Potentiometers are widely seen as the low cost solution for many position sensing applications. However, here Mark Howard of Zettlex questions whether their reputation for low cost is justified
Potentiometers are generally seen as the first choice for most applications because they are viewed 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, this doesn’t tell the whole story. 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.
There is a massive swing towards the use of non-contact sensing which 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. However, there are also stories of potentiometers failing, causing down-time and disruption. So why is it that in some cases potentiometers work perfectly well and in others they fail?
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.
By far the most common failures for potentiometers 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.
Foreign matter
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.
Vibration
Although more subtle, the effects of vibration on a potentiometer are just as devastating. Typically, a potentiometer’s life will be rated by a number of 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 ten times per second over perhaps a few microns. Such regimes are not only present in 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.
A 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.
Also, buyers of equipment are increasingly aware of the reputation that potentiometers have. Although this is somewhat unfair there is nevertheless a tendency for equipment that uses potentiometers to be more closely scrutinised and hence, there are more pressures 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. Consequently, many equipment builders are looking to replace potentiometers with non-contact solutions for marketing, rather than strictly technical reasons.
An awkward transition
Nevertheless, not everyone is changing from potentiometers to non-contact solutions, due to changeover being far from straightforward. 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.
Non-contact devices also consume more power than a potentiometer and tend to produce a digital electrical output compared to a potentiometer’s analogue output. Lastly, 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.
To minimise the impact of changing to a non-contact alternative, a shortcut is to use a new generation inductive sensor. These 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. 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°C to 230°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.
Zettlex
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