An adaptive fast fixed-time guidance law with an impact angle constraint for intercepting maneuvering targets

Sliding mode guidance laws based on a conventional terminal sliding mode guarantees only finite-time convergence, which verifies that the settling time is required to be estimated by selecting appropriate initial launched conditions. However, rapid convergence to a desired impact angle within a uniform bounded finite time is important in most practical guidance applications. A uniformly finite-time/fixed-time convergent guidance law means that the convergence (settling) time is predefined independently on initial conditions, that is, a closed-loop convergence time can be estimated a priori by guidance parameters. In this paper, a novel adaptive fast fixed-time sliding mode guidance law to intercept maneuver targets at a desired impact angle from any initial heading angle, with no problems of singularity and chattering, is designed. The proposed guidance law achieves system stabilization within bounded settling time independent on initial conditions and achieves more rapid convergence than those of fixed-time stable control methods by accelerating the convergence rate when the system is close to the origin. The achieved acceleration-magnitude constraints are rigorously enforced, and the chattering-free property is guaranteed by adaptive switching gains. Extensive numerical simulations are presented to validate the efficiency and superiority of the proposed guidance law for different initial engagement geometries and impact angles.     

飞机电静液作动器滑模 PID控制器设计

在多电飞机应用环境中,由于电静液作动器（Electro Hydrostatic Actuator,以下简称EHA）系统本身的强非线性与承载交变动载荷的不确定性,简单PID控制无法达到理想控制效果。提出了滑模 PID复合控制,电机电流环和转速环构成控制系统内环,以PI控制器实现电机调速;作动筒位置环为外环,以滑模控制提升系统的快速性和鲁棒性。建立了EHA数学模型,并设计了滑模控制器结构。仿真结果表明,滑模 PID复合控制方法能有效地消除超调和减小跟随误差,实现对EHA位置的精确控制。     

Fundamental (f) oscillations in a magnetically coupled solar interior-atmosphere system – An analytical approach

Solar fundamental (f) acoustic mode oscillations are investigated analytically in a magnetohydrodynamic (MHD) model. The model consists of three layers in planar geometry, representing the solar interior, the magnetic atmosphere, and a transitional layer sandwiched between them. Since we focus on the fundamental mode here, we assume the plasma is incompressible. A horizontal, canopy-like, magnetic field is introduced to the atmosphere, in which degenerated slow MHD waves can exist. The global (f-mode) oscillations can couple to local atmospheric Alfvén waves, resulting, e.g., in a frequency shift of the oscillations. The dispersion relation of the global oscillation mode is derived, and is solved analytically for the thin-transitional layer approximation and for the weak-field approximation. Analytical formulae are also provided for the frequency shifts due to the presence of a thin transitional layer and a weak atmospheric magnetic field. The analytical results generally indicate that, compared to the fundamental value ($\omega =\sqrt{\mathit{gk}}$), the mode frequency is reduced by the presence of an atmosphere by a few per cent. A thin transitional layer reduces the eigen-frequencies further by about an additional hundred microhertz. Finally, a weak atmospheric magnetic field can slightly, by a few percent, increase the frequency of the eigen-mode. Stronger magnetic fields, however, can increase the f-mode frequency by even up to ten per cent, which cannot be seen in observed data. The presence of a magnetic atmosphere in the three-layer model also introduces non-permitted propagation windows in the frequency spectrum; here, f-mode oscillations cannot exist with certain values of the harmonic degree. The eigen-frequencies can be sensitive to the background physical parameters, such as an atmospheric density scale-height or the rate of the plasma density drop at the photosphere. Such information, if ever observed with high-resolution instrumentation and inverted, could help to gain further insight into solar magnetic structures by means of solar magneto-seismology, and could provide further insight into the role of magnetism in solar oscillations.     

Kink mode m = 1 in magnetic tube with discontinuous magnetic field

In this paper, we study conditions of realization and stability of kink modes with azimuthal wave numbers $m=\pm 1$ in cylindrical plasma flex with twisted magnetic field and homogeneous current along its axis. We assume permanent axial magnetic field both inside and outside the flex, surrounded by currentless plasma. Azimuthal magnetic field decreases inversely proportional to the distance from the boundary beyond the flex. We derived dispersion equations for stable and unstable modes in approximation of “thin” plasma flex. The analysis of equations has been provided for the case of discontinuous axial magnetic field on flex’s boundary. Conditions of propagation of wave modes have been defined. It was shown, that unstable modes can be implemented in certain interval of longitudinal wavenumbers. Results can be applied for the interpretation of solar magnetic tubes behavior, using measurements, provided by spacecrafts.     

