A servo is a device used to provide mechanical movement through an electrical control interface. Unlike a motor, which rotates at a required speed, a servo rotates to a required position.
Servos used for RC are comprised of a small electric motor typically packaged in a small rectangular case with an attached wire through which power and control signals are passed. A potentiometer (pot) in the case senses the position of the output shaft and a small circuit board compares this to the required position and drives the motor to minimise the difference.
The drive shaft of the motor is attached to an arm that can be used to actuate motion, usually through a stiff rod connected to a control surface such as an elevator, rudder, or aileron. Servos can also be used to control the steering mechanism in the nose or tail of an airplane, the swashplate in helicopters, and other mechanical tasks such as retractable landing gear or as part of a headtracking system to aim an FPV camera. Typically servo output arms rotate about 120 degrees.
Types of servo
Servos are chosen for a given application based on weight, size, speed, and torque.
Metal gear servos
Some models have metal gears that make them more resistant to damage in a crash, or ball bearings that reduce wear and slop.
Servos are also available in digital and analog varieties, this refers to the design of their circuit board, which in digital servos contains a microprocessor-based control circuit. Digital and analog servos share the same motor, potentiometer and gears of a standard servo, but the electronics that compare the input signal, the current position and control the motor voltage are different. All things being equal, digital servos generally have faster response, more accurate centering and higher holding torque. They also require more power than analog servos, which increases the risk of receiver brownout and may affect the choice of BEC. Some digital servos can also be programmed to rotate at a given rate or range of motion. This usually requires a special programming device to make the configuration changes.
Retract servos are usually lower geared and cannot be positioned anywhere other than at the extremes of their travel.
Sail winches are fitted with a small drum instead of an output arm and can rotate several times.