The theory of both the monofrequency and multifrequency modes of ultrasonic holographic imaging as suitable for the requirements of object recognition in robot assembly tasks is described. The theory was implemented experimentally. Results show that the monofrequency approach provides fairly good lateral but insufficient depth resolution. By contrast, with multifrequency holography, depth resolutions of better than 3 mm and lateral resolutions of about 10 mm are attainable. New ultrasonic transducers, combined with a special signal preprocessing procedure, are a prerequisite for resolutions of this order. Typical images as obtained from several test scenes are presented. Suitable applications and possible future research are briefly outlined.
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