The use of ultrasonics for gauging and proximity sensing in air

The use of ultrasonics is investigated for gauging and proximity sensing of distances of about 1 m and less in air. Because strong attenuation due to absorption in air limits usable frequencies to less than about 1 MHz, ultrasonic methods are inherently less accurate than optical methods. However, in tests reported here with a commercially available ultrasonic system, it is shown that perpendicular distances up to 250 mm can be measured with an accuracy of ±0.1 mm, provided there is adequate compensation for changes in the speed of sound due to temperature variation. This accuracy in distance measurement, coupled with the fact that it is possible to have a minimum beam diameter, or spot size, as small as 1 mm, means that ultrasonic methods are sufficiently accurate for many applications in manufacturing and robot control. In addition, ultrasonic systems are generally cheaper, lighter, more compact, and more durable than equivalent optical systems. Results are reported here on the effect of air currents, surface temperature, and background noise. Different aspects of ultrasonic gauging are discussed, including absorption in air and beam forming and focusing. Factors affecting the precision of ultrasonic gauging are emphasized.