Fill by weight

[Clay B.]

think I see why you have a ramp function versus a "trickle" function. You are feeding from a vibrating bed. When this is done a ramp down function is prefered because you can better control the feed rate. Vibration tables tend to be non-linear to the speed of the vibration motor. So be ramping the speed down and monitoring the rate of change on the weight you end up with a more accurate final weight.

If you were using a fill valve the most common method is to have a trickle position. This way your flowing at a slow enough speed that you can stop accuratly.

Also Note: If you take the PLC route. Not all PLC's have load cell inputs. You may still need a controller.

 

[Longhorn]

Longhorn - just curious, why the initial 'fine fill'?

We use plastic bottles and this initial weight stabilises the bottle. Coarse fill would make the bottle move due to the initial high flow!

 

[spaderkung]

I think the key issue here is to separate the filling into coarse/fine regardless of method to make it fast _and_ accurate.

Filling to 90% of the set-point is probably possible with a fixed filling time. And if it is possible to reach 100% within tolerance after calibration then problem solved with one parameter - fill time.

A "PID" that is fast could also have overshoot. And in this application that is not allowed. If "PID" is used at all then two sets of parameters are necessary - fast/slow (equivalent of coarse/fine using different size of pipes).

An easily adjusted compensation time (see in-flight) will be helpful.

 

[Longhorn]

It must be the opposite first coarse fill and fine fill at the end.

Fine fill first - to stabilise bottle or container
Coarse fill - to fill bulk volume
Fine fill finish - to trickle increase to target weight.

We use in-flight correction on one of our fillers, the weight of the container at the point of the valve closing is taken and subtracted from the final weight to get in-flight weight.

Next revolution round this is subtracted from the final fine fill weight e.g 200g changes to 195g.

hope this helps!

 

[CharlesM]

We use in-flight correction on one of our fillers, the weight of the container at the point of the valve closing is taken and subtracted from the final weight to get in-flight weight.

Next revolution round this is subtracted from the final fine fill weight e.g 200g changes to 195g.

Good info Thanks

 

[SMOKE]

We use in-flight correction on one of our fillers, the weight of the container at the point of the valve closing is taken and subtracted from the final weight to get in-flight weight.

Next revolution round this is subtracted from the final fine fill weight e.g 200g changes to 195g.

That would only work with a single speed fill No PID. You would need a very consistent feed rate to make that work.

 

[Longhorn]

Smoke - You are absolutely correct, and this is exactly what happens. This is achieved by a seperate PID Loop to control the pressure inside the "bowl" above the fluid. Keep in mind i am talking about a liquid filler here.

LH

 

[widelto]

Fine fill first - to stabilise bottle or container
Coarse fill - to fill bulk volume
Fine fill finish - to trickle increase to target weight.

We use in-flight correction on one of our fillers, the weight of the container at the point of the valve closing is taken and subtracted from the final weight to get in-flight weight.

Next revolution round this is subtracted from the final fine fill weight e.g 200g changes to 195g.

hope this helps!

I like it, good point, i´ll try it in my design.

 

[Peter Nachtwey]

I once ask Pete if Delta made a scale interface. He said that they had made some customs. I would talk to him.

There are others that have off the shelf systems. We are not a good option unless one needs a custom system. However, my business part did design some excellent strain gauge amplifiers in the early 90s.

However, if I had to get the job done quickly I would use a small single axis motion controller with analog inputs, but that is just because they seem to grow around here. Faster scans, faster analog faster digital, easy to use Ethernet interface, better diagnostics. How can a PLC compete with that?
It wouldn't be the first time a motion controller got used for something else.

 

[CharlesM]

However, if I had to get the job done quickly I would use a small single axis motion controller with analog inputs, but that is just because they seem to grow around here. Faster scans, faster analog faster digital, easy to use Ethernet interface, better diagnostics. How can a PLC compete with that?

I might be able to do that. I just happen to have a RMC150 with a scale attached. I need to find something to feed the material then I could test it. Would you use quick moves for the fill?

 

[Peter Nachtwey]

My reply was half in jest. I would use anything before suffering with an S7

A quick move would probably work well but I would try using taking advantage of the raw speed and go as fast as I can in open loop. I would compensate for the in-flight material. Eventually you will approach a time where the in flight material plus what you have filled will be very close to the desired fill weight. Think of the in-flight time as a dead time and the in flight weight is proportional to the control output. When getting close to the fill weight I would ramp down quickly to about 10% or so and top it off.

The in-flight material would be proportional to the sum of control output signals for the in-flight time. Also note that if the material drops 1 ft then the in-flight time is 250 milliseconds and if the sample rate is 500 micro seconds then the in-flight buffer would need to be 500 control output readings long.

Don't sum the whole array each scan.
Total:=Total+New-ControlOuputQ;
ControlOuputQ:=New;
I:=I+1;
If I>=LengthOfControlOutputQ THEN
I:=0;
END_IF
EstimatedWeight:=MeasuredWeight+K*Total;
Where K is the relationship of the control output history to the in-flight material. You should be able to calculate K from the feeder rate as a function of the control signal and time.

A RMC150 seems like gross overkill and do you have the analog inputs already? At least you can do a proof of concept quickly.

 

[Thim]

What is "in-flight material"
I checked the net but didn't found anything relevant.

 

[Peter Nachtwey]

What is "in-flight material"
I checked the net but didn't found anything relevant.

In flight time is similar to dead time. Material that is in the process of dropping on the scale but hasn't hit the scale yet so isn't being measured. If you dropped material from 16ft or 4.9m it would take a second for material to hit the scale after you stopped the feeder or conveyor. If you stopped the feeder only when the fill weight was reached you would find that you have one extra second worth of material on the scale. It takes a surprising long time for things to fall when dealing with computers.

 

[CharlesM]

My reply was half in jest. I would use anything before suffering with an S7

I wanted to say something but I let it pass. I was once asked "Why would you do that in the motion controller when you can do it in the PLC" I get along fine with S7 but I don't get to advanced. Have you looked at the S7-1200? It's their new low end controller but I think it will replace the 300 & 400 in a few years. It is only ladder now but they do have blocks for working with arrays.

A RMC150 seems like gross overkill and do you have the analog inputs already? At least you can do a proof of concept quickly.

Yes the 150 is in my demo machine and I have a scale connected and working. If I can find something for a feeder I will give it a test.

 

[Thim]

Thx Peter,

So it is what we call Afterfall, i wasn't familiar with the term "In-Flight"

Maybe stupid remark, isn't the vibrating feeder a slow stopping something?