et’s discuss ultrasonic methods for object
detection by going over some
fundamentals. We’ll start by talking
about the general situation of detecting objects.
Humans (and some machines) use vision and to a
lesser extent hearing to avoid collisions or otherwise
detect objects in their path. In that case, the system is
using passive light energy or passive acoustic energy to
Most machines performing obstacle detection rely on
active sensing. That is, they project energy of some type —
whether light, RF, or acoustic — in the direction of the
possible target and measure the reflected energy returned
to a receiver.
When the energy is in the RF region of the
electromagnetic spectrum, this type of system is called
RADAR (RAdio Detection And Ranging). The term radar is
also sometimes used when the energy is in the light part
of the electromagnetic spectrum; in this case, it’s
sometimes called LIDAR (LIght Detection And Ranging) or
LADAR (LAser Detection And Ranging).
When the energy is in the ultrasonic region of the
acoustic spectrum, the term “ultrasonic radar” is used.
That’s what we’ll be talking about here.
Just What is Ultrasonic
Ultrasound is acoustic energy propagated at
frequencies above human hearing; generally 20 kHz to
several GHz. Let’s break that down a little more.
Acoustic propagation refers to the transmission of
sound through changes in the pressure levels as the
energy travels through some type of medium such as
air or water (we’ll concentrate on air).
It travels through the air at the speed of sound,
which is on the order of 343 meters per second at
standard temperature and pressure ( 20°C at sea level).
To illustrate what is meant by frequency, we can
use the example of a piano. You know that the keys
at the extreme left of an acoustic piano are referred
to as the “low notes.” Their very deep bass tone is the
result of the metal strings that the associated
hammers strike, causing them to oscillate at “low
This means the string vibrates from peak to peak
a very small number of times per second. The lowest
of a concert piano’s notes are vibrations that oscillate
only about 30 times per second; these are referred to
as 30 Hz.
At the extreme right side of the piano keyboard
are the “high notes.” These notes have strings that —
People with normal hearing can detect the entire
range of piano notes, and some folks can even detect
frequencies as high as 20,000 Hz.
Beyond this frequency of 20,000 Hz, humans do not
hear the sound (many animals, however, have hearing to
frequencies far beyond this limit). Because the sound
cannot be heard by humans, it’s possible to propagate
considerable energy at these frequencies without annoying
people or otherwise interfering with their normal activities.
So, how do we create ultrasonic waves? Typically, this
is done with a transducer, which takes electrical energy
modulated at the appropriate frequency and converts this
to mechanical energy that moves a surface, creating the
A common type of hobbyist ultrasonic transmitter is a
piezoelectric ceramic transducer, typically operating at 40
kHz. Such a piezoelectric device has the unusual property
that it expands or contracts as the voltage across it
increases or decreases.
Therefore, by driving it with an AC voltage, the
device’s surface in a particular axis moves with respect to
the opposite surface.
Such a device can be made to change its dimensions
at 40 kHz and above. In addition, the device can be
manufactured so that it is “resonant” at a specific
Figure 1. Swinging requires the use of resonance.
54 SERVO 02.2018