An automatic controller, in general, compares actual output of a plant with that of the reference input (desired value) and produces a control signal that will reduce the deviation between the actual and desired values to zero or a small value. Figure 2.1 presents a block diagram of a typical control system that is integrated with magnetorheological (MR) devices. The controller detects the actuating error signal, which is usually a very low power level, and ampli„es it to a suf„ciently high level via the current ampli„er for MR devices. The actuator is a power device that produces plant input according to the control signal.