Rotor rotating speed and angular feature information detection method based on vortex light

Abstract

The invention relates to a rotor rotating speed and angular feature information detection method based on vortex light. According to the basic principle that vortex light has a rotating Doppler effect, a novel rotor object detection method is provided. Different from an existing traditional method for rotating speed measurement and object feature recognition through passive optical imaging, the method adopts a laser active detection mode, and detection of the rotating speed and angular direction features of a rotor wing object is achieved through superposition state vortex light. Compared with a traditional method, the method has the advantages that a good effect is achieved under the conditions that the rotating speed of the measured object is high and light is low. The method is novel in principle, simple in structure and high in sensitivity.

Description

Technical Field

[0001] The present invention relates to a method for detecting rotor speed and angular characteristic information based on vortex light, and mainly involves the fields of optics, physics, photoelectric conversion, and signal extraction, especially technical methods such as light field control of light beams, laser radar, and remote sensing. Background Art

[0002] The photons in the light beam have not only spin angular momentum (SAM) but also orbital angular momentum (OAM). The topological charge determines the size of OAM. The vortex phenomenon in the light field was first discovered near the focal plane of the lens group in 1967. In 1992, Allen discovered that under paraxial conditions, the vortex with a phase factor The vortex beam has orbital angular momentum, where l is the topological charge of the vortex light orbital angular momentum, is the azimuth angle; each photon carries The orbital angular momentum of To reduce Planck's constant, the phase factor indicates that during the propagation of vortex light, if the light beam propagates for one cycle, the wavefront rotates around the optical axis, and the phase changes by 2πl accordingly. This light beam whose phase changes with the phase angle is a new type of structured light beam with a spiral wavefront, and has important application value in the fields of optical communication, particle micro-manipulation, motion detection, optical micro-measurement, etc. In 1994, he predicted in his article that a light beam carrying a phase factor The vortex beam has a rotational Doppler effect. Subsequent studies have also proved the existence of this effect. The rotational Doppler effect of vortex light is due to the fact that the direction of the Poynting vector of the vortex beam is different from that of a general Gaussian beam. It is no longer parallel to the propagation direction of the beam, but has an angle rotating around the optical axis. Therefore, the direction can be decomposed into two components along the propagation direction of the beam and perpendicular to the propagation direction of the beam. According to the principle of the Doppler effect, vortex light has the ability to interact with rotating objects to produce the Doppler effect, so this effect is called the rotational Doppler effect.

[0003] Laguerre-Gaussian light is a typical vortex light. A complete single-state Laguerre-Gaussian beam has a circular intensity distribution and a hollow dark core. The area with zero intensity in the center of the beam is defined as a phase singularity. This beam will produce Δf when irradiating a general object without obvious angular features. 1 = lΩ frequency shift, where l is the topological charge of the light beam and Ω is the rotation speed of the object. This principle is the rotational Doppler effect of vortex light. However, since the frequency of light itself is 10 14There is no device that can sense the frequency change of this order of magnitude, which makes it impossible to directly detect the change in the frequency of the light itself, limiting the detection and application of the rotational Doppler effect. In 2013, Lavery et al. from the University of Glasgow published a paper in Science that used the superposition of two vortex light beams with opposite topological charges as detection light to achieve the measurement of vortex light on actual rotating objects. This superposition state vortex light will produce a light spot with a petal number of 2I, which has the property of eliminating the frequency shift of the linear Doppler effect caused by linear motion parallel to the optical axis. At the same time, this superposition state vortex light can use the collinear beat frequency method to test the frequency difference of the two beams of light, that is, after the light beams with similar frequencies are superimposed, the envelope forms an intensity change with the frequency difference between the two beams of light as the frequency. Compared with the heterodyne method, this method simplifies the experimental optical path and can eliminate the change in light frequency caused by non-rotational motion. According to theoretical derivation, the frequency shift of this light beam after irradiating a rotating object is: Summary of the invention

[0004] The technical problem solved by the present invention is: for aircraft that rely on rotors as lift, the present invention proposes using superposition vortex light as detection light to detect rotation speed and other characteristic information. The device is small in size, simple in structure, and sensitive in response, and has broad prospects in feature recognition and micro-motion detection.

