SERVOMOTORS
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CONTROL SEQUENCE
UNDERSTANDING
SERVOS
R/C servomotors (or just “servos”)
are packed with technology, including
a DC motor, gear train, sensor, and
control electronics. They are a kind of
servomechanism. Servomechanisms
use a feedback control loop to adjust
how a mechanism functions. One
example of a servomechanism is a
thermostat-controlled heating system.
The temperature-sensing thermostat
is the feedback-providing sensor, and
the heating element is the output. The
heater gets turned on and off based
on the temperature sensing.
For R/C servos, the sensor input is
a potentiometer (or “pot” for short)
that’s used to measure the amount of
rotation from the output motor. Control
electronics read the electrical resistance of the pot and adjust the speed
and direction of the motor to spin it
toward the commanded position.
Figure A shows an exploded view of a
standard servo and the workings of the
servo’s closed feedback control loop. A
Read
position
(pot)
Make
motion
(motor)
Couple
rotation
(gear train)
Drive motor
(control
electronics)
Drive
shaft
Gear train
Pot
Motor
Control electronics
Control input
1
SELECTING SERVOS
Servos come in a variety of shapes and
sizes (Figure B). The most common
class of servo is the standard servo
( 1). The smallest servos are the micro
class ( 2, 3), and the largest are the
high-torque/winch class ( 4). All of these
have the same 3-wire control, so it’s
easy to move to a bigger or smaller
servo as your needs change.
Besides their dimensions and weight,
servos are characterized by two main
attributes: torque and speed, determined by the gearing and the motor
in the servo.
The torque is essentially the strength
of the servo. A standard servo torque
value is 5.5kg/cm (75oz/in) when B
2
3
4
Illustrations by Damien Scogin
142 Make: Volume 19
