Simplifying the kinematic calibration of parallel mechanisms using vision-based metrology

In this paper, a vision-based measuring device is proposed and experimentally demonstrated to be an accurate, flexible, and low-cost tool for the kinematic calibration of parallel mechanisms. The accuracy and ease of use of the proposed vision sensor are outlined, with the suppression of the need for an accurate calibration target, and adequacy to the kinematic calibration process is investigated. In particular, identifiability conditions with the use of such an exteroceptive sensor are derived, considering the calibration with inverse or implicit models. Extensive results are given, with the evaluation of the measuring device and the calibration of an H4 robot. Using the full-pose measurement, an experimental analysis of the optimal calibration model is achieved, with study of the kinematic behavior of the mechanism. The efficiency of the provided method is thus evaluated, and the applicability of vision-based measuring devices to the context of kinematic calibration of parallel mechanisms is discussed.

I. Introduction

Parallel mechanisms are emerging in the industry (e.g., machine tools, high-speed pick-and-place robots, flight simulators, medical robots). Indeed, these mechanisms have the main property of having their end-effectors connected with several kinematic chains to their base, rather than one for the standard serial mechanisms. This allows parallel mechanisms to bear higher loads, at higher speed and often with a higher repeatability [1]. However, their large number of links and passive joints often limits their performance in terms of accuracy [2]. A kinematic calibration is thus needed.