Measurement accuracy in control of segmented-mirror telescopes

Design concepts for future large optical telescopes have highly segmented primary mirrors, with the out-of-plane degrees of freedom actively controlled. We estimate the contribution to errors in controlling the primary mirror that results from sensor noise and, in particular, compare mechanical measurements of relative segment motion with optical wave-front information. Data from the Keck telescopes are used to obtain realistic estimates of the achievable noise due to mechanical sensors. On the basis of these estimates, mechanical sensors will be more accurate than wave-front information for any of the telescope design concepts currently under consideration, and therefore supplemental wave-front sensors are not required for real-time figure control. Furthermore, control system errors due to sensor noise will not significantly degrade either seeing-limited or diffraction-limited observations.     

Calibration of a Novel MEMS Inertial Reference Unit

This paper describes a novel low-cost inertial reference unit (IRU) developed for spin-stabilized sounding rockets, as well as a method to calibrate the IRU using a one-axis rate table. The IRU contains three orthogonally mounted microelectromechanical system (MEMS) gyros and is intended for use in a low-cost system for attitude determination of sounding rockets. The high spin velocity of the rockets (4–6 r/s) causes sensor misalignments and scale factor errors to have a large effect on the gyro outputs. The performance of the IRU is considerably improved by detailed calibration, including temperature compensation. For sounding rocket applications, the total rate error is typically reduced by more than one order of magnitude. The calibration procedure uses a ramp profile calibration maneuver input at all three sensor axes and a Kalman filter to estimate the 12 error parameters.      

A Small Low-Cost Hybrid Orientation System and Its Error Analysis

Inertial orientation systems have many potential uses beyond air- and spacecraft applications. Compared to the conventional large-scale system, the novel aspects of an orientation system based on micro electromechanical systems (MEMS) are small size, lightweight, low-power consumption, and low cost by the inclusion of microsensors with on-chip functionality, such as micromachined rate gyroscopes, accelerometers, and etc. However, the performance associated with these low-cost MEMS sensors is generally not satisfied. A conventional strapdown method based on integral of angular rate to figure out attitudes will inevitably induce long-term drift. For eliminating this accumulative error and thus using the low-cost navigation system for a long-duration mission, a hybrid configuration by combining a MEMS-based azimuth-level detector with the conventional strapdown system is proposed in this paper. The azimuth-level detector is composed of three-axis MEMS accelerometers and three-axis MEMS magnetometers. With an absolute solution based on the gravity and the earth magnetic field rather than the integral algorithm, attitude and heading estimation without drift errors can be obtained so as to facilitate adjusting the attitude and heading for the conventional strapdown system. The hierarchical computation process for estimating the attitude and heading using the hybrid system is depicted first. Then, comprehensive system errors including transformation errors of the computation, such as skew, scale, and drift errors, are analyzed. Finally, it is verified by both of formula analysis and experimental test that the accumulative errors can be effectively eliminated via this hybrid scheme.      

视轴偏心三轴跟踪机架指向精度分析

本文分析了实现大范围空间光电跟踪的视轴偏心三轴跟踪机架的轴系误差.在刚体动力学的基础上,利用姿态变换的方法对视轴偏心的这种新型跟踪机架进行分析,推导出了机架指向精度的具体算法,探索了计算视轴偏心的三轴跟踪机架误差及误差分配的新方法,并针对一跟踪架的具体误差进行了指向精度计算.其中,垂直度误差对指向精度影响最大.     

Attitude and orbit control for satellite broadcasting missions

This paper gives a broad introduction to the problems of attitude and orbit control of geostationary communications satellites. It specifically discusses the relationships between the satellite user’s requirements for a broadcasting mission and the design of the attitude and orbit control system. To put the subject in perspective, a brief review of past and present satellites is presented first. Then orbit control is described in terms of the forces that act on a satellite in geostationary orbit and the necessary station-keeping strategies. The design of attitude control systems for three-axis stabilised satellites is presented by considering the disturbance torques, attitude sensors and actuators and by identifying the various system problems and their solutions. Sources of error in pointing the satellite towards the earth are discussed together with the formulation of error budgets. Finally, the design approach for missions that require extremely accurate pointing is considered, and some remarks are given regarding the achievable accuracy for this class of satellite missions.     

