Fixed-Order Output Feedback Control and Anti-Windup Compensation for Active Suspension Systems

Abstract

In this study, we apply an anti-windup scheme to the vehicle active suspension system. A fixed-order output feedback controller which does not explicitly take into account the actuator saturation constraint is first designed. Then, an anti-windup compensator is designed to handle the saturation constraint. Local control design technique based on the circle criterion and L 2 gain performance is used for the anti-windup compensator synthesis. A quarter car model is considered in this study and the effectiveness of the proposed approach is shown by a numerical example.The use of static-output feedback control for vehicle active suspension is proposed in Ref. (13) where genetic algorithm is utilized for the controller synthesis. In Ref. (14), the design of dynamic-output feedback controller for vehicle active suspension with the mixed H 2 /generalized H 2 based on LMI optimization (15) is presented. However, the controller's order is as high as the generalized plant. Recently, a fixed-order H ∞ control design using descent method in the controller gain space is presented in Ref. (16). This method treats the bilinear matrix inequality (BMI) that contains a Lyapunov matrix and the controller gain as variables directly. In addition, the effectiveness of the descent method compared with D-K iteration is shown in Ref. (17).In this study, we propose an alternative approach for the vehicle active suspension system. We perform some separation in the controller such that one part is devoted to achieve nominal performance and the other part is devoted to constraint handling. This is the approach taken in anti-windup compensation (18) . Anti-windup has been studied extensively over the past decades (see e.g., the survey in Refs. (18) -(20)). Originally, windup problems were encountered when using PI/PID controllers (19) where the integral states of the controller are wrongly updated. Later on, it is well known that windup not only occurs in integral controller, but whenever the controller contains badly damped or unstable modes. In addition, even in the absence of dynamic controller elements, saturation nonlinearities can also trigger undesired oscillations due to inappropriate plant state (21) . Therefore, by considering the behavior of the controller and plant states has potential for the performance improvement of the anti-windup compensated system (22), (23) . Furthermore, an attempt to use the anti-windup scheme for active vibration control is given in Refs. (24), (25) which shows the effectiveness of the anti-windup scheme for vibration isolation system. Because the improvement of the ride comfort can be seen as a vibration isolation problem, we aim to study the application of anti-windup scheme on the vehicle active suspension system.The anti-windup compensation involves a two-step design procedure. Namely, a controller is first synthesized for a nominal, usually linear system ignoring saturation. Then, the anti-windup compensator is designed such that, when saturation occurs, this compensator becomes active and leads t...

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