Waveform Distortion Research Articles

Source-independent least-squares reverse time migration in vertical transversely isotropic media based on the Student’s t-distribution

Abstract Seismic anisotropy exists in various types of strata and should be considered in seismic imaging schemes. Seismic imaging algorithms based on isotropic assumption neglect the effects of anisotropy on seismic data, which cause migration artifacts and waveform distortion. To correct the effects of anisotropy on seismic wave propagation, we propose an imaging algorithm that performs least-squares reverse time migration in vertical transversely isotropic acoustic media. We derive the following operators to implement this algorithm, the de-migration operator, its adjoint migration operator, and the corresponding gradient. However, an inaccurate estimated source wavelet will introduce error in the seismic simulation, and thus increase the mismatch between observed and synthetic data for least-squares reverse time migration. In addition, the noises, especially the noises with abnormal amplitudes in the seismic data, damage the inversion convergence and reduce the imaging resolution. To improve the image quality, we propose to use convolved wavefields between observed and synthetic data so that such a mismatch can be independent of the source wavelets. Also, we use the Student’s t-distribution instead of L2 norm in our inversion scheme to better handle the seismic noise. Its implementation only modifies the gradient of the conventional least-square reverse time migration scheme. Our numerical tests show a clear improvement using our proposed imaging algorithm when compared with the conventional isotropic migration scheme for the anisotropic data. Also, the synthetic examples demonstrate the feasibility and effectiveness of our proposed source-independent algorithm using the Student’s t-distribution.

Multi-Parameter Optimization of Stator Coreless Disc Motor Based on Orthogonal Response Surface Method

In response to the structural optimization problem of PCB stator coreless disc motors, the orthogonal response surface method was used to optimize the motor structure, preliminarily determine the basic parameters of the motor, and conduct orthogonal experiments on the motor parameters based on the optimization design objectives. The optimization factors were the quantity of the magnetic pole of the motor rotor p, the ratio of the main/auxiliary pole sizes Rnd, the thickness Tm of the permanent magnet, and the air gap length δ. The motor torque Td, the amplitude of the magnetic density of the air gap Bδ, and the waveform distortion rate THD were used as optimization objectives. The motor parameters that cause the motor torque to reach a maximum cause the air gap magnetic density to reach a maximum and the waveform distortion rate to reach a minimum. Due to the use of PCB plates instead of motor stator cores in the PCB coreless disc motors, the service life of the PCB board during motor operation will be reduced with the temperature increase generated by the stator winding. To solve this problem, a response surface analysis of the motor was carried out to reduce the increase in the temperature of the stator windings during the operation of the motor. The PCB board and stator winding are the main factors affecting the motor’s temperature increase. Taking the thickness of the PCB board, the hole diameter on the board, and the uneven width of the stator winding as optimization factors, the motor parameters with the lowest increase in the temperature of the motor winding were obtained. A simulation analysis was conducted using Ansys/Maxwell software, and the results prove the feasibility of the optimization.

A study on the evolution mechanism of small diameter thin-walled stainless steel bellows during a bending process

In order to accurately construct the evolution mechanism of the matrix during the bending process, this paper combines finite element simulation and material characterization methods to study the evolution process of the matrix in detail. Firstly, a repeated bending model of the bellows was constructed by ABAQUS finite element software. The deformation hardening, geometry evolution, stress and strain evolution of the matrix were analyzed in depth. At the same time, by characterizing the microstructure of the trough under different bending times, the evolution of the microstructure during the fracture process was obtained. Combined with the fracture characteristics, it can be seen that the deformation hardening of the trough is significant and the stress is large, so it is most likely to become the failure zone. The waveform distortion increases the deformation inhomogeneity of the trough at different positions, resulting in local large strain accumulation and further aggravating the fracture failure of the trough. During the bending process, a large number of martensite tissue appears in the central area of the bellow wall thickness, and the dislocation gradually moves from the inner and outer surfaces to the center. The crack initiates first at the bellow outer surface and propagates along the circumference and wall thickness direction. Bending four times is the key node of crack propagation. Moreover, the crack propagation models of the trough in the wall thickness and circumferential direction are established, and the fracture failure mechanism of the trough during repeated bending is revealed.

