The development of force actuators using ER and MR fluid technology

The most serious drawback of hydraulic actuators is the relatively high cost of the valves which control fluid flow. One possible solution to this problem is to exploit so-called electro-rheological (ER) fluids whose resistance to flow can be increased significantly through the application of an electric field. The increase in flow resistance is rapid (typically measured in milliseconds) and reversible upon removal of the electric field. Moreover, an ER flow-control valve involves no moving parts and does not require the fine tolerances associated with conventional hydraulic valves. One serious drawback associated with ER fluids is the relatively large voltages (typically up to 8 kV) which are needed to establish an electric field of sufficient intensity. One way of overcoming this problem is to utilise a so-called magneto-rheological (MR) fluids. These fluids consist of magnetizable solid particle in a carrier fluid of hydraulic oil. Not only do MR fluids offer yield strengths typically 20 times greater than those available from ER fluids but require only a low voltage supply (typically of 12 volts) to energise the magnetic circuit.