Fiber optic oxygen sensor leak detection system for space applications

This paper describes recent progress towards the development and qualification of a highly distributed, multi-point, all optical pressure and temperature compensated, fiber optic oxygen sensor (FOxSense™) system for closed-loop monitoring and safety of the oxygen ullage environment inside fuel tanks of military and commercial aircraft. The alloptical FOxSense™ system uses a passive, multi-parameter (O2/T&P) fiber optic sensor probe with no electrical connections leading to the sensors install within the fuel tanks of an aircraft. The all optical sensor consists of an integrated multi-parameter fiber optic sensor probe that integrates a fuel insensitive fluorescence based optical oxygen optrode with built-in temperature and pressure optical optrodes for compensation of temperature and pressure variants induced in the fluorescence response of the oxygen optrode. The distributed (O2/T&P) fiber optic sensors installed in the fuel tanks of the aircraft are connected to the FOxSense optoelectronic system via a fiber optic cable conduit reaching to each fuel tank in the aircraft. A multichannel frequency-domain fiber optic sensor read-out (FOxSense™) system is used to interrogate the optical signal of all three sensors in real-time and to display the fuel tank oxygen environment suitable for aircraft status and alarm applications. Preliminary testing of the all optical fiber optic oxygen sensor have demonstrated the ability to monitor the oxygen environment inside a simulated fuel tank in the range of 0% O2 to 40% O2 concentrations, temperatures from (-) 40°C to (+) 60°C, and altitudes from 0-ft to 40,000-ft.

A fiber optic multisensor methane detection system matched to topology and environment of a coal mine is reported. The system involves time domain multiplexed (TDM) methane sensors based on selective absorption of source radiation by atomic or molecular species in the gas sensing heads. A two-wavelength ratiometric approach allows simple self referencing, cancels out errors arising from other contaminants, and improves the measurement contrast. The laboratory system consists of a high radiance LED source, multimode fiber, optical sensing head, optical bandpass filters, and involves synchronous detection with low noise photodiodes and a lock-in amplifier. Detection sensitivity upon spectral resolution of the optical filters has also been investigated and described. The system performance was evaluated and the results are presented.

As an important member of optical fiber sensor, fiber optic Extrinsic Fabry-Perot Interferometric (EFPI) sensor has prospects for a wide range of industrial applications due to its small size, compact configuration, good reliability and flexibility. In a white light based fiber optic EFPI sensor system, which has the advantages of large dynamic range and high resolution, spectral analysis equipment is the most important part which takes the major cost of the system. Therefore, multiplexing spectral analysis equipment to measure multiple sensors simultaneously is an effective and necessary way that can save the cost of the whole sensor system. In this paper, a scheme based on the frequency division multiplexing of EFPI optical fiber sensors is proposed. Two EFPI sensors with different gap lengths are parallely connected through the optical fiber coupler. The overlapped interference spectrum of the two EFPI sensors is measured by using an optical spectrum analyzer, which is separated into two individual interference spectrums with a Finite Impuse Response (FIR) band-pass filter. Their envelope components are extracted with Hilbert transform, and then the interference spectrums are normalized by the envelope components. The respective gap lengths of the two EFPI sensors are achieved with the cross correlation calculation. Using this frequency division spectral analysis method, a two-channel multiplexing EFPI sensor system is implemented in the laboratory. The deployment of this frequency division multiplexing technology can greatly reduce the cost of the whole system which has extensive potential applications for distributed fiber optic EFPI sensor system.

Optical hydrogen sensors are intrinsically safe since they produce no arc or spark in an explosive environment caused by the leakage of hydrogen. Safety remains a top priority since leakage of hydrogen in air during production, storage, transfer and distribution creates an explosive atmosphere for concentrations between 4% (v/v) - the lower explosive limit (LEL) and 74.5% (v/v) - the upper explosive limit (UEL) at room temperature and pressure. Being a very small molecule, hydrogen is prone to leakage through seals and micro-cracks. Hydrogen detection in space application is very challenging; public acceptance of hydrogen fuel would require the integration of a reliable hydrogen safety sensor. For detecting leakage of cryogenic fluids in spaceport facilities, Launch vehicle industry and aerospace agencies are currently relying heavily on the bulky mass spectrometers, which fill one or more equipment racks, and weigh several hundred kilograms. This paper describes the successful development and test of a multi-point fiber optic hydrogen sensor system during the static firing of an Evolved Expandable Launch Vehicle at NASA's Stennis Space Center. The system consisted of microsensors (optrodes) using hydrogen gas sensitive indicator incorporated onto an optically transparent porous substrate. The modular optoelectronics and multiplexing network system was designed and assembled utilizing a multi-channel optoelectronic sensor readout unit that monitored the hydrogen and temperature response of the individual optrodes in real-time and communicated this information via a serial communication port to a remote laptop computer. The paper would discuss the sensor design and performance data under field deployment conditions.

