基于卡尔曼滤波的网络精确时钟同步研究

在网络化分布式测试与控制系统中,时钟同步是一项重要的课题研究指标.在基于IEEE 1588协议主从时钟的时钟同步中,时钟偏差和时钟漂移的精确测量是主从时钟同步的重要保证.提出了基于卡尔曼滤波的时钟同步方法,该方法不仅能对主从节点之间的时钟漂移进行估计、优化时钟模型,还能实现对时钟偏差的估计,消除传输网络中的干扰.实验结果表明,在时钟同步中引入卡尔曼滤波算法能显著提高时钟同步精度.     

带有倾斜因子的摄像机自标定方法

摄像机自标定是三维重建技术的基本问题 ,得到许多学者的大力研究 .为了简化摄像机自标定过程 ,一般假设摄像机内参数中的倾斜因子为零 ,然后对主点和焦距进行自标定 .但在摄像机模型为完全的射影模型时 ,即当倾斜因子 (Skew Factor)值较大时 ,则使用上述假设得到的自标定参数误差较大 ,有时甚至无法得到结果 .为了对倾斜因子值较大的摄像机进行准确标定 ,提出了一种当摄像机的倾斜因子已知但不为零时的摄像机自标定方法 ,试验结果证明该方法可以得到比较准确的摄像机内参数 ,并可使得后续的三维重建得到较好的结果 .     

Semiparametric stochastic frontier models for clustered data

The mixed model approach to semiparametric regression is considered for stochastic frontier models, with focus on clustered data. Standard assumptions about the model component representing the inefficiency effect lead to a closed skew normal distribution for the response. Model parameters are estimated by a generalization of restricted maximum likelihood, and random effects are estimated by an orthodox best linear unbiased prediction procedure. The method is assessed by means of Monte Carlo studies, and illustrated by an empirical application on hospital productivity.     

A robust probabilistic Braille recognition system

As a structured document, Braille is the most common means of reading and study for visually handicapped people. The need for converting Braille documents into a computer-readable format has motivated research into the implementation of Braille recognition systems. The main theme of this research is to propose robust probabilistic approaches to different steps of Braille Recognition. The method is meant to be very general in terms of being independent of those parameters of the Braille document such as skewness, scale, and spacing of the page, lines, and characters. For a given Braille document, a statistical method is proposed for estimating the scaling, spacing, and skewness parameters, whereby the detected dots of the Braille document are modeled using a parameterized probability density function. Skewness, scaling, and line spacing are estimated as a solution of a maximum-likelihood (ML) problem, which is solved using expectation maximization. Based on those parameters, each line of the Braille document is extracted, and each of three rows of individual lines is separated based on the vertical projection of the Braille dots. Finally, a scale-independent automatic document gridding procedure is proposed for dot localization and character detection based on a hidden Markov model.     

Modeling multivariate time series on manifolds with skew radial basis functions

We present an approach for constructing nonlinear empirical mappings from high-dimensional domains to multivariate ranges. We employ radial basis functions and skew radial basis functions for constructing a model using data that are potentially scattered or sparse. The algorithm progresses iteratively, adding a new function at each step to refine the model. The placement of the functions is driven by a statistical hypothesis test that accounts for correlation in the multivariate range variables. The test is applied on training and validation data and reveals nonstatistical or geometric structure when it fails. At each step, the added function is fit to data contained in a spatiotemporally defined local region to determine the parameters--in particular, the scale of the local model. The scale of the function is determined by the zero crossings of the autocorrelation function of the residuals. The model parameters and the number of basis functions are determined automatically from the given data, and there is no need to initialize any ad hoc parameters save for the selection of the skew radial basis functions. Compactly supported skew radial basis functions are employed to improve model accuracy, order, and convergence properties. The extension of the algorithm to higher-dimensional ranges produces reduced-order models by exploiting the existence of correlation in the range variable data. Structure is tested not just in a single time series but between all pairs of time series. We illustrate the new methodologies using several illustrative problems, including modeling data on manifolds and the prediction of chaotic time series.     

Calibrating effective focal length for central catadioptric cameras using one space line

In camera calibration, focal length is the most important parameter to be estimated, while other parameters can be obtained by prior information about scene or system configuration. In this paper, we present a polynomial constraint on the effective focal length with the condition that all the other parameters are known. The polynomial degree is 4 for paracatadioptric cameras and 16 for other catadioptric cameras. However, if the skew is 0 or the ratio between the skew and effective focal length is known, the constraint becomes a linear one or a polynomial one with degree 4 on the effective focal length square for paracatadioptric cameras and other catadioptric cameras, respectively. Based on this constraint, we propose a simple method for estimation of the effective focal length of central catadioptric cameras. Unlike many approaches using lines in literature, the proposed method needs no conic fitting of line images, which is error-prone and highly affects the calibration accuracy. It is easy to implement, and only a single view of one space line is enough with no other space information needed. Experiments on simulated and real data show this method is robust and effective.     

