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Strain-Based Measurement Introduction:strains. This characteristic allows high-output fully-active strain bridges to beimplemented.Other mechanical considerations of the cantilevered beam, used for eitherforce or acceleration measurement, relate to the resonant frequency of suchstructures. The cantilevered beam, used in the fabrication of the strain-gagedaccelerometer, will be designed to minimize the beam length. The resonant frequencyof the seismically-loaded beam, having negligible beam mass, is proportionalto the root of 1/L 3 . This means that the shorter beam lengths willyield much higher resonant frequencies than longer beam lengths. The piezoresistivestrain gages of the ÒUÓ type will provide minimum gage lengths thereforepermitting minimum beam lengths.The curvilinear motion of the loading point implies reduced perpendicularity atthe point of load application to the cantilevered beam at high acceleration inputlevels. Although small strains, on the order of 500 microstrain, are required bythe piezoresistive strain gage accelerometers, the curvilinear motion of the loadpoint will typically limit the nonlinearity performance of the device to between±.5 to ±1% of the full-rated output of the device. Additionally, the mechanicalstiffness and symmetry of the clamp structure will directly impact the symmetryof the strain field in positive- versus-negative bending of the beam. Theintegrally-machined clamp of Figure 1-36 will provide mechanical performancesuperior to that of the assembled clamp structure. The multi-componentclamp assemblies will contribute friction or hysteretic loss as well as assemblytolerances influencing the effective length dimension of the beam.CHAPTER 1-68 The Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999