Research On Key Technologies Of Non-contact Surface Measurement For Optical Reflectors

Aspheric and freeform optical reflectors are essential components of optical systems.By applying optical reflectors,the aberrations can be reduced while at the same time also reducing the required number of components,the energy loss,the size and the weight of the system,which is the reason that optical reflectors are widely used in applications such as large aperture telescope,space remote sensing,manufacturing of complex freeform surfaces and precision engineering.While,fabrication of large(off-axis)aspheric and freeform optical reflectors is time-consuming and high-cost.With the development of advanced optical manufacturing,especially the emergence of computer controlled local grinding and polishing technology,deterministic optical machining method in the mode of "coarse grinding(or milling)-fine grinding-polishing" are wide used optical shops.Although the deterministic grinding and polishing can ensure the high machining precision of optical reflectors,in every fabrication process the achievable precision is only as good as the measurement method.Suitable metrology is key in increasing machining efficiency and ensuring a smooth transition from coarse grinding(or milling)to fine grinding,but not available.Coordinate measuring machine(CMM)is widely used to measure surfaces with large form error and rough surface for its high universality,which is the most likely method for transitioning from coarse grinding to fine grinding.But traditional CMM faces a number of new challenges.Firstly,although CMM equipped with non-contact probes can overcome the problem of low efficiency when using contact probes,large measuring range and high precision cannot be obtained both.Secondly,Although it is much more time-saving to use spiral scanning mode rather than traditional raster scanning mode,the centering problem which is to align the probe with the rotation center of the spindle need to be solved.Thirdly,measuring the edge of a workpiece and planning a scanning trajectory are very difficult for traditional CMM.Full area covered profile measurement of irregular-shaped reflectors is a new challenge.At last,manual measurement or special equipment is needed when measuring surface defects for the reason that almost all the profile measurement systems don't have the mentioned function.It is of great value to integrate the function of surface defect detection into the surface measurement system.With the aim of giving effective solutions to the above issues,this thesis proposed several methods and key technologies on non-contact surface measurement of optical reflectors.The contents of this thesis including six chapters are summarized as below:In chapter 1,an introduction of the research background of surface metrology methods for optical reflectors is given.The application prospect of optical reflectors and fundamental theory of profile measurement are also introduced.Besides,various methods that are currently adopted in surface measurement are presented,together with their advantages and disadvantages.Technical difficulties encountered and measurement requirements are discussed,followed by the development of non-contact coordinated measuring technology,the comparison of domestic technology and advanced foreign technology,and the deficiency of current technology.Finally,the main content and significance of this paper are sketched.In chapter 2,auto-focusing technology based on voice coli motor(VCM)motion stage is presented.High precision probes currently use in generally have the disadvantage of short measuring range,which leads to the difficulty in measuring surface shape with high PV.Surface tracking method using VCM motion stage based auto-focusing technology is an effective way to solve this problem.Thus,two auto-focusing system with different precision and tracking speed,i.e.mechanical guide and air-bearing guide,are put forward based on the development of VCM motion system and the realization of auto-focusing technology.Details about the development of components from mechanical guide subordinated devices to the whole system are discussed,including design and manufacture of VCM,design of interpolator for encoders,control system and system debugging strategy.Then the advantages of air-bearing system and the difficulty in developing are introduced.And the assembling technologies of air-bearing guide are presented.A cheaper and easier technology which can achieve better positioning accuracy and higher tracking speed is presented.Besides,control strategy,debugging strategy and performance evaluating method that can be applied to similar systems are presented.In chapter 3,a new surface measuring system is put forward.And the development of a prototype of orthogonal CMM including the design,building and control of motion systems,measurement and compensation of motion error,design and development of high precision surface measuring system,strategy of measuring signal acquisition,anti-interference method and software development is presented.After the prototype was completed,system performance parameters such as resolution,drift and repeatability are measured.In chapter 4,a novel center alignment method of the measuring probe is proposed.Spiral scanning is a high-efficiency scanning mode for surface profile measurement systems.The most important priority to realize the spiral scanning mode is to accurately align the measuring probe with the rotational center of the spindle.This paper proposes a novel center alignment method of the measuring probe,which is considered to be suitable for any type of spiral scanning surface measurement systems.The proposed method,which only needs a tilted flat mirror as the artefact, makes the time-consuming center alignment process of the measuring probe become much easier and faster.The operational steps of the proposed method are presented.Experiments have also been carried out based on a self-developed optical profiler with spiral scanning operation to verity the feasibility of the proposed method.The experimental results show that the proposed method is capable of conducting a fast alignment(only takes 3 mins)while maintaining a high alignment accuracy.Evaluation of the alignment accuracy shows that the centering error is less than 10?m on the mechanical guide rail stage and about 1.7?m on the air-bearing stage.In chapter 5,technologies of full area covered measurement of irregular-shaped reflectors and surface deflect measurement are presented.The working principle of the chromatic confocal displacement sensor(CCS)is introduced firstly,after which the engineering application value of the signal intensity of the CCS is analyzed.Then optimization of the self-developed surface measurement is proposed to carry out the full area covered measurement of irregular-shaped reflectors.The proposed method was verified by a measurement experiment.To integrate the function of surface deflect measurement into the proposed surface measurement system,the mathematical model of "slope angle-displacement-signal intensity" is established.A newmethod of measuring the surface profile and the surface deflect simultaneously is proposed.A workpiece with surface deflect of sub-millimeter-scale was successfully measured.In chapter 6,the research contents and innovation points of this thesis is summarized,and some future works are discussed.