CAPACITANCE PICK-OFF FOR FORCE REBALANCE ACCELEROMETER

Abstract: An improved capacitor plate geometry for a force rebalance accelerometer. The accelerometer (10) has a reed (14) that includes a support (32, 82, 142), a paddle (34, 80, 140) suspended from the support for rotational movement with respect thereto, and a force balancing coil (24, 88, 152, 162). The coil is mounted on a surface (86, 150, 160) of the paddle such that it surrounds an area of that surface. The accelerometer also includes a position detecting circuit that provides an indication of the relative position of the paddle with respect to the support. The position detecting circuit includes a capacitor plate (60, 90, 154, 164) on the paddle. The improvement provided by the invention lies in the fact that at least a portion of the capacitor plate is positioned in the area of the paddle surface surrounded by the coil.

Description: CAPACITANCE PICK-OFF FOR FORCE REBALANCE ACCELEROMETER Field of the Invention The present invention relates to force rebalance accelerometers in which an acceleration-sensitive movable element is maintained in a neutral position. Background of the Invent i o n A prior art accelerometer with high performance potential is described in U.S.P. 3,702,073. The accelerometer comprises three components, a reed, and upper and lower stators or magnetic circuits between which the reed is supported. The reed includes a movable paddle that is suspended via flexures to an outer annular support ring, such that the paddle can pivot with respect to the support ring. The paddle, flexures and support ring are commonly provided as a unitary structure composed of fused quartz. Both upper and lower surfaces of the paddle include capacitor plates and force balancing coils. Each force balancing coil is positioned on the paddle such that the central axis of the coil is normal to the top and bottom surfaces of the paddle, and parallel to the sensing axis of the accelerometer. A plurality of mounting pads are formed at spaced apart positions around the upper and lower surfaces of the annular support ring. These mounting pads mate with inwardly facing surfaces of the upper and lower stators when the accelerometer is assembled. Each stator is generally cylindrical, and has a bore provided in its inwardly- facing surface. Contained within the bore is a permanent magnet. The bore and permanent magnet are configured such that an associated one of the force balancing coils mounted to the paddle- fits within the bore, with the permanent magnet being positioned within the cylindrical core of the coil. Current flowing through the coil therefore produces a magnetic field that interacts with the permanent magnet to produce a force on the paddle. Also provided on the inwardly facing surfaces of the stators are capacitor plates configured to form capacitors with the capacitor plates on the top and bottom surfaces of the paddle. Thus movement of the paddle with respect to the upper and lower stators results in a differential capacitance change. In operation, the accelerometer is affixed to an object whose acceleration is to be measured. Acceleration of the object along the sensing axis results in pendulous, rotational displacement of the paddle with respect to the support ring and the stators. The resulting differential capacitance change caused by this displacement is sensed by a feedback circuit. In response, the feedback circuit 10 produces a current that, when applied to the force balancing coils, tends to return the paddle to its neutral position. The magnitude of the current required to maintain the paddle in its neutral position is directly related to the acceleration along the sensing axis. One difficulty with force rebalance accelerometers is that higher g-range .D designs require proportionately higher servo force capacity to keep the paddle at its neutral position. The magnet strength or the torque coil current can be increased in order to increase the servo force, but both of these increases have undesirable side effects. Increasing the torque coil current produces both increased power consumption and heating effects which degrade performance, and 20 these effects are proportional to the square of the servo current. Similarly, magnetic strength can be increased by making the magnet larger, but if its diameter is constrained by the coil size, as in prior art designs, it can only be made longer. If the magnet design is already near the maximum energy slope, increasing the magnet length will reduce its efficiency, and it will be found that L5 the magnet size increases much more rapidly than its strength. Summary of the Invention The present invention provides a force rebalance accelerometer that has an increased servo force capacity relative to proof mass size. The invention thereby increases g-range without increasing either the size or the power requirements for 30 the accelerometer. An additional advantage of the invention is that it allows alignment of the center of mass of the proof mass and the force rebalance center without sacrificing servo force capacity or pick-off sensitivity. The accelerometer has a reed that includes a support, a paddle, and a force balancing coil. The paddle has a paddle_surface, and is suspended from the support 35 by flexure means for rotational movement with respect to the support. The coil is mounted on the paddle surface, such that the coil surrounds an area of the paddle surface. The term eoil as used herein may include an associated structure for mounting the coil to the paddle. - The accelerometer also includes position detecting means for providing an indication of the relative position of the paddle with respect to the support. The position detecting means includes a capacitor plate on the paddle. The improvement provided by the present invention lies in the fact that at least a portion of the capacitor plate is positioned in the area of the paddle surface surrounded by the coil. In a preferred