thumb|right|200px|Industrial servomotor The grey/green cylinder is the brush-type DC motor. The black section at the bottom contains the planetary reduction gear, and the black object atop the motor is the optical
encoderfor position feedback. This is the steering actuator of a large robot vehicle.]
A servomechanism, or servo is an automatic device which uses error-sensing
feedbackto correct the performance of a mechanism. The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position or other parameters. For example an automotive power window control is not a servomechanism, as there is no automatic feedback which controls position—the operator does this by observation. By contrast the car's cruise controluses closed loop feedback, which classifies it as a servomechanism.
Servomechanisms may or may not use a servomotor. For example a household furnace controlled by thermostat is a servomechanism, yet there is no closed-loop control of a servomotor.
A common type of servo provides "position control". Servos are commonly electrical or partially electronic in nature, using an
electric motoras the primary means of creating mechanical force. Other types of servos use hydraulics, pneumatics, or magneticprinciples. Usually, servos 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 sort of transducerat the output. Any difference between the actual and wanted values (an "error signal") is amplified and used to drive the system in the direction necessary to reduce or eliminate the error. An entire science known as control theoryhas been developed on this type of system.
Servomechanisms were first used in military
fire-controland marine navigationequipment. Today servomechanisms are used in automatic machine tools, satellite-tracking antennas, automatic navigation systems on boats and planes, and antiaircraft-gun control systems. Other examples are fly-by-wiresystems in aircraftwhich use servos to actuate the aircraft's control surfaces, and radio-controlled models which use RC servos for the same purpose. Many autofocuscameras also use a servomechanism to accurately move the lens, and thus adjust the focus. A modern hard disk drivehas a magnetic servo system with sub-micrometre positioning accuracy.
Typical servos give a rotary (angular) output. Linear types are common as well, using a screw thread or a
linear motorto give linear motion.
Another device commonly referred to as a servo is used in
automobiles to amplify the steering or braking forceapplied by the driver. However, these devices are not true servos, but rather mechanical amplifiers. (See also Power steeringor Vacuum servo.)
In industrial machines, servos are used to perform complex motion.
James Watt's steam enginegovernor, an automatic speed control, is generally considered the first powered feedback system. The windmill fantailis an earlier example of automatic control, but since it does not have an amplifieror gain, it is not usually considered a servomechanism.
The first feedback position control device was the ship
steering engine, used to position the rudder of large ships based on the position of ship's wheel. This technology was first used on the SS Great Easternin 1866. 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.
Electrical servomechanisms require a power amplifier.
World War IIsaw the development of electrical fire controlservomechanisms, using an amplidyneas the power amplifier. Vacuum tube amplifiers were used in the UNISERVOtape drive for the UNIVAC Icomputer.
Modern servomechanisms use solid state power amplifiers, usually built from
MOSFETor thyristordevices. Small servos may use power transistors.
The origin of the word is believed to come from the french “Le-Servomoteur” or slavemotor, first used by Farcot in 1868 to describe hydraulic and steam engines for use in ship steering. IEEE Industry Applications Magazine March/April 1996, pg 74 ]
2. position feedback
actuatorarm] RC servos are hobbyist remote control devices servos typically employed in radio-controlled models, where they are used to provide actuationfor various mechanical systems such as the steering of a car, the flaps on a plane, or the rudder of a boat.
RC servos are composed of a DC motor mechanically linked to a potentiometer.
Pulse-width modulation(PWM) signals sent to the servo are translated into position commands by electronics inside the servo. When the servo is commanded to rotate, the DC motor is powered until the potentiometer reaches the value corresponding to the commanded position.
