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Proximity sensors: there is more choice than you think

You like the robust nature of modern inductive proximity sensors, but when you need more range without increasing size then you might think your options are limited. Tim Baker of IFM wonders if you've considered solid state, magnetic proximity sensors.

When sensing metal parts on moving machinery the choice of sensor is usually limited to "which inductive proximity switch should I choose?" Understandably so, since inductive proximity switches have become well established over decades and perform a sterling job. After all, they will operate in the dirtiest of environments. If they are made of the correct materials (and yes, there is a choice) it doesn't matter if they are drenched in oil, or hosed down regularly, even operating under water or at extreme or greatly fluctuating temperatures.

Inductive sensors are robust - they'll take the knocks, even more so now that most leading manufacturers offer devices with a steel sensing face. The sensing performance has improved over the years, too, with a typical M18 inductive easily detecting a target in excess of 10mm away. Technologies have also been introduced to allow the range to be maintained when the target is made of a different metal. There is no shortage of different types of the market and many manufacturers from which to choose.

So what's the problem? The issue is this: what if the movement of the target cannot be kept repeatably within a few millimetres? There are indeed larger inductive proximity sensors on the market which can detect in excess of 100mm, but the technology to do this demands a bigger coil, which in turn needs a larger housing. The result is a solution that won't always fit into the available space and the cost escalates.  

So why not use photocell? The attainable range of a photocell - and let's take the example of a diffuse unit in the common M18 housing - may well be many hundreds of millimetres. Just as easy to install as an inductive sensor, no more space required, and comparable pricing. But in a dirty environment? Lots of water or oil splashing over the sensor? Looking at a dark target? The potential problems start to add up.

What about ultrasonic? Again range may not be an issue, but the dirty environment will take its toll on the reliability. There is another option which is not often considered, using the same manufacturing techniques as inductive proximity sensors, and therefore robust, independent of sensor soiling, temperature fluctuations and yet no more expensive. The answer? A proximity switch that uses a magnet as its target.

The technology is not particularly new. Reed switches have been used in many industries for years, but a reed is still an electromechanical component that is subject to the limitations of wear. Modern electronics long since came up with better magnet-sensing technologies for use in computer hard drives. That solid-state technology is used in a proximity sensor that will operate in exactly the same circumstances as an inductive sensor, in a comparable housing for a comparable price, yet give much greater sensing range, as long as the target is a magnet.

So what's the catch? Only one: the target needs to be a magnet. The sensing range is determined by the field strength of the magnet, but even on a small and relatively weak magnet it will equate to three times the range of an equivalent inductive sensor.
There are further advantages, too. As a magnetic field will pass unhindered through most materials, including non-ferrous metals, the magnet and sensor can be on separate sides of an enclosure. This leads to the most common use of magnet sensors, that of product recovery or clean-in-place systems where a 'pig' is propelled through stainless steel pipework. The pig contains a magnet, so its position can be detected from outside using a magnet sensor.

There are many more areas though where they can be used; lifting mechanisms, especially larger installations, are often subject to mechanical movement taking them beyond the reach of an inductive sensor, as are tailgates on trailers, which may become deformed in use.

Consider mobile machinery as well. Mobile machinery is often subject to the kind of wear and tear that causes damage to moving parts and leads to wide tolerances.  The conditions of usage can be unpredictable, in particular in the agriculture and construction industries.

So if you have sensing requirements where the environment is not conducive to optical or ultrasonic sensors, but an inductive proximity just doesn't have the reach, the ideal alternative is a magnetic sensor. IFM Electronic offers a wide range of magnetic sensors, in standard industrial M12 and M18 sizes constructed in fully stainless steel housings, including the front face.
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