selecting the proper inductive proximity sensor


I would like to know what kind of inductive proximity sensor to use. My project is to count revolutions per minute to calculate cm per minute travel of the conveyor attached to the drum (see attached). I don’t expect the drum to rotate any more than two to three revolutions per second. I want the distance per minute to read out on an LCD monitor.


Do you mean magnetic sensor or inductive?

Mark T,

I'm not an electronics guy, so I'm not sure of the difference. I just want it to work and was hoping someone could recommend a component that would work. Can you recommend a component?

MarkT: Do you mean magnetic sensor or inductive?


I'm thinking his sounds easier then it is because there are a lot of details that I may not know but it sounds inexpensive. I'm not sure exactly how to go about making it and not sure how to hook it up to the Arduino. Any suggestions? or can you post a link to a similar project?

KE7GKP: Yes it is possible to do with several different methods. One would be a "reluctance" sensor where you put a coil around a permanent magnet. Then when a magnetic object (like your steel screw) passes across the end, a pulse is generated in the coil. This pulse can be easily detected by Arduino.

You might want to put two steel screws opposite each other to maintain balance.

There are many approaches that can be called inductive or magnetic, and they split into two categories, static and dynamic. Static sensors will work no matter how slowly the drum rotates, which may be important to your application - these measure magnetic field and the field strength at the sensor is arranged to vary as the drum rotates - simplest example is using a magnet on the drum and a Hall sensor to pick up the field. This can use a switching Hall sensor (which outputs a logic level dependent on the sign of the field - these sensors have hysteresis and are easy to interface to).

Alternatively the Hall sensor can be linear type, a magnet is placed on one side of the hall sensor and the moving part has a protruding iron/steel part - as this moves past the sensor it concentrates the magnetic field and the output of the hall sensor (proportional to the local field) picks up a blip. The microcontroller has to decode this analog signal (the size of which depends on the magnet and the geometry). This can be useful to add to an existing machine without alteration (for instance gear teeth can be counted this way).

Dynamic sensors use electromagnetic induction where the rate-of-change of magnetic field induces a voltage in a coil - the signal strength depends on the velocity so this doesn't work when there very slow motion or with a wide range of velocities. This also requires a permanent magnet (either on the moving part or with the coil)

Metal detectors use a technique that is much more sensitive - a coil is used as an inductor in a resonant LC circuit - any magnetic or conducting material nearby changes the inductance of the coil slightly and this affects the resonant frequency (which can be measured very sensitively). This doesn't require a permanent magnet and can work statically as well, but is more complex and expensive, overkill for this application.