WO1992008139A1

WO1992008139A1 - Translational accelerometer with motion constraints

Info

Publication number: WO1992008139A1
Application number: PCT/US1991/007857
Authority: WO
Inventors: Stephen F. Becka; Brian L. Norling
Original assignee: Sundstrand Data Control, Inc.
Priority date: 1990-11-05
Filing date: 1991-10-21
Publication date: 1992-05-14
Also published as: IL99959A0

Abstract

A constraining system for an accelerometer of the type that comprises flexures (66, 68) or the like for mounting a proof mass (64, 124, 150) to a support (62, 122, 164) for translational motion along a sensing axis (SA) and rotational motion about a hinge axis (HA) perpendicular to the sensing axis. The system provides constraining rods (90, 92, 130, 132, 160, 162, 170) for constraining rotational motion of the proof mass about a pendulous axis (PA), the pendulous axis being normal to the sensing and hinge axes. The constraining rods may also limit proof mass translation along the hinge axis.

Description

TRANSLATIONAL ACCELEROMETER WITH MOTION CONSTRAINTS

License Rights

The U.S. Government has certain rights in this invention, as provided for by the terms of Contract No. F04704-86-C-0160 awarded by the Department of the Air Force.

Field of the Invention

The present invention relates to accelerometers and, in particular, to a high-performance accelerometer suitable for use in inertial navigation systems.

Background of the Invention

An example of a prior accelerometer design with high-performance potential is described in U.S. Patent No. 4,872,342. This accelerometer design includes a proof mass that is mounted by a pair of flexures so as to permit translational motion of the proof mass with respect to the accelerometer body along the sensing (i.e., input) axis, and to also permit rotational motion of the proof mass about a hinge axis that is perpendicular to the sensing axis. The flexures are positioned on opposite sides of a third, pendulous axis that is normal to the hinge axis and to the sensing axis, and that passes through the center of mass of the proof mass. Translational motion of the proof mass in response to acceleration is sensed by a pair of force transducers, and converted into an indication of acceleration along the sensing axis.

For optimum performance, accelerometers of the general type described above should have flexures that are highly compliant for translational motion of the proof mass along the sensing axis, and for rotation about the hinge axis. However, an ideal flexure mechanism would also be stiff to all other translations and rotations, thereby providing well-defined proof mass motion and high natural frequencies. However, the. accelerometer design described in U.S. Patent No. 4,872,342 is not fully optimized in this respect. In particular, the compliant flexures are the primary restraint for proof mass rotation about the pendulous

SUBSTITUTE SHEET axis. This compliance to pendulous axis rotation, when coupled with the inertia of the proof mass, leads to unacceptably low rotational resonances about the pendulous axis. Similar problems also occur with respect to translational motion along the hinge axis.

Summary of the Invention

The problem described above is solved by the present invention. In particular, the present invention provides means, separate from the proof mass mounting means and force transducers, for constraining rotation of the proof mass about the pendulous axis.

In a preferred embodiment, the invention is for use in an accelerometer that comprises a support, a proof mass, and mounting means for mounting the proof mass to the support. The mounting means permits translational motion of the proof mass along the sensing axis, and rotational motion of the proof mass about a hinge axis perpendicular to the sensing axis. Typically, such a mounting means does not supply a rigid constraint for proof mass rotation about the pendulous axis, i.e., the axis that is normal to the hinge and sensing axes, and that passes through the center of mass of the proof mass. The accelerometer also includes sensing means for reacting to proof mass motion by producing a signal indicative of acceleration along the sensing axis.

The present invention provides constraining means separate from the mounting means and from the sensing means, for constraining rotational motion of the proof mass about the pendulous axis. In one preferred embodiment, the constraining means comprises one or more rods connected between the proof mass and the support. Each rod is connected to the proof mass at a point offset from the center of mass along the accelero meter's sensing axis. Each rod is relatively stiff for tension or compression along its longitudinal axis, and relatively compliant for S bending in planes normal to its longitudinal axis. The rods also preferably constrain proof mass translation along the hinge axis. In one embodiment, two such rods are positioned on opposite sides of the proof mass from one another.

Brief Description of the Drawings FIGURE 1 is a partially schematic view of a translational accelerometer. FIGURE 2 is a partially schematic view of an accelerometer in accordance with the present invention.

FIGURES 3 and 4 illustrate two modes of movement of the proof mass tor the accelerometer of FIGURE 2.

