| A real-time acoustic imaging system is developed as a prototype electronic travel aid (ETA) device. The design is implemented on a field programmable gate array (FPGA). A 6 channel transmit and 4 channel receive digital beamforming algorithm with dynamic focusing is accommodated in a FPGA. The developed system consists of a FPGA, pulser and receiver circuitry and separate transmitter and receiver arrays, which are constructed by using commercially available transducers. The transducer elements have a physical dimension of 1.9 wavelengths and a half-power beamwidth of 43º at 40.8 kHz center frequency. The transmitter array is formed by aligning the transducers with minimum spacing between the elements, which is 2 wavelengths. Obviously, more than one wavelength inter-element spacing leads to the occurrence of grating lobes in the array response and decreases the Field of View (FOV) below the half-power beamwidth of transducers. To extend the FOV and eliminate the grating lobe, the receiver array is formed with 3 wavelength inter-element spacing. The non-identical element spacing makes the grating lobes of transmitter and receiver array to appear at different places. The described placement strategy and the functionality of the system is tested with several experiments. The results of these experiments prove the grating lobe suppression capability of the applied placement strategy. |