A method for identifying the rotation axis direction of a rotating target based on non-circularly symmetric vortex light

Abstract

The present invention relates to a method for identifying the rotation axis direction of a rotating target based on non-circularly symmetric vortex light. Vortex light is a structured light beam with a spiral phase and annular light intensity distribution, which carries orbital angular momentum and has good circular symmetry, and can be used to measure the rotation speed of a rotating target. Circularly symmetric vortex light does not have the ability to detect the target rotation axis. This paper proposes a method for breaking the circular symmetry of the target and realizing the identification of the target rotation axis direction. First, a plane rectangular coordinate system is established with the center of the vortex light as the coordinate origin as a reference, the vortex light field is divided into four parts to design the corresponding hologram, and a non-circularly symmetric vortex light field is prepared by a spatial light modulator; secondly, the generated non-circularly symmetric vortex light is vertically irradiated to any position on the surface of the object four times in sequence, and a photoelectric detector is used to receive the scattered light signal for time-frequency analysis; finally, a smoothing filter with an adaptive window length is designed and the frequency information of the four results is processed, and the direction of the target rotation axis can be determined according to the frequency of the four detection signals. This method is simple to operate and can realize the determination of the direction of the rotation axis of a non-cooperative target.

Description

Technical Field

[0001] The present invention mainly relates to the fields of optics, physics, photoelectric conversion, and signal detection, especially technical methods such as phase modulation and amplitude modulation of light beams. Technical Background

[0002] The technical background of the present invention is mainly based on the optical rotational Doppler effect. Rotational motion is ubiquitous in nature, ranging from cosmic bodies to electrons outside the nucleus. Accurate measurement of target rotational motion parameters has always been a research hotspot. At present, non-contact rotational target speed measurement mainly includes measurement based on Doppler radar and measurement based on visual imaging, but there are not many measurement methods for the rotation axis orientation of the rotating target.

[0003] Research in recent years has found that a new detection mechanism called the rotational Doppler effect has great application prospects in rotating target detection. Similar to the classical linear Doppler effect, the rotational Doppler frequency shift is also generated from the relative motion between the wave source and the rotating object. The difference is that the classical linear Doppler effect is caused by the movement of the object along the propagation direction of the wave source, while the rotational Doppler effect is caused by the relative rotational motion of the object in a section perpendicular to the propagation direction of the wave source. The early discovery of the classical Doppler effect came from people's observation of moving sound sources. Later, it was discovered that the Doppler effect exists in various types of beams. Especially since the invention of lasers in the 1960s, the Doppler effect of light waves has gradually been taken seriously, and various types of laser Doppler meters have been developed. After a beam of ordinary laser irradiates a rotating target, the frequency shift Δf produced by the frequency of the beam received by the rotating target compared to the wave source can be expressed as,

[0004] Δf=f0vcosθ / c (1)

[0005] Among them, f0 represents the frequency of the laser light source, v represents the speed of the object, c represents the speed of light, and α is the angle between the direction of the object's speed and the direction of wave source propagation. Obviously, when the relative motion angle between the object's motion direction and the wave source's propagation direction is 90°, the linear Doppler shift generated is zero. When a classical plane wave is vertically irradiated on the surface of a rotating object, since each tiny scattering point is perpendicular to the plane wave propagation direction, the Doppler shift generated is zero, so the classical linear Doppler effect is difficult to achieve the measurement of the object's rotation speed.

[0006] Since the discovery of vortex light carrying orbital angular momentum, it has been shown that vortex light can be used for non-contact measurement of the rotational speed of a rotating target due to the natural coupling between its spiral phase structure and rotational motion. The direction of the beam energy flow is no longer along the beam propagation direction, but has an angle with the propagation direction. The size of this angle is β = λ / 2πr. Since the movement direction of each tiny scattering point in the target rotation plane is exactly perpendicular to the beam propagation direction, the angle between the energy flow direction of the final beam and the direction of the scattering point movement speed has the following relationship:

[0007] α=π / 2-β (2)

[0008] Substituting the above formula into formula (1) and using the approximation that sinθ≈θ when θ is very small, we can obtain:

[0009] Δf=Ω / 2π (3)

[0010] Here is the expression of the rotational Doppler shift. In actual measurement, there are many ways to measure the rotational Doppler shift Δf. 14 Hz, which is difficult to measure directly. Therefore, the common frequency shift measurement method is to use the beat frequency detection method of the light beam to couple the signal light and the reference light to achieve the detection of the frequency shift. According to the actual frequency shift measurement value Δf, combined with the light beam topological charge information l, the rotation speed of the rotating target can be measured.

