Kinematic calibration of parallel mechanisms: a novel approach using legs observation

In this paper, a novel approach is proposed for the kinematic calibration of parallel mechanisms with linear actuators at the base. The originality of the approach lies in the observation of the mechanism legs with a camera, without any mechanism modification. The calibration can hence be achieved online, as no calibration device is linked to the end-effector, on any mechanism since no additional proprioceptive sensor installation is necessary. Because of the conditions of leg observability, several camera locations may be needed during the experimentation. The associated calibration method does not however require any accurate knowledge of the successive camera positions. The experimental procedure is therefore easy to perform. The method is developed theoretically in the context of mechanisms with legs linearly actuated at the base, giving the necessary conditions of identifiability. Application to an I4 mechanism is achieved with experimental results.

I. Introduction

Among the proposed architectures for parallel mechanisms, structures with linear actuators at the base are of great interest: inertial effects are lowered, and the use of modern linear actuators enables one to build mechanisms with high dynamics. Consequently, many recently developed structures belong to this class of mechanisms such as Hexaglide [2], I4 [1], and Orthoglide [3]. Their use is highly dependent on their accuracy. Compared to serial mechanisms, they may exhibit a much better repeatability [4], but not necessarily a better accuracy, because of their large numbers of links and passive joints [5]. A kinematic calibration is thus needed for these structures.