Abstract:
A method and system for a sensor system of a device is disclosed. The sensor system includes a first MEMS sensor (FMEMS), a second MEMS sensor (SMEMS) and a signal processor (SP). An excitation is imparted to the device along a first axis (FA). The FMEMS has a first primary sense axis (FPSA), moves in response to a component of the excitation along the FA aligned with the FPSA and outputs a first signal proportional to an excitation along the FPSA. The SMEMS has a second primary sense axis (SPSA), moves in response to a component of the excitation along the FA aligned with the SPSA and outputs a second signal proportional to an excitation along the SPSA. The SP combines the first signal and the second signal to output a third signal proportional to the excitation along the FA. The FA, the FPSA and the SPSA have different orientations.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to microelectromechanical systems (MEMS) device and more particularly, to a MEMS device configured to selectively measure excitation in different direction. 
       DESCRIPTION OF RELATED ART 
       [0002]    MEMS devices are formed using various semiconductor manufacturing processes. MEMS devices may have fixed and movable portions. MEMS force sensors have one or more sense material, which react to an external influence imparting a force onto the movable portions. The sense material can be the MEMS structural layer or a deposited layer. The MEMS force sensor may be configured to measure these movements induced by the external influence to determine the type and extent of the external influence. 
         [0003]    MEMS devices may include one or more movable portions coupled to one or more springs. Sometimes, large external acceleration or shock may impart undesirable movements of the movable portions. These undesirable movements may induce false measurements or introduce errors into the measurement capabilities of the MEMS device. It may be desirable to minimize the impact of extraneous forces or stress on operation of the MEMS device. 
         [0004]    With these needs in mind, the current disclosure arises. This brief summary has been provided so that the nature of the disclosure may be understood quickly. A more complete understanding of the disclosure can be obtained by reference to the following detailed description of the various embodiments thereof in connection with the attached drawings. 
       SUMMARY OF THE INVENTION 
       [0005]    In one embodiment, a method for determining excitation imparted to a device is disclosed. A sensor system with a first MEMS sensor, a second MEMS sensor and a signal processor is provided. An excitation is imparted to the sensor system along a first axis. The first MEMS sensor has a first primary sense axis and is configured to output a first signal proportional to the excitation of the sensor system along the first primary sense axis. The first sensor moves in response to a component of the excitation along the first axis that is aligned with the first primary sense axis. The second MEMS sensor has a second primary sense axis and is configured to output a second signal proportional to the excitation of the sensor system along the second primary sense axis. The second sensor moves in response to a component of the excitation along the first axis that is aligned with the second primary sense axis. The first signal and the second signal are combined by the signal processor to output a third signal proportional to the excitation of the sensor system along the first axis, wherein the first axis, the first primary axis and the second primary axis have different orientations. 
         [0006]    In yet another embodiment, a method for a sensor system is disclosed. A first MEMS sensor, a second MEMS sensor and a signal processor is provided. The sensor system is excited along a first axis. The first MEMS sensor has at least a first sense axis and a second sense axis. The first MEMS sensor also has a first plurality of sensing devices that is selectively configured to output a signal that is proportional to excitation along one of the at least the first sense axis and the second sense axis. The second MEMS sensor has at least a third sense axis and a fourth sense axis. The second MEMS sensor also has a second plurality of sensing devices that is selectively configured to output a signal that is proportional to excitation along one of the at least the third sense axis and the fourth sense axis. The first axis, the first sense axis and the third sense axis all have different orientations or different physical excitations. The first plurality of sensing devices and the second plurality of sensing devices are configured by the signal processor to output a first signal proportional to the excitation of the sensor system along the first axis. 
         [0007]    In yet another embodiment, a system is disclosed. The system includes a sensor system with a first MEMS sensor, a second MEMS sensor and a signal processor is provided. An excitation is imparted to the sensor system along a first axis. The first MEMS sensor has a first primary sense axis and is configured to output a first signal proportional to the excitation of the sensor system along the first primary sense axis. The first sensor moves in response to a component of the excitation along the first axis that is aligned with the first primary sense axis. The second MEMS sensor has a second primary sense axis and is configured to output a second signal proportional to the excitation of the sensor system along the second primary sense axis. The second sensor moves in response to a component of the excitation along the first axis that is aligned with the second primary sense axis. The first signal and the second signal are combined by the signal processor to output a third signal proportional to the excitation of the sensor system along the first axis, wherein the first axis, the first primary axis and the second primary axis have different orientations. 
         [0008]    In yet another embodiment, a sensor system is disclosed. The sensor system includes a first MEMS sensor, a second MEMS sensor and a signal processor. The sensor system is excited along a first axis. The first MEMS sensor has at least a first sense axis and a second sense axis. The first MEMS sensor also has a first plurality of sensing devices that is selectively configured to output a signal that is proportional to excitation along one of the at least the first sense axis and the second sense axis. The second MEMS sensor has at least a third sense axis and a fourth sense axis. The second MEMS sensor also has a second plurality of sensing devices that is selectively configured to output a signal that is proportional to excitation along one of the at least the third sense axis and the fourth sense axis. The first axis, the first sense axis and the third sense axis all have different orientations or different physical excitations. The first plurality of sensing devices and the second plurality of sensing devices are configured by the signal processor to output a first signal proportional to the excitation of the sensor system along the first axis. 
         [0009]    This brief summary is provided so that the nature of the disclosure may be understood quickly. A more complete understanding of the disclosure can be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The foregoing and other features of several embodiments are described with reference to the drawings. In the drawings, the same components have the same reference numerals. The illustrated embodiments are intended to illustrate but not limit the invention. The drawings include the following Figures: 
           [0011]      FIG. 1  shows a MEMS device, according to one aspect of the present disclosure; 
           [0012]      FIG. 1A  shows an example first sense circuit to measure excitation along X axis, according to one aspect of the present disclosure; 
           [0013]      FIG. 1B  shows an example second sense circuit to measure excitation along Y axis, according to one aspect of the present disclosure; 
           [0014]      FIG. 1C  shows an example Wheatstone bridge configuration of sensors, according to one aspect of the present disclosure; 
           [0015]      FIG. 1D  shows an example modified Wheatstone bridge configuration of sensors, according to one aspect of the present disclosure; 
           [0016]      FIG. 2A  shows an example sensor assembly, according to one aspect of the present disclosure; 
           [0017]      FIG. 2B  shows an example half Wheatstone bridge configuration for sensors of sensor assembly of  FIG. 2A , according to one aspect of the present disclosure; 
           [0018]      FIG. 3A  shows an example two sensor assemblies, according, according to one aspect of the present disclosure; 
           [0019]      FIG. 3B  shows an example full Wheatstone bridge configuration for sensors of sensor assembly of  FIG. 3A , according to one aspect of the present disclosure; 
           [0020]      FIG. 3C  shows a table showing various example sensor configurations for two sensor assembly of  FIG. 3A ; 
           [0021]      FIG. 3D  shows an example third sense circuit to measure acceleration along Z axis, according to one aspect of the present disclosure; 
           [0022]      FIG. 3E  shows an example fourth sense circuit to measure magnetic field along Z axis, according to one aspect of the present disclosure; and 
           [0023]      FIG. 4  shows an example sensing circuit, according to one aspect of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    To facilitate an understanding of the adaptive aspects of the present disclosure, an example MEMS device configured to measure excitation in at least one axis is described. Adaptive aspects of this example is further described with respect to another example MEMS device which may be configured to measure excitation in more than one axis. 
