Sensorless variable admittance control for human-robot interaction of a dual-arm social robot

These days, physical human-robot interaction (pHRI) of social robots has received a lot of attention. This paper proposes a method for variable admittance control (VAC) based pHRI to enable social robots to perform various social gestures. The proposed method includes several schemes. Firstly, a scheme to adjust the robot's damping is proposed, which takes into account the external torques exerted by the user and the robot's pose to control movement speed, reflecting the workspaces of both the human and the robot. Secondly, a scheme to change the joint stiffness is proposed, considering the reference angle and the external forces from the user to generate active motions for cooperative pHRI. Additionally, a scheme to change the robot reference point is proposed, which adjusts based on the user's external forces and the robot's posture for pHRI in various situations. The proposed methods are implemented with a dual-arm social robot. To realize pHRI based on the proposed schemes, a generalized momentum-based disturbance observer, one of the sensorless disturbance observers, is employed to estimate the exerted torque. To verify the performance of the proposed methods, experiments are carried out for handshaking and hugging.