Microaccelerometer employing resonant circuit detection of seismic mass displacement

• US 5,801,309 A
• Filed: 11/30/1995
• Issued: 09/01/1998
• Est. Priority Date: 03/18/1994
• Status: Expired due to Fees

• Current Assignee: New Jersey Institute of Technology, Princeton University
• Original Assignee: New Jersey Institute of Technology, Princeton University
• Inventors: Carr, William N., Cho, Dong-Il
• Primary Examiner(s): Williams, Hezron E.
• Assistant Examiner(s): NOT, DEFINED
• Application Number:
  US08/565,325
• Time in Patent Office: 1,006 Days
• Field of Search: 73/514.29, 73/514.16, 73/514.17, 73/514.18, 73/514.32, 73/514.33, 73/514.35, 310/329
• US Class Current: 73/514.29
• CPC Class Codes: G01P 15/097 by vibratory elements
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/132 with electromagnetic counterbalancing means
• CPC Class Codes: G01P 2015/0828 the mass being of the paddle type being suspended at one of its longitudinal ends

Abstract

A first embodiment of an improved microaccelerometer includes a seismic mass, a support wafer, a cover wafer and a beam (or beams) for flexibly mounting a seismic mass between the support and cover wafers. A first oscillator includes a resonant circuit whose capacitance comprises conductive plates on one surface of the seismic mass and a conductive coating on an opposed surface of the support wafer. A second oscillator includes a resonant circuit whose capacitance is comprised of conductive coatings on another surface of the seismic mass and on an opposed surface of the cover wafer. A difference circuit provides an acceleration output that is dependent on a difference in oscillation frequencies between the first and second oscillators, when the accelerometer is subjected to an acceleration event. A second embodiment includes a structure similar to the aforedescribed, however, the second oscillator is replaced by an ac levitation circuit that exerts a single direction restoring force on the seismic mass during an acceleration event. A third embodiment provides ac levitational restoring forces when the seismic mass is subject to acceleration in either of two opposed directions. In the latter embodiment, ac levitating circuits are disposed on opposed surfaces of the support and cover wafers.

Claims

1. An accelerometer comprising:

a seismic mass having upper and lower surfaces;

a support wafer positioned below said lower surface of said seismic mass and having an upper surface separated from and opposed to the lower surface of said seismic mass;

a cover wafer positioned above said upper surface of said seismic mass and having a lower surface separated from and opposed to the upper surface of said seismic mass;

beam means for flexibly mounting said seismic mass between said support wafer and said cover wafer;

a first oscillator having a resonant circuit, including inductance and first capacitance, said first capacitance comprising a dielectric positioned between a conductive plate arrangement covering a substantial area of said upper surface of said seismic mass and a conductive plate arrangement on said lower surface of said cover wafer;

a second oscillator having a resonant circuit including inductance and second capacitance, said second capacitance comprising a dielectric positioned between a conductive plate arrangement covering a substantial area of said lower surface of said seismic mass and a conductive plate arrangement on said upper surface of said support wafer; and

frequency difference means coupled to said first oscillator and said second oscillator for providing an output indicative of a difference in oscillation frequencies of said first and second oscillators when said accelerometer is subjected to an acceleration event that positionally displaces said seismic mass, said output of the difference in oscillation frequencies being a measure of said acceleration event.

2. The accelerometer of claim 1 wherein said first oscillator, including said inductance of said resonant circuit thereof, is positioned on said upper surface of said support wafer.

3. The accelerometer of claim 2 wherein said second oscillator, including said inductance of said resonant circuit thereof, is positioned on said lower surface of said cover wafer.

4. The accelerometer of claim 3 wherein composition of said cover wafer, said support wafer and said seismic mass is selected from a group comprising silicon and gallium arsenide compounds.