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Proximity sensors: you do have more choice

06 March 2013

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 from appropriate materials 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. But, as Tim Baker explains, they do have a problem, and it’s a question of range.

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 sensor easily detecting a target at a distance greater than 10mm from its face. Technologies have also been introduced that enables range to be maintained even when the target is made from different metals.

If you are in the market for such sensors, you are spoilt for choice, in terms of type, technology or supplier. So what’s the problem? The issue is this: what if the movement of the target cannot be kept repeatably within a few millimetres?

Larger inductive proximity sensors are available that can detect targets at distances in excess of 100mm, but in order to do this you need a bigger coil, which, in turn, needs a larger housing. The result is a sensor that may be difficult or impossible to accommodate within the available space and, naturally, the cost escalates. 

The alternatives
So why not use photocell? The attainable range of a photocell (taking the example of a diffuse unit in a common M18 housing format) may well be many hundreds of millimetres. It will be just as easy to install as an inductive sensor, with no more space required, and pricing is likely to be comparable.

But whereas an inductive sensor is able to function in a dirty environment, with lots of water or oil splashing over it, or when the target is dark, problems start to mount when you try to subject a photocell based sensor to the same conditions. And 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. It uses the same manufacturing techniques as inductive proximity sensors, and is therefore robust, independent of sensor soiling and temperature fluctuations, and is no more expensive. This is the proximity switch that relies on the target being magnetic.

The technology is not particularly new. Reed switches have been used across many industries for years, but a reed is still an electromechanical device with moving parts that are subject to wear. Modern electronics has long since came up with better magnet-sensing technologies; you only have to look at computer hard drives for an example of these advances.

This solid-state technology is now exploited by proximity sensors 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 magnetic. The sensing range is determined by the field strength of the magnetic target, but even when this is relatively weak 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 magnetic target and sensor can be on separate sides of an enclosure. This opens up a lot of opportunities for proximity sensors that rely on magnetic targets.

Common among these are product recovery or clean-in-place systems where a ‘pig’ is propelled through stainless steel pipework to flush out a system between production batches. The pig is fitted with a magnet, so its position can be detected from outside the stainless steel pipework using a magnet sensor. 

There are other examples too; lifting mechanisms, especially larger installations, are often subject to mechanical movement that takes them beyond the reach of an inductive sensor - tailgates on trailers, for example, which may become damaged, if their movement and position are not constantly monitored during operation.

Similarly, for mobile machinery subject to the kind of wear and tear that could cause damage to its moving parts, magnetic sensors can play an important role as long-range limit switches to prevent such damage. Detecting the relative positions of the articulated arms of an hydraulic excavator or mechanical handling system is one example.

So, if the production environment is not conducive to optical or ultrasonic sensors, or where an inductive proximity just doesn’t have the required range, so long as the target is magnetic, there is always the magnetic sensor to consider. The author’s company can supply magnetic sensors in standard industrial M12 and M18 sizes, in all-stainless steel housings, including the front face.

Tim Baker is with ifm electronic

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