Just wondering about weighing hoppers

First, Happy Thanksgiving!

Earlier today I watched a tour of the Hershey chocolate factory. Near the end, after the chocolate's been made into kisses, the pieces are dropped into several weighing hoppers and some controller chooses from among these which to drop into a bag to arrive at the exact desired weight. My question is, what's the algorithm to do this? Is it something in PLC logic, something purpose made by the hopper builder, what?

Thanks

it is purpose made weighing computer, choosing the weights to get nearest to the setpoint, without underfilling.
If not possible to be in margin it flushes all weighers and starts again.

You seem very sure of that shooter...what is your source?

Shooter is correct. I dont know what happens when a product with what should be pretty nearly identical piece weights cant find a weight within the low and high limits, but with products that have random piece weights the feeders feed the weigh head that has the lowest weight and tries to choose a combination that falls closesst to the target weight without going under. Some places allow underweights if there is a sufficient number of weights over declared weight

The scales will allow the user to program a target weight a high limit and what to do if it cant find a combination within the two I would say the most efficient thing to do would be to dump the closest combination above target and get on with life

I was more focusing on this part of the question:

Doug-P said:

Is it something in PLC logic, something purpose made by the hopper builder, what?

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To which shooter answered

shooter said:

it is purpose made weighing computer

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No matter what controls it the logic would be similar, but I would have thought it would be done in a PLC. Or at least, it easily could be, and I was wondering why shooter seemed so certain that it's a purpose made weighing computer rather than a PLC?

I mean these day the lines are starting to blur, so it's probably all academic at this point, but...

ASF said:

I was more focusing on this part of the question:



To which shooter answered



No matter what controls it the logic would be similar, but I would have thought it would be done in a PLC. Or at least, it easily could be, and I was wondering why shooter seemed so certain that it's a purpose made weighing computer rather than a PLC?

I mean these day the lines are starting to blur, so it's probably all academic at this point, but...

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I don't know if you could do it with a PLC or not... I have never seen a scale made with a PLC. One of the main obstacles is the filtering of vibration. they used to use a sort of a shock absorber full of oil to dampen the vibration ... a dashpot. These days they do it electronically .




It may be more involved than it seems... It is common for these scales to weigh within 0.1 gram at 120 cycles per minute.I think the actual circuitry of the individual weigh head uses current, but I believe it amounts to about .0004 volt per gram

Plus if they made it from PLCs they couldn't gouge you for replacement parts...

Plus if they made it from PLCs they couldn't gouge you for replacement parts...

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You got that right.
We have special built "Flow Scales" on our vessels that every fish must pass over, so our total quota can be calculated.
These scales have to work even with the motion of the ocean, (an at times thats a lot of motion)
This is all regulated and mandated by the Federal Government so these have to be pretty accurate and they get big $$$$ for the scales, parts and certification every year.

BCS

OK, it seems the hopper has built in logic for selecting the right combination of items to drop to get a desired weight. This question still remains - what sort of algorithm is used?

Is there more than one possible algorithm?

You could still interface a high resolution weigh head to a PLC with a serial connection (or even probably ethernet if it's reasonably new), and do all the thinking in the PLC. Not to say that it definitely is or isn't, but I can't see anything that would automatically preclude a PLC from being the brains behind it

Multi-Head Weighers and Form-Fill packing machines are a mainstay of the global food industry and are very complex very high speed systems which have to meet all sorts of regulatory as well as engineering standards. Almost all of the machines I have worked with come from a couple of Japanese companies and use custom embedded control systems. I have never seen a multi-head machine controlled by a conventional PLC though it may have been done. Bear in mind that the control system handles the product distribution as well as the weighing functions and there can be very large numbers of individual weighing pans on a single machine which can be carrying out weighing operations at rates of more than one a second.

Ishida makes an awesome multi-head weigh feeder. There is a vibrating "hat" on top and a bunch of "bukets" with load cells. The hat vibrates until there are enough scales with the right total to make the final batch weight, then they dump into the container or a 2nd set of scale buckets and the process repeats. They are fascinating to watch and I never really had to work on one in detail enough to analyse the details of exactly how the logic inside the thing works.

