Cost-Effective Calibration of Collaborative Robot Arm with Single Wire Encoder

With its flexibility of a collaboration robot and demands for maintaining virtual model’s fidelity in cyber-physical system of robotic manufacturing, frequent calibration is required. This paper proposes a cost-effective calibration method of a collaborative robot using a single wire encoder. First, DH (Denavit-Hartenberg) convention-based kinematics of the robot are analyzed. Then, a mathematical relation between distance errors between robot’s end effector and wire encoder and the kinematic parameters are presented. In the experiment, turning-in-place-based measurement methodology is applied. Also, in order to reduce error caused by wire bending at the point where a wire is hooked in previous studies, a rotatable wire hanger which has 2-DoF (Degree of Freedom) rotary joint is fabricated and applied. Next, identifiable parameters are analyzed through SVD (Singular value decomposition), then 21 parameters are calculated via Levenberg–Marquardt algorithm. The estimated parameters were validated by comparing fidelity of the calibrated and nominal model against actual physical system measured by a robot arm type measurement device. As a result, feasibility of the proposed method was checked by confirming 64.3% better fidelity of the calibrated model using the proposed method. Furthermore, the performance of the method also shows better results than camera-based robot’s internal calibration function provided by manufacturer.