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Rickard Lindhé

Metrology

Using a Laser Tracker to Improve Accuracy of Industrial Robots

Error compensation with a laser tracker increases robot accuracy to 0.5 mm.

Published: Wednesday, October 28, 2009 - 14:39

ABB of Västerås, Sweden, is a pioneer among world industrial robot manufacturers when it comes to using laser-based measuring technology to ensure robot precision. ABB, with an installed base of 125,000 robots, stands out as the world’s largest in the arena of industrial automation. And Leica Geosystems’ equipment is instrumental in a unique calibration method that offers customers exact robot positioning accuracy throughout the entire life cycle of their robots.

Absolute Accuracy in a Nutshell

 

Absolute Accuracy is a process that ensures that a robot will retain its accuracy throughout its entire life cycle. The method bridges the gap between the CAD system’s virtual robot’s precision and the work done by the actual robot on the factory floor. Using a laser tracker; a compensation parameter is established that corrects the positioning and the robot’s movements. These parameters take into account both the mechanical imperfections in the pattern of movements and the bending downwards caused by the loads.

 

For the company using the robot, Absolute Accuracy means that it can install and run a robot directly with the greatest degree of precision possible. Upgrading an existing robot in the field to Absolute Accuracy is also straightforward. The same tool configuration is used throughout, from calibration and verification at ABB in Västerås to installation, operation, and maintenance. ABB’s CalibWare, Calibration Pendulum and operating system algorithms are the same at ABB as at the customers’ premises. That reduces the risk of incompatibility to zero.

Known as Absolute Accuracy, ABB’s calibration method turns a standard robot into an exact robot by applying software-controlled correction for errors that arise in normal use caused by overload, kinematics, and dynamics. “A really great industrial robot is one that maintains its accuracy throughout its entire life cycle, including factory installation, motor output, regrouping of fixtures and everything else a robot might experience,” says Peter Fixell, product manager responsible for calibration and quality assurance at ABB in Västerås. “The difference in accuracy between a virtual‚ ideal robot and an actual robot is usually between 8–15 mm. The difference stems from mechanical tolerances and load. With Absolute Accuracy, we reduce the gap to an average 0.5 mm.”

ABB has two adjacent work cells for calibrating robots, each of them measuring 10 x 10 meters. Between the cells stands a Leica Laser Tracker. The operator controls the measurement phase from a PC that is situated to allow him a full view of both cells. While he is measuring a robot in one cell, his colleagues can use the time rigging up or putting away a robot in the other cell. After the robots are measured and calibrated, they are packed and ready for delivery to the customer



A Leica Laser Tracker can follow a movement of 6 m per second with an acceleration of 2G and measure 1,000 points per second. The accuracy of 0.01 mm per meter suits ABB’s work in supplying robots with lifelong position accuracy.

A dream come true

ABB’s Absolute Accuracy method eliminates the differences between the virtual robot’s precision in the CAD system and the work done by the actual robot on the factory floor.

The robot is instructed to move to 100 coordinate locations in the work area. The actual positions are determined by the laser tracker. Then, by comparing the theoretical and actual positions, ABB creates a set of compensatory parameters that correct the robot’s positioning and thereby its movements. The parameters take into account both the mechanical imperfections in the pattern of the movements and the bending or downward distortions caused by the loads. For companies using the robot, Absolute Accuracy means that they can install and run a robot directly with the greatest degree of precision possible. The same tool configuration is used throughout the process from calibration and verification to installation, operation, and maintenance. ABB’s CalibWare, Calibration Pendulum, and operating system algorithms are the same at ABB as at the customers' premises, which essentially reduces the risk of incompatibility to zero.

 “For companies using a single robot, this means that they have a robot that maintains high and consistent precision year in and year out,” says Fixell. “The company can download, without any difficulty, new simulated programs that the robot will work to without any corrections.”

Home and away

Helping customers achieve robot-supported production with a minimum amount of delays is key for ABB. For that reason, Fixell and his team also offer Absolute Accuracy calibration at the customer site.

“Customers who invest in robots without Absolute Accuracy need not think they have gone down a blind alley,” he says. “We have equipment that enables us to calibrate the robots effectively at the customers’ premises.

“What is required is measuring equipment that can deliver measurement data in (x, y, z) so that it can be imported into our CalibWare calibrating tool. For measurements at customers’ premises we use Leica’s TDA 5005 total station. It is lighter to carry than the LTD500 laser tracker, which is the model used in our production facilities”.

With a total station in one hand and the stand in the other, it is easy for an ABB engineer to move around any of its customers’ production environments. Battery operation provides flexibility and it takes just a few minutes to get it up and running. Measuring a position takes just three to five seconds, with an accuracy of 0.2–0.3 mm.

Speaking the same language

Regardless of whether measurement is done at ABB with the laser tracker or at the customers’ premises with a total station, the compatible measurement data is exported to ABB’s CalibWare, an add-on to the Robot Studio development and simulation environment. This is how the robot’s cell is simulated and displayed; and where the 100 calibration location coordinates in the robot’s working area are generated. The robot’s compensation parameters, unique to each single robot, are calculated on the basis of these locations, which are integrated into the robot’s operating system. Performance is further verified by checks in 50 new locations, in which the Robot Tool Centre Point position accuracy is fixed and a certification ticket issued.

CalibWare ensures consistency between ABB and customer production environments and produces a checklist of simple calibration instructions and an accuracy menu that links the customer’s maintenance measures.

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About The Author

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Rickard Lindhé

Rickard Lindhé is an independent journalist.