[0011] The above is the basic principle of measuring the target rotation speed based on the rotational Doppler effect. Research in recent years has shown that the rotational Doppler effect can not only measure the rotation speed of macroscopic objects, but also the rotation speed of microscopic particles. In addition, after the corresponding system design, in addition to measuring the target rotation speed, the target rotation direction and acceleration can also be measured. This technology has broad application prospects in the measurement of rotating target motion parameters. Summary of the invention

[0012] The technical problem solved by the present invention is: for high-precision and high-speed rotating objects such as machine tool shafts, engine rotors, turbine blades, etc., it is necessary to accurately find the rotation center or rotation axis of the object in certain situations, but the existing non-contact measurement methods are difficult to effectively measure the orientation of the rotating target shaft. Based on the Doppler effect of non-complete vortex light, the present invention proposes a measurement method for identifying the direction of the rotating target shaft based on multiple measurements. The orientation information of the target rotation axis can be obtained by comparing the rotation Doppler frequency shift size of the multiple measurement results. The device has a simple structure, a small size, can be integrated, does not require complex optical devices, and has broad application prospects in high-sensitivity measurement of micro-vibrations of rotating objects.

[0013] The technical solution of the present invention is:

[0014] The present invention relates to a method for identifying the direction of the rotating axis of a rotating target based on non-circularly symmetric vortex light, which mainly includes the steps of hologram design, incomplete light beam preparation, irradiation of the rotating axis target to be measured, echo signal processing, frequency shift peak size comparison, etc., and finally the acquisition of the rotating axis azimuth information of the rotating target can be realized. First, a partial vortex light hologram segmented on a computer is designed by computational holography technology, a plane rectangular coordinate system is established with the center of the vortex light as the origin to divide the light field into n parts, and the phase and amplitude information of the incomplete light field are integrated into a hologram by using a complex amplitude modulation method. In the second step, a beam of ordinary laser is generated by a laser and then irradiated on the surface of a spatial light modulator after beam expansion and collimation to prepare incomplete vortex light. In the third step, the generated incomplete vortex is directly irradiated on the surface of the rotating object to be measured, and a telescope and other devices are used to collect the scattered light signal. In the fourth step, the scattered echo signal is processed by Fourier transform to extract the rotation Doppler frequency shift signal generated by the partial vortex light. The fifth step is to repeat the third and fourth steps for a corresponding number of times according to the number of vortex light field divisions, and compare the obtained rotational Doppler frequency shifts. Finally, the relative position of the target rotation axis with respect to the vortex light propagation axis can be determined based on the frequency shift comparison result.

[0015] The principle of the present invention is:

[0016] (1) Principle of non-holonomic vortex optical axis orientation identification

[0017] From complete vortex light to incomplete vortex light, on the one hand, the pattern composition of the vortex light is changed; on the other hand, while breaking the integrity of the vortex light, the circular symmetry of the vortex light is also broken, providing a gap direction information in the light beam detection. Therefore, through a simple design, the orientation of the target rotation axis can be identified under non-coaxial conditions. The principle will be explained below.

[0018] When the vortex beam irradiates a rotating target with a rotation speed of Ω, a rotational Doppler effect will occur. Figure 2 As shown in the figure, the angle between the Poynting vector and the linear velocity of the small scattering point on the surface of the object is α, and the size of this angle can be expressed as α = π / 2-β. The angle relationship between the linear velocity direction of the tiny scattering point on the surface of the object and the expression of optical linear Doppler frequency shift Δf=2f0vcosθ / c, is substituted into the velocity of the tiny scatterer on the surface of the object. in represents the distance from each scattering point to the center of the rotating axis. The frequency shift caused by the relative motion of the tiny scattering point and the vortex light is f RDS=lΩ / 2π. The same frequency shift will occur at every tiny scattering point within the light field coverage. Therefore, the light beam scattered back from the surface of the object will produce a uniform frequency shift, which is also called the rotational Doppler shift, and will form a unique peak in the frequency signal spectrum of the response. However, when the vortex light propagation axis does not coincide with the object's rotation axis, it is another case.

