Translational accelerometer and accelerometer assembly method

• US 4,872,342 A
• Filed: 07/21/1988
• Issued: 10/10/1989
• Est. Priority Date: 06/27/1986
• Status: Expired due to Fees

• Current Assignee: Alliedsignal Inc.
• Original Assignee: Sundstrand Data Control, Inc.
• Inventors: Hanson, Richard A., Urbach, Edward A., Peters, Rex B., Norling, Brian L.
• Primary Examiner(s): Chapman, John
• Application Number:
  US07/222,680
• Time in Patent Office: 446 Days
• Field of Search: 73/510, 73/514, 73/517 R, 73/517 AV, 73/517 B, 73/517 A, 73/497, 310/329, 29/25, 29/35, 29/418, 29/423, 29/595, 29/610 SG
• US Class Current: 73/514.29
• CPC Class Codes: G01P 15/0888 for indicating angular acceleration
• CPC Class Codes: G01P 15/097 by vibratory elements
• CPC Class Codes: G01P 2015/0828 the mass being of the paddle type being suspended at one of its longitudinal ends
• CPC Class Codes: Y10T 29/49007 Indicating transducer

Abstract

An accelerometer comprising a body (10, 16, 12), a proof mass (18, 30, 32), a mounting strucutre comprising flexures (20, 22) for mounting the proof mass to the body, and force sensing elements (34, 38). The flexures permit translational motion of the proof mass with respect to the body along a sensitive axis SA and rotation of the proof mass with respect to the body about a hinge axis HA that is perpendicular to the sensitive axis. Acceleration of the accelerometer along the sensitive axis results in translational motion of the proof mass along the sensitive axis. The force sensing elements reacts to such translational motion by producing a signal indicative of acceleration along the sensitive axis. In a preferred embodiment, the mounting structure comprises a pair of fused quartz flexures that are oppositely directed with respect to one another, and the force sensing elements comprise a pair of vibrating beam force transducers that are connected to the proof mass on opposite sides of the hinge axis from one another. The described assembly method for accelerometers comprises forming bridges between the proof mass and body, the bridges being removed after attachment of the force sensing elements.

Claims

1. An accelerometer for measuring acceleration along a sensitive axis, comprising:

a body;

a proof mass;

mounting means for mounting the proof mass to the body, the mounting means comprising first and second flexures connected to the proof mass and arranged to permit substantially uncoupled translational and rotational motion of the proof mass with respect to the body such that the translational motion is along the sensitive axis and such that the rotational motion is about a single hinge axis perpendicular to the sensitive axis, the first and second flexures being positioned on opposite sides of a pendulous axis that is normal to the hinge axis and to the sensitive axis and that passes through the center of mass of the proof mass, each flexure having a hinge axis aligned along said single hinge axis, whereby acceleration ofthe accelerometer along the sensitive axis results in said translational motion of the proof mass along the sensitive axis; and

sensing means for reacting to said translational motion by producing a signal indicative of acceleration along the sensitive axis.

2. The accelerometer of claim 1, wherein each flexure comprises a leaf flexure formed so as to undergo angular bending as a result of rotation of the proof mass about the single hinge axis, and so as to undergo S-bending as a result of translation of the proof mass along the sensitive axis.

3. The accelerometer of claim 2, wherein the mounting means further comprises a support, and wherein each flexure comprises a leaf member extending between the support and the proof mass, the leaf members of the flexures lying in a common plane normal to the sensitive axis and parallel to the single hinge axis.

4. The accelerometer of claim 1, wherein the mounting means further comprises a support, wherein the first flexure extends from a first point of connection to the support to a first point of connection to the proof mass in a first direction along the pendulous axis, and wherein the second flexure extends from a second point of connection to the support to a second point of connection to the proof mass in a second direction along the pendulous axis opposite to the first direction.

5. The accelerometer of claim 4, wherein each flexure comprises a leaf flexure formed so as to undergo angular bending that results in rotation of the proof mass about the single hinge axis, and so as to undergo S-bending that results in translation of the proof mass along the sensitive axis.

6. The accelerometer of claim 5, wherein each flexure comprises a leaf member extending between the support and the proof mass, the leaf members of the flexures lying in a common plane normal to the sensitive axis.

7. The accelerometer of claim 4, wherein the flexures are radially symmetric with respect to a point between the flexures.

8. The accelerometer of claim 4, wherein the proof mass is symmetric along any line passing through a center of the proof mass.

