A high-precision laser three-dimensional scanning method based on apFFT

Abstract

The present invention discloses a high-precision laser three-dimensional scanning method based on apFFT, which includes steps such as a light source emitting intensity-modulated laser, a receiver receiving and sampling the reflected echo signal of the target to obtain M (odd number) discrete signals, forming an echo signal matrix, multiplying the all-phase processing matrix by the echo signal matrix, performing Fourier transform on the obtained column vector to obtain the apFFT transform spectrum, etc. The apFFT used in the present invention can prevent the fence effect and spectral leakage, has good performance in suppressing spectral leakage and resisting Gaussian white noise, improves the signal-to-noise ratio, the apFFT has phase invariance, does not require additional phase correction measures, reduces the algorithm complexity, improves the frequency measurement accuracy of the echo signal by correcting the echo frequency offset, and thus improves the point cloud acquisition accuracy.

Description

Technical Field

[0001] The present invention relates to the field of laser three-dimensional scanning, and specifically to a high-precision laser three-dimensional scanning method based on apFFT. Background Art

[0002] Phase method laser three-dimensional scanners have the advantages of fast continuous measurement speed and high measurement accuracy, and are widely used in fields such as industrial design, reverse engineering, and architectural surveying that require high precision and large-range distance measurement. The factors determining the distance measurement accuracy of three-dimensional scanners are mainly the measurement accuracy of the optical path difference, including the measurement of the optical wavelength and the phase discrimination accuracy. Since the optical wavelength is usually given, the error size of the phase measurement directly determines the distance measurement accuracy, so the performance of the phase discrimination module is particularly important.

[0003] Analog phase discrimination has been replaced by digital phase discrimination due to disadvantages such as slow measurement speed, low measurement accuracy, and complex circuits. Among digital phase discrimination techniques, methods such as the maximum likelihood estimation method, high-order statistic estimation method, and signal phase matching method have large measurement and calculation amounts and do not meet the high-speed measurement requirements of three-dimensional scanning applications. While methods such as the digital orthogonal transformation method, synchronous demodulation phase measurement method, and FFT method are relatively simple in calculation, the ability of the digital orthogonal transformation method and synchronous demodulation phase measurement method to suppress Gaussian white noise is significantly weaker than that of the FFT method. And the FFT method has large errors in phase difference measurement due to spectral leakage and "fence effect". In addition, the FFT method requires additional phase correction, increasing its algorithm complexity.

[0004] In summary, the prior art has the following technical problems:

[0005] 1) The ranging accuracy is relatively low

[0006] 2) The ranging speed is relatively slow

[0007] 3) The algorithm complexity is relatively high Summary of the Invention

[0008] In view of the problem of poor accuracy in obtaining the target distance by three-dimensional scanners in the prior art, the present invention proposes a high-precision laser three-dimensional scanning method based on apFFT.

[0009] The technical solution adopted by the present invention to solve the above technical problems is:

[0010] The present invention discloses a high-precision laser three-dimensional scanning method based on apFFT, including the following steps:

[0011] Step 1: A light source emits intensity-modulated laser light, and a receiver receives and samples the return light signal reflected by the target to obtain M (odd number) discrete signals, forming an echo signal matrix;

[0012] Step 2: Multiply the all-phase processing matrix by the echo signal matrix, perform Fourier transform on the obtained column vector, and obtain the apFFT transform spectrum;

[0013] Step 3: Use the obtained apFFT transform spectrum to perform second-order series offset frequency estimation and discrete spectrum correction on the echo frequency;

[0014] Step 4: According to the corrected echo signal frequency spectrum, obtain the phase difference information through arctangent phase calculation;

[0015] Step 5: Calculate the distance of the target based on the phase difference information, and combine it with the angle information obtained by the high-speed biaxial rotation subsystem to obtain the position information of the target relative to the 3D scanner;

[0016] Step 6: Rotate the reflecting mirror and repeat Steps 1 to 5 until the point cloud information of the space to be measured is obtained.

[0017] As a further improvement, the echo signal matrix of the present invention is expressed as:

[0018] [s(-N + 1), s(-N + 2),..., s(-1), s(0), s(1),..., s(N - 2), s(N - 1)] T

[0019] where N = (M + 1) / 2, a is the amplitude of the echo signal, f0 is the carrier frequency, f s is the sampling frequency, and θ0 is the initial phase of the signal.