embodiment, the coil has an outer circumference a portion of which is coextensive with the outer circumference of the paddle, a second portion of the coil overhangs the flexure means, and the capacitor plate is positioned entirely within and substantially fills the surrounded area. The result is an accelerometer in which the capacitor pick-off system utilizes space on the paddle that is already used by the force rebalance system, as well as an accelerometer in which the center of mass of the proof mass and the force rebalance center can be aligned so as to minimize nonlinear errors without sacrificing pick-off system sensitivity. Brief Description of the Drawings FIGURE 1 is a cross-sectional view of an accelerometer that includes the capacitor plate and coil positioned in accordance with the present invention; FIGURE 2 is a top plan view of the reed of the accelerometer of FIGURE 1; FIGURE 3 is a cross-sectional view taken along the line 3-3 of FIGURE 2; FIGURE 4 is a schematic view of a single coil accelerometer constructed in accordance with the present invention; FIGURE 5 is a schematic view of a single coil accelerometer using a capacitor plate arrangement in accordance with the prior art; and FIGURE 6 is a schematic view of a two coil accelerometer constructed in accordance with the present invention. Detailed Description of the Invention FIGURES 1-3 illustrate an accelerometer that includes an improved coil and capacitor plate geometry in accordance with a preferred embodiment of the present invention. The accelerometer 10 measures acceleration along sensing axis SA, and includes stator 12, reed 14, ceramic plate 16 and electronics assembly 18, all mounted within an enclosure formed by mounting flange 20 and case 22. Reed 14 is held between ceramic plate 16 and stator 12, and has coil 24 positioned on its upper surface. Stator 12 in turn bears against case 22 via positioning ring 26 and spring washer 28. The stator comprises excitation ring 42, magnet 44 and pole piece 46. The stator is shaped so that coil 24 occupies a comparatively narrow gap between pole piece 46 and excitation ring 42, to provide the force balancing function well known to those skilled in the art. Ceramic plate 16 is held against the reed by inner shoulder 30 of mounting flange 20, and the mounting flange and case 22 are interconnected by welding or by any other suitable process. Means (not shown) are provided for electrically interconnecting electronics assembly 18 with reed 14, and for providing connections between the electronics assembly and an electrical connector on the outer surface of mounting flange 20. Reed 14 is shown in greater detail in FIGURES 2 and 3. The reed has an overall disk-like shape, and includes annular support ring 32 and paddle 34 connected to one another via a pair of flexures 36 between which opening 40 is formed. For most of its perimeter, paddle 34 is separated from support ring 32 by circular gap 38. However, on the side of paddle 34 opposite to flexures 36, a portion of the paddle is cut away, in the area indicated by reference numeral 48, to provide alignment between the center of mass of the proof mass and the force rebalance center, as described in greater detail below. Three raised mounting pads 50-52 are located at approximately equally spaced positions around support ring 32, and three similar mounting " pads (not shown) are located immediately beneath mounting pads 50-52 on the lower surface of the support ring. When the accelerometer is assembled, the upper mounting pads 50-52 contact stator 12, while the lower mounting pads contact ceramic plate 16. Paddle 34 is mounted via flexures 36 such that the paddle can pivot with respect to support ring 32 about hinge axis HA that passes through the midpoints of the flexures and that is horizontal and in the plane of the drawing in FIGURE 2. Coil 24 is mounted on the upper surface of paddle 34. The outer edge of the coil is approximately coextensive with the outer edge of the paddle, except adjacent flexures 36 and cut away area 48. A thin shim (not shown) is positioned between coil 24 and paddle 34, such that the coil is positioned a short distance above flexures 36, to avoid interference during paddle movement. Capacitor plate 60 is positioned on paddle 34 entirely within the area of the paddle surface surrounded by coil 24, and forms a capacitor with the adjacent surface of pole piece 46, or with a second capacitor plate located on the pole piece surface. This capacitor forms a portion of a pick-off circuit that measures the capacitance between capacitor plate 60 and pole piece 46, or between capacitor plate 60 and the second capacitor plate, to thereby detect movement of the paddle from its null position. Capacitor plate 60 and the adjacent surface of pole piece 46 also provide squeeze film damping for the accelerometer. The pick-off circuit may also include a second capacitor formed between plates (not -3- shown) on the lower surface of paddle 34 and on the upper surface of ceramic plate 16. A portion of support ring 32 adjacent to flexures 36 may be divided by slot 70 into inner ring 72 and outer ring 74. Mounting pad 50 is positioned on outer ring 74 only, and the flexures are connected to inner ring 72. As a result of this arrangement, flexures 36 are isolated from stress coupled into reed 14 via mounting pad 50. The coil and capacitor plate may be electrically coupled to electronics module 18 via conductive strips (not shown) extending across flexures 36 and along each side of inner ring 72 to contacts on mounting pads 51 and 52. Corresponding conductors in ceramic plate 16 electrically connect these contacts to the electronics assembly. The described split support ring allows mounting pad 50 to be located near flexures 36, on outer ring 74, without creating direct mechanical coupling of the mounting pad to the flexure area of the support ring. The advantages of the present invention can be described with reference to the schematic