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Strain-Based Measurement Introduction:The Sensor Transfer Function Measurement:The measurement of the sensor transfer function is the determination of themagnitude and phase response of the sensor when it is presented with a knownparameter input function. In the determination of the transfer function of anysensor, it is necessary that a dynamic parameter input be provided in order thatthe frequency-domain response of the device can be established. In the case ofboth load cells and strain-gaged pressure sensors, it is most common that thesedevices are calibrated using static or deadweight methods that will provideonly zero-hertz information and provide inadequate information to establishthe entire transfer function of the sensor. In measurement environments wheredynamic inputs are expected, choose a sensor having a flat frequency responsethat is at least equal to or greater than the highest expected frequency present inthe measurand. Dynamic calibration methods are implemented in the calibrationof the self-generating class of sensors, piezoelectrics for instance, due tothe difficulties and inaccuracies that arise when attempting high precisionstatic calibrations of inherently dynamic devices. In this case high precisionmeans calibrations performed to better than ±1% full scale output (FSO) nonlinearityand hysteresis.The objective in calibrating any sensor is to establish the ÒsensitivityÓ of thesensor to a physical input. The problem with this is that this sensitivity numb erchanges depending upon the frequency of the input. In the simplest case, wewould prefer to model the output of a sensor as a linear line or y=mx + b relationshipwhere y is the output, m is the sensitivity, x is the input and b is whatyou get out with no input. When the linear model would allow excessive uncertaintyto result, we are forced to use more complex and typically polynomialrelationships to model the sensor output. The sensitivity of any sensor may bequoted in a number of different ways as follows:1. millivolts per unit of input ie: mV/microstrain at some defined excitation (What you get outper unit of input)2. millivolts per Volt of input ie: mV/V (What you get as a full scale per unit of excitation)3. millivolts per Volt of excitation per unit of input ie: mV/V/lbf (output per unit of excitationper unit of input)4. millivolts per milliamp ie: mV/ma (Common with constant -current driven sensors)CHAPTER 1-24 The Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999
Applications Note 1-6:5. millivolts per milliwatt ie: mV/mW (Common with optically-based sensors)Be aware that transducers with sensitivities quoted as full scale values in mV/Vmay meet all other specifications at only one calibrated excitation level. You donot necessarily have the latitude to select any arbitrary level of excitation. This isparticularly true with resistance based sensors where thermal specifications mayonly be valid at some defined level of input excitation. For instance, in the case ofstrain gage based sensors, a doubling of the input excitation implies a 400%increase in the power dissipation. This results from the fact that power, in the voltage-excitedcase, is equal to V 2 /R and in the current-excited case, equals I 2 R.Additionally, sensitivities quoted in mV/V/unit of input can result in extremelysmall numbers where significant digits can mean a great deal. For example a50,000 lbf load cell at 2mV/V of input results in .00004 mV/V/lbf as a sensitivity.The various test methods that are listed in the following text briefly describe sometechniques by which the transfer function of a sensor may be defined, where themost common methods in use are described in detail in following chapters:Accelerometers:The calibration of strain-based accelerometers is performed to determine the transferfunction of the device and is usually made by one of the following techniques.Sinusoidal Discrete Frequency:To perform discrete-frequency sinusoidal calibrations, the sensor ismounted to an electrodynamic shaker system and is driven to vibrate at aselection of discrete frequencies at a calibrated and known peak accelerationlevel. The output of the device is then compared to the output of a calibratedreference accelerometer mounted inside the shaker armature. Thephase difference between the reference accelerometer and the accelerometerbeing tested can also be measured and reported during this processThe Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999 CHAPTER 1-25
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
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