Series Elastic Force Control for Soft Robotic Fluid Actuators
This work addresses force control challenges for lightweight and human-safe robots, representing an incremental improvement by applying series-elastic actuation principles to soft actuators.
The paper tackled the problem of achieving precise force control in soft robotic fluid actuators by using internal-pressure force feedback with a disturbance observer and model-based feedforward compensation, resulting in an active low-frequency endpoint impedance range of 50dB.
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.