DESCRIPTION AND OPERATION
HSS MODULE CIRCUITRY
I-E96-322A
2 - 9
and minimum differential voltage indicating a fully open or
fully closed valve. A valve half (50 percent) open is at the null
position (differential voltage is zero). The demodulator sums
the LVDT secondary inputs (which are out of phase) and the
result is a DC offset that represents the actuator position. The
position feedback signal passes from the demodulator output
to the sample hold circuit.
The sample hold circuit samples the demodulator output 1,000
times per second. The position feedback travels directly from
the sample hold output to the position error circuit, completing
the control portion of the feedback loop.
ANALOG-TO-DIGITAL CONVERTER
This portion of the position feedback circuitry converts the
analog feedback signal to a digital value. The position feedback
passes from the A/D converter to the data buffers, to the
microprocessor, to the I/O expander bus and finally to the MFP
module.
LVDT OSCILLATOR
The LVDT oscillator (frequency selectable) supplies an excita-
tion voltage to the LVDT primary. This excitation voltage
induces a differential voltage on the LVDT secondary that indi-
cates actuator position. This output has selectable gains. Set-
ting a dipswitch adjusts the operating range of the LVDT
excitation voltage to allow a maximum range of output voltage
on the LVDT secondaries.
Figure 2-4. Position Feedback Circuit
400 Hz TO
15 kHz
LVDT
OSCILLATOR
LVDT
DEMODULATOR 1
DEMODULATOR
GAIN
POSITION
FEEDBACK
TO MICRO-
PROCESSOR
ADC
TO POSITION
ERROR CIRCUIT
(PROPORTIONAL AMP)
TP25408A
SUMMING
AMP
DEMODULATOR 2
ACTUATOR
S/H
S/H
BUFFER
BUFFER
CORE
+
+
+
+
–
–
–
–
