Adaptive Control Of Pneumatic Position Servo System

Pneumatic system has been widely applied in industrial automation fields such as medicine,micro-electric,petrochemical industry for its advantages of simple structure,high power-to-weight ratio,safety,cleanness.However,due to the special characteristics of pneumatic system,such as the condensability of air,friction of pneumatic cylinder,nonlinear gas flow in the servo valve,etc.,it is difficult to achieve high performance tracking control,as a result,its usage in the high-performance servo drive area is limited.Along with the performance improvement of servo valves and the microprocessors,and their cost reduction,it is possible to obtain higher precision,the research on using control methods to improve the tracking performance is meaningful for expanding the application field of pneumatic systems.Due to manufacturing error and inaccurate measurement,it is difficult to obtain precise model parameters of pneumatic servo system.It is a research focus for high performance control to reduce dependence on model(and parameters)and enhance the capacity of adaptation.Pneumatic system demonstrates the characteristics of fractional order system,so fractional order controller is expected to get a better control performance.In order to improve the tracking precision,and reduce the energy consumption of pneumatic position servo system,three controllers are proposed:All existing pneumatic system control methods must assume that the control gain sign of system is known,and most of the current adaptive control methods would not work when the sign of control gain is unknown or changeable.An adaptive control is designed for pneumatic servo system with unknown control gain by using backstepping methods and Nussbaum function.The proposed adaptive method doesn't require the model parameters and the control gain direction.Using Lyapunov stability theorem and related lemmas,the stability of designed adaptive controller is proved,and the tracking error is bounded and convergent.The experimental results demonstrate the effectiveness of the proposed approach.Pneumatic system involves gas dynamic,and researches show that fluid dynamics involves the characteristics of the fractional order,consequently,a hypothesis that a fractional order controller would obtain better performance than the integer order controller.To testify this hypothesis,a fractional order sliding mode controller based on exponential approach law is proposed for pneumatic position servo system.The proposed method uses the fractional order calculus theory to design the sliding mode surface.The stability of the controller is proved by using Lyapunov theorem.The experimental results demonstrate the effectiveness of the proposed approach.Fractional order PID controller is an extension of the traditional integer order PID controller.It is more flexible than the integer order PID controller,and it is expected better performances,especially for the fractional order system.However fractional order PID controller has more parameters,the adjustment of parameters is more difficult,however,the existing methods seldom treat this difficulty.In order to solve this problem,a genetic algorithm method based on Pareto rank is applied to optimize the parameters of the fractional order PID controllers online.The fractional order PID controller with optimized parameters is used to control pneumatic servo system.The experimental results demonstrate the effectiveness of the proposed approach.Finally,the quantitative comparison of the proposed method and the methods in existing references is conducted.The experimental results show that the adaptive controller system can effectively track the three kinds of reference signals when the control direction of pneumatic position servo system is changed.The tracking errors of all proposed methods are relatively small,as compared with some existing methods.The tracking precision of the fractional order sliding mode controller and fractional order PID controller are improved significantly,at the same time,the energy consumption reduced substantially.The fractional order PID controller with the optimized control parameters derived by the proposed genetic algorithm can obtain better control performance,in the sense of higher accuracy and less energy consumption,as compared with integer order PID controller with the optimized parameters obtained by the same genetic algorithm.These results have important application potential in the field of high performance tracking control of pneumatic servo systems.