Showing papers on "Fundamental frequency published in 2020"

Modified Second-Order Generalized Integrators With Modified Frequency Locked Loop for Fast Harmonics Estimation of Distorted Single-Phase Signals

Abstract: This article proposes modified second-order generalized integrators (mSOGIs) for a fast estimation of all harmonic components of arbitrarily distorted single-phase signals, such as voltages or currents in power systems. The estimation is based on the internal model principle leading to an overall observer consisting of parallelized mSOGIs. The observer is tuned by pole placement. For a constant fundamental frequency, the observer is capable of estimating all harmonic components with prescribed settling time by choosing the observer poles appropriately. For time-varying fundamental frequencies, the harmonic estimation is combined with a modified frequency locked loop (mFLL) with gain normalization, sign-correct antiwindup, and rate limitation. The estimation performances of the proposed parallelized mSOGIs with and without mFLL are illustrated and validated by measurement results. The results are compared to standard approaches such as parallelized standard SOGIs (sSOGIs) and adaptive notch filters (ANFs).

Small-Signal Modeling and Stability Prediction of Parallel Droop-Controlled Inverters Based on Terminal Characteristics of Individual Inverters

Abstract: Parallel droop-controlled inverters with renewable energy sources are widely employed in islanded ac microgrids, where dynamic interactions among them may cause small-signal stability issues. Since the active power-frequency droop scheme is applied, the dynamic interactions among inverters exist not only in bus voltage and transmitted current, but also in variable system fundamental frequency. This paper introduces a novel small-signal terminal characteristic model for droop-controlled inverter. Besides conventional impedance and admittance, a new set of terminal characteristics is proposed to characterize the dynamics of fundamental frequency. Furthermore, the small-signal model of parallel inverters is constructed based on the terminal characteristics of individual inverters. Covering the fundamental frequency interactions, a stability prediction approach based on generalized Nyquist criterion is proposed for parallel droop-controlled inverters. Besides the product of impedance and admittance, an additional term is added in the system return ratio, which consists of the proposed terminal characteristics associated with the fundamental frequency. Finally, experimental results validate the effectiveness of this proposed stability prediction approach.

29.9 A 4×4 Distributed Multi-Layer Oscillator Network for Harmonic Injection and THz Beamforming with 14dBm EIRP at 416GHz in a Lensless 65nm CMOS IC

Abstract: Integrated high-power THz arrays with beamforming ability can enable new applications in communication, sensing, imaging, and spectroscopy [1]. However, due to the limited power-generation capability of a single source above the device f max [2], efficient spatial power combining from multiple coherent sources becomes necessary to generate mW level of power. To create this 2D array of distributed frequency and phase-locked sources, prior works have shown central LO-signal distribution with local harmonic upconversion [3]. However, this requires high power consumption in the LO distribution. In addition, phase-matching with PVT variations across the sources at the harmonic-radiating THz frequency can be quite challenging. A small ∆θ perturbation at the fundamental frequency translates to N∆θ at the radiated Nth harmonic, thus corrupting the array beam pattern. Another method to synchronize multiple distributed radiating sources (ƛ/2 spaced at Nfo) is through a mutual coupling network with active/passive elements in a coupled oscillator array [4], [5]. However, the locking range in these methods is typically narrow (∆f locking ~ f 0 /20 to f 0 /10) and PVT variations can easily cause desynchronization. In such a network, each cell is a self-sustaining oscillator, and the coupling network tries to establish injection signals to force synchronization between these individual free-running oscillators. In this paper, we used a 2D oscillating network with negative G m (–G m ) cells at each node that do not oscillate individually but only collectively, establishing a robust frequency and phase distribution network across the chip for high THz-power generation. By making this network as the lowest layer, we can now separate the locking mechanism and the power-generation sources. This avoids loading and sub-optimal operation of the power sources. The distributed oscillating network at the lowest layer operates at 69.3GHz, and multi-layer local harmonic generation produces a radiated power of −3dBm and +14dBm EIRP at 416GHz in a 4×4 array.

