Flexure assemblies and methods for manufacturing and using the same

• US 8,528,405 B2
• Filed: 12/04/2009
• Issued: 09/10/2013
• Est. Priority Date: 12/04/2009
• Status: Active Grant

• Current Assignee: Charles Stark Draper Laboratory Incorporated
• Original Assignee: Charles Stark Draper Laboratory Incorporated
• Inventors: Jenkins, Lyle J., Bernstein, Jonathan J., Fyler, Donald C.
• Primary Examiner(s): Macchiarolo, Peter
• Assistant Examiner(s): Shah, Samir M
• Application Number:
  US12/631,398
• Publication Number:
  US 20110132088A1
• Time in Patent Office: 1,376 Days
• Field of Search: 735/142.9, 735/143.2, 735/041.2, 734/88, 735/140.1, 735/142.4, 438/50, 257/E21.214
• US Class Current: 73/514.29
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 2015/0817 for pivoting movement of the mass, e.g. in-plane pendulum
• CPC Class Codes: G01P 2015/0857 using a particular shape of the suspension spring

Abstract

In one embodiment, an accelerometer includes a suspension frame, a proof mass, and a plurality of flexures suspending the proof mass from the suspension frame. The flexures allow the proof mass to deflect in response to an acceleration along a sensitive axis of the accelerometer. Each flexure exhibits an initial spring rate along the sensitive axis of substantially zero.

Claims

1. An accelerometer, comprising:

a suspension frame;

a proof mass;

a plurality of flexures suspending the proof mass from the suspension frame and allowing the proof mass to deflect in response to an acceleration along a sensitive axis of the accelerometer, each flexure exhibiting an initial spring rate along the sensitive axis of substantially zero; and

circuitry for ascertaining the acceleration influencing the proof mass.

2. The accelerometer of claim 1, wherein each flexure is essentially rigid along at least one axis orthogonal to the sensitive axis.

3. The accelerometer of claim 1, wherein each flexure is pre-stressed.

4. The accelerometer of claim 3 further comprising a pair of capacitive plates for compensating for errors in the pre-stressing of the plurality of flexures.

5. The accelerometer of claim 4, wherein a first capacitive plate is located in proximity to a first surface of the proof mass and a second capacitive plate is located in proximity to a second surface of the proof mass.

6. The accelerometer of claim 4, wherein the capacitive plates create a negative electrostatic spring having a force-displacement curve substantially equal in magnitude to and opposite in direction from a force-displacement curve of the plurality of flexures.

7. The accelerometer of claim 1, wherein the suspension frame is made from a first material and the plurality of flexures are made from a second material different from the first material.

8. The accelerometer of claim 7, wherein the first material has a greater coefficient of thermal expansion than the second material.

9. The accelerometer of claim 7, wherein the first material is aluminum and the second material is silicon.

10. The accelerometer of claim 1, wherein the suspension frame and the plurality of flexures are made from the same material.

11. The accelerometer of claim 10, wherein the suspension frame exerts a compressive force on the plurality of flexures.

12. The accelerometer of claim 1 further comprising at least one mechanical forcing mechanism for controllably compressing the plurality of flexures.

13. The accelerometer of claim 12, wherein the at least one mechanical forcing mechanism is an adjustment screw.

14. The accelerometer of claim 1, wherein the plurality of flexures are made from single-crystal silicon and are doped with impurities that put the flexures into compression.

15. The accelerometer of claim 1, wherein each flexure comprises an epitaxial layer of a silicon-germanium alloy on a silicon layer.

16. The accelerometer of claim 1, wherein each flexure comprises an epitaxial layer of a silicon-germanium-boron alloy on a silicon layer.

17. The accelerometer of claim 1, wherein each flexure comprises a silicon dioxide layer on a silicon layer.

18. A flexural pivot, comprising:

a substantially ring-shaped flange; and

a plurality of radially-spaced flexures extending inwardly and continuously from the flange such that the plurality of radially-spaced flexures couple to one another at a center of the flange, wherein the flexural pivot exhibits a torsional spring rate of substantially zero.

19. The flexural pivot of claim 18, wherein each flexure is pre-stressed.

20. The flexural pivot of claim 18, wherein the flange is compressed.

21. The flexural pivot of claim 18 further comprising i) a base pivot positioned in proximity to a first side of the flange, and ii) a rotatable element, free to rotate relative to the base pivot, coupled to a second side of the flange.

22. The flexural pivot of claim 18 further comprising a base pivot positioned in proximity to a first side of the flange, the base pivot comprising a plurality of connection pins extending therefrom.

23. The flexural pivot of claim 22 further comprising a plurality of connectors, each radially-spaced flexure being coupled to one of the connectors and each connector being coupled to one of the connection pins.

24. A method for fabricating a proof mass assembly, the method comprising:

epitaxially growing, on at least one side of a crystalline material, an alloy having a lattice constant greater than that of the crystalline material, thereby forming a starting wafer; and

etching the starting wafer to define a suspension frame, a plurality of flexures extending therefrom, and a proof mass suspended by the flexures, each flexure being stressed by the lattice mismatch between the epitaxially grown alloy and the crystalline material such that its initial spring rate along a first axis is substantially zero.

25. The method of claim 24, wherein each flexure is compressively stressed.

26. The method of claim 24, wherein the crystalline material is silicon.

27. The method of claim 24, wherein the alloy comprises silicon and germanium.

28. The method of claim 27, wherein the alloy further comprises boron.

29. The method of claim 24, wherein etching the starting wafer comprises employing an inductively coupled plasma etch.

30. The method of claim 24, wherein etching the starting wafer comprises employing a selective etch to undercut each flexure.

31. The method of claim 30, wherein the selective etch is selected from the group consisting of an ethylene-diamine pyrocatechol etch, a tetra-methyl ammonium hydroxide etch, and a potassium hydroxide etch.

32. A method for fabricating a proof mass assembly, the method comprising:

providing a wafer having an isolated silicon layer proximate at least one surface thereof;

forming an oxide on at least a portion of each silicon layer; and

etching the wafer to define a suspension frame, a plurality of flexures extending therefrom, and a proof mass suspended by the flexures, the silicon exerting a stress upon the oxide such that an initial spring rate along a first axis of each flexure is substantially zero.

33. The method of claim 32, wherein the silicon exerts a compressive stress upon the oxide.

34. The method of claim 32, wherein the oxide is thermally grown on the silicon.

35. The method of claim 32, wherein the oxide is deposited on the silicon by chemical vapor deposition.

36. The method of claim 32, wherein etching the wafer comprises employing an inductively coupled plasma etch.

37. The method of claim 32, wherein etching the wafer comprises employing a selective etch to undercut each flexure.

38. The method of claim 37, wherein the selective etch is selected from the group consisting of an ethylene-diamine pyrocatechol etch, a tetra-methyl ammonium hydroxide etch, a potassium hydroxide etch, and a xenon difluoride etch.

39. The method of claim 32, wherein the wafer is a silicon wafer.

40. The method of claim 32, wherein each silicon layer is isolated from the wafer by an additional oxide layer therebetween.