Research On Model Identification And Sliding Mode Variable Structure Control Of Hydraulic Position Servo System

Hydraulic servo system has been widely used in the fields of robotics, automobile,aerospace and defense industry because of its high response speed, large power volume, strong carrying capacity of heavy load. Hydraulic systems are often used as actuators, such as CNC machine motion control, rolling mill pressure control systems, and vehicle active suspension systems. However,due to the change of oil temperature,the non-linearity of the proportional valve and the friction force of the hydraulic cylinder, the high performance tracking control of the hydraulic servo system is a challenge task, which limits the application of the hydraulic system in the high performance servo field.Due to the uncertainties caused by time-varying parameters and external perturbation, the sy stem parameters are difficult to determine and are varying with the operating conditions.These time-varying uncertain parameters require the controller to have strong robustness and adaptability. Therefore, reducing the dependence of model parameters and improving the adaptive ability to time varying factors is the key task of high performance controller design.The mathematical modeling and model identification of Festo hydraulic position servo system are carried out. The model parameters identified are used as the nominal parameters in the subsequent controller design. For the Festo hydraulic position servo system, three kinds of PID parameters engineering tuning methods are used to initialize the PID parameters of the system. Firstly,the analog PID parameters of the system are preliminarily adjusted using the engineering methods. Then, according to the different control degree (i.e., sampling time), the parameter tuning of the digital PID controller is carried out, and the expanded engineering tuning table for the hydraulic position servo system is obtained. And the PID parameters obtained from the engineering tuning table can track three reference signals well.Both control law and the sliding surface of the traditional sliding mode control are investigated. A robust term based on the boundary layer is introduced in the control law to weaken the buffeting. Both linear sliding mode surface and nonlinear sliding mode surface are investigated. For the linear sliding mode surface, a sliding mode controller based on reaching law is designed. For the nonlinear sliding mode surface, two kinds of fast terminal sliding mode controllers are proposed for the hydraulic position servo control system. The Lyapunov function and the related lemma are used to prove the asymptotic convergence of tracking error and the stability of the designed controllers. And the effectiveness of the variable structure control methods is verified by using experiments.The combination of adaptive backstepping and sliding mode control method effectively overcomes the difficulty caused by parameter uncertainty, and improves the robustness of the system. In the design of adaptive sliding mode controller, the adaptive law of uncertain parameters is given, and the stability of the system and the convergence of the tracking error are proved by Lyapunov method. Compared with PID controller and sliding mode controller, this method has the advantages of high tracking accuracy, fast response.An output feedback adaptive control based on pole assignment is proposed for the linearized model of asymmetric hydraulic cylinders with proportional valve in the case of only output displacements being measurable. The global convergence of the tracking error can be ensured by constructing the observer and introducing the adaptive law. Experimental results based on the Festo hydraulic system show that the method only uses the system output information to realize the tracking control in the presence of measurement noise and unknown model parameters. The output feedback adaptive control method has higher tracking accuracy and faster response speed.Finally, the six control methods proposed in this paper are compared and the tracking performance is quantitatively compared. The experimental results show that the output feedback adaptive control method causes the output to oscillate due to the adjustment transition process, but the control precision is the highest after the transient procedure. The control accuracy of the backstepping adaptive sliding mode control method is preceded only by the output feedback adaptive control method. Due to the high frequency band of the hydraulic system, the high precision and the weak chattering of the sliding mode control cannot be achieved simultaneously. In order to suppress the chattering, the tracking error would increase.The tracking response of PID control method is slow and the tracking error is relatively large.