Development of normally closed type of magnetorheological clutch and its application to safe torque control system of human-collaborative robot

In this study, a normally closed (NC) type clutch using magnetorheological (MR) suspension and a rare-earth permanent magnet is proposed as a safety device for human-collaborative robots. This clutch can transmit an effective torque under various conditions and function as a holding brake and a changeable torque limiter. To design this clutch to achieve both good controllable characteristics and safety performance, the normally closed structure with a permanent magnet was realized by placing two facing magnetic poles asymmetrically for efficient generation of the magnetic field, and by adopting the multi-sheet rotor for the improvement of the torque output. As the analysis results show, it was confirmed that the three-sheet rotor obtained about 1.5 times torque output as much as the single-sheet rotor. When the exciting current was 0.8A, the transmitted torque marked the minimum; when the exiting current was 0A, the torque output of approx. 41 Nm was generated with the time constant of 0.15 s.