[0005] The technical solution of the present invention is:

[0006] The present invention relates to a method for detecting rotor speed and angular characteristic information based on vortex light, which mainly comprises the following steps:

[0007] (1) Holograms of conjugate superposition state vortex light with different topological charges are designed using the complex amplitude modulation method and loaded onto a spatial light modulator as needed. Linearly polarized Gaussian light is irradiated onto the spatial light modulator, and the modulated light is filtered out through a 4f system to obtain the conjugate superposition state vortex light.

[0008] (2) The superposition state vortex light generated after modulation by the spatial light modulator is filtered to select the first-order diffraction light and expand the beam to adjust the beam size and collimate it before irradiating it on the rotor object. The scattered light is collected by a lens and irradiated on the light-sensitive component of the photodetector to detect the light intensity information of the scattered light. The photodetector converts the light intensity signal into an electrical signal and transmits it to the computer. The fast Fourier transform program processes the information of the light intensity change over time in real time, obtains the frequency information in the light intensity information, and draws a Fourier spectrum. The interval between the higher peaks in a cluster of peaks generated in the Fourier spectrum can extract the information of the number of rotors.

[0009] (3) The primary Fourier spectrum is Fourier transformed again and a secondary spectrum is plotted. The period value T of the signal peak with the highest intensity is selected from the secondary spectrum. 1 / T is the rotation speed of the rotating object.

[0010] (4) Based on the peak information of the low-frequency part of the primary spectrum, find out the frequency with the strongest signal peak value, which is mΩ. Then, based on the changes of other peaks in the low-frequency part, determine the number of rotors and the rotation speed of the object.

[0011] The principle of the present invention is:

[0012] The light field of Laguerre-Gaussian (LG) light can be expressed as:

[0013]

[0014] Where E is the wave vector of the LG beam, is the cylindrical coordinate, z R is the Rayleigh length, ω(z) is the beam radius, l is the topological charge, p is the radial node, k is the wave number, and R(z) is the radius of curvature. This formula can be simplified to:

[0015]

[0016] in, is the complex coefficient of each LG mode. 0 When the conjugate superposition state vortex light of is used as the detection light, considering that the center of the beam and the rotor shaft cannot be accurately aligned in the actual detection situation and the blade surface is not perpendicular to the optical axis, the beam irradiating the rotor can be decomposed into two groups of composite vortex beams according to the mode decomposition theory. Each group of composite vortex beams is a series of LG beams with a topological charge difference of 1, which can generally be expressed as:

[0017]

[0018] When l=±l 0 The OAM mode component accounts for a high proportion. The topological charge rotation Doppler frequency shift signal is mainly affected by the OAM mode of the detection light and the mode of the rotating object. Since the actual rotor object has obvious angular symmetry characteristics and there is a certain angle between the blades and the shaft, the mode component with a mode number equal to the blade number n and its multiple mn accounts for a high proportion; the modulation function of the object on light can be expressed as:

[0019]

[0020] Since the mode components with mode number equal to the number of blades n and its multiple mn account for a high proportion, when k = mn, B k >>B k-1 And B k >>Bk+1 When the rotating object rotates, the modulation function of the rotor object becomes:

[0021]

[0022] Where Ω is the rotation speed of the rotor object. Then the light field of the scattered light can be expressed as:

[0023]

[0024] in Because the scattered light is obtained by reflecting the detection light, the phase of the light beam changes, that is, After the beat frequency, the intensity of the scattered light can be derived as:

[0025]

