Fundamental frequency

About: Fundamental frequency is a research topic. Over the lifetime, 8941 publications have been published within this topic receiving 131583 citations.

Pulse-inversion-based fundamental imaging for contrast detection

Abstract: Pulse-inversion-based fundamental imaging was experimentally investigated for the enhancement of contrast detection. The pulse-inversion technique involves two firings with inverted waveforms. When the returning echoes from the two firings are summed, the residue signal is limited to even-order harmonics for tissue. However, when the returning echoes are from microbubbles, the fundamental signal is not completely cancelled because the reaction of the bubbles under compression is different from that under rarefaction. Thus, with the application of pulse-inversion technique, the fundamental signal can be used to enhance the contrast-to-tissue ratio. In this paper, B-mode, pulse-inversion-based fundamental images were constructed with various transmit waveforms. Motion artifacts also were studied. The results indicate that the contrast-to-tissue ratio was significantly enhanced compared to that obtained using either conventional, fundamental imaging or second-harmonic imaging. Longer transmit pulses resulted in a better signal-to-noise ratio, but did not noticeably affect the nonlinear response of the bubbles. In addition, the optimal ratio of the magnitude of the positive pulse to that of the negative pulse was unity, in terms of avoiding the uncancelled, third-order response in the fundamental frequency range. It also was found that the pulse-inversion fundamental technique is highly sensitive to tissue motion because the fundamental tissue signal is not cancelled when motion is present.

Vibration prediction of thin-walled composite I-beams using scaled models

Abstract: Scaled models of large and expensive structures facilitate in understanding the physical behavior of the large structure during operation but on a smaller scale in both size and cost. These reduced-sized models also expedite in tuning designs and material properties, but also could be used for certification of the full-scale structure (referred to as the prototype). Within this study, the applicability of structural similitude theory in design of partially similar composite structures is demonstrated. Particular emphasis is placed on the design of scaled-down composite I-beams that can predict the fundamental frequency of their corresponding prototype. Composite I-beams are frequently used in the aerospace industry and are referred to as the back bone of large wind turbine blades. In this study, the governing equations of motion for free vibration of a shear deformable composite I-beam are analyzed using similarity transformation to derive scaling laws. Derived scaling laws are used as design criteria to develop scaled-down models. Both complete and partial similarity is discussed. A systematic approach is proposed to design partially similar scaled-down models with totally different layup from those of the full-scale I-beam. Based on the results, the designed scaled-down I-beams using the proposed technique show very good accuracy in predicting the fundamental frequency of their prototype.

Current Differential Protection of Transmission Line Using the Moving Window Averaging Technique

Abstract: We propose a new approach to current differential protection of transmission lines. In this approach, we transform the instantaneous line current(s) by using a moving window averaging technique. If the time span of moving window is equal to one-cycle time, then the steady-state value of the transformed current is zero for a periodic signal which is composed of fundamental and harmonic frequencies. Signal distortions (e.g., a fault) cause the transformed currents to deviate from the nominal zero value. This permits the development of a sensitive, secure, fast, and yet simple current differential protection scheme. The scheme can be applied in toto to series-compensated transmission lines. Results on a four-machine ten-bus system and comparative evaluation with state-of-the-art methods brings out promise of the proposed method.

Discrete-Time Domain Modeling and Design for AC Machine Current Regulation

Abstract: Complex vector current regulators provide robust performance at high fundamental frequency operation, but there are difficulties in increasing the current regulation bandwidth and in operating at very high fundamental frequencies In this paper, a discrete-time domain model of general AC machines is proposed Based on this model, a discrete-time domain current regulator design methodology is proposed This method is a generalization of the complex vector current regulator's pole- zero cancellation approach and it furthermore explicitly incorporates and compensates the control delay common in discrete-time implementations