Time Domain Research Articles

A stochastic averaging mathematical framework for design and optimization of nonlinear energy harvesters with several electrical DOFs

Energy harvesters for mechanical vibrations are electro-mechanical systems designed to capture ambient dispersed kinetic energy, and to convert it into usable electrical power. The random nature of mechanical vibrations, combined with the intrinsic non-linearity of the harvester, implies that long, time domain Monte-Carlo simulations are required to assess the device performance, making the analysis burdensome when a large parameter space must be explored. Therefore a simplified, albeit approximate, semi-analytical analysis technique is of paramount importance. In this work we present a methodology for the analysis and design of nonlinear piezoelectric energy harvesters for random mechanical vibrations. The methodology is based on the combined application of model order reduction, to project the dynamics onto a lower dimensional space, and of stochastic averaging, to calculate the stationary probability density function of the reduced variables. The probability distribution is used to calculate expectations of the most relevant quantities, like output voltage, harvested power and power efficiency. Based on our previous works, we consider an energy harvester with a matching network, interposed between the harvester and the load, that reduces the impedance mismatch between the two stages. The methodology is applied to the optimization of the matching network, allowing to maximize the global harvested power and the conversion efficiency. We show that the proposed methodology gives accurate predictions of the harvester’s performance, and that it can be used to significantly simplify the analysis, design and optimization of the device.

Performance of differently arranged double-tuned mass dampers for structural seismic response control including soil-structure interaction

Reducing vibrations occurring in structures under environmental loads makes a significant contribution to the development of structural safety. Tuned mass dampers (TMD) are one of the systems commonly used to control structural vibrations. Since TMD is adjusted according to the dominant frequency of the structure, factors that may cause changes in the frequency of the structure can significantly reduce the effectiveness of TMD. One of the main factors that can cause changes in the frequency values of the structure is the interaction between the structure and the soil. To overcome this situation, it is recommended to apply control systems consisting of multiple TMDs (MTMD) to the main structure instead of a single TMD. MTMD systems are arranged in two different types, serial and parallel. In this paper, a methodology that considers the structure-soil interaction (SSI) is presented for the optimum design of control systems with different configurations placed on a main structure under the influence of ground motion. To research the SSI effects, the TMDs-structure coupled system is supported by rocking and swaying springs and the corresponding dashpots, and then the equations of motions of the structural system are derived. In the optimization process based on particle swarm optimization in the frequency domain, different soil types, configuration types, and mass ratios are considered. The efficiency of optimum TMDs arranged in series and parallel is also evaluated using near and far fault ground motions in the time domain. Numerical results show that if structures are constructed on soils with low shear wave velocities, ignoring SSI can significantly change the design parameters and effectiveness of the control system.

On an isotropic porous solid cylinder: the analytical solution and sensitivity analysis of the pressure

Within this work, we perform a sensitivity analysis to determine the influence of the material input parameters on the pressure in an isotropic porous solid cylinder. We provide a step-by-step guide to obtain the analytical solution for a porous isotropic elastic cylinder in terms of the pressure, stresses, and elastic displacement. We obtain the solution by performing a Laplace transform on the governing equations, which are those of Biot’s poroelasticity in cylindrical polar coordinates. We enforce radial boundary conditions and obtain the solution in the Laplace transformed domain before reverting back to the time domain. The sensitivity analysis is then carried out, considering only the derived pressure solution. This analysis finds that the time t, Biot’s modulus M, and Poisson’s ratio v have the highest influence on the pressure whereas the initial value of pressure P0 plays a very little role.

