Uncertainty analysis of high frequency image-based vibration measurements

Image-based vibration measurement techniques allow to remotely measuring the displacement of multiple targets in the field of view, without the need to mount anything on the measurand. In this paper the uncertainty budget of vision systems has been performed in order to both optimize the measurement procedure and identify the potential application fields. Two different types of camera are used in this work, both of them equipped with a 1280 × 1024 px sensor but with two different maximum frame rates at full resolution: 25fps and 2000fps respectively. The uncertainty analysis proposed here is based on a careful identification of the uncertainty sources and on experimental tests on an electro-magnetic shaker, where the displacement measured with the cameras are calibrated by means of the reference measurements provided by state-of-the-art traditional techniques.     

Ionospheric delay contribution to the uncertainty of time and frequency measurements by one-way satellite time transfer method

The results of international time and frequency comparisons between INPL reference and BIPM using GPS satellite system are presented. The uncertainty of time and frequency comparison by the one-way satellite time transfer method is discussed. Ionospheric delay contribution to this uncertainty is evaluated. The diurnal, seasonal and long-term variations of ionospheric delay are presented and discussed. Ionospheric delay has daily and seasonal oscillations with amplitudes of about 20–60 and 20 ns, respectively. These oscillations are due to variations of solar activity and the variations of total electron content in ionosphere. Long-term variations of ionospheric delay correlate with 11-year sunspot cycle. The difference between ionospheric delay values at minimum (in 1996) and maximum (in 2002) solar activity is 3 times. The increased values of ionospheric delay have caused the increase of the standard deviation of the time comparison results in the same proportion.     

光声成像反射伪影的机理分析和控制

在光声成像中,重建图像经常会受到图像伪影的影响而降低图像质量.为了解决图像伪影的问题,从超声波在水中的传播特性出发,分析了光声图像中伪影的产生机理.分析表明:伪影的产生是由超声信号在水面的反射延迟造成的.进而提出了通过延迟相减来控制反射伪影的方法,并给出了在实际应用中控制水面高度的依据.通过构建的光声成像平台进行实验验证,结果表明:提出的方法能有效地消除重建图像中的伪影,提高了重建图像的质量.     

Method for measuring the resonant frequency of photoacoustic detector in the real-time mode

A method for measuring the resonant frequency of a photoacoustic detector (PAD) in the realtime mode in a wide temperature and gas-mixture-composition range is proposed. The method is based on measurements of natural frequencies of the resonance PADs, which are excited using an auxiliary acoustic emitter built into the PAD. The measurement procedure takes ≤0.1 s. When the PAD is filled with air or nitrogen, the high accuracy and reproducibility of the measurement results is experimentally shown. The relative measurement error of the PAD resonant frequency (~1700 Hz) is approximately 6 × 10^–5.     

Analysis of jitter influence in fast frequency measurements

This paper presents a theoretical analysis of possible jitter impact in application of numeric criterion for fast measurement of frequency by coincidence principle. The primary goal is the generation of a signal containing a known amount of each jitter components. This signal was used for testing signals with regular pulse trains. Initially, jitter components are analyzed and modeled individually. Next, sequences for combining different kinds of jitter are modeled, simulated and evaluated. Jitter model simulation in Matlab is utilized to show the independence of frequency measurement results on the total jitter present in the reference and desired pulse trains independently. A good agreement between previously introduced theory of fast measurement of frequency and simulation in jitter presence is verified; these results allows to engineers use the numeric criterion for fast measurement of frequency in spite to interactions among jitter components in various applications for frequency domain sensors.     

High-pressure apparatus for dielectric measurements in high frequency

The present article describes an experimental apparatus for the measurement of low-loss dielectric material under conditions of high pressures (maximum pressure 1500 bars) and high frequencies (1-15 MHz.) The measurements of these losses are based on the classical method of the Q-meter with a general Radio type 1690-A sample holder, located in a high-pressure bomb. All the manual operations made on the holder during the measurements are controlled by dc motors. The first results have shown that the dielectric losses of polyethylene (PE) vary with the pressure. This apparatus will later be used in the measurement of the dielectric losses of the insulating materials used for submarine telecommunication cables.     

Lock-in amplifier error prediction and correction in frequency sweep measurements

This article proposes an analytical algorithm for predicting errors in lock-in amplifiers (LIAs) working with time-varying reference frequency. Furthermore, a simple method for correcting such errors is presented. The reference frequency can be swept in order to measure the frequency response of a system within a given spectrum. The continuous variation of the reference frequency produces a measurement error that depends on three factors: the sweep speed, the LIA low-pass filters, and the frequency response of the measured system. The proposed error prediction algorithm is based on the final value theorem of the Laplace transform. The correction method uses a double-sweep measurement. A mathematical analysis is presented and validated with computational simulations and experimental measurements.     

