Angular velocity vector measurement system and method based on dynamic holographic grating

Abstract

The invention discloses an angular velocity vector measurement system and method based on a dynamic holographic grating, and belongs to the technical field of intelligent space, the system comprises a laser, a spatial light modulator, a first 4f system, a Ronchi grating, a reflector, a second 4f system and a photoelectric detector, an adjusting system is arranged between the laser and the spatial light modulator, and the Ronchi grating is arranged between the laser and the Ronchi grating. A measured target is arranged on the rear focal plane of the second 4f system, a fifth lens is arranged between the measured target and the photoelectric detector, and the measured target, the fifth lens and the photoelectric detector are sequentially connected. By adopting the system and the method, the calculated amount is small, and the rotation angular velocity value and the rotation direction can be determined at the same time only through single signal acquisition.

Description

Technical Field

[0001] The present invention relates to the field of intelligent space technology, and in particular to an angular velocity vector measurement system and method based on a dynamic holographic grating. Background Art

[0002] With the continuous increase of human space activities, the number of man-made objects in space has increased dramatically, including satellites in orbit, failed spacecraft and a large amount of space debris. The existence of these non-cooperative objects (i.e., targets that cannot be controlled or their attitude information obtained through conventional communication means) poses a serious threat to existing and future space missions. In order to ensure space safety and promote the sustainable use of outer space resources, it is necessary to measure the angular velocity vector of non-cooperative objects in space. The current measurement methods include model-based point cloud registration methods, methods based on strong geometric features, methods based on image recognition, methods based on optical observations and methods based on traditional Doppler detection.

[0003] The model-based point cloud registration method first uses a binocular camera to collect the target's image, processes the image to obtain the target's 3D point cloud information, and then uses the ICP matching algorithm to register the obtained 3D point cloud with the target's 3D model to estimate the target's motion parameters. However, this method is too computationally intensive and is more applicable to space detectors or larger non-cooperative targets in space, and is poor at detecting small non-cooperative targets in space.

[0004] Based on the method of strong geometric features, a photometric measurement model based on the reflection of the target shape and the motion relationship between the sun, the earth and the space target is established. The influence of the attitude angle, angular velocity and target shape on the photometric observation data is analyzed. By establishing the kinematic model of the target attitude angle and angular velocity, the joint estimation of the attitude and angular velocity is realized. However, in the process of image measurement, the relative positions of the detector, the detected object and the sun cannot change too much. Therefore, for objects with low rotation speed, the measurement error is large. In addition, it is also required that the measured target cannot be a shape without significant length, width and height contrast, such as a sphere.

[0005] Method based on image recognition. First, the optical axis of the visual camera is on the same horizontal line with the center of mass of the spatial non-cooperative target. The spatial non-cooperative target spins around the principal axis of maximum inertia. Next, the sequence of images is continuously acquired, the key frames are selected using the time constraint and motion constraint methods, and the current key frame is multi-scale random fern encoded and stored in the random fern library. Then, the closed-loop detection shows that when the encoding of the key frame is consistent with the code value in the random fern library, it means that the spatial non-cooperative target has completed a cycle of rotation. Finally, the difference between the timestamps of the closed-loop frame and the starting frame is used to obtain the spin period of the spatial non-cooperative target. However, the amount of calculation is large. If key frames are used for recognition, the optical axis of the visual camera and the center of mass of the spatial non-cooperative target must be on the same horizontal line. If the measurement deviates, it will introduce greater errors in the subsequent monitoring calculation process.

[0006] Based on the optical observation method, the image is directly processed for attitude estimation. For the processing of photometric data, traditional international research focuses on obtaining characteristic information such as the working state and structure of the target through inversion methods. The rotation axis and periodicity of the target are extracted through time series analysis methods. However, when observing irregular objects, its measurement error increases and the convergence speed of the algorithm slows down.

[0007] Based on the traditional Doppler detection method, the rotational Doppler signal can only obtain the angular velocity, but cannot obtain the angular velocity direction associated with the rotation direction. Summary of the invention

[0008] The purpose of the present invention is to provide an angular velocity vector measurement system and method based on a dynamic holographic grating, which relaxes the requirements on the illumination alignment and roughness of the detected surface, does not need to track the rotation axis of a non-cooperative target in space, and only needs to collect and process the scattered signals. The angular velocity can be accurately extracted based on the statistical results of the peak frequency interval of the received signal peak. On this basis, the angular velocity and rotation direction can be simultaneously determined only by single signal acquisition.

