PATENT DOCUMENT

Publication Number: US-10837796-B2
Application Number: US-201816147088-A
Country: US
Kind Code: B2

Title: Gyroscope sensitivity calibration

Abstract:
An in-situ calibration system, method and apparatus is disclosed that uses test electrodes to stimulate a proof-mass of a MEMS based gyroscope at a drive frequency as quasi-Coriolis forces to extract the electromechanical gain, and uses a non-resonant carrier signal on the proof-mass to extract the additional changes in the sense and drive capacitance. Additionally, an in-situ calibration system, method and apparatus is disclosed that uses quadrature electrodes to apply a known force stimulus to the proof-mass as part of a calibration procedure, where the known force is applied again after installation into a system or further into the life of the gyroscope. Differences in the proof-mass response to the force are proportional to changes in sensitivity, which allows the sensitivity to be corrected in-field.

Claims:
What is claimed is: 
     
       1. A method of calibrating sensitivity of a micro-electrical-mechanical system (MEMS) based gyroscope, comprising:
 driving a proof-mass of the MEMS based gyroscope using signals from a drive capacitance-to-voltage (C2V) converter output to generate quasi-Coriolis forces on the proof-mass; 
 extracting electromechanical gain from the MEMS based gyroscope; and 
 using the gain to recalibrate a sensitivity of the MEMS based gyroscope. 
 
     
     
       2. The method of  claim 1  further comprising:
 injecting a non-resonant carrier signal into the proof-mass of the MEMS based gyroscope to extract additional changes in a sense capacitance and a drive capacitance of the MEMS based gyroscope; and 
 using the gain and the additional changes to recalibrate sensitivity of the MEMS based gyroscope. 
 
     
     
       3. A method of calibrating sensitivity of a micro-electrical-mechanical system (MEMS) based gyroscope, comprising:
 at a first time:
 driving, by quadrature test electrodes, a proof-mass of the MEMS based gyroscope at a drive frequency to apply a known force stimulus to the proof-mass; 
 determining a first response of the proof-mass to the known force stimulus; 
 
 at a second time after the first time:
 driving, by the quadrature test electrodes, the proof-mass of the MEMS based gyroscope at the drive frequency to apply the known force stimulus to the proof-mass; 
 determining a second response of the proof-mass to the known force stimulus; 
 determining a difference in first and second responses of the proof-mass to the know force stimulus; and 
 
 using the difference to recalibrate sensitivity of the MEMS based gyroscope. 
 
     
     
       4. The method of  claim 3  further comprising:
 injecting a non-resonant carrier signal into the proof-mass of the MEMS based gyroscope to extract additional changes in a sense capacitance and a drive capacitance of the MEMS based gyroscope; and 
 using the difference and the additional changes to recalibrate sensitivity of the MEMS based gyroscope. 
 
     
     
       5. A gyroscope sensitivity calibration system, comprising:
 a micro-electrical-mechanical system (MEMS) based gyroscope including:
 a drive mass; 
 a sense mass disposed within an opening of the drive mass and configured to move within the opening in response to the drive mass moving along a drive axis; 
 test electrodes configured to excite the drive mass during a calibration mode of operation; 
 
