Abstract:
An accelerometer with improved immunity to sensitivity drift is disclosed. In some embodiments, the accelerometer comprises an actuator that induces a known acceleration on a reference frame. A signal based on this known acceleration is used to calibrate the accelerometer to mitigate the effects due to at least one of sensitivity drift, D.C. bias drift, sense laser wavelength drift, and resonant frequency drift.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This case claims priority to: U.S. Provisional Patent Application Ser. No. 61/034,418, filed Mar. 6, 2008, which is incorporated by reference. 
     If there are any contradictions or inconsistencies in language between this application and one or more of the cases that have been incorporated by reference that might affect the interpretation of the claims in this case, the claims in this case should be interpreted to be consistent with the language in this case. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to sensors in general, and, more particularly, to accelerometers. 
     BACKGROUND OF THE INVENTION 
     An accelerometer provides an output signal based on an acceleration that acts upon it. Accelerometers are used in a wide variety of applications, such as inertial guidance systems, automobile crash detection systems, video game controllers, and shipping container shock sensors. 
     An accelerometer with very high sensitivity can be used as a gravity sensor. Gravity sensors are used in such area as geological surveying, oil field exploration, homeland security, and seismology. 
     The advent of Micro-Electro Mechanical Systems (MEMS) technology has ushered in a new era of accelerometers. A typical MEMS-based accelerometer includes a sensing element that is based on a proof mass attached to a reference frame by means of resilient tether (e.g., a spring element or spring system). An acceleration of the reference frame or the presence of a gravitational field causes a displacement of the proof mass relative to the reference frame. A transducer converts this displacement into an output signal. 
     The sensitivity of an accelerometer (expressed in terms of Volts or Amperes per unit of acceleration) is dependent upon its signal-to-noise ratio (SNR) and output drift. In the absence of an acceleration, the output signal of the accelerometer typically exhibits a constant voltage or current level. This steady-state output value is referred to as a D.C. bias. Unfortunately, DC bias is subject to drift over time and temperature. In addition, accelerometers are subject to sensitivity drift with temperature. Such drifts contribute to the noise on the output signal, thereby decreasing the signal-to-noise ratio of the accelerometer. This adversely affects the overall sensitivity of the device. 
     Approaches to mitigating the effects of bias stability have been developed in the prior art. In some cases, temperature sensors and microcontrollers are integrated with the accelerometer. During operation, the temperature-induced drift can be removed from the output signal by means of a look-up table or mathematical solution. Unfortunately, this leads to significant added cost and complexity. 
     Other approaches rely on the integration of analog correction circuitry with the accelerometer. Unfortunately, it is very hard to tune the correction circuitry to accommodate variations from accelerometer to accelerometer. 
     Still other approaches employ active temperature stabilization. In such approaches, the accelerometer is held at a temperature above the ambient by means of a heating element. This dramatically increases power consumption for the accelerometer however. Low power consumption is particularly desirable for mobile applications. 
     In some cases, a user-employable “reset” button is provided to enable a user to zero the accelerometer when it is in a state of zero acceleration. This, however, is impractical in many applications. 
     Finally, AC-coupling an accelerometer can eliminate bias drift all-together. Unfortunately, in many applications DC-coupling is required or highly desirable (e.g., tilt sensors, geological surveying, inertial navigation, etc.). 
     SUMMARY OF THE INVENTION 
     The present invention provides an accelerometer with high SNR and high sensitivity without some of the disadvantages of the prior art. Some embodiments of the present invention are particularly well-suited for use in devices such as gravity sensors and accelerometers in applications such as geological surveying, oil field exploration, inertial navigation, homeland defense, and shock detection. 
     Sensors in accordance with the present invention provide measurement sensitivity that is improved over the prior art by including a reference signal suitable for sensor calibration. As a result, embodiments of the present invention can mitigate the effects of:
         i. D.C. bias drift; or   ii. resonant frequency drift; or   iii. sensitivity drift; or   iv. wavelength noise associated with displacement sensors used to monitor acceleration; or   v. any combination of i, ii, iii, and iv.       