Inertial sensor technology trends

This paper presents an overview of how inertial sensor technology is applied in current applications and how it is expected to be applied in nearand far-term applications. The ongoing trends in inertial sensor technology development are discussed, namely interferometric fiber-optic gyros, micro-mechanical gyros and accelerometers, and micro-optical sensors. Micromechanical sensors and improved fiber-optic gyros are expected to replace many of the current systems using ring laser gyroscopes or mechanical sensors. The successful introduction of the new technologies is primarily driven by cost and cost projections for systems using these new technologies are presented. Externally aiding the inertial navigation system (INS) with the global positioning system (GPS) has opened up the ability to navigate a wide variety of new large-volume applications, such as guided artillery shells. These new applications are driving the need for extremely low-cost, batch-producible sensors     

基于MEMS惯性传感器的微型姿态测量系统

提出了一种基于低成本MEMS惯性传感器的微型姿态测量系统,包括MEMS速率陀螺、MEMS磁强计、单轴MEMS加速度传感器.重点研究了基于扩展Kalman滤波（EKF）的姿态估计创新算法,通过速率陀螺更新误差状态四元数计算姿态角,并通过飞行方向的加速度传感器和三轴磁强计来补偿陀螺漂移和姿态角误差,利用扩展卡尔曼滤波方程消除瞬时干扰,实现高动态姿态测量.系统的仿真和高动态实验表明,姿态测量动态精度低于5°,静态精度低于0.7°.     

Vision-based horizon extraction for micro air vehicle flight control

Recently, more and more research has been done on micro air vehicles (MAVs). An autonomous flight control system is necessary for developing practical MAVs to be used for a wide array of missions. Due to the limitations of size, weight, and power, MAVs have the very low payload capacity and moments of inertia. The current technologies with rate and acceleration sensors applied on larger aircrafts are impractical to MAVs, and they are difficult to be scaled down to satisfy the demands of MAVs. Since surveillance has been considered as the primary mission of MAVs, it is essential for MAVs to be equipped with on-board imaging sensors such as cameras, which have rich information content. So vision-based techniques, without increasing the MAVs payload, may be a feasible idea for flight autonomy of MAVs. In this paper, a new robust horizon extraction algorithm based on the orientation projection method is proposed, which is the foundation of a vision-based flight control system. The horizon extraction algorithm is effective for both color images and gray images. The horizon can be extracted not only from fine images captured in fair conditions but also from blurred images captured in cloudy, even foggy days. In order to raise the computational speed to meet real-time requirements, the algorithmic optimization is also discussed in the paper, which is timesaving by narrowing the seeking scope of orientations and adopting the table look-up method. According to the orientation and position of the horizon in the image, two important angular attitude parameters for stability and control, the roll angle and the pitch angle, could be calculated. Several experimental results demonstrate the feasibility and robustness of the algorithm.     

Bayesian Information for Sensors

An engineering company manufacturing high‐precision sensors had accumulated huge historical databases of information on a type of sensors, which had been tested. The aim of the company was not to use this historical data to improve estimation of future individual sensor parameters, but rather to use it to reduce the number of measurements needed per sensor, guaranteeing a required level of accuracy. In the paper, we show how this can be performed, using Bayesian ideas, and introduce the novel theory for linear regression models, which determines how the reduction in individual sensor measurements can be achieved. Specifically, for estimating parameters of closely related sensors, an estimate can be thought of as comprising a global component, that is, the mean of all the sensors, and a local component, which is a shift from the mean. The historical data can, in a Bayesian framework, provide the global component and hence all that is needed from an individual sensor is the local component. In non‐Bayesian estimation methods, both components are required, and hence, many measurements are needed. On the other hand, with Bayesian methods, only the local fit is needed, and hence, fewer measurements per sensor are required. We provide the supporting theory and demonstrate on a real‐life application with real data. Copyright © 2014 John Wiley & Sons, Ltd.     

Self-calibration and compensation of residual gyro drifts for rotation inertial navigation system with fibre optic gyro

Fibre optic gyros (FOGs) are widely used in the rotation inertial navigation system (RINS). However, FOGs are sensitive to surrounding environments, and thus the drifts of FOGs would vary during the rotation of the gyros. This phenomenon may bring about residual gyro drifts, which are the primary cause for the accumulated navigation errors in the RINS. A recursion least square estimation method based on position errors is proposed to obtain residual drifts. A multi-position calibration method is presented to separate the residual drifts along the body axis. Static experiment results show that the positioning accuracy of a 12-h navigation is improved significantly from 2.24 to 0.54 n mile by compensating the residual drifts along the body axis. A vehicle-mounted motional experiment is conducted to verify the proposed calibration method, and the positioning accuracy is improved from 1.38 to 0.66 n mile.     

