Natural Resonance Frequency Research Articles

Miniaturized active stepped impedance planar inverted-F antenna using common ground

This paper presents a compact active integrated antenna (AIA) comprising of class-A power amplifier (PA) and stepped impedance planar inverted-F antenna (PIFA). In the proposed design, a common ground is used for both PA and PIFA, resulting a compact antenna of size 0.14λ0×0.11λ0×0.01λ0 (λ0 is free space wavelength at 0.85GHz). Moreover, it is demonstrated that by using the stepped impedance radiator the operating frequency of the active PIFA is shifted down from its natural resonant frequency of 1.36GHz to 0.85GHz, offering an extensive size reduction of 80%. This active integration increases the passive antenna gain through the effective loading of the antenna to the power amplifier. The measured result indicates that the active and passive antennas achieved the gain of 15.7dB and 3.81dBi, respectively after the integration. In addition, the maximum SAR value of antenna is found to be 0.64W/kg.

An Optimization and Comparative Study of Air-Coupled CMUT Cells With Circular and Annular Geometries.

Air-coupled capacitive micromachined ultrasonic transducers (CMUTs) with annular cell geometry have recently been reported to have a promising transmit sensitivity. This paper reports three optimization schemes, which further improve the transmit sensitivity and also help achieve a reasonable comparison between the novel annular and conventional circular cells. Lumped element models of both cell types with laminate plate structures are presented. Based on these models, a design optimization flowchart was constructed to facilitate analytical optimization on the three schemes. Circular and annular CMUTs with a common 97-kHz natural resonance frequency were fabricated and characterized to verify the efficacy of the optimization principle. Using the optimization flowchart, annular and circular cells with frequencies ranging from 100 to 300 kHz were analytically optimized and then compared. The comparison results demonstrate that, given the same dc bias and ac excitation voltage, the output power density at the plate surface of the optimized annular cell is double that of the optimized circular cell. Additionally, when generating the same surface power density, an optimized annular cell requires either half the dc bias or half the ac excitation voltage of an optimized circular cell. This paper provides a practical optimization framework for CMUT cell design and demonstrates the superiority of annular cells for air-coupled applications.

OWC WEC integrated within a breakwater versus isolated: Experimental and numerical theoretical study

Oscillating Water Column (OWC) devices are one of the most promising technologies to be implemented into already existing or in-development ocean structures, such as breakwaters. All moving components to retrieve power from the waves are well above the waterline and the still structure can be easily incorporated within solid edifices. In this paper, we investigate the hydrodynamic response of two types of Bent Duct OWC devices with different inlet geometries, both in isolation and when implemented in a flat-faced breakwater. A rectangular and circular cross-sectional shaped OWC device are used for comparison. Numerical method using a FEM based frequency domain model and experimental investigation using the Australian Maritime College wave basin are applied and the results evaluated across a range of frequencies, 0.5Hz–1.2Hz. Both the capture width and volume flux resulting from the numerical method match accurately with those resulting from the experiment. Discrepancies only arise around the natural resonance frequency where the assumptions of small amplitudes become erroneous. The implementation of the device within the breakwater was found to significantly enhance the capture width of each device, while the variation in inlet geometry provided low deviations in the results.

High-frequency properties of [Co90Nb10/Ni0.45Zn0.55Fe2O4(t)]6 multilayer films for application in GHz range

A series of [Co90Nb10/Ni0.45Zn0.55Fe2O4(t)]6 multilayer films were fabricated on Si(111) substrates by means of radio frequency magnetron sputtering. The influences of Ni-Zn ferrite interlayer thicknesses (t) on the static and high-frequency soft magnetic properties of multilayer films have been investigated. As increasing the ferrite thickness (t) from 0 to 6 nm, the saturation magnetization Ms decreased from 876 emu/cm3 to 488 emu/cm3, but the in-plane uniaxial magnetic anisotropy effective field Hk and electrical resistivity ρ varied from 17 Oe to 52 Oe and from 170 µΩ cm to 3670 µΩ cm respectively. As a consequence, the resonance frequency fr of the multilayer films were continuously increased from 1.45 GHz to 2.7 GHz. The results showed that the interlayer ferrite thickness had an important effect on magnetic properties of the multilayer films and that it can be convenient to adjust the natural resonance frequency.

