Instrumentation • Electronics
4 Potentiometers have been around for a long time and are still, by far, the most commonly used position sensor. So why does every design engineer seem to be looking for a non-contact alternative? Mark Howard examines this phenomenon and explains the pros and cons of potentiometers.
4 Les potentiomètres ont été pendant longtemps et demeurent toujours, de loin, le capteur de position le plus utilisé. Pourquoi chaque ingénieur concepteur semble-t-il alors chercher une alternative sans contact ? Mark Howard étudie ce phénomène et explique les avantages et les inconvénients des potentiomètres.
4 Potentiometer gibt es schon sehr lange und sind noch immer bei weitem die am häufigsten verwendeten Wegmesssysteme. Warum scheint daher jeder Konstrukteur nach einer kontaktfreien Alternative zu suchen? Mark Howard untersucht dieses Phänomen und erläutert die Vor- und Nachteile von Potentiometern.
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here are more potentiometers sold than any other form of position sensor. They are simple, inexpensive, widely available and compact. Laser trimmed potentiometers offer high
accuracy measurement and, unlike their more complex, non-contact counterparts, there are practically no issues of long term component obsolescence. In terms of value for money, potentiometers - usually referred to as ‘pots’ - are an excellent choice for many applications and this is likely to remain the case for the foreseeable future. So why is it that every design engineer seems to
be looking for a non-contact alternative? Certainly, over the past 20 years, there has
been a massive swing towards non-contact position sensing. This has increased to such an extent that when potentiometers are nowadays proposed as part a technical solution, there is a good chance of raised eyebrows, sucking of teeth and pointed questions about reliability and lifetime. So what’s caused this? Is this some mild form of mass hysteria among engineers or is the swing to non-contact position sensing really justified? Clearly, there are
Fig. 1. Potentiometers can work well and offer long periods of trouble-free operation.
many applications where potentiometers will work perfectly well and offer trouble free operation over long periods. The very desk on which this article was written carries a vintage radio from the 1970s, whose volume is elegantly and smoothly controlled with a good old potentiometer behind the Bakelite fascia. But now consider a potentiometer measuring a linear displacement once every five minutes or so – the kind of typical application and duty cycle for a piece of factory automation such as an actuator or valve. A good quality potentiometer might typically be rated for 500 000 cycles. At 500 000 cycles this potentiometer should be good for five years even with constant
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use 24 hours a day, seven days a week. It appears that all potentiometers have been classed as unreliable because of a relatively small number, but seemingly notorious, failures in harsh environments. Now ‘harsh environments’ come in all sorts but there seems to be three particular aspects that cause problems for pots – vibration, foreign matter and extreme climates. Potentiometers are vulnerable in applications
with any significant vibration. Let us consider the previous application more closely but in a vibrating environment such as a road vehicle, heavy plant or aircraft system. When we look closely at the displacement
we can see that there are frequent ‘micro displacements’ caused by the vibration. At this microscopic level, the potentiometer’s conductive
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