Strain Guage Summing

For hopper weighing, if more than one load cell is mounted then a summing circuit is used and output of this circuit is given to indicator or controller. I am not clear about the working of the summing circuit. How does it exactly work?

SVN

svn said:

For hopper weighing, if more than one load cell is mounted then a summing circuit is used and output of this circuit is given to indicator or controller. I am not clear about the working of the summing circuit. How does it exactly work?

SVN

Click to expand...

In platform scales that have load cells in all four corners, a summation board is required. The purpose of the summation board is to equalize the amount of signal contributed by each load cell. If you look at the load cell, it will have a mv/V specifaction stamped on it. Typically this will be 3mv/V but in reality each load cell may be, as an example, 2.999mv/V , 3.001mv/V etc. The summation board compensates for the slight difference in load cell ratings.
On a platform scale, the load can be positioned anywhere on the scale, and if the load is placed near a load cell that has a 3.001mv/V rating and then the load is moved closer to a 2.999mv/V rating load cell, two different weight readings will be observed.
Since the load to measured on a platform scale can be placed anywhere on the platform, the summation board is essential to compensate for this.


On a hopper scale, a summation board is usually not necessary as the load is distributed evenly to all 3 or 4 load cells supporting the hopper. Even if the load cells are not exactly matched mv/V , each load cell contributes the same ratio of signal from empty weight to full hopper weight. A summation board on a hopper is usually a convenient method of physically tying all load cells together.

Ian

For a hopper weight, I have tied all four load cell outputs together with a wire nut (without a summing board) and run directly to the weight indicator. It worked okay.

Replace 'strain gauge' with 'load cell' and Google 'load cell summing', or 'load cell summing box', or 'load cell summing box schematic'.

Loadcell Summing

curleyandshemp, thanks for the clarification.

Lancie1, thanks for the reply.

DougP, thanks for the reply. I already tried to google, but I was sure somebody on this forum will be able to give better information. PLCs.net rocks!

Lancie1 said:

For a hopper weight, I have tied all four load cell outputs together with a wire nut (without a summing board) and run directly to the weight indicator. It worked okay.

Click to expand...

I am amazed! I can't see how that would possibly work!

With my quick calculations it looks like tying the load cell outputs together would average the signals.

I read this post with interest because I had a project with Load Cells. This application weighs a 55 gallon drum to check level.
I was confused about Summings boards. Searched the Internet for a schematic without success. A schematic for an amplifier that worked on 1 or 2 Load Cells had the cells in parallel, so that's what I did. The results were confusing, so I contacted Omega Engineering for help.
Bernie's answer is correct. 2 cells in parallel average. A summing board does the same thing - average, not add. It has pots to adjust the Excitation output to balance them, but that's all.

A proper setup for a weighing platform would be a cell on each corner, but this was expensive and accuracy wasn't important. The platforms are rollers, slightly tilted back to keep the barrel in place. There wasn't a good way to do it with 1 cell, so we used 2 - one on each of the rear corners. The front corners are pivot points.

A 55 gallon drum of water is 459 pounds. The Load Cells are rated for 1000 pounds. 2 millivolts per volt with 10 volts Excitation. 20 millivolts full scale.

I was hoping I could set this up to have 10 millivolts at 500 pounds, but Load Cells don't work that way. With 500 pounds on the platform, each corner has one fourth the weight - 125 pounds. Quick thinking was a 1000 pound cells gives a 2x safety margin for a 500 pound weight, but results are actually 8x!

Take a platform with four Load Cells, one on each corner. The millivolt output range will be the same if you read 1 cell, or 2, 3 or 4 in parallel. From what I understand, the results will be the same with a Summing box. A platform designed for 1000 pounds should have 250 pound cells on each of the 4 corners. I was only using 2, but the math is the same - 250 on each corner.
In my case, the 1000 pound cells are too large. I should have used 250 or 500 pound units. With 1000 pound cells, a full barrel outputs only 2.3 millivolts. (459 / 4 = 114.75 pounds per corner. 114.75 x 20 millivolts = 2.295). The meter is scaled 0-4000 pounds.
Analog output from the meter is 0-10 volts to the PLC. This is where I lucked out. I can scale the 0-10 volt output in the meter to anything I want. Resolution (millivolts per bit) is the only part that suffers.
I knew weight would be distributed across the 4 corners, but worried what would happen as someone was unloading a full barrel - one corner might see the full weight. As it is, this could never happen, because cells are only at the rear, and unloading is from the front.

During my search for an amplifier, I found meters from Omega are cheaper. CNiS3253 ($240.00) has 10 volt Excitation (60 ma max), 0-10 volt output (for PLC), and a relay that I used for an alarm.

It's ok to parallel load cells, a lot of weigh suppliers use this method, nobel, Applied weighing etc, most only use a block of 4 terminals (+ex -ex +out -out).

One thing to note if mounting load cells on vessels it's better to use 3 rather than 4 as this gives more even distribution, 4 needs to be located very accurately or you get what they call stiction i.e. inconsistant weights, think about it a vessel will sit on 3 legs & all will have a load bearing on them, however 4 legs & it's like the preverbial coffee table you tried to level with a saw.