Time-domain Expressions Research Articles

Integrated design optimization of actuator layout and structural ply parameters for the dynamic shape control of piezoelectric laminated curved shell structures

Compared with static shape control, dynamic shape control is more difficult due to its time-varying nature. To improve the control precision and reduce the number of actuators needed, an integrated design optimization model of structure and control is proposed. The following three sets of design variables are considered to minimize the time domain variance between the controlled and desired dynamic shapes: the time-varying actuator voltages, the actuator layout and the structural ply parameters. To satisfy the engineering performance requirements to the greatest extent possible, constraints on the structural mass, the energy, the maximum transient voltage and the number of actuators are considered. Because the control voltages are varying in time, a two-level optimization strategy is adopted. In the inner optimization problem, the Newmark integral method is applied to derive discrete time domain expressions for the shape control equations, and then, the Kuhn-Tucker condition is introduced to calculate the optimal time-varying voltage distribution. To solve the outer optimization problem, because of the coexistence of discrete variables and continuous variables, a simulated annealing algorithm is used. To address the shape control of complicated curved shells, a finite element formulation for an eight-node laminated curved shell element with piezoelectric actuators is derived. Numerical examples show that the proposed integrated design optimization method can significantly improve the control effect and that the optimization of the structural ply parameters plays an important role. Moreover, the control system can be simplified by taking the minimum number of actuators as the objective function when the control accuracy allows.

Linear Equivalent Model for VHF Class Φ2 Inverter Based on Spectrum Quantification Method to Reduce GaN Reverse Conduction Loss

Reverse conduction loss of GaN high electron mobility transistors (HEMTs) in very high frequency (VHF) converters is non-neglectable due to the absence of body diode. To reduce the reverse conduction time, this paper proposes a linear equivalent model for class <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi _{2}$ </tex-math></inline-formula> inverter to derive optimal duty cycles for different working conditions. The proposed model is derived from frequency-domain perspective, which simplifies the derivation process and provides an intuitive physical insight for class <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi _{2}$ </tex-math></inline-formula> inverter. Firstly, the power switch is modeled as a current source according to time-domain expressions of inductor current, which simplifies the circuit to a linear network. Based on quantitative analysis of the current source spectrum characteristics, the linear network response to the current excitation is derived, which forms the linear equivalent model. Then, drain voltage of the main switch under different working conditions is obtained. With the zero-crossing point of the drain voltage, numerical solutions of the optimal duty cycles are calculated. Finally, the optimal duty cycles are implemented with a high-resolution duty cycle generation circuit in a 27.12MHz prototype, which achieves not only a peak efficiency of 93.6% at full load, but also higher efficiency over the whole load and input voltage range compared to conventional fixed duty cycle.

Fast evaluation of lightning electromagnetic fields based on matrix pencil method in time domain

AbstractThe time domain expression of the lightning electromagnetic fields (LEMF) contains multiple integration processes, and the direct solution tends to take up a lot of calculation time. In this paper, the structure of the time domain expression of the LEMF is analyzed, the idea that using the matrix pencil method to realize the fast evaluation of the LEMF is proposed, and the feasibility of this method for calculating the LEMF is discussed. Besides, several numerical examples are given to prove the efficiency and correctness of the proposed method when the parameters of the observation point and ground are varied.

An improved zero-voltage zero-current transition boost converter employing L-C-S resonant network

An improved zero-voltage zero-current transition boost converter (IZVZCTBC) is introduced. This converter is basically a fourth-order DC-DC converter wherein a L-C-S (Inductor–Capacitor–Switch) resonant circuit is embedded for soft-switching. L-C-S tank network is the modified version of conventional ZVZCT switch cell. The main feature of L-C-S tank circuit is to enhance the performance of zero-voltage zero-current transition boost converter in terms of eliminating the high current stress, decreasing the switching losses and increasing the efficiency of converter. This converter exhibits both zero-voltage turn on and zero-current turn off switching characteristics based on the gating signals applied to switches. The principle of operation and time domain expressions of IZVZCT boost converter with L-C-S cell are presented. For the closed loop operation, digital controller is designed and the performance of the controller has been validated through simulation for different line and load variations. The mathematical and theoretical analysis is verified accurately by a 12-24 V, 30 W converter through PSIM simulation software and the results ensures that overall efficiency of the converter has improved to 97% along with elimination of current stress.