 a drive system configured to generate quasi-Coriolis excitation signals and apply the excitation signals to the test electrodes, wherein the quasi-Coriolis excitation signals excite the drive mass with quasi-Coriolis forces to induce quasi-Coriolis excitation forces on the sense mass; and 
 a sense system configured to extract a varying capacitance due to displacement of the sense mass in response to the induced quasi-Coriolis excitation forces, and to determine a mechanical sensitivity of the MEMS based gyroscope based on the varying capacitance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 62/639,458, filed Mar. 6, 2018, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to calibrating micro-electrical-mechanical system (MEMS) based gyroscopes. 
     BACKGROUND 
     The sensitivity of gyroscopes is normally calibrated by the gyroscope supplier using a known sequence of rotary stimuli. The sensitivity, however, is dependent on a number of electromechanical properties that are strain sensitive and/or change over the life of the gyroscope. As a result, the sensitivity shifts after installation of the gyroscope into the end-user&#39;s system or drifts over the life of the gyroscope. Because the true input stimuli into the gyroscope after installation are not observable, it is difficult to conveniently recalibrate the sensitivity of the gyroscope when the gyroscope is deployed in the field. 
     SUMMARY 
     An in-situ calibration system, method and apparatus are disclosed for motion-free calibration of a MEMS based gyroscope in-field. In an embodiment, a method of calibrating sensitivity of a MEMS based gyroscope comprises: driving a sense proof-mass of the MEMS based gyroscope with forces at the gyroscope drive frequency to generate quasi-Coriolis forces on the proof-mass; extracting electromechanical gain from MEMS based gyroscope; injecting a non-resonant carrier signal onto the proof-mass to extract additional changes in a sense capacitance and a drive capacitance of the MEMS based gyroscope; and using the additional changes to recalibrate sensitivity of the MEMS based gyroscope. 
     In an embodiment, a method of calibrating sensitivity of a MEMS based gyroscope comprises: at a first time: driving, by test electrodes, a sense proof-mass of the MEMS based gyroscope with a known force stimulus at the drive frequency; determining a first response of the proof-mass to the known force stimulus; at a second time after the first time: driving, by the test electrodes, the sense proof-mass of the MEMS based gyroscope with a known force stimulus at the drive frequency; determining a second response of the proof-mass to the known force stimulus; determining a difference in first and second responses of the proof-mass to the known force stimulus; and using the difference to recalibrate sensitivity of the MEMS based gyroscope. 
     Particular implementations disclosed herein provide one or more of the following advantages. The disclosed system and method allows for motion-free, recalibration of MEMS based gyroscope sensitivity over the life of a MEMS based gyroscope in-field. 
     The details of the disclosed implementations are set forth in the accompanying drawings and the description below. Other features, objects and advantages are apparent from the description, drawings and claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is an electromechanical model of a gyroscope with additional test electrodes (Cori-P and Cori-N) for calibrating sensitivity of a MEMS based gyroscope using drive feedthrough excitation and capacitance extraction, according to an embodiment. 
         FIG. 2  is a conceptual diagram illustrating a gyroscope drive circuit, according to an embodiment. 
         FIG. 3  is a diagram illustrating a gyroscope sense circuit, according to an embodiment. 
         FIGS. 4A and 4B  illustrates drive feedthrough calibration using a drive sense capacitance-to-voltage (C2V) converter output to generate the quasi-Coriolis excitation forces on the test electrodes, according to an embodiment. 
         FIG. 5  illustrates identifying unknown sense capacitance, according to an embodiment. 
         FIG. 6  illustrates identifying unknown drive capacitance, according to an embodiment. 
         FIGS. 7A and 7B  illustrate an example embodiment that uses the drive signal output of the drive a phase-locked loop (PLL) to generate the quasi-Coriolis excitation forces on the test electrodes, according to an embodiment. 
         FIG. 8  is an electromechanical model of a gyroscope with additional test electrodes (Quad-P and Quad-N) for calibrating sensitivity of a MEMS based gyroscope using electrostatically induced quadrature and capacitance extraction, according to an embodiment. 
         FIG. 9  is a conceptual diagram illustrating a gyroscope drive circuit, according to an embodiment. 
         FIG. 10  is a conceptual diagram illustrating a gyroscope sense circuit, according to an embodiment. 
         FIGS. 11A and 11B  illustrate quadrature tuning, according to an embodiment. 
         FIG. 12  is flow diagram of a process for calibrating a MEMS based gyroscope using drive feedthrough excitation and capacitance extraction, according to an embodiment 
         FIG. 13  is flow diagram of a process for calibrating a MEMS based gyroscope using electrostatically induced quadrature, according to an embodiment. 
         FIG. 14  is a block diagram of an electronic system architecture for implementing the MEMS based gyroscope described in reference to  FIGS. 1-6 , according to an embodiment. 
     
    
    
