control engineering Control engineering or control systems engineering is an engineering discipline that deals with control systems, applying control theory to design equipment and systems with desired behaviors in control environments. The discipline of controls o ...

a servomechanism, usually shortened to servo, is an automatic device that uses error-sensing

negative feedback Negative feedback (or balancing feedback) occurs when some function (Mathematics), function of the output of a system, process, or mechanism is feedback, fed back in a manner that tends to reduce the fluctuations in the output, whether caused by ...

to correct the action of a mechanism. On displacement-controlled applications, it usually includes a built-in encoder or other position feedback mechanism to ensure the output is achieving the desired effect. The term correctly applies only to systems where the

feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...

or error-correction signals help control mechanical position, speed, attitude or any other measurable variables. For example, an automotive

power window Power windows or electric windows are automobile windows which can be raised and lowered by pressing a button or switch, as opposed to using a crank handle. History Packard had introduced hydraulic window lifts (power windows) in fall of ...

control is not a servomechanism, as there is no automatic feedback that controls position—the operator does this by observation. By contrast a car's

cruise control Cruise control (also known as speed control, cruise command, autocruise, or tempomat) is a system that automatically controls the speed of a motor vehicle. The system is a servomechanism that takes over the throttle of the car to maintain a ste ...

uses closed-loop feedback, which classifies it as a servomechanism.

Applications

Position control

A common type of servo provides ''position control''. Commonly, servos are

electric Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...

hydraulic Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counter ...

, or

pneumatic Pneumatics (from Greek ‘wind, breath’) is a branch of engineering that makes use of gas or pressurized air. Pneumatic systems used in Industrial sector, industry are commonly powered by compressed air or compressed inert gases. A central ...

. 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 of

transducer A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, measurement, and contr ...

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 of

control theory Control theory is a field of mathematics that deals with the control of dynamical systems in engineered processes and machines. The objective is to develop a model or algorithm governing the application of system inputs to drive the system to a ...

. Typical servos can give a rotary (angular) or linear output.

Speed control

Speed control via a

governor A governor is an administrative leader and head of a polity or political region, ranking under the head of state and in some cases, such as governors-general, as the head of state's official representative. Depending on the type of political ...

is another type of servomechanism. The

steam engine A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be trans ...

uses mechanical governors; another early application was to govern the speed of

water wheels A water wheel is a machine for converting the energy of flowing or falling water into useful forms of power, often in a watermill. A water wheel consists of a wheel (usually constructed from wood or metal), with a number of blades or bucket ...

. Prior to World War II the

constant speed propeller In aeronautics, a variable-pitch propeller is a type of propeller (airscrew) with blades that can be rotated around their long axis to change the blade pitch. A controllable-pitch propeller is one where the pitch is controlled manually by the ...

was developed to control engine speed for maneuvering aircraft. Fuel controls for

gas turbine A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...

engines employ either hydromechanical or electronic governing.

Others

Positioning servomechanisms were first used in military

fire-control A fire-control system (FCS) is a number of components working together, usually a gun data computer, a Director (military), director, and radar, which is designed to assist a ranged weapon system to target, track, and hit a target. It performs ...

and

marine navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...

equipment. Today servomechanisms are used in automatic machine tools, satellite-tracking antennas, remote control airplanes, automatic navigation systems on boats and planes, and

antiaircraft Anti-aircraft warfare, counter-air or air defence forces is the battlespace response to aerial warfare, defined by NATO as "all measures designed to nullify or reduce the effectiveness of hostile air action".AAP-6 It includes surface based, ...

