Evaluation of Series Clutch Actuators With a High Torque-to-Weight Ratio for Open-Loop Torque Control and Collision Safety

For robots that act outside of tightly controlled environments, force control is often better suited than position control. Series clutch actuators, used without an elastic element, can achieve excellent position control or can be used as a force/torque source. Clutches based on different physical principles can be employed; while recently a lot of research has been performed on clutches based on magnetorheological fluid (MR fluid), they still have a relatively low torque-to-weight ratio, which makes the integration into robots difficult. In industrial applications magnetic particle clutches are preferred, as they have proven their reliability, but they are even heavier for the same torque. Friction clutches have a higher torque-to-weight ratio, but are generally considered to be difficult to control. The goal of this research is to evaluate series clutch actuators with a high torque-to-weight ratio for open-loop torque control and collision safety. First, the characteristics of electromagnetically controlled clutches based on friction, MR fluid, and magnetic particles are profiled. Subsequently, simple model-based open loop torque control is implemented and the hysteresis, step response, and frequency response are evaluated. The friction clutch has a higher hysteresis and torque variation than the other clutches, but a faster step response and a comparable frequency response. Collision experiments are conducted and the results show that also for collisions the torque can be controlled. Furthermore, in contact free motion, by using clutch settings that fulfill the dynamic torque requirements, unhindered position control is feasible, and by using torque limits that are only slighter higher than the dynamic requirements, safer actuation can be achieved. In conclusion, electromagnetically controlled friction clutches not only have a high torque-to-weight ratio (in our case 8 N \cdotm/0.32 kg), but are also a viable option for torque control.