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Feature Measurement solutions Understanding base strain sensitivity


Base bending, or base strain sensitivity, and its effects upon piezoelectric accelerometer signal output is an important performance consideration for any transducer user. Joseph Que, applications engineer, Meggitt Sensing Systems – Measurement Group, provides a basic understanding of the role and importance of base bending as it relates to transducer signal output, and offers guidelines for the proper assessment of its potential contributions to measurement errors


ase strain sensitivity is largely a function of the interaction between the transducer and test article. Only a negligible magni- tude of force need be applied to a quartz crystal (piezoelectric) sensing element to produce a detectable output signal. Ideally, only the force of applied acceleration should impart such displacements to the crystal sensing ele- ment. However, influences from various external forces may cause certain defor- mations to the sensing element, result- ing in the generation of outputs which can contaminate true desired signal. Examples include pyroelectric excitations, high sound pressure levels and specimen strain. Pyroelectric errors tend to occur at very low frequencies – consequently, they can generally be ignored, if the intended measured fre- quency is greater than 2Hz. Typical errors caused by high sound pressure levels are also relatively small under normal measurement situations and can be ignored, unless the intended measur- able acceleration levels are very low. High sound pressure levels are usually accompanied by high vibration levels. When a test article is subjected to vibration, both bending and transverse strain may be induced at certain frequencies. Because of the close attach- ment of the accelerometer to the test article, the accelerometer base is sub- jected, to a degree, to the same bending mode. Figures 1 and 2 illustrate test article-induced base bending, in which the accelerometer will transmit its deformation to the piezoelectric sensing element. Since only minute displace- ment levels are required to generate piezoelectric accelerometer output signals, this output represents an error signal which might occur, even at 0Hz. It is this error, which can be quite high, that is referred to as base bending, or base strain sensitivity.


B


Base strain A piezoelectric accelerometer with shear mode construction tends to be less susceptible to base bending errors than one designed and operating in compres- sion mode (see Fig 1 & 2). As shear mode designs call for the quartz crystal sensing element to be detached from close


contact with the base, deformations are transmitted to the crystal only via secondary modes in the crystal mount- ing post. The crystal sensing element of a normal compression mode unit, however, will be subjected to the direct primary base bending mode.


Although shear mode transducers have lower base bending sensitivity than those operating in compression mode, in addition to smaller mass, there are certain applications in which these cannot be used, especially in cryogenic and higher temperature environments. It is logical to question whether the use of a shear mode piezoelectric accelerometer might be less preferable than a compression mode version. In general, the amount of base strain sensitivity present within a piezoelec- tric accelerometer will depend primar- ily upon its basic construction. One will usually find greater levels within a family of units which have lower sensi- tivities and higher natural frequencies. Strain sensitivity variations can exist within general accelerometer product families, including among units of the same model number and type. In some cases, a variation may also be present due to strain input direction. A typical single ended compression mode accelerometer can exhibit base strain sensitivities of as high as 25 equivalent


g at 250 micro strains (µε), while higher performance shear mode units may run


as low as 0.5 equivalent g at 250µε. The figure of 250µε is a general indus- try reference standard, set forth by the


Instrument Society of America (ISA). Meggitt specifies the base strain sensitiv- ity of its Endevco accelerometers as an


equivalent g per µε, data also collected in accordance with this standard. For example, the ISOBASE Model 2271A series is a compression mode transducer family, the patented design of which reduces base strain sensitivity to less than 0.5 equivalent g at


250µε, with reliable product performance over the -269˚C to +260˚C range.


A typical piezoelectric accelerometer test set-up is shown in Figure 3. It cannot be assumed that base strain sensitivity is a linear function of applied strain up to this level. Experience has


measurement & sensors directory 2012-2013 Figure 3 Figure 1


shown that the interface of the accelerometer with the test structure creates slip planes. The variation of the static coefficient of friction along these slip planes creates the non- linearity in the transmission of strain


to the crystal. The figure of 250µε is purely an arbitrary reference upon which a user can exercise an engi- neering-based judgment. Otherwise, a rigorous application of strain sensitivity data to error analysis would require the testing of each accelerometer at its actual level of strain within the applica- tion, as well as its actual orientation with respect to strain inputs.


Figure 2


The susceptibility of a unit to base strain sensitivity can vary by a ratio of as much as 5:1 or 10:1, as compared to other units of the same model and type. At Meggitt, the initial production lot of each Endevco piezoelectric trans- ducer is tested to establish the physical operating characteristics. The published base strain sensitivity specification of corresponding data sheets is a conserva- tive value representing average data for that accelerometer, taken under stan- dardized test conditions, representative of the most unfavourable possible trans- ducer orientation. With the design of the ISOBASE series, bending relief has been incorpo- rated directly into the transducer mechanical configuration to isolate base bending deformations from the compression seismic system, while compensating for the effects of strain upon a compression mode device. If an application calls for the measurement of relatively low acceleration levels and base bending effect reduction is of concern, the approach is to use a shear mode accelerometer, or ISOBASE compression mode design, to minimise signal output errors. A strain survey should be conducted on the mounting location through- out the frequency band of inter- est. Strain data can then be correlated against the published strain sensitivity information.


Meggitt Sensing Systems www.meggittsensingsystems.com


Contributing editor: Molly Bakewell Chamberlin, president Embassy Global, LLC


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