Inexpensive, high-performance, optically coupled temperature and strain gauges based on a combination of advanced optoelectronic and microelectromechanical concepts have been proposed. The sensors would contain vibrating beams with submillimeter dimensions, made primarily of silicon by microfabrication techniques. The beams would be designed so that (1) their resonance frequencies would vary with strain and temperature, respectively, and (2) their vibrations would be both excited and measured by use of light. The sensors would be impervious to mechanical shocks, would have masses in the microgram range, and would consume only femtowatts of power. Unlike electrically coupled sensors, the proposed sensors would be immune to (and would not generate) electromagnetic interference at suboptical frequencies.
These sensors were conceived for original application in measuring strains and temperatures on a canister that would be sealed pyrotechnically on Mars to bring a sample of the Martian atmosphere back to Earth. The strain measurements would be converted to readings of the pressure of the enclosed gas sample. In that application, there is a requirement for noninvasiveness; one must not create a potential leak by penetrating the canister to insert instrumentation for monitoring the enclosed gas. There is also a requirement to be able to separate the canister from, and connect the canister to, different instrumentation systems without having to make and break sensor electrical contacts. The same features that make the proposed sensors attractive for the original application also make these sensors attractive for terrestrial applications for monitoring temperatures and strains in sealed gas containers and in other structures.
A sensor of this type would include either a cantilever or a double-pinned microbeam in a polycrystalline silicon vacuum enclosure. The sensor would include an integral photodiode, and in the presence of optical excitation, the electrical output of the photodiode would cause bending of the beam via electrostatic attraction. The vibrations would give rise to modulation of the incident light reflected from the beam. Optical excitation and readout would be accomplished via an optical fiber, which could be terminated in the sensor body or, if necessary, at a distance of as much as a few millimeters.
A double-pinned microbeam would be particularly suitable for measuring axial strain; if a sensor containing such a microbeam were intimately coupled to, and suitably oriented on, a structural member, the stress in the member would alter the tension in the beam, thereby altering its resonance frequency. A cantilever microbeam would be well suited for measuring temperature; the coefficient of thermal expansion of a bimorph coating can be made to differ from that of silicon, so that thermally induced stress in the beam would change its resonance frequency. Moreover, with a typical resonance quality factor (Q) of about 105 and power dissipation of about 10 -15 W, a cantilever-microbeam temperature sensor would not thermally contaminate its environment.
This work was done by Frank Hartley of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category, or circle no. 172 on the TSP Order Card in this issue to receive a copy by mail ($5 charge). NPO-20223