In order to meet the demands of calibration on the ground for optical navigation sensor with high precision,a method is given to design precise star charts simulator based on the characteristics of conventional technique by combined the OLED light source with fiber-optic light guide technology.The general structure of star charts simulator is presented after the analysis of its design.The coupling efficiency between the OLED and fiber is improved by a rebuilding of the coupling structure between the optical fiber and the light source as well as the fiber holding plate.By optimizing the design of the self-focusing lens in coupling system and the detail design of filters in stellar magnitude simulation system,aprecise control is achieved to 5~10 for the stars positional accuracy.It is found that the design for star charts simulator approaches the requirement for the calibration of precise optical navigation star sensor through the theoretical analysis and the measurements for the magnitude and the stars positional accuracy.
With the development of deep space exploration projects, conventional star simulators used for calibration of near-Earth detection sensors are no longer able to meet the ground calibration and detection requirements of deep space detection star sensors. From existing relevant information, it is known that there is currently no high-precision star simulator developed in China with performance indicators (inter star angular distance accuracy not greater than 1 “, star magnitude not less than 10 class stars). The star simulator consists of two parts: a target standard source and a collimating optical system, where the target standard source is a device designed to provide a standard star map for the calibrated sensor. The existing target source designs mostly use two star pattern simulation methods: liquid crystal display (LCD) backlight plate combined with fixed star point plate and liquid crystal light valve control backlight plate. However, for deep space navigation sensors that need to capture higher star levels and high-precision inter star angular distance performance, both methods have drawbacks in providing high-precision calibration parameters. For example, when using a fixed star chart board for simulation, different star charts require the replacement of targets; When using liquid crystal light valve control, the stability of lower brightness is greatly affected by dynamic noise, which makes it impossible to achieve accurate simulation of high stars, and rectangular pixels cannot meet the roundness requirements of star points for sub pixel calibration accuracy. Therefore, in response to the calibration requirements of a certain star sensor, this article designs a high-precision target standard source that can achieve indicators such as high star magnitude, high star point spacing accuracy, and single star real-time control.
(1) Main technical indicators of target standard source
According to the calibration requirements of high-precision star sensors, the main technical indicators of the designed target standard source are shown in Table 1.
(2) Design scheme and working principle of target standard source
To eliminate the technical shortcomings of traditional target sources, a high-precision star point target method is used for star map simulation by combining organic light-emitting device (OLED) array light source and fiber optic beam coupling mechanism. The use of optical fibers is considered to have a circular emitting end face, which is easy to improve the accuracy of the centroid method in calculating star point positions; The use of OLED is due to its controllable brightness of a single pixel, as well as its higher contrast and illumination uniformity compared to LCD backlight boards, making it possible to simulate high stars and other phenomena. The designed target standard source consists of two parts: a variable star and other target simulator illumination system and a high-precision target, as shown in Figure 1.
The OLED is controlled by a precision controllable power supply to emit light from pixel points at predetermined positions. The emitted light enters the optical coupling system composed of a self focusing lens group and an optical fiber in/out plate, and is projected onto the star map target at the focal plane of the optical collimation system of the star simulator. A specific star map is simulated by several transparent micropores on it. The brightness of each individual star point is controlled by each coupled beam, thus forming a static variable star and other target standard sources.
(1) Target standard source structure design
The overall structure of the target standard source is shown in Figure 2, consisting of high-precision target assembly lenses, optical coupling structures, and OLED light sources. To ensure the stability of the coupling optical path, the coupling mechanism and OLED light source are fixed in the same metal bracket; To avoid damaging the optical fiber during fiber replacement, the fiber inlet and outlet plates are locked with bolts; At the same time, in order to ensure that the end face of the optical fiber outlet plate corresponds to the position of the target star point, the target and the optical fiber outlet plate are placed in the same sleeve, and installed in the assembled mirror tube with the optical attenuation plate and filter; The target standard source is equipped with a collimating optical system interface, which provides high-precision star maps for calibration after assembly.
(2) Structural Design of Optocoupled System
From the perspective of collimation and luminous area, the OLED light source used differs significantly from commonly used fiber coupled light sources (such as lasers), making it impossible to achieve direct light coupling. Therefore, it is necessary to design an optical coupling mechanism to improve coupling efficiency. As shown in Figure 3, the optical coupling mechanism adopts a self focusing lens array structure, where each lens receives light from the fixed area of the OLED and couples with the corresponding optical fiber to form a fiber light source coupling system. Figure 4 shows the dual coupling structure diagram.
