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6.5 Proximity Sensors 6 Common Sensing Techniques for Reactive Robots
Proximity sensors measure the relative distance (range) between the sensor
and objects in the environment. Since the sensor is mounted on the robot, it is
a straightforward computation to translate a range relative to the sensor to a
range relative to the robot at large. Most proximity sensors are active. Sonar,
also called ultrasonics, is the most popular proximity sensor, with infrared,
bump, and feeler sensors not far behind.

6.5.1 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 “ul-
trasonics” are used interchangeably when discussing extracting range from
acoustic energy.
Ultrasonics is possibly the most common sensor on commercial robots op-
erating indoors and on research robots. They are active sensors which emit a
TIME OF FLIGHT sound and measure the 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 provid-
ing direct range measurements, the transducers were cheap, fast, and had
terrific coverage. In the early 1980’s, the Polaroid Land Corporation had de-
veloped 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

afield of viewof 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.