Research On Deflection Measurement Method Based On Optical Image And Its Application

Bridges and other large structures will be damaged due to safety hazards in the early design or construction process,overload,aging and other problems;leading to various catastrophic accidents,causing significant property losses and casualties.So it is necessary to carry out health monitoring of large structures.Deflection is an important parameter in the health monitoring of bridge structures,which can effectively prevent the collapse and deterioration of the bridge.The traditional deflection measurement method often requires multiple sensors to be placed on the bridge,which costs a lot in use and maintenance.Therefore,the off-axis digital image correlation method and UAV deflection measurement method are developed in this paper to be suitable for far/near distance non-contact deflection measurement.Finally,the measured deflection is applied to damage detection.The main research contents and conclusions of this paper are as follows:1.In this paper,an off-axis digital image correlation method considering off-axis angle is established,and the effects of camera parameters and environmental factors on the deflection measurement accuracy are systematically analyzed,which can realize the long-distance deflection measurement of bridges.The results show that :(1)The deflection measurement accuracy decreases with the increase of the measurement distance,when the measurement distance is far(such as more than 200 m),the measurement error will exceed 1 mm;(2)Using a lens with a larger focal length can improve the deflection measurement accuracy;(3)The change of the camera pitch angle(within 15°)has almost no effect on the measurement accuracy;(4)When the light intensity in the field is weak(less than 500 lux),the deflection measurement error is large;(5)Measurement accuracy is almost unaffected by changes in ambient temperature.2.An image correction method based on SIFT algorithm is established,which can effectively eliminate the measurement error caused by the change of UAV’s own attitude when measuring the bridge deflection,and can realize the bridge deflection measurement in close range.The results show that : The image correction method can effectively eliminate the influence of UAV attitude change,and the measurement error is about 1 pixel.When measured at a relatively close distance(such as 10 m),the deflection measurement accuracy of the bridge can be controlled within 1mm.3.An autocorrelation function damage detection method based on deflection measurement is developed and successfully applied to the damage assessment of multistorey frame structures.The experimental results show that the damage location identified is consistent with the preset damage location under the steady-state excitation frequency,and the damage index can quantitatively reflect the degree of damage.The research results of this paper can develop a set of structural health monitoring system based on optical images,which can be applied to displacement measurement and damage detection of different structures.