Analytical Expressions for Lightning Electromagnetic Fields With Arbitrary Channel-Base Current—Part I: Theory

The article provides analytical expressions for the electromagnetic fields generated by a lightning return stroke characterized by a channel-base current with arbitrary time waveform, in presence of either a perfectly conducting or a lossy ground, assuming the transmission line model for the current along the channel. In this second case, a time-domain analytical expression for the Cooray-Rubinstein formula is presented. The main idea that leads to the derivation of analytical formulas consists of dividing the channel into intervals in which the distance between the field source point and the observation point can be approximated with a linear function of the time, and of the spatial coordinates of both points. In the companion aticle, a detailed comparison is proposed with the classical (numerical) approach highlighting an excellent agreement both at close and far distances, considering all the values of practical interest for the ground conductivity. Moreover, the method guarantees a meaningful improvement in computational performance.

Extending SFR model to incorporate the influence of thermal states on primary frequency response

Due to the increasing penetration of variable renewable energies, the primary frequency response (PFR) capability of thermal power units is becoming more and more significant for power systems to tackle abrupt active power disturbances. Moreover, the actual thermal states of boilers that are usually assumed to keep rated and constant may degrade PFR capability, and incur unexpected severer frequency deviations and even instability. This study proposes an extended system frequency response (ESFR) model incorporating the influence of thermal states on PFR. The ESFR model is developed by extending the system frequency response (SFR) model with a boiler thermodynamic sub-model, of which a low-order and linear time-domain expression is derived. Then, a quantitative assessment framework is established to assess the influence of thermal states on PFR capability, in which the instantaneous and accumulative quantities are calculated via the expressions of the ESFR and SFR models. Furthermore, based on the developed ESFR model, an improved estimation approach of dynamic frequency response considering thermal states is provided for the preliminary application. Case studies are conducted to validate the proposed model and approach.

A New Calculation Method of the Lightning Electromagnetic Field Considering Variable Return Stroke Velocity

The article proposes an improved method for the calculation of the electromagnetic field radiated by a lightning return stroke over a perfectly conducting ground. Specifically, the proposed time domain expressions allow a closed-form solution for the field produced by an impulse-current propagating upward along a vertical path with a given velocity. The expressions are valid for a wide class of engineering return stroke models and can be adopted also in case the return stroke velocity varies along the channel. The solution for an arbitrary current waveform can be obtained by the convolution of the current at the channel base with the electromagnetic field due to the impulse-current along the channel. The accuracy and advantages of the method are assessed by the comparison with those obtained by a finite-element method computer model. The expressions are implemented in the LIOV-EMTP-RV code for the calculation of the induced voltages in an overhead distribution line.

Inversion Method in Pulsed Eddy Current Testing for Wall Thickness of Ferromagnetic Pipes

Based on time-domain analytical expression to pulsed eddy current field of a ferromagnetic pipe, a least squares inverse problem, between the calculated and measured values of induced voltage, is established to determine the wall thickness, conductivity, and relative permeability. And then, the coupling relationship among parameters in the inverse problem is investigated. Furthermore, the inverse problem is reestablished and the method for calculating the relative variation of wall thickness is proposed via taking the ratio of the inversion results between two testing points. Finally, the experiment results on a steel tube indicate that inversion results are independent on the initial value, and can effectively suppress the influence of the magnetic permeability variation, thus the reliability of the thickness testing get improved. Based on the above, a noncontact pulsed eddy current testing method is presented for measuring the relative wall thickness of ferromagnetic pipes. It can be used for nondestructive evaluation of wall-thinning corrosion of in-service coated ferromagnetic tube.

