Low Frequency Bandwidth Research Articles

Application Status of Left and Right Handed Transmission Lines in Broadband Antenna Units

In recent years, high standards and strict requirements have been put forward on the structure and performance of antennas, especially bandwidth, gain and directivity have become the most important performance indicators of antennas. How to broaden bandwidth has become a hot topic. The left and right hand transmission line structure has the advantages of low loss, frequency bandwidth, small size and easy fabrication, and is more suitable for the design of a wideband antenna. This paper summarizes the application of the left and right hand transmission lines in the broadband antenna unit.

Monitoring of Wound Rotor Induction Machines by Means of Discrete Wavelet Transform

This article studies the efficacy of a time-scale technique based on DWT for both stator and rotor asymmetries faults detection in WRIMs. The preliminary effort on the RAF detection in IMs relied mainly on the energy evaluation of a known low frequency bandwidth through DWT analysis. It was illustrated that the SCSVM and the IMSC are both good indicators for the RAF monitoring in steady state working conditions of WRIMs. In this work, it will be shown that in the case of any load variation, there is a time-varying DC component that may affect the energy evaluation criterion, resulting in a false alarm. Thus, an efficient method is proposed to estimate and then, to remove this DC component from both SCSVM and IMSC, before the energy evaluation through DWT analysis. Furthermore, by introducing a new frequency bandwidth, associated with SAF in the rotor current signature, this method can be extended to a universal slip independent stator and rotor asymmetries faults detection technique in WRIMs. The proposed approach is validated by experimental tests using a 7.5kW WRIM test rig.

Compensating Earth Ionosphere Phase Distortion in Spaceborne VHF Radar Sounders for Subsurface Investigations

Spaceborne low-frequency and wide bandwidth radar sounders are a promising technology to regularly investigate at global-scale Earth’s icy and arid regions. However, Earth ionosphere distorts the radar signal impacting performance parameters, such as subsurface resolution, of the radar system. One of the most relevant distortions that a sounder signal in the lower part of the very high-frequency (VHF) band (e.g., 40–50 MHz) encounters is the distortion of the phase component that could become mission critical if not properly compensated. Low-frequency and high fractional bandwidth radar systems are particularly affected by this issue. Previous works on radar sounder ionosphere phase distortion compensation addressed the Martian ionosphere and used techniques based on the Taylor series expansion. In this letter, we focus on the Earth ionosphere and we exploit a recently proposed ionosphere compensation technique based on the Legendre orthogonal polynomials expansion, which proved to be more accurate than the compensation based on Taylor expansion. Simulations show that the method allows a nominal compensation of the phase distortions under realistic ionosphere scenarios expected during the acquisitions. Furthermore, it proved to be accurate and robust for total electron content conditions expected during nighttime for all the geomagnetic latitudes. The results confirm that the method can accurately compensate the distorting effects on the phase component of a spaceborne VHF radar sounder.

Lossless medical image watermarking method based on significant difference of cellular automata transform coefficient

Conventional medical image watermarking techniques focus on improving invisibility and robustness of the watermarking mechanism to prevent medical disputes. This paper proposes a medical image watermarking algorithm based on the significant difference of cellular automata transform (CAT) for copyright protection. The medical image is firstly subsampled into four subimages, and two images are randomly chosen to obtain two low-frequency bandwidths using CAT. Coefficients within a low-frequency bandwidth are important information in an image. Hence, the difference between two low-frequency bandwidths is used as an important feature in the medical image. From these important features, watermarks and cover images can be used to generate an ownership share image (OSI) used for verifying the medical image. Besides appearing like cover images, the OSI will also be able to register with a third party. When a suspected medical image requires verification, the important features from the suspected medical image are first extracted. The master share image (MSI) can be generated from the important features from the suspected medical image. Lastly, the OSI and MSI can be combined to extract the watermark to verify the suspected medical image. The advantage of our proposed method is that the medical image does not require alteration to protect the copyright of the medical image. This means that while the image is protected, medical disputes will be unlikely and the appearance of the registered OSI will carry significant data to make management more convenient. Lastly, the proposed method has the features of having better security, invisibility, and robustness. Moreover, experimental results have demonstrated that our method results in good performance.

