The Vehicle Dynamics Group (VDG) at the University of Pretoria has developed a semi-active hydro-pneumatic suspension system for an off road vehicle. The suspension system can switch its characteristics between two discrete spring characteristics as well as two discrete damping characteristics all incorporated in a single suspension strut. This original 4-State Semi-active Suspension System (or 4S4), switches between discrete characteristics through the control a set of solenoid valves. Recently, the 4S4 was further developed with the aim of extending its damping characteristics to be continuously variable through the use of Magneto-rheological (MR) technology. The newly developed MR4S4 prototype received a re-designed flow path which channels the MR suspension fluid through two independent magnetic valves (flow orifice enclosed by coils) in parallel. The damping characteristics of each of the valves are controlled independently by the application of electric currents through damper coils. These valves are also able to block flow completely to achieve the discrete spring characteristics through switching flow to the independent accumulators.

However, in order to ensure that this new technology could be effectively applied and controlled a model of the MR4S4 needed to be developed.

This paper describes the development and validation of a physics based model which is able to capture the overall dynamics and properties of the MR4S4 suspension system. Importantly, the aim of the research was to appropriately capture the physical properties of both the gas as well as the MR fluid as it interacts with the suspension displacements and forces. This model would aid further research in the development of control strategies and provide insight through simulation studies on the systems’ influence on vehicle dynamics.

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