Study On Key Technique Of High-precision Dynamics Star Simulator Based On LCOS Splicing Technology

Star sensor is a device taking stars as references to measure attitude and position.Accurate,highly independent and tiny,it is currently used widely in aerospace,guidance and navigation,etc.Due to the important usage of star sensor,its function of on-orbit working performance requires massive on-ground tests.And thus the star simulator is particularly important as on-ground testequipment of the star sensor.In terms of tests on star sensors,the star simulators can be divided into two categories: standardized static ones and function-testing dynamic ones.Researches on technology of dynamic star simulator have been deepened over years in China.But the lagging display device has constrained its overall development.Elements such as large size and gap of pixel and low contrast have slowed down the enhancement of performance of dynamic star simulator.This paper briefly introduces types of celestial sensors and summarizes the features of the star sensors with the highest accuracy among them,and it thereby clarifies the specific requirements of the star simulator in on-ground test.Based on the contrast and analysis of the current development of the dynamic star simulators at home and abroad,the main development direction of the dynamic star simulator is summed up: high precision,miniaturization and high dynamic.Reflective liquid crystal on silicon(abbr.LCOS)due to its own characteristics fits into to aforementioned development trend.Besides,the use of optical splicing method can effectively improve the accuracy of the star position.The dynamic star simulator being researched is designed based on LCOS splicing.But as a newly-born micro-display component,LCOS technology is not yet mature.In the stitching environment of the LCOS display background exists excessive stray light,resulting in degradation of star sensor's accuracy of centroid extraction and consequently a failure to guarantee precision of star location displayed.In order to improve the extraction and location accuracy of centroid of star spot,the key technology of restraining stray light and position correction of star spot applied in LCOS splicing dynamic star simulator is studied in this paper.The main function of star simulator is achieved by star navigation simulation on-ground,in order todetermine the spatial position between the simulated celestial bodies,the basic theory of star navigation has been summarized.In this paper,the overall design scheme of high precision dynamic star simulator based on LCOS is presented according to the function of the dynamic star simulator,and the structure layout is described.According to the test conditions of star sensor,dynamic star simulator selects the corresponding simulation program and test adjustment mechanism.Then the algorithm with partition searchingof star image simulation program and the main design parameter adjustment mechanism are discussed in detail.Through the research on the influence of stray light disturbance on the extraction precision of star sensor,a conclusion can be secured that the star spot centroid is deviated due to the influence of optical noise on diffuse star spot,thenthe necessity of stray light suppression is verified.The paper analyzes the causes of stray lightand probes the influence of the degree of polarization on stray light,and therebyformulates the mathematicbetween the angle of incidence and the degree of polarization.And based on the research result,the method that suppresses stray light by optimizing LCOS splicing optical engine is identified,specifically,optimizing the splicing optical structure by replacing a single polarized beam splitter(abbr.PBS)with a prism combination consists of two pieces of PBS and a set of transflective lens to improve the consistency of spliced images and setting a 1/4 wave plate in light path of optical system to eliminate stray light outside the system;optimizing lighting mode byilluminating LCOS with narrow field angle light source,increasing the polarizing efficiency of the illuminating ray and reducing the stray light caused by insufficient degree of polarizationto improve the extraction accuracy of star spot centroid.The performance evaluation onthe stray light suppression of LCOS splicing dynamic star simulator is carried outin order to verify the feasibility the suppression method applied,which shows that the stray light of optimized LCOS splicing dynamic star simulator is 2.93 times lower and at the same time the extraction accuracy is 2.46 times higher than before.Opto-mechanical structure and its illuminating optical engine are optimized according to the theory of stray light suppression.LCOS splicing structure adopts the design principle of orientable adjustment.Through analyzing the precision of the optimized splicing structure,the tolerance range of each component in the design process is obtained.According to the adjustment dimension that the splicing system needs to realize,the constraints of the degrees of freedom are summarized and the corresponding design is carried out.Finally,the adjustment accuracy of relative displacement and rotation of two pieces of LCOS is improved by constraining the degree of freedom independently.In this paper,the deviation brought about by splicing is discussed,and it turns out that the main source of deviation is the alignment deviation in the splicing process.Finally,the conclusion that the splicing deviation is less than 1/3 pixel is obtained by adopting the cross-hair alignment method.As for the optimization design of the illumination optical system,the implementation method of narrow field angle light source is mainly described,specifically,re-distributing light of white LED through the compound parabolic concentrator(abbr.CPC),zooming the field angle of the beam emitted by CPC through telescope system and expanding the irradiation surface according to the principle of laser beam expansion.In order to increase the uniformity of illumination,two sets of compound eye lenses are set as optical-uniform device on the focal plane of the telephoto system.Parameters of the main optical components are calculated according to the design requirements.Tracepro is used to simulate lighting optical system.It has been finally verified that divergence angle of designed light source is not greater than°± 5.In addition,non-homogeneity on the irradiated surface should not be lower than 7.3%,which meets the simulation results.The selection of the imaging optical system and the influence of aberrations on the accuracy of the star spot position of the LCOS splicing dynamic star simulator are studied and the selection principle of the initial structure is analyzed in detail.The enhanced structure of Erfle eyepiece with large interpupillary distance,low distortion,and achromatism is adopted as initial structure according to the optical structure characteristic and the required aberration characteristic of the LCOS splicing dynamic star simulator.The aberration of the designed imaging optical system is estimated and the initial structure is modified according to the relationship between the field of view,relative aperture and aberration.After the ZEMAX optimization,the eyepiece system is designed with a field angle of ?11.1oand a focal length of 79.2mm.The image quality is evaluated by the designed optical system,which yields the result that all the main design parameters are better than those of expected.The actual MTF is tested to be ? 0.49 at v = 60 lp / mm,which is better than the required system parameters.In order to realize the requirement of the high precision display of the star spot in the dynamic star simulator,the main factors influencing the precision of the star light emission are analyzed,and the method of improving the precision of the starlight emission by the aberration correction theory is studied.The fixed-point aberration compensation to pixel point facilitated with the pixel compensation technique based on the wave-front aberration theory is discussed in detail.The accuracy of aberration correction is verified to be improved by the aberration compensation technique,and the emitting accuracy of star light is better than ± 15 ".After analyzing the precision of the system,the conclusion is secured that the deviation revised by with aberration compensation technique is 7.8".