Bayesian skew selection for multivariate models

We develop a Bayesian approach for the selection of skew in multivariate skew t distributions constructed through hidden conditioning in the manners suggested by either Azzalini and Capitanio (2003) or Sahu et al. (2003). We show that the skew coefficients for each margin are the same for the standardized versions of both distributions. We introduce binary indicators to denote whether there is symmetry, or skew, in each dimension. We adopt a proper beta prior on each non-zero skew coefficient, and derive the corresponding prior on the skew parameters. In both distributions we show that as the degrees of freedom increases, the prior smoothly bounds the non-zero skew parameters away from zero and identifies the posterior. We estimate the model using Markov chain Monte Carlo (MCMC) methods by exploiting the conditionally Gaussian representation of the skew t distributions. This allows for the search through the posterior space of all possible combinations of skew and symmetry in each dimension. We show that the proposed method works well in a simulation setting, and employ it in two multivariate econometric examples. The first involves the modeling of foreign exchange rates and the second is a vector autoregression for intra-day electricity spot prices. The approach selects skew along the original coordinates of the data, which proves insightful in both examples.     

LED PWM dimming linearity investigation

LED PWM dimming application for large scale LED video displays is analyzed. The need for short light pulse duration is outlined. PWM dimming with short driving pulses is investigated experimentally. The LED response time skew introduces the nonlinearity for PWM dimming. For LED response time skew estimation, a method is suggested that has been successfully applied to measure some of today’s market representative LEDs. PWM dimming nonlinearity can be forecasted using the estimated skew. For a particular driving configuration, it is indicated that LED PWM dimming fails to satisfy the required 14 bit output coding together with the image refresh frequency of 400 Hz. A rough investigation demonstrates that the skew is quite stable. Therefore, the nonlinearity correction for the PWM pulse durations shorter than the skew value should be possible.     

e-PCP: A robust skew detection method for scanned document images

We present here an enhanced algorithm (e-PCP) for skew detection in scanned documents, based on the work on Piecewise Covering by Parallelogram (PCP) for robust determination of skew angles [C.-H. Chou, S.-Y. Chu, F. Chang, Estimation of skew angles for scanned documents based on piecewise covering by parallelograms, Pattern Recognition 40 (2007) 443-455]. Our algorithm achieves even better robustness for detection of skew angle than the original PCP algorithm. We have shown accurate determination of skew angles in document images where the original PCP algorithm fails. Further, the increased robustness of performance is achieved with reduced number of computation compared to the originally proposed PCP algorithm. The e-PCP algorithm also outputs a confidence measure which is important in automated systems to filter cases where the estimated skew angle may not be very accurate and thus can be handled by manual intervention. The proposed algorithm was tested extensively on all categories of real time documents and comparisons with PCP method is also provided. Useful details regarding faster execution of the proposed algorithm is provided in Appendix.     

Reliable buffered clock tree routing algorithm with process variation tolerance

1 Introduction The clock distribution network design is a very challenging task, because the per-formance and functionality of the whole synchronous system directly depend upon the clock signals. Clock skew is manifested by a lead/lag relationship between the clock signals. Conventional clock designs always demand a zero clock skew system, since they think that clock skew may limit the maximum operation frequency. Exact zero skew was first accomplished in ref. [1], and then DME (Deferred-Mer…     

依赖ER模型的多关系频繁模式发现方法

为了解决多关系频繁模式挖掘面临的统计偏斜问题和效率问题,提出了基于ER(实体-联系)概念模型的方法。其以ER模型的联系集为核心,利用扩展的关系数据库SQL统计原语,在用户给定数据约束和兴趣度约束的情况下,减少多关系频繁模式的产生数量,既不需要将相关关系表做物理连接,也不会产生统计偏斜。与相关研究工作的比较,说明了利用关系数据库管理系统和ER模型实现多关系频繁模式挖掘的有效性及正确性。     

Self-Calibration of Rotating and Zooming Cameras

In this paper we describe the theory and practice of self-calibration of cameras which are fixed in location and may freely rotate while changing their internal parameters by zooming. The basis of our approach is to make use of the so-called infinite homography constraint which relates the unknown calibration matrices to the computed inter-image homographies. In order for the calibration to be possible some constraints must be placed on the internal parameters of the camera.We present various self-calibration methods. First an iterative non-linear method is described which is very versatile in terms of the constraints that may be imposed on the camera calibration: each of the camera parameters may be assumed to be known, constant throughout the sequence but unknown, or free to vary. Secondly, we describe a fast linear method which works under the minimal assumption of zero camera skew or the more restrictive conditions of square pixels (zero skew and known aspect ratio) or known principal point. We show experimental results on both synthetic and real image sequences (where ground truth data was available) to assess the accuracy and the stability of the algorithms and to compare the result of applying different constraints on the camera parameters. We also derive an optimal Maximum Likelihood estimator for the calibration and the motion parameters. Prior knowledge about the distribution of the estimated parameters (such as the location of the principal point) may also be incorporated via Maximum a Posteriori estimation.We then identify some near-ambiguities that arise under rotational motions showing that coupled changes of certain parameters are barely observable making them indistinguishable. Finally we study the negative effect of radial distortion in the self-calibration process and point out some possible solutions to it.An erratum to this article can be found at     

