RC Servo
From Wikipedia, the free encyclopedia
RC servos are servos (position controlled DC motors) typically employed in radio-controlled models. They are used to provide actuation for mechanical systems such as the steering of a car, the flaps on a plane, or the rudder of a boat.
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[edit] Overview
RC servos are comprised of a DC motor mechanically linked to a potentiometer. Pulse-width modulation (PWM) signals sent to the servo are translated into position commands by electronics inside the servo. When the servo is commanded to rotate, the DC motor is powered until the potentiometer reaches the value corresponding to the commanded position that it's ordered.
Due to their affordability, reliability, and simplicity of control by modern microprocessors, they are often found being used in small-scale robotics applications.
[edit] Communication
The servo is controlled by three wires: ground (black), power (red) and control (any other color) and will move based on the pulses sent over the control wire. The pulses sent over the control wire set the angle of the servo horn. The servo expects a pulse every 20 ms, or 0.02 s in order to gain correct information about the angle. The width of the servo pulse dictates the range of the servo's angular motion.
A servo pulse of 1.5 ms width will set the servo to its "neutral" position, or 90°. For example a servo pulse of 1.25 ms could set the servo to 0° and a pulse of 1.75 ms could set the servo to 180°. The physical limits and timings of the servo hardware varies between brands and models, but a general servo's angular motion will travel somewhere in the range of 180° - 210° and the neutral position is almost always at 1.5ms.
[edit] Power
Servo motors are often powered from nickel-cadmium battery packs common to most RC devices. Voltage ratings vary from product to product, but most servos are operated at 4.8 V DC or 6.0 V DC (a 4 or 5 cell battery).
