  Fluid-based soft actuators are an attractive option for lightweight and
human-safe robots. These actuators, combined with fluid pressure force
feedback, are in principle a form of series-elastic actuation (SEA), in which
nearly all driving-point (e.g. motor/gearbox) friction can be eliminated.
Fiber-elastomer soft actuators offer unique low-friction and low-hysteresis
mechanical properties which are particularly suited to force-control based on
internal pressure force feedback, rather than traditional external force
feedback using force/tactile sensing, since discontinuous (Coulomb) endpoint
friction is unobservable to internal fluid pressure. However, compensation of
endpoint smooth hysteresis through a model-based feedforward term is possible.
We report on internal-pressure force feedback through a disturbance observer
(DOB) and model-based feedforward compensation of endpoint friction and
nonlinear hysteresis for a 2-DOF lightweight robotic gripper driven by
rolling-diaphragm linear actuators coupled to direct-drive brushless motors,
achieving an active low-frequency endpoint impedance range ("Z-width") of 50dB.
