Broad Frequency Bandwidth Research Articles

Improving performance: recent progress on vibration-driven locomotion systems

Improving locomotion performance is the primary objective in studying vibration-driven locomotion systems. In this paper, we report out recent progress within this topic from three aspects. First, dry friction-induced stick-slip motions, which are typical non-smooth dynamical behaviors, are analyzed and categorized based on sliding bifurcation analysis, and the sticking feature of the system is utilized to achieve directional locomotion as well as to increase average locomotion speed without additional energy input. Second, a strong global nonlinearity, bistability, is introduced into the internal oscillation, with which, unique benefits are obtained, including broad frequency bandwidth for high average locomotion speed and multiple locomotion modes. Third, by incorporating two internal oscillators into the system, planar locomotion capability is acquired; various trajectories can be executed by simply tuning the frequencies of the two internal oscillators. In addition to theoretical and numerical efforts, based on our designed prototypes, the proposed strategies and the achieved performance boosts are experimentally verified. We believe that this progress report, along with the discussions on challenging issues and directions worth exploring, could serve as a solid foundation and useful guideline of future research.

Graphene nanoflakes with optimized nitrogen doping fabricated by arc discharge as highly efficient absorbers toward microwave absorption

Abundant resources, light weight as well as stable physicochemical properties enable carbon nanomaterials promising candidates toward microwave absorption. However, it is still a great challenge for carbon-based absorbers to achieve broad frequency bandwidth and strong absorption, which fundamentally ascribes to the poor impedance matching resulted by their comparatively high electrical conductivities. Herein, a feasible and high-yield method has been employed to the in-situ synthesis of N-doped graphene nanoflakes which can effectively address interfacial impedance mismatching, and realize the majority of microwaves penetration into the interior of absorber. The incorporation of substitutional N atoms into graphene lattices markedly weaken crystallization degree and introduce masses of defects, directly leading to the decline of electric conductivity, meanwhile benefiting the improvement of static magnetization. Our findings indicate that compared with pure graphene nanoflakes, the counterpart containing 4.6 at.% of nitrogen exhibits an excellent absorption capability, in which more than 99% of microwave energy can be quantitatively attenuated at 5–18 GHz. Experimental results coupled with theory calculations further elucidate that such high performance essentially originates from the proper impedance matching constructed in the N-doped graphene nanoflakes.

Self-aligned and hierarchically porous graphene-polyurethane foams for acoustic wave absorption

Manipulating the morphology of three-dimensional (3D) cellular structures in both macroscale and microscale is very important in designing a sound absorbing material effective in a broad frequency bandwidth. However, it is extremely difficult to synthesize the hierarchical porous structures having nano- and micro-pores in a self-assembled or controlled manner. Herein, we report an efficient strategy to fabricate the self-aligned and hierarchically porous graphene-polyurethane foams as a sound absorbing material via electrostatic repulsion mechanism of graphene oxide and potassium hydroxide activation. Two unique microscopic cellular structures are designed by controlling the morphology of graphene layers inside the polyurethane backbone using spontaneous self-alignment and stochastic disruptive methods, respectively. The obtained heterogeneous graphene microstructure network provides enhanced mechanically load-bearing ability and significantly improves the sound energy attenuation performance over 312% compared to pristine sound absorber depending on the existence of ordered or disordered graphene lattices, resulting in an efficient pathway for rapidly decaying out acoustic wave energy.

Wide bandwidth acoustic transmission via coiled-up metamaterial with impedance matching layers

Many acoustic metamaterials suffer from a narrow bandwidth transmission because of the impedance mismatch at the air-metamaterial interface. In this paper, a two-dimensional impedance-matched metamaterial with broadband transmission performance is investigated. The impedance matching layer is introduced for a gradient variation of effective impedance from the inlet of the unit to the outlet. The effective medium theory and corresponding effective model are used to explain the underlying mechanism. The improved energy transmission of our designs is demonstrated by experiment and numerical simulation within a broad frequency bandwidth over 6 kHz. Our impedance-matched design can be used to enhance sound absorption, which is expected to present improved acoustic performance in the applications of acoustic damper and muffler.

