The Current SHRP-LTPP

FWD Calibration Pooled Fund Study Initial MeetingMay 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

Time

Lo

ad Reference

FWDMeasured Load

Actual Load

11

Load Cell Calibration – Calibration Curve

FWD Load

Ref

eren

ce L

oad

slope * old FWD gain = new FWD gain

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

Time

Def

lect

ion Reference

FWD

BeamMeasured Deflection

Actual Deflection

20

Geophone Calibration –Calibration Curve

FWD Deflection

Ref

eren

ce D

efle

ctio

n

slope * old FWD gain = new FWD gain

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?