A Wheatstone bridge is an electrical circuit that can measure very small variations in electrical resistance. This technique is commonly used for strain gauge sensors because it accurately measures relative deformations of a few mm /m (see the attached diagrams). If no force is applied to the sensor, the four gauges have the same resistance and the V.out output signal is zero.
If a force is applied in compression, gauges 1 and 4 become longer and consequently their resistance increases. Conversely, gauges 2 and 3 become shorter and their resistance decreases. As a result, the Wheatstone bridge is unbalanced and a positive V.out voltage can be measured. Conversely, this voltage becomes negative if a tension force is applied. If these relative variations are small (<1%), we can consider that the relation (Force => Deformation => Resistance of the gauges => Electrical signal) is linear.
Furthermore, it should be noted that the output signal is also proportional to the supply voltage V.in. The sensitivity of the sensor is therefore expressed in mV / V where the denominator is equal to V.in. Thus, for a sensor whose sensitivity is 2 mV / V and is supplied with 10 V, the output signal will evolve from 20 mV between zero and full scale.
Typical sensitivities for metal gauge sensors change from 0.5 to 4 mV / V depending on:
• Principle of operation
For some sensors, the four gauges are deformed in the same proportion; for others, some gauges operate at 100% and others at 30% (Poisson's ratio).
• The required overload capacity. e.g. the breaking capacity must be 500% for hoisting device.
• Fatigue resistance.
• The type of material selected for the proof body.