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A servomotor (or servo motor or simply servo) is a rotary or
linear actuator A linear actuator is an actuator that creates linear motion (i.e., in a straight line), in contrast to the circular motion of a conventional electric motor. Linear actuators are used in machine tools and industrial machinery, in computer perip ...
that allows for precise control of angular or linear position, velocity, and acceleration in 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 constitutes part of a
servomechanism In mechanical and control engineering, a servomechanism (also called servo system, or simply servo) is a control system for the position and its time derivatives, such as velocity, of a mechanical system. It often includes a servomotor, and ...
, and consists of a suitable
motor An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gene ...
coupled to a
sensor A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...
for position
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 ...
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
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 ...
system. Servomotors are used in applications such as
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 ...
, 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 In mechanical engineering, mechanical and control engineering, a servomechanism (also called servo system, or simply servo) is a control system for the position and its time derivatives, such as velocity, of a mechanical system. It often includes ...
, 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 A potentiometer is a three- terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. The measuring instrum ...
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 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, includi ...
, but it forms the basis of the simple and cheap servos used for
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s. More sophisticated servomotors make use of an absolute encoder (a type of
rotary encoder A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angle, angular position or motion of a shaft or axle to Analog signal, analog or Digital signal, digital output signals. There are two main types of ...
) to calculate the shaft's position and infer the speed of the output shaft. A variable-speed drive is used to control the motor speed. Both of these enhancements, usually in combination with a
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algorithm, allow the servomotor to be brought to its commanded position more quickly and more precisely, with less overshooting.


Servomotors vs. stepper motors

Servomotors are generally used as a high-performance alternative to the
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 ...
. 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 Inkjet printing is a type of printer (computing), computer printing that recreates a digital image by propelling droplets of ink onto paper or plastic substrates. Inkjet printers were the most commonly used type of printer in 2008, and range f ...
; 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. 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
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on a closed loop stepper system.


Encoders

The first servomotors were developed with
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s as their encoders. Much work was done with these systems in the development of
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and
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during
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. 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 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 ...
s that can make use of such a speed signal, generally warrant a more precise encoder. Modern servomotors use
rotary encoder A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angle, angular position or motion of a shaft or axle to Analog signal, analog or Digital signal, digital output signals. There are two main types of ...
s, either
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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. 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. 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. For large industrial servomotors,
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are typically used, often with
variable frequency drive A variable-frequency drive (VFD, or adjustable-frequency drive, adjustable-speed drive, variable-speed drive, AC drive, micro drive, inverter drive, variable voltage variable frequency drive, or drive) is a type of AC motor drive (system inco ...
s 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. Drive modules for servomotors are a standard industrial component. Their design is a branch of
power electronics Power electronics is the application of electronics to the control and conversion of electric power. The first high-power electronic devices were made using mercury-arc valves. In modern systems, the conversion is performed with semiconduct ...
, usually based on a three-phase
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- ...
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. 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
microcontroller A microcontroller (MC, uC, or μC) or microcontroller unit (MCU) is a small computer on a single integrated circuit. A microcontroller contains one or more CPUs (processor cores) along with memory and programmable input/output peripherals. Pro ...
s in order to reduce cost for large-volume applications.


Integrated servomotors

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


See also

* Direct-drive sim racing wheel


References


External links

* {{Authority control Actuators Control devices Electric motors Motion control