Coherent resonance in klystron oscillator at self-excitation threshold

The phenomenon of coherent resonance in a nonlinear dynamical system based on a delayed-feedback klystron (DFK) oscillator has been experimentally studied. The application of an external white noise signal to the DFK oscillator at the self-excitation threshold leads to the appearance of a resonant response in the system, the maximum of which is attained at a definite power of the external noise. It is demonstrated for the first time that the coherent resonance in response to an external harmonic signal in the microwave range allows the signal-to-noise ratio at the output of a regenerative ring amplifier to be significantly increased.     

Generation of chaotic microwave pulses in a ring system based on a klystron power amplifier and a nonlinear delay line on magnetostatic waves

The autonomous generation of stationary chaotic microwave pulse trains in a self-oscillating ring system with a multicavity klystron power amplifier operating in a small-signal regime and a wideband non-linear delay line on surface magnetostatic waves has been experimentally studied. It is established that the characteristics of generated chaotic microwave pulses can be controlled by varying the electron beam current and accelerating voltage in the klystron.     

基于混沌理论的海底混响中目标回波提取

提出了基于混沌理论的混响中目标回波提取新方法。该方法主要得益于一种新的预测模型,该模型基于径向基函数神经网络,综合利用了时间序列的前向和后向预测,解释了该模型用于混沌信号分离的基本原理,用几种混沌时间序列分析了该模型用于混沌信号建模和谐波信号提取的性能。该方法用于湖试混响中目标回波提取的结果表明:该模型可以用于提取信混比不小于1dB的目标回波。     

The mean Poincaré return time locking: A criterion of chaos induced synchronization

A new method for diagnostics of the induced synchronization of chaotic self-oscillation is proposed, which is based on the local theory of Poincaré recurrences. It is established that, in the region of synchronization, the mean Poincaré return time is locked by an external harmonic signal.     

Features of induced synchronization of a bistable oscillator with chaotic dynamics

The features of induced synchronization of a Chua oscillator with a nonlinear element admitting cubic approximation have been numerically simulated for a system where the autonomous operation admits regular or chaotic regimes determined by losses in the oscillatory circuit. The cases of deterministic synchronization (under the action of a harmonic external driving signal) and chaotic synchronization (chaotic control) have been considered. It is established that, outside the band of synchronous oscillations that corresponds to the deterministic synchronization, the external harmonic signal stimulates the transformation of regular oscillations into chaos with the motions switched between two attractors. In cases of chaotic synchronization, there is a residual “noise” in the form of differential chaotic oscillations, which grows with increasing non-identity of the driving and driven signals.     

Dynamic S-Box Generation Using Novel Chaotic Map with Nonlinearity Tweaking

A substitution box (S-Box) is a crucial component of contemporary cryptosystems that provide data protection in block ciphers. At the moment, chaotic maps are being created and extensively used to generate these S-Boxes as a chaotic map assists in providing disorder and resistance to combat cryptanalytical attempts. In this paper, the construction of a dynamic S-Box using a cipher key is proposed using a novel chaotic map and an innovative tweaking approach. The projected chaotic map and the proposed tweak approach are presented for the first time and the use of parameters in their working makes both of these dynamic in nature. The tweak approach employs cubic polynomials while permuting the values of an initial S-Box to enhance its cryptographic fort. Values of the parameters are provided using the cipher key and a small variation in values of these parameters results in a completely different unique S-Box. Comparative analysis and exploration confirmed that the projected chaotic map exhibits a significant amount of chaotic complexity. The security assessment in terms of bijectivity, nonlinearity, bits independence, strict avalanche, linear approximation probability, and differential probability criteria are utilized to critically investigate the effectiveness of the proposed S-Box against several assaults. The proposed S-Box’s cryptographic performance is comparable to those of recently projected S-Boxes for its adaption in real-world security applications. The comparative scrutiny pacifies the genuine potential of the proposed S-Box in terms of its applicability for data security.     

