Monday, September 3, 2012


by Alan Lee

A servo motor forms part of a servomechanism. A servomechanism is an automated device that uses error‐sensing negative feedback to correct the performance of a mechanism. The control input signal is compared to the actual position of the mechanical output. Where there is a difference, it is known as an error. The value difference between the new incoming signal and existing position is amplified and is used to drive the system – to reduce and eliminate the error.

Typically, a servo motor is a small mechanical device that has a shaft which represents the output. The shaft's
angular moment is controlled by an input signal which is sent directly to the servo. This input signal is analyzed with reference to the existing position of the output and angular movement is made if there exists an error. The output angular position of the shaft will be maintained until the servo receives a new input signal.

James Watts (1736 – 1819), a Scottish mechanical engineer is considered the first person to develop a
servomechanism. His first feedback control system was a steam steering engine used in large ships. This
technology was first used on the SS Great Eastern in 1866. The position of the ship’s rudder (output) was
controlled based on the position of the steering wheel (input).

RC servomechanisms (servos) are commonly found in remote control devices such a radio controlled cars, planes and boats. Bigger systems are also used to control original sized vehicles.

How it works
Servos are controlled by sending a signal pulse with variable width. The control wire is used to send this signal. Each pulse signal contains three parts of information: minimum pulse, maximum pulse and repetition rate. The duration of the positive pulse determines the position of the servo output shaft.

Each servo has a different pulse width as its center position. For example, a servo motor’s neutral position
corresponds to a 1.50ms signal. When a longer pulse signal is delivered, for example 2.00ms, the output shaft will rotate clockwise from the center. When a shorter pulse signal is delivered, for example 1.00ms, the output shaft will rotate anti‐clockwise from the center.

The servo control pulse is repeated typically every 20 milliseconds to check for a new updated signal. The output shaft’s rotation angle is therefore determined by the length of this pulse width modulation.