-gun control systems. Other examples are

fly-by-wire Fly-by-wire (FBW) is a system that replaces the conventional manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires, and flight control co ...

systems in

aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines ...

which use servos to actuate the aircraft's control surfaces, and

radio-controlled model A radio-controlled model (or RC model) is a Physical model, model that is steering, steerable with the use of radio control. All types of model vehicles have had RC systems installed in them, including radio-controlled car, ground vehicles, radio ...

s which use RC servos for the same purpose. Many

autofocus An autofocus (or AF) optical system uses a sensor, a control system and a motor to focus on an automatically or manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system ...

cameras also use a servomechanism to accurately move the lens. A

hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...

has a magnetic servo system with sub-micrometer positioning accuracy. In industrial machines, servos are used to perform complex motion, in many applications.

Servomotor

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4. actuator arm A ''servomotor'' is a specific type of motor that is combined with a rotary encoder or a potentiometer 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 a

PID controller A proportional–integral–derivative controller (PID controller or three-term controller) is a control loop mechanism employing feedback that is widely used in industrial control systems and a variety of other applications requiring continuou ...

allow more precise control of position and thus faster achievement of a stable position (for a given motor power). Potentiometers are subject to

drift Drift or Drifts may refer to: Geography * Drift or ford (crossing) of a river * Drift, Kentucky, unincorporated community in the United States * In Cornwall, England: ** Drift, Cornwall, village ** Drift Reservoir, associated with the village ...

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 inexpensive radio control servos (RC servos) used in

s which use a free-running motor and a simple potentiometer position sensor with an embedded controller. The term ''servomotor'' generally refers to a high-end industrial component while the term ''servo'' is most often used to describe the inexpensive devices that employ a potentiometer.

Stepper motor A stepper motor, also known as step motor or stepping motor, is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can be commanded to move and hold at one of these steps without any pos ...

s 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. 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-scale

robotics Robotics is an interdisciplinary branch of computer science and engineering. Robotics involves design, construction, operation, and use of robots. The goal of robotics is to design machines that can help and assist humans. Robotics integrat ...

applications. A standard RC receiver (or a microcontroller) sends

pulse-width modulation Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts. The average value of voltage (and current) fed ...

(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.

History

James Watt James Watt (; 30 January 1736 (19 January 1736 OS) – 25 August 1819) was a Scottish inventor, mechanical engineer, and chemist who improved on Thomas Newcomen's 1712 Newcomen steam engine with his Watt steam engine in 1776, which was fun ...

is generally considered the first powered feedback system. The

windmill fantail A fantail is a small windmill mounted at right angles to the sails, at the rear of the windmill, and which turns the cap automatically to bring it into the wind. The fantail was patented in 1745 by Edmund Lee, a blacksmith working at Brockmill For ...

is an earlier example of automatic control, but since it does not have an

amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...

gain Gain or GAIN may refer to: Science and technology * Gain (electronics), an electronics and signal processing term * Antenna gain * Gain (laser), the amplification involved in laser emission * Gain (projection screens) * Information gain in de ...

, it is not usually considered a servomechanism. The first feedback position control device was the ship

steering engine A steering engine is a power steering device for ships. History The first steering engine with feedback was installed on Isambard Kingdom Brunel's ''Great Eastern'' in 1866. Designed by Scottish engineer John McFarlane Gray and built by George ...

, used to position the rudder of large ships based on the position of the ship's wheel.

John McFarlane Gray John McFarlane Gray (7 April 1831 – 14 January 1908), also known as J. Macfarlane Gray, was a Scottish engineer who invented a portable steam riveting machine and a steam steering mechanism for Isambard Kingdom Brunel's famous ''SS Great Easte ...

was a pioneer. His patented design was used on the

SS Great Eastern SS ''Great Eastern'' was an iron sail-powered, paddle wheel and screw-propelled steamship designed by Isambard Kingdom Brunel, and built by John Scott Russell & Co. at Millwall Iron Works on the River Thames, London. She was the largest ship ev ...

in 1866.

