Abstract:
An arcuate reed for use with a capacitance type accelerometer proof mass. The reed includes grooves for improved gas damping and reduced latch-up without a reduction in damping effects.

Description:
[0001]    This application is a Continuation in Part of U.S. patent application Ser. No. 09/188,865 filed Nov. 9, 1998 entitled “Star Patterned Accelerometer Reed” which is a Continuation in Part of U.S. patent application Ser. No. 08/835,872 filed Apr. 8, 1997 having the same title which is a continuation of Ser. No. 08/515,442 having the same title and filed Aug. 15, 1995. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to accelerometers and, more specifically, to force rebalance accelerometers.  
         BACKGROUND OF THE INVENTION  
         [0003]    Force rebalance accelerometers which include a proof mass suspended between one or more magnet assemblies are generally known in the art. Examples of such accelerometers are disclosed in U.S. Pat. Nos. 4,182,187; 4,250,757; 4,399,700; 4,400,979, 4,441,366; 4,555,944; 4,555,945; 4,592,234; 4,620,442; 4,697,455; 4,726,288; 4,932,258; 4,944,184; 5,024,089; 5,085,079; 5,090,243; 5,097,172; 5,111,694; 5,182,949; 5,203,210; 5,212,984; 5,220,831; and Re. 34,631 all of which are incorporated herein by reference.  
           [0004]    Such force rebalance accelerometers normally include a proof mass having a reed or flapper formed from amorphous quartz, suspended by one or more flexures between stators having permanent magnets to enable the proof mass to defect in response to forces or accelerations along a sensitive axis, generally perpendicular to the plane of the proof mass. The proof mass also typically includes at least one torquer coil secured to the reed which functions as an electromagnet. At rest, the proof mass is normally suspended equidistantly between upper and lower excitation rings. Electrically conductive material forming pick-off capacitance plates, is disposed on opposing sides of the proof mass to form capacitive elements with the excitation rings. An acceleration or force applied along the sensitive axis causes the proof mass to deflect either upwardly or downwardly which causes the distance between the pick-off capacitance plates and the upper and lower excitation rings to vary. This change in the distance between the pick-off capacitance plates and the upper and lower excitation rings causes a change in the capacitance of the capacitive elements. The difference in the capacitances of the capacitive elements is thus representative of the displacement of the proof mass along the sensitive axis. This displacement signal is applied to a servo system that includes the torquer coils which function, in combination with a current applied to the torquer coils and the permanent magnets, to return the proof mass to its null or at-rest position. The magnitude of the drive currents applied to the torquer coils, in turn, is representative of the acceleration of force along the sensitive axis.  
           [0005]    One problem encountered in this type of accelerometer results from the use of a gas, usually a mixture of neutral gases such as helium and nitrogen, utilized to fill the accelerometer to provide gas damping for the proof mass. Under extreme acceleration conditions, it has been found that the gas damping can result in an overshoot condition when the extreme acceleration condition is removed.  
           [0006]    In addition, it has been discovered that in certain power off situations, the reed, which has a highly polished surface will have a tendency to stick or latch up to the highly polished surface of the stator thereby increasing turn on times for the accelerometer. One approach to solving this problem is described in U.S. Pat. No. 4,825,335 wherein a rectangular moveable capacitor plate, which is suspended on each side by fingers, is provided with a number of air passages extending through the plate along with grooves in the plate leading up to the air passages to facilitate the flow of air. However, etching holes in the flapper of the above described accelerometer is not a practical solution to this problem.  
           [0007]    In currently known low frequency force rebalance accelerometers, the component of damping of a pendulum suspended between two capacitor plates was essentially described by theoretical flat plate dumping according to the equation:  
           damping coefficient=βA 2 /h 3    (1)  
           [0008]    where: A is the area of the plate,  
           [0009]    h is the distance between the pendulum and the relevant capacitor plate; and  
           [0010]    β is a constant dependent upon the shape of the area.  
           [0011]    This theoretical flat plate damping equation applied when three conditions pertained. First, when the distance, h, was small relative to the pendulum dimensions. For example, when the ratio of the distance, h, to the pendulum diameter was on the order of 1/1000. Second, when the displacements of the pendulum were small compared with the distance, h, i.e., when the accelerometer was in servo. Third, when the accelerometer responded to low frequency input, for example, a vibration or shock environment below 10 KHz.  
