Improved Joint Zero Offset Calibration for Industrial Cobot Arms

Title:  Improved Joint Zero Offset Calibration for Industrial Cobot Arms
Author: Dante Neckmar
Date & Time: June 4th, 9:15–10:00
Location: Seminar Room M 3170-73 in the M-building, LTH
Supervisor: Yiannis Karayiannidis, Kristian Kassow
Examiner:  Bo Bernhardsson
Opponents: Jacob Heyden and Georg Emmanouilidis 


Abstract:
Collaborative robots, or cobots, are increasingly used in industrial automation, where they work in flexible, semi-modular configurations while still being expected to meet demanding positioning-accuracy requirements. After a joint is replaced on such a robot, the resulting zero offset typically forces a full kinematic recalibration before the robot can return to service, causing downtime and requiring specialized equipment. Existing solutions are either simple manual references that lack the accuracy needed, or external optical systems that are expensive and complex to set up. This thesis develops a low-complexity, joint-

level zeroing method for the cobot arms produced by Kassow Robots, with the goal of avoiding that recalibration step. A sensitivity analysis showed that a pin-level repeatability of around 20μm is enough to keep the tool center point (TCP) positioning error below 1 mm across the full product range. Three concepts were considered: a mechanical fixture, a laser triangulation system, and a vision-based solution. The mechanical concept was chosen for its lower complexity and cost, and was designed and built as a cup-shaped fixture with a self-locking block that engages the joint output through well-defined contact. A 3D-printed prototype was used to validate the principle, and a machined stainless steel version was then tested over 96 zeroing cycles. The proposed prototype achieved a TCP standard deviation of 0.18 mm and a pin-level repeatability of about 3μm, well within the requirement. The results show that a passive mechanical fixture can deliver the precision needed for joint replacement without optical hardware or firmware modifications, removing the need for full kinematic recalibration after joint maintenance.