Recognizing Retro-Reflectors with an Obliquely-Oriented Multi-Point Sonar and Acoustic Flow

This paper applies acoustic principles and a novel scanning method motivated by acoustic flow to recognize retro-reflectors directly from sonar echoes while performing drive-by scanning. Right-angle corners and cylinders form specular retro-reflectors that produce strong echoes whose features can be easily identified. A multi-point sonar produces a point process whose density encodes the echo amplitude and allows strong echoes to be isolated. In drive-by scanning an obliquely-oriented sonar beam passes over a retro-reflector which then exhibits a sequence of strong echoes having a pattern predicted by a forward model. Drive-by scans of a simple retro-reflector, a complex hallway environment and distant objects illustrate the method. An algorithm recognizes and localizes the retro-reflector by performing a two-dimensional search to find coordinates producing range readings matching the data in a weighted-least-squared error sense. A conventional Polaroid 6500 ranging module produced 5,000 sonar points in a drive-by scan of a hallway, which the algorithm converted into six corner locations. Interfering reflectors were detected by large deviations from the predicted template. Echoes from occluded retroreflectors up to 7 m range were analyzed. Recognizing objects directly from echoes extends sonar sensing from data acquisition to landmark identification for robot navigation. KEY WORDS—acoustic flow, drive-by sensing, landmark recognition, object recognition, pattern classification, Polaroid transducer, robot navigation, sonar