9. The accelerometer of claim 1, wherein the sensing means comprises first and second force sensing elements between the body and the proof mass, the first and second force sensing elements being connected to the proof mass at respective first and second connection points, the first and second connection points being located on opposite sides of the single hinge axis from one another.

10. The accelerometer of claim 9, wherein the force sensing elements are connected such that differential thermal expansion and contraction of the force sensing elements with respect to the other components of the accelerometer results primarily in rotation of the proof mass about the single hinge axis.

11. The accelerometer of claim 10, wherein the first force sensing element extends from the first connection point in one direction along the sensitive axis, and wherein the second force sensing element extends from the second connection point in a direction opposite to that of the first force sensing element.

12. The accelerometer of claim 11, wherein each force sensing element is a vibrating beam force transducer.

13. The accelerometer of claim 12, wherein each force sensing element comprises a double-ended tuning fork.

14. The accelerometer of claim 13, wherein the proof mass comprises a proof mass base, wherein the mounting means further comprises a support, wherein each flexure extends between the proof mass base and the support, and wherein the support, flexures and proof mass base comprise a unitary, fused quartz structure.

15. The accelerometer of claim 14, wherein the proof mass further comprises weights positioned on opposite sides of the proof mass base from one another.

16. The accelerometer of claim 9, wherein the first and second connection points lie along the pendulous axis.

17. The accelerometer of claim 16, wherein the center of gravity of the proof mass is positioned midway between the connection points.

18. The accelerometer of claim 9, wherein the proof mass is characterized by a center and a radius of gyration, and wherein the force sensing elements are attached to the proof mass at connection points that are spaced from the center by a distance greater than the radius of gyration.

19. The accelerometer of claim 16, further comprising upper and lower frames between which the mounting means is supported, wherein the proof mass has upper and lower surfaces on which shock pads are positioned, each shock pad being positioned between the first and second connection points, such that rotation of the proof mass about the single hinge axis does not significantly vary the distance between each shock pad and the frame adjacent thereto.

20. The accelerometer of claim 1, wherein the flexures comprise fused quartz.

21. The accelerometer of claim 1, wherein the proof mass comprises a proof mass base, wherein the mounting means further comprises a support, wherein each flexure extends between the proof mass base and the support, and wherein the accelerometer further comprises upper and lower frames between which the support is fastened.

22. The accelerometer of claim 21, wherein the support comprises a ring encircling the proof mass base, the ring having an upper surface in which two upper mounting pads are formed, and a lower surface on which two lower mounting pads are formed, the upper and lower mounting pads being directly above and below one another, the upper and lower mounting pads being adapted to abut the upper and lower frames, respectively, to secure the support between the frames.

23. The accelerometer of claim 22, wherein each mounting pad is relatively elongated in a circumferential direction and relatively narrow in a radial direction.

24. The accelerometer of claim 23, wherein the two upper mounting pads are radially symmetric with respect to one another, and form the only point of contact between the support and the upper frame, and wherein the lower mounting pads are radially symmetric with respect to one another, and form the only points of contact between the support and the lower frame.

25. The accelerometer of claim 24, wherein the upper and lower mounting pads do not intersect either the single hinge axis or the pendulous axis.

26. The accelerometer of claim 1, wherein the mounting means further comprises a support and an outer and inner arm for each flexure, each outer arm being connected between the support and the respective flexure, each inner arm being connected between the respective flexure and the proof mass, each arm being compliant about an axis parallel to the sensitive axis.

27. The accelerometer of claim 1, wherein the proof mass comprises a proof mass base and a weight secured thereto, one end of the weight being secured to the proof mass base through a compliant attachment element.

28. The accelerometer of claim 27, wherein the attachment element is compliant along the pendulous axis.

29. The accelerometer of claim 28, wherein the attachment element comprises a plate oriented normal to the pendulous axis.

30. The accelerometer of claim 1, wherein the sensing means comprises first and second force sensing elements connected between the body and the proof mass, each force sensing element comprising a vibrating beam force transducer, the first and second force sensing elements being connected to the proof mass at respective first and second connection points located on opposite sides of the single hinge axis from one another, the first force sensing element extending from the first connection point in one direction along the sensitive axis, and the second force sensing element extending from the second connection point in a direction opposite to that of the first force sensing element, the first and second force sensing elements including means for producing respective first and second output signals indicative of the force on the force sensing elements, the sensing means further comprising processing means for producing a signal indicative of acceleration along the sensitive axis as a function of the difference between the first and second output signals, and for producing a signal indicative of angular acceleration as a function of the sum of the output signals.