     In some embodiments, the accelerometer provides two signals: (1) a first signal based on motion of a mass that is resiliently connected to a first reference frame; and (2) a second signal based on an induced motion of the first reference frame relative to a second reference frame (e.g., a substrate). To generate the second signal, a known motion that exhibits a known acceleration is induced on the first reference frame. This known motion is then detected directly and forms the basis of the second signal. The accelerometer can then be calibrated based on the second signal. In some embodiments, calibration of the accelerometer is enabled by inducing a periodic motion of the first reference frame at a frequency that is outside the expected range of frequencies of the target acceleration sensed by the accelerometer. 
     In some embodiments, the second signal is generated by detecting motion of the first reference frame using an optically resonant cavity. The first reference frame is oscillated along a direction with amplitude that exceeds the optical response of the cavity. In other words, the first reference frame translates a distance that results in at least two optical resonant transmission peaks being detected. This enables more accurate calibration of the sensor. 
     In some embodiments, motion of the first reference frame is induced by an impulse of force. The impulse response of the first reference frame is measured periodically, and the resonant frequency is observed. In some embodiments, a swept-sine measurement is used to detect drift in the resonant frequency of the mass. 
     An embodiment of the present invention comprises: a substrate; a first element; an actuator, wherein the actuator induces a first motion of the first element with respect to the substrate; a second element, wherein the second element comprises a first physical adaption for moving with a second motion in response to an acceleration; a first sensor, wherein the first sensor provides a first signal that is based on the first motion; and a second sensor, wherein the second sensor provides a second signal that is based on the second motion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a block diagram of details of an accelerometer in accordance with an illustrative embodiment of the present invention. 
         FIGS. 2A and 2B  depict a top and side view, respectively, of details of accelerometer  100 . 
         FIG. 3  depicts a method for sensing acceleration in accordance with the illustrative embodiment of the present invention. 
         FIG. 4  depicts a cross-sectional view of details of an accelerometer in accordance with an alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a block diagram of details of an accelerometer in accordance with an illustrative embodiment of the present invention. Accelerometer  100  comprises acceleration sensor  102 , substrate  106 , actuator  108 , sensor  110 , and processor  112 . 
     Acceleration sensor  102  is an accelerometer that provides signal  110 . Signal  110  is an electrical signal that is based on the motion of a proof mass relative to a reference frame. Acceleration sensor  102  is described in more detail below and with respect to  FIGS. 2A and 2B . 
     Substrate  104  is a rigid platform suitable for providing a reference frame for motion of acceleration sensor  102 . Acceleration sensor  102  is supported above substrate  104  by tether  106 . Tether  106  has sufficient mechanical strength to support acceleration sensor  102 , but is resilient to enable motion of accelerometer  102  with respect to substrate  104 . Suitable materials for substrate  104  and tether  106  include, without limitation, semiconductors, semiconductor compounds, dielectrics, glasses, polymers, ceramics, metals, and composite materials. 
     Actuator  108  is an electrostatic actuator suitable for inducing motion of acceleration sensor  102 , relative to substrate  104 . Alternative actuators suitable for use in actuator  108  include, without limitation, piezoelectric, magnetic, thermal, microfluidic, pneumatic, hydraulic, shape memory alloy, and magnetostrictive actuators. 
     Sensor  110  is a sensor that provides signal  118  based on the motion of acceleration sensor  102  relative to substrate  104 . Sensors suitable for use as sensor  110  include, without limitation, optical, magnetic, electrostatic, capacitive, induction, piezoelectric, and piezoresistive sensors. 
     Processor  112  is a general purpose processor capable of executing software routines, computation, providing drive signal  114 , and computing a value for acceleration based on received signals  110  and  118 . 
       FIGS. 2A and 2B  depict a top and side view, respectively, of details of accelerometer  100 . Accelerometer  100  comprises acceleration sensor  102 , tethers  106 , anchors  208 , actuators  108 , and sensor  110 . 
     Acceleration sensor  102  comprises mass  202 , frame  204 , tether  206 , and mass sensor  210 . Acceleration sensor  102  is supported above substrate  104  by virtue of tethers  106 , which extend from supports  208 . 
     Mass  202  is a rigid block of material having a known mass. Mass  202  is attached to frame  204  by tether  206 . 
     Tether  206  is a resilient element that enables motion of mass  202  along the x-direction, as shown. 
     Frame  204  is an annulus of rigid material. Frame  204  is supported above substrate  104  via tethers  106 . In some embodiments, frame  204  does not surround mass  202 . In some embodiments, frame  204  is a simple shape, such as a block having a square, circular, elliptical, or irregular shape. 
