Abstract:
Apparatus and associated methods relate to maximizing a signal to noise ratio of an accelerometer by inhibiting signals arising from movements of a proofmass in directions perpendicular to a direction of intended sensitivity. The direction of intended sensitivity of the accelerometer is along an axis of the proofmass. The accelerometer is rendered substantially insensitive to lateral accelerations of the proofmass by making the accelerometer axially symmetric. Two axially-asymmetric acceleration sensing devices are axially engaged in such a manner as to render the coupled sensing devices substantially axially-symmetric. In some embodiments, each acceleration sensor has an axially-thin membrane portion extending from a proofmass portion. The two acceleration sensors can be engaged in an antiparallel fashion at projecting ends of the proofmass portions. An engagement surface will be located about halfway between the axially-thin membrane portions of the two acceleration sensors, thereby causing mechanical symmetry about the engagement surface.

Description:
BACKGROUND 
       [0001]    Piezoelectric accelerometers may be made using various components and geometries. Some piezoelectric accelerometers use piezoelectric sensing elements mounted to an elastic membrane. Piezoelectric transducers are used to sense a mechanical deformation of the elastic membrane and generate an electrical signal indicative of the mechanical deformation. Piezoelectric transducers can produce a voltage, a current, or a charge in response to changes in pressure, acceleration, temperature, strain, or force, etc. Such transducers can be used to monitor processes or deformable members. Some piezoelectric transducers are configured to generate an electric signal only in response to accelerations in a specific direction. 
         [0002]    Piezoelectric accelerometers may have a proofmass attached to an elastic membrane. The elastic membrane is configured to span from the proofmass to a device to be acceleration tested. When the device under test is accelerated, inertia of the proofmass will cause the proofmass to move in a dissimilar fashion than the movement of the accelerating device under test. Because of this dissimilar movement, the elastic membrane spanning between the proofmass and the device under test, may distort. This distortion may be sensed by the piezoelectric transducer and converted into an electric signal indicative of such a distortion. 
         [0003]    Accelerometers may be axially asymmetric, even for devices intended to sense only or primarily axial accelerations. For example, an axially-thin elastic membrane may laterally extend from a first axial end of the proofmass. A second axial end of the proofmass may project away from the axially-thin elastic member. A lateral force acting upon the accelerometer can cause the proofmass to become canted with respect to the axially-thin elastic membrane, to which it is attached. Such canting of the proofmass can cause the axially-thin elastic membrane to deform. Such a deformation may be detected by a piezoelectric transducer mounted on the axially-thin elastic membrane, thereby causing a signal indicative of acceleration, albeit a lateral acceleration. In some applications, suppressing lateral accelerations may improve the signal-to-noise ration of axial acceleration measurements. 
       SUMMARY 
       [0004]    A piezoelectric accelerometer includes a first acceleration sensor. The first acceleration sensor includes a first proofmass. The first acceleration sensor includes a first axially-thin membrane portion coupled to the first proofmass. The first acceleration sensor also includes a first piezoelectric transducer on the first membrane portion. The first acceleration sensor is axially asymmetric. The piezoelectric accelerometer also includes s second acceleration sensor axially engaged with the first acceleration sensor. The second acceleration sensor includes a second proofmass. The second acceleration sensor also includes a second axially-thin membrane portion coupled to the second proofmass. The second acceleration sensor is axially asymmetric. 
         [0005]    A method for maximizing a lateral movement rejection ratio of an accelerometer includes affixing a device under test to a first axial end of a proofmass via an axially-deformable member. The method includes inhibiting a movement in directions parallel to a first lateral line of the first axial end of a proofmass. The method includes inhibiting a movement in directions parallel to a second lateral line of a second axial end of a proofmass. The method includes sensing a deformation of the axially-deformable member. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a perspective view of half of an exemplary accelerometer that is configured to be axially coupled to an identical half accelerometer. 
