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The Current SHRP-LTPP FWD Calibration Procedure 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 2 What We Calibrate Load Cell Deflection Transducers Other transducers are conventional, typically easy to calibrate, but not covered by current procedure 3 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 4 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 5 Typical FWD Load Cell 6 Load Cell Calibration – Reference Device Because load cells are simple and wellunderstood, 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 7 Reference Load Cell 8 Load Cell Calibration – Test Configuration 9 Load Load Cell Calibration – Device Outputs Reference Measured Load Actual Load FWD Time 10 Load Cell Calibration – Calibration Curve Reference Load slope * old FWD gain = new FWD gain FWD Load 11 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 12 Deflector Calibration – Typical Geophone 13 Deflector Calibration – Geophone Schematic 14 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 15 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 16 Deflector Calibration – Stable Reference System geophone reference beam inertial block LVDT test slab isolator pads 17 Deflector Calibration – Stable Reference System 18 Deflection Deflector Calibration – Device Outputs Reference Measured Deflection Actual Deflection FWD Beam Time 19 Geophone Calibration – Calibration Curve Reference Deflection slope * old FWD gain = new FWD gain FWD Deflection 20 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 21 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 22 Deflector CalibrationRelative Calibration Cartoon rel. cal. stack FWD geophones 23 Deflector Calibration – Typical Relative Calibration 24 Deflector Calibration – Typical Relative Calibration 25 Deflector Calibration – Other Rel Cal Stacks 26 Any Questions? 27
