Automatic Weighing dosing control?

We have a new machine at work that is supposed to automatically weigh correct amounts of chemicals being dispensed in a bucket. The hoppers that
dispense the chemicals use screw auger or vibratory feeders. The control has a coarse dosing and a fine dosing. The programmers are having difficulty getting the system tuned to work for all the different weights. For example, one recipe may require 1 lb of a certian chemical and another recipe may require 10 lbs.
They are trying to program the thing in percents, for example a 10lb weightment may work for 80% feed to 70% of the weight and 20% feed for 97% of the weight, and the inflight material makes up the other 3%. The programmers are wanting to save all 4 of these parameters for every weight for every chemical. Shouldn't this be scalable? I thought it might be better to set up coarse dosing to 70% fill at high speed and perhaps 97% at low speed. Perhaps low speed would be 20% of high speed and high speed would be proportional to the weight of the dosing.
Anyone here know rules of thumb or better algorithms for weightment control?
Thanks!
Roger_N
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Well it aint rocket science. Working in percentages of the total weight is very dumb. If the important stuff happens in the last half kilo having filled 1, 10 or 1000 kilos previously makes sod all difference.
The important thing is the fill rate and how long it takes to switch to a lower fill rate (including 0) and how much is in flight at whatever fill rate.
If you are really writing software to control the machine then just make a guess at how much is in flight when you cut off the slow speed fill - you will know how far out you were after the first fill so you can improve your guess and carry on modifying your guess to compensate for variation in the machine and product.
For the high speed fill guess the same erring on the short side then use the duration of the low speed fill to adjust your guess. Aim for minimum duration of the low speed fill which is long enough to correct the worst variation in the high speed fill.
I did systems like this 30 years ago without any software. A big auger did the bulk fill counting auger turns, the container then indexed to a small top up auger and load cell. The operator manually set the cut off weight for the top up fill. A timer was triggered with the top up fill. If the top up fill finished before the timer the bulk fill count was nudged down, if it finished after the timer it was nudged up. The operator tweaked the timer downwards to optimise speed while ensuring at least a bit of top up always occurred.
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nospam wrote:

About all I can add to this is "yea verily". There may be better ways to solve this problem, but I can't think of them.
The idea of using system behavior in prior runs to determine current behavior is dead clever - I've used this sort of thing before in very different circumstances (not weighing, for one thing) and it has worked out well, as long as I paid attention to the run-run variability.
--

Tim Wescott
Wescott Design Services
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Sorry about the slow reply, I wasn't getting my messages from this group and some of the others. Yeah, we are mixing up to 9 chemicals in a bag. We have some recipes that call for as low as 0.07 lbs and the same station may have to dose 4 or 5 lbs for a different recipe. If a weight comes out wrong, it stops at that station and we can add or take out to get the weight right.
Thanks RogerN
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