This paper describes the successful development and test of a multipoint fiber optic hydrogen microsensors system during the static firing of an Evolved Expandable Launch Vehicle (EELV)/Delta's common booster core (CBC) rocket engine at NASA's Stennis Space Center. The hydrogen sensitive chemistry is fully reversible and has demonstrated a response to hydrogen gas in the range of 0% to 10% with a resolution of 0.1% and a response time of ≤5 seconds measured at a gas flow rate of 1 cc/min. The system consisted of a reversible chemical interaction causing a change in reflective of a thin film of coated Palladium. The sensor using a passive element consisting of chemically reactive microcoatings deposited on the surface of a glass microlens, which is then bonded to an optical fiber. The system uses a multiplexing technique with a fiber optic driver-receiver consisting of a modulated LED source that is launched into the sensor, and photodiode detector that synchronously measures the reflected signal. The system incorporates a microprocessor to perform the data analysis and storage, as well as trending and set alarm function. The paper illustrates the sensor design and performance data under field deployment conditions.

Water quality testing is of great significance for daily water use or water use in high precision manufacturing. The use of single-photon detection technology to detect water particles is one of the important methods of physical water quality detection. However, the current water quality particle detection technology needs to be improved in the range and accuracy of turbidity detection. The paper aims to use single-photon detection technology to design a fiber optic turbidity detection system to solve the current problems of small turbidity detection range and low accuracy in a large range. In response to this, this paper has made an in-depth understanding of single-photon detection technology and designed two sets of detection schemes using the performance of photons. The first set is the transmission optical fiber turbidity detection, which can effectively detect the ultralow turbidity range and a wide range of water quality particles. The second set is a scattering optical fiber turbidity detection system, which can effectively detect water particles in the low and medium turbidity range. In this paper, the two systems were integrated into a whole system, so the results of the two detection methods can also be compared, and the detected turbidity can be obtained more accurately. The experimental results showed that for the transmission optical fiber turbidity detection system, the detection effect of the system was the best under the incident light intensity of 11 wm, and the fitting value was 0.99; for the scattering optical fiber turbidity detection system, the detection effect of the system was the best under the incident light intensity of 4 wm, and the fitting value was 0.99.

The paper discusses a fiber optic multisensor methane detection system matched to topology and environment of the underground mine. The system involves time domain multiplexed (TDM) methane sensors based on selective absorption of source radiation by atomic/molecular species in the gas sensing heads. A two-wavelength ratiometric approach allows simple self-referencing, cancels out errors arising from other contaminants, and improves the measurement contrast. The laboratory system consists of a high radiance LED source, multimode fiber, optical sensing head, optical bandpass filters, and involves synchronous detection with low noise photodiodes and a lock-in amplifier. Detection sensitivity versus spectral resolution of the optical filters has also been investigated and described. The system performance was evaluated and the results are presented.

In this paper, a novel liquid level sensing system is proposed to enhance the capacity of the sensing system, as well as reduce the cost and increase the sensing accuracy. The proposed sensing system can monitor the liquid level of several points at the same time in the sensing unit. Additionally, for cost efficiency, the proposed system employs only one sensor at each spot and all the sensors are multiplexed. In multiplexed systems, when changing the liquid level inside the container, the float position is changed and leads to an overlap or cross-talk between two sensors. To solve this overlap problem and to accurately predict the liquid level of each container, we proposed a deep neural network (DNN) approach to properly identify the water level. The performance of the proposed DNN model is evaluated via two different scenarios and the result proves that the proposed DNN model can accurately predict the liquid level of each point. Furthermore, when comparing the DNN model with the conventional machine learning schemes, including random forest (RF) and support vector machines (SVM), the DNN model exhibits the best performance.

: This report describes the work performed with Production Products Manufacturing & Sales (PPMS), Inc., under the Liquid Molded Composite Armor Smart Structures Using Embedded Sensors Small Business Innovative Research (SBlR) Program sponsored by the U.S. Army Research Laboratory (ARL). In the Phase I effort, fiber optic sensor systems were investigated for in-process cure monitoring and in-service health and dynamic response monitoring of monocoque and hybrid liquid molded composite armor structural parts. Sensor embedding techniques during resin infusion molding were developed, thick panels with varying residual stress characteristics were fabricated, two fiber optic sensor types were utilized and compared, several demodulation techniques were studied, and information processing programs were written for converting the sensor signal to engineering data. Bragg grating and fluorescence optrode fiber optic sensors were selected and embedded in armor panels to monitor the cure of the systems and health of the panels during impact and four-point bend tests. During panel curing, the Bragg gratings detected the change in strain in the material at the onset of cross-linking as well as the strain changes (residual stress) resulting from panel processing. The Bragg gratings, serving a dual purpose, measured the resulting strain from the external loads applied due to impact and bending. The fluorescence optrode was used during the cure process only to measure the changes in the material as it cured (degree of cure). Stitching of thick-section fiberglass preforms was also investigated.

