Safely Rendering Small Impedances in Admittance-Controlled Haptic Devices

This paper introduces a simple and minimal admittance controller capable of rendering small impedances. The proposed controller ensures stable human-robot interactions by dynamically modifying and limiting the commanded acceleration of a haptic device. A Lyapunov stability analysis demonstrates that these dynamic acceleration modifications yield a stable inner position control loop with bounded tracking errors. This paper also discusses results from several experiments performed by healthy subjects who used a one degree-of-freedom haptic device to assist forearm flexion and extension. The results show that the controller is capable of rendering apparent inertia reductions as high as 99.3% while remaining robust to varying operator inputs and dynamics.