Sonar or ultrasonics

Sonar or ultrasonics

Sonar refers to any system for using sound to measure range. Sonars for
different applications operate at different frequencies; for example, a sonar
for underwater vehicles would use a frequency appropriate for traveling
through water, while a ground vehicle would use a frequency more suited
for air. Ground vehicles commonly use sonars with an ultrasonic frequency,
just at the edge of human hearing. As a result the terms “sonar” and “ultrasonics”
are used interchangeably when discussing extracting range from
acoustic energy.
Ultrasonics is possibly the most common sensor on commercial robots operating
indoors and on research robots. They are active sensors which emit a
sound and measure the TIME OF FLIGHT time it takes for the sound to bounce back. The time
of flight (time from emission to bounce back) along with the speed of sound
in that environment (remember, even air changes density with altitude) is
sufficient to compute the range of the object.
Ultrasonics is common for several reasons. Its evolution paralleled the rise
of the Reactive Paradigm. In the mid-1980’s, Hans Moravec did impressive
robot navigation with a ring of sonars. The ring configuration gave a 360
coverage as a polar plot. This ring was developed by one of the first mobile
robot manufacturers, Denning Robotics, and since then sonar rings are often
referred to as “Denning rings,” regardless of manufacturer. Besides providing
direct range measurements, the transducers were cheap, fast, and had
terrific coverage. In the early 1980’s, the Polaroid Land Corporation had developed
small, inexpensive sonars for use as camera range finders. A bigger
version, the Polaroid Lab Grade ultrasonic transducer, costs on the order of
$30 USD and can measure ranges from 1 to 25 feet with inch resolution over
a field of view of 30. Furthermore, the measurement time was on the order
of seconds versus hours for computer vision. Ultrasonics became the sensor
of choice for behavior-based robots.

to 3 meter range. As a result many algorithms only treat the lobe as being
between 8 and 15 wide depending on how reliable the range readings are
in a particular environment. Ch. 11 will go over this in more detail.
The strength of the main lobe in the environment determines the maximum
range that the sonar can extract reliability. In ideal indoor venues, a
sonar might return ranges of up to 25 feet, while in the outdoors, the same
sonarmight go no further than 8 feet with any repeatability. So while the upper
limit of the range reading depends on the sensor and the environment,
the lower limit does not. Ultrasonic transducers have a “dead time” immediately
following emission while the membrane vibration decays. The decay
time translates into an inability to sense objects within 11 inches because
measurements made during this period are unreliable because the membrane
may not have stopped ringing.
Regardless of the maximum allowed range return (i.e., does the program
ignore any reading over 3 meters?) and the width of the lobe, most computer
programs divide the area covered by a sonar into the three regions shown in
Fig. 6.6. Region I is the region associated with the range reading. It is an
arc, because the object that returned the sound could be anywhere in the
beam. The arc has a width, because there are some resolution and measurement
errors; the width of Region I is the tolerance. Region II is the area that
is empty. If that area was not empty, the range reading would have been
shorter. Region III is the area that is theoretically covered by the sonar beam,
but is unknown whether it is occupied or empty because it is in the shadow
of whatever was in Region I. Region IV is outside of the beam and not of
interest.
Although they are inexpensive, fast, and have a large operating range, ultrasonic
sensors have many shortcomings and limitations which a designer
should be aware of. Ultrasonic sensors rely on reflection, and so are susceptible
to specular reflection. Specular SPECULAR REFLECTION reflection is when the wave form hits a
surface at an acute angle and the wave bounces away from the transducer.
Ideally all objects would have a flat surface perpendicular to the transducer,
but of course, this rarely happens. To make matters worse, the reflected signal
may bounce off of a second object, and so on, until by coincidence return
some energy back to the transducer. In that case, the time of flight will not
correspond to the true relative range.
Even with severely acute angles, the surface is usually rough enough to
send some amount of sound energy back. An exception to this is glass, which
is very common in hospitals and offices where mail robots operate, but induces
serious specular reflection. Fortunately this energy is often sufficiently

strong to pass the thresholding in the transducer circuit. However, a new
problem, foreshortening, may occur. Recall FORESHORTENING that a sonar has a 30 field of view.
This means that sound is being broadcast in a 30 wide cone. If the surface is
not perpendicular to the transducer, one side of the cone will reach the object
first and return a range first. Most software assumes the reading is along the
axis of the sound wave. If it uses the reading (which is really the reading for
15), the robot will respond to erroneous data. There is no solution to this
problem.
SPECULAR REFLECTION Specular reflection is not only by itself a significant source of erroneous readings;
it can introduce a new type of error in rings of sonars. Consider a ring
of multiple sonars. Suppose the sonars fire (emit a sound) at about the same
time. Even though they are each covering a different region around the robot,
some specularly reflected sound from a sonar might wind up getting
received by a completely different sonar. The receiving sonar is unable to tell
the difference between sound generated by itself or by its peers. This source
CROSS-TALK of wrong reading is called cross-talk, because the sound waves are getting
crossed. Most robot systems stagger the firing of the sonars in a fixed pattern
of four sonars, one from each quadrant of the ring) at a time. This helps some
with cross-talk, but is not a complete or reliable solution. If the sonar sound
frequency and firing rate can be changed (which is generally not the case),
then sophisticated aliasing techniques can be applied. These techniques are
outside the scope of this book.
One researcher,Monet Soldo, told a story of when she developed a reactive
mobile robot for IBM’s T.J.Watson Laboratories during the late 1980’s. The
robot used sonar as its primary sensors, and she had written behaviors to
guide the robot through doors, rooms, and hall successfully at reasonable
speeds. The day came for the big demonstration, which was to be held not
in the hallways of the laboratory but in the front reception area. The robot
navigated successfully out of the lab, down the halls, and then went berserk
when it got to the atrium. She rebooted, and tried again, but with the same
result. After days of trying to debug the code, she realized it wasn’t a code
problem, it was an environment problem: most of the atrium reception area
consisted of glass partitions. The specular reflection and cross-talk caused
the robot to hallucinate, although in different ways each time.
I had a similar problem when my robot started navigating in an office environment.
In that environment, the robot was expected to navigate among
office cubes, or work areas delimited by partitions. The partitions were covered
with cloth to dampen the sound from the workers. Unfortunately, the
cloth also absorbed the sound from the sonars! These stories emphasize the