A field-extended solar sensor based on diffraction order sub-multiplexing

Abstract

This invention discloses a field-of-view extended solar sensor based on diffraction order multiplexing, comprising a dual-band bandpass filter, a diffractive light introducer, and a color CMOS image sensor. The dual-band bandpass filter selects two separate, preset wavelengths of sunlight for incidence, while the diffractive light introducer diffracts the transmitted sunlight into multi-order beams. The color CMOS image sensor acquires diffraction spot images containing spectral information from both wavelengths. The current effective diffraction order is determined by the mapping relationship between the centroid positions and spot spacing of the spots in the two corresponding wavelength channels and the diffraction order. Finally, the solar vector is calculated using a diffraction optics model. This invention resolves the inherent contradiction between large field of view and high precision in traditional digital solar sensors, effectively overcoming the order ambiguity problem in diffraction-extended field of view. Its lightweight design adapts to high-density integration requirements, providing technical support for spacecraft attitude measurement.

Description

Technical Field

[0001] This invention relates to solar sensors, and more specifically to a field-extended solar sensor based on diffraction order multiplexing. Background Technology

[0002] Sun sensors provide a reference for a spacecraft's orientation relative to the sun by detecting the direction of the sun's vector. Traditional sun sensors are mainly divided into two categories: analog and digital. Analog sun sensors estimate the sun's angle by measuring the difference in photocurrent; however, due to limitations in sensor noise and operating principles, their accuracy is usually low. Digital sun sensors, on the other hand, form a solar spot on an image sensor using a light introducer, and calculate the angle by measuring the offset of the spot's center. This method offers higher accuracy than analog sun sensors and is currently the mainstream solution.

[0003] like Figure 1 As shown, digital sun sensors are typically based on the pinhole imaging principle, that is, when the distance between the plane of the light introducer 2 and the plane of the CMOS image sensor 4 is... When the angle of incidence of sunlight is At that time, the distance the sunspot has moved compared to when it was directly incident is According to The estimated value of the angle of incidence of the light ray is calculated. Then for pixel size The size of the imaging area is The CMOS image sensor 4 has a solar incidence angle resolution of The single-axis field of view of the solar sensor is Therefore, its performance improvement is always limited by the core contradiction of the difficulty in achieving both a large field of view and high precision, namely: to achieve high precision, the distance between the light introducer 2 and the CMOS image sensor 4 needs to be increased. To improve angular resolution, but this directly leads to a narrowing of the field of view; conversely, to obtain a large field of view, the distance between the light introducer 2 and the CMOS image sensor 4 is reduced. If a fisheye lens is introduced, the angular resolution will decrease due to reduced resolution or severe aberrations.

[0004] In summary, traditional digital sun sensors are limited by optical principles and cannot achieve both a large field of view and high precision. This results in problems such as large size and low integration, making it difficult to meet the urgent needs of micro and nano satellite platforms for lightweight, high-performance sun sensors. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a wide-field-of-view, high-precision, and miniaturized field-of-view extended solar sensor based on diffraction order multiplexing.

[0006] Technical Solution: This invention provides a field-of-view extended solar sensor based on diffraction order multiplexing, comprising a signal processing unit, and a dual-band bandpass filter, a diffractive light introducer, and a color CMOS image sensor arranged sequentially along the optical path. The dual-band bandpass filter transmits sunlight within two preset wavelength bands, the spectra of which are mutually isolated. The diffractive light introducer diffracts the transmitted sunlight into a beam containing multiple diffraction orders. The color CMOS image sensor acquires diffraction spot images corresponding to the two preset wavelength bands. The two preset wavelength bands of the dual-band bandpass filter correspond one-to-one with the two different spectral response channels of the color CMOS image sensor, enabling the formation of a pair of double spots on the color CMOS image sensor for sunlight with a single incident direction. The relative position of these double spots serves as an order code, uniquely mapping to the diffraction order that generates the double spots. The signal processing unit processes the images acquired by the color CMOS image sensor to calculate the angular information of the solar vector.

