A new method for DAC corrections for application in nuclear ADC

The paper describes a new method for measurement and correction of errors associated with DACs. Very small errors of a few μV can be measured and corrected. The method enables us to correct commercially available monolithic DACs, so that they can be used in nuclear ADCs where high integral accuracy and low differential nonlinearity (DNL) are required. This cuts down the circuit size and cost for DACs applied in nuclear ADCs.The paper proposes a “theorem of DAC code correction” which is based on the relationship existing between the errors associated with different bits of a DAC. Complete mathematical proof is given to establish the correctness of the method. The “theorem” can be implemented if the correct value of any bit or of the full scale voltage is known.The method also provides a check on whether errors are correctly measured and whether they are perfectly compensated.The illustration of the relationship between errors associated with different bits also reveals that the conventional ways of adjusting DNL for 16-bit and 18-bit (0.5 LSB) DACs followed by commercial manufacturers are not accurate. These corrections can be implemented more effectively if the procedure described is followed.     

Digital-to-analog conversion by pulse-count modulation methods

Three low-cost digital-to-analog converters (DACs) are described and compared. These designs can easily be implemented in an integrated circuit: the conventional pulse-width modulation (PWM) DAC, the new pulse-count modulation (PCM) DAC and the first-order noise shaping (FONS) DAC. All three methods control the ratio of the sum of all pulse durations to the constant total period. As the pulse durations are integral multiples of a unit pulse, all three can be classified as pulse-count modulation methods. Block diagrams of all three DACs consisting of a simple digital circuit and a low-pass filter are presented. For a constant digital input value the worst case ripple of the filter output is used to calculate the cutoff frequency of the low-pass filter. Approximations for the 3 dB cutoff frequency of first-order, second-order and fourth-order Butterworth low-pass filters are given. The dynamic properties are analyzed in the time domain (settling time) and in the frequency domain (unfiltered output spectrum of a full-scale sine wave input). The main influences on the static accuracy are analyzed. A case study demonstrates the abilities of PCM and FONS     

Testing of Precision DAC Using Low-Resolution ADC With Wobbling

Testing of high-resolution, digital-to-analog converters (DACs) with gigahertz clock rates is a challenging problem. The bottleneck is fast and accurate output measurement. This paper presents a novel high-performance DAC testing approach that uses a flash analog-to-digital converter (ADC) to achieve highspeed data acquisition, adopts the wobbling technique to provide a sufficient resolution, and processes the data with a sophisticated algorithm to guarantee high test accuracy. Simulation results show that, by using a 6-bit ADC and wobbling, the static linearity of 14-bit DACs can be tested to better than 1-LSB accuracy. The experimental results that are included in the paper also affirm the performance of the algorithm. This method provides a solution to both the production and on-chip testing problems of high-performance DACs.     

前后向时间序列模型联合估计的时变结构模态参数辨识

为提高时变结构模态参数辨识精度和抗噪声能力,提出一种前后向泛函向量时变自回归滑动平均（FS-VTARMA）时间序列模型联合估计的模态参数辨识方法。首先建立前后向FS-VTARMA模型联合估计的均方误差形式的费用函数,其次引入非平稳信号中前向模型和后向模型估计系数的近似共轭关系,再利用两步最小二乘法（2SLS）得到时变模型系数,最后把时变模型特征方程转换为广义特征值问题提取出模态参数。利用时变刚度系统非平稳振动信号验证该方法,结果表明：能有效地克服前向模型估计中模态参数一步延迟以及起始时刻无法准确获得,以及后向模型估计中模态参数一步超前以及终止时刻无法准确获得的缺点,具有更高的模态参数辨识精度和更强的抗噪声能力。     

Simultaneous identification of structural parameters and input time history from output-only measurements

