Low-Cost FMCW Radar for Full-Field Vibration Monitoring of Bridges

Abstract

Many existing methods for vibration-based monitoring of bridges rely on expensive sensor networks or require direct physical contact with the structure, which limits their widespread adoption, especially across large networks of ageing infrastructures. Recent advancements in non-contact radar technology enable precise and multichannel vibration measurements from a distance, allowing operation safely without interfering with traffic or requiring complex installation. This feature offers a practical and scalable approach to monitoring hard-to-access bridges or structures. However, commercial solutions remain expensive and are often constrained by limited beam coverage, focusing on single-point measurements. Addressing the existing drawbacks and augmented past research, this study presents an application of low-cost frequency modulated continuous wave (FMCW) radar-based vibration monitoring bridges through experimental validation. This research also investigates multi-point vibration measurements, thanks to the use of multiple transmitting and receiving antennas in radar enabling full-field vibration measurements of the structure. The fundamental working principle of the radar systems is by emitting electromagnetic waves continuously with a linearly changing frequency and comparing its frequency to the reflected waves from the target. To monitor sub-millimetre-level displacements, which are more often observed in the real-world infrastructure systems, the radar systems use the interferometry technique to analyse the phase shift in the reflected signal to accurately estimate the small displacements. The present study adopts the two-step strategy to validate the predictive capabilities of the FMCW radar system, as shown in Figure. The initial step involves the validation of low-cost sensors against commercial solutions on a corner reflector attached to a shake table, enhancing the signal-to-noise ratio and thereby avoiding interference from static clutter. Subsequently, validation of multiple-point vibration measurements for different targets located within the same range is carried out using multiple transmitting and receiving antennas. This approach increases confidence in the radar system's capability to evaluate the full-field vibration of a structure, rather than relying solely on single-point measurements. Second, the radars are tested for the real-world application on a bridge under the traffic or railway loadings. The field campaign is carried out to evaluate the dynamic behaviour of the bridge, such as its modal properties under ambient traffic conditions. The results reveal that the low-cost radar system effectively captures the dynamic behaviour of both the corner reflectors in the laboratory setup and the bridge during the field campaign. This enables informed decision-making for the stakeholder and asset managers for timely, actionable insight for maintenance and safety interventions by relying on the low-cost FMCW radar systems.