SPICE: Self-Supervised Pitch Estimation

Abstract: We propose a model to estimate the fundamental frequency in monophonic audio, often referred to as pitch estimation. We acknowledge the fact that obtaining ground truth annotations at the required temporal and frequency resolution is a particularly daunting task. Therefore, we propose to adopt a self-supervised learning technique, which is able to estimate pitch without any form of supervision. The key observation is that pitch shift maps to a simple translation when the audio signal is analysed through the lens of the constant-Q transform (CQT). We design a self-supervised task by feeding two shifted slices of the CQT to the same convolutional encoder, and require that the difference in the outputs is proportional to the corresponding difference in pitch. In addition, we introduce a small model head on top of the encoder, which is able to determine the confidence of the pitch estimate, so as to distinguish between voiced and unvoiced audio. Our results show that the proposed method is able to estimate pitch at a level of accuracy comparable to fully supervised models, both on clean and noisy audio samples, although it does not require access to large labeled datasets.

Phase relations in resonant circuits with a wide falling section on the amplitude characteristic

Abstract: This article examines the features of circuits in order to determine the possibility of their use for controlling thyristors when building new, simple and reliable circuits for controlling boost voltage stabilizers. In ferroresonant circuits connected to a voltage source with a low internal resistance, with a certain combination of parameters, oscillations are excited at the fundamental frequency, the initial phase of which has a shift with respect to the phase of the applied voltage. Moreover, the phase of the excited oscillations depends on the magnitude of the applied emf.

Analysis and Assessment of Modular Multilevel Converter Internal Control Schemes

Abstract: Adoption of distributed submodule (SM) capacitors in a modular multilevel converter (MMC) necessitates complex controllers to ensure the stability of its internal dynamics. This paper presents comprehensive analysis and assessment of different proportional resonant (PR)-based control schemes proposed to stabilize the internal dynamics and ensure ac and dc sides power quality of the MMC within a dc transmission system. With the consideration of passive component tolerances, different energy- and voltage-based control schemes under various conditions are analyzed. It has been established that without vertical voltage balance control, unequal passive component values in the upper and lower arms of the same phase leg may cause: unbalanced fundamental currents in the arms, unequal dc voltage across the arms, and fundamental oscillations in the common-mode currents that lead to fundamental frequency ripple in the dc-link current. The theoretical analysis that explains this mechanism is presented, and is used to show that vertical voltage balancing is necessary for the nullification of arm voltage difference and suppression of odd oscillations caused by capacitive/inductive asymmetry between arms of the same phase leg. Simulations support the theoretical analysis and the effectiveness of voltage balancing in ensuring correct operation, independent of tolerances of the MMC passive elements, and operating conditions. A new direct method for elimination of fundamental oscillations in the common-mode and dc-link current is proposed. Experimental results from a single-phase MMC prototype validate the presented theoretical discussions and simulations.

Vibration behavior of timber-concrete composite floors under human-induced excitation

Abstract: The design of timber-concrete composite (TCC) floors is usually subject to the requirement of serviceability rather than strength. Most studies reported so far focus on the dynamic characteristics of the TCC floor. However, there are few studies conducted on the vibration comfort of TCC floors under human-induced excitation. In this paper, dynamic experiments were performed to obtain the fundamental frequency and the damping ratio of a glulam-concrete composite floor under simply supported and fixed support boundary conditions, respectively. Besides, an in-depth investigation was carried out considering the influences of walking path, step frequency, number of pedestrians, arrangement of walking loads and walking mode on the vibration response of the composite floor. Test results showed that the peak acceleration of the floor increased with an increase in step frequency and the number of pedestrians. Regular walking caused a higher peak acceleration compared to irregular walking. At the same step frequency, marking time caused a larger peak acceleration than normal walking. Three existing analytical models were applied to predict the fundamental frequency of the floor, which conformed to the experimental results. In addition, a calculation model and finite element (FE) models were proposed to predict the peak floor acceleration under a single person walking, and the numerical model results agree well with the test results. A double-index was proposed to evaluate the vibration serviceability of TCC floors; the fundamental frequency of the floor is required to exceed 15 Hz and the peak acceleration ought to be less than 0.15 m/s2.

