Experimental Waveforms Research Articles

A thermodynamically-based viscoelastic-viscoplastic model for the high temperature cyclic behaviour of 9–12% Cr steels

Improving the understanding of the long term rate dependent behaviour of materials is of critical importance in many engineering applications. Without this understanding, it is potentially difficult to ensure safe and effective plant operation while simultaneously satisfying requirements for sustainability and responsible resource management. In the present work, a thermodynamically-based constitutive model is proposed to capture the rate sensitivity, the stress relaxation and the accelerated cyclic softening observed during cyclic deformation of a P91 steel at an elevated temperature (600°C). The model is developed within the framework of Thermodynamics of Irreversible Processes and Generalized Standard Materials formalism, thereby offering a thermodynamically grounded coupling of both viscoelasticity (semi-recoverable strain accumulation at vastly different time scales) and viscoplasticity (irreversible strain observed above the stress threshold). The later part combines a hyperbolic sine-power flow rule with non-linear isothermal cyclic evolution of isotropic and kinematic hardening. The applicability of the model to various mechanical loadings (e.g., cyclic tensile-compression tests, fatigue-relaxation tests, anhysteretic tests) is validated by designing a heuristic optimisation program based on a nonlinear least-squares function coupled with the Levenberg-Marquardt algorithm. The optimisation procedure is informed (through the estimation of initial material parameter estimates and objective function evaluation) by anhysteretic type experiential data only (wherein long term load hold periods are introduced at various points in the waveform). Rate dependency is determined and validated by considering experimental waveforms with different loading (strain) rates (0.1%.s−1, 0.01%.s−1 and 0.001%.s−1) and ranges (0.25%, 0.4% and 0.5%) to highlight most of the deformation mechanisms involved during the fatigue and relaxation processes. By comparing predicted and experimentally observed material responses, it is demonstrated in the present work that the viscoelastic-viscoplastic strain decomposition has the ability to capture the accelerated cyclic softening and the uncoupled stress relaxation behaviour (below and above yielding) for a P91 steel at elevated temperatures.

Development and modelling of a new type of sensor for detecting current transients for power system protection

A new type of sensor is proposed to detect high frequency transients in currents and their initial polarities. The proposed sensing system replaces both high frequency signal sampling and processing by a simple detection coil wound on a ferrite core and an analogue electronic circuit. It is verified through laboratory experiments that transient occurrence, occurrence time, and initial transient polarity can be determined accurately. A detailed model of the sensor, including the dynamic hysteresis characteristics of the ferrite core, is developed and implemented on an electromagnetic transient simulation software, and verified through experimental measurements. Application of the developed model to simulate a transient current polarity comparison based protection scheme is demonstrated. The versatility of the proposed sensor compared to common digital signal processing based approaches, namely discrete wavelet transform and mathematical morphology, is highlighted using experimental waveforms.

Defect feature extraction of marine protective coatings by terahertz pulsed imaging

Feature extraction of marine protective coatings using Hilbert transform (HT) and wavelet transform modulus maximum (WTMM) on terahertz pulsed imaging (TPI) of the coatings was proposed. For TPI-based marine protective coating detection, it is difficult to locate exactly the reflected echoes form internal structure of coating system by the faint feature in the time domain due to the interference of background noise. However, those faint features were usually caused by the interface between two different medium layers whose refractive indices are very similar or caused by micro defects beneath the coating. The proposed algorithm was validated by simulated and experimental TPI waveform obtained from marine paint samples with or without defects. To extract the structure feature more clearly and intuitively, Hilbert transform procedure was carried out on detected terahertz signal to get Hilbert envelope for further processing. Subsequently, the modulus maximum of the stationary wavelet transform approximation coefficients were employed as the criteria for feature extraction of internal interfaces and defect features, according to the relationship between WTMM and signal singularity. The results demonstrated that the combination of HT and WTMM algorithms could be used to exactly extract the structure feature and to evaluate the position of defects beneath coatings.

