Page 306 - Concise Encyclopedia of Robotics

Sonar
15 mW of power for each square centimeter of surface area exposed to
bright sunlight. Solar cells produce direct current (DC), which is used by
most electronic systems. Solar power is ideal for use in mobile robots that
function outdoors, especially in environments that get plenty of sunshine.
The stand-alone solar power system is most suitable for mobile robots.
It uses banks of rechargeable batteries, such as the lead–acid type, to store
electric energy as it is supplied by photovoltaics during hours of bright
sunshine. The energy is released by the batteries at night or in gloomy
daytime weather. This system is independent of the electric utility. The
main limitation of the stand-alone solar power system for use in mobile
robots is the fact that the solar batteries (combinations of solar cells in series
and/or parallel) must have significant surface area exposed to the sun in
order to generate enough power to operate robotic propulsion motors.
This can pose a design problem.
An interactive solar power system is connected to the utility power grid.
This type of system does not normally use storage batteries. Any excess
energy is sold to utility companies during times of daylight and mini-
mum usage. Energy is bought from the utility at night, during gloomy
daytime weather, or during times of heavy usage. This scheme can be
used with fixed robots, or with computers intended to control fleets of
mobile robots.
See also ELECTROCHEMICAL POWER and POWER SUPPLY.
SONAR
Sonar is a medium-range and short-range method of distance measure-
ment. The term is an acronym that stands for sound detection and ranging.
The basic principle is simple: bounce acoustic waves off of objects, and
measure the time it takes for the echoes to return. In practice, sonar systems
can be made so sophisticated that they rival vision systems for getting
pictures of the environment.
Audible versus ultrasonic
Sonar can make use of audible sound waves, but there are advantages to
using ultrasound instead. Ultrasound has a frequency too high for humans
to hear, ranging from about 20 kHz to more than 100 kHz. (One kilo-
hertz, kHz, is 1000 cycles per second.)
An obvious advantage of ultrasound in robotics is that the acoustic-
wave bursts are not heard by people working around the robot. These
waves, if audible, can be annoying.Another advantage of ultrasound over
audible sound is the fact that a system using ultrasound is less likely to be
fooled by people talking,machinery operating,and other common noises.
At frequencies higher than the range of human hearing, acoustic distur-

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