Logix 5000 Question - PID loop or no?

cpfanstiehl

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Join Date
Jul 2019
Location
RHODE ISLAND
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Good morning guys,


I have a machine that fills bags of prepared food at a variable speed, and a machine that checks the weight of the bag. What I am trying to do is to use the weight checker to speed up or slow down the bag filling machine so that it will automatically adjust the speed for the set weight.

The idea is that both machines have Allen Bradley controllers with ethernet comms and I use either on board PLC for the control or i use an external one i will be adding.

I'm trying to think how i would do this in ladder logic and a PID loop comes to mind but I wouldn't even know where to start to set it up...

Help/Ideas?
 
Honestly, get yourself a dedicated weight controller and interface the PLC with it rather than attempting this on a PLC. Nowadays weight controllers have some "learning abilities" and can tweak themselves into a perfect weigh everytime.
 
A PID is generally not the right solution here, since most of the time, you want the bag to fill as quickly as possible.



A more typical arrangement, is to have a full-fill feeder, and then another dosing type feeder. When the weight reaches some amount from full (depending on feed system and product, might be 95%, might be 80%, that depends), the full feed shuts off, and you pulse the dosing feeder (or modulate the full feeder) to achieve the final weight, giving time between doses for the scale to stabilize.
 
cardosocea said:
Honestly, get yourself a dedicated weight controller and interface the PLC with it rather than attempting this on a PLC. Nowadays weight controllers have some "learning abilities" and can tweak themselves into a perfect weigh everytime.
Click to expand...
What is a dedicated weight controller? do you have one you recommend? or are you saying the weight checking machine should be able to adjust the fill?


Also, the weight checker I'm using is a mettler toledo C3350 if that helps.

Thank you for the response!
 
rdrast said:
A PID is generally not the right solution here, since most of the time, you want the bag to fill as quickly as possible.



A more typical arrangement, is to have a full-fill feeder, and then another dosing type feeder. When the weight reaches some amount from full (depending on feed system and product, might be 95%, might be 80%, that depends), the full feed shuts off, and you pulse the dosing feeder (or modulate the full feeder) to achieve the final weight, giving time between doses for the scale to stabilize.
Click to expand...
A more typical arrangement, is to have a full-fill feeder, and then another dosing type feeder. When the weight reaches some amount from full (depending on feed system and product, might be 95%, might be 80%, that depends), the full feed shuts off, and you pulse the dosing feeder (or modulate the full feeder) to achieve the final weight, giving time between doses for the scale to stabilize.

I may be reading this wrong but I'm not sure that will work as the bag filler is a Vertical bag filler and sealer. the machine calls for product into a chute. volume is adjusted by adjusting pump speed(from the machine). machine is a rovema BVC-310
 
I agree with all the others, PID is not required, just subtract a given amount off the required fill weight & use this to slow the feeder to a trickle, PID will really slow the fill down, as the idea is as it gets closer to set point it slows the feed. this will slow the filling process to a very small trickle when approaching the set point. Batch filling is not a PID operation, if required you could have more than one set point for slow down i.e. at 80% of fill weight, run speed at half, at 95% run at quarter speed or any combination.
 
cpfanstiehl said:
A more typical arrangement, is to have a full-fill feeder, and then another dosing type feeder. When the weight reaches some amount from full (depending on feed system and product, might be 95%, might be 80%, that depends), the full feed shuts off, and you pulse the dosing feeder (or modulate the full feeder) to achieve the final weight, giving time between doses for the scale to stabilize.

I may be reading this wrong but I'm not sure that will work as the bag filler is a Vertical bag filler and sealer. the machine calls for product into a chute. volume is adjusted by adjusting pump speed(from the machine). machine is a rovema BVC-310
Click to expand...
Also, they want least amount of pumps possible to cut down on solids degradation.
 
parky said:
I agree with all the others, PID is not required, just subtract a given amount off the required fill weight & use this to slow the feeder to a trickle, PID will really slow the fill down, as the idea is as it gets closer to set point it slows the feed. this will slow the filling process to a very small trickle when approaching the set point. Batch filling is not a PID operation, if required you could have more than one set point for slow down i.e. at 80% of fill weight, run speed at half, at 95% run at quarter speed or any combination.
Click to expand...
This was the other end of what I was thinking. I wasn't sure if there was a specific instruction or AOI that already exists for something like this.


Thank you everyone for the quick responses! What a helpful community.
 
Why an AOI it's a simple bit of code. two compares & moves.
so when the actual is less than set point minus the trickle weight move high speed to drive, when greater than set point minus trickle move low speed to drive.
 
Calculate the "Spill" every load and adjust on the fly as this will change with humidity and temperature.

Keep an average, and use this spill to shut the valve off on the next fill cycle.

Can you fill at different rates, full speed and trickle?
 
cpfanstiehl said:
The bag is being weighed after the its filled and sealed. The final product is what we are checking and adjusting for.

The infeed flow and timing of fill/seal are the factors of how much the bag will weigh. Also, type of product as well.
Click to expand...

I would store in a FIFO buffer the negative and positive "spill" values after weighing and keep an average in the PLC per product type.

I would average the spills and apply the average to my new cutoff time, or what ever you are using to stop the fill sequence and start the seal function.
 

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