Ectron 451, Instruction Manual

Page: 43 / 74

SECTION V
THEORY of OPERATION
ANALOG INPUT
The two signal inputs from the LVDT feed a differential, programmable-gain amplifier stage,
which can be set to any of three gains. This is followed by a demodulator that is synchronized
to the phase of the input signal. This synchronozation allows the Model 451 to operate with
a wide variety of LVDT’s and cabling arrangements without the need for phase adjustment.
The output of the demodulator is voltage proportional to the difference between the two
LVDT secondary voltages multiplied by the gain of the programmable-gain amplifier.
Following the demodulator is a two-pole, selectable, low-pass filter with bandwidths of
1 Hz, 10 Hz, 100 Hz, and a wideband position (which filters the signal to about 600 Hz).
Following the filter is a 14-bit analog-to-digital converter (ADC). This ADC operates at
approximately 2000 samples per second and feeds its output to the microcontroller.
FIRMWARE
The Model 451 firmware provides the front-panel user interface, calculation of the constants
needed for real-time operation, processing of the real-time input-to-output data stream, and
monitoring for out-of-range and other error conditions. The Model 451 is programmed to
provide extensive configuration capabilities that allow the user to adjust many parameters
used in operating an LVDT, as well as calibration of its own input and output circuitry.
The Model 451 has an overall gain range of 1 to 300, both positive and negative. Specifying
negative gain will result in inversion of the output signal, which is useful since it is often
rather tedious to predict the polarity of an LVDT system in advance. The input program-
mable-gain amplifier provides part of the needed gain, and real-time multiplication of the
signal-in firmware provides the remainder.
Real-time operation involves reading the input voltage x from the ADC, multiplying that
input voltage by a gain factor m, adding an offset value b, and feeding the result y to the
output DAC. Calculation of the m and b values is done during setup, so the Model 451
only evaluates the equation y = mx + b in real time, about 2000 times per second.
The front-panel display shows LVDT displacement, updated about three times per second.
This display is calculated from the input voltage and the transducer sensitivity and depends
on the accuracy of the transducer sensitivity. In GAIN and EXAMPLE modes, the transducer
sensitivity is not needed for calculation of the output, so the display of displacement may
not agree with the output if the transducer sensitivity does not match the transducer in use.
Three modes of operation are provided:
GAIN
,
EXAMPLE
, and
SCALE
. The operation of
each will be described separately.
GAIN MODE
In the gain mode, m and b (from above) are the gain and offset, respectively, entered from
the front panel. The range of gain is 1 to 300, positive and negative, where negative gain
will invert polarity of the output compared to the input. In
GAIN
mode, the Model 451
operates much like a conventional LVDT conditioner.
Model 451 Alignment
5-1