Page 229 - Introduction to AI Robotics

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6 Common Sensing Techniques for Reactive 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 maxi-
mum 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
sonar might go no further than 8 feet with any repeatability. So while the up-
per limit of the range reading depends on the sensor and the environment,
the lower limit does not. Ultrasonic transducers have a “dead time” imme-
diately 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 mem-
brane 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 measure-
ment 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, ul-
trasonic sensors have many shortcomings and limitations which a designer
should be aware of. Ultrasonic sensors rely on reflection, and so are suscep-
SPECULAR REFLECTION tible to specular reflection. Specular 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 sig-
nal 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 in-
duces serious specular reflection. Fortunately this energy is often sufficiently