Impedance Control on Arbitrary Surfaces for Ultrasound Scanning using Discrete Differential Geometry

Abstract

We propose an approach to robotic ultrasound scanning and interaction control with arbitrary surfaces using a passivity-based impedance control scheme. First, we introduce task coordinates depending on the geometry of the surface, which enable hands-on guidance of the robot along the surface, as well as teleoperated and autonomous ultrasound image acquisition. Our coordinates allow controlling the signed distance of the robot to the surface and alignment of the tool to the surface normal using classical impedance control. This corresponds to implicitly obtaining a foliation of parallel surfaces. By setting the desired signed distance negative, i.e., into the surface, we obtain passive contact forces and simultaneously provide an intuitive way to control the maximum penetration depth into the surface. We extend the approach to also incorporate coordinates allowing to control the specific point on the surface and, automatically, on all parallel surfaces. Finally, we demonstrate the performance of the controller on the seven degrees of freedom lightweight robot DLR Miro: the robot tracks complex trajectories while accurately keeping the desired distance to the surface and applying an almost constant contact force. Finally, we compare the approach to the state of the art.