Null Voltage Research Articles

X-ray Induced Electric Currents in Anodized Ta2O5: Towards a Large-Area Thin-Film Sensor.

We investigated the characteristics of radiation-induced current in nano-porous pellet and thin-film anodized tantalum exposed to kVp X-ray beams. We aim at developing a large area (≫cm2) thin-film radiation sensor for medical, national security and space applications. Large area (few cm2) micro-thin Ta foils were anodized and coated with a counter electrode made of conductive polymer. In addition, several types of commercial electrolytic porous tantalum capacitors were assembled and prepared for irradiation with kVp X-rays. We measured dark current (leakage) as well as transient radiation-induced currents as a function of external voltage bias. Large transient currents (up to 50 nA) under X-ray irradiation (dose rate of about 3 cGy/s) were measured in Ta2O5 capacitors. Small nano-porous Ta and large-area flat Ta foil capacitors show similar current-voltage characteristic curve after accounting for different X-ray attenuation in capacitor geometry. The signal is larger for thicker capacitor oxide. A non-negligible signal for null external voltage bias is observed, which is explained by fast electron production in Ta foils. Anodized tantalum is a promising material for use in large-area, self-powered radiation sensors for X-ray detection and for energy harvesting.

Three-Dimension Space Vector Modulation for Three-Phase Series-End Winding Voltage-Source Inverters

Series-end winding voltage-source inverters (SEW-VSIs) can effectively improve the dc-link voltage utilization and provide the capabilities to handle the zero-sequence loop. However, the existing modulation schemes for SEW-VSIs cannot fully utilize the output capability of SEW-VSIs while guaranteeing simple implementation. To this end, a three-dimensional space vector modulation (3D-SVM) scheme is proposed for driving three-phase SEW-VSIs in this paper. The modulation is conducted in the 3D-space made up of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">α-β</i> plane and the zero-sequence-axis, and the reference voltage vector in the 3D-space is synthesized by three adjacent basic voltage vectors and the null voltage vector, which can fully utilize the output capability of three-phase SEW-VSI. Then, a convenient implementation method based on a look-up table is developed to find three basic voltage vectors adjacent to the reference voltage vector. Moreover, in the 3D-space, the feasible region and the linear modulation region of the proposed 3D-SVM are investigated. Accordingly, intuitive overmodulation between different subspaces and the switching sequence generation are developed. A series of simulation and experimental results are presented to validate the effectiveness of the proposed 3D-SVM for three-phase SEW-VSI drives.

Multivector-Based Model Predictive Current Control With Zero-Sequence Current Suppression for Three-Phase Series-End Winding Permanent Magnet Synchronous Motor Drives

To suppress the zero-sequence current and enhance the steady-state performance of the three-phase series-end winding permanent magnet synchronous motor (TPSW-PMSM) drives, a multi-vector-based model predictive current control scheme with zero-sequence current suppression is developed in this paper. The concept of the desired voltage is utilized to preselect the candidate voltage vectors and offer the guidance of the geometric division of the sector. In one sampling period, multi-voltage vectors are employed, and the combinations include one active voltage vector and one null voltage vector, and two active voltage vectors, which improves the steady-state performance. Meanwhile, the duty cycle of each voltage vector is derived based on an intuitive geometric expression of the cost function. Further, the relationship between the zero-sequence voltage injection and the switching sequence is revealed, and it is employed to suppress the zero-sequence current actively. In addition, overmodulation operation is also taken into consideration in the proposed scheme. Finally, the experimental study validates the effectiveness of the proposed scheme.

Electrically Tunable Defect-Mode Wavelengths in a Liquid-Crystal-in-Cavity Hybrid Structure in the Near-Infrared Range.

This work proposes a novel approach to developing a core component for a near-infrared (NIR) spectrometer with wavelength tunability, which is based on a liquid crystal (LC)-in-cavity structure as a hybrid photonic crystal (PC). By electrically altering the tilt angle of the LC molecules under applied voltage, the proposed PC/LC photonic structure consisting of an LC layer sandwiched between two multilayer films generates transmitted photons at specific wavelengths as defect modes within the photonic bandgap (PBG). The relationship between the number of defect-mode peaks and the cell thickness is investigated using a simulated approach based on the 4 × 4 Berreman numerical method. Furthermore, the defect-mode wavelength shifts driven by various applied voltages are studied experimentally. To minimize the power consumption of the optical module for spectrometric application, cells of different thicknesses are explored for the wavelength-tunability performance of the defect modes scanning through the entire free spectral ranges to the wavelengths of their next higher orders at null voltage. A 7.9 μm thick PC/LC cell is verified to attain the low operating voltage of merely 2.5 Vrms required to successfully cover the entire NIR spectral range between 1250 and 1650 nm. The proposed PBG structure is thus an excellent candidate for application in monochromator or spectrometer development.