Vibration separation method for permanent magnet guideway irregularity measurement

High-temperature superconducting (HTS) pinning maglev has the potential to achieve high operational speed while having a low maintenance cost, attracting a lot of attention. The HTS pinning maglev system realizes the levitation of the vehicle through the flux pinning characteristics between the high-temperature superconducting bulk and the magnetic field of the permanent magnet guideway (PMG). Thus, the profile of the magnetic field of the PMG can significantly affect the levitation status of the vehicle, which makes the PMG irregularity become one of the main excitation sources of the system. The magnetic induction intensity on the surface of the PMG can be used to characterize the PMG irregularity, but the non-contact dynamic measurement may produce large errors by the vibration of the measurement system during the process. In order to evaluate the PMG irregularity, the vibration component needs to be separated from the overall signal. In this paper, through the synchronous measurement of the Hall sensor and accelerometer, the vibration separation method of the PMG irregularity measurement is simplified to the optimal filter estimation. The particle swarm optimization (PSO) algorithm is used to calculate the parameters of the optimal filter based on the correlation coefficient criterion. The effectiveness of the proposed method is verified by the test. The separated signal is more accurate in both time domain and frequency domain for the characterization of PMG irregularity.

Design of robust active suspension system for performance improvement and vibration control of railway vehicles: An H∞andμ-synthesis perspective

The ride comfort of a high-speed railway vehicle is a significant factor in assessing its performance. Despite having sound suspension systems and efficient control algorithms, the suspension system performance deteriorates with time. Hence, in this paper, a robust active vibration control system used in railway vehicles has been proposed. For analysis, a 27-degree of freedom (DOF) model consisting of structured and unstructured uncertainties has been considered. Two different robust control techniques, [Formula: see text] and [Formula: see text]-synthesis integrated with the Kalman estimator, have been adopted. The robust stability and the robust performance of the proposed control strategies have been evaluated using the structured singular values. The time and frequency domains of the closed loop perturbed responses to random track disturbances are shown. The root mean square (RMS) values of the accelerations of the car bodies are used to compare the suggested control schemes to the passive system. The primary focus of this study is to examine the level of ride comfort provided by the railway vehicle. To achieve this, simulated findings are compared and verified against experimental data collected and reported by the Research Designs and Standards Organization (RDSO). The observational data demonstrates that the results obtained from the suggested control methods are in close agreement with the experimental findings, with a minimal disparity ranging from 2.36% to 8.81% for lateral motion and 2.84% to 6.30% for vertical motion. Also, the % improvement of RMS values with [Formula: see text]-synthesis controller confirms the enhancement of the ride comfort as compared to other techniques.

Global insights on flood risk mitigation in arid regions using geomorphological and geophysical modeling from a local case study

This research provides a comprehensive examination of flood risk mitigation in Saudi Arabia, with a focus on Wadi Al-Laith. It highlights the critical importance of addressing flood risks in arid regions, given their profound impact on communities, infrastructure, and the economy. Analysis of morphometric parameters ((drainage density (Dd), stream frequency (Fs), drainage intensity (Di), and infiltration number (If)) reveals a complex hydrological landscape, indicating elevated flood risk. due to low drainage density, low stream frequency, high bifurcation ratio, and low infiltration number. Effective mitigation strategies are imperative to protect both communities and infrastructure in Wadi Al-Laith. Geophysical investigations, using specialized software, improve the quality of the dataset by addressing irregularities in field data. A multi-layer geoelectric model, derived from vertical electrical sounding (VES) and time domain electromagnetic (TDEM) surveys, provides precise information about the geoelectric strata parameters such as electrical resistivity, layer thicknesses, and depths in the study area. This identifies a well-saturated sedimentary layer and a cracked rocky layer containing water content. The second region, proposed for a new dam, scores significantly higher at 56% in suitability compared to the first region’s 44%. The study advocates for the construction of a supporting dam in the second region with a height between 230 and 280 m and 800 m in length. This new dam can play a crucial role in mitigating flash flood risks, considering various design parameters. This research contributes to flood risk management in Saudi Arabia by offering innovative dam site selection approaches. It provides insights for policymakers, researchers, and practitioners involved in flood risk reduction, water resource management, and sustainable development in arid regions globally.

Polynomial modal method for crossed slanted gratings

Slanted gratings have emerged as a promising area of research due to their distinct properties, such as polarization control, beam steering, and enhanced interactions between light and matter. However, accurately and efficiently modeling these structures, particularly in the case of two-dimensional (2D) slanted gratings, has proven to be challenging. Traditional methods like the Fourier modal method (FMM or RCWA) and finite difference time domain (FDTD) are commonly used but involve approximations of the geometry to accommodate the slant effect. In this study, we address these challenges by employing the polynomial modal method (PMM) for 2D slanted gratings, which, to our knowledge, is a novel approach not previously explored for this type of grating. We introduce a 2D slanted coordinate system to rigorously handle the grating profile. For 2D slanted gratings, the PMM offers several advantages over the FMM, as it overcomes limitations associated with factorization rules and/or staircase approximation of the profile.

