Distributed wide field electromagnetic method based on high-order 2n sequence pseudo random signal

To make three-dimensional electromagnetic exploration achievable, the distributed wide field electro- magnetic method (WFEM) based on the high-order 2ⁿ sequence pseudo-random signal is proposed and realized. In this method, only one set of high-order pseudo-random waveforms, which contains all target frequencies, is needed. Based on high-order sequence pseudo-random signal construction algorithm, the waveform can be customized according to different exploration tasks. And the receivers are independent with each other and dynamically adjust the acquisition parameters according to different requirements. A field test in the deep iron ore of Qihe?Yucheng showed that the distributed WFEM based on high-order pseudo-random signal realizes the high-efficiency acquisition of massive electromagnetic data in quite a short time. Compared with traditional controlled-source electromagnetic methods, the distributed WFEM is much more efficient. Distributed WFEM can be applied to the large scale and high-resolution exploration for deep resources and minerals.     

Chatter identification in end milling process based on EEMD and nonlinear dimensionless indicators

Vibration analysis is widely used to reveal the fundamental cutting mechanics in machining condition monitoring. In this work, vibration signals generated in different chatter conditions as well as stable cutting are studied to understand chatter characteristics. Considering the nonlinear and non-stationary properties of chatter vibration in milling process, a self-adaptive analysis method named ensemble empirical mode decomposition (EEMD) is adopted to analyze vibration signals and two nonlinear indices are extracted as chatter indicators. Firstly, the vibration signal is preprocessed with a comb filter to eliminate the interference of rotation frequency, tooth passing frequency and their harmonics. Secondly, EEMD is applied to decompose the filtered signal into a set of intrinsic mode functions (IMFs). Sensitive IMFs containing rich chatter information are selected. With the development of chatter, an accumulation phenomenon appears in the spectrum of sensitive IMFs and chatter frequencies are modulated by the rotation frequency and tooth passing frequency. Finally, two nonlinear dimensionless indices within the range of [0, 1], i.e., C₀ complexity and power spectral entropy, are extracted from the sensitive IMFs in both time domain and frequency domain. The proposed method is verified with well-designed cutting tests. It is found that, the stochastic noise dominates in the sensitive IMFs of stable cutting and both the C₀ complexity value and power spectral entropy are the largest; with the increase of chatter severity level, the periodic chatter components dominate gradually and the proportion of stochastic noise decreases, and thus these two indicators decrease.     

基于Root MUSIC法的轧辊偏心参数估计

针对轧辊偏心信号是混杂在各种随机干扰中的复杂高频周期信号,因FFT法对信号分析的局限性,提出了一种Root MUSIC(Multiple Signal Classification)法和Prony法相结合的轧辊偏心信号估计新方法。利用Root MUSIC法准确估计出偏心谐波的频率及谐波的个数,同时使用Prony方法估计出偏心信号的各次谐波幅值和相位。仿真结果验证了可行性和有效性,在信噪比较低的情况下仍能准确地同时估计出偏心谐波的频率、幅值及相位,尤其在频率分辨率和抗噪声上比FFT法具有优越性     

谱峭度和Vold-kalman阶比跟踪在风电机组齿轮箱故障诊断中的应用

针对风电机组齿轮箱在时变工况下的振动信号具有非平稳特性,提出一种谱峭度和Vold-kalman阶比跟踪(Vold-Kalman Filter Based Order Tracking,VKF-OT)相结合的故障特征提取方法。以转频和啮合频率作为VKF-OT的提取频率,获得随转速变化的阶比信号,通过阶比信号复包络直接求两种频率分量的幅值、相位,经实验分析这种方法能保留齿轮箱的瞬变信息。而后计算两种频率分量的谱峭度,以最大谱峭度对应的频率带能量与原阶比信号总能量之比作为故障特征,最后采用高斯混合模型对风电机组齿轮箱在不同工况下的150组振动信号进行特征描述,运用最大贝叶斯分类器实现故障识别。故障识别率表明该方法可有效地识别任意时变工况下的齿轮早期局部微弱故障。     

Influence of source frequency on impact-echo data quality for testing concrete structures

In impact-echo data, a correct interpretation of the measured frequencies may become a problem when in presence of a multitude of peaks in the spectra. An important evaluation criterion is then the magnitude of the measured peaks. This signal parameter is strongly influenced by the frequency components of the wave generated at the impact point. On-site measurements showed that, in particular cases, the wave centre frequency might have greater importance than the maximum usable frequency of the signal. Values of impact contact time, wave centre and maximum frequency in function of the impact source size were calculated from on-site data and from calibration measurements in the laboratory. The consequences for the planning of the tests and for a correct data interpretation are discussed. It was concluded that the expected measured frequency should be excited with maximum energy in the input signal in order to achieve an optimum quality of the data and ease data interpretation.     

