The Shape And Profile Precision Measurement Of Object Based On Fringe Structured Light Projection

With the development of our nation’s aerospace science and technology,the high performance aero-engine is an increasingly urgent need.At the same time,the aircraft engine blade with super high bearing temperature is one of the key technologies which can improve aero-engine performance.The hollow inner cavity composed of the ceramic core can enable the blade to endure higher temperature by raising cooling efficiency.Preparing enough ceramic cores of superb quality rapidly is a choke point to fabricate marvelous blades.The defects including flashings and blind holes of cores manufactured by current processes always exist,which need to be detected and pruned as the complicated surface.However,the available method and facility could not inspect these defects rapidly and precisely.This dissertation proposes the method based on fringe structured light projection for detecting defects of the ceramic core.The 3D shape and profile measurement technique by casting surface fringe structured light to the cores calculates the phase value loaded the shape and profile information,acquire the surface point cloud data and extract the detects location by comparing the measured point cloud with the ideal point cloud.This dissertation mainly studies the key technology of 3D measurement by fringe structured light projection,and focuses on adjusting phase error,calibration internal and external parameters of the measuring system and the processing of points cloud affecting the shape and profile measurement.Main achievements are as follows:1、 The current measuring development of the ceramic core is investigated,the low-accuracy,slow speed and complex structure of existing methods are pointed out,which lead to the urgency of the defects detection requirement for ceramic core.The status of three dimensional measurement based on fringe projection is investigated,and the advantage of shape and profile measurement based on structured light fringe projection for detection of ceramic core is analyzed.2、 The basic principle and technical essentials of each step in the active binocular stereo vision is introduced.The main methods of phase distribution calculation are described in detail,and the advantages and disadvantages of major methods in resolving principle phase value and unwrapping wrapped phase map is analyzed.The experimental platform is set up to verify the feasibility of the technique employed in this paper to obtain the intricate surface phase.3、 For the nonlinear light intensity responses of the system which affects the accuracy of the phase distribution computation,a theoretical model is established according to the operating principle,and the phase error caused by nonlinear response is simulated and calculated,which disprove the influence of fringe frequency.The methods of eliminating phase error in hand is presented,this paper proposed a novel method to rectify the deviation by calibration amplitude referring to the error model and simulation result.This method could reduce the deviation amplitude effectively.4、 To calibrate the intrinsic and extrinsic of the system with high precision,the central perspective imaging mathematical model of the camera is presented particularly,the application scenarios of available calibrating methods is analyzed contrastively,and Zhang Z’s flexible calibrating theory is applied.To solve the mismatching problem of the circular markers on the target board between the marker and its image,this paper proposes an method by utilizing the pose relation to achieve the matching inerrably.At the same time,the deviation of pixel coordinates caused by affine transformation is corrected,and the calculation process of parameters is also optimized.The internal and external parameters of the projection module are calibrated by taking advantage of the relationship between the phase distribution value and the projection chip array with the projecting process regarded as inverse imaging process.The experimental result demonstrates that these work can promote the calibration accuracy observably.The 3D measurement precision of the system is tested.The factors affecting the interference fringes drift of hologram are analyzed,and the characteristics of the periodic and continuous disturbances are pointed out.The method of frequency domain filtering for single pixel points is proposed,which eliminates the influence of the disturbance of mechanical vibration and the change of air refraction on phase-shifting digital holography and enhances the adaptability of phase-shifting digital holography in industrial measurement.5、 Several important error sources of 3D coordinate calculation accuracy are analyzed,including the pixel coordinate location error and the phase calculation error,and there reason-generating,influence mechanism and expression form are described.The error model is established which provides the ultimate coordinate deviation caused by these error sources.6、 The main techniques in point cloud processing are introduced in detail.The topological organization structure of the discrete point cloud is established.For the noise as a consequence of the shelter,stray light and device defects,this paper proposes a eliminating solution by filtering the dull spot of the phase map and utilizing radius outlier removal which is effective through testifying.As the complicacy of the ceramic core surface,this paper propose a method that coalesce modifying the intrinsic shape signatures and local surface patches by utilizing the differential geometrical information to extract the characteristic points.The random sample consensus and iterative closest point are employed to achieve the coarse and precise registration.To locate the defects of the core,the solution of searching disparate geometric characteristic(mean curvature)points is put forward.The result demonstrates that this solution can identify the defects location precisely.