[Paper Review] Series Elastic Force Control for Soft Robotic Fluid Actuators

Chunpeng Wang, John P. Whitney|arXiv (Cornell University)|Apr 2, 2020

Prosthetics and Rehabilitation Robotics34 references4 citations

TL;DR

This paper presents a series elastic force control framework for soft robotic fluid actuators using internal pressure feedback and a disturbance observer (DOB) to achieve high Z-width impedance control. By combining DOB-based force feedback with model-based compensation for smooth hysteresis and friction, the system achieves a 50 dB Z-width up to 3 rad/s, enabling both high stiffness and near-zero impedance operation without external force sensors.

ABSTRACT

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.

Motivation & Objective

• To enable wide-range endpoint impedance control in lightweight soft robotic grippers using fluid-actuated series-elastic systems.
• To eliminate reliance on external force sensors by using internal fluid pressure as the primary feedback signal for force control.
• To compensate for smooth endpoint friction and hysteresis without endpoint state or external force sensing.
• To achieve high Z-width (impedance dynamic range) for safe, compliant interaction with delicate environments.
• To validate the effectiveness of internal pressure feedback and model-based compensation in a 2-DOF robotic gripper with fiber-elastomer actuators.

Proposed method

• Internal fluid pressure is used as a proxy for joint torque, enabling force feedback without external sensors.
• A disturbance observer (DOB) is applied to estimate and reject disturbances using internal pressure and motor state feedback.
• A model-based feedforward term compensates for smooth hysteresis and viscous friction at the endpoint, derived from system identification data.
• The system uses direct-drive brushless motors coupled to rolling-diaphragm linear actuators with purposeful fluid/hose compliance to create series elasticity.
• The DOB is adapted for fluid-actuated systems to estimate external forces and endpoint impedance in real time.
• System identification is performed to model the actuator dynamics, including nonlinear friction and hysteresis, for accurate feedforward compensation.

Experimental results

Research questions

• RQ1 Can internal fluid pressure alone provide sufficient force feedback for high-performance impedance control in soft robotic actuators?
• RQ2 To what extent can smooth endpoint friction and hysteresis be compensated without external force or position sensors?
• RQ3 What is the achievable Z-width of endpoint impedance using internal pressure feedback and DOB-based control?
• RQ4 How does the combination of series elasticity and model-based compensation enable both high stiffness and near-zero impedance operation?
• RQ5 Can the system achieve backdrivability comparable to ideal zero-impedance systems using only internal pressure feedback?

Key findings

• The system achieves a Z-width of approximately 50 dB up to 3 rad/s, demonstrating a wide dynamic range of endpoint impedance control.
• The maximum Z-width is limited to ~70 dB at DC in the rigid-hose case, but the compliant-hose system achieves 50 dB due to volumetric compliance.
• The gripper exhibits passive backdrivability with motors off, achieving an impedance level consistent with near-zero stiffness (yellow curve in Fig. 7-C).
• Active zero-impedance operation is achieved via force feedback, enabling backdriving with fingertip forces within ±5 grams over a 10 cm range.
• The system successfully crushes an aluminum beverage can under maximum PD gains, demonstrating high stiffness capability (red curve in Fig. 7-C).
• The method enables accurate endpoint state estimation and force control even without explicit endpoint sensing, relying on internal pressure and motor feedback.