1
LPF
φ
1
φ
2
VCO
Instrumentation
Amplifier
and
Schmitt Trigger
Phase Comparator
Output Buffer
VW Gage
Coil
Microprocessor
V+
1. INTRODUCTION
The Geokon 8020-42CPR is a device that allows single coil vibrating wire gages to be driven in
the “Autoresonant” mode, instead of the standard “Pluck and Read” mode, resulting in a greater
dynamic range (up to 20Hz). The benefits of Autoresonant vs. Pluck and Read topologies are
many, including greater reading stability and wider dynamic bandwidth. In addition, since there
is no asynchronous swept frequency or pulse pluck excitation to interfere with the vibrating wire
signal, there is the ability to read the gage frequency with a general-purpose frequency counter or
low cost datalogger, instead of a complex dedicated readout device or datalogger. In addition to
the gage frequency output, the 8020-42CPR also provides two 4-20mA signals that are
proportional to “digits” (frequency²). These 4-20mA outputs are factory set for the range of 2250
digits (1500Hz – 20mA) to 12250 digits (3500Hz - 4mA), but may be adjusted for other ranges.
One of the 4-20mA outputs is a non-isolated current source (I_out, / I_out). The other 4-20mA
output (LOOP_+, LOOP_-) is a current sink designed to be used in applications that require an
isolated controller. Typically, the 4-20mA output is adjusted to cover the full range span of a
single transducer.
Historically, autoresonant vibrating wire gages have employed two coils. The first is the
Transmit (excitation) coil that provides a phase synchronous pulse (pluck) to maintain
oscillation, while the second is the Receive (reading) coil that recovers the vibrating wire signal.
The two-coil approach, while dependable, adds to the cost and imposes a considerable
mechanical limitation to the design and construction of the gage. Since the SCA is designed to
operate as a “transceiver” using only one coil, these limitations are eliminated while providing
the benefits of the autoresonant mode.
Figure 1 - Block Diagram
Frequency
Output
Enable
PLL_OUT
VCO_HOLD
T/R
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Summary of Contents for 8020-42CPR
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