1046_0=PhidgetBridge 4 ABCD inputs: Math for Load-Cell Sum?

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Dan
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1046_0=PhidgetBridge 4 ABCD inputs: Math for Load-Cell Sum?

Postby Dan » Sun Jan 11, 2015 9:51 am

Horizontal, rectangular platform with one load cell at each corner,
i.e. 4 independent load cells: A,B,C,D.
Off-platform-center, variable vertical load (z axis), stationary load-point position (x,y) located at all times within the platform's perimeter.
Note: likely each A,B,C,D load cell will be sensing a different vertical load.

Question: How to measure (the math for) the total A+B+C+D combined, z-axis, vertical loading?

I am assuming this is a simple case of adding up (arithmetic sum) all A+B+C+D vertical contributions/effects.

Welcome comments/corrections if this A+B+C+D assumption is wrong.
This seems to me straight forward, as this project is similar to a bathroom floor scale, with some expected amount of x,y,z vibrations (10Hz max), and wanting maximum readings per second possible (100reads/sec to 120 reads/sec).

Only z-axis data is important.

Thank you,
Dan

bradyg
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Re: 1046_0=PhidgetBridge 4 ABCD inputs: Math for Load-Cell S

Postby bradyg » Thu Aug 27, 2015 8:44 am

Hi Dan,
I just stumbled across this post and am working on a similar project myself. You are correct that taking the arithmetic sum of all 4 sensors will give you the correct total force.

I have a question for you: I am in the process of trying to sum the forces from two separate load cells, and can not seem to get it to work properly. If you could post your code showing how you were able to do this, it would be much appreciated!

Thanks in advance.

Dan Hariton
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Re: 1046_0=PhidgetBridge 4 ABCD inputs: Math for Load-Cell S

Postby Dan Hariton » Mon Oct 19, 2015 8:49 am

bradyg wrote:>> I just stumbled across this post and am working on a similar project myself.
>> You are correct ... the arithmetic sum of all 4 sensors will give you the correct total force.


Thanks for confirming this math is correct A+B+C+D=load cell sum.
Did experiment with 4 "identical" load cells and the lab bench results confirmed it as well.

bradyg wrote:>> question for you:
>> I am in the process of trying to sum the forces from two separate load cells,
>> and can not seem to get it to work properly.
>> If you could post your code showing how you were able to do this ...


In my application there is no (software) code for the sum.
I electrically connected in parallel all 4 load cells.

May use an external 5V DC supply.
May use an external 15V DC supply, made-up from 3 5V_USB_AC-power-cubes connected in series (they are 5V floating voltages). A voltage stack made up from three 5V cubes.
The Wheatstone bridge top DC voltage is +10V (stack top), bottom is -5V (stack bottom), and the 1040 GND is 0V (one third up).

I am doing (instant/no CPU latency) analog summation.
Load-cell differences are minor, in my case they do not adversely impact the test results.
The whole 4-load-cell set is being seen (by the 1040 USB Phidgets Bridge) as one BIG load cell.
Using only one analog, differential-input, sense-port of the 1040.
Needed to re-calibrate the resulting BIG (composite) load cell sensitivity.

PS
AliExpreess sells 5kg-10kg-20kg load cells at a comparable price to Phidgets load cells.
Caveat: AliExpress load cells may or may not come with the extension-spring for (tail) cable stress relief.


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