[0007] In this invention, a diffractive light introducer achieves field-of-view expansion through a diffraction order multiplexing mechanism. Specifically, when sunlight strikes at a large angle, its zero-order diffraction spot will move out of the detector's field of view. A diffraction grating is used to introduce specific higher-order diffraction spots into the effective imaging area of ​​the CMOS sensor. This maps large-angle incident light, which traditional methods cannot detect, onto the finite-sized color CMOS image sensor, thus achieving substantial field-of-view expansion without increasing the area of ​​the color CMOS image sensor. Simultaneously, to address the order ambiguity caused by diffraction, this invention introduces a spectral discrimination mechanism. Two preset wavelengths selected by a dual-band bandpass filter are mutually isolated and correspond one-to-one with two different spectral response channels of the color CMOS image sensor. This allows for the formation of a pair of spatially distinguishable double spots with a specific relative positional relationship on the color CMOS image sensor for sunlight from a single incident direction. The relative position of these double spots has a unique mapping relationship with the diffraction order that generated the spot pair. This positional relationship based on spectral dispersion serves as an order encoding and is the fundamental mechanism for achieving unambiguous recognition of multiple diffraction orders.

[0008] Furthermore, the signal processing unit is configured to perform the following steps: extracting the centroid positions of the diffraction spots in the corresponding spectral channels of two preset bands from the image acquired by the color CMOS image sensor; calculating the distance between the centroids of the spots between the two spectral channels; determining the effective diffraction order corresponding to the current solar incidence angle based on the distance between the centroids of the two spots and the preset unique mapping relationship between them and the diffraction order; and calculating the angle information of the solar vector through a diffraction optics model by combining the effective diffraction order, the parameters of the diffraction ray introducer, and the distance between the diffraction ray introducer and the color CMOS image sensor.

[0009] Furthermore, the dual light spots are selected from the pair of dual light spots closest to the center of the color CMOS image sensor.

[0010] Furthermore, the diffractive light introducer includes a light-shielding mask with a central light-transmitting aperture and a diffraction grating integrated into the light-transmitting aperture region.

[0011] Furthermore, the diffraction grating uses two orthogonal one-dimensional gratings superimposed.

[0012] Furthermore, the diffraction grating is a two-dimensional diffraction grating.

[0013] Furthermore, a dual-band bandpass filter and a diffractive light introducer are positioned on the upper side of the precision 3D-printed support structure, while a color CMOS image sensor is positioned on the lower side of the support structure, achieving integrated design.

[0014] Furthermore, the relative distance between the diffractive light introducer and the color CMOS image sensor can be finely adjusted for spacing calibration.

[0015] Furthermore, the color CMOS image sensor is connected to the support structure by screws, which can convert the rotation into axial displacement of the support structure, thereby enabling continuous adjustment of the distance between the diffractive light introducer and the color CMOS image sensor.

[0016] Furthermore, the color CMOS image sensor is a Thorlabs CS505CU1.

[0017] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0018] (1) This invention innovatively adopts a diffraction order multiplexing mechanism, which introduces light rays that are traditionally undetectable due to large angles into the detection field of view through specific higher-order diffraction, thereby expanding the angular detection range of the system without sacrificing angular resolution. Furthermore, to address the order ambiguity problem introduced by diffraction, the centroid spacing of the dual-band light spots enables unambiguous identification of the diffraction order corresponding to the light spots, ensuring the uniqueness and high accuracy of the measurement results under large field of view conditions.

[0019] (2) This invention abandons the mechanical, energy-intensive, or complex optical auxiliary means used to solve order ambiguity, and instead relies on static spectral filtering and image processing algorithms. This design strategy not only greatly simplifies the complexity of the optical-mechanical structure and reduces the size and weight of the system, but also significantly enhances its adaptability and long-term reliability in harsh aerospace environments such as mechanical vibration, thermal cycling, and changes in lighting conditions due to the absence of moving parts and insensitivity to absolute light intensity. It is highly compatible with the urgent needs of modern micro-miniature aerospace platforms for payload, high functional density, and high robustness.

[0020] (3) The core optical components used in this invention, such as diffraction gratings and bandpass filters, can all be mass-produced and standardized using mature optical thin film and micro-nano fabrication processes. The color CMOS image sensor is a mature commercial device, and the signal processing algorithm can be implemented on a general embedded platform. Therefore, the entire system has excellent manufacturability, consistency, and cost control potential, providing a highly competitive high-performance solution for future applications requiring large-scale, low-cost deployment, such as large constellations and distributed detection systems. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating the working principle of a traditional digital sun sensor.

