Further studies on efficient AFEC schemes for Ka-bandsatellite systems

Adaptive fade margin is required to counter the severe but varying rain attenuation in Ka satellite communications. In searching for a suitable rain countermeasure, the effectiveness of the adaptive forward error control (AFEC) scheme is studied. Two AFEC schemes using convolutional codes and concatenated codes are proposed and their performance analyzed. The schemes can provide a progressively adaptive fade margin of 10.1 and 10.4 dB, respectively, in excess of the systems fixed fade margin. To improve the efficient use of shared resources of the system, an AFEC scheme using double coding is introduced, and its performance is analyzed. In this scheme a single codec is used repeatedly, and as a result the hardware cost is reduced and the utilization increased. The effective usable capacity of a conceptual AFEC resource sharing scheme using Golay double coding is also analyzed     

Frequency diversity in millimeter wave satellite communications

Dual-band frequency diversity seems to be an effective fade countermeasure to rain-induced attenuation in satellite communications above 20 GHz. This method, particularly suited for satellites operating in two frequency bands, typically K_a band and C or K_u band, achieves very low levels of outage probability especially where the rain fades are severe. A theoretical analysis of a frequency diversity system is performed to evaluate the amount of reserve capacity needed to counteract fading in a satellite network. The problems that arise in implementing adaptive fade countermeasures because of the dynamic characteristics of fading are discussed, and the results of the simulations performed using the attenuation time series at 11.6 GHz, measured with the Sirio satellite throughout four years, are presented     

Friction compensation for low velocity control of hydraulic flight motion simulator: A simple adaptive robust approach

Low-velocity tracking capability is a key performance of flight motion simulator (FMS), which is mainly affected by the nonlinear friction force. Though many compensation schemes with ad hoc friction models have been proposed, this paper deals with low-velocity control without friction model, since it is easy to be implemented in practice. Firstly, a nonlinear model of the FMS middle frame, which is driven by a hydraulic rotary actuator, is built. Noting that in the low velocity region, the unmodeled friction force is mainly characterized by a changing-slowly part, thus a simple adaptive law can be employed to learn this changing-slowly part and compensate it. To guarantee the boundedness of adaptation process, a discontinuous projection is utilized and then a robust scheme is proposed. The controller achieves a prescribed output tracking transient performance and final tracking accuracy in general while obtaining asymptotic output tracking in the absence of modeling errors. In addition, a saturated projection adaptive scheme is proposed to improve the globally learning capability when the velocity becomes large, which might make the previous proposed projection-based adaptive law be unstable. Theoretical and extensive experimental results are obtained to verify the high-performance nature of the proposed adaptive robust control strategy.     

单外涵变循环发动机变几何特性仿真

为了研究单外涵变循环发动机变几何性能收益,建立了一种单外涵变循环发动机总体性能仿真模型,并通过算例验证 了仿真模型的计算精度。根据不同飞行状态的发动机控制规律和最优控制目标,模拟生成3种变几何方案最佳变几何参数以及 最佳节流特性和高度-速度特性。结果表明：在设定的控制规律下,相对发动机常规变几何方案（方案1）,尾喷管、混合器与低压 涡轮导向器可调的变几何方案（方案3）使发动机地面节流状态耗油率降低1.7%～3.0%,超声速巡航推力增大14%～29%,亚声速 巡航耗油率降低0.9%～3.1%,在3种变几何方案中性能收益最大;尾喷管与混合器可调的变几何方案（方案2）使发动机地面节流 状态耗油率降低1.2%～2.2%,超声速巡航推力增大3%～17%,亚声速巡航耗油率降低0.9%～1.2%,在3种变几何方案中性能收 益居中。发动机变几何方案的选择应综合考虑结构复杂度、可靠性、质量等方面的代价与基于特定任务需求的总体性能收益的 平衡。     

Robust and adaptive actuator failure compensation designs for a rocket fairing structural-acoustic model

The actuator failure compensation problem is formulated for active vibration control of a rocket fairing structural-acoustic model with unknown actuator failures. Performance of a nominal optimal control scheme in the presence of actuator failures is studied to show the need of effective failure compensation. A robust control scheme and two adaptive control schemes are developed, which are able to ensure the closed-loop system signal boundedness in the presence of actuator failures whose failure pattern and values are unknown. The adaptive scheme for parameterizable failures ensures asymptotic stability despite failure uncertainties. Simulation results verified their failure compensation effectiveness.     