Attach an insulating sheet with a circular hole array (paired with a self focusing lens group) to the OLED light source surface to segment the entire emitting surface, and a single emitting circular hole will serve as the light source corresponding to the self focusing lens; In response to the small size and large number of lenses in the lens group, a pressing plate with a through hole array is used to replace a separate pressing ring for axial fixation of the lens group; Considering the processing error of the lens, a coupling distance adjustment plate is installed between the lens and the fiber incident plate. Grinding its thickness can correct the coupling rate, ensuring that the focusing spot strictly covers the end face of the fiber core to meet the requirements of star point non-uniformity; To ensure the assembly accuracy of the optical coupling mechanism and the perpendicularity of the lens optical axis and the end face of the coupling fiber, the components are positioned and locked with pins, and then the lens group pressing plate, coupling distance adjustment pad, and the through hole array of the fiber incident plate are sequentially processed; After the optical fiber is glued to the optical fiber inlet/outlet plate, the surface is then precision ground.
(3) Design of high-precision target and fiber input/output plate
The star point spacing accuracy of the designed target standard source is ensured by the machining accuracy of the target target and the fiber input/output plate. The target is made of quartz glass with high transmittance and low linear expansion coefficient as the substrate. One side is coated with a metal film, and the other side is coated with a reduced reflection film. Laser direct writing lithography technology is used to process 121 circular through-holes at a certain coordinate position. The dots without metal shading film represent star points, and the black opaque area represents the actual star sky. Combined with the light beam guided by each fiber, simulation of the position and illumination of star points in the sky will be achieved. The effective diameter D of the target can be determined by the focal length f of the collimating optical system and the field of view a used × The size of the image plane determined by a. The fiber optic input/output plate used in conjunction with the array light source and target is made of titanium alloy material with a smaller expansion coefficient. Referring to the diameter of the optical fiber and the coordinate conversion ratio between the effective size of the OLED light source and the target size, circular holes are machined on the optical fiber input/output plate, and optical fibers are inserted into the holes for adhesive fixation. Then, the surface is precision ground and polished,
The optical system of the standard target source includes two parts: a beam coupling system and a star point output simulation system. Its function is to couple the light source into the optical fiber and output it to the target, and after filtering and attenuation, it is emitted from the star simulator collimation optical system. The principle is shown in Figure 6,
(1) Design of Light Source and Fiber Optic Coupling System
The OLED used in the design is a self luminescent material with built-in electronic circuit system, and each pixel is independently driven by the corresponding circuit. As a surface light source, it has high luminous efficiency and good uniformity, meeting the design requirements of the target source. The specifications are determined by the contrast, brightness, and size of the star map.
Given that the brightness difference between each magnitude is 2.51 times, the contrast M of the selected target source for simulating 5-10 magnitude stars should meet 100
(2) Selection of light sources and optical fibers
In the practical use of optical fibers, the efficient coupling between the fiber and the beam is the main problem that needs to be solved. The efficiency depends on the matching of the luminous area and the fiber core area, as well as the matching of the light source divergence angle and the fiber numerical aperture angle. Select fiber optic parameters based on indicator requirements: fiber core diameter Φ 60 μ m. A multimode fiber with a numerical aperture NA of 0.6, with an aperture angle of 2 θ C is approximately 62 °.
Based on the radiation flux of 0 magnitude stars, 6.87 × 10-13W/cm ²， It can be calculated that the radiation flux of Class 5 stars is 6.87 × 10-15W/cm ²， The radiation flux provided by the array OLED for each coupled system should be greater than this value after various attenuation factors. Due to the requirement of having 100 star points in the star map, the single star size ranges from 60 to 100 μ If one fiber corresponds to one star point between m, the OLED surface size can be determined based on the calculated effective target size and fiber spacing.
(3) Design and optimization of self focusing lenses
Usually, end face spherical lenses, cylindrical lenses, or conical lenses are used in coupling systems to improve coupling efficiency, but it is difficult to apply high-precision fiber end face processing technology. This article adopts the convex lens pairing method and selects a self focusing lens with the advantages of small size, easy processing and assembly, easy adjustment and alignment, and high coupling efficiency to replace the lens combination of first collimating and then converging. The designed self focusing lens structure is shown in Figure 7, and its parameters are given in Table 2.
