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Most mobile exoskeletons use DC motors as actuators and batteries as power sources. While DC motor consumes a lot of power and limits the working time of the exoskeleton, it produces large impedance that would cause discomfort and danger to people. Magnetorheological (MR) fluid is a smart fluid that can change its rheological behavior under applied magnetic field. Devices using MR fluids have the benefits of high torque, good controllability, low power requirement, and safety. In this research, we develop a new MR actuator for leg exoskeleton. The MR actuator consists of a DC motor and an MR brake/clutch. When active torque is desired, the DC motor works and the MR actuator functions as a clutch to transfer the torque generated by the motor to the leg; when passive torque is desired, the DC motor is turned off and the MR actuator functions as a brake to provide controllable passive torque. The results show that the adaptive control can track the needed torque/power of the knee joint very well, even under the variation of parameters.