Measuring and testing – Speed – velocity – or acceleration – Angular rate using gyroscopic or coriolis effect
Reexamination Certificate
1999-09-24
2001-07-10
Moller, Richard A. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Angular rate using gyroscopic or coriolis effect
Reexamination Certificate
active
06257059
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to microfabricated tuning fork gyroscopes.
Microfabrication enables mechanical gyroscopes and other devices to be constructed using batch fabrication techniques known for fabricating solid state electronics. These techniques result in instruments of smaller size, lower cost, and greater reliability than those constructed by conventional techniques.
Micromechanical tuning fork structures are known for use as inertial rate sensors. Known tuning fork structures typically sense angular rate along an axis in-plane with a major planar surface of a substrate on or in which the device is constructed. One such device is an in-plane tuning fork gyroscope, which employs meshing drive and driven finger electrodes or combs associated with two vibrating tuning fork elements or proof masses.
The fabrication of such known devices is fairly straightforward, involving photolithographic and other semiconductor fabrication techniques. For damping and cross-coupling reasons, the plates of such known devices may be made with holes or apertures. Some fabrication sequences such as polysilicon and bulk silicon require the holes to enhance under cut etching. However, such devices are known and configured to sense only angular rates imposed in the plane of the major planar surface of the proof mass(es), and not for sensing angular rate about an axis perpendicular to the major plane of the substrate. Mechanical fixturing and wire bonding to sense angular rate about an axis perpendicular to the major plane of the substrate, and/or to realize a three axis system from known in-plane tuning fork gyroscope configurations is expensive and cumbersome.
Other relatively sophisticated micromechanical configurations are known for sensing out-of-plane angular rates. U.S. Pat. No. 5,016,072 to Greiff describes a double gimbal gyroscope structure which senses out-of-plane angular rates. However, the processing required to achieve such a double gimbal structure is not compatible with the processing required to achieve the referenced in-plane structures. Thus, realization of a three axis inertial measurement unit on a single chip would be difficult and perhaps commercially impracticable.
BRIEF SUMMARY OF THE INVENTION
The present invention defines the structure for a microfabricated out-of-plane tuning fork gyroscope which senses angular rate about an axis perpendicular to a major plane of the substrate upon which the device is constructed. The out-of-plane tuning fork gyroscope is fabricated by processing similar to and compatible with that of the referenced in-plane tuning fork gyroscope, making construction of a three axis angular rate sensor on a single chip viable.
In a first embodiment, the out-of-plane tuning fork gyroscope incorporates a striped capacitor readout having two apertured proof masses and electrodes in the form of first and second sets of strips deposited on a substrate below (or above) the apertures in the proof masses. A comb drive causes each proof mass to vibrate in the major plane, typically in opposition. The vibrating proof masses are compliant in an axis parallel to the major plane of the substrate but different from the vibration axis, and translate along the axis in response to an angular rate or acceleration input about an axis normal to the substrate. As the proof masses translate, the apertures cover the electrode strips to varying relative degrees, so that the capacitance between the proof masses and each set of electrodes increases and decreases in proportion to the differential axial position of the proof masses and hence to the input angular rate.
In a further embodiment, the meshing finger electrodes of the comb drive are used for both drive and angular rate sensing. Fixed combs are arranged in electrically isolated pairs 180° out of phase. As the proof masses translate in response to an out-of-plane angular rate input, the distance between the combs on the proof masses and the fixed combs varies, varying the capacitance. The combs can be driven with a voltage at the drive axis resonance frequency to provide both drive and sense operation.
The sensitivity of the out-of-plane tuning fork gyroscope approaches that of the in-plane tuning fork gyroscope for a given proof mass size and separation of resonant frequencies.
In a further embodiment, the out-of-plane tuning fork gyroscope incorporates a center motor that is split into two halves for common mode rejection of electrical coupling, which can cause gyroscope errors.
The microfabrication process of the out-of-plane tuning fork gyroscope is compatible with that of the in-plane tuning fork gyroscope, so that both types of devices can be made on the same silicon wafer or even the same chip. Thus, a complete inertial measurement unit, having three axes of rate and three axes of acceleration, can be built on a single silicon substrate.
REFERENCES:
patent: 5016072 (1991-05-01), Greiff
patent: 5757103 (1998-05-01), Lee et al.
Bernstein Jonathan J.
Kirkos Gregory A.
Lee Tommy W.
Petrovich Anthony
Weinberg Marc S.
Moller Richard A.
The Charles Stark Draper Laboratory Inc.
Weingarten, Schurgin Gagnebin & Hayes LLP
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