Resonance frequency control of RF normal conducting cavity using gradient estimator of reflected power

Control of the natural resonance frequency of an RF cavity is essential for accelerator structures due to their high cavity sensitivity to internal and external vibrations and the dependency of resonant frequency on temperature changes. Due to the relatively high radio frequencies involved (MHz to GHz), direct measurement of the resonant frequency for real-time control is not possible by using conventional microcontroller hardware. So far, all operational cavities are tuned using phase comparison techniques. The temperature dependent phase measurements render this technique labor and time intensive. To eliminate the phase measurement, reduce man hours and speed up cavity start up time, this paper presents a control theme that relies solely on the reflected power measurement. The control algorithm for the nonlinear system is developed through Lyapunov’s method. The controller stabilizes the resonance frequency of the cavity using a nonlinear control algorithm in combination with a gradient estimation method. Experimental results of the proposed system on a test cavity show that the resonance frequency can be tuned to its optimum operating point while the start up time of a single cavity and the accompanied man hours are significantly decreased. A test result of the fully commissioned control system on one of TRIUMF’s DTL tanks verifies its performance under real environmental conditions.

Analysis and Damping of Mechanical Resonance of Wind Power Generators Contributing to Frequency Regulation

Wind power generators will become increasingly useful in modern power systems; therefore, they should be able to support the system frequency in a way similar to that of conventional generators. Droop and virtual inertia are the most reported methods in the literature. However, their impact on the generator and system frequency stability, when the double-mass mechanical dynamics of a wind power generator are considered, has not been addressed thoroughly. In this paper, small-signal modeling, analysis, and eigenvalues studies are used to show that incorporating a wind power generator in the frequency regulation can expose its shaft to forces stimulating its natural resonance frequency dynamics and lead to instability. This paper shows that the mechanical resonance of frequency-regulating wind generators must be studied and enhanced not individually but as a part of the whole power system stability analyses. To overcome the stability problem caused by implementing frequency regulation in wind generators, this paper investigates different alternatives to stabilize the generator dynamics and, at the same time, minimize undesirable interference in the conventional wind power generator controllers. Time-domain simulation results validate the analytical results and discussions.

Epitaxially stabilized thin films of \u03b5-Fe2O3 (001) grown on YSZ (100)

Epsilon ferrite (ε-Fe2O3) is a metastable phase of iron(III) oxide, intermediate between maghemite and hematite. It has recently attracted interest because of its magnetocrystalline anisotropy, which distinguishes it from the other polymorphs, and results in a gigantic coercive field and a natural ferromagnetic resonance frequency in the THz range. Moreover, it possesses a polar crystal structure, making it a potential ferroelectric, hence a potential multiferroic. Due to the need of size confinement to stabilize the metastable phase, ε-Fe2O3 has been synthesized mainly as nanoparticles. However, to favor integration in devices, and take advantage of its unique functional properties, synthesis as epitaxial thin films is desirable. In this paper, we report the growth of ε-Fe2O3 as epitaxial thin films on (100)-oriented yttrium-stabilized zirconia substrates. Structural characterization outlined the formation of multiple in-plane twins, with two different epitaxial relations to the substrate. Transmission electron microscopy showed how such twins develop in a pillar-like structure from the interface to the surface. Magnetic characterization confirmed the high magnetocrystalline anisotropy of our film and revealed the presence of a secondary phase which was identified as the well-known magnetite. Finally, angular analysis of the magnetic properties revealed how the presence of twins impacts their azimuthal dependence.

Self-Tuning Variable Frequency Controller for Inductive Electric Vehicle Charging With Multiple Power Levels

A self-tuning controller for contactless electric vehicle (EV) charging systems based on inductive power transfer (IPT) with multiple power levels is proposed. The multiple charging levels (consisting of ten charging levels) are achieved by controlling the energy injection frequency of the transmitter coil of the IPT system. The proposed controller is capable of self-tuning the switching operations to the natural resonance frequency of the IPT system, and it benefits from soft-switching operations (zero-current switching), which ensures the maximum performance of the IPT system. The proposed controller has such a simple design, which can be implemented based on a simplified control circuit. The simulation of the proposed controller for an inductive charging system at different charging levels is carried out in MATLAB/Simulink. Also the proposed controller with an ac/dc/ac converter is implemented experimentally on an IPT charging system to verify the effectiveness of the controller at different charging levels. The experimental test results conform with the simulation results and verify that the proposed controller effectively enables self-tuning capability and soft-switching operations at different charging levels for IPT-based contactless EV charging systems.

