Low Frequency Operation Research Articles

Notice of Violation of IEEE Publication Principles: A thin broadband microwave absorber with hexagonal resistive frequency selective surface

Notice of Violation of IEEE Publication Principles A Thin Broadband Microwave Absorber with Hexagonal Resistive Frequency Selective Surface by Feifei Li, Chao Meng, Fan-guang Meng, Digang Fan, Ping Chen, and Rui-xin Wu in the IEEE Antennas and Wireless Propagation Letters (Early Access) April 2018 After careful and considered review of the content and authorship of this paper by a duly constituted expert committee, this paper has been found to be in violation of IEEE’s Publication Principles. This paper contains significant portions of work presented in the paper cited below. The original work was translated and copied without attribution (including appropriate references to the original author(s) and/or paper title) and without permission. Design of a Wide‐band Metamaterial Absorber Based on Resistance Films by Gu Chao, Qu Shao‐Bo, Pei Zhi‐Bin, Xu Zhuo, Lin Bao‐Qin, Zhou Hang, Bai Pen, Gu Wei, Peng Wei‐Dong, and Ma Hua in Acta Physica Sinica, Vol 60, No. 8, August 2011 We realized a thin, broadband absorber based on resistive frequency selective surface (FSS). By introducing a design parameter, p-factor, we successfully designed a microwave absorber, whose reflectivity below -10dB covers 6.4-24.9 GHz. The thickness of the absorber is only 0.087-L at the lowest operating frequency. We show that broadband absorption can be achieved by carefully balancing susceptances of resistive FSS and other parts of the absorber. Furthermore, we find that lossless dielectric slabs with lower dielectric constant are the best choices for broadband absorber. Experimentally, the samples were fabricated by using silk-screen printing technique and their absorbing performance was measured. Experimental results are in good agreement with simulated ones. Our work provides a method to realize broadband absorber in a simple way, which may be used for the absorbers working in THz, infrared or even optical band.

Modified Single-stage Single-phase to Multi-phase Matrix Converter for Extended Low Output Power Frequency Operation

Modified Single-stage Single-phase to Multi-phase Matrix Converter for Extended Low Output Power Frequency Operation

SRNSD: Structure-Regularized Night-Time Self-Supervised Monocular Depth Estimation for Outdoor Scenes.

Deep CNNs have achieved impressive improvements for night-time self-supervised depth estimation form a monocular image. However, the performance degrades considerably compared to day-time depth estimation due to significant domain gaps, low visibility, and varying illuminations between day and night images. To address these challenges, we propose a novel night-time self-supervised monocular depth estimation framework with structure regularization, i.e., SRNSD, which incorporates three aspects of constraints for better performance, including feature and depth domain adaptation, image perspective constraint, and cropped multi-scale consistency loss. Specifically, we utilize adaptations of both feature and depth output spaces for better night-time feature extraction and depth map prediction, along with high- and low-frequency decoupling operations for better depth structure and texture recovery. Meanwhile, we employ an image perspective constraint to enhance the smoothness and obtain better depth maps in areas where the luminosity jumps change. Furthermore, we introduce a simple yet effective cropped multi-scale consistency loss that utilizes consistency among different scales of depth outputs for further optimization, refining the detailed textures and structures of predicted depth. Experimental results on different benchmarks with depth ranges of 40m and 60m, including Oxford RobotCar dataset, nuScenes dataset and CARLA-EPE dataset, demonstrate the superiority of our approach over state-of-the-art night-time self-supervised depth estimation approaches across multiple metrics, proving our effectiveness.

Model Predictive Control With Reduced Common-Mode Voltage Based on Optimal Switching Sequences for Nested Neutral Point Clamped Inverter

This paper proposes a new model predictive control strategy with reduced common-mode voltage (CMV) based on optimal switching sequences (OSS-MPC) for a three-phase (3PH) four-level nested neutral point clamped (4L-NNPC) inverter. The proposed method controls load current, mitigates the voltage fluctuations of flying capacitors (FCs) in the entire operating frequency range, and provides a constant inverter switching frequency. In this work, the switching sequences (SSs) are configured so that the peak CMV is limited to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V_{dc}/6$</tex-math></inline-formula> . By involving time in the optimization process, the balance of FCs voltages is realized in the entire operating frequency, especially in low-frequency operating mode. Also, the voltage fluctuations of FCs are further mitigated through a mathematical-logic technique. To implement the proposed method on the digital signal processor, the deadbeat control technique is also used to reduce the search space and thus, computation burden. The performance of the proposed method is verified in MATLAB/SIMULINK. The experimental results are carried out based on a scaled-down hardware prototype validating the theoretical claims.

