Implementing contact tasks involving position uncertainty with impedance controlled robots

This paper will discuss a high bandwidth, six degree of freedom impedance controller for a Puma 562 robot which is used to perform contact tasks. We will focus on how impedance control can be exploited to perform contact tasks with rigid objects in the presence of positioning uncertainty. Our work has shown that the permissible deviation between the nominal and actual trajectory in performing these tasks is significantly greater with impedance control. By using impedance control in such cases, the interface forces between the manipulator and its environment are kept well within acceptable levels. 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 along these axis. The implementation scheme is such that the specified robot impedance along specific constrained or unconstrained directions is defined with respect to a task frame and is independent of the location and orientation of the base of the robot. Nominal trajectories are formulated by mimicking the gross movements of how a person would perform the same task. Our work has concentrated on tasks associated with workpiece and tool manipulation at a machining center using a robot mounted on a wheeled mobile platform which moves from workcell to workcell. This task environment provides an experimental testbed for demonstrating how significant positioning uncertainty can be handled. The result of our work implies that the location of the base of the robot does not have to be highly repeatable when performing tasks in a dynamic environment. Experimental results for turning a kinematically constrained crank (which might be used to close a vice, for example) are described.