Optical fibre damage detection for an aircraft composite leading edge

We report the monitoring of oxygen partial pressure (pO2) in the liquid phase in cell cultures with a luminescence quenching based fiber optic oxygen sensor. The sensitivity of the fiber optic oxygen sensor was evaluated in the gas phase and the sensor was calibrated in the liquid phase before monitoring cell culture pO2 with a luminescence phase detection system. In these experiments, porcine kidney (PK-15) cells were used as the cell samples and the experimental results showed that the sensor is suitable for monitoring of pO2 in gas and liquid phases. In addition, we found that in cell cultures exposed to ambient air, measured pO2 values in the liquid phase differed according to the different cell densities in the culture. Under conditions of low pO2 in the gas phase and high cell density, the cell metabolism reduces the pO2 in the liquid phase to values lower than those of the gas headspace, indicating that the cellular oxygen uptake rate exceeds the oxygen diffusion rate. Therefore there is a need for direct pO2 monitoring in a cell culture, particularly in static high density cell culture systems where oxygen mass transfer is limited.

In this paper, the analysis of sensor results from a light emitting diode (LED) based optical fiber sensor (OFS) system is presented. A tri-color RGB (Red-Green-Blue) LED is used to provide three stimulus colors to stimulate a Surface Plasmon Resonance (SPR) sensor which is optically coupled to the system electronics. Analysis of the sensor results under different light conditions is used to identify a particular chemical under test. In the analysis approach undertaken, a combination of the CURE (Clustering Using REpresentatives) data clustering method (algorithm) and the k-nearest neighbor (kNN) algorithm is used for classifying the chemical under test. The system hardware is based on the field programmable gate array (FPGA) and the classification is undertaken on a personal computer (PC) using the Python open source programming language.

In this work, we analyze a remote optical sensor system based on Raman amplification, composed by one Long Period Grating (LPG) as a sensor head, separated by 50 km from the optical source, and the interrogation unit composed by two Fiber Bragg Gratings (FBGs) modulated by two Piezoelectrics Transductors (PZTs). Optical fiber sensor systems are typically limited to operate at distances of only few kilometers due to the attenuation effects and noise present in the optical fiber that adversely affects the performance of the sensor interrogation process. We present experimental and simulated results obtained in the context of the analysis of remote optical fiber sensors. The simulation models compute the Raman interaction between the pumps and the sensor signals and allow speeding up the analyses regarding the setup to be experimentally implemented in order to measure/monitor environmental temperature. Experimental results obtained in the implemented setup show that under Raman amplification the power ratio between the two central wavelengths of the FBGs has a linear relation with the change of the LPG resonance induced by environmental temperature.

A strain sensor system for smart structures and skins has been developed. The sensor system uses a two-mode elliptical core fiber sensor and two diode lasers, operating at nominal wavelengths of 750 nm and 780 nm, respectively. The lasers are frequency-modulated such that periodic mode hopping is induced. Data is sampled for each laser mode. A least-absolute-deviation method has been devised for calculating the sensor phases at both wavelengths. The ambiguity arising from the periodicity of the interferometer response is resolved by comparing all possible optical path differences corresponding to the sensor phases at the two wavelengths. The system is able to measure 0.01 of a fringe over a range of ten fringes.

The objective of this work is to present a conceptual design and the modelling of a distributed sensor system based on fiber optic devices (Fiber Bragg Grating, FBG), aimed at measuring span-wise and chord-wise variations of an adaptive (morphing) trailing edge. The network is made of two different integrated solutions for revealing deformations of the reference morphing structure. Strains are confined to typical values along the span (length) but they are expected to overcome standard ranges along the chord (width), up to almost 10%. In this case, suitable architectures may introduce proper modulations to keep the measured deformation low while preserving the information content. In the current paper, the designed monitoring system combines the use of a span-wise fiber reinforced patch with a chord-wise sliding beam. The two elements make up a closed grid, allowing the reconstruction of the complete deformed shape under the acceptable assumption that the transformation refers to regular geometry variations. Herein, the design logic and some integration issues are reported. Preliminary experimental test results are finally presented.