Low-frequency Response Research Articles

Transforming Energy and Pedagogy

Transforming Energy and Pedagogy

A Comprehensive Study of Cyber Attack Mitigation with the Exchange of Frequency Containment Reserves Control in a Multi-Infeed Direct Current Power System.

By 2040, the Korean government aims for a penetration rate of 30-35% of the total power from renewable sources. Due to a lack of inertia, particularly in remote systems such as those on Jeju Island, these circumstances will reduce network stability. To maintain the diversity and unpredictability of RES penetration, HVDC systems with an exchange of frequency containment reserve control are utilized. An exchange of frequency containment reserves control (E-FCR) is one of the balancing arrangement concepts of HVDC systems. However, the development of E-FCR concepts is vulnerable to cyber attacks because this concept only considers one wide-area measurement for data exchange. This study established a simultaneous cyber attack operation, i.e., an attack was set at the same time as a contingency operation that affects the balancing arrangement between two regions. Multiple possibilities of cyber attack and mitigation operations were suggested according to their ability to access information in the MIDC system. Then, a cyber detection strategy was proposed through a normalized correlation concept to activate mitigation control that could enhance the frequency stability by adjusting the value of the ramp-rate deviation between two HVDC types. By simulating the Korean power system model that was implemented in PSS/E, along with a Python script, simulation results demonstrated that a cyber attack on missing data can cause severe low-frequency nadir responses, and the proposed methodology can practically detect and mitigate cyber attacks.

The novel paste electrolyte measuring cell for EIS testing of the commonly used surface protection on bronze

The cell was tested on polished and patinated bronzes of different compositions, on sulfide patina/Paraloid B-72-coated bronzes with different numbers of coating layers, on protected bronze samples with different degrees of damage to the protective layers, and on uncoated and coated patinated bronzes after 24 h conditioning in the condensation chamber. A quantitative criterion of Phase Angle at 0.1 Hz was deduced and explained. The systems show two or three time constants, depending on the number and integrity of the layers. Capacitances of the high-frequency response that are attributed to the coating range from 1.8 to 31 nF/cm2 and capacitances of the medium-frequency response attributed to the patina range from 1.1 F/cm2 to 2.3 nF/cm2, indicating Paraloid B-72 penetration into the pores of the patina. Log ∣Z∣ at 0.1 Hz decreases with mechanical damage to the protective layers. Damage also leads to the occurrence of the low-frequency response attributed to the Faradaic process on the substrate. The three layers of Paraloid B-72 on the sulfide patina had Log ∣Z∣ at 0.1 Hz around 8 and were insensitive to the 24-h condensation, indicating good protection. The cell was further applied to different sides of bronze coins, so no electrical contact with the substrate was required. The Kramers-Kronig test was applied to confirm the validity of the EIS results.

Drive-by bridge damage detection using Mel-frequency cepstral coefficients and support vector machine

Bridge damage detection using vibration data has been confirmed as a promising approach. Compared to the traditional method that typically needs to install sensors or systems directly on bridges, the drive-by bridge damage detection method has gained increasing attention worldwide since it just needs one or a few sensors instrumented on the passing vehicle. Bridge frequencies extracted from the vehicle’s vibrations can be good references for damage detection. However, extant literature considered mainly low-frequency responses of the vehicle, while the high-frequency responses that also contained the bridge’s damage information were often ignored. To fill this gap, this paper developed a damage detection approach that utilized both low and high-frequency responses of the passing vehicle. Mel-frequency cepstral coefficients (MFCCs) and support vector machine (SVM) were employed to classify damage severity. Firstly, the vehicle’s frequency responses are utilized as input features to train SVM models to identify the bridge’s condition. Then, to reduce dimensions of inputs and improve training efficiency, frequency responses are projected from the Hertz scale into the Mel scale, and two means using MFCCs are used to feed different SVM models. A laboratory experiment with a U-shaped continuous beam and a model car was used to verify the effectiveness of the proposed method. Results showed that high-frequency responses contain much information about the bridge’s conditions, and using MFCCs could apparently improve computational efficiency. The errors of damage detection when a heavy car was employed were within 5%.