Cosine Windows in Interpolated DFT-based Method for an Accurate High-Frequency Distortion Assessment in Power Systems

The transformation of electrical networks in the context of the new smart grid paradigm unavoidably involves new challenges regarding Power Quality (PQ) disturbances not only for customers but also for all the other involved stakeholders. Among PQ disturbances, waveform distortions have recently gained growing interest due to the massive presence of new technologies in distributed energy resources, in modern loads and in advanced smart metering systems. The presence of these devices determines arduous electromagnetic compatibility problems since the current and voltage waveform distortions in smart grids are characterized by spectral components above the traditional 2 kHz frequency limit, in a range extended up to 150 kHz. In this paper, an interpolated DFT-based (IDFT) method, recently proposed in the relevant literature in the field of signal processing, is properly extended for an accurate and fast assessment of power system waveform distortions in the frequency range from 2 to 150 kHz. Since DFT-based methods can suffer well-known spectral leakage problems, in this paper the IDFT is applied using cosine windows that minimize interference conditions among spectral components and maximise the estimation accuracy of the spectral component amplitude, phase angle and frequency. An optimal number of cosine window terms is also searched to improve the spectral analysis of high-frequency power system waveforms. Numerical applications on synthetic test signals and measured waveforms are carried out to quantify the accuracy and computational efforts of the proposed approach and to select the cosine window terms that better optimize the waveform distortion assessment.

Accurate Spatial-Frequency Response Controllable Frequency Invariant Beampattern Synthesis With Minimum Beamwidth

This communication proposes a new relaxation variable based cost function for the problem of frequency invariant (FI) beampattern synthesis. Two constraints in the spatial and frequency domains are imposed on the optimization problem. The objective of sparsifying the relaxation variable is exploited in constructing the constraint in the spatial domain, and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> -spatial response variation (SRV) is introduced in the frequency domain constraint to reduce frequency variance. These constraints enable a desirable trade-off between beamwidth and FI properties while achieving accurate spatial-frequency response control. Since the resulting optimization problem is sparse and non-convex, it is then proposed to be solved by exploiting the reweighted- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> norm in this paper. The remarked feature of the new approach is the improved target detection and anti-interference performance in radar and communication systems without waveform distortion. Simulation results validate the effectiveness of the proposed method.

Analytical model OFDM-MIMO signal in a radio channel with frequency-time selective fading

One of the main problems in modern wireless electronic communication networks is the transmission and reception of signals in multipath propagation. Due to the imperfect impulse response of the communication channel, frequency-selective fading of the transmitted signal occurs. This can cause waveform distortion and inter-symbol interference, which can seriously degrade the performance of the communication system. Thus, increasing the noise immunity of wireless electronic communication networks in real-world conditions is one of the most important tasks of modern communication technologies. Orthogonal frequency division multiplexing is known for its resistance to multipath fading and is used to reduce the impact of inter-symbol interference. Multi-antenna systems are a key technology for increasing bandwidth and improving communication reliability. The combination of these two technologies in wireless electronic communication networks can reduce the impact of negative phenomena on the transmitted signal in difficult interference conditions. An analytical model of the signal transmitted by a nonstationary frequency-selective radio channel in a wireless electronic communication network with orthogonal frequency multiplexing and a multi-antenna system is proposed. Analytical expressions are presented that allow us to jointly take into account the influence of interchannel and inter-symbol interference (the effect of multipath signal propagation and Doppler shift on the transmitted signal) against the background of fluctuating noise. Taking into account the factors affecting the transmitted signal makes it possible to bring the model closer to the real processes that occur in the vast majority of radio channels of wireless electronic communication networks.

Cavitation Detection in a Tonpilz-Type Transducer for Active SONAR Transmission System

The active sound navigation and ranging (SONAR) transmission system emits acoustic pulses underwater using a wave generator, a SONAR power amplifier (SPA), and a projector. The acoustic pulse travel in the direction of the target and return as an echo to a hydrophone to learn the range or speed of the object. Often the same device is used as a hydrophone and a projector; in this context, it is known as a transducer. In order to obtain a maximum range of detection in the SONAR, it is desirable to generate the maximum amount of acoustic power until the point in which the echo can be detectable in an atmosphere with non-wished noise. Therefore, a high value of source level (SL) is required that depends largely on the value of electrical power applied to the transducer (Pe). However, when trying to obtain the maximum range of detection in the SONAR system there are the following three peculiar limitations that affect performance: The cavitation, the reverberation, and the effect of interaction in the near field. In this paper, an experimental measurement methodology is presented to detect the cavitation effects in a tonpilz-type transducer for an active SONAR transmission system using a transducer as a projector and a calibrated hydrophone in a hydroacoustic tank by measuring the parameters of total harmonic distortion of the fundamental waveform (THD-F) of the generated acoustic pulse, transmitting voltage response (TVR) to characterize the system and sound pressure level (SPL) that indicates the intensity of sound at a given distance. Whereas the reverberation and the interaction effect in the near field are objects of other study cases. A 570.21 W and THD-F &lt; 5% switched-mode power amplifier (SMPA) prototype was developed to excite the electroacoustic transducer employing a full-bridge inverter (FBI) topology and a digital controller using a field-programmable gate array (FPGA) for unipolar sine pulse width modulation (SPWM) to generate a continuous wave (CW) acoustic pulse at a frequency 11.6 kHz. The results obtained show that from the level of Pe=196.05 W with the transducer at 1 m of depth, the value of THD-F increases significantly while the behavior of the TVR and SPL parameters is affected since it is not as expected and is attributed when cavitation occurs.