         [0025]      FIG. 1  shows a MEMS device  100 , in accordance with an embodiment of this disclosure. The MEMS device  100  includes a first MEMS sensor  102  and a second MEMS sensor  104 . Preferred measurement of an excitation for the MEMS device  100  is along a first axis and a fourth axis orthogonal to the first axis. In some example, the first axis may correspond to an X axis and the fourth axis may correspond to a Y axis. In one example, the first MEMS sensor  102  is configured to measure an excitation along a second axis, second axis different than the first axis and the fourth axis. Sometimes, the second axis may be referred to as a first primary sense axis. The second MEMS sensor  104  is configured to measure an excitation along a third axis, the third axis different than the first axis, second axis and the fourth axis. Sometimes, the third axis may be referred to as a second primary sense axis. 
         [0026]    The first MEMS sensor  102  includes a first proof mass  106 , a first sensing device  108  and a second sensing device  110 . The first proof mass  106  is movably coupled to a first anchor  112 . For example, a first spring  114  couples the first proof mass  106  to the first anchor  112 . Upon imparting an excitation, for example, along the second axis, the first proof mass  106  may move along the second axis. In one example, the first sensing device  108  and the second sensing device  110  may be capacitive sensing device. One of the electrodes of the first sensing device  108  and the second sensing device  110  may be stationary and another of the electrodes of the first sensing device  108  and the second sensing device  110  may be formed over a portion of the first proof mass  106 . Upon imparting excitation, for example, along the second axis, a gap between the electrodes of the first sensing device  108  may increase and a gap between the electrodes of the second sensing device  110  may reduce. As one skilled in the art appreciates, a change in the gap between electrodes of a capacitive sensor changes corresponding capacitance value of the capacitive sensor. 
         [0027]    In one example, the first MEMS sensor  102  is configured such that an excitation B along the second axis has a corresponding excitation component along the X axis and negative Y axis. 
         [0028]    The second MEMS sensor  104  includes a second proof mass  116 , a third sensing device  118  and a fourth sensing device  120 . The second proof mass  116  is movably coupled to a second anchor  122 . For example, a second spring  124  couples the second proof mass  116  to the second anchor  122 . Upon imparting an excitation, for example, along the third axis, the second proof mass  116  may move along the third axis. In one example, the third sensing device  118  and the fourth sensing device  120  may be capacitive sensing device. One of the electrodes of the third sensing device  118  and the fourth sensing device  120  may be stationary and another of the electrodes of the third sensing device  118  and the fourth sensing device  120  may be formed over a portion of the second proof mass  116 . Upon imparting excitation, for example, along the third axis, a gap between the electrodes of the third sensing device  118  may increase and a gap between the electrodes of the fourth sensing device  120  may reduce. As one skilled in the art appreciates, a change in the gap between electrodes of a capacitive sensor changes corresponding capacitance value of the capacitive sensor. 
         [0029]    In one example, the first MEMS sensor  102  is configured such that an excitation B along the second axis has a corresponding excitation component along the positive X axis and negative Y axis. The second MEMS sensor  104  is configured such that an excitation A along the third axis has a corresponding excitation component along the positive X axis and positive Y axis. As one skilled in the art appreciates, by selectively configuring the first MEMS sensor  102  and second MEMS sensor  104  in this manner may be advantageously used to measure excitation along two axes, for example, X axis and Y axis. In one example, the X axis corresponds to the first axis and the Y axis corresponds to the fourth axis. An example first sense circuit  130 - 1  will now be described with reference to  FIG. 1A  to measure excitation along X axis. Later, another example second sense circuit  130 - 2  will be described with reference to  FIG. 1B  to measure excitation along Y axis. 
         [0030]    Now, referring to  FIG. 1A , an example first sense circuit  130 - 1  to measure excitation along X axis is described. In this example, the first sensing device  108 , second sensing device  110 , third sensing device  118  and the fourth sensing device  120  are selectively coupled to form a Wheatstone bridge configuration. More specifically, a first end  108 - 1  of the first sensing device  108  and a first end  110 - 1  of the second sensing device  110  are coupled together at a first junction  132 . The first end  118 - 1  of the third sensing device  118  and the first end  120 - 1  of the fourth sensing device  120  are coupled together at a second junction  134 . A second end  108 - 2  of the first sensing device  108  and a second end  118 - 2  of the third sensing device  118  are coupled together at a third junction  136 . A second end  110 - 2  of the second sensing device  110  and a second end  120 - 2  of the fourth sensing device  120  are coupled together at a fourth junction  138 . An input voltage Vin is applied between the third junction  136  and the fourth junction  138 . An output voltage Vout is measured between the first junction  132  and the second junction  134 . A first signal generated by the first MEMS sensor  102  is indicative of the excitation B, with a positive X component and a negative Y component. A second signal generated by the second MEMS sensor  104  is indicative of the excitation A, with a positive X component and a positive Y component. A ratio of the output voltage Vout to input voltage Vin will provide a third signal, the value of which is indicative of the excitation along the X axis. As one skilled in the art appreciates, in this configuration, the ratio of Vout to input voltage Vin is indicative of the sum of the excitation A and B. In other words, addition of the first signal and the second signal. As excitation A has positive Y component and excitation B has a negative Y component, the sum of excitation A and B (third signal) will result in a value indicative of the excitation along the X axis. 
         [0031]    Now, referring to  FIG. 1B , an example second sense circuit  130 - 2  to measure excitation along Y axis is described. In this example, the first sensing device  108 , second sensing device  110 , third sensing device  118  and the fourth sensing device  120  are selectively coupled to form a Wheatstone bridge configuration. More specifically, a first end  108 - 1  of the first sensing device  108  and a first end  110 - 1  of the second sensing device  110  are coupled together at a fifth junction  140 . A first end  118 - 1  of the third sensing device  118  and a first end  120 - 1  of the fourth sensing device  120  are coupled together at a sixth junction  142 . A second end  110 - 2  of the second sensing device  110  and a second end  118 - 2  of the third sensing device  118  are coupled together at a seventh junction  144 . A second end  108 - 2  of the first sensing device  108  and a second end  120 - 2  of the fourth sensing device  120  are coupled together at an eighth junction  146 . An input voltage Vin is applied between the seventh junction  144  and the eighth junction  146 . An output voltage Vout is measured between the fifth junction  140  and the sixth junction  142 . As previously described, the first signal generated by the first MEMS sensor  102  is indicative of the excitation B, with a positive X component and a negative Y component. The second signal generated by the second MEMS sensor  104  is indicative of the excitation A, with a positive X component and a positive Y component. A ratio of the output voltage Vout to input voltage Vin will provide a fourth signal, the value of which is indicative of the excitation along the Y axis. As one skilled in the art appreciates, in this configuration, the ratio of Vout to input voltage Vin is indicative of the difference between the excitation A and B. As excitation A and excitation B both have positive X components, the difference between excitation A and B will result in a value indicative of the excitation along the Y axis, with the cancellation of X component. 
         [0032]    When constructing a MEMS sensor, for example, using single crystal silicon, it is advantageous to construct springs on the low Elastic modulus axis. A spring with a certain stiffness, constructed on the low Elastic modulus axis will have a wider width than the same stiffness spring on the high Elastic modulus axis. The wider width spring is preferable, as it is more robust to fabrication imperfections. Further, in some examples, wider width spring may survive shock better, for example, shock caused when the sensor is dropped. It is common in MEMS applications to align the high Elastic modulus axis of single crystal silicon with sensor axes X and Y. Such a configuration results in a low Elastic modulus along an axis that is at +45 degree and −45 degree with respect to the sensor axes. In some examples, it is advantageous to construct MEMS sensors to sense the +45 degree and −45 degree axes, as they may be more robust to fabrication imperfections and survive excessive shock. 
         [0033]    As an example, referring back to  FIG. 1 , second axis may correspond to an axis that is −45 degrees with respect to X axis and third axis may correspond to an axis that is +45 degrees with respect to X axis. Now, referring to  FIG. 1C , for capacitors C 1 , C 2 , C 3  and C 4  configured in a Wheatstone bridge configuration  130 - 3  as shown, the following equation applies: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       out 
                     