Design of two-loop FOPID-FOPI controller for inverter cart-pendulum system

Abstract The inverted cart-pendulum system (ICPS) consists in having a pendulum
mounted on a sliding cart, with the pivot point fixed. This real time experiment
indeed looks like a rocket and its functionality is akin to a rocket. These are the
launchers and the missile guidance and control as well as construction anti-seismic
measures also. The control aim in these systems is to maintain the inverted pendulum
vertically stable. The system is causal but unstable and, therefore, has no
minimum phase. Therefore, the right half plane pole and zero are close to each
other. Therefore, the stability of the system can be considered as problematic
at some points. Unfortunately, linear time- invariant (LTI) classical controllers
are incapable of offering suffient loop robustness for such systems. This paper
aims to project a two-loop fractional order controller (2-LFOC) design to stabilize
a higher-order nonlinear ICPS. The modeling, linearization, and control
of ICPS are demonstrated in this work. The control target is adjusted so that
the inverted pendulum stabilizes in its upright state when the cart reaches the
required point. To fulfill the control objective, two-loop FOPID-FOPI controllers
are proposed, and the Levenberg Marquardt Algorithm (LMA) is utilized to
tune the controllers. A novel nonlinear integral of time-associated absolute-error
(ITAE) based fitness formula considering the settling time and rise time is used
to fit the controller parameters for 2-LFOC. A performance comparison with the
PID controller in terms of different time domain parameters such as risetime(TR),
peaktime(TP ), settlingtime(TS), maximum overshoot (OSM), maximum undershoot
(USM) and steady-state error (ESS) are investigated. Stability analysis
using Riemann surface observation of the system compensated with the proposed
controller is presented in this work. The robust behavior of the two-loop FOPIDFOPI
controller is verified by the application of disturbances in the system and
the Reimann surface observation.

Heterodyne coherent detection of the electric field temporal trace emitted by frequency-modulated comb lasers

Frequency-modulated (FM) combs are produced by mode-locked lasers in which the electric field has a linearly chirped frequency and nearly constant amplitude. This regime of operation occurs naturally in certain laser systems and constitutes a valuable alternative to generate spectra with equidistant modes. Here, we use a low-noise fs-pulse comb as the local oscillator and combine dual comb heterodyne detection with time domain analysis of the multi-heterodyne signal to reveal the temporal trace of both amplitude and phase quadratures of FM comb lasers’ electric field. This technique is applied to both a dense and a harmonic mid-infrared free-running quantum cascade laser frequency comb and shows direct evidence of the FM behavior together with the high degree of coherence of these sources. Our results furnish a deeper insight on the origin of the FM combs and pave the way to further improvement and optimization of these devices.

Testing for finite variance with applications to vibration signals from rotating machines

In this paper we propose an algorithm for testing whether the independent observations come from finite-variance distribution. The preliminary knowledge about the data properties may be crucial for its further analysis and selection of the appropriate model. The idea of the testing procedure is based on the simple observation that the empirical cumulative even moment (ECEM) for data from finite-moments distribution tends to some constant whereas for data coming from heavy-tailed distribution, the ECEM exhibits irregular chaotic behavior. Based on this fact, in this paper we parameterize the regular/irregular behavior of the ECEM and construct a new test statistic. The efficiency of the testing procedure is verified for simulated data from three heavy-tailed distributions with possible finite and infinite variances. The effectiveness is analyzed for data represented in time domain. The simulation study is supported by analysis of real vibration signals from rotating machines. Here, the analyses are provided for data in both the time and time-frequency domains.

