Surface micromachined silicon carbide accelerometers are undergoing development for eventual use in high-temperature environments like those inside turbines, internal-combustion engines, and other machines. These accelerometers would be used to measure vibrations indicative of deterioration of mechanical components; as such, they would be valuable diagnostic tools that could give advance warnings of failures or for the need to perform maintenance. They would even be small enough to fit into turbine blades.
Like similar accelerometers micromachined out of silicon, surface micromachined silicon carbide accelerometers are based on the concept of displacements of proof masses suspended on springs and they include electrodes for (1) capacitive sensing of displacements of proof masses and (2) electrostatic feedback for centering and for nulling displacements caused by relatively steady forces like those caused by gravitation and centripetal acceleration. The figure depicts an experimental surface micromachined silicon carbide accelerometer in which the proof mass (also called the "shuttle") is suspended at opposite ends by springs in the form of folded beams (pairs of beams joined at trusses). At each end, the outer two beams are attached to a substrate at four anchor locations.
The particular folded-beam structure offers low stiffness (and thus high sensitivity to acceleration) along both directions perpendicular to the nominal longitudinal axes of the beams and high stiffness (and thus low sensitivity to acceleration) parallel to the nominal longitudinal axes of the beams. If an accelerometer of this or a similar configuration were to be used to measure turbine-blade vibrations, then it would be mounted with the nominal longitudinal axis oriented in the radial direction to minimize the undesired response to centripetal acceleration. The folded-beam structure also helps to minimize the undesired differential thermal expansion between the substrate and the shuttle.
Electrodes for capacitive sensing of displacement and electrostatic actuation of the shuttle are located along the sides of the shuttle; two sets of these electrodes are on the shuttle and are interdigitated with adjacent sets of electrodes on the substrate. The gaps between the shuttle and the proximate substrate electrodes are typically 1 µm wide, and the electrode fingers are typically between 10 and 50 µm long. Inasmuch as capacitance between the substrate end shuttle is inversely proportional to the gap for small displacements, maximum sensitivity requires minimum gap. On the other hand, the gap must be made large enough to prevent touching of the substrate and shuttle electrodes at extreme excursions of the shuttle.
The main advantage of the silicon carbide accelerometers over silicon ones is the ability of any corresponding electronics (e.g., capacitance sensing circuits) to function at higher temperatures: Whereas silicon devices must generally be maintained at temperatures below 250 °C, silicon carbide devices can function at temperatures as high as 600 °C. Like silicon devices, silicon carbide devices can be fabricated inexpensively in batches with a high degree of repeatability.
This work was done by Russell G. DeAnna of the U. S. Army Research Laboratory for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Mechanics category. Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4—8, 21000 Brookpark Road, Cleveland, Ohio 44135.
Refer to LEW-17002.