Instrumentation • Electronics
4 Potentiometers are still, by far, the most commonly used position sensor. They are widely seen as the low cost solution for many position sensing applications. But is their reputation for low cost justified? Mark Howard takes a holistic view on the cost of potentiometers.
4 Les potentiomètres sont encore de loin les détecteurs de positions les plus communément utilisés. Ils sont largement perçus comme une solution bon marché pour de nombreux programmes de détection de position. Mais leur réputation bon marché est-elle justifiée? Mark Howard propose une approche holistique du coût des potentiomètres.
4 Potentiometer sind immer noch mit großem Abstand die meistverwendeten Positionssensoren. Sie gelten gemeinhin als kostengünstige Lösung für viele Anwendungen mit Positionsabfrage. Aber ist ihr Ruf als
Billiglösung gerechtfertigt? Mark Howard mit einer umfassenden Analyse der Potentiometerkosten.
The true cost of potentiometers
D
espite a massive swing towards non- contact position sensing, design engineers still choose potentiometers more frequently than any other form of position sensor. They are generally
seen as the first choice for most applications because they are also seen as the lowest cost solution to position sensing. In most cases a simple comparison based on bill of material (BOM) costs is likely to show that potentiometers are less costly than any non-contact alternative. However, such a comparison does not tell the whole story. On closer inspection, the more complex the picture becomes. This article discusses a more holistic cost analysis, outlining some of the difficulties in changing from potentiometers to non-contact solutions and proposes some options. The trend towards non-contact sensing is
fuelled by the belief that potentiometers are not as reliable as non-contact position sensors. Clearly, there are many applications where potentiometers work perfectly well and offer trouble-free operation over long periods. There are also stories of potentiometers failing, causing downtime and disruption. So why is it that in some cases potentiometers work perfectly well and in others they fail? The answer lies in the basic physics of how a potentiometer works. Essentially, a potentiometer divides an electrical potential in proportion to distance travelled. In other words, a resistor with one or more electrical pick-offs or contacts sliding along its resistive track. These contacts are typically small, pressed metal parts sliding over a printed track of electrically resistive ink. The further the contact travels along the track, the greater the drop in voltage to the contact. Potentiometers fail for a variety of reasons but
by far the most common failures occur at the sliding contact and specifically at the interface of electrical contact and track. These failures can be attributed to two main factors - foreign matter and vibration. Add a tiny piece of foreign matter - such
Fig. 1. Zettlex manufactures sensor solutions that have no contacts, no bearings, no delicate parts and zero maintenance, just accurate measurements.
as sand, grit or dirt - between the contact and track and the resulting abrasion has a dramatic effect on a potentiometer’s lifetime and reliability. Unfortunately, such foreign matter can be attracted to the contact area due to micro-climates caused by humidity, moisture, condensation or static electricity. Lubrication does not necessarily help because lubricant can bind the foreign matter and exacerbate the problems. Certainly, seals and baffles can reduce or mitigate the ingress of foreign
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matter but the particle size required for abrasive action need only be microscopic. High quality seals are invariably expensive - therefore reducing or eradicating the potentiometer’s cost advantage. The effects of vibration on a potentiometer are
subtler, but nonetheless just as devastating. Typically, a potentiometer’s life will be rated by a number of cycles - normally between 100,000 and 5 million cycles. At the microscopic level, a potentiometer’s sliding contact and track cannot differentiate between a full cycle and a vibration-induced ‘micro cycle’. When a machine is vibrating at 10Hz, for
example, this will cause the sliding contact to displace 10 times per second over perhaps a few microns. Such regimes are not only present in obviously harsh vibration environments such as mining, quarrying or aerospace equipment - but can also be present in seemingly benign applications where pumps, motors or turbulent fluid flow in a pipe generate vibration. One day’s operation at 10Hz vibration is equivalent to almost one million cycles. The vibration effect is exacerbated if the potentiometer’s contact is at one particular position for extended periods - for example a ‘fully closed’ or ‘fully open’ position - since most of the wear is concentrated in that one spot. The contact effectively wears a hole in the resistive track and the potentiometer develops a dead spot or becomes unreadable. Once such failures start to occur in the field,
much larger effects dominate any financial analysis - service call outs, repairs, replacements, product returns or even product recall. Given the consequential impact that an unreliable product can generate, only a relatively small percentage of failures are necessary to trigger a product recall
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