The large-size blade is the core component of nuclear power and ultra-supercritical steam turbine, and its existing dimension detection methods generally have the problems of low detection efficiency or insufficient accuracy. So, an efficient and high-precision measurement technology of full profile based on multi view structured light 3D measurement is proposed in this paper. And several key factors affecting the measurement accuracy are analyzed. Then a highlight elimination technology combining multi view and phase shift is proposed to eliminate the effect of specular on the smooth metal surface of the blade. A non-overlapping point cloud registration method based on feature constraints is presented to solve the difficulty caused by the lack of overlapping point cloud on the thin inlet and outlet edges of blade and other areas. Finally, a platform for the whole profile of large-size turbine blade is constructed to realize the full profile measurement and evaluation of large-size turbine blades. The measurement results show that the full surface point cloud of an about 900mm long blade is completely obtained by the measurement platform, and the measurement accuracy of 70μm is achieved through the comparison test of high-precision gauge blocks and some standard balls.
Against the overall data layering problem caused by the different precision of each component in the multi-visual structured light measurement system, a compensation method based on spatial interpolation with an error library composed of 3D point clouds is proposed. First, the corresponding relationship between each monocular structured light system and the reference binocular structured light system is established by the unique code value, which is determined by the orthogonal coded fringe patterns projected by different systems. And an error library is constructed by comparing the measuring coordinate values of datum points in different measurement systems. Then an Inverse Distance Weighted method (IDW) based on the K-D tree neighborhood is proposed to complete the error compensation of measuring points in the measurement space, so as to improve the data quality of the whole system after complementary fusion. Different from the traditional error compensation method, this paper focuses on the multi-view structured light measurement system, from the point cloud data end for the measurement end compensation, to achieve the comprehensive compensation of complex error. Finally, experimental results show that the proposed method has outstanding effect and practicability.
As one of the core components of a turbine, the quality of the blade manufacturing has a strong impact on the energy conversion efficiency of the turbine, where the key technology of quality evaluation of blades is point cloud registration. However, with the application of structured light three-dimensional measurement technology in full profile measurement, the typical point cloud registration methods only focus on the minimization of surface profile error, ignoring the position error relative to reference datum, which can easily lead to the misjudgment of qualified blades. In this paper, a new blade error evaluation method is presented to register the point cloud data scanned from a physical blade to its theoretical CAD model, which fits the two surfaces based on parameter priority. Firstly, qualified blades are quickly selected after global fine registration using the best-fit algorithm. Subsequently, based on the priority of position error parameters, the coordinate descent algorithm combined with the minimum zone criterion is adopted for local fine registration, which guarantees accurate evaluation results. Finally, the shape and position error of the actual blade is obtained accurately by calculating transformation parameters of registration and the deviation between the registered point cloud and its CAD model of the blade. Experimental results show that compared with state-of-the-art registration methods, the presented method gives higher priority to the parameters which are difficult to finish or repair by machining, and the position errors are controlled in the tolerance area, which effectively reduces the misjudgment. In addition, evaluation results of blade errors with the method are mainly reflected in profile, which is valuable for guiding the blade finishing or trimming in practice.
The linear translation stages play an important role in computer numerical control machine tools, coordinate measuring machines and semiconductor lithograph equipment. Six geometric motion errors are associated with a precision linear stage. High precision simultaneous measurement of multiple degrees of freedom is essential for the multiple-dimension motion calibration of the ultra-precision linear translation stage. In this paper, a laser heterodyne interferometer for simultaneous measurement of displacement and roll angle based on the acousto-optic modulator is proposed. In this optical configuration, a stabilize single-frequency laser (f0) and two acousto-optic modulators are used to generate the laser source. The frequency shifts of the two AOMs are f1 and f2, respectively. The positive first order diffraction beams of two AOMs, f0+f1 and f0+f2, are merged into one orthogonally and linearly polarized beam with a frequency offset of (f1-f2) as the laser source for the roll measurement-unit. A half wave plate (HWP) is utilized as the roll sensing plate to magnify the tiny roll angle into an observable phase shift based on the change of polarization state. The resolution of the roll measurement system is determined by the amplification factor which can be enhanced by a multi-reflection cavity made of a specifically arranged assembly square pyramidal mirror and a folding mirror. The zeroth beam f0 and the positive first order beam f0+f1 provided by two AOMs forms two space separated beams for the displacement measurement, which can eliminate the periodic nonlinearity error. The optical system of the simultaneous measurement of displacement and roll angle were setup. The experimental results showed good repeatability and good consistency.
In fringe projection profilometry, highlight usually causes the saturation and blooming in captured fringes and reduces the measurement accuracy. To solve the problem, a regional-projection fringe projection (RP-FP) method is proposed. Regional projection patterns (RP patterns) are projected onto the tested object surface to avoid the saturation and blooming. Then, an image inpainting technique is employed to reconstruct the missing phases in the captured RP patterns and a complete surface of the tested object is obtained. Experiments verified the effectiveness of the proposed method. The method can be widely used in industrial inspections and quality controlling in mechanical and manufacturing industries.
