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An innovative bolt loosening detection method based on double-layer slotted circular patch antenna is proposed. By integrating the patch antenna with the bolt, the longitudinal length variation of the bolt can be transformed into a change in the air gap thickness of the antenna and cause the resonant frequency to vary. Thus, we can detect the bolt loosening through the resonant frequency shift of the double-layer circular patch antenna. Furthermore, the circular radiation patch of the antenna is slotted to increase the effective length of current flow, thus achieving the miniaturization of the antenna sensor for bolt loosening detection. The proposed double-layer slotted circular patch antenna sensor can be as small as a coin with a high sensing sensitivity of 626.57 MHz/mm, and it can be interrogated either wired or wirelessly. A series of simulations and experimental tests demonstrate the feasibility and effectiveness of the proposed antenna sensor for bolt loosening detection. The experimental results show that the relationship between the bolt preload and the antenna resonant frequency is relatively linear, and the resonant frequency of the antenna shifts 18 MHz for an M24 bolt from completely loosened to fully fastened.
Xue et al. (Mon,) studied this question.