Proximity switch turns 50
The inductive proximity switch - indispensable for modern automation
How did it all begin?
Fifty years ago, when Walter Pepperl and his colleague Wilfried Gehl were commissioned by BASF to find an alternative to mechanical contacts, they had no idea that they would set a milestone in the development of automation technology. The challenge was to develop a robust component that would operate reliably after many thousands of switching cycles at very low switching currents, in the corrosive atmosphere of a chemical plant. As trained radio engineers, they knew what happens when a metallic object approaches a coil system. With the bi-polar transistor, which William B. Shockley had invented 10 years earlier, the two engineers had at their disposal a new compact component with which the damping of an oscillating circuit could be easily evaluated and converted into a switching signal. The invention of the proximity switch is consequently an early example of how communications engineering expertise advances automation technology.
In the early years, application of the inductive proximity switch was restricted to the chemical industry, where problems with mechanical contact wear due to the low currents used and the resulting absence of cleansing through contact erosion, were the most severe. However, in the early sixties, people also began to appreciate the practically unlimited service life of these switches in other automation applications. So it was no surprise when Pepperl+Fuchs introduced an inductive version of the DIN 43 694-compliant roller lever limit switch on the market in 1968. This device shared mounting compatibility with its mechanical counterpart and had the option of 5 different positions for the active sensor surface, so that any possible travel direction of the mechanical switch could be replicated. Furthermore, in those days it was not yet possible to reproduce all the different voltage ranges electronically. Nevertheless, the 60 different versions of proximity switch required as a result could not prevent its success on the market. The absence of wear was a major advantage for the reliability of automation systems.
Ten years later, the next-generation device was presented, which simplified applications. The user could now change the active sensor surface himself and the amplifiers with different voltage ranges could be easily replaced by insertion. This proximity switch design continues to be one of the highest selling, but the development of this technology has resulted in much greater variety.
Versatility of the proximity switch
The most familiar proximity switch design is the cylindrical threaded bushing. Its design resembles a threaded stud and, consequently, it can be easily mounted on any machine. The sturdy brass or stainless steel housing protects the electronics against all types of environmental influence and, of course, mechanical destruction. Moreover, there are hundreds of other designs available on the market today, taking account of the various installation requirements in machine manufacturing. This sensor is probably one of the few electronic components whose housing design must be adapted to the application rather than to the packaged electronics. Versatility is therefore a matter of course.
In the 1990s, the proximity switch electronics were also fundamentally further developed. Integrated circuitry was introduced, which extended functionality and improved EMC interference immunity. New oscillator concepts enabled switches with a reduction factor of 1 to have the same switching distances for different metals. For applications with especially high durability requirements, proximity switches are available with active sensor surfaces made of stainless steel. Extremely sensitive evaluation is required for such applications, as the pre-damping of the oscillator through the end surface of the housing is quite significant.
Micro-controllers have now also become established in proximity switches. These enable even higher switching distances to be achieved and provide for simplified adjustment of components in production, in addition to improved functionality. All this in a device which is subject to constant price pressure owing to the very high unit volume requirements.
These facts show that the development of the proximity switch has never stopped over the last fifty years. Driven on by incessant new requests and requirements from machine manufacturing and plant engineering, the proximity switch has been continuously reinvented and this trend is set to continue into the future.
The future of the proximity switch
There are presumably several reasons for the market success of the proximity switch. On the one hand, the coil as sensor element is cost-effective to produce and evaluate. On the other hand, these switches are extremely sturdy, easy to use and reliable. Soiling or other environmental influences hardly effect their function. These properties distinguish the proximity switch favorably from other types of sensor. It is therefore no surprise that the quantities sold have increased dramatically in recent years, with no sign of a downturn currently in sight.
And there are some interesting applications in automation technology that exploit the basic principle of the proximity switch and provide the same benefits to the user. Analog position measurement is a good example of such an application.
For this purpose, multiple coils are arranged in a row to precisely measure the horizontal displacement of a metal target to a tenth of a millimeter. A micro-controller evaluates the damping of the different coils by the target and thereby calculates the exact position. The measurement result is independent of the precision of the vertical guidance of the target. This is a good example of how a fifty-year-old principle can be transformed into a reliable and easy-to-use displacement measurement system with the aid of modern electronic technology.
Sensors as the sensory organs of machines are indispensable in the field of automation. In principle, it is merely a matter of transforming physical events into electrical ones. The coil as a sensor element for the proximity switch has established itself as an almost perfect device for position recognition. As long as machines are constructed primarily from metallic materials, the further success of the proximity switch is assured.
Dr. Peter Adolphs
General Manager Factory
Tel: 0161 633 6431
Published in Valve User Magazine Issue 9
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