[0020] As a further improvement, the all-phase processing matrix of the present invention is expressed as:

[0021]

[0022] As a further improvement, the apFFT transform spectrum Y(k) of the present invention is expressed as:

[0023]

[0024] As a further improvement, the second-order series offset frequency σ of the echo of the present invention is expressed as:

[0025]

[0026] where k0 is the frequency at the peak spectral line of the apFFT transform spectrum, and Y(k0), Y(k0 - 1), Y(k0 + 1) are the spectral line amplitudes corresponding to the apFFT transform spectrum at k0, k0 - 1, and k0 + 1.

[0027] As a further improvement, the corrected echo signal frequency f′ of the present invention is expressed as:

[0028] The corrected target distance is: where K is the number of full waves, c is the speed of light, is the phase difference.

[0029] The beneficial effects of the present invention are:

[0030] (1) The apFFT used in the present invention can prevent the fence effect and spectral leakage, has good performance in suppressing spectral leakage and anti-Gaussian white noise, and improves the signal-to-noise ratio.

[0031] (2) The apFFT used in the present invention has phase invariance, does not require additional phase correction measures, and reduces the algorithm complexity.

[0032] (3) By correcting the echo frequency offset, the present invention improves the measurement accuracy of the echo signal frequency, thereby improving the point cloud acquisition accuracy. Description of the Drawings

[0033] Figure 1 is a schematic diagram of the principle of a laser three-dimensional scanner;

[0034] Figure 2 is a flowchart of laser three-dimensional scanning based on the apFFT algorithm. Detailed Embodiments

[0035] The following further describes the present invention in conjunction with specific embodiments. The following embodiments are intended to illustrate the present invention rather than further limit the present invention.

[0036] In view of the introduction of the background technology, it is necessary to improve the performance of the phase discrimination module at the current stage to improve the ranging accuracy. Based on this, the present invention provides a high-precision laser three-dimensional scanning method based on apFFT, which reduces the algorithm complexity and improves the point cloud acquisition accuracy.

[0037] Referring to Figure 1 and Figure 2 Specifically illustrate this embodiment. A high-precision laser three-dimensional scanning method based on apFFT described in this embodiment includes the following steps:

[0038] Step 1: Use a laser light source with a central wavelength of 1550 nm (model FPL1009P) to emit laser light. Select the DDS chip AD9914 with 12-bit and a maximum speed of 3.5 GHz as the modulation signal generation unit to modulate the laser light generated by the laser light source through an intensity modulator to generate a sine signal with a modulation frequency of 100 MHz. After hitting the target, an echo signal is generated and reflected back to the receiver. Select a 12-bit ADC with a sampling speed of 3.2 GHz as the receiver to receive and sample the reflected light signal from the target. The core component of the receiver is a photodetector, model IAG200H3. The detector converts the optical signal into an electrical signal, which is amplified by an amplifier. The amplified electrical signal is converted into a digital signal by an ADC (model: BAL-0006) digital acquisition card to obtain M (odd) discrete signals, forming an echo signal matrix.

[0039] The signal after the intensity modulator from the light source can be expressed as:

[0040] s(t) = a cos(2πf0t + θ0)

[0041] where a is the signal amplitude, f0 is the signal frequency, and θ0 is the signal initial phase.

[0042] After reflection from the target and sampling by the receiver, M (odd) echo signals are obtained, which can be represented by a matrix as:

[0043] [s(-N + 1), s(-N + 2),..., s(-1), s(0), s(1),..., s(N - 2), s(N - 1)] T , T is the transpose

[0044] where N = (M + 1) / 2, a is the echo signal amplitude, f0 is the carrier frequency, f s is the sampling frequency, and θ0 is the signal initial phase.

[0045] Step 2: Multiply the all-phase processing matrix by the echo signal matrix, and perform a Fourier transform on the resulting column vector to obtain the apFFT transform spectrum.

[0046] Cross-multiply the all-phase processing matrix by the echo signal matrix, and perform a Fourier transform on the resulting column vector to obtain the apFFT transform spectrum of the echo signal.

[0047] Among them, the all-phase processing matrix is expressed as:

[0048]

[0049] The apFFT transform spectrum Y(k) is expressed as:

[0050]

[0051] Denote the frequency at the peak spectral line of Y(k) as k0, and the frequencies at the spectral lines adjacent to it on the left and right can be expressed as k0 - 1 and k0 + 1.

