Research On Thickness Detection System Based On Spectral Confocal

With the rapid development of advanced precision manufacturing in china,the industry community's demand for precision measurement of thickness and other parameters has increased.In many fields,such as mobile phone glass panels,optical lens processing,and the processing of opaque materials such as aviation aluminum,there are strict requirements for thickness parameters.Because some products have high temperature and other characteristics that are not easy to contact during the production process and contact detection will cause damage to the product surface,non-contact thickness measuring system research is very necessary.Spectral confocal technology is suitable for thickness measurement due to its high accuracy,high detection speed and wide application range.Spectral confocal system can realize white light and encode the functional relationship between the focus distance and focus wavelength.This feature is the basis of the spectral confocal thickness detection system.Spectral confocal overcomes many limitations of the detection system and is widely used in glass processing,mobile phone appearance detection,semiconductor chips and other fieldsThe main research content of this thesis includes the following aspects:The principle of spectral confocal and the feasibility of using spectral confocal for thickness measurement are discussed.The research situation of spectral confocal at home and abroad is introduced.The measurement scheme of spectral confocal system is proposed and various performance indicators of the system are analyzed.In this thesis,the parameter model of confocal probe and the mathematical model of thickness measurement of single confocal probe system are established.Aiming at the problem that the classical compensation model does not consider the relationship between the refractive index and the wavelength of the plate material,it is proposed to use a reference plate to calibrate the relationship between refractive index and the wavelength,and use polynomial fitting and solve the Sellmeier parameter to obtain the refractive index with respect to the wavelength functionA mathematical model is established when the surface of the tested plate is tilted,the influence of the tilt of the plate on the thickness measurement error is analyzed,and the drift phenomenon caused by the tilt of the plate on the peak wavelength of the surface return spectrum signal is analyzed.A method of fixed peak wavelength and maximum it's light intensity is proposed to calibrate the parallelism between the confocal probe and the plate.A mathematical model for measuring the thickness of an opaque plate of a two-way confocal spectral confocal thickness detection system is established.Aiming at how to determine the confocal distance in the model,a reference plate is proposed to calibrate the confocal distance.Analyzing the influence of the tilt of the plate relative to the optical axis on the thickness measurement error,and the amplification effect of the two confocal probes' misalignment of the optical axis on the error,it is proposed to use the confocal probe' s maximum received light intensity method to align the system' s optical axis.The modulation effect of the light source spectral characteristics on the received spectral signal and its influence on the peak wavelength decoding are analyzed,and the difference method is proposed to remove the spectral characteristics of the light source to eliminate the effect of the light source spectrum on the spectral signal.A two-way confocal spectral confocal thickness detection system was built using dispersive confocal probes and spectrometers,wide-spectrum light sources,Y-shaped optical fibers,and five-axis displacement platforms.The system measures Scope reached 1.992mm.Based on the QT platform,the measurement software is developed for the system using C++language,which integrates various measurement functions of the system.To solve the problem of inaccurate optical axis of the system,four steps of optical axis calibration are proposed.The two confocal probes were calibrated with a laser interferometer,and the actual coding relationship between the focus distance and the focus wavelength was obtained.Use a reference plate with a thickness of 0.552mm to calibrate the confocal distance of the system that has been calibrated by the optical axis,and measure the thickness of a plate with a thickness of 0.700mm.The average measurement error of 10 different positions within the measurement range is 3.57um;using a single confocal probe Calibrate the material's refractive index for the reference plate with a thickness of 0.542mm,and use it to detect the thickness of a transparent plate of the same material with a thickness of 0.53mm.The average measurement error is 2.66um.