Inmechanical andcontrol engineering, aservomechanism (also calledservo system, or simplyservo) is acontrol system for the position and itstime derivatives, such asvelocity, of amechanical system. It often includes aservomotor, and usesclosed-loop control to reducesteady-state error and improve dynamic response.^[1] In closed-loop control, error-sensingnegative feedback is used to correct the action of the mechanism.^[2] In displacement-controlled applications, it usually includes a built-inencoder or other position feedback mechanism to ensure the output is achieving the desired effect.^[3] Following a specified motion trajectory is calledservoing,^[4] where "servo" is used as averb. Theservo prefix originates from theLatin wordservus meaning slave.^[1]

The term correctly applies only to systems where thefeedback or error-correction signals help control mechanical position, speed, attitude or any other measurable variables.^[5] For example, an automotivepower window control is not a servomechanism, as there is no automatic feedback that controls position—the operator does this by observation. By contrast a car'scruise control uses closed-loop feedback, which classifies it as a servomechanism.

A common type of servo providesposition control. Commonly, servos areelectric,hydraulic, orpneumatic. They operate on the principle of negative feedback, where the control input is compared to the actual position of the mechanical system as measured by some type oftransducer at the output. Any difference between the actual and wanted values (an "error signal") is amplified (and converted) and used to drive the system in the direction necessary to reduce or eliminate the error. This procedure is one widely used application ofcontrol theory. Typical servos can give a rotary (angular) or linear output.

Speed control via agovernor is another type of servomechanism. Thesteam engine uses mechanical governors; another early application was to govern the speed ofwater wheels. Prior to World War II theconstant speed propeller was developed to control engine speed for maneuvering aircraft. Fuel controls forgas turbine engines employ either hydromechanical or electronic governing.

Positioning servomechanisms were first used in militaryfire-control andmarine navigation equipment. Today servomechanisms are used inautomatic machine tools, satellite-tracking antennas, remote control airplanes, automatic navigation systems on boats and planes, andantiaircraft-gun control systems. Other examples arefly-by-wire systems inaircraft which use servos to actuate the aircraft's control surfaces, andradio-controlled models which use RC servos for the same purpose. Manyautofocus cameras also use a servomechanism to accurately move the lens. Ahard disk drive has a magnetic servo system with sub-micrometer positioning accuracy. In industrial machines, servos are used to perform complex motion, in many applications.

Aservomotor is a specific type of motor that is combined with arotary encoder or apotentiometer to form a servomechanism. This assembly may in turn form part of another servomechanism. A potentiometer provides a simple analog signal to indicate position, while an encoder provides position and usually speed feedback, which by the use of aPID controller allow more precise control of position and thus faster achievement of a stable position (for a given motor power). Potentiometers are subject todrift when the temperature changes whereas encoders are more stable and accurate.

Servomotors are used for both high-end and low-end applications. On the high end are precision industrial components that use a rotary encoder. On the low end are inexpensiveradio control servos (RC servos) used inradio-controlled models which use a free-running motor and a simple potentiometer position sensor with an embedded controller. The termservomotor generally refers to a high-end industrial component while the termservo is most often used to describe the inexpensive devices that employ a potentiometer.Stepper motors are not considered to be servomotors, although they too are used to construct larger servomechanisms. Stepper motors have inherent angular positioning, owing to their construction, and this is generally used in an open-loop manner without feedback. They are generally used for medium-precision applications.^[6]

RC servos are used to provide actuation for various mechanical systems such as the steering of a car, the control surfaces on a plane, or the rudder of a boat. Due to their affordability, reliability, and simplicity of control by microprocessors, they are often used in small-scalerobotics applications. A standard RC receiver (or a microcontroller) sendspulse-width modulation (PWM) signals to the servo. The electronics inside the servo translate the width of the pulse into a position. When the servo is commanded to rotate, the motor is powered until the potentiometer reaches the value corresponding to the commanded position.

James Watt'ssteam enginegovernor is generally considered the first powered feedback system. Thewindmill fantail is an earlier example of automatic control, but since it does not have anamplifier orgain, it is not usually considered a servomechanism.

The first feedback position control device was the shipsteering engine, used to position the rudder of large ships based on the position of the ship's wheel.John McFarlane Gray was a pioneer. His patented design was used on theSS Great Eastern in 1866.Joseph Farcot may deserve equal credit for the feedback concept, with several patents between 1862 and 1868.^[7]

The telemotor was invented around 1872 byAndrew Betts Brown, allowing elaborate mechanisms between the control room and the engine to be greatly simplified.^[8] Steam steering engines had the characteristics of a modern servomechanism: an input, an output, an error signal, and a means for amplifying the error signal used for negative feedback to drive the error towards zero. The Ragonnetpower reverse mechanism was a general purpose air or steam-powered servo amplifier for linear motion patented in 1909.^[9]

Electrical servomechanisms were used as early as 1888 inElisha Gray'sTelautograph.

Electrical servomechanisms require a power amplifier.World War II saw the development of electricalfire-control servomechanisms, using anamplidyne as the power amplifier.Vacuum tube amplifiers were used in theUNISERVO tape drive for theUNIVAC I computer. The Royal Navy began experimenting with Remote Power Control (RPC) onHMS Champion in 1928 and began using RPC to control searchlights in the early 1930s. During WW2 RPC was used to control gun mounts and gun directors.

Modern servomechanisms use solid state power amplifiers, usually built fromMOSFET orthyristor devices. Small servos may use powertransistors.

The origin of the word is believed to come from the French "Le Servomoteur" or the slavemotor, first used by J. J. L. Farcot in 1868 to describe hydraulic and steam engines for use in ship steering.^[10]

The simplest kind of servos usebang–bang control. More complex control systems use proportional control,PID control, and state space control, which are studied inmodern control theory.

Servos can be classified by means of their feedback control systems:^[11]

• type 0 servos: under steady-state conditions they produce a constant value of the output with a constant error signal;
• type 1 servos: under steady-state conditions they produce a constant value of the output with null error signal, but a constant rate of change of the reference implies a constant error in tracking the reference;
• type 2 servos: under steady-state conditions they produce a constant value of the output with null error signal. A constant rate of change of the reference implies a null error in tracking the reference. A constant rate of acceleration of the reference implies a constant error in tracking the reference.

Theservo bandwidth indicates the capability of the servo to follow rapid changes in the commanded input.

• Fractional horsepower motor
• Motion control – Field of automation which studies how to precisely move parts of machines
• Servo control – Aspect of the operation of a servo
• Synchro – Variable transformers used in control systems

• Bennett, S. (1993).A History of Control Engineering 1930–1955. London: Peter Peregrinus Ltd. On behalf of the Institution of Electrical Engineers.ISBN 0-86341-280-7.
• Hsue-Shen Tsien (1954)Engineering Cybernetics,McGraw Hill, link fromHathiTrust

• Ontario News "pioneer in servo technology"
• Rane Pro Audio Reference definition of "servo-loop"
• Seattle Robotics Society's "What is a Servo?"Archived 2007-07-11 at theWayback Machine
• different types of servo motors"