[0019] As attached Figure 2 As shown in the figure, when there is a lateral displacement between the beam propagation axis and the object rotation axis, the size of the rotational Doppler frequency shift of each tiny scattering point will also change accordingly. Here, a plane rectangular coordinate system is established with the center of the vortex spot on the surface of the object as the coordinate origin. In the plane rectangular coordinate system, by converting and The relationship between the two can be obtained. The Doppler frequency shift under the condition of the beam lateral displacement irradiating the rotating target can be expressed as:

[0020]

[0021] in represents the distance between the center of rotation and the beam propagation axis, and θ refers to the distance between the x-axis and the scattering point radius. The angle between them can accurately give the position of each small scattering point; γ is the angle in the coordinate system and The angle between them can be used to determine the corresponding target rotation axis orientation. Formula (4) needs to be discussed in several cases. In previous work, the case where the vortex light field always contains the target rotation axis, that is, d≤r, was discussed, and a method for measuring the target rotation speed based on spectrum broadening was proposed. Under actual measurement conditions, it is more common for the vortex light field to not contain the rotation axis of the object, that is, d>r. It is not difficult to see from formula (4) that f RDS The magnitude of the frequency shift depends not only on the topological charge and rotation speed of the light beam, but also on the position of the scattering point (angular position θ and radial position d) and the relative orientation of the rotation axis γ. When the topological charge of the detection beam and the target rotation motion parameters remain unchanged, the magnitude of the rotational Doppler frequency shift will be completely determined by the position of the scattering point and the relative orientation of the rotation axis. Therefore, when the position of the target scattering point is determined, by observing the distribution of the rotational Doppler frequency shift, it is expected to achieve sensitive detection of the orientation of the target rotation axis.

[0022] For a complete vortex light ring with good circular symmetry, since the signal finally reflected in the spectrum is the sum of the frequency shifts generated by each tiny scattering point on the vortex light field ring, the observed frequency shift is integrated along the beam propagation axis (0,2π), so the final rotation target axis azimuth γ has no effect on the rotation Doppler frequency shift, so it is impossible to obtain the position information of the rotation axis from the Doppler spectrum. Based on the theoretical analysis shown in equation (4), a new method is proposed here to achieve the identification of the target axis of rotation by breaking the circular symmetry of the vortex light beam. When the vortex light beam non-coaxially illuminates the surface of the rotating body, a complete annular light field will appear on the surface of the object. A plane rectangular coordinate system is established with the center of the light spot as the origin to divide the vortex light field into four parts, and the four parts of the light field in the four quadrants are numbered A, B, C, and D respectively. After dividing the entire annular light field into four parts, one of the parts is used in turn to detect the rotating object. Since the incomplete light field at this time only appears in the area of ​​θ0<θ<(θ+π / 2), the Doppler frequency shift signal finally reflected in the spectrum is related to the axis of rotation. The corresponding frequency shift appears in a spectrum range determined by the rotation direction and the detection light area. When the offset distance between the propagation direction of the target detection light and the target axis remains unchanged, the rotational Doppler frequency shifts generated by the four vortex light parts are different. Therefore, the direction of the axis can be distinguished through four independent measurements. The above is an explanation of the basic principle of detecting the direction of the target axis using incomplete vortex light.

[0023] (2) Principle of coherent superposition of light beams

[0024] It can be seen from the basic principle of this patented method that the final detection of the rotational Doppler frequency of the light beam needs to be obtained by coherent superposition of the signal light and the reference light. In the specific implementation process, due to the use of positive and negative superposition state conjugate vortex light, the two components will have mutual beat frequency coupling. The basic principle of coherent superposition between vortex lights is as follows:

[0025] The expressions of local oscillator light and detection light are respectively and Where A0 represents the light intensity, w represents the angular frequency of the light wave, and t represents the time. (k is the wave number, is the distance of the light beam along the propagation direction, is the initial phase) represents the initial phase and the phase change caused by the propagation distance. After the experimental light beam is reflected by the object and superimposed with the detection light, the light intensity at the photodetector can be expressed as:

[0026]

[0027] The frequency of the laser is generally around 10 14The magnitude of the difference frequency term is far beyond the response frequency of the photodetector. Therefore, the double frequency term in equation (5) will become a DC component, while the difference frequency term can generally fall within the response range of the photodetector according to its size range. Therefore, the periodic size change of the difference frequency term is reflected in the signal of the photodetector.

[0028] Since the local oscillator light and the signal light originally come from a beam of light with the same frequency, but the signal light is modulated and changes its frequency after interacting with the object, a frequency difference occurs when it is mixed again. This frequency difference is often directly related to the movement of the object, so the movement information of the object can be obtained. This detection method that uses the coherence of the light beam, removes the common mode part and retains the differential mode part is widely used in high-frequency electromagnetic wave detection.

[0029] The main advantages of the present invention are:

[0030] (1) This method is novel in technology, simple in operation, easy to implement, and has strong adaptability under practical conditions.

[0031] (2) This method has high reliability in detecting the axis of a rotating target. According to the principle of the method, the final axis orientation of the method is determined by the broadened Doppler frequency shift signal, which will not be affected by the image of slight disturbance.