Nonlinear and Transient Stability Analysis of Phase-Locked Loops in Grid-Connected Converters

The undesired nonlinear operation of phase-locked loops (PLL) is one of the main causes of transient instability in grid-connected converters. However, the stable operating region of PLL, knowing which is helpful for protection purposes, is hard to find due to the nonlinear characteristic of the transient process. In this article, the nonlinear characteristic of the PLL control loop relevant to the grid-connected converter operation is identified. The averaging method is applied to derive a time-domain expression for the PLL operation under disturbance. Based on the analytical expression, the transient response and related stability criterion are established, and the PLL design is improved while targeted to focus on various ac grid faults or disturbances. The results, applicable to transient stability enhancement, are verified by circuit simulations and experiments.

Study on Vibration Characteristics of Polishing Rod for Polishing Aeroengine Blade with Abrasive Cloth Wheel

The vibration of grinding tools is very essential to the performance of parts and the safety of machine tools. However, the vibrationis rarely studied in the process of polishing aeroengine blade with abrasive cloth wheel. Polishing force is measured by dynamometer in the process of polishing aeroengine blade with abrasive cloth wheel, and the time-domain expressions of polishing force are obtained. Time-domain and frequency-domain vibration solutions of position at which the abrasive cloth wheel is installed under the coupling of axial force and normal force are obtained by the simulation method. The results show that the total displacement is approximately equal to the displacement in the normal direction. The frequency-domain characteristics of axial force and normal force are similar. The frequency-domain characteristics of vibration in x, y, and z directions are similar. The component frequencies of polishing force are not the same as the component frequencies of rod vibration, which means the vibration is not stable. The range of spindle speed to avoid resonance in polishing process is calculated.

Synaptic Channel Modeling for DMC: Neurotransmitter Uptake and Spillover in the Tripartite Synapse

In Diffusive Molecular Communication (DMC), information is transmitted by diffusing molecules. Synaptic signaling, as a natural implementation of this paradigm, encompasses functional components that, once understood, can facilitate the development of synthetic DMC systems. To unleash this potential, however, a thorough understanding of the synaptic communication channel based on biophysical principles is needed. Since synaptic transmission critically depends also on non-neural cells, such understanding requires the consideration of the so-called tripartite synapse. In this paper, we develop a comprehensive channel model of the tripartite synapse encompassing a three-dimensional, finite-size spatial model of the synaptic cleft, molecule uptake at the presynaptic neuron and at glial cells, reversible binding to individual receptors at the postsynaptic neuron, and spillover to the extrasynaptic space. Based on this model, we derive analytical time domain expressions for the channel impulse response (CIR) of the synaptic DMC system and for the number of molecules taken up at the presynaptic neuron and at glial cells, respectively. These expressions provide insight into the impact of macroscopic physical channel parameters on the decay rate of the CIR and the reuptake rate, and reveal fundamental limits for synaptic signal transmission induced by chemical reaction kinetics and the channel geometry. Adapted to realistic parameters, our model produces plausible results when compared to experimental and simulation studies and we provide results from particle-based computer simulations to further validate the analytical model. The proposed comprehensive channel model admits a wide range of synaptic configurations making it suitable for the investigation of many practically relevant questions, such as the impact of glial cell uptake and spillover on signal transmission in the tripartite synapse.

Horizon instability of the extremal BTZ black hole

We study real-time propagation of a massive scalar field on the extremal BTZ black hole spacetime, focusing on the Aretakis instability of the event horizon. We obtain a simple time-domain expression for the AdS3 retarded Green function with Dirichlet boundary conditions and construct the corresponding time-domain BTZ retarded Green function using the method of images. The field decays at different rates on and off the horizon, indicating that transverse derivatives grow with time on the horizon (Aretakis instability). We solve the null geodesic equation in full generality and show that the instability is associated with a class of null geodesics that orbit near the event horizon arbitrarily many times before falling in. In an appendix we also treat the problem in the frequency domain, finding consistency between the methods.

