Given the Project requirements, the BOFDA technology was chosen for the realization of the interrogation unit. For the latter, a Vector Network Analyzer (VNA) operating over the frequency range 300 kHz - 20 GHz was chosen, allowing a spatial resolution as high as 5 mm. The VNA is based on the PXIe architecture, therefore being ideally suited for applications requiring the integration in a compact and low-weight prototype.
While spatial resolution and sensing range constraints were easily met using the BOFDA technology, a major concern, since the beginning of the Project activities, was the further requirement of a sub-second acquisition time. Several strategies have been adopted in order to reduce the acquisition time, by acting both on the hardware and software of the developed prototype. From the hardware point of view, the optimization regarded the choice of a low-phase noise laser source, as laser phase noise was early recognized as one of the main noise sources. Furthermore, a fast polarization-diversity scheme was adopted, for polarization management of the Brillouin acquisitions. As regards signal processing, a novel processing technique for the extraction of temperature&strain profiles has been developed. The technique removes the distorting terms affecting BOFDA measurements, while adding no relevant complexity in the reconstruction of the temperature (or strain) distributions from the acquired data.
The prototype has been assembled in a compact rack unit, controlled by an external PC through Ethernet connection and supplied from a single +12V power supply. The prototype has been also thermally tested, in order to verify its capability to operate for long periods and over an extended temperature range.
The prototype has been finally tested in the laboratory, in order to assess its performance. The results confirmed that the prototype was compliant with the Project’s requirements.
During the second part of the Project, a few experimental tests were carried out at the Topic Manager facilities in Getafe (Spain). Temperature tests have been carried out in the typical conditions of composite manufacturing, up to 200°C, as well as in in-service conditions, with temperatures down to -50°C. The temperatures provided by the prototype were compared to those provided by FBGs and thermocouples. Further tests have been carried out over composite plates subjected to tensile and compressive loads. In such cases, the accuracy of the developed unit was assessed against strain measurements provided by conventional FBGs and foil gauges.
As regard dissemination of the results, a special section was created on the Research Gate social networking site, in order to facilitate dissemination of the results of the Project and discussion with researchers. Furthermore, the technologies exploited for the development of the Prototype have been disseminated to master’s degree and PhD students at University of Campania and Rey Juan Carlos University. Some of the experimental results have been presented during the conference I3S MDPI, held in Naples in May 2019. For this contribution, a permission from the Topic Manager was achieved in advance. Some of the results will be also presented at the ISROS 2019 conference to be held in Airbus facilities in Toulouse (France), in November 2019.
