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.
Construction
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.
Operation
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.
Connection
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.
RC servos use a three-pin 0.1" spacing jack (female) which mates to standard 0.025" square pins. The most common order is signal, +voltage, ground. The standard voltage is 4.8 V DC, however 6 V and 12 V is also used on a few servos. The control signal is a digital PWM signal with a 50 Hz frame rate. Within each 20 ms timeframe, an active-high digital pulse controls the position. The pulse nominally ranges from 1.0 ms to 2.0 ms with 1.5 ms always being center of range. Pulse widths outside this range can be used for "overtravel" - moving the servo beyond its normal range.
There are two general types of PWM. Each PWM defines a value that is used by the servo to determine its expected position. The first type is "absolute" and defines the value by the width of the active-high time pulse with an arbitrarily long period of low time. The second type is "relative" and defines the value by the percentage of time the control is active-high versus low-time. The "absolute" type allows up to eight servos to share one communication channel by multiplexing control signals using relatively simple electronics and is the basis of modern RC servos. The "relative" type is the more traditional usage of PWM whereby a simple low-pass filter converts a "relative" PWM signal into an analog voltage. The two types are both PWM because the servo responds to the width of the pulse. However, in the first case a servo may also be sensitive to pulse order.
The servo is controlled by three wires: ground, power, and control. The servo will move based on the pulses sent over the control wire, which set the angle of the actuator arm. The servo expects a pulse every 20 ms in order to gain correct information about the angle. The width of the servo pulse dictates the range of the servo's angular motion.
A servo pulse of 1.5 ms width will typically set the servo to its "neutral" position (typically half of the specified full range), a pulse of 1.0 ms will set it to 0°, and a pulse of 2.0 ms to 90° (for a 90° servo). The physical limits and timings of the servo hardware varies between brands and models, but a general servo's full angular motion will travel somewhere in the range of 90° – 180° and the neutral position (45° or 90°) is almost always at 1.5 ms. This is the "standard pulse servo mode" used by all hobby analog servos.