Abstract:
Systems and methods for improving common mode cancelation in a vibrating beam accelerometer (VBA) by using multiple resonant modes. The VBA includes two double-ended tuning forks (DETF). Additional oscillators drive the DETFs into the extra resonant modes. This increases common mode rejection from two modes to four modes. In addition the scale factor of the additional mode may provide a greater scale factor than prior designs.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Vibrating beam accelerometer (VBA) devices use two dual beam resonators that operate in an in-plane, out-of-phase mode. The difference between the two resonator frequencies is used to measure force or acceleration. The sum of the resonator frequencies is use to track extraneous forces created by temperature, radiation, humidity, aging, static charge, etc. This difference is typically called common mode, which is used to reduce non acceleration (g) errors. For some applications this does not provide sufficient accuracy. 
       SUMMARY OF THE INVENTION 
       [0002]    The present invention provides systems and methods for improving common mode cancelation in a vibrating beam accelerometer (VBA) by using multiple resonant modes. The VBA includes two double-ended tuning forks (DETF). Additional oscillators drive the DETFs into the extra resonant modes. This increases common mode rejection from two modes to four modes. In addition the scale factor of the additional mode may provide a greater scale factor than prior designs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0004]      FIG. 1  is a partial perspective view of an exemplary vibrating beam accelerometer (VBA) formed in accordance with an embodiment of the present invention; 
           [0005]      FIG. 2  is a flowchart of an exemplary process for operating the VBA shown in  FIG. 1 ; and 
           [0006]      FIG. 3  is a block diagram of an exemplary system for performing the process shown in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0007]      FIG. 1  is a partial perspective view of an exemplary vibrating beam accelerometer (VBA)  20  that provides improved common mode cancellation. The VBA  20  includes a proof mass  30  connected to a base section  32  via a flexure (not shown). Two double-ended tuning forks (DETFs)  36 ,  38  are attached between the proof mass  30  and the base section  32 . One DETF  36  is attached to a top of the proof mass  30  and the base section  32  and the other DETF  38  is attached to a bottom of the proof mass  30  and the base section  32 . The DETFs  36 ,  38  are designed to resonant at different frequencies. The DETFs  36 ,  38  are driven at two different resonant modes (e.g. mode 1 and 3), thereby producing output signals in the two resonant modes but at slightly different frequencies in order to avoid interference between the modes. These outputted signals are used to produce a force/acceleration signal having greater common mode rejection, such as temperature, radiation, humidity, aging, static charge, and other common mode anomalies. 
         [0008]      FIG. 2  illustrates a flowchart of an exemplary process  40  used to operate the VBA  20 . First at a block  44 , the first DETF  36  is driven at two different resonant modes. Simultaneously at a block  46 , the second DETF  38  is also driven at two different resonant modes. Frequencies used to drive the two resonant modes of the second DETF  38  are different than the frequencies used to drive the two resonant modes of the second DETF  36 . The two resonant modes are the same—e.g. mode 1 and 3. Other resonant mode may be used. 
         [0009]    At a block  50 , during an acceleration event (i.e., a moment when a sensor reading is desired), a sample of the resonant frequencies for the two modes for each DETF  36 ,  38  is taken. At a block  52 , an improved acceleration value is generated based on the four sampled resonant frequencies. A more detailed example is shown below. 
         [0010]      FIG. 3  is a block diagram of an exemplary VBA system  120 . The system  120  includes two DETFs  136 ,  138 . In one embodiment, the DETFs  136 ,  138  include a pattern of electrodes on the tines of the DETFs  136 ,  138 . Driving electrode pads  146 ,  158  are located adjacent to the electrode patterns on the DETFs  136 ,  138 . Each of the driving electrode pads  146 ,  158  receive input signals from two oscillators  140 ,  142 ,  150 ,  152 . The first oscillators  140 ,  150  provide first signals to the pads  146 ,  158  that cause the DETFs  136 ,  138  to oscillate at a first resonant mode. The second oscillators  142 ,  152  provide second signals to the pads  146 ,  158  that cause the DETFs  136 ,  138  to oscillate at a second resonant mode. The frequencies of the first signals are different and the frequencies of the second signals are different. The DETFs  136 ,  138  are configured (e.g., slightly different beam widths) to resonate at slightly different frequencies. 
         [0011]    Each of the analog signals outputted by the DETFs  36 ,  38  are filtered by two bandpass filters  160 ,  162 ,  172 ,  174 . The bandpass filters  160 ,  162 ,  172 ,  174  are chosen according to the two resonant frequency modes experienced by the DETFs  136 ,  138 . Outputs from the filters  160 ,  162 ,  172 ,  174  are turned into digital frequency values by analog-to-digital converters (ADC) with digital counters  164 ,  166 ,  176 ,  178 . The generated digital frequency values are then sent to a processor  170 . The processor  170  generates an acceleration value based on the digital frequency values and predefined coefficients that are prestored in system memory  180 . 
         [0012]    In one embodiment, consider the following::
       the first mode output of the A/D  164  as f 11      the second mode output of the A/D  166  as f 12      the first mode output of the A/D  176  as f 21      the second mode output of the A/D  178  as f 22 .       
 
         [0017]    The processor  170  performs the following operations on the digital frequency values: 
         [0000]    
       
      
       F 
       mod1 
       =a*f 
       11 
       +b*af 
       12  
      
     
         [0000]    
       
      
       F 
       mod2 
       =c*f 
       21 
       +d*af 
       22  
      
     
         [0000]      Common Mode Acceleration estimate= u*F   mod1   +v*F   mod2   +q.    
         [0018]    The coefficients a, b, c, d, u, v, q are tabulated calibration coefficients, stored in the system memory  180 . In one embodiment, the coefficients are determined in a set of calibration tests, prior to connecting instrument to the system  20 . In one embodiment, an initial guess to the values of these coefficients is made and a Kalman filter is used to adapt those values over the course of calibration tumble tests. 
         [0019]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.