Third harmonic transmit phasing for tissue harmonic generation

Generation of tissue harmonic signals during acoustic propagation is based on the combined effect of two different spectral interactions of the transmit signal. One produces harmonic whose frequency is the sum of transmit frequencies. The other results in harmonic at difference frequency of the transmit signals. Both the frequency-sum component and the frequency-difference component are sensitive to the phase of their constitutive spectral signals. In this study, a novel approach for modifying the amplitude of tissue harmonic signal is proposed based on phasing these two components to achieve either harmonic enhancement or suppression. Both experiments and simulations were performed to investigate the effects of 3f0 transmit phasing on tissue harmonic generation. Results indicate that the relative phasing between the frequency-sum component and the frequency-difference component markedly changes the amplitude of the second harmonic signal. For harmonic enhancement, approximate 6 dB increase of second harmonic amplitude can be achieved while the lateral harmonic beam pattern also is improved as compared to conventional situations in which only the frequency-sum component is considered. For harmonic suppression, the second harmonic signal also could be significantly reduced by about 11 dB when the frequency-difference component is out of phase with the frequency-sum component. Hence, the method of 3f0 transmit phasing has potentials for both improving signal-to-noise ratio in tissue harmonic imaging and enhancing image contrast in contrast-agent imaging by suppression of tissue harmonic background.     

基于动态可变参数的复合混沌系统的语音加密算法研究

利用混沌理论,基于Hénon映射和Logistic映射设计了一个复合混沌系统,并证明了其具有更好的初值敏感性。对产生的混沌二值序列进行美国国家标准与技术研究所(National Institute of Standard and Technology,NIST)随机数测试,分析得出该序列具有较好的随机性,可用于加密。依此设计了一个语音加密算法,实现了语音信号的加/解密功能。仿真分析表明,该混沌语音加密算法具有较高的安全性,能产生足够的密钥空间,有较强的保密性能。     

Generalized synchronization in a system of coupled klystron chaotic oscillators

The phenomenon of generalized chaotic synchronization has been studied in a system of two unidirectionally coupled chaotic oscillators modeling two-sresonator klystron autooscillators. A mechanism explaining the observed behavior is presented.     

A consistent tissue attenuation coefficient estimator using bubble harmonic echoes

The ultrasonic property of soft tissue can be quantified by its attenuation coefficient α. Traditionally the backscattering signal of tissue is used to estimate α. To improve precision, a large number of spatially independent samples of tissue echoes are required for averaging. In this paper, we propose a new estimation method, which makes use of microbubbles to provide temporally independent samples for averaging. It is easier for temporal sampling to maintain ergodicity and provide a large number of independent samples for statistical averaging. A stochastic model for the harmonic signals of an ideal bubble attenuated by tissue is derived based on Kuc's and Miller's works. An estimator of α is then presented. This estimator is consistent and could be biased because of the unknown squarelaw relation between the second and fundamental harmonics for non-ideal bubble oscillation. In experimental works, we design a simplified phantom for demonstrating the performance of the proposed estimator. It is shown that both first and second harmonics can estimate α consistently. However, the interference of the tissue backscattering signal may cause additional estimation error using the first harmonic.     

Adaptive chaotic maps for identification of complex targets

Identification of complex targets in radar or sonar is a difficult problem. The author shows how the parameters of a simple chaotic map may be optimised to increase the cross correlation of a transmitted signal with the reflection from one target while decreasing the cross correlation with another target. The targets consist of simple arrays of point scatterers. The author shows that the optimisation process produces a chaotic signal with a power spectrum that maximises the ratio of the cross correlation with one target to the cross correlation with a different target, however the author also shows that the specific time ordering of the chaotic signal is also important in determining the cross correlation. The performance of the chaotic signal is better than a random signal with the same power spectrum. The author also shows an example of applying these methods to targets that are more complex.     

