Suppressing Delay-Induced Oscillations in Physical Human-Robot Interaction with an Upper-Limb Exoskeleton using Rate-Limiting

In physical human-robot interaction (pHRI) enabled by admittance control, delay-induced oscillations arising from both the neuromuscular time-delays of the human and electromechanical delays of the robot can cause unsafe instability in the system. This study presents and evaluates rate-limiting as a means to overcome such instability, and provides a new perspective on how rate-limiting can benefit pHRI. Specifically, a rate-limited and time-delayed human-in-the-loop (HITL) model is analyzed to show not only how the rate-limiter can transform an unstable equilibrium (due to time-delay) into a stable limit-cycle, but also how a desired upper-bound on the range of persistent oscillations can be achieved by appropriately setting the rate-limiter threshold. In addition, a study involving 10 subjects and the EXO-UL8 upper-limb exoskeleton, and consisting of 16 trials - 4 rate-limiter thresholds by 4 time-delays - is performed to: (1) validate the relationships between time-delays, rate-limits, and position bounds on persistent oscillations, and (2) demonstrate the effectiveness of rate-limiting for recovery from delay-induced oscillations without interfering with regular operation. Agreement of experimental results with the theoretical developments supports the feasibility of incorporating rate-limiting in admittance-controlled pHRI systems as a safety mechanism.