RC servos are electro-mechanical devices that respond to a control signal, which instructs them to move their output shaft to a certain position. A servo is normally plugged into a radio receiver with a three pin connector. The three wires are power (usually 4.8V to 6.0V), a ground wire, and a signal wire. The signal wire carries a PWM (Pulse-Width Modulation) signal consisting of a 1-2ms pulse repeated 50 times a second. A 1.5ms pulse will tell the servo to move to its output shaft to the center position, 0 degrees. For a servo with a 90 degree range of motion, a 1ms pulse will move the servo to -45 degrees, and a 2ms pulse will move the servo to +45 degrees.
The torque rating specifies how much force the servo can exert. It is typically expressed in units of ounce-inches (oz-in) or kilogram-centimeters (kg-cm). The higher the number, the more force the servo can exert. If you know the length of the servo arm that will be used, you can use this measurement and the servo torque rating to calculate how much force end of the arm can exert. A long arm will reduce the maximum possible force, and a short arm will increase it.
The speed of a servo is measured in the number of seconds it takes to move a certain amount of rotation, usually 60 degrees. The smaller the number, the faster the servo is. A servo that is rated 0.15 seconds is able to rotate 60 degrees in 0.15 seconds.
Size and Weight
The size and weight of a servo are important considerations when used in a small airplanes or other RC devices where there is not much room, or weight is an issue. Typically, smaller servos will have lower torque ratings.
Standard servos have bushings supporting the main shaft, heavy duty servos typically have one or two ball bearings supporting the main shaft.
The standard motor used in an RC servo is a three pole ferrite motor. Five pole coreless motors are used in some high speed servos, and heavy duty coreless motors are using in some high end heavy duty RC servos.