Sensing Distance and Flush Mounting - What Matters?

What does rated sensing distance mean?

The rated sensing distance sn is the ideal value stated in the datasheet at which a proximity sensor responds to a standard target plate. It applies under laboratory conditions and is not a value you should design with directly.

Several practical figures derive from sn. The effective sensing distance sr accounts for manufacturing tolerances (typically 0.9 to 1.1 x sn), the usable sensing distance su also adds temperature and voltage variation. For safe design the assured sensing distance sa matters, at which switching is guaranteed.

Flush or non-flush mounting?

For inductive proximity sensors the housing type decides whether the sensor may be mounted flush (embeddable) or non-flush (non-embeddable) in metal. Flush sensors carry a metal ring around the active face that focuses the field forward and shields surrounding metal.

• Flush (embeddable): the sensor can sit level with the metal surface without the surrounding material triggering it. The trade-off is a shorter range.
• Non-flush (non-embeddable): larger range up to roughly double the sensing distance, but a clearance zone around the head is mandatory.
• Quasi-flush: a compromise with slight protrusion and moderate clearance requirement.
• Rule of thumb: at the same diameter a non-flush sensor switches about twice as far as a flush one.

Which clearances and reduction factors apply?

The sensing distance only holds for the standard target of mild steel (St37/Fe360). Other metals cut the range via the reduction factor. In addition you must respect side clearance, opposing sensors and the free zone so that nearby metal does not switch the sensor by mistake.

• Flush sensors usually need no side clearance, but keep a minimum spacing to the next sensor against mutual interference.
• Non-flush sensors need a metal-free ring around the head, often three times the sensor diameter.
• With factor-1 types the reduction factor is material-independent - handy for mixed materials.
• Set opposing sensors at least twice the sensing distance apart.

How does mounting affect interference immunity?

The mounting type directly affects electromagnetic compatibility. Flush, shielded sensors are less sensitive to lateral metal disturbance and neighbouring fields because the metal ring guides the field. Open, non-flush heads react more strongly to surrounding metal and adjacent sensors.

• Route sensor cables separately from power and motor lines to avoid coupling.
• Keep minimum spacing between sensors so their fields do not overlap.
• Shielded cables and defined grounding reduce interference pulses.
• Choose enough margin to sa so that vibration and thermal drift cause no false switching.

Frequently asked questions