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
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Micromachined force-balance anemometer
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Overview
The document presents a technical overview of a micromachined force-balance anemometer developed by Frank T. Hartley and David Crisp at NASA's Jet Propulsion Laboratory. This innovative device is designed to measure wind velocity with high sensitivity and a wide dynamic range, making it suitable for various applications, including aerospace.
The anemometer is constructed using a unique design that incorporates two proof mass force plates and two perforated quad plates, which are eutectically bonded together. The device operates by measuring the capacitance between the quad plates and the force plates, allowing it to determine the pitot static force (pressure multiplied by aperture area) applied by the wind. The anemometer's dimensions are compact, measuring less than 2 cm square and 2 mm thick, and it can be mounted in various orientations to measure different wind directions.
One of the key features of this anemometer is its hi-polar electrostatic force feedback control system, which maintains the proof mass in a centered position during operation. This system applies voltages to the quad platens and force plates, ensuring that the proof mass remains stable and sensitive to changes in wind force. The device is designed to be robust and athermal, meaning it can operate effectively across a range of temperatures without significant performance degradation.
The anemometer boasts an impressive dynamic range of 10^6 and a frequency response that extends from sub-Hertz to hundreds of Hertz, making it capable of detecting both low and high-frequency wind fluctuations. The design also includes a zero flexure deflection mechanism, which enhances its sensitivity and accuracy.
Additionally, the document outlines the manufacturing process, which involves precise etching and deposition techniques to create the quad platens and force plates. The use of submicron-thick insulating layers prevents electrical contact between the electrodes while allowing for the necessary electric fields to be generated for operation.
Overall, this micromachined force-balance anemometer represents a significant advancement in sensor technology, combining miniaturization, sensitivity, and robustness, making it a valuable tool for measuring wind forces in various scientific and engineering applications.