The roll angle measurement method based on a heterodyne interferometer is an efficient technique for its high precision and environmental noise immunity. The optical layout bases on a polarization-assisted conversion of the roll angle into an optical phase shift, read by a beam passing through the objective plate actuated by the roll rotation. The measurement sensitivity or the gain coefficient G is calibrated before. However, a relative tilt between the laser and objective plate always exist due to the tilt of the laser and the roll of the guide in the field long rail measurement. The relative tilt affect the value of G, thus result in the roll angle measurement error. In this paper, a method for field calibration of G is presented to eliminate the measurement error above. The field calibration layout turns the roll angle into an optical path change (OPC) by a rotary table. Thus, the roll angle can be obtained from the OPC read by a two-frequency interferometer. Together with the phase shift, an accurate G in field measurement can be obtained and the measurement error can be corrected. The optical system of the field calibration method is set up and the experiment results are given. Contrasted with the Renishaw XL-80 for calibration, the proposed field calibration method can obtain the accurate G in the field rail roll angle measurement.
Measuring objects with high dynamic range (HDR) reflectivity by coded structured-light, captured stripes are usually seriously distorted by reflectivity, causing inaccurate measurement results. A stripe enhancement method is proposed to deal with the problem. The method is based on the correspondence between phase and intensity of the stripe. First, the phase map of the captured stripe pattern is retrieved by phase-shift algorithm and multiexposure method, where saturation and low contrast of the stripe are eliminated; then, the modulation of stripes is normalized to eliminate the influence of reflectivity; finally, the enhanced stripe is obtained by assembling the modulation and the phase map. Experimental results demonstrate that the method is efficient for objects with HDR reflectivity and achieves high accuracy.
The coherent noise appears in the constructed image of digital holographic microscopy due to the laser source; thus, the imaging quality is degraded. A method of coherent noise reduction using a laterally shifting hologram aperture is presented. An original hologram with coherent noise is captured by a camera first. A series of holograms are sampled by laterally shifting the digital aperture in the original hologram. Instead of extracting the specimen’s part information, each sampled hologram, which includes the whole specimen, is reconstructed. The coherent noise is reduced by averaging the different reconstructed images. The experiment demonstrates the feasibility of the approach. The presented approach with a single recorded hologram realizes the coherent noise reduction without loss of spatial resolution, which is useful for real-time measurement.
The adaptive spatial filtering method is commonly adopted to extract the +1 term spectrum in digital holography for real-time dynamic analysis. However, the typical filtering method is not satisfactory for automatic analysis, because the reset of the filtering window is needed to extract the area of the +1 term spectrum. Therefore, an adaptive spatial filtering method based on region growing and the characteristic of the spectrum separation is proposed. Its filtering window is automatically formed by region growing. The key parameters, including threshold and seed point, are set by the intensity distribution of the hologram spectrum. Then the adaptive filtering extracting the +1 term spectrum is realized by multiplying the hologram spectrum by the filtering window. Compared to the typical filtering method, the experimental results of a microhole array and a phase step show that the proposed method has better adaptability and a higher precision. Moreover, the applicability of this method for different uses is also demonstrated by experiments with a microhole array and a phase step.
The telecentric arrangement in digital holographic microscopy (DHM), considered to be a pure-physical compensation for defocus aberration introduced by microscope objective (MO), shows shift-invariant behavior. Its optical arrangement requires precise adjustment of the distance between MO aperture stop and collimated lens. However, it is difficult to measure and quantify the distance even by monitoring the spatial frequency spectrum of recorded hologram in the absence of object. Thus the misalignment results in the residual defocus aberration in the telecentric arrangement. The total aberrations compensation for misalignment of telecentric arrangement in DHM is presented, in which a posteriori surface fitting method based on Zernike polynomials is performed to eliminate the residual defocus aberration as well as other primary aberrations. The approach reduces the difficulty in precise alignment of the telecentric arrangement and decreases the measurement error caused by aberrations in construction. Three-dimensional retrieval of the height for micro-hole arrays with high-spatial-frequency content demonstrates the feasibility of the method.
A method for improving the measuring accuracy of structured light measurement system, which adopts projecting stripe pattern to measure the three-dimensional profile, is presented. Based on the evaluation of the reliability of center extraction results, the improvement of accuracy is achieved by identifying and rejecting the stripe center extraction results with large error. Two parameters are used to evaluate the reliability of center extraction results. The first parameter is the average energy of the stripe, which is used to analyze and establish the relationship between the extraction accuracy and the signal-to-noise ratio through a statistical method. The second parameter is the asymmetric degree of the stripe gray distribution which introduces error into the center extraction, and a new method is proposed for measuring the asymmetric degree. Then, the criteria of the data rejection defined by the thresholds are presented, and large error data with low reliability are identified according to the thresholds. Higher measuring accuracy is achieved by rejecting the identified data. The validity of the method has been proved by experiments.
