Using Six Degree of Freedom Impedance Controlled Robots to Perform Contact Tasks in a Workcell

Abstract

Abstract This paper discusses the use of a high bandwidth, six degree of freedom impedance controlled robot to perform tasks involving both limited apriori positioning information and contact with the components of the task. We will focus on choosing the task frame, selecting impedances and exploiting impedance control in order to perform contact based tasks. We will examine two tasks that are associated with loading and unloading tooling and workpieces into and out of a machining center in the presence of positioning uncertainty. Tasks are based on cranking, sliding and screwing primitives. Selection of appropriate impedance elements and nominal trajectory generation are discussed. Based on a knowledge of the task, we define constrained and unconstrained subspaces and select the robot impedance to complement the environment’s impedance along each axis. Nominal trajectories are formulated by mimicking the gross movements of a person performing the same task. Our work has shown that the permissible positioning deviation between the nominal and actual trajectory, with impedance control, is significantly greater than if impedance control were not being used. In our experiments, the resulting interface forces between the manipulator and its environment are kept well within acceptable levels despite intentional inaccurate positioning. The results presented here represent a significant milestone in demonstrating the utility of impedance controlled robots in practical tasks since these experiments were conducted using real, six degree of freedom, off the shelf robots interacting with ordinary, unmodified tooling and fixtures. The experiments demonstrate that impedance controlled robots can be readily integrated with the types of equipment and environments typically found on factory floors, while simultaneously reducing the accuracy required of the robot or of the tooling setup, thus leading to lower costs.