PATENT DOCUMENT

Publication Number: US-10732199-B2
Application Number: US-201715849542-A
Country: US
Kind Code: B2

Title: Multi-stage MEMS accelerometer for mixed g-level operation

Abstract:
A multi-stage MEMS accelerometer is disclosed that includes a MEMS sensor that has two suspended structures (proof masses) suspended by suspension members. The suspended structures move together in response to input acceleration when less the acceleration is less than a threshold value. When the input acceleration is greater than the threshold value, one of the suspended structures makes contact with a mechanical stop while the other suspended structure continues to move with increased stiffness due to the combined stiffness of the suspension members. The contact with the mechanical stop contributes a nonlinear mechanical stiffening effect that counteracts the nonlinear capacitive effect inherent in capacitive based MEMS accelerometers. In some embodiments, more than two suspended structures can be used to allow for optimization of sensitivity for multiple full-scale ranges, and for higher fidelity tuning of mechanical sensitivity with nonlinear capacitance. In some embodiments, compliant mechanical stops are used.

Claims:
What is claimed is: 
     
       1. A micro-electromechanical systems (MEMS) accelerometer comprising:
 a substrate; 
 one or more mechanical stops; 
 a first suspended structure attached to, or formed in, the substrate by a first suspension member having a first stiffness and configured to move in response to input acceleration being less than a first threshold level; 
 a second suspended structure attached to the first suspended structure by a second suspension member having a second stiffness, the second suspended structure configured to move in response to the input acceleration being less than the first threshold level and to make contact with the one or more mechanical stops when the input acceleration is more than the first threshold level, and wherein the first suspended structure is configured to continue to move on the first and second suspension members with increased stiffness after the contact is made; and 
 a readout circuit configured to measure the input acceleration based on movement of the first suspended structure. 
 
     
     
       2. The MEMS accelerometer of  claim 1 , wherein the first suspended structure is suspended in an opening of the second suspended structure. 
     
     
       3. The MEMS accelerometer of  claim 1 , wherein the one or more mechanical stops are mechanically compliant to reduce rebound effects after the contact is made. 
     
     
       4. The MEMS accelerometer of  claim 1 , wherein the first or second suspension member is a torsion bar. 
     
     
       5. The MEMS accelerometer of  claim 1 , wherein MEMS accelerometer is a capacitive based MEMS accelerometer and the contact of the second suspended structure with the one or more mechanical stops counteracts a nonlinear capacitive effect inherent in the MEMS accelerometer. 
     
     
       6. The MEMS accelerometer of  claim 1 , wherein the first and second stiffness are different. 
     
     
       7. The MEMS accelerometer of  claim 1 , wherein the input acceleration is angular acceleration. 
     
     
       8. The MEMS accelerometer of  claim 1 , further comprising:
 N additional suspended structures attached to the first and second suspended structures by N additional suspension members, the N additional suspended structures configured to move in response to the input acceleration being less than N different threshold levels and to make contact with the one or more mechanical stops when the input acceleration is more than any one of the N threshold levels, and wherein the first suspended structure is configured to continue to move on the first and second suspension members with increased stiffness after the contact is made, where N is a positive integer greater than two. 
 
     
     
       9. The MEMS accelerometer of  claim 1 , wherein the MEMS accelerometer is a capacitive based MEMS accelerometer and the first suspended structure is one electrode of a capacitor. 
     
     
       10. The MEMS accelerometer of  claim 1 , wherein the MEMS accelerometer is a capacitive based MEMS accelerometer and electrodes of the capacitor are attached to, or formed in, the substrate and positioned under the first and second suspended structures. 
     