           [0007]      FIG. 2A  is a side elevation view of the exemplary accelerometer depicted in FIG. 
           [0008]      FIG. 2B  is a side elevation view of the exemplary accelerometer depicted in  FIG. 1  during an axial acceleration event. 
           [0009]      FIG. 2C  is a side elevation view of the exemplary accelerometer depicted in  FIG. 1  during a lateral acceleration event. 
           [0010]      FIG. 3  is a perspective view of an exemplary accelerometer configured to suppress signals resulting from lateral forces. 
           [0011]      FIG. 4A  is a side elevation view of the exemplary accelerometer depicted in  FIG. 3 . 
           [0012]      FIG. 4B  is a side elevation view of the exemplary accelerometer depicted in  FIG. 3  during an axial acceleration event. 
           [0013]      FIG. 4C  is a side elevation view of the exemplary accelerometer depicted in  FIG. 3  during a lateral acceleration event. 
           [0014]      FIGS. 5A-5D  are perspective views of the exemplary accelerometers depicted in  FIGS. 1-2  shown with peripheral attachment regions. 
           [0015]      FIG. 6  is a perspective view of an exemplary accelerometer configured to suppress signals resulting from lateral forces. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  is a perspective view of half of an exemplary accelerometer that is configured to be axially coupled to an identical half accelerometer. In  FIG. 1 , axially-asymmetric accelerometer  10  includes proofmass  12  and axially-thin elastic membranes  14   a - d . Piezoelectric transducers  16   a - d  are affixed to axially-thin elastic membranes  14   a - d . Proofmass  12  has center of mass  18  along an axis  20  extending between a first axial end  22  and a second axial end  24 . 
         [0017]    Axially-thin elastic membranes  14   a  and  14   c  extend in opposite lateral directions along first lateral line  26  from first axial end  22  of proofmass  12 . Axially-thin elastic membranes  14   b  and  14   d  extend in opposite lateral directions along second lateral line  28  from first axial end  22  of proofmass  12 . Accelerometer  10  is configured to measure accelerations parallel to axis  20  of proofmass  12 . 
         [0018]      FIG. 2A  is a side elevation view of the exemplary accelerometer depicted in  FIG. 1 . In  FIG. 2A , piezoelectric sensing film  16   a ,  16   c  are deposited on top of elastic membranes  14   a ,  14   c , respectively. Piezoelectric sensing film  16   a ,  16   c  can be patterned such that they are located at locations, such as at locations A-D, where stress can result from axial accelerations. Superimposed on accelerometer  10  of  FIG. 2A  are lateral axis L and axial axis A. Lateral axis L defines a lateral dimension parallel to first lateral line  26  (shown in  FIG. 1 ) which defines the directions that axially-thin elastic membranes  14   a ,  14   c  extend from proofmass  12 . Axial axis A is collinear with axis  20  depicted in  FIG. 1 . Axially-thin elastic membranes  14   a ,  14   c  are attached to device under test  30  at lateral ends  32   a ,  32   c  of axially-thin elastic membranes  14   a ,  14   c , respectively. Axially-thin elastic membranes  14   a ,  14   c  span a space between device under test  30  and proofmass  12 . 
         [0019]      FIG. 2B  is a side elevation view of the exemplary accelerometer depicted in  FIG. 1  during an axial acceleration event. In  FIG. 2B , proofmass  12  is shown responding to an axial acceleration, indicated by axial acceleration vector  31 . Because elastic membranes  14   a ,  14   c  are axially thin compared to the lateral dimensions, an axial acceleration can create large bending stresses at certain locations of axially-thin elastic membranes  14   a ,  14   c , such as at locations A-D. These bending stresses can create electrical signal outputs by piezoelectric sensing film  16   a ,  16   c.    
