Micromachined sensors are a gem in jet engines

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Oxfordshire-based micromachined sensor firm Oxsensis expects to have sensor prototypes operating inside jet engines before the end of this year.

“We will have lab products later this year and expect production in 2007-08,” chief executive offcier David Gahan told Electronics Weekly.

Oxsensis is a spin-out from the central microstructure facility of the UK’s Rutherford Appleton Laboratory. Its creation was prompted in part by ex-Bookham Technology optical fibre engineers.

The intellectual property revolves around micromachining sapphire, from which its pressure and temperature sensors are made.

“If you want to stick something inside a jet or rocket engine, you really want something that is as inert as possible,” said Gahan.

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Testing a sensor at 1,200°C
         

Sapphire (single-crystal Al2O3) fits the bill up to 1,000°C, but its very inertness makes it tricky to micromachine. “To etch it you have to use reactive ion etch with some fairly aggressive ion species,” said Gahan.

What the species are, and the etch resist used, are closely-guarded secrets. “There are some American companies that would love to know,” said Gahan. “People have tried to use sapphire in the past but haven’t put any sensible sensors on the market.”

The structures used by Oxsensis have also not been revealed yet. According to Gahan, the process is somewhere between surface and bulk micromachining, and results in extremely thin sapphire membranes.

Optical fibres somehow coupled into the membranes allow movement to be measured. “The basic principle is the same as colours in soap bubbles or oil on water – iridescent interference patterns,” said Gahan. “You can measure the thickness of a film very very accurately by the spectral distribution.”

In the Oxsensis sensor, a broad spectrum is sent down the fibre into the sensor, and the spectrum that returns contains the pressure or temperature information.

Sapphire melts at 2,050°C and glass fibre at 1,500°C, allowing operation at a significant proportion of this. The photo above shows a sensor under test at 1,200°C, but Gahan suggest lifetime could be limited at this extreme.

The firm is aiming its sensors at jet and car engines – initially as development instrumentation and later as permanent installations. “To achieve emission targets, jet engines will have to use burn-by-wire control loops to stabilise unstable combustion and achieve higher operating temperatures,” said Gahan.

The technology is too expensive to fit cars at the moment. “We hope to cost-reduce to the point it can be used in automotive,” said Gahan.

www.oxsensis.com

Tags: Rutherford Appleton Laboratory, temperature sensors

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