Research On 3D Optical Non-contact Measurement Technology For Dynamic Deformation Of Composite Propellers

Marine propellers are common propulsion devices for ships.The structural design of the propeller blades directly determines efficiencies of power.Therefore,it is of great significance to measure the deformation of the propeller in work conditions.Since marine propellers often work underwater,it becomes a challenge for full-field threedimensional deformation measurement.Due to advantages of simple optical arrangement and non-contact and full-field measurement,three-dimensional digital image correlation(3D-DIC)has been applied to dynamic and static deformation measurement in air.In this thesis,a comprehensive study,which includes image acquisition,image matching,underwater refraction three-dimensional reconstruction,experimental verification,has been conducted on the underwater three-dimensional deformation measurement of marine propellers.The work mainly includes the following parts:1.A calibration method for stereo camera system in underwater situation has been proposed.A two-step calibration method for determination of internal and external parameters of cameras has been developed.The Zhang’s calibration method is used to calculate the internal parameters,and the self-calibration method based on epipolar constraint is used to calculate the external parameters.Based on the comparative experiment,the proposed method has the advantages of simple operation and strong adaptability.It is concluded that the accuracy of the proposed method is equivalent to the accuracy of the Zhang’s calibration method.2.Underwater image matching is implemented using Digital Image Correlation(DIC)algorithm.After matching between the left and right images is performed in the initial stage,image coordinates of the selected points are tracked in the left and right image sequences,respectively.The three-dimensional shape of the object is calculated based on the principle of triangulation,and the deformation respect to the initial position is obtained by substraction.3.A refraction stereo vision reconstruction method is proposed to correct the measurement deviation caused by refraction.Through the internal and external parameters,combined with the coordinates of points to be measured obtained by direct reconstruction,equations of the incident and refracted light are constructed according to the geometric constraints.The refraction correction is then derived in two cases.The validity and accuracy of the proposed refraction correction method are verified in experiments.4.Three-dimensional deformation measurement of the propeller blades in the cavitation tunnel has been conducted with a dual-camera system.The full-field threedimensional deformations of the marine composite propeller prototype at a fixed rotating phase at different angular speeds were obtained.The point cloud registration method was used to eliminate rigid-body displacement of the blade relative to the initial position caused by the vibration of the propeller shaft.