Analog Proximity Sensors: 0 to 10V, 4 to 20mA and When You Need One

Almost every proximity sensor sold is a switch. It tells you that something is present or it is not. An analogue proximity sensor does something different: it reports how far away the target is, continuously, as a voltage or a current.

That is a different product solving a different problem, and the most useful thing this page can do is help you work out which of the two you actually need, because the answer is often not the one people arrive asking for.

Switching or measuring: the question that decides everything

Ask what the control system does with the information.

If it acts on a threshold such as stop when the part arrives, or alarm if the gate is open, you need a switching sensor. Cheaper, simpler, more robust, and the whole D30C range is built for exactly this.

If it needs the actual distance to close a loop, control a position, measure thickness or detect gradual drift, you need continuous measurement.

A good number of analogue enquiries turn out to be threshold problems where someone wanted margin. If two or three switching sensors at the heights that matter would do the job, that is usually the better engineering and certainly the better price.

0 to 10V or 4 to 20mA

Where continuous measurement is genuinely needed, the output choice is mostly about cable.

0 to 10V is simple and feeds most PLC analogue input cards directly. Its weakness is cable length and electrical noise. Voltage drops along a long run and the reading drifts with it, and a signal cable running beside VFD output cabling picks up interference readily.

4 to 20mA is a current loop, and current does not change along the cable. It is immune to voltage drop, far more tolerant of noise, and it has a diagnostic property voltage does not: zero current is not a valid reading, so a broken wire is distinguishable from a genuine zero. On a voltage output, a severed cable reads as zero volts, which may be a perfectly plausible measurement.

For anything beyond a few metres, in any Indian plant with drives in it, specify 4 to 20mA. The only reason to prefer voltage is an input card that only accepts it.

The limitation nobody mentions

An analogue inductive sensor measures over a very short span, typically a few millimetres, and the output is not linear across it. It is also affected by target material exactly as a switching sensor is, so an aluminium target reads differently from a steel one at the same distance.

That makes analogue inductive sensing appropriate for a narrow set of jobs: measuring a small gap, detecting runout or vibration on a shaft, checking thickness, or monitoring wear. It is not a general distance measurement device, and buyers who arrive wanting one usually want something else.

What they usually actually need

If the measurement is over more than a few millimetres, the right product is a position transducer rather than a proximity sensor. This is the part of the range we supply as sole authorised distributor in India and SAARC for Novotechnik and Contelec.

• Linear travel, from short strokes to long ones, with the right mounting for the mechanism. See the linear transducer comparison.
• Rotary angle, contacting or touchless, up to 360 degrees. The RFC-4800 and RSC-2800 are the most specified.
• More than one revolution, where absolute position has to survive a power cycle. See the multi-turn range.
• Inside a hydraulic cylinder, where nothing can be exposed outside.

These measure over real distances, output 0 to 10V or 4 to 20mA as standard, and several offer CANopen or IO-Link if the machine is bus-based.

How to get the right answer

Tell us what the controller does with the number rather than what output you want. The range of movement, the resolution you genuinely need as a figure, what the target is, the environment, and whether this is a new design or a replacement. We will tell you whether it is a switching job, an analogue proximity job or a position transducer job, and we would rather say so than sell you the wrong category.