Application Of Digital Image Processing In Strain Measurement Of Structure

Structural strain is one of the main physical quantities that should be observed in the civil engineering structural monitoring. The common method is to affix the bended strain gage to the pre-monitored site of the component, to stick the gage, to weld the wire and seal the gage etc, which often be carried out at high altitudes, it is unsafe, inconvenient, inefficient. In addition, the environmental temperature and humidity level affect the measured value sharply, so the measured value is short of reliability. In connection with all these limitations, a distant and discontiguous strain measurement system will be researched.The main content and objective is to accomplish strain measurement used the image processing techniques. The mathematical model of digital image is established, the collection, preprocessing, sub-pixels position are researched as well as the system calibration of the image. Combined with the characteristic of the strain measurement and the perfect optical system image-forming principle, considered the axisymmetrical and nonaxisymmetrical aberration, we adopt the automatic calibration method based on region growing and surface fitting. Cubic polynomial is used in the arithmetic to fit the corresponding relationship between the object space and pixel coordinate system. As a result, the actual position in the object space is obtained by measuring the pixel position in the target image used the relationship. The scenes are more complicated in the fields. To achieve the goal of real-time dynamic tracking, the algorithm should be simple and efficient, and the stability and precision of the tracking must be guaranteed. For this reason, the window-centroid tracking algorithm and the centroid searching method are employed.This strain measurement system is realized through image processing in portable computer with the help of image capture case and the corresponding USB 2.0 digital interface to obtain image message based on the cheap CCD camera, so this system can be operated easily and conveniently. In the Windows XP, Visual C++6.0 programming language is used to develop the strain measurement software system which based on the digital image processing techniques. Take the strain measurement of a projecting steel beam for example, the theoretic validity and practical feasibility of the system is proved.