A gravitational deflection compensation strategy for HPKMs

Hybrid parallel kinematic machines (HPKM) are prone to structural deflection under the influence of gravity, both in the serial wrist joints and in the upper tripod structure. The deflection results in errors at the tool tip, leading to manufacturing inaccuracies and performance outside the manufacturer's quoted accuracy. To further increase the take-up of HPKMs robust online compensation strategies are required that return and maintain the HPKM to the quoted accuracy level under production conditions.This paper addresses the problem of mass-induced errors in the positional accuracy of hybrid parallel kinematic machines (HPKMs) and describes the development of a novel robust polar compensation methodology for the reduction of these errors. The strategy developed significantly reduces the level of error observed. A full implementation of the gravitational error compensation strategy on the Neos Robotics TR600 at the University of Nottingham has seen the measured system’s gravitational error reduced by 70% and the system performance returned to the quoted manufacturer’s accuracy level.