Volterra小波变换最优阈值的混沌去噪方法

针对混沌信号小波去噪中难以确定最优阈值的问题,提出一种Volterra小波变换最优阈值的判定方法。利用小波变换将混沌信号分解,对不同尺度下的小波信号设定浮动因子以调节阈值大小,最后根据混沌序列Volterra自适应预测的相对误差选取最优阈值。利用该方法对不同维度的Lorenz混沌时间序列进行了去噪研究,结果表明所提方法是有效的。     

Precompensated excitation waveforms to suppress harmonic generation in MEMS electrostatic transducers

Microelectromechanical systems (MEMS) electrostatic-based transducers inherently produce harmonics as the electrostatic force generated in the transmit mode is approximately proportional to the square of the applied voltage signal. This characteristic precludes them from being effectively used for harmonic imaging (either with or without the addition of microbubble-based contrast agents). The harmonic signal that is nonlinearly generated by tissue (or contrast agent) cannot be distinguished from the inherent transmitted harmonic signal. We investigated two precompensation methods to cancel this inherent harmonic generation in electrostatic transducers. A combination of finite element analysis (FEA) and experimental results are presented. The first approach relies on a calculation, or measurement, of the transducer's linear transfer function, which is valid for small signal levels. Using this transfer function and a measurement of the undesired harmonic signal, a predistorted transmit signal was calculated to cancel the harmonic inherently generated by the transducer. Due to the lack of perfect linearity, the approach does hot work completely in a single iteration. However, with subsequent iterations, the problem becomes more linear and converges toward a very satisfactory result (a 18.6 dB harmonic reduction was achieved in FEA simulations and a 20.7 dB reduction was measured in a prototype experiment). The second approach tested involves defining a desired function [including a direct current (DC) offset], then taking the square root of this function to determine the shape of the required input function. A 5.5 dB reduction of transmitted harmonic was obtained in both FEA simulation and experimental prototypes test.     

Spectra and autocorrelation functions of a geometrically nonlinear beam

Abstract

The aim of this paper is to explore the Fourier spectra and autocorrelation functions of a geometrically nonlinear elastic beam with supports subjected to harmonic excitation. By using the large deformation theory and considering the deflection caused by both moment and shear as well as the inertia force induced by both displacement and rotation, the equation of motion is derived, which turns out to be a nonlinear fourth‐order partial differential equation. Because of the complexity of the partial differential equation, this paper first employs the Galerkin method to reduce it into a nonlinear second‐order ordinary differential equation. Then the Runge‐Kutta‐Verner numerical method is applied to find the solution. Due to the nonlinearity of the equation, chaotic motion is found to exist in this nonlinear elastic beam system. This paper draws time histories and Poincaré maps to show that chaotic motion indeed exists in the geometrically nonlinear elastic beam. For both regular and chaotic motions, Fourier spectra and autocorrelation functions are then plotted, which can be used as important criteria to diagnose whether the geometrically nonlinear beam will be chaotic in the future because of the significantly different results from those two kinds of motions.     

Adaptive resonator-based method for power system harmonic analysis

A simple algorithm for the harmonic estimation, in a wide range of frequency changes, with benefits in a reduced complexity and computational efforts is prescribed. This implementation is based on a recently introduced common structure for recursive discrete transforms and contemplated as an implementation of finite-impulse-response (FIR) and infinite-impulse-response (MR) filter transfer functions to reduce computational efforts. This structure consists of digital resonators in a common negative feedback loop. The structure of the estimation algorithm consists of two decoupled modules: the first one for an adaptive filter of input signal with harmonic amplitude and phase calculation, the second one for an external frequency estimation. A very suitable algorithm for frequency and harmonic phasor estimations is obtained. To demonstrate the performance of the developed algorithm, computer-simulated data records are processed. Simulation results show that this algorithm is applicable to detect the harmonic amplitudes of steady-state, varying and decaying sinusoidal signals. It has been found that the proposed method really meets the needs of online applications. This technique provides accurate amplitude estimates in about one period.     

