Digital Signal Processing Reference
In-Depth Information
remote, the IR LED and sensor must be facing each other. Bandwidth is limited
by the 38kHz modulation on the IR signal and the filters inside the IR detector.
(An IR sensor strip that converts IR to visible light is available from Radio
Shack. This sensor can be used to confirm the operation of IR LEDs.)
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Wheel Encoder
The Nubotics WW01 WheelWatcher incremental quadrature encoder system
from Acroname is shown in Figure 13.7. This low-cost electronics board bolts
onto the top of a standard-size R/C servo. The adhesive-backed codewheel
attaches to a wheel mounted on the servo's output shaft. Two pairs of optical
emitters and receivers bounce light beams off of the codewheel.
Note that there are 32 black stripes on the reflective codewheel. When the
wheel is rotating, the encoder produces two series of digital pulses that are 90
degrees out of phase. When one of the pulses changes twice before the other
pulse changes, the direction has been reversed. 128 clock pulses per revolution
are produced and a separate direction signal indicates the current direction of
rotation. By counting pulses with a counter or by accurately measuring the time
between individual pulses using a fast hardware counter on the FPGA, it is
possible to more accurately control the position and velocity of the servo motor.
When used on robot drive motors, this optical encoder feedback provides more
accurate position and speed control for the robot.
Figure 13.7
Nubotics WW-01 Wheel Watcher Incremental Encoder System.
