PREVIEW EXHIBITOR Getting tough with electrolytic tilt sensors
Despite the growth in the use of MEMS tilt sensors, in harsh applications electrolytic tilt sensors can provide significant benefits, as Jeff Gurr, electromechanical product specialist at Charcroft Electronics, explains
M
EMS tilt sensors are smaller and lower cost than electrolytic tilt
sensors, so it would not be surprising to expect the rise in MEMS technology to signal the end of the electrolytic alternatives. The reality, however, is that in harsh applications the balance is firmly tipped in favour of electrolytic tilt sensors. In applications where drift and repeatability are of crucial importance and in which product lifetimes are measured in years, not months, then electrolytic tilt sensing can often be the preferred technology.
THE COST OF HIGH REPEATABILITY The factors which are most important for measuring the performance of a tilt sensor are drift, repeatability and environmental durability. There are considerable differences in
the repeatability offered by off-the-shelf MEMS and electrolytic tilt sensors. Even low-cost off-the-shelf electrolytics can deliver repeatability of five arc-seconds, and high-end versions can achieve sub-arc-second repeatability. In comparison, this level of performance is only typically available on the highest- cost MEMS sensors and the need for additional support electronics adds to the total bill-of-materials cost. For example, a metal dual-axis ±65° electrolytic sensor generally costs under $5, whereas achieving similar performance with a MEMS device and its supporting electronics can cost between three- and-a-half to five times this price.
THE COST OF CONTROLLING VOLTAGE VARIATIONS Cost is also a factor in controlling drift which causes readings to become increasingly inaccurate over time, necessitating frequent re-calibration. To achieve optimum performance, MEMS sensors need a stable voltage supply over both time and temperature, with high-repeatability versions needing control within the microvolt range. This usually requires the use of high-precision, and therefore high-cost, power supplies. In comparison, the ratiometric measurement used by electrolytic tilt sensors makes these devices immune to variations in the voltage supply and allows for the use of lower cost power supplies.
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LONGEVITY AND DURABILITY Another factor which can significantly affect sensor performance is environmental durability. Here, MEMS sensors can be constrained by their operating temperature range and by their packaging options. Off-the-shelf electrolytic tilt sensors cover an operating temperature range from -60˚C to 120˚C and are hermetically sealed to enable them to withstand humidity and other environmental conditions. The fact that electrolytic sensors have no moving parts also helps them to handle vibration and shock to ensure a long operational lifetime. Packaging and mounting options
for electrolytic tilt sensors include a patented metal can for soldering directly onto a PCB, or ceramic versions with mounting holes for easier fixing into a rugged assembly. They can even be mounted remotely without the need for a custom PCB to carry the support electronics which would be needed by a MEMS sensor. Encapsulation can add a stable base with minimal thermal changes to ensure high repeatability and optimum performance over temperature.
A SIMPLE DESIGN PROCESS Like MEMS sensors, electrolytic tilt sensors
Jeff Gurr
can be mounted onto a circuit board, but rather than needing complex support electronics, all they need is a power source; a microcontroller with two digital ports and an A/D converter; and a few passive components. Simply reading an analogue voltage provides the ratiometric measurements which are proportional to the angle being measured. Because the reading is ratiometric, a
perfectly stable voltage is not required if the A/D reference voltage is the same as that of the tilt sensor drive voltage. The electronics are further simplified by the electrolytic sensor’s ability to operate at any
voltage and its low power requirements.
THE FUTURE OF ELECTROLYTIC TILT SENSORS Despite the fact that electrolytic sensors from the 1960s are still in production, manufacturers such as Fredericks continue to make innovations in this technology. Glass tilt sensors based on thick-film
technology, for example, will measure with sub-arc second repeatability at significantly lower cost, whilst ceramic sensors will operate at even higher temperatures. In addition, the latest generation of metal sensors will deliver high repeatability with larger measuring angles and a smaller size.
Fredericks TrueTILT narrow range electrolytic sensors
Charcroft Electronics
www.charcroft.com
Electrolytic tilt sensors and other components for sensing flow, pressure, position and temperature in harsh environments, will be featured on Charcroft Electronics’ stand, C11, at Sensors & Instrumentation 2016
SEPTEMBER 2016 | SUPPLEMENT: SENSORS & INSTRUMENTATION 2016
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