In this program, Foster-Miller applied finite element analysis (FEA) for the initial design of a single transducer, micro-machined from silicon nitride, for use in gas turbine engines at temperatures over 300(C to measure vibration, pressure and temperature. We considered two different transducer geometries to demonstrate measurement performance. The finite element analyses were conducted to evaluate the static deflections, resonance frequencies and the amplitude at the resonant frequencies due to external excitation. Both of the devices were 0.18 microns thick. Finite element models were generated using commercially available software and mechanical properties of silicon nitride were used as input for the FEA program.

Static and dynamic analyses (frequency response based on modal superposition) were conducted on the two designs. For static loading, 1g and 5g gravity loading were applied to the structure. Both the devices are fixed at the ends.

The resonant frequencies of the above design were obtained by conducting an eigenvalue analysis. A subspace iteration technique was used in obtaining the first six eigenvalues. The results of our analyses showed resonant frequencies at 1400 to 8300 Hz. These vibration modes were investigated to further determine the influence of ambient temperature on the operation of the transducer.

Based on the modal analyses, a modal superposition method was used to obtain the maximum amplitude due to sinusoidal excitations at the base of the structure over the range of the first six natural frequencies. The dynamic responses of the MEMS transducers were modeled using maximum amplitude of 1mil (25.4 microns) over a frequency range from 500 Hz to 20,000 Hz. The model also analyzed the effect of damping of the structure.  The results of these analyses clearly showed that damping plays a critical part in the design. The devices have to be heavily damped to attenuate the maximum resonance amplitudes due to expected typical vibration amplitudes from stationary components in a gas turbine engine.

Based on the FEA, actual MEMS accelerometers were built and tested, showing that silicon nitride can be used to make practical high temperature transducers.

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