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Strain-Based Measurement Introduction:The hysteresis in the output of any sensor is the difference between theoutput of the sensor when inputs are monotonically increased and theoutput of the sensor when inputs are monotonically decreased. Industrystandard procedures typically require that loads be applied in 11, 20%of full scale input increments/decrements ie: 0%, 20%, 40%, 60%,80%, 100%, 80%, 60%, 40%, 20%, 0% of input. The objective in determininghysteresis loss is to find the magnitude of input where the maximumdifference exists between the Òup-loadÓ response and the ÒdownloadÓresponse. This is not trivial in the case of high performance calibrationsperformed on comparison calibration systems due to theimpossibility of exactly repeating the up-load points on the unload portionof the test. To alleviate this problem it is recommended that the unloadresponse be characterized with a three-coefficient polynomialcurve fit and the outputs then computed for the average inputs achievedon the up-load line. In this way, true hysteresis can be measured.FIGURE 1-11b: The Hydraulic Tester, Transfer Calibration: or Transfer Standard) Changeable Spacerto accommodatevarious sensor styles Reference Sensor (Proving RingHydraulic PumpHydraulic RamLocation of Sensor forCompression LoadingLocation of Sensor forTension LoadingCHAPTER 1-32 The Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999
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
Page 1 and 2: The Art of Practical andPrecise Str
Page 3 and 4: Special Credits:Mrs. Helen Pierson,
Page 5 and 6: Table of ContentsBy James G. Pierso
Page 8 and 9: ContentsApplications Note 4-10:Lead
Page 10: ContentsCHAPTER 8Electrical Conside
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Page 15 and 16: Applications Note 1-1: Introduction
Page 17 and 18: Applications Note 1-1: Introduction
Page 19 and 20: Applications Note 1-2:The Art of Ap
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