Error correction of glass mediums in high-temperature digital image correlation deformation measurement
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摘要: 为了校正玻璃介质在高温变形测量中引起的测量误差，本文将玻璃介质作为相机标定模型的一部分，基于摄影测量技术和数字图像相关法，提出一种复杂环境下的双目相机标定方法，将其应用在高温变形测量中。首先，针对复杂环境下图像质量差引起的标定困难问题，采用带畸变校正的相机成像模型，通过捆绑调整的相机标定方法完成双目相机标定，提高了标定成功率和稳定性。其次，针对复杂环境下双目相机标定精度低的问题，分析镜头焦距、环境光干扰和玻璃与相机距离等因素对标定结果的影响，给出最佳标定参数，使得标定重投影误差由0.832个像素减少到0.132个像素。最后，采用有玻璃介质的测量环境，比较标定时有无玻璃两种情况下的测量误差，结果表明：本文方法能大幅减少测量误差。试验结果表明，该方法能够有效减少高温环境下玻璃介质导致的位移场测量误差，X，Y和Z轴位移场平均测量误差分别减少70.16%，76.51%和40.05%。本文方法能够实现复杂环境下相机高精度标定，标定稳定性好，是实现高温变形准确测量的有效途径。Abstract: In order to correct the measurement error caused by a glass medium in high-temperature deformation measurement, we take a glass medium as a part of the camera calibration model. Based on photogrammetry technology and digital image correlation, a binocular camera calibration method in a complex environment is proposed and applied to high-temperature deformation measurements. Firstly, aiming at the calibration difficulty caused by the poor image quality in complex environments, the camera imaging model with distortion correction is adopted to achieve binocular camera calibration by bundle adjustment camera calibration method, which improves the success rate and stability of calibration. Secondly, to solve the problem of low calibration accuracy of binocular cameras in complex environments, the influence of lens focal length, ambient light interference and the distance between glass and camera on the calibration results are analyzed, and the optimal calibration parameters are given, so that the calibration reprojection error is reduced from 0.832 pixels to 0.132 pixels. Finally, the measurement error of the calibration method with and without glass is compared by using the measurement environment with a glass medium, which proves that this method can greatly reduce the measurement error. The test results show that this method can effectively reduce the measurement error of a displacement field caused by glass medium in a high-temperature environment. The maximum decrease of measurement error of the displacement field in the X, Y and Z axes is 70.16%, 76.51% and 40.05%, respectively. The method in this paper can achieve high-precision camera calibration in complex environments, and has good calibration stability. It is an effective way of realizing accurate measurement of high-temperature deformation.
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图 6 玻璃与相机距离对标定精度的影响
Figure 6. Influence of the distance between the glass and the camera on calibration accuracy
图 9 X，Y和Z轴方向的位移均值和标准差
Figure 9. Mean value and standard deviation of displacement in X, Y and Z axes
图 11 X，Y轴方向的应变均值和标准差
Figure 11. Strain mean value and standard deviation in the X and Y directions
图 12 试件50°到120°X，Y，Z轴方向位移场变化图像
Figure 12. Displacement variation in the X, Y and Z directions for the calibration with and without glass with temperature from 50 °C to 120 °C
图 14 不同方向平行线位移均值
Figure 14. The mean displacement of parallel lines in different directions
表 1 不同焦距下的重投影误差
Table 1. The reprojection error at different focal lengths
焦距/mm 8 12 16 25 50 75 Sigma/pixel 0.024 0.030 0.039 0.024 0.040 0.048 
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