Self-calibration of a general hexapod manipulator with enhanced precision in 5-DOF motions

This paper presents a self-calibration strategy for a general hexapod manipulator to overcome shortages and embrace merits of existing approaches. The calibrated system has 4-degrees-of-freedom (DOF) and one redundant sensor, while the algorithm is formulated to solve a linear system, which takes all measurement errors into account, using the ordinary least-squares method iteratively. The results reveal that the proposed algorithm possesses the following advantages: (1) it is capable of directly calibrating the position and orientation of the tool on the end-effector, and thereby totally removes trajectory errors which can not be eliminated by touching-off; (2) the unobservability of the parameters due to mobility constraints on the passive joints is completely avoided; (3) the algorithm is numerically robust, efficient and accurate that the calibrated parameter errors is kept to the same order of the measurement errors; and (4) the calibrated procedure can be performed automatically. Due to these merits, the present scheme is attractive for an autonomous hexapod manipulator when a great precision is required in a workspace of 5-DOF.