Kinematic Calibration of a Six-Legged Walking Machine Tool

Abstract This paper presents the kinematic calibration of a novel six-legged walking machine tool comprising a six-legged mobile robot integrated with a parallel manipulator on the body. Each leg of the robot is a 2-universal-prismatic-spherical (UPS) and UP parallel mechanism, and the manipulator is a 6-PSU parallel mechanism. The error models of both subsystems are derived according to their inverse kinematics. The objective function for each kinematic limb is formulated as the inverse kinematic residual, i.e., the deviation between the actual and computed joint coordinates. The hip center of each leg is first identified via sphere fitting, and the other kinematic parameters are identified by solving the objective function for each limb individually using the least-squares method. Thus, the kinematic parameters are partially decoupled, and the complexities of the error models are reduced. A calibration method is proposed for the legged robot to overcome the lack of a fixed base on the ground. A calibration experiment is conducted to validate the proposed method, where a laser tracker is used as the measurement equipment. The kinematic parameters of the entire robot are identified, and the motion accuracy of each leg and that of the manipulator are significantly improved after calibration. Validation experiments are performed to evaluate the positioning and trajectory errors of the six-legged walking machine tool. The results indicate that the kinematic calibration of the legs and manipulator improves not only the motion accuracy of each individual subsystem but also the cooperative motion accuracy among the subsystems.