Image-based Displacement Measurement Methods For Bridge Structures Considering The Camera Motion

The bridge structure engineering plays an important role in the traffic construction of a nation.The static and dynamic response of the bridges are the main measurement content and critical data source for the structural safety assessment,reconstruction of dangerous bridges,and acceptance of new bridges.To overcome the difficulties in the deformation measurement of long-span bridges,advanced noncontact technologies have been developed,such as the microwave radar,measurement robot,and 3D laser scanning and machine vision.In contrast,optical technology shows significant value and attracts much attention due to its advantages of fine positioning,multi-point synchronization,non-contact and non-destructive implementation.Although the optical method has been widely used,there are still some field limitations and key problems to be solved,for example,fast and accurate pixel displacement extraction,off-axis displacement conversion,and camera disturbance.Centering this topic,this paper carried out the following innovation work:First,for pixel displacement extraction,two target tracing and location methods with higher efficiency and accuracy were proposed.(1)SURF-BRISK was improved from three aspects:downscale feature detection,prejudgment model optimization and adding the constraint of geometric consistency.(2)To weaken the positioning fluctuation caused by brightness change in outdoor measurement,a gray centroid location method with pixel area consistency constraint was proposed.Second,in order to solve the difficulties of the existing scale factor calibration,this study made two types of work.(1)To reduce the displacement conversing error caused by the camera angle,a new conversion formula considering the coordinate change of the measured point was developed.(2)In order to solve the difficulties of remote scale factors calibration though a laser rangefinder,this paper proposes to calibrate the object distance and inclination angle of multi-point by using the captured image,the whole process of which does not require any additional tools.Third,the camera disturbance is very common and directly affects the measurement accuracy.On one hand,the correction effect of the relative displacement method is limited.On the other hand,the accuracy of rotation attitude extraction by images and the translation tracking through an accelerometer can not meet the engineering requirements.Then two new solutions for accurate displacement measurement under camera disturbance were developed.(1)In this study,it is found that the correction effect of the relative displacement method is directly related to the distance of reference points.Then,a relative displacement method based on two reference points with different object distance was developed,in which the reference point does not have to be close to the measuring point close to the measuring point.(2)A bridge deflection measurement method based on the combination of camera and angle sensor is proposed.This method possesses the full-field measurement capability of the camera and the stable-precision advantage of the contact-type sensor.A multi-target,real-time synchronous measurement system based on a standing camera is developed.This system has the following advantages.(1)Multiple functions including image acquisition,ranging and positioning,camera angle measurement and real-time image processing are integrated together.(2)In terms of hardware,the functions of long-distance ranging and target positioning are improved through the combination of a high power laser indicator and rangefinder.(3)Based on the research work in this paper,the developed realtime processing software provides different matching algorithms and calibration methods for users to select according to measurement conditions.In this study,the measurement accuracy of the system was evaluated under different inclination angles and object distance.Finally,camera-based deflection measurement of Nash Bridge in the load test was carried out.Two feasible measurement schemes integrated with the theoretical research in the above are proposed and used for the same loading cases.Both methods can realize multi-point synchronous measurement,and their reliability was proved by comparing with the results of the traditional displacement measurement equipment.