Numerical simulation of electromagnetic coupling in explicitly meshed wiring looms and bundles

In this paper, the Unstructured Transmission Line Modelling (UTLM) method based on a tetrahedral mesh is applied to model the electromagnetic coupling into wire looms and bundles with multiple cores that are typical of an aircraft system, when they are exposed to plane wave illuminations. The impact on the electromagnetic coupling into wires of both bundle configuration and the positioning of the bundle relative to simple structures are investigated using the UTLM method with explicit meshing of the wires. The work not only confirms that UTLM method as a powerful tool for dealing with wire looms and bundles but provides invaluable information on the margins to be expected in key experimental waveform parameters such as peak amplitude and frequency response.

Experimental analysis of normal spark discharge voltage and current with a RC-type generator in micro-EDM

This paper focused on the experimental analysis of the discharge voltage and current waveforms in micro-EDM with a RC-type generator. A quasi-maintaining voltage assumption was proposed, and the spark gap discharge voltage was modeled as a voltage source in a first-order liner form. The discharge current and the discharge capacitor voltage under different machining conditions were calculated and compared with the real experimental waveforms of good and bad conductive workpiece, respectively. For the good conductive materials, the discharge voltage looked more like a constant maintaining voltage, and for the bad conductive materials, a low resistive ceramics composite and P-type monocrystalline silicon, the gap discharge voltage featured a relatively high average value and a relatively large variation slope rate. Based on the comparisons, the proposed discharge gap model and its simplification simulated the real discharge waveform and can be used for pulse discrimination and pulse energy prediction in micro-EDM.

Simulation and experimental analysis of alternating-current phenomenon in micro-EDM with a RC-type generator

In micro electrical discharge machining (micro-EDM) with a RC-type generator, due to the oscillation voltage on a sparking gap caused by the parasitic parameters in a discharge loop and the smaller discharge capacitor, the normal-discharge is usually accompanied by the alternating-current. In this research, the alternating-current phenomenon was analyzed comprehensively through theoretical analysis, electrical simulation and experimental verification. An electrical discharge model based on real experimental waveforms was provided, and the machining influences of alternating-current were investigated. It was found that the alternating-current was likely occur when the discharge capacitor restored significant energy at the end of the first normal-discharge, and it remarkably contributed to material removal on the workpiece under some conditions, which improved the machining efficiency at a limited expense. So, the process servo control and the strategy for tool wear prediction and compensation should consider the alternating-current effects or utilize it in the finishing regime and high-accuracy machining applications.

PFM and PWM Hybrid Mode Control Strategy for Switching Power Supply and Its Implementation

A PFM and PWM hybrid control strategy for high frequency switching power supply is designed in the article. Power supply working in heavy load mode uses PWM, and power supply at startup or working in light load mode uses PFM. The TIDM-PSFB-DCDC dc regulated power supply produced by TI company is used for experimental verification in hybrid control model. Programming by the STMS320F28335 chip goes on the hardware implementation of control strategy, and a detailed introduction and key statements of the procedure are given. Through the final experimental waveform, the power supply can work smoothly under the mixed control mode, and working conditions of heavy load and light load can be distinguished, and which improves the power supply’s working efficiency.

Reanalysis of Discharge Voltage of RC-type Generator in Micro-EDM

In micro electrical discharge machining (micro-EDM), RC-type generator is popular because of the short pulse duration with low pulse energy. According to the experimental waveforms captured in micro-EDM processing with a commercial RC-type generator, the discharge voltage had a relatively slow slope rate section during the discharge period, so a quasi maintaining-voltage assumption was proposed. Then, an electrical circuit model, considering the parasitic parameters, was established and the discharge process was analyzed theoretically. The calculated waveforms matched the experimental waveforms well, which supported the validation of the proposed electrical circuit model. The proposed electrical circuit model was also simulated and simulation results showed that the parasitic parameters would indeed have great effects on the discharge voltage.

