Robotics Class: Using Infrared Headlights to "See" the Road
Today's hottest products seem to have one thing in common:
wireless communication. Personal organizers beam data into desktop computers, and
wireless remotes let us channel surf. Many remote controls and PDA’s use signals in
the infrared frequency range to communicate, below the visible light spectrum. With
a few inexpensive and widely available parts, the BASIC Stamp can also receive and
transmit infrared light signals.
Infrared: Infra means below, so Infra-red is light (or
electromagnetic radiation) that has lower frequency, or longer wavelength than red
light. Students were instructed to use infrared light to illuminate the robot’s path and determine when the light reflects off an
object.
The infrared object detection system we’ll build on the
Boe-Bot is like a car’s headlights in several respects. When the light from a car’s headlights
reflects off obstacles, your eyes detect the obstacles and your brain processes them and
makes your body guide the car accordingly. The Boe-Bot uses infrared LEDs for
headlights. They emit infrared, and in some cases, the infrared reflects
off objects and bounces back in the direction of the Boe-Bot. The eyes of the Boe-Bot are
the infrared detectors. The infrared detectors send signals indicating whether or not
they detect infrared reflected off an object. The brain of the Boe-Bot, the BASIC Stamp, makes
decisions and operates the servo motors based on this sensor input.
The IR detectors have built-in optical filters that allow
very little light except the 980 nm infrared that we want to detect with its internal photodiode
sensor. The infrared detector also has an electronic filter that only allows signals
around 38.5 kHz to pass through. In other words, the detector is only looking for infrared
that’s flashing on and off 38,500 times per second. This prevents IR interference from common
sources such as sunlight and indoor lighting. Sunlight is DC interference (0 Hz), and
indoor lighting tends to flash on and off at either 100 or 120 Hz, depending on the main
power source in the region. Since 120 Hz is outside the electronic filter’s 38.5 kHz
band pass frequency, it is completely ignored by the IR detectors.