Fabrication and characterization of miniature nonlinear piezoelectric harvester applied for low frequency and weak vibration

Abstract In order to collect energy from human activities, piezoelectric energy harvesters should have a flexible structure, low resonant frequency and wide frequency bandwidth. In this paper, the fabrication and characterization of a metal-based Pb(Zr,Ti)O3 (PZT) bimorph thick film energy harvester. PZT thick films were deposited on cantilever-shaped titanium (Ti) substrate via a dip-coating process. The electrical properties of the PZT films sintered at different temperatures were investigated. The nonlinear piezoelectric energy harvester has the following advantages: it is suitable for low frequency, weak vibration, and wide frequency bandwidth. The maximum open-circuit voltage of the piezoelectric energy harvester is 1.6 V and the output power is 0.85 μW with 150 kΩ load resistance at low resonant frequency of 17.6 Hz when the input vibration acceleration was 0.12 g. In addition, the frequency bandwidth is broadened from 0 to 8.4 Hz as the input acceleration increases from 0.05 g to 0.12 g.

Front-End Module of 18–40 GHz Ultra-Wideband Receiver for Electronic Warfare System

In this study, we propose an approach for the design and satisfy the requirements of the fabrication of a small, lightweight, reliable, and stable ultra-wideband receiver for millimeter-wave bands and the contents of the approach. In this paper, we designed and fabricated a stable receiver with having low noise figure, flat gain characteristics, and low noise characteristics, suitable for millimeter-wave bands. The method uses the chip-and-wire process for the assembly and operation of a bare MMIC device. In order to compensate for the mismatch between the components used in the receiver, an amplifier, mixer, multiplier, and filter suitable for wideband frequency characteristics were designed and applied to the receiver. To improve the low frequency and narrow bandwidth of existing products, mathematical modeling of the wideband receiver was performed and based on this spurious signals generated from complex local oscillation signals were designed so as not to affect the RF path. In the ultra-wideband receiver, the gain was between 22.2 dB and 28.5 dB at Band A (input frequency, 18–26 GHz) with a flatness of approximately 6.3 dB, while the gain was between 21.9 dB and 26.0 dB at Band B (input frequency, 26–40 GHz) with a flatness of approximately 4.1 dB. The measured value of the noise figure at Band A was 7.92 dB and the maximum value of noise figure, measured at Band B was 8.58 dB. The leakage signal of the local oscillator (LO) was –97.3 dBm and –90 dBm at the 33 GHz and 44 GHz path, respectively. Measurement was made at the 15 GHz IF output of band A (LO, 33 GHz) and the suppression characteristic obtained through the measurement was approximately 30 dBc.

Fault diagnosis in yaw drive induction motor for wind turbine

Wind turbines are widely exploited throughout the world. The availability and reliability of offshore wind turbines are asking to impose a constant maintenance strategy. In this work, we propose a method that allows filtering the signal of the frequency inverter that feeds the yaw drive used in wind turbine. The redundant information is eliminated via discrete wavelet transform and empirical modal decomposition. The two types of faults are detected from the envelope of the Hilbert transform. The magnitude imbalance detection is carried out in the time domain. The root mean square values of the envelopes of the three-phase system have a good indicator for the fuzzy system to evaluate the severity of the defect. In the frequency domain, the signature of the broken bar fault is located in the low-frequency bandwidth. The harmonics appeared in the spectrum sensitive in amplitude and frequency to the variation of load. Experimental results have demonstrated the accuracy of the proposed method.

