Time delay in a semi-active damper: modelling the bypass valve

Ride comfort and handling of off-road vehicles can be significantly improved by replacing the normal passive dampers in the vehicle suspension system with controllable, two-state, semi-active dampers. The hydraulic valve, which enables the semi-active damper characteristics to be controlled, is a critical component of a semi-active damper and has a marked influence on suspension performance. Models of the dynamics of a hydraulic bypass valve used on semi-active suspension systems for heavy vehicles were investigated. It is envisaged that similar models will eventually be incorporated into a full vehicle, three-dimensional simulation study. Valve response time (or time delay) is used as a measure of model accuracy because it is an important parameter in the performance of a semi-active damper. Models were created with AMESim, a commercial fluid power simulation environment, and MATLAB. AMESim was found to be capable of dealing with detailed and complex fluid power models. Attempts to solve models of similar complexity in the MATLAB environment were unsuccessful due to numerical stiffness. Experimental work was conducted to obtain dynamic performance data with which to validate model integrity. Several external factors influenced the valve behaviour during experiments. Test bench dynamics significantly influences results and obscures the absolute accuracy of the models and the experimental data. The investigation demonstrated an approach to creating fluid power models for this application that can be used in simulation, but also indicated that substantial effort is required in the process. The accuracy of the current model is not sufficient for design purposes.