FIGURE 5 illustrates a preferred embodiment of the invention. FIGURE 6 is a cross-sectional view of the accelerometer of FIGURE 5, taken along hinge axis HA.

FIGURE 7 is a cross-sectional view of another preferred embodiment of the invention.

FIGURE 8 is a schematic cross-sectional view of a further embodiment of the invention.

Detailed Description of the Invention

FIGURE 1 schematically illustrates an accelerometer of the general type described in U.S. Patent No. 4,872,342. The accelerometer measures acceleration along sensing axis SA, and comprises proof mass assembly 10 that has an overall disk-like shape, and that is typically clamped between upper and lower plates (not shown). Proof mass assembly 10 comprises support ring 12 from which proof mass 14 is suspended by flexures 16 and 18. The flexures permit the proof mass to move translationally along sensing axis SA, and to rotate with respect to the support about hinge axis HA that lies in the plane of the proof mass assembly, and that is normal to sensing axis SA.

A pair of force transducers 20 and 22 are connected between proof mass 14 and supports 24 and 26, respectively, the supports being fixed in position with respect to support ring 12. In the illustrated embodiment, proof mass 14 has a center of mass 30 that is located on hinge axis HA. The accelerometer has a pendulous axis PA that is defined to be an axis passing through center of mass 30, and normal to the hinge and sensing axes. Force sensing elements 20 and 22 are located in the SA-PA plane, and their longitudinal axes are aligned with the sensing axis.

Force transducers 20 and 22 are electrically coupled to a suitable sensing circuit. The sensing circuit determines acceleration by determining the difference between the forces sensed by the force transducers. A tension force exerted along a force transducer is viewed as having a positive sign, while a compression force has a negative sign (or vice versa). An acceleration along sensing axis SA causes the proof mass to translate, without rotation, along the sensing axis. This movement exerts a compression force on one of the force transducers, and an equal tension force on the other force transducer. Thus when the difference between the sensed forces is taken, the magnitudes add. However, differential thermal expansion between the force transducers and the other accelerometer components causes proof mass 14 to rotate about hinge axis HA, and the resistance to this movement by the flexures results in equal forces sensed bv the force transducers. When the difference between the sensed forces is taken, this thermally-induced signal cancels. This feature renders the accelerometer relatively insensitive to temperature-induced errors.

For improved performance, an accelerometer of the type shown in FIGURE 1 should be fabricated with flexures that are highly compliant for the described translational and rotational motions. However, flexures 16 and 18 also form the primary restraint for rotation of the proof mass about pendulous axis PA. As a result, the accelerometer is also relatively compliant for pendulous axis rotations. This leads to an unacceptably low frequency rotational resonance of the proof mass about the pendulous axis. A similar problem exists with respect to translational motion of the proof mass along hinge axis HA. Both of these problems are addressed by the present invention.

FIGURE 2 presents a schematic view of an accelerometer constructed in accordance with the present invention. The accelerometer of FIGURE 2 includes all of the components shown in FIGURE 1, and such components are therefore referenced with the same reference numerals. In addition, the accelerometer of FIGURE 2 includes constraining rods or beams 40 and 42 connected between the proof mass and mechanical ground. In particular, constraining rod 40 has one end connected to post 44 that extends upward from the upper surface of proof mass 14, and has its other end connected to support 46 that is fixed in position with respect to support ring 12. Constraining rod 42 has one end connected to post 50 that extends from the lower surface of proof mass 14, and has its other end connected to support 52 that is fixed in position with respect to support ring 12. Preferably, the central axes of posts 44 and 50 are aligned with one another, and pass through the center of mass 30 of the proof mass. Constraining rods 40 and 42 are parallel to one another and to the hinge axis, and offset from center of mass 30, as well from the hinge axis, by equal amounts in opposite directions. In general, the greater the offset, the greater constraint provided against pendulous axis rotation.

Constraining rods 40 and 42 are dimensioned such that acting together, they are relatively compliant for the two desired proof mass movements, and relatively uncompliant for other proof mass movements. In particular, the rods are fabricated such that they are relatively stiff (i.e., noncompliant) with respect to tension and compression forces, and such that they resist buckling under compressive loads. The rods therefore prevent rotation of the proof mass in either direction about pendulous axis PA, and also prevent translational movement of the proof mass along hinge axis HA. The two proof mass movements to which the constraining rods are compliant are the two desired motions, i.e., translation along sensing axis SA and rotation about hinge axis HA. These two movements are shown in FIGURES 3 and 4, respectively. In particular, FIGURE 3 illustrates that when proof mass 14 translates along sensing axis SA, constraining rods 40 and 42 S bend in a plane perpendicular to the pendulous axis to accommodate such movement. FIGURE 4 illustrates that when proof mass 14 rotates about hinge axis HA, constraining rods 40 and 42 S bend in separate planes normal to sensing axis SA, and also twist about their longitudinal axes. Long thin rods are highly compliant for both of the required types of movements, i.e., for twisting about their longitudinal axes, and for S bending in directions normal to their longitudinal axes.

