Study And Implementation Of Focused Beam-forming Technology For Imaging Sonar

Beam-forming algorithm is one of the key for the signal processing system of imaging sonar. Its effect is to strengthen the signal in a certain direction, to suppress interference in other directions. In the near-field environment, sound wave which is approximately spherical wave form spreads to the array. If the beam-former is designed by far-field plane wave mode in this situation, beam performance will seriously deteriorate. According to the geometry of spherical wave, the focused beam-forming based on the semicircle array has been deeply studied in this paper.Firstly, the development and the research of imaging sonar and beam-forming algorithm has been outlined in this paper. The active sonar signal has been analyzed, and the beam-forming scheme combined with bandpass sampling and complex baseband signal has been proposed to reduce the calculation of the algorithm. the principle of focused beam-forming based on the semicircle array of the system has been detailed analyzed and a focused compensation scheme in different distances has been determined. Phase compensation coefficient is a two-dimensional function which changes with the distance and direction. The beam pattern of focused beam-forming has been obtained by Matlab program simulation. Compared with the beam pattern of conventional methods, the beam width is shaped 0.25 degree and the sidelobe is decreased 6.55 d B.Secondly, a real time processing structure of digital focusing beam-former is designed based on FPGA in this paper. Each software module is designed under Quartus II development environment by VHDL language. The beam-forming module has 73 units. The processing speed is increased and 73 beams are generated in real time through ping-pong option and parallel structure. The focused module uses 32 sets of weight coefficients to achieve the improvement of resolution in the near-field environment. The reliability and real time of the system has been verified by the timing simulation and the chip test.Finally, the measurement experiment of beam pattern and the static imaging experiment have been done in the pool. The experimental results testify that the focusing beamformer designed in this paper makes a great improvement in the performance of the beam-pattern and in the near-field resolution of the imaging sonar.