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What You Need to Know
About Radio Control Servo Motors
Since the early 1990s, servos for radio
controlled airplanes and cars have
been a preferred method of motorizing
a robot. Many of the benefits of servo
motors are obvious: They’re small,
relatively inexpensive, and for the most
part easy to use with most any robotic
control system. Radio control (R/C)
servos combine a DC motor, gearing,
and control electronics in one compact
package. Plus, most servos are
engineered for convenient mounting.
Just a couple of screws and the motor
is tightly secured to your bot.
Some of the benefits are less
obvious, but just as important. Though
the typical R/C servo is designed for
imitated rotation — to control a
steering mechanism on a model car or
a wing flap on a model airplane —
with some basic hardware hacking, it’s
possible to convert most servos to
rotate continuously. In this fashion, the
same small, inexpensive, and easy
motor solution can be used for moving
your robot creation across the floor.
Servos are also common finds,
both locally and through mail order.
Many areas of the country have at
least one neighborhood hobby store
that caters to the radio control
enthusiast. So if you find your latest
robot needs one more motor, there’s
a fair chance you can pick one up
some late Saturday afternoon. And in
those areas where a good hobby
store is hard to find, or when you’re
needing something special, you can
always turn to mail order, and the
literally hundreds of online retail stores
that carry all types and sizes of R/C
62 SERVO 06.2008
servos. In this installment of Robotics
Resources, we’ll take a closer look at
R/C servos. We last looked at them in
July 2005, and given the importance
these motors have in building the
typical desktop robot, we’re due for
another look.
What Makes Up a
Radio Control Servo
The vast majority of servo motors
for model radio control applications
follow the same design principles. In
fact, there’s even some standardization
— in both electrical interfacing and
sizing — that allows you to freely
interchange one servo for another.
More about that later.
The R/C servo consists of a DC
motor, a series of gears to reduce the
speed of the motor, a control board,
and a potentiometer. The motor and
potentiometer are connected to the
control board, all three of which form
a closed feedback loop. Both control
board and motor are powered by a
constant DC voltage, usually between
4. 8 and 6 volts. (A few servo brands
can tolerate voltages of up to 7.2, but
this is dependent on the make and
model, and may cause excessive heat
build-up in the motor housing which
can lead to a shortened life-span.)
To operate the motor, a digital
signal is sent by some outboard
electronics to the control board. In the
typical R/C application, the outboard
electronics is a radio control receiver.
The receiver picks up signals from its
nearby transmitter, which is being
humanly operated. For the typical
robot, the outboard electronics can
also be an R/C receiver or, more often,
a microcontroller that is programmed
to provide the same kind of control
signal provided by the receiver.
The control signal used by R/C
servos is nearly universal in concept.
Specifically, the servo responds to a
signal made up of short pulses;
the pulses vary from about one
millisecond (one thousandth of a
second, or ms) to about 2 ms. These
pulses are sent approximately 50
times each second. The exact length
of the pulse — in fractions of a
millisecond — determines the position
of the servo. With a 1 ms duration,
the servo is commanded to turn all
the way in one direction. At 2 ms, the
servo is commanded to turn all the way
in the other direction. Logically, at 1.5
milliseconds, the servo is commanded
to turn to its center — or neutral —
position. Most any in-between angle is
accommodated by using pulse values
between 1 and 2 ms.
Many people refer to this control
signal as pulse width modulation, but
perhaps a more accurate term is pulse
duration modulation, as the servo
responds to the specific duration of
the pulse, rather than the ratio of on
and off times, as is the case with
pulse width modulation. Regardless of
what you call it, the signal is relatively
simple to reproduce. Most popular
microcontrollers used with robotics —
such as the Parallax BASIC Stamp,
Netmedia BasicX, OOPic, and others —
have built-in commands that make