Design of a High Performance Phase-Locked Loop With DC Offset Rejection Capability Under Adverse Grid Condition

Abstract: In the new energy grid-connected power generation system, accurately extracting the grid synchronization signals such as the frequency, phase and amplitude of the grid voltage is the basis for effective control. Aiming at the requirements for detecting grid synchronization signals under unbalanced, harmonics and DC offset voltage mixed conditions, a dual second-order complex coefficient filter with DC offset rejection capability (DSOCCF $_{dc}$ ) is proposed, combining the approach of moving average filter (MAF), a novel hybrid filter in dq -frame is designed and on the basis of this design a new synchronous reference frame phase locked loop (SRF-PLL) design approach based on the hybrid filter is proposed. The proposed approach employs moving average filter (MAF) to block the high-frequency harmonics in the grid voltage, and uses DSOCCF $_{dc}$ to separate the fundamental frequency positive and negative sequence and reject DC offset. It can accurately extract the synchronization information of the grid fundamental frequency positive sequence. After simulation and experiment verification, it can be confirmed that the proposed PLL can quickly and accurately lock the properties of the grid voltage under adverse grid condition, and also have high detection accuracy and strong robustness to frequency fluctuations.

Multi-Scale Modeling and Simulation of DFIG-Based Wind Energy Conversion System

Abstract: A multi-scale transients model of a doubly fed induction generator (DFIG)-based wind energy conversion system (WECS) is developed, implemented, and validated. All ac circuit and control quantities of the electrical part are modeled by analytic signals rather than just real signals. In addition to the real parts, the analytic signals also comprise orthogonal imaginary parts. While measured Fourier spectra of real ac quantities involve positive and negative frequency components, they only involve positive frequency components when extended and represented as analytic signals. With the introduced shift frequency operator, the analytic signal can now be shifted in the frequency domain to reduce its maximum frequency contents and thus allow for a larger time-step size in accordance with the Nyquist criterion. If the shift frequency is set equal to the ac fundamental frequency, then the affiliated ac voltages and currents become dynamic phasors. At a zero shift, however, instantaneous signals can be tracked as in an electromagnetic transients program (EMTP). This is illustrated here for the voltage sourced converters (VSC) of the WECS. By appropriate selection of the shift frequency, both electromagnetic and electromechanical transients are simulated efficiently. Studies involving wind power fluctuations, three-phase-to-ground fault and single-phase-to-ground fault confirm these claims.

Measuring the frequency response of the honeybee thorax

Abstract: Insects with asynchronous flight muscles are believed to flap at the effective fundamental frequency of their thorax-wing system. Flapping in this manner leverages the natural elasticity of the thorax to reduce the energetic requirements of flight. However, to the best of our knowledge, the fundamental frequency of the insect wing-muscle-thorax system has not been measured. Here, we measure the linear frequency response function (FRF) of honeybee Apis mellifera thoraxes about their equilibrium state in order to determine their fundamental frequencies. FRFs relate the input force to output acceleration at the insect tergum and are acquired via a mechanical vibration shaker assembly. When compressed 50 μm, the thorax fundamental frequency averaged across all subjects was about 50% higher than reported wingbeat frequencies. We suspect that the measured fundamental frequencies are higher in the experiment than during flight due to boundary conditions and posthumous muscle stiffening. Next, we compress the thorax between 100-300 μm in 50 μm intervals to assess the sensitivity of the fundamental frequency to geometric modifications. For all specimens considered, the thorax fundamental frequency increased nearly monotonically with respect to level of compression. This implies that the thorax behaves as a nonlinear hardening spring when subject to large displacements, which we confirmed via static force-displacement testing. While there is little evidence that insects utilize this non-linearity during flight, the hardening characteristic may be emulated by small resonant-type flapping wing micro air vehicles to increase flapping frequency bandwidth. Overall, methods established through this work provide a foundation for further dynamical studies on insect thoraxes moving forward.