Design of a delay locked loop with low power and high operating frequency range characteristics in 180-nm CMOS process

Design of a delay locked loop with low power and high operating frequency range characteristics in 180-nm CMOS process

Advanced magnetocaloric microwires: What does the future hold?

Magnetic refrigeration (MR) based on the magnetocaloric effect (MCE) is a promising alternative to conventional vapor compression refrigeration techniques. The cooling efficiency of a magnetic refrigerator depends on its refrigeration capacity and operation frequency. Existing refrigerators possess limited cooling efficiency due to the low operating frequency (around tens of Hz). Theory predicts that reducing geometrical effects can increase the operation frequency by reducing the relaxation time of a magnetic material. As compared to other shapes, magnetocaloric wires transfer heat most effectively to a surrounding environment, due to their enhanced surface area. The wire shape also yields a good mechanical response, reducing the relaxation time and consequently increasing the operation frequency of the cooling device. Experiments have validated the theoretical predictions. By assembling microwires with different magnetocaloric properties and Curie temperatures into a laminate structure, a table-like magnetocaloric bed can be created and used as an active cooling device for micro-electro-mechanical system (MEMS) and nano-electro-mechanical system (NEMS). This paper assesses recent progress in the development of magnetocaloric microwires and sheds light on the important factors affecting the magnetocaloric behavior and cooling efficiency in microwire systems. Challenges, opportunities, and strategies regarding the development of advanced magnetocaloric microwires are also discussed.

Realization of an ultra-thin absorber with fragmented magnetic structure at L-, S-, and partial C-bands

In this paper, an ultra-thin absorber with a total thickness of 9.2 mm is designed and verified at the frequency band of 1–5.34 GHz. The absorber is composed of a layer of metasurface, multi-layered magnetic substrate, a layer of fragmented magnetic structure obtained by improved MOEA/D-GO (Multi-Objective Evolutionary Algorithm based on Decomposition combined with Enhanced Genetic Operators), and a copper back plate. The absorber is achieved by two steps. First, we designed and measured an ultra-thin absorber at 0.78–2.04 GHz by adding a layer metasurface onto the top of a basic multi-layer absorber composed of magnetic materials. The fractional bandwidth (FBW) of the absorber is 89.4%, and the electrical thickness is only 0.024λ0 at the lowest operating frequency. Second, to broaden the bandwidth, we use an improved MOEA/D-GO to optimize one magnetic layer of the absorber. The working frequency band of the optimized absorber is 1–5.34 GHz, covering L- (1–2 GHz), S- (2–4 GHz), and partial C-bands (4–8 GHz). Furthermore, we modified the structure of the metasurface to make the absorber polarization-independent. The FBW of the final absorber is 136.4%, and the electrical thickness is 0.031λ0 at the lowest operating frequency. The prototype of the absorber is measured, and the experimental results agree well with the simulated performance. The results show that the improved MOEA/D-GO can be used to design and optimize sophisticated electromagnetic (EM) structures with the predesigned properties, and the absorber with ultrathin thickness and light weight verified in this paper have great application potentials in EM compatibility, EM shielding, and radar cross section reduction at the low bands of the microwave spectrum.

A comparative analysis of power pulsating buffer architectures for mitigating high‐frequency and low‐frequency ripple in PV microinverter applications

AbstractThis study provides a comparative analysis of feasible architectures of Power Pulsating Buffer (PPB) as an actively controlled energy storage solution alternative to the electrolytic capacitors in a single‐stage isolated, single‐phase 120 V AC 60Hz grid‐connected photovoltaic (PV) microinverter for a 400 W rated system with 20–40 V input range. The comparison encompasses key factors including energy density, volume reduction, efficiency, frequency operation, and system stability. The findings shed light on the primary advantages and disadvantages of the buck‐ and boost‐derived PPB under study. The results indicate that the boost PPB exhibits enhanced stability, a more compact design, and higher power efficiency compared to the buck PPB. However, for low switching frequency operations, the buck PPB proves to be more effective in mitigating DC port voltage ripple and ensuring controller over microinverter load variation.

