Passive Wireless Bolt Preload Monitoring Based on Magnetic Resonance Coupled Ultrasonic Time of Flight

The real-time monitoring of bolt preload is vital for the predictive maintenance of critical infrastructures. Conventional approaches often rely on wired or battery-powered sensors, making them unsuitable for large-scale distributed deployments. This article presents a novel passive wireless bolt preload monitoring system based on the ultrasonic time of flight (ToF). The system includes a sensor and a reader coupled through magnetic resonance coils. The sensor, consisting of a piezoelectric transducer and a resonant coil, is installed directly on the bolt head. The reader includes a 4.5-MHz radio frequency (RF) signal transceiver and a ToF measurement circuit. RF energy is wirelessly delivered to the sensor through the resonant coupling network to excite longitudinal ultrasonic waves within the bolt. Echoes from internal interfaces are coupled back through the same path for ToF information extraction. Experimental results show a strong linear correlation (R² > 0.999) between ToF and preload across various bolt sizes (M16, M18, and M20), with measurement errors below 2 % compared with commercial sensors. The system demonstrates robustness against spatial misalignment, fixture variability, temperature shifts, and humidity variations. This passive wireless, noninvasive, and high-precision system holds strong potential for long-term structural health monitoring (SHM) with minimal maintenance requirements.