Characteristics of the D-Dot Sensor for Measurement of High Voltage Pulse of a Water-Insulated Transmission Line

The characteristics of the differential-output D-dot sensor, which is employed to diagnose the voltage pulse across the water-insulated component of the pulsed power accelerator, are investigated by theory, circuit simulation, and experiments. The leakage current from the D-dot sensor to the inner electrode of the measured component will degrade the low-frequency response property of the monitor. The lower level of the water medium resistivity the worse performance of the sensor’s low-frequency response property will be. The actual lower-limited response frequency of the D-dot sensor is not related to the sensor itself but is mainly determined by the resistivity of the water. These are not only deduced from theory but also approved by the experimental results. Circuit simulation results suggest that the D-dot sensor without consideration of the leakage current will over-evaluate the amplitude and rise time of the voltage pulse across the water-filled element. We have proposed a method to make compensation for such an effect by using a passive resistor–capacitor (RC) integrator instead of the numerical process suggested previously. It is approved both by circuit simulation and experimental results that the output of the D-dot sensor post-processed by the RC integrator could give accurate measurement results providing that the time constant of the RC integrator is identical to the characteristic time of the D-dot sensor.

Bias-Independent Inter-Modulation Method for Simultaneously Measuring Low-Frequency Modulation and Bias Half-Wave Voltages of Mach-Zehnder Modulators

A bias-independent inter-modulation method is proposed and demonstrated for measuring low-frequency modulation and bias half-wave voltages of Mach-Zehnder modulators (MZMs). The method consists of simultaneous sinusoidal modulation on the modulation and bias ports of the MZM under test. Sinusoidal-modulated sidebands heterodyne with each other and generate the desired inter-modulation products after photodetection, which allows extracting both the modulation depth and half-wave voltage for the modulation and bias ports of the MZM. Our method is independent of bias voltages of the MZM, which can be canceled out by carefully choosing the sinusoidal-modulation frequencies. Moreover, the proposed method enables the low swing voltage for measuring both the modulation depth and half-wave voltage of MZMs. Experiments indicate that the proposed method features the simple setup and high accuracy for low-frequency response measurement ranging from 1 Hz to 1 MHz.

Vocal Tract Resonance Detection at Low Frequencies: Improving Physical and Transducer Configurations

Broadband excitation introduced at the speaker's lips and the evaluation of its corresponding relative acoustic impedance spectrum allow for fast, accurate and non-invasive estimations of vocal tract resonances during speech and singing. However, due to radiation impedance interactions at the lips at low frequencies, it is challenging to make reliable measurements of resonances lower than 500 Hz due to poor signal to noise ratios, limiting investigations of the first vocal tract resonance using such a method. In this paper, various physical configurations which may optimize the acoustic coupling between transducers and the vocal tract are investigated and the practical arrangement which yields the optimal vocal tract resonance detection sensitivity at low frequencies is identified. To support the investigation, two quantitative analysis methods are proposed to facilitate comparison of the sensitivity and quality of resonances identified. Accordingly, the optimal configuration identified has better acoustic coupling and low-frequency response compared with existing arrangements and is shown to reliably detect resonances down to 350 Hz (and possibly lower), thereby allowing the first resonance of a wide range of vowel articulations to be estimated with confidence.

Dependence of strategic solar climate intervention on background scenario and model physics

Abstract. Model dependence in simulated responses to stratospheric aerosol injection (SAI) is a major uncertainty surrounding the potential implementation of this solar climate intervention strategy. We identify and aim to understand the drivers of large differences in the aerosol mass latitudinal distributions between two recently produced climate model SAI large ensembles using two models from the same modeling center despite using similar climate targets and controller algorithms. Using a hierarchy of recently produced simulations, we identify three main contributors to the differences including (1) the rapid adjustment of clouds and rainfall to elevated levels of carbon dioxide, (2) the low-frequency dynamical responses in the Atlantic meridional overturning circulation, and (3) the contrasts in future background forcing scenarios. Each uncertainty is unlikely to be significantly narrowed over the likely timeframe of a potential SAI deployment if a 1.5 ∘C target of global warming over preindustrial conditions is to be met.

Nonlinear Low-Frequency Response of a Floating Offshore Wind Turbine Integrated With a Steel Fish Farming Cage

In this article, the effects of Morison drag force and the second-order hydrodynamics on a state-of-the-art floating system integrating a floating offshore wind turbine with a steel fish farming cage (FOWT-SFFC) are studied. To numerically solve the second-order hydrodynamic problem with ease, a simplified structure is adopted. Convergence study is carried out on the full difference-frequency quadratic transfer function (QTF). The middle-field method is recommended to be used in the calculation of QTF. For the aero-hydro-servo-elastic simulations of FOWT-SFFC, the time-domain solver OrcaFlex is used. The effects of the Morison drag force and second-order hydrodynamics on the dynamic responses under a variety of conditions are discussed. The comparison between the results with and without Morison drag force shows that Morison drag force has twofold effects on the low-frequency responses, making the excitation and damping level both larger. The results obtained by including first-order wave loads only and involving both first-order and second-order wave loads are also compared. The comparison reveals that the low-frequency contents of the dynamic responses, particularly heave and pitch, can be boosted when the second-order hydrodynamic forces are included. In contrast to the full QTF method, the known Newman's approximation leads to underprediction of the low-frequency responses of FOWT-SFFC.

