DEVELOPMENT AND VALIDATION OF REACTION WHEEL DISTURBANCE MODELS: EMPIRICAL MODEL

Accurate disturbance models are necessary to predict the effects of vibrations on the performance of precision space-based telescopes, such as the Space Interferometry Mission (SIM). There are many possible disturbance sources on such spacecraft, but mechanical jitter from the reaction wheel assembly (RWA) is anticipated to be the largest. A method has been developed and implemented in the form of a MATLAB toolbox to extract parameters for an empirical disturbance model from RWA micro-vibration data. The disturbance model is based on one that was used to predict the vibration behaviour of the Hubble Space Telescope (HST) wheels and assumes that RWA disturbances consist of discrete harmonics of the wheel speed with amplitudes proportional to the wheel speed squared. The MATLAB toolbox allows the extension of this empirical disturbance model for application to any reaction wheel given steady state vibration data. The toolbox functions are useful for analyzing RWA vibration data, and the model provides a good estimate of the disturbances over most wheel speeds. However, it is shown that the disturbances are under-predicted by a model of this form over some wheel speed ranges. The poor correlation is due to the fact that the empirical model does not account for disturbance amplifications caused by interactions between the harmonics and the structural modes of the wheel. Experimental data from an ITHACO Space Systems E-type reaction wheel are used to illustrate the model development and validation process.     

Passive vibration isolation of reaction wheel disturbances using a low frequency flexible space platform

Reaction wheel assemblies (RWAs) are momentum exchange devices used in fine pointing control of spacecrafts. Even though the spinning rotor of the reaction wheel is precisely balanced to minimize emitted vibration due to static and dynamic imbalances, precision instrument payloads placed in the neighborhood can always be severely impacted by residual vibration forces emitted by reaction wheel assemblies. The reduction of the vibration level at sensitive payloads can be achieved by placing the RWA on appropriate mountings. A low frequency flexible space platform consisting of folded continuous beams has been designed to serve as a mount for isolating a disturbance source in precision payloads equipped spacecrafts.Analytical and experimental investigations have been carried out to test the usefulness of the low frequency flexible platform as a vibration isolator for RWAs. Measurements and tests have been conducted at varying wheel speeds, to quantify and characterize the amount of isolation obtained from the reaction wheel generated vibration. These tests are further extended to other variants of similar design in order to bring out the best isolation for given disturbance loads. Both time and frequency domain analysis of test data show that the flexible beam platform as a mount for reaction wheels is quite effective and can be used in spacecrafts for passive vibration control.     

Design and analysis of an intelligent vibration isolation platform for reaction/momentum wheel assemblies

This study focuses on design and analysis of an intelligent vibration isolation platform for reaction wheel assemblies (RWAs) and momentum wheel assemblies (MWAs). A passive platform consisting of four folded beams is designed and analysed for MWAs. A simple and effective mathematical model is developed for the system consisting of the platform and MWAs, and this model is used to investigate the passive vibration isolation performance. Further development is performed to produce an intelligent platform for RWAs, with piezoelectric sensors and actuators bonded to the vertical beams. The flywheel imbalance and impulse load are assumed to be input disturbances for the investigation of the active vibration isolation performance by the finite element method (FEM). The simulation results show that the passive vibration isolation platform is particularly effective for the suppression of a high frequency range vibration for MWAs, and the intelligent platform using velocity feedback control effectively attenuates the dynamic amplification of amplitude at resonance for RWAs. Thus, it is concluded that the passive platform can be used as a vibration isolation platform for MWAs and that the intelligent one can be used for RWAs.     

空间目标抖动状态红外图像仿真建模研究北大核心CSCD

空间目标容易受到太空垃圾碰撞及干扰从而产生抖动。针对空间抖动目标红外图像的模型构建问题,对天基空间红外成像系统的主要噪声进行分析并考虑杂散光影响,基于Creator与Vega软件平台相结合,提出抖动状态下空间目标表面缺陷的红外图像的建模方法。依据空间目标基本特征分析其红外辐射特性,在Creator中对空间目标缺陷进行三维建模;根据目标以及背景红外辐射特性对三维模型进行温度场分析,将分析所得结果与Vega红外模块相结合获得红外图像模型;确定抖动图像数学模型并对仿真图像施加抖动影响,然后施加杂散光影响获得最终模拟图像。实验结果表明:该方法生成的抖动状态下空间目标红外图像与实验图像相似程度高,能为空间目标探测与态势感知提供一种有效的模拟系统。     

Consideration of piezoceramic actuator nonlinearity in the active isolation of deterministic vibration

There is an increasing need to effectively control micro-vibration in such fields as metrology, optics and micro-electronics. This paper describes the design of an adaptive feedforward strategy for vibration isolation of harmonic disturbance using a piezoelectric actuator with hysteretic behavior. A nonlinear analytical model of the piezoelectric actuator including a ferroelectric-like behavior is built using a Preisach model of hysteresis. Pre-multiplication of a single-frequency reference signal by the nonlinear model of the stack is investigated in order to effectively compensate the actuator nonlinearity. It is observed that a simple linear model of the stack is sufficient in the adaptation of a filtered-X LMS feedforward controller to effectively compensate the actuator nonlinearity, provided the reference signal has frequency components at the disturbance frequency and its higher harmonics.     