(4) Design of Star Point Output Simulation System
The simulator requires a spectral range of 500-800nm and uses optical filters to correct the spectral characteristics of the light source. Due to the fact that the receiver of the sensor is a CCD, the cut-off wavelength is generally up to 1200nm. Therefore, the filter is designed with a cutoff wavelength of below 500nm and a cutoff wavelength of 800-1200nm, with a center wavelength of 650nm, to achieve spectral irradiation of 500-00nm. Considering that the brightness of stars 5 to 10 is very low, and the OLED luminescence stability is better when displaying brightness in normal state, optical attenuation plates can be installed at the higher brightness output end to attenuate the energy of star points to achieve precise brightness control of stars 5 to 10. The attenuation plates can also be replaced according to practical applications.
For the high-precision standard target source designed, the simulated magnitude accuracy and star point spacing position accuracy are key technical indicators that determine the calibration level of the star sensor by the standard target source. However, the design difficulty lies in the precision control of the fiber optic spot brightness.
(1) Analysis and Testing of Magnitude Brightness
From the design principle, it can be seen that coupling efficiency is an important indicator of the coupling mechanism designed in this article. Its influencing factors include the longitudinal accuracy, lateral spacing error, and fiber end face tilt error of the mechanical alignment between the fiber incident plate and the self focusing lens group. Through the overall coordination and processing of the optical coupling mechanism, the grinding of the coupling distance adjustment plate, and the polishing and polishing of the input/output plate, the coupling efficiency (including 5% reflection of the fiber end face and 5% fiber loss) can be controlled above 65%.
The control of the brightness of each star point requires testing the actual brightness of the star point on the target and the corresponding OLED luminous position driving current value, establishing the actual functional relationship between the magnitude and current, and achieving precise control of the star point brightness through control software. The BM-5A brightness meter (with brightness ranging from 0.001 to 1200000cd/m2 and accuracy of ± 4%) was used for actual testing of the target star map. Figure 9 shows a comparison between the theoretical brightness of the input star magnitude and the corresponding measured star point brightness.
(2) Analysis and testing of star point spacing accuracy
Star point spacing accuracy Δ S refers to the linear displacement error between any two star points in the target star map of the target standard source. When calibrating the sensor parameters in the star simulator, it is converted into the inter star angular distance accuracy and given. In small field of view and large focal length optical systems Δ Interstellar angular distance error caused by s Δ S is the star point spacing accuracy of the target standard source, and f is the focal length of the collimating optical system of the star simulator.
To meet the calibration requirements of high-precision navigation sensor inter star angular distance accuracy, it is necessary to strictly control and reasonably allocate the collimation optical system error of the star simulator and the design accuracy of the target standard source. Considering that the straightness of the x-axis and y-axis within a 200mm stroke of the laser direct writing equipment used for target star map preparation is not greater than 0.15 μ m. Therefore, control the accuracy of star point spacing on the target panel Δ S ≤ 1 μ M. A precision microscope was used to measure the designed target. Considering that the deformation of the target under the extreme temperature of the calibration environment has little impact on the accuracy of star point spacing, only the accuracy of the center star point position of each point in the target star map at room temperature is given. Taking the actual design as an example, when the calibration accuracy of the inter star angular distance of the optical navigation sensor is not greater than 1 ″, and the focal length of the collimating optical system of the star simulator is f=5000mm, the designed target standard source can ensure the accuracy of the inter star distance σ≤ 0.14 ″.
This article designs a high-precision target standard source composed of a variable star and other target simulator illumination system and high-precision targets. Compared with existing related equipment, it has advantages such as standard star size, high uniformity, and variable magnitude. According to the calibration index requirements, detailed designs were carried out on the optical and mechanical structures of the fiber optic light source coupling mechanism, fiber optic input/output plate, and target. After theoretical analysis and practical testing, the designed target standard source can not only ensure the roundness and non-uniformity indicators of star points required for star sensor calibration, but also achieve an accuracy of star point spacing not exceeding 1 μ m. The simulation of 5-10 stars meets the calibration requirements of high-precision sensors.
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