Identifying Internal Parts of a Load Cell

[_greenhorn_]

Hi everyone,

I was recently disassembling a load cell and was checking the parts inside. I expected 4 strain gauges or some variation to form the wheatstone bridge setup. What i found is somewhat true...

On one side is as expected, two strain gauges (1 active and I think the other one is for temperature compensation). I was expecting the same on the other side but....i found more. The is a PCB with resistors and underneath is the other strain gauge as expected.

Does anyone know what this circuit and resistors are for? Is there a name for this circuit? I tried checking and all I can seem to find is the simplied wheatstone bridge.....just 4 resistors.

[Zero999]

The top photograph shows two strain gauges which change their resistance, depending how much they're deformed.

The bottom picture shows some resistors which will form a Wheatstone bridge with the strain gauges.

[_greenhorn_]

Thanks for the feedback Zero999.

I kept dug a bit deeper and it seems that there were indeed additional strain gauges under the PCB. This bring up the total to  2 active strain gauges, 2 compensation strain gauge plus 4 resistors on the PCB.

I will try to draw up the schematic and hopefully get a better understanding for the setup. Unfortunately the glue used for encapsulation is a bit of pain to remove and I broke a couple of wires in the process. 

That said, I am curious as to why there are so many and what is the reason for doing so. Maybe it is for calibration or balancing output. 

[David Hess]

I think the two half bridges are at 45 degrees so they measure shear in the web.

The single gauge off to the side is in series with the excitation so is a modulus compensation (mod comp) resistor.  The modulus of elasticity of the load cell material changes with temperature causing a gain error so the extra gauge in series with the excitation compensates for that; think of it as a precision low value RTD (resistance temperature device).  Some designs might use two modulus compensation gauges for symmetry but I do not think that is strictly necessary.  The gauge materials and backing and adhesive also affect the modulus among other things.

It looks like the board contains a resistor divider to trim the zero and somewhere there should also be a resistor or length of resistance wire to temperature compensate the change in zero over temperature.  This is added after temperature cycling the load cell to measure its temperature coefficient.

[Doctorandus_P]

Start by looking again.
Your first picture does not have 2 but 3 strain gauges.

The 2 Strain gauge in the middle may be on the same piece of mylar, but that's just for positioning and ease of mounting. The top half has the strain gauge wires in this /////// orientation, while the one on the bottom is in the \\\\\\\\\\ orientation.
This means that is one of the strain gauge is under tension (resistance gets higher) the under is under compression (resistance gets lower). These 2 form one leg of the Wheatstone bridge.
Note there is a wire attached to the midpoint between these 2 strain gauges


The other side of the load cell looks similar. So there are 4 strain gauges for your Wheatstone bridge, and 2 for some compensation and sensitivity adjustments.