Potentiometers suffering because of the failure of a few in harsh environments

19 November 2010

Potentiometers have been around for many years and are still the most commonly used position sensor. So, why do design engineers always look for a non-contact alternative? Mark Howard of Zettlex examines this question and explains the pros and cons of potentiometers

There are more potentiometers sold than any other form of position sensor. They are simple, inexpensive and compact. Laser trimmed potentiometers offer high accuracy measurement and, unlike their more complex, non-contact counterparts, there are few issues of long-term component obsolescence. In terms of value for money, potentiometers offer a good solution for many applications.

Over the past 20 years there has, however, been a move 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. But what has caused this? Is the move to non-contact position sensing justified?

Clearly, there are many applications where potentiometers will work perfectly well and offer trouble free operation over long periods. But, consider a potentiometer measuring a linear displacement once every 5 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 use 24 hours a day, 7 days a week.

Where they might let you down
It would appear that all potentiometers have been classed as unreliable because of a relatively small number of 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 track cannot differentiate between a full cycle and a vibration induced ‘micro cycle’. Furthermore, because the potentiometer’s wiper is at the same point for most of the time, the same part of the track is subject to most of the wear. A microscopic wear point on a potentiometer’s tracks grows quickly – resulting in a discontinuity or ‘flat spot’ with no electrical response. Operation is severely, usually terminally, effected. Whereas 500,000 cycles previously equated to a lifetime of five years, in this example even at a modest vibration cycle of 1Hz, the lifetime reduces to less than 10 days!

Ingress of foreign matter can also be a source of accelerated failure. Again, at a microscopic level the potentiometer’s wiper should normally ride over the conductive track’s molecular surface. When it’s just the track and the wiper this works well. Introduce even tiny particulates between track and wiper and the effect is the same as an abrasive – rapidly accelerating the wear of the conductive track surface. Wind blown desert sand is notoriously abrasive and problematic. Unfortunately, the application of a lubricant can bring the law of unintended consequences into play, since the lubricant can act as an attractant or binder to the particulates and so accelerate still further the rate of wear.

Extreme environments per se, are not a root cause of failure for potentiometers but rather the generation of tiny micro climates in the immediate vicinity of the wiper and tracks. For example, humid air, when cooled, may result in condensation on the wiper and ultimately corrosion or, as with some lubricants, the condensation attracts and retains foreign particles.

In summary, there are some applications where potentiometers will work well and there are others – notably in harsh environments – where potentiometers can prove unreliable. The unfortunate consequence of these high profile, harsh environment failures is that they have overshadowed the more benign applications where potentiometer operation has been reliable. This has led to a widespread perception that potentiometers are the cheap, low-quality option for position measurement. This widespread perception can put equipment manufacturers on the back foot when they are selling equipment that relies on potentiometers – since they are often forced to defend or justify the reliability and quality of their product. Consequently, many equipment builders are looking to replace potentiometers with non-contact solutions for marketing, rather than strictly technical, reasons.

However, not everyone is changing to non-contact Why not? Firstly, there is the issue of cost. Most of industry still works from simplistic bill of material costings and these will always favour potentiometers over non-contact. It takes a more sophisticated cost analysis to include breakdowns, warranty, spares, maintenance and service costs, to show that non-contact solutions are the less costly alternative in harsh environments. Similarly, the sophistication required to show that product sales prices can be maintained when non-contact solutions are used, rather than a potentiometer, is usually beyond most hard-nosed industrial companies.

Just as importantly, there is the knock-on engineering caused by replacing potentiometers with a non-contact alternative. Non-contact devices tend to produce a digital electrical output, whereas potentiometers produce a simple analogue output. Changing from analogue to digital will require the host electrical system to be re-engineered, re-tested and re-qualified. Similarly, potentiometers are compact and so the space previously occupied by a potentiometer will usually be too small or not quite the right shape for its non-contact alternative. Such a change may require a complete mechanical redesign and hence re-testing and re-qualification.

New generation Inductive
Where potentiometers are being swapped for a non-contact alternative, a common replacement is one of the new generation inductive sensors. These new sensors work in a similar way to traditional resolvers or linear transformers but are just as compact as a potentiometer. Rather than a traditional inductive sensor’s wire spools, the new generation devices use printed, laminar windings to generate the inductive fields. They can also generate a high accuracy voltage or current analogue output to mimic a potentiometer and hence avoid re-engineering the host control system. They are also suited to use in harsh environments with operating temperatures between –55°C to +230°C and can be encapsulated for long term submersion or operation in explosive environments. Finally, because they are lightweight and non-contact, vibration and shock have negligible effect.

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