[0022] Figure 2 This is a schematic diagram of a field-of-view extended solar sensor based on diffraction order multiplexing provided in an embodiment of the present invention;

[0023] Figure 3 This is a schematic diagram of the principle of a field-of-view extended solar sensor based on diffraction order multiplexing in an embodiment of the present invention;

[0024] Figure 4 This is a schematic diagram of the processing flow of the diffractive light introducer in an embodiment of the present invention. Detailed Implementation

[0025] The invention will now be further described with reference to the accompanying drawings.

[0026] Appendix Figures 1 to 4 The accompanying figure labels are as follows:

[0027] 1. Dual-band bandpass filter; 2. Light introducer; 201. Light-shielding mask; 202. Diffraction grating; 3. Support structure; 4. CMOS image sensor; 5. Signal processing unit.

[0028] like Figure 2As shown, this embodiment of the invention provides a field-of-view extended solar sensor based on diffraction order multiplexing, including a dual-band bandpass filter 1, a light inlet 2, a support structure 3, a CMOS image sensor 4, and a signal processing unit 5. The light inlet 2 is a diffractive light inlet, and the CMOS image sensor 4 is a color CMOS image sensor. The dual-band bandpass filter 1, the diffractive light inlet, and the color CMOS image sensor are sequentially mounted on the support structure 3 along the optical path. The dual-band bandpass filter 1 transmits sunlight within two preset wavelength bands, with the spectra of the two preset wavelength bands being mutually isolated. The diffractive light inlet, as the core light modulation device, diffracts the transmitted sunlight into a beam containing multiple diffraction orders, and the color CMOS image sensor acquires the diffraction spot images corresponding to the two preset wavelength bands. The two preset bands of the dual-band bandpass filter 1 correspond one-to-one with the two different spectral response channels of the color CMOS image sensor (i.e., the spectra of the two isolated preset bands are only effectively detected by the corresponding response channels). This allows a pair of double spots to be formed on the color CMOS image sensor for sunlight with a single incident direction. The relative position of these double spots serves as an order code, uniquely mapping to the diffraction order that generated them. The signal processing unit 5 is electrically connected to the color CMOS image sensor and is used to process the images acquired by the color CMOS image sensor and calculate the angular information of the solar vector.

[0029] Here is a specific example.

[0030] (a) Dual-band bandpass filter

[0031] The dual-band bandpass filter 1 is selected as MBP-A435-600, with center wavelengths of 435nm and 600nm, and bandwidths of 30nm and 90nm, respectively.

[0032] (ii) Color CMOS Image Sensor

[0033] The selected color CMOS image sensor is the Thorlabs CS505CU1, with an imaging area size of 9.28mm × 5.90mm and a single pixel size of [missing information]. The pixel size is 3.45μm, and the red and blue pixel response ranges are approximately 400nm-525nm and 575nm-925nm, respectively.

[0034] (III) Supporting Structure

[0035] Support structure 3 is fabricated from PLA material using precision 3D printing technology (using a Bamboo Lab X1 Carbon printer with a precision of 0.08 mm). This support structure 3 is used to fix the dual-band bandpass filter 1, the diffractive light introducer, and the color CMOS image sensor. The relative distance between the diffractive light introducer and the color CMOS image sensor is... The design is 20mm. The relative distance between the diffractive light introducer and the color CMOS image sensor is finely adjustable for spacing calibration. Specifically, the backplate of the color CMOS image sensor and the support structure 3 are machined with metric M2 threaded holes, and the two are connected by M2 screws. By tightening the M2 screws, the rotation of the screws can be precisely converted into the axial displacement of the support structure 3, thereby achieving micron-level continuous adjustment of the spacing between the diffractive light introducer and the color CMOS image sensor.

[0036] (iv) Diffractive light introducer

[0037] Combination Figure 3 The diffractive light introducer consists of a light-shielding mask 201 (made of metal) with a central light-transmitting hole of 200μm×200μm on a fused silica substrate (BK7), and a grating constant integrated in the light-transmitting hole region. It consists of a diffraction grating 202 with a diameter of 2.1 μm.