A multi-modal energy harvesting device for low-frequency vibrations

This paper presents an innovative design of a low-frequency multi-modal system vibration-based energy harvester (VEH) for powering wireless autonomous monitoring systems wind turbines of 30 kW. The main objective is to design an energy harvesting device capable to operate in a very low-frequency bandwidth (3 to 10 Hz) increasing as much as possible the operational bandwidth by enhancing the amplitude of the second mode of vibration. The electrical power performance is evaluated for four different energy harvesting designs, which are mainly composed of multi-beams cantilevers with tip masses. For the harvesting system with two multiple-beams trident, a rigid beam is selected to join them. This versatile geometric configuration offers the possibility to modify the vibration characteristics of the harvester in several alternative ways, not only by increasing the tip mass which may be not favorable from a structural viewpoint. The resonant frequencies values, the time voltage signals and the electric power are obtained through a finite element beam formulation early proposed by the authors, capable to modeling three dimensional systems. The numerical results are validated through experimental tests. Regarding the output power, the most promising design with two multiple-beams trident with a tip mass delivers 36.48 μW and 96.04 μW in the proposed range of operation (first two resonance frequencies 4.76 and 7.91 Hz, respectively) excited by 0.1 g of base acceleration. This clearly indicates that the device is a very good candidate for the proposed application of autonomous wireless monitoring, since the output power is larger than the minimum of 20 μW required.

Sound absorption of a new oblique-section acoustic metamaterial with nested resonator

This study designs and investigates high-efficiency sound absorption of new oblique-section nested resonators. Impedance tube experiment results show that different combinations of oblique-section nest resonators have tunable low-frequency bandwidth characteristics. The sound absorption mechanism is due to air friction losses in the slotted region and the sample structure resonance. The acousto-electric analogy model demonstrates that the sound absorption peak and bandwidth can be modulated over an even wider frequency range by changing the geometric size and combinations of structures. The proposed structure can be easily fabricated and used in low-frequency sound absorption applications.

Quasi-fractal axial-symmetric structure feeded by segment of nongrounded coplanar line

Background. Developments in telecommunication technologies and global implementation of wireless systems calls far new types of antenna systems. Microstrip antennas look the most promising in this field since they possess multiband operation capability with small dimensions and low weight, high degree of integration with other microstrip circuits and can be used as elements of phased antenna arrays. Objectives. The goal of this paper was to simulate electrodynamic characteristics of quasi-fractal radiating structure with complex composition based on a microstrip monopole, to optimize parameters of the excitation element in the form of unshielded coplanar waveguide segment. Matherials and methods. Quasi-fractal radiating structure with complex composition based on a microstrip monopole consists of dielectric substrate, microstrip monopole with quasy-fractal aperture, an unshielded coplanar waveguide segment. The analysis was based on the Finite Elements Method with Integral Equation Method used to account for electromagnetic fields at infinity. Results. Within scope of numerical simulation spectral characteristics of the antenna were analyzed. Comparative analysis of spectral characteristics of canonical disk resonator excited with a microstrip line segment and canonical disk resonator excited with a segment of a coplanar line with eigenmode spectrum of a quasi-fractal excited with a segment of a coplanar waveguide was performed. Influence of dielectric substrate permittivity value on frequency and power characteristics of the radiating structure was studied. It was shown that variations in dielectric permittivity value lead to significant changes of frequency response in the higher frequency band only, wherein increase of dielectric permittivity value leads to a shift of frequency response minima towards lower frequencies of the given frequency band. It was determined that radiating structure is dualband and bandwidth of operating bands was different. Lower frequency bandwidth on dB was , and higher frequency bandwidth was . Numerical simulation of power characteristics was performed with the radiation patterns presented as elevation patterns with fixed azimuthal angle at various frequencies. Possible methods of radiating structure parameters correction with the goal of improving power characteristics and external circuits matching were given. Conclusions. Within scope of numerical simulation was determined that the given quasi-fractal radiating structure with complex composition based on a microstrip monopole can provide effective radiation with acceptable parasitic loss rate. The structure can provide acceptable level of external circuits matching in wide enough frequency band. The ability to form radiated fields with given characteristics in two operating bands.