     The specific shapes and sizes of mass  202 , frame  204  and tether  206  are design considerations that are application dependent. For most applications, mass  202  and/or frame  204  would have a circular or square shape, wherein mass  202  has a diameter or width within the range of approximately 0.5 millimeters (mm) to approximately 20 mm, while frame  204  would be within the range of approximately 3 mm to approximately 22 mm on a side. The illustrative embodiment depicts one exemplary design comprising: a mass  202  having a square shape of approximately 4 mm on a side and a thickness of approximately 0.5 mm; a frame  204  having a square annular shape of approximately 6 millimeters on a side with an annular width of approximately 0.5 mm and a thickness of approximately 0.5 mm; and four tethers  206 , each having a length of approximately 0.5 mm and a thickness of 0.1 mm. Further, although the illustrative embodiment comprises four tethers  106 , it will be clear to one skilled in the art, after reading this specification, how to specify, make, and use embodiments of the present invention that comprise any number of tethers that support acceleration sensor  102  above substrate  104 . 
     Suitable materials for mass  202 , frame  204 , and tether  206  include, without limitation, semiconductors, semiconductor compounds, dielectrics, glasses, polymers, ceramics, metals, and composite materials. In some embodiments, mass  202 , frame  204 , and tether  206  are formed from a continuous layer of material. 
       FIG. 3  depicts a method for sensing acceleration in accordance with the illustrative embodiment of the present invention. Method  300  is described with continuing reference to  FIGS. 1 ,  2 A, and  2 B. 
     Method  300  begins with operation  301 , wherein an oscillation of frame  204  with respect to substrate  104  is induced along the x-direction by periodic excitation of actuators  108 . 
     Each of actuators  108  comprises a lower electrode  212  and an upper electrode  214 . Upon application of a suitable voltage between lower electrodes  212  and upper electrodes  214 , frame  204  is attracted toward substrate  104 . In other words, frame  204  moves in the negative x-direction, as shown. When frame  204  moves along the x-direction, tethers  106  are stretched and a tensile strain is induced in the tethers. 
     Upon removal of the voltage applied between electrodes  212  and  214 , the tension in tethers  106  acts as a restoring force that pulls frame  204  away from substrate  104 . In other words, frame  204  moves in the positive x-direction, as shown, when the voltage is removed from electrodes  212  and  214 . By repeatedly applying and removing the voltage between electrodes  212  and  214 , therefore, oscillation of frame  204  along the x-direction is induced. This oscillation imparts a known acceleration on frame  204  in the x-direction. 
     In some embodiments, actuator  108  oscillates frame  204  with a frequency that is well below the resonant frequency of acceleration sensor  102 . This resonant frequency is defined by the combined mechanical and material characteristics of mass  202 , frame  204 , and tether  206 . Well below this resonant frequency, the amount of deflection of mass  202  along the x-direction, Δx m , is determined by 
                       Δ   ⁢           ⁢     x   m       =     a     ω   0   2         ,           (   1   )               
where α is the applied acceleration and ω 0  is the resonant frequency.
 
     For a sinusoidal displacement of frame  204 , x e =x e0  sin ωt, where x e0  is the maximum deflection and ω is the frequency of excitation, an applied acceleration of α=−ω 2 x e  is produced. The displacement of mass  202 , therefore, can be described as: 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     x 
                   
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                         - 
                         
                           ω 
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                         ω 
                         0 
                         2 
                       
                     
                     ⁢ 
                     
                       
                         x 
                         e 
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   2 
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     For operation of the accelerometer at a frequency below the resonance of the tether-mass system, the displacement of frame  204  is always larger than that of mass  202 . In fact, for some embodiments, for frequencies below 100 Hz, arbitrarily low accelerations can be applied and sensed. 
     In some embodiments, actuators  108  apply an impulse function to frame  204 . As a result, accelerometer  100  exhibits a “ring-down behavior.” From this ring-down behavior, the resonant frequency of the accelerometer can be computed. This enables a mitigation of the effect of resonant frequency drift on the sensitivity of the accelerometer. 
     In some alternative embodiments, motion of acceleration sensor  102  relative to substrate  104  is induced by an actuator that is not directly connected to acceleration sensor  102 . In such embodiments, frame  204  is driven via an actuator that is operatively coupled to frame  204  through a motion transducer, such as a lever arm. In some embodiments, this motion transducer provides de-amplification of the actuator motion. As a result, effects of actuator noise on the performance of the accelerometer can be mitigated. 