Compensation of Current Measurement Offset Error for Permanent Magnet Synchronous Machines

Abstract: Thermal drift-caused current measurement offset error usually results in unfavorable torque ripples of fundamental frequency, and hence a compensation method for current offset error is proposed in this article, which is initially derived based on the nonsalient-pole machines and then extended to the salient-pole machines. By filtering the difference between the predicted currents from a simple nominal model and the measured currents, the offset error is directly estimated and then compensated with minimal effects on the current dynamics. It is proved that the estimation is accurate if the filter has unity gain at the frequency of the offset error. Therefore, the proposed method is free of the current regulator type and shows strong robustness against machine parameter deviations. It can also stably work even when the output voltage is saturated. Two kinds of filters are designed in the stationary reference frame, and the low-pass filter with low bandwidth is recommended for its easy implementation. Finally, the effectiveness of the proposed method is verified by simulations and experiments.

Low-Frequency Stability Analysis of Inverter-Based Islanded Multiple-Bus AC Microgrids Based on Terminal Characteristics

Abstract: For system planning of three-phase inverter-based islanded ac microgrids, the low frequency instability issue caused by interactions of inverter droop controllers is a major concern. When internal control information of procured commercial inverters is unknown, impedance-based small-signal stability criteria facilitate prediction of resonances in medium and high frequency ranges, but they usually assume the grid fundamental frequency as constant and thus they are incapable of analyzing the low-frequency oscillation of the fundamental frequency in islanded microgrids. Aiming at solving this issue, this paper proposes two stability analysis methods based on terminal characteristics of inverters and passive connection network including the dynamics of the fundamental frequency for analysis of low-frequency stability in islanded multiple-bus microgrids. Based on the Component Connection Method (CCM) to systematically separate inverters from the passive connection network, a general approach is developed to model the microgrid as a multiple-input-multiple-output (MIMO) negative feedback system in the common system d-q reference frame. By applying the generalized Nyquist stability criterion (GNC) to the return-ratio and return-difference matrices of the MIMO system model, the low-frequency stability related to the fundamental frequency can be analyzed using the measured terminal characteristics of inverters. Analysis and simulation of a 37-bus microgrid verify the effectiveness of the proposed stability analysis methods.

Enhanced logical vibrational resonance in a two-well potential system

Abstract: Various external driving forces can induce logical stochastic or vibrational resonance, such as noise, harmonics, and the combination of noise and harmonics In engineering, using harmonics as driving force is more conducive to the control of logic operations, while a wider optimal parameter region and a shorter switching time are expected in practice to improve the robustness and response speed of system Here, we report the logical vibrational resonance in a two-well potential system subjected to biharmonics Our results show that the variable frequency (VF) (one harmonic) could broaden the optimal parameter region when an appropriate weak long-period signal is chosen as the fundamental frequency (FF) (the other harmonic) An intuitive interpretation for LVR is given by means of bifurcation and potential well diagrams In addition, according to dynamic potential wells varying with input signal, four different kinds of switching modes are presented, and the switching time presents differences for different switching modes There may be a trade-off between fast response and the robustness of system Noise obviously affects the optimal parameter region of VF and the switching time Finally, some results are further verified by circuit simulation

