Servomotor

servomotor [′sər·vō‚mōd·ər]

(control systems)

The electric, hydraulic, or other type of motor that serves as the final control element in a servomechanism; it receives power from the amplifier element and drives the load with a linear or rotary motion. Also known as servo.

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Servomotor 

a servomechanism that reproduces certain mechanical displacements of a controlled object by means of an actuating electric motor.

A servomotor includes a reference device, a measuring transducer, a comparator, an amplifier, and an actuating electric motor. The reference device produces the initial signal, which usually varies according to some arbitrary relationship. The measuring transducer continuously measures the actual value of the quantity being reproduced at the controlled object; this quantity is compared with the reference value by the comparator. The measuring transducer and the comparator are usually combined in a single unit that produces a deviation signal proportional to the difference between the reference value and the actual value of the quantity being reproduced. The deviation signal, in the form of a voltage or a current, then passes to the amplifier input and on to the actuating motor. The actuating motor moves the controlled object in such a way that the deviation signal is reduced. In the absence of a deviation signal, the rotor of the electric motor is at rest.

Servomotors provide continuous or discrete control. Continuous control affords an uninterrupted regulation of the voltage, or power, of the actuating motor as a function of the deviation signal. In the simplest case, this relationship is linear. Servomotors designed for discrete control are subdivided into relay and pulse drives. In relay drives, contactless relays serve as an amplifier. At a certain value of the deviation signal, the relays connect the actuating motor at the full power rating. In pulse drives, the actuating motor is connected to the circuit periodically at specified equal time intervals by pulses of the control current. The amplitude, duration, or frequency of these pulses are a function of the deviation signal.

In most servomotors the reference quantity is the rotation angle of the input shaft, and the controlled quantity is the rotation angle of the output shaft to which the controlled object is linked. In such systems the most widely used types of measuring transducers are potentiometers and alternating-current induction machines of the selsyn or rotary-transformer type. Servomotors are used in various control, data transmission, and telemetry systems.

REFERENCES

Elektroprivodsistem upravleniia letatel’nykh apparatov. Moscow, 1973.

Osnovy avtomatizirovannogo elektroprivoda. Moscow, 1974.

IU. A. KHOKHLOV