A Lightweight Force-Controllable Wearable Arm Based on Magnetorheological-Hydrostatic Actuators

Supernumerary Robotic Limbs (SRLs) are wearable robots augmenting human capabilities by acting as a co-worker, reaching objects, support human arms, etc. However, existing SRLs lack the mechanical backdrivability and bandwidth required for tasks where the interaction forces must be controllable such as painting, manipulating fragile objects, etc. Being highly backdrivable with a high bandwidth while minimizing weight presents a major technological challenge imposed by the limited performances of conventional electromagnetic actuators. This paper studies the feasibility of using magnetorheological (MR) clutches coupled to a low-friction hydrostatic transmission to provide a highly capable, but yet lightweight, force-controllable SRL. A 2.7 kg 2-DOFs wearable robotic arm is designed and built. Shoulder and elbow joints are designed to deliver 39 and 25 Nm, with 115 and 180{\deg} of range of motion. Experimental studies conducted on a one-DOF test bench and validated analytically demonstrate a high force bandwidth (>25 Hz) and a good ability to control interaction forces even when interacting with an external impedance. Furthermore, three force-control approaches are studied and demonstrated experimentally: open-loop, closed-loop on force, and closed-loop on pressure. All three methods are shown to be effective. Overall, the proposed MR-Hydrostatic actuation system is well-suited for a lightweight SRL interacting with both human and environment that add unpredictable disturbances.

Performance Study of Low Inertia Magnetorheological Actuators for Kinesthetic Haptic Devices

A challenge to high quality virtual reality (VR) simulations is the development of high-fidelity haptic devices that can render a wide range of impedances at both low and high frequencies. To this end, a thorough analytical and experimental assessment of the performance of magnetorheological (MR) actuators is performed and compared to electric motor (EM) actuation. A 2 degrees-of-freedom dynamic model of a kinesthetic haptic device is used to conduct the analytical study comparing the rendering area, rendering bandwidth, gearing and scaling of both technologies. Simulation predictions are corroborated by experimental validation over a wide range of operating conditions. Results show that, for a same output force, MR actuators can render a bandwidth over 52.9% higher than electric motors due to their low inertia. Unlike electric motors, the performance of MR actuators for use in haptic devices are not limited by their output inertia but by their viscous damping, which must be carefully addressed at the design stage.