Step Changes In Frequency Research Articles

A New THD Measurement Method With Small Computational Burden Using a SOGI-FLL Grid Monitoring System

This article proposes a new method for obtaining the total harmonic distortion (THD) using few math operations, a low-pass filter (LPF), and a grid monitoring system able to provide the fundamental and harmonics components of the grid voltage. The method is particularly developed by using the second-order generalized integrator (SOGI) error signal notch filter transfer function characteristics of a standard SOGI-FLL monitoring system. The method is accurate and has a small computational burden, so it is suitable for the online assessment of the grid voltage or current THD and can easily be implemented into a digital signal processor. The accuracy and transient response of the system have been analyzed, showing that they can easily be determined by the tuning of the SOGI filter and the LPF. The method also shows to be robust to grid perturbations, such us voltage sags, or swells, and frequency step changes. Simulations and experimental results are provided to validate the proposed THD method. Moreover, comparison with a fast Fourier transform (FFT)-based THD is also presented, which shows that the proposed THD method results to be faster, more accurate, and simpler than the FFT-based one.

Performance Comparison of Active Rectifier Control Schemes in More Electric Aircraft Applications

In a more electric aircraft (MEA) design, the onboard electric generators are mechanically coupled directly to the jet engines, thus producing a variable-frequency constant magnitude voltage. This wild-frequency characteristic imposes a new design challenge on the onboard power converters. In this article, an experimental performance comparison between the model predictive and the proportional–integral (PI)-based control schemes for active rectifiers in MEA applications is presented. The comparison of the aforementioned controllers is executed under the MEA variable input frequency perspective for the first time and, therefore, constitutes the main contribution of this article. This comparison mainly addresses the dynamic response of the controllers in terms of dc-bus voltage regulation and unity power factor operation under frequency ramping, frequency step changes, and load step variations. Furthermore, the robustness of the controllers is analyzed in terms of their performance degradation in the presence of parameter mismatch. Both the model predictive and PI-based controllers are experimentally tested using a 1-kW SiC-MOSFET two-level voltage-source-converter topology.

A new nonlinear Type3-PLL with noise rejection and fast locking performance in tracking telemetry and command systems

Type3-PLLs are widely used for carrier synchronization in tracking telemetry and command (TT&C) systems. A major challenge with the traditional Type3-PLL is how to further improve their acquisition and tracking speed without compromising the noise suppression ability and stability. In this article, drawn lessons from the nonlinear control theory, a new structural Type3-PLL is presented. The idea of the proposed Type3-PLL, which can be denoted by N-Type3-PLL, is to couple with a designed nonlinear element in the traditional Type3-PLL. The loop noise bandwidth is adaptively adjusted by the designed nonlinear element on the basis of the loop filter output, which is determined by the frequency-phase error. The output noise of the nonlinear element is then suppressed by the consequent low-pass filter. The influence of the nonlinear element parameters on the N-Type3-PLL is quantitatively analyzed and discussed. Simulation results indicate that the N-Type3-PLLs under various parameters can achieve locking faster than the traditional Type3-PLL in response to a step change in frequency. The nonlinear element parameters constraint the comprehensive performance of the N-Type3-PLL. The optimal parameters of the nonlinear element are solved via a suggested iterative feedback method. Compared with the traditional Type3-PLL, the output frequency of the N-Type3-PLL under the optimal parameters jitters much less and the standard deviation of frequency-phase error is reduced from 0.6198 (rad/s) 2 to 0.2264 (rad/s) 2 when the input SNR equals −15 dB. The lock time after the step is extended from 0.22 s to 0.24 s, which is within an acceptable range. The loss threshold of the proposed N-Type3-PLL is reduced from −20 dB to −23 dB.

