Bridge Signal
An electrical bridge circuit consisting of four resistive arms produces a non-zero differential output voltage when the resistance ratios of the arms are unequal. This Wheatstone bridge unbalance is the primary mechanism used to convert physical changes like strain or pressure into measurable electrical signals. It is directly proportional to the change in resistance of the active sensing elements within the bridge.
Measurement Process
In a typical strain gauge sensor, mechanical strain changes the resistance of the active gauge, causing a proportional change in the output. This Wheatstone bridge unbalance is measured as a differential voltage across the output terminals when an excitation voltage is applied. Differential voltage measurements provide high sensitivity while canceling out common-mode errors like temperature-induced resistance changes.
This structural stability makes the bridge circuit ideal for micro-scale sensors operating in fluctuating industrial environments.
Signal Conditioning
High-gain instrumentation amplifiers are used to boost the millivolt-level output of the unbalanced bridge. If the initial Wheatstone bridge unbalance is too large, it can saturate the amplifier or reduce the available dynamic range of the measurement system. Precision trim resistors or digital-to-analog converters are used to balance the bridge under zero-load conditions.
System Tolerance
Slight variations in the nominal resistance of the bridge resistors can introduce a small residual unbalance. This residual Wheatstone bridge unbalance is measured during final sensor assembly and stored in the sensor’s non-volatile memory for digital offset correction. This ensures that the instrument starts from a perfectly balanced baseline before each measurement cycle.
