standard servos, but differ internally. They have no
mechanical stops and rotate continuously in both directions,
making them suitable for driving wheels on a robot.
They have no position sensor to provide feedback to
the internal control circuit, so this is an “open loop” control
system. The servo will turn forward and reverse at different
speeds based on the control signal, but there is no built-in
way to verify rotation or distance.
CR servos should stop at a specified control signal (1.5
millisecond pulse) but not all do, mainly because of
manufacturing tolerances, temperature stability, etc. Some
CR servos can be “nulled” (adjusted) for a precise stop by
turning an internal potentiometer with a small screwdriver.
Good quality servos are consistent over time; cheaper ones
drift and need to be re-nulled occasionally.
It’s noteworthy that when a CR servo is nulled and
receiving a stop command (1.5 ms pulses), it is not simply
“doing nothing.” It is applying dynamic braking and will
resist turning more so than if the servo was unpowered.
CR servos are not terribly fast; 40-50 RPM is typical. Not
surprisingly, their top speed increases when powered by
higher voltages. I coaxed 60 RPM out of a large CR-modified VS- 11 servo by using 7. 2 volts, but that risked
servo damage. Stick with six volts max. If you need more
speed or higher voltage, use an H-bridge with a gearmotor
instead of a servo.
Even lacking feedback and being slow and drift-prone,
CR servos continue to be popular in hobby robotics. They
are fairly cheap, ready to go with a built-in control circuit,
and have the familiar servo form factor. Many standard
servos can be modified for continuous rotation. Some are
easier to modify than others, and some give better fine
To modify a servo, the internal feedback potentiometer
must be disconnected or replaced, and the output gear
(and/or case) modified to remove the mechanical stops.
Many examples of this popular “hack” can be found on the
Finding wheels to mount on a CR servo can be a
challenge. There are some premade wheels which fit
directly onto a servo spline (Pololu and Solarbotics sell
Futaba spline wheels). Some have the matching female
spline molded into the plastic wheel; others have a servo
horn attached. You can make your own wheels by
attaching a servo horn to some existing wheels using either
screws or glue as previously described.
Accuracy is naturally required to center the horn on the
wheel. Otherwise, the wheel will be eccentric and make the
robot wobble. In the case of gluing servo horns to a wheel,
epoxy would be a better choice than CA, since the curing
time is minutes instead of seconds.
As the epoxy is curing, you can spin the servo using a
servo tester to verify concentricity and nudge the wheel into
running true. Be sure to rough up the nylon servo horn to
promote good adhesion (also described previously).
If you are handy and have access to some basic shop
tools, you can make your own wheels out of a flat plate of
plastic, wood, or other soft material. Cut out an oversized
circle with a bandsaw, then glue the servo horn firmly to
the circle as described, eyeballing the center. Now, mount
the wheel on the CR servo and spin it with the servo tester,
using a pencil or marker to draw a circle of the right
diameter. Next, use a disk or belt sander to make the wheel
round — just barely oversize.
Finally, spin the servo/wheel with the servo tester while
CAREFULLY holding it against the sander, making it
perfectly circular at the desired size. Watch that servo cable
and do NOT let it get pulled into the sander!
Two servos I know of come with a 1.75” diameter
circular horn — a small but free and instant wheel. This
diameter “wheel” gives a forward speed of 5-7
inches/second. Drive wheels need rubber tires for traction,
which are often O-rings or rubber bands. A handy source
for custom-width rubber band tires is to cut a loop out of a
bicycle inner tube, which is what I used on that 1.75” horn.
For any given servo and RPM, larger wheels will give higher
I have even seen peanut butter jar lids screwed to servo
horns, using rubber band tires to give a good advantage on
mobile robots (Figure 6). Get creative!
72 SERVO 07.2015
Figure 6a. Figure 6b.