Research on bolt group loosening detection based on traveling-wave RFID displacement sensor tag

Bolt connections widely exist in various engineering structures. During long-term usage, bolts are frequently subjected to loading forces. The bolt looseness leads to structural damage, consequently affecting the safety and stability of the system. Therefore, bolt loosening detection is an important engineering issue in the field of Structural Health Monitoring (SHM). This paper proposes a highly sensitive, wireless, passive Ultra High Frequency (UHF) Radio Frequency Identification (RFID) traveling-wave displacement sensor tag. The displacement detection part of the sensor tag utilizes a microstrip line-excited Complementary Split Ring Resonator (CSRR) structure, which generates a sharp stopband due to the proximity coupling effect, offering high sensitivity in small displacement changes. By integrating an RFID tag antenna and chip at both ends of the microstrip line, a traveling-wave sensor tag is formed, and the displacement changes are characterized by the signal of the threshold power to activate the tag on the reader side, enabling reliable wireless measurement. Simulation and experimental results demonstrate that within the UHF range, the proposed traveling-wave RFID displacement sensor tag can reach a detection sensitivity of 204.91 MHz/mm, a minimum resolvable displacement of 0.01 mm, and a relative precision of 2.2%. Meanwhile, this paper validates the feasibility of using the displacement sensor for detecting bolt and bolt group loosening. For M30 bolts, the angular resolution for loosening detection can reach 1°. The proposed bolt group loosening detection scheme is wireless, passive, highly sensitive, and low-cost.