Split diaphragm capacitive sensor for 3-D ranging

The development of a novel capacitive sensor for 3D airborne ultrasonic ranging is described. The sensor provides, in addition to the conventional range measurement, angular information about the signal bearing. The sensor is derived from a widely used Polaroid-type electrostatic transducer that features 1% accuracy in ranging within a 15° detection cone. The transducer consists of a thin polymer foil metallized on one side, the diaphragm, that is stretched over a roughened metallic back plate where the dielectric consists of two layers, the foil and the layer of air. The metallized coating has been split in half by chemical etching in order to obtain a two element array for measuring the bearing angles associated with the range finders. The crosscorrelation technique has been applied to process the raw signal data in order to obtain the desired phase delays. As a result, a nearly linear agreement between the phase delays and the bearing angles in a 20° cone has been measured.