                     
                       V 
                       in 
                     
                   
                   = 
                   
                     
                       
                         C 
                         1 
                       
                       
                         
                           C 
                           1 
                         
                         + 
                         
                           C 
                           3 
                         
                       
                     
                     - 
                     
                       
                         C 
                         2 
                       
                       
                         
                           C 
                           2 
                         
                         + 
                         
                           C 
                           4 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0034]    Now, referring back to  FIG. 1 , if the capacitance values for first sensing device is represented as B−, second sensing device is represented as B+, third sensing device is represented as A+ and fourth sensing device is represented as A− and nominal capacitance value for these capacitors is C (with no external influence), then due to an external influence, the capacitance values for capacitance A+, A−, B+ and B− would change as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     
                       A 
                       + 
                     
                   
                   = 
                   
                     C 
                     + 
                     
                       
                         dC 
                         dX 
                       
                        
                       X 
                     
                     + 
                     
                       
                         dC 
                         dY 
                       
                        
                       Y 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                 
               
             
             
               
                 
                   
                     C 
                     
                       A 
                       - 
                     
                   
                   = 
                   
                     C 
                     - 
                     
                       
                         dC 
                         dX 
                       
                        
                       X 
                     
                     - 
                     
                       
                         dC 
                         dY 
                       
                        
                       Y 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   3 
                 
               
             
             
               
                 
                   
                     C 
                     
                       B 
                       + 
                     
                   
                   = 
                   
                     C 
                     + 
                     
                       
                         dC 
                         dX 
                       
                        
                       X 
                     
                     - 
                     
                       
                         dC 
                         dY 
                       
                        
                       Y 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   4 
                 
               
             
             
               
                 
                   
                     C 
                     
                       B 
                       - 
                     
                   
                   = 
                   
                     C 
                     - 
                     
                       
                         dC 
                         dX 
                       
                        
                       X 
                     
                     + 
                     
                       
                         dC 
                         dY 
                       
                        
                       Y 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   5 
                 
               
             
           
         
       
     
         [0035]    Now, referring to Wheatstone bridge configuration of  FIG. 1A  and comparing with the Wheatstone bridge configuration shown in  FIG. 1C , CA+ corresponds to C 1 , CB− corresponds to C 2 , CA− corresponds to C 3  and CB+ corresponds to C 4 . 
         [0036]    Applying the corresponding values for C 1 , C 2 , C 3  and C 4  in Equation 1, we derive that 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       out 
                     
                     
                       V 
                       in 
                     
                   
                   = 
                   
                     
                       1 
                       C 
                     
                      
                     
                       dC 
                       dX 
                     
                      
                     X 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   6 
                 
               
             
           
         
       
     
         [0037]    In other words, for the Wheatstone bridge configuration shown in  FIG. 1A , the Vout/Vin is proportional to the excitation along the X axis. 
         [0038]    Now, referring to Wheatstone bridge configuration of  FIG. 1B  and comparing with the Wheatstone bridge configuration shown in  FIG. 1C , CA+ corresponds to C 1 , CB+ corresponds to C 2 , CA− corresponds to C 3  and CB− corresponds to C 4 . 
         [0039]    Applying the corresponding values for C 1 , C 2 , C 3  and C 4  in Equation 1, we derive that 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       out 
                     
                     
                       V 
                       in 
                     
                   
                   = 
                   
                     
                       1 
                       C 
                     
                      
                     
                       dC 
                       dY 
                     
                      
                     Y 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   7 
                 
               
             
           
         
       
     
         [0040]    In other words, for the Wheatstone bridge configuration shown in  FIG. 1B , the Vout/Vin is proportional to the excitation along the Y axis. 
         [0041]    As one skilled in the art appreciates, in the example equations described above, the ratio dC/dX and dC/dY (rate of change of capacitance due to component of excitation along X axis and Y axis respectively) is assumed to be same for all the capacitors. If there is a mismatch in the rate of change of capacitance, then an error in measurement is introduced. If we assume a difference in sensitivity of a between capacitors, then, 
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     
                       A 
                       + 
                     
                   
                   = 
                   
                     C 
                     + 
                     
                       
                         ( 
                         
                           
                             1 
                             + 
                           
                           ∝ 
                           
                             / 
                             2 
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               dC 
                               dX 
                             
                              
                             X 
                           
                           + 
                           
                             
                               dC 
                               dY 
                             
                              
                             Y 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   8 
                 
               
             
           
         
       
     
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     
                       A 
                       - 
                     
                   
                   = 
                   
                     C 
                     - 
                     
                       
                         ( 
                         
                           
                             1 
                             + 
                           
                           ∝ 
                           
                             / 
                             2 
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               dC 
                               dX 
                             
                              
                             X 
                           
                           + 
                           
                             
                               dC 
                               dY 
                             
                              
                             Y 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   9 
                 
               
             
             
               
                 
                   
                     C 
                     
                       B 
                       + 
                     
                   
                   = 
                   
                     C 
                     + 
                     
                       
                         ( 
                         
                           
                             1 
                             - 
                           
                           ∝ 
                           
                             / 
                             2 
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               dC 
                               dX 
                             
                              
                             X 
                           
                           - 
                           
                             
                               dC 
                               dY 
                             
                              
                             Y 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   10 
                 
               
             
             
               
                 
                   
                     C 
                     
                       B 
                       - 
                     
                   
                   = 
                   
                     C 
                     - 
                     
                       
                         ( 
                         
                           
                             1 
                             - 
                           
                           ∝ 
                           
                             / 
                             2 
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               dC 
                               dX 
                             
                              
                             X 
                           
                           - 
                           
                             
                               dC 
                               dY 
                             
                              
                             Y 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   11 
                 
               
             
           
         
       
     
         [0042]    For C 1 =C A+ , C 3 =C A− , C 2 =C B− , C 4 =C B+  for a Wheatstone bridge as configured in  FIG. 1B , 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       out 
                     
                     
                       V 
                       in 
                     
                   
                   = 
                   
                     
                       1 
                       C 
                     
                      
                     
                       ( 
                       
                         
                           
                             dC 
                             dX 
                           
                            
                           X 
                         
                         + 
                         
                           
                             ∝ 
                             2 
                           
                            
                           
                             dC 
                             dY 
                           
                            
                           Y 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   12 
                 
               
             
           
         
       
     
         [0043]    For C 1 =C A+ , C 3 =C A− , C 2 =C B+ , C 4 =C B−  for a Wheatstone bridge as configured in  FIG. 1C , 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       out 
                     