Improved Model Predictive Current Control for Linear Vernier Permanent-Magnet Motor With Efficient Voltage Vectors Selection

In order to relieve the computational burden and reduce the ripples in thrust force and current of linear vernier permanent-magnet motors, this article proposes three-vector-based model predictive current control (MPCC) with efficient voltage vectors selection. The proposed method avoids the traversal of all the possible voltage vectors. Only three nonadjacent active voltage vectors are predicted and evaluated by the cost function. Two active voltage vectors can be precisely determined according to the relationship of three cost function values. The prediction workload can be reduced with the efficient voltage vectors selection. Also, the two active voltage vectors along with the null voltage vector are used to synthesize the final applied voltage vector based on deadbeat principle. The proposed three-vector-based MPCC method extends the modulation area from discrete points to the whole regular hexagon. Thus, the ripples in thrust force and current can be minimized. Theoretical analyses and experimental results are given to verify the effectiveness of the proposed MPCC.

A Closed-Loop Signal Conditioning Scheme for Core-Less Planar LVDT

In this paper, we present an efficient signal conditioning scheme for a previously developed planar core-less Linear Variable Differential Transformer. The stationary coils are excited with variable amplitude complementary sinusoidal current signals through a Howland source using a closed loop controller in such a way that the moving coil voltage is maintained close to zero. The ratio of the difference to the sum of the amplitudes of the applied voltages is taken as the output, which is proportional to the moving coil position. The highlight is that any external interference can be detected by monitoring the null voltage, which allows taking remedial measures like switching of operating frequency. Tests on a prototype sensor with this scheme implemented shows improvement in positioning performance by more than one order of magnitude under strong interference near the operating frequency. Other benefits include insensitivity to variations in the forward path gain and better update rate. The new scheme also retains the characteristics of tolerance to vertical displacement, good linearity and high resolution previously expounded.

Control of PMSM Drive Using Lookup Table Based Compensated Duty Ratio Optimized Direct Torque Control (DTC)

There are several ways to reduce the ripple magnitude for direct torque-controlled permanent magnet synchronous motor (PMSM) drives. In this work, the compensated duty ratio optimized direct torque control (DDTC-TC) method was proposed to reduce the ripple for three-phase PMSM. The proposed control strategy employs the simplicity of conventional DTC (CDTC) and compensation of the reference torque to enforce the developed torque to oscillate around the required value. A torque error square minimization technique is employed to determine the duty ratio. The magnitude of the developed torque by the PMSM depends on the torque slope during the active and null voltage periods. The slope of the torque during each period is computed online. Based on the online computed slopes, the magnitude of the switching period and compensation torque were calculated. The ripple reduction capability of the proposed control scheme is tested using Matlab Simulink and a hardware in loop system. Results indicate that the control scheme is effective. Specifically, the torque and speed ripples are very small. The torque ripple during speed fluctuation is less for open-end winding PMSM. In addition, the proposed scheme is tested for four-quadrant operation, and the scheme is effective for four quadrant operation. Finally, verification of the effectiveness of the proposed scheme is performed using OPAL-RT-RT (OP4500). Verification indicates that the proposed scheme is effective.

Model Predictive Torque Control for a Dual Three-phase PMSM Using Modified Dual Virtual Vector Modulation Method

Single voltage vectors applied in the conventional model predictive torque control (MPTC) for multiphase motors do not only suffer from serious torque and stator flux ripples but also cause the large harmonic current. To address the aforementioned challenges, an MPTC using a modified dual virtual vector modulation method is proposed to improve the operational performance of a dual three-phase permanent magnet synchronous motor. Virtual voltage vectors are synthesized as the candidate control set to restrain the harmonic current. A transformation method is introduced to consider both the stator flux and torque in the duty cycle modulation. The torque and stator flux ripples are simultaneously reduced by addressing the limitations of nonuniform units. Furthermore, the null voltage vector is then inserted to expand the modulation range and improve the steady-state performance. Moreover, the sawtooth carrier is adopted to address the challenge of the asymmetric switch sequence caused by the modified modulation. Finally, the experimental results are presented to verify the effectiveness and superiority of the proposed MPTC method.

