Papers by Keyword: Phase Spectrum

Abstract: The phase spectrum time delay estimation (TDE) method is being used widely in trajectory tracking and target location, which due to its superiority in computation load and accuracy. However, the precision of TDE had declined severely as the result of noise. This paper plans to analyze the factor of noise, which has effects on the performance of phase spectrum TDE algorithm based on the difference principle. Theoretical analysis and computer simulation show that the fluctuation of phase caused by noise leads to poor performance. Furthermore, the extent of the influence relates to the time delay and signal frequency.

Abstract: Analyzed the importance of selecting reasonably ground motion input in seismic fragility analysis of the bridge. Based on the common methods of synthesis artificial seismic wave which used trigonometric series and according to the characters of Fourier series, extracted phase spectrum from practical seismic waves ,through modulating the Fourier amplitude spectrum, obtained the artificial seismic wave which fitted compatible with the design response spectrum. Similar to nature small earthquake records, it has the character of non-stationary on time-domain and frequency-domain. Synthesize a series of artificial seismic waves in this method, can improve the accuracy and pertinence of the bridge seismic fragility.

Abstract: Ultrasonic phase spectrum of reflection coefficient has been used to nondestructively characterize the density, porosity, and microcracks of plasma sprayed Cr2O3 coatings irradiated by high-intensity pulsed ion beam (HIPIB). The ultrasonic measurement was investigated using immersion focusing pulse echo method with a 25 MHz transducer and the phase spectrum of reflection coefficient has been experimentally obtained. The relationship among velocity, density, attenuation coefficient and the phase spectrum was analyzed based on an acoustic transmission model in a multi-layered structure. The velocity relates to the harmonic frequency of the phase spectrum, and the density and attenuation coefficient play roles on the amplitude of the maximum and the shape of the spectrum. The effects of the three parameters (velocity, density and attenuation coefficient) on the amplitude of the maximum of the phase are similar. Some critical value exists which change the relationship between the individual parameter and the maximum of the phase spectrum. When one of the three parameters exceeds the corresponded critical value, the amplitude of the maximum decreases in the form of exp( ) n y = A Bx . For the as-sprayed and irradiated coatings with the thickness of 50 0m, the velocity calculated from the phase spectrum is 2522 and 2772 m/s, harmonic frequency corresponding to 12.61 and 13.86 MHz, respectively. These ultrasonic determinations demonstrated that the coating becomes more densified after irradiation by HIPIB, in agreement with SEM observations. Ultrasonic phase spectrum itself is a good indicator to reveal differences between density and microstructures of coating prepared with various conditions as well as to follow their evolution after HIPIB irradiation.