Vibration of stiffened skew plates by using B-spline functions

This paper presents a general procedure for calculating the free vibration of stiffened skew plates by the Rayleigh-Ritz method with B-spline functions as coordinate functions. The stiffened skew plates are modelled as the skew plate with a number of stiffening beams.The results are compared with existing values based on other numerical methods. Vibration characteristics of stiffened skew plates are also studied with changing the arrangements of stiffening beams, the stiffness parameters of beams, skew angle and aspect ratio.     

Large elastic deformations of clamped skewed plates by dynamic relaxation

This investigation is concerned with the nonlinear behaviour of clamped Isotropic skew plates of constant thickness subjected to a uniformly distributed transverse load. The recently developed numerical technique of dynamic relaxation has been adopted for the analysis. A detailed study of the large deflection behaviour of skew plates has been made by varying three parameters, viz. skew angle, load, and aspect ratio. Numerical results have been compared with the available solutions. Representative nondimensional solutions are presented in the form of graphs to elucidate the nonlinear effect due to large deflection at higher loads.     

工艺参数变化下的基于统计时序分析的时钟偏差安排

针对工艺参数变化的情况,提出一种成品率驱动的时钟偏差安排算法.提出统计时序约束图的概念,利用统计时序分析的结果将时序电路转换为统计时序约束图;将寻找关键环问题转换为最小费用/时间比值环问题,并按比例分配关键环中的时钟偏差的安全余量.实验结果表明,该算法有助于提高集成电路的成品率.     

Finite element flutter analysis of composite skew panels

A finite element method is used to study the dynamic instability of laminated composite skew panels subjected to supersonic flow. The FEM employs eight-noded isoparametric elements which take into account transverse shear deformation. The linearized Piston theory is applied to assess the aerodynamic loads. The effects of skew angle on critical aerodynamic parameters are investigated for different aspect ratios, boundary constraints, fibre orientations and lamination schemes. It is observed that the skew angle has a stabililzing effect on the flutter boundary, whereas couplings have a destabilizing effect. The higher aspect ratio is also found to exhibit a stabilizing effect on the flutter boundary.     

On the Recursive Adaptive Control for Free-floating Space Manipulators

This paper is devoted to investigating the recursive implementation schemes of adaptive control for free-floating space manipulators. Using spatial vector tool and some physical properties that free-floating space manipulators enjoy, we establish a general framework on the seeking of the centripetal and Coriolis matrix that satisfies the skew symmetry requirement. Under this general framework, we propose a recursive adaptive algorithm for free-floating manipulators, which is composed of two parts: the first part is the recursive derivation of the required manipulator control torques, and the second part is the recursive updating of the spacecraft reference velocity and acceleration. To guarantee the uniform positive definiteness of the estimated spacecraft inertia, we present a parameter projection algorithm to project the estimated parameters into some pre-specified parameter region. In the next, we extend the proposed recursive adaptive algorithm to task-space control of free-floating space manipulators. We examine the performance of the proposed recursive adaptive algorithms via numerical simulation on a six-DOF space manipulator.     

基于主方向检测的畸变车牌字符图像校正方法

本文提出了一种对畸变字符图像快速变形校正的算法,它主要是通过对字符主方向的搜索来求取倾斜图像的有关参数。该算法计算形变区域的水平及垂直差分并设计出判别函数,进一步搜索出使判别函数达到最大的主字符方向并进行校正,最后给出了一种图像毛刺的剔除方法。实验表明,本文介绍的算法充分利用了畸变字符图像的特征,同时使用了较优化的快速搜索策略,取得了较好的实际结果。     

Linear free flexural vibration of cracked functionally graded plates in thermal environment

In this paper, the linear free flexural vibrations of functionally graded material plates with a through center crack is studied using an 8-noded shear flexible element. The material properties are assumed to be temperature dependent and graded in the thickness direction. The effective material properties are estimated using the Mori–Tanaka homogenization scheme. The formulation is developed based on first-order shear deformation theory. The shear correction factors are evaluated employing the energy equivalence principle. The variation of the plates natural frequency is studied considering various parameters such as the crack length, plate aspect ratio, skew angle, temperature, thickness and boundary conditions. The results obtained here reveal that the natural frequency of the plate decreases with increase in temperature gradient, crack length and gradient index.