     The same reference symbol used in various drawings indicates like elements. 
     DETAILED DESCRIPTION 
     Overview 
     In a first embodiment, a calibration system and method is disclosed that uses test electrodes to stimulate a proof-mass of a MEMS based gyroscope at the gyroscope drive frequency as quasi-Coriolis forces to extract the electromechanical gain, and uses a non-resonant carrier signal on the proof-mass to extract the additional changes in the sense and drive capacitance. The supplier of the part would exercise the part in a calibration station to extract baseline gyroscope sensitivity values. After the calibration and before removing the part, the supplier would then run a calibration sequence to measure the electromechanical transfer function of the gyroscope using in-phase test electrodes and the sense and drive-sense capacitances. After installation into a system and/or sometime later in-field, the test sequences described above would be run again, and changes (deltas) to the output would be used to recalibrate the sensitivity of the gyroscope. 
     In a second embodiment, a calibration system and method is disclosed that uses quadrature electrodes to apply a known force stimulus to a gyroscope proof-mass as part of a calibration procedure, where the known force is applied again after installation into a system or further into the life of the gyroscope. Differences in the proof-mass response to the force are proportional to changes in sensitivity, which allows the sensitivity to be corrected in the field. As with the first embodiment, the calibration may also include a calibration sequence to measure the sense and drive-sense capacitances. 
     Gyroscope Sensitivity Calibration Using Drive Feedthrough Excitation and Capacitance Extraction 
       FIG. 1  is an electromechanical model of a calibration system  100  for calibrating sensitivity of a MEMS based gyroscope using drive feedthrough excitation and capacitance extraction, according to an embodiment. Model  100  includes drive mass  101  (m D ), drive-drive lateral comb electrodes  102 , sense (proof) mass  103  (m S ), sense springs  104   a - 104   d  (k S ), drive springs  105   a - 105   d  (k D ), drive-sense lateral comb electrodes  106 , sense electrodes  107   a,    107   b  and test electrodes  108   a - 108   d  (Cori-P, Cori-N). 
     In normal operation, lateral comb electrodes  102  apply linear electrostatic forces to drive mass  101 . For example, a DC bias voltage is applied to drive mass  101  and when an AC drive voltage is applied to lateral combs  102  at the drive frequency of drive mass  101 , the linear electrostatic forces excite drive mass  101  to oscillate along the drive axis (X) at the gyroscope&#39;s resonant frequency. Drive-sense lateral comb electrodes  106  sense output current from drive mass  101  moving on drive springs  105   a - 105   d.  This output current is proportional to the velocity of drive mass  101 . Sense mass  103  is disposed within an opening in drive mass  101  and moves on sense springs  104   a - 104   d.  Sense electrodes  107   a,    107   b  sense the sense direction (Y) displacement/deflection of sense mass  103  from its baseline position. Electrodes  108   a - 108   d  are used to excite drive mass  101  with quasi-Coriolis force, as described further below. 
       FIG. 2  is a conceptual diagram illustrating a gyroscope drive system  200 , according to an embodiment. Drive system  200  includes a closed-feedback control loop that includes automatic gain control (AGC)  201 , phase-locked loop (PLL)  205  and capacitance-to-voltage (C2V) converter  207 . The control loop is configured to maintain a constant velocity of drive mass  101  and to set the amplitude of the drive voltage. 
     AGC  201  is a closed-loop feedback regulating circuit that is configured to provide a controlled drive voltage (V DRV ) amplitude at its output. In an embodiment, AGC  201  includes integration amplifier  202  (CINT) and synchronous demodulators  203   a,    203   b.  Integration amplifier  202  is coupled to bias resistors  204   a,    204   b.  PLL  205  generates and provides clock signals clk_vel and clk_pos to demodulators  203   a,    203   b,  respectively. C2V converter  207  converts a varying capacitive charge q x  (coulombs) to a voltage signal (VDS), where q x  is caused by displacement/deflection x of sense mass  103  from its baseline position. C2V converter  207  includes feedback capacitor  206  (C CVD ) that integrates motion induced current from the drive-sense combs into voltage VDS. VDS is provided to an input of PLL  205  as a reference signal for generating clocks clk_vel, clk_pos. Demodulator  203   b  demodulates the displacement/deflection of sense mass  103  using clk_pos. The output of demodulator  203   b  is compared to V REF  to produce an error signal. This error signal is integrated with integrator  202  and then remodulated at drive frequency by mixing with the PLL clk_vel signal to produce drive voltage V DRV . 
     In normal operation, system  200  provides a force F DRV  on drive mass  101  given by: 
                         F   DRV     ~   2     ⁢   VD   *     V   DRV     *       dC   D     dX       ,           [   1   ]               
where C D  is the drive capacitance, x is the displacement of drive mass  101  from its baseline position, and VD is the bias voltage on the proof mass.
 
     The varying capacitive charge q x  (coulombs) due to the displacement/deflection x of sense mass  103  is given by: 
                       q   x     =     x   *     dC   dx     *   VD       ,           [   2   ]               
where,
 
               dC   dx     ,         
is the change in the drive-sense capacitance C due to the displacement/deflection x of drive mass  101 .
 
     Equations [3]-[6] further describe drive system  200  and the mechanical sensitivity of the gyroscope as follows: 
                       y   Ω     =     X     4   ⁢     π   2     ⁢   Δ   ⁢           ⁢     f   ⁡     (     1   -       Δ   ⁢           ⁢   f       f   DRV         )             ,           [   3   ]                   i   REF     =     i   x       ,           [   4   ]                   V   REF     =         R   REF       R   x       ⁢     V   x         ,           [   5   ]                   V   REF     =         R   REF       R   x       ⁢       q   x       C   CVD           ,           [   6   ]                   V   REF     =         R   REF       R   x       ⁢     X     C   CVD       ⁢     dC   dx     ⁢   VD       ,           [   7   ]                 x   =         R   x       R   REF       ⁢       C   CVD       dC   dx       ⁢       V   REF     VD         ,           [   8   ]               
where Ω is the actual angular rate, f DRV  is the gyroscope drive frequency, Δf is the delta frequency between drive and sense modes, and y is the sense displacement.
 
