Bridge Structural Displacement Measurement and Modal Parameter Identification Using Microwave Radar

Structural displacement of bridges is a key indicator for evaluating their in-service performance; however, achieving high-precision non-contact measurements in complex environments remains challenging. To address the issues of uneven signal-to-noise ratio (SNR) and limited field of view in multi-target monitoring using microwave radar, this paper proposes an adaptive Variational Mode Decomposition (VMD) denoising method that integrates echo energy information, combined with a multi-reference-point method to identify structural modal parameters. This method uses distance cells with high signal-to-noise ratios as references and adaptively determines the number of decomposition layers based on a spectral correlation coefficient threshold (0.01), thereby improving the consistency and accuracy of signal processing across multiple measurement points. Experiments were conducted on a three-span prestressed concrete beam bridge. The results show that after VMD denoising, the residual noise at measurement points No. 3 to No. 8 was less than 5%, indicating a significant improvement in signal quality; the residual noise at measurement point No. 9 was approximately 25.3%, which was significantly affected by sidelobe interference and was therefore excluded from subsequent analysis; the radar displacement data showed good agreement with long-span strain measurements, with relative errors at typical measurement points less than 3%; the first two modal shapes were consistent with the finite element results, with a correlation coefficient greater than 0.9. The study demonstrates that the proposed method can achieve high-precision measurement of bridge displacement and reliable identification of modal parameters.