Discussion on Electro-Hydraulic Loading System with Negative Elastic Stiffness Based on Mixed Sensitivity Method

High-precision loading control of an electrohydraulic load simulator is affected not only by some unfavorable factors such as the nonlinear of flow-pressure of the servo valve and the parameter uncertainty, but also by the displacement of the load. In the condition of the negative elastic stiffness, the characteristics of system are affected by the variations of stiffness, both in its magnitude and sign. These variations may also make the system unable to work steadily. To solve these, firstly, the dynamic model of the electro-hydraulic system under position disturbance is established, the disturbance force expression is decomposed, and the compensator is used to reduce the effects of the disturbance force. Secondly, a mixed sensitivity method of using H
_∞
robust control theory is used for the compensation to the unstructured uncertainty of the system, and a robust controller is obtained by the help of the Robust Control Toolbox in Matlab. Finally, the simulation is implemented by combining the robust controller and the compound control strategy of the PID controller. The results show that the composite control strategy makes the system have good robustness and dynamic tracking performance. The amplitude error of output force tracking is not more than 3.2% and the maximum phase difference is -3.4°. Under the condition of the changes of hydraulic stiffness, the composite control strategy can quickly track the desired output force and the maximum steady-state error is within 1 %.