Load Cell Application Q&A: Single Cell for Loss in Weight Feeder

[ghaulk]

I have a question regarding a load cell application I am working on. I am designing a loss in weight feeder system and would like to use a single load cell for my hopper weight. Upon preliminary trials, I mounted the load cell on one side of the hopper and hinged the other. This does not give me repeatable measurements. I would hang the hopper, but I can't because of design constraints. I am doing this in an effort to save $. I have the load cell connected to a signal conditioner which sends info. to a PLC. I know I can put 3 load cells on the hopper with a summing board, but I was wondering if anyone had any suggestions on how to use one load cell for this application?
Thanks

 

[ghaulk]

I noticed that there has been no replies to my question. I'm new in this forum. Is this the right place to ask these types of questions?

 

[TVP45]

Your problem will always be the x-y components of the weight as measured by a load cell. Typically, this sort of measurement is done with a single ball swivel and two load cells, one at x and one at y.

If your hinge joint were perfect, you should get repeatable measurements; are you using a ground shaft and bearings?

Further, how are you transmitting load to the load cell; sounds like you might be getting some tensile component at one edge. A smooth, domed button (no screws please) works best.

 

[Danger]

Welcome to PF, Ghaulk.
This might be better-suited for Engineering than here, but I'm not sure. Anyhow, your question is a bit vague. For instance, why do your design constraints forbid suspending the hopper? That would definitely be my first approach to the problem. I'm also a bit confused about why your hinged apparatus isn't consistent. It sounds like a good idea to me. Are your hinges well lubricated?

edit: Hi, Tvp. You sneaked in while I was composing.

 

[ghaulk]

Yes, I am using a polished shaft and a linear bearing, but the fit is not perfect (it was ordered by someone else). While waiting on a response I did more experimenting and found that I may be able to hang the hopper. This would definitely be the best way to go since the center of gravity would remain constant. I really appreciate the advice. Thanks.

 

[Danger]

Good luck with it, then. Keep us posted as to your results.

 

[mgb_phys]

I'm also a bit confused about why your hinged apparatus isn't consistent. It sounds like a good idea to me.

Problem is that if the hinge is slightly off level then depending where the c of g of the load in the hopper is, you will have some sideways load on the hinge.
You could do it with 2 point contact pivots and a single load cell, or as TVP45 said with a single pivot point and 2 load cells.

Are your hinges well lubricated?

I propose that as the engineering forum secret greeting!

 

[TVP45]

Yes, I am using a polished shaft and a linear bearing,

Ach, I missed that. By chance do you have a plastic or composite linear bearing rather than recirculating steel balls? If so, and if your load is fairly small, you may be getting significant stiction.

 

[Danger]

Problem is that if the hinge is slightly off level then depending where the c of g of the load in the hopper is, you will have some sideways load on the hinge.

Good point. I had assumed an ideal setup, which is unlikely.

I propose that as the engineering forum secret greeting!

 

[FredGarvin]

One other thing you may want to look at is the stiffness of the hopper. Depending on the loads, you may be deflecting the hopper enough to also introduce uncertainties.

 

[what-how-why]

It does not make much sense to dig into structural details before the very basic mechanics is examined - The weight of hopper and material inside the hopper is shared between the hinge and the load cell. If you can make sure the distribution of wieght between the hinge and load cell is even all the time, you get reproducible result by multiplication of load cell reading. If hte distribution of weight between the hinge and load cell is sort of ramdom, you get random result.

As material is discharge from the hopper continuously, suppose the centre of gravity of hopper + material cannot stay stable. Suggest to make a simple test - install another load cell - just for test purpose - to measure the weight going the to hinged side, and compare reading from both load cell to determine whether distribution between the hinge and the working load cell is consistently even.