[0026] Where k = -, and k' = -l'. From formula (7), it can be seen that the intensity signal of the scattered light can be equivalent to a series of sinusoidal signals with frequencies around 2l0Ω and a frequency difference of Ω. Therefore, after the light intensity signal is converted into an electrical signal and Fourier transformed by a computer, 2l 0 An envelope of peak intensity with intervals of Ω is formed near Ω, and the relative intensity of the peak with frequency mnΩ in the envelope is relatively high. 0 The change in peak intensity near Ω can be used to analyze the number of blades of the rotor object. Since the peak interval in the primary spectrum is Ω, the primary spectrum is Fourier transformed again, that is, the peak interval in the primary spectrum is extracted to obtain the secondary spectrum. The rotation speed of the object can be calculated by observing the peak position with the highest intensity.

[0027] Compared with the existing solutions, the main advantages of the solution of the present invention are:

[0028] (1) It has a wide range of applications and strong flexibility. It can measure the rotation speed and angular characteristics of rotor objects under certain offset and oblique incidence conditions.

[0029] (2) Reduce costs and save space. The vortex light control method is simple, requires fewer devices and does not require high-precision optical path construction.

[0030] (3) There is no need to place any markers on the surface of the rotor object. It is only necessary to irradiate the conjugate superposition state vortex light onto the surface of the rotor object, collect the reflected light for primary and secondary spectrum analysis, and extract the rotation speed and object angular characteristic information from it. The data volume is small and the processing method is simple. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is an application flow chart for detecting rotor objects based on superposition vortex light;

[0032] Figure 2 This is a schematic diagram of the detection optical path and device;

[0033] Figure 3 This is a schematic diagram of the high frequency part of the primary spectrum;

[0034] Figure 4 is a schematic diagram of the secondary spectrum;

[0035] Figure 5 This is a schematic diagram of the low-frequency part of the primary spectrum. Specific implementation plan

[0036] The present invention uses conjugate superposition state vortex light as the detection beam, and the implementation object is a rotor object. The specific implementation steps are as follows:

[0037] First, a phase diagram of a conjugate superposition Laguerre-Gaussian beam is prepared and superimposed with a blazed grating to obtain a holographic pattern that can be precisely controlled. The holographic pattern is loaded into a spatial light modulator (6). A continuous laser (1) generates stable Gaussian light, which is sequentially transmitted through a linear polarizer (2), a bandpass filter (3), and then through a beam collimation and expansion system composed of a lens (4) and a lens (5). The light spot is adjusted to a suitable size and irradiated onto the spatial light modulator (6). After complex amplitude modulation, the emitted light is a superposition Laguerre-Gaussian beam. After passing through a spatial filtering system composed of a lens (7), an aperture (8), and a lens (9), the light is irradiated onto the surface of a rotor object (10). The light scattered from the surface of the object is converged by a lens (11) and received by a photodetector (12). The light intensity signal is converted into a voltage signal and input into a signal processing device for two Fourier transforms. The primary and secondary spectra are analyzed respectively, as shown in FIG. Figure 3 , Figure 4 and Figure 5 shown.

[0038] For example, by loading a hologram on a spatial light modulator, a conjugate superposition state vortex light with a topological charge of ±19 is modulated and irradiated onto the surface of a three-rotor object. The radius of the beam irradiated onto the three-rotor object is 25 mm. The rotation speed of the rotor object is adjusted to 43.3 Hz. After receiving the scattered light, the photodetector performs primary and secondary spectrum analysis, and the results are as follows: Figure 3 , Figure 4 and Figure 5 shown.

[0039] from Figure 3 The high frequency part of the primary spectrum shown shows that within the frequency range of 1000-2300Hz, the interval between each peak is the rotor speed, forming an envelope. It can be seen from the figure that the peak intensity is higher when the frequency is a multiple of three times the speed, which is consistent with the angular characteristics of the detected object being a three-rotor object.

[0040] from Figure 4It can be seen from the secondary spectrum shown that the period T of the highest intensity peak within the period of 0-0.1s is 0.02306s, and the measured rotation speed 1 / T can be calculated to be 43.365Hz, with an error of about 0.85%.

