Experimental investigations of sensor-based surface following tasks by a mobile manipulator

Abstract

integrated: while instantaneous position estimates are direct functions of the joint sensor data for This paper discusses experimental investigations the manipulator subsystem, they are functionals of of the feasibility and requirements of simultaneous the wheel rotation and steering sensor data for the external-sensor-based-control of the wheeled platform platform, typically requiring time integration of these and the manipulator of a mobile robot. The values over the entire trajectory history due to the experiments involve 3-D arbitrary surface following non-holonomic nature of the wheeled system. As by the manipulator while the platform moves along a a result, motion accuracies of human-size platforms predefined trajectory, are generally two or three orders of magnitude A variety of concave and convex surfaces were worse than the accuracies of common manipulators. used in the experiments, during which target and This paper discusses experiments performed with measured values of the platform and arm positions the HERMIES-III autonomous mobile manipulator, and orientations,together with the surface absolute location and normal estimates, were logged at. 10 Hz. For all experiments, the data logs showed significant noise, at high frequency, in the calculated surface normal values despite smooth tracking of their target values by the arm and the platform, with typical :+ closed loop delays between target and achieved values of the order of 100 msec. This high-frequency noise in the calculated values is conjectured to result mainly from the arm's transmission cables compliance and backlash in the spherical wrist gears. On the other hand, the end-effector distance to the surface showed some low frequency errors of the order of :t=20%. The two major sources of these low frequency errors 0 . appeared to reside respectively in the low values ., of the velocity bound and gain parameters utilized '' to filter the high frequency noise in the calculated .... normal values prior to using them as input to the arm control, and in the rolling contact of the platform's rubber-coated wheels on the ground where significant errors in the platform's positions and orientations can accumulate.