MR Devices with Advanced Magnetic Circuits
Magnetorheological (MR) devices such as dampers and clutches are usually driven by an electromagnet as the source of the magnetic field assuring a simple control of the damping force or the transmitted torque. However, the design of hybrid magnetic circuits containing additional permanent magnets or switchable hard magnets offers far-reaching perspectives in terms of fail-safe behaviour and energy efficiency. In this chapter, after a short introduction to the principles of MR fluids and magnetic circuits, MR devices with different magnetic circuit designs are described. The key feature of the design concepts is represented by the superposition of magnetic fields from different magnetic sources in the MR fluid. The combination of magnetic sources allows for pre-setting of the base damping force in a powerless operational state of the damper or to supply electric power only for an alteration of the damping force. According to these concepts of advanced magnetic circuits, MR dampers were designed, manufactured and investigated. The special damping characteristics of these new MR dampers are described and the influence of the MR fluid composition in terms of the concentration of magnetic particles is also reflected. The concept of hybrid magnetic circuits was extended also to rotational devices like MR clutches, whose transmitted torque is controlled by the magnetic field. The integration of permanent magnets in addition to the electromagnet allows it to exchange the operating states of engagement and disengagement of the clutch, which could increase the energy efficiency. MR clutches with such hybrid magnetic circuits were also developed and the results of experimental investigations are described.