Common sub-millimeter particle impact phenomena range from zero to thousands of joules of impact energy. The physics of impacts are associated with a wide variety of physical phenomena, including the generation of heat, light, and sound. Although higher energy impact events may result in vaporization of the impacted material and other easily detectable effects, lower energy level impacts of interest may occur with little obvious physical effect. Preliminary research with capacitative sensors provided encouraging results for detecting low-energy impacts. However, vibration within the sensor mounting structure interfered with the detection of impact events. Research on triboluminescent phosphors indicated that a thin layer of material could be used to form the basis of an optical sensor to detect small particle impacts without interference from structural vibrations. A ZnS:Mn phosphor was used as the basis for developing a triboluminescent fiber optic sensor to detect small particle impact events. Detection of impacts is accomplished by detecting the optical pulse that is generated by the abrupt charge separation caused by the particle impact within the phosphor. Laboratory-based experiments were performed to capture the operational characteristics of the sensor. The data are used to study the characteristic response, sensor repeatability, and spatial homogeneity of the detection surface. Tests were also performed to identify the energy detection boundary and to assess environmental survivability. Results of these tests are reported in this paper.
Linear unmixing is a method of decomposing a mixed signature to determine the component materials that are present in sensor's field of view, along with the abundances at which they occur. Linear unmixing assumes that energy from the materials in the field of view is mixed in a linear fashion across the spectrum of interest. Traditional unmixing methods can take advantage of adjacent pixels in the decomposition algorithm, but is not the case for point sensors. This paper explores several iterative and non-iterative methods for linear unmixing, and examines their effectiveness at identifying the individual signatures that make up simulated single pixel mixed signatures, along with their corresponding abundances. The major hurdle addressed in the proposed method is that no neighboring pixel information is available for the spectral signature of interest. Testing is performed using two collections of spectral signatures from the Johns Hopkins University Applied Physics Laboratory's Signatures Database software (SigDB): a hand-selected small dataset of 25 distinct signatures from a larger dataset of approximately 1600 pure visible/near-infrared/short-wave-infrared (VIS/NIR/SWIR) spectra. Simulated spectra are created with three and four material mixtures randomly drawn from a dataset originating from SigDB, where the abundance of one material is swept in 10% increments from 10% to 90% with the abundances of the other materials equally divided amongst the remainder. For the smaller dataset of 25 signatures, all combinations of three or four materials are used to create simulated spectra, from which the accuracy of materials returned, as well as the correctness of the abundances, is compared to the inputs. The experiment is expanded to include the signatures from the larger dataset of almost 1600 signatures evaluated using a Monte Carlo scheme with 5000 draws of three or four materials to create the simulated mixed signatures. The spectral similarity of the inputs to the output component signatures is calculated using the spectral angle mapper. Results show that iterative methods significantly outperform the traditional methods under the given test conditions. *
Fiber optic sensors offer many advantages over electrical sensors for use in harsh environments. One advantage over distributed electrical sensors is the elimination of the need to route electrical power and wiring to the sensors, which, in general, improves safety and reduces power consumption. Another advantage is that the optical sensors are immune to electromagnetic interference that may be caused by radio frequency signals used for communications. Another benefit of using an optical approach for impact detectors is the implicit immunity from false detections that may otherwise be caused by unrelated mechanical shock or vibration events. Previous studies have documented the characteristics of the Optical Debris Impact Sensor (ODIS). With the ODIS, the impacts are inferred by detecting the brief triboluminescent optical pulses generated by the abrupt charge separation within a phosphor that is caused by the particle impacts. The main limitations of the ODIS are the small detection area and the limited sensitivity. This paper describes a method for extending the ODIS to accomplish broad area detection on a surface with potentially higher sensitivity. The sensing element is comprised of a stack of planar optical waveguides with phosphor-coated strips. The geometry of the design ensures optical pulses are automatically captured by the waveguides and routed to a fiber optic cable that transports the signal to a remote high-speed photodetector. Background light levels in the vicinity of the detector are filtered out by the tailored frequency response of the photodetector.
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