Environmental technology
Sensing change M
Potentiometers are the workhorses of position sensing. But many engineers are looking to find alternative technologies. What are the pros and cons? By Mark Howard of Zettlex
ore potentiometers are sold than any other formof position sensor. They are simple, cheap, widely available and compact. Laser-trimmed potentiometers
offer high accuracymeasurement and, unlikemore complex, non-contact devices, there are practically no issues of long-term component obsolescence. In terms of value formoney, pots are excellent formany applications and likely to remain so. So why are design engineers apparently
looking for a non-contact alternative? Over the past 20 years, there has been a
massive swing towards non-contact position sensing. Now when pots are proposed, there is a good chance of raised eyebrows, sucking of teeth and questions about reliability and lifetime.Why? Clearly, inmany applications pots work
perfectly well and offer trouble free operation over long periods. But consider a potmeasuring a linear displacement once every fiveminutes or so – a typical application and duty cycle for a piece of factory automation such as an actuator or valve. A good quality potmight typically be rated for 500,000 cycles, so should be good for five years even with constant use. It appears that pots have been classed as
unreliable because of a relatively small, but seemingly notorious, number of failures in “harsh environments”. Three aspects cause problems – vibration, foreignmatter and extreme climates. Pots are vulnerable in applications with
significant vibration. In a vibrating environment, such as a road vehicle, heavy plant or aircraft system, vibration causes frequent “micro displacements”. At this microscopic level, the pot’s conductive track cannot differentiate between a full cycle and a vibration induced “micro cycle”. Furthermore, because the pot’s wiper is
at the same point formost of the time a microscopic wear point on the track can quickly grow – resulting in a discontinuity or “flat spot” with no electrical response. Operation is severely, usually terminally, affected.Whereas 500,000 cycles previously equated to a life of five years, in
Choices: Conventional pots (left) work in many applications, while inductive sensors (above) are a newer option
even at amodest vibration cycle of 1Hz, lifetime can reduce to less than 10 days. Foreignmatter can also accelerate
failure. Again, atmicroscopic level the pot’s wiper should normally ride over the conductive track’smolecular surface.When it’s just track and wiper this works well. Introduce even tiny particulates and the effect is the same as an abrasive – rapidly accelerating the wear of the conductive track surface. A lubricant can act as an attractant or binder to the particulates and accelerate still further the rate of wear. Extreme environments are not in
themselves a root cause of pot failure but they generate tinymicro climates around the wiper and tracks. For example, humid air when cooledmay result in condensation on the wiper and ultimately corrosion or, as with some lubricants, the condensation attracts and retains foreign particles. The unfortunate consequence of these
high profile, harsh environment failures is that they have overshadowed themore benign applications where pot operation has been reliable and given the impression that pots are the cheap, low quality option for positionmeasurement. This puts
equipmentmanufacturers on the back foot so they look to replace pots with non- contact solutions formarketing, rather than technical, reasons. But not everyone is changing to non-
contact. First, there is the issue of cost. Bill ofmaterial costings will always favour pots and it takesmore sophisticated cost analysis to show that non-contact solutions are less costly in harsh environments. Just as importantly, there is the knock-on
engineering caused by replacing pots with a non-contact alternative. Non-contact devices tend to produce a digital electrical output, where pots produce a simple analogue output. Changing fromanalogue to digital willmean re-engineering, re- testing and requlifying the host electrical system. And space considerationsmay require a completemechanical redesign, re- testing and re-qualification, too. Where pots are being swapped for a non-
contact alternative, a common replacement is one of the new generation inductive sensors. These work in a similar way to traditional resolvers or linear transformers but are as compact as a pot. Rather than a traditional inductive sensor’s wire spools, they use printed, laminar windings to generate the inductive fields. They can also generate a high accuracy
voltage or current analogue output to mimic a pot and hence avoid re-engineering the host control system. They suit operating temperatures between –55 and +230°C and can be encapsulated for long term submersion or operation in explosive environments. And because they are lightweight and non-contact, vibration and shock have negligible effect. ■
February 2011 ◆ Environmental Engineering ◆ 43
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