(A) study on improving the response of electro-hydrostatic actuation system based on hybrid actuation control

Abstract

In this dissertation, a new strategy for high performance of electro-hydrostatic actuation (EHA) system is studied in view of both control and circuit design. Recently, an electro-hydrostatic actuation system has been developed for mobile purpose from the advantage of high efficiency compared to conventional servo-valve controlled hydraulic actuation system. Nevertheless, due to the absence of pre-pressurized fluid, the basic hydrostatic actuation systems employing a single variable-speed prime mover and a single fixed-displacement pump have inherent limitations with relatively low response. On the other hand, the flow-source actuation systems inevitably involve mismatched disturbances as well as non-linear functions and system uncertainties. To address the problems, the dissertation proposes the EHA circuit with high response and high efficiency simultaneously using an additional high-pressure accumulator and presents a robust control approach by combining disturbance observer and backstepping controller based on identified nonlinear hydraulic system. First, the conditions for high response of a hydrostatic actuation system with passive auxiliary circuit for effectively compensating the volume difference between both chambers of a double-acting single-rod hydraulic cylinder is analyzed by bond graph approach. Accordingly, a hydrostatic closed circuit with an additional pressurized flow source is suggested. To identify the proposed circuit, with the aid of the augmented force state, a nonlinear dynamic model of minimum-phase is derived by the connection rule. Since the proposed circuit has three operation modes (EHA mode, hybrid actuation mode, charge mode) according to the position of each directional valve, the operational principle of each mode is analyzed and, especially, the operation of the high-pressure accumulator in the hybrid actuation mode is considered. Then, the validity of the induced mathematical model is verified by comparison with the experimental system. In the EHA mode of the proposed EHA circuit, a model-based robust control – a disturbance observer based robust backstepping control – is designed in state space. To begin with, a full-state disturbance observer is independently designed to estimate disturbances that include friction, load force, and parameter uncertainties. Motivated by the fact that the nonlinear system is well-identified in the conducted study, the backstepping approach is considered for reference tracking. Consequently, to reject the mismatched disturbance while guaranteeing the tracking performance, the disturbance-compensated backstepping controller is designed by adopting the Lyapunov function of the closed-loop system. As a result, it is shown that, when the disturbance estimation error is bounded to some extent, the proposed approach reveals stronger Lyapunov stability with a simpler design than ARC, and has stronger Lyapunov stability than that of DOB-SMC in case of successful derivative of the mismatched disturbance estimate. The effectiveness of the proposed approach is demonstrated by an application to the pump-controlled EHA system. In addition, to maximize the efficiency of the hybrid actuation mode considered as a multiple-input single-output (MISO) control system, a reference-based proportional-differential (PD) add-on controller is applied that provides an additional flow rate to achieve the required performance. Although the charge mode is essentially required to charge the discharged flow rate of the high-pressure accumulator, it is shown that the proposed EHA system is applicable to the mobile system that performs periodic movement such as gait of the lower-limb exoskeleton robot.