drawings in FIGURES 4 and 5. FIGURE 4 illustrates an accelerometer constructed in accordance with the present invention, while FIGURE 5 illustrates a prior art capacitor plate geometry. The accelerometer shown in FIGURE 4 comprises paddle 80 connected to support 82 by flexure 84, such that the paddle can rotate with respect to the support. The paddle has an upper surface 86 on which coil 88 is mounted. Capacitor plate 90 is secured to surface 86 in the area of surface 86 surrounded by the coil. The adjacent surface 92 of stator 94 forms a capacitor with plate 90. FIGURE 5 schematically illustrates an accelerometer that utilizes a prior art arrangement of the type shown in U.S.P. 3,702,073. This accelerometer includes paddle 100 mounted to support 102 by flexure 104, such that the paddle can rotate with respect to the support. The paddle includes an upper surface 106 on which coil 108 and capacitor plate 110 are mounted. The stator includes a portion 114 extending within coil 108, to provide the force rebalancing function. A second capacitor plate 112 is mounted to the portion of the stator (not shown) opposite capacitor plate 110. Typically, capacitor plates 110 and 112 have crescent shapes, and extend around the outside of coil 108, except adjacent to flexure 104. One important advantage of the present invention, as compared to prior art arrangements, is that the capaeitive pick-off system utilizes space on the paddle that is already used by the force rebalance system, i.e., by the coil. Thus the capacitor plates do not add any additional size to the accelerometer, or any additional weight to the paddle. In addition, comparison of FIGURES 4 and 5 illustrates that for a given paddle size, the arrangement of the present invention permits use of a larger magnet and a larger coil, without losing capacitance or damping area. This feature permits an increase of the g-range of the accelerometer, without an increase in the accelerometer's overall size. A further advantage of the invention is that the roughly circular damping areas provided by the capacitor plate and the adjacent pole piece surface provide greater damping per unit area than elongated narrow shapes such as an arc. The present invention also provides another important advantage relating to force alignment. In general, in a force rebalance accelerometer, it is desirable to align the center of the acceleration force, i.e., the center of mass of the proof mass, with the center of rebalance force. If these force centers are not aligned, acceleration will produce a net torque on the proof mass that tends to rotate it, via S-bending of the flexures, about an axis parallel to, but displaced from, the hinge axis. This unwanted rotation produces errors due to nonlinearities, especially in the form of cross-coupling errors. In general, the center of rebalance force will lie along the symmetry axis of the magnet and coil, e.g., at centerline 62 shown in FIGURE 2. However, because of the presence of the flexures, the center of mass of the proof mass will typically not lie at centerline 62, but will be displaced away from the flexures relative to the centerline, i.e., in the downward direction shown in FIGURE 2. The present invention provides a way of eliminating this misalignment without affecting the sensitivity of the pick-off system or the efficiency of the magnetic circuit. In particular, in the embodiment of the invention set forth in FIGURES 1-3, a portion of the paddle indicated by reference numeral 48 has been removed, to shift the center of mass of the proof mass toward the flexures, such that it is positioned at centerline 62 and therefore coincides with the center of rebalance force. This removal of paddle material can be accomplished without significant loss of capacitor plate area or damping area. In contrast, with reference to FIGURE 5, it may be seen that in a prior art accelerometer, removal of the outer lip of the paddle would have a significant effect on the amount of capacitor plate area that can be used for the pick-off function. Thus, since the outer lip cannot readily be removed in prior art designs, such designs require that the center of magnetic force be moved to coincide approximately with the center of mass, by widening a portion of the gap in the vicinity of the flexures. This operation is difficult to accomplish accurately, and in any case decreases efficiency by increasing stray magnetic flux. FIGURE 6 schematically illustrates a second preferred embodiment of the invention. In this embodiment, paddle 140 is suspended from support ring 142 by one or more flexures 144. The flexures define a hinge axis HA about which paddle 140 can rotate with respect to support ring 142. Hinge axis HA is normal to the plane of the drawing in FIGURE 2. Paddle 140 includes parallel, opposed faces 150 and 160 to which coils 152 and 162, respectively, are mounted. Capacitor plate 154 is positioned on that portion of surface 150 enclosed by coil 152, and capacitor plate 164 is positioned on that portion of surface 160 enclosed by coil 162. The upper stator of the accelerometer illustrated in FIGURE 6 includes magnet 172 and pole piece 174. The remaining components of the upper stator are conventional and are omitted for ease of illustration. However, such components function to provide a return path for magnetic flux lines passing from pole piece 174 through coil 152, and to provide support for support ring 142. Similarly, the lower stator comprises magnet 182 and pole piece 184. The inner faces of pole pieces 174 and 184 form capacitors with capacitor plates 154 and 164, respectively. The embodiment of FIGURE 6 includes all of the advantages discussed above in connection with the single coil embodiment. While the preferred embodiments of the invention have been illustrated and described, variations will be apparent to those skilled in the art. Accordingly, the scope of the invention is to be determined by reference to the following claims.

