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Old April 28th, 2021, 04:52 AM   #1
MayadaAhmed
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Pencil Belt feeder design

hello,
i am trying to design a conveyor belt feeder , to output mass flow rate of my bulk material (ore) , we have a belt conveyor and we should install a VFD to the motor and a scale or load cells to the belt, by multiplying speed by weight we get the rate. doing this through the plc and trying to maintain the setpoint by manipulating the speed of the belt. (20 meter belt)
my question is where to install the load cells , should we take a known length segment and weight it , or should we distribute load cells , i heard something about a 2 dimensional load cells array but i didn't get it
please help
thank you.
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Old April 28th, 2021, 07:43 AM   #2
drbitboy
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I am not an expert in this type of thing, but from first principles I am am wondering such a system will work. The approach described is analogous to having a tank with no control of the inlet and and a flow control on the outlet: the tank may either overflow or go empty.


For example, assume, a priori, that
  • the setpoint is 100kg/s,
  • the feeder is loading material at a constant rate of 100kg/s onto the conveyor at the upstream end,
  • the conveyor speed is 1ms/, so there are are 100kg/m of material along the entire conveyor,
  • the proposed weigh scale is measuring 100kg of material over a 1m section of conveyor
    • For now assume said scale is at the downstream end,
  • the control scheme has the conveyor moving at a constant speed of 1m/s, so the setpoint of 100kg/s is being met
Then the feeder changes to loading material at a rate of 80kg/s:
  • The conveyor will continue to run at 1m/s for 19s i.e. until the first bit of conveyor that received the changed feed rate reaches the weigh scale, because until then the scale will still be measuring 100kg/s ove the 1m section at the end.
    • During those 19s (=19m/(1m/s)), there will be 80kg/m (=(80kg/s)/(1ms/)) of material added to the conveyor.
    • After those 19s, the scale will measure 80kg over the 1m section,
    • and the control scheme will raise the speed to 1.25m/s (=(100kg/s)/(80kg/m)).
  • At 1.25m/s and a feed rate of 80kg/s, there will be 64kg/m (=((80kg/s)/(1.25ms)) added at the upstream end.
    • After 15.2s (=19m/(1.25m/s)), the scale will measure 64kg,
    • and the control scheme will raise the speed to 1.5625m/s (=(100kg/s)/(64kg/m))
    • At 1.25ms/ and a feed rate of 80kg/s, there will be 51.2kg/m (=(80kg/s)/(1.5625m/s)) added at the upstream end.
  • Etc.
It does not matter where the scale is, that only changes the delay between changes in feed rate and control scheme's reaction to same.
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Last edited by drbitboy; April 28th, 2021 at 07:45 AM.
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Old April 28th, 2021, 08:12 AM   #3
Ken Moore
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Purchase a dedicated weigh belt scale. They come with the load cells, rollers, and encoder for belt speed. They also have built in calibration options. Then use the analog output from the scale to control your drive speed, via pid loop.

I have used Milltronics in mine material handling with good results. The zero does drift and we calibrate weekly.

https://new.siemens.com/global/en/pr...onics-msi.html
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Old April 28th, 2021, 09:12 AM   #4
HoldenC
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I also agree with the Milltronics.

Another great option is the Ramsey Micro-tech 2000. They offer whole packages as well. I like their built in weight system. It allows you to simply turn a wrench to drop known weights on the belt line for easy calibration of your span.
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Old April 28th, 2021, 12:00 PM   #5
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I agree with Ken Moore and HoldenC.

Simply purchase a weighfeeder. I don't see where you stand to gain anything by reinventing the wheel.
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Old April 28th, 2021, 01:15 PM   #6
drbitboy
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Ignore Post #2; I'm an idiot and I made bad, ignorant assumptions.
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Old May 4th, 2021, 07:13 AM   #7
MayadaAhmed
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Quote:
Originally Posted by Bit_Bucket_07 View Post
I agree with Ken Moore and HoldenC.

Simply purchase a weighfeeder. I don't see where you stand to gain anything by reinventing the wheel.
The point is, there is no space available to install a weight feeder, so we want to create the fiction of the feeder by our already there belt conveyor
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Old May 4th, 2021, 07:48 AM   #8
Corsair
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Belt Feeders

Don't beat yourself up, drbitboy. We appreciate you.

The assumption with the feed systems that I have done is that the input to the belt is 'choke fed'. This term is usually applied to crushers meaning that the input stream keeps the crusher full all the time. In the case of a feeder it means that the product is in a column over one end of the belt and some sort of gate restricts the height of the product over the belt to a fixed position. That means the faster the belt goes the higher the feed rate. If the gate is set too high the belt will need to run too slow, etc. The chute feeding the belt extends down close to it so the angle of repose of the material will stop the flow when the belt is stopped.

The flow rate is a product of the weight on the load cell and the speed of the belt. The scale will have an encoder to input the belt speed.

IF the system doesn't need extreme accuracy and IF the geometry causes the stream of product over the belt to be consistent and IF you don't need to detect a feed break from being out of incoming product then you MAY be able to experiment and do an open-loop system that correlates speed to tons per hour without load cells. This would NOT be my first choice. It would have issues with wear on the chute surfaces changing the geometry.

I've used several Milltronics scales and Ramsey belt feeders and would strongly echo the previous posts.
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Old May 4th, 2021, 09:22 AM   #9
drbitboy
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Quote:
Originally Posted by MayadaAhmed View Post
The point is, there is no space available to install a weight feeder, so we want to create the fiction of the feeder by our already there belt conveyor

[Thanks, @Corsair; my brother explained about the gate/inverted weir. Anyway, getting back on the horse ...]


If OP is looking for a fiction, then assume the belt is at constant tension, and a measurement of the sag between two widely-spaced rollers is proportional to weight, and run the belt speed in approximately inverse proportion to the sag. Who knows: with regular calibration, it might even meet a target accuracy spec?


I would think the sag-measurement point should be near the exit of the conveyor, otherwise the speed control would have to be delayed by the distance of the measurement from the exit.
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Old May 4th, 2021, 12:58 PM   #10
mylespetro
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I've only ever worked on packaged systems that do this (feeding coal to a boiler in a power plant), but if I recall correctly, I think there were two load cells, one on either side of the belt up near the head end of the conveyor.


What you're referring to is a gravimetric feeder by the way, you may be able to find more information if you Google that.
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Old May 4th, 2021, 04:00 PM   #11
Bit_Bucket_07
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Quote:
Originally Posted by MayadaAhmed View Post
The point is, there is no space available to install a weight feeder, so we want to create the fiction of the feeder by our already there belt conveyor
Well then, install a belt scale and an encoder to measure belt speed. Surely, you don't need to build your own belt scale with individual load cells?
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