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Real-Time Measurement of Subgrade Settlement in High-Speed Railways with a Resolution of 0.25 mm Using a Laser Imaging Method
S. Yang, Y-Y. Yang, L. Zhang, J-Y. Huang, Y-R. Bai and X-C. Lin

A new technique based on the property of laser is developed to measure subgrade settlement with high accuracy and resolution. In this technique a laser mounted on the subgrade produces a horizontal beam and is received and imaged by a charge-coupled device (CCD). When the subgrade occurs, it results in a displacement of the image on the CCD. By measuring the displacement one can monitor the settlement in situ with high accuracy. Using a laser beam rather than a light-emitting diode (LED), the CCD can receive a high brightness spot. This makes the system working effectively during daytime despite the interference of sunlight and makes it possible to detect the settlement when the distance between the laser and the CCD is 40 m. By laser beam shaping, the image on the CCD is nearly rounded so that the CCD can read its position easily improve the resolution of the system. The system is designed to work in the field and it works normally under moderate atmospheric turbulence both experimentally and theoretically. So, the system enjoys some promising merits such as high accuracy, high resolution, low environmental influence factor and high signal-to-noise ratio (SNR). The resolution of the system depends on the focal length of the lens used. With a cemented doublet of focal length 1000 mm, the resolution is 0.25 mm or better and the theoretical calculations agree well with the experimental data.

Keywords: Laser beam, charge-coupled device (CCD), railway, laser measurement, subgrade settlement, high brightness, displacement distance, safety

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