Transducers and Sensors

Sensor and transducer roles

A sensor responds to a physical quantity; a transducer converts one form of information or energy into another. Measurement chains often include a sensor, excitation, conditioning, conversion and calibration. Sensitivity, range, linearity, hysteresis, drift, dynamic response and loading all affect performance. Examples include resistive temperature detectors, thermocouples, strain gauges, LVDTs and Hall sensors. A thermocouple produces a temperature-dependent voltage difference and needs reference-junction treatment. An RTD changes resistance and requires excitation, which can cause self-heating. An LVDT uses AC excitation and differential secondary signals to infer displacement.

Strain-gauge model

Gauge factor GF = (ΔR/R)/ε for small strain ε. For an ideal quarter bridge with one active gauge, small output magnitude is approximately Vex GF ε/4. The sign depends on which arm contains the gauge and the output polarity. Temperature and lead effects require appropriate compensation.

Worked example

A 120 Ω strain gauge has GF = 2 and tensile strain 500 microstrain = 500 × 10⁻⁶. ΔR = R GF ε = 120 × 2 × 500 × 10⁻⁶ = 0.12 Ω. With 5 V bridge excitation, small-signal output magnitude is 5 × 2 × 500 × 10⁻⁶/4 = 1.25 mV. If a linear conditioning stage of gain 100 is used within its permitted input/output range, output magnitude is 125 mV. This calculation neglects offset, temperature effects and bridge nonlinearity.

Selection and conditioning

Match sensor range and bandwidth to the quantity. Evaluate excitation power, common-mode voltage, noise, calibration and environmental conditions. A more sensitive sensor is not automatically better if it saturates, loads the system or drifts excessively.

Quick check

1. What is microstrain? Strain multiplied by 10⁶ in reporting; 1 microstrain is 10⁻⁶ dimensionless strain.
2. Does an RTD require excitation? Yes, for resistance measurement.
3. Can the conditioner introduce error? Yes, through gain, offset, noise and loading.