Duty-Ratio-Based Direct Torque Control With Enhanced Harmonic Current Suppression for Dual-Three-Phase Permanent Magnet Motor

This article proposes a new duty-ratio-based direct torque control (DR-DTC) with enhanced harmonic current suppression for a dual-three-phase permanent magnet (DTP-PM) motor. First, a new virtual voltage vector construction method based on the carrier-based modulation technique is proposed. Twelve virtual voltage vectors with null average voltage values in two harmonic subspaces are constructed. The sequences of the constructed virtual voltages can be calculated directly in the natural coordinate system. In addition, the whole sequences are standard symmetrical, which can be easily generated by a digital controller. After that, an online harmonic current compensation method is investigated. Additional controllers compensate for the harmonic currents induced by the back electromotive force and nonlinear factors. Since the virtual voltage vectors have null average voltage values in two harmonic subspaces and the online harmonic current compensation method is designed, the proposed DR-DTC can be introduced to the DTP-PM motor with a single neutral point and two neural points. With the proposed strategy, the capability of harmonic current suppression can be significantly improved, and the processes of virtual voltage construction can be simplified. In addition, the advantages of the conventional DR-DTC can be inherited. Finally, experiments are carried out to validate the feasibility of the proposed strategy.

Multimodal spectrometric and dielectric biosensing with an ionic-surfactant-doped liquid crystal

The conventional principle of liquid-crystal (LC)-based biosensing stems from change in LC orientation caused by biomolecule-induced disruption of otherwise ordered LC alignment so that the resulting textural change or light leakage can be detected under a polarizing optical microscope. In this study, a cationic surfactant, cetyltrimethylammonium bromide (CTAB) was doped into LCs to take advantage of the complex formed between the cation and the analyte; i.e., bovine serum albumin (BSA), in biodetection. The optical response of immobilized BSA becomes discernable by applying DC voltage across the LC cell thickness to transduce the disturbance in LC alignment into detectable optical signal and enable quantitative bioassays through transmission spectrometry. On the other hand, dielectric analysis readily permits excellent BSA quantitation without the need of DC voltage, ensuring a limit of detection of 2.7 × 10–11 g/ml of BSA at null bias voltage, which greatly outperforms textural brightness analysis leading to a limit of detection of 2.9 × 10–9 g/ml and transmission spectrometric analysis with a limit of detection of 6.6 × 10–10 g/ml at a bias voltage of 2.3 V. The LC-based protein assay platform disclosed in this work opens a new doorway toward label-free biosensing for multimode quantification.

Proportional Usage of Low-Level Actions in Model Predictive Control for Six-Phase Electric Drives

Finite Control-Set Model Predictive Control (FCS-MPC) appears as an interesting alternative to regulate multiphase electric drives, thanks to inherent advantages such as its capability to include new restrictions and fast-transient response. Nevertheless, in industrial applications, FCS-MPC is typically discarded to control multiphase motors because the absence of a modulation stage produces a high harmonic content. In this regard, multi-vectorial approaches are an innovative solution to improve the electric drive performance taking advantage of the implicit modulator flexibility of Model Predictive Control (MPC) strategies. This work proposes the definition of a new multi-vectorial set of control actions formed by a couple of adjacent large voltage vectors and a null voltage vector with an adaptative application ratio. The combination of two large voltage vectors provides minimum x-y current injection whereas the application of a null voltage vector reduces the active voltage production. Moreover, the optimum selection of the null voltage vector for each couple of large voltage vectors permits reducing the switching frequency. On the other hand, the active application time for this couple is estimated through an analytic function based on the operating point. This procedure avoids the use of an iterative process to define the duty cycles, hence significatively decreasing the computational burden.

Експериментальне дослідження частотного дискримінатора з компенсації зміни рівня вхідного сигналу

Subject and Purpose. Answering the need for powerful high-frequency signal generation and desired radar range provision, pulsed magnetrons hold the current lead as part of radar transmitters. Yet there are some disadvantages, such as significant noise level and technological difficulties concerning the frequency tuning implementation. In this connection, a direct control of the pulsed magnetron frequency with the use of the Automatic Frequency Control (AFC) causes problems. And in the receiver, the AFC of the heterodyne rather than of the magnetron is more frequently used. The heterodyne frequency is maintained at such a level that the receiver is always tuned to the frequency of signal to receive. For various signal generation arrangements, the existing methods of frequency tuning with a frequency discriminator will be analyzed towards the following development of a principal electrical schematic diagram with gaining the operating characteristics of the frequency discriminator prototype on that diagram basis. Methods and methodology. The electrical schematic diagram is developed in terms of intuitive approach and analyzed by the mathematical modeling method. Results. The functional scheme of a frequency discriminator combining advantages of both single-cycle circuit (design simplicity and ease of tuning) and two-cycle circuit (null output voltage at a given frequency of the input signal and compensation possibilities as to the input voltage amplitude varying and interference) has been developed. The principal electrical schematic diagram of this new frequency discriminator has been reported together with the measured amplitude and frequency characteristics, dynamical figures, etc. Conclusions. The novelty of the proposed concept of a heterodyne AFC system built around a frequency discriminator has been confirmed. Its advantages over the known solutions have been shown.