Design of an FPGA based DPLL with fuzzy logic controllable loop filters with application customization capability

The carrier recovery loops are important in carrier tracking approaches particularly in the presence of high dynamic stress on user receivers and noisy environment applications. The precise carrier tracking techniques are proposed in systems that are sensitive to carrier mismatches, such as terrestrial or satellite tracking systems. The fading phenomenon, phase and frequency step changes and high user dynamics are currently most important challenges in the development of robust carrier tracking systems. In this work, a novel Digital Phase Locked Loop (DPLL) is proposed using type-2 fuzzy logic controller to improve noise immunity and handling user dynamic in digital receivers with application customization capability. Due to fast and accurate decision-making by proposed fuzzy logic controller, optimal loop filter coefficients are generated for DPLL. The proposed DPLL is simulated with Xilinx System Generator Software and can be implemented on FPGA. In comparison to traditional approaches, proposed new DPLL shows better performance in response to phase step, frequency step and frequency ramp signals with acceptable settling time alongside minimum complexity in implementation and customization.

High-Quality Sine Wave Generation Using a Differential Boost Inverter at Higher Operating Frequency

Buck-converter-based topologies are used to generate high-frequency sinusoidal outputs. Buck-based inversion circuits such as voltage source inverters or class-D amplifiers have inherent control-to-output linearity in large-signal sense. However, in these topologies, the instantaneous output is always smaller than the dc-input during linear modulation. A differential boost inverter (DBI) is a boost-based topology that is used to generate a sinusoidal output. In DBI, the instantaneous output can be higher or lower than the dc-input voltage. DBI exhibits nonlinear control-to-output behavior in large-signal dynamic sense. Therefore, generating a high-frequency sinusoidal output using this topology is a challenge. The issues associated with a DBI for high-frequency sine wave generation are characterized in this paper. Conventional linear and nonlinear control techniques fail to produce a high-quality sine wave output at higher operating frequency. A nonlinear feedback linearization technique is proposed, which forces the output to be linear with respect to the reference even at higher operating frequency. This leads to a high-frequency high-quality sine wave generation using a DBI. The proposed modulator is verified using a laboratory prototype to generate a sine wave up to 2 kHz. A triangular wave of 100-Hz frequency is also generated by the proposed technique. Superior dynamic responses of a dynamic linearizing modulator controlled DBI for a step change in frequency, load current, input voltage, and reference are also experimentally verified.

Fast and robust software‐based digital phase‐locked loop for power electronics applications

In this study, a fast and fully software-based algorithm for digital phase-locked loop (PLL) is proposed via a new hybrid approach in software and hardware by using an advanced digital signal processor architecture. The proposed algorithm is robust against line disturbances such as phase-angle jump, voltage sag, third harmonic injection, multi-zero crossing and step change in frequency at the input voltage. Performance and robustness of the proposed method are investigated through experimental studies. Furthermore, it is compared with three different PLL algorithms in detail to show its superiority over existing methods.

Dissociation Between Force and Calcium after a Step Change in Frequency

The force frequency response (FFR) is one of the major physiological regulators which govern alterations in contractile function in the body in which an increase in frequency of pacing leads to an increase in force production and a decrease in rate of contractile kinetics. Typically studies on FFR have been conducted at steady state where the changes in contractile function have been elicited by changes in myofilament phosphorylation. Here we aim to determine the role of alterations in calcium homeostasis and myofilament sensitization during the transition from one steady state of contractile function to another.Trabeculae, excised from the right ventricular free wall of New Zealand white rabbits were loaded with Rhod-AM calcium indicator dye and stimulated to contract from 1 to 4 Hz. Data was analyzed through customized Labview software.We hypothesize that the transition from one steady state to another occurs in two phases; the rapid/early phase is due to increases in calcium transient amplitude and diastolic calcium levels, and the gradual/latent phase is due to alterations in phosphorylation state of myofilament proteins such as troponin I (TnI) and myosin light chain 2 (MLC2). During our experiments we are able to measure calcium and developed force simultaneously. Despite previous studies which show alterations in myofilament calcium sensitivity occurring at steady state, we are able to show that during the immediate phase calcium and force change with each other, while at the latent phase there is an increase in diastolic calcium level that is not paralleled by the diastolic force. This suggests a myofilament desensitization during this latent phase that is not seen in the early transitory period as originally thought.

