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Mark Howard from Zettlex examines the most suitable sensor options for use in harsh environments in sectors such as aerospace, defence, heavy industrial, utility and oil and gas.

Harsh environments come in many forms, but their common feature is that they place heavy demands on control equipment.

The failure of position or speed sensors in the field can have a massive technical or commercial impact.

According to Howard, there is no hard and fast rule as to what constitutes a ‘harsh environment’ but, for the purposes of this article, a harsh environment can be defined as one containing one or more of the following factors: high temperatures (more than 85C), low temperatures (less than -20C), thermal cycling, high vacuum, high pressure (more than 10bar), vibration, shock, AC/DC noise, radiation, water, dirt, aggressive chemicals, long-term immersion, extended life (more than 10 years) and explosive dust and gas (Atex-rated environments).

Despite their variety, such environments invariably make the selection of appropriate equipment both difficult and critical because performance and reliability will surely be tested.

This is especially true when it comes to the selection of electrical control equipment.

Failures in the electrical systems of modern machinery typically account for more than 80 per cent of all failures.

Howard suggests taking a motorcar as an example; of all the breakdowns experienced by an individual, around 80 per cent are related to the ‘electrics’ compared with the main mechanical components.

Other than cables and connectors, position and speed sensors are the most common elements in electrical control systems.

This means that selecting the right position sensor – one that will operate accurately and reliably – in a particular harsh environment is crucial.

One option is to always specify high-reliability, fully qualified, redundant sensors with military-style connectors, heavy-duty housings and cables.

It is certainly one way to ensure high reliability but, unless the user is making civil aircraft or jet engines, it is seldom an economically viable approach.

The smart approach, according to Howard, is to specify a sensor that is reliable and cost efficient.

Across all sectors, potentiometers are the most ubiquitous position sensors.

This is because they are simple, compact and lightweight and offer good value for money.

In harsh environments, however, they have a poor reputation and are rarely chosen because they are susceptible to wear.

Wear rates accelerate rapidly with vibration or the ingress of foreign matter such as sand or grit.

The basic materials of most potentiometers are not generally well suited to extreme temperatures.

Similarly, across all sectors, optical position sensors are a common choice, but are seldom chosen for harsh environments.

The reason is that their optical path is susceptible to obscuration by foreign matter, especially in higher-resolution devices (more than 8 bits) where optical feature sizes compare with dust particles, fibres or hair.

A further limiting factor is that silicon-based electronics are required at the sensing point, effectively limiting the operating temperatures.

There are a variety of magnetic sensors, ranging from simple switch devices to high-accuracy, long-length, magnetostrictive sensors.

Although they all operate using a magnet, a variety of techniques are used.

While simple devices typically use the Hall effect, others use the time-of-flight principle – measuring the time taken for an ultrasonic pulse to travel and return along a magnetostrictive strip.

As the magnet moves along the strip, the flight time increases and vice versa.

Magnetostrictive devices are best suited to long linear displacements of more than 100mm, whereas Hall-effect devices are more suited to rotary arrangements.

Hall-effect devices are widely used, especially in automotive applications, but require fine-tolerance mechanical engineering for accurate measurement.

At first glance, magnetic devices offer a good solution to sensing position in harsh environments but, as with many things, the devil is in the detail, said Howard.

He added that practical experience has shown there are significant issues limiting their use.

First, as with optical devices, their performance in extreme temperatures is limited, because they require silicon-based electronics at the sensing point.

Second, magnetic hysteresis – inherent in any magnetic measurement – limits measurement performance to relatively crude applications.

Third, there are significant temperature coefficients and all magnets are limited by their Curie point, where their magnetic fields distort.

There is also the issue of batch-to-batch variability – since the field generated by one magnet is never the same as that of a second owing to small differences in composition.

Shock and impact must be strictly limited since magnets are brittle.

One particularly nasty failure mode is the gradual build-up of magnetic particulates at the magnet (such as ferrous swarf, dust or particulates in engine oil), which effectively destroys measurement performance by distorting the magnetic field.

All magnetic devices are susceptible to the effect of stray magnetic fields and even DC magnetic devices will also pick up lower-frequency AC effects.

These will be exhibited as noise on the sensor readings – for example, a nearby mains cable is likely to induce 50Hz AC noise source.

Capacitive position sensors are not widely used in extreme environments.

Although they are resilient to wear, shock and vibration, generally, they suffer from drift owing to variation in temperature or humidity and are susceptible to foreign matter.

This is because the capacitive sensing principle is fundamentally unable to differentiate between the capacitive target object and foreign material such as a grease smear, ice or water condensation.

Inductive devices such as resolvers and linearly variable differential transformers (LVDTs) are the traditional choice for the ‘high-integrity’ applications in the aerospace, defence and oil and gas sectors.

Such devices are known to be reliable and safe and have been in widespread use since World War II.

In some cases, the use of inductive devices is mandated by regulations relating to safety-critical or safety-related applications.

The textbook application is aerospace actuators for ailerons – a harsh environment with extreme temperatures and vibration, where reliable, accurate operation is essential.

Inductive position and speed sensors work on the same fundamental principles as a transformer, which means that no silicon-based electronics are necessary at the sensing point.

This means that any associated electronics can be displaced some distance away from the sensing point – typically, a more benign environment where temperature extremes are unlikely to exceed the -40C to +125C limits for most commercial electronic components.

Nevertheless, the transformer windings of traditional inductive devices will tend to make them bulky and heavy.

High-accuracy devices require precision-wound spools, and these are difficult and expensive to manufacture.

One inductive product – the Inductosyn, from Farrand Controls in the US – is widely used in the aerospace and defence sectors thanks to its high levels of accuracy (less than 1 arc-second accuracy per rev), stability and reliability, even in tough environments such as those found on spacecraft.

Instead of the traditional wire windings, Inductosyn systems use a laminar winding arrangement.

However, the price of these devices is out of the range of most mainstream applications.

A budget form of the laminar, inductive position and speed sensor has emerged in recent years in a new generation of inductive devices such as those made by Zettlex.

These devices use compact, lightweight printed circuit boards rather than the traditional bulky transformer windings.

This approach has led to a variety of shapes and sizes of sensor, ranging from less than 1mm to more than 1,000mm and covering linear, rotary, curvi-linear, 2D and 3D forms.

According to Howard, high levels of accuracy can be achieved without finely toleranced mechanical installation.

The fundamental operating principles are similar to the traditional inductive devices and, similarly, any associated electronics can be located away from the harsh conditions surrounding the sensing point.

The performance of the new generation of device is at least as good in harsh environments as the traditional inductive devices, offering up to 24-bit resolution and accuracies of less than 20 arc-seconds over 360deg.

Encapsulation of the main printed components makes the sensors suitable for intrinsically safe (Atex) applications.

Since there are no electrical contacts and the main components are lightweight, they are claimed to offer excellent performance in extreme vibration and shock environments.

The aerospace and defence sectors have been quick to capitalise on this new technology, and Zettlex devices are used for flight controls on several unmanned aerial vehicles, as well as equipment platforms in active service with UK, US and Allied armed forces.

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