Non-classical electron transport in the cathode plume of a Hall effect thruster

Abstract: An experimental investigation is presented into the wave-driven electron transport in the near-field plume of a hollow cathode operating in a 300 V, 4.5 kW magnetically shielded Hall thruster. Correlational analysis of probe measurements in the cathode plume shows two types of electrostatic waves: ion acoustic turbulence propagating along the applied longitudinal magnetic field at frequencies from 500 to 1250 kHz and coherent, azimuthal anti-drift waves with a fundamental frequency of 95 kHz and mode numbers from m = 1 − 4. A quasilinear analysis is applied to quantify the impact of each wave on the electron transport in the near-field plume. It is found that the ion acoustic modes give rise to an enhanced effective collision frequency in the direction parallel to the applied magnetic field that exceeds the classical collision frequency by two orders of magnitude. The anti-drift waves promote an anisotropic collision frequency that depends on the direction of the electron drift. While the enhanced collision frequency from these waves is comparable to the classical frequency for motion along the applied magnetic field, the effective collision frequency in the azimuthal direction exceeds the classical by three orders of magnitude. These results are discussed in the context of their impact on the steady-state plasma gradients in the near-field cathode plume. Closure models for incorporating the effective collision frequencies from both types of waves into fluid-based codes are derived and shown to agree with the measured wave-driven collision frequencies.

SnS nanosheets for harmonic pulses generation in near infrared region.

Abstract: Two-dimensional materials have attracted increasing attention because of their excellent mechanical, thermodynamic, magnetic, electrical and optical properties. Here, a new two-dimensional material of tin sulfide (SnS) is experimentally prepared. It is layered like black phosphorus and owns distinct optoelectronic properties, but eliminates the disadvantage of instability. The nonlinear saturable absorption characteristics of the SnS nanosheets is investigated at 1563.3 nm by the double-balanced detection method. The obtained modulation depth and saturation intensity are 5.4% and 66.3 MW/cm2, respectively. A passively harmonic mode-locked erbium-doped fiber laser based on the SnS saturable absorber (SA) has been demonstrated. The results show that mode-locking with fundamental frequency of 5.47 MHz is realized at pump power of 28.38 mW. With the increase of pump power, the laser can operate from fundamental frequency to high-order harmonic mode-locking. The maximum repetition rate of 412.73 MHz has been obtained, which is equivalent to the 76th harmonic mode-locking. This work reveals that SnS nanosheets is a novel and efficient SA with high damage threshold, which will find potential applications in optical communication, photoelectric detection, laser medicine, etc.

Frequency Adaptive Parameter Estimation of Unbalanced and Distorted Power Grid

Abstract: Grid synchronization plays an important role in the grid integration of renewable energy sources. To achieve grid synchronization, accurate information of the grid voltage signal parameters are needed. Motivated by this important practical application, this paper proposes a state observer-based approach for the parameter estimation of unbalanced three-phase grid voltage signal. The proposed technique can extract the frequency of the distorted grid voltage signal and is able to quantify the grid unbalances. First, a dynamical model of the grid voltage signal is developed considering the disturbances. In the model, frequency of the grid is considered as a constant and/or slowly-varying but unknown quantity. Based on the developed dynamical model, a state observer is proposed. Then using Lyapunov function-based approach, a frequency adaptation law is proposed. The chosen frequency adaptation law guarantees the global convergence of the estimation error dynamics and as a consequence, ensures the global asymptotic convergence of the estimated parameters in the fundamental frequency case. Gain tuning of the proposed state observer is very simple and can be done using Matlab commands. Some guidelines are also provided in this regard. Matlab/Simulink based numerical simulation results and dSPACE 1104 board-based experimental results are provided. Test results demonstrate the superiority and effectiveness of the proposed approach over another state-of-the art technique.

Robust Frequency, Phase, and Amplitude Estimation in Power Systems Considering Harmonics

Abstract: Efficient and accurate estimation of voltages and currents is essential for the control of electrical grids, particularly microgrids. In this paper, we present a novel two-step framework to estimate these parameters for both single-phase and three-phase power systems including harmonics and interharmonics. After employing a harmonic version of the Aboutanios and Mulgrew (HAM) estimator, the fundamental frequency and phase estimates are further refined by a weighted least squares (WLS) estimator. The WLS refinement step takes advantage of the harmonic and interharmonic relationships to further improve the parameter estimation from the HAM step. Consequently, the proposed HAM-WLS method maintains accuracy even in electrically-noisy environments that are also contaminated with both harmonics and interharmonics. Results from both simulated and real data under various power system conditions verify that the HAM-WLS estimator outperforms state-of-the-art estimation methods in terms of accuracy, complexity and convergence.