The stabilization system of primary oscillation for a micromechanical gyroscope

The mode of primary oscillations of a micromechanical gyroscope (MMG) sensor is provided by an electrostatic comb-drive actuator in which the interaction between the micromechanical structures and electronics occurs by means of a single or differential capacitive sensor. Two pairs of capacitive sensors are traditionally used for frequency stabilization of MMG primary oscillations. The first pair of capacitive sensors excites primary oscillations, while the second measures the amplitude of primary oscillations. The stabilization system provides a continuous frequency tuning of primary oscillations that increases the duration of transition processes, the time of operational readiness, and the instability of the output signal from the secondary oscillation channel of the MMGs. This paper presents a new approach to the primary oscillation control system of the two-component MMG. The method of calculating the natural resonant frequency is based on measurements of the total current passing through the comb-driver actuator capacitances, and a lock-in detection is suggested. This paper consists of the results of the numerical analysis, the description of the proposed approach to the frequency control of the primary MMG oscillations, and the Simulink model of the behaviour of the MMG stabilization system, depending on its mechanical-and-physical properties with regard to a 2% shift of the natural resonant frequency. The frequency control of the primary oscillations at 2% frequency detuning is performed within 0.11 s.

A benchtop biorobotic platform for in vitro observation of muscle-tendon dynamics with parallel mechanical assistance from an elastic exoskeleton

We present a novel biorobotic framework comprised of a biological muscle-tendon unit (MTU) mechanically coupled to a feedback controlled robotic environment simulation that mimics in vivo inertial/gravitational loading and mechanical assistance from a parallel elastic exoskeleton. Using this system, we applied select combinations of biological muscle activation (modulated with rate-coded direct neural stimulation) and parallel elastic assistance (applied via closed-loop mechanical environment simulation) hypothesized to mimic human behavior based on previously published modeling studies. These conditions resulted in constant system-level force-length dynamics (i.e., stiffness), reduced biological loads, increased muscle excursion, and constant muscle average positive power output—all consistent with laboratory experiments on intact humans during exoskeleton assisted hopping. Mechanical assistance led to reduced estimated metabolic cost and MTU apparent efficiency, but increased apparent efficiency for the MTU+Exo system as a whole. Findings from this study suggest that the increased natural resonant frequency of the artificially stiffened MTU+Exo system, along with invariant movement frequencies, may underlie observed limits on the benefits of exoskeleton assistance. Our novel approach demonstrates that it is possible to capture the salient features of human locomotion with exoskeleton assistance in an isolated muscle-tendon preparation, and introduces a powerful new tool for detailed, direct examination of how assistive devices affect muscle-level neuromechanics and energetics.

Experimental and Numerical Study on Pressure Pulsations Under Various Acoustic Boundary Conditions in Piping Systems

To improve design and troubleshooting techniques of piping systems for operating power plants, it is necessary to investigate, by experiment and simulation, the behavior of fluid inside the piping system in detail. This study was conducted using full-scale piping system under conditions that could seriously threaten the plant operation, by matching pressure pulsations, acoustic resonance, and piping natural frequency. Although piping vibration is reported to influence fluid pressure pulsations, there were no such examples of influence in this experiment. Knowing that the opening ratio of the pressure control valve affects the boundary condition for acoustic resonance, experiment and simulation at different opening ratios were conducted. It has been suggested that the cases in which a valve partially open at 25% or less should not be taken as a closed end. This finding conflicts with such a widespread design assumption.

Post annealing induced magnetic anisotropy in CoFeSi thin films on MgO(0 0 1)

Co61Fe26Si13 (CoFeSi) thin films were deposited on MgO(0 0 1) substrates by magnetron sputtering and were annealed at different temperatures (Ta). X-ray diffraction revealed that the crystal structure varies from the as-deposited amorphous phase to the cubic phase when Ta = 800 °C, indicating the re-crystallization process after annealing treatment. The static magnetic property measurements show a gradually four-fold cubic anisotropy dominating the magnetic anisotropy with the increasing Ta. The ferromagnetic resonance technique based on vector network analyzer was utilized to analyze the dynamic magnetic properties. CoFeSi film annealed at 600 °C presents an obvious in-plane uniaxial magnetic anisotropy, and it presents a low effective damping constant of around 0.011 with a natural resonance frequency of around 3.4 GHz. However, the CoFeSi film annealed at 800 °C showed a strong cubic magnetocrystalline anisotropy and full-in-plane-direction workable natural resonance frequency at around 2.5 GHz.

A MAC based excitation frequency increasing method for structural topology optimization under harmonic excitations

This work is focused on the topology optimization of structures that are subjected to harmonic force excitation with prescribed frequency and amplitude. As an important objective of such a design problem, the natural resonance frequency of the structure is driven far away from the prescribed excitation frequency for the purpose of avoiding resonance and reducing the vibration level. Therefore when the excitation frequency is higher than the natural resonance frequency of the structure, the natural resonance frequency will decrease, then the optimum topology configuration will be distorted with large amount of gray elements. A MAC (Modal Assurance Criteria) based excitation frequency increasing method is proposed to obtain a desired configuration. MAC is adopted here to track the natural resonance frequency which can provide the baseline reference for the current excitation frequency during the optimum iterative procedure. Then the excitation frequency increases progressively up to its originally prescribed value. By means of numerical examples, the proposed formulation can generate effective topology configurations which can avoid resonance.