An Improved Direct Torque Control of Three-Level Dual Inverter Fed Open-Ended Winding Induction Motor Drive Based on Modified Look-Up Table

The induction motor (IM) drive with direct torque control scheme (DTC) turns prominent for industrial applications. However, it retains disadvantage of high torque ripples. The conventional look-up table for DTC scheme is structured with selection of reverse voltage vectors (VVs) when flux and torque errors crosses lower hysteresis boundary condition. This results in higher torque ripples and increase in switching frequency. In this article, modified look-up table for DTC of three-level dual voltage source inverter fed open-ended winding IM drive is proposed, where the VVs selection for lower hysteresis boundary conditions of torque and flux are restructured with null voltage states. The selection of suitable null voltage switching states from the various possible combinations of dual inverter switchings is based on minimization of switching state transitions. To nullify flux instability at zero speed, proper active VVs are placed at hysteresis flux +1 and torque 0 condition in the modified look-up table. These overall effective modifications in the proposed DTC scheme enjoys the benefits of reduction in torque ripple, switching frequency, and stable flux maintenance. The MATLAB simulations and practical investigations are conducted for existing and proposed DTC schemes. From these results, the benefits of the proposed DTC over existing DTC schemes are verified.

Investigation of a Via-Loaded Microstrip Magnetic Dipole Antenna With Enhanced Bandwidth and Gain

A via-loaded microstrip magnetic dipole antenna with enhanced bandwidth and gain is proposed and investigated. The antenna is constructed on a long microstrip antenna with three edges shorted and one edge opened. The open aperture is equivalent to a magnetic dipole, which serves as the radiation source. In the long cavity, two modes are generated, in which the lower one is half-TE110 mode and the higher one is half-TE310 mode. By loading two sets of shorting vias around the null voltage points of the half-TE310 mode, the resonant frequency of the half-TE110 mode is shifted upward. Then, the two modes are merged to achieve an enhanced bandwidth. Moreover, the superposition of the far fields of the two modes results in a narrowed unidirectional beam, a reduced sidelobe level (SLL), and therefore, an improved gain. The theoretical investigation is validated with simulation results. A prototype of the antenna is fabricated and measured. Measured results show that the proposed antenna with a low profile of $0.027\lambda _{0}$ ( $\lambda _{0}$ is the wavelength in free space) has an impedance bandwidth of 9.1%, and a peak gain of 10.2 dBi.

Vertex collisions in 3-periodic nets of genus 4.

Unstable nets, by definition, display vertex collisions in any barycentric representation, among which are approximate models for the associated crystal structures. This means that different vertex lattices happen to superimpose when every vertex of a periodic net is located at the centre of gravity of its first neighbours. Non-crystallographic nets are known to be unstable, but crystallographic nets can also be unstable and general conditions for instability are not known. Moreover, examples of unstable nets are still scarce. This article presents a systematic analysis of unstable 3-periodic nets of genus 4, satisfying the restrictions that, in a suitable basis, (i) their labelled quotient graph contains a spanning tree with zero voltage and (ii) voltage coordinates belong to the set {-1, 0, 1}. These nets have been defined by a unique circuit of null voltage in the quotient graph. They have been characterized through a shortest path between colliding vertices. The quotient graph and the nature of the net obtained after identification of colliding vertices, if known, are also provided. The complete list of the respective unstable nets, with a detailed description of the results, can be found in the supporting information.

Dead‐beat predictive direct power control of voltage source inverters with optimised switching patterns

This study presents a novel dead-beat predictive direct power control (PDPC) strategy working at nearly constant switching frequency for the voltage source inverter. The proposed strategy employs two active voltage vectors and one null voltage vector during every control period. To solve the problem of negative predicted duration time of active voltage vectors in the conventional dead-beat PDPC strategy, an automatic updating mechanism is designed. When the predicted duration time is negative, the applied voltage vectors will be updated automatically without the need of reselecting voltage vectors or recalculation of the duration time. Accurate tracking of both active and reactive power can be achieved with small power ripples using the strategy proposed in this study. The proposed PDPC strategy shows excellent steady performance and fast dynamic responses. High sampling frequency can also be avoided and the calculation process is simple enough to be implemented on normal DSPs. Simulation and Experimental results are presented to validate the feasibility of the proposed dead-beat PDPC strategy.