       FIG. 3  is a diagram illustrating gyroscope sense system  300 , according to an embodiment. Sense system  300  includes PLL  301 , analog-to-digital (ADC)  302 , demodulator  303  and C2V converter  304 . C2V converter  304  further includes feedback capacitors  305   a,    305   b  (C CVS ). 
     The varying capacitive charge q y  (coulombs) due to the displacement/deflection y of sense mass  103  is given by: 
                       q   y     =     y   *       dC   S     dy     *   VD       ,           [   9   ]               
where C S  is the sense capacitance from the sense electrodes  107   a,    107   b.  
 
     Equations [10]-[12] further describe sense system  300  and electromechanical sensitivity as follows: 
                       y   Ω     =     X     4   ⁢     π   2     ⁢   Δ   ⁢           ⁢     f   ⁡     (     1   -       Δ   ⁢           ⁢   f       f   DRV         )             ,           [   10   ]                   Rate_Out   Ω     =       X     4   ⁢     π   2     ⁢   Δ   ⁢           ⁢     f   ⁡     (     1   -       Δ   ⁢           ⁢   f       f   DRV         )           ⁢       dC   S     dy     ⁢   VD   ⁢     1     C   CVS           ,           [   11   ]                   Rate_Out   Ω     =       X     4   ⁢     π   2     ⁢   Δ   ⁢           ⁢     f   ⁡     (     1   -       Δ   ⁢           ⁢   f       f   DRV         )           ⁢         dC   S     dy       dC   dx       ⁢           ⁢       C   CVD       C   CVS       ⁢           ⁢       R   x       R   REF       ⁢     V   REF         ,           [   12   ]               
where Rate_Out is the measured angular rate, C CVS  is the offset sense capacitance and Ω is the actual angular rate. Note that the gyro sensitivity
 
             Rate_Out   Ω         
depends on many parameters and drift in any one of these parameters causes a sensitivity drift.
 
     Sense system  300  is configured to extract varying capacitance q y  due to y displacement/deflection of sense mass  103 . Differential sense electrodes  107   a,    107   b  generate varying capacitive charge q y , which is converted to an analog voltage signal by C2V converter  304 . The analog voltage signal is demodulated by synchronous demodulator  303 , which is coupled to clock signal clk_vel generated and provided by PLL  301 . Demodulator  303  extracts an analog rate signal from the analog voltage signal. The analog rate signal is converted to a digital rate signal by ADC  302  to produce the measured digital rate signal Rate_Out. 
       FIGS. 4A and 4B  illustrate drive feedthrough quasi-Coriolis excitation, according to an embodiment. Differential Coriolis excitation signals (Cori_P, Cori_N) are applied to differential test electrodes  108   a - 108   d  to generate quasi-Coriolis excitation forces on sense mass  103 . In an embodiment, the Coriolis excitation signals (Cori_P, Cori_N) can be generated from drive system  200 . For example, switch  401  can be controlled by switch selection signal (cal_sel) to generate Coriolis excitation signals that have 90° and 270° phases relative to the phase of the output voltage VDS of C2V  207 . In an embodiment, cal_sel can be generated by logic implemented in hardware (e.g., an ASIC) and/or software. 
     Equations [13]-[15] further describe the generation of a Rate_Out when quasi-Coriolis excitation is applied: 
                       V   DS     =         R   x       R   REF       ⁢     V   REF         ,           [   13   ]                   F   Cori     =     4   ⁢       dC   S     dy     ⁢     V   D     ⁢     V   DS         ,           [   14   ]                 Rate_Out   ∼             4   ⁢           ⁢     dC   S           f     DRVm   S       ⁢           ⁢   dy       ⁢     V   D     ⁢     V   DS         4   ⁢     π   2     ⁢   Δ   ⁢           ⁢     f   ⁡     (     1   -       Δ   ⁢           ⁢   f       f   DRV         )           ⁢       dC   S     dy     ⁢     V   D     ⁢     1     C   CVS           ,           [   15   ]               
where F Cori  is the electrostatic force generated by the quasi-Coriolis excitation and m S  is the sense mass  103 .
 