Stochastic models of jitter

This study presents stochastic models of jitter. Jitter designates small, random, involuntary perturbations of the glottal cycle lengths. Jitter is a base-line phenomenon that may be observed in all voiced speech sounds. Knowledge of its properties is therefore relevant to the acoustic modeling, analysis, and synthesis of voice quality. Also, models of jitter are conceptual frameworks that enable experimenters and clinicians to distinguish jitter in particular from aperiodic cycle length patterns in general. Vocal jitter is modeled by means of the ribbon model of the glottal vibration combined with stochastic models of the disturbances of the instantaneous frequency. The disturbance model comprises correlation-free noise and vocal microtremor. Properties of jitter that are simulated are the stochasticity, stationarity, and normality of the decorrelated cycle length perturbations, the size of decorrelated jitter, the correlation between the perturbations of neighboring glottal cycles, the modulation level and modulation frequency owing to microtremor, the asynchrony between external disturbances and glottal cycles, the dependence of the size of jitter on the average glottal cycle length, and the relation between jitter and laryngeal pathologies. Modeled jitter is discussed in the light of measured jitter, as well as the physiological and statistical plausibility of the model parameters.     

Multi-parameter identification from scalar time series generated by a Malkus-Lorenz water wheel

We address the issue of multi-parameter estimation from scalar outputs of chaotic systems, using the dynamics of a Malkus water wheel and simulations of the corresponding Lorenz-equations model as an example. We discuss and compare two estimators: one is based on a globally convergent adaptive observer and the second is an extended Kalman filter (EKF). Both estimators can identify all three unknown parameters of the model. We find that the estimated parameter values are in agreement with those obtained from direct measurements on the experimental system. In addition, we explore the question of how to distinguish the impact of noise from those of model imperfections by investigating a model generalization and the use of uncertainty estimates provided by the extended Kalman filter. Although we are able to exclude asymmetric inflow as a possible unmodeled effect, our results indicate that the Lorenz-equations do not perfectly describe the water wheel dynamics.     

Lissajous figures in the application of micro-vibration measurement

Based on Michelson interferometer and phase generated carrier (PGC) homodyne demodulation technique, an optical interferometer system is built, and a novel method using the central angles of Lissajous figures to measure micro-vibration displacement is proposed. The Lissajous figures are obtained by synthesizing two interferometric signals that their vibration directions are orthogonal and their intensities are equal. Through theoretical analysis, software simulation and experimental test, it is concluded that the central angles of Lissajous figures have a linear relationship with the phase modulation coefficients and the micro-vibration displacements when the phase modulation coefficients are small. So the micro-vibration displacements can be directly obtained by measuring the central angles of Lissajous figures. The method is simple and convenient. The experimental results indicate that the micro-vibration displacements measuring range can reach 5-775 nm, with a resolution of about 5 nm.     

一种低能耗高稳定性的制冷控制策略

针对制冷系统智能稳定控制的需求,提出了一种基于输入关系式建模分析和稳态预测的控制策略。该策略通过对基于静态模型的系统控制优化点搜索方式,寻找系统控制过程中最佳的控制点。并对制冷系统的工作机制进行建模,给出了主要的参数关系式和参数估计方法,在此基础上,建立了制冷系统的稳态预测方法。经过实验仿真测试表明,所设计的制冷系统智能控制策略具有高稳定性和低能耗的特点。     

Numerical and Analytical Approaches to Modeling Critical Two-Phase Flow with Granular Layer

Based on data obtained in the previous experimental study conducted by the authors, two approaches are proposed for analytical and numerical modeling of a critical two-phase flow in a pipe with a granular layer. An analytical approach is based on a polytrophic model, while a numerical approach was developed using a smoothed particle hydrodynamics method. A model of isenthalpic flow of vapor–water mixture in a fixed bed of solid particles is considered is this study. The mixture expansion process is considered to be polytropic. Similarly to the known problem of gas dynamics of a granular bed, an analytical relationship for calculation of a critical mass velocity was obtained. The results of the calculation based on the analytical and numerical models were compared with the experimental data and agreement between analytical and numerical data and the experiment was observed.     

