Abstract:
The present invention provides a MEMS vibratory type inertial sensor that has some level of built in test to help improve the reliability by helping to identify erroneous or misleading data provided by the inertial sensor. In one illustrative embodiment, a test signal is injected into one or more of the inputs of the MEMS vibratory type inertial sensor, where the test signal produces a test signal component at one or more of the MEMS vibratory type inertial sensor outputs. The test signal component is then monitored at one or more of the outputs. If the test signal component matches at least predetermined characteristics of the original test signal, it is more likely that the MEMS vibratory type inertial sensor is operating properly and not producing erroneous or misleading data. In some embodiments, the test signal is provided and monitored during the normal functional operation of the MEMS vibratory type inertial sensor, thereby providing on-going built in test.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to MEMS vibratory type inertial sensors such as MEMS gyros and MEMS accelerometers, and more specifically to MEMS vibratory inertial sensors with build in test.  
       BACKGROUND  
       [0002]     MEMS vibratory type inertial sensors are used in a wide variety of applications. For many of these applications, a high degree of reliability is desired. For example, in automotive stability control systems, reliable inertial sensors are desirable to reduce erroneous or misleading data, which in some cases, could lead to loss of control of the automobile. What would be desirable is a MEMS vibratory type inertial sensor that has some level of built in test to help improve the reliability by helping to identify erroneous or misleading data provided by the inertial sensor.  
       SUMMARY  
       [0003]     The present invention provides a MEMS vibratory type inertial sensor that has some level of built in test to help improve the reliability by helping to identify erroneous or misleading data provided by the inertial sensor. In one illustrative embodiment, a test signal is injected into one or more of the inputs of the MEMS vibratory type inertial sensor, where the test signal produces a test signal component at one or more of the MEMS vibratory type inertial sensor outputs. The test signal component is then monitored at one or more of the outputs. If the test signal component matches at least predetermined characteristics of the original test signal, it is more likely that the MEMS vibratory type inertial sensor is operating properly and not producing erroneous or misleading data. In some embodiments, the test signal is provided and monitored during the normal functional operation of the MEMS vibratory type inertial sensor, thereby providing on-going built in test. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  is a schematic view of a MEMS-type gyroscope in accordance with the present invention; and  
         [0005]      FIG. 2  is a schematic view of an illustrative MEMS-type gyroscope with a level of build in test. 
     
    
     DESCRIPTION  
       [0006]     The following description should be read with reference to the drawings, in which like elements in different drawings are numbered in like fashion. The drawings, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. Although examples of construction, dimensions, and materials may be illustrated for the various elements, those skilled in the art will recognize that many of the examples provided have suitable alternatives that may be utilized.  
         [0007]     For illustrative purposes, and referring to  FIG. 1 , a MEMS-type gyroscope  10  will be described in detail. However, it should be recognized that the present invention can be applied to a wide variety of MEMS vibratory type inertial sensors such as MEMS gyros and MEMS accelerometers, as desired.  
         [0008]     Gyroscope  10 , illustratively a vibratory rate gyroscope, includes a first proof mass  12  and second proof mass  14 , each of which are adapted to oscillate back and forth above an underlying support substrate  16  in a drive plane orthogonal to an input or “rate” axis  18  of the gyroscope in which inertial motion is to be determined. As indicated generally by the right/left set of arrows  20 , the first proof mass  12  can be configured to oscillate back and forth above the support substrate  16  between a first shuttle mass  22  and first drive electrode  24 , both of which remain stationary above the support substrate  16  to limit movement of the first proof mass  12 . The second proof mass  14 , in turn, can be configured to oscillate back and forth above the support substrate  16  in a similar manner between a second shuttle mass  26  and second drive electrode  28 , but in most cases 180° degrees out-of-phase with the first proof mass  12 , as indicated generally by the left/right set of arrows  30 .  
