Abstract:
The apparatus in one embodiment may have: a beam having at least first and second mounting points that are operatively coupled to the beam by first and second flexures, respectivly; the beam having first and second nodal points; the beam having a center area; and an altered mass content in a vicinity of the center area, the altered mass content being such that the first and second nodal points are substantially aligned with the first and second flexures. The beam is therefore supported at the nodal points for the fundamental mode of vibration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application contains subject matter that is related to the subject matter of the following application, which is assigned to the same assignee as this application. The below-listed application is hereby incorporated herein by reference in its entirety.  
         [0002]     “Vibratory Gyro Bias Error Cancellation Using Mode Reversal”, Ser. No. ______, filed (NGC-275/001000-199).  
         [0003]     “Bias and Quadrature Reduction in Class II Coriolis Vibratory Gyros”, Ser. No. ______, filed (NGC-279/001039-199).  
         [0004]     “Oscillation of Vibrating Beam in a First Direction for a First Time Period and a Second Direction for a Second Time Period to Sense Angular Rate of the Vibrating Beam,” by Robert E. Stewart, application Ser. No. 11/057,324, filed Feb. 11, 2005. 
     
    
     TECHNICAL FIELD  
       [0005]     The invention relates generally to electromechanical systems and more particularly to vibrating beams.  
       BACKGROUND  
       [0006]     An electromechanical system in one example measures a parameter. The electromechanical system may comprise a micro-electromechanical system (“MEMS”) accelerometer or gyroscope that measures the parameter. For example, the accelerometer measures an acceleration and the gyroscope measures an angular rate (e.g., rotation). The gyroscope in one example comprises a vibrating beam with high Q degenerate fundamental modes of vibration. For example, high Q vibrating beams require little energy to sustain vibration. The vibrating beam in one example is employable for high performance closed loop angular rate sensing. The vibrating beam in another example is employable for lower performance open loop angular rate sensing. The mathematical model of the symmetrical vibrating beam is in many aspects similar to a vibrating ring or hemispherical resonator gyroscope (“HRG”). The analytical similarity to the hemispherical resonator gyroscope indicates that the vibrating beam gyroscope has the potential of achieving similar performance.  
         [0007]     Drive components coupled with the vibrating beam cause a first oscillation of the vibrating beam. An angular rate of the vibrating beam and the first oscillation induce a Coriolis force on the vibrating beam. For example, the angular rate is about the longitudinal axis of the vibrating beam. The Coriolis force causes a second oscillation of the vibrating beam. The second oscillation is substantially perpendicular to the first oscillation. Feedback components in one example provide feedback on a magnitude of the first oscillation to the drive components for regulation of the first oscillation. Pickoff sensor components sense the second oscillations and apply control signals to null the pickoff signal. The control signals are a measure of the magnitude and polarity of the angular rate of the vibrating beam.  
         [0008]     Coriolis Vibratory Gyros (CVG) based on the free-free vibrating beam are currently used to sense angular rotation rate. The beams are typically supported at the nodal points of the beam to minimize the coupling of the vibration energy to the mounting base. The coupling of vibration energy to the mounting base reduces the Q of the resonant frequency of the beam and introduces sensitivity to variation in the mechanical impedance of the mounting.  
         [0009]     There is a need in the art for an improved CVG that minimizes coupling of vibration energy to a mounting base for use in high performance vibrating beam gyros which require high Q and low mounting sensitivity.  
       SUMMARY  
       [0010]     One embodiment of the present method and apparatus encompasses an apparatus. The apparatus may comprise: a beam having at least first and second mounting points that are operatively coupled to first and second flexures; the beam having first and second nodal points; the beam having a center area; and an altered mass content in a vicinity of the center area, the altered mass content being such that the first and second nodal points are substantially aligned with the first and second flexures.  
         [0011]     Another embodiment of the present method and apparatus encompasses a method. The method may comprises: providing a beam having at least first and second mounting points that are operatively coupled to first and second flexures, first and second nodal points, and a center area; and altering mass content of the center area such that the first and second nodal points are substantially aligned with the first and second flexures. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0012]     Features of exemplary implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which:  
         [0013]      FIG. 1 , depicts an apparatus having a micro-electromechanical system (“MEMS”) gyroscope;  
         [0014]      FIG. 2  is a graph of reaction force as a function of mass added to the center of the beam;  
         [0015]      FIG. 3  depicts an embodiment of a method according to the present method; and  
         [0016]      FIG. 4  depicts another embodiment of a method according to the present method. 
     
