Admittance control of a redundant industrial manipulator without using force/torque sensors

Nowadays robotic manipulators are used as multi-purpose tools and must be able to complete various tasks. Pure position control schemes are often not sufficient to fulfill the requirements of these tasks. Interaction with the environment requires an extension of the conventional position control in order to achieve a desired compliance, and thus to limit the impact in order to avoid damaging of involved objects. This paper presents an admittance control scheme applicable to kinematically redundant manipulators without using joint torque sensors or wrist-mounted force/torque sensors. By using motor current measurements an estimation of the external forces acting on the manipulator can be obtained and allows for a compliant behavior. Joint friction effects are overcome by superposing an additional movement in the null-space of the end-effector task. This control scheme is applied to an industrial manipulator, namely a Staubli TX90L mounted on a linear axis (constituting a redundant 7-DOF manipulator) and experimental results are provided.