         [0009]     The first proof mass  12  can include a thin plate or other suitable structure having a first end  32 , a second end  34 , a first side  36 , and a second side  38 . Extending outwardly from each end  32 , 34  of the first proof mass  12  are a number of comb fingers  40 , 42 . Some of the comb fingers can be used to electrostatically drive the first proof mass  12  in the direction indicated by the right/left set of arrows  20 . In the illustrative gyroscope  10  depicted in  FIG. 1 , for example, a first set of comb fingers  40  extending outwardly from the first end  32  of the first proof mass  12  can be interdigitated with a corresponding set of drive comb drive fingers  44  formed on the first drive electrode  24 . A second set of comb fingers  42  extending outwardly from the second end  34  of the first proof mass  12 , in turn, can be interdigitated with a corresponding set of comb fingers  46  formed on the first shuttle mass  22 . In some embodiments, the set of comb fingers  46  may be used to sense the motion of the first proof mass  12 .  
         [0010]     The second proof mass  14  can be configured similar to the first proof mass  12 , having a first end  48 , a second end  50 , a first side  52 , and a second side  54 . A first set of comb fingers  56  extending outwardly from the first end  48  of the second proof mass  16  can be interdigitated with a corresponding set of comb fingers  58  formed on the second shuttle mass  26 . In some embodiments, the set of comb fingers  58  may be used to sense the motion of the second proof mass  14 . A second set of comb fingers  60  extending outwardly from the second end  50  of the second proof mass  14 , in turn, can be interdigitated with a corresponding set of drive comb fingers  62  formed on the second drive electrode  28 .  
         [0011]     The first and second proof masses  12 , 14  can be constrained in one or more directions above the underlying support structure  16  using one or more suspension springs. As shown in  FIG. 1 , for example, the first proof mass  12  can be anchored or otherwise coupled to the support substrate  16  using a first set of four suspension springs  64 , which can be connected at each end  66  to the four corners of the first proof mass  12 . In similar fashion, the second proof mass  14  can be anchored to the underlying support substrate  16  using a second set of four springs  68 , which can be connected at each end  70  to the four corners of the second proof mass  14 .  
         [0012]     In use, the suspension springs  64 , 68  can be configured to isolate oscillatory movement of the first and second proof masses  12 , 14  to the direction indicated generally by the right/left set of arrows  20 , 30  to reduce undesired perpendicular motion in the direction of the rate axis  18 , and to reduce quadrature motion in the direction of the sensing motion  72 . In addition to supporting the proof masses  12 , 14  above the support substrate  16 , the suspension springs  64 , 68  can also be configured to provide a restorative force when the drive voltage signal passes through the zero point during each actuation cycle.  
         [0013]     A drive voltage V D  can be applied to the first and second drive electrodes  24 , 28 , inducing an electrostatic force between the interdigitated comb fingers that causes the comb fingers to electrostatically move with respect to each other. The drive voltage V D  can be configured to output a time-varying voltage signal to alternate the charge delivered to the comb fingers, which in conjunction with the suspension springs  64 , 68 , causes the first and second proof masses  12 , 14  to oscillate back and forth in a particular manner above the support substrate  16 . Typically, the drive voltage V D  will have a frequency that corresponds with the resonant frequency of the first and second proof masses  12 , 14  (e.g. 10 KHz), although other desired drive frequencies can be employed, if desired.  
         [0014]     A pair of sense electrodes  74 , 76  can be provided as part of the sensing system to detect and measure the out-of-plane deflection of the first and second proof masses  12 , 14  in the sense motion direction  72  as a result of gyroscopic movement about the rate axis  18 . As shown by the dashed lines in  FIG. 1 , the illustrative sense electrodes  74 , 76  can include a thin, rectangular (or other) shaped electrode plate positioned underneath the proof masses  12 , 14  and oriented in a manner such that an upper face of each sense electrode  74 , 76  is positioned vertically adjacent to and parallel with the underside of the respective proof mass  12 , 14 . The sense electrodes  74 , 76  can be configured in size and shape to minimize electrical interference with the surrounding comb fingers  40 , 42 , 56 , 60  to prevent leakage of the drive voltage source V D  into the sense signal.  
         [0015]     A sense bias voltage V S  applied to each of the sense electrodes  74 , 76  can be utilized to induce a charge on the first and second proof masses  12 , 14  proportional to the capacitance between the respective sense electrode  74 , 76  and proof mass  12 , 14 . The sense electrode  74 , 76  and the first and second proof masses  12 , 14  preferably include a conductive material (e.g. a silicon-doped conductor, metal or any other suitable material), allowing the charge produced on the sense electrode  74 , 76  vis-à-vis the sense bias voltage V S  to be transmitted to the proof mass  12 , 14 .  