Note: Observation and analysis of the aberration in a photoacoustic imaging system

Acoustic lens is applied in the research of photoacoustic imaging. The aberration is always inevitable due to the existence of the acoustic lens, and it influences the resolution of the imaging system severely. However, the imaging system based on an acoustic lens also provides a new method for observing the aberration of a system and testing the quality of the acoustic lens. In this paper, a method of observing the aberration of photoacoustic imaging system is reported, and the causes of aberrations are discussed based on the theory of lens aberration. All these would be helpful to correct the aberration and improve the resolving power of acoustic lens.     

Analysis of tiltrotor whirl flutter in time and frequency domain

The whirl flutter phenomenon in a rotor is induced by in-plane hub forces, and imposes a serious limit on the forward speed. In this paper, based on Greenberg’s model, quasi-steady and unsteady aerodynamic forces are formulated to examine the whirl flutter stability for a three-bladed rotor without flexible wing modes. Numerical results are obtained in both time and frequency domains. Generalized eigenvalue solution is utilized to estimate the whirl flutter stability in the frequency domain, and Runge-Kutta method is used to analyze it in time domain. The effects of varying the pylon spring stiffness and the swashplate geometric control coupling upon the flutter boundary are investigated. An optimum pitch-flap coupling parameter is discovered through the parametric study. Aeroelastic stability boundaries are estimated with the three different aerodynamic models. It is found that the analysis with the full unsteady aerodynamics predicts the highest flutter speed.     

A precise scheme for detection of current transformer saturation based on time frequency analysis

Current Transformers (CTs) are prone to saturation due to large amplitude of fault current and existence of decaying Direct Current (DC) offset. Since the CT saturation leads to mal-operation of protective relays, detection and correction of saturated currents is one of the most important challenges in the power systems protection. In this paper, a new algorithm is presented for detection of CT saturation time interval by using the Improved S-Transform (IST). Simultaneously, IST can yield a complete visualization of signal in both time and frequency domains. An index based on instantaneous power of the CT secondary current is calculated using the IST output matrix. Then, the saturation region is estimated by determining the maximum and minimum values of the proposed index in the time domain. Comprehensive simulations are implemented using PSCAD/EMTDC software. Main parameters which have direct effect on the saturation levels are considered in simulation studies. As extremum points of the proposed index are determined for detection of saturation regions, the proposed method is not affected by CT parameters. The obtained results show that the proposed algorithm can precisely detect CT saturation time intervals even in noisy conditions.     

基于空域与频域稳定特征融合的离焦虹膜识别

针对扩大景深的定焦虹膜识别系统易引入不同程度的离焦虹膜图像,增加识别错误率的问题,提出稳定特征融合解决方案.以基于序列图像的离焦虹膜识别方法为分析和验证基础,通过对不同图像离焦程度下稳定特征在空域和频域分布规律的统计研究,确定基于单幅图像注册的稳定深点区域特征与稳定傅里叶低频相位特征的提取规则,并将二者在匹配分数级进行融合完成识别.在SUT-DI离焦虹膜数据库上的实验结果表明,与融合前基于单特征的以及基于序列图像的识别方法相比,该方法取得了更佳的识别性能,等误率最高降低7.77％,证明了该方法对于图像遭受光学离焦具有鲁棒性,能够为便捷式虹膜识别系统的开发提供技术支持.     

Hilbert-Huang transformation-based time-frequency analysis methods in biomedical signal applications

Hilbert-Huang transformation, wavelet transformation, and Fourier transformation are the principal time-frequency analysis methods. These transformations can be used to discuss the frequency characteristics of linear and stationary signals, the time-frequency features of linear and non-stationary signals, the time-frequency features of non-linear and non-stationary signals, respectively. The Hilbert-Huang transformation is a combination of empirical mode decomposition and Hilbert spectral analysis. The empirical mode decomposition uses the characteristics of signals to adaptively decompose them to several intrinsic mode functions. Hilbert transforms are then used to transform the intrinsic mode functions into instantaneous frequencies, to obtain the signal's time-frequency-energy distributions and features. Hilbert-Huang transformation-based time-frequency analysis can be applied to natural physical signals such as earthquake waves, winds, ocean acoustic signals, mechanical diagnosis signals, and biomedical signals. In previous studies, we examined Hilbert-Huang transformation-based time-frequency analysis of the electroencephalogram FPI signals of clinical alcoholics, and 'sharp I' wave-based Hilbert-Huang transformation time-frequency features. In this paper, we discuss the application of Hilbert-Huang transformation-based time-frequency analysis to biomedical signals, such as electroencephalogram, electrocardiogram signals, electrogastrogram recordings, and speech signals.     