Temporal multilayer structures in discrete physical systems towards arbitrary-dimensional non-Abelian Aharonov-Bohm interferences

Temporal modulation recently draws great attentions in wave manipulations, with which one can introduce the concept of temporal multilayer structure, a temporal counterpart of spatially multilayer configurations. This kind of multilayer structure holds temporal interfaces in the time domain, which provides additional flexibility in temporal operations. Here we take this opportunity and propose to simulate a non-Abelian gauge field with a temporal multilayer structure in the discrete physical system. Two basic temporal operations, i.e., the folding/unfolding operation and the phase shift operation are used to design such a temporal multilayer structure, which hence can support noncommutative operations to realize the non-Abelian Aharonov-Bohm interference in the time domain. A two-/three-dimensional non-Abelian gauge field can be built, which may be further extended to higher dimensions. Our work therefore provides a unique platform enabling generalization of non-Abelian physics to arbitrary dimensions and offers a method for wave manipulations with temporal band engineering.

A new frequency domain method to solve the potential flow problem

The potential flow theory is a reliable tool widely-used in ocean engineering to solve the fluid hydrodynamic problems such as the wave hydrodynamic problems. The Rankine source and its image about the seabed is often chosen as the Green's function to solve the potential flow problems for its simple form, easy singular-integral treatment and few boundary conditions in the boundary integral equations. However, traditionally, the boundary element method based on it is only executed in the time domain by time stepping technique which requires lots of computation due to the time domain form of the damping zone's boundary condition. To apply this Green's function in frequency domain, this paper derived a parameterized frequency-domain formula for the boundary condition of the damping zone, and successfully applied the Rankine source and its image about the seabed in frequency domain. This paper's work possesses a great potential to promote the efficiency in solving the potential flow problems.

Femtosecond Terahertz Pulse Propagation Measurements and Simulations of Dewetting Kinetics in Real Time.

We report a terahertz time-domain study of dewetting kinetics on two time scales: femtoseconds and in real time (on the order of minutes). We recorded full electric field terahertz time domain signals with femtosecond time resolution during dewetting of water in cellulose. The femtosecond time-domain signals were analyzed with respect to the amplitude and signal emission times and how these two quantities changed over the course of dewetting kinetics. The femtosecond time-domain signals were modeled by a combination of finite-difference time-domain electromagnetic simulations of linear terahertz pulse propagation and an effective medium description of the dielectric permittivity. A logistic regression mechanism was incorporated into the electrodynamics simulations to account for the observed kinetics. In addition, analysis of the area-normalized, real-time time-dependent terahertz spectra was used to identify broad regions in the terahertz spectral range that correlated with the kinetic process. Real-time two-dimensional terahertz correlation maps were used to identify the pure rotational spectrum of water vapor, thus characterizing the evaporation part of the dewetting kinetics problem. We conclude with a kinetic model that accounts for our observations through a microscopic mechanism involving the interconversion among bulk water, water clusters, and individual water molecules in the gas phase. Overall, the approach presented here illustrates an application of femtosecond time-resolved experiments in the terahertz spectral range to the study of kinetic processes.

Root phenotype detection of rice seedling under nitrogen conditions based on terahertz imaging technique

Root phenotype detection is the basis for screening of dominant root and improving of seed breeding. The efficiency of root phenotype detection is restricted by the concealment and complexity of crop roots. In this paper, a new method based on terahertz imaging technology was proposed to detect the phenotypic characteristics of rice root and quantitatively predict root nitrogen content. First, the terahertz imaging spectral data of rice root were preprocessed with de-overlapping, reconstruction, enhancement, and segmentation. Then, the phenotypic characteristics of root length, diameter and surface area of rice roots were extracted according to the refinement algorithm. In addition, three linear regression models were fitted between root phenotype and root nitrogen content. Finally, Convolutional Neural Network (CNN), Genetic Algorithm-Back Propagation Neural Network (GA-BPNN), and Sparrow Search Algorithm-Support Vector Regression (SSA-SVR) were used to predict nitrogen content in rice roots by training and testing terahertz time domain data. Compared with two kinds of root phenotypic analysis software, the average errors of root length, diameter and root surface area calculated in this paper were 10.68 %, 7.29 % and 3.49 %, respectively. The fitting determination coefficients between root length, root surface area and the root nitrogen content were 0.88 and 0.87 after removing three groups of high-nitrogen data and one group of contrast data. The prediction accuracy of SSA-SVR model was 0.99 and the root mean square error was 0.05. Studies show that terahertz imaging technique can provide an effective analytical way for qualitative analysis of root phenotype.

