Equivalent Time-Invariant Modelling of Electrohydraulic Actuators with Application to Robust Control Synthesis

An important aspect of robust control development around hydraulic actuators is establishing a set of equivalent linear time-invariant (LTI) models that describe the dynamics of the system over the desired envelope of operation. The nonlinearities inherent in the hydraulic functions must be recast into an equivalent linear form in order to make the robust control problem amenable to solution by linear techniques. This paper develops a simple model-based approach for evaluating equivalent LTI frequency response functions of an electrohydraulic actuator by Fourier transformation of acceptable actuator input-output data. The efficacy of the numerical procedure is compared with two other available methods, namely small-signal analysis and Golubev's least-squares approach. It is shown that the proposed approach can describe large signal effects and at the same time properly characterize the features of the hydraulic actuator frequency response that are important for robust control design, without the need for a priori information about the asymptotic behaviour or structure of the equivalent LTI transfer function. The applicability of the proposed numerical technique towards development of practical controllers for fluid power systems is demonstrated by the results of a typical robust control design example for an experimental electrohydraulic positioning system.