                     
                       V 
                       in 
                     
                   
                   = 
                   
                     
                       1 
                       C 
                     
                      
                     
                       ( 
                       
                         
                           
                             dC 
                             dY 
                           
                            
                           Y 
                         
                         + 
                         
                           
                             ∝ 
                             2 
                           
                            
                           
                             dC 
                             dX 
                           
                            
                           X 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   13 
                 
               
             
           
         
       
     
         [0044]    In Equation 12, the term 
         [0000]    
       
         
           
             
               ∝ 
               2 
             
              
             
               dC 
               dX 
             
           
         
       
     
         [0000]    and in Equation 13, the term 
         [0000]    
       
         
           
             
               ∝ 
               2 
             
              
             
               dC 
               dY 
             
           
         
       
     
         [0000]    may be referred to as a cross axis term. If the capacitors were perfectly matched, then, the cross axis term would be zero. To remove the cross axis term, in one example, a modified Wheatstone bridge  130 - 4  as shown in  FIG. 1D  is used, where each side of the bridge is driven with different voltage, for example, V in1  and V in2 . Then, 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     out 
                   
                   = 
                   
                     
                       
                         
                           C 
                           1 
                         
                         
                           
                             C 
                             1 
                           
                           + 
                           
                             C 
                             3 
                           
                         
                       
                        
                       
                         V 
                         
                           in 
                            
                           
                               
                           
                            
                           1 
                         
                       
                     
                     - 
                     
                       
                         
                           C 
                           2 
                         
                         
                           
                             C 
                             2 
                           
                           + 
                           
                             C 
                             4 
                           
                         
                       
                        
                       
                         V 
                         
                           in 
                            
                           
                               
                           
                            
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   14 
                 
               
             
           
         
       
     
         [0045]    By selecting V in1  and V in2  to be 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     
                       in 
                        
                       
                           
                       
                        
                       1 
                     
                   
                   = 
                   
                     
                       V 
                       in 
                     
                     
                       ( 
                       
                         
                           1 
                           + 
                         
                         ∝ 
                         
                           / 
                           2 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   15 
                 
               
             
             
               
                 
                   
                     V 
                     
                       in 
                        
                       
                           
                       
                        
                       2 
                     
                   
                   = 
                   
                     
                       V 
                       in 
                     
                     
                       ( 
                       
                         
                           1 
                           - 
                         
                         ∝ 
                         
                           / 
                           2 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   16 
                 
               
             
           
         
       
     
         [0046]    and applying and to the modified Wheatstone bridge  130 - 3  of  FIG. 1D , 
         [0047]    For C 4 =C A+ , C 3 =C A− , C 2 =C B− , C 4 =C B+  (as shown in  FIG. 1B  configuration) 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       out 
                     
                     
                       V 
                       in 
                     
                   
                   = 
                   
                     
                       1 
                       C 
                     
                      
                     
                       ( 
                       
                         
                           
                             dC 
                             dX 
                           
                            
                           X 
                         
                         + 
                         
                           
                             2 
                              
                             α 
                           
                           
                             
                               α 
                               2 
                             
                             + 
                             4 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   17 
                 
               
             
           
         
       
     
         [0048]    For C 1 =C A+ , C 3 =C A− , C 2 =C B+ , C 4 =C B−  (as shown in  FIG. 1C  configuration) 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       out 
                     
                     
                       V 
                       in 
                     
                   
                   = 
                   
                     
                       1 
                       C 
                     
                      
                     
                       ( 
                       
                         
                           
                             dC 
                             dY 
                           
                            
                           Y 
                         
                         + 
                         
                           
                             2 
                              
                             α 
                           
                           
                             
                               α 
                               2 
                             
                              
                             4 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   18 
                 
               
             
           
         
       
     
         [0049]    Now, referring to Equation 17 and Equation 18, the output of the modified Wheatstone bridge has a component that is proportional to excitation and a constant term 
         [0000]    
       
         
           
             
               
                 2 
                  
                 α 
               
               
                 
                   α 
                   2 
                 
                 + 
                 4 
               
             
             . 
           
         
       
     
         [0000]    The constant term 
         [0000]    
       
         
           
             
               2 
                
               α 
             
             
               
                 α 
                 2 
               
               + 
               4 
             
           
         
       
     
         [0000]    is preferable over the cross axis term 
         [0000]    
       
         
           
             
               ∝ 
               2 
             
              
             
               dC 
               dX 
             
              
             
                 
             
              
             and 
              
             
                 
             
              
             
               ∝ 
               2 
             
              
             
               dC 
               dY 
             
           
         
       
     
         [0000]    if the cross axis term is large. In some examples, the cross axis sensitivity may be less than a threshold value. If the cross axis term is greater than a threshold value, then the modified Wheatstone bridge configuration shown in  FIG. 1E  may be preferable. In some examples, the threshold value may be less than or equal to 1%. 
         [0050]    Now, referring to  FIG. 1D , an example modified Wheatstone bridge  130 - 4  is shown with two input voltages, V in1  and V in2 . The voltages V in1  and V in2  may be generated from a single voltage source, for example, Vin, by using a voltage divider  150 . The voltage divider  150  may include a variable resistor divider, with a first resistor portion  152  and a second resistor portion  154  coupled in series at junction  156 . Voltage Vin is applied across the voltage divider  150 . Voltage Vin is applied as V in2  across capacitors C 2  and C 4 . A portion of the voltage Vin is applied as V in1  across capacitors C 1  and C 3 . For example, a voltage across the first tap  158  of first resistor portion  152  and the second tap  160  of second resistor portion  154  is applied as V in1  across capacitors C 1  and C 3 . In one example, the first tap  158  and the second tap  160  are chosen such that the voltage between the junction  156  and the first tap  158  is same as the voltage between the junction  156  and the second tap  160 . In other words, the junction  156  has half the voltage of V in1 . 
         [0051]    Now, referring to  FIG. 2A , an example sensor assembly  200  is shown. The sensor assembly  200  is configured to be a magnetometer and accelerometer combination sensor. The sensor assembly  200  measures both acceleration in the Y axis and magnetic field in the X axis and Z axis. In this example, for an acceleration Ay along Y axis, the proof mass  202  is configured to move up, along positive Y direction. For a magnetic excitation Bx along the X axis, the proof mass  202  rotates in plane about the Z axis. For a magnetic excitation Bz along the Z axis, the proof mass  202  rotates out of plane about the X axis. 
         [0052]    The sensor assembly  200  is movably coupled to an anchor  204  via spring  206 . Four capacitors CTL  208 , CBL  210 , CTR  212  and CBR  214  are formed in the sensor assembly  200 . Further two capacitors CTM  216  and CBM  218  are formed in the sensor assembly  200 . A first permanent magnet  220  and a second permanent magnet  222  are disposed over the proof mass  202 . Due to external influence, the values of the capacitors CTL  208 , CBL  210 , CTR  212 , CBR  214 , CTM  216  and CBM  218  selectively change. Change in the capacitance value is measured to determine the extent of the external influence. For example, the capacitors may be selectively configured as part of a Wheatstone bridge, an input voltage is applied and output voltage is measured to determine the value of the excitation, as previously described. 
         [0053]    Due to manufacturing variations, the rotation center of the proof mass  202  may not align with the center of the capacitors, which means capacitors CTL  208  and CBL  210  see a different response than the capacitors CTR  212  and CBR  214 . In other words, an acceleration in the Y axis may indicate a false X axis magnetic signal. And, a magnetic field along X axis may indicate a false Y axis acceleration signal. In some examples, this error may be referred to as a cross domain error. 
         [0054]      FIG. 2B  shows an example half Wheatstone bridge  224 . A second half Wheatstone bridge may be used to configure a complete Wheatstone bridge (not shown). In some examples, the second half Wheatstone bridge may include capacitors formed on another sensor. In some examples, the second half Wheatstone bridge may include capacitors formed on a separate substrate, for example, a CMOS substrate. Now, if all the capacitors have a nominal capacitance value of C, then, 
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     TL 
                   
