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The FBA-3 Force
Balance Accelerometer is a high-sensitivity, low-frequency triaxial device suitable for a variety of seismic and structural
applications. It is an economical instrument characterized by
high reliability, ruggedness and low current drain.
Designed to meet the stringent requirements of
USNRC Regulatory Guide 1.12 for nuclear power plants, the FBA-3
is deployed in over 100 nuclear power plants in the United
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States
and abroad. Frequency response is flat from dc to 50 Hz. Nominal
full-scale range is ±1 g, but optional full-scale ranges are
available.
The FBA-3 is packaged in a cast aluminum base
and cover, sealed to prevent the entrance of moisture and dirt.
The three accelerometers are orthogonally mounted on an internal
deck plate.
Technical Description
The FBA-3 is a spring-mass device which uses
variable capacitance transduction, as shown in the schematic
below.
The output is fed back to the parallel combination of capacitor
C0 and the torquer coil, which is an integral part of the mass.
From the coil the feedback loop is completed through resistors
R0 and Rh. This has the effect of stiffening the system, thus
increasing the natural frequency to 50 Hz. Resistor R0 (with C0)
controls the damping, which normally is adjusted to 70%
critical. The acceleration sensitivity is controlled by the gain
Kp of the post-amplifier.
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Technical Specification |
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Full Scale Range
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1 ,3( .g0.1± & )lanoitpo g 2 |
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Natural Frequency
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50 Hz* |
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Bandwidth |
DC to 50 Hz (3dB point) |
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Damping |
70% critical* |
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Operating temperature range
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-20º to 70ºC (0º to 160ºF) |
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Output (full-scale)
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±2.5V* into 50,000 ohms |
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Zero offset |
Less than 25 mV* |
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Cross-axis sensitivity |
Less than .03 g/g* |
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Linearity |
Less than 1% of full-scale |
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Noise (0 to 50 Hz) |
Less than ±25μV |
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Dynamic Range (0 to 50 Hz) |
100dB |
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Temperature effects (zero drift an
sensitivity) |
Less than 2% of full-scale |
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Supply voltage
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±12 Vdc |
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Turn-on time |
Operational within 0.1 second after power
applied |
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Calibration |
Electrical commands can be applied to
produce damping and natural frequency outputs |
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*Measured values furnished with each
sensor. |
Seismic Qualifications
Kinemetrics Inc has provided strong motion accelerograph systems
to over 100 nuclear power plants throughout the world for over
10 years. Kinemetrics products have been carefully tested for
generic qualification to meet most existing and future
requirements.
Kinemetrics seismic test characteristics for the FBA-3
accelerometer have the following general characteristics:
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General Characteristics |
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Biaxial: horizontal and vertical rotated,
and repeated at 90 degrees. |
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Five OBE’s (Operating Base Earthquake)
followed by one SSE (Safe Shutdown Earthquake) in each
direction. |
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Random excitation controlled at
2
octave intervals with incoherent phasing between axes,
over the range 1 to 40 Hz. |
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Test duration of thirty seconds minimum. |
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Minimum SSE RRS ZPA (Required Response
Spectrum Zero Period Acceleration) of 2 g with 3% damped
response accelerations exceeding 6g in the range 2 to 20
Hz. |
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Sensor SSE RRS ZPA of 6g with 1% damped
response accelerations exceeding 14g in the range 2 to
30 Hz. |
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Functional testing conducted on devices
prior to, during and following seismic tests. |
In addition,
Kinemetrics has performed RIM (Required Input Motion) testing of
pipe-mounted sensors. The FBA-3 accelerometer has been qualified
as follows:
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General Qualifications |
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Biaxial: horizontal and vertical rotated,
and repeated at 50 degrees. |
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Steady-state sinusoidal dwell tests at
1
octave intervals from 1 to 45 Hz. |
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Dwell duration of 30 seconds at each
frequency. |
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0-Peak input acceleration at each
frequency of 4.5g except limited by displacement (below
5 Hz). |
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Functional tests conducted on devices
prior to, during and following seismic tests. |
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Physical Characteristics |
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Dimensions |
200mm X 200mm X 200mm water tight |
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Enclosure |
water tight (8” cube) |
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Weight |
7 kg (15 pounds) |
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