A micromachined force-balance anemometer has been developed by modifying the design of a micromachined force-balance accelerometer that responds to accelerations as small as 109 × normal Earth gravitation (about 10 - 8 m/s2). The anemometer thus offers the advantages of the accelerometer; namely, high sensitivity, wide dynamic range, bipolar response, athermality, robustness, compactness, and low power consumption.
Both the accelerometer and anemometer versions of the design include a proof mass suspended on springs in a housing. The proof mass is in the form of two square plates, called "force plates," that are bonded together to form a single plate. The springs are thin beams (flexure springs) that lie alongside the edges of the proof mass (see figure). The springs are flexible enough to allow displacement of the proof mass along the z axis, but stiff enough to resist significant displacement of the proof mass along the x and y axes.
The housing includes two plates, called "quad platens," between which the proof mass is suspended on the spring flexures. In its equilibrium (non-spring-deflection) position, the proof-mass force plates lie parallel to the quad platens and about midway between them. Patterned metal coatings on the faces of the force plates and on the quad platens serve as electrodes for controlled electrostatic deflection of the proof mass and as electrodes of capacitive proximity sensors for measuring the z displacement of the proof mass. The quad platens are so named because each one is divided into four electrode areas. In the anemometer version, quad platens are perforated (the central half of each electrode area is removed) to allow gas to flow.
In operation, the outputs of the capacitive displacement sensors are processed through a feedback control system that applies voltages between the quad platens and force plates to keep the proof mass centered at or near the equilibrium position. These voltages serve as measures of the force with which the proof mass is deflected by acceleration (in the case of the accelerometer) or by pitot static force (in the case of the anemometer).
During handling, the proof mass can be "caged" to protect its delicate spring suspension. This is accomplished by applying an electrostatic-deflection voltage to clamp the proof mass against one of the quad platens. Submicron-thick electrically insulating surface layers prevent electrical contact between facing electrodes while allowing the interelectrode gap to become small enough to enable a small battery to generate an electric field sufficient to maintain clamping.
The overall dimensions of the micromachined anemometer are less than 2 by 2 by 0.2 cm. The dynamic range is 106. The frequency band of high sensitivity ranges from less than 1 to hundreds of hertz.
This work was done by Frank T. Hartley and David Crisp of Caltech for NASA's Jet Propulsion Laboratory. NPO-20129