Servomotor

Source: Wikipedia, the free encyclopedia.
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 on top of the motor is the optical rotary encoder for position feedback. This is the steering actuator of a large robot vehicle.
flange mountings
for interchangeability

A servomotor (or servo motor or simply servo)

motor coupled to a sensor for position feedback
and a controller (often a dedicated module designed specifically for servomotors).

Servomotors are not a specific class of motor, although the term servomotor is often used to refer to a motor suitable for use in a

automated manufacturing
.

Mechanism

A servomotor is a closed-loop servomechanism that uses position feedback (either linear or rotational position) to control its motion and final position. The input to its control is a signal (either analog or digital) representing the desired position of the output shaft.

The motor is paired with some type of

error signal
, which when fed back causes the motor to rotate in the direction needed to bring the shaft to the desired position. The error signal reduces to zero as the desired position is approached, stopping the motor.

Simple servomotors use position-only sensing via a

bang-bang control of their motor; the motor only rotates at full speed or is stopped. This type of servomotor is not widely used in industrial motion control, but it forms the basis of the simple and cheap servos used for radio-controlled models
.

More sophisticated servomotors make use of an

PID control algorithm, allow the servomotor to be brought to its commanded position more quickly and more precisely, with less overshooting.[5]

Servomotors vs. stepper motors

Servomotors are generally used as a high-performance alternative to the

absolute encoder
is used.

The lack of feedback of a stepper motor limits its performance, as the stepper motor can only drive a load that is well within its capacity, otherwise missed steps under load may lead to positioning errors and the system may have to be restarted or recalibrated. The encoder and controller of a servomotor are an additional cost, but they optimize the performance of the overall system (for all of speed, power, and accuracy) relative to the capacity of the basic motor. With larger systems, where a powerful motor represents an increasing proportion of the system cost, servomotors have the advantage.

There has been increasing popularity in closed-loop stepper motors in recent years.[

PID controller on a closed loop stepper system.[6]

Encoders

The first servomotors were developed with

anti-aircraft artillery during World War II.[8]

Simple servomotors may use

PID controllers
that can make use of such a speed signal, generally warrant a more precise encoder.

Modern servomotors use

absolute or incremental
. Absolute encoders can determine their position at power-on but are more complicated and expensive. Incremental encoders are simpler, cheaper, and work at faster speeds. Incremental systems, like stepper motors, often combine their inherent ability to measure intervals of rotation with a simple zero-position sensor to set their position at start-up.

Instead of servomotors, sometimes a motor with a separate, external linear encoder is used.[9] These motor + linear encoder systems avoid inaccuracies in the drivetrain between the motor and linear carriage, but their design is made more complicated as they are no longer a pre-packaged factory-made system.

Motors

The type of motor is not critical to a servomotor, and different types may be used.

variable frequency drives to allow control of their speed. For ultimate performance in a compact package, brushless AC motors with permanent magnet fields are used, effectively large versions of Brushless DC electric motors.[12]

Drive modules for servomotors are a standard industrial component. Their design is a branch of

H bridge. These standard modules accept a single direction and pulse count (rotation distance) as input. They may also include over-temperature monitoring, over-torque, and stall detection features.[13]
As the encoder type, gearhead ratio, and overall system dynamics are application specific, it is more difficult to produce the overall controller as an off-the-shelf module, and so these are often implemented as part of the main controller.

Control

Most modern servomotors are designed and supplied around a dedicated controller module from the same manufacturer. Controllers may also be developed around microcontrollers in order to reduce cost for large-volume applications.[14]

Integrated servomotors

Integrated servomotors are designed to include the motor, driver, encoder, and associated electronics into a single package.[15][16]

See also

References

  1. ^
    ISBN 978-0-19-883210-2. {{cite journal}}: Cite journal requires |journal= (help
    )
  2. ^ Sawicz, Darren. "Hobby Servo Fundamentals" (PDF). Archived (PDF) from the original on 2012-09-07. Retrieved 2012-10-12.
  3. ISBN 978-1-84800-336-1. Archived
    from the original on 21 March 2017.
  4. ISBN 978-1-4398-5901-8. Archived
    from the original on 21 March 2017.
  5. ISBN 978-3-642-20253-7. Archived
    from the original on 20 March 2017.
  6. ^ "Fastech Closed Loop Stepper Motors". Fastech Korea. Archived from the original on 2015-03-17.
  7. ^ Upson, A.R.; Batchelor, J.H. (1978) [1965]. Synchro Engineering Handbook. Beckenham: Muirhead Vactric Components. pp. 7, 67–90.
  8. ^ "Chapter 10". Naval Ordnance and Gunnery. Vol. 1. US Navy. 1957. Archived from the original on 2007-12-02.
  9. ^ "Accupoint™ Linear Encoders". Epilog Laser. Archived from the original on 2012-10-07.
  10. ^ "How to drive a servo motor & its industrial applications". Components CSE. Retrieved 31 January 2023.
  11. ^ "Brushless DC motor cores for servomotors". Maxon Motor. Archived from the original on 2013-12-25.
  12. Moog Inc. Archived
    from the original on 2012-10-13.
  13. ^ "Brushless PWM Servo Amplifiers" (PDF). Advanced Motion Control. Archived from the original (PDF) on 2014-11-27.
  14. ^ Chowdhury, Rasel. "Color detector and separator device".
  15. ISBN 978-1-60021-097-6. Archived
    from the original on 2018-05-13.
  16. ISBN 978-0-8247-4394-9. Archived
    from the original on 13 May 2018.

External links
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