Forced vibration of fluid conveying carbon nanotubes considering thermal effect and nonlinear foundations

Nonlinear and linear vibrations of fluid conveying carbon nanotubes considering thermal effects are studied. It is assumed that the nanotube conveying fluid flow has a general type of boundary conditions and is supported on a nonlinear Winkler-Pasternak foundation. The nonlocal Euler-Bernoulli beam theory is implemented to develop the governing differential equation of motion of the nanotube system. Furthermore, the surface effect is considered in the vibration modelling of the carbon nanotubes. The influence of the thermal effect combined with the applied longitudinal force on the vibration performance of the system is examined. The Galerkin procedure is employed to obtain the nonlinear ordinary differential equation and the multiple time-scales method is utilized to study the primary vibration resonance of the system. A parametric sensitivity analysis is carried out to reveal the influence of different parameters on the vibration primary resonance and linear natural frequency. The effects of temperature variation, boundary conditions, foundation coefficients and the fluid flow velocity on the primary resonance and natural frequency of the nanotube are investigated. The effect of temperature on the material properties of carbon nanotube is originally combined with the vibration model. Different types of Zigzag carbon nanotubes are considered and the effects of different parameters, namely, the fluid flow velocity and the temperature variations are compared.

An analytical and numerical investigation of acoustic attenuation by a finite sonic crystal

Sonic crystals are scatterers arranged periodically in a homogeneous fluid medium, for which sound does not transmit through the crystal in certain frequency bands known as stop bands. Acoustic wave transmission through a two-dimensional sonic crystal composed of a finite array of scatterers is investigated. Two types of scatterers are considered: sound-hard cylinders and C-shaped locally resonant scatterers. An analytical method is devised to solve the corresponding multiple scattering problems. The method combines an integral equation technique for the single scatterer with an enhanced multipole method using domain decomposition into slabs. A numerical approach using commercial software is also considered for validation and is based on the finite element method. Simulations of sound transmission through an array of 5 by 51 scatterers show remarkably good agreement with the corresponding infinite system. For an array comprising locally resonant scatterers, an approximate band gap around the resonator natural frequency is observed in addition to the band gap due to the overall periodicity of the finite sonic crystal.

Investigation of block foundations resting on soil–rock and rock–rock media under coupled vibrations

In the present study, the dynamic response of block foundations of different equivalent radius to mass (Ro/m) ratios under coupled vibrations is investigated for various homogeneous and layered systems. The frequency-dependent stiffness and damping of foundation resting on homogeneous soils and rocks are determined using the half-space theory. The dynamic response characteristics of foundation resting on the layered system considering rock–rock combination are evaluated using finite element program with transmitting boundaries. Frequencies versus amplitude responses of block foundation are obtained for both translational and rotational motion. A new methodology is proposed for determination of dynamic response of block foundations resting on soil–rock and weathered rock–rock system in the form of equations and graphs. The variations of dimensionless natural frequency and dimensionless resonant amplitude with shear wave velocity ratio are investigated for different thicknesses of top soil/weathered rock layer. The dynamic behaviors of block foundations are also analyzed for different rock–rock systems by considering sandstone, shale and limestone underlain by basalt. The variations of stiffness, damping and amplitudes of block foundations with frequency are shown in this study for various rock–rock combinations. In the analysis, two resonant peaks are observed at two different frequencies for both translational and rotational motion. It is observed that the dimensionless resonant amplitudes decrease and natural frequencies increase with increase in shear wave velocity ratio. Finally, the parametric study is performed for block foundations with dimensions of 4 m × 3 m × 2 m and 8 m × 5 m × 2 m by using generalized graphs. The variations of natural frequency and peak displacement amplitude are also studied for different top layer thicknesses and eccentric moments.