The low-frequency seismic vibrator: design and experimental verification

Extending the Vibroseis acquisition bandwidth towards low frequencies has become an increasing trend in land seismic exploration. It is clear that adding low-frequency content to seismic data can be beneficial to enabling waveform inversion for velocity model determination or refinement, improving the resolution of seismic data and obtaining structural information of deeper reservoirs. These geophysical benefits have been highlighted by many authors, for example Baeten et al. (2013) and Mahrooqi et al. (2012). However, using the Vibroseis method to acquire low frequency seismic data becomes very challenging. Owing to mechanical and hydraulic limitations, most conventional seismic vibrators cannot produce sufficient low-frequency force for transmission of seismic energy into deep ground below 10 Hz. Because of this fact many sweep design techniques aimed at enhancing the Vibroseis low-frequency contents have been developed (Bagaini 2006, 2008; Sallas 2010; Baeten 2011). These low frequency sweeps can enable conventional vibrators to shake as close as possible to their low-frequency mechanical limitations. Unfortunately, this approach requires a lower drive level and hence a slower sweep rate resulting in a longer sweep length. Therefore, the generation of extra low-frequency bandwidth with low frequency sweep methods usually has an impact on productivity.

An exploratory study examining factors underpinning postural instability in older adults with idiopathic neck pain

There is limited understanding of potential mechanisms underpinning postural control deficits in people with neck pain. This study examined several characteristics that might explain impaired postural stability in a group of older adults with neck pain, and compared the results in this cohort with that of asymptomatic controls. In this cross-sectional study we compared physical activity, lower limb motor and sensory function, vestibular and visual function, falls efficacy and dizziness handicap in 84 older adults with (n=35, age 69.6±6.3) and without (n=49, age 69.4±4.7) idiopathic neck-pain. Additionally, dynamic balance was assessed using the dynamic gait index (DGI) and standard and wavelet analysis of static balance was computed after data capture. Physical activity levels, lower limb motor and sensory function, vestibular function and visual contrast sensitivity were not different between groups (p>0.05). The neck-pain group demonstrated higher falls efficacy (p=0.01), greater levels of dizziness handicap (p<0.01), and higher CoP velocity measures in the moderate (1.56–6.25Hz) and low (0.39–1.56Hz) frequency bandwidths. Our results suggest that neck-pain induced postural control deficits in older adults may not be associated with the physical activity levels, lower limb motor and sensory function, or vestibular and visual function. Inferring from wavelet analysis results, we speculated that sensory re-weighting may have occurred to compensate for the deficits in neck proprioception. Further research is warranted to determine neck specific mechanisms underpinning postural control dysfunction in neck pain.

Low Side Lobe Tapered Slot Antenna with High Gain Using Gradient Refractive Index Metamaterial for Ultra Wideband Application

A broadband gradient refractive index (GRIN) metamaterial is used to improve the gain of the tapered slot antenna. The proposed metamaterial is capable of reducing the side lobe level of the antenna. The gradient refractive index (GRIN) metamaterial is constructed by using non-resonant parallel-line unit cells with different refractive index. Due to the non-resonant structure, the proposed unit cell exhibits low loss and large frequency bandwidth. The metamaterial, whose effective refractive index is lower than that of the substrate on which the antenna is printed. Therefore, the proposed metamaterial is act as a regular lens in beam focusing. The GRIN metamaterial is integrated in front of the antenna which has the capability to manipulate electromagnetic wave accurately. The measurement results indicate that the reflection coefficient of the antenna is below -10 dB over the frequency band from 3 to 11 GHz. The radiation pattern of the antenna shows the beam width becomes narrow and directive with low side lobe level. The peak gain is increased by 2.1 dB at 9.5 GHz.

The performance of delay compensation in real-time dynamic substructuring

Real-time dynamic substructuring (RTDS) is a state-of-the-art experimental technique for evaluating the dynamic performance of a structural system subjected to time-varying loads in civil engineering. The accuracy and stability of RTDS is affected by the natural dynamics of the constituent transfer system. Of various control strategies, and due to the merits of simple implementation and low computational cost, delay-compensation methods have become most pervasive. In this paper, the performance of delay compensation based methods for RTDS is assessed in terms of accuracy and stability. Three commonly-used delay compensation schemes are considered, two of which are time variant and one time invariant. Stability is assessed analytically, numerically, and experimentally. Accuracy is assessed numerically and experimentally. To provide a suitable test for the delay compensation control schemes, a shaking table is adopted as the RTDS transfer system. It is demonstrated numerically, analytically, and experimentally that when applied to transfer systems such as these, delay compensation can work to the detriment of test accuracy and test stability. Adequate performance of delay compensated, shaking-table based RTDS is confined to a narrow low frequency bandwidth, which severely restricts the range of potential application.

