Data Acquisition

Analog and digital signals 29The gauge factor, provided by manufacturers for a particular strain gauge, typically liesbetween 2 and 4 for commonly used metal foil gauges with nominal resistance of 120 Ω,350 Ω and 1 kΩ. Thus, if a 350 Ω gauge with a gauge factor of 2.0 is stretched by 1%,then its resistance will change by 2% or 0.57 Ωs.2.8 Wheatstone bridges2.8.1 General characteristicsDue to its sensitivity, the Wheatstone bridge circuit is a commonly used circuit for themeasurement of small changes in electrical resistance, particularly for strain gauges. Itcomprises four resistive elements and can be excited by either a voltage or current source.The standard Wheatstone bridge configuration is shown in Figure 2.16.Figure 2.16Standard Wheatstone bridge configurationWhen excited by an input voltage V EX it can be shown that the output voltage V 0 isgiven by the equation:V0R1R3= −VEX R1 + R2 R3 + R4When the ratio of resistances R 1 to R 2 is equal to the ratio of resistances R 3 to R 4 , thenthe measured output voltage is 0 V, and the bridge is said to be balanced.When a resistive element changes its resistance in response to the physical parameterbeing measured (e.g. a strain gauge) it is called the active element, while the remainingresistors are called completion resistors. If R 1 is an active element, then an increase in theresistance of the active element R 1 increases the output voltage. A decrease in thisresistance will decrease the voltage appearing at the output. It is conversely true that if R 2is an active element, then an increase in its resistance would result in a reduction of thevoltage appearing at the output, while a decrease in this resistance would result in theoutput voltage increasing.