An automated long-term bridge deflection monitoring system based on subpixel laser spot localization

Bridge deflection monitoring is essential for construction control and long-term structural health monitoring. Conventional surveying methods are labor-intensive, while existing automated laser-based monitoring systems are often constrained by pixel-level spot localization accuracy. Moreover, installation tilt and target misalignment, which frequently occur in bridge construction environments, are seldom explicitly considered, leading to systematic measurement errors and reduced field reliability. To address these challenges, a geometric coupling compensation method and a robust subpixel laser spot localization strategy are developed and integrated into an autonomous multi-target bridge deflection monitoring framework. The geometric coupling model compensates for installation tilt and laser-to-inclinometer misalignment, thereby reducing systematic errors introduced during field deployment. The subpixel localization strategy improves laser spot center extraction accuracy and robustness. In addition, ultra-wideband (UWB) positioning is combined with automated pointing control to enable unattended sequential polling of multiple monitoring targets. The proposed system was validated through a nine-month continuous deployment on an active bridge construction site. Experimental results demonstrate that, within a measurement range of 48 m, the system achieves a deflection accuracy of ± 1.5 mm with an average polling time of 30 s per point. Long-term field operation further confirms stable performance under practical construction conditions. The proposed approach provides an effective solution for autonomous bridge deflection monitoring and supports the transition from construction-stage alignment control to long-term structural health monitoring throughout the bridge lifecycle.