Sensors, actuators, and industrial control devices
This domain tests how sensing devices connect to controllers and how actuators respond. Expect discrete and analog sensors, PNP/NPN outputs, sinking/sourcing inputs, relays, solenoids, and signal scaling.
Core rules
- Discrete sensors give on/off states; analog sensors give a continuous signal, commonly 4–20 mA or 0–10 V.
- PNP (sourcing) sensor output switches +24 V to the load; NPN (sinking) output switches the load to 0 V. Match the sensor to the PLC input common.
- Inductive sensors detect metal; capacitive sensors detect most materials; photoelectric sensors use light and can be diffuse, retro-reflective, or through-beam.
- 4–20 mA uses a live zero: 4 mA represents the lower range value, so a broken wire reads below 4 mA and can be detected.
- Actuators include relay/contactor coils, solenoid valves, and motor starters; inductive loads need suppression.
Worked example
A pressure transmitter is ranged 0–100 bar and outputs 4–20 mA. At 12 mA, the process value is (12 − 4) ÷ (20 − 4) × 100 = 50 bar. If the PLC analog input is configured 0–20 mA instead of 4–20 mA, the same 12 mA would be scaled as 60 bar, a 10 bar error. The reasoning: scale from the live zero, not from 0 mA, and verify the input configuration against the transmitter range.
Traps
Wiring a PNP sensor to a sinking input without matching commons; assuming an NPN output supplies +24 V; using a 2-wire sensor as if it were 3-wire; ignoring the live zero; and grounding a shield at both ends causing noise. These errors produce false signals, not just unreliable readings.