Contact and Physical Interaction

This article reviews approaches to controlling robots undergoing physical contact and dynamic interaction with objects in the world. Conventional motion control is compared with a hybrid combination of position and force control. Several challenges are reviewed, most importantly the problems of instability: dynamic instability due to coupling, and static instability due to exerting force. Energetically passive interactive dynamics addresses the former; a minimum stiffness proportional to the force exerted addresses the latter. Actuators, which dominate the robot's interactive dynamics, are briefly surveyed, including series elastic, variable-stiffness, and emerging designs. A comparison with human performance is made. A bioinspired approach to controlling interactive dynamics (mechanical impedance or admittance) is reviewed. Robot configuration profoundly modulates apparent inertia, whereas force feedback control has minimal influence. Superimposing first-order mechanical impedances simplifies controlling many degrees of freedom. It manages redundancy while preserving passivity (unlike null-space projection methods) and enables seamless operation into and out of singular configurations.