基于DT-CWT和能量算子解调的滚动轴承故障诊断研究

滚动轴承出现局部损伤时会产生周期性冲击,振动信号往往具有调制特征,为了能够准确、有效地检测出轴承故障,提出了双树复小波和能量算子解调的诊断方法。首先运用双树复小波对采集到的轴承振动信号进行分解,得到若干个不同频带的分量,提取信号中能量集中的高频调制频带进行信号重构。然后采用能量算子的方法对重构信号进行解调。最后对解调得到的瞬时幅值进行频谱分析便能准确提取故障特征频率。通过信号仿真和实验数据处理结果证明了该方法的有效性。     

Cross flow around a cylinder at periodic disturbances in the boundary layer by electrolysis bubbles

Particularities of dynamic flow control in an open-loop system around a cylinder with a boundary layer by means of periodic generation of microbubbles are presented. Experiments were carried out in the range of Reynolds numbers of Re = (0.3?1.2) × 10⁵. It is shown that this method provides very similar results as in the case of continuous generation of microbubbles; however, the energy expenditure is two times less. The effect can be explained by the delayed response of the boundary layer separation on the disturbance cutoff. Lock-in modes of the natural vortex shedding frequency f _N and the frequency of forced oscillations f _E, as well as superposition of these frequencies, are registered. Maximal values of the amplitude of the drag and lift oscillations are registered in the range of low frequencies (f _E/f _N < 0.07).     

Evaluation of ultrasonic aluminium degassing by piezoelectric sensor

The purpose of this work was the development of a reliable technique to evaluate the intensity of acoustic cavitation during degassing of aluminium melts and to use it to select the optimum processing time for an envisaged degassing efficiency.A high sensitivity piezoelectric disk type device was used as a sensing feedback in water and liquid AlSi9Cu3 alloy. The signal acquisition and processing was carried out on a dedicated LabVIEW^® based application which allowed real-time monitoring of the piezoelectric sensor's data and ultrasonic parameters. Standard Fast Fourier Transform was applied to obtain the dominant frequencies, as well as the sub and ultra-harmonics. It was found that the amplitude of the FFT sub-harmonic (f/2) was the best indicator to evaluate the process degassing efficiency, and it could be used to select the optimal processing time, independently of other variables.The developed methodology was applied to several AlSi9Cu3 melts, and validated by measuring the final alloy densities and the volume fraction of porosities, revealing that it is an efficient, fast and cost effective technique to evaluate the degassing treatment of aluminium alloys. Experimental curves of AlSi9Cu3 alloy degassing efficiency as a function of f/2 amplitude are presented for different degassing times.     

Programming spindle speed variation for machine tool chatter suppression

This paper presents a novel method for programming spindle speed variation for machine tool chatter suppression. This method is based on varying the spindle speed for minimum energy input by the cutting process. The work done by the cutting force during sinusoidal spindle speed variation S³V is solved numerically over a wide range of spindle speeds to study the effect of S³V on stable and unstable systems and to generate charts by which the optimum S³V amplitude ratio can be selected. For on-line application, a simple criterion for computing the optimal S³V amplitude ratio is presented. Also, a heuristic criterion for selecting the frequency of the forcing speed signal is developed so that the resulting signal ensures fast stabilization of the machining process. The proposed criteria are suitable for on-line chatter suppression, since they only require knowledge of the chatter frequency and spindle speed. The effectiveness of the developed S³V programming method is verified experimentally.     

A cutter tool monitoring in machining process using Hilbert–Huang transform

This paper presents the use of accelerometer signals in order to detect cutter tool wear and tool fault under different cutting conditions. The new method, Hilbert–Huang transform (HHT), was applied to the cutter tool wear and tool fault monitoring and compared to Fourier transform. In the results processed by short-time Fourier transform the cutter tool wear or tool fault is detected by increasing the power in the power spectral density.The acceleration signals (processed by using HHT) change the frequency in the marginal spectra as a result of geometric change of the cutter tool. Results are presented in both frequency domain and time–frequency domain in the case of HHT. The HHT presents data locally without harmonics. This fact permits overcoming of the nonlinearity and nonstationarity of the acceleration signal. Axes x and z are the most sensitive to both HHT and Fourier transform.     