Uncertainty analysis in matched-field geoacoustic inversions

Quantifying uncertainty for parameter estimates obtained from matched-field geoacoustic inversions using a Bayesian approach requires estimation of the uncertainties in the data due to ambient noise as well as modeling errors. In this study, the variance parameter of the Gaussian error model, hereafter called error variance, is assumed to describe the data uncertainty. In practice, this parameter is not known a priori, and choosing a particular value is often difficult. Hence, to account for the uncertainty in error variance, several methods are introduced for implementing both the full and empirical Bayesian approaches. A full Bayesian approach that permits uncertainty of the error variance to propagate through the parameter estimation processes is a natural way of incorporating the uncertainty of error variance. Due to the large number of unknown parameters in the full Bayesian uncertainty analysis, an alternative, the empirical Bayesian approach, is developed, in which the posterior distributions of model parameters are conditioned on a point estimate of the error variance. Comparisons between the full and empirical Bayesian inferences of model parameters are presented using both synthetic and experimental data.     

Laser transmission welding of composites – Part B: Experimental validation of numerical model

In this paper, experimental measurements are performed to confirm the global analytical model (refraction and absorption phenomena) presented in the previous work. Initially, an experimental approach to the estimation of the laser beam spread in a semi-transparent composite and at interface (width of the output beam) subjected to an incident heat flux, was presented. This parameter represents a fundamental input data for the global analytical model (refraction and absorption) during the numerical simulation of transmission infrared welding. Then, an experimental setup for the temperature measurement is performed using infrared camera, during infrared transmission welding of materials joints to validate the welding simulation results (a transient numerical model, based both on conduction and radiation mode heat transfer) with the developed analytical model. The commercial FEM software COMSOL Multiphysics® is used to compute temperature distribution by implementing a radiative source term. Numerical simulations are compared with experimental data. The agreement between simulations and experiments is fair, which gives confidence to use the developed model with acceptable accuracy.     

基于Boltzmann输运方程的SPECT系统解析建模方法

在单光子发射断层成像(SPECT)中, 为了校正劣化因素的影响, 提高图像质量, 需要对SPECT成像的物理过程进行准确建模. 本文提出了基于Boltzmann输运方程及其Neumann级数解理论的SPECT系统解析建模方法, 并采用数论高维数值积分算法对解析建模公式进行数值求解. 分别对点源、均匀圆柱体模型和NCAT模型进行SPECT投影过程计算, 将其结果与传统的Monte Carlo建模方法进行比较. 结果表明解析建模方法的计算速度和精度综合性能优于Monte Carlo建模方法, 且具有不受统计噪声影响的优点, 因而更适于进行SPECT成像过程的建模.     

光纤水听器相位生成载波解调非线性因素的抑制方法

为了消除非线性因素对相位生成载波解调结果的干扰,降低相位生成载波解调过程中低通滤波器的频响特性对相位生成载波解调结果的影响,该文提出了一种基于扩展卡尔曼滤波椭圆参数估计的相位生成载波微分交叉相乘解调方法。该方法在综合考虑各种非线性因素对相位生成载波微分交叉相乘解调结果的影响的基础上,对传统的相位生成载波微分交叉相乘解调过程进行了改进。经过计算机仿真和实验验证,该文所提出的扩展卡尔曼滤波微分交叉相乘方法能够有效地抑制非线性因素对相位生成载波解调结果的干扰,相比传统的相位生成载波微分交叉相乘方法,其信噪比提高了35.0 d B,总谐波失真降低了30.7 dB,信噪谐波比提高了31.0 dB,且扩展卡尔曼滤波微分交叉相乘方法受解调过程中低通滤波器频响特性的影响较小。     

平行磨削非轴对称非球面光学元件表面形貌

结合砂轮表面仿真及磨削过程的运动学仿真获得工件表面轮廓、形貌和粗糙度预计,可以作为磨削过程中的理论依据,是精密磨削加工技术中主要的研究内容之一。平行磨削技术是加工非轴对称非球面光学元件的重要手段,而相关的仿真过程报道还很少。提出一种基于平行磨削的精密磨削加工非球面表面生成的仿真方法,该方法主要包含使用高斯方法生成具有不同统计学特征的随机砂轮表面形貌,建立单磨粒运动轨迹方程和圆弧砂轮细分后与工件表面点接触的运动关系,据此给出平行磨削加工表面生成的数值算法,并对不同加工参数下的工件表面形貌进行仿真。仿真结果和测量结果的一致性验证了所给算法的正确性和有效性     

