Transcript Deflector Calibration

The Current SHRP-LTPP
FWD Calibration Procedure
FWD Calibration Pooled Fund Study Initial Meeting
May 21-22, 2003
College Station Texas
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 wellunderstood, 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
Load Cell Calibration –
Device Outputs
Reference
Measured
Load
Actual
Load
FWD
Time
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Load Cell Calibration –
Calibration Curve
Reference Load
slope * old FWD gain = new FWD gain
FWD Load
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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
reference beam
inertial block
LVDT
test slab
isolator pads
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Deflector Calibration –
Stable Reference System
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Deflection
Deflector Calibration –
Device Outputs
Reference
Measured
Deflection
Actual
Deflection
FWD
Beam
Time
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Geophone Calibration –
Calibration Curve
Reference Deflection
slope * old FWD gain = new FWD gain
FWD Deflection
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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 CalibrationRelative Calibration Cartoon
rel. cal. stack
FWD
geophones
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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?
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