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在激光制导导弹(或者炸弹)中,激光信息处理器的原理框图如图 3所示。以现场可编程门阵列(field-programmable gate array,FPGA)为核心处理器,为激光探测器提供控制信号,并将其输出的4路单端模拟信号经高通滤波和电压驱动后,送入可变增益放大器(variable gain amplifier, VGA)进行放大,再由模数转换器采样并送入FPGA,最终将处理结果通过RS422输出。
按照激光信号的传输路径和处理流程,可将影响激光制导寻的器测角精度的因素分为外部环境因素和系统内部因素[3]。其中外部环境因素主要为大气湍流引起测角误差[5-6]。系统内部因素主要包括探测器安装误差、探测器性能引起的误差以及激光信号处理电路导致的误差[3]。这些误差会导致导引头的线性度变差,从而降低激光导引头的制导精度和打击目标时的命中精度。下面主要讨论激光信息处理电路引起的制导误差。
假设UA,UB,UC,UD分别为入射光斑在激光探测器A, B, C, D 4个象限产生的光电压,由激光制导的基本原理可知,如果4个通道对漫反射激光的响应度是完全一致的,则导引头在俯仰和偏航方向相对光轴的偏差可由以下公式计算得到[4]:
$ \Delta x=\frac{\left(U_A+U_B\right)-\left(U_C+U_D\right)}{U_A+U_B+U_C+U_D} $
(1) $ \Delta y=\frac{\left(U_A+U_D\right)-\left(U_B+U_C\right)}{U_A+U_B+U_C+U_D} $
(2) 信息处理电路中导致采集通道响应存在差异的主要原因包括:(1)模拟信号调理通道,包括放大器和滤波器等环节中分立器件(放大器、电阻、电容和电感等)的误差造成的响应不一致性;(2)数模转换过程中对模拟信号的量化过程中导致的不一致性。
以上两个环节产生的响应不一致性,会导致4个采集通道对相同特性输入信号的响应度产生差异,从而致使根据(1)式、(2)式计算出来的光轴在俯仰、偏航两个方向上与目标的夹角存在一个固定的偏移量,最终降低激光制导导弹(或者炸弹)的打击精度。按照传统的不一致性校正方法,在信息处理器中,对4个采集通道采用系数修正的方式进行校正。假设以A路为基准,这时计算两个方向偏差的公式就变为:
$ \begin{gathered} \Delta x= \\ \frac{\left(U_A+k_B U_B\right)-\left(k_C U_C+k_D U_D\right)}{U_A+k_B U_B+k_C U_C+k_D U_D} \end{gathered} $
(3) $ \begin{gathered} \Delta y= \\ \frac{\left(U_A+k_D U_D\right)-\left(k_B U_B+k_C U_C\right)}{U_A+k_B U_B+k_C U_C+k_D U_D} \end{gathered} $
(4) 式中, kB, kC和kD分别是B, C, D 3个通道的校正系数。
然而,(3)式和(4)式为理论计算方法,由于激光信息处理电路中,在导弹(或者炸弹)飞行过程中,需要根据接收激光能量的强弱,通过VGA电路动态调整放大电路的增益,而在不同增益点,4个信号通道的响应也存在差异。图 4为未经过校正的4个采集通道在不同增益点的响应不一致性曲线。从图中可以看出,在不同增益点,4个通道的响应存在明显差异,所以要在全接收能量范围都保持4个采集通道具有较好的响应一致性,就需要对不同增益点分别计算一组校正系数,而对于采用FPGA作为处理器的系统,(3)式和(4)式中的小数乘法运算不仅耗费逻辑资源,还会增加处理延时,所以在真正的工程实践中很难实现。
经过验证,在FPGA程序中每增加一个16位定点乘法器,综合后,占用的逻辑资源增加0.1%,功耗增加约0.04%,而增加的处理延时约为200ns(假设输入时钟频率为150MHz),所以如果在每个离散增益点都采用传统的系数校正方法进行校正,产生的逻辑资源、功耗和处理延时的开销对系统应用极为不利。
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对于激光制导导引头而言,线性区是衡量其制导性能的一项重要指标,线性区曲线越是接近于直线,则表明其制导精度越高[7-10]。为了验证该校正方法的效果,对一个视场范围为±4°的激光导引头进行校正前后线性区的测试,测试结果如图 8和图 9所示。对比图中的线性区曲线可以看出,经校正后的线性区曲线更接近于直线,表明其线性度越好,测角精度更高,对于伺服平台而言,可明显提高控制精度。
一般采用圆概率偏差(circular error probable,CEP) 描述命中精度[11-12]。为了更加充分地验证本文中提出的不一致性校正方法对制导精度的影响,通过仿真进行了验证,采用该校正方法分别进行了3次仿真,CEP统计见表 1。在仿真过程中,为了排除信噪比变化对仿真结果产生的影响,使激光目标模拟器的能量保持恒定,从而保证信息处理电路的VGA处于相同的增益点。
Table 1. Statistical table of miss distance in semi-physical simulation
CEP before correction/m CEP after correction/m the first time 0.45 0.16 the second time 0.38 0.21 the third time 0.41 0.20 由于本文中提出的校正方法,针对实际使用的各离散增益点,对4个采集通道的输出响应进行精确校正,从而最大程度地减小了导引头输出的俯仰、偏航两个方向上与目标夹角的偏移量,最终达到提高制导精度的目的。
通过表中数据可以看出,校正后可以将CEP数值降低约50%, 理论上可以明显提高制导精度。
基于增益控制电路的响应不一致性校正方法
Correction method of response inconsistency based on gain control circuit
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摘要: 为了研究半主动激光制导导引头信息处理电路中,4个采集通道响应不一致性导致的制导精度降低的问题,对造成采集通道间响应度存在差异的原因进行了理论分析和研究,研究了信息处理电路中的增益控制电路,分析了其设计要点,并给出了一种增益控制电路的设计原理图。采用一种基于该增益控制电路的通道间响应度不一致性校正方法,针对导弹(或者炸弹)实际飞行过程中使用的各个离散增益点,进行了不一致性校正;通过对比校正前后的线性区曲线和半实物仿真数据,验证了该校正方法的有益效果。结果表明,利用该校正方法校正后,导引头线性区曲线精度更高,并且可以将半实物仿真圆概率偏差数值降低约50%。该研究为提高半主动激光制导精度提供了一种有效的方法。Abstract: In order to study the problem of the guidance accuracy reduction caused by the inconsistency of the four acquisition channels in the information processing circuit of the semi-active laser guidance seeker, the reason of the difference of the response between the sampling channels was analyzed and studied. The gain control circuit in the information processing circuit was studied. The key points of the design were analyzed, and the design principle of a gain control circuit was given. An inter-channel response inconsistency correction method based on the gain control circuit was used to correct the inconsistency of discrete gain points used in the actual flight of a missile (or a bomb). The effectiveness of the correction method was verified by comparing the linear curve before and after correction with the simulation data of hardware-in-the-loop. The experimental results show that the linear circular error probable (CEP) of the seeker is more accurate and the CEP of the semi-physical simulation can be reduced by about 50%. This study provides an effective method for improving the precision of semi-active laser guidance.
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Table 1. Statistical table of miss distance in semi-physical simulation
CEP before correction/m CEP after correction/m the first time 0.45 0.16 the second time 0.38 0.21 the third time 0.41 0.20 -
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