Development of the photogrammetric crack measurer Technology

KEY WORDS: deformation monitoring, engineering geodesy, photogrammetry, crack measurer, block of photogrammetric marks, strain gauge.

ABSTRACT: Presents main research results carried out during development of photogrammetric crack measurer. Examines the basic operation principles of this device.

The observation of dynamics of crack formation in buildings, as well as the values of the disclosures of expansion joints in engineering structures is an important task for engineering surveys and geodetic monitoring. Along with the geodetic methods, different types of crack measurers and strain gauges are widely used to solve these problems. The most effective are electronic linear displacement sensors. These devices provide the highest accuracy (up to several microns and above) and allow organizing permanent monitoring system through electronic reading of the results. However, due to the high cost of used electronic sensors, they are usually used on the most important and dangerous objects or the most critical areas. Other types of crack measurers (mechanical, scale), and geophysical methods, rarely provide accuracy better than 0.1 mm. In addition, as a general rule, most types of crack measurers are arranged and installed so that they track movement only in one direction (usually perpendicular to the direction of the crack or seam). If you want to monitor the spatial deformation, you have to install not one, but three such sensors or crack measurers, which complicate the monitoring system and leads to additional costs.

All of these considerations, along with experience of scientific and technical developments in the field of digital photogrammetry have prompted us to begin to create of a new type of crack measurer based on photogrammetric techniques. Study starts in 2015. We proceeded from the following basic principles underlying the work on creating of the photogrammetric crack measurer:

1. Photogrammetric crack measurer consists of of 2 blocks of photogrammetric marks, which are fixed on both sides of the expansion joint or cracks, cameras and special software.
2. The principle of operation is based on the fact that the marks block is periodically photographed by the camera, and the results of strict photogrammetric processing of the images is determined by the relative position of the centers of the two blocks of marks arranged on both sides of the crack. Deformations are determined from comparisons of the processing results of a photo shooting conducted at different times.
3. Technology needs to be available for using of non-specialists in photogrammetry, therefore should be used usual digital camera, and all the complexity of photogrammetric processing of images (from calibration to calculation of the coordinates and deformation) should be hidden behind a high level process automatization of the software.

Today research and development is nearing completion and we can say that the main tasks achieved:

• technology allows to get the accuracy of determining of the linear displacements at the level of a few microns at distances of photo shooting up to 20 meters
• technology allows to determine the linear motion in three axis
• in addition to the linear deformations can be defined the angular deformations.

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