Using Passive End-Point Motion Constraints to Calibrate Robot Manipulators

The concept of using a method to constrain the endpoint of a serial linkage (such as a robot manipulator) in order to identify the kinematic parameters is appealing, since it no longer becomes necessary to use ancillary equipment to measure the pose, partial or otherwise. This means that the joint angles obtained from the manipulator and knowledge of the type of constraint, are all that is needed to perform the calibration. This concept is applied to the calibration of a six-degree-of-freedom manipulator by connecting its end point to a table by means of a ball bar. The kinematic equations of the manipulator and constraint are developed, and then used to simulate the calibration process in order to determine whether the noise level present in the measurements disrupts the convergence of the process, the effect of measurement noise on the accuracy of identification and the number of experimental observations needed. The acquisition of experimental data is then described, and the results of the identification are discussed. It is concluded that for certain types of kinematic structures, this method of calibration is particularly attractive since it is rapid, simple to perform, and requires very little precision equipment.