Characterization of repeatability of XY-Theta platform held by robotic manipulator arms using a camera

This paper presents a XY-Theta micrometric platform, which is extremely compact and offers a wide 300 × 300 mm workspace. This platform is held by a serial kinematic chain of four revolute joints, constituting a redundant robot. Each point of the horizontal platform can be positioned under a vertical axis in a two-step approach: in a coarse positioning mode, the four axes are controlled to position and orientate the object with a position error less than 7 µm; in a fine mode, two axes are mechanically blocked while two others are controlled to reduce the final position error below 2 µm. The choice of the blocked and moving axes depends on the lever arm length and the mechanism is designed to optimize the link lengths to reduce the final position error. The aim of the paper is to characterize the platform repeatability performances. An estimation of the repeatability is performed with a camera. These results are then compared to previous results based on the stationary cube method. The two measurements methods lead to similar results with a repeatability close to 2 µm showing a significant improvement of the performances.