Page 179 - Build Your Own Combat Robot

Build Your Own Combat Robot
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The R/C Servo
The basic building block of R/C models is the R/C servo. Usually packaged along
with a radio transmitter and receiver set, an R/C servo is a miniature electronics
device that includes an electronic motor-controller board, a motor, a geartrain,
and a position-feedback sensor all in one small plastic case. The servo contains a
simple electronic circuit.
Originally made from discrete components but now packaged in a single cus-
tom integrated circuit, the servo converts the length of the input pulse into a volt-
age level, compares the voltage level to the signal from the position sensor on the
output shaft, and drives the motor appropriately depending on the difference. The
effect is that the signal from the radio controls the position of the output shaft of
the servo. Typical R/C servos have a range of travel from 90 to 120 degrees, with a
2.0-millisecond pulse driving the shaft fully clockwise and a 1.0-millisecond pulse
driving it fully counterclockwise. Most servos have a maximum range of travel of
about 180 degrees, but the pulse-width range will be from 0.8 to 2.2 milliseconds
to achieve this range of motion.
In the early days of R/C hobbies, all controls worked through mechanical ser-
vos. R/C servos directly drove steering links in cars and control surfaces on model
airplanes. Throttle control of motors was also accomplished with servos. A servo
would open and control the intake valve on a gas engine to control its power.
When electric motors became popular in R/C cars, the same hobby control servos
were used to control them; but instead of opening and closing a throttle valve, the
servo arm would slide along a set of contacts to make or break the power circuit to
the motor. When Field Effect Transistor (FET)–type electronic speed controllers
entered the market, they duplicated the interface of the earlier mechanical speed
controllers, with what had been a position control signal to control a servo’s output
shaft now being a speed and direction control for an electric motor.

C ontrol Channels

Traditional R/C systems are rated by the number of channels they can control.
Channels refer to the number of independent servo signals the system can send si-
multaneously to the receiver. Most of the low-cost radio sets meant for R/C cars are
two-channel radios. The radio transmitter can send command information for two
separate servo positions at once to the receiver to control both steering and motor
speed (or throttle) simultaneously. The next level for R/C cars is three-channel ra-
dios; the third channel is intended to control a gearshift, air horn, lights, or other
on-board accessories. Most of these radio transmitters use a pistol-grip configura-
tion, in which a gun-style finger trigger controls the throttle channel and a miniature
wheel on the side of the transmitter controls the steering channel. A pistol-grip
transmitter is shown in Figure 8-2.