Servos have countless applications in the operation of electrical and electronic equipment. In working with radar and antennas, directors, computing devices, ship's communications, aircraft control, and many other equipments, it is often necessary to operate a mechanical load that is remote from its source of control.

To obtain smooth, continuous, and accurate operation, these loads are normally best controlled by synchros. You may already know, the big problem here is that synchros are not powerful enough to do any great amount of work. This is where servos come into use.

A servo system uses a weak control signal to move large loads to a desired position with great accuracy. The key words in this definition are move and great accuracy. They may be found in such varied applications as moving the rudder and elevators of a model airplane in radio-controlled flight, to controlling the diving planes and rudders of nuclear submarines.

They also very powerful. They can move heavy loads and be remotely controlled with great precision by synchro devices. They take many forms. These systems are either electromechanical, electrohydraulic, hydraulic, or pneumatic. Whatever the form, a relatively weak signal that represents a desired movement of the load is generated, controlled, amplified, and fed to a servo motor that does the work of moving the heavy load.


A control system is a group of components that are linked together to perform a specific purpose. Generally, a control system has a large power gain between input and output. The components used in the system and the complexity of the system are directly related to the requirements of the system's application.

Control systems are broadly classified as either CLOSED-LOOP or OPEN-LOOP.

Closed-loop control systems are the type most commonly used in the Navy because they respond and move the loads they are controlling quicker and with greater accuracy than open-loop systems.

The reason for quicker response and greater accuracy is that an automatic feedback system informs the input that the desired movement has taken place. Upon receipt of this feedback information, the system stops the motor, and motion of the load ceases until another movement is ordered by the input.

This is similar to the system that controls heat in many homes. The thermostat (input) calls for heat. The furnace (output) produces heat and distributes it. Some of the heat is "fed back" to the thermostat. When this "feedback" raises the temperature of the room to that of the thermostat setting, the thermostat responds by shutting the system down until heat is again required.

In such a system, the feedback path, input to output and back to input, forms what is called a "closed loop." This is a term you will hear and use often in discussions of control systems. Because closed-loop control systems are automatic in nature, they are further classified by the function they serve (e.g., controlling the position, the velocity, or the acceleration of the load being driven).

An open-loop control system is controlled directly, and only, by an input signal, without the benefit of feedback. The basic units of this system are an amplifier and a motor. The amplifier receives a low- level input signal and amplifies it enough to drive the motor to perform the desired job. Open-loop control systems are not as commonly used as closed-loop control systems because they are less accurate

Open and closed loop servo systems

Basic Servos System Operation

Functional Loops beginning with the Position Loop

Velocity Loop


Clutch Damping systems

Frequency Response

Components and Circuits

Error detectors, summing network, and transformer types

Tachometer or (Rate Generator)

Modulators in the system

Demodulators in the system


Magnetic Amplifiers

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