Emission spectrum of a qubit in Rabi model in strong coupling regime

The analytical eigenenergies and eigenstates of the Rabi model are obtained approximately based on a unitary transformation and a generalized rotating-wave approximation (GRWA). Using these analytical expressions without the rotating wave approximation (RWA), we generalize the definition of the physical emission spectrum valid with the RWA in order to meet without the RWA with some modifications. Taking into account the counter-rotating wave terms and the intercrossing of energy level in the strong coupling regime, the physical emission spectrum of qubit is investigated. Different from the case with RWA, the multi-peak vacuum Rabi splitting, even when the qubit initially in its ground state and the bosonic field initially in vacuum, can emerge. These new features of physical emission spectrum originate from the effect of counter-rotating wave terms. Moreover, the intercrossing of energy level can also be observed in the strong coupling regime.     

飞轮微振动与整星耦合特性参数补偿算法研究

为了提高飞轮与整星耦合特性影响成像像移理论预估的精度,提出了一种星上飞轮安装刚度和阻尼参数的数值补偿算法.首先,建立补偿算法数学模型,基于飞轮径向摇摆模态以及线性整星微振动传递函数模型,通过联合飞轮在专用测量平台上的扰振实测数据以及星上安装后所致成像像移实测数据构建优化函数,实现对星上飞轮安装刚度以及阻尼参数的补偿.其次,采用该算法对某型号卫星上的飞轮安装刚度以及阻尼进行了补偿和估计,最后对参数补偿前后的飞轮扰振所致像移分别进行理论预估与对比.对比结果表明:参数补偿后的预估像移数据较补偿前预估像移数据更接近实测像移数据且径向摇摆模态固有频率曲线更加明显,证明此参数补偿算法可行.本研究显著降低了整星微振动分析过程中飞轮在星上安装刚度与测试平台安装刚度不一致所带来的分析误差,为飞轮与整星耦合特性安装等效参数求解提供了一种思路.     

Exact eigenstates of the two-photon Jaynes-Cummings model with the counter-rotating term

We have investigated the eigenenergy spectrum of the two-photon Jaynes-Cummings (JC) model with and without the rotating-wave approximation (RWA). Our analysis has indicated that the counter-rotating term dramatically changes the nature of the RWA energy spectrum and that the non-RWA spectrum can be approximated by the RWA spectrum only in the range of a sufficiently small coupling constant. Furthermore, unlike the one-photon counterpart, the two-photon JC model without the RWA is well defined only if the coupling parameter is below a certain critical value. As a result, the dynamics of the two-photon JC model without the RWA is significantly different from its RWA counterpart. For instance, the counter-rotating term can dramatically enhance the field squeezing effect. Besides, we would expect that the quantum dynamics of the two-photon JC model without the RWA is qualitatively different from that of the usual one-photon case.     

深海远程正交频分复用水声通信块间迭代稀疏信道估计方法

在深海远程正交频分复用(OFDM)水声通信中,信道时延长、频率选择性衰落严重,传统的块独立压缩感知稀疏估计需要较高导频插入密度才能保证一定的估计性能,通信频谱利用率较低。提出了一种基于信道稀疏时变建模的块间迭代信道估计方法,利用深海信道在两个相邻OFDM数据块之间的时间相关性建立块间信道稀疏多途结构的时变关系,在此基础上,对传统稀疏信道估计算法中的候选字典矩阵的字典原子进行删减并改进优化方程,实现了对前一数据块所估信道信息的有效利用,显著降低了信道估计所需的导频插入密度。在深海不同接收深度、不同距离条件下开展了海试验证,实验结果表明,与传统稀疏信道估计方法相比,本方法在导频插入密度减半的条件下可达到优于传统方法的估计性能。     

面向语音增强的序贯隐马尔可夫模型时频语音存在概率估计

语音存在概率的估计是语音增强的核心技术之一,针对传统的存在概率估计方法是启发式的,没有把存在概率的估计统一到一个理论框架之中,不能保证估计最优,提出了一种基于序贯隐马尔可夫模型(SHMM)的存在概率估计方法,在每一子带上构建一个SHMM模型描述对数功率谱包络的时间序列,把谱包络序列看作一个在语音和噪声状态之间转移的动态一阶马尔可夫链,采用单高斯函数构建每一状态的概率模型,语音状态的后验概率即为语音信号的存在概率。为了满足算法实时性要求,SHMM参数估计简化为一阶回归过程,根据极大似然准则逐帧更新模型参数。实验表明:SHMM所描述的时序相关性对存在概率的估计起到关键作用,它优于一般的启发式估计方法;SHMM算法的语音增强分段信噪比(SegSNR)和对数谱失真(LSD)性能优于经典的改进型最小统计量控制递归平均(IMCRA)算法。