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Strain-Based Measurement Introduction:erence Figure 1-32). It is critically important that the pre- and post-thermalcycling sweeps be performed at close to the same laboratorytemperature due to the strong dependency of damping factor withrespect to temperature.Regardless of the type of damping used, the flat frequency responsebandwidth of the accelerometer will be greater in the damped case thanin the undamped. In the undamped case the frequency response will beflat, ± 5%, to approximately 20% of the resonant frequency. In the critically-dampedcase (z =.7), the accelerometer will be flat to as much as60% of the resonant frequency at one temperature as shown in Figure1-33.FIGURE 1-33. The Effect of Damping on the Second-Order SystemResponse:Critically-Damped:Flat (± 5%) to 60%of Resonant FrequencyMagnitude ofResponse432Undamped:Flat (±5%) to 20%of ResonantFrequencyz = .11.5z = .7.125.2 .5 1 2 3w/w 0The Second-Order Magnitude ResponselogIn flight test environments it is not uncommon for the flight test accelerometersto experience very different temperatures at different pointson the air-frame. Some accelerometers may be mounted out in a coldCHAPTER 1-62 The Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999
Applications Note 1-10:wing tip and others mounted next to hot propulsion systems. This variationin temperature implies that the damping factor will vary from unit-to-unitwith the associated loss of phase data from channel to channel. It is for thisreason that the piezoelectric-based undamped accelerometer is most oftenused for this type of modal analysis. If damped accelerometers are mountedinto thermal controller and heater assemblies, the accelerometers can beprecisely phased from unit-to-unit.Just as an automotive shock absorber will become warm when absorbingenergy, the damping fluid will also heat in measurement environments thatcontinuously present repetitive high-level inputs. The mechanical energyabsorbed will be converted into heat in the damping fluid causing thedamping fluid viscosity to decrease, reducing the fluidÕs ability to absorbadditional energy. It has been observed in this type of measurement environmentthat the damping factors can be reduced to such a low level thatthe accelerometer is often over-ranged after having operated successfullyfor several minutes under high-level input conditions.Gas damping:Gas damping is used to achieve the same end result as fluidic damping buttends to be much less sensitive to temperature variations. Gas damping isachieved by pre-charging the interior of the accelerometer with a dry puregas to a predetermined pressure. The technique of gas damping is mostoften used with the micro-machined silicon piezoresistive accelerometer.The relatively-low density of the silicon seismic mass results in a largeseismic-massarea in order to achieve significant damping factors. Gasdamping is achieved by forming the accelerometer structure such that precisiongaps and orifices are formed between the moving seismic elementand the outer housing of the device. The motion of the seismic elementcombined with the precision-damping gaps and orifices result in the gasbeing squeezed into a film as shown in Figure 1-34.The internal pressure will increase as temperature is increased but not tothe extent found in the fluid-damped versions. Although the internal pres-The Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999 CHAPTER 1-63
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The Art of Practical andPrecise Str
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Special Credits:Mrs. Helen Pierson,
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Table of ContentsBy James G. Pierso
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ContentsApplications Note 4-10:Lead
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ContentsCHAPTER 8Electrical Conside
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CHAPTER 1Strain-BasedMeasurementInt
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Applications Note 1-1: Introduction
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Applications Note 1-1: Introduction
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Applications Note 1-2:The Art of Ap
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Applications Note 1-31. The presenc
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