mechanical Mechanical may refer to: Machine * Machine (mechanical), a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement * Mechanical calculator, a device used to perform the basic operations o ...

and

control engineering Control engineering, also known as control systems engineering and, in some European countries, automation engineering, is an engineering discipline that deals with control systems, applying control theory to design equipment and systems with d ...

, a servomechanism (also called servo system, or simply servo) is a

control system A control system manages, commands, directs, or regulates the behavior of other devices or systems using control loops. It can range from a single home heating controller using a thermostat controlling a domestic boiler to large industrial ...

for the position and its

time derivative A time derivative is a derivative of a function with respect to time, usually interpreted as the rate of change of the value of the function. The variable denoting time is usually written as t. Notation A variety of notations are used to denote th ...

s, such as

velocity Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...

, of a

mechanical system A machine is a physical system that uses power to apply forces and control movement to perform an action. The term is commonly applied to artificial devices, such as those employing engines or motors, but also to natural biological macromolec ...

. It often includes a

servomotor A servomotor (or servo motor or simply servo) is a rotary or linear actuator that allows for precise control of angular or linear position, velocity, and acceleration in a mechanical system. It constitutes part of a servomechanism, and consi ...

, and uses

closed-loop control A closed-loop controller or feedback controller is a control loop which incorporates feedback, in contrast to an ''open-loop controller'' or ''non-feedback controller''. A closed-loop controller uses feedback to control states or outputs of a dy ...

to reduce

steady-state error In systems theory, a system or a process is in a steady state if the variables (called state variables) which define the behavior of the system or the process are unchanging in time. In continuous time, this means that for those properties ''p'' ...

and improve dynamic response. In closed-loop control, 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 ...

is used to correct the action of the mechanism. In displacement-controlled applications, it usually includes a built-in encoder or other position feedback mechanism to ensure the output is achieving the desired effect. Following a specified motion trajectory is called servoing, where "servo" is used as a

verb A verb is a word that generally conveys an action (''bring'', ''read'', ''walk'', ''run'', ''learn''), an occurrence (''happen'', ''become''), or a state of being (''be'', ''exist'', ''stand''). In the usual description of English, the basic f ...

. The ''servo'' prefix originates from the

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

word ''servus'' meaning slave. 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 handle ...

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 A small number of cars in the 1920s, such as the 1925 Flint Model E-55, f ...

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 an automobile. The system is a servomechanism that takes over the car's throttle to maintain a steady sp ...

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 possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

hydraulic Hydraulics () 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 counterpart of pneumatics, which concer ...

, or

pneumatic Pneumatics (from Greek 'wind, breath') is the use of gas or pressurized air in mechanical systems. Pneumatic systems used in Industrial sector, industry are commonly powered by compressed air or compressed inert gases. A centrally located a ...

. 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 Energy transformation, 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, M ...

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 control engineering and applied mathematics that deals with the control system, control of dynamical systems in engineered processes and machines. The objective is to develop a model or algorithm governing the applic ...

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

Speed control

Speed control via a

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

is another type of servomechanism. The

steam engine A steam engine is a heat engine that performs Work (physics), 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 (locomotive), cyl ...

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

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

. Prior to World War II the

constant speed propeller Constant or The Constant may refer to: Mathematics * Constant (mathematics), a non-varying value * Mathematical constant, a special number that arises naturally in mathematics, such as or Other concepts * Control variable or scientific con ...

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

gas turbine A gas turbine or gas turbine engine is a type of Internal combustion engine#Continuous combustion, continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas gene ...

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 th ...

and

marine navigation Marine navigation is the art and science of steering a ship from a starting point (sailing) to a destination, efficiently and responsibly. It is an art because of the skill that the navigator must have to avoid the dangers of navigation, and it ...

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 (AAW) is the counter to aerial warfare and includes "all measures designed to nullify or reduce the effectiveness of hostile air action".AAP-6 It encompasses surface-based, subsurface ( submarine-launched), and air-ba ...

-gun control systems. Other examples are

fly-by-wire Fly-by-wire (FBW) is a system that replaces the conventional aircraft flight control system#Hydro-mechanical, manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted to electronic sig ...

systems in

aircraft An aircraft ( aircraft) is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or the Lift (force), dynamic lift of an airfoil, or, i ...

