Servomechanism
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In
The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position, speed, attitude or any other measurable variables.[5] For example, an automotive power 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's cruise control uses closed-loop feedback, which classifies it as a servomechanism.
Applications
Position control
A common type of servo provides position control. Commonly, servos are
Speed control
Speed control via a
Others
Positioning servomechanisms were first used in military
Servomotor
A servomotor is a specific type of motor that is combined with a
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 radio-controlled models 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 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-scale robotics applications. A standard RC receiver (or a microcontroller) sends pulse-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.
History
James Watt's steam engine governor is generally considered the first powered feedback system. The windmill fantail is an earlier example of automatic control, but since it does not have an amplifier or 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 the ship's wheel. John McFarlane Gray was a pioneer. His patented design was used on the SS 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 by
Electrical servomechanisms were used as early as 1888 in Elisha Gray's Telautograph.
Electrical servomechanisms require a power amplifier.
Modern servomechanisms use solid state power amplifiers, usually built from MOSFET or thyristor devices. Small servos may use power transistors.
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 use
Types of performances
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.
The servo bandwidth indicates the capability of the servo to follow rapid changes in the commanded input.
See also
- Fractional horsepower motor
- Motion control
- Servo control
- Synchro, a form of transmitter and receiver motor used in servomechanisms
Further reading
- 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.
- McGraw Hill, link from HathiTrust
References
- ^ ISBN 978-0-19-883210-2.
- ^ Baldor Electric Company – Servo Control Facts. Accessed 25 September 2013
- ^ Anaheim Automation: Servo Motor Guide. Accessed 25 September 2013
- ^ Clarence W. de Silva. Mechatronics: An Integrated Approach (2005). CRC Press. p. 787.
- ^ BusinessDictionary.com definition Archived 2017-03-27 at the Wayback Machine. Accessed 25 September 2013
- ^ "How to drive a servo motor & its industrial applications". Components CSE. Retrieved 31 January 2023.
- ISBN 978-0-86341-047-5.
- ^ Andrew Betts Brown
- ^ Eugine L. Ragonnet, Controlling Mechanism for Locomotives, U.S. Patent 930,225, Aug. 9, 1909.
- ^ IEEE Industry Applications Magazine March/April 1996, pg 74
- ^ G. W. Younkin, Industrial Servo Control Systems – Fundamentals and Applications – Second Edition, Taylor and Francis, 2007.