Waveform distortion of off-axis fishbone instability in the nonlinear magnetohydrodynamic evolution in tokamak plasmas

We have investigated the waveform distortion of energetic particle driven off-axis fishbone mode (OFM) in tokamak plasmas with kinetic magnetohydrodynamic (MHD) hybrid simulations. We extended our previous simulations (Li et al 2022 Nucl. Fusion 62 026013) by considering higher-n harmonics in the MHD fluid, where n is toroidal mode number. The waveform distortion is successfully reproduced in the simulation for both magnetic fluctuations and temperature fluctuations. It is clarified that the waveform distortion arises from the superposition of the n = 2 harmonics on the fundamental n = 1 harmonics of OFM, where the n = 2 harmonics are generated by the MHD nonlinearity from the n = 1 OFM. Two types of waveform distortion can occur depending on the phase relationship between the n = 1 and n = 2 harmonics and the relative amplitude of the n = 2 harmonics to the n = 1 harmonics. Lissajous curve analyses indicate that the wave couplings between the n = 1 and n = 2 harmonics with phase-lock and lead to ‘rising distortion’ and ‘falling distortion’, respectively. The two types of waveform distortion can be attributed to the strong shearing profile of radial MHD velocity with n = 2 around the q = 2 magnetic flux surface. The dependence of waveform distortion on viscosity is investigated. It is found that the viscosity which is needed to reproduce the waveform distortion is larger than that in the experiment.

A Machine Learning-Assisted Inversion Method for Solving Biomedical Imaging Based on Semi-Experimental Data

Machine learning approaches have been extensively utilized in the field of inverse scattering problems. Typically, the training dataset is generated synthetically using ideal radiation sources such as plane waves or cylindrical waves. However, the testing data often consist of experimental data that take into account the antenna port couplings and waveform distortions within the system. While noise can be artificially added to synthetic data, it may not accurately represent the real experimental noise. Consequently, the application of machine learning-assisted inversion techniques may encounter challenges when the training dataset differs significantly from the experimental data. In this paper, we propose an experimental system specifically designed for human body imaging. A semi-experimental training dataset is constructed using full-wave simulation software, incorporating the relative permittivities of common human tissues. Furthermore, the system noise is meticulously considered through full-wave simulation, enhancing the authenticity of the dataset. A back-propagation scheme is firstly employed to obtain the rough reconstructed images. Then, the U-net convolutional neural network (CNN) is employed to map these rough images to high-resolution ones. Numerical results demonstrate that, in comparison to networks trained solely on synthetic data, the network trained using semi-experimental data achieves superior reconstruction results with lower errors and improved image quality.

Tortuosity of Lightning Current Arc Channel and the Resultant Distortion of Luminosity Waveform

Light emissions in two optical bands are simultaneously observed from arc discharges produced by three laboratory impulse current generators, representing three current components (return stroke, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M$</tex-math> </inline-formula> component, and continuing current) of lightning, applied across an air gap between two round graphite electrodes in a light-tight environment. The results demonstrate that if a tortuosity of the discharge channel did occur, the luminosity waveforms would exhibit distortions. However, these luminosity distortions, often found in naturally-occurred lightning arcs, previously attributed to the spectroscopic wavelength transfer or the increase of neutral spectral lines, are hardly observed in a normal straight laboratory-generated lightning arc, confirming that the distortions in luminosity waveform should be attributed to the tortuosity of lightning channel. The distortions of luminosity waveforms, corresponding to three types of simulated lightning-current components, are featured with various fine structures, which have also been observed in other investigations for electromagnetic fields and acoustic signatures, if a tortuous channel is involved.