                   = 
                   
                     C 
                     + 
                     
                       
                         ( 
                         
                           
                             1 
                             + 
                           
                           ∝ 
                           
                             / 
                             2 
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               dC 
                               dY 
                             
                              
                             Y 
                           
                           - 
                           
                             
                               dC 
                               
                                 dB 
                                 x 
                               
                             
                              
                             
                               B 
                               x 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   19 
                 
               
             
             
               
                 
                   
                     C 
                     TR 
                   
                   = 
                   
                     C 
                     - 
                     
                       
                         ( 
                         
                           
                             1 
                             + 
                           
                           ∝ 
                           
                             / 
                             2 
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               dC 
                               dY 
                             
                              
                             Y 
                           
                           + 
                           
                             
                               dC 
                               
                                 dB 
                                 x 
                               
                             
                              
                             
                               B 
                               x 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   20 
                 
               
             
             
               
                 
                   
                     C 
                     BL 
                   
                   = 
                   
                     C 
                     + 
                     
                       
                         ( 
                         
                           
                             1 
                             - 
                           
                           ∝ 
                           
                             / 
                             2 
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               dC 
                               dY 
                             
                              
                             Y 
                           
                           - 
                           
                             
                               dC 
                               
                                 dB 
                                 x 
                               
                             
                              
                             
                               B 
                               x 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   21 
                 
               
             
             
               
                 
                   
                     C 
                     BR 
                   
                   = 
                   
                     C 
                     - 
                     
                       
                         ( 
                         
                           
                             1 
                             - 
                           
                           ∝ 
                           
                             / 
                             2 
                           
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           
                             
                               dC 
                               dY 
                             
                              
                             Y 
                           
                           + 
                           
                             
                               dC 
                               
                                 dB 
                                 x 
                               
                             
                              
                             
                               B 
                               x 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   22 
                 
               
             
           
         
       
     
         [0055]    Selecting V in1  and V hi  to be the same V in , 
         [0056]    For C 1 =C TL , C 3 =C BL , C 2 =C TR , C 4 =C BR   
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       out 
                     
                     
                       V 
                       in 
                     
                   
                   = 
                   
                     
                       
                         1 
                         
                           2 
                            
                           
                               
                           
                            
                           C 
                         
                       
                        
                       
                         ( 
                         
                           
                             
                               dC 
                               dY 
                             
                              
                             Y 
                           
                           - 
                           
                             
                               α 
                               2 
                             
                              
                             
                               dC 
                               
                                 dB 
                                 x 
                               
                             
                              
                             
                               B 
                               x 
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       1 
                       2 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   23 
                 
               
             
           
         
       
     
         [0057]    For C 1 =C BL , C 3 =C TL , C 2 =C TR , C 4 =C BR   
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       out 
                     
                     
                       V 
                       in 
                     
                   
                   = 
                   
                     
                       
                         1 
                         
                           2 
                            
                           
                               
                           
                            
                           C 
                         
                       
                        
                       
                         ( 
                         
                           
                             
                               dC 
                               
                                 dB 
                                 x 
                               
                             
                              
                             
                               B 
                               x 
                             
                           
                           - 
                           
                             
                               α 
                               2 
                             
                              
                             
                               dC 
                               dY 
                             
                              
                             Y 
                           
                         
                         ) 
                       
                     
                     + 
                     
                       1 
                       2 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   24 
                 
               
             
           
         
       
     
         [0058]    If by selecting different voltages V in1  and V in2 , as previously discussed, 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     
                       in 
                        
                       
                           
                       
                        
                       1 
                     
                   
                   = 
                   
                     
                       V 
                       in 
                     
                     
                       ( 
                       
                         
                           1 
                           + 
                         
                         ∝ 
                         
                           / 
                           2 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   25 
                 
               
             
             
               
                 
                   
                     V 
                     
                       in 
                        
                       
                           
                       
                        
                       2 
                     
                   
                   = 
                   
                     
                       V 
                       in 
                     
                     
                       ( 
                       
                         
                           1 
                           - 
                         
                         ∝ 
                         
                           / 
                           2 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   26 
                 
               
             
           
         
       
     
         [0059]    For C 1 =C TL , C 3 =C BL , C 2 =C TR , C 4 =C BR   
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       out 
                     
                     
                       V 
                       in 
                     
                   
                   = 
                   
                     
                       
                         1 
                         
                           2 
                            
                           
                               
                           
                            
                           C 
                         
                       
                        
                       
                         dC 
                         dY 
                       
                        
                       Y 
                     
                     + 
                     
                       2 
                       
                         4 
                         - 
                         
                           α 
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   27 
                 
               
             
           
         
       
     
         [0060]    For C 1 =C BL , C 3 =C TL , C 2 =C TR , C 4 =C BR   
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       out 
                     
                     
                       V 
                       in 
                     
                   
                   = 
                   
                     
                       
                         1 
                         
                           2 
                            
                           C 
                         
                       
                        
                       
                         dC 
                         
                           dB 
                           x 
                         
                       
                        
                       
                         B 
                         x 
                       
                     
                     + 
                     
                       2 
                       
                         4 
                         - 
                         
                           α 
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   28 
                 
               
             
           
         
       
     
         [0061]    Now, referring to Equation 27 and Equation 28, we see that by selectively selecting the capacitors for the construction of the Wheatstone bridge, we can measure the acceleration component along Y axis or magnetic field Bx respectively, with the addition of constant 
         [0000]    
       
         
           
             
               2 
               
                 4 
                 - 
                 
                   α 
                   2 
                 
               
             
             . 
           
         
       
     
         [0000]    If the cross domain component is greater than a threshold value, then, it is preferable to have the constant term in the equation rather than a cross domain component. In some examples, the cross domain component threshold value may be less than or equal to 1%. 
         [0062]    Now, referring back to  FIG. 2A , due to a magnetic field along Z axis, the sensor assembly  200  will rotate about the X axis, causing capacitors CTM and CBM to change. In some examples, the proof mass  202  may have a mass imbalance about the X axis. For example, the permanent magnet  220  and permanent magnet  222  may be so formed to induce a mass imbalance. When an external acceleration is induced along the Y axis, the proof mass  202  will rotate about the X axis, due to the mass imbalance. This rotation causes capacitors CTM and CBM to change, indicating a spurious magnetic influence along the Z axis. This may be sometimes referred to as cross domain coupling. In one example, to eliminate the cross domain coupling from acceleration along Y axis to Z axis magnetic sensors, a Wheatstone circuit similar to the Wheatstone circuit described with reference to  FIG. 1E  may be used. 
         [0063]    For a nominal capacitance value of C, 
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     TLR 
                   
                   = 
                   
                     
                       