A new time-domain method is suggested in this paper for simultaneous identification of the structural parameters and the time history of the input excitation using output-only measurements. The proposed method is based on an iterative identification procedure consisting of the least-squares identification technique and a modification process between each iterative step. The modification process is introduced to convert the spatial information of the external excitation into mathematical conditions. First, the unknown force vector is conjectured through the equation of motion using the initial guess of the structural parameters and the measured structural responses. The estimated input force vector is then modified to force it comply with the spatial distribution of the external excitations. The modified input force vector is further used to provide new estimation of structural parameters. Repeat the aforementioned procedure until the structural parameters satisfy the preset convergence criterion. Numerical examples as shear building and truss bridge model are employed to evaluate the feasibility of the proposed method. In the numerical examples, typical scenario of complete and noise-free as well as incomplete and noise-contaminated output measurements are considered. The results demonstrate that the proposed method can accurately identify both the structural parameters and the input time history for the cases that the structural responses are not polluted or slightly contaminated by measurement noise.The writers are grateful for the financial support from the National Natural Science Foundation of China (NSFC) through its Project of Young Scientists Fund (No. 50308020) awarded to the first author.     

DAC阶梯信号频谱的新简化分析及其基波的非滤波测量

提出一个新的简化方法分析了DAC生成的阶梯波频谱,包括幅值和相角。以正弦数字信号的离散值为输入,DAC将生成一个阶梯信号,其中的基波分量与输入的正弦信号之间幅值差为-1. 644 9/n²,相位差为-p/n,n为一个周期内离散点的个数;而阶梯信号中的谐波位于(tn±1)阶次上,其幅值是基波分量的1/(tn±1)。设计了一个不需滤波器而仅对阶梯波中基波分量作测量的实验,实验结果证实了上述关于基波的分析结论。讨论了各种可能的应用。     

基于扩展卡尔曼滤波的结构参数和荷载识别研究

经典的扩展卡尔曼滤波（Extend Kalman Filter,EKF）方法可有效识别结构参数,但却需要已知外部激励,然而,在工程实际中,有些外激励往往难以实时获取。为此,该文提出了一种基于EKF的未知激励下的结构参数和荷载识别方法。通过在观测方程中引入投影矩阵,实现了结构参数的识别,同时,利用最小二乘估计实时识别了未知的外激励。为了验证该方法的有效性和鲁棒性,文中采用了三个数值算例：四层的Benchmark模型、分段线性系统和非线性Duffing系统。数值分析的结果表明,该方法不仅能够准确识别线性和非线性结构的参数,还能有效识别作用于这些结构的外激励。     

Shanks' Method for Dynamic Phasor Estimation

A new algorithm for phasor estimation is proposed. It is based on a signal model that allows amplitude and phase dynamic variations. An autoregressive moving average (ARMA) model is assumed for the oscillating signal. Its autoregressive part is fixed, and it is defined only by the nominal fundamental frequency. Its best moving average parameters are estimated with Shanks' method. These parameters provide the key information from which the phasor state vector is estimated through the partial fraction expansion of the ARMA rational polynomial. These estimates could be useful, not only for the monitoring and controlling of the power system, but also for discriminating between a fault and an oscillation state.     

Assessing and ensuring parameter identifiability for a physically-based strain hardening model for twinning-induced plasticity

Physically-based strain hardening models have become important ingredients in metal forming simulations over the last years, since they allow for the modeling of multi-stage forming processes based on the evolution of physically meaningful internal variables. Although these models are physically-based, there are still many fitting parameters involved which have to be identified from experiments. As a matter of fact, for each physical effect that is included in the model, a separate equation with new fitting parameters is introduced, such that physically-based models tend to contain a large number of fitting parameters. Parameter estimation is often based on the macroscopic response of a specimen which is tested in compression, tension or shear at various strain rates and temperatures. It is not guaranteed that this macroscopic information suffices to estimate parameters in model equations that describe (sub-) microscopic phenomena, since the effect of one parameter on the course of strain hardening can be compensated by other parameters. Since such parameter correlations are hard to detect from the model equations alone, the parameter estimation process may be ill-conditioned, i.e. numerous parameter sets can be found for such models that deliver almost the same minimum value of the error function in the parameter identification process. Given that parameter estimation involves a series of costly experiments, methods are needed that allow for analyzing the identifiability of the model parameters before costly experiments are performed. In this paper, an approach is presented that analyzes model parameter dependencies and quantifies the identifiability of the model parameters. The model considered in this study calculates the flow stress based on the evolution of three dislocation densities and the evolution of deformation twins. The analysis shows that correlations between the model parameters exist and that it is not possible to determine all model parameters based on an experimental set of flow curves in a single curve fitting procedure. An adapted fitting strategy is presented in which fitting is performed step-wise so that in each fitting step, only identifiable parameters are estimated, allowing for successful parameter identification.     

