Abstract:
An accelerometer device for reducing stress on the sensor resulting from temperature extremes and multiple coefficients of thermal expansion. An exemplary accelerometer device includes upper and lower stators and a reed. The reed includes a support ring and a paddle that is flexibly connected to the support ring. The support ring includes a ring section and at least two mounting devices. The mounting devices are at least partially mechanically isolated from the ring section. The ring section flexibly receives the paddle. The mounting devices include a pad area and a neck area that connect the pad area to the ring section. The neck area includes a width dimension that is narrower than a diameter dimension of the pad area.

Description:
[0001]    This application is a continuation of U.S. patent application Ser. No. 13/656,600 by Roehnelt et al., filed Oct. 19, 2012 and entitled, “STRESS REDUCTION COMPONENTS FOR SENSORS,” the entire content of which is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    As shown in  FIGS. 1 and 2 , a reed is mounted to a magnetic circuit by compression. As temperature changes the much-thicker magnetic circuit component, it deforms the reed into a novel shape, which causes the paddle to deflect. Additionally, any slip of the mounting points causes error. 
         [0003]    The slip potential increases at high temperature, as the clamping force decreases and shear stress between the reed and the magnetic circuit increases. 
         [0004]    The coefficient of thermal expansion (CTE) (α) of the excitation ring  13  is higher than a of the attached reed  16 . As the excitation ring  13  is also considerably stronger, it will pull mounting points  18  radially, which will also cause a compression or tensile stress as the excitation ring  13  attempts to move the mounting points  18  to a smaller or larger radius. 
         [0005]    Because the fused silica (commonly referred to as quartz) of the reed  16  is a highly elastic material, the reed  16  does not plastically deform to accommodate the metal of the excitation ring  13 . Instead, some other mechanism of stress accommodation occurs, Possibilities are: a) slip of the mounting points  18 ; b) local yielding of metal part; and c) the rim of the reed  16  becomes an oval shape, which forces the paddle  19  out of plane. Any one of these, or a combination thereof, will cause sensor error that is made worse by temperature extremes. 
         [0006]      FIGS. 1 and 2  illustrate an accelerometer that includes the asymmetric flexure arrangement of the present invention. The accelerometer measures acceleration along sensing axis SA, and includes stator, reed, and mounting members  18 . The reed is held between mounting member  18  and stator, and has a coil positioned on its upper surface. The excitation ring (e-ring) comprises stator  13 , magnet and pole piece. The e-ring is shaped so that the coil occupies a comparatively narrow gat between pole piece and stator  13 , to provide the force balancing function well known to those skilled in the art. 
         [0007]    The reed has an overall disk-like shape, and includes annular support ring and paddle connected to one another via a pair of flexures between Which an opening is farmed. For most of its perimeter, the paddle is separated from the support ring by a circular gap. Raised mounting pads  18  are located at approximately equally spaced positions around support ring. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention reduces stress on the sensor resulting from temperature extremes and multiple coefficients of thermal expansion and also to assist in maintaining co-axiality between sense elements and the return path. The present invention is particularly useful for down hole use Where the environment requires use of materials with non-ideal coefficient of thermal expansion match. The present invention reduces the stress on the sense element, increasing accuracy over temperature by including flexure(s) for the mounting points between the sense element and the return path of a quartz flexure accelerometer. The arrangement of the flexures not only reduces stress but assists in maintaining co-axiality between the sense element and the return path. 
         [0009]    An exemplary accelerometer device includes upper and lower stators and a reed. The inwardly facing surface of a least one stator includes a bore within which is positioned a permanent magnet capped by a pole piece. The reed includes a support ring and a paddle that is flexibly connected to the support ring via flexures that are compliant out of plane. The support ring includes a ring section and at least two mounting devices. The mounting devices are at least partially mechanically isolated from the ring section. 
         [0010]    In one aspect of the invention, the mounting devices include a pad area and a neck area that connect the pad area to the ring section. The neck area includes a width dimension that is narrower than a diameter dimension of the pad area. 
         [0011]    In another aspect of the invention, the pad area and the neck area are defined by an outer edge of the reed and a cavity linking the first and second sides. 
