time-of-flight ends? Perhaps we should use the start of the
reflected waveform. Or, the peak.
Issues with Ultrasonic
We’ve already mentioned the ambiguity in
determining TOF, but this is a relatively minor issue, since
it generally results in an uncertainty of less than an inch
which — for many hobbyist robot applications — is
Other issues, however, are more serious. These include
clutter, a return signal that falls off rapidly with distance,
and transmitter ringdown.
Clutter is a potential problem in any radar application,
but particularly a problem with ultrasonic applications
because of the wide field of view (FOV) of the ultrasonic
Figure 3 shows a directivity graph that is similar to
the charts for most ultrasonic transmitters. The figure
shows that the transmitter still has about 1/10 its power
at ± 30° from boresight.
Such a large FOV means that the device can “see” not
only pertinent objects in front of it but also irrelevant
objects far from the robot.
In fact, the ultrasonic response shown back in Figure
2 is an example of this. In that particular application, the
transmitter/receiver was mounted about four feet above
It was pointed directly at a target about five feet away
(the second of the two largest responses).
A fluorescent light fixture hanging from the ceiling a
couple of feet above the transmitter/receiver was actually
causing the first large response. The transmitter/receiver
responded to this light fixture because it was within its
field of view.
Such “false alarms” reduce the reliability of the
ultrasonic sensor’s results and often lead to the conclusion
that ultrasonic sensors make poor ranging devices.
Another problem plaguing ultrasonic transmitters is
the fact that they cannot immediately stop their
movement when the voltage stops changing.
This is related to their resonant nature. Just as
a child swinging cannot immediately stop but must
instead let the swing slowly lose energy, the piezo
transmitter must likewise slowly stop its movement.
Figure 4 shows an ultrasonic transducer
subjected to a 0 to 30V square wave and then —
mid-screen — switched to a high-impedance
connection. The transmitter continues to oscillate,
generating voltage as it slowly “winds down.”
The reason this is important is that ultrasonic
receivers often have a difficult time distinguishing
between the signal produced by ringdown and an
actual return from a very close object.
As a result, most ultrasonic receivers establish
a “dead time” during which the receiver does not
attempt to identify targets. Thus, most ultrasonic
sensor systems cannot identify objects that are very
56 SERVO 02.2018
Figure 3. Directivity graph for a typical
Figure 4. Transmitter subjected to 16 periods of
square wave showing ringdown.