Enhancing In-Situ Structural Health Monitoring through RF Energy-Powered Sensor Nodes and Mobile Platform

TL;DR

This paper tackles the power constraints of in-situ SHM by introducing RF energy-powered sensor nodes embedded in concrete and powered by a mobile RF transmitter. It develops two architectures: an active RF-SN for thin concrete that transmits data via ZigBee and a passive RF-SN for thick concrete that uses backscatter with square-chirp modulation to cope with high attenuation. Experimental results show robust operation: active nodes at 13.5 cm depth can start with a 915 MHz transmitter at 32.5 dBm EIRP and deliver data rapidly, while passive nodes achieve continuous 224 bps transmission at 13.5 cm depth with 3% BER at 23.6 dBm EIRP, thanks to energy-efficient backscatter and modulation. Overall, the work demonstrates a viable battery-free, long-term SHM approach using mobile energy delivery, enabling scalable monitoring across diverse concrete structures with practical energy and communication performance.

Abstract

This research contributes to long-term structural health monitoring (SHM) by exploring radio frequency energy-powered sensor nodes (RF-SNs) embedded in concrete. Unlike traditional in-situ monitoring systems relying on batteries or wire-connected power sources, the RF-SN captures radio energy from a mobile radio transmitter for sensing and communication. This offers a cost-effective solution for consistent in-situ perception. To optimize the system performance across various situations, we've explored both active and passive communication methods. For the active RF-SN, we implement a specialized control circuit enabling the node to transmit data through ZigBee protocol at low incident power. For the passive RF-SN, radio energy is not only for power but also as a carrier signal, with data conveyed by modulating the amplitude of the backscattered radio wave. To address the challenge of significant attenuation of the backscattering signal in concrete, we utilize a square chirp-based modulation scheme for passive communication. This scheme allows the receiver to successfully decode the data even under a negative signal-to-noise ratio (SNR) condition. The experimental results indicate that an active RF-SN embedded in concrete at a depth of 13.5 cm can be effectively powered by a 915MHz mobile radio transmitter with an effective isotropic radiated power (EIRP) of 32.5dBm. This setup allows the RF-SN to send over 1 kilobyte of data within 10 seconds, with an additional 1.7 kilobytes every 1.6 seconds of extra charging. For the passive RF-SN buried at the same depth, continuous data transmission at a rate of 224 bps with a 3% bit error rate (BER) is achieved when the EIRP of the transmitter is 23.6 dBm.

Figures (17)

• Figure 1: Application scenarios of RF-SNs with mobile platform. (a) Health check of bearing wall with active RF-SNs. (b) Overpass health monitoring with passive RF-SNs.
• Figure 2: $S_{11}$ and $S_{21}$ measurements.
• Figure 3: $S_{11}$ measurements for dipole antennas at various depths and moistures in concrete. (a) $h_c\!=\!3.5$ cm and $f_r\!=\!915$ MHz. (b) $h_c\!=\!10$ cm and $f_r\!=\!915$ MHz. (c) $h_c\!=\!3.5$ cm and $f_r\!=\!2.4$ GHz. (d) $h_c\!=\!10$ cm and $f_r\!=\!2.4$ GHz.
• Figure 4: $S_{21}$ measurements for dipole antennas at various depths and moistures in concrete. (a) $h_c\!=\!3.5$ cm and $f_r\!=\!915$ MHz. (b) $h_c\!=\!10$ cm and $f_r\!=\!915$ MHz. (c) $h_c\!=\!3.5$ cm and $f_r\!=\!2.4$ GHz. (d) $h_c\!=\!10$ cm and $f_r\!=\!2.4$ GHz.
• Figure 5: Architecture of the active RF-SN, where $C_s$ is the energy storage capacitor and $V_{dd}$ is the supply voltage for the MCU.