Post on 17-Jan-2016
description
The Current SHRP-LTPP
FWD Calibration Pooled Fund Study Initial MeetingMay 21-22, 2003
College Station Texas
FWD Calibration Procedure
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Transducers On an FWD
Load Deflection (typically 7 or 9) Distance Temperature GPS ??? – New ones being added all the time
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What We Calibrate
Load Cell Deflection Transducers
Other transducers are conventional, typically easy to calibrate, but not covered by current procedure
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Basic Calibration Methodology
Mount FWD transducer and independent reference device in series
Apply input Compare outputs
Seems simple enough, but the devil’s in the details
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Load Cell Calibration
Load Cell: Device for measuring force Typical FWD load cells strain-gauge based Output a voltage proportional to applied load Well-understood and used in a variety of
applications
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Typical FWD Load Cell
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Load Cell Calibration – Reference Device
Because load cells are simple and well-understood, we can use another load cell as a reference device
Reference load cell should have precision 1 order of magnitude greater than FWD load cell
Reference load cell is NIST traceable
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Reference Load Cell
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Load Cell Calibration – Test Configuration
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Load Cell Calibration – Device Outputs
Time
Lo
ad Reference
FWDMeasured Load
Actual Load
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Load Cell Calibration – Calibration Curve
FWD Load
Ref
eren
ce L
oad
slope * old FWD gain = new FWD gain
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Deflector Calibration
Deflection sensors are not as simple as load cells Most FWDs use geophones
Advantage: Cheap, robust Disadvantage: Difficult to calibrate
Some FWDs use seismometers Advantage: Can be statically calibrated Disadvantage: Expensive, delicate
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Deflector Calibration –Typical Geophone
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Deflector Calibration –Geophone Schematic
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Deflector Calibration – Geophones Continued
Geophones output a voltage proportional to the velocity of the coil relative to the magnet
How this voltage is translated into deflection is a proprietary secret
We assume: Factory calibration of geophones is performed on a “shaker table” at a variety of frequencies, frequency-specific calibration factors are applied to raw data using Fourier transform
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Deflector Calibration – Reference Device
We do not use a reference deflector Difficult and expensive to calibrate
We use a reference LVDT Widely used, well understood, easy to calibrate
device Measures distance between two points Requires stable reference point
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Deflector Calibration –Stable Reference System
geophone
LVDT
reference beam
inertial block
test slab isolator pads
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Deflector Calibration –Stable Reference System
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Deflector Calibration – Device Outputs
Time
Def
lect
ion Reference
FWD
BeamMeasured Deflection
Actual Deflection
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Geophone Calibration –Calibration Curve
FWD Deflection
Ref
eren
ce D
efle
ctio
n
slope * old FWD gain = new FWD gain
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Geophone Calibration –Relative Calibration
Are we done yet? – Not quite ... Reference LVDT is not sufficiently precise
Precision should be ~ 1 order of magnitude greater than FWD deflector
No practical device is known with such a precision
Relative calibration reduces subsequent random error
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Deflector Calibration –Relative Calibration
Deflectors are all placed in rigid “relative calibration stack”
Deflectors all undergo the same deflection Average of all measurements used as a
virtual reference device
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Deflector Calibration-Relative Calibration Cartoon
geophonesFWD
rel. cal. stack
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Deflector Calibration –Typical Relative Calibration
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Deflector Calibration –Typical Relative Calibration
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Deflector Calibration –Other Rel Cal Stacks
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Any Questions?