Analysis of Cartesian Force Sensitivity for Joint Torque Controlled Robots

Kurzfassung

Compliant, joint torque controlled robots measure the forces and torques at the tool center point indirectly using their built-in joint torque sensors. This allows robust impedance control but has drawbacks regarding the force sensitivity, which denotes the measurable force changes in relation to the external applied force variation. Depending on the robot configuration, torque inaccuracies and joint hysteresis have varying impact on the quality of the measured forces and torques. Joint torque offsets are influencing the measurement statically, whereas friction related joint hysteresis is changing the detected torques dynamically. Therefore, a bandwidth is introduced, which specifies the measured torque deviation based on joint hysteresis. Since the hysteresis gives a statement about the possible torque variation, it can be used to define the sensitivity. A method is described, to detect these influences and transform the insights into Cartesian space. This leads to an improvement of the force measurement accuracy due to offset compensation and the force sensitivity for the manipulator can be estimated according to the hysteresis. An accurate analysis of the Cartesian force sensitivity for impedance controlled robots is inalienable when it comes to applications like automated assembly tasks, where tactile force feedback is used to compensate position inaccuracies of objects. The Cartesian force sensitivity of redundant manipulators is determined using force ellipsoids, which are capable of showing the sensitivity of the force measurement according to the spatial directions at the tool center point. Hence, a statement about the expected sensitivity in different poses and configurations is generated and verified with a proposed robotic measurement setup.