                         
                           C 
                           TL 
                         
                         + 
                         
                           C 
                           TR 
                         
                       
                       2 
                     
                     = 
                     
                       C 
                       + 
                       
                         
                           dC 
                           dY 
                         
                          
                         Y 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   29 
                 
               
             
           
         
       
     
         [0064]    where C TLR  is the effective capacitance of capacitors CTL and CTR. 
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     BLR 
                   
                   = 
                   
                     
                       
                         
                           C 
                           BL 
                         
                         + 
                         
                           C 
                           BR 
                         
                       
                       2 
                     
                     = 
                     
                       C 
                       - 
                       
                         
                           dC 
                           dY 
                         
                          
                         Y 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   30 
                 
               
             
           
         
       
     
         [0065]    where C BLR  is the effective capacitance of capacitors CBL and CBR. 
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     TM 
                   
                   = 
                   
                     C 
                     + 
                     
                       ( 
                       
                         
                           
                             dC 
                             
                               dB 
                               z 
                             
                           
                            
                           
                             B 
                             z 
                           
                         
                         + 
                         
                           β 
                            
                           
                             dC 
                             dY 
                           
                            
                           Y 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   31 
                 
               
             
             
               
                 
                   
                     C 
                     BM 
                   
                   = 
                   
                     C 
                     - 
                     
                       ( 
                       
                         
                           
                             dC 
                             
                               dB 
                               z 
                             
                           
                            
                           
                             B 
                             z 
                           
                         
                         + 
                         
                           β 
                            
                           
                             dC 
                             dY 
                           
                            
                           Y 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   32 
                 
               
             
           
         
       
     
         [0066]    In the above equation, β corresponds to the cross domain component of Y acceleration causing an RX rotation and that is detected by the C TM  and C BM  capacitors. Now, using two different voltages V in1  and V in2 , as previously described with reference to  FIG. 1E , 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     out 
                   
                   = 
                   
                     
                       
                         
                           C 
                           1 
                         
                          
                         
                           V 
                           
                             in 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                       
                       + 
                       
                         
                           C 
                           2 
                         
                          
                         
                           V 
                           
                             in 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                     
                     
                       
                         C 
                         1 
                       
                       + 
                       
                         C 
                         2 
                       
                       + 
                       
                         C 
                         3 
                       
                       + 
                       
                         C 
                         4 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   33 
                 
               
             
           
         
       
     
         [0067]    and selecting V in1 =βV in , V in2 =V in   
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       out 
                     
                     
                       V 
                       in 
                     
                   
                   = 
                   
                     
                       1 
                       2 
                     
                      
                     
                       ( 
                       
                         
                           
                             dC 
                             
                               dB 
                               z 
                             
                           
                            
                           
                             B 
                             z 
                           
                         
                         + 
                         1 
                         - 
                         β 
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   34 
                 
               
             
           
         
       
     