Due to their affordability, reliability, and simplicity of control by microprocessors, RC servos are often used in small-scale
The servo is controlled by three wires: ground (usually black/orange), power (red) and control (brown/other colour). This wiring sequence is not true for all servos, for example the S03NXF Std. Servo is wired as brown(negative), red (positive) and orange (signal). 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 set the servo to its "neutral" position, or 90°. For example a servo pulse of 1.25 ms could set the servo to 0° and a pulse of 1.75 ms could set the servo to 180°. The physical limits and timings of the servo hardware varies between brands and models, but a general servo's angular motion will travel somewhere in the range of 180° - 210° and the neutral position is almost always at 1.5 ms.
Servo motors are usually powered from either NiCad or the more environmentally friendly NiMH packs common to most RC devices. Voltage ratings vary from product to product, but most servos are operated at 4.8 V or 6 V DC from a 4 or 5 cell battery.
Synchro, a form of transmitter and receiver motor used in servomechanisms
Stepper motor- a similar brushless, synchronous electric motor that can divide a full rotation into a large number of steps
* [http://www.rane.com/par-s.html#servo_loop Rane Pro Audio Reference definition of "servo-loop"]
* [http://www.seattlerobotics.org/guide/servos.html Seattle Robotics Society's "What is a Servo?"]
* [http://www.openservo.com/ Community-based project for creating a low-cost digital servo]
* [http://www.societyofrobots.com/actuators_servos.shtml Servo Tutorial for Robotics]
* [http://www.societyofrobots.com/actuators_modifyservo.shtml Tutorial on how to modify a servo for full 360 degree rotation]
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Look at other dictionaries:
Servomechanism — Ser vo*mech an*ism, n. an electronically controlled mechanical or hydraulic device permitting a large action or strong forces to be controlled by a small electrical signal. [PJC] … The Collaborative International Dictionary of English
servomechanism — ► NOUN ▪ a powered mechanism producing motion or forces at a higher level of energy than the input level, e.g. in the brakes and steering of large motor vehicles … English terms dictionary
servomechanism — [sʉr΄vō mek′ə niz΄əm] n. [< SERVOMOTOR + MECHANISM] an automatic control system in which the output is constantly or intermittently compared with the input through feedback so that the error or difference between the two quantities can be used … English World dictionary
servomechanism — servomechanical /serr voh meuh kan i keuhl/, adj. servomechanically, adv. /serr voh mek euh niz euhm, serr voh mek /, n. an electronic control system in which a hydraulic, pneumatic, or other type of controlling mechanism is actuated and… … Universalium
servomechanism — A type of control system in which a small signal or a small force is used to control a much larger force and in which output accurately follows the input even if it is varying rapidly. The system constantly compares the input and the output until … Aviation dictionary
servomechanism — sekimo mechanizmas statusas T sritis automatika atitikmenys: angl. servomechanism vok. Folgemechanismus, m; Folgewerk, n; Nachlaufsteuerung, f; Nachlaufwerk, n rus. следящий механизм, m pranc. dispositif d asservissement, m … Automatikos terminų žodynas
servomechanism — vykdiklis statusas T sritis automatika atitikmenys: angl. servodrive; servogear; servomechanism vok. Servomechanismus, m; Stellantrieb, m rus. исполнительный механизм, m; сервомеханизм, m; сервопривод, m pranc. mécanisme d asservissement, m;… … Automatikos terminų žodynas
servomechanism — valdomasis mechanizmas statusas T sritis automatika atitikmenys: angl. servomechanism vok. Servomechanismus, m rus. сервомеханизм, m pranc. servomécanisme, m … Automatikos terminų žodynas
servomechanism — vykdymo mechanizmas statusas T sritis automatika atitikmenys: angl. final controlling drive; servogear; servomechanism vok. Stellantrieb, m rus. исполнительный механизм, m; сервомеханизм, m pranc. mécanisme exécutif, m … Automatikos terminų žodynas
servomechanism — noun Etymology: servo (as in servomotor) + mechanism Date: 1926 an automatic device for controlling large amounts of power by means of very small amounts of power and automatically correcting the performance of a mechanism … New Collegiate Dictionary