make to another, the basics are about
1) Open the servo by removing its
case screws. You’ll need a miniature
Philips-head screw driver.
2) Carefully remove the output gear,
and file or cut off its stop. This is
easier with plastic gears, but you must
be careful that when you cut off the
stop, you don’t break the gear. It’s
usually better to file the stop down.
3) Examine the bottom of the output
gear, which is engaged with the
servo’s potentiometer. You’ll find two
general designs: a molded-in slot that
slips around the shaft of the potentiometer; or a small piece of plastic
that engages the potentiometer with
the output gear. The plastic piece is
easier to work with, as you can just
slip it off. If the gear uses a molded-in
slot, you’ll have to drill it out.
4) With the servo still open, apply 1.5
ms pulses to the servo, and manually
adjust the potentiometer so that the
Bear in mind that the average
servo is not engineered for lots and
lots of continual use. The mechanics
of the servo are likely to wear out
after perhaps as little as 25 hours
(that’s elapsed time), depending on
the amount of load on the servos.
Models with metal gears and/or brass
bushings or ball bearings will last
longer. Also consider that the control
electronics of a servo are made for
intermittent duty. Servos used to power
a robot across the floor may be used
minutes or even hours at a time, and
tend to be under additional mechanical
stress because of the weight of the
robot. Though not exactly common, it
is possible to burn out the control
circuitry in the servo by overdriving it.
Standard size servos are not
particularly strong when compared to
many other DC motors with gear-heads.
Don’t expect a servo to move a five or
10 pound robot. If your robot is heavy,
64 SERVO 06.2008
consider using either larger, higher output
servos (such as 1/4-scale or sail winch)
or DC motors with built-in gear heads.
Finally, keep in mind that
modifying a servo voids its warranty.
You’ll want to test the servo before
you modify it to ensure that it works.
Before moving on ... if you’re not
interested in modifying a servo yourself,
you can buy them pre-modified from
a couple of sources. One is Parallax,
who offers an already modified
version of its standard servo. This modified servo is the same as is used on the
company’s popular BOE-Bot robot.
Also, my own small mail order outfit,
Budget Robotics, routinely offers the
GWS S35 continuous rotation servo.
Servos of Different
Sizes and Types
As noted above, R/C servos follow
some standards, including size. By
using (more or less) the same sizes,
servos are interchangeable within a
model airplane or car. Exceptions
exist, of course, but for the most part,
you’ll find R/C servos in the following
• Standard, which measure about 1-1/2”
by 3/4” by 1-3/8” (case dimensions).
Most use a flange mounting with four
holes spaced within a rectangle of
approximately 1-7/8” by 3/8”.
• Quarter-scale (or large-scale) servos
are about twice the size of standard
servos, and are significantly more powerful. Quarter-scale servos are designed
to be used in large model airplanes,
but they also make perfect power
motors for a robot. Typical size for a
quarter-scale servo is 2” by 1-1/8” by
2-3/8”. Most larger scale servos can
be modified for continuous rotation,
but it may require extra effort due to
the increased size of the components.
• Mini/micro servos are about half the
size (and smaller!) of standard servos,
and are designed to be used in tight
spaces in a model airplane or car. They
aren’t as strong as standard servos,
however. A typical size for a mini servo
is 1-1/8” by 5/8” by 1”. Typical for a
micro servo is 7/8” by 3/8” by 5/8”. Most
micro and mini servos are not easily
adapted to continuous rotation, unless
you have good mechanical skills.
A relative newcomer on the servo
scene is the specialty robotics motor,
designed for the special requirements
of desktop rolling and walking bots.
Most of the large servo manufacturers
offer at least one or two such servos,
which typically offer higher torque and
other features that make them ideal
for robotics use. Some of these servos
— such as the Hitec HSR-8498HB —
are provided in their own unique form
factor. They are not sized along the
same lines the standard servos
mentioned above are. For these special-purpose robotics servos, be prepared
for a little bit of sticker shock.
A special mention is also due to
the Dynamixel servos provided with the
Robotis robot construction sets — see
robotis.com for more information. These
unique servos — like several robotics-specific models from other companies
— offer high torque, internal feedback,
and serial communication. The Dynamixel
servos also support software
changeable operation from angular to
continuous rotation, and back again.
Manufacturers list several
specifications that are unique to R/C
servo motors. One is transit time,
which is the approximate time it takes
for the servo to rotate the shaft X
degrees — usually specified as 60
degrees. Small servos turn at about a
quarter of a second per 60 degrees,
while larger servos tend to be a bit
slower. The faster the transit time, the
“faster acting” the servo will be.
If you’ve modified the servo for
continuous rotation, you can calculate
equivalent RPM by multiplying the 60
degree transit time by six (to get full
360 degree rotation), then dividing
the result into 60. For example, if a
servo motor has a 60 degree transit
time of 0.20 seconds, that’s one
revolution in 1.2 seconds (0.2 6
= 1.2), or 50 RPM ( 60 / 1.2 = 50).