GPS多径信号的自适应滤波估计方法

 研究全球定位系统(GPS)多径信号估计的问题。通过分析自适应滤波器的原理,建立了数字中频信号处理的数学模型,提出一种用自适应滤波实现GPS多径幅度、码相位和载波相位估计的方法。该方法采用不同延迟的伪随机序列对信号进行解扩、解调和累加,得到了作为期望信号的系列自适应滤波相关值。对该方法与其他3种方法进行了理论上的分析比较,得出本方法具有信噪比高、自适应滤波性能好、带有码相位信息和不存在载波模糊度问题等优点。根据各种滤波器算法的特点和本应用的需求,给出了选用递归最小二乘算法实现的方法。通过计算机仿真,验证了提出的方法能够在14 dB的信噪比下,以1个采样间隔的时间延迟分辨率和0.005周的载波相位估计精度估计出GPS L1的多径信号。     

高精度有限差分格式的色散优化及耗散控制

本文综述了我们在高精度有限差分格式的色散优化和耗散控制方面的研究进展。首先,我们提出了半离散有限差分格式色散和耗散相互独立的充分条件,实现了差分格式色散和耗散特性的独立调节。在此基础上,提出了色散最小、耗散可控的高精度差分格式,称为MDCD格式。MDCD格式已经得到了广泛应用,取得了很好的计算效果,但其主要缺点是耗散的调节依赖于经验。为了解决这一问题,我们进一步提出耗散的自适应调节方法。具有自适应耗散特性的高精度有限差分格式的基本特征是,差分格式的耗散能够随解的局部尺度自适应调节。为了构造这类格式,我们提出了一种新型的尺度识别器,它能够以等效无量纲波数的形式来定量衡量数值解的局部长度尺度。在此基础上,设计差分格式耗散参数与尺度识别器得到的等效无量纲波数之间的关系,从而构造了一类色散最小、耗散自适应的差分格式,称为MDAD格式。为了计算含有间断的问题,同时保持在光滑区的良好耗散特性,我们利用尺度识别器对一种经典的激波探测器进行改进,提出了一种新的激波探测器,并将自适应耗散格式与对应的WENO格式相混合,得到自适应耗散混合格式。近似色散关系显示该混合格式兼具高分辨率和鲁棒性。多个含间断流场的标准算例测试结果显示,自适应耗散混合格式具有良好的分辨率和激波捕捉能力。     

Predicting and adapting satellite channels with weather-induced impairments

Efficiency improvements using predictive and adaptive methods over satellite channels with weather-induced impairments are presented. Scintillation and rain attenuation are the two dominant factors for signal fading over satellite-Earth paths at operating frequencies over 10 GHz. We develop statistical and spectral analyses of these processes, and obtain simple linear predictors for received signal attenuation using autoregressive (AR) models. For adaptation, we propose changing signal transmission power, modulation symbol size, and/or code rate as the state of the channel changes. In particular, we introduce a continuous power control and discrete rate control strategy. Quantitative analyses of power consumption and channel capacity indicate that there can be a substantial gain in performance with such adaptive schemes.     

LU-AUSMPW混合格式在可压缩无粘和粘性流动数值分析中的应用

本文首先将 AUSMPW格式与三阶 MUSCL格式融合, 给出了其在任意曲线坐标下的三维形式, 并将其与 LU-SGS格式结合, 应用于可压缩 Euler和 Navier-Stokes方程的求解。其次, 构造了一种新的通量限制器。最后, 为了验证 LU -AUSMPW混合格式的性能, 将平面叶栅跨音速无粘流动以及喷管超音速粘性流动作为算例。本文计算结果与文献计算结果和实验数据相符很好, 表明采用 LU-AUSMPW混合格式数值模拟可压缩流场, 具有较高的计算精度、较快的收敛速度和良好的稳定性。      

