Robot Servo Errors Were Only Human

While I worked at the General Motors Scarborough Van Plant, I was a supervisor at Ladder II, which was also called the CarTrac line. I built the frame of the 'G' van. The frame was built according to the options list in the production build sequence document. The different options could include short, standard, or extended frame length; second, third, and fourth row seats; standard or large gasoline tank; and diesel battery option.

A matching floor was built in Ladder II. The floor was built upside-down and transferred to a carrier. The built frame from Ladder I was then turned upside-down and transferred and set on top of the floor. The floor and frame were located by fixture pins and clamps and secured to the carrier. The carrier then advanced into the robotic welding stations.

The CarTrac line had two groups of four ABB IRB 90 spot welding robots, and one group of five ABB IRB 2000 MIG welding robots. The build data was transferred as the carrier progressed through the welding stations. The robots welded the floor and frame assembly according to the build data. If Ladder I made a mistake in the frame build that was not caught by the operator when the floor and frame were loaded, the robots would likely crash.

One of the mistakes that would cause a robot crash was the diesel battery plate. When the van had a diesel battery option, the robot program selected had a modified path to move the spot welding gun around the extra plate. If the frame had the plate, but the build data didn't call for one, the spot welding gun would hit the plate and the robot would stop on a servo error. The robot carrying the MIG welding gun also had to avoid hitting the diesel battery plate. Another problem was the gasoline tank option.

The cross rails were different according to the gasoline tank size. If the cross rails installed did not match the build data, the selected robot path could crash the spot welding gun into the cross rails. Even if the built frame matched all of the options, there could still be problems. The frame rails were U-shaped channels. If the open end was too wide, the spot weld gun might hit the side. If a part identifying tag was left on the frame and got between the frame and floor, the weld current path would be blocked with a resulting weld error. Each error was displayed on several message screens so that maintenance and production workers could correct the problem. All of these problems were well known to the staff at the CarTrac line.

As part of process improvement, an engineer tracked the errors' display. The errors were tabulated, and the frequency of robot servo errors was identified as a line stoppage issue. The engineer presented the numbers to me, and wanted to know what was wrong with the robots that they had so many servo errors, and what needed to be done to fix the problem.

This was where my experience with robots conflicted with perceptions. The robot is usually the most reliable piece of equipment in the work cell. Usually the problems would be with part fit and fixturing, the welder, or the weld gun. My reply was that there was nothing wrong with the robots, and to ask if he had investigated the root cause of the robot servo errors. I then explained that the robot servo error usually meant a mistake in the build from Ladder I, and if he wanted to reduce the line stops due to robot servo errors, he would have to go to Ladder I, find out why they made so many mistakes, and correct those mistakes. I didn't hear any more about that study.

This entry was submitted by Glenn Aitchison and edited by Rob Spiegel.

Glenn Aitchison's first field service job was in 1987. Since then he has worked in robotics, automotive, as well as industrial automation and machinery. He received his Certificates of Qualification as an Industrial Electrician and as an Industrial Mechanic (Millwright).

Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.

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