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.
