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.
Wow, sounds like a lot of hardship could be avoided by a conference between Ladder I and Ladder II as to coordinating build orders. Sort of reminds me of "how spec'd vs. how built".
Agreed. It also sounds like there could be more automation in the coordination of the "Ladders". Another alternative is to tag each assembly so that the stations can get the data from a central repository.
The coordination was in the build order documents. Each van was listed in order, and from Ladder I, through all of the Body Shop, Paint Shop, and Final Assembly, the build order document was the 'bible'. The problems happened when the 'bible' was not followed exactly. The spacing between frames was about 90 seconds. It took about 150 seconds for a crew to build a frame in Ladder I, so there were 2 build crews. If those 2 crews got out of sequence, either by skipping a frame or building a duplicate, problems happened. It was easy to spot a long frame / short floor or short frame / long floor mismatch, but others were not so easy.
It is interesting to see that the engineer came to you with the question about the robots. The engineer seemed to only pull some data from a server and did no real root cause analysis. Data is not always the only answer.
Sounds like this plant needed to tag/code every sub-part as it was made so that the next station could read that tag and confirm tha the correct pieces were in place before beginning the next step in the welding process. A bar code tag in a known spot on every piece that would have to be read before the welding operation could be started. Could easil be done as the assembly was being moved into the first welding station. Make sure the correct parts are in place before slamming the robot into things and slowing down the process.
Hopefully thy did something like that on the next version of the line. Would really speed things up because there would be a record of the wrong parts being placed on the line, and the corresponding effort to ruduce the human errors and improve quality.
Tim; My experience with robots is that the robot is usually blamed first for any problem. Since I was usually the robot tech, I had to find the real problem, which usually was not the robot. It was not unusual for a problem investigation to be cursory, and stop at blaming the robot.
kf2qd; The Scarborough Van Plant built the 'G' van until 1993. Then the equipment was dismantled and moved to Flint Michigan. I think the 'G' van was discontinued entirely in 1995. The Scarborough plant had painting robots in the Paint Shop, and welding robots in the Body Shop - CarTrac. The rest of the plant was primarily manual assembly operations.
Every part had a part number, but that number was on the box or bin of many individual unlabeled parts. The parts were manually selected and positioned for assembly, and initially manually welded. The robots then welded the assembled frame to the assembled floor. The build information was in the PLC and was shifted to the welding stations as the carrier advanced into the station. There was no inspection capability to automatically identify the assembly to verify the manually selected parts.
"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."
The numbers the engineer presented should have included the number of times the crash was related to an incorrect bracket having been present... Ya think? At which point that becomes the issue.
And my experience with robots has given me this as a starting point as to their 'intelligence'... they are as dumb as a box of rocks.
Where a human would side-step the inconsistent configuration 99% of the time; the robot will crash into it 100% of the time.
So when a human puts on the wrong bracket and the robot trips over it... no surprise.
As much as possible sensors/identifiers could help. Idiot proofing applies to robotic assembly lines just as it does to human assembly lines. It depends how much you want to spend to achieve zero-error production.
We do a lot of fixturing that restricts against incorrect assembly, and some of our assemblies lines have vision, bar code scans, or checklists to keep both the robots and the humans on track... i.e. if they had scanned the brackets as they were applied, and that info when into that frames live tracking info...
So the process "allow" human error to create robot error.
The lesson learned, but the fix is not discussed. Can we assume a better computer tracking regimen was implemented?
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