基于半导体激光器的乙炔气体光声光谱检测及其定量分析

油中溶解乙炔是变压器等油浸式电气设备早期放电性故障的重要特征气体。基于半导体激光器的光声光谱传感技术具有灵敏度高,选择性好等优点,能很好地应用于微弱气体检测中。论文构建了基于分布反馈半导体激光器光声光谱检测装置,并分析了光声池的特性参数;实验研究了光声信号与激光功率、乙炔气体浓度之间的关系;并借助激光器的波长调制特性,研究了乙炔分子在近红外区第一泛音带1.5μm附近的光声光谱;提出了一种基于最小二乘回归的光声光谱定量分析方法。理论和实验结果为乙炔的光声光谱在线检测及高灵敏度可调谐光声光谱仪的设计提供了参考。     

Traceability and uncertainty analysis in volume measurements

Laboratories must ensure that obtained results are reliable, for that it is necessary to calibrate glassware and proving tanks using the correct method and estimate of uncertainties. The aim of this work is to present the gravimetric method as a suitable and traceable method to calibrate glassware according to laboratory requirements. Moreover, the model for the calculation of the measurement uncertainty is described and it is proved to be completely suitable for the calibration by gravimetric method through a numerical example drawn from the INRIM uncertainty budget in the CCM.FF-K4 key comparison.     

Real time measurements of hysteresis in a piezoelectric nanopositioner stage

This paper aims to develop a hysteresis model for a single axis piezoelectric nanopositioner stage at different operating points. The adopted methodology compares results based on the developed analytical simulation model using MATLAB with real time results using LabVIEW. The experimental results for real time measurements had been acquired for the piezoelectric nanopositioner stage using data acquisition system from National Instruments (NI). The experimental setup is described. The simulation and real time measurements results are compared to validate the adopted approach.     

卫星双向法时间频率传递中Sagnac效应的计算分析

结合卫星双向法时间传递(TWSTFT),介绍了Sagnac效应原理,推导了Sagnac效应的基本计算公式,并给出详细计算过程.对Sagnac效应在常见几种情况下的应用进行了讨论.这对于卫星时间同步和卫星导航定位具有重要的参考价值和实用意义.     

A firmware system for phase measurements in extremely low and ultralow frequency ranges

In experimental investigations of artificial and natural signals in extremely-low frequency (ELF) and ultra-low frequency (ULF) ranges, it is necessary to measure amplitudes and relative phases of the electromagnetic field components in a network of stations. The phase measurement includes accurate synchronization of data with the universal time. In this work, the geophysical data acquisition system, which allows one to synchronize measurement results and the coordinated universal time with an accuracy of a few microseconds, is described. Methods for processing the electromagnetic field components of the harmonic signal source, recorded by this data acquisition system, are proposed.     

Nanoscale potential measurements in liquid by frequency modulation atomic force microscopy

We have developed a method for local potential measurements in liquid using frequency modulation atomic force microscopy. In this method, local potential is calculated from the first and second harmonic vibrations of a cantilever induced by applying an ac bias voltage between a tip and a sample. The use of an ac bias voltage with a relatively high frequency prevents uncontrolled electrochemical reactions and redistribution of ions and water. The nanoscale resolution of the method is demonstrated by imaging potential distribution of a dodecylamine thin film deposited on a graphite surface in 1 mM NaCl solution.     

Locating damage in waveguides from the phase of point frequency response measurements

A method for detecting damage in uniform waveguide structures from two or more point frequency response functions (FRFs) is described. Attention is focussed on bending waves in beams although the method can in principle be applied to any waveguide structure. The input FRF is the superposition of directly injected waves and waves reflected from the damage and from other scattering regions in the structure. The phase of this FRF modulates with wavenumber, with the period of modulation in wavenumber space being related to the distances between the excitation point and scattering locations. The phase spectrum of the input FRF is found: the phase is determined, dispersive effects are removed by transforming from the frequency domain to the wavenumber domain, and the inverse Fourier transform from the wavenumber to the space domain found. Peaks in this phase spectrum indicate the distance to the scatterer. Two (or more) input FRFs can be used to determine the location of the scatterer unambiguously. Signal processing issues are discussed. Numerical results for a uniform beam with a breathing crack are given and experimental results for beams with a slot cut into them are presented. The approach lies in the middle ground between low frequency, modal methods and high frequency, ultrasonic methods. It allows one to interrogate a region of a structure rather than the complete structure and there is no requirement for a validated model of the structure in order to locate damage, apart from an estimate of the dispersion relation.     

Enhanced wavelet-based methods for reducing complexity and calculation time in sonar measurements

We introduce two enhanced wavelet-based methods to improve accuracy and optimize calculation efficiency for sonar time delay estimation. Existing time delay estimations provide poor results because of many environmental effects such as noise, multipath, and crosstalk. While advanced digital signal processing (DSP) techniques have been proposed to overcome these problems, they entail increased complexity and calculation time. We use prediction in the methods we propose; the position where the reflected wave starts to occur is predicted by a recognition technique. The results are much better and the calculation time is shorter than other methods that use DSP techniques. In the first method, the optimization procedure is applied in the time domain, while in the second method, the optimization is calculated in the wavelet domain. Numerical comparisons and simulations using synthetic signals are provided to demonstrate the effectiveness of the proposed enhanced methods. We also demonstrate that our new algorithms are more stable than the existing ones and that the calculation time is reduced dramatically while maintaining increased accuracy, especially in a high-noise environment.