Build Your Own Combat Robot

242 Build Your Own Combat Robot
Passive Sensors
Passive sensors monitor some condition in the environment. They don’t introduce
anything into the environment; they simply sense what is happening around them.
A thermometer and a photocell are everyday examples of passive sensors. If connected
to a household heating system, a thermometer’s findings are reported to a
simple circuit in a household thermostat to tell the heater when to turn on or off.
Similar circuits are used to control air conditioners in warm climates. Photocells
monitor ambient light to sense how bright it is. These are used in street lights to
sense when the lights are no longer needed—a circuit turns off the lights when the
sun comes up. Similarly, when the sun goes down at night, the light level drops to a
predetermined level and a circuit turns the lights back on.
Another type of passive sensor is the passive infrared (PIR) sensor, sometimes
called a pyroelectric sensor. These sensors are commonly used to detect the presence
of a person and activate a circuit. They can control lights within a room or
outside a house, or they can be used as a burglar alarm. The sensor is a small
crystal mounted within the housing that can sense the infrared radiation emitted
by a person. The sensor has a circuit that charges the crystal, and the presence of
the radiation discharges the crystal, which is detected by another circuit. This is
called the pyroelectric effect. The radiation is focused upon the crystal by a row
of Fresnel lenses that cause a series of signal peaks as a person moves by. Several
autonomous robots have used these sensors to detect heat emissions from their
surroundings.
For combat robots, an electronic thermometer can be used to monitor the internal
temperature, the temperature of the motors, or batteries. If the temperature
gets too high, cooling motors can turn on or the power requirements can be reduced
to avoid overheating.
A tilt sensor can be used to monitor whether the robot gets flipped upside
down. Once the sensor detects a flip, it can initiate an arm or piston that will flip
the robot right side up. Or, if the robot was designed to run upside down, the tilt
sensor can be used to reverse steering controls, since an upside-down robot will
turn in opposite directions than a right-side-up robot. Another type of passive sensors
that can be used are acoustic sensors that can listen for the motors of the opponent
robot. These sensors can help guide your robot toward its opponent.
The most complex-passive sensor is a charged coupled device (CCD) camera
that is used to “see” the environment. CCD cameras are part of a vision system.
When used alone, they require advanced object-recognition software and usually
a dedicated computer. They can also be used with active sensors to help simplify
the computational software. Vision systems are most commonly found at robot
soccer events. Recently, CCD cameras have been used to detect flames in the Trinity
College Fire Fighting contest, and some members of the Seattle Robotics Society
have developed methods to use CCD cameras and simple microcontrollers to see
the lines in line-following contests.