Servo (radio control)

Servos (also RC servos) are small, cheap, mass-produced servomotors or other actuators used for radio control and small-scale robotics. Most servos are rotary actuators although other types are available. Linear actuators are sometimes used, although it is more common to use a rotary actuator with a bellcrank and pushrod. Some types, originally used as sail winches for model yachting, can rotate continuously. A typical servo consists of a small electric motor driving a train of reduction gears. A potentiometer is connected to the output shaft. Some simple electronics provide a closed-loop servomechanism. The position of the output, measured by the potentiometer, is continually compared to the commanded position from the control (i.e., the radio control). Any difference gives rise to an error signal in the appropriate direction, which drives the electric motor either forwards or backwards, and moving the output shaft to the commanded position. When the servo reaches this position, the error signal reduces and then becomes zero, at which point the servo stops moving. If the servo position changes from that commanded, whether this is because the command changes, or because the servo is mechanically pushed from its set position, the error signal will re-appear and cause the motor to restore the servo output shaft to the position needed. Almost all modern servos are proportional servos, where this commanded position can be anywhere within the range of movement. Early servos, and a precursor device called an escapement, could only move to a limited number of set positions. Radio control servos are connected through a standard three-wire connection: two wires for a DC power supply and one for control, carrying a pulse-width modulation (PWM) signal. Each servo has a separate connection and PWM signal from the radio control receiver. This signal is easily generated by simple electronics, or by microcontrollers such as the Arduino. This, together with their low-cost, has led to their wide adoption for robotics and physical computing.