Joseph Farcot Jean Joseph Léon Farcot (23 June 1824 – 19 March 1908) was a French engineer and industrialist whose factories employed up to 700 workers. He was also a prolific inventor. He was one of the pioneers of the servomechanism, where a feedback loop ...

may deserve equal credit for the feedback concept, with several patents between 1862 and 1868. The telemotor was invented around 1872 by

Andrew Betts Brown Andrew Betts Brown MICE MINA (1841–1906) was a Scottish engineer and inventor. He invented the hydraulic crane and overhead travelling crane. He founded several companies including the Vauxhall Iron Works, which later evolved into Vauxhal ...

, allowing elaborate mechanisms between the control room and the engine to be greatly simplified. 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 Ragonnet

power reverse On a steam locomotive, the reversing gear is used to control the direction of travel of the locomotive. It also adjusts the cutoff of the steam locomotive. Reversing lever This is the most common form of reverser. It consists of a long lever moun ...

mechanism was a general purpose air or steam-powered servo amplifier for linear motion patented in 1909. Electrical servomechanisms were used as early as 1888 in

Elisha Gray Elisha Gray (August 2, 1835 – January 21, 1901) was an American electrical engineer who co-founded the Western Electric Manufacturing Company. Gray is best known for his development of a telephone prototype in 1876 in Highland Park, Illinois. ...

Telautograph The telautograph is an analog precursor to the modern fax machine. It transmits electrical impulses recorded by potentiometers at the sending station to servomechanisms attached to a pen at the receiving station, thus reproducing at the receivin ...

. Electrical servomechanisms require a power amplifier.

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

saw the development of electrical

servomechanisms, using an

amplidyne An amplidyne is an obsolete electromechanical amplifier invented prior to World War II by Ernst Alexanderson. It consists of an electric motor driving a DC generator. The signal to be amplified is applied to the generator's field winding, and ...

as the power amplifier.

Vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...

amplifiers were used in the

UNISERVO The UNISERVO tape drive was the primary I/O device on the UNIVAC I computer. It was the first tape drive for a commercially sold computer. The UNISERVO used metal tape: a thin strip of nickel-plated phosphor bronze (called Vicalloy) 1200 feet lon ...

tape drive for the

UNIVAC I The UNIVAC I (Universal Automatic Computer I) was the first general-purpose electronic digital computer design for business application produced in the United States. It was designed principally by J. Presper Eckert and John Mauchly, the invento ...

computer. The Royal Navy began experimenting with Remote Power Control ( RPC) on

HMS Champion Five ships of the Royal Navy have borne the name HMS ''Champion'': * was a 24-gun sixth rate launched in 1779. She was reassigned to harbour service in 1810 and sold in 1816. * was an 18-gun sloop launched in 1824. She was reassigned to harb ...

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 from

MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...

thyristor A thyristor () is a solid-state semiconductor device with four layers of alternating P- and N-type materials used for high-power applications. It acts exclusively as a bistable switch (or a latch), conducting when the gate receives a current ...

devices. Small servos may use power

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...

s. 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.IEEE Industry Applications Magazine March/April 1996, pg 74 The simplest kind of servos use

bang–bang control In control theory, a bang–bang controller (2 step or on–off controller), is a feedback controller that switches abruptly between two states. These controllers may be realized in terms of any element that provides hysteresis. They are often use ...

. More complex control systems use proportional control,

PID control A proportional–integral–derivative controller (PID controller or three-term controller) is a control loop mechanism employing feedback that is widely used in industrial control systems and a variety of other applications requiring continuous ...

, and state space control, which are studied in modern control theory.

Types of performances

Servos can be classified by means of their feedback control systems:G. W. Younkin, Industrial Servo Control Systems – Fundamentals and Applications – Second Edition, Taylor and Francis, 2007. * 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. The

servo bandwidth Servo bandwidth is the maximum trackable sinusoidal frequency of amplitude A, with tracking achieved at or before 10% of A amplitude is reached. The servo bandwidth indicates the capability of the servo to follow rapid changes in the commanded inpu ...

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

References

External links

Ontario News "pioneer in servo technology"

{{Authority control Control theory Control devices Mechanical amplifiers