         [0020]      FIG. 2C  is a side elevation view of the exemplary accelerometer depicted in  FIG. 1  during a lateral acceleration event, indicated by lateral acceleration vector  33 . In  FIG. 2C , proofmass  12  is shown responding to a lateral acceleration. Superimposed on accelerometer  10  of  FIG. 2C  is a vector depicting inertia vector  33  originating at center of mass  18  of proofmass  12  and extending along lateral axis  26  (depicted in  FIG. 2A ). Because center of mass  18  is not coplanar with axially-thin elastic membranes  14   a ,  14   b , a lateral acceleration of proofmass  12  can cause bending stresses of top of elastic membranes  14   a - c . These bending stresses can create electrical signal outputs by piezoelectric sensing film  16   a - c.    
         [0021]    For accelerometers designed for measuring accelerations in directions parallel to axial axis  20  in  FIG. 2B , electrical signal outputs by piezoelelctric sensing film  16   a - c  can be considered as noise. Therefore, minimizing such electrical signal outputs can improve sensing of axial accelerations. Simulations for a specific configuration of an axially-asymmetric accelerometer were performed. Simulations of both a lateral acceleration and an axial acceleration were conducted. Results of these simulations on such an axially-asymmetric accelerometer, such as accelerometer  10 , yield the following sensitivities: i) piezoelectric sensor  16   a ,  16   c  will generate a 1.03 mVolt signal in response to a 1 gram axial acceleration; ii) piezoelectric sensors  16   a ,  16   c  will generate a 0.26 mVolt signal in response to a 1 gram lateral acceleration. This simulation represents only a modest 75% lateral rejection ratio. Various specific axially-asymmetric accelerometer geometries will result in various different specific sensitivities. 
         [0022]      FIG. 3  is a perspective view of an exemplary accelerometer configured to suppress signals resulting from lateral acceleration. In  FIG. 3 , axially-symmetric accelerometer  34  which is made by axially connecting two of accelerometers  10  in an antiparallel fashion at second axial ends  24  of proofmasses  12  (as depicted in  FIG. 1 ). Each of accelerometer  10  will now be considered one of accelerometer half  36   a  or  36   b . Various means for axially engaging second axial ends  24  of proofmasses  12  from accelerometer halves  36   a ,  36   b  may be used. For example, an adhesive may be used to axially engage second axial ends  24  of proofmasses  12 . In an exemplary embodiment, second axial end  24  may engage one another due to affixing lateral ends of axially-thin elastic membranes  16   a - h  to a device under test in locations that maintain engagement of second axial ends  24  of proofmasses  12 . 
         [0023]    Axially-symmetric accelerometer  34  includes proofmass  38 , which includes proofmasses  12  of accelerometer halves  36   a ,  36   b . Axially-symmetric accelerometer  34  also includes axially-thin elastic membranes  14   a - h . Piezoelectric transducers  16   a - h  are affixed to axially-thin elastic membranes  14   a - h . Proofmass  38  has center of mass  19  along an axis  20  extending between a first axial end  22  and a second axial end  25 . 
         [0024]    Axially-thin elastic membranes  14   a  and  14   c  extend in opposite lateral directions along first lateral line  26  from first end  22  of the proofmass  12 . Axially-thin elastic membranes  14   b  and  14   d  extend in opposite lateral directions along second lateral line  28  from first axial end  22  of proofmass  38 . Accelerometer  34  is configured to measure accelerations parallel to axis  20  of proofmass  38 . 
         [0025]      FIG. 4A  is a side elevation view of the exemplary accelerometer depicted in  FIG. 3 . Superimposed on accelerometer  34  of  FIG. 4A  are lateral axis L and axial axis A. Lateral axis L defines a lateral dimension parallel to first lateral line  26  (shown in  FIG. 1 ) which defines the directions that axially-thin elastic membranes  14   a ,  14   c ,  14   e ,  14   g  extend from proofmass  38 . Axial axis A is collinear with axis  20  depicted in  FIG. 1 . Axially-thin elastic membranes  14   a ,  14   c ,  14   e ,  14   g  are attached to device under test  30  at lateral ends  32   a ,  32   c ,  32   e ,  32   g  of axially-thin elastic membranes  14   a ,  14   c ,  14   e ,  14   g  respectively. Axially-thin elastic membranes  14   a ,  14   c ,  14   e ,  14   g  span a space between device under test  30  and proofmass  38 . 
