Study On The Combining White Light Interferometry And Laser Scanning System For Topography Measurement

With the rapid development of the communication,aerospace,and machinery manufacturing industries,the demand for ultra-precision machining parts is dramatically increasing,and the surface morphology and structure of the developed products are becoming more complex.Therefore,there is a need to develop approaches to inspect the surface topography of ultra-precision machined mechanical parts and optical components in the field of manufacturing.Previous studies obtained the basic shape of the surface topography and the roughness information which are only suitable for collecting the surface contour line.However,a white light interference(WLI)method suitable for a surface detail analysis is applied in local measurement,limitting the performance analysis of an optical workpiece surface morphology.Therefore,it is necessary to carry out a more in-depth study on a wide range of full-frequency surface topography measurement methods.This research is focused on both WLI and laser scanning measurement approaches.A laser scanning method can be used to pre-measure the measured a surface quickly,and the feature information of the surface to be measured can be fully understood through an analysis,and the local detail information can be obtained using the WLI method.Finally,a high-precision miniaturized measurement system can be developed and integrated,which can be used for both in-line measurement and off-line measurements independently.The approaches described in this thesis can be summarized as follows:1)A new white light scanning interferometry(WLSI)measurement algorithm is proposed,which combines white light interferometry phase shift and Fourier transform(FFT)coherent peak sensing technology to effectively solve the problem of positioning errors in the maximum modulation,eliminate the bat wing effect of step edge,and avoid the error detection of a coherent peak.With this algorithm,high-speed WLPSI measurements are applied to determine the maximum modulation of the WLI interference signal and to identify the coarse profile of the test surface regions separated by a large step,based on a determination of the surface discontinuities,which is calculated to correct for slight displacements and phase shifts that may have occurred.Within the range of surface discontinuities,through a correlation analysis of the WLI envelope curves,the FFT method can be used to reposition the maximum modulation.Finally,by using a polynomial threepoint fitting method,the estimated Fourier peak positions are accurately determined,a series of height values based on these reference positions are obtained,and the final surface height is determined from the average of these height values.To a certain extent,this method effectively overcomes the disadvantages of the traditional WLPSI and FFT methods,and can improve the accuracy of the height measurements in steep regions and in areas with large inter-pixel steps on the test surface.Based on this,a high-precision and miniaturized of WLSI measurement probe is developed.Through the design and modeling of the WLSI system,including the hardware and software design of the system,the accuracy and stability of the developed system are tested.2)This auto laser focusing measuring system aims at combining a high-precision nano-displacement PZT device with a micro-objective lens,allowing the recognition and tracking of the focusing error signal,and realizing continuous monitoring and automatic surface measurement through a high-precision displacement.During the process of continuous surface tracking measurements,the linear range of the 4QD is used.In addition,the micro objective lens is driven by a nano-displacement device allowing a corresponding adjustment such that the light spot projected onto the tested surface.A computer is employed to record and save the location of the nano-displacement device at multiple focusing positions in real-time,indirectly obtaining the contour information of the object to be tested.In this way,a traditional focusing laser measurement system can be adjusted,overcoming the traditional linear range,and expanding the application of the focusing laser measurement system.The miniaturized focusing laser probe includes the following contents:(1)The measurement range of the laser head module should be enlarged.It is found that different multiple microscopic objective lenses in a commercial laser reading head module can be replaced,and different measurement ranges can be realized,along with a larger range of surface profile measurements.(2)By designing a stable focusing error signal conditioning circuit,the noise of the measuring focusing error signal is reduced.Thus,the sensitivity and stability of the system is proved.(3)The performance of a new modified polynomial fitting algorithm in the microdisplacement measuring system is reported,which can effectively extend the linear measurement range of 4QD.The miniaturized laser measurement system can break through the limitations of a traditional non-contact measurement system.It has the advantages of a small volume,low cost,and nanoscale vertical resolution measurement capability.3)A new method for a laser focusing probe that does not require a calibration of the linear range measurement was developed.A PZT is used to realize a closedloop system for high-speed measurements.The measurement process does not require a linear measurement range of the FES to be calibrated,and is suitable for measuring the vibration desensitization in a harsh environments.The proposed algorithm combines PZT periodic scanning least squares fitting with a regression analysis to find the best focus position in terms of probability.This measurement method does not require linear calibration of the FES curve,and the flexibility of the PZT stroke extends the longitudinal measurement range of the laser probe.The new fast autofocus algorithm based on the principle of laser astigmatism and PZT periodic scanning is proposed,including that(1)through PZT vertical scanning,the FES curve of the surface of the object to be measured is obtained;(2)the two-dimensional initial points of FES voltage and PZT voltage are determined at the same time when they are close to the focus by using fast sorting and the functional properties of FES curve;(3)the PZT voltage is obtained by the least square fitting or regression analysis based on the Levenberg Marquardt algorithm using the two-dimensional data points collected in the second step;(4)obtain the relative height of the surface to be measured along the optical axis.4)Aiming at applying WLSI and a laser auto focusing scanning(LAFS)method as the core,a WLSI-LAFS integrated composite measurement system is proposed.An optical path of the integrated composite system is designed,through which the fast laser point measurement method assists with the fast positioning of the WLI measurements,and can be used to carry out a full frequency analysis of the surface data.Using a reasonable structure and an optical path design,the unity in the coordinate system data of the two measurement methods can be achieved,and thereby ensure that they can coordinate with each other during the measurement process.The optimization design of the measurement path based on the construction of the measurement system is studied.The critical points in this research are a unification of the optical axis,the alignment method of the initial measurement benchmark,and an improvement in the measurement efficiency.Among these,the WLI method can quickly realize the nano-precision measurements of the surface topography within a small field of view,because there are numerous sampling points in the local measurement.Owning to the large number of sampling points,a complete full-frequency surface information acquisition can be achieved.For large-scale surface topography,it is necessary to merge multiple single-view data.Thus,the laser scanning method introduced in this measurement system can be used to quickly predict the measured surface.In addition,the characteristic information of the surface to be measured is comprehensively understood based on an analysis.Furthermore,a fast measurement and high precision mosaic are both realized when employing WLI to achieve a wide range of surface topography measurements.5)The integrated WLSI-LAFS measurement scheme and a path planning of fullfrequency 3D surface measurement are studied.The miniaturized in-line measurement system and ultra-precision machine tool are established.Among these,measurement path planning is realized through a linkage control of a multi-axis motion system.To improve the measurement efficiency and accuracy,the LAFS part was applied as a useful supplement to a WLSI measurement.After an ultra-precision machining process,the spindle and guide rail of the machine tool drives the in-line measurement system to move along the designed measurement path.Therefore,an inline measurement of the freeform optical surface of the PMMA is conducted.In addition,an optical path system is integrated to realize both miniaturization and inline measurements of the freeform optical surface and improve the manufacturing accuracy.The experimental results show that the measurement system can achieve repeated nano-scale measurements of optical freeform surfaces with low reflectivity.