SPICE-Based Circuit Modeling and Validation of an NFC Energy-Harvesting Strain Sensor for Structural Health Monitoring

Structural Health Monitoring (SHM) demands advanced and reliable systems and circuits capable of continuously detecting environmental and mechanical stressors that may impair the integrity, durability, and stability of critical infrastructure. This study introduces an innovative, untethered, and battery-free solution that combines Near Field Communication (NFC)-powered circuits with flexible strain gauge (SG) sensors for non-invasive SHM, specifically targeting surface-level cracks and deformations. The system is based on a specially designed NFC circuit that handles both energy harvesting and wireless data transmission, enabling seamless deployment in structurally sensitive, remote, or hard-to-reach areas without the need for wiring or extensive modifications. Flexible SGs, configured in quarter- and half-Wheatstone-Bridge arrangements, are bonded to critical components to measure strain and detect crack propagation. The complete system was modelled and simulated using SPICE to evaluate the behaviour of both the strain sensing and the NFC-based wireless energy transfer. Hardware implementation closely matched simulation results, demonstrating full system functionality using a 13.56 MHz NFC link, with optimized power delivery at 2.7 V across an operational distance of up to 4 cm. Both simulations and experimental tests were conducted to investigate the transient and steady-state behaviour of the circuit. The NFC energy harvesting circuit delivered up to 12.5 mW under load conditions. A functional hardware prototype, built using off-the-shelf components, demonstrated the feasibility of the system for autonomous and long-term SHM applications.