Influence of fluidelastic vibration frequency on predicting damping controlled instability using a quasi-steady model in a normal triangular tube array

The impact of gradient imperfections on UTE images and UTE image-derived bone water quantification was investigated at 3 T field strength. The effects of simple gradient time delays and eddy currents on UTE images, as well as the effects of gradient error corrections, were studied with simulation and phantom experiments. The k-space trajectory was mapped with a 2D sequence with phase encoding on both spatial axes by measuring the phase of the signal in small time increments during ramp-up of the read gradient. In vivo 3D UTE images were reconstructed with and without gradient error compensation to determine the bias in bone water quantification. Finally, imaging was performed on 2 equally configured Siemens TIM Trio systems (Siemens Medical Solutions, Erlangen, Germany) to investigate the impact of such gradient imperfections on inter-scanner measurement bias. Compared to values derived from UTE images with full gradient error compensation, total bone water was found to deviate substantially with no (up to 17%) or partial (delay-only) compensation (up to 10.8%). Bound water, obtained with inversion recovery-prepared UTE, was somewhat less susceptible to gradient errors (up to 2.2% for both correction strategies). Inter-scannercomparisonindicated a statistically significant bias between measurements from the 2 MR systems for both total and bound water, which either vanished or was substantially reduced following gradient error correction. Gradient imperfections impose spatially dependent artifacts on UTE images, which compromise not only bone water quantification accuracy but also inter-scanner measurement agreement if left uncompensated.

Current methods to infer an agent’s state of attentional focus rely on scalp potential recordings and pupil diameter measurements, both of which are unrealistic in many real-world situations, and are also prone to movement and electrical artifacts. Being able to predict attentional performance from a simple and noninvasive measure, such as respiration, could have obvious potential benefit for simplifying measurement and improving task performance in many settings, and could also be employed clinically with attentionally compromised populations for training and rehabilitation. It has been suggested that respiration and attention comprise a neuro-physiologically coupled system, and behavioral data has indicated that attentional performance, including reaction time and reaction time variability (RTV), covary with respiratory dynamics. In the present study, we tested several neural network configurations for the prediction of attentional control state (RTV) from respiratory parameters. We observed significant predictive power derived solely from respiratory input, and conclude that a robust and portable feedback device utilizing soft computation is feasible for this purpose. We suggest specific model and data source improvements to potentially further reduce errors in prediction.

Low complexity clutter removal in GPR images via lattice filters

We present a laser phase noise (PN) induced effect of a phase-modulation-to-intensity-modulation conversion noise and noise pedestals underneath each of the orthogonal frequency division multiplexing (OFDM) subcarriers in a self-coherent optical OFDM transmission using a self-homodyne technique. We provide a statistical analysis on the received symbols using a histogram to demonstrate the effect of a phase rotation term and inter-subcarrier interference individually and collectively. The PN is then compensated using a simple time delay to realign the phase walk-off of the subcarriers relative to the carrier. Significant quadrature improvements of 6.82 dB using 5 MHz laser linewidth over a 720 km transmission length and 5.38 dB using 20 MHz over 240 km have been obtained with 16 quadrature amplitude modulation (QAM) over 15 GHz OFDM signal bandwidth. The technique also significantly reduced an optical-signal-to-noise ratio requirement at the bit error rate of 1×10-3 by 16.15 dB for 64-QAM over 160 km. With the delay, the system can tolerate three times the chromatic dispersion-length product.

