Sensors, Transducers and Instrumentation
Sensors, Transducers and Instrumentation
The exam tests selection and wiring of industrial sensors, signal conditioning and calibration: inductive, capacitive, photoelectric, magnetic and Hall sensors, encoders, LVDT, strain gauges, RTD, thermocouple and pressure transmitters. Expect a signal-scaling calculation and a wiring question.
Core rules
- Inductive sensors detect metal only; capacitive sensors detect most materials including liquid level through a wall; photoelectric sensors use through-beam, retro-reflective or diffuse modes.
- A PNP (sourcing) sensor switches +24 V to the input and needs a sinking input card, whose common is 0 V. An NPN (sinking) sensor switches 0 V and needs a sourcing input card, common +24 V. Three-wire sensors use brown +24 V, blue 0 V, black signal.
- A 4–20 mA loop has a live zero: 4 mA represents the lower range value, so a broken wire reads below 4 mA and can be detected.
- Pt100 is 100 Ω at 0 °C, changing about 0.385 Ω/°C; thermocouples rely on the Seebeck effect and need cold-junction compensation; encoders give counts per revolution, so resolution = 360°/counts per rev.
- Accuracy is closeness to true value; repeatability is closeness of repeated readings. A gauge can be repeatable but inaccurate.
Worked example
A pressure transmitter covers 0–25 bar with a 4–20 mA output into a 12-bit analogue input (0–4095 counts). It reads 16 mA. Fraction of span = (16 − 4)/16 = 0.75, so pressure = 0.75 × 25 = 18.75 bar. The count value is 0.75 × 4095 ≈ 3071. A Pt100 reading 119.25 Ω gives (119.25 − 100)/0.385 ≈ 50 °C.
Traps
Wiring a PNP sensor to a sourcing input; treating 4 mA as zero bar; forgetting lead resistance in two-wire RTDs; confusing repeatability with accuracy.