Abstract: The deformation measurement of rotating helicopter high-speed rotor blades is usually based on circular markers. However, circular markers are usually with problems, such as low exposure, small target region, and asymmetric projection, which easily lead to detection missing of markers and location error of circle centers. In order to avoid these problems, a detection method based on circular markers in helicopter rotor blade images is proposed in this paper. Firstly, pixel coordinates of local gray extreme value centers in the image are extracted, and interferences are removed according to array arrangement structure, then pixel coordinates of all circular marker extreme values are obtained. Secondly, the circular region of interest (ROI) is established with each extremum coordinate as the center and the minimum distance from the adjacent extremum as the diameter. Within the ROI, the circle center is obtained by parallel watershed transformation and least square circle fitting. Thirdly, based on perspective transformation, the projection mapping relationship between the image and another helicopter rotor blade image, which with the same phase and perpendicular to the camera optical axis is established. And the projection mapping matrix is optimized by levenberg-marquardt (LM). Finally, the images is converted to a positive image for circle center fitting, and exact circle center coordinates are obtained by inverse transformation of circle center coordinates. Experimental results show that the accuracy and the precision of the proposed method are 98.89% and 0.191 mm, and it has been applied in the high-precision visual measurement of the motion trajectory and deformation of the helicopter high-speed rotor baled.

Key words: helicopter blade image, circular marker, asymmetric projection, watershed transformation