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

radio-controlled model A radio-controlled model (or RC model) is a model that is steerable with the use of radio control (RC). All types of model vehicles have had RC systems installed in them, including ground vehicles, boats, planes, helicopters and even submarine ...

s which use RC servos for the same purpose. Many

autofocus An autofocus (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 h ...

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 hard disk drive platter, pla ...

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

Servomotor

200px, Small R/C servo mechanism.
1. electric motor
2. position feedback potentiometer
3. reduction gear">potentiometer.html" ;"title="electric motor
2. position feedback potentiometer">electric motor
2. position feedback potentiometer
3. reduction gear
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 PID or Pid may refer to: Medicine * Pelvic inflammatory disease or pelvic inflammatory disorder, an infection of the upper part of the female reproductive system * Primary immune deficiency, disorders in which part of the body's immune system is ...

allow more precise control of position and thus faster achievement of a stable position (for a given motor power). Potentiometers are subject to drift 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,Clarence W. de Silva. Mechatronics: An Integrated Approach (2005). CRC Press. p. 675. "The terms ''stepper motor'', ''stepping motor'', and ''step motor'' are synonymous and are often u ...

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 the interdisciplinary study and practice of the design, construction, operation, and use of robots. Within mechanical engineering, robotics is the design and construction of the physical structures of robots, while in computer s ...

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

pulse-width modulation Pulse-width modulation (PWM), also known as pulse-duration modulation (PDM) or pulse-length modulation (PLM), is any method of representing a signal as a rectangular wave with a varying duty cycle (and for some methods also a varying peri ...

(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 f ...

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 is a two-port electronic circuit that uses electric power from a power su ...

or gain, 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 Prior to the invention of the steering engine, large steam-powered warships with manual steering needed huge crews to turn the rudder rapidly. The Royal Navy once used 78 men hauli ...

, 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-hull (watercraft), hulled steamship designed by Isambard Kingdom Brunel, and built by John Scott Russell & Co. at Millwall Iron Works on the River Thames, London, England. Powered by both sidewheels and a screw ...

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 Vauxhall ...

, 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 Reversing gear is a mechanism used to both control the direction of travel of a steam locomotive and adjust its engine's steam cutoff. Reversing lever The most common form of reversing gear uses a lever to engage (known as a ''Johnson bar'' in ...

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 engineering, electrical engineer who co-founded the Western Electric, Western Electric Manufacturing Company. Gray is best known for his Invention of the telephone, dev ...

Telautograph The telautograph is an ancestor of the modern fax machine. It transmits electrical signals representing the position of a pen or tracer at the sending station to repeating mechanisms attached to a pen at the receiving station, thus reproducing at ...

. Electrical servomechanisms require a power amplifier.

World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

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, thermionic 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. It ...

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 lo ...

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 RPC may refer to: Science and technology * Rational polynomial coefficient * Reactive Plastic Curtain, a carbon-dioxide-absorbing device used in some rebreather breathing sets * Regional Playback Control, a regional lockout technology for DVDs ...

) on HMS 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 from

MOSFET upright=1.3, Two power MOSFETs in amperes">A in the ''on'' state, dissipating up to about 100 watt">W and controlling a load of over 2000 W. A matchstick is pictured for scale. In electronics, the metal–oxide–semiconductor field- ...

thyristor A thyristor (, from a combination of Greek language ''θύρα'', meaning "door" or "valve", and ''transistor'' ) is a solid-state semiconductor device which can be thought of as being a highly robust and switchable diode, allowing the passage ...

devices. Small servos may use power

transistor A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...

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 (hysteresis, 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 ...

. More complex control systems use proportional control,

PID control PID or Pid may refer to: Medicine * Pelvic inflammatory disease or pelvic inflammatory disorder, an infection of the upper part of the female reproductive system * Primary immune deficiency, disorders in which part of the body's immune system i ...

, and state space control, which are studied in

modern control theory Modern may refer to: History *Modern history ** Early Modern period ** Late Modern period *** 18th century *** 19th century *** 20th century ** Contemporary history * Moderns, a faction of Freemasonry that existed in the 18th century Philosoph ...

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