This paper proposes an efficient and accurate scattered field prediction method based on Kirchhoff Approximation called `Mirror Kirchhoff Approximation' (MKA) which is suitable for evaluating the shadowing effect by a metal cuboid. The disadvantages of conventional methods, such as low accuracy of Kirchhoff Approximation (KA) for metal cuboid and high computational complexity of Method of Moment (MoM) for a shadowing object at millimeter wave (mmWave), have motivated the establishment of an efficient and accurate prediction method for a metal cuboid at mmWave. The proposed method solves the previous issues by introducing the ray-based reflection into conventional KA. The idea and detail formulations of the proposed method are presented. The proposed method is validated by comparing with MoM and KA in terms of complexity and accuracy. The results imply that the proposed method presents good accuracy with low calculation time. The MKA has a maximum 8.3 dB improvement compared with conventional KA. Calculating time is improved by 392-915 times compared with MoM.
Transcranial magnetic stimulation (TMS) has been widely used in the treatment of varied physical and neuropsychiatric disorders, especially in major depression. The intracranial electromagnetic field is generated by the the time-varying current in the stimulation coil to change the potential of targeted neurons during the treatment. Since different mental disorders correspond to specific stimulation targets and broad stimulation range might raise serious side effects, stimulation focalization is very important in TMS. To achieve focalized stimulation, a novel magnetic stimulation coil with the field shaper and the crescent ferromagnetic core (the FSMC coil) is proposed and optimized in this study. The Finite-Element Method (FEM) is adopted to analyze the relationships between the design parameters of the field shaper and crescent ferromagnetic core and the characteristics of the intracranial electromagnetic field. Compared to traditional single circular coil, the focalization of the intracranial electromagnetic field generated by the optimized FSMC coil can be significantly improved both from 2D and 3D levels. To verify our method, an anatomically realistic human head model with different electrical properties assigned to each tissue of the brain is employed in this paper. We also checked the maximum induced charge density on the targeted plane generated by the optimized coil to make sure that it will not cause any induced neurologic damage.
In this paper, we present a general solution for time-harmonic electromagnetic fields with its electric and magnetic fields parallel to each other (E || B fields) in source-free vacuum and demonstrate that every time-harmonic E || B field is composed of the superposition of two counter-propagating Beltrami fields. We show that every E || B field can be categorized into one of two cases depending on the time dependence of the function that describes the proportionality between the electric and magnetic fields. After presenting the mathematical definition of a Beltrami field in electromagnetism and its handedness, we perform a detailed analysis of time-harmonic E || B fields for each case. For the first case, we find the general solution for the E || B fields using the angular-spectrum method and prove that every first-case E || B field can be generated by superposing two oppositely traveling Beltrami fields with the same handedness. For the second case, we deduce the general solution for the E || B fields by employing complex analysis and demonstrate that every time-harmonic E || B field is composed of two counter-propagating planar Beltrami fields with opposite handedness.
The research goal of low-resolution radar aircraft target classification is to analyze the category of the given low-resolution radar aircraft target echo. In existing solutions, the feature extraction methods based on rotating modulation spectrum have good performance, such as the complex cepstrum method, autocorrelation method, cycle diagram method, autoregressive model power spectrum method, and singular value decomposition method. Most of these methods are more complicated in calculations, and practical applications often require higher pulse frequencies and longer observation times, which are greatly restricted. In this paper, a classification method based on ensemble empirical mode decomposition and multifractal correlation (CMEEMDMFC) is proposed. The basic design idea is to obtain the intrinsic mode functions (IMFs) by using the signal decomposition ability of ensemble empirical mode decomposition (EEMD) and select some components which are beneficial for improving the signal-to-noise ratio (SNR) for recombination. Then extract the corresponding multifractal correlation (MFC) features from the new signals for recognition. For verifying the validity of the model, a comparison model was selected to test on the same data set. Experimental results show that the proposed model performs well in classification accuracy.
We studied electromagnetic wave propagation in a system that is periodic in both space and time, namely a discrete 2D transmission line (TL) with capacitors modulated in tandem externally. Kirchhoff's laws lead to an eigenvalue equation whose solutions yield a band structure (BS) for the circular frequency ω as function of the phase advances kxa and kya in the plane of the TL. The surfaces ω(kxa, kya) display exotic behavior like forbidden ω bands, forbidden k bands, both, or neither. Certain critical combinations of the modulation strength mc and the modulation frequency Ω mark transitions from ω stopbands to forbidden k bands, corresponding to phase transitions from no propagation to propagation of waves. Such behavior is found invariably at the high symmetry X and M points of the spatial Brillouin zone (BZ) and at the boundary ω = (1/2)Ω of the temporal BZ. At such boundaries the ω(kxa, kya) surfaces in neighboring BZs assume conical forms that just touch, resembling a South American toy ``diábolo''; the point of contact is thus called a ``diabolic point''. Our investigation reveals interesting interplay among geometry, critical points, and phase transitions.