Resonant inverters for photovoltaic array to utility interface

A number of high-frequency link power converter schemes suitable for interconnecting a photovoltaic (PV) array to the utility line using resonant inverters are presented. Use of high-frequency resonant inverters (HFRI) results in reduced size, weight and cost. Scheme 1 uses the HFRI-cycloconverter, scheme 2 uses the HFRI-rectifier pulsewidth-modulated voltage source inverter, scheme 3 uses the HFRI-rectifier line commutated inverter working with a fixed maximum possible inverting fringe angle, and scheme 4 uses the HFRI-rectifier line-connected inverter (with line current modulation). These schemes are compared. Based on the comparison, scheme 3 was selected for an initial study. To overcome various problems associated with scheme 3, the line current is modulated sinusoidally in scheme 4. In this scheme, the HFRI is controlled appropriately to obtain a rectified sine wave of 120 Hz in the DC link and the function of the line-connected inverter is to unfold this waveform on alternate half cycles to obtain a sinusoidal line current with the line voltage. Methods of commutation of the line connected inverter are explained. Experimental results obtained with scheme 4 are presented     

超级燃烧室混合室掺混特性数值研究

为了满足TBCC超级燃烧室的各工作模式下的低阻高效混合,设计了两种采用方形波瓣混合器和结构可调的导流片相结合的强化掺混方案。方案A由上下扩张角不等的波瓣混合器和处于波谷上方的离散状导流片组成,波瓣混合器正对于分流环;方案B由全环形导流片和上下等扩张角的波瓣混合器组成,波瓣混合器的中径小于分流环直径。通过数值研究对比分析了两种混合方案的流动特征发现,两种方案都未出现倒流和流动分离现象;除冲压模态时的波峰截面方案B的流线分布较为理想外,其它工作状态下方案A波峰和波谷截面流线分布均优越方案B。通过对总压恢复系数、热混合效率及动量混合系数的对比发现,混合方案A在飞行Ma03的各工作状态下的总压恢复系数均高于方案B最多高3.5%且能在较短的掺混距离内使内外涵气流的温度和速度混合均匀。考虑到兼顾飞行Ma03范围内的低阻高效混合要求,方案A具有良好的强化掺混特性。     

高速直升机方案研究

对目前三种主流高速直升机——复合式、倾转旋翼／机翼式和转换式进行了综合分析；根据发展态势，比较了它们的构型、性能及关键技术等方面的优缺点，并对可行性进行了讨论。在此基础上．提出了一种旋翼／机翼转换式高速直升机方案，并对此方案进行了旋翼系统的原理性试验，对关键技术——模式转换过程进行了理论计算和试验研究。验证了直升机模式和飞机模式在相互转化过程中升力、功率和操纵的平滑连续性及保持操稳性；最后，根据研究结果，经过计算得出了此方案的总体参数。     

组合导航系统中一种基于特征值分解的自适应信息融合滤波算法

基于分散滤波理论的联合滤波算法,可以有效地降低组合导航系统的计算负担,并且增强系统的容错性能。给出了一种联合滤波算法中信息分配系数的自适应计算方法,能够使联合系统根据导航过程中各传感器的信息质量的变化合理地反馈全局信息。仿真结果表明,该算法可以有效地降低由于导航子系统降级带来的滤波误差。     

连续纤维增强金属基复合材料涡轮轴结构承扭特性分析

为实现对该类复合材料部件结构完整性的设计分析,以连续纤维增强复合材料轴结构为研究对象,对比分析4种细观力学模型计算连续纤维增强金属基复合材料的力学性能参数;在此基础上,采用RVE(代表体积元)有限元模型计算的复合材料力学性能参数作为输入,建立连续纤维增强金属基复合材料轴结构力学分析模型.计算与对比分析不同材料方案下纤维增强金属基复合材料轴结构在扭转载荷作用下的变形量及承扭能力,当纤维体积分数一定时,方案4的变形量最小,方案2的临界屈曲转矩最大.      

基于边界变差最小的高精度有限差分格式构造

从加权紧致非线性（WCNS）高精度有限差分格式出发，在其重构形式的基础上，根据边界变差最小（BVD）原理，遵循单元边界两侧重构物理量值之差最小的准则，在每个单元内通过两步空间重构，构造了一种新的高精度有限差分格式。一般对WCNS等加权非线性格式的改进都是基于改善色散耗散特性、优化非线性权、提高分辨率等单一途径，将它们作为重构候补函数进行结合，既保持了各自优势所在，又能控制格式整体黏性，所得格式具有丰富的应用场景。通过数值实验，将结果与单一格式进行对比，新格式既能在流场光滑区保证设计的精度，对激波等间断附近的振荡也有很好的抑制作用，提高了对高波数区的分辨率，而且在长时间计算后也有较为精确的结果。面向广泛发展的数值格式，还可以构造出其他新方法，对包含强间断和多尺度的流动问题可以获得更好的结果。     

