Research On The Control Strategy Of Electro-hydraulic Position Servo System With Variable Spring Stiffness Load

The electro-hydraulic position servo system will be essentially unstable under the spring load with a negative value.Although the existing correction strategies can achieve the stability of the system under the influence of the spring load with a negative value,the system still cannot achieve the quick and accurate tracking of the output signal to the command signal in the case of large changes in the spring load.In this paper,the electro-hydraulic position servo system is taken as the research object,which mainly deals with the problems of poor system stability,narrow bandwidth and low control precision caused by load spring variation and disturbance of external load force.The Lyapunov direct method is adopted to make the system stably track the command signal stably under the condition of the change of load spring and the disturbance of the external load.The main work of this paper is as follow:(1)The flow in the large-diameter standard butterfly valve was taken as the research object,the aerodynamic torque characteristic of valve plate during the dynamic process of the opening of the valve plate was solved by means of CFD software and its dynamic mesh technology: With the opening of the butterfly valve,the characteristic curves of aerodynamic torque of the valve plate under different inlet pressure conditions are monotonically increasing first,after reaching the maximum magnitude,the aero-dynamic torque characteristic curve began to monotonically decrease.Then the conclusion that the load spring is a parametric variable related to the actuator displacement was gotten by this typical torque characteristic.(2)The working principle of the electro-hydraulic position servo system was described,and the mathematical model of the whole system was sort out by combining the mathematical equations of the hydraulic power mechanism and the electrical control part of the system.Through proper linearization of the system,the electro-hydraulic position Closed-loop transfer function of servo system and system control block diagram were summarized.(3)The mathematical model of the electro-hydraulic position servo system was analyzed,and the mathematical model was factorized into a standard form,and the expressions of important parameters such as the integrated natural frequency and the comprehensive damping ratio of the system were derived;The effects of changes in load spring and the disturbance of the external load on the system characteristics were discussed.(4)The state space expression of the system was derived based on the closed-loop transfer function.The asymptotical stability condition of the system was solved through the inversion of Lyapunov direct method,and the generality of the control strategy was given.Based on the parameters of a typical electro-hydraulic system,a differential control compensator in series in forward channel combine with a mechanical-hydraulic compensator with adjustable hydraulic resistance attaching to actuator were constructed with the control strategy obtained by the Lyapunov direct method.The control strategy proposed in this paper was realized by comparing the order of magnitude and simplifying the controller parameters.(5)The numerical simulation of this typical hydraulic system under different conditions was performed by employ of Simulink software.The results show that,compared with the dynamic pressure feedback correction,after adopting the composite control including the differential control compensator in series in forward channel and the mechanical-hydraulic compensator with adjustable hydraulic resistance attaching to actuator,the amplitude margin of the system was increased to 45.6dB and the phase margin was increased to 88.9°.After the system adopted composite control,the idling steady-state error was less than 0.01%,and the response of the system under different characteristic loads was almost the same.The adjustment time was not more than 0.03 s,and the steady-state error was not more than 0.15%;under positive and negative alternating load,the system could reach steady state within 0.02 s,the steady state error was less than 0.183%,and the dynamic characteristics and stability margin of the system were greatly improved.