Objective In the rail profile full section measurement system, the laser on both sides of the rail is not coplanar, resulting in the measurement profile distortion and profile measurement error. Presently, few studies are available on the noncoplanar problem of laser on both sides in rail profile measurement. The research focus is primarily on how to adjust the laser to meet the coplanar condition. When the laser from the structured light sensor on both sides of the rail is not coplanar, some distortion occurs in the measurement profile, resulting in rail profile measurement error. This contour measurement error caused by laser noncoplanar installation is referred to as laser noncoplanar error for ease of expression. To ensure the accuracy of rail profile measurement results, the lasers on both sides of the rail should be coplanar. However, in practical application, owing to the limitations of component processing accuracy and field installation environment, ensuring that the laser on both sides of the rail are accurately coplanar is difficult. If the precise coplanarity of the laser planes on both sides of the rail is pursued unilaterally through the precise assembly and adjustment device, it will not only result in a substantial increase in cost but will also complicate the assembly and adjustment, which is not conducive to on-site installation and operation of the railway. Presently, few reports are available on the method of correcting laser noncoplanar error on both sides of rail in full section rail profile measurement. To address the aforementioned issues, this paper proposes a laser noncoplanar error correction model of rail profile measurement system based on projection transformation. This method does not need the laser planes on both sides of the rail to be accurately coplanar but only needs to be roughly aligned, which greatly reduces the requirements of on-site installation. Methods First, the internal and external parameters of the left and right cameras and the laser plane parameters on both sides are obtained through the traditional calibration method, and then the longitudinal parameters of the rail are calibrated using the plane calibration plate (Fig. 7). On this basis, the reference coordinate system is established using the world coordinate system’s origin and the longitudinal parameters of the rail (Fig. 6), and the auxiliary plane perpendicular to the longitudinal direction of the rail is established using the longitudinal parameters of the rail (Fig. 10). The half section data of the profile on both sides of the rail are projected onto the auxiliary plane to obtain the projected profile (Fig. 5). The projected profile is used as the final measurement result of the rail profile to correct the laser noncoplanar error. Results and Discussions To verify the effectiveness of the proposed method, three groups of comparative experiments of rail profile measurement are designed (Fig. 9). Experiment 1: the lasers on both sides are coplanar and perpendicular to the longitudinal direction of the rail on both sides. Experiment 2: the lasers on both sides are not coplanar, and the laser plane on the left is not perpendicular to the rail’s longitudinal direction. Experiment 3: the lasers on both sides are neither coplanar and both planes are not perpendicular to the rail’s longitudinal direction. The results show that the proposed method can reduce the error in rail profile measurement caused by laser noncoplanar installation to less than 0.10 mm (Figs. 11-13). Furthermore, if the proposed error correction method is used, even if the laser plane is not perpendicular to the longitudinal direction of the rail after the measurement system is installed, it can ensure that the measurement result is the cross-section profile data of the rail rather than the oblique section profile data, ensuring the measurement accuracy of the rail profile. Conclusions Aiming at the problem of laser noncoplanar error encountered in the full section measurement of rail profile, a laser noncoplanar error correction model based on projection transformation is proposed, the principle and steps of error correction are described in detail, and the effectiveness and accuracy of the proposed method are verified by experiments. The proposed method reduces the rail profile measurement error caused by noncoplanar laser installation to less than 0.10 mm. This method does not require the laser planes on the left and right sides of the rail to be perfectly coplanar, but only that they be roughly aligned. It not only ensures rail profile measurement accuracy, but also greatly reduces the requirements for component processing accuracy and on-site installation environment, and avoids the time-consuming and labor-intensive laser plane fine adjustment process on site. © 2022 Science Press. All rights reserved.
