Design and experimental evaluation of a robust force controller for an electro-hydraulic actuator via quantitative feedback theory

This paper presents the design and experimental evaluation of an explicit f orce controller for a hydraulic actuator in the presence of significant sys tem uncertainties and nonlinearities. The nonlinear version of quantitative feedback theory (QFT) is employed to design a robust time-invariant contro ller. Two approaches are developed to identify linear time-invariant equiva lent model that can precisely represent the nonlinear plant, operating over a wide range. The first approach is based on experimental input-output mea surements, obtained directly from the actual system. The second approach is model-based, and utilizes the general nonlinear mathematical model of a hy draulic actuator interacting with an uncertain environment. Given the equiv alent models, a controller is then designed to satisfy a priori specified t racking and stability specifications. The controller enjoys the simplicity of fixed-gain controllers while exhibiting robustness. Experimental tests a re performed on a hydraulic actuator equipped with a low-cost proportional valve. The results show that the compensated system is not sensitive to the variation of parameters such as environmental stiffness or supply pressure and can equally work well for various set-point forces. (C) 2000 Elsevier Science Ltd. All rights reserved.