    
     DETAILED DESCRIPTION  
       [0017]     Vibratory gyros may be a collective name for mechanical devices that in various ways use Coriolis acceleration to sense rotation. These gyros appear in a large number of shapes and are also known as tuning forks, vibrating disks, vibrating wine glass etc.  
         [0018]     Turning to  FIG. 1 , an apparatus  100  in one embodiment comprises a micro-electromechanical system (“MEMS”) gyroscope. The gyroscope is employable for high accuracy navigation angular rate sensing. The apparatus  100  in one example has a vibrating beam  102  and a plurality of drive/sensor components  105 ,  106 ,  110 , and  112 . Depending on an oscillation mode of the vibrating beam  102 , a first subgroup of the drive/sensor components  105 ,  106 ,  110 , and  112  drive a first oscillation of the vibrating beam  102  and a second subgroup of the drive/sensor components  105 ,  106 ,  110 , and  112  sense a second Coriolis induced oscillation of the vibrating beam  102 .  
         [0019]     In one example, the drive oscillation of the vibrating beam  102  is along a first direction (e.g., out-of-plane). So, the drive/sensor components  105  and  106  serve as drive components for the vibrating beam  102  and the drive/sensor components  110  and  112  serve as pickoff sensors for the vibrating beam  102 . In another example, the drive oscillation of the vibrating beam  102  is along a second direction (e.g., in-plane). So, the drive/sensor components  110  and  112  serve as drive components for the vibrating beam  102  and the drive/sensor components  105  and  106  serve as pickoff sensors for the vibrating beam  102 .  
         [0020]     The vibrating beam  102  comprises one or more nodal axes for vertical oscillation (e.g., vertical vibration). For example, vertical oscillation of the vibrating beam  102  may occur about the nodal axes. The vibrating beam  102  may also have one or more nodal axes for horizontal oscillation (e.g., horizontal vibration). For example, horizontal oscillation of the vibrating beam  102  may occur about the nodal axes. The nodal axis for vertical oscillation and the nodal axis for horizontal oscillation in one example intersect at a nodal point. The nodal point remains substantially still for oscillation in any one or more of a plurality of substantially perpendicular directions. For example, the nodal point remains substantially still during both vertical oscillation and horizontal oscillation. The vibrating beam  102  may have one or more connection components  120  that serve to connect a frame  118  with the vibrating beam  102  at the nodal point of the vibrating beam  102 .  
         [0021]     The vibrating beam  102  in one example is supported by a flexure component  122  connected to the frame  118  through top and bottom covers (not shown). The flexure component  122  allows movement of the vibrating beam  102  about the nodal point upon occurrence of an angular rate. For example, the angular rate is about the longitudinal axis of the vibrating beam. Pickoff sensors and processing components measure and translate the movement of the vibrating beam  102  into a signal representing the direction and magnitude of the angular rate.  
         [0022]     The flexure component  122  allows horizontal and vertical oscillation of the vibrating beam  102  about the nodal point. The flexure component  122  may have a reduced section of the vibrating beam  102  that lies along a center line of a longitudinal axis of the vibrating beam  102 . The nodal point in one example is internal to the vibrating beam  102 . For example, the vibrating beam  102  may have one or more apertures  124  that expose an area around the nodal point. The aperture  124  in one example passes through the vibrating beam  102 . The aperture  124  surrounds the nodal point and the flexure component  122  to provide space for movement of the vibrating beam  102 . The aperture  124  is near the nodal point. The aperture  124  allows the frame  118  to support the vibrating beam substantially near the nodal point. The aperture  124  may have an open space etched from a surface of the vibrating beam  102  to expose the nodal point and form the flexure component  122 .  
         [0023]     To initialize an angular rate sensing gyroscope, the drive components of the drive/sensor components  105 ,  106 ,  110 , and  112  cause a first oscillation of the vibrating beam  102 . An angular rate of the vibrating beams  102  about its longitudinal axes and the first oscillation induce a Coriolis force on the vibrating beam  102 . The Coriolis force causes a second oscillation of the vibrating beam  102 . The second oscillation is substantially perpendicular to the first oscillation. Feedback components in one example provide feedback on a magnitude of the first oscillation to the drive component  106  for regulation of the first oscillation. The sensor components of the drive/sensor components  105 ,  106 ,  110 , and  112  pickoff the second oscillations and apply control signals to null the pickoff signal. The control signals are a measure of the magnitude and polarity of the angular rate of the vibrating beam  102 .  
         [0024]     The drive components of the drive/sensor components  105 ,  106 ,  110 , and  112  in one example may have electrostatic drive components, magnetic drive and/or piezoelectric drive components. The sensor components of the drive/sensor components  105 ,  106 ,  110 , and  112  in one example may have capacitive pickoff sensors, magnetic pickoff sensors, piezoresistive sensors, and/or piezoelectric pickoff sensors.  
         [0025]     A beam may be supported at the nodal points for the fundamental mode of vibration. Since achieving high Q is important to achieving high performance of the gyro, in one example it is desirable to minimize the vibration energy lost through coupling to the mounting structure.  
         [0026]     The location of the nodal points of a beam with a rectangular crossection may be given by: 
 
