Abstract:
Systems and methods for minimizing vibration rectification error (VRE) for a closed-loop accelerometer. In one embodiment, the method includes applying a known vibration signal to the closed-loop accelerometer along a first axis and adjusting a feedback gain setting until VRE is below a first threshold value. After the feedback gain setting has been adjusted, applying a random vibration signal to the closed-loop accelerometer along the first axis and adjusting a servo system proportional gain value until VRE is below a second threshold value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This invention claims the benefit of U.S. Provisional Application Ser. No. 60/613,864 filed Sep. 28, 2004, the contents of which are incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Vibration Rectification Error (VRE) is common in accelerometers. If the VRE is not compensated for, performance of the accelerometer is degraded.  
         [0003]     Therefore, there exists a need for methods and apparatus for minimizing VRE for closed-loop accelerometers.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     The present invention provides systems and methods for minimizing vibration rectification error (VRE) for a closed-loop accelerometer. In one embodiment, the method includes applying a known vibration signal to the closed-loop accelerometer along a first axis and adjusting a feedback gain setting until VRE is below a first threshold value. After the feedback gain setting has been adjusted, applying a random vibration signal to the closed-loop accelerometer along the first axis and adjusting a servo system proportional gain value until VRE is below a second threshold value. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0005]     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0006]      FIG. 1  illustrates an example of a closed-loop accelerometer;  
         [0007]      FIGS. 2-4  illustrate flow diagrams that show an example process for minimizing vibration rectification error of the closed-loop accelerometer shown in  FIG. 1 ; and  
         [0008]      FIG. 5  illustrates response of a closed-loop accelerometer before and after adjustments to vibration rectification error according to the process shown in  FIGS. 2-4 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]      FIG. 1  illustrates a closed-loop accelerometer system  20 . The system  20  includes an accelerometer  30 , a position measurement circuit  32 , a controller  34 , switching logic  36 , a K L  multiplier  48 , and a K U  multiplier  50 . The accelerometer  30  includes position detectors  40  that are in signal communication with the position measurement circuit  32 . The accelerometer  30  also includes upper and lower restoring force actuators  42  and  44  that are coupled to the multipliers  50  and  48 , respectively. The accelerometer  30  also includes a proof mass  52  attached to stationary substrate  54  via a flexible beam  56 .  
         [0010]     During operation of the system  20 , an acceleration load is applied to the accelerometer  30 , whereby the proof mass  52  flexes in the direction of the experienced acceleration load. The position measurement circuit  32  determines the position of the proof mass  52  based on signal sent from the detectors  40 . The controller  34  generates a restoring force signal based on the determined position of the proof mass  52 .  
         [0011]     The switching logic  36  sends the restoring force signal to one of the multipliers  48  or  50  depending upon the signal outputted by the controller  34 . Then actuator  42  or  44  applies a restoring force to the proof mass  52  in order to center the proof mass  52  within the accelerometer  30 . The system  20  reports the acceleration load applied to the proof mass  52  based on either one of the voltage or current level of the restoring force signal.  
         [0012]     The present invention includes methods and systems for adjusting the value of K L  or K U , then adjusting the value of K P  used in the controller  34  for minimizing the effects of vibration rectification error (VRE). K P  is the proportional gain value and K I  is the servo system integral gain value. The output of the controller  34  is shown by Equation (1): 
 
 K   I ∫ xdt+   K   P     x      (1) 
 
         [0013]     x—amount of proof mass displacement  
         [0014]     The controller  34  converts the sensed distance value into the restoring force signal.  
         [0015]     In one embodiment, an automatic gain value adjusting device  60  is in data communication with the controller  34  and multipliers  48  and  50 . The device  60  includes a processor and memory for executing program code that analyzes the output of the controller  34  and automatically adjusts the gain values in order to minimize VRE. The device  60  performs the process shown in  FIGS. 2-4 . In another embodiment, adjusting the gain values is performed manually by an operator upon analysis of determined VRE (see the detailed example below).  
         [0016]      FIG. 2  illustrates an example process  70  for minimizing the effects of VRE such as that experienced by the accelerometer  30  as shown in  FIG. 1 . The process  70  first begins at a block  72  whereby the accelerometer  30  is mounted to a vibration table. Next at a block  80 , K L  or K U  is adjusted using a known vibration signal. This is described in more detail below in  FIG. 3 .  
         [0017]     Next at a block  86 , K P  is adjusted using a random vibration signal. This is described in more detail below in  FIG. 4 .  
         [0018]      FIG. 3  illustrates an example process for performing the adjustment of K L  or K U  as described in the block  80  of the process  70  shown in  FIG. 2 . First, at a block  106 , the accelerometer  30  is vibrated at a known sinusoidal frequency of frequency f having an amplitude A. Next at a block  110 , VRE is measured. Then, at a decision block  112 , the process  80  determines if the measured VRE is below an acceptable threshold. If at the decision block  112 , the VRE is not below an acceptable threshold, then the value of K L  is changed, see block  114 . The process then returns to the block  106 . If at the decision block  112 , the VRE was measured to be below the acceptable threshold, then this part of process  70  is complete.  
         [0019]      FIG. 4  illustrates the process performed at the block  86  of the process  70  shown in  FIG. 2 . First at a block  118 , a random vibration is applied to the accelerometer  30 . Then, at a block  120 , the VRE of the accelerometer  30  is measured. If at a decision block  122 , the VRE is not below an acceptable threshold, then at a block  126 , the value of KP is changed and the process returns to the block  118 . If, however, at the decision block  122 , the VRE is below the acceptable threshold, this part of the process  70  is complete.  
         [0020]     Upon completion of the process  70 , VRE has been minimized because of adjusting KP after adjustments have been made to KL or KU. In systems with more than one accelerometer, the process  70  is repeated for the untested accelerometers.  
         [0021]      FIG. 5  illustrates an example response of an accelerometer before and after the process  70  shown in  FIG. 2  has been performed.  
         [0022]     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. For example, the accelerometer  30  are MEMS or non-MEMS devices that can be made from silicon, quartz, fused silica, or any other material that develops its rebalancing force using electrostatics, magnetic fields, piezo-electric effects, or any other means. 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.