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全回路跟踪系统由预瞄模型、主轴系统和从轴系统组成,系统原理图如图1所示。
控制结构图如图2所示。从图2可以看出,主轴系统由滞后环节、主轴探测器模型、电机模型、负载模型、角速度传感器模型、角度传感器模型及角度控制器和角速度控制器组成;从轴系统由从轴探测器模型、坐标变换、快反镜模型及角度控制器组成。这里,控制器均选用经典PID控制方法,角度传感器和角速度传感器的传递函数假设为1,即认为实际系统输出与测量系统输出一致。其他环节数学模型依据常规器件性能选择。
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电机选用直流力矩电机,系统采用PWM电压调速,因此可以得到系统的电平衡、力平衡方程,如公式(1)~(5)所示:
$$ u = {R_0} \cdot I + L\dfrac{{\rm{d}}I}{{{\rm{d}}t}} + E $$ (1) $$ J\dfrac{{{\rm{d}}\omega }}{{{\rm{d}}t}} + {f_m} \cdot \omega = M + {T_f}\left( \omega \right) $$ (2) $$ E = {C_e} \cdot \omega $$ (3) $$ M = {C_m} \cdot I $$ (4) $$ {R_0} = R + {R_q} $$ (5) 式中:u为调制电压;R0为电机驱动总电阻;I表示流过电机绕组电流;L为电机绕组电感;
${{\rm{d}}I / {{\rm{d}}t}}$ 为微分算子;E为电机绕组反电动式;J为电机及负载转动惯量;ω为电机负载角速度;fm为机械阻尼;M为电机输出力矩;Tf(ω)为摩擦力矩;Ce为反电势常数;Cm为力矩常数;R为电机绕组电阻;Rq为驱动芯片内阻。在此基础上进行线性化处理,假设摩擦总阻系数fm是不变的,进行拉式变换可得频域方程:$$U\left( s \right) = ({R_0} + Ls)I\left( s \right) + E\left( s \right)$$ (6) $$(Js + {f_m}) \cdot \omega \left( s \right) = M\left( s \right)$$ (7) $$ E\left( s \right) = {C_e} \cdot \omega \left( s \right) $$ (8) $$ M\left( s \right) = {C_m} \cdot I\left( s \right) $$ (9) 由系统框图求得系统闭环传递函数为:
$${G_0} = \frac{{Js + {f_m}}}{{\left( {Js + {f_m}} \right)\left( {Ls + R} \right) + {C_m}{C_e}}} \cdot \frac{{{C_m}}}{{Js + {f_m}}}$$ (10) 因为电机电气时间常数
${T_L} = {L / R}$ ,电机机械时间常数${T_M} = {{JR}/ {{C_e}{C_m}}}$ 且电机机械时间常数一般远远大于电气时间常数,可将公式(10)简化为:$${G_0} \approx \frac{1}{{\left( {{T_L}s + 1} \right)\left( {{T_M}s + 1} \right)}}$$ (11) -
预瞄模型为目标进入粗跟踪视场前的预先指向过程。机载激光武器跟踪系统实际工作中,由雷达等上游机构发送目标角度,该角度往往大于主轴探测器视场角,即此时目标处于主轴探测器视场外,需要控制机构旋转一定角度使目标进入主轴视场内后再进行复合轴控制。控制机构闭环传递函数
${G_{VB}}\left( s \right)$ 为:$${G_{VB}}\left( s \right) = \frac{{{K_{si}} + {K_{sp}}s}}{{{T_M}{T_L}{s^3} + \left( {{T_M} + {T_L}} \right){s^2} + \left( {{K_{sp}} + 1} \right)s + {K_{si}}}}$$ (12) 式中:
${K_{si}}$ ${K_{sp}}$ 为PI控制参数。预瞄模块的输出为
${\theta _p}$ ,其表达式为:$${\theta _p} = \left\{ {\begin{array}{*{20}{l}} \theta &{\left| {\theta - {G_{VB}}\left( s \right) \cdot \theta } \right| < {{10}^{ - 3}}} \\ {{G_{VB}}\left( s \right) \cdot \theta }&{\left| {\theta - {G_{VB}}\left( s \right) \cdot \theta } \right| \geqslant {{10}^{ - 3}}} \end{array}} \right.$$ (13) 式中:
$\theta $ 为输入角度值。图3(a)和(b)为预瞄模块功能对比图,目标初始位置为1.57 rad,预瞄过程时间为0.6 s。不采用预瞄模块的仿真系统在初期超调为38.71%,而采用预瞄模块的仿真系统没有超调,更符合实际工程情况。
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由于脱靶量的输出中要经过光电扫描、A/D、D/A、图像识别算法等,使得输出的脱靶量滞后目标成像1~2帧,这里以2帧延迟为例,滞后时间约为20 ms。仿真平台中,直接使用延迟函数,作为滞后环节。
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从轴系统是由压电陶瓷驱动的一块快速反射镜,通过光束在空间上的转动,来实现主控系统残余误差的补偿。仿真假定从轴系统探测器选用1 kHz的高帧频CCD,采用压电陶瓷驱动,传递函数的形式如标准二阶系统,根据实际工程经验可以将快反镜的数学模型
