Many vehicle suspension systems utilize damping devices or shock absorbers for controlling the vibrations or oscillations of the body and wheel due to road disturbances imposed on the mass-spring system of the vehicle body/wheel and suspension springs. A vehicle suspension damping device typically provides a resistive force proportional to the relative translational velocity between the body and the wheel. High performance controlled damping applications, such as those used in vehicle suspension systems, preferably provide a relatively low damping force at low speeds for comfort, and provide a relatively high damping force at high speeds for improved vehicle handling. It is known that such response characteristics can be provided by semi-active or active suspension systems, wherein the damping response of the systems can be continuously varied in real time in response to the dynamic conditions experienced by the vehicle using continuously variable real-time damping (CV-RTD) actuators. The use of CV-RTD dampers employing “smart fluids” (e.g., electro-rheological (ER) and magneto-rheological (MR) fluids) with continuously variable and controllable rheology and a fixed flow portion instead of moving mechanical valves with a variable flow portion have been proposed.
Magneto-rheological (MR) fluids consist of magnetizable particles (e.g., iron and/or iron alloy powders) suspended in an inert base fluid (e.g., synthetic oil). MR fluids typically exhibit Newtonian flow characteristics, with negligible yield stress when there is no external magnetic field. However, the yield stress of a MR fluid can be increased by several orders of magnitude by subjecting it to a magnetic field perpendicular to the flow direction of the fluid. This Bingham plastic behavior of MR fluid in an activated state is advantageous in creating actuators with controllable force or torque characteristics such as vibration dampers and clutches, without employing movable valves. Recent advances in material technology and electronics have renewed the interest in MR fluids for applications in smart dampers for fast and efficient control of force or torque (e.g., damping) in a mechanical system.