  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.
