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Parallel kinematic mechanisms (PKMs) offer advantages of high stiffness to mass ratios, greater potential for accuracy and repeatability, and lower cost when compared to traditional assembly machines. Because of this, there is a strong interest in using PKMs for aerospace assembly and joining operations. This paper looks at the calibration of a prototype Gantry TAU robot by extending the higher-order implicit loop calibration techniques developed for serial link mechanisms to parallel link mechanisms. The kinematic model is based on the geometric model proposed by Dressler et al., augmented with a cubic spline error model of the motion errors for each of the three translation actuators resulting in 185 parameters. Measurements are taken with a 6-DOF laser tracker, and the kinematic parameters are solved as the maximum likelihood parameter estimate.

Current methods of machine calibration and software compensation focus on either the joint motion errors (classic machine tool software compensation) or the geometric errors between the joints (robot calibration). However, both types of errors have a significant impact on the volumetric accuracy of a machine tool or robot. We have developed a calibration method that simultaneously identifies joint motion errors and geometric errors in a machine or robot with an arbitrary number and arrangement of links using a laser tracker. The simultaneous identification of all error sources decreases measurement time, with a typical calibration for a moderate sized machine taking about four hours and 200-500 measurements. The model presented is based on a mathematically minimal parametric model of the machine. Parameter identification is done in a statistically significant way, resulting in both the “best-fit” values for the parameters and the statistical confidence in those values.