Key Technology Of Free-form High-efficient Measurement Based On Confocal Scanning

Optical free-form surfaces can significantly improve the design freedom and the imaging quality of modern optical systems,and provide necessary technical support for the development of modern optical systems in lightweight and refinement.Therefore,they are widely used in space optics,augmented reality,and other fields.With the increasing demand for free-form surfaces in modern optical systems and the continuous improvement machining efficiency of free-form surfaces in intelligent manufacturing,higher demand for free-form surfaces measurement efficiency is put forward.The coordinate measuring machine(CMM)and profilometer based on confocal sensors have become the main measurement technology for the three-dimensional topography measurement of large-diameter free-form surfaces due to the advantages of nondestructive and high versatility measurement.However,the low free-form surfaces measurement efficiency caused by the confocal axial tomography mechanism limits the further development and application of confocal scanning instruments.In addition,the long measurement time of free-form surface will lead to the increase of measurement error caused by temperature change,mechanism drift,and other factors,and then affects the measurement accuracy of free-form surfaces.Therefore,improving the confocal scanning measurement efficiency of free-form surfaces is an important issue to be solved urgently in the surface metrology field and the development of scanning optical probe measuring instruments.The purpose of the study on “Key technology of free-form high-efficient measurement based on confocal scanning” is to solve the low scanning measurement efficiency problem of CMM and profilometer based on confocal sensors and to provide key technology and theoretical support for realizing high-efficiency and non-destructive scanning measurement of large-aperture elements represented by free-form surfaces.The major issues addressed in this study are as follows:Firstly,the research of confocal dynamic tomography scanning technology was carried out to solve the low scanning measurement efficiency of free-form surfaces caused by the traditional confocal axial tomography scanning mechanism.The imaging characteristics of traditional confocal axial tomography were analyzed based on scalar diffraction theory,and a dynamic tomography scanning model was established for simultaneously axial and transverse scanning.The dynamic tomography scanning measurement is realized by tracking the best focus position on the measurement surface,and the influence regularity of the axial and transverse scanning speed ratio and the surface form of the measurement sample is discussed.The experimental results show that compared with traditional axial tomography technology,the measurement time of the optical element profile measurement based on confocal dynamic tomography technology is reduced by 90%.Secondly,to realize the high-efficiency scanning measurement of the three-dimensional topography of the free-form surface,the study of the scanning trajectory prediction method for the measurement of the unknown model free-form surface is carried out.In the framework of Bayesian theory,Gaussian process regression is used for mathematical modeling to achieve free-form surface reconstruction.This method can not only give the results of free-form surface reconstruction under the condition of a small number of sampling points but also give the uncertainty of free-form surface reconstruction results.According to the uncertainty of reconstruction results,the sampling point distribution of free-form surface measurement is guided to realize the global dynamic sampling of free-form surface,and the approximate surface of the original surface is obtained,which provides the scanning trajectory for the subsequent efficient scanning measurement of free-form surface.Thirdly,aiming at the problem of low scanning measurement efficiency caused by data redundancy of traditional free-form surface uniform sampling strategy,the sparse sampling strategy and surface reconstruction method of the free-form surface were studied.The random sparse sampling strategy was used to plan the number and distribution of sampling points of free-form surface measurement at one time,and the scanning path of sparse sampling points was optimized by the genetic algorithm.Then,the low-rank regular objective function was established based on the inherent low-rank characteristics of the free-form surface and solved by the alternating RMSprop algorithm.The reconstruction of the free-form surface under the Nyquist sampling rate was realized,to improve the efficiency of three-dimensional scanning measurement of free-form surface.The experimental results show that,compared with the traditional uniform sampling strategy,the sparse sampling strategy based on low-rank matrix recovery theory can effectively reduce the number of sampling points,and the three-dimensional scanning measurement time of free-form based on confocal dynamic tomography scanning technology can be reduced by 36%.Lastly,the integration research of the three-coordinate scanning measuring instrument based on the confocal sensor is carried out,which provides profile and surface detection means for large aperture optical elements,especially for complex free-form surfaces.The realization method of confocal dynamic tomographic scanning measurement based on a resonant mirror scanning device and the key technology of confocal optical system design is described.A comprehensive device of a multi-function three-coordinate scanning measurement instrument based on confocal sensing is designed and integrated.The instrument measures the generatrix profile of large-aperture optical components and the three-dimensional topography of typical free-form surface components.