handbook of modern sensors

7.6 Ultrasonic Sensors 287
(A)
(B)
Fig. 7.38. Equivalent circuits for the (A) one- and (B) two-dimensional position-sensitive
detectors. (Courtesy of Hamamatsu Photonics K.K., Japan.)
optical range (as in a PSD) electromagnetic in the microwave range, acoustic, and so
forth. Transmission and reception of the ultrasonic energy is a basis for very popular
ultrasonic-range meters, and velocity detectors. Ultrasonic waves are mechanical
acoustic waves covering the frequency range well beyond the capabilities of human
ears (i.e., over 20 kHz). However, these frequencies may be quite perceptive by
smaller animals, like dogs, cats, rodents, and insects. Indeed, the ultrasonic detectors
are the biological ranging devices for bats and dolphins.
When the waves are incident on an object, part of their energy is reflected. In many
practical cases, the ultrasonic energy is reflected in a diffuse manner; that is, regardless
of the direction from which the energy comes, it is reflected almost uniformly within
a wide solid angle, which may approach 180 ◦ . If an object moves, the frequency of
the reflected waves will differ from the transmitted waves. This is called the Doppler
effect. 6
6 See Section 6.2 of Chapter 6 for the description of the Doppler effect for the microwaves. The
effect is fully applicable to the propagation of any energy having a wave nature, including
ultrasonic.