Low-loss and low-cross talk polarization-insensitive multimode silicon waveguide crossing.

A polarization-insensitive multimode silicon waveguide crossing is investigated and experimentally characterized in this Letter. By employing the particle swarm optimization (PSO) algorithm and finite difference time domain (FDTD) method, the lengths and widths of the waveguides in the proposed device are optimized for attaining wide bandwidth, small insertion loss (IL), low cross talk (CT), and compact size. Measurement results reveal that the footprint of the presented device is 11.92 μm × 11.92 μm. From 1520 to 1600 nm, the measured insertion loss and cross talk are smaller than 0.67 dB and -28.6 dB in the case of the TE0 mode, lower than 0.65 dB and -28.7 dB in the case of the TE1 mode, less than 0.48 dB and -36.3 dB in the case of the TM0 mode, and lower than 0.62 dB and -28 dB in the case of the TM1 mode.

Static and dynamic stabilities of modified gradient elastic Kirchhoff–Love plates

The static and dynamic stabilities of modified gradient elastic Kirchhoff–Love plates (MGEKLPs), which incorporate two length-scale parameters related to strain gradient and rotation gradient effects, are comprehensively analyzed under various load forms and boundary conditions (BCs). The study of static stability employs static balance method and an improved energy method by introducing higher-order deformation gradients and corresponding energy terms. Utilizing the variational method, a sixth-order fundamental buckling differential equation for MGEKLPs under both transverse and in-plane loads is derived, serving as the foundation for the static balance method. The static stability analysis of MGEKLPs examines the combined effects of strain and rotation gradients on size-dependent critical buckling loads. Building on generalized strain energy with higher-order deformation energy, the energy method of classical elastic thin plate model is enhanced and applied to the static stability analysis of MGEKLPs. This approach enables the investigation of static stability without being constrained by the need to solve complex differential equations, making it applicable to various BCs and load scenarios. While static stability provides a description of stable state of an elastic system, dynamic stability offers a more scientific and rigorous analysis. The dynamic stability of simplified gradient elastic Kirchhoff–Love plates (SGEKLPs) with curved edges and different BCs is further investigated by combining the generalized strain energy with Lyapunov’s second stability method, presenting the dynamic stability criterion in the form of norms. A strict description of the dynamic stability of a SGEKLP over the entire time domain is provided for different supporting conditions, including case where all edges are supported and case with free edges. The analysis of size-dependent static and dynamic stabilities offers theoretical guidance for designing elastic thin plates with microstructures.

Coherent Feature Extraction with Swarm Intelligence Based Hybrid Adaboost Weighted ELM Classification for Snoring Sound Classification.

For patients suffering from obstructive sleep apnea and sleep-related breathing disorders, snoring is quite common, and it greatly interferes with the quality of life for them and for the people surrounding them. For diagnosing obstructive sleep apnea, snoring is used as a screening parameter, so the exact detection and classification of snoring sounds are quite important. Therefore, automated and very high precision snoring analysis and classification algorithms are required. In this work, initially the features are extracted from six different domains, such as time domain, frequency domain, Discrete Wavelet Transform (DWT) domain, sparse domain, eigen value domain, and cepstral domain. The extracted features are then selected using three efficient feature selection techniques, such as Golden Eagle Optimization (GEO), Salp Swarm Algorithm (SSA), and Refined SSA. The selected features are finally classified with the help of eight traditional machine learning classifiers and two proposed classifiers, such as the Firefly Algorithm-Weighted Extreme Learning Machine hybrid with Adaboost model (FA-WELM-Adaboost) and the Capuchin Search Algorithm-Weighted Extreme Learning Machine hybrid with Adaboost model (CSA-WELM-Adaboost). The analysis is performed on the MPSSC Interspeech dataset, and the best results are obtained when the DWT features with the refined SSA feature selection technique and FA-WELM-Adaboost hybrid classifier are utilized, reporting an Unweighted Average Recall (UAR) of 74.23%. The second-best results are obtained when DWT features are selected with the GEO feature selection technique and a CSA-WELM-Adaboost hybrid classifier is utilized, reporting an UAR of 73.86%.

