Fifty years old: the proximity switch

29 November 2008

In 1958—exactly 50 years ago—the proximity switch was invented in a Mannheim laboratory owned by Pepperl+Fuchs. What was originally conceived as a customer-specific solution for an intrinsically safe current circuit in the chemical industry, has since become the universally recognised industry standard for non-contact switching.

The first proximity switch
The first proximity switch

The proximity switch is one of the oldest electronic components in automation. It has been continuously reinvented over the years to keep pace with ever-changing requirements.

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 1960s, people 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 roller lever limit switch on the market in 1968. This device shared mounting compatibility with its mechanical counterpart and had the option of five different positions for the active sensor surface, so that any possible travel direction of the mechanical switch could be replicated.

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.


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, proximity switch electronics were 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.

Microcontrollers 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.

Prox switch in 1968 (left)
Prox switch in 1968 (left)

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.


There are 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. Analogue 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.


A good example of the latest in proximity sensor design is one created by the inventor himself: the VariKont®. These new representatives of Pepperl+Fuchs proximity switches feature a revised modern housing design, identical standard dimensions, and technical state-of-the-art inner workings. The new generation of inductive proximity sensors combines innovation with tried and tested technologies. The connectors at the lower housing sections are compatible with previous models, so users will not have troublesome adjustments and installations.

The VariKont design is a classic in the history of the inductive proximity switch. The housing shape developed by Pepperl+Fuchs prevailed on the market and was even standardised according to ISO norm.


The inductive sensors were literally perfected across all corners and edges. Four of the eight rounded corners are equipped with display LEDs. The operating and switching states are displayed on two green and two yellow LEDs arranged cross-corner. This makes it possible for users to see all status information regardless of the angle of view, which is a clear advantage during start-up, servicing, or troubleshooting. The layout of the LEDs is patented, assuring that competitors will not reach the same high degree of functionality.

The corner LEDS were introduced in 2003 with the Varikont L sensor, the smaller sensor designed for interior spaces and without need for a separate terminal compartment.

VariKont prox switch
VariKont prox switch

The sensor head houses all sensor system components and analysis electronics while the base section features the elements for the mechanical fastening as well as the terminal compartment for electrical installation. Three screws are used to connect the function section with the pre-mounted and electrically connected mounting unit. Easy installation and set-up and speedy replacement, especially in case of servicing, without electrical connection work are the result.


Among the secrets of the success of the VariKont ’variable contact switch‘ are its ruggedness and reliability under all conceivable operating conditions concerning interior and exterior applications. Pepperl+Fuchs has improved these essential properties for the new VariKont inductive sensor. The impermeability and ruggedness of the housing now meets the requirements of protection class IP67 and IP69k. This means the new sensors are even more weather resistant, water-proof, heat-resistant, and impact-resistant. Improved UV-resistance also protects the units from the damaging rays of the sun. Together with the new sealing concept, VariKont proximity switches are even better protected from moisture when used outside.

The new sensor now offers all switching distances from 20 mm to 40 mm in standard-sized housing. Previously, switches with 40-mm switching distance were only available with larger housings. Also new are inductive sensors with reduction factor 1. They offer an ideal solution if frequently changing workpieces made from iron, aluminium, or other light metals are processed, like in the automotive industry. In systems where many electronic devices work together in a confined space, the optimised electromagnetic behaviour makes the proximity switches resistant to external interferences. VariKont sensors are npn-switching, pnp-switching as NC or NO contact in 2-wire, 3-wire, or 4-wire variants usually available on the market.


A modern design and extensive enhancements concerning the electrical and mechanical properties as well as a cleverly designed "all-angle" status display mean the new VariKont inductive proximity sensor generation offers users the latest maximum of functionality and user benefit. The properties of the sensors and switches enable a reliable and stress-free operation and come one step closer to the zero-fault principle. Since the tried-and-tested connection system between upper and lower sensor section was also applied to the new models, users are easily able to replace old sensors with the new counterparts to upgrade their systems without complex installation work. The existing wiring and connection technology can be maintained unchanged.

—Dr. Peter Adolphs, General Manager Factory Automation, Pepperl + Fuchs

Dipl.-Wirtsch.-Ing. (FH) Thomas Hensler contributed to the writing of this article.


Figure 1: The first inductive proximity switch, manufactured in 1958

Figure 2: A proximity switch from 1968. It is functionally compatible with the roller lever switch shown on the right.

Figure 3: The 360 degree all-angles view function is one of the highlights of the new VariKont generation. This function makes it possible to see all operating and switching states during installation and set-up as well as during normal operating mode. Improved UV-resistance with the proper material selection and a revised sealing concept reliably keep moisture away from the inside of the sensor even after many years of outside use.

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