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At Hannover Fair: The Latest in Proximity Sensors

23 June 2009

It was over 50 years ago that two radio engineers, Walter Pepperl and Wilfried Gehl, were commissioned by BASF to find an alternative to mechanical contact switches. The challenge was to develop a robust component that would operate reliably after many thousands of cycles at very low currents, in the corrosive atmosphere of a chemical plant. What they came up with was the first inductive proximity switch, and their company Pepperl+Fuchs has gone on to become a leader in the technology.

Lighting up their corners, these VariKont sensors on display in the Pepperl+Fuchs stand show high diagnostic visibility from any direction.
Lighting up their corners, these VariKont sensors on display in the Pepperl+Fuchs stand show high diagnostic visibility from any direction.

One of the oldest electronic components in the field of automation, the proximity switch has been continuously reinvented over the years to keep pace with ever-changing requirements. To read an online version of Control Engineering Europe’s article published last year, “Fifty years old: the proximity switch,” click here.

There are four main types of proximity switches: sonar, photoelectric, capacitive electric, and inductive electric. The latter are the most cost-effective and the most widely used in industry.

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Lighting up their corners, these VariKont sensors on display in the Pepperl+Fuchs stand show high diagnostic visibility from any direction.

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The way an inductive proximity switch works is, it sends out a high frequency alternating field through its sensing face. The range of the device is determined from the size of the alternating field but it usually is several millimetres, or at the most a few tens of millimetres, from the end of the device. When any metal that is a good conductor of electricity, such as iron, copper, or aluminium comes within the range of the alternating field, the field is damped. The switch senses this changed condition and sends out a signal.

The switches are highly reliable with good repeat accuracy. Because they never have to come into contact with the material they are detecting, they have a long service life and are relatively immune to noise, temperature, light, or water.

As simple as they are, even after 50 years, innovation continues with the proximity switch. The best way to see the latest results is to visit Hannover Messe.

In the Pepperl+Fuchs stand the company was showing its new generation of VariKont sensors (first photo), with a reworked design, and new internal assemblies.

The enclosure of the sensor has become an ISO standard, accepted worldwide, boasts Thomas Hensler, P+F’s product manager for inductive sensor systems. So his company’s designers are not going to change that. But, other than that, switch designers have been busy and he says their designs have been “perfected down to the very last detail.

“When a product has successfully proved itself day after day for 30 years in the harsh industrial environment, it is a challenge for the inventor and the manufacturer to find a worthy successor,” he says. A nice thing to complain about.

“Only convincing improvements for the user will justify the step into a new generation of devices.”

Pepperl+Fuchs has made improvements in the electrical characteristics, robustness, and reliability under extreme conditions. With improved sealing they are now rated IP67 and IP69k. A special coating gives UV protection, so the switches can work outside in the sunshine with maximum protection.

The most noticeable feature of the VariKont switches are the indicator LEDs on four of the eight corners. This is what Mr. Hensler calls the “all around view” status display. Two are green and two are yellow; for operating status and the switching state. They are diagonally arranged so that the user can see them from any direction.

The corner LEDs were introduced six years ago and are now on all the VariKont sensors—and you won’t see other vendors imitating them anytime soon, because the arrangement is patented.

The new VariKont now provides switching distances of 20 mm to 40 mm in the standard enclosure size. Previously, the versions with a 40 mm switching distance were only available in a larger unit.

Another new feature is the reduction factor of 1, necessary for frequently changing work pieces in iron, aluminium, or other light metals that have to be machined in the automobile industry.

A new non-adhesive coating material developed by ifm to withstand welding applications is ‘tougher than Teflon.’
A new non-adhesive coating material developed by ifm to withstand welding applications is ‘tougher than Teflon.’

Another important factor is tolerance to EMC. In plants where many electronic devices are operating in a small space, the optimised electromagnetic compatibility (EMC) of the proximity switch renders it insensitive to external interference.

The makers of VariKont were wise enough to keep the base of the enclosure, with its connections, compatible with predecessor models. This way users can upgrade to the latest sensor without facing the prospect of expensive adaptation and installation work.

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A new non-adhesive coating material developed by ifm to withstand welding applications is said to be "tougher than Teflon." It shows up as a light gray coating on this sensor.