     
       11. The MEMS accelerometer of  claim 1 , wherein the readout circuit measures differential capacitance.

Description:
TECHNICAL FIELD 
     This disclosure relates generally to capacitive based micro-electromechanical systems (MEMS) accelerometers. 
     BACKGROUND 
     Higher full-scale ranges in capacitive based MEMS accelerometers are desirable for applications that involve high dynamics. In some cases, these high dynamics only occur on rare occasions. The accelerometer designer, however, typically has to design the MEMS sensor to accommodate the highest dynamics regardless of how often it occurs. Enabling these higher full-scale ranges normally requires MEMS sensor design changes, such as making the MEMS sensor stiffer or increasing the capacitive gap. These MEMS sensor design changes may result in degraded sensitivity of the MEMS sensor, resulting in higher signal-to-noise (SNR) or worse strain immunity. 
     SUMMARY 
     A multi-stage MEMS accelerometer for mixed g-level operation is disclosed. In an embodiment, a micro-electromechanical systems (MEMS) accelerometer comprises: a substrate; one or more mechanical stops; a first suspended structure attached to, or formed in, the substrate by a first suspension member having a first stiffness and configured to move in response to input acceleration being less than a first threshold level; a second suspended structure attached to the first suspended structure by a second suspension member having a second stiffness, the second suspended structure configured to move in response to the input acceleration being less than the first threshold level and to make contact with the one or more mechanical stops when the input acceleration is more than the first threshold level, and wherein the first suspended structure is configured to continue to move on the first and second suspension members with increased stiffness after the contact is made; and a readout circuit configured to measure the input acceleration based on movement of the first suspended structure. 
     In an embodiment, an electronic system comprises: a multi-stage micro-electromechanical systems (MEMS) accelerometer that includes a MEMS sensor that has two structures suspended by suspension members, the suspended structures configured to move together in response to input acceleration when the input acceleration is less than a threshold value, and when the input acceleration is greater than the threshold value, one of the suspended structures makes contact with one or more mechanical stops while the other suspended structure continues to move with increased stiffness due to the combined stiffness of the suspension members; a readout circuit configured to: convert a change in differential capacitance between electrodes of the MEMS sensor to differential voltage signals; and calculate acceleration data based on the differential voltage signals; one or more processors; memory coupled to the one or more processors and storing instructions that when executed by the one or more processors, cause the one or more processors to perform operations comprising: obtaining the acceleration data from the MEMS accelerometer; calculating a location of the electronic system using the acceleration data; and displaying the location on a display device of the electronic system. 
     Particular implementations disclosed herein provide one or more of the following advantages. A multi-stage MEMS accelerometer allows for higher full-scale ranges for applications that involve high dynamics (e.g., &gt;32 g, 1 g=9.8 m/s 2 ) without degrading the sensitivity of the MEMS sensor or worsening the immunity of the MEMS sensor to strain. Moreover, the contact of one or more suspended structures during high dynamics contributes to a mechanical stiffening of the other sense mass that counteracts the nonlinear capacitive effect inherent in capacitive based MEMS accelerometers. 
     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 
         FIGS. 1A and 1B  are conceptual drawings illustrating the dynamics of a two-stage MEMS accelerometer for mixed g-level operations, according to an embodiment. 
         FIGS. 2A and 2B  are plots illustrating the stabilizing effect that mechanical stiffening nonlinearity has on capacitive nonlinearity, according to an embodiment. 
         FIGS. 3A and 3B  are top views of an out-of-plane and in-plane, respectively, two-stage, capacitive based MEMS sensor, according to an embodiment. 
         FIG. 4  is a conceptual drawing illustrating the dynamics of a multi-stage MEMS accelerometer for mixed g-level operations, according to an embodiment. 
         FIG. 5  is a conceptual drawing illustrating the dynamics of a two-stage MEMS accelerometer for mixed g-level operations with a compliant mechanical stop that reduces a rebound effect after contact, according to an embodiment. 
         FIG. 6  is a conceptual drawing illustrating the dynamics of an angular MEMS accelerometer for mixed g-level operations, according to an embodiment. 
         FIG. 7  is a flow diagram of a readout circuit for processing differential capacitance of a MEMS accelerometer for mixed g-level operations, according to an embodiment. 
         FIG. 8  is architecture for an electronic system that uses a multi-stage, MEMS accelerometer for mixed g-level operations, according to an embodiment. 
     
    
    
