Sensors, Actuators, and Transducers
Core concepts
Candidates must know sensor operating principles: resistive (thermistor, strain gauge, potentiometer), capacitive, inductive, piezoelectric, Hall-effect, optical (photodiode, LDR), and thermal (thermocouple, RTD). Understand transfer functions, sensitivity, linearity, hysteresis, resolution, accuracy versus precision, and drift. Signal conditioning-Wheatstone bridges, instrumentation amplifiers, filtering, and analogue-to-digital conversion-is central. Know actuator types: DC and stepper motors, servos, solenoids, relays, and piezo actuators, including drive requirements and flyback protection. Understand calibration, ratiometric measurement, and the difference between open-loop and closed-loop actuation.
Traps
A frequent error is confusing accuracy with precision, or resolution with accuracy. Questions may ask about thermocouple cold-junction compensation-remember the reference junction must be compensated. Watch for sensors with non-linear outputs where a simple gain calculation is invalid. In motor questions, inductive kickback and the need for a flyback diode are commonly tested. Another trap is ignoring loading effects: a high-impedance sensor connected to a low-impedance input will read incorrectly.
Worked example
A strain gauge in a quarter bridge with gauge factor 2 and 1000 µε strain changes resistance by ΔR = GF × ε × R = 2 × 0.001 × 350 Ω = 0.7 Ω. Rule of thumb: bridge output is roughly (ΔR/R) × V_excitation / 4, so tiny changes require amplification.