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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

Load cell calibration test configuration
Load Cell Calibration – Test Configuration

Load cell calibration calibration curve
Load Cell Calibration – Calibration Curve

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 typical geophone
Deflector Calibration –Typical Geophone

Deflector calibration geophone schematic
Deflector Calibration –Geophone Schematic

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

Deflector calibration stable reference system1
Deflector Calibration –Stable Reference System

Geophone calibration calibration curve
Geophone Calibration –Calibration Curve

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

Deflector calibration relative calibration cartoon
Deflector Calibration-Relative Calibration Cartoon

rel. cal. stack



Deflector calibration typical relative calibration
Deflector Calibration –Typical Relative Calibration

Deflector calibration typical relative calibration1
Deflector Calibration –Typical Relative Calibration

Deflector calibration other rel cal stacks
Deflector Calibration –Other Rel Cal Stacks