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摘要
To achieve high-speed online detection of banknote surface coatings, a high-speed and real-time design method for a near-infrared (NIR) weak signal detection system based on high-speed banknote quality inspection machines is proposed. First, according to the characteristics of coating materials, driving and weak signal acquisition circuits are developed by using NIR light-emitting diodes and photodiodes to achieve high-speed real-time weak signal detection. Second, the real-time data transmission and process are realized by using a field programmable gate array (FPGA) and a graphical virtual instrument platform. Finally, a peak standard deviation method for calculating the peak intensity of the signal is proposed, and the real-time performance of the algorithm is analyzed. The detection of the coating on the banknote surface and the real-time simulation experiment of the algorithm is carried out. The experimental results indicate that when the paper speed reaches 22 m/s, whether the substrate is simple or complex, the signal from the uncoated banknotes is significantly higher than that of the coated banknotes, and the differences are about 7000, 4000, and 1400, respectively. The more complex the substrate, the smaller the signal difference. The time from the beginning to the end of the algorithm is about 2-3 ms, which is far less than the time interval of 22 ms. The speed of the algorithm meets the real-time requirements. The system developed in this paper is high-speed, real-time, and stable；it can effectively detect whether the coating on the surface of the paper is available. It is expected to provide a new method for online quality detection of coatings.
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