Controllable Electrorheological Fluid

A mechanical actuator system has variable and controllable mechanical impedance. Such a mechanical actuator system may be used to effectuate a degree of freedom in a robot, i.e., to control speed, output torque and direction of movement of a robotic component, such as a joint, wheel, arm, wrist or grabber. Mechanical impedance, i.e., an amount of “resistance” the robot presents to a human user, can be controlled for safety and rehabilitation purposes. The mechanical actuator system includes a mechanical differential and two adjustable-engagement clutches driven by motor. Advantageously, the motor may turn at a constant speed and direction, yet the mechanical actuator system can be controlled to turn in either direction and at a desired speed. The adjustable-engagement clutches may be electrorheological (ER) fluid clutches, magnetorheological (MR) fluid clutches, conventional dry friction clutches or any other type of clutch whose degrees of engagement can be controlled.