${G_m}\left( s \right)$ 设为:$${G_m}\left( s \right) = \frac{{5.12}}{{\left( {0.175s + 1} \right)\left( {0.000\;406s + 1} \right)}}$$ (14) 另外采用高速处理器后,图像跟踪器的纯滞后时间τ远小于采样周期T,所以可以忽略系统的纯滞后时间。
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若假设驱动器与反射镜的接触点与镜面中心的距离为l,驱动器运动距离为d,镜面的旋转角度为
${\theta _d}$ ,那么经过镜面的光线改变角度为2${\theta _d}$ ,所以反射比为2。坐标变换公式为:$$d = l\sin \left( {{\theta _d}} \right)$$ (15) 由于进入子控系统的脱靶量非常小,一般在500 μrad以下,所以可将公式(15)近似为公式(16):
$$d = l{\theta _d}$$ (16) -
探测器模型主要考虑最小分辨率因素,采用阶梯函数使得主系统探测器最小分辨率为50 μrad,子系统探测器最小分辨率5 μrad。数学公式为:
$$\frac{{{\theta _r}}}{\theta } = f\!f \times {\rm{sign}}(\theta ) \times \left| {fix({\theta / {f\!f}})} \right| + ceil(\boldsymbolod (\theta ,f\!f)/f\!f)$$ (17) 式中:ff为最小分辨率;θ为输入角度;θr为输出角度。
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建立ADRC控制器。跟踪微分器方程表达式为:
$$\left\{ \begin{array}{l} fs = - r(r({v_1} - u(t)) + 2{v_2}) \\ {v_1} = {v_1} + h \cdot {v_2} \\ {v_2} = {v_2} + h \cdot fs \end{array} \right.$$ (18) 式中:
$h$ 为仿真时间间隔;${v_1}$ 为系统输入的跟踪信号;${v_2}$ 为输入的1阶微分信号;${f_s}$ 为系统的2阶微分信号;$u(t)$ 为输入信号;$r$ 为跟踪微分器的快速因子。5阶扩张状态观测器表达式为:
$$\left\{ \begin{array}{l} e = {{\textit{z}}_1} - y \\ {{\textit{z}}_1} = {{\textit{z}}_1} + h\left( {{{\textit{z}}_2} - {\beta _{01}}e} \right) \\ {{\textit{z}}_2} = {{\textit{z}}_2} + h\left( {{{\textit{z}}_3} - {\beta _{02}}e + 0.005u(t)} \right) \\ {{\textit{z}}_3} = {{\textit{z}}_3} + h\left( { - {\beta _{03}}e} \right) \end{array} \right.$$ (19) 式中:y为实际输出;
${{\textit{z}}_1}$ 为对系统输出的估计;${{\textit{z}}_2}$ 为对系统输出1阶的估计;${{\textit{z}}_3}$ 为对系统扰动的估计;${\; \beta _{01}}、 {\; \beta _{02}}、 $ $ {\; \beta _{03}}$ 为权重因子。非线性控制器表达式为:
$$\left\{ \begin{array}{l} {e_1} = {v_1} - {{\textit{z}}_1} \\ {e_2} = {v_2} - {{\textit{z}}_2} \\ {U_0} = {b_{01}} \cdot fal({e_1},\!{\alpha _1},theta) \!+\! {b_{02}} \cdot fal({e_2},{\alpha _2},\!theta) \!-\! 200{{\textit{z}}_{\rm{3}}} \end{array} \right.$$ (20) 式中:
${b_{01}},{b_{02}}$ 为非线性控制器的权重因子;${\alpha _1},{\alpha _2}$ 为非线性饱和因子;$theta$ 为切换阈值。$fal(e,\alpha ,theta)$ 的表达式为:$$fal(e,\alpha ,theta) = \left\{ {\begin{array}{*{20}{c}} {{{\left| e \right|}^\alpha }{\rm{sgn}} (e)}&{\left| e \right| \geqslant theta} \\ {\dfrac{e}{{thet{a^{1 - \alpha }}}}}&{\left| e \right| < theta} \end{array}} \right.$$ (21) -
因为复合轴系统的主轴系统和从轴系统的输出是线性累加的,故两个系统的稳定性互不干涉,可分别验证主轴系统和从轴系统的稳定性。并且由于主轴系统中包含非线性函数,文中采用系统辨识后得到的近似线性系统,并通过计算系统零极点来判断系统稳定性。采用下文模型参数,利用最小二乘法可辨识出系统的参数模型,系统参数模型表达式为:
$$\left\{ {\begin{array}{*{20}{l}} {A\left( q \right) = 1 \!-\! 3.422{q^{ \!-\! 1}} \!+\! 4.386{q^{ \!- \!2}} \!-\! 2.478{q^{\! - \!3}}{\rm{ \!+\! }}0.514\;2{q^{ \!-\! 4}}} \\ {B\left( q \right) = - 0.000\;179\;5 + 0.000\;660\;5{q^{ - 1}}} \end{array}} \right.$$ (22) 在将离散的参数模型,转换为连续模型,可以得到如公式(23)所示的连续参数模型:
$$G(s) \!\!=\!\! \frac{{ \!\!-\! 0.000\;\!179\;\!5{s^4} \!-\!\! 1.136{s^3} \!+\!\! 1\;\!214{s^2} \!\!+\! 4.556{e^7}s \!\!+ \!\!1.701{e^{11}}}}{{{s^4} \!+\! 2\;660{s^3} \!\!+\! 1.7{e^6}{s^2} \!+\!\! 2.385{e^9}s + 2.084{e^{10}}}}$$ (23) 据此,可以得到公式(23)所示系统和原始辨识系统的Bode对比图,如图4所示。
图 4 辨识系统与原系统对比Bode图
Figure 4. Comparison of Bode diagram between identification system and original system
由Bode图可以看出,辨识精度较高,再通过仿真软件分析,辨识精度达到99.29%,几乎和原系统在2~200 Hz段内的频率特性一致。连续参数模型经过化简可得:
$$G(s) = \frac{{ - 1.136{s^3} + 1\;214{s^2} + 4.556{e^7}s + 1.701{e^{11}}}}{{{s^4} + 2\;660{s^3} + 1.7{e^6}{s^2} + 2.385{e^9}s + 2.084{e^{10}}}}$$ (24) 公式简化系统的闭环极点为:
$$\left\{ {\begin{array}{*{20}{l}} {{p_1} = - 0.008\;8{e^3}} \\ {{p_2} = - 2.366{e^3}} \\ {{p_3} = \left( {{\rm{ - 0}}{\rm{.142\;6 + 0}}{\rm{.990\;7i}}} \right){e^3}} \\ {{p_4} = \left( {{\rm{ - 0}}{\rm{.142\;6 - 0}}{\rm{.990\;7i}}} \right){e^3}} \end{array}} \right.$$ (25) 由系统闭环极点可以得出系统稳定。
Research on high precision tracking control technology of airborne laser weapon
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摘要: 机载激光武器系统是一种定向能武器系统,对跟踪精度要求较高,传统PID控制无法满足其高精度跟踪需求。建立了预瞄模型、探测器模型、快速反射镜模型、时滞模型等数学模型,搭建完整的仿真系统,并创新性地采用自抗扰控制算法和复合轴控制结构相结合的控制方式,以提高控制精度。通过功能验证试验验证文中搭建的仿真系统,加入实际采集的某运输机扰动,其跟踪精度为5.16 μrad,相较于传统PID控制,跟踪精度提高25倍。同时给出一种虚拟战场场景,经文中搭建的仿真模型验证,其俯仰轴和偏航轴的跟踪精度均小于10 μrad。Abstract: The airborne laser weapon system is a kind of directional energy weapon system, which has a higher tracking accuracy requirement than other weapon systems, so that the traditional PID control cannot meet the requirements of high precision control. A compound axis servomechanism with preview model was used to build a complete simulation system, including detector model, fast mirror model and time delay model, etc. In order to improve the control accuracy, the active disturbance rejection control algorithm and the compound axis control structure were combined. The simulation system built in the article was verified by the function verification test of some transport disturbance data collected in practice. The results show that the tracking accuracy of the system is 5.16 μrad, which is 25 times of the traditional PID control. At the same time, a virtual battlefield scene was given, which was verified by the simulation system built in this paper. The tracking accuracy of pitch axis and yaw axis were both less than 10 μrad.
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