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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. 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

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  1. FWD Calibration Procedure The Current SHRP-LTPP FWD Calibration Pooled Fund Study Initial Meeting May 21-22, 2003 College Station Texas

  2. Transducers On an FWD • Load • Deflection (typically 7 or 9) • Distance • Temperature • GPS • ??? – New ones being added all the time

  3. What We Calibrate • Load Cell • Deflection Transducers • Other transducers are conventional, typically easy to calibrate, but not covered by current procedure

  4. 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

  5. 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

  6. Typical FWD Load Cell

  7. 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

  8. Reference Load Cell

  9. Load Cell Calibration – Test Configuration

  10. Load Cell Calibration – Device Outputs

  11. Load Cell Calibration – Calibration Curve

  12. 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

  13. Deflector Calibration –Typical Geophone

  14. Deflector Calibration –Geophone Schematic

  15. 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

  16. 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

  17. reference beam geophone inertial block LVDT test slab isolator pads Deflector Calibration –Stable Reference System

  18. Deflector Calibration –Stable Reference System

  19. Deflector Calibration – Device Outputs

  20. Geophone Calibration –Calibration Curve

  21. 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

  22. 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

  23. Deflector Calibration-Relative Calibration Cartoon rel. cal. stack FWD geophones

  24. Deflector Calibration –Typical Relative Calibration

  25. Deflector Calibration –Typical Relative Calibration

  26. Deflector Calibration –Other Rel Cal Stacks

  27. Any Questions?

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