A typical quarter bridge strain measurement solution comprises
When an object is stretched due to an external force and the length of the object increases from L to L+ΔL, the ratio ΔL/L is called strain.
ε = ΔL/L
As the ratio of deformation is often very small, it is often represented in a units of 10-6 or μstrain
A strain gauge (gage) can be used to measure the strain of this object. The most common type of strain gauge (gage) consists of a flexible backing which supports a metallic foil pattern etched onto the backing. As the object is deformed, the foil pattern is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge circuit, can be used to calculate the exact amount of deformation by means of the quantity known as the gauge (gage) factor.
The gauge(gage) factor of a strain gauge (gage) relates strain to change in electrical resistance. The gauge (gage) factor GF is defined by the formula
where RG is the resistance of the undeformed gauge, ΔR is the change in resistance caused by strain, and ε is strain.
In our example, we will use one strain gauge (gage) with GF=2, RG = 120 ohms and connected in a quarter bridge configuration. The bridge is excited at VEXC=3.3V.
The voltage output of the wheatstone bridge VO (seen at the differential input AIN3, AIN2) is given by
If the bridge is balanced VO =0 since all the resistances are equal. When a strain is applied, RG becomes R + ΔR and substituting R to all the other resistances, the equation becomes
If we assume 2 ΔR << 4R
Finally substituting VEXC=3.3V, GF=2, we obtain (in ustrains)
ε = -ΔVO * 1000000 / 1.66