Dual-rotor permanent magnet motor has the characteristics of high torque density and high efficiency and has a wide range of application prospects in many fields. However, the double air-gap structure also makes the internal magnetic field distribution more complicated and torque fluctuation more serious. To improve the double-layer air-gap magnetic field distribution and reduced torque pulsation, based on the Halbach array magnetization, the inner and outer irregular Halbach array dual-rotor permanent magnet motor model was established to obtain the ideal one-sided magnetic field. By comparing the magnetic field distribution of the inner and outer layers, the no-load back-EMF, and the cogging torque, it is proved that the motor with the proposed structure can optimize the air-gap magnetic density and no-load back-EMF and reduce the cogging torque; at the same time, the torque ripple is also significantly reduced to ensure the stability of the motor operation.
A multi-band cylindrical conformal endoscopy antenna with low specific absorption rate (SAR) is designed for endoscope applications, which covers the Industrial, Scientific and Medical bands (ISM 902-928 MHz, 2.4-2.4835 GHz), Medical Device Radio Communications Service band (MedRadio 401-406 MHz) and Wireless Medical Telemetry Service (WMTS 608-614 MHz). The proposed antenna radiates as a symmetrical meanderline structure with a center loaded parasitic opened-loop element, which is bent into the cylindrical conformal shape and wrapped onto the inner wall of the capsule shell. The parasitic opened-loop element excites low frequency resonance at 403 MHz and reduces the SAR values of the antenna. The measured relative bandwidth (|S11| < -10 dB) of the antenna implements 133% ultra-wideband, ranging from 0.35 GHz to 1.76 GHz, and 39% wideband, ranging from 2.01 GHz to 3 GHz. The peak gains and the peak 1 g SAR values at 403 MHz, 611 MHz, 915 MHz, 2.4 GHz are -26.6 dBi, -18.9 dBi, -11.8 dBi, -11.3 dBi, and 83, 82, 94, 153 W/kg, respectively. The results indicate that the proposed antenna complies well with the human safety standards.
In this paper, a direct instantaneous torque control (DITC) optimization scheme based on adaptive dynamic hysteresis (ADH) strategy is proposed for switched reluctance motor (SRM) drive system. This method can further improve the torque tracking accuracy, reduce torque ripple and solve the problem of smooth transition between SRM phases. According to the torque generation characteristics and hysteresis rule of DITC, the traditional hysteresis rule is modified, and the sampled discrete torque slope data are compensated online. Taking the slope curve after compensation as the standard, the upper and lower limits of the hysteresis controller are assigned to achieve the control effect of hysteresis dynamic regulation. The effectiveness of the method is verified by simulation under diferent operating conditions.
In this paper, an S/C/Ku triple-band (TB) dual-polarization (DP) shared aperture array (SAA) with an approximate frequency ratio of 1:1.8:4.5 is proposed. An S/C dual-band dual-polarized (DBDP) perforated patch antenna is designed as a shared element, and the S-band array consists of S-band-working shared elements. The C-band array interlaces with C-band-working shared element and C-band cross-patches with phase compensation. The Ku band array consists of dual-polarization rectangular dielectric resonator antennas (DRAs) for its small section area and high design freedom. In order to ensure the symmetry of the structure, all the ports adopt a vertical welding structure. A 1 × 3 prototype array is fabricated and measured, showing that the S-, C-, and Ku-bands obtain the bandwidths of 2%, 2.1%, and 1.8%, respectively. The polarization isolation is better than 20 dB in all three bands, while the cross-polarization is lower than -20 dB. The proposed array has the advantages of low cost and high integration; moreover, the proposed array owns excellent potential for expansion to large aperture benefited from its symmetry.
A scheme for radar cross section (RCS) reduction of microstrip antenna array in wideband using artificial magnetic conductors (AMC), without compromising the radiation characteristics of the antenna array, is proposed. This design is based on the principle of passive cancellation. The novelty is that the reflection characteristics of the microstrip antenna array are also taken into consideration during the design process of AMCs. The aperiodic configuration is composed of three kinds of AMC lattices with selected dimensions and is applied to the design of microstrip antenna array for the purpose of RCS reduction. The simulated results show that the monostatic RCS is reduced over a wideband from 15.2 to 35 GHz (about 79% relative bandwidth), covering the operation band (20-20.75 GHz) of the antenna array. In addition, compared with the periodic configuration, it has about 4 dB lower maximum bistatic RCS.
Electromagnetic properties of a planar metallic metasurface with the design inspired by Babinet's principle are numerically studied. The metasurface is constructed from a metal plate perforated by coaxial-sector apertures. It is shown that the chosen coaxial-sector apertures make it possible to obtain a wider operating range of the metasurface than those composed of apertures of other shapes (e.g. round or rectangle). Moreover, the proposed metasurface performs an efficient polarization conversion of the linearly polarized wave to elliptically and circularly polarized ones in the reflected field.
This paper presents a method for rapid microwave imaging of traumatic brain injury based on scattering parameters. The algorithm uses the integer order Bessel function and Born approximation, which converts nonlinear inverse scattering problem into linear problem. After truncated singular value decomposition, imaging can be performed without iteration. Simulations and experiments show that the algorithm can not only reduce the amount of calculation for fast imaging, but also accurately image a brain hematoma or foreign body.
