Observer-based Impedance Control in Robotics

Abstract

In situations where robots are performing manipulation tasks, e.g. welding, grinding, and drilling, it is necessary to avoid excessive forces in the contact with the environment. Because of geometric uncertainty and finite positioning accuracy this usually cannot be solved by position control alone. The subject of this thesis is robot force control, with particular interest devoted to impedance control. Two other force control strategies are also treated to some extent. These are compliance control and direct force control. Impedance control is a technique aimed at controlling the dynamic relation between position error and force error in interaction similar to Newton's second law of motion. The velocity is used to modify the damping in the impedance relation. A problem, however, is that the velocity is not available for measurement. For this reason an observer is designed to reconstruct the velocity signal. A good model of the robot joint used is obtained by system identification. Another important issue is stability, and two different approaches are used for the stability analysis. The first one uses theory for piecewise linear control systems, motivated by the fact that the contact force non-linearity can be modeled as piecewise linear. The other approach is based on results on observer-based SPR feedback and stability is proved by the Popov criterion. Experiments are carried out on an ABB industrial robot 2000 to verify the theoretical results.