Accuracy improvement for 6-axis serial robot using double ball-bar

To increase productivity in manufacturing using a robot, it is required to monitor its performance periodically. Circular test using a double ball-bar (DBB) is one of the simple, and reasonable method to check the robot's performance. For this reason, if the error compensation can be done by the circular test, the productivity can be increased by reducing the costs for maintaining accuracy performance. This work is targeting to improve accuracy of a 6-axis serial robot using a single DBB. This process consists of 3 steps, (1) measuring tool centre point (TCP), (2) estimating kinematic parameters, and (3) verification of the obtained values. At first, the position of the TCP with respect to the end-effector is measured using geometry-based method using DBB. Secondly, a mathematical relation of the kinematic parameters and the distance error is derived. Then the parameters which are redundant or merely affect distance errors are eliminated, and the identifiable parameters are analysed through the mathematical model. The measurement is carried out on a nominal circular path, then the kinematic parameters are calculated from the above method. The validity of the obtained values is determined by checking error reduction after re-measuring with the calibrated values applied. The contribution of this work is suggesting a rapid, simple method to improve the accuracy of a 6-axis serial robot. As this work does not require additional devices for measuring TCP, the whole process can be done only using a single DBB.