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在激光半主动寻的制导中,根据分布在探测器不同象限的光斑能量进行信号处理,读取角度偏差值,对信号的角度偏差进行计算,进而得到信号与光轴的夹角,调整导弹飞行方向,对准目标。
角度偏差是通过对四象限探测器输出的各象限信号幅值进行运算得出,且采用对数相减的运算方式,其运算原理见图1,以下两式用于计算角度偏差:
$${E_{{x}}} = \lg \left( {{S_{\rm A}} - {S_{\rm B}}} \right) - \lg \left( {{S_{\rm C}} - {S_{\rm D}}} \right)$$ (1) $${E_{{y}}} = \lg \left( {{S_{\rm A}} - {S_{\rm D}}} \right) - \lg \left( {{S_{\rm C}} - {S_{\rm B}}} \right)$$ (2) 式中:SA、SB、SC、SD分别为光斑在四象限探测器中A、B、C、D象限对应的能量值强度;当目标位于视场中心时,Ex、Ey相等,导弹对准目标,可准确地飞向攻击目标。当Ex、Ey不等时,说明导弹与目标具有角度误差,通过实时处理Ex、Ey两者差值,不断自动修正导弹飞行方向,直至导弹准确对准目标,进行攻击。此运算方式可以实现高精度测角,对光斑大小及均匀性要求较高;光斑的大小及均匀性不符合要求将导致制导武器的制导效果变差。
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激光半主动寻的制导中所接收的指示激光波长漂移较小,光学系统设计过程中仅需考虑单色像差对系统跟踪精度的影响,结合1.1节的分析,主要考虑像差对光斑能量均匀性分布与光斑大小的影响[9]。根据以上两方面,将像差分为非对称、对称像差。非对称像差主要影响轴外视场光斑能量均匀性分布,间接影响光斑大小,包含彗差、像散、场曲、畸变;对称像差主要影响光斑大小,主要为球差。
明确单色像差对半主动光学系统的影响后,可得到构建其光学系统的原则:(1)根据轴上光线球差与目标光斑大小确定光学系统的初始结构与调焦量;(2)在得到光学系统初始结构后,优化非对称像差提高光斑能量均匀性,得到满足设计要求的光学系统。
等效焦距是光学系统的重要参数,也是光学系统设计的关键参数之一。对于远物距激光半主动光学系统,按公式(3)确定等效焦距
$f'$ :$$f' = \frac{D}{{2 \cdot \tan ({{\rm FOV} / {2)}}}}$$ (3) 式中:D为探测器光敏面直径;FOV为全视场角。
图2为激光半主动光学系统原理图,其中,光学系统是由多个透镜组组合而成的,
$f'$ 为系统等效焦距,d为各个透镜组之间的间隔,l′为系统的近轴像距,L′为边缘光线的像面距离,Δy′为垂轴球差,要保证半主动系统的光斑大小,透镜组的光焦度分配以及轴向球差与离焦量的对应关系应满足以下公式:$$ \sum\limits_{{{i}} = 1}^{{j}} {{{{h}}_{{i}}}{\varPhi _{{i}}}} = \varPhi $$ (4) $${\rm{tan}} (U') = \frac{R}{{A{\rm{ + }}\left( { - \delta L'} \right)}} = {h_1} \cdot \varPhi $$ (5) 式中:hi为入射光线在各透镜组的高度;R为像面的光斑半径;Φ为系统光焦度;Φi为各个透镜组的光焦度;U′为像方孔径角;δL′为轴向球差;A为后工作距调节量。
在激光半主动光学系统设计时,可通过赋予不同的球差与离焦量设计出不同初始结构的光学系统,最终光学系统结构形式也不尽相同。
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相对于激光光谱响应探测器近15万元的售价,可见光COMS探测器价格不足千元,价格相差约150倍,为了实现对镜头低成本检测,激光半主动光学系统设计时,要同时对系统进行谱段迁移的可视化检测设计。可视化检测方案选用可见光探测器、632.8 nm光源搭建低成本的可视化检测系统。
(1)光斑大小检测原理:由于折射率是光波波长的函数,光学材料对短波长的折射率要比长波长大,造成短波长在透镜的表面有更强的折射。见图3,选用可见光波段波长(632.8 nm,图3中红色光线),对应产生该波长下的球差和该波长与激光波长(1064 nm,图3中蓝色光线)产生的色差Δlλ1λ2′,进而得到632.8 nm光谱对应离焦位置的光斑大小。同时根据公式(5)可以得出对应波长(632.8 nm)相同光斑口径的理论离焦量,从而明确可见光探测器与镜头的相对位置,进行光斑大小的间接检测。
(2)光斑能量均匀性检测原理:像差改变及镀膜会影响光斑汇聚状态,对光斑均匀性带来影响。因此,需采用间接测量方法。首先,在仿真软件中分析1064 nm波长下光斑均匀的光学系统在632.8 nm波长下光斑均匀性;之后对632.8 nm波长下光斑均匀性进行测试,与仿真分析结果进行对比;若两者一致,则可代表光学系统在1064 nm波长下光斑均匀。
(3)透镜倾斜光斑状态检测原理:通常光学系统存在鬼像通道,透镜倾斜后,鬼像通道形成光斑,偏离光斑中心。通过镀制在1064 nm波段处反射率低、在632.8 nm波段处反射率高的膜层,提高关注鬼像光斑的能量,依据该现象,可作为检测镜头透镜倾斜的依据,进而快速筛选不合格镜头。
Design of laser semi-active optical system and visual testing
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摘要: 激光制导是当今最常用的制导方式之一,激光半主动光学系统性能的优劣直接影响其制导精度。提出了一种激光半主动光学系统像差优化设计方法,通过赋予不同的球差与离焦量实现激光半主动光学系统初始结构设计,通过对非对称像差优化实现光斑均匀化设计,设计并研制了折射式激光半主动光学镜头,光学系统工作波段1064 nm,视场为±9.2°,光斑大小5 mm,能量分布均匀;为解决激光半主动镜头不能单独检测的问题,提出了利用色差特性实现镜头低成本可视化检测的原理,并搭建了激光半主动光学镜头的可视化检测系统。镜头的测试结果表明,光斑大小满足设计要求,低成本可视化检测系统大幅提高了镜头检测效率,并易于工程化,批量化生产。Abstract: Laser guidance is one of the most commonly used guidance methods nowadays, and the performance of laser semi-active optical system directly affects its guidance accuracy. The method for the aberration optimization design of laser semi-active optical system was discussed, the initial structure of the optical system was realized by giving different spherical aberration and defocus values, and the spot uniformity design was realized by controlling the asymmetric aberration. A refractive laser semi-active lens was designed and fabricated, in 1064 nm working wavelength, with ± 9.2° field of view, 5 mm spot size, uniform energy distribution. In order to solve the problem that laser semi-active lens can not be detected separately, the principle of a low-cost visual testing of lens was proposed, which based on the chromatic aberration characteristics, and the visual testing system for laser semi-active optical lens was built. Test results of the lens show that the spot size satisfies the design requirements, and the low-cost visual testing system significantly improves the efficiency of lens testing and is easy to engineering and mass-produce.
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Key words:
- laser semi-active seeking guidance /
- optical design /
- visual testing /
- aberration
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