Analysis of low frequency response characteristics of multi-inertia channel hydraulic mounts

Analysis of low frequency response characteristics of multi-inertia channel hydraulic mounts

Two-Dimensional Low-Frequency Optical Accelerometer With High Accuracy and Low Cross-Axis Sensitivity

Two-dimensional (2D) accelerometer is a commonly used sensor for detecting horizontal vibrations. However, their low frequency response characteristics are limited by high cross-axis sensitivity and low resolution. To address this challenge, this study proposes a new 2D optical accelerometer based on a symmetric 2D vibration pickup unit and a high-sensitivity 2D angle sensor modified from a DVD pickup head. The parameters of the symmetrical 2D pickup unit were determined through modeling and simulation, and the performance of the accelerometer was tested experimentally. Experimental data show that the accelerometer has good low frequency response characteristics in the range from 1 to 10 Hz, sensitivity is better than 2.5 V/(m/s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), cross-axis sensitivity is less than 0.075 V/(m/s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), and the resolution is better than 0.4 mm/s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Moreover, the sources of uncertainty of the accelerometer were analyzed in detail and the synthetic uncertainty of the accelerometer was analyzed based on experiments. Compared with conventional techniques in this area, the proposed 2D optical accelerometer has featured the advantages, such as high accuracy and low cross-axis sensitivity, and can be widely used for low-frequency vibration monitoring in engineering applications.

Analysis of low frequency response characteristics of multi-inertia channel hydraulic mounts

Analysis of low frequency response characteristics of multi-inertia channel hydraulic mounts

Improving low-frequency response of sonic boom measurements through digital filtering.

High-fidelity measurement of sonic boom waveforms requires microphones and data acquisition hardware with flat frequency responses extending below 1 Hz. Hardware limitations can pose challenges meeting these requirements. This letter describes an engineering method involving digital pole-shift filtering that can be used in post-processing to extend effective hardware bandwidth. This approach is evaluated for sonic boom recordings from NASA's Quiet Supersonic Flights 2018 measurement campaign. Recordings of several booms at multiple measurement sites using different hardware/microphone combinations were used to design filters. Results demonstrate that the measurement-designed filters significantly reduce the mean square error between original and benchmark waveforms. Digital filter designs based on hardware manufacturer specifications also reduce error, but not as much. Residual errors after filtering, method limitations, and transferability to a launch vehicle reentry boom measurement are discussed.

A Real-Time Non-Implantation Bi-Directional Brain-Computer Interface Solution without Stimulation Artifacts.

The non-implantation bi-directional brain-computer interface (BCI) is a neural interface technology that enables direct two-way communication between the brain and the external world by both "reading" neural signals and "writing" stimulation patterns to the brain. This technology has vast potential applications, such as improving the quality of life for individuals with neurological and mental illnesses and even expanding the boundaries of human capabilities. Nonetheless, non-implantation bi-directional BCIs face challenges in generating real-time feedback and achieving compatibility between stimulation and recording. These issues arise due to the considerable overlap between electrical stimulation frequencies and electrophysiological recording frequencies, as well as the impediment caused by the skull to the interaction of external and internal currents. To address those challenges, this work proposes a novel solution that combines the temporal interference stimulation paradigm and minimally invasive skull modification. A longitudinal animal experiment has preliminarily validated the feasibility of the proposed method. In signal recording experiments, the average impedance of our scheme decreased by 4.59 kΩ, about 67%, compared to the conventional technique at 18 points. The peak-to-peak value of the Somatosensory Evoked Potential increased by 8%. Meanwhile, the signal-to-noise ratio of Steady-State Visual Evoked Potential increased by 5.13 dB, and its classification accuracy increased by 44%. The maximum bandwidth of the resting state rose by 63%. In electrical stimulation experiments, the signal-to-noise ratio of the low-frequency response evoked by our scheme rose by 8.04 dB, and no stimulation artifacts were generated. The experimental results show that signal quality in acquisition has significantly improved, and frequency-band isolation eliminates stimulation artifacts at the source. The acquisition and stimulation pathways are real-time compatible in this non-implantation bi-directional BCI solution, which can provide technical support and theoretical guidance for creating closed-loop adaptive systems coupled with particular application scenarios in the future.

