User`s manual

ROBOT . HEAD to TOE

Product User’s Manual – SC08A

3. How RC Servo Motor works

Radio Control (RC) hobby servos are small actuators designed for remotely operating model

vehicles such as cars, airplanes, and boats. Nowadays, servos are become more popular in

robotics, creating humanoid robot, biologically inspired robot, robotic arm and etc. This is

because its’ ability to rotate and maintain and certain location, position or angle according to

control pulses from a single wire. Inside a typical servo contains a small motor and gearbox

to do the work, a potentiometer to measure the position of the output gear, and an electronic

circuit that controls the motor to make the output gear move to the desired position. Because

all of these components are packaged into a compact, low-cost unit, servos are great actuators

for robots.

Besides signal wire, a RC servo has other two leads: power and ground. The control signal is

a continuous stream of pulses that are 1 to 2 milliseconds long, repeated approximately fifty

times per second, as shown below.

The width of the pulses determines the position to which the servo moves. The servo moves

to its neutral, or middle, position when the signal pulse width is 1.5 ms. As the pulse gets

wider, the servo turns one way; if the pulse gets shorter, the servo moves the other way.

Typically, a servo will move approximately 90 degrees for a 1 ms change in pulse width.

However, the exact correspondence between pulse width and servo varies from one servo

manufacturer to another.