Vibratory digital integrating accelerometer

• US 4,699,006 A
• Filed: 02/04/1986
• Issued: 10/13/1987
• Est. Priority Date: 03/19/1984
• Status: Expired due to Fees

• Current Assignee: Charles Stark Draper Laboratory Incorporated
• Original Assignee: Charles Stark Draper Laboratory Incorporated
• Inventors: Boxenhorn, Burton
• Primary Examiner(s): CHAPMAN JR, JOHN E
• Application Number:
  US06/826,133
• Time in Patent Office: 616 Days
• Field of Search: 73/517 R, 73/517 B, 73/517 AV, 310/329
• US Class Current: 73/514.15
• CPC Class Codes: G01C 19/5719 using planar vibrating masses driven in a translation vibration along an axis
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/097 by vibratory elements
• CPC Class Codes: G01P 2015/0831 the mass being of the paddle type having the pivot axis between the longitudinal ends of the mass, e.g. see-saw configuration
• CPC Class Codes: G01P 2015/084 the mass being suspended at more than one of its sides, e.g. membrane-type suspension, so as to permit multi-axis movement of the mass

Abstract

An accelerometer transducer has a base holding a sheet normal to an input axis. The sheet is suspended from the base by a first spring arrangement for oscillatory motion in the way of limited angular excursions about a central axis lying in the sheet. A weight is spring-suspended from the sheet in a manner permitting displacement of the center of gravity of the weight along the input axis. Acceleration along the input axis varies the moment of inertia of the assembly consisting of the sheet and the weight about the central axis, altering the frequency of oscillatory motion by an amount functionally related to the magnitude of the acceleration. The sheet is driven at its characteristic frequency, and its frequency of oscillation is measured. In a preferred embodiment, a first transducer includes a weight suspended offset from the sheet in a first direction along the input axis, and a second transducer includes a weight offset from the sheet in the opposite direction along the input axis. The change of oscillatory frequency of the second transducer is subtracted from that of the first transducer to develop an output quantity linearly related to the acceleration. In a preferred embodiment, the sheets, springs and weights are micromechanical devices fabricated by semiconductor fabrication techniques. Electrostatic driving elements for driving the sheet, and a capacitive pick-off for sensing its vibration are also shown.

Claims

1. Apparatus for sensing the magnitude of acceleration along an input axis, such apparatus comprising:

a base, defining an opening transverse to the input axis;

a sheet member, pivotally suspended from the base over the opening by at least one torsion spring for reciprocating angular motion about a first axis lying in the sheet member;

a weight, having a center of gravity offset from the sheet member along an input axis;

suspension means, suspending the weight to the sheet member for displacement with respect to the first axis, such displacement being responsive to acceleration along the input axis;

the movable system consisting of the sheet member, the suspension means, the weight and the at least one torsion spring being characterized by a natural frequency of angular motion about the first axis, said natural frequency varying subject to displacement of the weight, whereby the natural frequency of angular motion of such movable system is representative of the acceleration along the input axis; and

means for sensing said natural frequency.

2. A sensor according to claim 1, wherein the sheet member has an inner contour defining an aperture therein, and wherein the weight comprises a second sheet member suspended in the aperture, said second sheet member having a center of gravity offset from the first axis, and wherein the suspension means comprises at least one second torsional spring element suspending the second sheet member for torsional motion about a second axis orthogonal to the input axis and the first axis.

3. A sensor according to claim 1, wherein the suspension means comprises:

post means attached to the sheet member and protruding therefrom; and

beam means, extending from the post means to the weight, for permitting said displacement of the weight by deflection of the beam means.

4. A sensor according to claim 1, wherein the suspension means comprises:

a diaphragm means, having the weight centrally located thereon, for permitting said displacement by deformation of the diaphragm; and

spacer means attached to the sheet and to the diaphragm for holding said diaphragm means spaced apart from the sheet member.

5. A sensor according to claim 1, further including:

means for driving the movable system at its natural frequency; and

means for detecting the frequency of motion of the movable system.

6. A sensor according to claim 5, wherein the means for detecting includes means for counting pulses manifesting the frequency of motion.

7. An accelerometer assembly comprising a pair of first and second sensors according to claim 1, wherein the center of gravity of the weight of the first sensor is offset in a first direction along the input axis, and the center of gravity of the weight of the second sensor is offset from the sheet member of the second sensor in a direction opposite to the first direction along the input axis.

