Distributed optical fibre smart sensors for acoustic sensing in the structural health monitoring of robust aerospace vehicles

The use of distributed optical fibre smart sensors for the detection of acoustic signals in the Structural Health Monitoring (SHM) of robust aerospace vehicles has been demonstrated. Current distributed optical fibre sensors are multiplexed along a single fibre. Inherent problems exist with a multiplexed architecture. Two significant issues are; the possibility of fibre breakage, and the possibility of failure of the single transmitter, the single receiver, or the single processor. In a ‘smart’ architecture, the intelligence, as well as the sensors, is distributed. Hence, if destructive damage occurs, then the SHM system can continue to operate in all other locations on the vehicle, making the system robust. Work on the optical fibre sensors was limited to acoustic signals. This included acoustic emissions, acousto-ultrasonics, acoustic transmissions and other dynamic strain signals. Fibre Bragg Gratings (FBGs) were chosen as the optical fibre sensor for the detection of the acoustic signals. FBGs offer significant advantages over other types of optical fibre sensors. The most significant of these is the ease of multiplexing and their versatility, i.e. the ability of FBGs to detect a significant number of measurands. In the work on optical fibre sensing, we showed the implementation of an innovative detection system. This Transmit Reflect Detection System (TRDS) made use of both the transmitted and reflected signals from the FBG. The TRDS is an improvement on conventional power detection where either the transmitted or reflected component is used. The TRDS was used to successfully detect all types of dynamic and static signals, the most significant being the acoustic emission from a lead pencil break test. The use of the FBG sensor as a receiver for acoustic communications was also shown. Acoustic communications have been proposed for use in the SHM of robust aerospace vehicles with the use of autonomous agents, e.g. inspection or repair robots. The FBG receivers were compared with PZT receivers. When communicating through aluminium, the FBG performance was not as good as the PZT receiver, specifically due to the properties of the FBG which limit the frequency response. However, in Carbon Fibre Composites (CFC), the FBG outperformed the PZT due to the properties of the CFC. We also note that when contained within the thermal packaging the FBG had a very interesting frequency response, likely due to the suspended beam nature of the structure. This type of packaging could be used to tune the response of the FBG sensor. The work on the distributed optical fibre smart sensors showed the implementation of a Smart Transducer Interface Module (STIM), which used the TRDS with a Digital Signal Processor (DSP). The output of the TRDS was differentially amplified with a high speed amplifier, and the output was passed to the ADC onboard the DSP. The DSP was also used to toggle on and off output, including closed loop actuation, and controlling a 1550nm laser, which would represent the source used in the implemented system. The use of STIM to form a distributed optical fibre sensor network was also shown in principle