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Metal faces

Traditionally, cylindrical inductive sensors have a plastic cap on the front face and a nickel plated brass enclosure, and that is ideal for most applications.

However, in very harsh industrial applications, the mechanical stresses encountered can result in a reduced service life on plastic faced sensors.

So metal faces on proximity sensors are becoming popular and the sensor manufacturers are learning how to overcome the challenges that this form of construction brings.

Metal face sensors are useful when the application requires extreme robustness, resistance to corrosion, good sealing, resistance to wear, and a long service life, says Pepperl+Fuchs product manager Armin Hornberger. He says the food industry is a good example. There the metal face is well received because it is easily cleaned.

In the machine tool industry, the surface in stainless steel affords protection from metal swarf and is electrically unaffected by the same. In outdoor applications, metal is the natural choice thanks to its insensitivity to environmental influences, such as humidity and mechanical shock.

Another area of application is the automobile industry, due in particular to the sensor’s robustness in basic assembly operations. And metal face sensors are available with specially treated enclosures that are resistant to weld spatter.

Full-metal inductive sensors were also a big item in the ifm electronic stand, where product manager Mario Holt was showing some new versions to fair visitors.

The company was presenting, for the first time, its third series of full-metal sensors, called the C series. They were designed especially for the metalworking industry.

The C series has a reinforced sensing face to withstand mechanical stress, such as impacts, much more often than a sensor with a plastic face. Vibration, shock, impact and coolant tests confirm this in rough operating conditions.

ifm has also developed a new anti-adhesive coating material that is “tougher than Teflon.” It is a light gray coating on the sensor (second photo) that is especially immune to weld splatter.

There are some interesting flash videos on ifm’s web site, in the full metal inductive sensor section, that show what it does to test the sensors.

Siemens’ Simatic PXI920 inductive proximity switch for does precise analogue and dynamic measurements of distances from metal objects.
Siemens’ Simatic PXI920 inductive proximity switch for does precise analogue and dynamic measurements of distances from metal objects.

In one case, the prox sensors are in boiling water for a few minutes and then lifted out and placed in ice cold water. The torture test was repeated 50 times with no harm to the half dozen sensors.

Analogue distance measurement

Siemens's Industry Automation Division has produced a new inductive proximity switch, called the Simatic PXI920, but it does a lot more than just detect the presence or absence of a metal object nearby.

It can make the precise analogue and dynamic measurement of distances from metal objects. It measures precise object distances from 0 to 3 millimetres at a frequency of 250 hertz.

The output signal is a value between 0 and 10 volts, proportional to the measured distances of the object.

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RIGHT: Siemens’ Simatic PXI920 inductive proximity switch measures precise distances from metal objects, so it can be used in web applications for dynamic measurements.

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The proximity switch, which can be mounted flush, is said to be especially suitable for dynamic fine positioning and controlling feeders. One application is, for example, so-called ‘dancer controls,’ which are used for continuous products, such as foils, plastics, paper, sheet metal and textiles. The PXI920 can give an output signal to let the control system know just how close the continuous web of material, usually travelling at a high speed, is getting to the sensor.

In automated production, analogue distance measurement also plays a part in the precise evaluation of wear and fracture, and in making the simple distinction between tool and workpiece. This switch has advantages over binary proximity switches, especially under difficult environmental conditions. For example, it can detect the presence of swarf and slag before a possible plant failure occurs.

Two-wire switches

At the Turck stand, product Manager Oliver Marks was happy that his company had recently completed development of a new 2-wire “uprox” proximity switch, developed especially for the auto industry in France.

It’s a rather special product, to be sure, but there are specific applications where automation engineers insist they be used.

Most prox switch applications use 3-wire technology; this is probably 85 to 90% of the world market, says Mr. Marks.

But the French, for their own reasons—they say it has the advantage that you can immediately detect a circuit break—want 2-wire technology.

Providing a separate mounting bracket, the new Bi10-QN26 allows the user to position the sensors’ active face in four directions. In this way, the sensor can be utilised for various applications in the vicinity of the French car manufacturers Citreon, Peugeot and Renault—like position detection at conveyor belts or part-inspection in assembling processes.

The cuboidal two-wire sensor has a height of 26 mm and provides a switching distance of up to 10 mm. A clearly visible LED enables the user to check the sensor’s proper operation directly on-site.


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