Servomotor

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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. Servomotor.jpg
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.
Industrial servomotors and gearboxes, with standardised flange mountings for interchangeability 3417 09 Servogetriebe.jpg
Industrial servomotors and gearboxes, with standardised flange mountings for interchangeability

A servomotor (or servo motor or simply servo) [1] is a rotary or linear actuator that allows for precise control of angular or linear position, velocity, and acceleration in a mechanical system. [1] [2] It constitutes part of a servomechanism, and consists of a suitable motor coupled to a sensor for position feedback and a controller (often a dedicated module designed specifically for servomotors).

Contents

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 closed-loop control system. Servomotors are used in applications such as robotics, CNC machinery, and 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 position encoder to provide position feedback (and potentially also speed feedback in more sophisticated designs). The controller compares the measured position with the desired position to generate an 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 potentiometer and 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. [3]

More sophisticated servomotors make use of an absolute encoder (a type of rotary encoder) to calculate the shaft's position and infer the speed of the output shaft. [4] A variable-speed drive is used to control the motor speed. [5] Both of these enhancements, usually in combination with a PID control algorithm, allow the servomotor to be brought to its commanded position more quickly and more precisely, with less overshooting. [6]

Servomotors vs. stepper motors

Servomotors are generally used as a high-performance alternative to the stepper motor. Stepper motors have some inherent ability to control position, as they have built-in output steps. This often allows them to be used as an open-loop position control, without any feedback encoder, as their drive signal specifies the number of steps of movement to rotate, but for this, the controller needs to 'know' the position of the stepper motor on power up. Therefore, on the first power-up, the controller will have to activate the stepper motor and turn it to a known position, e.g. until it activates an end limit switch. This can be observed when switching on an inkjet printer; the controller will move the ink jet carrier to the extreme left and right to establish the end positions. A servomotor can immediately turn to whatever angle the controller instructs it to, regardless of the initial position at power up if an 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. [7] They act like servomotors but have some differences in their software control to get smooth motion. The main benefit of a closed-loop stepper motor is its relatively low cost. There is also no need to tune the PID controller on a closed loop stepper system. [8]

Encoders

The first servomotors were developed with synchros as their encoders. [9] Much work was done with these systems in the development of radar and anti-aircraft artillery during World War II. [10]

Simple servomotors may use resistive potentiometers as their position encoder. These are only used at the very simplest and cheapest level and are in close competition with stepper motors. They suffer from wear and electrical noise in the potentiometer track. Although it would be possible to electrically differentiate their position signal to obtain a speed signal, PID controllers that can make use of such a speed signal, generally warrant a more precise encoder.

Modern servomotors use rotary encoders, either 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. [11] 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. [12] At the simplest, brushed permanent magnet DC motors are used, owing to their simplicity and low cost. Small industrial servomotors are typically electronically commutated brushless motors. [13] For large industrial servomotors, AC induction motors are typically used, often with 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. [14]

Drive modules for servomotors are a standard industrial component. Their design is a branch of power electronics, usually based on a three-phase MOSFET or IGBT 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. [15] 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. [16]

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. [17]

Integrated servomotors

Integrated servomotors are designed to include the motor, driver, encoder, and associated electronics into a single package. [18] [19]

See also

Related Research Articles

Control theory is a field of control engineering and applied 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 desired state, while minimizing any delay, overshoot, or steady-state error and ensuring a level of control stability; often with the aim to achieve a degree of optimality.

<span class="mw-page-title-main">Electric motor</span> Machine that converts electrical energy into mechanical energy

An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of torque applied on the motor's shaft. An electric generator is mechanically identical to an electric motor, but operates in reverse, converting mechanical energy into electrical energy.

<span class="mw-page-title-main">Stepper motor</span> Electric motor for discrete partial rotations

A stepper motor, also known as step motor or stepping motor, is a Brushless DC electric motor that rotates in a series of small and discrete angular steps. Stepper motors can be set to any given step position without needing a position sensor for feedback. The step position can be rapidly increased or decreased to create continuous rotation, or the motor can be ordered to actively hold its position at one given step. Motors vary in size, speed, step resolution, and torque.

In mechanical and control engineering, a servomechanism is a control system for the position and its time derivatives, such as velocity, of a mechanical system. It often includes a servomotor, and uses closed-loop control to reduce steady-state error and improve dynamic response. In closed-loop control, error-sensing negative feedback 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. The servo prefix originates from the Latin word servus meaning slave.