         SUMMARY OF THE INVENTION  
         [0012]    It is therefore an object of the invention to provide for improved gas damping and reduced lock up in an accelerometer having a proof mass that includes an arcuate reed suspended by hinges for rotation between two stator members.  
           [0013]    It is a further object of the invention to provide an arcuate reed for use with a proof mass of an accelerometer where the reed includes a number of radially extending grooves.  
           [0014]    Another object of the invention is to provide an arcuate reed for use with a proof mass, which is suspended by a pair of flexure hinges between a pair of stators, where the reed includes radially extending grooves etched on both sides and located at approximately 45 degree intervals on the side away from the hinges. In a reed approximately 0.030 inches thick, the grooves can be 0.020 inches wide and 0.0005 inches deep. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0016]    [0016]FIG. 1 is an exploded perspective view of a force rebalance accelerometer according to the invention;  
         [0017]    [0017]FIG. 2 is a top view of a reed having grooves for use with the accelerometer of FIG. 1; and  
         [0018]    [0018]FIG. 3 is a graph of damping coefficient according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    [0019]FIG. 1 illustrates in an exploded view form, an acceleration transducer of the type disclosed in detail in the aforementioned U.S. Pat. No. 4,250,757. In this embodiment, for descriptive purposes, the accelerometer includes an upper magnet or stator structure  10  and a lower magnet or stator structure  12 . Included in each of the upper  10  and lower  12  stator structures are permanent magnets as illustrated by a magnet  14  shown in the lower stator structure  12 . In addition, the lower stator structure includes support posts for electrical lead as illustrated at  16  and  18 . Also shown in FIG. 1 is a movable element assembly in form of a proof mass assembly, generally indicated at  20 . Included in the proof mass assembly is an outer annular support member or ring  22  which is supported between opposed planar surfaces  19  and  21  of the upper stator structure  10  and the lower stator structure  12  by pairs of spacer elements or mounting pads  24  on the member  22 . The lower pad of each pair of mounting pads is not shown in the drawing. As shown in FIG. 1, the location of each pair of mounting pads  24  is spaced apart from each other around the support ring  22 . Included in the proof mass assembly  20  is a movable flapper or reed  26  extending radially inward from the outer support ring  22 . The reed  26  has an electrically conductive material, preferably gold and having an arcuate shape, that serves as a capacitive pick-off area or plate  28 . The capacitive pick-off plates  28  on the upper and lower surfaces of the reed  26  cooperate with the opposed planar surfaces  19  and  21  of the upper and lower stator structures  10  and  12  to provide a capacitive pick-off system.  
         [0020]    Mounted on each side of the reed  26  is a force restoring coil  30 . As is well understood in the art, the force restoring or torquer coils  30  cooperate with the permanent magnets  14  to retain the reed  26  within a predetermined position with respect to the support ring  22 .  
         [0021]    The reed  26 , including the force restoring coils  30 , is connected to the support ring  22  by means of a pair of flexure elements  32  and  34 . The flexure elements  32  and  34  permit the proof mass assembly  20  including the reed  26  and the coil  30  to move in a rotational pendulous manner with respect to the annular support ring  22 . The reed  26  will move in response to forces along the sensitive axis  35  of the accelerometer. Also deposited on the support ring  22  and flexure elements  32  and  34  are thin film pick-off leads  36  and  38  which provide electrical connections to the capacitive pick-off plates  28  and the force restoring coils  30 .  
         [0022]    In order to increase damping efficiency and reduce lock-up, the reed  26 , as shown in the top view of FIG. 2, is configured with a set of radially extending grooves  40 - 48  in the upper surface. The drawing of FIG. 2 omits the torquer coils  30  shown in. FIG. 1 and illustrates a cut out area  50  on the reed  26  which permits electrical connections to the torquer coils  30 . In the preferred embodiment of the invention, the five grooves  40 - 48  are spaced at 45 degree intervals in a star pattern on the upper surface of the half of the reed  26 , which is opposite the flexure elements  32  and  34 . In one embodiment, corresponding grooves (not shown) are also etched in the lower surface of the reed. However, in another embodiment, the grooves are etched in only one of either the upper surface of the reed  26  or the lower surface of the reed  26 , as desired for a particular application. The grooves  40 - 48  extend radially from the cut out area  50  to the edges of the reed  26 . In the embodiment of the reed  26  shown in FIG. 2, the reed  26  is approximately 0.030 inches thick with a diameter of 0.642 inches and the grooves  40 - 48  etched into the reed  26  are approximately 0.020 inches wide with an approximate depth of 0.0005 inches which depth is less than the typical pick off gap of 0.00075 inches.  
