Ring-type passive loose-bolt sensing based on waveguide resonance for replacing conventional washer

Aging infrastructure has resulted in the demand for sensor-based remote monitoring. As loose bolts can lead to serious accidents, it is essential to properly monitor them. In this work, we present a ring-type passive sensor based on waveguide resonance for the purpose of replacing the conventional washer and monitoring the compressive tightening force. We propose the operating principle of the ring-type sensor in which specific waveguide resonance modes occur when the bolt is fastened, and the sensor passively detects bolt looseness by observing the resonance frequency changes of those modes. Simulations and experiments are performed to verify the principle of bolt loosening detection by the ring-type sensor and expand the application scope. On the basis of the results, we construct a model for achieving high sensitivity and determine the direction for improving sensitivity by clarifying the transition process from the initial fastened state to the final loosened state. Our findings demonstrate that the sensor is scalable enough to operate in various sizes and can replace the existing washer.