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靶盘式激光烧蚀微推力器的主要结构包括工质供给模块、激光器功能模块和控制模块三部分。激光器功能模块通过电光转化生成激光,激光聚焦在靶盘上形成烧蚀点和冲量;如图5所示,工质供给模块通过轴向和切向步进电机的运动实现激光聚焦点在靶盘上的运动,使推力产生连续独立的冲量;控制模块对内实现激光出光与靶盘转动的配合,监测推力器工作状态,对外通过接口实现能量、信息的交互。
推力器的单脉冲冲量
${I_s}$ 主要受激光脉冲能量${E_l}$ 和冲量耦合系数${C_m}$ 影响。$$ {I_s} = {E_l} \cdot {C_m} $$ (1) 对于一个工质成分均一、靶材涂覆均匀、聚焦光路固化的微推力器,单脉冲冲量基本稳定不变。总冲
${I_t}$ 是单脉冲冲量与烧蚀点数量之积,则:$$ {I_t} = {I_s} \cdot N $$ (2) 靶盘上每一个烧蚀点可看成是圆形的,靶盘利用率为:
$$ \eta = \frac{{N \cdot {\rm{\pi }}r_a^2}}{{{S_t}}} $$ (3) 式中:
${r_a}$ 为烧蚀点半径;${S_t}$ 为靶面总面积,对于圆环形靶面有:$$ {S_t} = {\rm{\pi }}\left( {r_o^2 - r_i^2} \right) $$ (4) 式中:
${r_i}$ 为靶盘内半径;${r_o}$ 为靶盘外半径。可见,如何合理设计靶盘上烧蚀点分布,使靶盘上容纳更多的烧蚀点,成为提高靶盘利用率和推力器总冲的关键。针对这一问题,文中提出了串珠法和套圈法两种分析方法,并设计了三种具体烧蚀点分布方式,以下具体说明。
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(1)根据靶面形状建立坐标系;
(2)确定烧蚀点运动的路径方程,可以是连续的,也可以是分段的;
(3)在路径方程上,第n个烧蚀点与第n+1,n-1个烧蚀点圆心之间的距离为ra,根据该约束条件和路径方程通过解析或迭代的方法得到第n个点的位置坐标。
(4)计算烧蚀点总数及靶面利用率。
这种方法关键在于先确定烧蚀点的运动路径,再在路径上确定每一个烧蚀点的位置坐标,就像先固定串线的骨架,把每个珠子串起来一样。下面通过圆圈路径和螺旋路径分别说明串珠法的具体实施过程。
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如图6所示,建立以靶盘中心为圆心的极坐标系,在烧蚀点一圈内沿圆路径运动,轴向静止,仅切向转动。相邻两圈的半径之差为2ra。
从外向里数,靶盘上能容纳的所有的圆圈半径分别为:
$$ {r_o} - {r_a},{r_o} - 3{r_a}, \ldots ,{r_o} - \left( {2n - 1} \right){r_a}, \ldots ,{r_o} - \left( {2m - 1} \right){r_a} $$ (5) $$ m = \left\lfloor {\frac{{{r_o} - {r_i}}}{{2{r_a}}}} \right\rfloor $$ (6) 第n圈上,相邻两个烧蚀点圆心之间的距离为2ra,对应的弧度为:
$$ {k_{na}} = \arcsin \left( {\frac{{2{r_a}}}{{{r_o} - \left( {2n - 1} \right){r_a}}}} \right) $$ (7) 则第n圈上能容纳的烧蚀点数量为:
$$ {N_n} = \left\lfloor {\frac{{2{\rm{\pi }}}}{{{k_{na}}}}} \right\rfloor $$ (8) 整个靶盘烧蚀点总数Nd及推力器的总冲It分别为:
$$ {N_d} = \sum\limits_{n = 1}^m {{N_n}} $$ (9) $$ {I_t} = {N_d} \cdot {I_s} $$ (10) 式中:Is为单烧蚀点形成的冲量。
需要指出的是,从里向外数的情况,即所有的路径圆圈半径分别为:
$$ {r_i} + {r_a},{r_i} + 3{r_a}, \cdots ,{r_i} + \left( {2n - 1} \right){r_a}, \cdots ,{r_i} + \left( {2m - 1} \right){r_a} $$ (11) $$ m = \left\lfloor {\frac{{{r_o} - {r_i}}}{{2{r_a}}}} \right\rfloor $$ (12) 总圈数与从外向里数一致,每一圈的半径相等或更小,因此从外向里数更加合理。
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采用圆圈路径时,轴向运动仅在烧蚀点跨圈运动时发生,在实际应用中对轴向电机瞬时加减速要求较高,且可能导致推力器本身角动量不稳定。可以考虑采用螺旋路径改善这一问题。如图7所示,轴向和径向电机逐点动作,当切向运动一圈后,轴向恰好也运动到了下一圈次。则烧蚀点运动的路径符合螺旋线方程,记烧蚀点圆心的位置矢量P在极坐标系下的位置坐标为(R, θ), R 为轴向位置坐标, θ 为切向位置坐标。则R与θ满足:
$$ R = {r_i} + \left( {\frac{\theta }{{\rm{\pi }}} + 1} \right){r_a} $$ (13) 螺旋线上的相邻两烧蚀点Pn,Pn+1之间的距离等于2倍烧蚀点半径,即