         [0026]      FIG. 4B  is a side elevation view of the exemplary accelerometer depicted in  FIG. 3  during an axial acceleration event. In  FIG. 4B , proofmass  12  is shown responding to an axial acceleration, as indicated by vector  31 . Because elastic membranes  14   a ,  14   c ,  14   e ,  14   g  are axially thin compared to the lateral dimensions, an axial acceleration can create large bending stresses at certain locations of axially-thin elastic membranes  14   a ,  14   c ,  14   e ,  14   g  such as at locations A-D, A′-D′. These bending stresses can create electrical signal outputs by piezoelectric sensing elements  16   a ,  16   c ,  16   e ,  16   g.    
         [0027]      FIG. 4C  is a side elevation view of the exemplary accelerometer depicted in  FIG. 3  during a lateral acceleration event. In  FIG. 4C , accelerometer  34  is again shown in side elevation view, but instead of an axial acceleration, proofmass  38  is shown responding to a lateral acceleration. Superimposed on accelerometer  34  of  FIG. 4C  is a vector depicting inertia vector  33  originating at center of mass  19  of proofmass  38  and extending in a direction parallel to lateral axis L (depicted in  FIG. 4A ). Because center of mass  19  is halfway between axially-thin elastic membranes  14   a ,  14   c  and axially-thin membranes  14   e ,  14   g , canting of proofmass  38  is inhibited in response to a lateral acceleration. Thus, little bending deformation of axially-thin elastic membranes  14   a ,  14   c ,  14   e ,  14   g  will result. 
         [0028]    Simulations for a specific configuration of an axially-symmetric accelerometer were performed. Simulations of both a lateral acceleration and an axial acceleration were conducted. Results of these simulations on such an axially-symmetric accelerometer, such as accelerometer  34 , yield the following sensitivities: i) piezoelectric sensor  16   a ,  16   c ,  16   e ,  16   g  will again generate a 1.03 mVolt signal in response to a 1 gram axial acceleration; ii) piezoelectric sensors  16   a ,  16   c ,  16   e ,  16   g  will generate a 0.016 mVolt signal in response to a 1 gram lateral acceleration. This simulation represents only a modest 98.5% lateral rejection ratio. Various specific axially-symmetric accelerometer geometries will result in various different specific sensitivities. 
         [0029]      FIGS. 5A-5D  are perspective views of the exemplary accelerometers depicted in  FIGS. 1-4  shown with peripheral attachment regions. In  FIGS. 5A-5B , accelerometer half  36   a  or  36   b  is depicted with peripheral attachment regions  40 . In  FIGS. 5C-5D  accelerometer  34  is depicted with peripheral attachment regions  40 . Peripheral attachment regions  40  can facilitate attachment of accelerometer  34  to a device under test. 
         [0030]      FIG. 6  is a perspective view of an exemplary accelerometer configured to suppress signals resulting from lateral acceleration. In  FIG. 6 , a cylindrically symmetrical embodiment of axially symmetric accelerometer  42  is shown. Axially-symmetric accelerometer  42  has both axis  20  of lateral symmetry and center or mass  19  of axial mirror symmetry. Axially-thin annular elastic membranes  44   a ,  44   b  laterally extend from proofmass  46 . Piezoelectric sensors  48   a ,  48   b  are attached to axially-thin annular elastic membranes  44   a ,  44   b.    
         [0031]    The following are non-exclusive descriptions of possible embodiments of the present invention. 