     Referring to Equation [15], there are several drawbacks with using drive feedthrough quasi-Coriolis excitation: 1) sensitivity change due to a change in drive capacitance (dC/dx) is not observable, 2) the change in sense capacitance (dC S /dy) varies to the second power and 3) output varies with bias voltage. 
       FIG. 5  illustrates a method for identifying unknown sense capacitance C S , between the proof mass and the sense electrodes according to an embodiment. In this mode, the gyroscope stops moving and the static capacitance is being measured. In this example embodiment, switch  500   c  switches the proof mass voltage from DC voltage VD to the output of PLL  301 , switch  500   a  disconnects the PLL from using the drive C2V output, and switch  500   b  switches PLL  301  to accept commands to set the VCO to a fixed frequency freq_tst. Charge is generated by the PLL voltage on the static capacitance C S  between proof mass  103  and sense electrodes  107   a  and  107   b.  The charge is converted to a voltage using sense C2V converter  304 . The output of sense C2V converter  304  is demodulated by rate demodulator  303 , which uses one of clock signal clk_pos or a frequency test signal (freq_tst) to extract an analog test signal proportional to the capacitance. The clock signal and frequency test signal are generated and provided by PLL  301 . The analog rate signal is converted by ADC  302  into a digital test signal (Rate_Out). In an embodiment, switches  500   a,    500   b,  and  500   c  are controlled by test signal tst_sel, which is generated by logic implemented in hardware (e.g., ASIC) and/or software. In an embodiment, the freq_test signal has a frequency that lies on the flat area (stable or constant frequency range) of a frequency response curve of drive mass  101 . 
     The configuration shown will determine changes in a delta gap g between sense electrodes  107   a,    107   b  (parallel plate electrodes) due to a position change of sense mass  103 . In an embodiment, if there is a suspected common mode gap change, sense C2V converter  304  is configured to switch from differential amplification to single-ended amplification and then switch between positive and negative electrodes. In an embodiment, an attenuation stage can be placed between PLL  301  and drive mass  101  (e.g., a divider). 
       FIG. 6  illustrates identifying unknown drive capacitance C, according to an embodiment. In this mode, the gyroscope is stationary and the static capacitance is being measured. In this example embodiment, switch  600   c  switches the proof mass voltage from DC voltage VD to the output of PLL  205 , switch  600   a  disconnects the PLL from using the drive C2V output, switch  600   b  switches PLL  205  to accept commands to set the VCO to a fixed frequency freq_tst, switch  600   d  disconnects the AGC from the drive loop, and switch  600   e  connects demodulator  203   b  output to an ADC. The PLL voltage on the static capacitance C between proof mass  103  and drive-sense electrode  106  generates charge. The charge is converted to a voltage using drive-sense C2V converter  207 . The output of drive-sense C2V converter  207  is demodulated by rate demodulator  203   b,  which uses one of clock signal clk_pos or a frequency test signal (freq_tst) to extract an analog test signal proportional to the capacitance. The analog rate signal is converted by ADC  600  into a digital test signal that is stored in memory for compensating sensitivity due to drive-sense capacitance change. Switches  600   a,    600   c,    600   d  and  600   e  are controlled by the test selection signal (tst_sel). In an embodiment, tst_sel is generated by logic implemented in hardware (e.g., ASIC) and/or software. In an embodiment, freq_test can have a frequency that lies on a flat area of a frequency response curve of drive mass  101 . 
       FIGS. 7A and 7B  illustrate an example embodiment that uses a PLL to generate velocity clock signals for quasi-Coriolis excitation, according to an embodiment. Referring to  FIG. 7A , clock signals (clk_vel_p, clk_vel_n, clk_pos_p, clk_pos_n) are shown being applied to differential electrodes  108   a - 108   d  to excite drive mass  101  with quasi-Coriolis forces to induce quasi-Coriolis excitation on sense mass  103 . Also shown is sense mass  103  and differential sense electrodes  107   a,    107   b  fore sensing displacement/deflection of sense mass  103 .  FIG. 7B  illustrates an embodiment for generating differential velocity clock signals (clk_vel_p, clk_vel_n) using PLL  301  and phase inverter  700 , such that clk_vel_n is 180° out of phase with clk_vel_p. A disadvantage with this design is that the quasi-Coriolis excitation signals are not proportional to drive amplitude. 
     Gyroscope Sensitivity Calibration Using Electrostatically Induced Quadrature 
     The embodiment described below uses quadrature test electrodes to apply a known force stimulus to the proof-mass as part of the normal calibration where the known force can then be applied again after installation of the gyroscope in a system or further into the life of the gyroscope. Differences in proof-mass response to the force will be proportional to the changes in sensitivity, which allows the sensitivity to be corrected in-field. 
       FIG. 8  is an electromechanical model  800  for calibrating a MEMS based gyroscope using electrostatically induced quadrature, according to an embodiment. Model  800  includes drive mass  801  (m D ), drive-drive lateral comb electrodes  802 , sense (proof) mass  803  (m S ), sense springs  804   a - 804   d  (k S ), drive springs  805   a - 805   d  (k D ), drive-sense lateral comb electrodes  806 , sense electrodes  807   a,    807   b  and test electrodes  808   a - 808   d  (Quad-P, Quad-N). 
     In normal operation, lateral comb electrodes  802  apply linear electrostatic forces to drive mass  801 . For example, a DC bias voltage is applied to drive mass  801  and when an AC drive voltage is applied to lateral combs  802  at the drive frequency of drive mass  801 , the linear electrostatic forces excite drive mass  801  to oscillate along the drive axis (X) at the gyroscope&#39;s resonant frequency. Drive-sense lateral comb electrodes  806  sense output current from drive mass  801  moving on drive springs  805   a - 805   d.  This output current is proportional to the velocity of drive mass  801 . Sense mass  803  is disposed within an opening in drive mass  801  and moves on sense springs  804   a - 804   d.  Sense electrodes  807   a,    807   b  sense the sense direction (Y) displacement/deflection of sense mass  103  from its baseline position. Electrodes  808   a - 808   d  are used to apply DC control voltages that are modulated by the motion of the drive mass, which applies quasi-Coriolis forces onto mass  803 , as described further below. 
       FIG. 9  is a conceptual diagram illustrating a gyroscope drive system  900 , according to an embodiment. Drive system  900  includes a closed-feedback control loop that includes automatic gain control (AGC)  901 , phase-locked loop (PLL)  905  and capacitance-to-voltage (C2V) converter  907 . The control loop is configured to maintain a constant velocity of drive mass  801  and to set the amplitude of the drive voltage. 
     AGC  901  is a closed-loop feedback regulating circuit that is configured to provide a controlled drive voltage (V DRV ) amplitude at its output. In an embodiment, AGC  901  includes integration amplifier  902  (CINT) and synchronous demodulators  903   a,    903   b.  Integration amplifier  902  is coupled to bias resistors  904   a,    904   b.  PLL  905  generates and provides clock signals clk_vel and clk_pos to demodulators  903   a,    903   b,  respectively. C2V converter  907  converts a varying capacitive charge q x  (coulombs) to a voltage signal (VDS), where q x  is caused by displacement/deflection x of sense mass  803  from its baseline position. C2V converter  907  includes feedback capacitor  906  (C CVD ) that integrates the input charge into a voltage. VDS is provided to an input of PLL  905  as a reference signal for generating clocks clk_vel, clk_pos. Demodulator  903   b  demodulates the displacement/deflection of drive mass  801  using clk_pos. The output of demodulator  903   b  is compared to V REF  to product an error signal. This error signal is integrated with integrator  902  and then remodulated at drive frequency by mixing with clk_vel to, produce drive voltage V DRV . 
     In operation, drive system  900  provides a force F DRV  on drive mass  101  given by: 
                       F   DRV     ∼     2   ⁢   VD   *     V   DRV     *       dC   D     dX         ,           [   16   ]               
where C D  is the drive capacitance and x is the displacement of drive mass  801  from its baseline position.
 