Motor Speed Signature Analysis for Local Bearing Fault Detection With Noise Cancellation Based on Improved Drive Algorithm

Abstract: Motor speed signature analysis provides a noninvasive method for bearing fault detection. However, for the vector-controlled ac motors, periodic speed ripples related to fundamental frequency $f_{e}$ and its twice harmonic $2f_{e}$ , which are caused by current measurement errors, are difficult to be attenuated by motor inertia or bandwidth of speed loop under low-speed conditions. The unwanted components would reduce the signal-to-noise ratio of motor speed and increase the difficulty of bearing fault detection. To solve the problem, this paper proposes a new noise cancellation strategy, which applies the improved drive algorithm instead of conventional signal processing schemes to cancel out the noise component before the data acquisition. Specifically, resonance controllers are introduced and set in parallel with the existed proportional–integral controller to suppress the speed ripples. Moreover, the envelope spectrum analysis is carried out to detect fault characteristic. The effectiveness of the proposed method is validated through simulation and experimental tests. Besides, its superiority under low-speed conditions is also demonstrated, compared with the spectral kurtosis of speed signal and three current-based methods.

DESMEX: A novel system development for semi-airborne electromagnetic exploration

Abstract: There is a clear demand to increase detection depths in the context of raw material exploration programs. Semi-airborne electromagnetic (semi-AEM) methods can address these demands by combining the advantages of powerful transmitters deployed on the ground with efficient helicopter-borne mapping of the magnetic field response in the air.The penetration depth can exceed those of classical airborne EM systems,since low frequencies and large transmitter-receiver offsets can be realized in practice. A novel system has been developed that combines high-moment horizontal electric bipole transmitters on the ground with low-noise three-axis induction coilmagnetometers, a three-axis fluxgate magnetometer and a laser gyroinertial measurement unit integrated within a helicopter-towed airborne platform. The attitude data are used to correct the time series for motional noise and subsequently to rotate into an Earth-fixed reference frame. In a second processing step, and as opposed to existing semi-airborne systems, we transform the data into the frequency domain and estimate the complex-valued transfer functions between the received magnetic field components and the synchronously recorded injection current by regression analysis. This approach is similar to the procedure employed in controlled-source EM. For typical source bipole moments of 20-40 kAm and for rectangular current waveforms with a fundamental frequency of about 10 Hz, we can estimate reliable three-component transfer functions in the frequency range from 10-5000 Hz over a measurement area of 4 x 5 km2 for a single source installation. The system has the potential to be used for focused exploration of deep targets. (Less)

Nonlinear responses and bifurcations of a rotor-bearing system supported by squeeze-film damper with retainer spring subjected to base excitations

Abstract: A high-speed rotating rotor system mounted on a moving vehicle is inevitably subjected to parametric excitations and exciting forces induced by base motions. Dynamic characteristics of a rotor-bearing system supported by squeeze-film damper with retainer spring subjected to unbalance and support motions are investigated. Using Lagrange’s principle, equations of motion for rotor system relative to a moving support are derived. Under base excitations, steady-state and transient responses are analyzed by frequency–amplitude curve, waveform, orbit, frequency spectrum, and Poincare map. Changing with rotating speed or base harmonic frequency, journal motions are analyzed by bifurcation diagram. The results indicate that under base axial rotation, increasing base angular velocity, first two critical speeds decrease but resonant amplitudes increase slightly. The journal whirls around the static eccentricity with noncircular orbit. Under base lateral rotation, critical speeds, and resonant amplitudes remain essentially unchanged, but orbit’s deviation is related to base angular velocity. Excited by base harmonic translation, the integral multiples of fundamental frequency $$k{\varOmega }\left( {k = 1,2} \right)$$ , base harmonic frequency $${\varOmega}^{z}$$ , and combined frequencies $$k{\varOmega } \pm j{\varOmega }^{z} { }\left( {k,j = 1,2} \right)$$ are stimulated, changing the motions from periodic to quasiperiodic. Overall, it provides a flexible approach with good expandability to predict dynamic characteristics of squeeze-film damped rotor system under base motions.

