基于广义回归神经网络集成的宽带波束形成算法

首先构造宽带波束成形所需要的协方差矩阵,利用基于粒子群优化算法对核主成分分析方法和广义回归神经网络进行了优化。在对神经网络的输入变量进行降维处理后,生成多个复杂度低的泛回归神经网络模型。利用提出的基于聚类启发式集成算法求出波束成形时的权系数,既考虑了网络的差异性,又考虑了网络的正确性。仿真结果表明,提出的基于聚类启发式神经网络集成的波束形成算法在网络结构十分简单的情况下,仍然具有较好的性能。     

一种基于粒子滤波的双目视觉惯导SLAM技术

针对智能无人系统的定位与地图构建问题,提出一种基于粒子滤波的双目视觉惯导SLAM（即时定位与地图构建）方法。算法基于传统粒子滤波思想设计实现,后端处理时仅对位姿状态量进行滤波,有效解决了传统算法的维度爆炸问题,减少计算量的同时保证一定的精确度,兼顾SLAM算法精确性和即时性的要求。实验结果表明,方法整体定位精度相对误差低于5%,在光照条件适宜的小型场景效果更佳,误差低于3%,能够达到分米级精度,证明了SLAM方法能够完成SLAM系统的要求,实现即时定位与地图构建功能,具有一定的准确性、稳定性和鲁棒性。粒子滤波能够应用于视觉惯导SLAM领域并达到较高的精度要求。     

On the ability of sliding mode and LQR controllers optimized with PSO in attitude control of a flexible 4-DOF satellite with time-varying payload

Precise pointing of the satellite and its payload is essential in the accurate accomplishment of a space mission. In this study, the system of a satellite and its payload are considered as 4-DOF equations of motion. The time-varying payload can observe one direction of the Earth independently, and the satellite can point to the Earth station by its 3-DOF motions simultaneously. Sliding mode and LQR controllers are designed for damping disturbances, and consequently high pointing accuracy. Environmental disturbances and the associated time delay of Low Earth Orbit (LEO) are applied to the system. An algorithm based on Particle Swarm Optimization (PSO) is proposed to find the optimum values of variables and Normalized Integral Square Error (NISE) of the two aforementioned controllers. Numerical simulations indicate the optimized magnitudes of target detection errors and control efforts in four directions. The results revealed that PSO-SMC can finely track the time-varying payload and has better efficiency in comparison with PSO-LQR.     

An efficient analytical homogenization technique for mechanical-hygrothermal responses of unidirectional composites with applications to optimization and multiscale analyses

The elasticity-based Locally Exact Homogenization Theory (LEHT) is extended to study the mechanical-hygrothermal behaviors of unidirectionally-reinforced composites. Based on the framework developed previously, thermal and moisture effects are incorporated into the LEHT to study the homogenized and localized responses of heterogeneous materials, which are validated using available analytical and numerical techniques. The LEHT programs are then encapsulated as subroutines with Input/Output (I/O) interfaces, to be readily applied in different computational scenarios. In order to illustrate the efficiency of the LEHT, the theory is firstly coupled to the Particle Swarm Optimization (PSO) algorithm in order to minimize the axial thermal expansion mismatch in hexagonal and square fiber arrays by tailoring the fiber volume fraction. The LEHT is then implemented into the lamination theory to study fabrication-induced residual stresses arising during the cool-down process which introduces local laminate stresses owing to thermo-mechanical property mismatch between plies. Both of these applications illustrate the efficiency and accuracy of the LEHT in generating effective properties and local stress distributions, making the theory a golden standard in validating other analytical or numerical techniques as well as a reliable tool in composite design and practice for professionals and non-professionals alike.     

Design and performance analysis of position-based impedance control for an electrohydrostatic actuation system

Electrohydrostatic actuator (EHA) is a type of power-by-wire actuator that is widely implemented in the aerospace industry for flight control, landing gears, thrust reversers, thrust vector control, and space robots. This paper presents the development and evaluation of position-based impedance control (PBIC) for an EHA. Impedance control provides the actuator with compliance and facilitates the interaction with the environment. Most impedance control applications utilize electrical or valve-controlled hydraulic actuators, whereas this work realizes impedance control via a compact and efficient EHA. The structures of the EHA and PBIC are firstly introduced. A mathematical model of the actuation system is established, and values of its coefficients are identified by particle swarm optimization. This model facilitates the development of a position controller and the selection of target impedance parameters. A nonlinear proportional-integral position controller is developed for the EHA to achieve the accurate positioning requirement of PBIC. The controller compensates for the adverse effect of stiction, and a position accuracy of 0.08 mm is attained. Various experimental results are presented to verify the applicability of PBIC to the EHA. The compliance of the actuator is demonstrated in an impact test.     