8. An accelerometer according to claim 7, further including:

means for driving the respective movable systems of the first and second sensors at their respective natural frequencies, and

means for detecting the frequency of motion of the respective movable systems.

9. An accelerometer according to claim 8, wherein the means for detecting includes:

means for counting pulses manifesting the respective frequencies of motion of the movable members.

10. An accelerometer according to claim 9 or 1, wherein the spring, the sheet and the weight are fabricated as micromechanical structures by semiconductor fabrication techniques.

11. An accelerometer according to claim 10, further including:

control circuitry, integrally fabricated on the base, for driving the movable system and for detecting the frequency of motion thereof.

12. An accelerometer transducer for sensing acceleration along an input axis, comprising:

a base;

a first spring element;

a first sheet member having a nominal plane and mounted by the first spring element to the base for reciprocating angular motion about a first axis lying in the plane at a natural frequency of motion, said base and first spring element determining an orientation of the first sheet member orthogonal to the input axis;

a second member, mounted to the first member by a second spring element for movement responsive to acceleration along the input axis, such movement causing a change of moment of inertia about the first axis, whereby an acceleration along the input axis causes the natural frequency to change by a quantity representative of the acceleration; and

means for sensing said natural frequency.

13. A planar intertial sensor for manifesting acceleration along an input (Z) axis, comprising a sheet member and associated base member, said base member including means for supporting the peripheral region of said sheet member in a plane perpendicular to said input (Z) axis, said plane including a first (X) reference axis and a second (Y) reference axis, said first and second reference axes being mutually perpendicular and intersecting said input axis at a common point,

wherein said sheet member encloses a first pair of opposed C-shaped void regions and at least one second pair of opposed C-shaped void regions, the void regions of said first pair being larger that the void regions of said second pair, and said first pair of void regions being disposed symmetrically about said second axis and said common point, and said second pair of void regions being interior to the void regions of said first pair and being disposed symmetrically about axes parallel to said first axis when said sheet member lies in said plane,

wherein the portions of said sheet member interior to said first pair of void regions are spaced apart from said base member,

wherein the portion of said sheet member between opposed ends of the first pair of void regions is flexible about said second axis,

wherein the portion of said sheet member between opposed ends of the second pair of void regions is imbalanced about said first axis such that acceleration along the input axis changes the moment of inertia of the said portion of the sheet member about said second axis, and further including means for sensing changes in the moment of inertia.

14. A force responsive assembly for use in apparatus for sensing acceleration along an input axis associated with a base member comprising:

A. a force responsive member including a substantially planar sheet portion,

B. a torsion spring pivotally coupling said planar sheet portion to said base member whereby said planar sheet portion is adapted for reciprocating angular excursions from a reference plane perpendicular to said input axis about a first reference axis lying in said plane, wherein said force responsive member further includes a mass element and a means for coupling said mass element to said sheet portion at a distance which varies according to acceleration along the input axis so that said force responsive member is characterized by a moment of inertia about said reference axis which is a locally monotonic function of the acceleration of said base member along said input axis.

15. An assembly according to claim 14 further comprising means for driving said force responsive member in reciprocating angular excursions about said reference axis.

16. An assembly according to claim 15 further comprising a feedback control means for adaptively controlling the rate of said excursions to be at the natural frequency of angular motion of said force responsive member about said first reference axis, said rate being representative of the acceleration of said base member along said reference axis.

17. An assembly according to claim 14 wherein said mass element coupling means includes

a planar member and associated torsion spring, said associated torsion spring pivotally coupling said planar member to said planar sheet portion about a second reference axis orthogonal to said input axis and said first reference axis, and

means for supporting said mass element such that said mass element is offset from the principal plane of said planar member, and is offset from said input axis.

18. An assembly according to claim 14 wherein said mass element coupling means includes means for supporting said mass element such that said mass element is centered along said input axis and is offset from the principal plane of said planar portion by a distance which is a monotonic function of acceleration of said base member along said input axis.

19. An assembly according to claim 18 wherein said supporting means comprises a diaphragm supporting said mass element, and wherein said diaphragm is peripherally supported by said sheet portion.

20. An assembly according to claim 18 wherein said supporting means comprises a plurality of deflection springs aligned in a plane parallel to the principal plane of said sheet portion, all of said springs being affixed at one end thereof to said mass element, and affixed at the other end thereof to support members extending from said sheet portion.