Efficient Narrowband Noise Cancellation System Using Adaptive Line Enhancer

Abstract: Rotating machines such as motors, generate noise at the fundamental frequency and its harmonics. The narrowband active noise control (NANC) algorithm is widely used to cancel such noise. Traditional feedforward NANC systems use non-acoustic sensors to measure rotation speeds, and then a bank of signal generators produce synchronized tonal signals as the reference signals according to the fundamental frequency of the undesired noise. However, the non-acoustic sensors such as encoders usually cost a lot and are not reliable. This study proposes using the feedback ANC structure and incorporates with several adaptive line enhancers (ALEs) to generate the reference signals. Without using non-acoustic sensors, the proposed system only updates the center frequency of the first ALE to track the fundamental frequency change, which greatly reduces complexity of the proposed ANC system. The frequency mismatch (FM) problem is overcome by properly setting the bandwidths of the ALE. Performance were verified by using both simulations and real-time experiments.

Joint Detection of Human and Object Motion Using Harmonic Micro-Doppler Radar and Harmonic Tags

Abstract: We present a new method for jointly detecting the motion of moving people and tagged objects using harmonic micro-Doppler radar and passive harmonic tags. Wirelessly measuring the motion of people and held objects is important for applications, including sensing in living spaces and industrial monitoring. A significant challenge in discriminating the motion of objects from that of people is the relatively low signal power generated by the radar return from a held object relative to the clutter return power. We overcome this limitation by using harmonic tags that generate a response at a harmonic of the incident radar signal. The radar emits a continuous-wave (CW) signal at a fundamental frequency of 2.5 GHz, which scatters off the person and is collected by a 2.5 GHz receiver. The harmonic tag collects the 2.5 GHz signal and generates a harmonic response at 5 GHz, which is collected by a 5 GHz receiver on the radar. Frequency modulation on the 5 GHz signal is thus generated only by the motion of the tag, while the 2.5 GHz modulation is predominantly due to the motion of the person. The fundamental and harmonic received signals can thus be processed separately for direct joint detection of the dynamic motion of people and held objects.

Bifurcation behavior for mass detection in nonlinear electrostatically coupled resonators

Abstract: The nonlinear coupled vibrations widely exist in coupled resonant structures, which can lead to complex dynamic bifurcation behavior and expand the research scope of fundamental physics. A new micro-mass detection method is proposed by using bifurcation jumping phenomenon in nonlinear electrostatically coupled resonators in this article. Considering the fundamental frequency excitation, the one-to-one internal resonance equations to describe electrostatically coupled resonant sensor are obtained by using Hamilton’s principle and Galerkin method. Then, the perturbation analysis method is introduced to study the response and stability of the system for small amplitude vibration. Through bifurcation analysis, it is found that the isolated response branches appear in nonlinear electrostatically coupled resonators and present the physical conditions of this phenomenon. Typically, we demonstrate the exploitation of the bifurcation jump phenomena of two electrostatically coupled microbeam resonators to realize the mass quantitative detection and threshold detection, which overcomes the detection inaccuracy caused by frequency drift in the nonlinear vibration. Finally, the numerical experiments verify the validity of the method. The results of this paper can be potentially useful in micro-mass detection.