     At operation  302 , sensor  110  provides signal  118 . Signal  118  is based on acceleration associated with the oscillation of frame  204  with respect to substrate  104 . Sensor  110  comprises an interferometric optical sensor including a first optically resonant cavity, laser  216  and detector  224 . The first optically resonant cavity is defined by mirrors  220  and  222 . Laser  216  emits light signal  218 , a portion of which is reflected by the first optically resonant cavity and detected by detector  224 . Detector  224  provides signal  118  (not shown for clarity), which is based on the intensity of detected light signal  218 . The intensity of detected light signal  218  is based on the amount of light reflected by the first optically resonant cavity. This amount is based on the spacing between mirrors  220  and  222  (i.e., the cavity length of the first optically resonant cavity). 
     In some embodiments, actuators  108  induce displacements of frame  204  that exceed an optical period of sensor  110 . As a result, multiple reflection peaks are available for the first optically resonant cavity. This enables accurate calibration of actuator  108 , which can be performed on a regular basis if desired (e.g., once per hour, etc.). 
     At operation  303 , mass sensor provides signal  116 . Signal  116  is based on motion of mass  202  with respect to substrate  104 . Mass  202  moves along the x-direction in a predictable manner in response to the oscillation of frame  204  along the x-direction. This predictable motion is perturbed, however, by any external acceleration along the x-direction imparted on accelerometer  100 . 
     Mass sensor  210  comprises an interferometric optical sensor including a second optically resonant cavity, laser  234  and detector  226 . The second optically resonant cavity is defined by mirrors  230  and  232 . Laser  234  emits light signal  228 , a portion of which is reflected by the second optically resonant cavity and detected by detector  226 . Detector  226  generates an electrical signal that is based on the intensity of detected light signal  228 . The intensity of detected light signal  228  is based the amount of light reflected by the second optically resonant cavity. This amount is based on the spacing between mirrors  230  and  232  (i.e., the cavity length of the second optically resonant cavity). 
     In some embodiments, lasers  216  and  234  and detectors  220  and  230  are located on top of substrate  104  (i.e., between frame  204  and substrate  104 ). In some embodiments, at least one of sensor  110  and mass sensor  210  comprises collimating and/or beam steering optics. 
     It will be clear to those of ordinary skill in the art that the paths of light signals  218  and  228  are merely representative of the operation of their respective optically resonant cavities, as the reflection and transmission characteristics of an optically resonant cavity are more complex that as depicted. 
     In some embodiments, high-resolution displacement sensors other than optically resonant cavities are used to sense the relative positions of the frame and mass with respect to the substrate. Suitable technologies for use as high-resolution sensors include, without limitation, optical, magnetic, electrostatic, capacitive, induction, piezoelectric, and piezoresistive sensors. 
     At operation  304 , acceleration of accelerometer  100  along the x-direction is computed by processor  112 . The acceleration is computed based on signals  116  and  118 . 
       FIG. 4  depicts a cross-sectional view of details of an accelerometer in accordance with an alternative embodiment of the present invention. Accelerometer  400  comprises acceleration sensor  402 , substrate  104 , actuators  108 , sensor  404 , and processor  112 . 
     Acceleration sensor  402  is analogous to acceleration sensor  102 ; however, acceleration sensor  402  comprises mass sensor  406  rather than mass sensor  210 . Mass sensor  406  is described in more detail below in conjunction with sensor  404 . 
     Operation of accelerometer  400  is analogous to operation of accelerometer  100 ; however, in accelerometer  400 , sensors  404  and  406  receive light signals  414  and  416 , which are generated by splitting light emitted by a single laser source. 
     Laser  408  emits light signal  410 , which is suitable for operation of sensor  404  and mass sensor  406 . Light signal  410  is received by beamsplitter  412 , which distributes it into light signals  414  and  416 . 
     Since both sensor  404  and mass sensor  406  receive light from the same source, accelerometer  400  is less sensitive to drift of the wavelength of light received by sensor  404  and mass sensor  406 . Such wavelength drift would affect the output of both sensors equally; therefore, a combination of these outputs can be used to effectively null the effects of wavelength noise. 
     It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.