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Polymer Optical Sensing System Innovation Benefits Leadership Education

Final Report Summary - POSSIBLE (Polymer Optical Sensing System Innovation Benefits Leadership Education)

POSSIBLE (Polymer Optical Sensing System Innovation Benefits Leadership Education) involves a collaboration with industry. Working with industry allowed the Fellow to understand the different perspective of industry on new technology. This experience was important for the future development of an independent research career in an applied subject where interaction with industry is critical. The two key research objectives were to develop optical fiber sensing systems for (i) fuel level monitoring in aircraft; (ii) acoustic and sensing for oil exploration and security applications. The project sought to take a significant step beyond the state of the art by taking advantage of recent rapid developments in the field of polymer optical fiber sensors.

We sought to address two pressing commercial requirements. The first concerns the monitoring of fuel level in civil aircraft. There is a strong motivation in the aerospace industry to move away from electrical sensors, especially in the fuel system. This is driven by the need to eliminate potential ignition hazards, the desire to reduce cabling weight and the need to mitigate the effects of lightning strikes in aircraft where the conventional metallic skin is increasingly being replaced by composite materials. The second requirement concerned the monitoring of acoustic signals and vibration in the subsea environment, for applications in geophysical surveying and security (detection of unwanted craft or personnel). There is strong motivation to move away from electrical sensors due to the bulk of the sensor and associated cabling and the impossibility of monitoring over large distances without electrical amplification. Optical approaches offered a means of overcoming these difficulties.

• The first part of project was focused on training, with the fellow learning to use modeling software for 3D structural analysis (SolidWorks), which permits the integration of different materials, including PMMA, glass, silicone rubber, acrylic, etc. The software, based on the finite element method, has a simulation package to simulate stress, displacement and strain in a specific prototype, in this case a liquid level sensor system. For that application, the sensor has the form of a diaphragm with embedded FBG strain sensor. A certain amount of optimization, by trading off diaphragm diameter and thickness, material properties, fiber properties (silica vs. POF - using fibers produced in the TRIPOD-ITN project) has been investigated. The final simulated design consists of an acrylic tube with windows drilled at equidistant positions and the optical pressure sensors (including optical fiber and diaphragms) positioned over holes spatially separated as in the proposed real device. The model is composed of five optical sensors and the performance was investigated applying different types of external loads such as gravity and different pressure values. Stress, displacement and strain results have been simulated and compared with experiments results, validating the model parameters.

Following the modelling, a complete liquid level gauging system has been designed and investigated. Several versions of two different configurations have been fabricated and analyzed. The first configuration is based on a single optical sensor placed at the bottom of the tank. The second one (operating in either the 850 or 1550 nm window) features five, multiplexed FBG based pressure sensors placed at different depths. This new liquid level sensor system was tested using silica and polymer optical Bragg gratings embedded in diaphragms, which can be use in different applications, like biochemical processing, floods, water and fuel tanks, aircrafts applications. First, the level gauging system was characterized in water, determining the pressure sensitivities of each sensor in each different configuration, the measurement resolution, the long term drift and stability, the repeatability and the temperature sensitivity. After preliminary results, the system was improved and refined to improve its performance. After that, experimental results showed that when using POFBGs the sensitivity is improved significantly when compared with silica FBGs. The final step involved testing in our facility using JET A-1 fuel.

The approach developed within POSSIBLE has several advantages:
i. Fault tolerance: malfunctioning sensors can be identified and their outputs ignored;
ii. Operation independent of fuel density: changing the density alters the slope of the fitted line, but not its intercept;
iii. Operation insensitive to g-force: this again changes the slope of the line but not its intercept;
iv. Temperature insensitivity: temperature induced shifts in both the nominal Bragg wavelengths of the sensors and the sensitivity of the sensors are compensated for.

It should be noted that the aircraft fuel gauging problem is particularly demanding since not only can the effective g-force vary due to acceleration, but the attitude of the plane to the effective gravitational force an also change: in other words the plan of the liquid surface can have different orientations with respect to the airframe structure. This is problem common to almost all gauging systems and is solved by having multiple level gauges coupled with appropriate signal processing.

Due to significant commercial interest in this system, greater effort was devoted to the liquid level sensing work than on the acoustic sensing described later. This shift in emphasis has proven to be justified as commercial funding for further development of this work has been obtained.

• The focus of other part of this project is to find new acoustic and sensing solutions for oil exploration and security applications collaborating with Kongsberg. An acoustic sensor for operation in water with frequencies up to 20 kHz was investigated. Current technology is predominantly based on piezoelectric strain sensors. They have satisfactory performance but suffer from other limitations: physical size; limited deployment range without local power and amplification (contributing further to physical size); lack of multiplexing capabilities. To solve these problems, there has been a steady move to introduce optical fiber based systems, firstly for surveillance and for geophysical surveying. Optimization involved choice of fiber material, mechanical amplification and the most important, the choice of signal processing scheme.

A heterodyne technique based on an unbalanced interferometric wavelength discriminator was experimental investigated and the performance of both types of fiber (silica and polymer fiber) containing FBGs was compared. Using this interferometric technique, preliminary results gave us a considerable sensitivity improvement (more than 6 times better) using a polymer FBG, which arises as a result of the much more compliant nature of POF compared to silica fiber (3 GPa vs 72 GPa, respectively). An interferometric system was constructed aiming to use the signals from the silica and polymer FBGs, showing promising results using polymer technology. These results are in preparation for publications in a conference and a journal paper.