[0038] The following is Let's take direction as an example to illustrate. The direction is based on a one-dimensional grating. By using the superposition of two orthogonal one-dimensional gratings, or by directly using a two-dimensional diffraction grating, the spectral identification and order unambiguation method described in this invention can be naturally extended to a two-dimensional plane, thereby realizing the determination of the solar vector in... and Angle measurement in two directions.

[0039] According to the grating equation, for an incident angle of... The incident light, its Diffraction angle It can be represented as:

[0040]

[0041] in, It is the wavelength of the incident light; It is the grating constant.

[0042] Therefore, the grating The offset of the diffraction spot on the color CMOS image sensor is .

[0043] For the center wavelength of the MBP-A435-600 dual-band bandpass filter 1 =435nm and =600nm, its Order diffraction spot offset and They can be represented as:

[0044]

[0045] in, and The center wavelengths are respectively and The m-th order diffraction angle at that location. The imaging area size of the color CMOS image sensor is 9.28mm × 5.90mm (here, the size of the color CMOS image sensor is taken as...). Given its shorter side is 5.90 mm, the following can be calculated to ensure the first wavelength at both wavelengths. The incident angles at which all diffraction spots fall on the imaging region satisfy the following conditions:

[0046]

[0047] Substituting the data, for the diffraction order The fields of view can be calculated as follows: =[-34.1 -25.2 ], =[-20.7 -8.0 ], =[-8.4 8.4 ], =[8.0 20.7 ], =[25.2 34.1 Therefore, it can be guaranteed that within any field of view within the overall ±34.1° field of view, at least one complete pattern will be imaged on the color CMOS image sensor. During the transition phase, before the image formed by one set of patterns leaves the color CMOS image sensor, another set of patterns will be imaged on the color CMOS image sensor, thus avoiding blind spots during field of view transition.

[0048] Furthermore, the blue channel on a color CMOS image sensor ( ) and the red channel ( ), Interval between diffraction spots for:

[0049]

[0050] Considering ,have ,therefore This can be simplified to: .

[0051] like Figure 4 As shown, the diffractive light introducer needs to be fabricated using micro-nano fabrication processes. The specific process includes: 1) cleaning the fused silica substrate and spin-coating SU8 photoresist; 2) using a laser direct writing system to expose the pattern of the light-transmitting aperture portion of the light-shielding mask and the overlay markings; removing the photoresist from the unexposed areas through development and fixing processes; 3) using a thermal evaporation process to deposit a Cr metal thin film as a light-shielding layer on the entire substrate surface; 4) using a stripping solution to remove the light-shielding mask from the substrate; 5) flipping the sample and spin-coating SU8 photoresist on the reverse side of the substrate; 6) using a laser direct writing system to expose the diffraction grating pattern; removing the photoresist from the unexposed areas through development and fixing processes, thereby forming an etching mask on the photoresist layer; 7) using an inductively coupled plasma (ICP) dry etching process to etch the exposed quartz substrate to form the required grating structure.

[0052] (v) Signal Processing Unit

[0053] The process by which signal processing unit 5 processes the image and calculates the angle information of the solar vector is as follows:

[0054] 1) Image Acquisition and Preprocessing: After sunlight passes through the dual-band bandpass filter 1 and the diffractive light introducer, it forms a diffractive light spot image on the color CMOS image sensor. The signal processing unit 5 reads the original image and first uses adaptive Wiener filtering to denoise the image in order to suppress noise and improve the signal-to-noise ratio, laying the foundation for subsequent accurate feature extraction.

[0055] 2) Spot Recognition and Centroid Localization: Utilizing the separation of the blue (B) and red (R) response channels of a color CMOS image sensor, the corresponding light spots are extracted respectively. =435nm and Two diffraction spot images with a wavelength of 600nm were used. A set of dual-spot images closest to the center of the color CMOS image sensor was selected, and edge detection was performed using the Sobel operator. A two-dimensional Gaussian surface fitting method based on least squares was employed to locate the centroid of each spot at the sub-pixel level, obtaining the centroid coordinates of the blue channel spot. Coordinates of the centroid of the red light channel spot .

[0056] 3) Field of view recognition: Calculate the centroids of the two light spots in... Spacing in direction , and the spacing By comparison, the effective diffraction order corresponding to the current solar incidence angle can be uniquely and unambiguously determined. .