Pulse Width Modulation Inverter Control Methods-A Comprehensive Study

Harmonics in electrical supply refers to the distortion of the normal electrical waveform due to the presence of frequencies that are multiples of the fundamental frequency. The presence of harmonics in electrical supply can lead to a number of problems, including: Overheating of equipment, Power quality issues, Extra energy consumption etc. This research aims to provide a cost-effective solution to reduce harmonics in three phase inverters and improve power factor without using any hardware components such as filters and multi-level inverters and by simply changing control pulses of the inverter switches. This research provides a comparison of three most popular inverter control techniques SPWM control, THPWM control and SVPWM control techniques of three phase inverters. All these techniques were compared with the help of MATLAB-SIMULINK and the best control technique with lower order harmonics and less power factor was determined.

Design of Rotor Magnetic Barrier Structure of Built-in Permanent Magnet Motor Based on Taguchi Method

In this paper, a 20kW vehicle built-in permanent magnet synchronous motor is taken as an example, and a magnetic barrier structure is added to the rotor of the motor to solve the uneven saturation problem of the rotor side magnetic bridge. This structure improves the air-gap flux density waveform of the motor by influencing the internal magnetic flux path of the motor rotor, thus improving the sine of the no-load back EMF waveform of the motor and reducing the torque ripple of the motor. At the same time, Taguchi method is used to optimize the structural parameters of the added magnetic barrier. In order to facilitate the analysis of its uneven saturation phenomenon and improve the optimization effect, a simple equivalent magnetic network (EMN) model considering the uneven saturation of rotor magnetic bridge is established in this paper, and the initial values of optimization factors are selected based on this model. Finally, the no-load back EMF waveform distortion rate, torque ripple and output torque of the optimized motor are compared and analyzed, and the influence of magnetic barrier structure parameters on the electromagnetic performance of the motor is also analyzed. The results show that the optimized motor can not change the output torque of the motor as much as possible on the basis of reducing the waveform distortion rate of no-load back EMF and torque ripple.

Synthesis of Behavioral Models for Circuits With Nonlinearity Less Than Model Error

A method for the parameter extraction of a nonlinear-dynamical behavioral model is considered. At this method, the model satisfactorily reflects the nonlinear signal distortions, the value of which is less than the model error. As an example, a digitizer was used. It is shown that a model in the form of a third-order nonlinear recursive filter with integrators in the feedbacks is sufficient. The maximum modeling error is 10% and the amplitude of nonlinear distortions is 3-7%. By including the difference between the calculated and measured nonlinear distortion signals in the optimization criteria when extracting of the model parameters, an error of no more than 29% in the representation of the amplitude of nonlinear distortions was achieved at the correct modeling of the distortion waveform. We explain this by the fact that most of the model error relates to the representation of linear dynamical properties. These properties are the same at any level of the test signal and, under the appropriate parameter’s extraction, do not affect to the modeling error for the circuit nonlinearity.

Reproduction of human blood pressure waveform using physiology-based cardiovascular simulator

This study presents a cardiovascular simulator that mimics the human cardiovascular system's physiological structure and properties to reproduce the human blood pressure waveform. Systolic, diastolic blood pressures, and its waveform are key indicators of cardiovascular health. The blood pressure waveform is closely related to the pulse wave velocity and the overlap of the forward and reflected pressure waves. The presented cardiovascular simulator includes an artificial aorta made of biomimetic silicone. The artificial aorta has the same shape and stiffness as the human standard and is encased with a compliance chamber. The compliance chamber prevents distortion of the blood pressure waveform from strain-softening by applying extravascular pressure. The blood pressure waveform reproduced by the simulator has a pressure range of 80–120 mmHg, a pulse wave velocity of 6.58 m/s, and an augmentation index of 13.3%. These values are in the middle of the human standard range, and the reproduced blood pressure waveform is similar to that of humans. The errors from the human standard values are less than 1 mmHg for blood pressure, 0.05 m/s for pulse wave velocity, and 3% for augmentation index. The changes in blood pressure waveform according to cardiovascular parameters, including heart rate, stroke volume, and peripheral resistance, were evaluated. The same pressure ranges and trends as in humans were observed for systolic and diastolic blood pressures according to cardiovascular parameters.

A control strategy of microgrid voltage source inverter based on self‐learning sliding mode control

AbstractIn island mode, voltage source inverter (VSI) supports the frequency and voltage of microgrid. After the complex load is connected, the VSI control performance is degraded, and the output voltage has deviation, negative sequence, waveform distortion and other problems, which further deteriorate the power quality of the microgrid. Different from the traditional strategy that only focus on a single problem, the strategy proposed in this paper can deal with these three power quality problems simultaneously. In this paper, a self‐learning sliding mode control strategy is proposed. First, a nonlinear smooth function is used to design an expansion observer, which can estimate the expansion state of the internal uncertainties and external disturbances of the control system. Second, the extended observer is combined with the self‐learning synovial control technology to realize the self‐learning synovial disturbance rejection control of VSI control strategy. This strategy can improve the stability of voltage control under various working conditions without precise mathematical model. The simulation results show that, compared with the traditional control strategy, this strategy has good robustness under different working conditions.