     The varying capacitive charge q x  due to the displacement/deflection x of drive mass  801  is given by: 
                       q   x     =     x   *     dC   dx     *   VD       ,           [   17   ]               
where
 
             dC   dx         
is the change in sense capacitance C due to the displacement/deflection x of drive mass  801 .
 
     Equations [18]-[23] further describe drive system  900  and the mechanical sensitivity of the gyroscope as follows: 
                       y   Ω     =     X     4   ⁢     π   2     ⁢   Δ   ⁢           ⁢     f   ⁡     (     1   -       Δ   ⁢           ⁢   f       f   DRV         )             ,           [   18   ]                   i   REF     =     i   x       ,           [   19   ]                   V   REF     =         R   REF       R   x       ⁢     V   x         ,           [   20   ]                   V   REF     =         R   REF       R   x       ⁢       q   x       C   CVD           ,           [   21   ]                   V   REF     =         R   REF       R   x       ⁢     X     C   CVD       ⁢     dC   dx     ⁢   VD       ,           [   22   ]                 x   =         R   x       R   REF       ⁢       C   CVD       dC   dx       ⁢       V   REF     VD         ,           [   23   ]               
where Ω is the actual angular rate, f DRV  is the gyroscope drive frequency, Δf is the delta frequency between drive and sense modes, and y is the sense displacement.
 
     Referring to Equations [18]-[23], it is a design goal for displacement/deflection x to remain constant. But the displacement/deflection x is inferred from other measurements that may be prone to error. 
       FIG. 10  is a diagram illustrating gyroscope sense system  1000 , according to an embodiment. Sense system  1000  includes PLL  1001 , analog-to-digital (ADC)  1002 , demodulator  1003  and C2V converter  1004 . C2V converter  1004  further includes capacitors  1005   a,    1005   b  (C CVS ). 
     The varying capacitive charge q y  (coulombs) due to the displacement/deflection x of sense mass  803  is given by: 
                       q   y     =     y   *       dC   S     dy     *   VD       ,           [   24   ]               
where C S  is the sense capacitance from the sense electrodes  107   a,    107   b.  
 