         [0016]     During operation, the Coriolis force resulting from rotational motion of the gyroscope  10  about the rate axis  18  causes the first and second proof masses  12 , 14  to move out-of-plane with respect to the sense electrodes  74 , 76 . When this occurs, the change in spacing between the each respective sense electrode  74 , 76  and proof mass  12 , 14  induces a change in the capacitance between the sense electrode  74 , 76  and proof mass  12 , 14 , which can be measured as a charge on the proof masses  12 , 14  using the formula:  
         [0017]     q=ε 0 AV s /D  
         [0018]     wherein A is the overlapping area of the sense electrode and proof mass, V S  is the sense bias voltage applied to the sense electrode, ε 0  the dielectric constant of the material (e.g. vacuum, air, etc.) between the sense electrodes and the proof masses, and D is the distance or spacing between the sense electrode  74 , 76  and respective proof mass  12 , 14 . The resultant charge received on the proof mass  12 , 14  may be fed through or across the various suspension springs  64 , 68  to a number of leads  78 . The leads  78 , in turn, can be electrically connected to a charge amplifier  80  that converts the charge signals, or currents, received from the first and second proof masses  12 , 14  into a corresponding rate signal  82  that is indicative of the Coriolis force.  
         [0019]     To help balance the input to the charge amplifier  80  at or about zero, the sense bias voltage V S  applied to the first proof mass  12  can have a polarity opposite to that of the sense bias voltage V S  applied to the second proof mass  14 . In certain designs, for example, a sense bias voltage V S  of +5V and −5V, respectively, can be applied to each of the sense electrodes  74 , 76  to prevent an imbalance current from flowing into the output node  84  of the charge amplifier  80 . To help maintain the voltage on the proof masses  12 , 14  at about virtual ground, a relatively large value resistor  86  can be connected across the input  88  and output nodes  86  of the charge amplifier  80 , if desired.  
         [0020]     A motor bias voltage V DC  can be provided across the first and second shuttle masses  22 , 26  to detect and/or measure displacement of the proof masses  12 , 14  induced via the drive voltage source V D . A motor pickoff voltage V PICK  resulting from movement of the comb fingers  42 , 56  on the first and second proof masses  12 , 14  relative to the comb fingers  46 , 58  on the first and second shuttle masses  22 , 26  can be used to detect/sense motion of the first and second proof masses  12 , 14 .  
         [0021]      FIG. 2  is a schematic view of an illustrative MEMS-type gyroscope with a certain level of build in test. The gyroscope of  FIG. 1  is shown in block form as gyro block  10 . In the illustrative embodiment, a drive oscillator  100  receives the motor pickoff voltage V PICK  discussed above with respect to  FIG. 1 . While only one motor pickoff voltage is shown in  FIG. 2 , it is contemplated that in some embodiments, the drive oscillator  100  may be configured to receive a motor pickoff voltage V PICK  from each of the proof masses of  FIG. 1 . However, for simplicity, the embodiment shown in  FIG. 2  only shows and discusses the operation of one of the proof masses.  
         [0022]     The drive oscillator  100  uses the motor pickoff voltage V PICK  to provide the next drive motor cycle. As noted above, the drive motor signal can be configured to output a time-varying voltage signal to alternate the charge delivered to the comb fingers, which in conjunction with the suspension springs  64 , 68 , causes the first and second proof masses  12 , 14  to oscillate back and forth in a particular manner above the support substrate  16  (see  FIG. 1 ). Typically, the drive voltage V D  will have a frequency that corresponds with the resonant frequency of the first and second proof masses  12 , 14  (e.g. 10 KHz), although other desired drive frequencies can be employed, if desired.  