A novel ZnO/CQDs/PVDF piezoelectric system for efficiently degradation of antibiotics by using water flow energy in pipeline: Performance and mechanism

Using clean and natural energy from the environment to treatment of organic pollutants is a sustainable approach to alleviate energy and environmental problem. This work prepared a novel ZnO/CQDs/PVDF pipe system with excellent piezoelectric properties for efficient tetracycline (TC) degradation. The piezoelectric catalytic activity of ZnO/CQDs/PVDF was excited by hydrodynamic force to achieve a removal rate of 95.17 % of TC within 70 min with a second-order kinetic constant of 0.0185 L∙(mg min)−1, it can still maintain high stability after 35 h cycles. Most importantly, the water flow-driven piezoelectric catalytic system has extremely low energy consumption, is only 17.57 % of that of the ultrasonic system. The CQDs promote the transfer rate of ZnO surfaces charges, and PVDF with flexible β-rich phases were more sensitive to low-frequency pressure responses. ∙OH and ∙O2- radicals were demonstrated to the main radicals for the degradation of tetracycline. Based on LC-MS results and DFT calculations, the degradation mechanisms of tetracycline molecules, including hydroxylation, C-C, and C-N bond cleavage processes, were successfully predicted. In addition, ZnO/CQDs/PVDF pipe have obvious inhibitory effect on the biotoxicity of intermediates generated during the degradation process and exhibit excellent antibacterial ability. This work demonstrated that ZnO/CQDs/PVDF pipe have efficient utilization of low-frequency water flow energy and shows great potential in practical applications.

Low-frequency ambient ocean-bottom node surface-wave seismology: A Gulf of Mexico case history

Long-time marine seismic recordings are becoming more common with the increased use of ocean-bottom nodes (OBNs), which can measure ambient seismic energy at frequencies lower than the typical minimum values in active-source compressed air-gun surveys. Interferometric processing on long-time ambient multicomponent data allows for the extraction of low-frequency (sub-2.0 Hz) responses in virtual source gathers (VSGs). Using 40 days of continuous OBN recordings acquired on a large dense array during a field experiment in the Gulf of Mexico, we find that sub-2.0 Hz surface-wave energy in the computed VSGs is strongly coherent and exhibits an identifiable spatially varying character. In particular, after rotating the data components from a Cartesian geographic into a polar wave-vector reference frame, we find that radial VSGs (i.e., oriented along the vector connecting the virtual source and receiver) clearly indicate that surface-wave propagation is influenced by salt bodies as identified in a colocated active-source survey situated at a minimum of 0.7 km depth below the seafloor, an observation consistent with calculated 0.25–0.50 Hz surface-wave sensitivity kernels. This suggests that low-frequency ambient OBN surface-wave seismology could be important for estimating the long-wavelength elastic material properties (particularly S-wave velocity) and identifying the lateral boundaries of salt bodies without any prior knowledge of subsurface geology.

The Effect of the Second-order Difference-frequency Wave Force on the Motions of an Underwater Vehicle Near the Surface

_ When an underwater vehicle performs tasks, such as power recharge, air exchange, deployment, and retrieval near the surface, the low-frequency suction force and pitch moment will interfere with its motion. This paper presents a study on the effect of second-order difference-frequency wave force on the motions of the DARPA SUBOFF model at various submergence depths and environmental conditions. Ignoring the second-order wave force component induced by the second-order difference-frequency force may underestimate the second-order effect in irregular waves in the time domain. Comparisons were made between Newman’s approximation, which sets the off-diagonal difference-frequency quadratic transfer function (QTF) value to the average over the corresponding diagonal values, and the full QTFs based on the Pinkster approximation in the frequency and time domains. The QTFs-based prediction is significantly larger values than Newman’s approximation. Time-domain simulations were conducted, to investigate the effect of the second-order difference-frequency wave loads on motion responses at different nondimensional submergence depths and various sea conditions. The results of the computation indicate that ignoring the second-order difference-frequency wave force may underestimate the motion and dynamic responses of underwater vehicles; in addition, the pitch and heave motion responses are dominated by the low-frequency response. Introduction When an underwater vehicle travels close to the water surface, it experiences an upward force and pitch moment, known as surface suction. This can significantly affect the underwater vehicle’s behavior when power recharge, air exchange, deployment, and retrieval are incorporated into the underwater vehicle simulations. Veillon et al. (1996) noted that for a 10,000-ton submarine at a depth of 50 m, the surface suction due to waves will require around 20–30 tons of compensation to stop the submarine from surfacing. In addition, Hirom (1974) indicated that in waves, the fluctuating forces on a submarine can be on the order of 1000 kN, with a steady component of approximately 10 kN. In calm water, suction occurs owing to a higher flow velocity and a lower pressure above an underwater vehicle when sailing near the surface, and the wave pattern generated by the underwater vehicle complicates the surface effect. The longitudinal position of the center of pressure is after the amidships, which induces the pitch moment. In the presence of waves, the first-order wave force, which is the oscillatory force at the wave frequency, acts on the underwater vehicle. In addition, waves and model motions induce low-frequency suction force and pitch moment. Using any internal compensation and control surfaces to offset the induced suction force and pitch moment requires a good and early understanding of the magnitude of these forces, for designing sufficient-capacity compensation tanks (Crossland 2013). Therefore, the surface suction and pitch moment effects are essential components of the hydrodynamic characteristics of underwater vehicles sailing near the water surface.