     Equations [25]-[27] further describe sense system  300  and the electromechanical sensitivity of the gyroscope as follows: 
                       y   Ω     =     X     4   ⁢     π   2     ⁢   Δ   ⁢           ⁢     f   ⁡     (     1   -       Δ   ⁢           ⁢   f       f   DRV         )             ,           [   25   ]                   Rate_Out   Ω     =       X     4   ⁢     π   2     ⁢   Δ   ⁢           ⁢     f   ⁡     (     1   -       Δ   ⁢           ⁢   f       f   DRV         )           ⁢       dC   S     dy     ⁢   VD   ⁢     1     C   CVS           ,           [   26   ]                   Rate_Out   Ω     =       X     4   ⁢     π   2     ⁢   Δ   ⁢           ⁢     f   ⁡     (     1   -       Δ   ⁢           ⁢   f       f   DRV         )           ⁢         dC   S     dy       dC   dx       ⁢       C   CVD       C   CVS       ⁢       R   x       R   REF       ⁢     V   REF         ,           [   27   ]               
where Rate_Out is the measured angular rate, C CVS  is the offset sense capacitance and Ω is the actual angular rate. Note that the gyro sensitivity
 
             Rate_Out   Ω         
depends on many parameters and drift in any one of these parameters causes a sensitivity drift.
 
     Sense system  1000  is configured to extract varying capacitance q y  due to y displacement/deflection of sense mass  803 . Differential sense electrodes  807   a,    807   b  generate varying capacitance q y , which is converted to an analog voltage signal by C2V converter  1004 . The analog voltage signal is demodulated by synchronous demodulator  1003 , which is coupled to clock signal clk_vel generated and provided by PLL  1001 . Demodulator  1003  extracts an analog rate signal from the analog voltage signal. The analog rate signal is converted to a digital rate signal by ADC  1002  to produce the measured digital rate signal Rate_Out. 
       FIGS. 11A and 11B  illustrate quadrature tuning, according to an embodiment. Differential DC voltages (V Q , −V Q ) are applied to differential electrodes  808   a - 808   d.    
     Equations [28]-[31] further describe the quadrature tuning by the test electrodes and the subsequent generation of a Rate_Out signal when quadrature tuning voltages V Q  and −V Q  are applied: 
                     K   =         [           k   xx           k   xy               k   xy           k   yy           ]     Mech     =       ∑     i   =   1     Nelec     ⁢       [             δ   dx     ⁢     (       C     s   ⁢   _   ⁢   i       dx     )               δ   dy     ⁢     (       C     s   ⁢   _   ⁢   i       dx     )                   δ   dx     ⁢     (       C     s   ⁢   _   ⁢   i       dx     )               δ   dy     ⁢     (       C     s   ⁢   _   ⁢   i       dx     )             ]     ⁢   Δ   ⁢           ⁢     V   i   2             ,           [   28   ]                 K   =         [           k   xx           k   xy               k   xy           k   yy           ]     Mech     +         4   ⁢     ɛ   0         ℊ   2       ⁡     [         0           V   Q     ⁢     V   D     ⁢   thk                 V   Q     ⁢     V   D     ⁢   thk             -       C   S     ℊ       ⁢     (       V   D   2     +     V   Q   2       )             ]           ,           [   29   ]                 y   ∼         (         -   j     ⁢           ⁢   Ω     +         k   xy     -         4   ⁢     ɛ   0     ⁢   thk       ℊ   2       ⁢     V   Q     ⁢     V   D             m   s     ⁢     f   DRV           )     ⁢   X       4   ⁢     π   2     ⁢   Δ   ⁢           ⁢     f   ⁡     (     1   -       Δ   ⁢           ⁢   f       f   DRV         )             ,           [   30   ]                   Δ   ⁢           ⁢   f     ∼       Δ   ⁢           ⁢     f   0       -     f   ⁡     (     (       V   D   2     +     V   Q   2       )     )           ,           [   31   ]               
where k xy  is the stiffness along the drive direction, k yy  is stiffness along the sense direction, and k xy  is a quadrature term due to stiffness coupling between drive and sense. In this approach, the quasi-Coriolis force is generated by the DC voltage and the motion of the drive mass.
 