Digital holographic microscopy (DHM) has been widely applied for the topography measurement of microscopic
specimen. A total surface fitting method based on Zernike polynomials is presented to remove aberrations in DHM, in
which Zernike polynomial coefficients enable to provide quantitative measurement of primary aberrations. The phase
free of aberrations is obtained by subtracting out the surface fitting result from the reconstructed phase. The method
carries out the total phase aberrations compensation automatically by only one hologram, instead of knowing the
physical parameters of optical setup and the aberration mathematical model in advance. The optical system of off-axis
DHM is set up and the experiment results are given. Compared with the double-exposure method, the Zernike surface
fitting method obtains better phase information owing to removing residual tilt aberration.
In order to improve the accuracy and efficiency of weld defect segmentation in automatic radiographic nondestructive
testing and evaluation(NDT&E), an effective weld defect segmentation algorithm based on flooding has been developed,
which has the self-adaptive characteristics. Firstly, the defect’s feature points are extracted from the scale space of
radiographic films. Based on the information of defect points, the seed points and seed domains of defect discrimination
are adaptively determined, in which the defect segmentation seed will be searched. Then, aiming at the sparsity of weld
defects and canyon characteristics of 3D topographic map of defect regions, the drip-watering and water flooding have
been used for reference. The flooding is carried out by using line-flooding algorithm, in which water starts from defect
seed points and flows to the neighbor regions in order. On the basis of the flooding-area change and flooding-level
ascending velocity, the defect segmentation threshold values are determined and the weld defects also are segmented
from the radiographic films. At last, the comparative experiments have been carried out to compare the proposed
algorithm with the watershed segmentation algorithm and background subtraction segmentation algorithm. And the
experiment results confirm that the proposed algorithm obviously improves the accuracy and efficiency of weld defect’s
segmentation.
In order to obtain the non-overlapping and high-quality reconstructed image, this paper analyzes the system parameters
in digital holographic microscopy. Nowadays a few scholars have analyzed the system parameters which need to satisfy
the sampling theorem and spectrum separation conditions. In this paper, not only the sampling theorem and spectrum
separation but also the size relationship between the reconstructed plane and the magnified image are studied. Then
relationships of system parameters are proposed. First, the maximum object size is directly proportional to the
wavelength and microscope objective focal length, inversely proportional to the sampling interval. Second, the minimum
magnification is described accurately. Finally, the paper gives the range of recoding distance. Experiments further
demonstrate the proposed conclusion’s validity.
In the structured light three-dimensional measurement system, calibration is the key to the measurement accuracy; however, conventional calibration methods for the projector are either too complex or inaccurate. Therefore, we propose a simple and accurate method to calibrate the projector. In this method, the calibration points in the camera image plane can be mapped to the projector according the homography of the planar projection. In addition, an error surface compensation method is developed to minimize mapping errors caused by lens distortion of the camera and projector. As a result, the projector can be calibrated with the same method as the camera. Experiments are conducted to verify the effectiveness of the proposed method.
In the calibration process of structured light three-dimensional (3D) measurement system, the accuracy of the calibration
points' image coordinates directly influences the system's measurement accuracy. Based on the analysis of errors in
calibration points' image coordinates, mathematical models are built. A solution to eliminate errors in those image
coordinates is proposed according to the further analysis of the models, and calibration points are designed to be circle
for high-precision and steady extraction. The solution contains procedures as following: 1) A novel and real-time
algorithm is proposed, which is used for the correction of the non-uniform intensity in image caused by non-uniform
illumination and the camera's parameters. Taking preliminary extracted elliptical center coordinates and average gray
value of the ellipses as known information, the intensity distribution of calibration images can be obtained by
interpolation. Then the non-uniform intensity of calibration images is corrected in accordance with the interpolation
results. 2) High frequency noise in the images is filtered. 3) At last, error of asymmetric perspective projection is also
compensated based on its model. Simulation and experiment results indicate that this solution can efficiently reduce the
calibration errors.
This paper presents a novel color coding method which is applied to structured light measurement using fringe-pattern
projections. The method is based on time coding, and reduces the projection times of fringe patterns with the same
number of fringes by adding colors into the conventional fringe patterns. But the introduction of colored fringes also
brings a difficulty in image processing, that is color crosstalk. To address this problem, sinusoidal fringe patterns with
four colors of white, red, green and blue are used to project to the measured object surface to reduce the edge effect.
Based on those ideas and methods, a three-dimensional measurement system is built, and the direct linear transform
(DLT) method of calibration with lens distortion corrected by cross ratio invariance principle is used to calibrate the
internal and external parameters of the system. Then the parameters of the system are optimized by the bundle
adjustment method. Finally, a standard metal hemisphere with spraying is measured, and the experimental results show
that with the same times of projection, the lateral measurement resolution is higher than the traditional method, and the
measurement accuracy of the depth direction is about 0.27mm.
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