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Servos are the end units in a radio control chain.  They are used to move the aircraft's control surfaces, the motor throttle and to actuate other devices such as retractable landing gears.

A servo consists basically in a motor, gearbox, feedback potentiometer and an electronic board inside a plastic case.  Outside are the servo arm and the servo cable and plug.

The servo arm is often a plastic piece with holes on it for attaching push rods or other mechanical linkages.


There are linear and rotary servos, but the most widespread today are the rotary servos whose arm rotates about 45 degrees left and right from its centre point.

The picture on right shows some servo hardware, such as mounting screws, rubber pads, and different sorts of servo arms.

The servo has an electronic circuit that compares the incoming control pulse with a local generated one whose width corresponds to the servo arm's actual position.  The servo's internal pulse width is determined by its feedback potentiometer whose slider moves together with the servo's arm.  When the width of the incoming control pulse is different from the local generated, the servo motor will rotate until the both pulses' width are equal.  The direction of rotation depends on whether the incoming pulse is wider or shorter than the local pulse.


There are two operating concepts: the conventional servo and the digital servo.  The conventional servo circuit uses a pulse stretcher to widen the pulse difference between the incoming pulse and the locally generated. Thereby a 1% pulse difference produces a 50% duty cycle for motor drive.  A continuous drive signal will be obtained when the pulse difference is over 10%.  Also a small dead band is provided to prevent the servo being in continuous state of motion when insignificant pulse differences occur.

The difference between the conventional and the digital servo is that the pulse drive to the motor occurs every 20mS with the conventional, whereas with the digital occurs (for example) every 3.3mS, which means that the digital servo sends pulses to the motor at a much higher frequency.  Digital servo incorporates a microprocessor, which receives the input pulse signal and generates power pulses to the servomotor based on preset values.  Some brands offer the possibility to program certain parameters such as Dead-Band Width, Direction of Rotation, Neutral Point, Servo Arm Throw and End Point.

The digital servo is supposed to have constant torque throughout the servo travel, faster control response and more accurate positioning, but at the expense of greater power consumption.

Servos are available in different shapes, sizes, weights and output torque.  Typically they may be classified as follows :

Giant - weights around 100gr (3.5oz)
Standard - 45gr (1.6oz)
Mini - 20gr (.70oz)
Micro - 8gr (.28oz)
Pico - 5.5gr (.18oz)

Further lighter systems use a coil/magnet concept, and may weight less than 1gr (.035oz).  However, they need a special tailored receiver.

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