A CROSSED-E TRANSFORMER (or pickoff) is two E-transformers placed at right angles to each
other. This type of error detector is capable of detecting error in both horizontal and vertical directions.
A CONTROL TRANSFORMER (CT), when used as a magnetic error detector, can rotate through
unlimited angles. The output of this type of CT is always an ac servo error signal that must be
demodulated if it is used with a dc servo motor.
A RATE GENERATOR (tachometer), when used in the velocity servo loop, is the heart of the
feedback loop. The tach senses velocity (speed) of the load. A tachometer can be either an ac or dc rate
generator. The output frequency of the ac tach is the same as the reference frequency, varying only in
phase depending on the direction of rotation.
MODULATORS are used to change a dc error signal into an ac input error signal for servo
amplifiers. This device is required when ac servo amplifiers are used instead of dc amplifiers.