Electrical Power Systems Technology

Mechanical Systems 395

conveyor-belt control units, in spindle-speed control in machine-tool op-

erations, and in disk or magnetic-tape drives for computers. As a general

rule, a servo system is a rather complex unit that follows the commands of

a closed-loop control path. Figure 14-33 shows the components of a typical

servo system.

Figure 14-33. Block diagram of a typical servo system

The input of a servo system serves as the reference source, or as a set

point to which the load element responds. When the input is changed in

some way, a command is applied to the error detector. This device receives

data from both the input source and from the controlled output device. If a

correction is needed, with reference to the input command, it is amplified

and applied to the actuator. The actuator is normally a servomotor that

produces controlled shaft displacements. The controlled output device is

usually a system that relays information back to the error detector for po-

sition comparison.

A servomotor is primarily responsible for producing mechanical

changes from an electromagnetic actuating device. A device of this type

is normally coupled to the work load by a gear train or some mechani-

cal linkage. Both AC and DC servomotors are used to achieve this opera-

tion. As a general rule, a servomotor is unique, when compared with other

electric motors, because a servomotor is a very special type of device that

is used to achieve a precise degree of rotary motion. Servomotors, for ex-

ample, are designed to do something other than change electrical energy

into rotating mechanical energy. Motors of this type must first be able to

respond accurately to signals developed by the amplifier of the system.

Second, they must be capable of reversing direction quickly when a spe-