A survey of some outstanding questions in dense hydrogen

Dense hydrogen, sometimes thought of as the simplest condensed matter system, continues to be a cautionary tale against overconfidence in the predictive powers of condensed matter theory. Recent experimental advances using diamond anvil cells (DACs), and single and multi-shock compression all present new challenges to theory, and old questions such as the ground state insulator to metal transition pressure remain almost as unsettled as when originally proposed by Wigner and Huntington 60 years ago. This article gives an overview of efforts to understand two areas of the hydrogen phase diagram: the low to room temperature region probed by DAC experiments and the 0.5–2.0 Mbar pressure, 0.5–3.0 e.v. temperature range now being explored in deuterium using laser-generated shocks. The aim is to provide nonspecialists with a guide to this interesting and active field. This revised version was published online in July 2006 with corrections to the Cover Date.     

Parametric identification of vehicle’s structural characteristics

A method for the estimation of structural parameters in commercial passenger vehicles is proposed in this paper. The novel scheme assumes that the vertical dynamics of the vehicle can be described by a seven degrees-of-freedom (pitch – bounce – roll) model and formulates the discrete-time equivalent of the analytical equations, using the bilinear (Tustin) approximation. A corresponding vector autoregressive-moving average with exogenous inputs model with special architecture is derived, for any case of the acquired vibration response data, which is estimated through the implementation of a hybrid identification method. Accurate structural, as well as modal parameters extraction is obtained by the discrete-to-continuous transformation of the estimated characteristic matrix polynomial.     

3D测量系统中的高精度摄像机标定算法

本文在分析完整的摄像机镜头畸变模型的基础上,提出了一种新的标定算法.该算法包括三个步骤,首先在不考虑镜头畸变的情况下利用标定块上的中间若干个点,采用线性优化方法求出除畸变系数以外的其他外部参数和主要的内部参数;然后固定上述已求得的参数,利用线性优化方法求解畸变系数;最后对所有内部参数和外部参数进行全局非线性优化.最后对本文的标定算法进行了标定实验,实验结果表明,本文算法的标定精度可以达到0.0367 mm,可以满足高精度三维测量及其他应用的要求.     

Identification of Heffron-Phillips model parameters for synchronous generators operating in closed loop

Heffron-Phillips model of a synchronous machine is commonly used in small signal stability analysis and for off-line design of power system stabilisers. The data used to determine the parameters of this model are either hard to measure or require the machine to be taken off-line to take the measurements which, in general, is inconvenient. identifying these parameters from online data measurements is important since it does not require any a priori knowledge of the machine data. the problem of closed-loop identification of the Heffron-Phillips model parameters is of practical importance since the data used for identification can be gathered when the machine is normally connected to the power system. the use of open-loop identification techniques using data gathered during closed-loop operation of synchronous generators leads to bias errors in the estimated parameters. motivated by the fact that the synchronous machine model is multivariable and is well defined in a state space structure, a closed-loop subspace parameter identification technique is proposed. consistency of the proposed approach is illustrated using Monte Carlo analysis. comparison of the proposed method with open-loop identification technique shows the superiority of this approach.     