Stabilizing the Buck Converter With a First-Order-Filter-Based Time Delay Feedback Controller

A first-order-filter-based time delay feedback (TDF) controller is proposed to stabilize the buck converter when operating with non-linear phenomena, such as bifurcation and chaos. Compared with the existing second-order filter method, the proposed controller needs only two parameters. This is similar to the ideal TDF controller and it makes the design of the controller simpler. The discrete model of the converter is used to compute one of the two parameters, which is known as the feedback gain. The frequency domain comparison of the proposed controller and the ideal TDF controller shows that the proposed controller is almost non-invasive, because it does not change the dc component of the converter and only changes slightly the harmonic at the switching frequency. The proposed controller has another structure called the extended controller, which needs three parameters. By increasing the third parameter, the feedback gain needs a larger value to stabilize the converter. The benefit of the proposed controller lies in the fact that it does not need the digital circuit that was thought to be necessary for the TDF control of the converter. Simulated and experimental waveforms verify the computation results. The added feedback signal is small enough when the switch turns on.

Radial density distribution of a warm dense plasma formed by underwater electrical explosion of a copper wire

Time- and space-resolved evolution of the density (down to 0.07 of solid state density) of a copper wire during its microsecond timescale electrical explosion in water was obtained by X-ray backlighting. In the present research, a flash X-ray source of 20 ns pulse-width and >60 keV photon energy was used. The conductivity of copper was evaluated for a temperature of 10 kK and found to be in good agreement with the data obtained in earlier experiments [DeSilva and Katsouros, Phys. Rev. E 57, 5945 (1998) and Sheftman and Krasik, Phys. Plasmas 18, 092704 (2011)] where only electrical and optical diagnostics were applied. Magneto-hydrodynamic simulation shows a good agreement between the simulated and experimental waveforms of the current and voltage and measured the radial expansion of the exploding wire. Also, the radial density distribution obtained by an inverse Abel transform analysis agrees with the results of these simulations. Thus, the validity of the equations of state for copper and the conductivity model used in the simulations was confirmed for the parameters of the exploding wire realized in the present research.

Semiconductor sharpeners providing a subnanosecond voltage rise time of GW-range pulses.

The article describes semiconductor sharpeners providing a subnanosecond voltage rise time of GW-range pulses. The sharpeners are made as stacks of series-connected dynistor structures built into an oil-filled coaxial line with 48 Ω wave impedance at the place of an inner conductor. Two sequential sections of pulse sharpening are used. An input voltage pulse has the amplitude of 540 kV with the rise time of ∼1.2 ns at 0.2-0.9 level from the amplitude and voltage rise rate of ∼0.3 MV/ns. After pulse propagation through the sharpening sections, its rise time is reduced down to 360 ps, and the voltage rise rate is increased up to ∼0.95 MV/ns. Peak power of the sharpened pulse is within the range of 4.5-5.5 GW. The sharpeners are tested at a pulse repetition frequency of up to 1 kHz. Sharpener operation is studied by numerical simulation methods. Experimental waveforms of output pulses and the corresponding calculated voltage-time dependences are in statistical agreement.

Reverse voltage generating circuit for rapid commutation of output current polarity in variable polarity arc welding power supply

The topology of variable polarity (VP) arc welding power supply is composed of a first inverting stage responsible for the adjustment of output current amplitude and a second inverting stage used for the commutation of output current polarity. A reverse voltage generating (RVG) circuit is proposed, which could be applied in the second inverting stage of a VP arc welding power supply to generate a stable reverse voltage with fixed amplitude during the commutation procedure and to make the commutation rate of output current more rapidly. First, a basic circuit unit is designed to illustrate the operating principle of the RVG circuit. Then, the Pspice simulations for the comparison between the RVG circuit and the conventional RC resonant circuit in the half-bridge applications of second inverting stage are carried out to illustrate the superiority of RVG circuit. Finally, an experimental prototype of VP arc welding power supply is established to test and verify the proposed RVG circuit. The experimental waveforms of output current and voltage demonstrate that the RVG circuit is effective and suitable for generating a stable reverse voltage with fixed amplitude, and realizing a rapid commutation of the output current polarity in VP arc welding power supply.