Detection of back-side pit on a ferrous plate by magnetic flux leakage method with analyzing magnetic field vector

We have developed a magnetic flux leakage (MFL) system using magnetic resistive (MR) sensors for detecting two dimensional magnetic field components, and an induction coil that generates low magnetic field strengths and extremely low frequencies. The signal at each scanned measurement point (i) was divided by the signal strength M_mes,i and phase α_i by a lock-in amplifier. Using the strength M_mes,i and phase α_i, we calculated the imaginary part of the signal using the common phase β. By optimization of the common phase β to the imaginary part, the analyzed scanning data curve was shown to be effective in estimating the size (depth and diameter) of back-side pits on a ferrous plate. Comparing the two dimensional magnetic field components of leakage, the imaginary part of the y-component parallel to the induced magnetic field was found to be suitable for detecting the back-side pits.     

Prediction of multiple dominant chatter frequencies in milling processes

Chatter frequencies of milling operations and their strengths are analysed using the semi-discretization method. It is known that milling processes, being parametrically excited systems, are theoretically associated with infinitely many chatter frequencies that are given as a base frequency plus an integer multiplier of the tooth passing frequency (or the characteristic frequency of the cutter) according to the Floquet theory [1]. The dominant vibration frequencies that are usually associated with chatter frequencies are, however, hidden among the infinitely many harmonics. In this paper, it is shown that the amplitudes corresponding to the individual frequency harmonics can be determined in a simple way by analysing the eigenvectors of the Floquet transition matrix obtained by the semi-discretization method without increasing the computational cost. The method can be used to determine the dominant frequency components that help in the identification of the interactions between different modes and the spindle speed.     

Frequency-dependent dispersion of high-frequency ground penetrating radar wave in concrete

This paper studies the dielectric dispersion of high frequency radar wave in concrete in early-aged and hardened concrete specimens. Frequency-dependent spectra of phase velocity v(ω) were measured to deduce the spectra of real part of dielectric permittivity ε′(ω). The dispersion was measured by three high nominal ground penetrating radar frequencies (1.5, 1.6 and 2.6 GHz), experimenting on two steel bars with concrete cover 50 and 100 mm. It was found that v(ω) and ε′(ω) dispersed at lower frequency, but became stable at high frequency regions, which agrees with the classical GPR plateau. The same frequency components at different nominal antenna frequencies show a close range of v(ω) and ε′(ω) in concrete of different ages. The results in this paper warrant further investigation of using GPR wave to study material properties.     

Steady-state creep of a particulate SiC/6061 Al composite

The creep behavior of a 10 vol.% silicon carbide particulate reinforced 6061 Al composite produced by powder metallurgy (PM) has been examined by creep tests in both tension and compression at 400°C. The tensile creep data covering minimum creep rates of the orders 10⁻⁹ to 10⁻⁴/s show an apparent stress exponent n_app≈13, but a comparison with compressive creep data reveals that some high strain-rate data in tension are due to the transition to the tertiary stage. Analysis of the data is made only for the steady-state creep rate, together with that for an unreinforced PM 6061 Al alloy, by incorporating a threshold stress. This gives a stress exponent n=3 for the matrix alloy, whereas the composite data show such a trend that the n value gradually changes from 3 to 1 as the effective stress increases. A new method of steady-state creep data analysis is formulated by taking account of the interface-confined diffusional flow and thereby the finding above is reasonably assessed.     

Terahertz dielectric properties of plasma-sprayed thermal-barrier coatings

We investigated the electromagnetic wave transmittance and dielectric properties of yttria partially stabilised zirconia thermal-barrier coatings by terahertz time-domain spectroscopy in which the samples were irradiated by a pulsed THz wave in the frequency range of 0.1-6.3 THz. The coating microstructure was varied by changing the spray conditions and the THz transmittance and dielectric constants were examined as functions of frequency. The coatings exhibited a high transmittance of 20%-80% at frequencies below 0.5 THz and almost zero transmittance above 1.5 THz. The refractive index n for different coatings ranges from 4 to 6, depending on the coating microstructure. For any given coating, n increases to some extent with increasing frequency. Unlike the imaginary part of the dielectric constant, the real part of the dielectric constant is strongly correlated with the porosity of the coatings, which suggests that terahertz spectroscopy may potentially be used to non-destructively evaluate the porosity of ceramic coatings.     

Nonlinear acoustic characteristics of Fatigue microcracks in al alloy plate

A system consisting of piezoelectric transducers, arbitrary waveform generator, power amplifier, laser vibrometer, digital oscilloscope, and a computer has been constructed to study the nonlinear acoustic characteristics of fatigue microcracks of 2024-T351 aluminum alloy plates. Using two different frequency signals (f_H=810 kHz, f_L=100 kHz), the intact and microcracked samples are experimented, respectively. The experimental results show that in the intact samples there are only two fundamental frequencies and no harmonics and sidebands occur. In the microcracked samples, there is an abundance of the harmonics and sidebands, and the frequencies f_H±f_L and f_H±2f_L rise. These new frequency components can be used to indicate the presence of a microcrack or damage.     

