A model analysis of an active vehicle suspension system using hydraullic control

Abstract

The aim of this study was build and evaluate the performance of a laboratory active suspension. Much has been published on active suspension theory but relatively few have actually been built and reported on. This study examines the performance of two types of active suspension controllers and the performance of a hydraulic control system used to generate the active force. A quarter car model is used to represent the major dynamic modes for a simple active suspension. A Linear Quadratic Regulator (LQR) controller is designed with a weighted performance index. Two variants of this controller are studied: an optimal and a sub-optimal type with reduced state variable feedback. Of particular note is that an optimal controller is able to be produced without measuring the road input to the suspension. The controllers are optimised by minimising the performance index using the Ricatti equation. A "frequency shaped" LQR controller is also studied. This employs a conceptual filter with an output made up from system state variables. The filter is included as a cost function in the weighted performance index. Thus the conceptual filter characteristics influence the calculated optimum feedback gains and effectively implement a filter without the physical hardware and without the disadvantages of time delays that a physical filter would introduce. A laboratory suspension was built and tested. A main area of interest was the performance of the inner loop, which generates the active force required by the controller. This loop must faithfully reproduce the required force from an electrical signal. The loop did not perform as expected due to inadequate system gain available from the servo valve. This is despite the valve being adequately sized for the pressure and maximum flow rate demands of the system. A model of the system was developed and used to predict the pcrf01mance and to analyse potential system improvements. The results show that a redesigned servo valve with much greater flow gain is required in order to achieve a satisfactory performance.