Metal resistance strain gauge bridge circuit diagram

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When a metal resistance strain gauge is used in mechanical measurement, it must be connected to a bridge circuit. The output signal from the strain gauge bridge is typically weak, and using a DC amplifier can lead to zero drift. To avoid this issue, an AC amplifier is often used for signal amplification. Therefore, the strain gauge bridge is usually powered by alternating current, forming an AC bridge. Depending on the reading method, the bridge can be classified into two types: balanced and unbalanced. Balanced bridges are suitable for measuring static parameters, while unbalanced bridges are ideal for dynamic measurements.

Although DC and AC bridges are similar in operation, for simplicity, only the DC unbalanced bridge will be discussed below.

The circuit shown represents a DC unbalanced bridge with a terminating amplifier. The first bridge arm is connected to the resistance strain gauge R1, while the other three arms are connected to fixed resistors. When no strain occurs, the bridge remains balanced, maintaining the initial condition R1.R4 = R2.R3, resulting in zero output voltage:

UOUT = A(R1.R4 - R2.R3) = 0

However, when the strain gauge is subjected to strain, its resistance changes by ΔR1, causing the bridge to become unbalanced.

Assuming and considering the initial balance of the bridge , we can neglect the small terms in the denominator , which simplifies the equation to:

From this equation, it is clear that the output voltage is directly proportional to the change in resistance caused by the strain on the gauge. This relationship makes the strain gauge bridge a reliable tool for measuring mechanical deformation in various engineering applications.