Impacts of Simulated Weightlessness by Dry Immersion on Optic Nerve Sheath Diameter and Cerebral Autoregulation.

Dry immersion (DI) is used to simulate weightlessness. We investigated in healthy volunteers if DI induces changes in ONSD, as a surrogate marker of intracranial pressure (ICP) and how these changes could affect cerebral autoregulation (CA). Changes in ICP were indirectly measured by changes in optic nerve sheath diameter (ONSD). 12 healthy male volunteers underwent 3 days of DI. ONSD was indirectly assessed by ocular ultrasonography. Cerebral blood flow velocity (CBFV) of the middle cerebral artery was gauged using transcranial Doppler ultrasonography. CA was evaluated by two methods: (1) transfer function analysis was calculated to determine the relationship between mean CBFV and mean arterial blood pressure (ABP) and (2) correlation index Mxa between mean CBFV and mean ABP.ONSD increased significantly during the first day, the third day and the first day of recovery of DI (P < 0.001).DI induced a reduction in Mxa index (P < 0.001) and an elevation in phase shift in low frequency bandwidth (P < 0.05). After DI, Mxa and coherence were strongly correlated with ONSD (P < 0.05) but not before DI. These results indicate that 3 days of DI induces significant changes in ONSD most likely reflecting an increase in ICP. CA was improved but also negatively correlated with ONSD suggesting that a persistent elevation ICP favors poor CA recovery after simulated microgravity.

An optimal discrete-time feedforward compensator for real-time hybrid simulation

Real-Time Hybrid Simulation (RTHS) is a powerful and cost-effective dynamic experimental technique. To implement a stable and accurate RTHS, time delay present in the experiment loop needs to be compensated. This delay is mostly introduced by servo-hydraulic actuator dynamics and can be reduced by applying appropriate compensators. Existing compensators have demonstrated effective performance in achieving good tracking performance. Most of them have been focused on their application in cases where the structure under investigation is subjected to inputs with relatively low frequency bandwidth such as earthquake excitations. To advance RTHS as an attractive technique for other engineering applications with broader excitation frequency, a discrete-time feedforward compensator is developed herein via various optimization techniques to enhance the performance of RTHS. The proposed compensator is unique as a discrete-time, model-based feedforward compensator. The feedforward control is chosen because it can substantially improve the reference tracking performance and speed when the plant dynamics is well-understood and modeled. The discrete-time formulation enables the use of inherently stable digital filters for compensator development, and avoids the error induced by continuous-time to discrete-time conversion during the compensator implementation in digital computer. This paper discusses the technical challenges in designing a discrete-time compensator, and proposes several optimal solutions to resolve these challenges. The effectiveness of compensators obtained via these optimal solutions is demonstrated through both numerical and experimental studies. Then, the proposed compensators have been successfully applied to RTHS tests. By comparing these results to results obtained using several existing feedforward compensators, the proposed compensator demonstrates superior performance in both time delay and Root-Mean-Square (RMS) error.

High speed acoustic communication with orbital angular momentum multiplexing

Acoustic communication is critical for underwater application such as deep ocean scientific explorations, off-shore industrial controls, and ocean environment monitoring. This is because other techniques using electromagnetic waves such as RF communications are difficult for underwater applications due to the strong absorption of water in such a frequency. Optical communication, on another hand, suffers from the light scattering from micro-particles or marine life making long range underwater optical communication very challenging. Therefore, using acoustic waves to transmit information is currently the dominate technique for underwater applications including data collection and remote control of off-shore benthic stations. However, the low frequency bandwidth available for acoustic communication limits the data transmission rate and information capacity or content. We propose and experimentally demonstrate here a new approach using the orbital angular momentum (OAM) of acoustic vortex beams which provides a new and independent channel that enhances the data transmission rate by eight fold. The OAM multiplexing method demonstrated here will impact significantly on future underwater communications.

