Experiments with Simplified Computed-Torque Controllers for Free-Flying Robots

Abstract

Base accelerations <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> and base angular velocity in free-flying robots introduce inertial forces not seen in-fixed-base robots. Consequently, dynamic models of free-flying robots possess more degrees of freedom than their fixed-base counterparts. In this paper the effect of neglecting base accelerations in an inertial space endpoint Computed-Torque (CT) controller is examined. Numerical simulations predict a rather wide set of conditions under which CT controllers can neglect base accelerations in their dynamic model with insignificant performance degradation. Two predictions are experimentally verified with a laboratory robot. These two experiments show that negligible additional endpoint controller error results with a simplified CT controller for the given robot mass parameters and manipulator articulation. Modeling error is less than that due to uncertainties in mass distribution and friction effects. A consequence of neglecting robot base accelerations in the simplified CT controller is that Jacobian augmentation terms for momentum control or base control are no longer included. This control will then need to be external to the manipulator controller. On the other hand, a potential saving of an order of magnitude in computation for manipulator control exists.