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

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

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 geophonesFWD rel. cal. stack

24 Deflector Calibration – Typical Relative Calibration

25 Deflector Calibration – Typical Relative Calibration

26 Deflector Calibration – Other Rel Cal Stacks

27 Any Questions?