$$ \sqrt{R_n^2 + R_{n + 1}^2 - 2{R_n}{R_{n + 1}}\cos \left( {{\theta _n} - {\theta _{n + 1}}} \right) }= 2{r_a}$$ (14) 起始烧蚀点P1和终止烧蚀点PM分别满足:
$$\left\{ \begin{aligned} &R_1=0\\ &R_M \leqslant r_o-r_a\\ &R_{M+1} > r_o-r_a \end{aligned} \right.$$ (15) 基于公式(14)的迭代关系和公式(15)的边界条件,可迭代计算找到终止烧蚀点PM,以及所有烧蚀点的位置坐标。
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串珠法基于平面几何方法,直接计算得到烧蚀点运动路径和所有烧蚀点位置坐标,对后续的工程机电控制设计比较友好。设当前路径的曲率半径为R,则当前路径上一个面积为
${\rm{\pi }}r_a^2$ 的圆形烧蚀点实际占据的面积为$$ {s_r} = \arcsin \left( {\frac{{2{r_a}}}{R}} \right) \cdot \left[ {{{\left( {R + {r_a}} \right)}^2} - {{\left( {R - {r_a}} \right)}^2}} \right] $$ (16) ${s_r}$ 随R增大而减小,如图8所示,当R趋向于无穷大时,${s_r}$ 有最小值$4r_a^2$ ,因此,串珠法分布的烧蚀靶平面利用率η的上限为:$$ {\eta _m} = \frac{{{\rm{\pi }}r_a^2}}{{4r_a^2}} \approx 78.54{\text{% }} $$ 此外,对于圆圈路径,靶面的内外半径之差不是2ra的整数倍时,会出现“空圈”,靶面一圈内烧蚀点之间弧度差不能被2π整除时,会存在“空点”。对于螺旋路径,在靶面内外边缘,都会存在狭长的空白区域,导致靶面的利用率进一步降低。为了优化靶平面利用率,该节介绍一种基于平面圆的密排原理的烧蚀点分布分析方法,可以在平面上实现靶平面的最大化利用。
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圆的平面密排[17]又称六角密排,即当平面上圆以图9所示的方式聚集时,平面上圆的排布达到最密。
此时,连接相邻六个圆的圆心相邻构成六边形,可以看成每个圆在平面上实际占据了一个外接正六边形的面积。无限多的密排圆对无限大的面积利用率
${\eta _h}$ 为:$$ {\eta _h} = \frac{{{\rm{\pi }}{r^2}}}{{6/\sqrt 3 {r^2}}} \approx 90.64 \text{% } $$ (17) 而有限数量的密排圆对于有边界平面的面积利用率与边界处形状有关,对于凸多边形或者圆形边界,其面积利用率η一般比ηh小。下面以靶面的实际形状为边界,说明套圈法的具体实施过程。
(1)建立计算域Ω。在直角坐标系上建立边长为ro+2ri的矩形计算域Ω,Ω可完全覆盖靶面。
(2)在Ω上密排圆。在坐标原点处以ra为半径建立中心圆,中心圆所在行为中心行,在中心行上自中心圆向x轴正方向密排圆,可容纳的圆数量Nr为:
$$ {N_r} = \left\lfloor {\frac{{{r_o} + {r_a}}}{{2{r_a}}}} \right\rfloor $$ (18) 中心行上可容纳的圆总数为2Nr+1。在Ω上自中心行向y轴正方向密排行,可容纳的行数量Nc为:
$$ {N_c} = \left\lfloor {\frac{{{r_o} + {r_a}}}{{\sqrt 3 {r_a}}}} \right\rfloor $$ (19) Ω上可容纳的行总数为2Nc+1。Ω所有密排圆的分布如图10所示。
(3)对Ω上的所有密排圆建立二维行列索引,并确定圆心坐标。中心行记为第0行,中心圆所在列记为第0列,中心圆索引为[0,0]。则第i行、第j列圆的索引为[i,j]。其圆心在直角坐标系上的坐标为(cxi,cyj),i为偶数时,有:
$$ \left\{ {\begin{array}{*{20}{l}} {c{x_i} = 2j \cdot {r_a}}\\ {c{x_j} = \sqrt 3 i \cdot {r_a}} \end{array}} \right. $$ (20) i为奇数时,有:
$$ \left\{ {\begin{array}{*{20}{l}} {c{x_i} = \left( {2j + 1} \right) \cdot {r_a}}\\ {c{x_j} = \sqrt 3 i \cdot {r_a}} \end{array}} \right. $$ (21) (4)建立可行域δ, δ为圆环内边界与外边界之间的区域。
(5)逐一判断密排圆是否在δ内,判别式为:
$$ \left\{ {\begin{array}{*{20}{l}} {cx_i^2 + cy_j^2 < = {{\left( {{r_o} - {r_a}} \right)}^2}}\\ {cx_i^2 + cy_j^2 > = {{\left( {{r_i} + {r_a}} \right)}^2}} \end{array}} \right. $$ (22) 所有δ内的密排圆即可视为可行的烧蚀点分布。
从上述套圈过程中可以看出,可行域的“圈”套出的六角密排的阵列,其内部已达到紧密排列,仅在可行域边界存在一些“毛刺”,可有效提高靶面利用率。
Optimization of target plate utilization of disk-type laser ablation microthrusters