Adaptive Driving Bus Voltage and Energy Recycling Control Schemes for Low-Power AC–DC RGB-LED Drivers

A low-power ac–dc light-emitting diode (LED) driver with adaptive driving bus voltage (ADBV) and energy recycling (ER) control schemes is proposed to drive red, green, and blue LEDs in series to achieve high efficiency. The bus voltage of the power factor correction flyback converter with the ADBV control is always adjusted, so as to reduce power loss on current regulators. However, since the small bandwidth of the flyback converter and large bus capacitor limit the transient response of the flyback converter, it is difficult to maintain the voltage across current regulators at the minimum value. The ER control is proposed to accelerate the transient response of the bus voltage, making the voltage across current regulators as close to the minimum voltage as possible. Furthermore, extra energy is stored and restored on the recycling capacitor when the bus voltage is switched between different supplying levels without consuming much power due to the ER control. The experimental waveforms of the laboratory prototype are shown to confirm the validity of the proposed schemes.

Low-Voltage-Ride-Through (LVRT) Control of an HVDC Transmission System Using Two Modular Multilevel DSCC Converters

This paper presents an intensive discussion on a high-voltage direct-current (HVDC) long-distance transmission system combining two modular multilevel double-star chopper-cells (DSCC) converters with dc power cables. Hereinafter, each converter is referred to simply as a DSCC converter, or just as a DSCC for more simplicity. Such an HVDC transmission system is required to provide low-voltage-ride-through (LVRT) capability to enhance system availability. This paper proposes a practical LVRT control characterized by the use of power-line communications between the two DSCC converters. The validity and effectiveness of the LVRT control is verified not only by simulated waveforms obtained from the software package “PSCAD/EMTDC” but also by experimental waveforms from a three-phase 200-V, 400-Vdc, 10-kW, 50-Hz downscaled system with 300-m-long dc power cables.

Towards the Identification of Hemodynamic Parameters Involved in Arteriovenous Fistula Maturation and Failure: A Review.

Native arteriovenous fistulas have a high failure rate mainly due to the lack of maturation and uncontrolled neo-intimal hyperplasia development. Newly established hemodynamics is thought to be central in driving the fistula fate, after surgical creation. To investigate the effects of realistic wall shear stress stimuli on endothelial cells, an in vitro approach is necessary in order to reduce the complexity of the in vivo environment. After a systematic review, realistic WSS waveforms were selected and analysed in terms of magnitude, temporal gradient, presence of reversing phases (oscillatory shear index, OSI) and frequency content (hemodynamics index, HI). The effects induced by these waveforms in cellular cultures were also considered, together with the materials and methods used to cultivate and expose cells to WSS stimuli. The results show a wide heterogeneity of experimental approaches and WSS waveform features that prevent a complete understanding of the mechanisms that regulate mechanotransduction. Furthermore, the hemodynamics derived from the carotid bifurcation is the most investigated (in vitro), while the AVF scenario remains poorly addressed. In conclusion, standardisation of the materials and methods employed, as well as the decomposition of realistic WSS profiles, are required for a better understanding of the hemodynamic effects on AVF outcomes. This standardisation may also lead to a new classification of WSS features according to the risk associated with vascular dysfunction.

A Discontinuous Current-Source Gate Driver With Gate Voltage Boosting Capability

In this paper, a novel discontinuous current-source driver (CSD) is proposed, in which the power MOSFET gate-source voltage is increased to more than the drive supply voltage. The proposed CSD is able to recover gate energy dissipated in a conventional driver. In comparison to the conventional gate driver, the proposed CSD achieves fast switching speed to reduce switching losses. A wide range of operating duty cycle, low circulating losses, and high Cdv/dt immunity are other features of the proposed CSD. In comparison to previous discontinuous CSDs, the special advantage of the proposed circuit is power MOSFET gate voltage boosting, which leads to reduction of Rds(on) and, thus, the conduction loss. The proposed circuit is appropriate for voltage regulators (VRs) with synchronous rectifier and also it is suitable for two-stage 48-V power pod applications and low-voltage converters. Two-stage VR for laptop computer CPUs is another application of this gate driver circuit to improve light load performance. A prototype of the circuit operating at 1 MHz is implemented, and the experimental waveforms justify the theoretical analysis.

