A compact, low-power device measures pressure via the pressure-induced damping of oscillation of a small mechanical resonator. To achieve compactness and low mass — and thus low-power consumption — the resonator is micromachined out of silicon. In addition to the resonator, the device includes an electronic circuit that drives the oscillation and effectively measures the resonance quality factor (Q), which is inversely proportional to the rate of damping.
The drive circuit generates a drive voltage that alternates at the frequency of the mechanical resonance. The circuit tracks the mechanical resonance to maintain the drive frequency at the resonance frequency, even when the resonance frequency drifts gradually with time or with changes in temperature. The drive voltage is applied to electrostatic-deflection electrodes to excite and maintain the oscillation. The circuit includes a feedback loop that measures the amplitude of the oscillation and adjusts the drive voltage to maintain the oscillation at a preset amplitude.
At a given time, the magnitude of the drive voltage needed to maintain the preset amplitude of oscillation depends on the rate of damping and thus on the pressure at that time. Accordingly, the magnitude of the drive voltage developed by the feedback loop is sampled and taken as an indication of pressure.
The overall sensitivity of the device depends partly on the intrinsic Q of the resonator (the Q that the resonator would exhibit during operation in a perfect vacuum) and partly on the sensitivity of the drive circuit. Inasmuch as the intrinsic Q of a micromachined resonator like the one used in this device typically ranges from 100,000 to 200,000, high sensitivity to pressure can be achieved readily.
The device can be designed to have a wide dynamic range. For example, an early version of the device was found to indicate pressures in the range of 10-6 to 10-2 torr (approximately 10-4 to 1 Pa) with nearly linear response over a large part of that range (see figure). It should be possible to extend the upper limit of this pressure range to as much as 10 atm (approximately 1 MPa) and the lower limit to <10–6 torr (<10–4 Pa) by slight modifications of the resonator and drive circuit. The pressure-measurement resolution is limited only by the capabilities of the drive circuit; it should be possible to design the circuit to achieve a resolution of < 10-6 torr (<10-4 Pa) over the extended pressure range.
This work was done by Roman C. Gutierrez, Tony K. Tang, Jaroslava Wilcox, William J. Kaiser, Christopher B. Stell, Vatche Vorperian, and Kirill V. Shcheglov of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Mechanics category.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
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Refer to NPO-20052.