The Art of Practical and Precise Strain Based ... - Webprofile.info

Applications Note 1-6:Phase information is not available in this type of static testing. In the caseof the piezoelectric load cell, very long time constants of the output do permitstatic calibration within defined precision limits.GravimetricThe ÒdrivenÓ gravimetric method of calibration utilizes an inertial massmounted to the load cell and driven by a shaker system where the frequencyis either set to discrete points or swept over a defined frequencyband and the magnitude and phase data are recorded. By measuring theapplied acceleration with an internally-mounted reference accelerometer,and in knowing the mass of the inertial weight, the applied force may becalculated by the relationship F = MA. The free-fall gravimetric methoduses a load cell as the reference sensor to calibrate an accelerometermounted on the opposite side of the inertial mass. The gravimetric methodis reciprocal in nature, as will be discussed, but is normally limited to ± 1%precision which is generally adequate for transfer function determinationbut inadequate for high-precision calibration. Both the free-fall gravimetricand driven systems are depicted in Figure 1-13.FIGURE 1-13. The Gravimetric Test System: Load Cell M Test Mass Accelerometer Elastic Suspension:Manually Deflectedto create momentary Free-FallconditionsPostGuide TubeBeamDamperBaseTie rodsMassElectrodynamicShakerLoad CellReference AccelerometerF = MAFoamThe Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999 CHAPTER 1-33