     In an embodiment, V Q  is applied during initial calibration of the gyroscope to calibrate the output response to voltage transfer function (dps/V Q ). Later after installation in a system or in the field, V Q  is applied again and the transfer function is re-calculated. Any changes from initial calibration are due to changes in sensitivity and this delta value is either applied immediately or stored in memory for compensation of the sensitivity. In another embodiment, this technique can be used to extract the phase or sensitivity through a delta quadrature measurement. 
     Example Processes 
       FIG. 12  is flow diagram of process  1200  for calibrating a MEMS based gyroscope using drive feedthrough excitation and capacitance extraction, according to an embodiment. Process  1200  can be implemented using, for example, electronic system  1400  shown in  FIG. 14 . 
     Process  1200  can begin by driving a proof-mass of a MEMS based gyroscope at the gyroscope drive frequency to generate quasi-Coriolis forces on the proof-mass ( 1201 ). Process  1200  continues by extracting electromechanical gain from the MEMS based gyroscope ( 1202 ). Process  1200  continues by driving the proof-mass with a non-resonant carrier signal to extract additional changes in a sense capacitance and a drive capacitance of the MEMS based gyroscope ( 1203 ). Process  1200  continues by using the gain and additional changes to recalibrate sensitivity of the MEMS based gyroscope ( 1204 ). 
       FIG. 13  is flow diagram of process  1300  for calibrating a MEMS based gyroscope using electrostatically induced quadrature, according to an embodiment. Process  1300  can be implemented using, for example, electronic system  1400  shown in  FIG. 14 . 
     Process  1300  can begin by, at a first time, driving, by quadrature test electrodes a proof-mass of a MEMS based gyroscope at a drive frequency to apply a known force stimulus to the proof-mass ( 1301 ). Process  1300  continues by determining a first response of the proof-mass to the known force stimulus ( 1302 ). Process  1300  continues by, at a second time after the first time, driving, by the quadrature test electrodes, the proof-mass of the MEMS based gyroscope at the drive frequency to apply the known force stimulus to the proof-mass ( 1303 ). Process  1300  continues by determining a second response of the proof-mass to the known force stimulus ( 1304 ). Process  1300  continues by determining a difference in first and second responses of the proof-mass to the know force stimulus ( 1305 ). Process  1300  continues by using the difference to recalibrate sensitivity of the MEMS based gyroscope ( 1306 ). 
     Example System Architecture 
       FIG. 14  is a block diagram of an electronic system architecture for implementing the MEMS based gyroscope described in reference to  FIGS. 1-13 , according to an embodiment. Architecture  1400  can be included in any electronic device that uses motion sensors, including but not limited to: smartphones, tablet computers, wearable devices (e.g., a smart watch) and automotive systems. 
     Architecture  1400  includes processor(s), memory interface  1402 , peripherals interface  1403 , motion sensors  1404   a . . .    1404   n,  display device  1405  (e.g., touch screen, LCD display, LED display), I/O interface  1406  and input devices  1407  (e.g., touch surface/screen, hardware buttons/switches/wheels, virtual or hardware keyboard, mouse). Memory  1412  can include high-speed random access memory and/or non-volatile memory, such as one or more magnetic disk storage devices, one or more optical storage devices and/or flash memory (e.g., NAND, NOR). 
     Memory  1412  stores operating system instructions  1408 , sensor processing instructions  1409  and application instructions  1412 . Operating system instructions  1408  include instructions for implementing an operating system on the device, such as iOS, Darwin, RTXC, LINUX, UNIX, WINDOWS, or an embedded operating system such as VxWorks. Operating system instructions  1408  may include instructions for handling basic system services and for performing hardware dependent tasks. Sensor-processing instructions  1409  perform post-processing on motion sensor data (e.g., averaging) and provide control signals to motion sensors. Application instructions  1410  implement software programs that use data from one or more motion sensors  1404   a . . .    1404   n,  such as navigation, digital pedometer, tracking or map applications. At least one motion sensor  1404   a  is a capacitive-based MEMS based gyroscope that operates as described in reference to  FIGS. 1-13 . 
     For example, in a navigation application executed on a smartphone, angular rate data is provided by the MEMS based gyroscope to processor(s)  1401  through peripheral interface  1403 . Processor(s)  1401  execute sensor-processing instructions  1409 , to perform further processing of the angular rate data (e.g., averaging). Processor(s)  1401  execute instructions for various applications running on the smartphone. For example, the angular rate data can be used to determine a more accurate orientation of the smartphone in a reference coordinate system (e.g., a direction heading or attitude). The more accurate orientation can be used by the navigation/compass application to perform a variety of navigation functions (e.g., direction heading, turn-by-turn instructions). Other applications are also possible (e.g., gaming applications, camera applications, beamforming, calibrating other sensors). 
     While this document contains many specific implementation details, these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a sub combination or variation of a sub combination. Logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. In addition, other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Accordingly, other implementations are within the scope of the following claims.

Metadata:
Filing Date: 20180928
Publication Date: 20201117
Grant Date: 20201117
Priority Date: 20180306
Inventors: PAINTER, CHRISTOPHER C.
TSANG, SEE-HO
Assignee: APPLE INC
CPC Classifications: [{"code": "G01C25/005", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01C25/005", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01C19/5719", "inventive": false, "first": false, "tree": "[]"}, {"code": "B81B2201/0242", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01C19/5712", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01C19/5776", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81B2203/04", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01C19/5712", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81B2203/04", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01C19/5776", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01C25/005", "inventive": true, "first": true, "tree": "[]"}, {"code": "B81B2201/0242", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 67843810