The Line's Too Fast



  • Something about today's WTF (One At a Time Please).... probably running databases with Excel spreadsheets.... reminded me of my first co-op placement years ago.

     We were an industrial automation shop, and I was working on SCADA interfaces. One of our customers, the plant engineer at the local auto factory, phoned up. We had installed a PLC system to control the electric drives pulling the whole assembly line, complete with a nice SCADA interface and emergency stop controls.

    The union boys were complaining that the assembly line was running too fast. You see, one of the displays that showed a schematic of the entire assembly line, along with some stats (AC drive status, line speed etc.) sat in the plant maintenance shop. The local union steward would check in once a day to see how many cars they'd made so far that year (counted by the SCADA app). Well, the display also showed the line speed down to the thousandth of an inch per hour.  The union contract said X inches / hr.  The display was showing X.001 inches / hr. 

     We could call up the actual values direct from the AC drives that showed (the equivalent of) exactly X, and we assured them that it was a harmless rounding error. These were the days of Windoze 3.1 after all... 16 bit stuff.  (The fact that the requirements from the customer had been to show up to 1 decimal place of accuracy and the additional two were .. uh... features,  was lost on everyone)  If you went to the line with a stopwatch and a ruler, you could measure that it moving at the correct speed.

    Now, the rational person would do the math and consider that this + .001 equated to something like an extra 8.76 inches ... per year. Not a big deal right? Well the union folks calculated that this would produce an extra car every 12.5 years. Thats right, the suits were taking advantage of the hard working folk again, and they insisted that we "slow that line down to exactly X inches / hr" as per the latest union contract.

     The tricky part was that the drives were calibrated to run the line in only 2-3 preset speeds for different shifts... getting all the drives calibrated for a new speed (or changing a preset speed) required several days of extensive recalibration, during which the line would be shut down - not an option for an auto plant running 3 shifts 7 days a week.  "Isn't there something you can do?" asked the tired plant engineer who really wanted the union steward to stop camping out in his office.

     So we opened up the SCADA application and just tweaked the scaling factor between the raw line speed (in millifleems per doozlehoofer) and the value displayed on the screen in inches / hr.  It was amazing how that value was always amazingly accurate from that point on..



  • I love pedants



  • Why not just truncate the output to what they requested? Altering the math would produce bugs in the future would it not?



  • @Lingerance said:

    Why not just truncate the output to what they requested? Altering the math would produce bugs in the future would it not?

    But these are millifleems.

    That's perfect. 



  • A millifleem is?



  • @Lingerance said:

    A millifleem is?

     An engineering unit. Often in SCADA applications what you actually read is a voltage input from an ADC or something. It often won't be precisely calibrated to an absolute value, so it's typical to have a conversion factor in the software somewhere. It's like having an amplifier go up to 10, the actual raw number you read off the dial doesn't mean a lot.
     



  • @tezoatlipoca said:

    Now, the rational person would do the math and consider that this + .001 equated to something like an extra 8.76 inches ... per year. Not a big deal right? Well the union folks calculated that this would produce an extra car every 12.5 years. Thats right, the suits were taking advantage of the hard working folk again, and they insisted that we "slow that line down to exactly X inches / hr" as per the latest union contract.

    This may sound pedantic, but I suspect they wanted to avoid the slippery slope. If an extra 0.001 inches per hour is irrelevant, then how about an extra 1 inch? 10 inches? Or how about an extra car every week? Every day? They want to be able to point at the measurement and wave the contract at the employer if/when the latter does attempt to insert a few extra cars per day hoping the employees won't notice. If they don't enforce the measurement very strictly, they may lose this argument.

    This sort of thing is common in many sectors where employees can work only X hours/day by law (eg. lorry drivers) to avoid sleeping on the job and the resulting accidents. The employer tells the employee to work X hours and 15 minutes; the employee decides 15 minutes isn't worth quitting and finding another job. After this, the employer raises the bar to X+30 minutes; and so on, until the employer ends up working day and night or finally quits.

    I'm not defending the union, because they completely missed the fact that the employer is in the position to tamper with the measurement without raising suspicion anyway. Who knows, the OP might have been asked to make the display go down by 1% every two weeks while the line goes faster correspondingly. The OP actually did fudge the calculations, and the union never knew or even considered the possibility.



  • @tezoatlipoca said:

    The union boys were complaining that the assembly line was running too fast. You see, one of the displays that showed a schematic of the entire assembly line, along with some stats (AC drive status, line speed etc.) sat in the plant maintenance shop. The local union steward would check in once a day to see how many cars they'd made so far that year (counted by the SCADA app). Well, the display also showed the line speed down to the thousandth of an inch per hour.  The union contract said X inches / hr.  The display was showing X.001 inches / hr.

    Why not just say “Well, the display’s wrong” and leave it at that?



  • @Brother Laz said:

    This may sound pedantic, but I suspect they wanted to avoid the slippery slope. If an extra 0.001 inches per hour is irrelevant, then how about an extra 1 inch? 10 inches? Or how about an extra car every week? Every day? They want to be able to point at the measurement and wave the contract at the employer if/when the latter does attempt to insert a few extra cars per day hoping the employees won't notice. If they don't enforce the measurement very strictly, they may lose this argument.

    This wouldn’t be a problem, since they measure cars per day directly. Measuring the line speed is foolish: if the employer really could get away with having them make more cars per day but for that measurement, they could just put the cars 0.001 inches closer together on the line.



  • It sounds like it would have been easier to just make the display read X inches / hr, regardless of the actual line speed.



  • @Random832 said:

    @Brother Laz said:

    This may sound pedantic, but I suspect they wanted to avoid the slippery slope. If an extra 0.001 inches per hour is irrelevant, then how about an extra 1 inch? 10 inches? Or how about an extra car every week? Every day? They want to be able to point at the measurement and wave the contract at the employer if/when the latter does attempt to insert a few extra cars per day hoping the employees won't notice. If they don't enforce the measurement very strictly, they may lose this argument.

    This wouldn’t be a problem, since they measure cars per day directly. Measuring the line speed is foolish: if the employer really could get away with having them make more cars per day but for that measurement, they could just put the cars 0.001 inches closer together on the line.

    I'm sure they produce an integer number of cars every day.  Oh, they don't....


    I'm sure quantifying fractions of a car is easier than quantifying inches per hour.  Oh, no, it isn't....



  • @poopdeville said:

    I'm sure they produce an integer number of cars every day. Oh, they don't....

    I'm sure quantifying fractions of a car is easier than quantifying inches per hour. Oh, no, it isn't....

    How about "if it's not done, it doesn't count for that day, it counts for the next day"? Or is that too simple for you? This was STATED in the original post, that they were looking at the number of cars produced (for the year so far, but you think that'd be enough to catch "a few extra cars per day" - or even per year - but you just won't admit that you* were talking out of your ass there.)

    *yes, not you you, but you're taking his side. 



  • I would have taken an alternative approach and said that we didn't believe that the line was running any faster, but if it did produce more cars (e.g. one extra car per 12 years or so) then that car would be donated to the union to be used as a prize in a raffle or some such good cause. Turning that down would be difficult to justify surely!


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