[0009] To achieve the above-mentioned purpose, the present invention provides an angular velocity vector measurement system based on a dynamic holographic grating, comprising a laser, a spatial light modulator, a first 4f system, a Ronchi grating, a reflector, a second 4f system and a photoelectric detector, wherein an adjustment system is arranged between the laser and the spatial light modulator, the spatial light modulator is arranged on the front focal plane of the first 4f system, the Ronchi grating is arranged on the back focal plane of the first 4f system, the reflector and the Ronchi grating are in the same straight line and are located on the front focal plane of the second 4f system, a measured target is arranged on the back focal plane of the second 4f system, a fifth lens is arranged between the measured target and the photoelectric detector, and the measured target, the fifth lens and the photoelectric detector are connected in sequence.

[0010] Preferably, the laser, the adjustment system and the spatial light modulator are connected in sequence, and the adjustment system includes a wave plate, an aperture stop 1, an optical isolator and a sixth lens which are arranged in sequence.

[0011] Preferably, the first 4f system comprises a first lens and a second lens which are arranged in sequence, and an aperture stop 2 is arranged at a position where the focal planes of the first lens and the second lens overlap.

[0012] Preferably, the second 4f system includes a third lens and a fourth lens which are arranged in sequence, and an aperture stop 3 is arranged at a position where the focal planes of the third lens and the fourth lens overlap.

[0013] Preferably, a speed-adjustable motor is connected to one side of the target to be measured.

[0014] The present invention also provides an angular velocity vector measurement method based on a dynamic holographic grating, the steps comprising: S1. Use a laser to generate a laser beam, and pass the output beam through an adjustment system to adjust and change the beam size while eliminating the influence of stray light.

[0015] S2, using MATLAB program to control the generation of the required time-varying phase modulated digital dynamic holographic grating image corresponding to the topological charge number ±m0, adjusting the parameters of the actual optical path components, projecting the grating image onto the spatial light modulator, and the light beam passing through the spatial light modulator generates light beams of different diffraction orders corresponding to the topological charge number ±m0; S3. As the light beam propagates, the light beams of different diffraction orders gradually separate and irradiate the first 4f system. After the light beam is modulated by the first 4f system, two light beams are obtained, and the two light beams converge onto the Ronchi grating. S4, Ronchi grating recombines the two beams to generate a superimposed vortex beam corresponding to the topological charge number ±m0, and the superimposed vortex beam passes through a reflector to irradiate the direction of the target to be measured; S5, the light beam in step S4 passes through the second 4f system, the size of the illumination light beam is adjusted to a suitable size, and an image is formed on the surface of the measured target; S6, after the light beam hits the target, it is scattered; S7, the scattered light is converged by the fifth lens and collected by the photodetector; S8, the photoelectric detector receives the collected scattering signal, the scattering signal is directly imported into the computer, the corresponding time domain intensity signal is generated according to the scattering signal, the signal is subjected to data extraction, Fourier transform is performed on it using MATLAB, the spectrum information of the collected signal is obtained, and normalization processing is performed; S9, according to the spectrum information obtained in S8, select an appropriate normalized power threshold and frequency region, calculate the frequency interval in the extraction region, and obtain the angular velocity of the target under test; S10. Based on the spectrum information obtained in S8, compare the actual measured extracted frequency value obtained in the high-frequency region (generally regarded as the angular velocity multiplied by the topological charge and then multiplied by 2) with the corresponding multiple value of the angular velocity calculated in step S9, and determine the rotation direction of the target by comparing the size relationship between the two.

[0016] Preferably, in step S2, while considering the detector frequency response range and the sampling rate, the illumination vob of the topological charge ±m0 is appropriately increased, so that the final RDE frequency component can be kept away from the low-frequency noise.

[0017] Preferably, step S3 specifically includes: light beams of different diffraction orders are gradually separated and irradiated onto the first lens, the first lens converges the light beams of each diffraction order onto the aperture stop 2 where the focal planes of the two lenses of the first 4f system coincide, the aperture stop only allows equal-order positive and negative diffraction light beams with opposite topological charges ±m0 to pass through, and the two equal-order positive and negative diffraction light beams converge onto the Ronchi grating after passing through the second lens.

[0018] Preferably, the superimposed vortex light beam in step S4 presents a petal-shaped intensity distribution with C2m0 rotational symmetry, which rotates as the holographic grating changes.

[0019] Preferably, in step S6, surfaces with different roughness are used as the target to be measured, and the rotation state is maintained by an adjustable speed motor. At the same time, the lateral offset of the center of the irradiation light beam relative to the rotation center of the target to be measured is appropriately increased, which will also be more conducive to increasing the proportion of the dominant frequency interval. The target to be measured can be in a superimposed motion state of lateral motion and spin at the same time.