空时分组编码的无人机中继通信航迹规划方法

无人机(UAV)中继通信是实现远距离点对点无线通信的一种重要技术手段。为提高无人机中继通信系统链路传输的可靠性,提出基于空时分组编码的无人机放大转发中继通信传输方案,并基于双跳链路遍历容量最大化的准则给出了无人机最佳航迹规划方法,并进一步利用FM-EM算法给出了基于空时分组编码无人机中继通信系统的中断概率及遍历信道容量计算公式。计算机仿真表明:提出的方法显著优于传统的单发单收(SISO)无人机中继通信系统。     

Development of swashplateless helicopter blade pitch control system using the limited angle direct-drive motor (LADDM)

It can be greatly beneficial to remove the swashplate of conventional helicopter, because the swashplate is usually complicated, aerodynamically resistive, and obstacle of more complex pitch control for improving performance. The present technologies for helicopter vibration reduction are usually narrow in effective range or requiring additional actuators and signal transfer links,and more effective technology is desired. Helicopter blade pitch control system, which is removed of swashplate and integrated high-frequency pitch control function for active vibration reduction, is likely the suitable solution at current technical level. Several potential implementation schemes are discussed, such as blades being directly or indirectly driven by actuators mounted in rotating frame and application of different types of actuators, especially implementation schemes of electro-mechanical actuator with or without gear reducer. It is found that swashplateless blade pitch control system based on specially designed limited angle direct-drive motor(LADDM) is a more practical implementation scheme. An experimental prototype of the finally selected implementation scheme has been designed, fabricated and tested on rotor tower. The test results show considerable feasibility of the swashplateless helicopter blade pitch control system using the LADDM.     

非结构网格上新型的NND有限元格式

张涵信发展的ＮＮＤ差分格式是由中心差分格式,二阶迎风格式和一阶迎风格式混合组成的杂交型格式。众所周知,和中心差分格式相对应的是Ｇａｌｅｒｋｉｎ有限元格式。通过对中心型有限元格式加修正项的方法本文成功地构造出二阶迎风型有限元格式和一阶迎风型有限元格式。     

Experimental research of active vibration and noise control of electrically controlled rotor

Electrically Controlled Rotor (ECR), also called as swashplateless rotor, applies blade pitch inputs via trailing-edge flaps system instead of traditional swashplate mechanism. In addition to primary control, rotor vibration reduction and noise alleviation are also achievable via applying higher harmonic control inputs with the trailing-edge flaps. In this paper, the feasibility of ECR to reduce vibration and noise actively is verified experimentally. Firstly, the test scheme of ECR active vibration and noise control is proposed, and the ECR test platform is modified according to the test scheme. Then, an adaptive control algorithm based on Kalman filter is developed. Lastly, hover and wind tunnel tests is performed to verify the feasibility of ECR active vibration and noise control. The results demonstrate that the ECR are effective for reducing rotor vibration and noise simultaneously. In the hover condition, the ECR can reduce the in-plane hub vibration by 42% and the in-plane noise by 4 dB. In wind tunnel condition, ECR can reduce the hub vibration by 75% and the BVI noise by 3 dB.     

A comparison of two adaptive detection schemes

Two schemes for adaptive detection are compared: Kelly's generalized likelihood ratio test (GLRT) and the mean level adaptive detector (MLAD). Detection performance, P_D, is predicted for the two schemes under the assumptions that the input noises are zero-mean complex Gaussian random variables that are temporally independent but spatially correlated; and the amplitude of the desired signal is Rayleigh distributed. P_D is computed as a function of the false alarm probability, the number of input channels, the number of independent samples per channel, and the matched filtered output signal-to-noise (S/N) power ratio. In this analysis the GLRT is shown to have better detection performance than the MLAD. The difference in detection performance increases as one uses fewer input samples. However, the required number of samples necessary to have only a 3 dB detection loss for both detection schemes is approximately the same. This is significant since for the present, the MLAD is considerably less complex to implement than the GLRT