Is the human operator in a teleoperation system passive?

Conventional approaches for stability analysis of bilateral teleoperation systems assume that the human operator does not inject energy into the system and behaves in a passive manner. Does this assumption hold for various tasks the human operator may execute in a teleoperation context? To answer this question, in this paper we measure the endpoint impedance (inertia, viscosity, and stiffness) of the human arm during two tasks: (1) relaxed grasping of a haptic device while the device imposes position perturbations, and (2) rigid grasping of a haptic device (posture maintenance) while the device imposes force perturbations. The human arm impedance is identified as a 2 × 2 transfer function matrix and assessed for passivity over the frequency range characteristic of human motion. Our results agree with previous findings that the relaxed human arm behaves as a passive system. However, whether the rigid arm behaves as an active or passive system is found to depend on the magnitude of the force perturbations. We discuss why the passivity of the human operator is task dependent.