• Infrared and Laser Engineering
  • Vol. 53, Issue 1, 20230444 (2024)
Chunyan Li, Danlin Li*, Jihong Liu, Chang Liu, Ke Li, and Jiewei Jiang
Author Affiliations
  • School of Electronic Engineering, Xi'an University of Posts & Telecommunications, Xi'an 710121, China
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    DOI: 10.3788/IRLA20230444 Cite this Article
    Chunyan Li, Danlin Li, Jihong Liu, Chang Liu, Ke Li, Jiewei Jiang. Research on jitter compensation algorithm in spectral confocal thickness measurement system[J]. Infrared and Laser Engineering, 2024, 53(1): 20230444 Copy Citation Text show less

    Abstract

    In order to obtain multi-point data of the sample, the spectral confocal displacement sensing system will produce jitter effect when moving the measurement, causing the drift of the measurement data. Based on the realized spectral confocal thickness measurement system, the effect of jitter is studied and the jitter compensation algorithm is explored; Firstly, based on the spectral confocal thickness measurement model and the presence of jitter when the probe is tilted to a certain extent relative to the optical axis, the relational model of the effect of jitter on the thickness measurement is deduced, and the thicknesses of four kinds of samples with different degrees of random jitter are analyzed by Monte Carlo simulation method. The analyzed results are compared with the Monte Carlo simulation results to verify the correctness of the expression of the thickness probability density function. The results show that the jitter effect leads to a degradation of the measurement performance, especially when the sample thickness is large; In the case of a large standard deviation of jitter, the measurement of thinner samples has a better anti-jitter performance. Then, in order to compensate the effect of jitter on the measurement results, it is proposed to use Savitzky-Golay filtering and Gaussian fitting to realize the filtering and the extraction of the peak wavelength of the spectral signal, and the jitter error compensation algorithm is established; Finally, experimental measurements were conducted on a sample with a thickness of (1.0±0.1) mm, and the average thickness was measured to be 1.064 0 mm. The compensated relative standard deviation was 0.29%, verifying the effectiveness of the jitter compensation algorithm. This research has some guiding significance to improve the system measurement stability and measurement accuracy.ObjectiveWith the development of miniature integrated optical instruments such as optical communications and optical sensing, the use of transparent materials is becoming more and more stringent. Highly accurate thickness measurement parameters help guide their precise application and control the performance of related ultra-precision optical instruments, making accurate thickness inspection necessary. Spectral confocal method uses a broad-spectrum light source to irradiate the surface of the object, uses the principle of optical dispersion to make the dispersive objective lens produce axial chromatic aberration, establishes the correspondence between the dispersive distance and the wavelength, and uses a spectrometer to detect the peak wavelength of the spectrum that is focused on the surface of the object and reflected back to get the accurate axial position or micro-displacement data. Such an approach allows to break through the diffraction limit of ordinary optical microscopes. It achieves ultra-high ranging resolution on the nanometer scale and has wide adaptability to environments and materials. When measuring the thickness of transparent materials using the spectral confocal method, the jitter effect alters the refractive properties of the beam entering the sample and random noise is present in the received spectral response curve reflected from the sample surface, which leads to drift of the measurement data. On this basis, the relationship model of the effect of jitter on the spectral confocal thickness measurement is firstly derived in this paper, and the distribution of the thickness probability density function of the sample under different degrees of random jitter is simulated and analyzed by Monte Carlo method. In order to compensate for the effect of jitter on the measurement results, it was proposed in this paper to use Savitzky-Golay filtering and Gaussian fitting to extract the peak wavelength of the spectral signal, and a jitter compensation algorithm was established. Finally, the stability of the measurement results is improved by experimental measurements, and the effectiveness of the algorithm is verified. MethodsThe effect of jitter on thickness measurement of transparent materials is studied in this paper. First, the thickness measurement models of the probe with respect to the optical axis were derived when the probe was not tilted (Fig.2) and tilted (Fig.3), and the influence of jitter on the thickness measurement results was characterized by the optical axis tilt, and simulation analysis was carried out (Fig.4). Then, by comparing thickness measurement errors and jitter standard deviation at different wavelengths (Fig.5), after comparative analysis of various algorithms, spectral noise was filtered by Savitzky-Golay filtering algorithm (Fig.7-8), and peak wavelength of spectral signal was extracted by Gaussian fitting algorithm (Tab.1). An optimized jitter compensation algorithm is constructed. Finally, the validity of Savitzky-Golay filtering algorithm and Gaussian fitting algorithm for jitter compensation in spectral confocal thickness measurement is verified (Tab.2). Results and Discussions The thickness of the sample under static conditions depends only on the focusing wavelength, the angle of incidence and the refractive index of the transparent material. Random jitter angle is the main source of thickness measurement error, and the thickness measurement error caused by sensor probe jitter should not be neglected. By analyzing the effect of random jitter angle on the measurement error, a jitter compensation mechanism is established to reduce the measurement error. Thickness measurement data is non-central cardinality distribution, the jitter effect will lead to the measurement performance degradation, especially in the case of larger sample thickness; In the case of larger jitter standard deviation, thinner samples have better anti-jitter performance. The peak wavelength of the spectral signal is extracted by S-G filtering and Gaussian fitting, which can reduce the error caused by the mechanical vibration of the probe and improve the measurement stability of the confocal spectroscopy measurement system. ConclusionsIn this paper, based on the spectral confocal method to realize the thickness measurement of transparent materials, the jitter effect generated by the movement will make the measurement data drift, and the influence of the jitter effect on the thickness measurement is systematically studied. Firstly, the relationship model of the effect of jitter on thickness measurement is established based on the principle of spectral confocal thickness measurement system, theoretical derivation is carried out, and MC simulation is used for simulation verification. Secondly, the thickness PDF and MC simulation results are compared and analyzed to verify the correctness of the thickness PDF expression. The results show that the jitter effect leads to a degradation of the measurement performance, especially when the sample thickness is large; In the case of a large standard deviation of jitter, the measurement of thinner samples has a better anti-jitter performance. In order to correct or compensate the effect of jitter on the measurement results, it is proposed to use S-G filtering and Gaussian fitting to realize the random noise filtering and the extraction of the peak wavelength of the spectral signal, and the jitter error compensation algorithm is modeled. Finally, experimental measurements were conducted on a sample with a thickness of (1.0±0.1) mm. Under stable conditions, the average thickness was measured to be 1.064 0 mm. The relative standard deviation of the moving measurement results was reduced from 1.86% before compensation to 0.29% after compensation, verifying the effectiveness of the jitter error compensation algorithm proposed in this paper and proposing improvement measures. The results of this paper have certain guiding significance for optimizing the system structure and further improving the performance of the system, and have certain advancing effect on the practical application of the spectral confocal displacement sensing system for stable measurement.
    Chunyan Li, Danlin Li, Jihong Liu, Chang Liu, Ke Li, Jiewei Jiang. Research on jitter compensation algorithm in spectral confocal thickness measurement system[J]. Infrared and Laser Engineering, 2024, 53(1): 20230444
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