The cross sectional shapes of the constraining rods may be selected to create different compliances in different direction. Thus constraining rods having round cross sectional shapes would have equal compliances for S bending in either the SA-HA plane (FIGURE 3) or the HA-PA plane (FIGURE 4). Rectangular rods of different aspect ratios could also be used, to create different compliances along different of these directions.

Constraining rods 40 and 42 will increase the stiffness of the accelerometer to translational motion along the sensing axis. This increased stiffness can be compensated by reducing the stiffness of the flexures for such motion. This reduction in flexure stiffness is made possible by the fact that the flexures are no longer required to restrain pendulous axis rotation.

A preferred embodiment of the accelerometer of the present invention is illustrated in FIGURES 5 and 6. This embodiment includes proof mass assembly 60 that comprises support ring 62 and proof mass 64 suspended from the support ring by flexures 66 and 68, substantially as shown in FIGURE 1. Force sensing elements 70 and 72 are connected between proof mass 64 and supports 74 and 76, respectively, the supports being fixed in position with respect to support ring 62. Proof mass 62 includes weights 80 and 82 positioned on its upper and lower surfaces, respectively, in order to provide an appropriate g-range for the accelerometer.

In the embodiment of FIGURES 5 and 6, the constraining means of the present invention comprises constraining rods 90 and 92. Constraining rod 90 has one end bonded to post 94 that extends upward from support ring 62, and has its other end bonded to post 96 that extends upwardly from proof mass 64, on the opposite side of weight 80 from post 94. In a complementary manner, constraining rod 92 has one end bonded to post 98 that extends downward from support ring 62, and has the other end bonded to post 100 that extends downward from proof mass 64 on the opposite side of weight 82. Each constraining rod together with its respective pair of posts could be formed as a single monolithic piece of fused quartz. In contrast to the arrangement of FIGURE 2 where posts 44 and 50 extend from the center of the proof mass, the embodiment of FIGURES 5-6 provides for relatively long constraining rods with respect to the accelerometer diameter. This is achieved by having each constraining rod pass through the SA-PA plane. Long constraining rods are preferred, because they minimize the effect on design compliances for a given rise in pendulous axis rotational stiffness.

The constraining rods are designed to resist buckling under axial compression loads, and therefore restrain the proof mass under hinge axis translation. This reduces the stress to the flexures as they S bend along the hinge axis. Since the flexures are no longer required to resist this load, the flexure width can be reduced, further reducing stress and allowing for increased proof mass damping area.

Another preferred embodiment of the invention is illustrated in FIGURE 7, in a cross sectional view similar to that of FIGURE 6. The accelerometer shown in FIGURE 7 includes support ring 122 from which proof mass 124 is suspended by flexures 126 and 128. Constraining rods 130 and 132 connect the proof mass to the support ring above and below the proof mass assembly respectively. Constraining rod 130 has its center connected to post 134 that extends upward from the center of proof mass 124, and has its ends connected to posts 136 and 138 that extend upwardly from the support ring on opposite sides of the proof mass along the hinge axis. In a similar manner, the center of constraining rod 132 is connected to post 140 that extends downwardly from the lower surface of proof mass 124, and has its ends connected to posts 142 and 144 that extend downwardly from the support ring on opposite sides of the proof mass.

In effect, in the embodiment of FIGURE 7, there are a total of four constraining rods. The advantage of this embodiment is that it is not necessary for the constraining rods to be able to withstand a full compressive load without buckling, since movement of the proof mass in either direction along the hinge axis will place portions of both constraining rods in tension.

A further embodiment of the invention is illustrated in schematic form in FIGURE 8. In this embodiment, proof mass 150 includes posts 152 and 154 that are aligned with center of gravity 156 of the proof mass. Constraining rods 160 and 162 are connected between posts 152 and 154 respectively and support 164. Constraining rod 170 is connected directly between proof mass 150 and support 164. As with the embodiment of FIGURE 7, no constraining rod will be subjected to a full compressive load, because movement of the proof mass in either direction along the hinge axis will place one or the other set of constraining rods in tension.