Two dominant timescales of cytoskeletal crosslinking in the viscoelastic response of the cytoplasm

Cells precisely regulate their frequency-dependent viscoelastic properties in response to chemical and mechanical cues. We use optical trap-based active microrheology using intracellular probes to measure the cytoplasmic mechanical response of fibroblast and macrophage cells over a broad frequency range ($\ensuremath{\sim}0.02--350$ Hz). Both cell types show similar frequency-dependent behavior, suggesting that the mechanisms that control the cell's mechanical response are general to many cell types. At frequencies above 1 Hz, the cytoplasmic mechanical behavior shows a broad distribution of relaxation timescales consistent with power-law mechanics. At low frequencies ($&lt;1$ Hz), cells exhibit fluidlike behavior with distinct relaxation timescales, similar to that observed in reconstituted networks of transiently crosslinked actin filaments. The response across all frequencies can be captured by a mathematical model combining a power-law term with two crosslinker-unbinding terms. The two unbinding rates required to describe the low-frequency response suggest that the viscoelastic relaxation of the cytoplasm is governed either by multiple dominant crosslinkers or by a single crosslinker with multiple states.

Low-frequency compensation of piezoelectric force sensors for servo systems

This paper presents a force/disturbance observer that compensates for the low-frequency response of a piezoelectric force sensor. Although the piezoelectric force sensor has many salient features, it is unable to measure dc and quasi-dc forces. The proposed observer compensates for this low-frequency deficiency, enabling the piezoelectric force sensor to be used in a servo system. Compared with previous studies, the proposed scheme requires redesigning neither the sensor structure nor sensing circuitry and can be easily integrated into an existing measurement system. As a by-product, the proposed observer also produces a disturbance estimate that can be used to enhance the robustness of a servo system. This paper reports experimental results of force control and position control for a linear motion platform equipped with a piezoelectric force sensor and a strain gauge-based load cell. The force control results indicate that the control system using the proposed observer can achieve a wider bandwidth than that using the load cell as a feedback-sensing device. In a comparative study, the position control results further demonstrate the proposed observer’s effectiveness in diminishing the piezoelectric sensor’s low-frequency deficiency. Moreover, the disturbance estimate produced by the proposed observer is experimentally introduced to the position control system, showing an enhanced tracking performance in terms of the root-mean-square error.

Analysis of High-Temperature Wideband Dielectric Properties of ATH-Filled Silicone Rubber Used for On-Site Insulation of Bare Overhead Conductors

Due to its excellent electrical and mechanical properties, and easy to cure at room temperature, room temperature vulcanizing silicone rubber is currently used in the on-site insulation of bare overhead conductors in the distribution system. The dielectric properties of silicone rubber materials have a great impact on power loss, which is vital to the transmission lines. Therefore, in this article, the dielectric properties of silicone rubber with the addition of different aluminum hydroxide contents are investigated by broadband dielectric spectroscopy in wide broadband range and wide temperature ranges, and the influence of aluminum hydroxide filler on the dielectric response is analyzed using a modified Cole–Cole model which takes the low-frequency dispersion process into account. It is found that the relationship of the dc conductivity and the low-frequency dispersion strength with temperature follows the Arrhenius equation law. The low-frequency dielectric response is influenced by dipoles’ polarization and electrode polarization. Moreover, the relaxation time decreases with the increase in temperature, and its mechanism can be revealed by the double-well model. The characteristic parameters obtained in this article can provide a reference for on-site insulating of bare overhead conductors.