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摘要: 碟片式激光烧蚀推力器的总冲与靶盘上烧蚀点数量线性相关,提高靶盘利用率有利于在有限的靶面上得到更多的烧蚀点。为了优化靶盘利用率,文中首先对激光烧蚀微推力器进行了结构设计和分析,对靶盘上的烧蚀点分布问题进行了理论建模,提出了串珠法和套圈法两种分析方法,分别设计了圆圈路径、螺旋路径和六角密排三种实际烧蚀点分布方式。通过计算,分析比较了三种分布方式下,靶盘尺寸和烧蚀点尺寸对靶盘利用率的影响规律。结果表明,六角密排的靶盘利用率理论最高可达90.64%,圆圈路径和螺旋路径的靶盘利用率理论最高可达78.54%;靶盘利用率受到靶盘尺寸和烧蚀点尺寸的影响,靶盘尺寸较小时,圆圈路径的靶盘利用率相对较大,靶盘尺寸较大时,六角密排的靶盘利用率较大;三种分布方式各有特点,在应用上各有侧重。该研究为充分利用碟片式激光烧蚀推力器的靶盘提供了理论指导和设计参考,对推力器的工程化设计有一定借鉴意义。Abstract: The total impulse of a disc-type laser ablation thruster is linearly related to the number of ablation points on the target disk, and improving the utilization rate of the target disk is beneficial to obtain more ablation points on the limited target disk surface. In this paper, to optimize the utilization rate of the target disk, the structure of the laser ablation microthrusters was designed and analyzed. The distribution of the ablation point on the target disk was theoretically modelled, two analytical principles, namely, the bead principle and the collar principle, were proposed, and three actual ablation point distribution methods, namely, the circle path, the spiral path and the hexagonal pack, were designed. Through calculation, the influence law on the utilization rate of the target disk by the size of the target disk and the size of the ablation point under the three distribution methods was analyzed and compared. The results show that the target disk utilization rate of the hexagonal pack can reach up to 90.64%, and the target disk utilization rate theory of the circle path and spiral path can reach up to 78.54%. The target disk utilization rate is affected by the size of the target disk and the size of the ablation point. When the size of the target disk is small, the target disk utilization rate of the circle path is relatively large. When the size of the target is large, the target disk utilization rate of the hexagonal pack is relatively large. Each of the three distribution methods has its own characteristics and has its own emphasis on application. The research provides theoretical guidance and a design reference for the full utilization of the target disk of the disc-type laser ablation thruster, and has some reference significance to the engineering design of the thruster.
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Key words:
- microthrusters of laser propulsion /
- hexagonal pack /
- utilization rate /
- path
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