具有分数导数本构关系的粘弹性浅拱的非线性动力学行为

利用分数导数本构模型描述材料的粘弹性特性,建立了粘弹性浅拱在横向荷载作用下的动力学方程。利用Galerkin截断法并结合边界条件分别得到了一阶和二阶Galerkin系统的控制微分方程。通过数值计算,分析了简谐激励下一阶Galerkin系统的非线动力学行为。研究表明:随着外激励幅值的变化,粘弹性浅拱系统可以通过倍周期分岔或阵发性两条路径进入混沌;固定外激励幅值、频率以及阻尼系数等状态参数,不同初始条件下,系统可以出现多周期解共存、周期解与混沌解共存的现象。     

Detecting Low Velocity Impact Damage in Composite Plate Using Nonlinear Acoustic/Ultrasound Methods

The objective of this work was to image the presence of impact damage by monitoring the nonlinear response of damaged carbon/epoxy composite samples. The presence of microcracks, debonding, delamination, etc… induce the material to behave in a nonlinear elastic fashion highlighted by the presence and amplitude of harmonics in the spectrum of the received signal when the sample is periodically excited at one of its resonance frequencies. The sensitivity of a second harmonic imaging technique (SEHIT) based on material nonlinear elastic effect known as second harmonic generation (SHG) was investigated. The proposed imaging process was used to detect barely visible impact damage (BVID) due to low velocity impact (<12 J). The results showed that the SEHIT methods appear to be highly accurate in assessing the presence and magnitude of damage with a very promising future for both NDT and possibly structural health monitoring (SHM) applications. Moreover the technique was validated with two conventional NDT techniques: pulse thermography and thermosonics. The first NDT method failed in detecting the damage on the impacted face. The second technique was capable of localising and quantifying the damage on the impacted surface agreeing well with the results obtained using the proposed nonlinear imaging method.     

Suppression of propagating second harmonic in ultrasound contrast imaging

Contrast agents, such as bubbles, are used in ultrasound to enhance backscatter from blood. To increase contrast between these agents and tissue, nonlinear methods such as harmonic imaging can be used. Contrast is limited, however, by tissue second harmonic signals. We show that a major source of this signal is nonlinear propagation through tissue. In addition, we present methods to suppress this second harmonic generation. One simple approach is to decrease the f/number of the imaging system. Simulations show that doubling the size of the array, while keeping total power output constant, decreases propagating second harmonic generation. A second approach uses active noise cancellation to suppress second harmonic generation. A specific method, the harmonic cancellation system (HCS), is developed and presented as an example. In simulations, HCS decreased second harmonic generation by over 30 dB. Using such methods, contrast can be improved between tissue and bubbles in harmonic imaging.     

Application of chaotic noise reduction techniques to chaotic data trained by ANN

We propose a novel method of combining artificial neural networks (ANNs) with chaotic noise reduction techniques that captures the metric and dynamic invariants of a chaotic time series, e.g. a time series obtained by iterating the logistic map in chaotic regimes. Our results indicate that while the feedforward neural network is capable of capturing the dynamical and metric invariants of chaotic time series within an error of about 25%, ANNs along with chaotic noise reduction techniques, such as Hammel’s method or the local projective method, can significantly improve these results. This further suggests that the effort on the ANN to train data corresponding to complex structures can be significantly reduced. This technique can be applied in areas like signal processing, data communication, image processing etc.     

改进谐波小波及其在振动信号时频分析中的应用

针对谐波小波在时域衰减速度慢的缺点,对其在频域进行光滑化处理,得到了性质更好的改进谐波小波。由于采用光谱化方法只能得到改进小波在频域的表达式,因而采用基于FFT的算法对它进行计算。针对振动信号一般舸样点较多的特点,采用计算等间隔小波系数的方法以减少计算量。以改进谐波小波对某新型导弹空中点火试验的振动信号进行了时频分析,并采用计算时频图等高图的方法以消除大部分无用的信息,得到了振动信号的高清晰时频图。