A Compact, Passive Frequency-Hopping Harmonic Sensor Based on a Microfluidic Reconfigurable Dual-Band Antenna

Abstract: We propose here a fully-passive wireless liquid sensor using a harmonic transponder, which comprises a dual-band microstrip antenna reconfigured by different types of liquids injected in a fluidic cavity. Different from traditional radio-frequency (RF) backscatter sensors, the proposed harmonic-transponder sensor (or harmonic sensor) receives frequency-hopped RF monotones and backscatters their second harmonics, with the peak frequency shifted by dielectric properties of liquid mixtures. This microstrip antenna has a hybrid-feed structure, of which an outer split-ring patch exhibits a narrow-band TM310 mode at the fundamental frequency ( ${f}_{0}$ ) and an inner elliptical patch displays a wideband resonance centered at the second-harmonic frequency ( $2{f}_{0}$ ), achieved with hybridization of TM $_{e110}$ and TM $_{o110}$ modes. In particular, the outer split-ring patch is loaded with a fluidic channel system to tune the resonance frequency of the TM $_{3}10$ mode ( ${f}_{0}$ ). We demonstrate that the type of liquid mixture filling in the fluidic cavity can be clearly perceived by reading the peak received signal strength indicator (RSSI) in the spectrum of second harmonics. Our results show the potential for deploying this passive wireless sensor in noisy environments that include clutters, multiple reflections, jamming, and crosstalks.

A nonlinear ultrasonic modulation method for crack detection in turbine blades

Abstract: In modern gas turbines, efforts are being made to improve efficiency even further. This is achieved primarily by increasing the generated pressure ratio in the compressor and by increasing the turbine inlet temperature. This leads to enormous loads on the components in the hot gas region in the turbine. As a result, non-destructive testing and structural health monitoring (SHM) processes are becoming increasingly important to gas turbine manufacturers. Initial cracks in the turbine blades must be identified before catastrophic events occur. A proven method is the linear ultrasound method. By monitoring the amplitude and phase fluctuations of the input signal, structural integrity of the components can be detected. However, closed cracks or small cracks cannot be easily detected due to a low impedance mismatch with the surrounding materials. By contrast, nonlinear ultrasound methods have shown that damages can be identified at an early stage by monitoring new signal components such as sub- and higher harmonics of the fundamental frequency in the frequency spectrum. These are generated by distortion of the elastic waveform due to damage/nonlinearity of the material. In this paper, new global nonlinear parameters were derived that result from the dual excitation of two different ultrasound frequencies. These nonlinear features were used to assess the presence of cracks as well as their qualitative sizes. The proposed approach was tested on several samples and turbine blades with artificial and real defects. The results were compared to samples without failure. Numerical simulations were conducted to investigate nonlinear elastic interaction of the stress waves with the damage regions. The results show a clear trend of nonlinear parameters changing as a function of the crack size, demonstrating the capability of the proposed approach to detect in-service cracks.

Theoretical Harmonic Spectra of PWM Waveforms Including DC Bus Voltage Ripple—Application to a Low-Capacitance Modular Multilevel Converter

Abstract: This article develops a new closed-form analytical solution to the harmonic spectrum of the pulsewidth modulated (PWM) output voltage of the single-phase inverter connected to the dc bus with considerable voltage ripple. The solution is based on a double Fourier series expansion in two variables. As the single-phase inverter forms the basic building block of most converters, the developed expressions can be applied to various topologies. This article first identifies the interactions of a single-frequency sinusoidal modulation signal and the carrier signal with the dc bus voltage harmonics. The dc bus voltage harmonics are constrained in this article to multiples of the inverter output voltage fundamental frequency. The analysis is then extended to include a multifrequency modulation signal. Starting from the general solution, new expressions for the output voltage harmonics of the modular multilevel converter (MMC) are developed. The low-capacitance MMC is chosen in this analysis due to inherent low-frequency voltage oscillations in the converter internal capacitors. The MMC analytical harmonic spectrum can incorporate the effects of the circulating current control and third harmonic injection PWM, by including the second- and third-order harmonic components in the modulation signal, respectively. The MMC analytical harmonic spectrum is benchmarked against the experimental results.