31. An accelerometer for measuring acceleration along a sensitive axis, comprising:

a body;

a proof mass;

mounting means for mounting the proof mass to the body, the mounting means comprising first and second flexures connected to the proof mass and arranged to permit translational motion of the proof mass with respect to the body along the sensitive axis and rotational motion of the proof mass with respect to the body about a single hinge axis perpendicular to the sensitive axis, the first and second flexures being positioned on opposite sides of a pendulous axis that is normal to the hinge axis and to the sensitive axis and that passes through the center of mass of the proof mass, the first and second flexures each having a hinge axis aligned along said single hinge axis, the center of mass of the proof mass being offset from the single hinge axis, the location of the center of mass of the proof mass and the net translational and rotational compliance of the mounting means being chosen so that acceleration of the accelerometer along the sensitive axis results in a combination of translational motion of the proof mass along the sensitive axis and rotational motion of the proof mass about the single hinge axis, such that the effective axis of rotation of the proof mass is external to the proof mass and to all structures directly attached to the proof mass, such as the flexures; and

sensing means for reacting to said translational motion by producing a signal indicative of acceleration along the sensitive axis. PG,28

32. An accelerometer for measuring acceleration along a sensitive axis, comprising:

a body;

a proof mass;

mounting means for mounting the proof mass to the body, the mounting means comprising first and second flexures connected to the proof mass and arranged to permit translational motion of the proof mass with respect to the body along the sensitive axis and rotational motion of the proof mass with respect to the body about a hinge axis perpendicular to the sensitive axis, the first and second flexures being positioned on opposite sides of a pendulous axis that is normal to the hinge axis and to the sensitive axis and that passes through the center of mass of the proof mass, whereby acceleration of the accelerometer along the sensitive axis results in a combination of translational motion of the proof mass along the sensitive axis and rotational motion of the proof mass about the hinge axis; and

sensing means for reacting to said translational motion by producing a signal indicative of acceleration along the sensitive axis, th sensing means comprising first and second force sensing elements connected between the body and the proof mass, the first and second force sensing elements being connected to the proof mass at respective first and second connection points, the first and second connection points being located on opposite side of the hinge axis from one another.

33. The accelerometer of claim 32, wherein each flexure comprises a leaf flexure formed so as to undergo angular bending as a result of of rotation of the proof mass about the single hinge axis, and so as to undergo S-bending as a result of the translation of the proof mass along the sensitive axis.

34. The accelerometer of claim 32, wherein the mounting means further comprises a support, wherein the first flexure extends from a first point of connection of the support to a first point of connection to the proof mass in a first direction along the pendulous axis, and wherein the second flexure extends from a second point of connection to the support to a second point of connection to the proof mass in a second direction along the pendulous axis opposite to the first direction.

35. The accelerometer of claim 32, wherein the flexures comprise fused quartz.

36. The accelerometer of claim 32, wherein the proof mass comprises a proof mass base, wherein the mounting means further comprises a support, wherein each flexure extends between the proof mass base and the support, and wherein the accelerometer further comprises upper and lower frames between which the support is fastened.

37. The accelerometer of claim 32, wherein the distance between the first connection point and the hinge axis is different from the distance between the second connection point and the hinge axis.

38. A method of assembling an accelerometer that is adapted for measuring acceleration along a sensitive axis, the method comprising:

forming at least two bridges between a proof mass and a body that comprise the accelerometer, said body including upper and lower frames, the accelerometer further comprising two force sensing elements connected between the body and the proof mass, and mounting means for mounting the proof mass to the body using flexures that are arranged to permit both translational and rotational motion of the proof mass with respect to the body, the mounting means permitting;

(a) rotation of the proof mass with respect to the body about a single hinge axis that is aligned with a hinge axis of each of the flexures and is perpendicular to the sensitive axis, and (b) translational motion of the proof mass with respect to the body along the sensitive axis;

assembling the accelerometer, the assembly step including the step of securing the mounting means between the upper and lower frames;

attaching the force sensing elements to the proof mass and body; and

removing both bridges.

39. The method of claim 38, wherein the bridges are formed on opposite sides of a single hinge axis from one another, such that a line between the bridges is substantially perpendicular to the single axis.

40. The method of claim 38, wherein the bridges, mounting means, and at least portions of the body and proof mass are formed from a single piece of amorphous quartz, and wherein each bridge extends between said proof mass portion and said body portion.