[0032] (3) This device is still applicable to extreme conditions such as high-speed rotation. This device uses light waves as the detection medium, has a long transmission distance, fast measurement speed, is not affected by factors such as temperature and environment, responds quickly, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Schematic diagram of the method system;

[0034] Figure 2 Schematic diagram of non-coaxial rotation Doppler effect;

[0035] Figure 3 Non-holonomic light field complex amplitude hologram;

[0036] Figure 4 The magnitude of the simulated frequency shift when the light field is divided into four parts;

[0037] Figure 5 Detection results when the rotation axis is placed in the second quadrant of the light field; Specific implementation plan

[0038] The present invention is based on the optical rotational Doppler effect and uses the incomplete vortex light as the detection carrier. The specific implementation steps are as follows:

[0039] The method mainly includes the steps of hologram design, incomplete beam preparation, irradiation of the rotating shaft target to be measured, echo signal processing, frequency shift peak size comparison, etc., and finally the rotation axis azimuth information of the rotating target can be obtained.

[0040] First, the segmented partial vortex light hologram is designed on the computer through computational holography technology. A plane rectangular coordinate system is established with the center of the vortex light as the origin to divide the light field into n parts. The phase and amplitude information of the incomplete light field are fused into a hologram using complex amplitude modulation for subsequent vortex light preparation. Taking the segmentation into 4 parts as an example, the complex amplitude hologram of the light field is shown in the attached figure. Figure 3 As shown.

[0041] In the second step, the prepared complex amplitude hologram is loaded on the spatial light modulator. Then, a beam of ordinary laser is generated by the laser and irradiated on the surface of the spatial light modulator after beam expansion and collimation. Under the modulation effect of the hologram, the light beam reflected from the surface of the spatial light modulator contains the required incomplete vortex light. After a simple 4f filtering, the incomplete vortex detection beam can be obtained.

[0042] The third step is to directly irradiate the generated incomplete vortex light onto the surface of the rotating object to be measured, and then use a telescope or other device to collect the scattered light signal. Repeat this step n times to collect a total of n groups of detection light signals.

[0043] The fourth step is to perform Fourier transform on the scattered echo signal, and then use data processing algorithms such as noise reduction, fitting, and smoothing to extract the rotational Doppler frequency shift generated by part of the vortex light.

[0044] The fifth step is to repeat the third and fourth steps n times to obtain n sets of rotational Doppler frequency shift data results generated by incomplete vortex light in different directions, and compare the obtained rotational Doppler frequency shift magnitudes, referring to the frequency shift magnitude relationship obtained by simulation, as shown in the attached figure. Figure 4 Finally, the relative position of the target axis relative to the vortex light propagation axis can be determined based on the comparison result of the frequency shift.

[0045] When the detection light topological charge is ±20, the light field is divided into 4 parts, and the measurement results obtained by rotating the target axis in the quadrant of the light field coordinate system are shown in the attached figure. Figure 5 shown.

[0046] 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 identifying the rotation axis direction of a rotating target based on non-circularly symmetric vortex light, comprising the steps of hologram design, non-complete vortex beam preparation, echo signal processing and frequency shift peak comparison, characterized in that: Firstly, a plane rectangular coordinate system is established with the center of the vortex beam as the origin, and this is used as a reference to describe the position of the target axis. Secondly, a hologram is designed to control the generation of incomplete vortex light, and incomplete beams in four quadrants are prepared respectively. Finally, when the beam propagation axis and the target axis have a lateral offset and their relative position remains unchanged, the rotating target is illuminated with incomplete vortex light in four quadrants in turn and the frequency analysis of the echo signal is performed. By comparing the frequencies of the four measurement results, the direction of the rotating target axis relative to the beam propagation axis can be determined.

2. A method for identifying the rotation axis direction of a rotating target based on non-circularly symmetric vortex light according to claim 1, characterized in that The circular symmetry of the annular vortex light is broken to transform it into a non-circularly symmetric vortex light, which provides a reference direction for identifying the target axis of rotation. The breaking of the non-circular symmetry is achieved by dividing the annular vortex light field into four or more parts.

3. The method for identifying the rotation axis direction of a rotating target based on non-circularly symmetric vortex light according to claim 1, characterized in that The propagation axis of the vortex light is not coaxial with the target rotation axis. The direction of the rotation axis can be described by establishing a plane rectangular coordinate system with the center of the vortex light as the coordinate origin. The judgment is achieved based on the relationship between the rotational Doppler frequency shifts generated under the measurement conditions of different parts of the vortex light field in several measurements.

4. The method for identifying the rotation axis direction of a rotating target based on non-circularly symmetric vortex light according to claim 1, characterized in that The rotation axis direction identification method is non-contact and can detect the rotation axis direction of a non-cooperative rotating target without measuring its rotation speed or shape information in advance.

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