Harmonic

About: Harmonic is a research topic. Over the lifetime, 44833 publications have been published within this topic receiving 495922 citations. The topic is also known as: overtone & partial.

The response of rectangular piezoelectric sensors to Rayleigh and Lamb ultrasonic waves

Abstract: A fundamental understanding of the response of piezoelectric transducer patches to ultrasonic waves is of increasing interest to the field of structural health monitoring While analytical solutions exist on the interaction of a piezoelectric actuator with the generated Lamb waves, the behavior of a piezoelectric sensor has only been examined for the limited case of a piezo-actuated Lamb wave in a pitch-catch configuration This paper focuses on the fundamental response of surface-bonded piezoelectric sensors to ultrasonic waves The response to both Rayleigh waves and Lamb waves is examined, starting with harmonic excitation fields and moving to broadband and narrowband excitation fields General oblique incidence of the wave on rectangular sensors is treated first; parallel incidence is then derived as a particular case The solutions are developed analytically for the harmonic and the narrowband excitations, and semianalytically for the broadband excitation The results obtained can be used to design u

Frictionless atom cooling in harmonic traps: A time-optimal approach

Abstract: In this article we formulate frictionless atom cooling in harmonic traps as a time-optimal control problem, permitting imaginary values of the trap frequency for transient time intervals during which the trap becomes an expulsive parabolic potential We show that the minimum time solution has a ``bang-bang'' form, where the frequency jumps suddenly at certain instants and then remains constant, and calculate estimates of the minimum cooling time for various numbers of such jumps A numerical optimization method based on pseudospectral approximations is used to obtain suboptimal realistic solutions without discontinuities, which may be implemented experimentally

A Nonlinear Acoustic Technique for Crack Detection in Metallic Structures

Abstract: A crack detection technique based on nonlinear acoustics is investigated in this study. Acoustic waves at a chosen frequency are generated using an actuating lead zirconate titanate (PZT) transducer, and they travel through the target structure before being received by a sensing PZT wafer. Unlike an undamaged medium, a cracked medium exhibits high acoustic nonlinearity which is manifested as harmonics in the power spectrum of the received signal. Experimental results also indicate that the harmonic components increase nonlinearly in magnitude with increasing amplitude of the input signal. The proposed technique identifies the presence of cracks by looking at the two aforementioned features: harmonics and their nonlinear relationship to the input amplitude. The effectiveness of the technique has been tested on aluminum and steel specimens. The behavior of these nonlinear features as crack propagates in the steel beam has also been studied.

Diagnosis of Induction Motor Faults in the Fractional Fourier Domain

Abstract: Motor current signature analysis (MCSA) is a well-established method for the diagnosis of induction motor faults. It is based on the analysis of the spectral content of a motor current, which is sampled while a motor runs in steady state, to detect the harmonic components that characterize each type of fault. The Fourier transform (FT) plays a prominent role as a tool for identifying these spectral components. Recently, MCSA has also been applied during the transient regime (TMCSA) using the whole transient speed range to create a unique stamp of each harmonic as it evolves in the time-frequency plane. This method greatly enhances the reliability of the diagnostic process compared with the traditional method, which relies on spectral analysis at a single speed. However, the FT cannot be used in this case because the fault harmonics are not stationary signals. This paper proposes the use of the fractional FT (FrFT) instead of the FT to perform TMCSA. This paper also proposes the optimization of the FrFT to generate a spectrum where the frequency-varying fault harmonics appear as single spectral lines and, therefore, facilitate the diagnostic process. A discrete wavelet transform (DWT) is used as a conditioning tool to filter the motor current prior to its processing by the FrFT. Experimental results that are obtained with a 1.1-kW three-phase squirrel-cage induction motor with broken bars are presented to validate the proposed method.