Tool_maker; I'm guessing you mean a tag applied to the part. For most of the parts, the part itself was not individually identified by a tag or code. Only the bin or box containing the parts had an identifying code.
There were several cross rails, and it could be a case of two cross rails in the wrong sequence - the parts were correct, but in swapped locations. In other cases it could be an extra, unnecessary part added.
When the operator at the station where the frame was mated to the floor identified an actual build vs. build data mismatch, an electrician could edit the PLC bit data to correct the build data in the PLC.
@kfqd: I have never worked in the sort of environment described so this question may be silly. If the operator can manually select the wrong part, is it not possible that another operator could apply the wrong tag/code?
"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."
Actually when I said crash I was thinking of this. I'm not sure about the robots on the GM line but ours don't always do well recovering from a crash...
We do that assembly line sharing stuff at times too... When a multiple use line is running, maintaining a clean separation between the small stuff, like optionals can be tough. I am not saying we do it perfectly, but we do strive to learn and improve. ; )
And I get what you're saying about quality. Robots are amazing at repeating the same process over and over in a very precise way (fast too the little buggers).
Ralphy Boy; The part tracking was not changed. The 'G' van was phased out about 2 years later, so no significant process improvements were planned or implemented.
One of the reasons that robotic automation improves quality, is robots will not work with 'junk'. A human welder can adjust the fit of a bad part and then weld the assembly. The robot welds where the part was supposed to be, and the assembly then fails.
"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?
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.
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.
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.
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