Identification and experimentation of an industrial robot operating in varying-impedance environments

It is well-known that iff the robot with its end-effector force control is passive, the closed loop system consisting of the robot and an arbitrary passive environment is asymptotically stable. Passive robot control, however, limits the achievable robot impedance reduction. Recently, investigated what performance can be achieved with nonpassive control when uncertainty bounds for the environment are known. The question of stability margins neglected in is treated in our companion paper. In this paper we report on experiments on a full scale industrial robot in order to identify six transfer functions from the velocity reference of the inner velocity loop to the end effector force output for six different environments, from solid wall to air. It is shown how the experimental data also makes it possible to compute the impedance of the controlled robot. Two controllers are designed: one marginally stable but with nonpassive and low-gain controlled robot impedance at low frequencies, and another controller respecting the stability margin of closed loop sensitivity gain less than 6 dB, but with a passive controlled robot impedance that has higher gain for low frequencies than that of the first controller. The designs are compared experimentally by having the robot interact with environments of different impedances, and with a human operator leading the robot The paper is concluded by a short discussion of the possible need of controller adaption when a robot operates in varying impedance environments, and interacts with a human operator.