         [0023]     To help control the amplitude of the voltage, the output of the drive oscillator  100  may be provided to an amplitude controller  102 . The amplitude controller  102  receives a reference amplitude from reference  104 . In a gyro that does not have built in test, the output of the amplitude controller  102  may be provided directly as the drive voltage V D  to one of the proof masses. However, and in accordance with one illustrative embodiment of the present invention, the output of the amplitude controller  102  may be provided to a modulator  105 , which modulates the output of the amplitude controller  102  and a test signal  106 . The test signal  106  may be a continuously running built in test (CBIT) AC signal, and may have a frequency that is substantially higher or substantially lower than the motor drive resonance frequency. In one illustrative embodiment, the test signal  106  has a frequency of 50 Hz and the motor drive signal has a frequency of about 10 KHz, however other frequencies may be used. In some cases, the amplitude of the test signal  106  may be made to substantially match an expected coriolis rate voltage (e.g. V RATE    82 ), but this is not required in all embodiments  
         [0024]     After the test signal  106  is modulated by modulator  105 , the result is provided to the gyro  10  as the drive voltage V D . The drive voltage V D  thus has a component that corresponds to the test signal  106  and a component that corresponds to the output of the amplitude controller  102 . The component of the drive voltage V D  that corresponds to the test signal  106  preferably has a frequency that is sufficiently off any resonant modes of the proof masses such that it has little or no effect on the electrostatic drive of the proof masses. However, in the illustrative embodiment, it is capacitively coupled to the proof masses (and in some cases to the sense plates), and ultimately to the charge amplifier  80 , which converts the charge signals, or currents, received from the first and second proof masses  12 , 14  into a corresponding rate signal  82  that is indicative of the Coriolis force. Thus, in the illustrative embodiment, the output  82  of the charge amplifier  80  may includes a component that corresponds to the experienced Coriolis force and a component that corresponds to the capacitively coupled test signal  106 .  
         [0025]     In the illustrative embodiment, the output  82  of the charge amplifier  80  may be provided to a rate amplifier  110 , which amplifies the signal. The output of the rate amplifier  110  may be provided to a filter amplifier  112 , which performs both a filtering and amplifying function. The output of the filter amplifier  112  may then be demodulated using the output of the amplitude controller  104 , as shown at  114 . In the illustrative embodiment, the test signal  106  is originally modulated using the output of the amplitude controller  104  as a reference, and the output of the filter amplifier  112  is demodulated using the same signal, as indicate by dashed line  116 . This may help keep the modulated test signal  106  relatively in phase with the rate signal  82  that is indicative of the Coriolis force.  
         [0026]     In the illustrative embodiment, the demodulated signal is provided to another filter amplifier  120 , and the result is a DC rate bias signal with a superimposed component of the test signal, as shown at  122 . This signal is passed to yet another filter  130  that separates the component of the test signal from the DC rate bias signal. If the component of the test signal  134  matches at least selected characteristics of the original test signal  106 , it is more likely that the gyro  10  is operating properly and not producing erroneous or misleading data.  
         [0027]     In some embodiments, the filter  130  may be simply a high pass filter and a low pass filter. For example, a high pass filter may pass the component of the test signal  134  while the low pass filter may pass the DC rate bias signal  132 . In other embodiments, however, the filter  130  may be a more sophisticated filter, such as an adaptive filter. One such adaptive filter is described in U.S. Pat. No. 5,331,402, which is assigned to the assignee of the present invention. In some embodiments, the adaptive filter may receive the original test signal  106  to help separate the component of the test signal  134  from the DC rate signal  132 . In some cases, the adaptive filter may be configured to “adapt” relatively slowly relative to the expected rate of change of the DC rate signal  132 . For example, the adaptive filter may have a time constant of 60 seconds, or any other suitable time constant as desired.  
         [0028]     The DC rate signal  132  and the separated component of the test signal  134  may be converted to digital signals for subsequent processing and/or analysis. In some embodiment, the DC rate signal  132  may be sampled at 100 Hz, and the separated component of the test signal  134  may be sampled at 200 Hz, although other sample rates may also be used if desired.  
         [0029]     As can be seen, the test signal  106  may be continuously supplied, and thus the operation of the gyro may be continuously monitored and/or tested. This may help improve the reliability of the gyro by helping to identify erroneous or misleading data provided by the gyro.  
         [0030]     Rather than injecting the test signal  106  into the motor drive signal, it is contemplated that the test signal  106  may be injected onto the sense plates, if desired. When so provided, the test signal  106  is capacitively coupled into the proof masses, and superimposed on the output  82  of the charge amplifier  80 , as described above. This configuration may also provide a certain level of built in test, and help improve the reliability of the gyro by helping to identify erroneous or misleading data provided by the gyro.