Pulsed Eddy Current Testing for Electromagnetic Parameters of a Spherical Conductor

This paper provides a pulsed eddy current testing method for electromagnetic parameters of a ferromagnetic spherical conductor. Firstly, the frequency domain solution of an eddy current field to the conductor is obtained by solving the electromagnetic boundary value problem. And then the time domain expression to the induced voltage across the pickup coil excited by a pulsed current in the drive coil is calculated by inverse Laplace transform based on residue theorem. Secondly, a least squares inverse problem, between the measured and analytical values of induced voltage, is established to determine the conductivity and relative permeability of the steel ball. And the coupling relationship between the two parameters of the steel ball is explored. Thirdly, the amplitude of the excitation current is changed to study the effect on the parameter inversion results caused by detection conditions. Finally, the time domain analytical formula and the parameters inversion method are verified, by comparing the theoretical induced voltage and conductivity inversion results with experimental voltage and conductivity test values by four-point method respectively.

Analytic formulation and optimization of weighted total harmonic distortion in single-phase staircase modulated multilevel inverters

Weighted total harmonic distortion (WTHD) is an important performance merit, representing either current or voltage quality of voltage source inverters. Unlike general (non-weighted) THD, which can be evaluated using a closed-form analytic time-domain expression, WTHD is typically calculated using approximate frequency-domain numerical approaches. The paper reveals a novel analytic WTHD expression for output voltage of Staircase Modulated (SCM) single-phase Multi-Level Inverters (MLI). The proposed formula, valid for SCM MLI of arbitrary topology with any number and parity of voltage levels, is a function of all switching angles (SA) only and may thus be easily implemented in common symbolic/numeric computing software. Due to its analytical nature, the WTHD expression yields highly accurate results, eliminating the risk of round-off and underestimation errors, associated with conventional frequency-based numerical approaches. Furthermore, based on the proposed formula, Optimal Minimization of WTHD was carried out for accurate calculation of optimum SA, yielding minimum WTHD for desired modulation index. Eight sets of high-resolution (1171 samples per level) look-up tables, linking desired modulation index to optimum SA values, were generated. A downloadable file, containing MATLAB coded WTHD functions and the eight look-up tables, are provided as a shortened URL. In order to verify the proposed expression, analytic WTHD values are compared with results obtained in previous works. In addition, the methodology was validated utilizing Processor-in-Loop simulation of 7 and 8 level SCM MLI.

Optimization of Extended Phase-Shift Control for Full-Bridge CLLC Resonant Converter With Improved Light-Load Efficiency

The capacitor–inductor–inductor–capacitor (CLLC) resonant converters are drawing more and more attention for their superiority in soft switching, wide output range, and symmetrically bidirectional operation. However, CLLC converter still suffers the problems of unsatisfactory voltage regulation and low efficiency under light-load conditions. Therefore, this article proposes an optimization for extended phase-shift (EPS) control in full-bridge CLLC resonant converter to improve the light-load efficiency. In order to study the relations between phase-shifts of EPS control and converter efficiency, a detailed circuit model is first established to solve the voltage gain, time-domain expressions of main circuit variables, and root mean square (rms) values of resonant currents. Then, a comprehensive loss evaluation is conducted by calculating and analyzing the main power losses, including conduction loss, switching loss, and core losses of magnetic components. Besides, zero voltage switching conditions of primary and rectifier switches are derived to define the range of soft switching. Finally, based on these analyses, the optimal combination of phase-shifts for EPS control is determined to achieve the maximum conversion efficiency of full-bridge CLLC converter at light-load conditions. The validity of the proposed optimized EPS control is verified on a 18∼25 V/400 V, 200 W GaN-based CLLC resonant converter prototype.

Design Method of 6-Element Boundary Gain for LLC Resonant Converter of Electric Vehicle

In the application of on-board chargers, LLC resonant converters need to work in multiple operating modes to achieve wide-range output requirements. For the most commonly used single-phase and three-phase LLC topologies, the time domain expressions of all possible operating modes are introduced to predict voltage and current behaviors. Based on the mode analysis, the 6-element boundary gain design method is proposed to design a wide gain range LLC resonant converter. The resonant current is used as the basis for ZVS judgment and the transformation trend of ZVS in the full operating range is given. Through reasonable distribution of the boost region and the buck region, the resonant devices can be designed more appropriately to achieve lower conduction loss and higher efficiency. Finally, the prototypes of 3.3kW and 6.6 kW are built to verify the theoretical analysis of the method in single-phase LLC and three-phase LLC.

