To reduce the impact of magnetization direction and stripe noise on the positioning accuracy of magnetic target and improve the positioning accuracy of underground shallow unexploded ordnance with different attitudes, this paper studies a normalized magnetic source intensity localization method based on Gaussian convolutional kernel. The traditional normalized magnetic source intensity method is not affected by the magnetization direction, but the stripe noise generated by the method itself can affect the positioning result. The use of Gaussian convolutional kernel can eliminate the influence of stripe noise. The actual flight test shows that the proposed method can eliminate stripe noise and is not affected by magnetization direction, and the maximum positioning error for five different attitudes of simulated unexploded ordnance on the ground is less than 15cm. Therefore, this method can be used for high-precision positioning of underground shallow unexploded ordnance with different attitudes.
For the impact of magnetic prospecting unmanned aerial vehicle route planning on the positioning accuracy of simulated unexploded ordnance, we conduct unmanned aeromagnetic prospecting flight tests and complete the localization of simulated unexploded ordnance using the normalized magnetic source intensity method, then analyze the localization results. The results indicate that as the survey line interval decreases to 0.5 m, the positioning error tends to stabilize. As the flight speed decreases from 5 m/s to 1 m/s, the positioning accuracy is significantly improved. As the flight altitude decreases from 3m to 1.5m, the positioning error decreases. By analyzing the impact of key parameters in magnetic prospecting unmanned aerial vehicle route planning on the positioning accuracy of simulated unexploded ordnance, we provide an empirical reference for the efficient and safe detection strategy of unexploded ordnance.
Hydrogen has a wide range of applications in various industries, including fuel cells, chemical processing, and electronics. However, hydrogen is highly flammable and explosive at a wide concentration range (4%~75%), which poses a significant safety concern. Hydrogen sensors with higher sensitivity, better selectivity, faster response, and wider dynamic range are of increasing importance in connection with the development. In this paper, a scheme of four-channel hydrogen sensor is proposed which is based on stimulated Raman spectroscopy and nanofiber. The tightly confined evanescent field associated with the nanofiber enhances the Raman gain per unit length by a factor of more than 104 over free-space beams. Time division multiplexing technology is used to implement multichannel detection. The whole sensing system consists of commercial optical modules such as DFB laser, EDFA and optical switch, and several homemade circuits instead of the commercial instruments for signal processing and controlling channel switching. Every channel can connect one probe which is based on nanofiber and packaged by special metal structure and method. The switching time can be millisecond order so that the signal period would not be influenced. The four-channel hydrogen sensor is realized in the experiment that demonstrates hydrogen detection from hundreds parts per million to 100% with each channel. The reported sensor could be used in the field of new energy, electric power and aerospace for detection of hydrogen leakage or monitoring of transformer health conditions with advantages of low cost, compact size and high resolution.
In order to monitor the safety of the whole cable in real time and effectively, this study introduces and adopts distributed optical fiber temperature sensing (DTS) technology as the method of cable safety monitoring. A variety of statistical results are used as the basis for judging the running state. Not only does the whole system have no blind zone of safety monitoring, but also provides accurate alarm information and fast response, which enables managers to deal with faults timely and avoid heavy losses. Therefore, the DTS technology has a high promotion value in the field of cable safety monitoring.
In this paper, we introduce our recent progress in resolution improvement of distributed temperature sensors by employing narrow pulse laser and wideband avalanche photoelectric detector (APD). The narrow pulse laser generates the light pulse with 4 ns with at its half maximum, which represents 40 cm spatial resolution. the wideband APD is employed to acquire the backscattering light at Raman wavelength and meet the need of spatial resolution of the narrow pulse laser. High spatial resolution is attributive to narrow pulse width, but also leads to the deterioration of the signal to noise ratio of demodulated temperature curve. Thus the APD with both high sensitivity and wideband width is employed to achieve high SNR and high spatial resolution. Temperature tests were taken out to evaluate the performance of proposed sensor. Results showed that the performance of our proposed spatial resolution enhanced distributed temperature sensor could achieve the temperature error of 1.2°C, temperature resolution of 0.49°C, sensing length of 1 km and spatial resolution of 0.5 m.
In this paper, fiber Bragg grating (FBG) sensors for strain monitoring at cryogenic temperatures were proposed and demonstrated. The strain sensitivity, strain repeatability and temperature compensation effect of FBG sensors were studied, within the temperature range of -196°C to 20°C. The strain sensitivity of FBG sensors at cryogenic temperatures was tested by quartz specimens with low thermal expansion coefficient. Experimental results show that the strain sensitivity of the relative change of the central wavelength for the FBG is 0.784, which almost does not change with the temperature. The repeatability of the adhesive used for FBG strain sensors at cryogenic temperatures was tested. Experimental results show that FBG sensors have good adaptability to cryogenic temperatures, and the strain repeatability is within 1.0% FS. The zero drift of the FBG strain sensor was compensated by using the FBG temperature sensor, and the strain error is less than 5% of the measured values with the compensation at cryogenic temperatures. Compared with the traditional electrical strain gauge used at cryogenic temperatures, FBG strain sensors have advantages in non-sensitivity drift and good zero drift compensation effect, and they can play an important role in the structural health monitoring at cryogenic temperatures such as spacecraft tanks, fuel pipelines, and so on.
In this paper, we introduce an optimal bias voltage searching strategy and maintaining method in BOTDA system based on dual sideband modulation. The system utilizes both up shifted and down shifted continuous wave light to generate Brillouin scattering light, while using the amplified pulsed light to boost the signal by stimulated Brillouin scatting effect. In order to obtain a clean Brillouin shifted curve along the fiber under test, the probe light must be in good signal to noise ratio. In addition, the intensity of shifted light should be stable as well. Therefore, we propose a novel control method of frequency shift in sideband modulation of a BOTDA system, which keeps the first order frequency-shifted light at its maximum signal to noise ratio and minimum light intensity fluctuation. We also implement static experiments to verify our proposed scheme. Strain/temperature tests were taken out to evaluate the performance of the BOTDA. Results showed that the resolution of BOTDA system could reach 13με/0.65°C at 50 km fiber cable with spatial resolution of 0.5m.
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