Vehicle Localization Using Sensors Data Fusion Via Integration of Covariance Intersection and Interval Analysis

Achieving an innovative integrated sensor fusion architecture with a robust vehicle navigation and localization using an extended Kalman filter, interval analysis and covariance intersection that can overcome the uncertainty in the system model and sensor noise statistics. There are various approaches to the problem, but here the focus is on an approach which can guaranteed performance of sensor-based navigation. The guaranteed performance is quantified by explicit bounds of position estimate of a ground vehicle. Ground vehicles generally carry dead reckoning sensors such as wheel encoders and inertial sensors, to measure acceleration and angle rate, while obstacle detection and mapmaking is done with time-of-flight ultrasonic sensors. Most of these sensors give overlapping or complementary information and sometimes are redundant as well, which offers scope for exploiting data fusion. The purpose here is to achieve data fusion for ground vehicles with low-cost sensors by forming an intelligent sensor system. This is accomplished by combining the sensors' measurements and processing these measurements with data fusion algorithms. The algorithms are complementary in the sense that they compensate for each other's limitations, so that the resulting performance of the sensor system is better than its individual components.     

Optimal experimental design in stochastic structural dynamics

This paper provides a methodology for optimal prediction of the response of randomly vibrating structures using information from a limited number of measurements. The objective is to optimize the locations of sensors for the purpose of making the most accurate predictions of the response at unmeasured locations in structural systems. The kriging method is used to find the response predictions and the corresponding mean-square errors at unmeasured locations. The mean-square errors in the predictions depend on the locations of sensors and the correlation characteristics of the response evaluated from the model of dynamics and the characteristics of the excitation. The response predictions depend also on the information contained in measurements. The optimal sensor locations are selected to minimize the total mean-square error of the response predictions at unmeasured points. This leads to a complicated non-convex optimization problem in which multiple local and global optima may exist. A hybrid optimization method based on evolution strategies is used to determine a global minimum. The optimal experimental design method presented in the paper is illustrated by designing the optimal sensor locations for an elastic beam and a plate subjected to a class of random stationary loads.     

Huber-based unscented filtering and its application to vision-based relative navigation

A new algorithm called Huber-based unscented filtering (UF) is derived and applied to estimate the precise relative position, velocity and attitude of two unmanned aerial vehicles in the formation flight. The relative states are estimated using line-of-sight measurements between the vehicles along with acceleration and angular rate measurements of the follower. By making use of the Huber technique to modify the measurement update equations of standard UF, the new filtering could exhibit robustness with respect to deviations from the commonly assumed Gaussian error probability, for which the standard unscented filtering would exhibit severe degradation in estimation accuracy. Furthermore, contrast to standard extended Kalman filtering, more accurate estimation and faster convergence could be achieved from inaccurate initial conditions. During filter design, the global attitude parameterisation is given by a quaternion, whereas a generalised three-dimensional attitude representation is used to define the local attitude error. A multiplicative quaternion-error approach is used to guarantee that quaternion normalisation is maintained in the filter. Simulation results are shown to compare the performance of the new filter with standard UF and standard extended Kalman filtering for non-Gaussian case.     

Configurable electrodes for capacitive-type sensors and chemical sensors

This paper describes a novel design for capacitive sensors or chemical sensors, which features configurable interdigitated electrodes: The electrode spacing can be varied by means of switches on the CMOS chip. This new design allows for performing two capacitive measurements with one single-sensor capacitor so that the number of sensors required to acquire a certain amount of information can be significantly reduced. The use of the same sensor and the same polymer layer for two measurements at a different electrode periodicity provides a better signal quality for the difference signal since detrimental influences, such as humidity and sensor drift, are similar for both electrode configurations and are strongly correlated. Such high signal quality is required for, e.g., the successful recognition of n-octane in the presence of tenfold larger background signals of humidity or, in general, for the determination of low analyte concentrations in humid air. The baseline drift in the concentration predictions based on the differential signal from the two electrode configurations was an order of magnitude lower than that for uncorrelated signals produced by two separate interdigitated capacitors on the same chip. Since the number of required sensors is reduced and, owing to the differential readout of two electrode configurations, reference capacitors are no longer necessary, the overall chip size and/or the number of sensor chips and, consequently, costs can be considerably reduced.     

