9 Hopper Potato Bagger

[ds7891]

Hey guys, I am looking at upgrading a machine from an old PCB controlled process with a membrane button control interface, to a PLC with a touchscreen.

Im in the stages of quoting and trying to figure out how I would tackle the logic to handle the 9 hopper sorting.

Currently, the machine works like this:

1. Potatoes roll down the main chute to a divider which rolls roughly 3 potatoes into each hopper.

2. Each hopper has a load cell on it and it displays the individual hopper weight on the LCD display

3. After all the hoppers have been filled, the machine pauses for a split second, then opens any combination of hoppers which equate to putting the combined weight just over the setpoint (i.e. - 10lbs if needing 10lb bags). It could be any random combination of hoppers.

4. The weight need to ALWAYS be equal or great to 10.00lbs and typically is only over by 20-50 grams. Of course depending on how the hoppers are stacked up.

5. I also need the ability to disable/enable hoppers if they are pulled for maintenance.

I have the load cell amplifier picked out, and have know how I will tackle the rest of the code on the machine. Its just the above I keep scratching my head on.

What would the process of logic look like for something like this?

No, there is no way to extract the code from the ROM chip on the big ol' PCB board.

To keep cost down, I am looking to use Automation Direct Cmore and BRX hardware. FYI.

 

[ds7891]

After using the right "keywords" in google, I came across this:

http://www.plctalk.net/qanda/archive/index.php/t-91753.html

Its been 7-8 years since I've been around a Multi-Head Weigher.

I may be in for a treat here. Or, not at all.

 

[GaryS]

Sounds like an interesting project
I have worked on a few of those in the past in candy factories
the biggest problem will be to come up with the necessary algorithm to the right combination to get the weight you need and to do it at high speed.
are you reading the 9 scales from a digital signal or an analog output
I would guess that you are using analog signals
keep us updated on you progress

 

[ds7891]

I would guess that you are using analog signals
keep us updated on you progress

Looking to put in a multi-channel Rice Lake's in with Modbus.

 

[GaryS]

Rice-Lake or Hardy both good choices
I thought you were going to reuse the original scales modules before
but you made a better choice by replacing them altogether if you can sell it the boss
I assume that it's Modbus over ethernet serial Modbus would be to slow for this application
I would still try to get away from Modbus altogether and see if they have ethernet IP
Good luck

 

[AustralIan]

Laumas out of Italy have good bang for buck and bang for cabinet space weigh cell to fieldbus units.

Brute force it is best.

So you have 2^9 possible combinations (512)

BestW:= 99999.99;
BestCombo:= 0;
MinW := 10.0 / lbsToKg;
For i := 1 to 511 do
  Weight := i.0 * hopper0W + i.1 * hopper1W
                + i.2 * hopper2W + i.3 * hopper3W
                + i.4 * hopper4W + i.5 * hopper5W
                + i.6 * hopper6W + i.7 * hopper7W
                + i.8 * hopper8W;
  Maint := (i.0 AND hopper0M) OR (i.1 AND hopper1M)
                OR (i.2 AND hopper2M) OR (i.3 AND hopper3M)
                OR (i.4 AND hopper4M) OR (i.5 AND hopper5M)
                OR (i.6 AND hopper6M) OR (i.7 AND hopper7M)
                OR (i.8 AND hopper8M);
  IF (NOT Maint) AND (Weight >= MinW) AND (Weight < BestW) THEN
    BestW := Weight;
    BestCombo := i;
  END_IF;
END_FOR;

// Error handling... If combo = 0, no possible combos above lim

// Actuations.. if BestCombo.0, activate hopper0, etc..

 

[parky]

I don't know how fast this system has to be, I did many projects interfacing with a well known weigher manufacturer. The speed that they worked at required specialised hardware & software, generally most PLC's were not fast enough for the application, this also applied to the load cells used. They were even looking at projecting the weight in each bucket by analising the wave form from the load cell to predict the possible final weight in each bucket (no idea if they actually got it to work), saying that, potatoes are large in comparison to any product I have witnessed on these weighers so perhaps speeds of 50-299 packs a minute is probably not possible. Also PLC's have come a long way since those days regarding speed & maths capability. Also you will to check the speed of the analogue conversion or communications if it is that type. But it sounds an interesting project, I would certainly make sure that their requirement can be met with the hardware/software you intend to use.

 

[ds7891]

I wonder if it there is someone out there that makes a controller which can be be installed with the everything else; handling the logic of the hopper selection.

I will ask Rice Lakes on Monday if they know.

 

[Toine]

One additional aspect you may want to take into account if you will be programming your own solution: some kind of maximum wait time per hopper. If one hopper has at some point a weight that is significantly different from a typical situation, you can imagine it never gets chosen to unload because it does not make the batch right. After some timeout is should be forced to unload anyway in order to avoid product waiting too long in one single hopper. Never a good thing in food applications.

I know a dutch company (US ownership) who have done this kind of solution before. If I am not mistaking it was a high speed multi hopper dispenser in the food industry. I believe it was for dosing potato crisps in bags of a fixed weight. They do all kinds of weighing and dosing solutions, have done things like yours too: dosing fruits, vegetables, eggs, fish in batches of same weight. We use some of their products, not of this kind though.