While the preferred embodiments of the invention have been illustrated and described, variations will be apparent to those skilled in the art. For example, the constraining means could comprise a single rod connected between the proof mass and the support. Such a rod could be similar to rod 40 of FIGURE 2, or to rod 90 of FIGURE 5, wherein the rod is connected to the support at a single point. Such a single rod could also be similar to rod 130 shown in FIGURE 7, wherein the rod is connected to the support at two points. The principles of the present invention are also applicable to a force rebalance accelerometer in which the proof mass is maintained in a null position by an electromagnetic, electrostatic, or other force rebalance mechanism. The invention is also applicable to accelerometers in which a different number of force transducers were used, or in which the force transducers are oriented in a manner different from that shown in the FIGURES. The invention is not limited to accelerometers that utilize flexures, but could be used in connection with any proof mass mounting technique in which the proof mass had the indicated degrees of freedom. Accordingly, the scope of the invention is to be determined from the following claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An accelerometer for measuring acceleration along a sensing axis, comprising: a support; a proof mass; mounting means for mounting the proof mass to the support, the mounting means permitting translational motion of the proof mass with respect to the support along the sensing axis and rotational motion of the proof mass with respect to the support about a hinge axis that is perpendicular to the sensing axis; sensing means for reacting to motion of the proof mass by producing a signal indicative of acceleration along the sensing axis; and constraining means separate from the mounting means and from the sensing means for constraining rotational motion of the proof mass about a pendulous axis, the pendulous axis being normal to the hinge axis and to the sensing axis and passing through a center of mass of the proof mass.

2. The accelerometer of Claim 1, wherein the constraining means comprises a constraining member connected between the proof mass and the support, the constraining member being connected to the proof mass at a point that is offset from the center of mass along the sensing axis, the constraining member having a constraining axis and being relatively stiff for tension or compression along its constraining axis, and relatively compliant for S bending in a plane normal to its constraining axis.

3. The accelerometer of Claim 1, wherein the constraining means comprises two or more constraining members connected between the proof mass and the support, each constraining member being connected to the proof mass at a point that is offset from the center of mass along the sensing axis, each constraining member having a constraining axis and being relatively stiff for tension or compression along its constraining axis, and relatively compliant for S bending in a plane normal to its constraining axis.

4. The accelerometer of Claim 3, wherein the constraining members are positioned in opposite sides of the proof mass from one another.

5. The accelerometer of Claim 4, wherein each constraining member extends through a plane that is normal to the hinge axis and that contains the center of mass of the proof mass.

6. The accelerometer of Claim 1, wherein the constraining means further includes means for constraining translational motion of the proof mass along the hinge axis.

7. The accelerometer of Claim 1, wherein the mounting means comprises first and second flexures positioned on opposite sides of the pendulous axis from one another.

8. The accelerometer of Claim 7, wherein the constraining means includes means for constraining translational motion of the proof mass along the hinge axis.

9. The accelerometer of Claim 7, wherein the constraining means comprises a rod connected between the proof mass and the support, the rod being connected to the proof mass at a point that is offset from the center of mass along the sensing axis, a rod having a longitudinal axis and being relatively stiff for tension or compression along its longitudinal axis, and relatively compliant for S bending in a plane normal to its longitudinal axis.

10. The accelerometer of Claim 7, wherein the constraining means comprises two or more rods connected between the proof mass and the support, each rod being connected to the proof mass at a point that is offset from the center of mass along the sensing axis, each rod having a longitudinal axis and being relatively stiff for tension or compression along its longitudinal axis, and relatively compliant for S bending in a plane normal to its longitudinal axis.

11. The acelerometer of Claim 10, wherein the rods are positioned on opposite sides of the proof mass from one another, and wherein the longitudinal axis of each rod is parallel to the hinge axis.

12. The accelerometer of Claim 11, wherein the flexures are positioned on opposite sides of the proof mass from one another along the hinge axis, wherein the support comprises first and second portions on opposites sides of the proof mass and flexures from another, wherein the proof mass comprises first and second portions adjacent to the first and second flexures respectively, wherein one of the rods is connected between the first portion of the support and the second portion of the proof mass, and wherein the other rod is connected between the first portion of the proof mass and the second portion of the support.