         [0068]    By selecting the correct voltages V in1  and V in2  a portion of the sense electrodes that are sensitive to Y acceleration can be used to cancel the cross domain if present in the C TM  and C BM  electrodes. In some examples, the cross-domain component may be eliminated, if the cross-domain component is above a threshold value. In some examples, the cross-domain component threshold value may be less than or equal to 1%. 
         [0069]    Having described a sensor assembly  200  with reference to  FIG. 2A  and a corresponding half Wheatstone bridge  224  with reference to  FIG. 2B , which can measure external influence in three axis, now a two sensor combination to measure external influence in six axis is described with reference to two sensor assemblies, for example, first sensor assembly  300 - 1  and second sensor assembly  300 - 2  as shown in  FIG. 3A  and a full Wheatstone bridge  324  as shown with reference to  FIG. 3B . 
         [0070]    Referring to  FIG. 3A , first sensor assembly  300 - 1  and second sensor assembly  300 - 2  are shown. First sensor assembly  300 - 1  and second sensor assembly  300 - 2  may be similar to sensor assembly  200 , except that first sensor assembly  300 - 1  is oriented along axis  1  and axis  2 , whereas second sensor assembly  300 - 2  is oriented along axis  3  and axis  4 . The first sensor assembly  300 - 1  is configured to be a magnetometer and accelerometer combination sensor. The first sensor assembly  300 - 1  measures both acceleration in the Y axis and Z axis and magnetic field in the X axis and Z axis. In this example, for an acceleration A 2  along axis  2 , the proof mass  302 - 1  is configured to move up, along positive axis  2  direction. For a magnetic excitation B 1  along axis  1 , the proof mass  302 - 1  rotates in plane about the Z axis. For a magnetic excitation Bz along the Z axis, the proof mass  202  rotates out of plane about the positive axis  1 . For an acceleration Az along the Z axis the proof mass  202  rotates about the positive axis  1 . 
         [0071]    The second sensor assembly  300 - 2  is also configured to be a magnetometer and accelerometer combination sensor. The second sensor assembly  300 - 2  measures both acceleration in the Y axis and magnetic field in the X axis and Z axis. In this example, for an acceleration A 4  along axis  4 , the proof mass  302 - 2  is configured to move up, along positive axis  4  direction. For a magnetic excitation B 3  along axis  3 , the proof mass  302 - 2  rotates in plane about the Z axis. For a magnetic excitation Bz along the Z axis, the proof mass  302 - 2  rotates out of plane about positive axis  3 . For an acceleration along the Z axis the proof mass  202  rotates in the negative axis  3 . 
         [0072]    The first sensor assembly  300 - 1  is movably coupled to an anchor  304 - 1  via spring  306 - 1 . Four capacitors C 13   308 - 1 , C 15   310 - 1 , C 14   312 - 1  and C 16   314 - 1  are formed in the first sensor assembly  300 - 1 . Further two capacitors C 11   316 - 1  and C 12   318 - 1  are formed in the first sensor assembly  300 - 1 . A first permanent magnet  320 - 1  and a second permanent magnet  322 - 1  are disposed over the proof mass  302 - 1 . Due to external influence, the values of the capacitors C 13   308 - 1 , C 15   310 - 1 , C 14   312 - 1 , C 16   314 - 1 , C 11   316 - 1  and C 12   318 - 1  selectively change. Change in the capacitance value is measured to determine the extent of the external influence. For example, the capacitors may be selectively configured as part of a Wheatstone bridge, an input voltage is applied and output voltage is measured to determine the value of the excitation, as previously described. 
         [0073]    In some examples, capacitors C 11   316 - 1  and C 12   318 - 1  of first sensor assembly  300 - 1  correspond to capacitors CTM  216  and CBM  218  of sensor assembly  200 - 1  respectively. Capacitors C 13   308 - 1  and C 14   312 - 1  of first sensor assembly  300 - 1  correspond to capacitors CTL  208  and CTR  212  of sensor assembly  200 - 1  respectively. Capacitors C 15   310 - 1  and C 16   314 - 1  of first sensor assembly  300 - 1  correspond to capacitors CBL  210  and CBR  214  of sensor assembly  200 - 1  respectively. 
         [0074]    The second sensor assembly  300 - 2  is movably coupled to an anchor  304 - 2  via spring  306 - 2 . Four capacitors C 23   308 - 2 , C 25   310 - 2 , C 24   312 - 2  and C 26   314 - 2  are formed in the second sensor assembly  300 - 2 . Further two capacitors C 21   316 - 2  and C 22   318 - 2  are formed in the sensor assembly  300 - 2 . A first permanent magnet  320 - 2  and a second permanent magnet  322 - 2  are disposed over the proof mass  302 - 2 . Due to external influence, the values of the capacitors C 23   308 - 2 , C 25   310 - 2 , C 24   312 - 2 , C 26   314 - 2 , C 21   316 - 2  and C 22   318 - 2  selectively change. Change in the capacitance value is measured to determine the extent of the external influence. For example, the capacitors may be selectively configured as part of a Wheatstone bridge, an input voltage is applied and output voltage is measured to determine the value of the excitation, as previously described. 
         [0075]    In some examples, capacitors C 21  and C 22  of second sensor assembly  300 - 2  correspond to capacitors CTM and CBM of sensor assembly  200 - 1  respectively. Capacitors C 23  and C 24  of second sensor assembly  300 - 2  correspond to capacitors CTL and CTR of sensor assembly  200 - 1  respectively. Capacitors C 25  and C 26  of second sensor assembly  300 - 2  correspond to capacitors CBL and CBR of sensor assembly  200 - 1  respectively. 
         [0076]    As one skilled in the art appreciates, the first sense assembly  300 - 1  is a rotated image of the second sense assembly  300 - 2 . For example, if the construction of the first sense assembly  300 - 1  is compared with the second sense assembly  300 - 2  with reference to the anchor  304 - 1  and  304 - 2 , the first sense assembly  300 - 1  is a rotated image of the second sense assembly  300 - 2 . In some examples, the first sense assembly  300 - 1  and the second sense assembly  300 - 2  may be constructed such that the first sense assembly  300 - 1  is a mirror image of the second sense assembly  300 - 2 . In other words, the location of the anchor  304 - 1 , spring  306 - 1 , anchor  304 - 2  and spring  306 - 2  will also form a mirror image. 
         [0077]    Now, referring to  FIG. 3B , a full Wheatstone bridge  324  is shown. In the full Wheatstone bridge, the capacitors C 1 , C 2 , C 3  and C 4  correspond to a half Wheatstone bridge  224  described with reference to  FIG. 2B . Capacitors C 1 ′, CT, C 3 ′ and C 4 ′ form the other half of the full Wheatstone bridge  324 . Capacitors of sensor assembly  300 - 1  are selectively configured to form a first subset of capacitors C 1 , C 2 , C 3  and C 4  of full Wheatstone bridge  324  and capacitors of sensor assembly  300 - 2  are selectively configured to form a second subset of capacitors C 1 ′, C 2 ′, C 3 ′ and C 4 ′ of full Wheatstone bridge  324 . A first voltage divider  326 - 1  and a second voltage divider  326 - 2  may be used to selectively apply a portion of the input voltage Vin to a subset of the capacitors, as previously described with reference to  FIG. 1E . 
         [0078]    For example, the first subset of capacitors C 1 , C 2 , C 3  and C 4  may form two pairs of capacitor C 1 , C 3  and C 2 , C 4 . The pair of capacitors C 1  and C 3  are connected in series. The pair of capacitors C 2  and C 4  are connected in series. Input voltage is applied across capacitors C 2  and C 4 . A portion of the input voltage is applied across capacitors C 1  and C 3 , using the first voltage divider  326 - 1 . For example, the second subset of capacitors C 1 ′, CT, C 3 ′ and C 4 ′ may form two pairs of capacitor C 1 ′, C 3 ′ and CT, C 4 ′. The pair of capacitors C 1 ′ and C 3 ′ are connected in series. The pair of capacitors CT and C 4 ′ are connected in series. Input voltage is applied across capacitors CT and C 4 ′. A portion of the input voltage is applied across capacitors C 1 ′ and C 3 ′, using the second voltage divider  326 - 2 . The first voltage divider  326 - 1  and second voltage divider  326 - 2  may be similar to voltage divider  150  described with reference to  FIG. 1D . 
         [0079]    In some examples, input voltage Vin may be applied across terminal junction T 1  and T 2  and output voltage Vout may be measured across terminal junction T 3  and T 4 . In this example, the first voltage divider  326 - 1  and second voltage divider  326 - 2  are between the input voltage source and the capacitor bridge. In some examples, the input voltage Vin may be applied across terminal junction T 3  and T 4  and output voltage Vout may be measured across terminal junction T 1  and T 2 . In this example, the first voltage divider  326 - 1  and second voltage divider  326 - 2  are between the capacitor bridge and terminal junction T 1  and T 2  across which the output voltage Vout is measured. 
         [0080]    Now, referring to table  330  of  FIG. 3C , various selective configuration of capacitors C 1 , C 2 , C 3  and C 4  and capacitors C 1 ′, C 2 ′, C 3 ′ and C 4 ′ to measure excitation in different direction is shown. As one skilled in the art appreciates, a switch circuit may selectively couple capacitors C 1 , C 2 , C 3  and C 4  and capacitors CV, C 2 ′, C 3 ′ and C 4 ′ to measure excitation in different directions. Column  332  shows excitation measured. Column  334  shows terminal junction to which input voltage Vin is applied. Column  336  shows terminal junction used to measure output voltage Vout. Column  338 - 352  shows selective capacitor configuration for capacitors C 1 , C 2 , C 3  and C 4  and capacitors C 1 ′, CT, C 3 ′ and C 4 ′ of the full Wheatstone bridge  324  shown in  FIG. 3B , to measure a specific excitation along a given axis. As previously described, a ratio of Vout/Vin provides a value proportional to excitation along the selected axis. 
