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FWD Calibration Procedure. The Current SHRP-LTPP. 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. What We Calibrate.

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the current shrp ltpp

FWD Calibration Procedure

The Current SHRP-LTPP

FWD Calibration Pooled Fund Study Initial Meeting

May 21-22, 2003

College Station Texas

transducers on an fwd
Transducers On an FWD
  • Load
  • Deflection (typically 7 or 9)
  • Distance
  • Temperature
  • GPS
  • ??? – New ones being added all the time
what we calibrate
What We Calibrate
  • Load Cell
  • Deflection Transducers
  • Other transducers are conventional, typically easy to calibrate, but not covered by current procedure
basic calibration methodology
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

load cell calibration
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
load cell calibration reference device
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
deflector calibration
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
deflector calibration geophones continued
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

deflector calibration reference device
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
deflector calibration stable reference system

reference beam


inertial block


test slab

isolator pads

Deflector Calibration –Stable Reference System
geophone calibration relative calibration
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
deflector calibration relative calibration
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