Abstract:
The analysis of the development context for a distributed temperature detection
system in an aircraft cargo compartment and the review of candidate technologies
stressed the advantages of using the spontaneous Raman scattering principle with a
signal processing in the time domain. Simultaneously, the wished detection
performance defines the hardware specifications of the system.
For the "Cargo Compartment Fire Detection and Extinguishing" application, several
design parameters were identified; namely: the spatial and sampling resolution, the
temperature accuracy and its resolution, the possible sensing element length for a
given peak optical power per pulse, the refreshment time to get a temperature profile
over the sensing element length, the operating/survival temperature ranges and the
mechanical constraints at the sensing element in operation or during installation and
the possibility to discriminate an optical fibre break from an increase in temperature.
To reject the noise generated and amplified by the Avalanche Photo-Diodes (APD)
and by the cascaded amplification modules, the numerical signals were . processed
using digital filters.
The detection system has also to early detect a fire while rejecting fire-like events. Its
detection performances are, namely: a temperature profile refreshment time of 3s, a
temperature resolution of± 1 °C around the alarm signal, a spatial resolution of 1.3m
for a sensing element length of 500m. The detection of a temperature increase is a
spatial analysis organized through specific algorithms. These algorithms involve
processing the temperature time derivative, the spatial gradient, the variation of
danger surfaces, an analysis of the temperature spatial frequencies and the
calculation of the power spectral density and of the temperature covariance matrix.
Fire tests were performed and the optical fibre within a protective stainless steel tube
was connected to a control unit to monitor the danger situation over an aircraft cargo
compartment.
