Servos are small, cheap, mass-produced actuators used for radio control and small 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 yachts, can rotate continuously.
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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 PWM (pulse-width modulation) 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.
The earliest form of sequential (although not proportional) actuator for radio control was the escapement.[1] Like the device used in clocks, this escapement controls the release of stored energy from a spring or rubber band. Each signal from the transmitter operates a small solenoid which then allows a two- or four-lobed pawl to rotate. The pawl, like a clock, has two pallets so that the pawl can only rotate by one lobe's position, per signal pulse. This mechanism allows a simple keyed transmitter to give sequential control, i.e. selection between a number of defined positions at the model.
A typical four-lobe escapement used for rudder control is arranged so that the first and third positions are "straight ahead", with positions two and four as "left" and "right" rudder. A single pulse from the first straight-ahead position allows it to move to left, or three pulses would select right. A further single pulse returns to straight-ahead.[2] Such a system is difficult to use, as it requires the operator to remember which position the escapement is in, and so whether a turn requires one or three pulses. A development of this was the two-lobe pawl, where keying the transmitter continuously (and thus holding the solenoid pallet in place) could be used to select the turn positions with the same keying sequence, no matter what the previous position.[2]
Escapements were low-powered, but light-weight. They were thus more popular for model aircraft than model boats.[1] Where a transmitter and receiver had multiple control channels (e.g. a frequency-keyed reed receiver), then multiple escapements could be used together, one for each channel.[1] Even with single channel radios, a sequence of escapements could sometimes be cascaded. Moving one escapement gave pulses that in turn drove a second, slower speed, escapement.[2] Escapements were disappearing from radio control, in favour of servos, by the early 1970s.[1]