So far, I’ve only talked about servos that are used when
precise angular positioning is required — like scanning a
sensor from side to side. Fortunately, R/C servos can also
rotate continuously, either by design or via modification
that you perform yourself. R/C servos make terrific drive
motors for your robot. They tend to be less expensive than
comparable DC gearmotors of the same specification, and
they come with their own driver electronics. They’re
definitely worth considering for your next robot. Servos that
rotate continuously act like an ordinary geared DC motor,
except it’s still controlled by sending it pulses.
• To make the motor go in one direction, send it
1.0 ms pulses.
• To make the motor go in the other direction, send
it 2.0 ms pulses.
• To make the motor slow to a stop, send it 1.5 ms
• To make the motor stop altogether, stop sending
Note that stopping the motor by ceasing the pulses
works for most servos except for digital servos. On most
digital servos, when pulses are stopped the servo will
merely continue with the last good position information it
received. You’re not likely to use digital servos for
continuous rotation, so this problem seldom comes up in
real life, but keep it in mind just in case.
There is a small handful of servos made for continuous
rotation. These include the GWS S- 35, the Parallax
continuous rotation servo, and the SpringRC SM-S4303R.
These are available from a variety of online resellers such as
Pololu, Parallax, and SparkFun.
You can also convert most any servo to continuous
rotation. Once modified, they are no longer capable of
precise angular rotation, but they’re perfectly suited as drive
motors for wheeled and tracked bots. There are a number
of methods you can use to modify an R/C servo for
continuous rotation. The basic process involves removing
the mechanical stops, and — for some types of servo
surgery — making a change in the electrical connections
inside. You’ll need the following tools to complete the
• #0 or #00 Phillips screwdriver
• 1/8” or smaller flat-bladed screwdriver
• “Nippy” cutters, X-ACTO blade, or razor saw
• Small flat jeweler’s file
Servo modification varies somewhat between makes
and models, but the basic steps are similar. The procedure
discussed here is common to those servos where the output
gear does not require to be physically mounted on the
potentiometer shaft. Rather, the gear sits atop a ball
bearing or a plastic or metal bushing.
Note! It’s possible to damage the servo and/or its main
output gear, so have a spare servo handy in case you make
a mistake. This is a learning process and requires some
patience and skill to get right.
1. Remove the case of the servo to expose the gear
train, motor, and potentiometer. This is accomplished by
removing the four screws on the back of the servo case and
separating the top and bottom.
62 SERVO 07.2014
Analog Versus Digital Servos
The most common — and most affordable — R/C servos are
analog, meaning their control circuitry uses traditional techniques
for controlling the motor. Digital servos use onboard
microcontrollers to enhance their operation.
Among the added features of digital servos are higher power
and programmable behavior. With the proper external programmer
(available separately at extra cost), it's possible to control the
maximum speed of the servo, for example, or make the servo
always start from power-up in a specific position.
Except for higher torques, from an applications standpoint
there is little difference between analog and digital servos. You
control both the same way. However, the higher torque of a digital
servo means the motor is drawing more current from its power
source, which means batteries tend not to last as long between
For most robotics applications, digital servos are not required.
You can get by with the less expensive standard analog servos. An
exception is when building a walking robot, where the extra
torque of digital servos comes in handy. Six-legged walking bots
may use 12 or even 18 servos just for the legs. The higher torque
helps to offset the added weight of all those servos.