         [0012]    In still another aspect of the invention, the pad area and the neck area are defined by an outer edge of the reed, a first cavity linking the first and second sides and a second cavity linking the first and second sides. The first and second cavities are at least partially circular. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0014]      FIGS. 1 and 2  illustrate cross-sectional views of a paddle-type accelerometer formed in accordance with the prior art; 
           [0015]      FIG. 3  illustrates a blown-up view of an accelerometer that uses the various stress relief components of the present invention; 
           [0016]      FIGS. 4-1  and  4 - 2  illustrate partial cross-sectional views of a stress relief structure formed into a reed of an accelerometer in accordance with an embodiment of the present invention; 
           [0017]      FIG. 5  illustrates thermal expansion induced force vectors experienced by mounting points of the accelerometer shown in  FIGS. 4-1  and  4 - 2 ; 
           [0018]      FIGS. 6-1  and  6 - 2  illustrate another isolation structure formed into the reed of an accelerometer in accordance with an embodiment of the present invention; 
           [0019]      FIGS. 7-1  and  7 - 2  illustrate different embodiments for the bottom (6 o&#39;clock) positioned attachment features for the devices shown in  FIGS. 4-1  and  6 - 1 ; 
           [0020]      FIG. 8-1  illustrates a partial cross-sectional view of an isolation device formed within a stator of a paddle-type accelerometer; 
           [0021]      FIG. 8-2  illustrates a cross-sectional view along a longitudinal axis of the accelerometer, partially shown in  FIG. 8-1 ; 
           [0022]      FIG. 9-1  illustrates a partial cross-sectional view of an isolation device formed within a stator of a paddle-type accelerometer; and 
           [0023]      FIG. 9-2  illustrates a cross-sectional view along a longitudinal axis of the accelerometer, partially shown in  FIG. 9-1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The present invention provides stress isolation/reduction features for avoiding plastic deformation, slip, or bending of a reed of an accelerometer e.g., Q-Flex made by Honeywell, Inc.). 
         [0025]      FIG. 3  illustrates a force rebalance accelerometer where the features of the present invention are used. This accelerometer includes an upper stator  20  and a lower stator  22 . The inwardly facing surface of a least one stator includes a bore within which is positioned a permanent magnet capped by a pole piece, as illustrated by pole piece  24  within a bore  26 . Also shown is reed assembly that is mounted between the upper and lower stators. The reed assembly includes a reed that includes an outer annular support ring  32  and a paddle  36  supported from the support ring by flexures. The reed is preferably fabricated from a single piece of fused silica. The support ring  32  includes three mounting locations. When the accelerometer is assembled, the mounting pads contact the upper and lower stators to provide support for the reed assembly. 
         [0026]    A capacitor plate is deposited on the upper surface of the paddle  36 , and a similar capacitor plate (not shown) is deposited on the lower surface of the paddle. The capacitor plates cooperate with the inwardly facing surfaces of upper and lower stators  20  and  22  to provide a capacitive pick-off system. Also mounted on either side of the paddle  36  are coil forms on which force-rebalance coils are mounted. As is well understood in the served instrument art, coils cooperate with the permanent magnets in the stators and with a suitable feedback circuit to retain the paddle  36  at a predetermined position with respect to the support ring  32 . Thin film pick-off leads, and similar leads (not shown) on the lower surface of the reed, provide electrical connections to the capacitor pick-off plates and force-rebalance coils. 
         [0027]    In the design of an accelerometer of the type shown in  FIG. 3 , it is nearly impossible to use the same material for all of the different components. For example, the reed is preferably composed of fused quartz, the coil is preferably composed of copper, and coil form (if used) is preferably made from aluminum. As a result, there will invariably be mismatches in the coefficients of thermal expansion (CTE) of adjacent components. Such mismatches can deform the components and cause errors in a number of different ways, depending on the type of suspension and displacement pick-off method used. 
         [0028]    The coil forms are typically mounted directly to the paddle  36  with a compliant elastomer. The mismatch in CTE between aluminum and fused quartz is large, and the compliant elastomer layer does not relieve all of the stress at this interface. The remaining stresses that are not cancelled by the opposing coil can deform the capacitor pick-off plates or the flexures. Either of these deformations can cause a bias in the accelerometer&#39;s output. In addition, distortions that change the position of the coil windings can cause scale-factor errors. These error sources are even more significant in a design in which only a single force-rebalance coil is used, because of the asymmetry of the resulting stress applied to the paddle. 
         [0029]    As shown in  FIG. 4-1 , a support ring  32 - 1  of the reed of an accelerometer includes multiple locations for mounting the support ring  32 - 1  to the other components of the accelerometer (stators). First mounting pads  50  mount to either side of the support ring  32 - 1 , The mounting pads  50  attach to the surface of the upper and lower stators. The mounting pads  50  are located along the support ring  32 - 1  approximately opposite flexures (not shown) for flexibly mounting a paddle proof-mass  36 - 1  to the support ring  32 - 1 . 
         [0030]    Mounting devices  52  and  54  are located along the support ring  32 - 1  approximately equidistant from the first mounting device  50 .  FIG. 4-2  illustrates a closer view of one of the mounting devices  54 . The mounting device  54  includes a mounting area  60 , The mounting area  60  (both sides) are raised above the rest of the support ring  32 - 1 . The raised area  60  are attached to the upper and lower stators, The mounting area  60  is formed by a cavity  64  that is etched around the mounting area  60  to isolate the mounting area  60  from the support ring  32 - 1 , except for a neck section  62  that attaches the mounting area  60  to the support ring  32 - 1 . The cavity  64  passes through the entire thickness of the support ring  32 - 1 . The cavity could be at least partially formed by machining or etching. 
