Weight Feeder Concept

[Frozzxx]

I'm just starting to learn about industrial automation processes, I want to know how a weight feeder unit works which consists of a load cell and speed sensor connected to a PLC that can control the VFD to regulate motor speed. is there a calculation that is done to find out how the calculation results of the load cell signal with the speed sensor signal will be used to control the VFD? thank you.

 

[strantor]

I don't see why a motor speed signal would be required or even useful.
I assume you refer to an auger-type feeding system, dry bulk solids?
I have designed bulk solids dosing systems based on a knife gate, no auger, so also no speed signal available at all, and also there is no consistent flow rate because it depends on how much product is in the hopper, granule size, coefficient of friction of the granules, and bulk density of the product. I originally tried to base my algorithm on the Beverloo equation but that was more trouble than it was worth and didn't work well. It required prior knowledge of, and inputting parameters of the product to be dispense. Dumbing it way down was the ticket to success running any product, without having to know anything about its properties beforehand. The way it works is:
1. Start opening the valve rapidly and keep opening until 1/3 target weight reached. Then dwell at current position until 50% target weight.
2. Capture current position and start closing the valve with quasi-logarithmic relationship between valve position and difference between current weight and target weight, such that it tapers off to just dribble at around 96% and dwell at current position until 100% target weight.
3. Close the valve.

In the system I just described, there is only one feedback: weight.
(Technically valve position is also available as feedback but it isn't used for anything but position control in a separate controller).
So I don't see where or why auger motor speed enters the equation.

If I was going to design an auger based system I would approach it the same way, except it would be even simpler. Run the auger at full speed until about 95% of target weight, then taper off to a dribble before stopping at 100%.

I don't know if this is an academic assignment or a real world application but assuming real world, my advice is that the "dribble mode" at the end is absolutely critical because load cell readings jump around when weight is changing, and the faster that weight is changing, the more they jump around. If extreme precision is required i would even recommend coming to a complete stop prior to 100%, average the weight value for a few seconds, then calculate the required rotations to reach 99%, do the rotations, dwell and average again, then proceed to appropriate rotations to hit 100% on the nose.

 

[drbitboy]

... want to know how a weight feeder unit works which consists of a load cell and speed sensor connected to a PLC that can control the VFD to regulate motor speed. ...

If this is a batch filling process, then @strantor's approach will work.

If it is a continuous process, and the load cell is part of a "check weigher" or "in-motion scale," that will require a different solution.

 

[Ken Moore]

I have been to a few sites that had bulk silos feeding a continuous process. We used loss in weight to calculate a flow (calculated once every minute), that was used to control a variable speed auger. Usually 2 to 3 silos going at once. It was not precise, but worked well enough to not overflow a feeder belt. The feeder belt had load cells and used speed to get a more exact flow rate to the process.

Example:
2 silos set at a rate of 25 tons/hr feeding a belt with a sepoint of 50 tons/hour.

 

[padees]

The OP is describing a Gravametric feeder system.


They are a bit more complicated than dumping a bunch of product on a conveyor with load cells...


To the OP. If you have a feeder system like this, you must understand the Mechanical Engineering behind this before you can even consider control equations.


I've worked with everything people have posted here. They are not all the same animal, and when you get to more precise feed controls, the variables increase in number as to what your control calculations will be. More variables, more sensors that are accurately monitored...

 

[drbitboy]

The OP is describing a Gravametric feeder system.

So it's continuous?

 

[robertmee]

I don't see why a motor speed signal would be required or even useful.
I assume you refer to an auger-type feeding system, dry bulk solids?
I have designed bulk solids dosing systems based on a knife gate, no auger, so also no speed signal available at all, and also there is no consistent flow rate because it depends on how much product is in the hopper, granule size, coefficient of friction of the granules, and bulk density of the product. I originally tried to base my algorithm on the Beverloo equation but that was more trouble than it was worth and didn't work well. It required prior knowledge of, and inputting parameters of the product to be dispense. Dumbing it way down was the ticket to success running any product, without having to know anything about its properties beforehand. The way it works is:
1. Start opening the valve rapidly and keep opening until 1/3 target weight reached. Then dwell at current position until 50% target weight.
2. Capture current position and start closing the valve with quasi-logarithmic relationship between valve position and difference between current weight and target weight, such that it tapers off to just dribble at around 96% and dwell at current position until 100% target weight.
3. Close the valve.

In the system I just described, there is only one feedback: weight.
(Technically valve position is also available as feedback but it isn't used for anything but position control in a separate controller).
So I don't see where or why auger motor speed enters the equation.

If I was going to design an auger based system I would approach it the same way, except it would be even simpler. Run the auger at full speed until about 95% of target weight, then taper off to a dribble before stopping at 100%.

I don't know if this is an academic assignment or a real world application but assuming real world, my advice is that the "dribble mode" at the end is absolutely critical because load cell readings jump around when weight is changing, and the faster that weight is changing, the more they jump around. If extreme precision is required i would even recommend coming to a complete stop prior to 100%, average the weight value for a few seconds, then calculate the required rotations to reach 99%, do the rotations, dwell and average again, then proceed to appropriate rotations to hit 100% on the nose.