Direct Current Tracking Using Boundary Control With Second-Order Switching Surface for Three-Phase Three-Wire Grid-Connected Inverter

Cascaded boundary-deadbeat controller can ensure system stability of single-phase grid-connected inverter operated under different grid conditions. However, it will require using numerous sensors and may experience performance degradation caused by unbalanced filter parameters in controlling a three-phase three-wire inverter. Furthermore, the system is in variable frequency operation. A boundary controller using second-order switching surface with direct current tracking capability, reduced number of current sensors, and fixed frequency operation of three-phase three-wire grid-connected inverter with inductive-capacitive-inductive ( LCL ) filter is presented. By applying the 60° discontinuous pulsewidth modulation scheme for a fictitious decoupled dual-buck structure in each operation sector, two separate sets of switching criteria for dictating the states of the switches of two half-bridge legs are formulated. Such technique can avoid dealing with the challenges caused by the interactions among three independent current regulators. A switching table with null voltage vectors excluded is designed to dictate the switching actions. Sensitivities of the system transfer characteristics to the parametric variations are investigated. A 3 kW prototype has been built and evaluated under stiff- and weak-grid conditions. The experimental results are favorably compared with theoretical predictions.

Dynamic Tuning and Memory Switching of Defect Modes in a Hybrid Photonic Structure

We propose a memorable and electrically tunable photonic device by infiltrating a dual-mode chiral-doped dual-frequency liquid crystal (LC) as the central defect layer in a one-dimensional photonic crystal (PC). According to the transmission properties of this structure, the wavelength tunability of defect modes is obtained by manipulating the LC layer in the dynamic mode due to the electrically controlled birefringence effect. Moreover, the switching between two memorable states, the splay and π-twist states, creates two distinct sets of defect modes at null voltage. The spectral characteristics of this device ensure its potential application as an energy-efficient multichannel wavelength filter.

Multiferroic behavior of heterostructures composed of lanthanum and bismuth ferrite

Heterostructured thin films of lanthanum ferrite (LFO) and bismuth ferrite (BFO) with different thicknesses were successfully obtained by a soft chemical method. The films were deposited by spin-coating and annealed at 500°C for 2h. The XRD pattern confirmed the purity of the thin films, where no additional peaks associated with impurity phases were present. The morphology analysis showed spherical grains with a random size distribution. The grain sizes increased with the number of BFO layers. The average grain size varied from 43nm to 68nm. The best dielectric results were obtained for the film with 6 LFO sublayers and 4 BFO top layers, in which the dielectric constant showed low dispersion. Since the capacitance-voltage curve for the film 6-LFO/4-BFO is symmetrical around null voltage, it can be inferred that this heterostructure has few mobile ions and accumulated charges on the film-substrate interface. In this film, polarization remains almost constant during 1012 cycles before the onset of degradation, which shows the very high resistance of the films to fatigue. Magnetoelectric coefficient measurements of the films revealed the formation of hysteresis loops, and a maximum value of 12V/cmOe was obtained for the magnetoelectric coefficient in the longitudinal direction; this value is much higher than that previously reported for pure BFO thin films.

A Highly Sensitive Voltage Interrogation Method Using Electro-Optically Tunable Waveguide Coupled Surface Plasmon Resonance Sensors

We present a highly sensitive detection method using electro-optically (EO) tunable waveguide coupled surface plasmon resonance (WCSPR) sensors by theoretical simulation. In this method, light is focused through a prism onto metal surface of the WCSPR sensor and the reflection is collected with a bicell photodetector. Time varied scanning DC electrical signal is applied on the waveguide layer of the WCSPR sensor and the ratio of the differential signal to the sum signal of the photodetector is calculated after each change. For each analyte, we get voltage-ratio curve after voltage scanning. Taking the ratio for a reference analyte calculated with null DC voltage applied as resonant ratio, different resonant voltages can be found in the voltage-ratio curves for different analytes. Theoretical simulation shows a high linearity between the resonant voltages and the refractive index (RI) of analytes. A sensitivity as high as 2.34 x 10(5) V/RIU can be expected in this method.