Abstract
This paper presents an automated calibration method for industrial robots, based on the use of (1) a novel, low-cost, wireless, 3D measuring device mounted on the robot end-effector and (2) a portable 3D ball artifact fixed with respect to the robot base. The new device, called TriCal, is essentially a fixture holding three digital indicators (plunger style), the axes of which are orthogonal and intersect at one point, considered to be the robot tool center point (TCP). The artifact contains four 1-inch datum balls, each mounted on a stem, with precisely known relative positions measured on a Coordinate Measuring Machine (CMM). The measurement procedure with the TriCal is fully automated and consists of the robot moving its end-effector in such as a way as to perfectly align its TCP with the center of each of the four datum balls, with multiple end-effector orientations. The calibration method and hardware were tested on a six-axis industrial robot (KUKA KR6 R700 sixx). The calibration model included all kinematic and joint stiffness parameters, which were identified using the least-squares method. The efficiency of the new calibration system was validated by measuring the accuracy of the robot after calibration in 500 nearly random end-effector poses using a laser tracker. The same validation was performed after the robot was calibrated using measurements from the laser tracker only. Results show that both measurement methods lead to similar accuracy improvements, with the TriCal yielding maximum position errors of 0.624 mm and mean position errors of 0.326 mm.
Highlights
The difference between robot repeatability and robot accuracy is well known in academia, but industrial clients of robot manufacturers often confound the two concepts, based on the personal experience of the third author, who is cofounder of Mecademic, a manufacturer of compact, high-precision industrial robots, and instigator of RoboDK, a software for offline programming and calibration of industrial robots
We proposed in [16] a measuring device used to automatically drive the robot tool center point (TCP) to coincide with the center of a datum ball
TriCal measuring device installed on a KUKA KR 6 R700 sixx industrial robot
Summary
The difference between robot repeatability and robot accuracy is well known in academia, but industrial clients of robot manufacturers often confound the two concepts, based on the personal experience of the third author, who is cofounder of Mecademic, a manufacturer of compact, high-precision industrial robots, and instigator of RoboDK, a software for offline programming and calibration of industrial robots. Is the accuracy of these three devices relatively poor (as low as 0.1 mm) but they are used as error measurement devices rather than replacements of a position’s physical constraint This means that the robot’s TCP is never exactly at the center of the tool tip, but as far away as 10 mm. The company IBS Precision Engineering (The Netherlands) offers the Trinity contact-free probe, which can measure the offset of the center of a special datum sphere (expensive and fragile) within a range of up to 3.5 mm (still too small for robotics) and with an accuracy of less than 0.001 mm All these devices are relatively expensive (the Trinity costs about USD 15,000), because they need to be built with great accuracy and calibrated before use. TriCal measuring device installed on a KUKA KR 6 R700 sixx industrial robot
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