Magneto-rheological (MR) Damper design for off-road vehicle suspensions with flow blocking ability

Abstract

This study presents the design, development and testing of a magneto-rheological (MR) damper for an off-road vehicle. The MR damper developed in this study is expected to enhance the capability of the suspension system by allowing variable damping due to inherent properties of the MR fluid. MR fluids exhibit a reversible behaviour that can be controlled with the intensity of an external magnetic field, causing a change in the effective viscosity and thereby also in the forcevelocity characteristics of the damper. The characteristic of this damper design distinguishes the damper developed in this study from the conventional damper used in the suspension system of a passenger car by having a controllable damping characteristic instead of a fixed one. The damper is designed to be used on the hydro-pneumatic 4 State Semi-active Suspension System (4S4) that was developed by the University of Pretoria. Another additional feature of the damper developed in this study is the ability of the damper to act as a fluid flow blocking device so as to enable semi-active control of the suspension springs as required for the 4S4 system. A mathematical model of the proposed damper has been developed using a modified Bingham plastic model. This model is used to determine the necessary geometry for the damper designed in this study, using the fluid flow rate and current to the electromagnet as the two main input variables. Furthermore, the model is used to compute the pressure drop over the MR valve as a function of the coil current and fluid flow rate. For manufacturing and size considerations, the proposed design incorporates a triple pass layout valve-mode damper with the MR fluid flowing through the three passages that are arranged in an S-shape so as to minimize the cross section of the electromagnet core while maintaining the required width and length of the valve to perform adequately. An experimental setup has been built to test the validity of the analytical model. Experimental results confirm the applicability of the analytical model to accurately predict the pressure drop over the MR valve for given operating temperature and fluid properties. Experimental results show that the original MR damper designed for this study provides the required range of damping as compared to the reference system used for this study, with the only error being that the values of damping, both in the on-and off-state, are lower than expected. By using the validated theoretical model it has been seen that the whole range of damping characteristics can be shifted upwards by reducing the MR fluid passage height inside the valve which should yield the required characteristics.