A comparative study on different power system frequency estimation techniques

This paper presents the estimation of frequency which is an important power system parameter by Extended Least Square (ELS) technique. The above technique is validated by comparing its performance with the existing techniques such as Kalman Filter (KF) and Least Mean Square (LMS) technique, etc. Using different simulation studies with signals having different signal to noise ratio values and with step change in frequency, it is observed that ELS technique outperforms over LMS and KF methods on power system frequency estimation. Initialisation of covariance matrix in KF method and complicacy due to incorporation of correlation matrix in LMS algorithm affect their convergence. But ELS algorithm becomes very simple and attractive due to the absence of covariance and correlation matrix.

Trends in frequency of precipitation extremes in the Yangtze River basin, China: 1960–2003 / Tendances d'évolution de la fréquence des précipitations extrêmes entre 1960 et 2003 dans le bassin versant du Fleuve Yangtze (Chine)

The spatial distribution and trends in the frequency of precipitation extremes over the last 44 years (1960–2003), especially since 1990, have been analysed using daily precipitation data from 147 stations in the Yangtze River basin. The research results are as follows: (1) The 15 mm precipitation isohyet approximately divides the precipitation extremes (corresponding to the 95th percentile) of the stations in the middle and lower Yangtze reaches (higher) from those of the upper Yangtze reaches (lower). Also the starting time of the precipitation extremes in the middle and lower Yangtze reaches is earlier than of those in the upper Yangtze reaches. Precipitation extremes are concentrated mostly in June in the middle and lower Yangtze reaches, and July in the upper Yangtze reaches. (2) During the period 1960–2003, the first two decades had fewer precipitation extremes than the last two decades. There have been significant increasing trends and step changes in frequency of annual total precipitation extremes and precipitation extremes with a 1–5 day gap in the middle and lower Yangtze reaches. Precipitation extremes occur more frequently in shorter periods, separated by a few days. Precipitation extremes are also becoming more concentrated in the month with the highest frequency of extremes (June) in the middle and lower Yangtze reaches. In the upper Yangtze reaches, there is an upward tendency of extreme events in June. Increasing precipitation extremes in June for both the middle and lower, and the upper Yangtze reaches will increase the probability of flooding if the observed trends of the last 40 years continue into the future.

Frequency shift keyed narrowband interference rejection: optimal exponential weighting factor for the RLS algorithm

Previous work has shown that co-channel narrowband interference can limit the performance of direct sequence spread spectrum (DSSS) and high frequency (HF) systems. Narrowband interference (NBI) can be single tone, chirped or frequency shift keyed (FSK) in nature and numerous techniques for its removal have been proposed. Linear adaptive prediction filters based on autoregressive modelling have been suggested owing to their ability to perform in a non-stationary environment. In the FSK narrowband interference case, adaptive filters are susceptible to excess residual errors owing to instantaneous frequency step changes and the finite convergence time required for the filter to adapt to a new interference frequency. The signal degradation owing to this type of interference becomes greater in high SNR regimes and has been found to be a function of the frequency parameters of the FSK interference signal. This paper discusses the convergence and frequency tracking properties of the recursive least squares (RLS) adaptive lattice filter using a posteriori estimation errors in the presence of FSK narrowband interference. An optimal exponential weighting factor that balances convergence time and steady state error is derived for this case of NBI. Results are compared to those of the previously proposed fast converging minimum frequency error (FCMFE) RLS lattice filter.

A Study of Acoustic Resonance in a Low-Speed Multistage Compressor

Measurements are presented of a resonant acoustic phenomenon occurring in a low-speed multistage compressor. The results show that this phenomenon shares many characteristics with acoustic resonance as measured in high-speed compressors. These similarities include a rotating pressure field, several acoustic frequencies corresponding to different circumferential modes, step changes in frequency as the flow rate is increased, and acoustic frequencies that are independent of flow coefficient, shaft speed, and the axial length of the compression system. The paper includes measurements of the helical structure of the rotating pressure field and of the variation in amplitude of the acoustic signal over a stator exit plane.