         [0023]    Grooves which are smaller than the gap between the pendulum and the capacitive pick-off plate  28  when the pendulum is positioned equidistant from each of the two capacitive pick-off plates  28  become significant when the pendulum approaches the capacitive pick-off plate  28  and the gap becomes smaller. Such a situation pertains when the accelerometer is no longer in servo but is operating beyond its design range, for example, when the input accelerometer is greater than the device input limits. When this situation pertains, the small grooves of the present invention significantly affect the component of damping described by theoretical flat plate damping. For example, when the pendulum is positioned near one of the two capacitive pick-off plates  28 , the coefficient of flat damping is given by the equation:  
         damping=1/( h+d ) 3 ≃1/(2 h ) 3    (2)  
         [0024]    where: h is the distance between the capacitive pick-off plate  28  and the pendulum; and  
         [0025]    d is the depth of the grooves.  
         [0026]    Grooves which are smaller than the gap between the pendulum and the capacitive pick-off plate  28  when the pendulum is positioned equidistant from each of the two capacitive pick-off plates  28  do not significantly affect the component of damping due to frictional gas glow across the pendulum surface when the pendulum is positioned equidistant from each of the two capacitive pick-off plates  28 , i.e., when the accelerometer is in servo. As the pendulum approaches the capacitive pick-off plate  28  and the gap becomes smaller, the grooves become significant avenues of escape for damping gas trapped between the pendulum and the capacitive pick-off plate  28 . Thus, small grooves in the pendulum significantly reduce the component of damping due to frictional gas flow across the pendulum surface as the pendulum approaches the capacitive pick-off plate  28 .  
         [0027]    It will be appreciated that the present invention advantageously provides a damping coefficient with greatly reduced non-linearity as a function of proof mass position relative to the capacitive pick-off plates  28 . Referring now to FIG. 3 and to equation (2), a graph  100  shows a plot of the relationship given in equation (1). The present invention improves the gas damping performance of the accelerometer by reducing the coefficient of damping at pendulum positions near the capacitive pick-off plate  28  where damping gas can cause overshoot in extreme acceleration conditions and smooth pendulum/capacitor surfaces can cause accelerometer lockup. By configuring the invention to delay or reduce only the rapid increase in the coefficient of damping as the pendulum approaches the capacitive pick-off plate  28 , as predicted by the cubic function, while leaving the coefficient of damping unaffected at the nominal servo position, the present invention significantly flattens the curve over its entire range of operation. The resulting response of the coefficient of damping to pendulum position is substantially reduced non-linearity.  
         [0028]    Non-linearity of the coefficient of damping curve is substantially reduced as follows. It will be appreciated that the groove features, with depth d significantly less than the servo-position gap h between the pendulum and capacitive pick-off plate  28 , have little effect on the coefficient of damping at the servo position, as can be seen by the governing equation CD=k×1/(h+d) 3 , where k is a constant. Near the servo position, the coefficient of damping can be approximated as CD=k×1/(h) 3 . At pendulum positions near the capacitive pick-off plate  28 , where h is small relative to d, the groove features become significant, and the coefficient of damping approaches CD=k×1(d) 3 .  
         [0029]    [0029]FIG. 3 illustrates the effect of the groove configuration on the coefficient of damping over the accelerometer&#39;s range of operation. The coefficient of damping for the flat plate case varies with pendulum position by the inverse cube function CD=k×1/(h) 3 . By way of contrast, a deeply grooved plate, such as that taught in U.S. Pat. No. 5,350,189 also varies with the inverse cube function, but at a reduced level across the entire operating range, CD=k 2 ×1/(h) 3 , where k 2  is lower than k. Advantageously, a shallowly grooved plate, such as that of the present invention, varies by the inverse cube function, CD=k×1/(h+d) 3 , where the influence of d flattens the curve as the reed/plate gap becomes smaller and non-linearity of the curve becomes greatly reduced.  
         [0030]    When the grooves formed in the pendulum are small compared with the distance h, the present invention does not significantly affect the assumption that the dependence of damping on the gap size or distance h, is described by theoretical flat plate damping or damping α1/h 3 . For example, when the pendulum is positioned equidistant from each of the two capacitive pick-off plates  28 , i.e., when the accelerometer is in servo, the dependence of the damping coefficient on the distance h lies along a continuum ranging from 1/(h+d) 3  to 1/h 3  depending upon the exact configuration of the grooves, where h=distance between the pendulum and the capacitive pick-off plate  28 , and d=depth of groove, that is the effect of a groove. Thus, the value of the damping coefficient is well approximated by 1/h 3  when d is small compared to h.  
