Resonant-bridge two axis microaccelerometer
First Claim
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1. An accelerometer comprising:
- a mass;
a support surrounding said mass with a gap provided substantially everywhere therebetween said support and said mass;
at least two bridges attached at one end to said support from which said mass is suspended, said bridges being attached to opposite sides of said mass such that the longitudinal axis through said bridges form a common axis through said mass;
means for vibrating said bridges at their respective resonant frequencies, their respective resonant frequencies change as a function of extension or compression of said bridges due to acceleration of said mass;
means for detecting a change in the difference of said resonant frequencies between both of said bridges, so as to provide an indication of acceleration in a direction along said common axis; and
wherein said means for detection is provided on said support and said bridges span over said means for detection such that a gap exists between said means for detection and said bridges.
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Abstract
A resonant bridge two-axis microaccelerometer is disclosed comprising polysilicon resonant bridges orthogonally attached to a silicon proof mass, such that the silicon proof mass is suspended by the resonant bridges. Acceleration in the plane of the substrate causes differential axial loads on the opposing microbridges in each pair, thereby shifting their resonant frequencies. The acceleration component aligned with a pair is measured by the difference in resonant frequencies.
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Citations
17 Claims
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1. An accelerometer comprising:
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a mass; a support surrounding said mass with a gap provided substantially everywhere therebetween said support and said mass; at least two bridges attached at one end to said support from which said mass is suspended, said bridges being attached to opposite sides of said mass such that the longitudinal axis through said bridges form a common axis through said mass; means for vibrating said bridges at their respective resonant frequencies, their respective resonant frequencies change as a function of extension or compression of said bridges due to acceleration of said mass; means for detecting a change in the difference of said resonant frequencies between both of said bridges, so as to provide an indication of acceleration in a direction along said common axis; and wherein said means for detection is provided on said support and said bridges span over said means for detection such that a gap exists between said means for detection and said bridges. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)
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15. An accelerometer comprising:
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a mass centrally positioned within a support with a gap substantially everywhere therebetween said mass and said support, at least two pairs of bridges for suspending said mass from said support, said bridges spanning over said gap between said mass and said support, each pair of said bridges having a first bridge with one end attached to one side of said mass and a second bridge attached to an opposite side of said mass and respective opposite ends of said first and second bridges attached to said support such that said first and second bridges form a common longitudinal axis through said mass, the formed axes of the two pairs of bridges being perpendicular to each other; a drive electrode for each bridge provided on said support, each drive electrode positioned such that a gap exists everywhere therebetween said respective bridge which spans over said drive electrode; a pair of sense electrodes for each bridge provided on said support, one sense electrode positioned adjacent to one side of the drive electrode and the other sense electrode positioned adjacent to an opposite side of the drive electrode, said pair of sense electrodes positioned such that a gap exists everywhere therebetween said respective bridge which spans over said drive and sense electrodes, vibration of said bridges producing a time varying capacitance across said gap; a feedback vibration detection circuit for each of said bridges connected to said drive electrodes and said pair of sense electrodes for each of said respective bridges, the circuit sensing through the pair of sense electrodes senses changes in the time varying capacitance and thereby the changes in vibration of said respective bridges, and the circuit causes the drive electrode to exert a force on said respective bridge as a function of the sensed changes in vibration to maintain vibration of each respective bridge at the respective resonant frequency; a plurality of stops to prevent deflection of said mass past a predetermined distance in a direction perpendicular to said common two axes, wherein one of said stops is attached to said mass and one of said stops is attached to said support; a plurality of constraining bridges attached to said mass to suppress rotation of said mass and to suppress displacement of said mass in a direction perpendicular to said two axes; and means for simultaneously measuring in each pair of bridges a change in the difference of said resonant frequencies between said first and second bridges of said pairs to provide an indication of acceleration in a direction along the respective common axes formed by said pair of bridges, the simultaneous measuring of said pairs providing an indication of orthogonal components of acceleration, said resonant frequency of each of said bridges changing as a function of extension or compression of said bridge due to acceleration of said mass.
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16. A method for fabricating a monolithic accelerometer comprising the steps of:
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forming drive and sense electrodes with a first structural layer on one side of a substrate; forming a body of a predetermined mass in said substrate, symmetrically aligned with said drive and sense electrodes; depositing a sacrificial layer over said drive and sense electrodes; forming bridges over said sacrificial layer; removing said sacrificial layer to form a gap therebetween said bridges and said drive and sense electrodes; and substantially isolating said mass from said substrate so as to suspend said mass from said bridges, and so as to minimally attach said mass to said substrate with a plurality of constraining bridges. - View Dependent Claims (17)
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Specification