[0041] from Figure 5 From the low-frequency part of the primary spectrum shown, it can be seen that within the frequency range of 0-500Hz, the peaks in the signal are spaced at intervals of the rotor speed. The peak intensity at 129.70Hz is the highest, and there are two lower signal peaks before it. It can be concluded that the object is a three-rotor and the speed is calculated to be 43.23Hz, which is consistent with Figure 3 and Figure 4 The obtained rotation speed is consistent with the rotor number information, and the verification result is correct.

[0042] In summary, this method can achieve high-precision measurement of the rotation speed of the rotor object and identification of the angular characteristics.

[0043] In addition, the spatial light modulator has certain limitations on the incident angle and power of the light beam, so a vortex half-wave plate can be used to modulate the light beam for high-power and long-distance detection. The specific optical path design needs to be improved based on the detection requirements and actual conditions.

[0044] The contents not described in detail in this specification belong to the prior art known to the professionals in this field.

Claims

1. A method for detecting rotor speed and angular characteristic information based on vortex light, characterized in that: The existing traditional methods are all based on image recognition or microwave radar detection. In order to cope with the environment with weak natural light, obtain the angular distribution information of the blades and accurately select specific objects, the rotation speed detection method of vortex light based on the rotational Doppler effect is adopted; the vortex light with different topological charges is irradiated on the rotor object by complex amplitude modulation. The light intensity signal of the scattered light received by the photoelectric detector is Fourier transformed, and the information such as the rotor speed and angular symmetry is obtained according to the spectrum.

2. A method for detecting the rotation speed and angular direction information of a rotor object according to claim 1, characterized in that: The detection beam is vortex light, and its light field distribution can be expressed as: Where A represents the amplitude of the vortex beam, represents the phase angle of the light beam, and l is the topological charge of the light beam. From the rotational Doppler effect, it can be obtained that the center frequency of the frequency shift of the scattered light is approximately located at 2lΩ and the frequency shift has a certain width, where is the topological charge and Ω is the blade speed.

3. The detection method according to claim 1 and claim 2, characterized in that: The primary and secondary spectra are obtained by performing the first and second Fourier transforms on the electrical signals converted from the scattered light signals received by the photoelectric detector. The angular characteristics of the rotor object can be analyzed based on the intensity characteristics of the signals of different frequencies in the primary spectrum. The rotation speed of the rotor object can be extracted based on the position of the peak with the highest intensity in the secondary spectrum. Thus, the rotation speed and angular characteristics of the rotating blades can be detected through the primary and secondary spectra.

4. The detection method according to claim 1 and claim 3, characterized in that: The collected scattered light intensity signal is transformed through Fourier transform to obtain a primary spectrum. In the high frequency part, it usually presents an envelope of peak intensity with intervals of Ω near the frequency of 2l0Ω. The relative intensity of the peak with a frequency of mnΩ in the envelope is relatively high. Where n is the number of blades and mn is its multiple. Therefore, the number of blades of the rotor object can be analyzed by the change of the peak intensity near 2l0Ω in the primary spectrum. And because the peak interval in the primary spectrum is Ω, the primary spectrum is Fourier transformed again, that is, the peak interval in the primary spectrum is extracted to obtain the secondary spectrum. The rotation speed of the object can be solved by observing the peak position with the highest intensity.

5. The detection method according to claim 1, claim 3 and claim 4, characterized in that: The collected scattered light intensity signal is transformed through Fourier transform to obtain a primary spectrum, which usually shows a peak at a frequency of mΩ in the low-frequency part, among which the peak at a frequency of nΩ is the highest and the peak at a frequency of mnΩ is also significantly higher than the adjacent peak. Therefore, the number of blades and the speed information of the rotor object can be obtained through the low-frequency part of the primary spectrum, and compared and verified with the results obtained by the speed and blade number detection method described in claim 4, to obtain a more accurate result.

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