A multilevel calibration technique for an industrial robot with parallelogram mechanism

• A multilevel calibration technique is proposed to compensate for the significant error sources with pertinence.
• The parallelogram structural error is modeled and calibrated.
• An absolute kinematic calibration model is established and resolved.
• The robot joint space is divided into cells for further error compensation.
• After the multilevel calibration, the maximum/mean point positioning errors are reduced to almost the same level as the robot bidirectional repeatability.

This paper presents a multilevel calibration technique for improving the absolute accuracy of an industrial robot with a parallelogram mechanism (ABB IRB2400). The parallelogram structural error is firstly modeled based on the partial differential of the position function of a general four-bar linkage and the linearization of the position constraints of the parallelogram mechanism, the model coefficients are fitted from experimental data. Secondly, an absolute kinematic calibration model is established and resolved as a linear function of all the kinematic parameters, as well as the base frame parameters and tool parameters. Finally, contrary to most other similar works, the robot joint space (rather than Cartesian space) is divided into a sequence of fan-shaped cells in order to compensate the non-geometric errors, the positioning errors on the grid points are measured and stored for the error compensation on the target points. After the multilevel calibration, the maximum/mean point positioning errors on 284 tested configurations (evenly distributed in the robot common workspace) are reduced from 1.583/0.420 mm to 0.172/0.066 mm respectively, which is almost the same level as the robot bidirectional repeatability.