The true cost of potentiometers
29 November 2012
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 from Zettlex UK takes a holistic view on the cost of potentiometers, outlining some of the difficulties in changing from potentiometers to non-contact solutions and proposes some options.
Despite 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.
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 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 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 decision. In such instances, when the financial impact on a producer’s brand or reputation is considered, the few pounds’ difference in BOM costs pales into insignificance.
There is a further aspect to be considered in any such financial analysis. Buyers of equipment are increasingly aware of the unfortunate reputation that potentiometers have. No doubt that this is somewhat unfair but nevertheless there is a tendency for equipment that uses potentiometers to be more closely scrutinised and hence there are more pressure on cost compared to alternatives that offer non-contact sensing solutions. 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.
Nevertheless, not everyone is changing from potentiometers to non-contact solutions. The reason is that the changeover is not at all straightforward. A major issue is that potentiometers are physically compact and so the space previously occupied by a potentiometer will usually be too small or not the correct shape for a non-contact replacement. The change to non-contact may require a complete mechanical redesign and hence re-testing and re-qualification of the host product.
Second, non-contact devices consume more power than a potentiometer and tend to produce a digital electrical output compared to a potentiometer’s analogue output. Similarly, such a changeover may require the host electrical system to be re-engineered, re-tested and re-qualified. Third, potentiometers are classed as ‘simple devices’ in safety-related or ATEX environments, whereas a non-contact device is unlikely to be classified as such and only infrequently are such devices ATEX-certified.
In order to minimise the impact of changing to a non-contact alternative, a common shortcut is to use a new generation inductive sensor. These sensors work in a similar way to traditional resolvers or linear transformers but crucially are just as compact as a potentiometer. Rather than a traditional inductive sensor’s wire spools, these new generation devices use printed circuits to generate the inductive fields.
This means that they can be readily arranged in a wide variety of compact shapes and sizes to suit the mechanical constraints of the host equipment. Such compactness and form flexibility can eradicate the need for a mechanical redesign to accommodate the changeover. These new sensors can also generate a high accuracy voltage or current analogue output to mimic a potentiometer and hence avoid re-engineering the host electrical system.
The sensors are well suited to harsh environments with operating temperatures between -55 deg C to +230 deg C and can be encapsulated for long-term submersion or operation in explosive environments. Since they are lightweight and non-contact, vibration and shock have negligible effect.
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