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Fourier transform siliconds (FTP) has the advantages of single frame acquisition, full analysis, and high resolution. With the development of computer technology hardware and software, the improvement of image acquisition equipment resolution, Fourier transform profile is subject to people's attention, and has become one of the most important and most active research fields in three-dimensional sensing.
The three-dimensional object contour measurement is performed by the principle of the grating projection method. The key is to select the appropriate parameters to ensure system measurement accuracy and measurement range, and ensure complete separation of Fourier spectrum. In this focus, the concept of equivalent wavelength is discussed, and the parameters in Fourier Transform Oil Translation (FTP) are selected and optimized. In-depth research in both theoretical and practical.
1. Optimization and choice of parameters in FTP
Fourier transform silicond artificial optical path diagram As shown in Figure 1, g represents a sinusoid grating, P represents the projection system, c represents the CCD camera, and S represents a monitor. In the case where the connection between the camera and the projector is parallel to the reference plane and the camera optical axis and the projector optical axis are parallel, the height and the phase difference between the measured objects and the phase difference is the mapping relationship. for:
In the formula, L is the distance from the CCD camera to the reference plane; D is the distance from the projection system to the camera into the camera; TGθ = D / L; θ is an angle between the projector optical axis and the optical axis of the camera. As can be seen from the formula (1), the accuracy of the object reconstruction depends on the measurement accuracy of the bit phase Δ (x, y), the base frequency F0 of the grating image, and the angle θ between the optical axis of the projector and the optical axis of the camera. Define equivalent wavelength: λc = p0 / tgθ, used to characterize the measurement accuracy of the system, the longer the equivalent wavelength, the lower the system's measurement accuracy.
The measurement accuracy of the system with λc and (DH / Dψ) is consistent. Define local spatial frequencies:
The (F0-FB) / F0 must be compared to 1/3 to determine the selection of θ. Regardless of the influence of the zero frequency, it is assumed that the angle angle when each spectrum is completely separated, then θc is determined by the following formula:
3, experimental verification of human face mask model
In order to verify the above problems, an actual measurement is performed on a face mask model. Its projection image is generated by a computer and projected from the liquid crystal projector (LCD) to the surface of the measured object. The system structure parameters are:
Here, the three-dimensional phase network of the parameter selection and optimization is compared to Fig. 11, which is clearly seen that the zero-frequency component and the fundamental component are avoided more effectively after parameter selection and optimization images. The spectral spectrum is mixed, and the measurement accuracy of FTP is improved, and Figure 10 is more accurate and clear, and the error has been significantly reduced.
In order to further illustrate the superiority of parameter selection and optimization, the following error diagram is obtained on the basis of the calibration: the calibration stroke is 10 cm, and the stepper motor carries the calibration plate to move 10 mm each time, so in the 10 cm stroke 11 sets of pictures were collected, 50, 40, 30, 20, 10, O, One 10, 12, One 30, One 40, One 50 (mm). Select the picture at 10 mm to analyze, obtain a standard difference diagram as shown in Fig. 13, found in the figure, the actual height is 10 mm, and its height is distributed at 9.97 ~ 10.005 mm under the measurement. There is an error within 0.03 mm. The experimental measurement standard deviation was 0.022 5 mm.
4, junction
During the parameter selection and optimization, you need to pay attention to the higher the angle, the higher the measurement accuracy, but the angle is too large, which makes the field of view become small, and the field of view will affect the measurement range, increase the error, therefore need to compress the angle . After multiple trials, the angle is preferably 20 ° ~ 30 °, the highest precision; the concept of equivalent wavelength, the compression angle, improve the spatial frequency of the projection grating, which can ensure the measurement accuracy of the system, but also guarantee The field of view of the system. At the same time, the reasonable selection of spatial band is performed, as well as filters, phase expansion, etc., further improves the measurement range of FTP, and the measurement accuracy of the system has also been guaranteed to achieve two full-specific effects.
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