Structural and magnetic properties of La3+ substituted barium−natural nanoferrites as microwave absorber in X-band

Selection of proper microwave absorbers in the X-band is vital to prevent the interference issues that often damage devices and cause signal degradation. In this spirit, we prepared three La3+ substituted barium-natural nanoferrites (BNFs) samples with chemical composition of BaO:(x)La2O3:(6-x)Fe2O3 (x=0, 0.1, 0.2 in mol) via solid-state reaction route. Synthesized samples were characterized via SEM, XRD, VSM, and VNA measurements to determine the La3+ ions concentration dependent variation in the structural, magnetic and microwave absorption properties. Transmission/reflection line (TRL) method was used to evaluate the samples reflection loss. La3+ free samples sintered at 1100°C revealed hexagonal BaFe12O19 and rhombohedral Fe2O3 phases. SEM images displayed the growth of new particle with the average size of 0.2 – 0.8µm as filler in BNFs. Furthermore, an incorporation of La3+ into the BNF system manifested the emergence of new BaLa2Fe2O7 tetragonal crystal phase. The average crystallite size of BNF was found to decrease with increasing La3+ ion concentrations. Conversely, substitution of La3+ in the BNF caused insignificant changes in the magnetic properties, the real part of the relative permittivity and the natural resonance frequency. Meanwhile, a reasonable shift in the microwave frequency absorption and enhancement in the reflection loss was evidenced due to the inclusion of La3+. BNF sample containing 0.2mol La2O3 exhibited a saturation magnetization and magnetic field anisotropy of 19.02 and 0.36T, respectively, where the maximum reflection loss is discerned to be −26.61dB at 10.87GHz with 1.25GHz bandwidth. This new class of ferrites may be prospective for microwave absorber in the X-band.

On the mechanism of bandgap formation in locally resonant finite elastic metamaterials

Elastic/acoustic metamaterials made from locally resonant arrays can exhibit bandgaps at wavelengths much longer than the lattice size for various applications spanning from low-frequency vibration/sound attenuation to wave guiding and filtering in mechanical and electromechanical devices. For an effective use of such locally resonant metamaterial concepts in finite structures, it is required to bridge the gap between the lattice dispersion characteristics and modal behavior of the host structure with its resonators. To this end, we develop a novel argument for bandgap formation in finite-length elastic metamaterial beams, relying on the modal analysis and the assumption of infinitely many resonators. We show that the dual problem to wave propagation through an infinite periodic beam is the modal analysis of a finite beam with an infinite number of resonators. A simple formula that depends only on the resonator natural frequency and total mass ratio is derived for placing the bandgap in a desired frequency range, yielding an analytical insight and a rule of thumb for design purposes. A method for understanding the importance of a resonator location and mass is discussed in the context of a Riemann sum approximation of an integral, and a method for determining the optimal number of resonators for a given set of boundary conditions and target frequency is introduced. The simulations of the theoretical framework are validated by experiments for bending vibrations of a locally resonant cantilever beam.

Canonical Model and Design Methodology for <italic>LLC</italic> DC/DC Converter With Constant Current Operation Capability Under Shorted Load

Different from the pulse width modulation dc/dc converters, short-circuit operation is one of the most unpredictable and challenging aspects in an LLC dc/dc converter. In order to reveal how the switching frequency affects the resonant voltage and current stress under shorted load, as well as associated output current, short-circuit operation of the LLC converter with and without diode clamping is analyzed, respectively, in this paper. A critical switching frequency, i.e., 1.5 × natural resonant frequency, is identified for the first time, which is the frequency boundary of the effectiveness of clamping diodes. Based on the derived mathematic model, canonical current gain curves and a generic voltage/current stress portrait ( Q–k map) under shorted load are proposed for the operation prediction. A new design methodology is further proposed for the design of the LLC dc/dc converter with constant current regulation capability under shorted load. The theoretical analysis and application of the proposed model are verified thoroughly by simulation and hardware experiment.

Frequency adjustable MEMS vibration energy harvester

Ambient mechanical vibrations offer an attractive solution for powering the wireless sensor nodes of the emerging “Internet-of-Things”. However, the wide-ranging variability of the ambient vibration frequencies pose a significant challenge to the efficient transduction of vibration into usable electrical energy. This work reports the development of a MEMS electromagnetic vibration energy harvester where the resonance frequency of the oscillator can be adjusted or tuned to adapt to the ambient vibrational frequency. Micro-fabricated silicon spring and double layer planar micro-coils along with sintered NdFeB micro-magnets are used to construct the electromagnetic transduction mechanism. Furthermore, another NdFeB magnet is adjustably assembled to induce variable magnetic interaction with the transducing magnet, leading to significant change in the spring stiffness and resonance frequency. Finite element analysis and numerical simulations exhibit substantial frequency tuning range (25% of natural resonance frequency) by appropriate adjustment of the repulsive magnetic interaction between the tuning and transducing magnet pair. This demonstrated method of frequency adjustment or tuning have potential applications in other MEMS vibration energy harvesters and micromechanical oscillators.