A Study on Control of an Electro-Hydraulic Servo Valve

Abstract

The performances of a hydraulic servo system are most influenced by the electro-hydraulic servo valves. Therefore, to improve performance of the system, it can be done by improving the performance of the servo valve. In the hydraulic control system, servo valves are classified into single-stage (direct drive servo valve (DDV)) and multi-stage servo valve. The DDV has a simple construction and inexpensive, but the capacity of this valve is limited by the steady state flow force in the spool. This problem can be solved by using a voice coil motor, but the total construction of this valve becomes bulky. Therefore the application of this valve in the mobile application, like in a mobile robot and an airplane is not convenience. To overcome this problem, two-stage servo valves are applied. Two-stage servo valves have a preamplifier (first-stage) which substantially multiplies the output force of the torque motor to overcome the total force in the spool-sleeve valve. The servo valve in this research is from SG servo with 63 lpm capacity. The frequency response of this valve is up to 60 Hz for 40% of full stroke. In this study, in order to increase the performance of the servo valve, the author designed and modified the servo valve by assembling an electrical position sensor to the servo valve. The electrical position sensor is assembled in order to eliminate the steady state error and to get a faster response of the valve. By using the sensor, a closed-loop control system to the valve can be realized. A dither is often used to improve the performance of the servo valve. The dither can reduce or remove the backlash of the servo valve. In the closed-loop control system, the dither signal can induce oscillation of the servo valve. To reduce or eliminate this phenomenon, a specific filter is needed to avoid dither signal feedback to the controller. Before applying a closed-loop control system to the servo valve, an appropriate model should be obtained. In 0, the model was built by using a second-order mathematical model and a third order mathematical model. The second and third-order mathematic models were built by using the experimental data of the servo valve. At first, a control strategy by using feedforward (FF) and input shaping control (ISF) controller to the PI-D controller is proposed. Application of the PI-D controller has a good performance of the servo valve. The response getting faster but the delay of the servo valve becomes higher. The feedforward controller is effective to eliminate the delay of the servo valve, but the feedforward controller stimulates a high gain in the control input of the servo valve. In this study, a high gain indication in the control input is eliminated by using the input shaping filter (ISF). The suggested controller is experimentally verified The second proposed controller is an integral sliding mode control (ISMC) combined Luenberger observer (LO). The ISMC is one of the most widely used because of its robustness to disturbance and parameter uncertainty. The servo valve itself has a big value of flow force as a disturbance and the servo valve model has a mismatch due to an uncertainty parameter of the servo valve. In order to realize the application of the ISMC in the servo valve, the state variable of speed and acceleration of the spool are needed. Measuring of all parameter is not a good choice to solve this problem. An observer can be applied as a mathematic sensor to this system. The observer in this paper is a Luenberger observer (LO). The suggested controller is verified experimentally. The second proposed controller is an integral sliding mode control (ISMC) combined the Luenberger observer (LO). The ISMC is one of the most widely used because of its robustness to disturbance and parameter uncertainty. The servo valve itself has a big value of flow force as a disturbance and the servo valve model has a mismatch due to an uncertainty parameter of the servo valve. The third proposed controller is a sensor-less controller by applying an input shaping filter (ISF) and a feedforward (FF) to the servo valve without taking feedback from the output system. In applying FF and ISF, state feedback from the system is not needed, therefore it will give a big advantage to the servo valve. The dynamic characteristics of the valves are generally evaluated by Bode diagram/ frequency response. Servo valve has a wide range of the frequency response, therefore a fast measuring method is needed (small sampling time). In order to get real-time and high-frequency data, a Simulink Real-Time (SLRT) is applied in this experiment.