     The same reference symbol used in various drawings indicates like elements. 
     DETAILED DESCRIPTION 
     System Overview 
     A multi-stage MEMS accelerometer is disclosed that includes a MEMS sensor that has two structures (proof masses) suspended by suspension members. When an input acceleration is less than a threshold value, the suspended structures move together. When the input acceleration is greater than the threshold value, one of the suspended structures makes contact with one or more mechanical stops while the other suspended structure continues to move with increased stiffness due to the combined stiffness of the suspension members. The contact with the one or more mechanical stops contributes a nonlinear mechanical stiffening effect that counteracts the nonlinear capacitive effect inherent in capacitive based MEMS accelerometers. In some embodiments, more than two suspended structures can be used to allow for optimization of sensitivity for multiple full-scale ranges, and for higher fidelity tuning of mechanical sensitivity with nonlinear capacitance. In some embodiments, gradual stopping is used (e.g., compliant mechanical stoppers) instead of abrupt stopping to reduce the rebound effect after a collision of a suspended structure with a mechanical stop. 
     System Dynamics 
       FIGS. 1A and 1B  are conceptual drawings illustrating the dynamics of a two-stage MEMS accelerometer for mixed g-level operations, according to an embodiment. System  100  includes substrate  101 , sense mass  102   a  (m 1 ), mass  102   b  (m 2 ), springs  104   a ,  104   b , mechanical stops  105   a ,  105   b  and electrodes,  106   a - 106   d.    
     Referring to  FIG. 1A , when the input acceleration ÿ is below a threshold value (e.g., ÿ&lt;32 g), both masses  102   a ,  102   b  move in response to the input acceleration. When mass  102   b  is not in contact with mechanical stop  105   a  the dynamics of system  100  are described by the following equations:
 
 m   1   {umlaut over (x)}   1   +k   1   x   1   +k   2 ( x   1   −x   2 )− k   1   y= 0,  [1]
 
 m   2   {umlaut over (x)}   1   +k   2   x   2   −k   1   y= 0,  [2]
 
 z=x   1   −y,   [3]
 
 w=x   1   −x   2 ,  [4]
 
 m   1   {umlaut over (z)}+k   1   z+k   2   w=−m   1   ÿ   [5]
 
 m   2   {umlaut over (z)}−m   2   {umlaut over (w)}−k   2   w=−m   2   ÿ,   [6]
 
where x 1 , {dot over (x)} 1 , {umlaut over (x)} 1  are the position, velocity and acceleration, respectively, of mass  102   a  x 2 , {dot over (x)} 2 , {umlaut over (x)} 2  are the position, velocity and acceleration, respectively, of mass  102   b  and  k   1  and k 2  are spring constants for springs  104   a ,  104   b , respectively, and where y, {dot over (y)}, ÿ are the position, velocity and acceleration, respectively, of substrate  101  rigidly attached to the system where acceleration is being measured, and z is the relative displacement of mass  102   a  from substrate  101 .
 
     When system  100  is operating in steady state, the dynamic equations for system  100  above reduce to:
 
 k   1   z+k   2   w=−m   2   ÿ   [7]
 
− k   2   w=−m   2   ÿ   [8]
 
     
       
         
           
             
               
                 
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     Referring to  FIG. 1B , when acceleration input ÿ is above the threshold value (e.g., ÿ&gt;32 g), mass  102   b  makes contact with mechanical stop  105   a , but sense mass  102   a  continues to move on springs  104   a ,  104   b  with increased mechanical stiffness. When mass  102   b  makes contact with mechanical stop  105   a  the dynamics of system  100  are described by the following equation:
 
 m   1   {umlaut over (z)} +( k   1   +k   2 ) z=−m   1   ÿ.   [10]
 
     When system  100  is operating in steady state, the dynamic equations for system  100  above reduce to: 
     
       
         
           
             
               
                 
                   
                     
                       
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     Accordingly, the acceleration scale factors (SF) are given by: 
     
       
         
           
             
               
                 
                   
                     