         [0031]    As shown in  FIG. 5 , the arrows indicate the direction in which stresses are applied to each of the mounting locations of the accelerometer shown in  FIG. 4-1 . These forces are due to a stress caused by differential thermal expansion of the parts of the accelerometer. The isolation mounts  52  and  54  (and mount  50  if it includes an isolation feature ( FIGS. 7-1 ,  7 - 2 ) mitigate some of the stresses shown by these arrows. The isolation mounts  52  and  54  allow the attached stators to expand or contract, without unduly affecting the support ring  32 - 1 . 
         [0032]    As shown in  FIGS. 6-1  and  6 - 2 , in one embodiment, a support ring  32 - 2  includes a first attachment point  50 - 2 , similar to first mounting device  50 , described and shown in  FIGS. 4-1  and  5 . The support ring  32 - 2  also includes spiral attachment devices  70 , located equidistant from the first attachment point  50 - 2 . Each of the spiral attachment devices  70  include an attachment area  74  that allows for mounting devices (not shown) to be attached on either side of the mounting area  74 . The mounting pads then attach to the respective upper or lower stator. The spiral attachment device  70  includes a first cavity  80  that passes all the way through the support ring  32 - 2 . The first cavity  80  starts at approximately a first radial projecting from the center of the mounting area  74 . The first cavity  80  curves in a counter-clockwise manner around the attachment area  74  and exits the support ring  32 - 2  at a second radial that is at least 270° from the first radial. A second cavity  78  begins at the edge of the support ring  32 - 2  at a third radial that is somewhere between the first and second radials. The second cavity  78  proceeds in a counterclockwise manner around the attachment area  74  and around the first cavity  80  until it reaches a location at a fourth radial that is between the first and second radials in a direction away from the first attachment point  50 - 2 . The second cavity  78  then straightens out or follows the curvature of the edge of the support ring  32 - 2 . Thus, the second cavity  78  forms a spiral neck  76  that attaches the attachment area  74  to the support ring  32 - 2 . The spiral attachment devices  70  allow for expansion and contraction of the stators while limiting stresses experienced at the support ring  32 - 2 . 
         [0033]      FIG. 7-1  shows an embodiment of a bottom (6 o&#39;clock) positioned attachment point  120  ( 50  or  50 - 2   FIGS. 4-1 ,  6 - 1 ). The attachment point  120  includes a cut out  122  that isolates a raised area  124  from the ring. 
         [0034]      FIG. 7-2  shows an embodiment of a bottom (6 o&#39;clock) positioned attachment point  130  ( 50  or  50 - 2   FIGS. 4-1 ,  6 - 1 ). The attachment point  130  includes two cut outs  132 ,  134  that isolate a raised area  136  and a shaft  138  from the ring. The shaft  138  zig-zags in a rounded and/or square pattern. Other shapes for the bottom (6 o&#39;clock) positioned attachment point are used provided rotation of the reed is minimal over temperature changes. 
         [0035]    As shown in  FIGS. 8-1  and  8 - 2 , a lower stator  90  has been machined to produce a plurality of pillars  92 . In one embodiment, the upper stator includes matching features to those shown on the lower stator  90 . The pillars  92  are located at approximately the circumferential edge of the stator  90 . The pillars  92  attach to opposing raised areas of a support ring  32 - 3  of the accelerometer reed or attach to mounting pads located on the support ring  32 - 3  of the accelerometer reed. The pillars  92  provide stress relief to the metal parts of the accelerometer. The stator  90  is machined away to expose the resulting pillar  92 . The taller the pillar  92  and the smaller the cross section, the greater the compliance of the pillar. An exemplary pillar includes compliance in both the radial and circumferential directions, which could be varied by the shape and cross section of the pillar. 
         [0036]      FIGS. 9-1  and  9 - 2  illustrate an embodiment in which a stator  100  includes a pillar  104  that has been machined from the stator material at the circumference of the stator at a mounting surface. The pillar  104  is defined by a first curved cavity  106 . An exemplary depth of the cavity  106  is 0.1-0.12″. The first curved cavity  106  is machined out of the metal (e.g., Invar) that forms the stator  100 . 
         [0037]    In an alternate embodiment, a second cavity  110  is etched below the pillar  104  from an exterior side of the stator  100 . The second cavity  110  provides more flexibility of the pillar  104 . 
         [0038]    In one embodiment, the pillars  92 ,  104  are used at all mounting locations. 
         [0039]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow. 
         [0040]    The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.