Presence detectors and limit switches
Also available in: Català · Castellano
Limit switch (contact)
The part physically presses a lever that closes an electrical contact. Digital signal: part present (1) or not (0).
In the cell, S1 checks that there is a part in position 1 after piston A has pushed it. If it doesn’t trigger, the magazine is empty and the cycle stops.
Reed (on the cylinders)
A switch that closes with the magnetic field of a magnet carried by the cylinder’s piston. That’s why each piston has signals such as A_0 (piston in) and A_1 (piston out): the PLC knows the movement has finished.
Optical (photoelectric)
It emits a light beam and detects whether the part interrupts or reflects it. No contact needed.
- S5: detects the part on the rotary table and stops conveyor 1.
- S7: detects the part at the inspection point and stops the conveyor so the colour sensor can read it.
Why they matter
A sequential automation advances step by step: each step waits for confirmation from a sensor before moving to the next. Without these detectors, the PLC would be acting blind.
Interactive demo
Related concepts
Inductive and capacitive proximity sensors
The capacitive sensor detects any material because it changes its capacitance; the inductive one only detects metals because currents are induced in them. Using both, you know whether a part is metallic.
Pneumatic cylinders and solenoid valves
Compressed air moves the cylinders; solenoid valves are the bridge to the PLC. A monostable valve springs back to rest on its own when it loses the signal; a bistable one needs a signal for each movement and remembers its last position.