It's not a motor speed sensor...it would be a speed sensor on the weight belt itself. It measures the speed of the actual dosing and with the loss in weight, you get a flow rate. While you could rely on just the motor speed and gearing to the belt, it doesn't account for any slippage of the belt due to increased weight on the belt during dosing. It would also detect overloading or jamming of the weigh belt. Just depends on what degree of accuracy you're trying to achieve. I've done various variations of these systems for 20+ years. If you're metering pharmaceutical compounds, accuracy matters. If you're dosing chemicals into rubber compounding, not as critical.

 

[padees]

So it's continuous?

Every feeder I have worked with like this has been 24/7/365 continuous. But doesn't necessarily need to be continuous as they are also built for batching operations.


Heck, this compounding company doesn't even shut down for holidays.

 

[robertmee]

I'm just starting to learn about industrial automation processes, I want to know how a weight feeder unit works which consists of a load cell and speed sensor connected to a PLC that can control the VFD to regulate motor speed. is there a calculation that is done to find out how the calculation results of the load cell signal with the speed sensor signal will be used to control the VFD? thank you.

In the case of a weight belt feeder, the speed feedback of the belt and the scale weight across the belt combine to equate to a flow rate in units/time. A closed loop PID control varies the speed setpoint of the belt to achieve the desired weight. There is a feed forward or predictive starting speed for a given rate as you need to establish flow and weight before enabling the PID.

Another style Is a loss in weight feeder. Similar to above, but the entire material hopper is on load cells. Flow rate is determine by loss in weight over time. In this case a speed sensor isn't critical. But the limitations are the system enters into volumetric mode instead of gravimetric during refill of the hopper. You lose some accuracy during this transition.

 

[padees]

It's not a motor speed sensor...it would be a speed sensor on the weight belt itself. It measures the speed of the actual dosing and with the loss in weight, you get a flow rate. While you could rely on just the motor speed and gearing to the belt, it doesn't account for any slippage of the belt due to increased weight on the belt during dosing. It would also detect overloading or jamming of the weigh belt. Just depends on what degree of accuracy you're trying to achieve. I've done various variations of these systems for 20+ years. If you're metering pharmaceutical compounds, accuracy matters. If you're dosing chemicals into rubber compounding, not as critical.

The ones I have worked with use encoders on the feed screw and measure "weight decay" of raw materials in the hopper to determine feed rate in a form of weight/unit time. Quite a bit more to it than this, but you get the gist.

 

[robertmee]

The ones I have worked with use encoders on the feed screw and measure "weight decay" of raw materials in the hopper to determine feed rate in a form of weight/unit time. Quite a bit more to it than this, but you get the gist.

Yes that's a loss in weight feeder or gravimetric feeder as I described above. I use them quite often from Kistler, Acrison, Merrick and the like.

But it sounds like the OP is describing a weight belt feeder. A bit different in principle...youre not weighing the material in the hopper, but material as it crosses a weigh bridge on a belt. I use Thayer units often for this application.

 

[padees]

I didn't see belt in the OP's post.


None the less, the OP may never fill us in.

 

[strantor]

It's not a motor speed sensor...it would be a speed sensor on the weight belt itself. It measures the speed of the actual dosing and with the loss in weight, you get a flow rate. While you could rely on just the motor speed and gearing to the belt, it doesn't account for any slippage of the belt due to increased weight on the belt during dosing. It would also detect overloading or jamming of the weigh belt. Just depends on what degree of accuracy you're trying to achieve. I've done various variations of these systems for 20+ years. If you're metering pharmaceutical compounds, accuracy matters. If you're dosing chemicals into rubber compounding, not as critical.

OP made no mention of a belt and these responses are making it clear to me that there are a lot more weighing applications than what I have experienced and a lot more room for interpretation in the wording than I realized. I don't have anything to add until the OP comes back and clarifies quite a few things.

 

[robertmee]

I didn't see belt in the OP's post.


None the less, the OP may never fill us in.

Didn't see hopper or auger or anything else either. We're all guessing, but likely between a gravimetric loss in weight or a weight belt feeder, bases are covered. The key for me was the speed sensor which is usually on a weight belt feeder. Most loss in weight hopper or auger feeders don't need speed sensors.

 

[padees]

Didn't see hopper or auger or anything else either. We're all guessing, but likely between a gravimetric loss in weight or a weight belt feeder, bases are covered. The key for me was the speed sensor which is usually on a weight belt feeder. Most loss in weight hopper or auger feeders don't need speed sensors.

Maybe so, but the feeders I have worked with use an encoder on the volumetric screw that determines screw speed as well. Also moisture sensors and stuff like that.


I have a small lab size extruder to upgrade controls on. It has 2 feeders on it, which I never looked at. All I have to do is enable operation in the program.


I'll look tomorrow to see what they are. One of the brands you mentioned sounds familiar.