Digital Signal Processing Reference
In-Depth Information
Sensors with a single output bit can utilize a simple control scheme, and for
basic tasks the robot can be controlled using hardware as simple as a state
machine. More advanced sensors that report actual distance, location, or
heading measurements will likely require a processor core on the FPGA
running a program that interprets sensor readings and implements the robot's
control algorithm.
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Line Tracker Sensor
A line tracker module from Lynxmotion is shown in Figure 13.3. This device
uses three pairs of red LEDs and infrared (IR) phototransistor sensors that
indicate the presence or absence of a black line below each sensor. When the
correct voltages are applied in a circuit, an IR phototransistor operates as a
switch. When IR is present the switch turns on and when no IR is present the
switch turns off. The LED transmits red light that contains enough IR to trigger
the phototransistor.
Each LED and phototransistor in a pair are mounted so that the light from the
LED bounces off the floor and back to the IR phototransistor. The LED and IR
sensor must be mounted very close to the floor for reliable operation. Black
tape or a black marker is used to draw a line on the floor. The black line does
not reflect light so no IR signal is returned. Three pairs of LEDs and IR
phototransistor sensors produce the three digital signals, left, center, and right.
The FPGA-bot can be programmed to follow a line on the floor by using these
three signals to steer the robot. The mail delivery robots used in large office
buildings use a similar technique to follow lines or signal cables in the floor.
Figure 13.3
- Three LEDs and phototransistors are mounted on bottom of the Line Tracker board.
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Infrared Proximity Detector
An IR proximity sensor module from Lynxmotion is seen in Figure 13.4. The
FPGA-bot can be outfitted with an infrared proximity detector that is activated
by two off-angle infrared transmitting LEDs. The circuit utilizes a center-
