The prediction result of Remaining Useful Life (RUL) of aero engine determines the timing of engine maintenance according to the condition, which is of great significance to the operation safety of the engine. In order to improve the prediction accuracy of aero-engine residual life, a chaotic genetic algorithm optimization sequential convolutional network (TGA-TCN) based residual life prediction method is proposed. The time dependence relationship of time series data is constructed based on time series convolutional network, and the optimal network structure is constructed by genetic algorithm and hyperparameter design. A turbofan engine degradation dataset (C-MPASS) is used to verify that the prediction accuracy of the model is significantly improved than that of CNN, LSTM, TCN, GA-TCN, etc.
Point diffraction interferometer (PDI) has become the most promising measurement tool since it is firstly proposed by Linnik and its key component used to generate the reference wave can be called as the reference wave source (RWS). A new RWS based on the silicon nitride (SiN) waveguide is now proposed, aimed at providing a spherical reference wave with high NA and high accuracy. Simulation results show that the PV and the RMS of the reference wave generated by the waveguide RWS are 2.86×10-4λ (λ=532nm) and 4.83×10-5λ respectively, and the maximum light transmittance of this RWS could reach 24%. In addition, the NA of the reference wave reaches up to 0.58.
This paper introduces a spherical optical surface defects evaluation system (SSDES) based on the dark-field microscopic scattering imaging (DFMSI) method. The specially designed annular illuminant with variable aperture angles ensures the condition of DFMSI for spherical optical components with variable surface shapes and radii of curvature. On account of the small imaging field of view (FOV) of the SSDES relative to the large spherical optical component under test, the scanning path for subaperture images is planned along longitudes and latitudes of the spherical surface to detect the whole surface. Besides, for avoiding the misplaced subaperture images stitching due to the decenter error, a centering system is utilized to perform the alignment of the optical axis of the spherical optics in relation to the reference axis before capturing subaperture images. Then we propose a defect evaluation method, primarily involving the threedimensional (3D) image reconstruction and global coordinate transformation, the projective stitching of 3D subaperture images, and the quantitative evaluation of defects, to process the captured spherical subaperture images. Experiments results are shown in good accordance with the OLYMPUS microscope for the relative error within 5%, and validate the SSDES to the micrometer resolution.
The point diffraction interferometer (PDI), emerging spherical wave used as the reference wave, is one of potential wavefront testing tools with very high accurate. However, it is not feasible in the in situ measurement of the surface error in the polishing process. For the test mirror with low reflectivity, the poor fringe contrast would be formed. It could increase the phase noise, which can’t be removed during phase retrieval, thus limiting the measurement accuracy. Therefore, the polarization point diffraction interferometry (PPDI) with even aspheric quarter-wave plate (EAQWP) is proposed for high-precision testing of the high-NA spherical mirror. The EAQWP is employed in the PPDI to transform the polarization states of the test and reference beams, then a good fringe contrast can be realized by adjusting the relative intensities of two interfering waves. Compared with the common quarter-wave plate (QWP), the substrate configuration of designed even aspheric surface reduces significantly the introduced wavefront distortion for large-NA measurement, at the same time, allows to use the same position and orientation of the EAQWP for testing mirrors with different NAs, which facilitates the system alignment in practical optical shop testing. Most importantly, the corresponding pose errors are corrected by computer-aided alignment with a difference restoration model. The viability of the new method and system are verified with an experiment for testing a spherical mirror with 0.04 reflectivity and 0.5 NA, and with a ZYGO interferometer match.
Point diffraction interferometer (PDI) uses the nearly ideal spherical wave front diffracted by the pinhole as the reference, thus its measurement accuracy depends mainly on the quality of the diffraction wave front. However, aberrations and numerical aperture (NA) of the microscope objective (micro-objective) used to illuminate the pinhole in PDI can affect the quality of the diffraction wave front and further reduce the measurement accuracy of PDI. It’s necessary to analyze the influence of the micro-objective on the quality of the diffraction wave front detailly. In this paper, numerical simulation of the propagation of the incident light through the pinhole is carried out with the method of the finite difference time domain (FDTD), vector diffraction integral and vector diffraction theory. Then energy transmittance, homogeneity and deformation are obtained, which are used to evaluate the influence of the micro-objective on the quality of the diffraction wave front. These simulation results will provide a reference for rational selection of the micro-objective parameters, contributing to establish PDI system with best measurement accuracy.
As a high accurate measurement for wavefront metrology, the point diffraction interferometer(PDI) has been developed to overcome the accuracy limitation in traditional interferometers enabling the measurement precision in the order of a subnanometer. The PDI employs a nearly ideal spherical wavefront generated by pinhole diffraction as the reference wavefront, and is expected to be a powerful tool for high-precision optical testing. The diffraction reference wavefront is the key factor which determines the achievable precision in the measurement. In order to achieve high measurement accuracy precise characterization of the properties of diffraction wave is required. The structure of the pinhole functions as a cylinder waveguide and the fields in the pinhole are described as sums over waveguide modes whose behavior are determined by the interaction between the sidewall and the light. The pinhole functions as a cylindrical waveguide, in the way it determines the light field in the hole and the properties of the diffraction wave. In the paper, we make clear the physical mechanism of pinhole diffraction. The vector diffraction theory and the field equivalence principle are discussed. The field in the pinhole is analyzed according to the waveguide theory, and the waveguide effect on pinhole diffraction transmittance is discussed. Our results provide an important theoretical reference for design of the PDI system.
In order to achieve the precision of sub-nanometer and even higher of surface testing, this paper presents a highprecision point diffraction interferometric system for high-NA spherical surface testing. The point diffraction mask as a key component is designed and analyzed in detail. The circularly polarized light is used as the light source of interferometer system to calibrate the oblique-reflection wavefront aberration introduced by the point diffraction mask. Besides, the calibration of adjustment error is a critical issue in the test of high-numerical-aperture spherical surface, and it is hard to separate the high-order aberrations introduced by wavefront defocus from the mearsured data. We present a novel calibration method based on the Zernike aberration coefficients introduced by wavefront tilt and defocus. This novel method can be carried out without knowing the actual adjustment error amount and just needs the numerical aperture of testing wavefront to solve the high-order aberrations. With the proposed calibration method, the requirement for accuracy of the mechanism and experience of operator is lowered. Experiment shows the accuracy of calibrated system is better than 0.001λ rms, which can realize the high-precision testing.
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