Research On The Key Techniques For Complex Curved Profile Measurement Of Aero-engine Blades

As the the 'heart' of aircraft,aero-engines are large and complex machines with highly requirements for reliability and efficiency.Blades are essential components in an aero-engine,which shape and size precision will greatly impact the performance of the aero-engine.Since each aero-engine contains lots of blades and the blade has high demands on shape and size precision,it thus is a hotspot research direction at present to rapidly and accurately measure the three dimentional(3D)profiles of aero-engine blades.Due to many advantages,such as high speed,dense point clouds and non-contact measurement,fringe projection profilometry(FPP)is selected to measure 3D profiles of aero-engine blades in our projects,In order to meet the measurement requirements of aero-engine blades,the measurement accuracy of FPP system need to be further improved.To this end,phase error calibration and compensation,precise projector calibration,simplification of high-order calibration mode and quick identification of valid points are thoroughly studied in this paper.The accomplished research works and novelties of this paper are summarized as follows:(1)By using the principal component analysis(PCA),a method to simplify the high-order polynomial calibration model is proposed.In practical applications,high-order polynomial calibration model has difficulty in determining parameters.To solve this proplem,a method to simplify the high-order polynomial calibration model is proposed based on the principal component analysis(PCA).In the proposed method,the significances of the model terms to the reconstructed results are computed and trival terms are identified and removed.With the proposed method,the stability of high-order polynomial calibration model can be greatly improved without significant expense on the reconstruction accuracy.(2)Nonlinear intensity response in FPP system can yield periodical phase errors in the absolute phase map.To reduce the periodical phase errors,it is theoretically analyzed by using Forier decomposition method.With the theoretical analysis,the special harmonics in the captured fringe images,which have the influence on the amplitude of the periodical phase errors,are determined.The specific result of the theoretical analysis is that,in N step phase-shifting method,the amplitude of the kN-order phase error is equal to the amplitude difference between the(KN-1)-order and the(kN+1)-order harmonics in the captured fringe images.Based on the analysis result,a black-box phase error compensation method is proposed in this paper.In this method,the periodical phase errors are greatly reduced by superimposing the special high-order harmonics upon the initial fringe patterns.The proposed method is more effective than traditional Gamma correction method.(3)In order to improve the accuracy of projector calibration,a novel projector calibration method is presented based on traditional calibration method and cross-ratio invariability.The traditional projector calibration method determines projector pixel coordinates in the accuracy of pixel level.Comparatively,the presented method can obtain the accuracy of sub-pixel level.(4)In order to identify the valid points and avoid the influence of invalid points in shadow areas and background areas on the measurement accuracy of FPP system,the modulation amplitude is taken as the criteria to distinguish valid points from invalid points.To automatically and precisely obtain modulation thresholds in practical applications,a rapid and adaptive modulation threshold determination method is given based on modulation histogram and image segmentation algorithm(Otsu algorithm).This method can quickly and effectively identify valid points in the complicated conditions.