基于nRF24E01的卫星三轴半物理仿真系统研究

针对利用反作用飞轮作为执行机构的微小卫星姿态控制系统单轴气浮台半物理仿真缺乏仿真实物的全面性问题,研究基于nRF24E01无线RS232串行通信模块的多单轴气浮转台联动运行、Matlab XPC实时仿真技术、星载计算机及反作用飞轮和光纤陀螺物理实物的卫星姿态控制系统三轴气浮台半物理仿真的方案设计;利用该仿真系统对以反作用飞轮为主要执行机构的卫星偏置动量三轴稳态控制模式进行了半物理仿真验证。仿真结果表明:半物理仿真平台工作效果良好,能够实现偏置动量三轴稳态控制算例的仿真过程,并观测到三轴敏感器和执行器的联动运行效果,验证了半物理仿真系统方案设计合理、可靠。     

Optimal fusion of multiple nonlinear sensor data

A framework for the detection of bandlimited signals by optimally fusing the multinonlinear sensor data is developed. Though most sensors used are assumed to be linear, none of them individually or in series gives the truly linear relationship, and errors are inevitable as a result of the assumption of linearity. A new approach, which takes the actual nonlinear characteristics of sensors into account, is advocated. Though the fusion of redundant information can reduce the overall uncertainty and, thus, serves to increase the accuracy of the process measurements, identifying the faulty readings and fusing only the reliable data are very difficult and challenging. An optimal multiple nonlinear sensor data fusion scheme in which multisensor data fusion is done by scheduling the sensor measurements is proposed. The main idea of the multisensor fusion schemes proposed in this paper is to pick only the reliable data for the fusion and disregard the rest. The proposed theoretical framework is supported by illustrative examples and simulation data.     

三轴激光陀螺放电状态监控

对三轴激光陀螺的结构及串扰现象作了简要介绍,提出了放电状态监控电路的必要性.基于所设计的高压稳流电源电路,提出在高压电源每一稳流支路加入一路模拟开关,来控制稳流和状态监控的时序,从而通过监测采样电阻的端电压来实现放电状态的监控,并详细阐述了三轴放电状态监控电路的软硬件实现.最后对电路进行了实验测试,三轴激光陀螺在5s内的点燃率达到100%,电流的长期稳定度优于1×10-4,验证了状态监控电路的可行性和可靠性.     

Local,hierarchic, and iterative reconstructors for adaptive optics

Adaptive optics systems for future large optical telescopes may require thousands of sensors and actuators. Optimal reconstruction of phase errors using relative measurements requires feedback from every sensor to each actuator, resulting in computational scaling for n actuators of n2. The optimum local reconstructor is investigated, wherein each actuator command depends only on sensor information in a neighboring region. The resulting performance degradation "global" modes is quantified analytically, and two approaches are considered for recovering global performance. Combining local and global estimators in a two-layer hierarchic architecture yields computations scaling with n(4/3); extending this approach to multiple layers yields linear scaling. An alternative approach that maintains a local structure is to allow actuator commands to depend on both local sensors and prior local estimates. This iterative approach is equivalent to a temporal low-pass filter on global information and gives a scaling of n(3/2). The algorithms are simulated by using data from the Palomar Observatory adaptive optics system. The analysis is general enough to also be applicable to active optics or other systems with many sensors and actuators.     

SINS/GPS组合导航的扩展Kalman滤波算法

针对捷联惯性导航系统(SINS)无法长时间单独工作和GPS卫星信号易失锁而无法定位的问题,分析了两种导航系统的优缺点,提出了SINS/GPS组合导航的方法.建立了陀螺和加速度计的误差模型,采用松耦合方式,设计了扩展Kalman滤波器.以姿态、速度、位置的误差以及陀螺、加速度计的误差作为状态变量,对姿态、速度、位置进行校正.运用Matlab对组合导航系统进行了仿真.结果表明,该算法简单,容易实现,能满足导航精度要求.     

Design Considerations for Two CCD-based Star Sensors

Solid-state image sensors such as charge-coupled devices (CCD) are being considered as the detectors in future star sensors required for the high-accuracy attitude control of spacecraft. Such detectors are adequate in respect of their electro-optical properties, and offer a fundamental advantage in geometrical stability in addition to their obvious practical advantages over other types of image scanner. However, imperfections inherent in present semiconductor processing technology necessitate complex calibration procedures if their full performance is to be realized. Other properties, such as the small image formats, complicate the optical design. Measures taken to overcome such design difficulties must be considered in relation to their impact on the strict weight, size, power and reliability specifications imposed on star sensors. These problems are illustrated with reference to two specific requirements: for a general-purpose star sensor with 1 arc second accuracy and for a 10 to 30 arc second Polaris sensor.