Microshutter Array Design of Field-of-view Gated Imaging Systems for All-time Star Sensor

Traditional all-time star sensors usually have a narrow Field of View （FOV） and adopt single star tracking mode. Because of only one target star in the FOV， the optical system should be installed on a two-dimensional rotation/scanning platform， or multiple telescopes are used to achieve synchronous measurement， which can lead to many shortcomings such as large volume， low reliability and poor autonomy. The optical imaging system based on FOV gated technology can obtain the wide FOV and strong sky background radiation suppression ability at the same time by combining a large total FOV with a narrow gated FOV， which is expected to achieve multi-star detection and star pattern recognition in the daytime. It has the advantages of small volume， light weight and good autonomy. In the FOV gated optical imaging system， it is necessary to use a key device of microshutter array to quickly switch the gated FOV.Microshutter arrays need to have the characteristics of large element size， high duty cycle and high response speed. At present， there is no microshutter array that meets the requirements can be used. In this paper， a short-wave infrared band FOV gated imaging system principal prototype is designed. In this system， two sets of microlens arrays are used， the aperture of the microlens element is 4 mm， the FOV gated imaging channel number is 7×7， and each imaging channel can obtain near diffraction limit imaging quality. The microshutter array is placed behind two sets of microlens arrays， and the size of the microshutter element is also 4 mm. The position of the microshutter element corresponds to that of the microlens element one by one. In order to meet the application requirements of the FOV gated imaging system， the main design parameters of the microshutter element are determined as follows. The area duty cycle is not less than 90%， the switching response time is not less than 25 ms， and the drive voltage is not more than 120 V. Based on the principle of electrostatic parallel plate capacitive drive and the MEMS bulk silicon process，two rectangular silicon thin plates are designed as light shields for microshutter elements. Furthermore，the light shields are also used as upper electrodes， and the lateral sides of silicon substrate are used as lower electrodes. By loading and removing the drive voltage， the switching of the open and closed states of a microshutter element can be realized. Considering the material characteristics， parameters such as electrode width， cantilever beam width and thickness are first determined. Then， according to the mathematical model of the microshutter element， the influence of different number and length of cantilever beam on the drive voltage and response time is analyzed. The calculation results show that the driving voltage increases with the number of cantilevers beams and decreases with its length. In order to minimize the driving voltage， the number of cantilevers beams should be 2， and the beam length should be greater than 200 μm. On the other hand， since the longer the cantilevers beam， the longer the response time of the microshutter element. In order to meet the 25 ms response time requirement， the beam length of the cantilevers beam is determined as 200 μm.The Ansoft Maxwell electromagnetic field simulation software is used to simulate the electrostatic moments of the microshutter at different torsion angles and then the Comsol Multiphysics finite element analysis software is used to calculate the torsional elastic coefficient of the cantilever beam. According to the balance conditions of the electrostatic moment and the elastic recovery moment，the variation of the torsion angle and the drive voltage is presented. The results show that the microshutter element can be opened when the drive voltage is 106.4 V， which is basically consistent with the theoretical calculation result of 114 V and demonstrates the feasibility of the design parameters of the microshutter array. In addition， since only one microshutter element is turned on under normal operating conditions，the light incident to the other unopened microshutter elements can be reflected or absorbed. The reflected light can form stray light within the optical system， which will affect the distribution of sky background radiation on the image surface of the detector. Therefore， according to the structural layout of the designed FOV gated imaging system， the effect of the surface reflectivity of the microshutter element on the stray light in the system is simulated and analyzed by using the advanced stray light analysis software ASAP. The results show that as the surface reflectivity of the microshutter element increases from 3% to 80%， the average illumination of the sky background radiation at the image plane on the surface of the detector increases very little， and the impact on stellar detection is almost negligible. Therefore， in the design of microshutter arrays， there is no special requirement for the reflectivity of the microshutter surface. This study provides a theoretical basis for the processing of microshutter array in FOV gated imaging system.

Based on the imaging principle and characteristics of the field of view gated imaging system for all day star sensors, an imaging channel of 7 has been preliminarily designed × The 7 field of view gated imaging system has an instantaneous gated field of view of 0.4 °, and the imaging channels of each gated field of view can achieve near-diffraction limit imaging. For this imaging system, a unit size of 4 mm, opening area duty cycle of 90%, response time of 25 ms, and number of units based on electrostatic drive have been designed × 7 microswitch array. Considering the large aperture of the microswitch unit, two rectangular thin plates were used as light blocking layers in the design. These two light blocking layers also serve as upper electrodes, and in conjunction with the lower electrode on the side of the substrate, the microswitch unit switches between on and off states by loading and removing the driving voltage. Starting from the material characteristics of bulk silicon processing technology, parameters such as electrode width, support beam width, and support beam thickness of the microswitch unit were first determined. Then, theoretical analysis was conducted on the number and length of support beams based on the mathematical model of the microswitch unit structure, and the main structural parameters of the microswitch array were optimized and designed. The driving characteristics of the microswitch unit were simulated and analyzed using Comsol Multiphysics and Ansoft Maxwell simulation analysis software. The results showed that the microswitch unit can be turned on when the driving voltage of the microswitch unit calculated by simulation is 106.4 V, which is basically consistent with the theoretical calculation result of 114 V, verifying the feasibility of the microswitch array design parameters. At the same time, the ASAP stray light analysis software was used to simulate and calculate the impact of surface reflectivity of 3%, 50%, and 80% microswitch arrays on the distribution of stray light in the introduced system. The results showed that the surface reflectivity of microswitch units had a small impact on the stray light in the system, and its impact on star detection under bright background conditions was almost negligible. Therefore, in the design and processing of microswitch arrays, there is no need to make special requirements for their surface reflectivity. This study provides an effective design scheme for microswitch arrays in field of view gated imaging systems, and provides a theoretical basis for the actual processing of microswitch arrays in field of view gated imaging systems.