[0020] Preferably, when measuring the target in step S6, the illumination beam is not required to be strictly aligned with the center and axis, and appropriately increasing the lateral offset of the illumination beam center relative to the rotation center of the target will also be more conducive to increasing the proportion of the dominant frequency interval.

[0021] Preferably, when making a judgment in step S10, if the actually measured extracted frequency value is less than the corresponding multiple of the angular velocity, it indicates that the rotation direction of the rotating object is the same as the preset direction of the time-varying phase modulation; if the actually measured extracted frequency value is greater than the corresponding multiple of the angular velocity, it indicates that the rotation direction of the rotating object is opposite to the preset direction of the time-varying phase modulation.

[0022] Therefore, the present invention adopts the above-mentioned angular velocity vector measurement system and method based on dynamic holographic grating, which has the following beneficial effects: (1) The amount of calculation is small. It only needs to collect and process the scattered signal. The angular velocity of the spin motion can be accurately extracted based on the statistical results of the peak frequency interval of the received signal peak. On this basis, the value of the angular velocity and the direction of rotation can be determined simultaneously by only collecting a single signal. (2) There are no strict requirements for the target to be measured. During measurement, it is only necessary that the surface roughness of the small area illuminated by the light beam is low. It is not required that the overall shape of the object is regular, which relaxes the requirements for the surface roughness of the detected surface; (3) During measurement, strict centration detection is not required, which relaxes the illumination alignment requirements for the detected surface, and there is no need to track the rotation axis of the non-cooperative target in space.

[0023] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a system structure diagram of an embodiment of the present invention; Figure 2 is a flow chart of a method according to an embodiment of the present invention; Figure 3 This is a time-varying effect diagram of a dynamic holographic grating projection diagram according to an embodiment of the present invention; Figure 4 The statistical results of frequency interval extraction of the embodiment of the present invention; Figure 5 It is a spectrum diagram of the high frequency domain of an embodiment of the present invention; Reference numerals 1. Laser; 2. Adjustment system; 3. Spatial light modulator; 4. First lens; 5. Aperture stop 2; 6. Second lens; 7. Ronchi grating; 8. Reflector; 9. Third lens; 10. Aperture stop 3; 11. Fourth lens; 12. Target to be measured; 13. Fifth lens; 14. Photodetector. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0026] Example Reference Figure 1The present invention provides an angular velocity vector measurement system based on a dynamic holographic grating, comprising a laser 1, a spatial light modulator 3, a first 4f system, a Ronchi grating 7, a reflector 8, a second 4f system, and a photodetector 14. An adjustment system 2 is arranged between the laser 1 and the spatial light modulator 3, and the laser 1, the adjustment system 2 and the spatial light modulator 3 are connected in sequence. The adjustment system 2 includes a wave plate, an aperture stop 1, an optical isolator and a sixth lens, etc., which are arranged in sequence. The spatial light modulator 3 is arranged on the front focal plane of the first 4f system, and the Ronchi grating 7 is arranged on the back focal plane of the first 4f system. The spatial light modulator 3 is a pure phase spatial light modulator (P-SLM) (Holoeye PLUTO-2-VIS-016). The first 4f system includes a first lens 4 and a second lens 6 which are arranged in sequence, and an aperture stop 2 5 is arranged at the overlap of the focal planes of the first lens 4 and the second lens 6. The reflector 8 is in the same straight line as the Ronchi grating 7 and is located on the front focal plane of the second 4f system, and a target 12 is arranged on the back focal plane of the second 4f system. The second 4f system includes a third lens 9 and a fourth lens 11 arranged in sequence, and an aperture stop 3 10 is arranged at the overlapped focal plane of the third lens 9 and the fourth lens 11. An adjustable speed motor is connected to one side of the target 12. A fifth lens 13 is arranged between the target 12 and the photodetector 14, and the target 12, the fifth lens 13 and the photodetector 14 are connected in sequence.

[0027] The present invention also provides a method for measuring angular velocity vector based on dynamic holographic grating, wherein the beam propagation path is referenced to Figure 1 As shown in the flow chart of the method Figure 2 As shown, the method steps include: S1. Use MGL-III-532-200mW laser 1 to output fundamental mode Gaussian light. Adjust system 2 to expand and collimate the laser beam to obtain a high-quality linearly polarized flat-top beam, and intercept part of the central beam to eliminate the influence of stray light.