         [0032]    A piezoelectric accelerometer includes a first acceleration sensor. The first acceleration sensor includes a first proofmass. The first acceleration sensor includes a first axially-thin membrane portion coupled to the first proofmass. The first acceleration sensor also includes a first piezoelectric transducer on the first membrane portion. The first acceleration sensor is axially asymmetric. The piezoelectric accelerometer also includes a second acceleration sensor axially engaged with the first acceleration sensor. The second acceleration sensor includes a second proofmass. The second acceleration sensor also includes a second axially-thin membrane portion coupled to the second proofmass. The second acceleration sensor is axially asymmetric. 
         [0033]    A further embodiment of the foregoing piezoelectric accelerometer, wherein the first proofmass of the first acceleration sensor can be securely engaged with the second proofmass of the second acceleration sensor so as to form a combined proofmass. A further embodiment of any of the foregoing piezoelectric accelerometers, wherein the first axially-thin membrane portion of the first acceleration sensor can define a first plane, and the second axially-thin membrane portion of the second acceleration sensor can define a second plane, and wherein a center of mass of the combined proofmass can be between the first plane and the second plane. A further embodiment of any of the foregoing piezoelectric accelerometers, wherein the combined proofmass is axially symmetric about the center of mass. 
         [0034]    A further embodiment of any of the foregoing piezoelectric accelerometers, wherein the second acceleration sensor further includes a second piezoelectric transducer on the second membrane portion. A further embodiment of any of the foregoing piezoelectric accelerometers, wherein the second acceleration sensor can be identical to the first accelerometer sensor. A further embodiment of any of the foregoing piezoelectric accelerometers, wherein the first and second acceleration sensors can be axially engaged in an anti-parallel orientation such that the piezoelectric accelerometer has symmetry about a symmetry point located at an interface between the first and second acceleration sensors. 
         [0035]    A further embodiment of any of the foregoing piezoelectric accelerometers, wherein the engagement of the first and second acceleration sensors can be realized by direct wafer bonding or chip bonding. A further embodiment of any of the foregoing piezoelectric accelerometers, wherein the engagement of the first and second acceleration sensors can be realized by wafer bonding or chip bonding with at least one intermedium layer. A further embodiment of any of the foregoing piezoelectric accelerometers, wherein the engagement of the first and second acceleration sensors can be realized by an adhesive. A further embodiment of any of the foregoing piezoelectric accelerometers, wherein each of the first and second axially-thin membrane portions can laterally project from the first and second proofmasses, respectively. 
         [0036]    A further embodiment of any of the foregoing piezoelectric accelerometers, wherein each of the first and second acceleration sensors have rotational symmetry. A further embodiment of any of the foregoing piezoelectric accelerometers, wherein rotational symmetry is 180 degree rotational symmetry. A further embodiment of any of the foregoing piezoelectric accelerometers, wherein rotational symmetry is 90 degree rotational symmetry. 
         [0037]    A method for maximizing an lateral movement rejection ratio of an accelerometer includes affixing a device under test to a first axial end of a proofmass via an axially-deformable member. The method includes inhibiting a movement in directions parallel to a first lateral line of the first axial end of a proofmass. The method includes inhibiting a movement in directions parallel to a second lateral line of a second axial end of a proofmass. The method includes sensing a deformation of the axially-deformable member. 
         [0038]    The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: i) generating a signal indicative of acceleration based on the sensed deformation of the axially-deformable member; and/or ii) engaging a first proofmass portion of a first axially-asymmetric accelerometer to a second proofmass portion of a second axially-asymmetric accelerometer. 
         [0039]    A further embodiment of any of the foregoing methods, wherein the affixing a device under test to a first axial end of a proofmass can include affixing laterally opposite attachment end portions of the axially-deformable member to the accelerating testing device. A further embodiment of any of the foregoing methods, wherein the axially deformable member can be a first axially deformable member, and the method can further includes affixing the device under test to a second axial end of the proofmass via an second axially-deformable member. A further embodiment of any of the foregoing methods, wherein affixing a device under test to a first axial end of a proofmass can include affixing a periphery of the axially-deformable member to the accelerating testing device. 
         [0040]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.