Don't let damaged proxies bring down the line

Modern proximity switches are extremely reliable, but they're not immune to mechanical damage which can lead to costly downtime.

Modern proximity switches are extremely reliable, but they're not immune to mechanical damage which can lead to costly downtime.

Tim Baker, of IFM Electronic, discusses why this damage occurs, and explains how a new design of proximity sensor is helping to solve the problem.

If a proximity switch fails, the plant stops, and downtime costs start to mount.

The most common cause of switch failure, by far, is mechanical damage and, strangely, this type of failure is often regarded as unavoidable, just something sensor users have to live with.

But, is this true? Can that expensive downtime be avoided? Let's see.

Most proximity switches are small devices - M12 and M18 versions are the most common - with sensing ranges of just a few millimetres.

A standard M18 flush-mountable sensor, for example, has a nominal range (Sn) of 5mm.

This, however, is what the sensor is designed to achieve, and it doesn't take into account things like manufacturing tolerances.

Sensor users, however, see this range in the catalogue, and this is the range they expect to get.

Unfortunately, things aren't that simple.

In mass-produced items, tolerances have to be accepted.

It is simply impossible to produce items which are identical with each other.

A proximity switch contains many mass-produced parts, all of which vary slightly from sample to sample.

These variations affect the performance of the sensor and, to take this into account, sensor standards allow a tolerance of ±10% on the nominal range.

Also, each sensor application differs from the next.

For example, supply voltages and ambient temperatures may vary.

These factors also affect sensing performance and, once again, the standards allow a variation of ±10% in sensing range.

Under worst-case conditions, therefore, the assured operating range (Sa) of the sensor is 81% of the nominal range quoted in the catalogue! Already, our 5mm range is down to just 4.05mm, and there's more to come.

So far, we haven't mentioned targets.

All proximity switches are designed for mild-steel targets of specified dimensions.

Real-world targets are very different.

Smaller targets mean reduced range, as do non-ferrous targets and targets with curved surfaces.

On top of this, in any real application, the target-to-sensor distance is sure to fluctuate a little.

Reputable proximity switch manufacturers, therefore, advise that, to ensure reliable operation, sensors should be used at 50% to 80% of nominal range.

It's these reduced ranges which raise the risk of mechanical damage.

When sensing ranges are small, a target which is only slightly out-of-position may rub and wear away the sensing face, or it may even hit the sensor.

Swarf may be trapped in the tiny gap between the sensor and target, again causing damage.

A partial solution is to increase the sensing range for a given size of sensor.

For example, extended-range flush-mounting M18 sensors are available with a nominal range of 8mm, compared with 5mm for standard types.

Unfortunately, the user is still left to guess the true reliable sensing range, and whether this will be enough to minimise the risk of mechanical damage.

There is, however, a better solution.

Quadronorm Plus sensors from IFM Electronic have extended sensing ranges, but they also have another very useful feature: a set-up LED.

This is lit when the target is at a distance between 80% and 100% of the real sensing range.

Users can, therefore, set sensors accurately at 80% of their real range, allowing an ample safety margin without wasting valuable operating range.

Let's look at some actual figures.

Typical real operating ranges for M12, M18 and M30 flush- mounting sensors are 1mm, 2.5mm and 5mm, respectively.

With extended-range sensors, the corresponding figures are 2mm, 5mm and 7.5mm, but the full potential of the sensors still cannot be used because, as we've seen, it's almost impossible to calculate the optimum working range.

The set-up LED on Quadronorm Plus sensors, however, shows optimum range clearly and unambiguously.

Reliable sensing can, therefore, be achieved at ranges between 140% and 220% greater than those achieved with standard sensors.

That extra range translates directly into reduced risk of mechanical damage to the sensor.

Returning to our original question of whether downtime due to sensor damage can be avoided, the answer is clearly and unambiguously yes.

It's simply a matter of choosing sensors from IFM Electronic's Quadronorm Plus range, which not only offer extended sensing ranges, but also allow those ranges to be used fully, without compromising reliable detection.

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