| Turbine blade film cooling holes are the key factors to improve the efficiency,reliability and high temperature resistance of aero engines.The cooling effect is greatly influenced by geometric parameters of film cooling holes such as the axes of cooling holes,diameter and spatial distribution.However,due to the factors such as blade casting deviations and EDM errors,some of the film cooling hole geometric parameters do not meet the design standard.A large number of cooling holes,with small diameters and variable axial directions,are distributed on the curved blade surface.The measurement of the geometrical characteristics of these cooling holes become the research focus for domestic aero engine manufacturing companies.In this thesis,the research on the measuring technology of geometric characteristics of film cooling holes by electrical discharge machining(EDM)is carried out.A measurement system based on a 3D laser point cloud is developed.This system mainly includes three modules: sensor calibration,measuring trajectory planning and point cloud data processing.The system uses a linear laser sensor as a measuring element and a five-axis machine tool as a motion platform to realize the measurement of geometric characteristics of film cooling holes.The sensor calibration module,whose function is to calibrate the origin and beam orientation vector of a laser sensor.The purpose of calibration is to realize the coordinate conversion among a multi-coordinate system of the measurement system.In order to address the problems in traditional calibration algorithms,such as difficulties in solving nonlinear equations,dependence on specific standard parts,and great influence of initial values,a calibration algorithm based on kinematic modeling and plane geometric constraints is proposed.The algorithm achieves sensor calibration by constructing a statically definite linear equation group.According to the experimental data,the calibration accuracy is 0.012 mm,which meets the measurement needs.The trajectory planning module,whose function is to plan a scanning trajectory,measure and avoid potential hardware collisions and beam interference,so as to determine an accurate,efficient and safe trajectory.In view of the problems of small measurement range,strict angle requirements,large number of measuring points,hardware collision,and beam interference in laser scanning,the following algorithms are studied: 1)measurement direction and coordinate distribution algorithm;2)collision detection and avoidance algorithm,based on a point cloud scanning body;3)beam interference detection algorithm based on point cloud monitoring;4)beam interference avoidance algorithm based on secondary planning of detection direction.In addition,the proposed algorithms are simulated and verified by VERICUT software,and finally the point cloud is obtained accurately,efficiently and safely.The point cloud data processing module,whose function is to determine the geometric parameters of film cooling holes by means of point cloud segmentation,hole axis extraction,diameter fitting and intersection position calculation.To solve the problem of low robustness in point cloud segmentation,a segmentation algorithm based on scan line fitting residuals is proposed,which converts a three-dimensional point cloud into a twodimensional scan line,and achieves target segmentation with the help of fitted residuals.To address the problems of low normal vector accuracy,small number of point clouds and large plane fitting errors in axis extraction,the axis extraction based on an improved Gaussian mapping transformation is studied,with the help of effect evaluation function,Gaussian mapping completion and random consistency sampling.To solve the problems of low calculation efficiency and large influence of initial value in diameter fitting,the diameter fitting algorithm based on axial projection is proposed,which converts the three-dimensional film cooling hole diameter into a twodimensional ring diameter to achieve qualitative detection of diameter.For the noise filtering after projection,a variable force iterative filtering algorithm based on error probability statistics is proposed.For the position of the film cooling hole,the spatial distribution of film cooling hole is determined by calculating the coordinates of the intersection point of the axis and the curved surface.In addition,based on the simulation of point cloud and EDM small hole machining samples,the principle feasibility and actual operability of the algorithm are verified.Through the actual measurement of film cooling holes,the measuring functionality of the system is verified,and the factors affecting the accuracy are analyzed.,Based on the measuring point cloud,the accuracy of proposed algorithms are verified with the help of Geomagic software.Through static,semi-dynamic and full-dynamic measurement experiments,decouple analysis of sensor measurement accuracy,blade clamping error,platform positioning error is carried out.The data shows that the measurement accuracy of the system in axial direction,position and diameter is 0.405°,0.038 mm and 0.024 mm respectively.Therefore,it provides an effective and feasible technical solution for the quantitative measurement of the axial and position and the qualitative pre-screening measurement of the hole diameter. |
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