Instrumentation • Electronics
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. Just like a pot hole (pun intended) in a road, 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 5 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. 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 more widespread perception by engineers and technical buyers 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. The unfortunate reality
is that partially ill-founded market perception as a driving force for change is just as real and just as brutal as any technical reason.
Why isn’t everyone changing to non-contact?
The reason that not everyone is changing to non-contact is that it’s not straightforward. Firstly, there is the issue of cost. Most of industry still works from simplistic bill of material costings and these will always favour pots 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 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. 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, these new generation devices use printed, laminar windings to generate the inductive fields. These sensors 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 well suited to harsh environments with operating temperatures between -55˚C to 230 ˚C and can be encapsulated for long term submersion or operation in explosive environments. l
Mark Howard is General Manager, Zettlex UK Ltd, Newton, Cambridge, UK.
www.zettlex.com
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www.engineerlive.com Fig. 2. Lightweight and non-contact potentiometers are negligibly affected by shock and vibration.
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