While I worked for ABB Flexible Automation, I provided support for ABB robots at the General Motors Oshawa Truck Plant, GMT 800 project. Part of the preventive maintenance at the plant was using a thermal imager to look for hot spots. A hot spot could include a loose electrical connection. During an inspection, they found one of the robots had hot servo motors. I was tasked to find out what was wrong with the robot and fix it.
During my investigation I found that the resistance welding gun the robot was carrying exceeded the rated payload. The perch position was programmed so that the minor axes -- the wrist axes, 4, 5, 6 -- were holding the weld gun at an awkward angle, causing each axis to carry a load against gravity. These three axes had the high temperatures. The servo-on time was set to maximum, effectively years. The robot would wait with the servos on, for years, before timing out, enabling the brakes, and turning off the servo power.
I presented these finding to the engineer in charge of that work cell. The options to get the servo motors to run cooler included:
- Reduce the weight of the weld gun to within the payload specification of the robot.
Replace the robot with a higher-payload unit to match the weight of the weld gun.
Change the perch position so that the wrist axes were not carrying a load. This could be done by removing the servo-on signal, causing the servo motors to power off, and the brakes to engage. Then release the brakes of the wrist axes, allowing the weld gun to hang in an orientation that would not load the wrist servo motors, and re-teach the perch position.
Change the servo time out to minutes, allowing the servo motors to turn off and the brakes to engage. This would cost a few milliseconds of cycle time each time the servo system was enabled and the brakes released.
Each of these options was rejected. When I was asked again about the hot servo motors, my response was that if they were not willing to accept any of the options, they would continue to have hot motors.
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