Hydraulic Actuator Control Using an Multi-Purpose Electronic Interface Card

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1 Hydraulic Actuator Control Using an Multi-Purpose Electronic Interface Card N. KORONEOS, G. DIKEAKOS, D. PAPACHRISTOS Department of Automation Technological Educational Institution of Halkida Psaxna 34400, Evia, Greece GREECE gedikeakos@gmail.com web address : Abstract: - The primary aim of this project is the implementation of a multi-purpose electronic board (card), which will host a controller and will act as an advanced interface design, between a user and a system. The last one is a hydraulic positioning system, which consists of a double acting hydraulic actuator operated by a hydraulic servo valve and an appropriate oil pump. The selection of the interface control card and its components are one of the most important issues of the project. Low cost electronics, reliability and versatility are basic criteria of the card design. The secondary aim of this project is to control the position of the hydraulic piston using the classical three term control method (PID). The details and characteristics of the system structure are given and experimentation process results conclude this paper. Key-Words: - Hydraulic Actuating System, Digital PID Controller, Steady State Error, Interface Control Card Design, A/D and D/A Conversion, Microprocessor Programming. 1 Introduction Since 1795 several engineers and inventors have added their contribution to this scientific field of science that deals with the subject of forces exerted on fluids or fluid dynamics. The history of hydraulic systems is found in dam design and engineering. It is found in the field of automobile, aviation, bicycles and railway. It is found in military applications and space exploration and in other disciplines where fluid circuitry is used such as turbines, pumps, and hydropower. The history of hydraulic systems is found in the current development of the computer where computational fluid dynamics is a buzz term. Hydraulic systems are used for providing high torques and high forces with a high level of control of the motion. Hydraulic fluid is virtually incompressible so controlling the flow of fluid provides accurate control of the motion of the relevant actuator. The advantages of hydraulic systems are the convenient power transfer, flexibility and variable speed control. The system has nonlinear dynamics, such as the pressure difference entering the cylinder chambers, friction forces and energy loses inside the cylinder body that make the performance of the system difficult to be perfectly controlled. The importance of the easiest controller implementation in order to achieve minimum overshooting and high position accuracy leads to the use of a PID controller. The most important part of the system is an interface board which will host the controller and all electronics peripherals. The interface card will be designed and assembled in order to be used for long-term operations of the system. The criteria for choosing these specific microprocessor and electronics are low cost, reliability and multitasking capabilities. The ease of use in terms of programming was taken under consideration since the hydraulic positioning system requires long control algorithms to be compiled by the microcontroller. One of the interface board main purposes is also data acquisition and real time data monitoring on a computer screen. This paper is organized as follows: Part 1 is an introduction to the hydraulic systems and interface control cards. In parts 2 & 3, the hydraulic positioning system description and the control method implementation are given, respectively. Finally, in parts 4 & 5 follows the electronic interface board parts and assembly details and the ISBN:

2 results with some further future applications will be discussed. 2 System Description The hydraulic positioning system under investigation consists of a double acting hydraulic cylinder (Differential cylinder 16/10/200), stroke of 200mm, combined with a hydraulic proportional control servo valve (4/3-way solenoid valve with closed mid-position). The micro controller will have to read the current position of the cylinder and send the result value to the computer, in order to minimize the error. It will also be responsible to convert the control signal from a digital word to an analog signal. The position sensor is a Linear Variable Differential Transformer, (model ACT1000C). In the following figure the main layout of the system with all its parts connected is provided. valve is shown on Figure 2. The mathematical model of the hydraulic cylinder and the valve consists of second order system equations and in fact they are also switching depending on the oil pressure, see [1]. Fig 2. The servo valve operation graph The mathematical model of the hydraulic positioning system is as follows: Equation of movement Continuity equation of the oil pressure in cylinders chambers Servo valve equation Fig 1. The system main layout It is obvious that the feedback signal of the system is the actual piston position provided by the LVDT. The output of it is the actual piston position and there is a control signal in Volts produced by the microprocessor as a result of the control algorithm implementation that drives the servovalve. The last one is the control device of the plant, which operates the whole system under the controller s commands. The valve opens and closes its ports in order to governs the oil flow in the two cylinder chambers and therefore move the piston. The Pressure-drop/flow-rate characteristic of the Where, m is the piston & rod mass, V is the velocity pf piston rod, A area of the piston, V 1 & V2 are the volumes of hydro cylinder chambers, K is the pump control element (swash plate) gain coefficient (m 3 /rad/deg), x is the displacement of piston, F F is the Friction Force (F F =b*dx/dt, b is the Coulomb friction constant coefficient), Y V is servo valve displacement, t time, C e & C i are the lumped external and internal leakage coefficients respectively, E is the liquid bulk modulus of elasticity (constant), P 1, P 2 are the pressure at the piston and rod chambers, Pa and finally A 1, A 2 are the areas of forward and return stroke of the piston (m 2 ). It must be stated that although 100% position accuracy of hydraulic piston displacement has been achieved in earlier research studies, there is still area for improvements like the one in this project. ISBN:

3 3 Control Method The aim of this project is to test the performance of the system while operating under the influence of the interface card, combined with a classical threeterm controller. The importance of the easiest controller implementation in order to achieve minimum overshooting and high position accuracy leads to the use of a PID controller. Depending on the requirements of the controlled system it can be used one, two or all three parameters of the controller. The input of the controller is the position error, the difference between the demanded input signal and the feedback signal. The output is the control action that is the input of the controlled system. Proportional controllers (P) just multiply error with a constant, called proportional gain (Kp). This gain leads the system to adjust faster into the demanded signal. It controls the speed of response. The control action is proportional to the actuating error signal. Proportional control gives speed to the system so as to react to any changes of the input. If the gain is high enough then the system oscillates before settling. To increase the damping of the system, the user must add the Derivative control term. This means that when the rising time is being smaller the settling time is greater, as the proportional gain is increased. So, D control is reducing the settling time. The transfer function of PD controller is a mutilation of the derivative gain and the difference between the previous and current error. The control action is proportional to the derivative of the actuating error signal. The only thing that the user can t achieve with a PD controller is to nullify error. That is why Integral controller is used. If the demanded accuracy is really high then an integrator must be used in order to have the provided control. The transfer function of PI controller is a mutilation of the integral gain and the addition of all errors. Because of the three parameters the tuning of the gains is really difficult. The control action is proportional to the derivative and the integral of the actuating error signal, see [2]. 4 The Interface Board In order to be able to control and record the behavior of the hydraulic plant, an interface card was designed and assembled. This should be able to convert the analog and continuous signal into digital words and also the digital input into an analog control signal to drive the hydraulic servo valve. Criteria like low cost electronics and multiple power supplies for the micro controller and its peripheral equipment should be kept. The hydraulic valve response time is need to be small enough so the interface card should be faster than that to drive the valve properly. The controller reads the current position of the hydraulic piston and corrects the input of the system according to the control algorithm in order to minimize the error. The position sensor, the LVDT, produces an analog signal of 0-5 Volt. An Analog to Digital Converter is used, specified to the highest accuracy for such a signal, see [3] and [6]. A Digital to Analog Converter transmits the digital control input into a continuous signal. The interface board consists of four main stages. The first is the micro controller (µc) stage, which is the AT89C51. The second stage is the 5V input Analog to Digital Converted (ADC). The third one is the 10V input Analog to Digital Converter. The fourth stage is the 10V output Digital to Analog Converter (DAC). The microprocessor, a flash AT89C51, is responsible to synchronize the communication between the sensors and the computer. The serial interface and the control of the converters are being controlled in this stage. The clock frequency is selected to be MHz so it can be divided exactly for the RS-232 serial communication. A circuit breaker, which is shorted on power up, is used for downloading every new program code to the micro controller. For the second and third design stages, the Analog to Digital Converters, a ADC bit µc compatible is used. This is a common ADC, with excellent characteristics. It s a successive approximation A/D converter that uses a potentiometric ladder. This converter appears as a memory location to the Input-Output ports of the µc and so no interfacing logic is needed. In addition to this, the voltage reference input can be adjusted to allow encoding any smaller analog voltage span to the full 8bit resolution. Another significant feature is that this converter has an on-chip clock generator and the conversion time is 100µsec. For the 10V input ADC stage the same converter is used with just a voltage divider added to the input of the converter, see [4]. The fourth stage of the interface control board is the DAC0830, which is used as a Digital to Analog converter in a voltage-switching configuration. In this configuration the ladder is operated as a voltage-switching network and not as the standard current switching. The reference voltage is ISBN:

4 connected to one of the current output terminals and the output voltage (Vref) is provided by the normal reference pin of the micro controller. The converter output is a voltage in the range from 0V to 255*Vref/256 as a function of the applied digital code. In this configuration the applied reference voltage must be always positive to prevent unacceptable behaviour. There is also a dependence of conversion linearity and gain error on the voltage difference between the supply voltage and the voltage applied to the normal current output terminals. This is a result of the voltage drive requirements of the ladder switches. Fig 3. The interface control card layout The power supplies voltages needed for the Interface Board are +5V, +15V and -15V. A power supply providing all these voltages is included on the card. If an external power supply is to be used then the card can be further minimized and the part with the power supply equipment can be removed. Then the voltages needed for the interface board can be supplied to it through a power supply connector. Assembling this kind of boards with ADCs and DACs, it is critical to design in a certain way the power lines, especially ground connections, to ensure proper operation. In this board, special care has been given to this. A brief description of the board layout is illustrated in Fig 3. At point 1 the microprocessor is shown and 2 is the RS-232 serial port that allows the card to computer communication. The point 3 of the card is the output control signal, which is sent to the valve while point 4 is the input of the board. Via this input all sensor signals and data are sent to the system controller for processing, see [5]. ISBN:

