Estimating Robot Body Torque for Two-Handed Cooperative Physical Human-Robot Interaction

Cooperative physical Human Robot Interaction (pHRI) aims to combine the best of human problem-solving skills with the strength, speed and accuracy of a robot. When humans and robots physically interact, there will be interaction Forces and Torques (F/Ts) that must be within safe limits to avoid threats to human life and unacceptable damage to equipment. When measured, these F/Ts can be limited by safety rated emergency stops, and one can design a compliant robot behavior to reduce interaction F/Ts, and avoid unnecessary emergency stops. Several recent collaborative robots offer measurements of interaction F/Ts by utilizing torque sensors in joints or observers for joint torque, and the classical end-effector F/T sensor can provide measurements of interaction at the working end of a robot. The end-effector wrench can be calculated from joint torques if and only if there is no interaction on the robot body. Typically, safety limits are evaluated around a single point of contact – on the end-effector or elsewhere on the robot body. This approach fails when a human uses both hands to interact with a robot, e.g. when hand guiding or otherwise cooperating with the robot placing one hand on the robot end-effector and the other hand on the robot elbow. Having two points of contact that are evaluated as one will limit the allowed F/Ts of the sum of the contacts rather than individually. In this paper, we introduce the body torque as the interaction on the body that is not the result of interactions on the end-effector. We then use this body torque, which is a more accurate representation of the forces applied to the robot body, to limit the body interaction F/Ts to ensure safe human-robot interaction. Furthermore, the body torque can be used to design null-space compliance for a redundant robot. Distinguishing body torque is a step towards safe cooperative pHRI, where body torque, unknown end-effector loads, and end-effector interaction F/T are all important measurements for safety, control and compliance.