Measurement method of helicopter blade root torsion angle based on perspective transformation

In the existing methods, images of markers are often taken from bottom to top. When the blade is twisted greatly, there is an obvious angle between the principal optic axis of the camera and the normal vector of a marker plane, resulting in perspective distortion of the captured images, which leads to relatively large measurement errors. In order to solve the problem raised above, a method of measuring the torsion angle of helicopter blade root based on the perspective transformation was proposed. Firstly, the target was vertically fixed under the blades, so that the camera was directly facing the propeller hub to capture images. Secondly, the four vertex coordinates of the blades in the target images of azimuth to be measured and positive azimuth were calculated when operating at the low-speed leveling state, and the perspective transformation matrix of the azimuth to be measured was solved. Moreover, the perspective transformation matrix was used to correct the distorted image of the azimuth to be measured under the high-speed motion state of the blades. Finally, the coordinates of the circle center in the reference image and the corrected image were obtained, and then the torsion angle of azimuth to be measured was calculated. The experimental results show that the measurement error of each azimuth is less than 0.2°. This method not only has high measurement accuracy, but also has the advantages of multi-azimuth dynamic measurement, which has a good application prospect in the field of motion parameters measurement of helicopter blade root.