         [0031]    When the depth d of the grooves formed in the pendulum is significant compared with the distance h, the present invention significantly affects the assumption that damping is described by theoretical flat plate damping. For example, when the pendulum is positioned near one of the two capacitive pick-off plates  28 , i.e., when the accelerometer is no longer in servo but is operating beyond its design range, such that h≈d, damping can be thought of as arising from two types of regions: one for the flat plate region where the coefficient varies as 1/h 3 , and one for the groove region where the coefficient varies as 1/(h+d) 3 . Thus, when significant grooves are formed in the pendulum, the result is a composite coefficient of damping which varies with an intermediate behavior. The exact details of the damping depend on the groove configuration, which determines the interaction between the airflows between the two types of regions, the flat plate area and the grooved area.  
         [0032]    When the depth d of grooves formed in the pendulum is small compared with the distance h, the grooves do not significantly affect the component of damping due to frictional gas flow across the pendulum surface. For example, when the pendulum is positioned equidistant from each of the two capacitive pick-off plates  28 , i.e., when the accelerometer is in servo, grooves which are smaller in depth than the distance separating the pendulum and the capacitive pick-off plates  28  do not significantly affect the restriction to flow of gas trapped between the pendulum and the capacitive pick-off plates  28 .  
         [0033]    When the depth d of grooves formed in the pendulum is significant compared with the distance h, the grooves significantly affect the component of damping due to frictional gas flow across the pendulum surface by reducing the restriction to gas flow. For example, when the pendulum is positioned near one of the two capacitive pick-off plates  28 , i.e., when the accelerometer is no longer in servo but is operating beyond its design range, a groove having a depth d equivalent to or greater than the reduced distance h provides a path for gas trapped between the pendulum and the capacitive pick-off plate  28  to escape. Thus, the grooves contribute to a reduction in the component of damping due to frictional gas flow across the pendulum surface.  
         [0034]    When the depth d of grooves formed in the pendulum is significant compared with the distance h, the grooves significantly affect the component of damping due to frictional gas flow across the pendulum surface by reducing the restriction to gas flow. For example, when the pendulum is positioned near one of the two capacitive pick-off plates  28 , i.e., when the accelerometer is in no longer in servo but is operating beyond its design range, a groove having a depth d equivalent to or greater than the reduced distance h provides a path for gas trapped between the pendulum and the capacitive pick-off plate  28  to escape. Thus, the grooves contribute to a reduction in the component of damping due to frictional gas flow across the pendulum surface.  
         [0035]    When the depth d of grooves formed in the pendulum is significant compared with the distance h, the grooves significantly affect the component of damping due to frictional gas flow across the pendulum surface. For example, when the pendulum is positioned equidistant between the two capacitive pick-off plates  28 , i.e., when the accelerometer is in servo, a groove having a depth d equivalent to the dimension h provides a significant path for trapped gas to escape and flow across the pendulum surface, which effectively reduces the component of damping due to frictional gas flow.  
         [0036]    When holes or passages are formed in the pendulum, either alone or in combination with grooves, the holes significantly affect the flat plate damping coefficient. For example, if a series of holes changes the effective damping area from a square area A to n smaller area squares, each smaller-area square having an area A/n, then the quadratic dependence of damping on area reduces the damping to n*(A/n) 2 =A 2 /n.  
         [0037]    When holes or passages are formed in the pendulum, either alone or in combination with grooves, the holes significantly affect the component of damping due to frictional gas flow across the pendulum surface. For example, when the pendulum is positioned equidistant between the two capacitive pick-off plates  28 , i.e., when the accelerometer is in servo, one or more holes remove a significant area of the pendulum surface, thereby providing a significant path for trapped gas to escape without flowing across the pendulum surface, which effectively reduces the damping due to frictional gas flow.  
         [0038]    The typical spacing between the pendulum and the capacitive pick-off plate  28  in a capacitive pick-off accelerometer is on the order of 0.000750 inches. The spacing dimension is driven by the capacitive nature of the device; the need to provide gas damping; and manufacturing tolerances inherent in the processes employed.  
         [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.