A servo motor electric motors are referred to that permit the control of the angular position of its motor shaft, and the rotational speed and acceleration. They consist of an electric motor, which is additionally equipped with a sensor for determining the position. Determined by the sensor of rotation position of the motor shaft is continuously transmitted to an attached mostly outside the actual motor control electronics, the servo controller so-called of the movement of the motor in accordance with one or more adjustable control variables - such as the target angular position of the shaft or the target rotational speed - in a closed loop regulates.


The combination of servo motor and servo controllers together forms the servo drive. Servo motors are operated in a closed loop. The operation may be moments, velocity or position controlled. Combinations are possible due to the nesting of loops. This allows to adapt to different applications.

The term servo motor comes from its earlier application as an auxiliary drive ( Latin servus a slave ) and describes in contrast to engine designations such as DC motor or induction motor no physical principle. A servo drive is a DC motor, an induction motor or a synchronous motor, that include any type electric motor. The distinction to other engines is therefore not in the motor itself, but only in its control, which is operated in a closed loop ( as opposed to the stepper motor or Zuschaltbetrieb the three-phase network, such as at the star -delta connection ).

Design and operation

For accurate detection of the rotor position of each servo motor is provided with a measuring device, which the current position ( eg, the angle of rotation covered with respect to an initial position) determines the motor. This measurement is via a rotary encoder, such as a resolver, incremental encoder or an absolute encoder.

The electronic control system compares the signal of this sensor with a predetermined position setpoint. If there is a deviation, the motor is rotated in that direction, which ensures a lower path to the setpoint. This means that the deviation is 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, the position control. Alternatively, the torque and speed can be controlled by this principle. Thus, for example, uniform traffic profiles with fluctuating loads are possible.

For synchronous or asynchronous motors, a vector controller is usually used to inject the torque below the position and speed controller.

Areas of application

Servo motors have a variety of fields of application. They are often used in industrial plants, but also in various machines, such as machine tools, packaging machines and industrial robots. Servo motors are also found in servos as vehicle, aircraft or ship models are used for example in modeling in radio-controlled. In a servo motor, however, other types can be fitted. In common parlance, however, are the " power " and the " servo motor " confused.

Due to their higher reliability compared to rotary coil instruments servo motors are also used in display devices of aviation and the military.

For the connection as well as for integration with process computer peripherals servo motors are also available in modules with fieldbus interfaces. Such modules are offered with the fieldbus interface CANopen communication profile according to DS 301 and DSP 402 device profile for years. They are intended to act for automation as compact drives. (Function blocks, which are downstream of a soft PLC in the process computer, take in such cases, the control of the motors. ) DC servo motors are equipped among other things with the Profibus DP interface after PROFIDrive profile. Such engines are available with power outputs from about 40 to 500 W available. They cover only from the low-power range.