ABSTRACTMarine seismic vibrators are generally considered to be less intrusive than airguns from an environmental perspective. This is because they emit their energy spread out in time, rather than in a single, high‐intensity pulse. There are also significant geophysical benefits associated with marine vibrators, and they stem from the ability to specify in detail the output acoustic waveform. The phase can be specified independently at each frequency. Such detailed control cannot be achieved with conventional airgun sources, where the phase can only be modified using simple overall time delays. The vibrator phase can be employed in several different ways: it can be applied to the overall source phase in a sequence so that it varies from one source point to the next; it can be applied to the individual vibrators within the source array so the source directivity is changed; it can be applied to the overall source phase of each source in a simultaneous source acquisition. Carefully designed phase sequences can attenuate the residual source noise, and this in turn allows extra source points to be interleaved between the conventional ones. For these extra source points, the relative phase of the vibrators within the array can be chosen to create a transverse gradient source, which illuminates the earth predominantly in directions out of the plane of the sail line without left/right ambiguity. If seismic vibrator data are acquired using interleaved conventional and transverse gradient sweeps, more information is collected per kilometre of vessel travel than is the case in conventional acquisition. This richer data acquisition leads to the possibility of acquiring all the necessary seismic data in a shorter time. Three‐dimensional reconstruction techniques are used to recover the same image quality that would have been obtained using the conventional, more time‐consuming acquisition. For a marine vibrator to be suitable for these techniques it must, in general terms, have ‘high fidelity’. The precise device specifications are defined through realistic end‐to‐end simulations of the physical systems and the processing. The specifications are somewhat more onerous than for a conventional vibrator, but they are achievable. A prototype vibrator that satisfies these requirements has been built. In a simulated case study of a three‐dimensional deep‐water ocean bottom node survey, the seismic data could have been acquired using marine vibrators in one third of the time that it would have taken using airguns.

A grid-connected inverter is equipped with an LCL filter at its output. This filter has a resonant characteristic and many active damping methods have been researched to suppress the resonant effects. The inverter is often controlled digitally and there is a control time delay, which is inevitable due to A/D conversion, computation times, etc. This delay degrades the stabilities and damping effects of the inverter because the frequency characteristics are different from the designed one, especially around the resonant frequency. This paper proposes a simple and effective compensation method of the control time delay to improve the damping effects. It is based on an estimation method of future state values advanced by the delay time. The proposed method is applied to the grid-connected inverter control and its effectiveness is investigated and validated through determining transfer functions of the regulated current errors by experiment.

The results are presented from computer simulations of acoustic pulse propagation in heterogenous media mimicking the human head in two-dimensional and three-dimensional geometries. In the three-dimensional experiment, the cranial bone is presented as a liquid layer with a speed of sound corresponding to that of longitudinal waves in the bone. In the two-dimensional experiment, both longitudinal and transverse waves are considered. Based on data obtained in the numerical experiments, the possibility of obtaining ultrasound images of point scatterers by compensating for aberrations introduced by cranial bones is studied. It is shown that even a simple time delay correction along straight rays greatly improves the quality of an ultrasound image obtained through a nonuniform-thickness solid layer.

Control of the LCL -type three-phase grid-connected converter is difficult due to high resonance peak of the LCL filter. Active damping is the state-of-the-art solution to this problem, but the damping performance will be affected by the inherent time delay of digital control, especially for high-power low switching frequency applications. Based on a discrete-time stability analysis of an LCL -type converter with capacitor-current-feedback active damping, a simple and effective time delay compensation method, which is based on area equalization concept, is proposed. The method can reduce the negative impact of the computation delay significantly. It has the potential to serve as a general solution to time delay compensation of a digitally controlled PWM converter. The validity of the proposed method is proved by experimental results.

A computationally efficient, time-domain Helmholtz-Kirchhoff (H-K) integral was derived and applied to reconstructing surface wave profiles from reflected acoustic pulses [Walstead and Deane, J. Acoust. Soc. Am. 133, 2597-2611 (2013)]. However, the final form of the integral equation incorporating a stationary phase approximation contained a complex phase term exp(iπ/4), which cannot be treated as a simple time delay. In this work, a real time-domain H-K integral is presented that includes an additional Hilbert transform of the time-derivative of the transmitted pulse. Numerical simulation with a sinusoidal surface shows good agreement between the real time-domain formulation and exact H-K integral, while achieving a significant improvement in computational speed (e.g., 2 orders of magnitude).

Abstract Soil pore water salinity plays an important role in the distribution of vegetation and biogeochemical processes in coastal floodplain ecosystems. In this study, artificial neural networks (ANNs) were applied to simulate the pore water salinity of a tidal floodplain in Florida. We present an approach based on embedding theory with mutual information to reconstruct ANN model input time series from one system state variable. Mutual information between system output and input was computed and the local minimum mutual information points were used to determine a time lag vector for time series embedding and reconstruction, with which the mutual information weighted average method was developed to compute the components of reconstructed time series. The optimal embedding dimension was obtained by optimizing model performance. The method was applied to simulate soil pore water salinity dynamics at 12 probe locations in the tidal floodplain influenced by saltwater intrusion using 4 years (2005–2008) data, in which adjacent river water salinity was used to reconstruct model input. The simulated electrical conductivity of the pore water showed close agreement with field observations (RMSEand), suggesting the reconstructed input by the proposed approach provided adequate input information for ANN modeling. Multiple linear regression model, partial mutual information algorithm for input variable selection,k‐NN algorithm, and simple time delay embedding were also used to further verify the merit of the proposed approach.

