Robust Velocity Control for Electromagnetic Friction Brake Based on Disturbance Observer

This paper presents a method for robust velocity control of electromagnetic friction brakes for constant velocity (isokinetic) training in rehabilitation. Rehabilitation treatments using robots have become an attractive option due to labor shortages. In robot rehabilitation, isokinetic exercises are a form of highly efficient training. In rehabilitation machines, passive machines (brakes and clutches) are commonly utilized because they are inexpensive and safe. We focus on electromagnetic friction (EF) brakes which have a higher torque-to-weight ratio than magneto-rheological and powder brakes. Owing to such characteristics, the size and weight of the rehabilitation robots can be reduced by using EF brakes. Consequently, it can reduce the burden on subjects. Although EF brakes generate high torque, they have hysteresis and offset torque, which deteriorates the control performance. This paper proposes three contributions of an EF brake. First, the analysis and modeling of the EF brake are proposed. Second, a disturbance observer is proposed for the EF brake. The proposed method can design the disturbance suppression characteristics independent of the tracking performance. Finally, a robust velocity control scheme that considers the sign of the velocity for isokinetic exercises is proposed. The proposed method is compared with the conventional proportional-integral controller and its effectiveness is experimentally confirmed.