From RC Helicopter Wiki
A shortened form of the word "servomotor", a servo is a small motor together with electronics that monitor and control its position precisely, that is used to transfer mechanical control commands to a remote location. In radio control systems, the servos attach to the receiver and through linkages, to the control surfaces of the model. Signals are received and translated by the receiver and are then sent to the servos as different length pulses, which cause the servos to move and actuate their associated control surface. The servo has a splined output shaft, onto which bolts a servo horn to which the linkage is attached.
Most RC model servos connect to the receiver via a three-pin connector; one wire (black or brown) supplies the 0V reference, one (red or orange) supplies the positive voltage, and one (white or yellow) supplies the pulse width modulation control signal.
A growing number of servos are termed digital servos; these maintain much tighter control of their output, at the expense of using more power. Conventional 'analog' servos apply a voltage to the motor proportional to the difference of the output's measured position and the commanded position; digital servos use digital electronics to allow the servo to use full power to correct even small position variations. Digital servos can also accept commands much more frequently, which is particularly useful for gyros and other stability augmentation systems.
Servos come in different sizes, suitable for different applications. Typically the manufacturer will specify a stall torque (in kg.cm or oz.in) and a rotation speed (in seconds per 60 degrees). Sometimes several similar models will exist, which have different gearings to trade torque for speed, or vice versa.
Retract servos are special in that they only have two positions: locked full one way or locked full the other way. They are used to move and lock retractable landing gear in place, and are often quite slow to appear realistic.
Metal geared servos use metal rather than plastic gears, improving drive train life and crash survivability.
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