불연속 변조 기법을 이용한 고속철도 추진제어장치용 단상 PWM 컨버터

본 논문에서는 불연속 변조 기법을 이용한 고속철도 추진제어장치용 단상 PWM 컨버터에 대해 연구하였다. 기존 PWM 컨버터는 대 전력 특성으로 인해 스위칭 손실을 줄이기 위해 주파수 변조지수를 10 이하로 낮게 사용함에 따라 제어의 대역폭이 감소하고, 안정적인 역률 제어를 위해 추가의 보상 기법을 필요로 하는 등의 문제점을 가지고 있다. 이를 해결하기 위해 3상 PWM 인버터에서 사용하는 불연속 변조 기법을 단상 PWM 컨버터에 적용하였다. 제안한 방식은 옵셋 전압 기법을 활용하여 간단히 구현될 수 있으며 동일 스위칭 손실 대비 주파수 변조 지수를 2배 증가시켜 제어 특성을 향상시키고 스위치 간의 동특성도 동일하게 가져갈 수 있다. 100 kW급 PWM 컨버터에 대한 시뮬레이션을 통해 제안한 불연속 변조 기법에 대한 타당성을 검증하였다. In this paper, for high speed railway propulsion systems, a single phase PWM Converter using discontinuous PWM (DPWM) was investigated. The conventional PWM Converter uses a low frequency modulation index of less than 10 to reduce switching losses due to high power characteristics, which results in low control frequency bandwidth and requires an additional compensation method. To solve these problems, the DPWM method, which is commonly used in three phase PWM Inverters, was adopted to a single phase PWM Converter. The proposed method was easily implemented using offset voltage techniques. Method can improve the control performance by doubling the frequency modulation index for the same switching loss, and can also bring the same dynamic characteristics among switches. Proposed DPWM method was verified by simulation of 100 kW PWM converter.

Random-vibration-based damage detection and precise localization on a lab–scale aircraft stabilizer structure via the Generalized Functional Model Based Method

The problem of random-vibration-based damage diagnosis, including detection and localization, for a lab–scale aircraft stabilizer structure is addressed. Diagnosis is based on the recently introduced Generalized Functional Model Based Method, which utilizes a simple data-based model – instead of large-scale finite element type models – in the inspection phase and is capable of estimating the damage coordinates and their uncertainty region. The focus of the work is on assessing the achievable detection performance and localization accuracy under constraints on the number of deployed sensors and the excitation bandwidth, both reflecting practical limitations relating to potential in-flight implementation. The damage scenarios simulate local stiffness reduction via the addition of a small mass at any point on the structure. While the method has been recently employed with structures consisting of one-dimensional elements, it is presently and for the first time challenged with the considerably more complex problem of damage localization on a two-dimensional structural element. The method’s details, including the transformation of functional boundary constraints into simple ones within the context of the associated optimization problem, are illustrated. The results obtained with a large number of experiments, using only two vibration sensors and limited, low-frequency signal bandwidth, demonstrate excellent damage detection and remarkably accurate localization, thus indicating the method’s high potential for effective damage diagnosis.

Non-minimum Phase Effects in Transformer Low-Frequency Bandwidth Responses

This paper presents the analysis and modeling of the non-minimum phase effects that can occur in the low-frequency response of transformers. This effect is highlighted when the phase response has angles outside the range –90° to +90° and the typical response shape, which consists of two resonance points for the lateral phases and one for the central phase, is not found. Analysis based on physical effects, particularly the grounding capacitances and their association with the response, is presented. A model is proposed for characterizing and reproducing transformer low-frequency response, including the non-minimum phase effect. Actual frequency response measurements were obtained experimentally to assess the modeling.