Precision Ultrasonic Meters of Fluid Level in Sealed Containers

The problems of measuring the fluid level in steel containers through a wall by acoustic methods are considered. When measuring the level by sound propagation time in a fluid in case of an inelastic wall the best accuracy is provided by the correlation-phase reception of compound signals. But for measurements in steel containers, such a technique is usually not used due to phase structure distortions of a signal with a broad frequency bandwidth when transmitting through an elastic wall (having multiple resonances in this band). The use of correlation-phase reception in such cases may be possible by choice a probing signal, a phase structure distortion of which will be small when transmitting through a wall. The aim of the work is to determine the possibilities of using the correlation-phase reception and the achievable accuracy of level measuring in the conditions of different wall thickness of steel containers. The significance of research is determined by the possibilities of a multiple increase in the measuring accuracy. Acoustic methods and level measuring devices are considered; the advantage of time-pulse (time-of-flight, TOF) methods over interferometric (including swept-frequency acoustic interferometry, SFAI) and other measuring methods in large containers is shown. A method of level measuring using the correlation-phase reception of compound signals and a scheme for calculating the broadband impulse signal transmission through an elastic wall (and building the amplitude-frequency and phase-frequency characteristics) taking into account the longitudinal and shear waves of thickness wall vibrations and standing waves of its resonant vibrations along the length (diameter) are proposed. A method for selecting the probing signal frequency range according to an amplitude-frequency characteristic of elastic wall which provide the low phase structure distortions when transmitting through a wall is suggested. The achievable accuracy estimates of measuring the sound velocity and the fluid level in rail tank wagons are obtained. Experimental works carried out on a thin-walled (0.8 mm) barrel with signals of the frequency range of 250-750 kHz (i.e. frequencies below the first thickness resonance frequency of barrel bottom) are confirmed the small phase structure distortion of signals when transmitting through the bottom and the high efficiency of the correlation-phase reception. With a signal-to-noise ratio of 0.4, a high time measuring accuracy of ~0.15 μs corresponding to a level measuring accuracy of ~0.1 mm is obtained. For signals of frequency range 250-750 kHz the interval of wall thickness is defined as 0.3-3.6 mm at which the efficiency of the correlation-phase reception and the level measuring accuracy should be high. The results showed that the use of correlation-phase reception of compound signals when measuring a fluid level through the bottom of a thin-walled steel barrel is possible and provides both high operating efficiency in the noise conditions and high measuring accuracy. It can be expected that the same accuracy factors and operating efficiency will be kept in a bottom thicknesses range at which the probing signal frequencies will be less than the first bottom thickness resonance frequency. In cases of large bottom thicknesses, if the first bottom thickness resonance frequency is low, the probing signal spectrum may include frequency regions between the thickness resonance frequencies. In this case, it becomes possible to use the correlation-phase reception for the level measuring in containers with a wide range of bottom thicknesses. Ref. 20, tab. 1, fig. 7.

Parameter study and optimization of a half-vehicle suspension system model integrated with an arm-teeth regenerative shock absorber using Taguchi method

Abstract This paper presents a novel regenerative shock absorber that utilizes an arm-teeth mechanism to convert linear to rotational motion and to amplify the generator input speed. The flywheel of the arm-teeth mechanism also acts as an energy-storing element, allowing for smooth operation especially on a rough and uneven road surface. The critical parameters are identified through experiments and the simulation model is established based on a half-vehicle suspension system model. The novelty of this work is the proposed arm-teeth mechanism and the optimization of the half-vehicle suspension system through the Taguchi method. In this method, the ratio of the peak output power over the squared excitation displacement amplitude and the frequency bandwidth of the front and rear shock absorbers are set as targets and evaluated respectively. The effects of each parameter on the targets and the optimal parameter combinations are identified through the Taguchi matrix calculations. The optimized system is evaluated under the excitation of the road surface profiles of Class A, C and E of ISO 8606. It is found that the optimized system is able to harvest more energy with a broader frequency bandwidth than the original system at a certain speed for all three random road profile classifications.

Side-Polished Fiber-Optic Line Sensor for High-Frequency Broadband Ultrasound Detection.

