DESCRIPTION AND OPERATION
HSS MODULE CIRCUITRY
2 - 8
I-E96-322A
®
DIGITAL-TO-ANALOG CONVERTER
The D/A converter receives the position demand from the
microprocessor. It converts the data from a digital word to an
equivalent analog signal. The D/A converter outputs the ana-
log position demand to the position error circuit.
POSITION ERROR
The position error circuit compares position demand with posi-
tion feedback. The output of this circuit is the difference
between the position feedback and the position demand signal.
This signal goes to the servo amplifier. The position error out-
put is zero when position demand and position feedback are
equal. When the amplifier output is zero, indicating that the
actuator is in the correct position, the servo amplifier holds the
servo valve in place and the actuator holds its position.
SERVO AMPLIFIER
The servo amplifier provides the power to drive the servo valve.
There are two servo drive outputs in parallel providing redun-
dancy in the control output. Each output connects to separate
coils. Servo valve operation requires only one functioning coil.
One output can short to ground or open without affecting the
other servo drive output. If one servo drive output fails, the
other output is able to drive the servo valve.
DITHER OSCILLATOR
The dither oscillator introduces an oscillation of small magni-
tude to the servo valve to help it overcome the effect of friction.
In systems where valve position rarely changes, the dither
oscillator prevents the valve from freezing in one position.
Position Feedback
There are three parts to the position feedback block (see
Figure
•
Demodulator.
•
A/D converter.
•
LVDT oscillator.
DEMODULATOR
The LVDT secondaries respond to the movement of the hydrau-
lic actuator by generating a differential voltage that is propor-
tional to the position of a transformer core that moves as the
actuator moves. Since the LVDT secondary is linear, the
induced voltage on the transformer secondary is calibrated
with the full range of the throttle valve. There is a maximum
