Research On Elimination Of Image Drift In Structural Displacement Photogrammetric Measurement Based On Diffusion Circle Spot

Dynamic displacement monitoring is an important part of structural health monitoring.Photogrammetric measurement has widely used in the field of civil engineering displacement monitoring due to its advantages of non-contact,long distance,and high precision.In recent years,the research on photogrammetry has gradually emerged in large numbers at home and abroad,and has become a popular research direction.However,photogrammetric measurement has long suffered from environmental effect,which alters the imaging geometry and leads to spurious image drift.Therefore,this paper explored the influence of environmental factors on the image drift of the photogrammetric measurement in the all-weather field measurement,and proposed an image drift elimination method based on diffusion circle spot.Firstly,the lens imaging model and the common camera calibration methods are briefly introduced.Particularly,the imaging principle of the diffuse circular spot is emphatically introduced.And the imaging diameter formula is deduced and verified by imaging experiments at different distances.Then the photogrammetric structure simulation dynamic displacement field monitoring experiment are first performed.The influence of environmental temperature and weather change on the image drift in the field photogrammetric measurement is discussed.Changes in temperature,precipitation,and air heat wave will all affect the accuracy in photogrammetric measurement.It is found that the environmental temperature change is the main factor that causes the image drift.The image drift increases with increasing temperature and decreases with decreasing temperature.However,this regularity is not always applicable,sometimes the opposite phenomenon occurs.In addition,precipitation and air heat waves will introduce high-frequency random errors in the photogrammetry system.Extreme weather conditions（rainstorm）may even change the camera’s pose and cause unpredictable image drift.Then,an image drift elimination method based on diffuse circular spot is proposed.Different from the existing use of the far-field reference object near the structure to be measured to obtain the image drift,the method proposed in this paper is based on the insensitivity of the sharpness of the diffuse circular spot,setting a luminous target near the camera as the near-field reference to obtain the image drift,which overcomes the difficulty that the far-field reference is difficult or even can’t be found in the actual measurement.After that,an improved image enhancement algorithm is used to eliminate image noise and redundant information for each frame of image collected.The Hough Transform is then exploited to identify the pixel coordinates of the diffuse circle spot,which are namely the measurement errors induced by varying environmental conditions.Finally,the pixel coordinates of the far-field target under different environmental conditions are normalized to the reference status in the initial frame by subtracting the pixel coordinates of the diffuse circle spot.By so doing,the image drift can be removed.After that,an improved image enhancement algorithm is proposed.The Tenengrad gradient is used as the image definition criterion.By so doing,the original manual intervention is converted into computer analysis,which greatly reduces the workload and human misjudgment and makes the operation easier and has better field application value.To exemplify the approach and algorithm,verification tests are carried out in outdoor environments to examine the performance of the approach for correcting the image drift.Virtual structural displacement is generated by making use of a tailor made motion simulation machine.A LED is placed near the camera so as to produce a diffuse circular spot on the image plane.Meanwhile,a fixed LED is attached on the motion simulation machine for comparison purpose.The results show that,in the long-term camera measurement field test,the collected image information will change with environmental factors.And the improved image enhancement algorithm proposed can effectively obtain the best image parameters at each moment.In addition,the image drift registered by the diffuse circular spot is similar to that recorded by the fixed LED.The RMSE of the differences between the image drifts is less than 4mm and the correlation coefficient between them is more than 0.9,whether in horizontal or vertical direction.After the rectification with the proposed approach,the RMS of the horizontal and vertical image drifts reduces by 84.54% and 84.79%respectively,implying that the approach achieves superior performance in correcting the image drift.