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17 Jun 2002
An optical sensor that measures hydraulic pressure in the landing gear of a commercial aeroplane has been developed by the UK-based company Druck.
Based on low-coherence-light interferometry, the sensor uses a dual superluminescent diode source coupled to a single mode optical fibre, and is immune to electromagnetic interference (EMI). An anisotropically etched silicon diaphragm is electrostatically bonded to a borosilicate glass insert and mounted in a titanium body (see figure). Project leader John Greenwood said: "Light from the fibre is collimated by a graded index lens and focused through the glass block. Hydraulic pressure deflects the silicon diaphragm towards the glass block, such that the gap between the glass and the silicon decreases in proportion to the pressure."
At the full scale pressure of 200 bar, the Fabry-Perot resonant gap reduces by 10 microns from the initial 80 microns.
The system has been installed on an Airbus test landing gear and calibrated over a range of temperatures between -40 and +80 deg C. Maximum deviation from linearity over the full pressure range was less than 0.6% at 20 deg C.
"Aircraft landing gear have around 70 sensors, but in contrast with all these other sensors, ours is immune to EMI, potentially lighter and also less hazardous," said Greenwood.
Druck's sensor uses Bookham Technology's ASOC technology for optical interrogation, which, according to Greenwood, has enabled cost-effective interfacing of instrumentation with the optical sensor.
Story courtesy of Opto and Laser Europe magazine
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