Low-frequency operation control method for medium-voltage high-capacity FC-MMC type frequency converter

The application of modular multilevel converters (MMCs) to large drive systems is subject to severe low-frequency operation restrictions. The fluctuation of capacitor voltage and the high amplitude of common mode voltage in sub-modules is a thorny problem. In this paper, a novel fly-across capacitor modular multilevel converter topology is adopted to eliminate low-frequency voltage ripples by using the fly-across capacitor as a power transfer channel between the upper and lower bridge arms of the MMC. A novel finite compensation method is proposed—instead of the traditional full compensation method—that introduces a real-time variable limiting factor to change the amplitude of the mixed injected high-frequency differential-mode voltage and high-frequency differential-mode current and reduce the amplitude of the common-mode voltage on the AC side while lowering the current stress of the power devices. Finally, a complete system simulation model is constructed, and the topology with the proposed control strategy are verified to have good output characteristics under different operating conditions; good results are achieved in suppressing the sub-module fluctuation and common-mode voltage.

Research on the element structure and surface current distribution of metasurface based on characteristic mode analysis

Scientific research has shown that the geometric structure of the metasurface has a significant impact on its ability to control the propagation of electromagnetic waves. The control of surface current can be achieved by designing the shape of the metasurface unit cell. In this paper, the traditional metasurface is studied using characteristic mode analysis, and a modified metasurface is proposed, which consists of 4 × 4 hexagonal rings with an asymmetric concave interior. The modified metasurface controls the distribution of surface current by increasing the path of surface current, thus lowering the frequency of modal significance. In addition, due to the special asymmetric concave interior, the frequency difference of different modal currents increases, leading to the expansion of the operating bandwidth of modal currents. A metasurface antenna is fabricated and measured for proof of concept. With a size of 0.69λL × 0.69λL × 0.06λL (λL is the wavelength in free space at the lowest operating frequency), the designed antenna achieves a −10 dB impedance band of 40%, 3 dB axial ratio band of 19.4%, and boresight gain of 8 dBi.

Compact folded‐shorted patch array offering dual‐band operation and dual‐circularly polarized radiation for picosatellites and other small satellites

AbstractThe development of a dual‐band folded shorted patch (FSP) antenna array is presented which offers dual‐circularly polarized radiation. Applications include small satellites like CubeSats and picosatellites. The proposed 2 × 2 FSP array design operates at 1.1 and 2.4 GHz and offers good radiation performance in terms of minimal reflection coefficient losses, broad beamwidths (which is generally required for low gain antenna structures ensuring link connectivity), as well as low cross‐polarization levels. The antenna array consists of a meandered layer defining the top of the FSPs. The shorting and folding technique as well as the noted meandering allows further size reduction of the quarter‐wavelength patch. In particular, the proposed antenna array is compact with a total size of 50 mm by 50 mm (or 0.18λ by 0.18λ at the lower operational frequency). An eight‐port coaxial and microstrip feed system is integrated onto the backside of the antenna ground plane, two ports for each element, and with external phase feeding of 90°, dual‐circularly polarized is made possible for the compact antenna unit. To the best knowledge of the authors, no similar antenna structure has been designed, manufactured and tested being useful for picosatellites and other small satellites.

Online Dynamic Optimization of Multi-Rate Processes with the Case of a Fluid Catalytic Cracking Unit

Due to operational limitations in the industrial field, the operating variables of fluid catalytic cracking units (FCCU) are of multiple operating frequencies, which are CO combustion promoter amount, recycle slurry flow rate, combustion air flow rate, heat escape, and reaction temperature, from low frequency to high frequency. There are usually two schemes for operation optimization of FCCU. The former is called single-rate, single-window optimization, whose operating variables are optimized only once in the whole operation cycle, which is easy to achieve, but the optimization effect is poor. The latter is called single-rate multi-window optimization, whose operating variables are optimized repeatedly and whose operation cycle is discretized into multiple optimization periods with the same frequency, which costs a heavy calculation burden and cannot adapt to the optimization variables with multiple operating frequencies. So, a multi-rate, variable-window online dynamic optimization method is proposed. In an operation cycle, the high-frequency operating variable is optimized in a short optimization period, and the low-frequency operating variable is optimized in a long optimization period; each optimization period has integer multiples to the minimum optimization period. Each optimized result for each optimization period is put into use online immediately. The optimization model involves the time domain differential equations, integral cost objective function, and measured disturbances. The experimental results show that compared with the single-rate, single-window optimization method and single-rate multi-window optimization method, the optimization effect of multi-rate, variable-window online dynamic optimization is better than single-rate, single-window optimization but worse than single-rate multi-window optimization. However, the optimization results are consistent with the operation frequency of each optimization variable, which can be implemented in complex chemical processes and increase certain economic benefits.

