Industrial sensors and actuators

Why this matters

The battery-swap station must know when a drone has landed, whether a battery is on the belt, and whether the charging cabinet is too hot. Choose the wrong sensor type, such as a metal-only sensor for a plastic battery case, and the system cannot “see” the battery; wire a sensor to the wrong PLC input type and no signal arrives even though the sensor works.

Presence sensors

Photoelectric sensors come in three types: through-beam, with separate transmitter and receiver and the object blocking the beam; retro-reflective, with transmitter and receiver in one housing and light returned from a reflector; and diffuse, with light reflected from the object itself.

PNP and NPN

A three-wire sensor has a positive supply wire, a negative supply wire and a signal wire. When it detects an object:

• PNP (sourcing) connects the signal wire to the positive supply; current flows out of the sensor into the PLC input
• NPN (sinking) connects the signal wire to the negative supply; current flows into the sensor

The PLC input must be the matching type: sinking inputs for PNP sensors, and sourcing inputs for NPN sensors. Always check the manuals of both the sensor and the input module, including the supply voltage in that sensor model’s datasheet.

The 4–20 mA signal

Sensors measuring continuous values, such as temperature or pressure, commonly send a 4–20 mA current in industry. Current is the same everywhere in a series loop, so long cables do not distort it as they do a voltage signal. It starts at 4 mA rather than 0 mA (a live zero) so a “true zero” can be told apart from a “broken wire”.

NAMUR recommendation NE 43 standardises signal levels for transmitter failure information. General sources describe values at or below 3.6 mA, or at or above 21 mA, as failures; check these figures against the manual of the transmitter model used.

def to_temperature(ma, lo=0.0, hi=100.0):
    if ma <= 3.6:
        return None, "fault: low (open wire or dead sensor)"
    if ma >= 21.0:
        return None, "fault: high (sensor failure)"
    value = lo + (ma - 4.0) / 16.0 * (hi - lo)
    note = "ok" if 4.0 <= ma <= 20.0 else "outside range, check sensor"
    return value, note


for ma in (4.0, 8.8, 12.0, 20.0, 2.1, 21.5, 3.8):
    value, note = to_temperature(ma)
    shown = f"{value:6.1f} °C" if value is not None else "   ---  "
    print(f"{ma:5.1f} mA -> {shown}  {note}")

  4.0 mA ->    0.0 °C  ok
  8.8 mA ->   30.0 °C  ok
 12.0 mA ->   50.0 °C  ok
 20.0 mA ->  100.0 °C  ok
  2.1 mA ->    ---    fault: low (open wire or dead sensor)
 21.5 mA ->    ---    fault: high (sensor failure)
  3.8 mA ->   -1.3 °C  outside range, check sensor

Actuators

• Solenoid valves and pneumatic cylinders: electricity switches a valve, which lets compressed air push a piston. A single-acting cylinder extends with air and returns by spring; a double-acting cylinder uses air both to extend and retract. The station’s battery gripper uses this
• Relays and contactors: a small signal switches a large circuit; a contactor is a large relay for high-power loads such as motors
• Stepper and servo: a stepper motor turns in steps according to pulses, usually without position feedback (open loop), while a servo has an encoder that measures position and corrects it (closed loop). Steppers are cheaper, but if overloaded they can lose steps without the controller knowing

Module lab

Common mistakes

Summary

• Inductive sensors detect metal, capacitive sensors both metal and non-metal, and photoelectric sensors use three beam arrangements
• PNP connects the signal to the positive supply and NPN to the negative, and must match the PLC input
• 4–20 mA starts at 4 mA to tell zero from a broken wire, and out-of-range signals must be reported as faults
• Choose actuators by load and whether position must be measured

Check your understanding

1. To detect a plastic battery case, should you use an inductive or a capacitive sensor?
2. A 0–100 °C transmitter sends 16 mA. What is the temperature?
3. A 0–10 bar transmitter sends 6 mA. What is the pressure?
4. How should a reading of 0.5 mA be interpreted?
5. When it detects an object, what does a PNP sensor connect its signal wire to?