[0028] S2. The collimated light beam is incident on the first 4f system, and the MATLAB program is used to control the generation of the required digital dynamic holographic grating image with time-varying phase modulation corresponding to the topological charge number ±18. The effect of the digital dynamic holographic grating image changing with time is shown in Figure 2. Figure 3 As shown, the period, size, position and other parameters of the actual optical path elements are adjusted and adapted, and the grating pattern is projected onto the spatial light modulator 3. The light beam passes through the spatial light modulator 3 to generate light beams of different diffraction orders corresponding to the topological charge number of ±18.

[0029] S3. As the light beam propagates, the light beams of different diffraction orders gradually separate and are irradiated onto the first lens 4. The first lens 4 converges the light beams of each diffraction order onto the aperture stop 5 where the focal planes of the two lenses of the first 4f system coincide. The aperture stop only allows the positive and negative first-order diffraction vortex light beams with opposite topological charges ±m0 to pass through, thereby achieving the effect of filtering. The two positive and negative first-order diffraction vortex light beams converge onto the Ronchi grating 7 after passing through the second lens 6, and the illumination light beam generated at the same time is further adjusted to a diameter of 3 mm.

[0030] S4 and Ronchi grating 7 recombine the two beams to generate a superimposed vortex beam with a topological charge of ±18. The superimposed vortex beam presents a petal-shaped intensity distribution with 36 petals of rotational symmetry, which rotates with the change of the holographic grating. Among them, a partial acquisition of it can be found that the frequency is 2f g =2.1Hz beat frequency signal, set the rotation direction to counterclockwise, and the superimposed vortex light beam passes through the reflector to irradiate in the direction of the target to be measured.

[0031] S5. In step S4, the light beam passes through the second 4f system, and the size of the illumination light beam is adjusted to a suitable size, and an image is formed on the surface of the target to be measured.

[0032] S6, using surfaces with different roughness as the measured target 12, keeping the rotating state through the adjustable speed motor, setting the speed of the adjustable speed motor to f Ω =33.8Hz. After the light beam is irradiated onto the target 12, it is scattered.

[0033] S7 . The scattered light irradiated onto the target 12 is converged by the fifth lens 13 and collected by the photoelectric detector 14 .

[0034] S8, the photoelectric detector 14 receives the collected scattering signal, and the scattering signal is directly imported into the computer, and a corresponding time domain intensity signal is generated according to the scattering signal. Data extraction is performed on the signal, and Fourier transform is performed on it using MATLAB to obtain the spectrum information of the collected signal, and then normalization processing is performed.

[0035] S9, according to the spectrum information obtained in S8, select the appropriate normalized power threshold and frequency region, without other preprocessing, calculate the frequency interval in the extraction region, and refer to the statistical results Figure 4 As shown, it can be obtained that the dominant frequency interval is 33.8 Hz, which is the angular velocity of the rotating object.

[0036] S10. Based on the spectrum information obtained in S8, the spectrum diagram of the high frequency domain is as follows: Figure 5 As shown, it can be observed that in the high frequency domain (f modThat is, the frequency component after the shift (the peak at 38 times 1264.4Hz is 1286.5Hz after the shift) is only the frequency component after the shift (the peak at 38 times 1216.8Hz is 1286.5Hz after the shift). The frequency component after the shift is greater than the angular velocity component, which means that it is opposite to the previously set rotation direction of the petals (counterclockwise), so it can be judged that the object's rotation direction is clockwise.

[0037] Therefore, the present invention adopts the above-mentioned angular velocity vector measurement system and method based on dynamic holographic grating, and obtains accurate dominant frequency interval according to the statistical results of the peak frequency interval of the received signal peak without strict requirements on light alignment and detection of surface roughness. On this basis, the angular velocity value and rotation direction can be simultaneously determined only by single signal acquisition.

[0038] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. An angular velocity vector measurement system based on a dynamic holographic grating, characterized in that: The invention comprises a laser, a spatial light modulator, a first 4f system, a Ronchi grating, a reflector, a second 4f system and a photoelectric detector. An adjustment system is arranged between the laser and the spatial light modulator. The spatial light modulator is arranged on the front focal plane of the first 4f system. The Ronchi grating is arranged on the back focal plane of the first 4f system. The reflector and the Ronchi grating are in the same straight line and are located on the front focal plane of the second 4f system. A measured target is arranged on the back focal plane of the second 4f system. A fifth lens is arranged between the measured target and the photoelectric detector. The measured target, the fifth lens and the photoelectric detector are connected in sequence.

