PURPOSE: To reduce an erroneous detection due to a wave, etc., by detecting collapse of a balance of a positive pressure and a negative pressure generated upon passage of a voyage ship hull. ;CONSTITUTION: When a waveshape of a wave is averaged at a center of a horizontal line, it is fluctuated equivalently at positive and negative sides. Accordingly, even if a large wave is generated and outputs are generated from positive pressure and negative pressure comparators 2, 3, a balance of positive pressure/negative pressure of this hydraulic signal is not collapsed. When a voyage ship hull is passed, a balance of positive and negative of the hydraulic signal detected by a hydraulic sensor 1 is collapsed, the output from the comparator 3 is increased, and a decision signal of passage of the hull is output from a discriminating comparator 5. Thus, sea noises of the hull and the wave, etc., can be distinctly detected, and an erroneous detection due to the wave, etc., can be reduced.;COPYRIGHT: (C)1993,JPO&Japio
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