You could do it with a PLC, but it might take a few years to adequately reinvent the mechanical parts of the wheel. In the end, you would have the advantage of common off the shelf replacement parts, but the mechanical design of Ishida would be hard to top and that is where the reliability needs to be. Their electronics are pretty salty, well protected and rarely fail even in extremely harsh wash-down environments.

Doug-P said:

OK, it seems the hopper has built in logic for selecting the right combination of items to drop to get a desired weight. This question still remains - what sort of algorithm is used?

Is there more than one possible algorithm?

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I am not going to tell you because I don't know... the manufacturers are not going to tell you because they don't want anyone to know. The factory reps only know so much ... I think there are two guys in Japan who know... Mr. Ishida and Mr. Yamato

If you propose to make one of these from a PLC, I would say you're gonna hafta figure it out yourself. I don't mean that to be a wise guy, but I think that's pretty much how it is. I also think that is what attracts guys like you to this stuff anyway, isn't it? You like figuring things out and making machines do what you want them to do.

It would be sort of like any other customer would tell you. They know what they want the machinery to do. It is up to you to figure out how to do it... You'd have to figure out your own algorithm

In the end, if your machine is going to be competitive, it is going to have to control the feeding conveyors, the buckets, and the packaging machine and weigh to 0.1 gram at 150 packages a minute.. and do it in about the least conducive environment possible. Vibrations, chemicals, water And to be honest... there are lots of products weighed at much slower speeds

If you have a reason beyond curiosity to be interested in doing this, please PM me. I would be interested to know what you're up to

realolman said:

I also think that is what attracts guys like you to this stuff anyway, isn't it? You like figuring things out and making machines do what you want them to do.

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It does have a certain appeal.

realolman said:

If you have a reason beyond curiosity to be interested in doing this, please PM me. I would be interested to know what you're up to

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Nope, not up to anything, just idle curiosity - like, is it something stunningly simple; or something deviously complex? Just how do they do it?

Just how do they do it?

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I can make a WAG.

Each bag is made up of candy kisses from several weighing hoppers (possibly 2 or 3 dumps from several weighing hoppers). Each hopper is calibrated to contain a certain percent (20 to 30 % maybe) of the total bag weight. Due to variations in the weight of each kiss, each hopper will have a range of possible weights. The logic selects the hoppers to dump the hard way: select a 1st test hopper (perhaps the heaviest hopper), then checks to see if a combination with the next hopper will be under or over the bag weight. If under, then the 3rd hopper is selected, and again the weigh is compared to find if it is Over or Under the desired weight. If over, the process next looks sequentiallly at all remaining hoppers. If none are found of the proper remaining needed weight, the process starts over with a lower-weight hopper now being first.

If under, the process continues selecting hoppers, until enough weight has accumulated to be the minimum required to be a "full" bag according to customer specifications (within certain tolerances +/- some % accuracy). When this combination is determined, only then is the physical dump made. It takes a long time to describe, but a PLC or computer can do calulations so fast that it appears to be instantanous.

Lancie1 said:

I can make a WAG.

Each bag is made up of candy kisses from several weighing hoppers (possibly 2 or 3 dumps from several weighing hoppers). Each hopper is calibrated to contain a certain percent (20 to 30 % maybe) of the total bag weight. Due to variations in the weight of each kiss, each hopper will have a range of possible weights. The logic selects the hoppers to dump the hard way: select a 1st test hopper (perhaps the heaviest hopper), then checks to see if a combination with the next hopper will be under or over the bag weight. If under, then the 3rd hopper is selected, and again the weigh is compared to find if it is Over or Under the desired weight. If over, the process next looks sequentiallly at all remaining hoppers. If none are found of the proper remaining needed weight, the process starts over with a lower-weight hopper now being first.

If under, the process continues selecting hoppers, until enough weight has accumulated to be the minimum required to be a "full" bag according to customer specifications (within certain tolerances +/- some % accuracy). When this combination is determined, only then is the physical dump made. It takes a long time to describe, but a PLC or computer can do calulations so fast that it appears to be instantanous.

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I like it.

I was toying with this idea: Since the weight difference between kisses would be miniscule a hopper could be weighed and the weight converted to an integer number of pieces. Knowing how many pieces make up the weight needed for the bag being processed, one could then proceed as Lancie1 describes. This would eliminate having to worry about fractions of an ounce. I don't think this would work for loose items like potato chips.