We demonstrate a side-polished fiber-optic ultrasound sensor (SPFS) with a broad frequency bandwidth (dc–46 MHz at 6-dB reduction) and a wide amplitude detection range from several kPa to 4.8 MPa. It also exhibits a high acoustic sensitivity of 426 mV/MPa with a signal-to-noise ratio of 35 dB and a noise-equivalent pressure of 6.6 kPa (over 1–50 MHz bandwidth) measured at 7-MHz frequency. The SPFS does not require multi-layer-coated structures that are used in other high-sensitivity optical detectors. Without any coating, this uses a microscale-roughened structure for evanescent-field interaction with an external medium acoustically modulated. Such unique structure allows significantly high sensitivity despite having a small detection area of only 0.016 mm2 as a narrow line sensor with a width of 8 μm. The SPFS performance is characterized in terms of acoustic frequency, amplitude responses, and sensitivities that are compared with those of a 1-mm diameter piezoelectric hydrophone used as a reference.

Near-Field Scanning and Propagation of Correlated Low-Frequency Radiated Emissions

Electromagnetic radiation from complex printed circuit boards can occur over a broad frequency bandwidth, ranging from hundreds of megahertz to tens of gigahertz. This is becoming a critical issue for assessment of EMC and interoperability as electronic components become more and more integrated. We use emissions from an enclosure with a single-slot aperture and equipped with operating electronics to exemplify and model such sources. Spatial correlation functions obtained from two-probe measurements are used both to characterize the source and to propagate the emissions. We examine emissions in the submicrowave frequency range, where evanescent decay dominates the measured correlation function at the distances measured. We find that an approximate, diffusion-like propagator describes the measured emissions well. A phase-space approach based on Wigner functions is exploited to develop this approximation and to provide enhanced understanding of the emissions.

Complex permittivity, permeability, magnetic and microwave absorbing properties of Ni2+ substituted mechanically milled U-type hexaferrites

The polycrystalline samples of Ni2+ substituted U-type hexaferrite; (Ba0.7Bi0.2)4(Co1−xNix)2Fe36O60 (0.25 ≤ x ≤ 1.00, in steps of 0.25) were prepared through solid state reaction method. XRD patterns, fitted with Le Bail method, revealed the formation of U-type hexaferrite phase in as-prepared samples. Scanning electron micrograph (SEM) indicates the presence of plate-like hexagonal particles with average sizes of ∼1–4 μm. In order to reduce the grain sizes down to nanoscale, two stage high energy mechanical jet milling was performed on x = 0.75 composition sample. The jet milled hexaferrite exhibits relatively smaller grain sizes (∼20–50 nm), lower saturation magnetisation (Ms∼38.6 emu/g) and higher coercivity (Hc∼590.2 Oe) in comparison to unmilled sample of same composition (x = 0.75). The jet milled sample, with nanoscale hexaferrite particles, exhibits excellent reflection loss (RL) of −43.8 dB (99.99% MW absorption) at 11.3 GHz frequency. In addition, the jet milled sample displays a broad frequency bandwidth (∼8.6 GHz) for RL ≤ −10 dB (90% MW absorption), extending over ∼9.1–17.7 GHz frequency range.

A low frequency underwater meta structure composed by helix metal and viscoelastic damping rubber

Many underwater acoustic absorption materials have been proposed, but good low frequency acoustic absorption remains a challenge. We report an underwater metastructure with excellent acoustic absorption that is constructed by perforating helixes of metal into viscoelastic damping rubber. Finite element analysis shows that this metastructure can achieve an acoustic absorption coefficient of 0.75 at about 100 Hz with a thickness of only 1/68 wavelength at the same frequency. Compared with a homogeneous viscoelastic rubber material, the addition of the helix structure improves the acoustic absorption ability in the range of 0–1,000 Hz. An analysis of vibration displacement maps indicates that waveform transformation, multiple scattering, and reflection energy dissipation mechanisms are the critical factors affecting the absorption performance. Different geometries and materials can adjust the sound absorption characteristics below 1,000 Hz. The proposed metastructure has the advantages of high acoustic absorption ability, broader frequency bandwidth, and regular geometry, providing more possibilities for underwater acoustic manipulation applications.

