Research On Diameter Measurement Technique Of Thin Cylinder By Laser Diffraction

Thin cylinders, such as micro-shaft and leg wird, are widely used in the fields of aviation, astronavigation, military industry and civil industry. The accuracy of manufacture of these parts can be increased by precison and high efficiency measurement of their diameters. So do the performance and producing efficiency of instrument or weapon, in which these parts are used. At the same time the defective rate, production costs and resource consumption can be also decreased. So the research on diameter measurement technique of thin cylinder is very important.In this dissertation, to impove the measurement accuracy of thin cylinder diameter is the primary object. A large quantity of references is consulted and the current situation of domestic and exernal research are studied. The main content and innovative achievements of this dissertation are as follows:At firs, the vector model of thin cylinder diffraction has been proposed. This model which considered the polarization character of incident light is on the basis of two theories: the vector diffraction field of half-infinite metal screen and Babinet's principle. And it consists of the diffraction model of E polarized incident field and H polarized incident field. Experiment result shows that the measurement of thin cylinder diameter can achieve higher measurement accuracy by vector model.Secondly, the measurement system of thin cylinder diameter by laser diffraction is developed. Semiconductor laser (LD), polarizing film and area CCD is used in the measurement system. LD is taken as incident source and polarizing film is as producer of linearly polarized light and area CCD is as receiver of diffraction pattern. The measurement range is from 0.05mm to 0.5mm.Thirdly, the formation of actual diffraction pattern of thin cylinder is studied systematicly. The difference between ideal diffraction pattern and that of actual measurement is considered and the actual diffraction pattern is studied on four aspects, such as white noise, characteristic of Gauss beam of LD, , spatial position and disturbance of thin cylinder and focal spot. The influence to actual diffraction pattern of all the influencing factors is studied in detail and the corresponding simulation of every influencing factor is also done. Then the simulation is compared with the actual diffraction pattern and they prove that the study is right.Forthly, the zero-phase filtering algorithm has been proposed to process the effective diffraction information. Considering the particularity of processing diffraction pattern of thin cylinder, the Chebyshevâ…¡filter is taken as the filter of zero-phase filtering algorithm based on analysis of usual filtering algorithm. At last, comparison among zero-phase filter, median filter and mean filter is done by simulation and actual measurement. Experiment result shows that it can achieve better filtering effect.Application of linear CCD and area CCD is analyzed in measurement of thin cylinder diameter and the problem that how to pick up effective diffraction information from diffraction pattern received by area CCD is studied. To pick up characteristic diffraction information accurately, the effective diffraction information filtered by zero-phase filter is subdivided by interpolating cubic spline function.Fifthly, a new weighted algorithm has been proposed. This algorithm is more rational by total effective diffraction information, which overcomes the shortage of picking up characteristic diffraction information by part effective diffraction informationFinaly, two important parameters of diffraction model and several thin cylinders of different diameters are measured. Then the influence of diffraction angle to measurement result is analyzed, so did theoretical error between different diffraction models and relative measurement error. Measurement result shows that the relative error of this measurement system is 0.4%.