Anders Grennberg

Estimation of subsample time delay differences in narrowband ultrasonic echoes using the Hilbert transform correlation

In many areas the time delay of arrival (TDOA) is desired. In the case of narrowband signals we propose a fast and simple method to estimate small time delays. This method is shown to have the same or better accuracy as the cross correlation methods for small delays in the order of fractions of the sample interval. It is based on using the Hilbert transform in correlation between two signals and consists of only one scalar product, which makes it fast. It may also be used in applications with narrowband signals where the measurements are repeatable, such as ultrasonic imaging and nondestructive testing. In ultrasonic applications, due to fluctuations in the insonified media, a small random time shift may be present causing the signals to be misaligned in time. Averaging signals under these conditions will result in a distortion of the signal shape. We propose an averaging method to avoid this and to accomplish a higher SNR without the distortion. Simulations and experiments from ultrasonic applications are presented.<<ETX>>

ESTIMATION OF THE SPATIAL IMPULSE RESPONSE OF AN ULTRASONIC TRANSDUCER USING A TOMOGRAPHIC APPROACH

The impulse response method is often used to study the ultrasonic field radiated by planar or gently curved transducers. These investigations can be extended to include the reflected echoes from point reflectors, which can be used in synthesizing echoes from reflectors of known shapes. The single point echo can be measured directly but there are however certain problems associated with this. This paper proposes an alternative method to estimate the echo from a pointlike reflector by measuring the echoes from sliding half‐planes. Using a tomographic approach, the single point echo is estimated from these measurements. To perform the tomographic inversion, a truncated weighted singular value decomposition (SVD) is used in order to catch essential features of the ultrasonic echoes, where the weight functions are chosen especially to suit this application. Numerical solutions are given for the chosen weight functions and an error analysis is carried out.

Experimental determination of the single point echo of an ultrasonic transducer using a tomographic approach

The impulse response method is often used to study the ultrasonic field radiated by planar or gently curved transducers. This, however, requires one transmitting and one receiving transducer which has to be placed with great accuracy in the ultrasonic field. Here we propose a measuring method with only one transducer which acts both as transmitter and receiver. Echoes from sliding halfplanes are registered and by using tomographic methods, the echo from a single point in the same plane as the halfplanes can then be calculated. Beside the ultrasonic field, these calculations will yield us the transducers characteristics. These can be used to extract further information from an echo of an arbitrary surface, such as the gradient of the surface.

Ultrasonic measurements of particle concentration in a multiphase flow

Non-invasive measurements of multiphase flows have several important applications in industry. In this paper we present a method that uses pulsed ultrasound and two small receivers to determine the mass concentration of iron ore particles in water. The proposed method is based on the assumption that when ultrasound is transmitted through a scattering medium, the shape of the energy lobe changes. In this paper we use two receivers to monitor how the lobe changes. We show with experiments how the proposed method can be used to determine particle mass fractions from 3 percent and up, with an accuracy of ±1 percent of the mass fraction. In the experiments we used a 3 MHz transmitter and two receivers, one along the acoustical axis and the other 6 mm off-axis, to measure the mass fraction of a polydisperse suspension of iron ore powder in water.

Feature extraction from multichannel EEG transients for clustering purposes

This paper treats the problem of clustering multichannel EEG transients occurring in epilepsy, so called spikes. The authors present a new method for feature extraction which is an expansion along an orthonormal set of matrices, obtained from truncated SVD and Gram-Schmidt orthogonalization. The method is compared to a method used earlier where each channel was Hermite function expanded. The result is that using the same expansion order the new method represents a set of spikes with higher fidelity than the Hermite function method. The output from the feature extraction is a vector for each spike in a set. The vectors from a set is clustered with the standard NM algorithm. Two sets of spikes have been investigated, both being clustered in appearance.