A novel indirect-drive regenerative shock absorber for energy harvesting and comparison with a conventional direct-drive regenerative shock absorber

Energy harvesting from the shock absorbers is now becoming an important technology in developing the electrical vehicles. Compared with other kinetic energy sources such as engine and brake systems which are continuous or periodic, the shock absorber is subjected to the fluctuated linear motion with relatively small displacement. This paper presents a novel indirect-drive regenerative shock absorber system that utilizes an arm-teeth mechanism to achieve linear to rotary motion conversion and to amplify its input speed for increasing energy harvesting output. The fluctuation of road randomness can be smoothed out through the flywheel. The proposed design has the advance of achieving all the targets with less number of components. Two prototypes of the direct-drive and indirect-drive regenerative shock absorbers have been built. The simulation models have been developed and validated by experimental results. The performance of this new indirect-drive system has been compared, through experimental testing and analytical modeling, with that of a conventional direct-drive system of the same generator configuration. The results show that this indirect-drive system can achieve greater peak power output and broader frequency bandwidth than the conventional direct-drive system. The indirect-drive system also presents a better ride comfort up to 13 Hz. Using Monte Carlo simulation, a parameter sensitivity analysis of both the indirect-drive and direct-drive systems has been carried out to compare their energy harvesting performances in terms of increasing the peak power output and broadening the energy harvesting frequency bandwidth. In both the systems, choosing the right tyre stiffness and electromechanical coupling constant is beneficial to increasing the peak power output ratio and the harvesting frequency bandwidth. The right choice of the gear ratio can further improve the peak power output ratio of the indirect-drive system. The variation of these parameters will allow for possibility of achieving higher power output when vehicle is driven on random road surfaces.

Complex-conductivity imaging for the understanding of landslide architecture

Geophysical methods are widely used for landslide investigation to delineate depth and geometry of the sliding plane. In particular, electrical resistivity tomography (ERT) is often used because both porosity and water saturation control the electrical resistivity of the subsurface materials and are critical for slope stability. Moreover, ERT can be employed to monitor changes in pore-fluid pressure which is an important factor triggering landslides. However, the interpretation of ERT results in clay-rich landslides can be challenging considering that high electrical conductivity values may not only be related to an increase in saturation but also to the surface conduction mechanism, which becomes dominant in the presence of clays. Recently, environmental investigations have demonstrated an improved subsurface characterization through induced polarization (IP) imaging, an extension of the ERT method, which permits to gain information about electrical conductive and capacitive (i.e., polarization effect) properties of the subsurface. As the polarization effect is mainly controlled by surface charge, which is large in clays, IP images are expected to improve the lithological interpretation and overcome the limitations of the ERT method. Additionally, measurements collected over a broad frequency bandwidth, the so-called spectral IP (SIP), have been successfully used in laboratory experiments to quantify textural and hydrogeological parameters. However, the application of SIP field measurements for the delineation of hydrogeological structures in landslides has not been addressed to date. To fill this gap, in this study we present SIP imaging results for data collected at the La Valette landslide (South East French Alps), where an existing geotechnical model of the landslide is available for evaluation. Moreover, our study provides a detailed revision on the collection and processing of SIP datasets, as well as a description of the diverse sources of error in IP surveys, to stress the importance of data-error quantification for a quantitative application of the SIP method. Our results demonstrate that adequate data processing allows obtaining consistent results at different frequencies and independently of the measuring protocol. Furthermore, the frequency dependence of the complex conductivity obtained in the field-scale SIP survey is consistent with earlier laboratory experiments. In conclusion, our study shows the potential of the SIP method to improve our understanding of subsurface properties, and an improved delineation of the contact between the mobilized material and the bedrock as well as variations in the clay content within the landslide and the bedrock.

Toward broadband vibration energy harvesting via mechanical motion-rectification induced inertia nonlinearity

Broad frequency bandwidth is a desired feature for most energy harvesting systems. Rotational electromagnetic generators are widely used in energy harvesting systems and the generator rotor is considered as an inerter. While a lot of research striving for increasing frequency bandwidth, we found out that the inerter makes the bandwidth narrow. To solve this problem, this paper proposes using inertia nonlinearity which is realized by mechanical motion rectification (MMR). The influence of the MMR on energy harvesting performance in inerter-based systems was numerically and experimentally investigated with harmonic excitations of constant displacement amplitude. Simulation is done by transforming the mechanical system to an analogous electrical system. The simulation results show that the bandwidth of the MMR based system is broader than that of the counterpart without MMR. System parameter was identified by parameter fitting and experiment was conducted to verify the numerical simulation. Moreover, in the MMR based system, the force transmitted from the harvester to the base was decreased compared to the counterpart without MMR. For excitations with constant force amplitude, MMR based energy harvesting systems also have much broader frequency bandwidth compared to the counterpart without MMR.

