Limitations of Ultrasonic
If you’ve used ultrasonic ranging modules in the past —
particularly the “cheapie” versions — you’re probably aware
that their performance is somewhat hit-or-miss. They can
produce both “false alarms” — indications of an object in
their path when none exists — as well as “missed
detections.” Some of this may have to do with the design
of the unit. However, at least some of it is inherent in how
ultrasonic transducers work.
False Alarms Caused by a
Because most ultrasonic ranging transducers have just a
small diaphragm for moving the air, the acoustic wave front
produced winds up traveling not just to the diaphragm but
to the sides as well (Figure 1). Typical transducers will
exhibit as much as half their power at angles of ± 30°.
As a result, the transducer will “illuminate” objects that
are not in the path of interest. These “objects” can include
such things as floor irregularities. Also, acoustic energy can
be reflected by one object to another — a phenomenon
referred to as multipath interference. The resulting time-of-flight will be meaningless or misleading.
Significant Fall-off in
Another challenge in ultrasonic ranging applications is
the rapid drop in amplitude as a function of range. The
reason for this can be seen in Figure 2. The energy scatters
from the transducer in both the horizontal and vertical axes.
Thus, it falls off at a 1/r2 rate, where r is the distance to the
object. Reflecting from the object, the echo also scatters
and falls off at a 1/r2 rate. The overall signal return
amplitude therefore changes as a function of 1/r4. As a
result, the dynamic of the signal over the working range of
the ultrasonic system is often huge.
Lack of Bearing Info
Simple ultrasonic ranging gives range information but
not bearing information. As a result, the robot or other user
of the information knows how far away the object is but
has no idea whether the object is to the left or to the right.
In many collision avoidance applications, such additional
information is needed to determine the proper diversionary
action to take.
Improving the Ultrasonic
How can we improve on the simple ultrasonic
transducer’s performance? We can take a hint from the
way in which RF (including microwave) radar makes
possible detection at very long distances: by using an
The Parabolic Antenna
Most commercial RF radar sets (for example, the
ones utilized at airports) use a parabolic reflector to focus
the energy from the transmitter to the object (usually
referred to as “the target” in radar applications), and use
the same or a similar parabolic reflector to focus
incoming energy from the target back onto a receiver.
Why a Parabola?
The parabola has the unique feature that any path
12 SERVO 05/06.2018
Figure 1. The wide field-of-view of a typical
ultrasonic transducer means that it will “see”
objects that are not in the path of the robot.
Figure 2. Both the originating energy and the
echo fall off at 1/r2 rates.