<span class="mw-page-title-main">Synchronous motor</span> Type of AC motor

A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integer number of AC cycles. Synchronous motors use electromagnets as the stator of the motor which create a magnetic field that rotates in time with the oscillations of the current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field. Doubly fed synchronous motors use independently-excited multiphase AC electromagnets for both rotor and stator.

In control theory, an open-loop controller, also called a non-feedback controller, is a control loop part of a control system in which the control action is independent of the "process output", which is the process variable that is being controlled. It does not use feedback to determine if its output has achieved the desired goal of the input command or process setpoint.

<span class="mw-page-title-main">Brushless DC electric motor</span> Synchronous electric motor powered by an electronic controller

A brushless DC electric motor (BLDC), also known as an electronically commutated motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic controller to switch DC currents to the motor windings producing magnetic fields that effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the current pulses that control the speed and torque of the motor. It is an improvement on the mechanical commutator (brushes) used in many conventional electric motors.

<span class="mw-page-title-main">Rotary encoder</span> Electromechanical device

A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to analog or digital output signals.

<span class="mw-page-title-main">Synchro</span> Variable transformers used in control systems

A synchro is, in effect, a transformer whose primary-to-secondary coupling may be varied by physically changing the relative orientation of the two windings. Synchros are often used for measuring the angle of a rotating machine such as an antenna platform or transmitting rotation. In its general physical construction, it is much like an electric motor. The primary winding of the transformer, fixed to the rotor, is excited by an alternating current, which by electromagnetic induction causes voltages to appear between the Y-connected secondary windings fixed at 120 degrees to each other on the stator. The voltages are measured and used to determine the angle of the rotor relative to the stator.

<span class="mw-page-title-main">Motion control</span> Field of automation which studies how to precisely move parts of machines

Motion control is a sub-field of automation, encompassing the systems or sub-systems involved in moving parts of machines in a controlled manner. Motion control systems are extensively used in a variety of fields for automation purposes, including precision engineering, micromanufacturing, biotechnology, and nanotechnology. The main components involved typically include a motion controller, an energy amplifier, and one or more prime movers or actuators. Motion control may be open loop or closed loop. In open loop systems, the controller sends a command through the amplifier to the prime mover or actuator, and does not know if the desired motion was actually achieved. Typical systems include stepper motor or fan control. For tighter control with more precision, a measuring device may be added to the system. When the measurement is converted to a signal that is sent back to the controller, and the controller compensates for any error, it becomes a Closed loop System.

An electronic speed control (ESC) is an electronic circuit that controls and regulates the speed of an electric motor. It may also provide reversing of the motor and dynamic braking. Miniature electronic speed controls are used in electrically powered radio controlled models. Full-size electric vehicles also have systems to control the speed of their drive motors.

<span class="mw-page-title-main">Servo drive</span> Electronic amplifier used to power electric servomechanisms

A servo drive is an electronic amplifier used to power electric servomechanisms.

Mitchell Electronics Incorporated, founded in 1979, is a manufacturer of equipment to test and run servomotors, encoders and resolvers as well as various third-party electronic devices. Corporate headquarters are located in Athens, Ohio, which is also where manufacturing takes place. The company is an associate member of the Electrical Apparatus Service Association (EASA), an international electromechanical trade organization.

ST Robotics is a company based in Cambridge, England, and Princeton, New Jersey, United States. The company designs and manufactures low-cost bench-top industrial robot arms and purpose built Cartesian robots. The company has no sales force and sells their robotic arm products mainly through the Internet as "boxed robots" with distributors around the world.

<span class="mw-page-title-main">Rotary actuator</span> AE motor

A rotary actuator is an actuator that produces a rotary motion or torque.

Minor loop feedback is a classical method used to design stable robust linear feedback control systems using feedback loops around sub-systems within the overall feedback loop. The method is sometimes called minor loop synthesis in college textbooks, some government documents.

<span class="mw-page-title-main">Servo (radio control)</span> Servomotor or other type of actuator used for radio control and small-scale robotics

Servos are small, cheap, mass-produced servomotors or other actuators used for radio control and small-scale robotics.

Classical control theory is a branch of control theory that deals with the behavior of dynamical systems with inputs, and how their behavior is modified by feedback, using the Laplace transform as a basic tool to model such systems.