Claims: The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 1. In an accelerometer for measuring acceleration along a sensing axis, the accelerometer comprising a reed that includes a support, a paddle, flexure means for supporting the paddle for rotational movement with respect to the support, and a force balancing coil, the paddle having a paddle surface, the force balancing coil being mounted on the paddle surface such tha't the coil surrounds an area of the paddle surface, the accelerometer further including position detecting means for providing an indication of the relative position of the paddle with respect to the support, the position detecting means including a capacitor plate on the paddle, the improvement wherein at least a portion of the capacitor plate is positioned within said area. 2. The improvement of Claim 1, wherein the coil has an outer circumference a substantial portion of which is coextensive with an outer circumference of the paddle, and wherein a second portion of the coil overhangs the flexure means. 3. The improvement of Claim 2, wherein the capacitor plate is positioned entirely within said area. 4. The improvement of Claim 3, wherein the capacitor plate substantially fills said area. 5. The improvement of Claim 4, wherein the accelerometer further comprises a magnetic circuit element extending towards the capacitor plate and forming a capacitor therewith, the magnetic circuit element substantially filling a volume of space encircled by the coil. 6. The improvement of Claim 5, wherein the magnetic circuit element and the capacitor plate are positioned with respect to one another so as to provide squeeze film gas damping for the paddle, and such that the magnetic circuit element forms a mechanical stop for the paddle. 7. The improvement of Claim 1, wherein a portion of the paddle opposite the flexure means is removed, the dimensions of said portion being selected such that the center of mass of the paddle and coil is aligned with a center of rebalance force. 8. In an accelerometer for measuring acceleration along a sensing axis, the accelerometer comprising a reed that includes a support, a paddle having two paddle surfaces, flexure means for supporting the paddle for rotational movement with respect to the support, and a force balancing coil mounted on each paddle surface such that each coil surrounds an area of the respective paddle surface, the accelerometer further including position detecting means for providing an indication of the relative position of the paddle with respect to the support, the position detecting means including a capacitor plate on each paddle surface, the improvement wherein at least a portion of each capacitor plate is positioned within said area of the respective paddle surface. 9. The improvement of Claim 8, wherein each coil has an outer circumference a substantial portion of which is coextensive with an outer circumference of the paddle, and wherein a second portion of each coil overhangs the flexure means. 10. The improvement of Claim 9, wherein each capacitor plate is positioned entirely within said area of the respective paddle surface. 11. The improvement of Claim 10, wherein each capacitor plate substantially fills said area of the respective paddle surface. 12. The improvement of Claim 11, wherein the accelerometer further comprises two magnetic circuit elements, one magnetic circuit element extending towards each capacitor plate and forming a capacitor therewith, each magnetic circuit element substantially filling a volume encircled by a respective one of the coils. 13. The improvement of Claim 12, wherein the magnetic circuit elements and capacitor plates are variably positioned so as to provide squeeze film gas damping and wherein the magnetic circuit elements provide mechanical stops for the paddle. 14. The improvement of Claim 8, wherein a portion of the paddle opposite the flexure means is removed, the dimensions of said portion being selected such that the center of mass of the paddle and coils is aligned with a center of rebalance force.

Inventors: BLAKE GRAEME A, HOLDREN FREDERICK V, NORLING BRIAN L, NOVACK MITCHELL J, BLAKE, GRAEME, A., HOLDREN, FREDERICK, V., NORLING, BRIAN, L., NOVACK, MITCHELL, J.
Applicants: SUNDSTRAND DATA CONTROL, SUNDSTRAND DATA CONTROL, INC.
Application year: 1991
Main classification: G01P15
IPC codes: G01P15