Feature extraction techniques for recognizing solid objects with an ultrasonic range sensor

Ultrasonic range sensing has attracted attention in the robotics community because of it simplicity in construction and low cost. However, determining range direction rather than just range magnitude is made difficult by the expanding signal beam of the sensor. This direction ambiguity can be reduced to some extent by increasing the operating frequency or diameter of the sensor, but some ambiguity will still remain. A technique is described for obtaining the true direction to a planar surface using three sensors or three positions of one sensor. A direct solution of the vector equation is discussed to illustrate the solution complexity in direct form. A simplifying transformation applied to the direct form and a further simplifying sensor configuration are described which greatly reduces the solution complexity. An alternate formulation of the problem is described from which a geometrical insight can be obtained. This alternate solution also lends itself to the generation of tangential constraint surfaces for bounding curved object surfaces. These local feature extraction techniques are extended to perform surface tracking for the extraction of global surface features. The connectivity of local features provides additional information. From this information classification of solid objects is possible.