1. Definition of Sensing Distance
Sensing distance is the distance at which the output changes state as a standard target, usually a mild steel square, approaches the sensing face. The rated value in the datasheet is guaranteed at nominal voltage and room temperature, so engineering practice applies a factor of 0.7 to 0.8 to allow for temperature drift, voltage variation and mounting conditions.
2. Factors That Change the Sensing Distance
- Target material: stainless steel, brass and aluminium reduce the distance and require a correction factor.
- Target size: a target smaller than the sensing face reduces the achievable distance.
- Mounting: a flush-mountable type can be embedded in metal, while a non-flush type needs an unshielded zone around the face.
- Ambient temperature and supply voltage: outside the specified range the switching point drifts.
3. Spacing and Mutual Interference
When switches are mounted side by side, keep at least the minimum spacing stated in the datasheet, generally a multiple of the sensing face diameter; insufficient spacing couples the magnetic fields and causes false switching. If the spacing cannot be increased, alternate models with different oscillator frequencies or offset the mounting positions.
4. Wiring and Interference Control
Confirm the output type (PNP or NPN) matches the PLC input. Route sensor cables separately from inverter and servo power cables, earth the shield at one end, and avoid sharing conduit with power lines. For intermittent false switching, check supply stability and earthing first, then look for welding equipment or strong magnetic fields nearby.
5. Commissioning and Acceptance
After installation, move the target by hand and confirm that the indicator and the PLC input change together. Use feeler gauges to establish the operate and release points and record the actual distance as a baseline for future checks. For batch installation, use the same production batch to reduce variation in switching points.
6. Field Response to False Switching
Check supply stability and earthing first, then whether cables share conduit with power lines. Metal chips collecting near the sensing face act as a false target, and insufficient bracket rigidity lets the switch oscillate around the switching point under vibration.
If adjustment does not solve it, increase the operating margin by setting the target at about 70 percent of rated distance, or filter short pulses in the input module. Record the new switching point afterwards as the reference for the next inspection.
7. Three Easily Overlooked Selection Parameters
Besides sensing distance, switching frequency, ingress protection and output current are often ignored. Switching frequency determines whether high-speed counting is possible, ingress protection governs life in coolant and dust, and output current must suit the load, including inrush when driving a relay directly.
- Confirm the switching frequency exceeds the target passing frequency in counting applications.
- Specify a higher ingress protection class and add a protective sleeve in washdown or oil-mist areas.
- Allow for voltage drop and interference on long cable runs, or add an isolation module.
References
- GB/T 14048.1-2012 Low-voltage switchgear and controlgear — Part 1: General rules
- IEC 60947-5-2:2019 Low-voltage switchgear and controlgear — Part 5-2: Proximity switches
- GB/T 4208-2017 Degrees of protection provided by enclosure (IP code)
- GB/T 17626.2-2018 Electromagnetic compatibility — Testing and measurement techniques — Electrostatic discharge immunity test
