Harsh environments are the norm in many sectors such as aerospace, defence, heavy industrial, utility, oil and gas. There is no hard and fast rule as to what constitutes a ‘harsh environment’ but, for the purposes of this article, we define a harsh environment as one containing one or more of the following factors: high temperatures (>85°C), low temperatures (<-20°C), thermal cycling, high vacuum, high pressure (>10bar), vibration, shock, AC/DC noise, radiation, water, dirt, aggressive chemicals, long term immersion, extended life (>10 years), explosive dusts & gases (ATEX rated environments).
Such environments invariably make the selection of appropriate equipment both difficult and critical because performance and reliability will surely be tested. Nowhere is this truer than in the selection of electrical control equipment. Failures in the electrical components of modern machinery typically account for >80% of all failures.
Across all sectors, potentiometers are the most ubiquitous position sensor. They are simple, compact, lightweight and low cost. A contact device, the potentiometer is 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 generally not well suited to extreme temperatures.
Optical encoders deliver the highest precision but are challenged in harsh environments. Transmissive optical encoders are particularly sensitive to shock and contaminant ingress. Optical encoders based on the principle of light wave interference are more rugged but the requirement for silicon based electronics at the sensing point inevitably limits the operating temperature.
Magnetic encoders, both Hall and magnetoresistive, are rugged, compact and can be very cost-effective. They are, however, susceptible to magnetic fields and the accuracy of the device changes over operating temperature range. The magnetic track is relatively brittle and can be susceptible to shock. Magnetic encoders can be submerged but only if the liquid is completely free of ferrous particles. Similar to optical encoders, magnetic encoders have electronics at the sensing point limiting operating temperature.
Capacitive position sensors are not widely used in extreme environments. Although they are resilient to wear, shock and vibration, they suffer from drift due 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.