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
This Brief includes a Technical Support Package (TSP).
Proximity measurement of pressure and temperature
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Overview
The document outlines a technical support package from NASA detailing the development of innovative proximity measurement sensors designed for monitoring pressure and temperature in hermetically sealed canisters, particularly for applications related to Martian gas sample return missions. The sensors utilize optically excited resonant cantilever microbeams, which are fabricated primarily from silicon using advanced microfabrication techniques. These microbeams are characterized by their submillimeter dimensions, high sensitivity, and ability to operate with extremely low power consumption (femtowatts).
Key features of these sensors include their resilience to mechanical shocks, withstanding forces exceeding 105g, and their capability to measure temperature with a resolution of 0.0001 K and pressure changes with sub-ppm accuracy. The sensors function by detecting changes in resonant frequency caused by thermally induced stress and strain, allowing for precise measurements of both temperature and pressure without the need for direct electrical connections. This is particularly advantageous in environments where maintaining a hermetic seal is critical, as conventional pressure transducers often require penetration of the seal, risking leaks.
The document emphasizes the importance of these sensors for the Mars missions, where they would be used to monitor the gas pressure (around 7 Torr) within sealed canisters during their return to Earth. The sensors are designed to be lightweight, low-volume, and capable of operating over a wide range of temperatures and pressures, making them ideal for space applications. Additionally, the use of optical excitation and measurement techniques ensures that the sensors are immune to electromagnetic interference, enhancing their reliability in challenging environments.
The proposed system allows for the multiplexing of multiple sensors over a single optical fiber, facilitating the measurement of various parameters such as hoop strain, ambient absolute pressure, and temperature across small gaps. The interface for these sensors consists of a monochromatic infrared laser diode and an infrared detector, which can be aligned with minimal spatial tolerances.
Overall, the document presents a comprehensive overview of a novel class of fiber-optic sensors that combine optoelectronic and microelectromechanical technologies, paving the way for advanced monitoring solutions in space exploration and other demanding applications.
