Servo motor

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Servo motor assembly integrated in a plotter

As a servomotor specific to electric motors referred to the control of the angular position of its motor shaft and the rotational speed and acceleration allow. They consist of an electric motor, which is also equipped with a sensor for determining the position. The rotational position of the motor shaft determined by the sensor is continuously transmitted to control electronics , the so-called servo controller, which are usually attached outside of the actual motor, which control the movement of the motor according to one or more adjustable setpoints - such as the setpoint angular position of the shaft or setpoint speed - in a control loop regulates.

An electric stepper motor can also be controlled in a targeted manner, but due to the lack of sensors and control circuits, it has various disadvantages compared to the servo motor, especially at high loads.


The combination of servo motor and servo controller together forms the servo drive . Servomotors are operated in a closed control loop. Operation can be torque, speed or position controlled. Combinations are possible by nesting the control loops. This enables adaptation to a wide variety of applications.

The term servo motor comes from its earlier field of application as an auxiliary drive (servus lat. Servant) and, in contrast to motor names such as DC motor or induction motor, does not describe a physical operating principle. A servo drive can contain a direct current motor , an asynchronous motor or a synchronous motor , i.e. any type of electric motor. The difference to other motors does not lie in the motor itself, but solely in its control, which is operated in a closed control loop (in contrast to a stepper motor or connection operation on the three-phase network such as star-delta connection ).

Structure and functionality

For the exact detection of the rotor position, each servomotor is provided with a measuring device which determines the current position (e.g. the covered angle of rotation with respect to an initial position) of the motor. This measurement takes place via a rotary encoder, e.g. B. via a resolver , an incremental encoder or an absolute encoder .

The electronic control compares the signal from this encoder with a specified position setpoint . If there is a discrepancy, the motor is turned in the direction that ensures a shorter travel distance to the setpoint. This leads to the deviation being reduced. The procedure is repeated until the current value is incrementally or via approximation within the tolerance limits of the setpoint. This is the simplest case, position control . Alternatively, this principle can also be used to regulate torque and speed. This enables, for example, uniform travel profiles with fluctuating loads.

In the case of synchronous or asynchronous motors , a vector controller is usually used below the position and speed controller to impress the torque .

application areas

Servomotors have a wide variety of fields of application. They are often used in industrial systems, but also in various machines , for example in machine tools , packaging machines or industrial robots . Servo
motors can also be found in servos , such as those used in model making in remote-controlled vehicle, aircraft or ship models. However, other types of motors can also be installed in a servo. In common usage, however, the “servo” and the “servo motor” are confused with one another.

Due to their higher reliability compared to moving coil instruments, servomotors are also used in display devices in aviation and the military.

Servomotors are also offered in modules with fieldbus interfaces for connection and integration in process computer peripherals. Such modules have been offered for years with the CANopen fieldbus interface according to the DS 301 communication profile and DSP 402 device profile. They are intended to act as compact drives for automation. (Function blocks that are connected to a soft PLC in the process computer take over the control of the motors in such cases.) DC servomotors are also equipped with the Profibus DP interface according to the PROFIdrive profile. Such motors are available with power outputs of around 40 to 500 W. They only cover the low-power range.

See also


  • Henneberger, G .: Modern servomotors for machine tools and handling devices: auxiliary sheets for the lecture . Institute for Electrical Machines, RWTH Aachen University, Aachen 1994.
  • Dote, Yasuhiko (Ed.): Brushless servomotors: fundamentals and applications. Clarendon Press, Oxford 1990, ISBN 0-19-859372-4 .
  • Moreton, Peter: Industrial brushless servomotors. Newnes, Oxford 2000, ISBN 0-7506-3931-8 .