|University of Illinois at Urbana-Champaign | Department of Electrical and Computer Engineering | Department of Bioengineering
Department of Statistics | Coordinated Science Laboratory | Beckman Institute | Food Science and Human Nutrition | College of Engineering
|Thursday, August 24th, 2017|
In 1976, Hughes and Thompson introduced a linear amplitude-steered array, which steered the maximum response of the array by amplitude weighting the output signals of the elements, thus eliminating the need for time-delays or phase-shift networks . The array was originally designed to operate with a narrowband signal. With a broadband signal, different frequencies are steered to different directions. Using this property, the linear version of the array can form an image using a single transmit pulse. Transmitting a linear FM chirp causes the main beam to be swept over a sector. An image is formed by time-frequency processing, where the elapsed time to the returned signal gives range information, and the frequency of the received signal provides information about the target's lateral position.
Currently, that amplitude-steering concept is being extended to a broadband two-dimensional array, which can be used for real-time three-dimensional imaging. In shifting the array's application from underwater communications to imaging, we must consider several issues: the design of the array, the field produced by the array, the axial and lateral resolution achieved, the signal processing methods to form an image, the display of three-dimensional data, and the effect of nonlinear propagation on the performance of the array.
The array has been designed for use in a diver held sonar camera. The field pattern of the linear array has been studied through simulation, and we have found that axial and lateral resolution both depend on the length of the array. Axial resolution improves with decreasing length, while lateral resolution improves with increasing length . A two-dimensional simulated image of seven point targets, formed using a constant-Q spectrogram , is shown. Higher frequencies are steered closer to broadside. Lower frequencies are steered to greater angles. A later study showed that time-frequency processing with the smoothed pseudo-Wigner distribution (SPWD) produced the best overall image results for a linear array imaging seven point targets .
The two-dimensional amplitude-steered array separates frequencies in the vertical direction by weighting the rows of the array. The weighting is implemented by assigning elements to one of four groups, rather than by scaling the signals from elements. In orderto avoid large grating lobes, a random assignment of elements isused within the rows of the array rather than a regular pattern . With the two-dimensional array, a linear FM chirp is transmitted from the center two columns of elements, giving a beam pattern that is narrow in the vertical direction and broad in the horizontal direction, at a particular frequency. Received signals from individual columns of elements are stored so that conventional phased array beamforming can be used to determine horizontal position of the target. Time-frequency processing is then used to determine the range and vertical position of the target.
The data are displayed as a set of three projection images. The first projection gives range versus horizontal position. The second projection shows vertical position versus horizontal position. And the third projection shows vertical position versus range. Simulations have been completed to show the results for a cylindrical target with bolt-like features .
 W. J. Hughes and W. Thompson, Jr., "Tilted directional response patterns formed by amplitude weighting and a single 90 degree phase shift," J. Acoust. Soc. Am., vol. 59, no. 5, pp. 1040-1045, May 1976.
 C. H. Frazier, W. J. Hughes, and W. D. O'Brien, Jr., "Analysis of resolution for an amplitude steered array," J. Acoust. Soc. Am., vol. 107, no. 5, Pt. 1, pp. 2430-2436, May 2000.
 J. C. Brown, "Calculation of a constant-Q spectral transform," J. Acoust. Soc. Am., vol. 89, no. 1, pp. 425-434, Jan. 1991.
 C. H. Frazier and W. D. O'Brien, Jr., "Image formation with the amplitude-steered array using time-frequency distributions," 25th International Acoustical Imaging Symposium, Bristol, UK, March 19-22, 2000.
[5a] C. H. Frazier, W. J. Hughes and W. D. O'Brien, Jr., A Two-Dimensional Amplitude-Steered Array for Real-time Volumetric Imaging, Part I: Theory. Journal of the Acoustical Society of America, submitted.
[5b] C. H. Frazier, W. J. Hughes and W. D. O'Brien, Jr. A Two-Dimensional Amplitude- Steered Array for Real-time Volumetric Imaging, Part II: Experiments. Journal of the Acoustical Society of America, submitted.
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