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Article

Cobot User Frame Calibration: Evaluation and Comparison between Positioning Repeatability Performances Achieved by Traditional and Vision-Based Methods

1
Department of Mechanical and Industrial Engineering, University of Brescia, Via Branze 38, 25125 Brescia, Italy
2
Department of Information Engineering, University of Brescia, Via Branze 38, 25125 Brescia, Italy
*
Authors to whom correspondence should be addressed.
Robotics 2021, 10(1), 45; https://doi.org/10.3390/robotics10010045
Received: 31 January 2021 / Revised: 4 March 2021 / Accepted: 6 March 2021 / Published: 8 March 2021
(This article belongs to the Special Issue Human–Robot Collaboration)
Since cobots are designed to be flexible, they are frequently repositioned to change the production line according to the needs; hence, their working area (user frame) needs to be often calibrated. Therefore, it is important to adopt a fast and intuitive user frame calibration method that allows even non-expert users to perform the procedure effectively, reducing the possible mistakes that may arise in such contexts. The aim of this work was to quantitatively assess the performance of different user frame calibration procedures in terms of accuracy, complexity, and calibration time, to allow a reliable choice of which calibration method to adopt and the number of calibration points to use, given the requirements of the specific application. This has been done by first analyzing the performances of a Rethink Robotics Sawyer robot built-in user frame calibration method (Robot Positioning System, RPS) based on the analysis of a fiducial marker distortion obtained from the image acquired by the wrist camera. This resulted in a quantitative analysis of the limitations of this approach that only computes local calibration planes, highlighting the reduction of performances observed. Hence, the analysis focused on the comparison between two traditional calibration methods involving rigid markers to determine the best number of calibration points to adopt to achieve good repeatability performances. The analysis shows that, among the three methods, the RPS one resulted in very poor repeatability performances (1.42 mm), while the three and five points calibration methods achieve lower values (0.33 mm and 0.12 mm, respectively) which are closer to the reference repeatability (0.08 mm). Moreover, comparing the overall calibration times achieved by the three methods, it is shown that, incrementing the number of calibration points to more than five, it is not suggested since it could lead to a plateau in the performances, while increasing the overall calibration time. View Full-Text
Keywords: user frame calibration; collaborative robots; robot positioning performances; fiducial markers; Sawyer; Rethink Robotics user frame calibration; collaborative robots; robot positioning performances; fiducial markers; Sawyer; Rethink Robotics
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MDPI and ACS Style

Pagani, R.; Nuzzi, C.; Ghidelli, M.; Borboni, A.; Lancini, M.; Legnani, G. Cobot User Frame Calibration: Evaluation and Comparison between Positioning Repeatability Performances Achieved by Traditional and Vision-Based Methods. Robotics 2021, 10, 45. https://doi.org/10.3390/robotics10010045

AMA Style

Pagani R, Nuzzi C, Ghidelli M, Borboni A, Lancini M, Legnani G. Cobot User Frame Calibration: Evaluation and Comparison between Positioning Repeatability Performances Achieved by Traditional and Vision-Based Methods. Robotics. 2021; 10(1):45. https://doi.org/10.3390/robotics10010045

Chicago/Turabian Style

Pagani, Roberto, Cristina Nuzzi, Marco Ghidelli, Alberto Borboni, Matteo Lancini, and Giovanni Legnani. 2021. "Cobot User Frame Calibration: Evaluation and Comparison between Positioning Repeatability Performances Achieved by Traditional and Vision-Based Methods" Robotics 10, no. 1: 45. https://doi.org/10.3390/robotics10010045

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