2. The angular velocity vector measurement system based on dynamic holographic grating according to claim 1, characterized in that: The laser, the adjustment system and the spatial light modulator are connected in sequence, and the adjustment system comprises a wave plate, an aperture stop 1, an optical isolator and a sixth lens which are arranged in sequence.

3. The angular velocity vector measurement system based on dynamic holographic grating according to claim 1, characterized in that: The first 4f system includes a first lens and a second lens which are arranged in sequence, and an aperture stop 2 is arranged at the overlapped position of the focal planes of the first lens and the second lens.

4. The angular velocity vector measurement system based on dynamic holographic grating according to claim 1, characterized in that: The second 4f system includes a third lens and a fourth lens which are arranged in sequence, and an aperture stop 3 is arranged at the overlapped focal plane of the third lens and the fourth lens.

5. The angular velocity vector measurement system based on dynamic holographic grating according to claim 1, characterized in that: One side of the target to be measured is connected with an adjustable speed motor.

6. A method for measuring angular velocity vector based on dynamic holographic grating, using an angular velocity vector measurement system based on dynamic holographic grating according to any one of claims 1 to 5, characterized in that the steps include: S1. Use a laser to generate a laser beam, and make the output beam pass through an adjustment system to adjust and change the beam size; S2, using MATLAB program to control the generation of the required time-varying phase modulated digital dynamic holographic grating image corresponding to the topological charge number ±m0, adjusting the parameters of the actual optical path components, projecting the grating image onto the spatial light modulator, and the light beam passing through the spatial light modulator generates light beams of different diffraction orders corresponding to the topological charge number ±m0; S3. As the light beam propagates, the light beams of different diffraction orders gradually separate and irradiate the first 4f system. After the light beam is modulated by the first 4f system, two light beams are obtained, and the two light beams converge onto the Ronchi grating. S4, Ronchi grating recombines the two beams to generate a superimposed vortex beam corresponding to the topological charge number ±m0, and the superimposed vortex beam passes through a reflector to irradiate the direction of the target to be measured; S5, the light beam in step S4 passes through the second 4f system, the size of the illumination light beam is adjusted to a suitable size, and an image is formed on the surface of the measured target; S6, after the light beam hits the target, it is scattered; S7, the scattered light is converged by the fifth lens and collected by the photodetector; S8, the photoelectric detector receives the collected scattering signal, the scattering signal is directly imported into the computer, the corresponding time domain intensity signal is generated according to the scattering signal, the signal is subjected to data extraction, Fourier transform is performed on it using MATLAB, the spectrum information of the collected signal is obtained, and normalization processing is performed; S9, according to the spectrum information obtained in S8, select an appropriate normalized power threshold and frequency region, calculate the frequency interval in the extraction region, and obtain the angular velocity of the target under test; S10. According to the spectrum information obtained in S8, the actual measured extracted frequency value obtained in the high frequency region is compared with the corresponding multiple value of the angular velocity calculated in step S9, and the rotation direction of the measured target is determined by comparing the magnitude relationship between the two.

7. The method for measuring angular velocity vector based on dynamic holographic grating according to claim 6, characterized in that: Step S3 specifically includes: light beams of different diffraction orders are gradually separated and irradiated onto the first lens, and the first lens converges the light beams of each diffraction order onto the aperture stop 2 where the focal planes of the two lenses of the first 4f system coincide, and the aperture stop only allows equal-order positive and negative diffraction light beams with opposite topological charges ±m0 to pass through, and the two equal-order positive and negative diffraction light beams converge onto the Ronchi grating after passing through the second lens.

8. The method for measuring angular velocity vector based on dynamic holographic grating according to claim 6, characterized in that: In step S4, the superimposed vortex beam presents a petal-shaped intensity distribution with C2m0 rotational symmetry, which rotates with the change of the holographic grating.

9. The method for measuring angular velocity vector based on dynamic holographic grating according to claim 6, characterized in that: In step S6, surfaces with different roughness are used as the measured target, and the rotating state is maintained by an adjustable speed motor.

10. The method for measuring angular velocity vector based on dynamic holographic grating according to claim 6, characterized in that: When making a judgment in step S10, if the actually measured extracted frequency value is less than the corresponding multiple of the angular velocity, it indicates that the rotation direction of the rotating object is the same as the preset direction of the time-varying phase modulation; if the actually measured extracted frequency value is greater than the corresponding multiple of the angular velocity, it indicates that the rotation direction of the rotating object is opposite to the preset direction of the time-varying phase modulation.

Images