Ascent guidance law for a horizontal take-off vehicle with a multi-combined cycle engine

This paper researches the ascent guidance law for the vehicle with a multi-combined cycle propulsion. The guidance law comprises two parts, namely, the off-line optimal trajectories generation and online guidance. With respect to the off-line part, disturbances are discretized and incorporated into the trajectory optimization problem; subsequently, a set of trajectories is calculated to constitute a database. To quickly obtain a database that comprises a large number of trajectories, a novel ascent profile is proposed with respect to height and velocity. Based on this profile, only inequity constraints exist in the optimization model, and the original optimization problem is converted to a parameter searching problem. The optimal trajectories are calculated using a hybrid optimization method that comprises a particle swarm optimization (PSO) method and the Hooke-Jeeves (HJ) method. With respect to online guidance, the profile is updated using a radial basis function neural network (RBFNN) based on the current flight states and the database. Simulation validates the efficiency of the proposed optimization method by comparing the method with the pseudospectral method; the robustness of the guidance law is also validated using Monte Carlo simulation.     

用于粒子分离器的砂粒反弹特性实验研究

针对砂粒反弹特性所面临的基础问题及其复杂特点,采用实验与理论分析的方法研究砂粒反弹机理。以砂粒与板面碰撞反弹为起点,自行设计了测量砂粒反弹系数的实验系统,并用高速摄影仪捕捉砂粒的运动轨迹,通过图像处理获得不同粒径的砂粒与不同材料的板面碰撞前后的速度矢量。以实验测量数据为基础,理论分析砂粒运动特性,总结砂粒反弹系数与入射角度之间的关系,建立不同材料的砂粒反弹系数的拟合公式,为进一步提高粒子分离器的分离效率提供研究基础与理论支撑。     

A Partially Orthogonal EnKF approach to atmospheric density estimation using orbital debris

A key requirement for accurate trajectory prediction and space situational awareness is knowledge of how non-conservative forces affect space object motion. These forces vary temporally and spatially, and are driven by the underlying behavior of space weather particularly in Low Earth Orbit (LEO). Existing trajectory prediction algorithms adjust space weather models based on calibration satellite observations. However, lack of sufficient data and mismodeling of non-conservative forces cause inaccuracies in space object motion prediction, especially for uncontrolled debris objects. The uncontrolled nature of debris objects makes them particularly sensitive to the variations in space weather. Our research takes advantage of this behavior by utilizing observations of debris objects to infer the space environment parameters influencing their motion.The hypothesis of this research is that it is possible to utilize debris objects as passive, indirect sensors of the space environment. We focus on estimating atmospheric density and its spatial variability to allow for more precise prediction of LEO object motion. The estimated density is parameterized as a grid of values, distributed by latitude and local sidereal time over a spherical shell encompassing Earth at a fixed altitude of 400 km. The position and velocity of each debris object are also estimated. A Partially Orthogonal Ensemble Kalman Filter (POEnKF) is used for assimilation of space object measurements to estimate density.For performance comparison, the scenario characteristics (number of objects, measurement cadence, etc.) are based on a sensor tasking campaign executed for the High Accuracy Satellite Drag Model project. The POEnKF analysis details spatial comparisons between the true and estimated density fields, and quantifies the improved accuracy in debris object motion predictions due to more accurate drag force models from density estimates. It is shown that there is an advantage to utilizing multiple debris objects instead of just one object. Although the work presented here explores the POEnKF performance when using information from only 16 debris objects, the research vision is to utilize information from all routinely observed debris objects. Overall, the filter demonstrates the ability to estimate density to within a threshold of accuracy dependent on measurement/sensor error. In the case of a geomagnetic storm, the filter is able to track the storm and provide more accurate density estimates than would be achieved using a simple exponential atmospheric density model or MSIS Atmospheric Model (when calm conditions are assumed).     

The dynamical environments analysis of surface particles for different shaped asteroids

Asteroids are coming to be a popular topic in the areas of astrophysical studies and deep space exploration recently. However, surface dynamics of asteroids is still a difficult problem. This paper aims at the motion analysis of surface particles for different asteroids. The dynamical analysis method of particles’ movement is given for three parts: global motion trend, local motion trend and static analysis. A dimensionless parameter ζ is defined to distinguish the predominant term to determine the distribution of effective potential. Three kinds of common asteroids: spheroidal asteroid, spindle-shaped asteroid and dumbbell-shaped asteroid are all discussed for those three parts with different parameter ζ. The motion trend of particles on the surface of each kind of asteroid is given. The static analysis of surface particles for different asteroids is also illustrated. Based on them, some common rules for different shaped asteroids are revealed. This paper could not only provide a reference for asteroid exploration missions but also be meaningful for the research of morphologic evolution of asteroids.     

固体火箭喷管颗粒尺寸分级两相跨音速流场计算

本文中对固体火箭喷管颗粒尺寸分级的两相跨音速流场作了计算.气相控制方程采用隐式近似因子分解法求解,尺寸分级的颗粒控制方程采用特征线求解,然后,二者进行充分的耦合,可以获得固体火箭发动机含有任意颗粒质量分数和不同颗粒尺寸时轴对称喷管跨音速流场的参数分布.文中讨论了不同颗粒半径和质量分数对流场的影响,对单一颗粒尺寸和颗粒尺寸分级的参数进行了比较.两相耦合计算的迭代收敛速度取决于气相,本文中气相方程求解的格式除部分边界外是隐式的,CFL数可取至6左右,收敛速度快.特别是对颗粒尺寸分级的计算,得益更大,其得益的倍数为颗粒的分级数.