Page 225 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

20ms

Pulse Signal to Servo Motor .75 to 2ms
9.3 Pulse width used to control a servo motor

An encoder chip in the transmitter modulates the pulse width on the
transmitter’s carrier signal. The pulse width is based on the position
(resistance) of the potentiometer’s shaft. The pulse widths are
varied between 1 and 2 milliseconds (ms) (see Fig. 9.3). When the
potentiometer is in its center position, the pulse width corre-
sponding to that channel is 1.5 ms. When the control is pushed to
one extreme, the pulse width increases to 2 ms. When pushed to the
opposite extreme, the pulse width shrinks to 1 ms.
The receiver decodes the pulses on the carrier signal and sends
them to their respective servo motors. The servo motor is an inte-
gral unit, containing a motor, gearbox, output shaft, and a printed
204 circuit board (PCB). The PCB on the inside of the servo motor gen-

erates a reference pulse that is based on the position of an internal
potentiometer connected to the output shaft. A decoder chip on
the internal PCB compares the incoming pulses from the receiver
to the reference pulses. The servo motor attempts to match the
pulse widths of the two signals by adjusting the position of the
servo motor’s output shaft. This is how the servo motor tracks and
holds its position based on the signal from the transmitter.

Eyes
The eye(s) for our T-bot is a miniature color video camera system
with audio (see Fig. 9.4). The color camera system includes both a
2.4-gigahertz (GHz) transmitter and receiver. The camera system
cost is approximately $99.95.
The overall size of the camera is small. It is mounted to the body of
the transmitter by a small angled bracket. The video camera is
small enough so that two video cameras are capable of being
mounted side by side and have the approximate interpupilary dis-
tance (IPD) of 63 millimeters (mm) between lenses. Mounting a
pair of cameras like this will enable the T-bot to transmit realistic
stereo pictures to the operator. For the prototype we will use just
one camera; later we will discuss improvements to the system that
Team LRN
Chapter nine