Roadmap › Industrial Automation — PLC, SCADA, Modbus and OPC UA › Module: Field Devices and Signals
Lessons in this course (10)
1. Sensors, actuators and signal types
Basic★ Must knowLesson 1 of 10
Sensors (inputs):
| Sensor | Measures / detects | Output |
|---|---|---|
| Inductive proximity | Metal object present | Digital (PNP/NPN) |
| Capacitive proximity | Any material (level in tank) | Digital |
| Photoelectric | Object breaks light beam | Digital |
| Limit switch | Mechanical position | Digital |
| RTD (PT100) / thermocouple | Temperature | Analog |
| Pressure / level / flow transmitter | Process values | Analog 4–20 mA, HART, fieldbus |
| Encoder | Speed / position | Pulses (high-speed counter) |
Actuators (outputs): contactors and motors, VFD (variable frequency drive — controls AC motor speed), solenoid valves, pneumatic cylinders, control valves, servo drives, indicator lamps, buzzers.
Signal types:
| Type | Range | Notes |
|---|---|---|
| Digital (discrete) | 24 V DC on/off | PNP (sourcing) vs NPN (sinking) sensors |
| Analog current | 4–20 mA | 4 mA = 0 %, 20 mA = 100 %; < ~3.8 mA means broken wire ("live zero"); immune to voltage drop on long cables |
| Analog voltage | 0–10 V | Simple, but affected by cable length/noise |
| Pulse | kHz | Flow meters, encoders |
Scaling an analog value: value = (raw − rawMin) / (rawMax − rawMin) × (engMax − engMin) + engMin. Example: a Siemens analog input gives 0–27648 for 4–20 mA; a 0–10 bar transmitter reading 13824 → 5 bar.