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The purpose of my project is to build a sufficiently accurate "Strain indicator" based on the use of a Wheatstone bridge using an Arduino. Currently I am using a HX711 as load cell amplifier which is working well since I use it with a full bridge.

Currently I want to connect only one strain gauge to a wheatstone bridge built using 3 resistances of 120Ohm (0.1%). I am able to measure something but I want to balance the Wheatstone bridge. My inexperienced question is it is possible to add an accurate balance system to my Wheatstone bridge?

I know that classical solution use potentiometer but the principle is based on a choice of a variable resistance with similar value of the others 3 resistances on the Wheatstone bridge (in my case 120Ohm). Unfortunately, finding potentiometers around this value is not possible. In addition the potentiometer must allow very small change of the resistance around 120Ohm.

I am from the mechanical field, sorry if my question seems to be very common...

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The way I do it is to use an instrumentation amplifier (TI, Analog devices etc.) that amplifies the difference signal and then apply a variable voltage to the reference input of the InAmp to balance the output to zero.

An InAmp basically comprises three op-amps as shown below: -

enter image description here

Note the "reference input" bottom right and the potential divider that sets the offset voltage on the InAmp output. This can be a potentiometer or even the output from a DAC. It all depends on what you want to do and how automated you want the zeroing process to be.

Some applications will allow a slow integrator to be fed from the InAmp output to the reference input and in this way you are always auto zeroing but at such a slow rate that the measurements you make are largely unaffected. This may or may not be of interest to you of course.

There may be a way of adjusting the HX711 (load cell amplifier) to do what I suggest above so if you want help in that direction, please link to a proper data sheet for the chip.

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There are two ways of using a Wheatstone Bridge.

The first classical way is to balance it, that is, adjust the arms until you get a zero output. Then you can assert that the ratios of arm impedances are equal, and calculate your unknowns from your knowns.

The second way, which is (I think) a slight abuse of the name Wheatstone Bridge, is to use the bridge configuration, often with strain gauges, Hall sensors, resistance thermometers, where there is a small variation to be measured in the face of a large resistance. The bridge is approximately balanced, to mostly remove the effect of the big static resistance, and then the residual output is measured with a sensitive voltmeter, like an HX711.

There is no need to balance the bridge precisely, and in fact, as the sensor changes resistance, this will unbalance it. The purpose of using the bridge is to reduce the dynamic range between the sensor static resistance, and the small measurement range. Once it has been reduced enough to match the voltmeter, and the system noise, there is little point in improving balance further.

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  • \$\begingroup\$ The first way is sometimes called the null mode of operation, whereas the second way is called the deflection mode. \$\endgroup\$ Jul 11, 2017 at 19:46
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There is no need to use equal resistances in each half of the bridge in most cases.

You can use something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

This gives you the equivalent of about +/-5 ohms adjustment in the reference resistor.

Pick R1/R2 to control the current through R3 depending on your excitation voltage.

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Unfortunately, finding potentiometers around this value is not possible.

For what you do the bridge needs only to be roughly balanced , not perfectly balanced.

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