         [0081]    For example, referring to row  354 , acceleration Ax along X axis is measured, by selectively configuring capacitors C 1 , C 2 , C 3  and C 4  and capacitors C 1 ′, CT, C 3 ′ and C 4 ′. For example, referring to row  354  and column  338 , capacitor C 23  of  FIG. 3A  is configured as capacitor C 1  in the full Wheatstone bridge  324  of  FIG. 3B . Similarly, referring to row  354  and column  348 , capacitor C 26  of  FIG. 3A  is configured as capacitor C 3  in the full Wheatstone bridge  324  of  FIG. 3B . Referring to columns  334  and  336 , the input voltage Vin is applied across terminal junctions T 1  and T 2  and output voltage Vout is measured across terminal junctions T 3  and T 4 . 
         [0082]    For example, referring to row  356 , acceleration Az along Z axis is measured, by selectively configuring capacitors C 1 , C 2 , C 3  and C 4  and capacitors C 1 ′, CT, C 3 ′ and C 4 ′. For example, referring to row  356  and column  338 , capacitor C 11  of  FIG. 3A  is configured as capacitor C 1  in the full Wheatstone bridge  324  of  FIG. 3B . Similarly, referring to row  356  and columns  340 , there is no connection (NC) for capacitor C 2  in the full Wheatstone bridge  324  of  FIG. 3B . Referring to columns  334  and  336 , the input voltage Vin is applied across terminal junctions T 1  and T 2  and output voltage Vout is measured across terminal junctions T 3  and T 4 . 
         [0083]    For example, referring to row  358 , magnetic excitation Bz along Z axis is measured, by selectively configuring capacitors C 1 , C 2 , C 3  and C 4  and capacitors C 1 ′, CT, C 3 ′ and C 4 ′. For example, referring to row  358  and column  338 , capacitor C 11  of  FIG. 3A  is configured as capacitor C 1  in the full Wheatstone bridge  324  of  FIG. 3B . Similarly, referring to row  358  and column  348 , there is no connection (NC) for capacitor C 3  in the full Wheatstone bridge  324  of  FIG. 3B . Referring to columns  334  and  336 , the input voltage Vin is applied across terminal junctions T 1  and T 2  and output voltage Vout is measured across terminal junctions T 3  and T 4 . 
         [0084]    Now, referring back to rows  356  and  358 , as one skilled in the art appreciates, by selectively coupling capacitors C 11 , C 12 , C 21  and C 22 , we can measure either Az or Bz. In other words, two different types of excitation may be measured along the same axis (Axis Z here), by selectively configuring same sensing devices. Now, referring to  FIG. 3D , a third sense circuit  370 - 1  to measure Az is described. Further, referring to  FIG. 3E , a fourth sense circuit  370 - 2  to measure Bz is described. In some examples, the third sense circuit  370 - 1  may be similar to first sense circuit  130 - 1 . In some examples, the fourth sense circuit  370 - 2  may be similar to second sense circuit  130 - 2 . 
         [0085]    Now, referring to  FIG. 3D , an example third sense circuit  370 - 1  to measure a first excitation along Z axis is described. In this example, the first excitation along Z axis corresponds to acceleration Az. In this example, the fifth sensing device  372 , sixth sensing device  374 , seventh sensing device  376  and the eighth sensing device  378  are selectively coupled to form a Wheatstone bridge configuration. In one example, referring back to row  356  of table  330  of  FIG. 3C , the fifth sensing device  372  corresponds to capacitor C 21 , sixth sensing device  374  corresponds to capacitor C 22 , seventh sensing device  376  corresponds to capacitor C 11  and eighth sensing device  378  corresponds to capacitor C 12 . 
         [0086]    More specifically, a first end  372 - 1  of the fifth sensing device  372  and a first end  374 - 1  of the sixth sensing device  374  are coupled together at a ninth junction  380 . The first end  376 - 1  of the seventh sensing device  376  and the first end  378 - 1  of the eighth sensing device  378  are coupled together at a tenth junction  382 . A second end  372 - 2  of the fifth sensing device  372  and a second end  376 - 2  of the seventh sensing device  372  are coupled together at eleventh junction  384 . A second end  374 - 2  of the sixth sensing device  374  and a second end  378 - 2  of the eighth sensing device  378  are coupled together at twelfth junction  386 . 
         [0087]    An input voltage Vin is applied between the eleventh junction  384  and the twelfth junction  386 . An output voltage Vout is measured between the ninth junction  380  and the tenth junction  382 . In some examples, the fifth sensing device  372  and the sixth sensing device  374  may be part of the first MEMS sensor  102 . In some examples, the seventh sensing device  376  and the eighth sensing device  378  may be part of the second MEMS sensor  104 . A ratio of the output voltage Vout to input voltage Vin will provide a fifth signal, the value of which is indicative of the first excitation along the Z axis. For example, acceleration along the Z axis. In some examples, the Z axis may correspond to a fifth axis. 
         [0088]    Now, referring to  FIG. 3E , an example fourth sense circuit  370 - 2  to measure a second excitation along Z axis is described. In this example, the second excitation corresponds to a magnetic excitation, Bz. In this example, the fifth sensing device  372 , sixth sensing device  374 , seventh sensing device  376  and the eighth sensing device  378  are selectively coupled to form a Wheatstone bridge configuration. In one example, referring back to row  356  of table  330  of  FIG. 3C , the fifth sensing device  372  corresponds to capacitor C 21 , sixth sensing device  374  corresponds to capacitor C 22 , seventh sensing device  376  corresponds to capacitor C 11  and eighth sensing device  378  corresponds to capacitor C 12 . 
         [0089]    More specifically, a first end  372 - 1  of the fifth sensing device  372  and a first end  374 - 1  of the sixth sensing device  374  are coupled together at a thirteenth junction  388 . A first end  376 - 1  of the seventh sensing device  376  and a first end  378 - 1  of the eighth sensing device  378  are coupled together at a fourteenth junction  390 . A second end  374 - 2  of the sixth sensing device  374  and a second end  376 - 2  of the seventh sensing device  376  are coupled together at a fifteenth junction  392 . A second end  372 - 2  of the fifth sensing device  372  and a second end  378 - 2  of the eighth sensing device  378  are coupled together at a sixteenth junction  394 . 
         [0090]    An input voltage Vin is applied between the fifteenth junction  392  and the sixteenth junction  394 . An output voltage Vout is measured between the thirteenth junction  388  and the fourteenth junction  390 . In some examples, the fifth sensing device  372  and the sixth sensing device  374  may be part of the first MEMS sensor  102 . In some examples, the seventh sensing device  376  and the eighth sensing device  378  may be part of the second MEMS sensor  104 . A ratio of the output voltage Vout to input voltage Vin will provide a sixth signal, the value of which is indicative of the second excitation along the Z axis. For example, magnetic field along the Z axis. In some examples, the Z axis may correspond to a fifth axis. 
         [0091]    Now, referring to  FIG. 4 , an example sensing circuit  400  is described. The sensing circuit  400  includes a capacitor bridge  402 , a capacitance to voltage amplifier  404 , an analog to digital converter (ADC)  406  and a digital electronics  408 . The bridge  402  is shown coupled to the amplifier  404 , which is shown coupled to the ADC  406 , which is shown coupled to the digital electronics  408 . The bridge  402  includes the sense capacitors  410 , switch circuit  412  and voltage divider  414 . The sense capacitors  410  may correspond to the capacitors, for example, shown with respect to  FIG. 3A . The switch circuit  412  may selectively couple the sense capacitors to form a Wheatstone bridge configuration, for example, as shown with respect to table  330  of  FIG. 3C . The voltage divider  414  applies a portion of the input voltage to the sense capacitors, for example, as described with reference to  FIG. 3B . 
         [0092]    In operation, out-of-phase voltage pulses are provided as input to the bridge  402  at Vin+ and Vin−. Output of the bridge  402  is converted to voltage by the amplifier  404  and the output of the amplifier  404  is converted from analog form to digital form by the ADC  406 . The resulting digital signal is processed by digital electronics  408  to produce values for both acceleration and magnetic field. For example, the digital electronics  408  may generate a value indicative of the ratio of Vout/Vin. 
         [0093]    In this disclosure, various sensing devices, for example, the first sensing device, second sensing device, third sensing device and fourth sensing device are described as a capacitive element, for example, variable capacitors. With variable capacitors, motion is detected as a change in capacitance. As one skilled in the art appreciates, in some examples, a different sensing device may be used to detect motion. For example, in some examples, a piezo-resistive element may be used as a sensing device. As one skilled in the art appreciates, piezo-resistive element may be configured to change its resistance upon detection of motion. These piezo-resistive elements may be configured as sensing devices and may be selectively configured in a Wheatstone bridge configuration to measure various excitations, as previously described in this disclosure. 
         [0094]    In this disclosure, various elements are disclosed using terms like first, second, third, fourth and the like, to assist in describing the function and features of various combinations of elements with reference to different configurations and arrangements. Depending upon the context and the corresponding configurations and arrangements, some of these elements may be similar, but referenced as elements with different terms, to assist in describing the functions and features of various combinations and arrangements. 
         [0095]    While embodiments of the present invention are described above with respect to what is currently considered its preferred embodiments, it is to be understood that the invention is not limited to that described above. To the contrary, the invention is intended to cover various modifications and equivalent arrangements within the spirit and scope of the appended claims.