Cardiac autonomic responses in relation to cognitive workload during simulated military flight

Understanding the operator's cognitive workload is crucial for efficiency and safety in human–machine systems. This study investigated how cognitive workload modulates cardiac autonomic regulation during a standardized military simulator flight. Military student pilots completed simulated flight tasks in a Hawk flight simulator. Continuous electrocardiography was recorded to analyze time and frequency domain heart rate variability (HRV). After the simulation, a flight instructor used a standardized method to evaluate student pilot's individual cognitive workload from video-recorded flight simulator data. Results indicated that HRV was able to differentiate flight phases that induced varying levels of cognitive workload; an increasing level of cognitive workload caused significant decreases in many HRV variables, mainly reflecting parasympathetic deactivation of cardiac autonomic regulation. In conclusion, autonomic physiological responses can be used to examine reactions to increased cognitive workload during simulated military flights. HRV could be beneficial in assessing individual responses to cognitive workload and pilot performance during simulator training.

Reconstruction of nonstationary sound field based on group sparsity time domain equivalent source method

To improve the accuracy and stability of reconstruction of nonstationary sound field, a novel time domain equivalent source method based on non-convex overlapping group sparsity (NOGS-TESM) is proposed. In the proposed NOGS-TESM, the time-varying pressure in the near field is first modeled by a set of equivalent sources positioned inside the vibrating structure. Then, the NOGS regularization is applied for solving the inverse problem, and a novel algorithm derived on the basis of Majorize-Minimization theory is utilized to obtain the optimal solution of equivalent sources strengths. Finally, the optimal equivalent sources strengths at all times are substituted into a time-domain convolution to calculate the time-varying pressure of reconstruction surface. The proposed NOGS-TESM enriches the prior information by considering the group sparsity of spatial pressure distribution, and enhances the accuracy of estimated equivalent sources strength via its alternative calculation of l0−norm. A vibrating plate to produce a nonstationary sound field, which is designed as a numerical simulation, and the time-varying pressures of measurement surface are input into the proposed NOGS-TESM. The quantitative and qualitative comparisons confirm that the proposed NOGS-TESM can greatly reconstruct the non-stationary sound field in time–space domains, and its reconstruction accuracy is higher than that of TESM with Tikhonov regularization (TR-TESM). Meanwhile, several parameters, for example, the noise and position of equivalent source surface, are discussed in detail, and the superiorities of the proposed NOGS-TESM in terms of stability and robustness are proved by comparing to TR-TESM. The predominance of the proposed NOGS-TESM is further validated by conducting one experiment on the vibrating steel plate.

Vibration control of the straight bladed vertical axis wind turbine structure using the leading-edge protuberanced blades

This study investigates the impact of structural responses resulting from alterations in aerodynamic characteristics caused by incorporating leading-edge protuberance (LEP) blades in vertical-axis wind turbines (VAWTs) under various wind speeds. The current experimental investigation was conducted on a scaled-down straight-blade VAWT with multiple wind speeds at a fixed pitch angle and uniform blade configuration. The unsteady structural excitation caused by the decay of vortices due to the different aerodynamic loads and its effect on the VAWT structure is examined in great detail. The presence of counter-rotating vortices from the LEP profile, however, enhances the aerodynamics of the blades at higher angles of attack. It is crucial to understand the structural characteristics resulting from changes in aerodynamics. The paper presents the effects of the LEP on the structural excitation of the VAWT by analyzing the acceleration and displacement data obtained from the static structure of VAWT for various operational wind speeds. The results showed decreased acceleration and displacement amplitudes for the VAWT with LEP at medium and higher wind speeds. Significant aerodynamic and structural damping was observed in the VAWT with LEP blades. The results obtained were in close agreement with the time and frequency domains of the measured displacement and acceleration from the structure. The bidirectional vibration control was observed for the VAWT with LEP, which was found to have a reduction ratio of 70 %.