有限时间收敛的阻力加速度跟踪进入制导律

摘要：针对地球或火星的大气层进入过程提出一种具有有限时间收敛性质的阻力加速度跟踪鲁棒制导律.将再入动力学方程抽象为一类带有不确定性及扰动的非线性系统,并针对该类系统提出一种基于滑模控制的有限时间鲁棒控制律.将所提出的理论方法应用于阻力加速度跟踪制导律的设计中,设计可使阻力加速度在有限时间内跟踪上标称值的制导律.仿真验证方法的有效性和先进性.     

基于有限时间理论的临近空间拦截器末制导律PWPF调节器研究

针对拦截临近空间高速机动目标,基于有限时间理论,设计了有限时间收敛末制导律和脉冲宽度脉冲频率（PWPF）调节器。首先,介绍了加入PWPF调节器的末制导数学模型及工作原理;其次,设计了有限时间收敛制导律,应用有限时间理论证明了收敛条件和性质,并推广到推力受限条件下;最后,考虑视线（LOS）角速率快速收敛和发动机最小工作时间要求,设计了PWPF调节器参数。仿真结果表明,视线角速率在制导结束前的有限时间收敛至零,对目标机动具有鲁棒性,具有较高的制导精度,避免了发动机频繁开启,节省了燃料。     

进口导叶形式对核心机驱动风扇级的影响

以某变循环发动机(VCE)所用核心机驱动风扇级(CDFS)为研究模型,数值模拟了CDFS在真实边界环境下的流动特性,分析了单/双涵道工作模式下,不同形式的可调进口导叶(IGV)对CDFS流动特性及性能的影响机理。结果表明:不同工作模式时,CDFS靠导叶角度的开闭实现大范围的流量调节。单涵道(SB)工作模式时,不同形式的可调进口导叶(VIGV)对CDFS性能的影响差异很小;双涵道(DB)工作模式时采用常规可调导叶(CIGV)会在其吸力面产生较大的流场分离,且流通能力和流量调节范围大大降低。可变弯度导叶通过可转动部分的开闭实现CDFS对流量调节的需求;通过固定部分保证CDFS导叶进口气流攻角基本不变,同时在固定部分和可转动部分连接处所形成的收缩通道的加速效应显著抑制了导叶吸力面的流动分离。可变弯度导叶是适用于CDFS在不同工作模式下性能参数及流量调节需求的可调导叶的形式。     

串联式TBCC发动机风车冲压模态性能模拟

建立了基于部件低转速特性匹配的串联式涡轮基组合循环（TBCC）发动机风车冲压模态性能计算模型,提出了压气机低转速大流量特性扩展方法,由换算扭矩代替等熵效率表示旋转部件特性,解决了低转速部件效率不连续相关问题。分析了冲压外涵面积变化和涡轮功率提取对风车冲压模态性能及部件匹配的影响规律,并基于推力、流量连续准则设计了涡轮模态至冲压模态转换过程（含风车冲压模态）的参数调节规律。计算结果表明:在风车冲压模态下,冲压外涵面积变化对风扇工作状态有显著影响,对压气机影响较小;涡轮可提取功率随着风车转速的减小先增加后降低,比冲随提取功率的增加基本呈线性降低趋势,功率提取位置（高、低压涡轮）对部件匹配有显著影响。      

基于有向图模型的民机航空电子系统故障诊断

结合民机航电系统外场可更换单元或模块（LRU/LRM）的故障模式及影响分析（FMEA）、故障树 分析和实时性要求,设计了基于时间故障传播图（TFPG）的系统故障诊断元模型和领域模型,研究了基于逻 辑和时间一致性检验的故障诊断算法,使用后向推理和可信度度量列出候选故障集。以飞行管理系统为例,构 造了其功能失效的TFPG 模型,并通过仿真验证了基于TFPG 的诊断在滤除级联效应和关联驾驶舱效应中的有 效性。