[0052] Step 3: Use the obtained apFFT-transformed spectrum to perform a second-order series offset frequency estimation and correct the discrete spectrum of the echo frequency.

[0053] The second-order series offset frequency σ of the echo can be expressed as:

[0054]

[0055] where Y(k0), Y(k0 - 1), and Y(k0 + 1) are the spectral line amplitudes corresponding to the apFFT-transformed spectrum at k0, k0 - 1, and k0 + 1.

[0056] According to the magnitude of the second-order series offset frequency σ, the frequency of the echo signal can be corrected.

[0057] The corrected echo signal frequency f′ is expressed as:

[0058]

[0059] The corrected target distance is: where K is the integer number of waves, c is the speed of light, is the phase difference.

[0060] Step 4: According to the corrected echo signal frequency spectrum, obtain the phase difference information through arctangent phase calculation.

[0061] Step 5: Calculate the distance of the target based on the phase difference, and combine it with the angle information obtained by the high-speed biaxial rotation subsystem to obtain the position information of the target relative to the three-dimensional scanner.

[0062] The target distance is: where K is the integer number of waves, c is the speed of light, is the phase difference.

[0063] Step 6: Rotate the reflecting mirror and repeat Steps 1 to 5 until the point cloud information of the space to be measured is obtained.

[0064] The object of the present invention is to suppress the interference of spectral leakage on the echo signal frequency and initial phase during the echo signal processing, and a frequency correction method is proposed. This method uses apFFT to obtain the spectrum of the echo signal, suppresses spectral leakage, and then uses the second-order series offset frequency estimation method to correct the echo frequency, thereby improving the accuracy of target distance measurement.

[0065] Those of ordinary skill in the art will understand that the above are only preferred examples of the invention and are not intended to limit the invention. Although the invention has been described in detail with reference to the foregoing examples, those skilled in the art can still modify the technical solutions described in the foregoing examples or make equivalent replacements for some of the technical features. Any modifications, equivalent replacements, etc. made within the spirit and principle of the invention shall be included within the protection scope of the invention.

Claims

1. A high-precision laser three-dimensional scanning method based on apFFT, characterized in that, It includes the following steps: Step 1: The light source emits intensity-modulated laser light. The receiver receives and samples the return light signal reflected by the target, obtaining M discrete signals (where M is an odd number), which form an echo signal matrix; Step 2: Multiply the all-phase processing matrix by the echo signal matrix, and perform a Fourier transform on the resulting column vector to obtain the apFFT transform spectrum; Step 3: Use the obtained apFFT transform spectrum to perform a second-order series offset frequency estimation to correct the discrete spectrum of the echo frequency; Step 4: According to the corrected echo signal frequency spectrum, calculate the phase difference information through arctangent phase calculation; Step 5: Calculate the distance of the target based on the phase difference information, and combine it with the angle information obtained by the high-speed biaxial rotation subsystem to obtain the position information of the target relative to the three-dimensional scanner; Step 6: Rotate the reflecting mirror, and repeat Steps 1 to 5 until the point cloud information of the space to be measured is obtained.

2. The high-precision laser three-dimensional scanning method based on apFFT according to claim 1, wherein The echo signal matrix is expressed as: [s(-N + 1), s(-N + 2),..., s(-1), s(0), s(1),..., s(N - 2), S(N - 1)] T where a is the amplitude of the echo signal, f0 is the carrier frequency, f s is the sampling frequency, and θ0 is the initial phase of the signal.

3. The high-precision laser three-dimensional scanning method based on apFFT according to claim 1, wherein The all-phase processing matrix is expressed as:

4. The high-precision laser three-dimensional scanning method based on apFFT according to claim 1 or 2 or 3, characterized in that The apFFT transform spectrum Y(k) is expressed as:

5. The high-precision laser three-dimensional scanning method based on apFFT according to claim 4, characterized in that The second-order series offset frequency σ is expressed as: where k0 is the frequency at the peak spectral line of the apFFT transform spectrum, and Y(k0), Y(k0 - 1), Y(k0 + 1) are the spectral line amplitudes corresponding to the apFFT transform spectrum at k0, k0 - 1, and k0 + 1 respectively.

6. The high-precision laser three-dimensional scanning method based on apFFT according to claim 1 or 2 or 3 or 5, characterized in that, The corrected echo signal frequency f′ is expressed as: The calibrated target distance is: where K is the integer number of waves, c is the speed of light, is the phase difference.

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