The challenge in using fully mechanized caving mining technology for thick coal seam mining is monitoring the mixing ratio of coal gangue during the top coal caving process. In this paper, we propose a passive wireless sensing system based on the second-order parity-time symmetric system operated around the exceptional point for the monitoring of coal gangue mixing, which has an LC resonance structure consisting of a spiral resonant coil and a parallel-plate capacitor at the wireless sensing end of the system. The parallel capacitor was designed as a sampling platform. When different proportions of the coal gangue mixture are placed on it, the capacitance value of the capacitor changes, changing the resonance frequency of the system. Our results indicated that the passive wireless sensing system could clearly distinguish the degree of mixing of the coal and gangue and has a higher sensitivity than the conventional system. The proposed system was compared with a mosquito resonant coil system of the same order, and the results indicated that the spiral resonant coil wireless sensing system has more significant advantages in the enhancement of sensitivity. This passive wireless sensing system provides an accurate method for the automatic identification of coal gangue in the top coal caving process; furthermore, it is inexpensive and highly operable.INDEX TERMS Coal gangue mixture, passive wireless sensing, second-order PT symmetry system.

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