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An investigation into the suitability of magneto-rheological (MR) clutches in the context of developing feasible actuation solutions for physical human-robot interaction is presented. Contact and collision forces pose great danger to humans, and thus, the primary criteria for actuator development is safety. While the majority of existing solutions make use of mechanical compliance in some form, in this paper, we will approach the problem by considering the use of MR clutches for coupling the motor drive to the joint. The suitability of MR actuators to provide an intrinsically safe actuation platform is investigated by modeling the torque to mass, and torque to inertia ratios, as well as output impedance of the MR clutch. These figures are compared to commercially available servo motors as well as mechanically compliant based human-safe actuator models. The MR clutch is analytically shown to have superior mass and inertia characteristics over servo motors while either matching or surpassing the intrinsic safety characteristics of the modeled compliant actuator. The implementation of MR-clutch-based actuation systems is investigated by examining the distributed active semiactive approach. The proposed approach is discussed in terms of mechanical as well controller complexity and relates the investigation to the feasibility of practical implementations. Performance characteristics are empirically investigated by experimentation with a prototype MR clutch constructed for this purpose. The prototype MR clutch can transmit torque up to 75 Nm and has a bandwidth of 30 Hz. Torque to mass and torque to inertia ratios of the prototype MR clutch are substantially greater than that of comparable servo motors. Conclusions drawn from this investigation indicate that indeed MR clutch actuation approaches can be developed to balance safety and performance while maintaining reasonable system complexity.
Date of Publication: Dec. 2011