Bidirectional Energy Flow Modulation for Passive Admittance Control

Admittance control is a control scheme to enable physical interactions of a robot, but it easily induces instability when the robot contacts a rigid surface. In this study, a passivity analysis was conducted on a robotic system with admittance control. The results showed that coupled stability with the environment can be ensured when the velocity error between the proxy and the real robot is eliminated. Thus, an adaptive structure modification method is proposed to suppress the possible source of instability. In addition, the energy tank method is combined with the proposed method to ensure system passivity. As a proof of concept, three robot experiments were performed, and the results of the proposed method were compared with those of conventional admittance control and impedance control (with friction compensation). The comparison showed that the proposed method could make the system passive while realizing the desired dynamics during the interaction.