Facile synthesis of rGO/SmFe5O12/CoFe2O4 ternary nanocomposites: Composition control for superior broadband microwave absorption performance

It is still a challenge to design and synthesize ferrite-based composites with strong absorption capability and broad frequency bandwidth. In this work, firstly, the binary Sm3Fe5O12/CoFe2O4 composites with different mass ratio of Sm3Fe5O12 to CoFe2O4 are synthesized by a facile hydrothermal process. Next, adding 10 mg graphite oxide (GO) into Sm3Fe5O12/CoFe2O4 solution, series of novel reduced graphene oxide (rGO)/Sm3Fe5O12/CoFe2O4 ternary nanocomposites are fabricated using a one-pot hydrothermal technique. The microstructure, morphology, chemical composition and magnetic property of as-synthesized nanocomposites are characterized by XRD, Raman, TEM, SEM, XPS and VSM. The magnetic performance of Sm3Fe5O12 changes with the addition of CoFe2O4 and rGO. The microwave absorption measurement is carried out by vector network analyzer, where the as-synthesized samples exhibit excellent and tunable microwave adsorption performance. Herein, introducing CoFe2O4 and rGO into Sm2Fe5O12 can effectively tune its dielectric and magnetic loss. Encouragingly, when the mass ratio of Sm3Fe5O12 to CoFe2O4 is 1:3, the rGO/Sm3Fe5O12/CoFe2O4 ternary nanocomposite possesses the maximum reflection loss (RL) of −73.71 dB at 14.88 GHz with a thickness of 2.09 mm. The effective bandwidth (RL < −10 dB) of the as-synthesized sample is 7.12 GHz (from 10.80 to 17.92 GHz) with a thickness of 2.18 mm, which is higher than that of most microwave absorbers reported previously. Noticeably, the superior microwave adsorption performance of as-synthesized samples can be mainly attributed to the synergistic effect of dipole polarization, interfacial polarization, natural resonance, eddy current loss, multiple scattering and reflection. These results show that the as-synthesized rGO/Sm3Fe5O12/CoFe2O4 ternary composites are promising candidates for the application in microwave adsorption with high reflection loss, broad bandwidth, lightweight and thin thickness.

Preparation of flower-like CoFe2O4@graphene composites and their microwave absorbing properties

Novel flower-like CoFe2O4@graphene composites were prepared using a spray drying technique in combination with a solvothermal method, followed by calcination. Hundreds of uniform CoFe2O4 microspheres are encapsulated with gauzy graphene to form large flower-like composites. This unique structure effectively reduces agglomeration of graphene and enhances microwave absorption performances. The minimum reflection loss of as-obtained flower-like CoFe2O4@graphene composites reaches −42 dB at 12.9 GHz with a thickness of 2 mm, and the effective absorption frequency (RL < −10 dB) is 4.59 GHz from 11.2 to 15.79 GHz. Flower-like CoFe2O4@graphene composites show better microwave absorbing performance than pure CoFe2O4 microspheres. These CoFe2O4@graphene composites are promising as light weight, broad frequency bandwidth and strong absorption microwave absorbers.

Broadband acoustic subwavelength imaging by rapidly modulated stratified media

An acoustic anisotropic lens (AAL) based on large mass-density modulation depth (LMMD) medium is proposed for subwavelength imaging. The underlying mechanism for converting evanescent components into propagating waves is attributed to the strong suppression of the transverse velocity field component in LMMD medium. In addition, the proposed lens can operate in a broadband manner, which is more flexible in practical applications. Both transfer matrix method and finite element method are used to corroborate the subwavelength imaging capabilities of the proposed lens. The numerical simulations demonstrate that the proposed lens can clearly distinguish two Gaussian sources with equal width of λ0/25 and separation of λ0/5 in a broad frequency bandwidth. Medium losses decrease the transmission but cannot compromise the resolution of the lens.