Admittance Remodeling Strategy of Grid-Connected Inverter Based on Improving GVF

With the continuous enhancement of weak grid characteristics, the negative effects of grid voltage feedforward (GVF) and PLL on grid-connected inverters become more and more serious and are coupled. Therefore, it is difficult to effectively solve the system stability problem by only improving the PLL structure. Firstly, based on an improved phase-locked loop structure (CCF-PLL) with complex coefficient filter and considering the influence of GVF, the output admittance model of a grid-connected inverter is established. Through stability analysis, it is found that conventional GVF leads the total output admittance phase of the inverter, thus reducing the system stability margin under the weak grid. Then, an improved admittance remodeling strategy of the grid-connected inverter is proposed. An all-pass filter is introduced into the GVF loop to correct the phase of the total output admittance of the inverter, and the phase margin is used as the constraint to design the control parameters, which effectively improves the stability of the system under the weak grid. Finally, the simulation results show that, compared with traditional GVF, the proposed strategy can obviously improve the distortion of grid-connected current waveforms and improve system stability.

Lightning VHF Radiation Mapping Method for an Irregular Short‐Baseline Array

AbstractLightning very high frequency interferometry is a crucial tool for studying the development of lightning leaders within thunderclouds, and the conventional antenna array is composed of two coplanar orthogonal baselines. In this study, we develop two new location methods for an irregular array based on the optimization algorithm, using all baselines in the calculation. Besides, we have analyzed the effects of two kinds of time uncertainty on the location result and provided a calibration method. The first type—the systematic time drift of antennas—can induce an offset on the location result, leading to a worse accuracy. We can calibrate the time drift of each antenna efficiently during the experiment. The second type—the waveform distortion between different antennas—can introduce the noise on time, leading to worse precision. The location precision improves when more baselines are used, whether they are independent or redundant. Our result shows that the non‐coplanar configuration can significantly improve the elevation uncertainty near the horizon and the azimuth uncertainty near the zenith after considering the angular uncertainty's theoretical distribution.

Research on Supraharmonics Suppression Strategy of Distribution Network based on Dynamic Cooperative Control of Multiple Harmonic Sources

In view of the increasing number of supraharmonic sources in distribution network and the increasing distortion of power grid waveform, a strategy of supraharmonic suppression in distribution network based on dynamic coordinated control of multi-harmonic sources is proposed. The characteristic harmonic current components related to carrier frequency in the output current of each supraharmonic source are shifted by several angles through carrier phase shifting, so that the total current harmonics of all supraharmonic sources merged into point of common coupling (PCC) are obviously reduced to meet the requirements of PCC supraharmonic control. This strategy dynamically adjusts the phase-shift angle according to the running number of supraharmonic sources to avoid the problem that the harmonics of the remaining supraharmonic sources cannot be cancelled after some supraharmonic sources exit the system. The simulation results show that the supraharmonic suppression strategy based on dynamic cooperative control of multi-harmonic sources in distribution network can significantly suppress the supraharmonic current component and voltage distortion rate of a certain number of parallel points.

Comparison of Temperature Rise Test Methods for Series Reactors Connected with High Voltage Power Capacitors

In substation, phase advance capacitors are used to improve the power factor. Series reactors are generally connected with the capacitors to reduce the waveform distortion and suppress the inrush current. The fifth harmonic current usually flows into the series reactors for a high voltage power capacitor. Japanese Industrial Standards provide three types of temperature rise test methods, and manufacturer chooses one of them. However, which test method should be selected has not been examined. In this paper, compare the advantages and disadvantages of the test methods using a prototype reactor.

A Proposal of a New Temperature Rise Test Method for Series Reactors Connected with High Voltage Power Capacitors

In substation, phase advance capacitors are used to improve the power factor. Series reactors are generally connected with the capacitors to reduce the waveform distortion and suppress the inrush current. The fifth harmonic current usually flows into the series reactors for a high voltage power capacitor. Japanese Industrial Standards provide three types of temperature rise test methods. Each test method has its own problems. To alleviate them, we have devised a new test method called “Middle Frequency Test” which uses a meddle frequency between the fundamental and fifth harmonics for excitation. This paper presents an implementation method and demonstrates the usefulness of the proposed test method.