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automatic device for the control of a large power output by means of a small power input or for maintaining correct operating conditions in a mechanism. It is a type of feedbackfeedback,
arrangement for the automatic self-regulation of an electrical, mechanical, or biological system by returning part of its output as input. A simple example of feedback is provided by a governor on an engine; if the speed of the engine exceeds a preset limit, the
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 control system. The constant speed control system of a DC motor is a servomechanism that monitors any variations in the motor's speed so that it can quickly and automatically return the speed to its correct value. Servomechanisms are also used for the control systems of guided missiles, aircraft, and manufacturing machinery.



a system for automatic regulation or control. In such a system a control input is reproduced as an output with a specified accuracy and modified according to some previously unknown relationship. The physical nature and design of a servomechanism may vary from case to case. The block diagram in Figure 1 demonstrates the general operating principle of a servomechanism.

Figure 1. Block diagram of a servomechanism: g(t) is the reference input value, n(t) is interference, ∊ is the deviation signal, u is the control signal, f(t) is the disturbance, and x is the output value; (1) comparator, (2) amplifier-converter, (3) actuating device, (4) main feedback loop, (5) auxiliary (local) feedback loop

One of the principal components of a servomechanism is the comparator, which compares the output value x actually obtained with the reference input value g(t) and produces a deviation signal ∊ = g(t) – x. The transfer of the value of x from the output to the input is made through a negative feedback loop, in which the sign of x is reversed. Since the specification of values calls for x = g(t), the deviation signal ∊ represents the error of the servomechanism. In a satisfactorily operating servo-mechanism, this error must be sufficiently small; therefore, the signal ∊ is first amplified and then converted to a new signal u, which operates the actuating device. This actuating device changes x in such a way as to null the deviation. However, because of the presence of various disturbances f(t) and interference n(t), the deviation signal is repeatedly reproduced; the servomechanism must thus operate continuously, attempting to null the deviation signal, that is, to “track” it and, as a consequence, to track the reference value g(t).

In order to implement the control process with sufficient accuracy, special compensators that constitute part of the amplifier-converter are used, as well as local feedback loops. As a result, the signal u depends in a fairly complicated way on ∊ and on the parameters of the actuating device itself. In some cases the servomechanism reproduces the input value g(t) on a different scale as x(f)= kg(t), where k is the scale coefficient. The

Figure 2. Diagram of a servomechanism in which the output shaft operation is a function of the rotation angle of the input shaft: θ1(t) and θ2 are the rotation angles of the input and output shafts, respectively; (SS) selsyn sensor, (SR) selsyn receiver, (∊) deviation signal, (AC) amplifier-converter, (G) generator, (M) motor, (RG) reduction gearing

value may also be reproduced according to a more complicated relationship: x(t) = F[g(t)].

An example of a servomechanism is shown in Figure 2, where the operating cycle of an output shaft is a function of an arbitrarily specified rotation angle at the input θ1(t). The deviation signal ∊ = – θ2 is produced by selsyns—a sensor and a receiver—connected in a transformer circuit, with the receiver connected to the output shaft. A generator-motor system equipped with reduction gearing functions as the actuating device. The disturbance in this case consists of any change in the load on the output shaft.

The operating principle of a servomechanism is also used in guidance systems. In the servomechanism of a radar antenna, the angular deviation between the radar beam and the direction to the target serves as the deviation signal; the actuating device is the electric drive of the antenna. The automatic pilot of a guided missile also operates on the same principle as a servomechanism; in this case the deviation signal is provided by the deviation of the missile from the direction of the beam, while the steering mechanism and rudders function as the actuating device. Many other remote-control and automatic guidance systems use the operating principle of a servomechanism.

Measuring instruments that use a compensation principle are also servomechanisms. In such instruments the difference between the reading of the instrument and the value measured at the input serves as the deviation signal. Some computer devices also use the servomechanism principle. Servomechanisms whose output value is a mechanical displacement are called positional servomechanisms. Some living organisms exhibit servo-mechanism processes.

In general, the design calculations for servomechanisms are based on the theory of automatic regulation and control. Servo-mechanisms may feature continuous linear or nonlinear control, or they may incorporate discrete control in a relay, pulse, or digital format. These differences affect the choice of the method used in calculating the dynamic parameters of the system. In addition, each unit and component is also subject to engineering calculations. One of the principal goals of these calculations is the synthesis of the compensators. Such synthesis is based on requirements that specify the quality of the control process.


Proektirovanie i raschel slediashchikh sistem. Leningrad, 1964.
Kochetkov, V. T., A. M. Polovko, and V. M. Ponomarev. Teoriia sistem teleupravleniia i samonavedeniia raket. Moscow, 1964.
Voronov, A. A. Osnovy teorii avtomaticheskogo upravleniia, parts 1–3. Moscow-Leningrad, 1965–70.
Besekerskii, V. A., and E. P. Popov. Teoriia sistem avtomaticheskogo regulirovaniia, 3rd ed. Moscow, 1975.


(control systems)
An automatic feedback control system for mechanical motion; it applies only to those systems in which the controlled quantity or output is mechanical position or one of its derivatives (velocity, acceleration, and so on). Also known as servo system.


A system for the automatic control of motion by means of feedback. The term servomechanism, or servo for short, is sometimes used interchangeably with feedback control system (servosystem). In a narrower sense, servomechanism refers to the feedback control of a single variable (feedback loop or servo loop). In the strictest sense, the term servomechanism is restricted to a feedback loop in which the controlled quantity or output is mechanical position or one of its derivatives (velocity and acceleration). See Control systems

The purpose of a servomechanism is to provide one or more of the following objectives: (1) ac­curate control of motion without the need for human attendants (automatic control); (2) maintenance of accuracy with mechanical load variations, changes in the environment, power supply fluctuations, and aging and deterioration of components (regulation and self-calibration); (3) control of a high-power load from a low-power command signal (power amplification); (4) control of an output from a remotely located input, without the use of mechanical linkages (remote control, shaft repeater).

The illustration shows the basic elements of a servomechanism and their interconnections; in this type of block diagram the connection between elements is such that only a unidirectional cause-and-effect action takes place in the direction shown by the arrows. The arrows form a closed path or loop; hence this is a single-loop servomechanism or, simply, a servo loop. More complex servomechanisms may have two or more loops (multiloop servo), and a complete control system may contain many servomechanisms. See Block diagram

Servomechanisms were first used in speed governing of engines, automatic steering of ships, automatic control of guns, and electromechanical analog computers. Today, servomechanisms are employed in almost every industrial field. Among the applications are cutting tools for discrete parts manufacturing, rollers in sheet and web processes, elevators, automobile and aircraft engines, robots, remote manipulators and teleoperators, telescopes, antennas, space vehicles, mechanical knee and arm prostheses, and tape, disk, and film drives. See Computer storage technology, Flight controls, Remote manipulators, Robotics


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A type of control system in which a small signal or a small force is used to control a much larger force and in which output accurately follows the input even if it is varying rapidly. The system constantly compares the input and the output until the error signal becomes zero.


a mechanical or electromechanical system for control of the position or speed of an output transducer. Negative feedback is incorporated to minimize discrepancies between the output state and the input control setting