Information from time-varying vibrotactile stimuli.

Experiments have been carried out to investigate the information transfer available via a single vibrator on the fingertip. In a first experiment, for stimuli with durations 80 to 320 ms, discrimination of a one-octave step change in frequency at the halfway point was investigated. Results were similar for three stimulus types--sinewave, monophasic pulse and tetraphasic pulse--suggesting temporal cues are more important than spectral cues in this task. In a second experiment, subjects were required to perceive changes in a sequence of stimulus elements. A presentation rate of 6.25 elements s-1 was found to give better results than a rate of 12.5 elements s-1. In the former case, the potential information transfer per element was estimated to be approximately 1.0 bits, corresponding to an information transfer rate of around 6 bits s-1. Implications for the design of a tactile aid to lipreading are discussed.

Transient Analysis of a Linear Induction Machine Using the d,q Pole-By-Pole Model

An evaluation of transient as well as steadystate behavior of linear induction machines (LIM's) is essential for high horsepower applications. The pole- by-pole model, a low order coupled circuit model derived for arbitrary excitation, provides a unique method of analyzing such behavior. In this paper a computer simulation algorithm suitable for either analog or digital computation is derived. To illustrate the flexibility of the approach practical application considerations such as acceleration response, behavior with an inverter supply and the effect of frequency step changes and SCR failure are modeled. Where feasible, computed waveforms are compared to test measurements from an actual prototype machine.

Logarithmic Ratemeter Response to a Step Change in Frequency

A formula is derived for the response time of a logarithmic ratemeter for a step change in periodic input frequency. It is shown that the formula is also valid for random inputs. A graph is included for the determination of 0%–90% rise and fall times as a function of the initial and final frequencies.

Optimum Strategies for Minimum Time Frequency Transitions in Phase-Locked Loops

Pontryagin's maximum principle is applied to minimize the time for a phase-locked loop to lock to a step change in frequency. In particular, a Type II phase-locked loop is considered in detail. Phase control and frequency control of the input signal are analyzed with the nonlinearity of the phase detector taken into consideration. It is shown that application of Pontryagin's maximum principle offers a decided advantage in shortening this time by proper control.

Important Feature of Time Modulation

TIME modulation, where information is conveyed by changing the time between similar parts of the modulated wave-train rather than by the frequency of occurrence, has a feature which is of great importance at the demodulation stage. This can best be demonstrated by considering a common method of de-modulating a frequency-modulated wave-train and comparing it with a method which is particularly simple to use where the wave-train is time-modulated. Fig. 1 shows a frequency modulated wave-train with a step change in frequency; a common and representative method of de-modulation is shown in Fig. 2. The box represents a device which produces a pulse of current (di) of short and controlled duration for each pulse of the input wave-train. The mean current flowing through R is, therefore, n × di where n is the pulse-recurrence frequency. The mean potential across R is n × di × R and is proportional to the mean value of the pulse frequency and hence represents the original modulation waveform. The value of c does not affect the mean value of current but has a marked effect on the transient performance of the circuit. If it is assumed that the current pulse has a negligible duration compared with the repetition frequency, then the waveform across the capacitor will be as shown in Fig. 1. The fall in potential between pulses is represented by the expression ν = V (e–t/cR) where ν is the capacitor potential at any time t after an input pulse at which time ν = V. The potential of the capacitor, therefore, falls exponentially with time with a time constant given by the product of c and R. In most situations it is desirable to recover the original modulating waveform undisturbed by components of the carrier, and the difficulty is to choose c.R so that the carrier frequency components (ripple) are at an acceptable level while maintaining a good transient performance of the de-modulated waveform. Often a simple resistance–capacitance filter will not give sufficiently good performance and elaborate low-pass filters must be used. The filters have, up to the present, always used inductive elements which are not as convenient to use as resistive–capacitance components (they must in general be specially wound) and cannot be made in ‘integrated circuit’ form. (This method of construction involves making the components and their interconnexions at the same time, often on a common surface.)