Analysis Methodology for Differential Mode Interference in Energy Supply System of Hybrid DC Breaker

High di/dt during the close-open operation process of hybrid dc breaker will cause differential mode interference (DMI) in the energy supply system of the breaker's driver units due to their inductive coupling. The generated DMI may lead to false action of the breaker, which is an important issue that should be carefully considered in design stage. A method to analyze DMI based on time-domain wideband circuit simulations is proposed in this article. First, a time-domain waveform expression of DMI voltage source is derived, and then wideband equivalent circuit models of major components in the energy supply system are established. Furthermore, a standardized design methodology to reduce DMI is suggested. Applying the method to a high-frequency energy supply system of a ±200 kV hybrid dc breaker, simulation results show that the DMI level for two DMI mitigation measures is reduced by about 90% and 80% of the initial value, respectively.

PML and CFS-PML boundary conditions for a mesh-free finite difference solution of the elastic wave equation

Mesh-free finite difference (FD) methods can improve the geometric flexibility of modeling without the need for lattice mapping or complex meshing process. Radial-basis-function-generated FD is among the most commonly used mesh-free FD methods and can accurately simulate seismic wave propagation in the non-rectangular computational domain. In this paper, we propose a perfectly matched layer (PML) boundary condition for a mesh-free FD solution of the elastic wave equation, which can be applied to the boundaries of the non-rectangular velocity model. The performance of the PML is, however, severely reduced for near-grazing incident waves and low-frequency waves. We thus also propose the complex-frequency-shifted PML (CFS-PML) boundary condition for a mesh-free FD solution of the elastic wave equation. For two PML boundary conditions, we derive unsplit time-domain expressions by constructing auxiliary differential equations, both of which require less memory and are easy for programming. Numerical experiments demonstrate that these two PML boundary conditions effectively eliminate artificial boundary reflections in mesh-free FD simulations. When compared with the PML boundary condition, the CFS-PML boundary condition results in better absorption for near-grazing incident waves and evanescent waves.

ENF Signal Enhancement in Audio Recordings

In electric network frequency (ENF) based audio forensics, the ENF signal captured in a questioned audio recording is estimated and analyzed for authentication purposes. However, the captured ENF signal is usually contaminated by very strong noise and interference. In this paper, we propose a robust filtering algorithm (RFA) for ENF signal enhancement in audio recordings, which could effectively suppress the additive noise and facilitate subsequent ENF estimation, especially in practical low signal-to-noise ratio (SNR) situations. The proposed algorithm encodes the time domain expression of the preprocessed audio signal (ENF signal plus noise) as the instantaneous frequencies (IFs) of an analytical sinusoidal frequency modulated (SFM) signal. Then, a kernel function is utilized to generate a sinusoidal time-frequency distribution (STFD) whose peaks correspond to the IFs of the analytical signal, i.e., the denoised ENF signal. It is then proven that finding the STFD peaks is equivalent to finding the averaged phases of the kernel function if the additive noise is a zero mean wide sense stationary (WSS) process. The RFA serves as a noise reduction mechanism yielding improved SNR at the filter output. Combined with generic frequency estimation methods, ENF extraction accuracy could be substantially improved with the use of the proposed RFA than without using it. Both synthetic and experimental results are provided to illustrate the effectiveness of our proposal. Reliable ENF extraction could be achieved under noise level down to −20 dB SNR, and the RFA is suitable for a wide range of ENF-based forensic applications.

Time-Domain Electromagnetic-Field Transmission Between Small-Loop Antennas on a Half-Space With Conductive and Dielectric Properties

The pulsed EM-field signal transfer between two co-planar small-loop antennas located on a half-space with dielectric and conductive properties is analyzed analytically with the help of the Cagniard–DeHoop technique and the Schouten–Van der Pol theorem. The analysis yields a closed-form time-domain expression for the open-circuit voltage induced across the ports of the receiving antenna. Limiting cases considering the mutual coupling between two loops placed in free-space and on a dielectric half-space are discussed. The obtained results are validated using analytical expressions for the special cases and with the aid of a 3-D EM computational tool.