0.2s). According to Figure 3., controller input e(k) is defined as follows:

e(k) = hr (k) - h(k) (2)

[FIGURE 3 OMITTED]

3.2 Control operating mode

In the second operating mode, i.e. control mode, the operator can to do everything he could do in simulation mode. Instead of mathematical model he uses real level measurement on laboratory setup, and activates its real elements. On the screen picture (Figure 2) of interactive graphical interface, in control mode the term SIMULATION is replaced with term CONTROL. After the stopping of control mode, operator can plot the graphs of level height h (mm) and water pump power P (%) with all monitored data from start to the end of each mode. Such graph is given in Figure 4.

[FIGURE 4 OMITTED]

4. CONCLUSION

An experimentally supported work on design, identification, mathematical modeling and control of an educational level regulation system is given in the paper. For that purpose an interactive graphical interface based on Matlab software was featured. The program source can easily be modified and represents a good basis for easy plug in any other control algorithm. In the future work some modern control techniques such are the fuzzy or neural network control algorithms will be implemented by students. Such experimental setup helps students on mechatronics courses to learn how to make the control unit, how to connect it with PC, how to build up the simple graphical interface and finally to understand modern control algorithms.

5. REFERENCE

Isermann, R. (1996). Modeling and Design Methodology for Mechatronic Systems, IEEE/ASME Transaction on Mechatronics, Vol. 1, No. 1, March 1996, pp. 16-28, ISSN 1083-4435

Kuo, C. B. & Golnaraghi, F. (2003). Automatic Control System, John Wiley & Sons, Inc., ISBN 0-471-13476-7, New York, USA

Lenart, Lj. (1995). System identification Toolbox--For Use with MatLab, The MathWorks, Inc., Natick, USA Matlab, Graphics (2006), The MathWorks, Inc., Natick, USA.

Masnjak, I. (2007). BSc. Thesis: Interactive Graphical Interface for Laboratory Setup of Water Level Control, Zagreb, Croatia

Petrlic, D.; Majetic, D.; Novakovic, B. & Brezak, D. (2008). Educatinal Systm for Water Level Regulation: Design and Identification, Proceedings of the 19th International DAAAM Symposium, Katalinic, B. (Ed.), pp. 1073-1074, ISNB 978-3-901509-68-1, Vienna, October 2008, DAAAM International, Vienna