Mr

Hello , does anyone have a generic plc program that I could look at which shows the logic that would be required to load a tote sitting on a floor scale? The floor scale has a 4-20ma signal. There’s one automatic on/off valve and there is one push button station that has momentary stop and start buttons. There is also a pump that is controlled by a VFD.
Thanks

I assume you mean the 4-20ma is from the scale or loadcells so detecting weight I assume you want to put an empty tote bin on the scales, snapshot the weght, run the pump to fill the tote to a weight then stop the pump is tht correct.
If so it is simple but what PLC are you going to use i.e. AB, Siemens, Mitsubishi etc.

Yes you are correct. We have controllogix and rslogix5000

Thanks

I do not have RSL installed at the moment but I can quickly make up some code in another PLC IDE, this is something I have done a number of times, do you have an HMI on the system for putting in the required weight ?
A bit more information would be helpful for example is the 4-20ma weight signal directly from the loadcells or from a weight indicator, if from a weight indicator/controller does it have an inputs to tare or gross the weight.
If not, then you will need some way of taring the standing weight of the weigh pan when the tote bin is in place, this can be done in the PLC simply by storing the un-filled weight from the scales then subtract that from the actual weight, when the required weight is reached that will be the actual weight.
This is how I see it working:
A tote is placed on the scales, a button (Start) is pressed, this zero's the scales or as above subtracts the weight from the start weight, after a short time the valve will open, pump start & ramp up to the required speed, assuming you have some sort of target weight (perhaps an HMI ) when the current fill weight reaches required weight less a pre-set slow down weight the pump slows down, when the weight in the tote reaches required fill weight (minus perhaps some in-flight weight to allow pump to ramp down) then stop the pump & close the valve. When you remove the tote bin the weight on the scales should go back down to 0, however, the weight on the reading in the plc will be negative as it is still tared, how you remove the tare weight will depend on how you want to handle it, you could just let it tare again when a new bin is placed or perhap it will automatically tare after the fill, but in that case the weight will be the total weight of the product plus the tote bin & any offset of the scales.
Without knowing what hardwareyou have what the exact control sequence you want it is difficult to give any code.
So perhaps write a little bit about how you see the operation of the bin filling giving details like how you determine the weight you want, how you wish to tare the bin weight, how you gros it again etc.

Yes, I’ve built the graphics on the hmi, it’s FactoryTalk. I have a field for the actual weight, one for the target weight and a field for the pre-act. Unfortunately the scale indicator is not that smart. I only have the 4-20mA card installed. No other inputs/outputs. The tare will be done manually at the indicator. Yes again, with the VFD, I can control the motor speed from the HMI. Say for example the target weight is 2400 and the pre-act is set to 200. Thanks for your detail.

Chris

Try one of these three links; there might be a program in one of the downstream URLs.

tail


dribble


spill

I couldn’t find anything there, thanks anyways

I have just coded something it took me about an hour, it uses a sequence step it is a bit crude & not tested yet & not in RSL but will give you an idea, I will give it a quick test tomorrow & post a pdf & screenshots.
This is my interpretation of it & it is a way I would do it if I could not tare the scales without going over to them, it is something similar to some I have done.

scstroud said:

I couldn’t find anything there, thanks anyways

Click to expand...

There was this: http://www.plctalk.net/qanda/showthread.php?p=41597#post41597

The next best thing to a coded example is a reasonably detailed description of an algorithm from a working system.

It's up to you to code the rest; I wouldn't think it would take more than a handful or two of statements/rungs.

The result was ±0.1%, which is probably dependent on the consistency of the feed system, but which I suspect is otherwise good enough for most situations.

Several other threads mentioned a dribble mode, which dropped the fill rate when near the setpoint; assuming you can reduce the feed rate, a dribble mode with the algorithm above might be good enough.

scstroud said:

Unfortunately the scale indicator is not that smart. I only have the 4-20mA card installed. No other inputs/outputs.

Click to expand...

"Unfortunately?" Do you mean you are not sure how to convert from the 4-20mA A/D digital value to get a weight with the same units as the values input from the HMI?

Here is a simple bit of logic, done in FBD & lad (tried to make it more like RSL but you can see the structure.
In this code, it is assumed that the 4-20ma has been converted to a float type, the process does not require a tare to be completed as it calculates the actual weight by subtracting the scales offset (usually is a small variation) plus the tote weight so effectively we get product weight of 0, the process is as follows.
On pressing the start button the system checks that the scales are empty (within for example +/- 10kg), if not a message is displayed that it is outside the start weight, once corrected, it requests a tote be placed, again it checks that the tote has been added (in my case the weight should be between 150 & 210kg), then it asks for the operator to confirm, the process opens the feed valve, starts the pump at high speed (100), when the weight gets within 50kg of target the pump slows to 50, when weight is reached it stops the pump & after a short delay closes the valve.
It then prompts the operator to remove the tote & accept, this then sets the process to idle.
I have used a sequence variable for the steps as the HMI should be able to use this as a text list to display the status based on the sequence number.
You will notice that normally I have the steps in multiples of 10, this is to allow extra steps to be added without having to move the compares of the step numbers to control the process, I have shown this by adding intermediate steps in this case 5 & 15, It is also useful if you wish to step backwards to a previous step.
Excuse the poor graphics but this is just to test & display the process.

What type of pump is it? Can it be dead headed without issue? Is this a new system or an existing one? Would it be possible to add a recirc line and valve?

For positive displacement pumps, dead heading is not an option. On systems I have used in the pass, we had a recirc line and valve.
When you reach preact, open recirc valve, wait for weight, then close fill valve. Pump never stops.

If you don't have a recirc, then you may need two preact settings.

Sequence:
1. start PB pressed
2. Tare scale
3. open valve, start pump at min speed.
4. pump set to max speed.
5. When preact one is reached, slow pump to min speed.
6. When preact two is reached, stop pump. wait x.x seconds, close valve.

Yes that's what my code does, the pump stops & short delay before closing the valve, I have deliberately left out a stage (in-flight) as stated, there is room to add extra stages that's the reason for steps of 10, those thoughts crossed my mind i.e. type of pump (PD/CF) possibly a divert valve rather than a simple open/close. There is little information regarding the load cell controller, most I have come across have digital inputs that can be configured for things like tare/gross, batch & remote print etc. I have done many systems like this for example, coms, digital control even soldering wires to the keypads so the buttons can be driven from volt free relays. one system I did, inserted a separate valve with a 10mm hole in the butterfly, this was normally open when the required weight minus say 20kg the valve closed to allow trickle the product before closing the main valve the product was fed from a ring system, many of these were to overcome the problems of remote control and accuracy at a lower cost (par for the course for some companies).