Design and experimental evaluation of robust controllers for electro-hydraulic actuators

This thesis presents the design and experimental evaluation of robust controllers for an electrohydraulic actuator that operates under significant system uncertainties and nonlinearities. The designed controllers allow the actuator (i) to follow a free space trajectory (motion control), (ii) to exert a desired force while in contact with an uncertain environment (force control), and (iii) to pass through the transition phase from free space to constrained space successfully and stably exert force on the environment (contact task control).; Firstly, a robust force controller is designed using linear and nonlinear approaches within the framework of Quantitative Feedback Theory (QFT). In the linear approach, the effects of nonlinearities and uncertainties, such as environmental stiffness and operating points, are accounted for by describing the linearized model parameters as structured uncertainties. The nonlinear approach is based on linear time-invariant equivalent models of the system that can precisely represent the nonlinear plant over a wide range of operation. The equivalent models can be generated by the nonlinear mathematical equations of the hydraulic actuator, or be obtained directly from input-output measurements of the actual system. Given the equivalent models/linearized model, a controller is designed to satisfy a priori specified stability, tracking and disturbance rejection specifications.; Secondly, the nonlinear QFT approach is used to design an explicit position controller to regulate the actuator in free space.; Finally, for contact task control, a simple switching condition is proposed based on robust position and force controllers, to make the transition from free space to a constrained one. The stability of the contact controller is shown using an extended version of Lyapunov's second method under the condition of existence and uniqueness of Filippov's solution.; The developed controllers enjoy the simplicity of fixed-gain controllers, are easy to implement, and at the same time are robust to the variation of hydraulic functions as well as environmental stiffness. Numerous experimental tests are performed on an industrial hydraulic actuator equipped with a servovalve and include motion through free space, contact with the environment and the transition between the two.