2.5维C/SiC复合材料弹性参数不确定性识别方法研究

提出了一种2.5维C/SiC编织复合材料弹性参数不确定性识别方法。采用刚度平均法获得复合材料等效弹性参数理论预测值。选取对结构动态特性影响较大的3个弹性参数E11,E22和G12作为待识别参数;在确定性识别结果基础上,采用拉丁超立方体采样构造随机试验样本,开展不确定性参数识别方法仿真研究。仿真结果表明,针对考虑弹性参数不确定性的2.5维C/SiC复合材料,采用所提出的方法能够准确识别材料弹性参数的均值与标准差,建立反映实际结构动态特性统计意义的精确动力学模型。     

基于左矩阵分式模型的短记录数据模态参数识别

在测量数据有限情况下,由于难以获得频响函数(FRF)的准确估计,使用FRF 作为原始数据的传统模态参数识别方法将不再适用。针对该问题,提出一种基于频响函数左矩阵分式模型的模态参数识别方法。该方法直接使用输入输出数据FFT 谱(IO 谱)作为原始数据,避免了频响函数估计。通过最小二乘估计在Z 域内求解模态参数,改善了矩阵的求解性态。针对左矩阵分式模型的特点,给出了一种通过主分量分析(PCA)建立稳定图的方法。最后采用GARTEUR 飞机模型建立仿真算例对所提出的方法进行了验证。     

A new method for high speed nuclear ADCs using commercially available ADCs and DACs

This paper describes a new and simple method for high speed nuclear ADCs using commercially available high speed ADCs and monolithic DACs. The digital code corresponding to the input analog voltage, which is first determined with an accuracy of $\text{±}\text{1}\text{2}$ LSB using a commercial ADC (conversion time 2.5 μs), is corrected using a corrected monolithic DAC and external comparators to yield a DNL of better than ± 2% (without Gatti's scheme) with a total conversion time of 3.5 μs. The advantage of the method is the smaller size of the circuit and higher speed of operation compared with other methods. The recent improvements in speed, size and cost of commercial ($\text{±}\text{1}\text{2}\text{LSB}$) ADCs are directly utilized to improve the corresponding factors in nuclear ADCs. Use of Gatti's sliding scheme can further reduce the DNL to 0.2%.     

A procedure for outlier identification in data sets from continuous distributions

We propose a procedure, based on sums of reciprocals ofp-values, for the identification of outliers in univariate or multivariate data sets coming from continuous distributions. Using results of Csörg? (1990), we find the limiting distribution of the relevant statistic for completely specified models. By simulations, we obtain approximate quantiles for the asymptotic distribution, (which does not depend on the specific model or the dimension where the data live) and for the finite sample distribution in different dimensions of our statistic when parameters are estimated, for the multivariate Gaussian model and a multivariate double exponential model with independent coordinates. Monte Carlo evaluation shows that the procedure proposed is effective in the identification of outliers, and that it is sensitive to sample size, a feature seldom found in outlier identification methods.     

Adaptive feed-forward controller of piezoelectric actuator for micro/nano-positioning

In this paper, design and implementation of an adaptive feed-forward controller for micro/nano-positioning control of piezoelectric actuator (PEA) is described. Discrete-time Dahl hysteresis-based mathematical model of the PEA is developed and the values of the model parameters are estimated through an autoregressive with exogenous terms (ARX) model identification technique using experimental input–output data. A recursive least-square estimator (RLSE)-based adaptive feed-forward (FF) controller is proposed, which takes into account the parameter uncertainty. The FF controller has also been implemented in a DsPIC30F4011 microcontroller. The established PEA model and the controller are validated by simulation and experimental results including parameter variation.     

非比例阻尼结构参数识别算法的研究

在地下结构的地震响应中,无限地基的辐射阻尼起着重要的影响,然而其阻尼形式同结构的比例阻尼假定不同。同样,对于施加隔振、减震措施的建筑结构,其系统阻尼也不再满足比例阻尼假定,在结构参数识别中将阻尼阵假定为与刚度阵形式相似的方法不能反映实际结构参数。从研究结构的阻尼模型发发,引入了节点堆积阻尼,这样形成的阻尼与刚度、质量既有一定的联系,又有一定的独立性。以剪切型结构为例对此做了详细说明,结果表明,引入堆积阻尼后,进行参数识别的结构动力学方程是一个非线性识别问题,在此基础上提出了基于最小二乘平均的非线性迭代识别算法。