[0057] 4) Solar vector calculation: combining known parameters =20mm, grating constant =2.1μm and wavelength =435nm and =600nm, calculate the incident angle of the solar vector according to the following diffraction optical model. :

[0058]

[0059] right and The angle of incidence of the solar vector can be obtained by averaging. .

[0060] Similarly, for two-dimensional gratings, it is possible to... The same calculation is performed in the direction to finally obtain the angle of incidence of the solar vector on the two-dimensional plane. .

[0061] The solar sensor in this embodiment achieves the following technical specifications on the prototype: the field of view is expanded from approximately ±8.4° in the traditional scheme to ±34.1°, and the angular resolution is better than 0.01° throughout the entire expanded field of view. The core optical head volume can be controlled to the cubic centimeter level, and the weight is only tens of grams. This embodiment fully verifies the excellent effects of the present invention in achieving a large field of view, high precision, and lightweight design.

Claims

1. A field-of-view extended solar sensor based on diffraction order multiplexing, characterized in that, The system includes a signal processing unit (5), a dual-band bandpass filter (1), a diffractive light introducer, and a color CMOS image sensor arranged sequentially along the optical path. The dual-band bandpass filter (1) is used to transmit sunlight within two preset wavelength bands, the spectra of which are mutually isolated. The diffractive light introducer is used to diffract the transmitted sunlight into a beam containing multiple diffraction orders. The color CMOS image sensor is used to acquire diffraction spot images corresponding to the two preset wavelength bands. The two preset wavelength bands of the dual-band bandpass filter (1) correspond one-to-one with the two different spectral response channels of the color CMOS image sensor, so that for sunlight with a single incident direction, a pair of double spots can be formed on the color CMOS image sensor. The relative position of the double spots is used as an order code, which has a unique mapping relationship with the diffraction order that generates the double spots. The signal processing unit (5) is used to process the images acquired by the color CMOS image sensor and calculate the angle information of the solar vector. The steps configured to be executed by the signal processing unit (5) include: extracting the centroid positions of the diffraction spots in the corresponding spectral channels of two preset bands from the image acquired by the color CMOS image sensor; calculating the distance between the centroids of the spots between the two spectral channels; determining the effective diffraction order corresponding to the current solar incidence angle based on the distance between the centroids of the two spots and the preset unique mapping relationship between them and the diffraction order; and calculating the angle information of the solar vector through the diffraction optical model by combining the effective diffraction order, the parameters of the diffraction ray introducer, and the distance between the diffraction ray introducer and the color CMOS image sensor.

2. The field-of-view extended solar sensor based on diffraction order multiplexing according to claim 1, characterized in that, The dual-spot selection is based on the pair of dual spots closest to the center of the color CMOS image sensor.

3. The field-of-view extended solar sensor based on diffraction order multiplexing according to claim 1, characterized in that, The diffractive light introducer includes a light-shielding mask (201) with a central light-transmitting aperture and a diffraction grating (202) integrated into the light-transmitting aperture region.

4. The field-of-view extended solar sensor based on diffraction order multiplexing according to claim 3, characterized in that, The diffraction grating (202) is made by superimposing two orthogonal one-dimensional gratings.

5. The field-of-view extended solar sensor based on diffraction order multiplexing according to claim 3, characterized in that, The diffraction grating (202) uses a two-dimensional diffraction grating.

6. The field-of-view extended solar sensor based on diffraction order multiplexing according to claim 1, characterized in that, A dual-band bandpass filter (1) and a diffractive light introducer are set on the upper side of the 3D printed support structure (3), and a color CMOS image sensor is set on the lower side of the support structure (3) to achieve integrated design.

7. The field-of-view extended solar sensor based on diffraction order multiplexing according to claim 6, characterized in that, The relative distance between the diffractive light introducer and the color CMOS image sensor can be finely adjusted for spacing calibration.

8. The field-of-view extended solar sensor based on diffraction order multiplexing according to claim 7, characterized in that, The color CMOS image sensor is connected to the support structure (3) by screws. The screws can convert the rotation into the axial displacement of the support structure (3), thereby realizing the continuous adjustment of the distance between the diffractive light introducer and the color CMOS image sensor.

Images