An integrated wireless impulse generator has been designed, simulated, fabricated and tested. Switched oscillator topology has been used as an impulse generator. A switched oscillator consists of a low impedance transmission line, which is charged by a DC source with a large input impedance. The transmission line is connected to a fast closing switch at one end and a high feed-point impedance antenna at the other end. After charging the transmission line, closing the fast switch short circuits the transmission line, resulting in a transient wave propagating toward the antenna. The mismatch between transmission line characteristic impedance and the antenna feed point impedance causes a reflection at the antenna terminal. Due to the short circuit at switch terminal, the reflected signal will reflect back at the switch terminal as well. This back and forth reflection generates a series of pulses at the antenna terminal which will be radiated by the antenna. The switched oscillator impulse generator is designed to operate in the industrial, scientific and medical (ISM) radio frequency band.
A bearingless induction motor (BL-IM) is a new type of motor integrating suspension and rotation functions. Higher requirements are put forward for its suspension performance. Due to the material advantages of a new type of amorphous alloy with high magnetic conductivity, low loss and low coercivity, it is considered to be used in the BL-IM rotor to reduce iron loss and improve the electromagnetic performance of the BL-IM. Finite element analysis software is used to analyze the performance of two different kinds of motors with the rotors made of conventional silicon steel and amorphous alloy respectively. The magnetic field density distribution, torque, speed, and radial force are compared between the two motors. The results show that the speed of amorphous alloy motor increases faster, and the rotor has better suspension characteristics. Moreover, the amorphous alloy material has a smaller density; the material properties can effectively reduce the weight of the motor; it is beneficial to the operation of the BL-IM in special environments.
A free-space and non-invasive measurement technique to characterize the dielectric properties of a non-magnetic NASA-developed composite material is presented. To estimate the dielectric properties of the composite material, the material under test is placed as a superstrate over a pre-characterized benchmark antenna. The reflection coefficients and gain of the superstrate-loaded antenna are then utilized to estimate the relative permittivity and loss tangent of the composite under test, respectively. Using numerical analyses and measurements of the benchmark antenna loaded with the superstrate, the aforementioned properties are estimated to be 6 and ~0.12, respectively. To validate the accuracy of the method, a square microstrip patch antenna is also designed on a grounded NASA-developed composite material at the ISM band.
When particle swarm optimization (PSO) is used to identify the parameters of permanent magnet synchronous motor (PMSM), the movement of particles is not selective, which makes the algorithm easy to fall into the local optimum, and the accuracy is poor. The simulated annealing particle swarm optimization (SAPSO) improves the accuracy and evolution speed, but SAPSO has redundant iteration problems. To solve these problems, a motor parameter identification method based on fast backfire double anneal particle swarm optimization (FBDAPSO) is proposed. By reducing the optimization time and quickly tempering and annealing the "misunderstood" difference, the motor adjustable model and fitness function are designed, and the number of iterations is constantly reset to achieve the effect of online identification. Under different working conditions, simulated and experimental results show that the proposed method can quickly and accurately identify the four parameters of the motor’s stator, winding resistance, stator winding d-axis inductance, stator winding q-axis inductance and permanent magnet flux linkage at the same time, compared with the traditional method of parameter identification, and it has better accuracy, rapidity, and robustness.
This paper deals with analyzing a novel eddy current damper for an axially magnetized multi-ring permanent magnet thrust bearing (MPMTB). Initially, the bearing is optimized for maximum axial force by selecting three general parameters (air gap, outer diameter of stator, and length) using a generalized optimization procedure. Then, the axial force of an optimized bearing is validated with the mathematical model results. Finally, the novel and conventional eddy current dampers (ECDs) for an optimized MPMTB are analyzed for damping forces and coefficients using three-dimensional (3D) finite element transient analysis in ANSYS. Based on the analysis results, the proposed novel structure could be selected to replace the conventional one for providing damping to MPMTB effectively without affecting the radial air gap between the rotor and stator rings.
Due to low power density, it is difficult for a single-band rectenna to harvest enough power for IoT devices like wireless sensors. Thus to supply these consuming devices, harvesting RF energy from multiple frequencies is a solution to enhance the amount of harvested DC power. In this work, we introduce a triple-band rectenna, working at 900 MHz, 1.8 GHz and 2.1 GHz, three readily available bands in the ambience, for energy harvesting application. The proposed rectenna consists of three monoband rectifiers connected to a multi-band receiving antenna via a highly efficient triplexer. The antenna is made by superposing two concentric rings and manipulating their radii to achieve the desirable operating frequencies, with antenna gains of respectively 2.5 dBi, 4.5 dBi, and 4 dBi. The contiguous triplexer is made by connecting open stubs band-reject filters and optimizing their positions, resulting in the triplexing efficiency higher than 75%. The measured RF-DC efficiency under -10 dBm triple-tone input power is 40%.