5 5 System Performance and Future Applications A very large number of experiments were designed in order to evaluate the benefits of such a technique in the hydraulic actuator position control problem presented here. The idea was to test the behaviour of the system over long time periods on different days. The subject of this investigation was to record how the optimal values for the three gain values (Kp, Ki, Kd) change over time so that the system operates with the best position accuracy. A large number of tests with the hydraulic actuator took place using the PID controller. In each different test the controller was manually retuned in order to record the values of the Kp, Ki and Kd gains that provided the best system response. Ten different experiments were designed to cover the range of the gain changes. The first experiment is starting with values P=0.61, I = 0.37 and D=1.28. It seems that the systems response has a very big error, which in order to be minimized the PID controller s parameters must be tuned, as illustrated in Fig 4. In the second experiment we increase the PD parameters because we aim in a better speed, smaller settling & rising time and the smallest overshoot. Also, these two parameters need to be properly combined in order to give a good control signal. In the third experiment we have a very small overshoot, and the system settles without error at 15.3msec. In the forth experiment the values at I and D parameters are lower. As a result the system has smaller overshoot and it settles at 15msec. In the fifth experiment it appears to have oscillation, an unstable response. When the oscillation is unreduced, then the gain is called critical. On the other examples we can see some typical calibration of the parameters, in order to find the best value match for the specific system. With small changes in the parameters the error range from -2mm to +3mm, and the settling time from 15msec to 19msec, as illustrated in Fig 5. Fig 4: Experiment 1 Fig 5: Experiment 10 During experimentation and in an attempt to summarize the system performance, it is obvious that the three-term (P-I-D) controller implemented here is not operating in highly satisfactory levels. The system non-linear dynamics, such as the pressure difference entering the cylinder chambers, friction forces and energy lose inside the cylinder body make the performance of the system difficult to be perfectly controlled. The overshooting behaviour and in some cases the unstable performance of the positioning system indicates the need for further control methods investigation Finally, but more importantly, a universal and multipurpose interface control board was implemented in the system. The interface card of this project has proven its reliability in long-term operations throughout experimentation process and therefore can be used in many positioning systems of this type. Its low cost electronic parts and assembly operated with zero problems in all regimes of the system performance. The ease of programming characteristics of the card track attention for further applications since the dimensions are small enough to be attached in various system constructions. At the top of all, not only a reliable and low cost interface card was designed but also a classical ISBN:

6 control approach was implemented in this combinational research work presented in this paper. The outcome, although acceptable, still leaves space for a different control method to be adopted, like, perhaps, an Intelligent Control Method. The implementation of such a method does not require an upgrade in the hardware of this specific interface card and this fact is the most important advantage of it. The micro controller source code, as the software part, is the only bit that needs to be re-designed in order to adopt the new control method algorithm. This fact proves the versatility of the interface board designed and built for the requirements of the present research project. References: 6 [1] Juma, Y, A, Mathematical Modeling for Pump Controlled System of Hydraulic Drive Unit of Single Bucket Excavator Digging Mechanism, Industrial Eng. Dept., Palestine, September, [2] Khol, R., editor, Electrical & Electronics Reference Issue, Machine Design, Vol 57, No 12, May 30, [3] Benjamin C. Kuo, Digital Control Systems, Second Edition, New York: Oxford University Press. [4] J. Proakis and D. Manolakis, Digital Signal Processing: Principles, Algorithms, and Applications, New York: Macmillan Publishing Company, [5] Katsuhiko Ogata, Discrete-Time Control Systems, Second Edition, University of Minnesota, Prentice-Hall International. [6] Charles L. Phillips & H. Troy Nagle, Digital Control System Analysis and Design, Third Edition, Prentice-Hall International. ISBN:

Position Control of a Hydraulic Servo System using PID Control

Position Control of a Hydraulic Servo System using PID Control ABSTRACT Dechrit Maneetham Mechatronics Engineering Program Rajamangala University of Technology Thanyaburi Pathumthani, THAIAND. (E-mail:Dechrit_m@hotmail.com)