INERTIAL SENSOR

• US 20080148851A1
• Filed: 11/26/2007
• Published: 06/26/2008
• Est. Priority Date: 11/28/2006
• Status: Active Grant

• Current Assignee: Nanometrics Incorporated (Onto Innovation Inc.)
• Original Assignee: Nanometrics Incorporated (Onto Innovation Inc.)
• Inventors: Ackerley, Nicholas Jason, Bainbridge, Geoffrey Stewart
• Granted Patent: US 7,594,438 B2
• Time in Patent Office: Days
• US Class Current: 73/514.38
• CPC Class Codes: G01P 15/08 with conversion into electric or magnetic values
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 2015/0814 for translational movement of the mass, e.g. shuttle type
• CPC Class Codes: G01P 2015/0828 the mass being of the paddle type being suspended at one of its longitudinal ends
• CPC Class Codes: G01V 1/181 Geophones

Abstract

A long-period weak-motion inertial sensor includes a frame having a frame mounting surface, a movable mass having a movable mass mounting surface, a transducer for sensing displacements of the movable mass with respect to the frame, and a monolithic flexure element for suspending the movable mass in the frame. The monolithic flexure element includes: a stiff frame integral clamp attachable to the frame mounting surface of the frame, a stiff movable mass integral clamp attachable to the movable mass mounting surface of the movable mass, and a stiffest flexible region for operatively connecting the frame integral clamp to the movable mass integral clamp. The frame and movable mass mounting surfaces do not overlap the stiffest flexible region, thereby minimizing the generation of creep and hysteresis noise. The variation in stiffness of the monolithic flexure element is controlled by varying thickness along the length of the flexure element.

Claims

1. A long-period weak-motion inertial sensor comprising:

a frame having a frame mounting surface;

a movable mass having a movable mass mounting surface;

a transducer for sensing displacements of the movable mass with respect to the frame; and

a monolithic flexure element for suspending the movable mass in the frame, the monolithic flexure element comprising;

a stiff frame integral clamp attachable to the frame mounting surface of the frame;

a stiff movable mass integral clamp attachable to the movable mass mounting surface of the movable mass; and

a stiffest flexible region for operatively connecting the frame integral clamp to the movable mass integral clamp;

wherein the frame and movable mass mounting surfaces do not overlap the stiffest flexible region, thereby minimizing the generation of creep and hysteresis noise, and

wherein variation in stiffness of the monolithic flexure element is controlled by varying thickness along the length of the monolithic flexure element.

2. The inertial sensor of claim 1, wherein the frame and movable mass integral clamps are at least thirty times as stiff as the flexible region.

3. The inertial sensor of claim 2, wherein the monolithic flexure element includes a material having a loss angle less than 10⁻

3 radians.

4. The inertial sensor of claim 3, wherein two or more monolithic flexure elements are arranged to constrain the movable mass to have a single effective degree of freedom, wherein the degree of freedom is rotational and has a predetermined axis.

5. The inertial sensor of claim 4, comprising two pairs of flexure elements, wherein a distance between the two pairs of flexure elements is substantially larger than a width of each pair, and wherein the two elements in each pair are not parallel to each other.

6. The inertial sensor of claim 5, wherein the flexible regions of the two pairs of flexure elements cross at their midpoints at right angles to each other.

7. The inertial sensor of claim 4, including a first and a second flexible region that are arranged to be coplanar and widely-spaced.

8. The inertial sensor of claim 7, including a third flexible region that is arranged to cross at right angles to and at the midpoint of the first and second regions.

9. The inertial sensor of claim 3, wherein two or more monolithic flexure elements are arranged to constrain the movable mass to have a single effective degree of freedom, wherein the degree of freedom is translational.

10. The inertial sensor of claim 9, including a first and second flexure element, the first and second flexure elements arranged to be substantially parallel, non-coplanar and spaced substantially further apart than a width of a flexible region.

11. The inertial sensor of claim 10, wherein the first flexure element includes a first and second flexible region wherein the first and second flexible regions are coplanar and a distance between the first and second flexible region is substantially larger than the width of either flexible region, and the second flexure element includes a third and a fourth flexible region wherein the third and fourth flexible regions are coplanar and a distance between the third and fourth flexible region is substantially larger than the width of either flexible region.

12. The inertial sensor of claim 9, comprising two widely spaced monolithic flexure elements, wherein:

the frame integral clamp region is substantially annular;

the movable mass integral clamp region is substantially annular and concentric with and circumscribed by the frame integral clamp region; and

each monolithic flexure element includes three or more flexible regions being symmetrically disposed around the center of the flexure element.

13. The inertial sensor of claim 3, wherein the monolithic flexure element supplies forces for balancing the effect of gravity on the movable mass.

14. The inertial sensor of claim 3, wherein the monolithic flexure element includes a datum plane defined by the frame and movable mass mounting surfaces and one surface of the flexible region.

15. The inertial sensor of claim 3, wherein the monolithic flexure element includes a plane of symmetry that bisects a thinnest dimension of the flexible region.

16. The inertial sensor of claim 3, wherein the monolithic flexure element includes sufficiently small residual internal stresses that it can be subjected to an appropriate aging heat treatment without changing shape by more than 1% of the length of the flexible region in a preinstalled state.

17. The inertial sensor of claim 3, wherein the monolithic flexure element includes a metal and a plating material, the plating material being dissimilar to the metal and chemically inert.

18. The inertial sensor of claim 3, wherein the monolithic flexure element is mounted to the frame and the movable mass by fasteners and load-spreading clamps for spreading stresses induced in the contact areas of the fasteners.

19. The inertial sensor of claim 3, wherein the frame and movable mass mounting surfaces are arranged to be at least twice as far from any surface of the flexible region as a thickness of the flexible region.

20. The inertial sensor of claim 3, wherein the material is selected from the group consisting of:

sapphire, silicon, high strength steels, beryllium copper, and titanium alloys.