The DC Rate Generator
The dc rate generator uses the same principles of magnetic coupling as the ac rate generator. The dc
rate generator, however, has a steady (nonfluctuating) primary magnetic field. This magnetic field is
usually supplied by permanent magnets. The amount of voltage induced in the rotor winding is
proportional to the number of magnetic flux lines cut. The polarity of the output voltage is determined by
the direction in which the rotor cuts the lines of magnetic flux.
The physical makeup and theory of operation of the dc rate generator (tach) is very similar to the dc
generator (NEETS, Module 5, Introduction to Generators and Motors). The only major differences are
size and the prime mover. The tach is much smaller and is linked mechanically to the servo motor or load
instead of to a prime mover.
Tachometer generators are used in servo systems to supply velocity or damping signals and are
sometimes mounted on or in the same housing as the servo motor.
Q-21. What is the basic difference between the primaries of ac and dc rate generators?
MODULATORS IN THE SERVO SYSTEM
Because of problems associated with dc amplifiers, such as drift (where the output varies with no
variation of the input signal), the ac amplifier is more widely used in servo applications. This creates a
need for a device to convert a dc error signal into an ac input for the servo amplifier. Such a device is
referred to as a MODULATOR.
Modulator and modulating techniques vary with different types of electronic equipment. The
modulator in the servo system performs a completely different function than its counterparts in radar or
The servo modulator converts a dc error signal into an ac error signal. The modulator uses two inputs
to produce the ac error signal. One input is the dc error signal (for example from an input potentiometer);
the other input is an ac reference voltage from some other source, such as the swp's ac supply system. The
ac output error signal must contain the same control information that is contained in the original dc error
signal. This is done in the following manner:
1. The phase between the ac output and the ac reference signal is determined by the polarity of the
dc input signal. The phase of the ac output indicates the direction of error (direction of the load
2. The amplitude of the ac output is proportional to the amplitude of the dc input signal and
indicates the amount of error signal (speed or angular displacement of the load).
These relationships of phase and amplitude must be maintained to ensure that the load will move the
desired amount, or the proper speed, and in the right direction.
A typical modulator that you will see in a servo system is the CRYSTAL DIODE MODULATOR.
The following paragraphs provide a brief explanation of how this modulator works.
Crystal Diode Modulators
The crystal diode modulator (fig. 2-17) consists of a diode bridge and a transformer network. When
the ac reference voltage is applied to transformer T1, diodes CR
and CR3 conduct during the negative
half-cycle. Conversely, diodes CR1 and CR4 conduct on the positive half-cycle. The diodes will conduct