In this paper the applications of acoustic surface wave device technology to electronic warfare systems have been reviewed. A variety of signal processing tasks such as simple time delay, frequency dependent time delay, programmable and discretely variable time delay, band pass filtering and generation and recognition of various coded waveforms can be performed by surface wave devices to improve the performance of EW system.

As an important part of the smart grid, a wide-area measurement system (WAMS) provides the key technical support for power system monitoring, protection and control. But 20 uncertainties in system parameters and signal transmission time delay could worsen the damping effect and deteriorate the system stability. In the presented study, the subspace system identification technique (SIT) is used to firstly derive a low-order linear model of a power system from the measurements. Then, a novel adaptive wide-area damping control scheme for online tuning of the wide-area damping controller (WADC) parameters using the residue method is proposed. In order to eliminate the effects of the time delay to the signal transmission, a simple and practical time delay compensation algorithm is proposed to compensate the time delay in each wide-area control signal. Detailed examples, inspired by the IEEE test system under various disturbance scenarios, have been used to verify the effectiveness of the proposed adaptive wide-area damping control scheme.

Remote signals available from the wide-area monitoring system could be used to enhance the stability of large interconnected power systems with the help of a wide-area damping controller (WADC). Uncertainty in system parameters and signal transmission time delay are the major factors, which could worsen the damping effect and deteriorate the system stability. This study presents a novel adaptive wide-area damping control scheme in which oscillation modes will be identified online using stochastic subspace identification for online tuning of the WADC structure and parameters using the residue method. A simple and practical signal time delay compensation algorithm will also be proposed to measure and compensate the time delay in each wide-area control signal so as to eliminate the effects of signal time delay on the proposed adaptive WADC. The effectiveness and robustness of the proposed adaptive wide-area damping control scheme have been verified with simulation studies on the IEEE 16-generator 5-area test system under various disturbance scenarios.

In this paper, we report a common phenomenon observed in chaotic systems linked by time delay. Recently, the Lorenz chaotic system has been extended to the family of Lorenz systems which includes the Chen and Lü systems. These three chaotic systems, corresponding to different sets of system parameter values, are topologically different. With the aid of numerical simulations, we have surprisingly found that a simple time delay, directly applied to one or more state variables, transforms the Lorenz system to the generalized Chen system or the generalized Lü system without any parameter changes. The existence of this phenomenon has also been found in other known chaotic systems: the Rössler system, the Chua's circuit and the 4-Liu system. This finding has shown a common characteristic of chaotic systems: a new chaotic "branch" can be created from a chaotic attractor by simply adding a time delay.

A simple time delay model for eukaryotic cell cycle

Accommodation as a function of age and the linearity of the response dynamics

In radio communications, a bandpass-to-lowpass transformation is needed to demodulate the received signal down to baseband. One crucial question in this context is how to effectively attenuate the image band signal. For this purpose, inphase/quadrature (I/Q) signal processing is widely utilized in today's radio receivers. In this paper, a novel structure for obtaining an image-free baseband observation of the received bandpass signal is presented. The starting point is to approximate the needed 90/spl deg/ phase difference between the I and Q branch signals using a simple time delay of one quarter of the carrier cycle. For narrowband signals, this approach can be used directly to attenuate the inherent self-image. By using an interference canceller-type of compensation technique, this concept is here generalized to cover also wideband multichannel signals. Furthermore, a closed-form expression to explicitly characterize the obtainable image attenuation is derived. Efficient implementation structures for digital radios utilizing periodically nonuniform subsampling are presented, and the validity of the proposed approach is further illustrated through simulation and design examples.

From Cauer synthesis to Legendre polynomials — The case of the lossless transmission line

Greenland—Antarctic phase relations and millennial time-scale climate fluctuations in the Greenland ice-cores