Deflection refers to the linear displacement of the axis of the member in the direction perpendicular to the axis or the line displacement of the middle surface of the plate and shell in the direction perpendicular to the middle plane when the force or temperature changes unevenly. The deflection of a beam or column refers to the displacement of a point on its axis in the normal plane of the axis at that point. The deflection of thin plate or shell refers to the displacement of each point on the middle plane on the axis of the central plane at that point. The deflection of each point on the object changes with the position and time and becomes the deflection function or displacement function. It is one of the research methods of solid mechanics to calculate strain and stress by calculating deflection function. Bridge deflection is the maximum deformation of bridge under load, and it is an important parameter of bridge structure. It is necessary to accurately measure the static and dynamic deflection values of bridges in the aspects of bridge detection, reconstruction of dangerous bridges and acceptance of new bridges. The traditional methods of deflection measurement include displacement meter method (displacement sensor method), level gauge method, total station method, dial indicator method, etc. Differential GPS method, photoelectric method, etc. With the development of measurement and control technology, inclinometer, accelerometer, connected tube method, strain gauge, fiber optic gyroscope, differential GPS, digital image method, collimating laser beam method and other new measurement components or measurement and control methods are gradually applied to deflection measurement.

The traditional bridge deflection measurement mainly adopts dial indicator or displacement meter method. This method has the advantages of simple equipment, high measurement accuracy and good stability. It can be measured at multiple points and directly obtain the deflection value of test points. At present, it is still mainly used in the fields of bridge maintenance, bridge safety assessment, bridge acceptance or composite plate bending test. The displacement meter method is more widely used because of its high degree of intelligence.

Deflectometer is mainly composed of power supply, displacement sensor, signal amplification circuit, data acquisition card, acquisition software and buzzer warning circuit. Select the appropriate voltage to supply power to the displacement sensor, the sensor output signal, after the signal amplification circuit conversion processing, access to the data acquisition card, by the acquisition software acquisition processing, real-time display, and judge the relationship between the measured value and the preset threshold. When the deflection detected by the displacement sensor is lower than the warning value, the system will work normally. If it is higher than the warning value, the warning circuit will send out a beep alarm to realize the alarm function. The data of the acquisition card can be transmitted to the computer, and the host computer of the same LAN can realize interactive communication and remote data communication.

The deflectometer designed based on displacement sensor needs to set up installation platform under the bridge of test point to facilitate the installation and fixation of displacement sensor. The deflection of bridges erected above rivers and lakes can not be detected by this method. However, the displacement meter method is simple, low cost and high precision. It can be used for both static and dynamic deflection detection. When multiple displacement sensors are installed under the bridge, the overall displacement (i.e. deflection) of the bridge can be detected. The displacement meter method is also often used in the comparison test of other measurement methods to test its measurement accuracy and feasibility.

Due to the limitation of the structure and installation requirements of displacement sensor, the displacement meter method is only applicable to the deflection measurement of simple test pieces in the vertical direction, but it is not accurate enough for the deflection measurement in the non vertical direction of some complex configurations, so it is rarely used.

Compared with displacement meter method, other deflection measurement methods have different advantages and limitations. For example: the level also has a high measurement accuracy, but it can only be used for static measurement, limited by manual reading and data sampling frequency, it is difficult to use for dynamic deflection measurement. Therefore, it is often necessary to close the traffic to ensure the measurement accuracy when used in bridge detection; the accelerometer method is only used for dynamic deflection measurement; the connected pipe method is simple in principle and easy to realize automatic measurement, but it can only be used for static measurement and low-frequency dynamic detection, and can not be used for high-precision dynamic deflection detection of bridges. At the same time, this method is not suitable for large span or large slope Bridge detection; the differential GPS method is easy to set up and has little impact on the environment, but the detection accuracy is low, which is only used for large bridge deflection observation and monitoring; the collimating laser beam method is simple in principle, easy to install and use, high precision, and easy to realize automatic measurement, but it is greatly affected by environmental factors, laser spot divergence or atmospheric turbulence will cause beam jitter and serious impact The accuracy of measurement is affected. At the same time, the laser light source should be installed at the stationary position, otherwise the beam tilt will directly affect the detection accuracy.