Nontarget-based structural displacement measurement using a digital camera with four laser rangefinders

Structural displacement is an important indicator for assessing the safety and integrity of structures. Among various assessment techniques, computer vision-based displacement measurement has been established as an effective method with the development of cost-effective and high-performance image sensors. However, the use of artificial targets attached to a structural surface or the need for known structural dimensions remains necessary, which is challenging in practical applications. Although nontarget-based methods have been explored to overcome the limitations, they can suffer from insufficient accuracy due to difficulties in integrating multiple devices and considering the relative position between the camera and structure. This study proposes a nontarget-based displacement measurement method that utilizes a vision system consisting of a digital camera and four laser rangefinders. A physical coordinate system which can accurately reflect the structural displacements is constructed based on the rotational alignment algorithm. Subsequently, the relationship between the image and obtained coordinate system is used for displacement calculation. A laboratory-scale test shows a maximum root mean square error (RMSE) of 0.33 mm under a maximum displacement of 8.48 mm at a distance of 25 m, while the conversion ratio yields that of 1.76 mm. Furthermore, a field application on a full-scale railway bridge indicates that the proposed method achieves the maximum RMSE of 0.03 mm, compared with 0.13 mm of the conversion ratio.