(The gauge factor for metallic strain gauges is typically around 2.)
We set up a stain gauge load cell and measure that change in resistance
and all is good, right? Not so fast. Strain measurements rarely involve
quantities larger than a few millistrain (fancy units for strain, but
still very small). So lets take an example: suppose you put a strain of
500me. A strain gauge with a gage factor of 2 will have a change in
electrical resistance of only
For a 120Ωgauge, this is a change of only 0.12Ω.
0.12Ωis a very small change, and, for most devices, couldn’t actually be
detected, let alone detected accurately. So we are going to need another
device that can either accurately measure super small changes in
resistance (spoiler: they are very expensive) or a device that can take that
very small change in resistance and turn it into something that we can
measure accurately.
This is where an amplifier, such as the HX711 comes in handy.
SparkFun’s HX711 Amplifier breakout board
A good way of taking small changes in resistance and turning it into
something more measurable is using a wheatstone bridge. A wheatstone
bridge is a configuration of four resistors with a known voltage applied like
this:
where Vin is a known constant voltage, and the resulting Vout is measured.
If then Vout is 0, but if there is a change to the value
of one of the resistors, Vout will have a resulting change that can be
measured and is governed by the following equation using ohms law:
By replacing one of the resistors in a wheatstone bridge with a strain
gauge, we can easily measure the change in Vout and use that to assess
the force applied.
Page 4 of 6
