A Novel Two-step Kinematics Calibration for The Hybrid Series-parallel Mechanism

As an important means to improve the kinematics accuracy, kinematics calibration is often used in the series and parallel robotics. However, since the complex kinematics in the hybrid series-parallel system, the general DH method cannot be directly used for the kinematics and error modeling. To solve this problem, a calibration method based on the equivalent kinematics model is proposed. Specifically, the parallel mechanisms in joint space are equivalent to series joints firstly, so that the kinematics and error models can be molded with the DH method. Then the kinematics error model of the hybrid system can be established by differentiating and substituting the error of parallel mechanism into the equivalent joints. Following that, the two-step calibration method is proposed based on the equivalent error model. The first step is used to keep the convergence of most calibrated parameters within a large scale, and the second step is made to improve the accuracy of all calibrated parameters. The simulation results show that the proposed calibration method ensures the convergence of the parameters during the whole iteration, and realizes the rapid and accurate identification of error parameters for the hybrid series-parallel system, which verifies the effectiveness of the proposed method.