Effect of surface free energy anisotropy on dendrite tip shape

In previous work, approximate solutions were found for paraboloids having perturbations with four-fold axial symmetry in order to model dendritic growth in cubic materials. These solutions provide self-consistent corrections through second order in a shape parameter ε to the Peclet number vs supercooling relation of the Ivantsov solution. The parameter ε is proportional to the amplitude of the four-fold correction to the dendrite shape, as measured from the Ivantsov paraboloid of revolution. The equilibrium shape for anisotropic surface free energy to second order in the anisotropy is calculated. The value of ε is determined by comparing the dendrite tip shape with the portion of the equilibrium shape near the growth direction, [001], for anisotropic surface free energy of the form γ=γ₀[1+4ε₄(n₁⁴+n₂⁴+n₃⁴)], where the n_i are components of the unit normal of the crystal surface. This comparison results in ε=−2ε₄−24ε₄²+O(ε₄³), independent of the Peclet number. From the experimental value of ε₄, it is found that ε≈−0.012±0.004, in good agreement with the measured value ε≈−0.008 of LaCombe et al. (Phys. Rev. E, 1995, 52, 2778)     

An evaluation of the electrochemical frequency modulation (EFM) technique

The EFM technique has been used to calculate anodic and cathodic Tafel slopes as well as corrosion current densities for the systems Cu/NaCl, Al 5083/NaCl, stainless steel/NaCl and mild steel in H₂SO₄ and NaCl. These values have been compared with those obtained from the analysis of polarization curves determined in the vicinity of the corrosion potential. In addition, polarization resistance values calculated based on the analysis of EFM data have been compared with values obtained from analysis of impedance data collected for the same systems. In most cases the corrosion rates determined with the EFM technique were much higher than the values determined with the other two techniques. This result is mainly due to the fact that for systems with low corrosion rates the modulation frequencies that are used in the EFM technique are in the capacitive region of the impedance spectra. Good agreement of corrosion rates obtained with the EFM and the polarization techniques was only observed for the mild steel/H₂SO₄ system that has very high corrosion rates. Analysis of EFM data assuming charge transfer or diffusion control produced different Tafel slopes and corrosion rates, but the same polarization resistance (R_p) values. When the EFM technique was applied to a resistor, the calculated R_p value equaled the value of the resistance which is further proof that the EFM technique measures the impedance of a corroding system in a narrow frequency band. The difference to the traditional EIS technique is that the amplitude of the ac signal can be different at different frequencies.     

Ultrasonic evaluation of TiAl and 40Cr diffusion bonding quality based on time-scale characteristics extraction

To solve the problem of ultrasonic pulse-echo method in the evaluation of kissing bond and unbond in TiAl and ⁴⁰Cr diffusion bonding, a characteristics extraction algorithm was proposed. The algorithm was based on continuous wavelet transform to convert ultrasonic TiAl and ⁴⁰Cr diffusion bonding interface signals into time-scale domain. The ultrasonic tests were performed by an ultrasonic C-scan imaging system using a 10 MHz focused transducer. The time-scale amplitude and phase of the interface signals were calculated and analyzed to distinguish the kissing bond and the unbond from the perfectly bonded interface. The kissing bond can be detected by the scale-dependent amplitude combined with phase variation and the unbond can be measured by the opposite phase. The amplitude and phase characteristics were extracted to reconstruct the amplitude and phase characteristics images for TiAl and ⁴⁰Cr diffusion bonding specimens evaluation. The amplitude and phase characteristics images are effective in the evaluation of bonding quality.     

13C CPMAS NMR,XRD, d.c. and a.c. electrical conductivity of aromatic acids doped polyaniline

Aromatic acids, para-toluene sulfonic acid (PTSA) and sulfosalicylic acid (SSA), doped polyanilines at different dopant levels and prepared by chemical and electrochemical methods are characterized by the high-resolution solid-state ¹³C CPMAS NMR technique. X-ray diffraction (XRD) patterns of highly protonated samples show a peak around 24 °. There is increase in crystallinity as the dopant concentration is increased. Temperature dependence of d.c. electrical conductivity measurements suggests that conduction takes place in an inhomogeneous medium consisting of ‘metallic’ polymer particles embedded in an insulating matrix. A.c. electrical conductivity measured on some of these samples shows that σ_ac ∝ ω^s in the frequency range 10 Hz to 100 kHz, and s is found to decrease as the dopant concentration and the temperature are increased. For higher dopant concentration samples, σ_ac is found to be temperature independent in the frequency range 10 Hz–100 kHz. The real part of the dielectric constant showed a steep increase at low frequencies. The results of a.c. transport are discussed in terms of hopping of charges among defect states.