Analysis of Control Strategy of Arc Plasma Power Supply Inverter Module

The inverter module serves as a critical component in the conversion of electrical energy within arc plasma power sources, exerting a profound influence on the overall performance and stability of the power supply. Consequently, the meticulous design and precise control of the inverter module are of paramount importance in ensuring the effective operation and application of arc plasma power sources. This paper introduces a dual-closed-loop control system, integrating a voltage outer loop with a current inner loop, as the cornerstone of its inverter module design. It also undertook a comprehensive comparative analysis of various voltage-control strategies, encompassing four control methods (PI, PID, PR, QPR) and two modulation techniques (bipolar modulation and unipolar, carrier-based modulation) under diverse operating conditions. Additionally, simulation experiments were conducted on a prototype 10 kW inverter module using the Matlab/Simulink simulation platform, with evaluation criteria including waveform tracking performance, voltage waveform distortion rate, and steady-state error. The results indicate that in low-frequency operating conditions, the voltage-control strategy employing QPR control plus unipolar, carrier-based modulation, and in high-frequency operating conditions, the voltage-control strategy utilizing PI control plus unipolar, carrier-based modulation exhibited superior waveform tracking performance. The waveform distortion rates were measured at below 0.47% and 4.2%, respectively, aligning perfectly with the stringent standards of IEEE 519. This research provides valuable theoretical support and practical guidance for future engineering endeavors in the field of inverters.

The Frequency-Variable Rotor-Blade-Based Two-Degree-of-Freedom Actuation Principle for Linear and Rotary Motion.

Piezoelectric accurate actuation plays an important role in industrial applications. The intrinsic frequency of previous actuators is invariable. However, variable frequency can approach the range near the low-intrinsic-frequency and realize a high actuation capability. The frequency-variable linear and rotary motion (FVLRM) principle is proposed for rotor-blade-based two-degree-of-freedom driving. Inertial force is generated by frequency-variable piezoelectric oscillators (FVPO), the base frequency and vibration modes of which are adjustable by the changeable mass and position of the mass block. The variable-frequency principle of FVPO and the FVLRM are recognized and verified by the simulations and experiments, respectively. The experiments show that the FVLRM prototype moves the fastest when the mass block is placed at the farthest position and the prototype is at the second-order intrinsic frequencies of 42 Hz and 43 Hz, achieving a linear motion of 3.52 mm/s and a rotary motion of 286.9 mrad/s. The actuator adopts a lower operating frequency of less than 60 Hz and has the function of adjusting the natural frequency. It can achieve linear and rotational motion with a larger working stroke with 140 mm linear movement and 360° rotation.

Maximum-Torque Optimization Control of Dual Three-Phase PMSM in Low-Frequency and Static Operations

Maximum-Torque Optimization Control of Dual Three-Phase PMSM in Low-Frequency and Static Operations

A Two-Step Continuous-Control-Set MPC for Modular Multilevel Converters Operating With Variable Output Voltage and Frequency

This paper presents a new enhanced single-stage two-step Continuous Control Set Model Predictive Control (CCS-MPC) algorithm, for Modular Multilevel Converters (MMCs) operating with variable output frequency at the AC port. The proposed two-step CCS-MPC allows the implementation of feed-forward compensation of the one-step ahead perturbations at the AC side, using an algorithm based on the Moore-Penrose pseudoinverse matrix. Limitations of the current and voltages in the MMC clusters are implemented to avoid overmodulation and overcurrents, being the solution of the constrained CCS-MPC online solved using the active-set method. Without losing generality, the experimental validation of the single-stage CCS-MPC is performed using an 18-cell MMC prototype, driving a 7.5 kW cage induction machine operating in a wide speed range. A dSPACE MicroLabBox platform is used to control the MMC-based drive by implementing the CCS-MPC algorithm, the modulation stage and the active-set method. It is experimentally demonstrated that the compensation of the low-frequency perturbations allows a considerable reduction in the required circulating currents at the low-frequency operation of the drive. Reduction in the capacitor voltage ripple, improved efficiency, and a smaller Total Harmonic Distortion (THD) in the output currents are also achieved with the proposed single-stage two-step CCS-MPC.