X node =0.267 L 
 
         [0027]     where: 
        X node =Distance from the center of the beam to each nodal point     L=Total length of the beam        
 
         [0030]     The location of the mounting points is determined by design of the beam. Manufacturing tolerances will cause the location of the mounting points to not be at the nodal points. In order to minimize the coupling of the vibration energy of the beam to the mounting structure it is desirable to compensate for the manufacturing tolerances and adjust the location of the nodal points to be at the mounting points. Embodiments according to the present method and apparatus provide a simple and controllable method of adjusting the location of the nodal points.  
         [0031]      FIG. 2  shows results of a computer simulation. These results show that the addition or removal of mass from the center of the beam changes the location of nodal points and that the translational force at the mounting point may be reduced to zero.  
         [0032]      FIG. 2  is a plot of the simulation results showing that adding 0.55 gm to the center of the beam reduces the translational force linearly from 0.67 lb to 0. Adding additional mass causes the translational force to increase as the nodal points are moved away from the mounting points. In this simulation it is assumed that the mounting points are located symmetrically about the center of the beam.  
         [0033]      FIG. 3  is a representation of an exemplary implementation of a beam  300  with mass removed from one or more sides  301 ,  302  of the beam  300  to adjust the location of the nodal point. The beam  300  may have mounting points  304 ,  308  located in an area of ends  306 ,  310  of the beam  300 . The mounting points  304 ,  308  are coupled to the beam  300  by flexures  301 ,  303 . In order to align the nodal points with the flexures  301 ,  303 , mass at the center area  312  of the beam  300  may be changed.  
         [0034]     More specifically, mass may be subtracted from the center area  312  from one or both of the sides  301 ,  302  of the beam  300 . Also, mass may be added to the center area  312  from one or both of the sides  301 ,  302  of the beam  300 . Furthermore, mass may be added to one side  301 ,  302  of the beam  300 , while mass may be removed from the other side  301 ,  302  of the beam  300 .  
         [0035]     The present apparatus in one example may comprise a plurality of components such as one or more of electronic components, hardware components, and computer software components. A number of such components may be combined or divided in the apparatus.  
         [0036]     The steps or operations described herein are just exemplary. There may be many variations to these steps or operations without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted, or modified.  
         [0037]     Although exemplary implementations of the invention have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims.