Build Your Own Combat Robot

Active Sensors
Chapter 11: Autonomous Robots 243
Active sensors often introduce a sound or light and look for how the introduced
energy reacts with the environment. Some examples of this type of sensing are sonar,
laser, and infrared reflective detectors.
Sonar detectors introduce a sound, typically higher in frequency than what humans
can hear, and listen for the echo. The bounced-back echo is used by sonar
range finders to send out a sound pulse and then compute the time it takes for the
sound to return. This time is directly proportional to the distance the sound must
travel to bounce off the nearest object and return. The speed of sound is around
770 mph, but it can easily be measured using a moderately fast microcontroller
chip like those found in many robots.
Infrared (IR) reflective sensor systems emit a specific wavelength of light and
look for a reflection of light. Since light travels so much faster than sound, it is difficult
to measure the time it takes to receive the reflected light. Infrared detectors are
typically used to detect whether an object is present within the range of the detector
rather than how far the object is from the detector. Some clever infrared detectors
use some simple geometry present in a triangle formed by the emitter that generates
the light, the reflected object, and the detector that senses the emitted light. Everyday
examples of infrared detectors can be found in modern bathroom stalls in public
places. The mechanism that automatically flushes the toilet typically uses an
infrared detector to detect the presence of a person using the toilet. The system is activated
when a person is in the stall for a predetermined period of time. Many systems
have a small flashing LED that speeds up its flashing when the time has
elapsed. When the person leaves the presence of the IR sensor, the toilet flushes.
Lasers can be used to detect where an opponent is located. This type of system is
fairly advanced and usually employs a CCD camera or a linear sensor array. A laser
beam is emitted from the robot, and the CCD camera is used to see the laser spot—
or line—on the opponent robot. Generally, a band pass filter is placed in front of
the camera to filter out all wavelengths of light except for the laser beam wavelength.
When the laser beam and the camera orientation is known, the range and
location of an object can be determined through mathematical triangulation. This
type of system is fairly complex, but not as complex as a true vision system, and it
is beginning to be seen in robotic applications using simple microcontrollers. This
type of system could be used in automatic weapons firing and assisted homing in
on an opponent, and it can be placed in fully autonomous robots.
Devantech SRF04 Ultrasonic Range Finder
The Devantech SRF04 Ultrasonic Range Finder (shown in Figure 11-1) is a
40-kHz ultrasonic range finder that can be used to determine the range of objects
from 1.2 inches to about 10 feet (or 3 cm to 3 meters).