선형화 모델을 사용한 유압 시스템의 제어에 관한 연구

Abstract

This study is largely categorized in two parts. The first part deals with a study on computational error problem of the conventional linearized equation for a servo valve. The second part handles a feedback control problem with a feedback linearization compensator. It is a specific feature of hydraulic control systems that the hydraulic systems have severe nonlinearity inherently compared to other systems like electric control systems. Therefore, precise modelling and analysis for the hydraulic control systems are not easy. The major reason for the nonlinearity is due to the nonlinearity of the servo valve in the systems. In the procedure of the hydraulic control system design, a linearized approximate equation described by the first order term of Taylor's series has been widely used. Such a linearized equation is effective just near the operating point. However, pressure and flowrate in actual hydraulic systems are usually not confined near an operating point but can vary in their magnitude and even in their sign according to the situation of load actuation. And, as of now, there are no general standard on how to determine the operating point of the servo valve or how to estimate computational errors in the process of applying the linearized equation. In the first stage of the first part of this study(in Chapter 2), the author evaluates the approximation errors of the existing linearized equation for a servovalve nonlinear flowrate characteristic. At first, the errors are evaluated on flowrate/pressure characteristics diagrams. Subsequently, they are investigated with time response simulation results for several hydraulic control systems. To enable systematic evaluation of computational error, the author proposes three kinds of equations with restructured forms of the existing linearized equation. As results of the evaluations, it is ascertained that comparatively good computational accuracy can be achieved with the existing linearized equation when both an operating point for the linearized equation and operating range of the hydraulic system stay near the flowrate axis of the flowrate/pressure characteristics diagram. In addition, the results show that comparatively big computational error may occur when operating range of a hydraulic system stay apart from the flowrate axis of the flowrate/pressure characteristics diagram. In the second stage of the first part of this study(in Chapter 3), the author suggests a new linearized flow equation for a servovalve as a modified form of the conventional linearized flow equation. Subsequently, a procedure to determine effective operating point for the new linearized equation is proposed. From the evaluations of time responses and frequency responses obtained from simulations for a hydraulic control system, the effectiveness of the new linearized equation and the procedure to determine effective operating point is confirmed. In the feedback control system with a hydraulic actuator, if the system is controlled using linear controllers, which are most common so far, it is not easy to achieve satisfactory control performances, as the linear controllers have to be dimensioned conservatively to ensure stability. As a countermeasure to overcome this difficulty due to hydraulic systems’ nonlinearities, applications of feedback linearization controllers to hydraulic control systems has been tried. But the research works with the controllers were not fully satisfactory in most cases, because the researchers did not consider the effects of disturbances and parameters' variations in systems. In the first stage of the second part of this study(in Chapter 4), the author applies a state feedback controllers incorporating a feedback linearization compensator to a hydraulic servo system. The focus of this study was set on the quantitative investigation of the effects(sensitivities) of disturbances and system parameters’ variation on control performances of the hydraulic control system. And, also, the author verifies the efficacy of a disturbance observer to overcome the control performances deteriorations due to system parameters' variations and disturbances in the control system. In the second stage of the second part of this study(in Chapter 5), the control strategies suggested in the first stage, using a feedback linearization compensator and a disturbance observer, are applied to a hydraulic control system for a vehicle suspension simulator. Although the hydraulic system has comparatively big external loads composed by constant and varying loads, it is ascertained that excellent control performances are obtained with the suggested control strategies.