Broadband acoustic focusing by Airy-like beams based on acoustic metasurfaces

An acoustic metasurface (AM) composed of space-coiling subunits is proposed to generate acoustic Airy-like beams (ALBs) by manipulating the transmitted acoustic phase. The self-accelerating, self-healing, and non-diffracting features of ALBs are demonstrated using finite element simulations. We further employ two symmetrical AMs to realize two symmetrical ALBs, resulting in highly efficient acoustic focusing. At the working frequency, the focal intensity can reach roughly 20 times that of the incident wave. It is found that the highly efficient acoustic focusing can circumvent obstacles in the propagating path and can be maintained in a broad frequency bandwidth. In addition, simply changing the separation between the two AMs can modulate the focal length of the proposed AM lens. ALBs generated by AMs and the corresponding AM lens may benefit applications in medical ultrasound imaging, biomedical therapy, and particle trapping and manipulation.

Numerical Modeling of Electromagnetic and Multi-point Impacts Stoppers Coupling for Low Frequency Energy Harvesting

We presented an electromagnetic and three oscillators multi-impact coupling device for a broad bandwidth frequency energy harvesting. The energy harvesting was achieved from low vibrations with limited amplitudes and quite low frequencies that were about 1/10th of the resonance frequency in harvester. When an excitation frequency approaching the oscillators’ resonant frequency, at least one of the oscillators will impact on a stiffer structure resulting in a higher “equivalent stiffness” resulting a much broader bandwidth with impacts. The Lorentz force was found that acted to counteract the cantilever free tip causing complicated dynamic responses, and this force was acting as energy dissipation. The inclusion of the three mechanical oscillators made energy harvesting possible when the excitation vibrations at quite low frequency as long as the high frequency cantilever was kept at a distance to prevent the Lorentz force to dominate. More specifically, the inclusion of the three mechanical impact oscillators made energy harvesting efficiently when excitation vibrations were at low frequency 5-70 Hz bandwidth.

Fabrication and evaluation of a thin film vibrotactile actuator made with silicon dioxide

One of the most important challenges in bringing haptic information to mobile devices is to develop a haptic actuator with a volume small enough that it can be easily inserted into mobile devices. The haptic information, which is created in the form of vibration, should have large enough magnitude to stimulate all mechanoreceptors in a wide frequency range for increasing the sense of reality. Therefore, in this paper, we propose a wideband film-type haptic actuator based on silicon dioxide (SiO2), designed to have sufficiently large magnitude to stimulate all mechanoreceptors in human skin in broad frequency bandwidth. We conduct experiments to measure the vibration strength of the proposed actuator in a wide frequency range and its displacement behavior according to the amount of voltage input. The results show that the actuator has sufficiently large vibrational force to stimulate human skin in a wide frequency range and proportional displacement behavior as a function of the applied input voltage. The obtained results indicate that the proposed actuator can selectively stimulate human’ mechanoreceptors and that it has feasibility for haptic actuators embedded into mobile devices.

Efficient and Lightweight Electromagnetic Wave Absorber Derived from Metal Organic Framework-Encapsulated Cobalt Nanoparticles

Porous-carbon-based nanocomposites are gaining tremendous interest because of good compatibility, lightweight, and strong electromagnetic wave absorption. However, it is still a great challenge to design and synthesize porous-carbon-based composites with strong absorption capability and broad frequency bandwidth. Herein, a facile and effective method was developed to synthesize Co magnetic nanoparticles/metal organic framework (MOF) (Co NPs/ZIF-67) nanocomposites. Co NPs/porous C composites were subsequently obtained by annealing Co NPs/ZIF-67 nanocomposites at different temperatures under an inert atmosphere. The carbonized nanocomposites showed highly efficient electromagnetic wave absorption capability. Specifically, the optimal composite (i.e., Co/C-700) possessed a maximum reflection loss (RL) value of -30.31 dB at 11.03 GHz with an effective absorption bandwidth (RL ≤ -10 dB) of 4.93 GHz. The electromagnetic parameters and the absorption performance of the composites are readily tunable by adjusting the carbonization temperature and the concentration of Co NPs in the composites. Because of the combination of good impedance matching, dual-loss mechanism, and the synergistic effect between Co NPs and porous carbon composites, these Co NPs/MOF-derived composites are attractive candidates for electromagnetic wave absorbers.