Chapter 7—AUXILIARY MACHINERY
most commonly used when fine control is desired
is the servovalve (servocontrol). Servocontrol may
be defined as a control actuated by a feedback
system which compares the output signal with the
input or reference signal and makes corrections
to reduce the differences. The feedback signal may
be provided by fluid pressure, mechanical linkage,
electrical signal, or a combination of the three.
One type of hydraulic servovalve is illustrated
in figure 7-4. The valve is controlled by two
solenoids through an amplifier which energizes
either the right or left solenoid, depending on the
input signal. The valve shown in figure 7-5 has
the right solenoid energized; this causes the reed
to block the right nozzle and causes a pressure
increase in chamber A. The increased pressure
causes the spool valve to start sliding to the left.
As the spool valve moves, it uncovers the high
pressure line to chamber D (right side of the
piston) and the return line from the left side of
the piston through chamber E (nonpressure side
of the hydraulic system). The synchromotor is
geared to the actuator shaft. As the actuator
moves to the left, the synchromotor rotates and
produces a feedback signal to the amplifier. When
the feedback signal and the input signal are
matched, the solenoid is deenergized and the
magnetic reed returns to the neutral position. With
Figure 7-5.—Servovalve—actuated.
7-21