Design of Control System for Battery Cascade Charging Power Supply

Electromagnetic launch (EML) system needs large energy to realize the high current discharge for launching, so that the requirement of pulse power energy increases rapidly in the EML experiments. In recent years, some scientists focus on provide enough energy for the EML system and a new method of charging the power module by the battery cascade charging power supply (BCCPS) is proposed. By using the BCCPS, the fast charging with high-voltage and large-current can be realized, but some restrictions of the BCCPS need to be dealt with, such as the fault protection, the sequential control, and the insulation of the battery. In this paper, a BCCPS’s control system is designed, which uses the digital signal processor as the main controller. The components of the control system, such as fiber communication system, voltage feedback system, protection circuits, loads grouping system, and sequence setting system, are used to enhance the performance of the BCCPS. The control system is used in a ten levels BCCPS and the experiment waveforms proved that it can work effectively.

A Broad Range of Speed Control of a Permanent Magnet Synchronous Motor Driven by a Modular Multilevel TSBC Converter

This paper provides a theoretical and experimental discussion on a modular multilevel triple-star bridge-cell converter with focus on mitigating the capacitor-voltage fluctuation contained in each bridge cell. A new control method proposed in this paper is characterized by properly superimposing both common-mode voltage and circulating currents in the whole speed range. Moreover, this paper optimizes the amplitude and frequency of the superimposed common-mode voltage on the basis of theoretical analysis, thus resulting in minimizing the voltage fluctuation. The optimized control method makes both voltage fluctuation and cluster-current amplitude smaller than do the existing methods. Experimental waveforms obtained from a three-phase downscaled system using a 370-V, 15-kW, 75-Hz, six-pole, permanent-magnet synchronous motor verify good start-up performance from a standstill to the rated speed, keeping both capacitor-voltage and cluster current within acceptable levels.

The Effect of Hardware-Computed Travel Time on Localization Accuracy in the Inversion of Experimental (Acoustic) Waveform Data

This study aims to advance hardware-level computations for travel-time tomography applications in which the wavelength is close to the diameter of the information that has to be recovered. Such can be the case, for example, in the imaging applications of (1) biomedical physics, (2) astro-geophysics and (3) civil engineering. Our aim is to shed light on the effect of that preprocessing the digital waveform signal has on the inversion results and to find computational solutions that guarantee robust inversion when there are incomplete and/or noisy measurements. We describe a hardware-level implementation for integrated and thresholded travel-time computation (ITT and TTT). We compare the ITT and TTT approaches in inversion analysis with experimental acoustic travel-time data recorded using a ring geometry for the transmission and measurement points. The results obtained suggest that ITT is essential for maintaining the robustness of the inversion with imperfect signal digitization and sparsity. In order to ensure the relevance of the results, the specifications of the test setup were related to those of applications (1)-(3).

Frequency Dependent Behavior of a Dynamo-Type HTS Flux Pump

The high-T c superconductor (HTS) dynamo is a type of superconducting flux pump that can inject a dc current into a superconducting magnet coil, without requiring physical connection to an external power supply. Here, we report experimental results from an HTS dynamo employing 46-mm wide coated conductor stator wire. We characterize the output of the device as a function of frequency, and show the effect of slitting the stator wire into two or three parallel equivalent strips. At operating frequencies above ~100 Hz, we observe that the internal resistance and open-circuit voltage ceases to increase linearly with frequency, and that the short-circuit current is greatly reduced. Experimental waveform data of the output open-circuit voltage enables us to identify three distinct and different signature waveforms, which mark the low-frequency, midfrequency, and high-frequency operating regimes. These regimes appear to arise due to the saturation of circulating currents within the stator wire, followed by a rise in temperature of the wire. We conclude that dissipation due to circulating currents plays an important role in depressing the output of this wide stator device at high frequencies. This effect is in marked contrast to results previously reported for a similar HTS dynamo employing narrower stator wires.