Figure 1 shows a typical
standard-sized R/C servo motor. It
measures about 1-1/2” x 3/4” x by
1-3/8”. For this style of servo, its size
and the way it’s mounted is the same,
regardless of the manufacturer. That
means you have your pick of a variety
of makers and can compare prices.
There are other common sizes of
servo motors besides that shown in
the figure which are used for
specialty applications. These include
model airplane landing gear
mechanisms. I won’t be talking about
these here, but just know they’re
available should you want to
Inside the servo is a motor and
various other components neatly
packaged (see Figure 2). While not
all servos are exactly alike, all have
these three major parts: motor,
reduction gear, and control circuitry.
• Motor. A DC motor capable of
reversing direction is at the
heart of the servo.
• Reduction gears. The high
speed output of the motor is reduced by a gearing
system. Many revolutions of the motor equal one
revolution of the output gear and shaft of the servo.
In most servos, the output gear turns no more than
about 180 degrees in either direction, and is limited
by mechanical stops.
• Control circuitry. The output gear is connected to a
potentiometer — a common electronic device similar
to the volume control on a radio. The potentiometer
connects with a control circuit. The position of the
potentiometer indicates the position of the output
The motor and potentiometer are connected to a
control board — all three of which form a closed feedback
loop. While 180° (half a circle) may not sound like much, in
actuality such control can be used to steer a robot, move
legs up and down, rotate a sensor to scan the room, and
more. The precise angular rotation of a servo in response to
a specific digital signal has enormous uses in all fields of
robotics. As you can surmise, standard radio control servo
motors are designed for limited rotation rather than for
continuous rotation, like a DC gear motor. There are servos
that rotate continuously — and you can modify one to freely
rotate (see later). This allows you to use a servo motor to
drive your robot around a room.
Powering and Controlling
an R/C Servo Motor
Most R/C servo motors are designed for operation at
4. 8 to 6 volts. Some can be powered with up to 7. 2 or
even 9. 6 volts, but these aren’t as common. In any case,
you should always check the specifications of your servos
before applying juice to them to make sure you are
operating them within safe limits.
You can’t run an R/C servo simply by connecting it to a
battery. It needs special control signals to operate. In a
radio control application, the receiver — mounted some
place in the airplane or vehicle — both powers and controls
the servo. For robotics, you replace the function of the
receiver with a microcontroller or other circuit. As it turns
out, it’s not terribly hard to control a servo using simple
programming. It’s easy stuff for all the popular
microcontrollers, such as the Parallax BASIC Stamp and
Propeller, the Arduino, and PICAXE.
The control signal that commands a servo to move to a
specific point is in the form of a steady stream of pulses.
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Figure 2. How an R/C servo works. A control signal causes the motor to turn one
direction or the other, depending on the current position of the output shaft.
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