  Precise and high-fidelity force control is critical for new generations of
robots that interact with humans and unknown environments. Mobile robots, such
as wearable devices and legged robots, must also be lightweight to accomplish
their function. Hydrostatic transmissions have been proposed as a promising
strategy for meeting these two challenging requirements. In previous
publications, it was shown that using magnetorheological (MR) actuators coupled
with hydrostatic transmissions provides high power density and great open-loop
human-robot interactions. Still, the open-loop force fidelity at low and high
frequencies are decreased by the transmission's dynamics and by nonlinear
friction. This letter compares control strategies for MR-hydrostatic actuator
systems to increase its torque fidelity, defined as the bandwidth (measured vs
desired torque reference) and transparency (minimizing the undesired forces
reflected to the end effector when backdriving the robot). Four control
approaches are developed and compared experimentally: (1) Open-loop control
with friction compensation; (2) non-collocated pressure feedback; (3)
collocated pressure feedback; (4) LQGI state feedback. A dither strategy is
also implemented to smoothen ball screw friction. Results show that approaches
(1), (2) and (3) can increase the performances but are facing compromises,
while approach (4) can simultaneously improve all metrics. These results show
the potential of using control schemes for improving the force control
performance of robots using tethered architectures, addressing issues such as
transmission dynamics and friction.
