Sensors & Instrumentation
Proximity sensor not detecting — sensor, wiring or PLC fault?
A missing sensor signal can start at the target and mounting, inside the sensor, along its cable, at the PLC input or in the machine sequence.
When a proximity sensor stops detecting, replacing the sensor immediately is tempting. The same visible symptom can, however, be caused by the target, mounting position, supply, cable, connector, PLC input channel or control logic.
Begin with the physical application
The target must be suitable for the sensor technology and pass within its reliable sensing range. A bracket can move, a stop can wear or contamination can change the effective gap. Flush and non-flush inductive sensors also have different mounting requirements, while capacitive and photoelectric devices respond to different materials and environmental conditions.
Sensor indication is only one clue
An indicator LED can show whether the sensor believes it has detected a target, but it does not prove that the output reaches the controller correctly. Conversely, no LED may indicate a missing supply rather than a failed sensing head. The device type, supply range and PNP/NPN output arrangement must match the circuit.
Cable and connector faults
Flexible cables near moving machinery suffer repeated bending, crushing and contamination. M12 connectors can admit moisture or develop poor pin contact. An intermittent conductor may pass a static check and fail only when the machine moves, so the physical route and timing of the fault matter.
PLC input and logic
If the electrical signal reaches the input terminal, the input channel and its common connection still need consideration. If the PLC input status changes correctly, the problem may instead be an interlock, timer, mode selection or sequence condition that prevents the expected action.
Useful observations
- Sensor part number and technology
- LED state with and without the target
- Target material, distance and alignment
- Whether moving the cable changes the symptom
- PLC input indication and machine mode
- Whether the fault is permanent or intermittent
Industrial control circuits may contain multiple supplies and automatic movement. Diagnosis should follow the machine’s isolation and safe-testing procedures.
When Truvo may help
Truvo Industrial investigates sensors, field wiring and PLC I/O faults on machinery across Sheffield and South Yorkshire. Clear photographs of the device, connector and input indication can help establish the right first checks before a visit.