<span class="mw-page-title-main">High performance positioning system</span> Industrial Engineering method

A high performance positioning system (HPPS) is a type of positioning system consisting of a piece of electromechanics equipment (e.g. an assembly of linear stages and rotary stages) that is capable of moving an object in a three-dimensional space within a work envelope. Positioning could be done point to point or along a desired path of motion. Position is typically defined in six degrees of freedom, including linear, in an x,y,z cartesian coordinate system, and angular orientation of yaw, pitch, roll. HPPS are used in many manufacturing processes to move an object (tool or part) smoothly and accurately in six degrees of freedom, along a desired path, at a desired orientation, with high acceleration, high deceleration, high velocity and low settling time. It is designed to quickly stop its motion and accurately place the moving object at its desired final position and orientation with minimal jittering.

A direct-drive simulator steering wheel is a simulator steering wheel with a direct-drive mechanism between the drive and output, i.e. without gearing, and is used similarly as with other simulator steering wheels for providing torque feedback so that the driver, through movement in the steering wheel, gets an interface for sensing what is happening to the car in the simulator. It is an example of human–computer interaction in driving simulators, racing simulators, and racing video games, and is an example of haptic technology

References

  1. 1 2 Escudier, Marcel; Atkins, Tony (2019). "A Dictionary of Mechanical Engineering". doi:10.1093/acref/9780198832102.001.0001. 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. ↑ "MKD Series". www.wakeindustrial.com. Retrieved 2024-06-01.
  4. ↑ Suk-Hwan Suh; Seong Kyoon Kang; Dae-Hyuk Chung; Ian Stroud (22 August 2008). Theory and Design of CNC Systems. Springer Science & Business Media. pp. 11–. ISBN   978-1-84800-336-1. Archived from the original on 21 March 2017.
  5. ↑ Jacek F. Gieras (3 June 2011). Permanent Magnet Motor Technology: Design and Applications, Third Edition. CRC Press. pp. 26–. ISBN   978-1-4398-5901-8. Archived from the original on 21 March 2017.
  6. ↑ Ralf Der; Georg Martius (11 January 2012). The Playful Machine: Theoretical Foundation and Practical Realization of Self-Organizing Robots. Springer Science & Business Media. pp. 302–. ISBN   978-3-642-20253-7. Archived from the original on 20 March 2017.
  7. ↑ "Servo Motor vs. Stepper Motor - What's the Difference?". This vs. That. Retrieved 2024-06-01.
  8. ↑ "Fastech Closed Loop Stepper Motors". Fastech Korea. Archived from the original on 2015-03-17.
  9. ↑ Upson, A.R.; Batchelor, J.H. (1978) [1965]. Synchro Engineering Handbook. Beckenham: Muirhead Vactric Components. pp. 7, 67–90.
  10. ↑ "Chapter 10". Naval Ordnance and Gunnery. Vol. 1. US Navy. 1957. Archived from the original on 2007-12-02.
  11. ↑ "Accupoint™ Linear Encoders". Epilog Laser. Archived from the original on 2012-10-07.
  12. ↑ "How to drive a servo motor & its industrial applications". Components CSE. Retrieved 31 January 2023.
  13. ↑ "Brushless DC motor cores for servomotors". Maxon Motor. Archived from the original on 2013-12-25.
  14. ↑ "Compact Dynamic Brushless Servo Motor". Moog Inc. Archived from the original on 2012-10-13.
  15. ↑ "Brushless PWM Servo Amplifiers" (PDF). Advanced Motion Control. Archived from the original (PDF) on 2014-11-27.
  16. ↑ 20240175678,Binder, Yehuda,"Method and Apparatus for Cooperative Usage of Multiple Distance Meters",issued 2024-05-30
  17. ↑ Chowdhury, Rasel. "Color detector and separator device".
  18. ↑ Max A. Denket (2006). Frontiers in Robotics Research. Nova Publishers. pp. 44–. ISBN   978-1-60021-097-6. Archived from the original on 2018-05-13.
  19. ↑ Jacek F. Gieras (22 January 2002). Permanent Magnet Motor Technology: Design and Applications, Second Edition. CRC Press. pp. 283–. ISBN   978-0-8247-4394-9. Archived from the original on 13 May 2018.
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