Broadband quasi-perfect sound absorption by a metasurface with coupled resonators at both low- and high-amplitude excitations

This study presents an acoustic metasurface (AM) consisting of coupled Helmholtz resonators with extended necks (HRENs) to effectively absorb broadband sound waves at both low and high amplitudes. A nonlinear acoustic model is proposed to characterize the AM performance based on the equivalent medium theory and the transfer matrix method. The nonlinear effects due to the high-amplitude excitation are considered by a velocity-dependence equivalent density of the air in the neck. The acoustic characteristics of an AM with two coupled HREN units at the sound pressure level (SPL) ranging from 100dB to 140dB are analytically, numerically, and experimentally investigated. Results from the three approaches show good agreement. The AM achieves quasi-perfect absorption (absorption coefficient α>0.9) in a wide range of SPL, while each HREN unit alone only performs well at around 130dB. The superior absorption performance of the AM at relatively low-amplitude excitation originates from the coherent coupling effects between adjacent units. With the increase of the sound level, the coupling effects tend to deteriorate; fortunately, the sound-vortex conversions occur, contributing to additional acoustic dissipation and thereby sustaining the quasi-perfect absorption capability. Based on the absorption mechanisms, a broadband AM with multiple coupled resonators is designed and manufactured. The measured averaged absorption coefficient of the AM is larger than 0.91 at the SPL below 140 dB (α>0.97 at 100dB) within [700,1100]Hz, and the compact thickness is of 1/18 wavelength at the lowest operating frequency. The findings in this study offer a promising approach to achieve highly efficient broadband sound absorption in a wide-ranging noise level, which holds great potentials in practical applications for noise control.

A novel capacitor voltage fluctuation suppression method for the low-frequency operating stage of MMC driven pumped storage system

The pumped storage power plant, which has the energy conversion and storage capacity, is an important solution for improving the power grid absorption ability to the unstable and fluctuating new energy from wind farms, as well as photovoltaic power stations. The modular multilevel converter (MMC) is a competitive topology for driving the pumped storage motor due to the advantages of high output power level and low output current harmonics. However, the voltages of the sub-module capacitors fluctuate largely when the MMC is operating at low-frequency stage, which would lead to an abnormal start-up process or an unstable low-speed running state of the variable speed pumped storage system. In order to suppress the low-frequency voltage fluctuation of the MMC sub-module capacitors, this paper proposes a charge channel topology including in-phase and inter-phase channel between the sub-modules for transferring the capacitor charge. Then, the reverse fluctuating fundamental component and third harmonic circulating component, as well as the synchronizing fluctuating second harmonic component, of the sub-module capacitor voltages could be counteracted, respectively. Eventually, the start-up operation performance, as well as the low-speed operation performance of the variable speed pumped storage system are enhanced greatly. The effectiveness of the proposed low-frequency capacitor voltage fluctuation suppression method is verified through simulation results.

Dynamic rate analysis for low frequency operation of chemical reactors

Periodic operation of chemical reactors can enhance reactant conversion and product selectivity. It is desirable to identify reaction kinetics and networks that may benefit from non-steady state operation and to quantify the extent of that enhancement without extensive computations. In this study, we describe a simple method to establish approximate functional dependencies of isothermal point reaction enhancement on dynamic and kinetic parameters during low frequency operation. Low frequency operation permits the use of steady state as opposed to transient conservation equations to identify limits of fractional rate enhancement. By approximating local dynamic rates as square waves modulating between two steady states, an analytical function can be derived from global rate expressions. The function relates fractional rate enhancement to concentration wave amplitudes, phase shifts, cyclic averages, duty cycles and reaction orders for one or more dynamically-fed reactants. We show that the method may be applied to predict integral conversion enhancement for ideal reactors. During integral operation, nonstoichiometric concentration waves may change phase through disproportionate consumption of reactants at wave maxima and minima, hence inverting waves out-of-phase (180°) from their initial waveform. This may cause local rate enhancement to transition to rate diminishment. Three experimental examples involving catalytic oxidations are described that illustrate selected theoretical aspects of the study.

Roton-Enabled Mechanical Diode at Extremely Low Frequency

Abstract This paper presents a refined model for a mechanical diode based on a mass-spring system. The proposed model utilizes a bilinear spring to construct a frequency converter, which effectively disrupts the reciprocal transmission of acoustic waves. By employing a mass-spring-mass system as a filter, a nonlocal connection is introduced to generate an extremely low-frequency band gap (2–4 Hz), thereby achieving a mechanical diode with a lower operating frequency. The feasibility of these low-frequency mechanical diodes is demonstrated through comprehensive numerical simulations and experimental analyses. In addition, we evaluated the effect of bilinear springs and nonlocal connection parameters on the diode performance.