Proximity Sensor

Abstract

A convenient principle for acoustic distance measurement is detecting the time of flight of an ultrasound pulse from the generation to the detection of the echo reflected by an object [79]. Existing ultrasound sensors typically use a single piezoelectric or electrostatic transducer element both for generation as well as detection of ultrasound pulses. The time of flight of an ultrasound pulse is determined by the distance to the object and the sound velocity of the medium in which the sound waves propagate. The time of flight is proportional to the object distance with 2x; the sound velocity as proportionality factor. To measure small distances, i.e., short times of flight, the sound pulse should be ideal, i.e., the pulse has to have the shape of an ideal step function. However, the quality factor of the ultrasound transducer does not allow the generation of such ideal pulses. Therefore, using conventional ultrasound transducer the time-of-flight measurement principle is limited up to now to distances larger than approximately 4cm. Similarly, the time-of-flight method in combination with micromachined silicon ultrasound transducer cannot be applied to measure small distances because the quality factor of approximately 100 limits the generation of short, well defined ultrasound pulses. The distance measurement methods presented in this Chapter are based on continuous sound generation and overcome the distance restriction of the conventional pulse-echo method. The measurement methods presented in Section 7.1 and 7.2 are based on amplitude measurements of the reflected sound pressure at different frequencies.