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     As shown by the scale factor equations above, when system  100  is detecting less than 32 g of acceleration, SF 1  is a function of the sum of the two masses m 1 , m 2  and the single spring constant k 1 . When system  100  is detecting more than 32 g, mass m 2  is in contact with mechanical stop  105   a , and SF 2  is a function of the sense mass m 1  and the two spring constants k 1 , k 2 , resulting in a mechanical stiffening of system  100 . 
     Note that the example above includes a single threshold value of 32 g. Those with ordinary skill in the art, however, will recognize that system  100  can be designed with any desired threshold value, and that multiple threshold values or threshold ranges can be used in place of the example threshold value of 32 g. Also, as described in reference to  FIG. 4 , any number of masses can be used in system  100 . In some embodiments, the actual values of the masses and spring constants can be determined through simulation or empirically based on the application and a desired acceleration threshold value. 
       FIGS. 2A and 2B  are plots illustrating the stabilizing effect that mechanical stiffening nonlinearity has on capacitive linearity, according to an embodiment. 
     Referring to  FIG. 2A , a plot of deflection versus acceleration with contact at 32 g is shown. The steady state deflection Z can be determined using the following equation: 
                   Z   =         -     (       m   1     +     m   2       )         k   1       ⁢           ⁢             a   in     +       [           (       m   1     +     m   2       )       k   1       ⁢     a   in       -         (       m   1     +     m   2       )       k   1       ⁢     (     32   ⁢           ⁢   g     )         ]     ⁢           ⁢     H   (           ⁢       a   in     -     32   ⁢           ⁢   g       )       +               [         -     m   1           k   1     +     k   2         ⁢     (       a   in     -     32   ⁢           ⁢   g       )       ]     ⁢     H   ⁡     (       a   in     -     32   ⁢           ⁢   g       )         ,                     [   15   ]               
where H is a Heaviside step function that is discontinuous and has a value of zero for negative argument and one for positive argument and a in  is the input acceleration. As can be observed in  FIG. 2A , after the input acceleration exceeds 32 g, the slope of deflection is reduced due to increased stiffening caused by the combined stiffness of the two springs when mass  102   b  makes contact with mechanical stop  105   a . The spring constants can be the same or they can be different based on different design tradeoffs (e.g., thresholds, sensitivity, etc.).
 
     Referring to  FIG. 2B , plots of capacitance change versus input acceleration with contact at 32 g is shown. The three plots shown are for multistage accelerometer output, a 3 rd  order fit to input acceleration typical of a conventional accelerometer and an ideal linear response to input acceleration. The capacitance change can be determined using the following equation derived from a Taylor series expansion carried out to the 3 rd  power: 
                       Δ   ⁢           ⁢   C     =           2   ⁢     C   0       d     ⁢   z     +         2   ⁢     C   0         d   3       ⁢     z   3           ,           [   16   ]               
where C 0  is the rest capacitance, z is the deflection and d is the gap spacing. As can be observed from the plots the mechanical stiffening nonlinearity has a stabilizing effect on the capacitive nonlinearity inherent in conventional accelerometers.
 
       FIGS. 3A and 3B  are top views of an out-of-plane and in-plane, respectively, two-stage, capacitive based MEMS sensor, according to an embodiment. Capacitive based MEMS accelerometers measure changes of the capacitance between a proof mass and a fixed conductive electrode separated by a narrow capacitive gap. 
       FIG. 3A  depicts an out-of-plane torsional type accelerometer where first suspended structure  301   a  has a mass m 1  and second suspended structure  302   a  has mass m 2 . The suspended structures are made of semiconductor material (e.g., polysilicon). Each structure  301   a ,  302   a  has a heavy side and a light side, which causes structures  301   a ,  302   a  to torsionally deflect in the z direction about anchor  304  in response to input acceleration.  FIG. 3B  depicts an in-plane linear motion type accelerometer where first suspended structure  301   b  has a mass m 3  and second suspended structure  302   b  has a mass m 4 . Each structure  301   b ,  302   b  deflects linearly in the x/y plane in response to input acceleration. 
     Structures  301   a ,  301   b  are attached to suspension members  305   a ,  305   b  and move within an opening (e.g., a window, cavity, recess or hole) formed in structures  302   a ,  302   b , respectively, in response to input acceleration being less than a threshold value. Suspension members  305   a ,  305   b  are attached to substrate  308  by anchor  304 . Suspension members  305   a ,  305   b  each have a stiffness that is represented by spring constants k 1 , k 2 , respectively, as described in reference to  FIGS. 1A and 1B . The stiffness can be the same or different. Out-of-plane mechanical stops  309   a ,  309   b  are attached to, or formed in substrate  308 . Out-of-plane electrodes  303   a ,  303   b  are attached to, or formed in, substrate  308 . In-plane electrodes  307   a ,  307   b  are attached to, or formed in, substrate  308 . In-plane mechanical stops  306   a ,  306   b  are attached to, or formed in, substrate  308 . 
     The dynamics of two-stage, MEMS sensor  300  is described in reference to  FIGS. 1A, 1B . When the input acceleration is less than a threshold value, (e.g., &lt;32 g) structures  301   a ,  301   b ,  302   a ,  302   b  move in response. When the input acceleration is greater than the threshold value (e.g., &gt;32 g), suspended structures  302   a  and  302   b  make contact with mechanical stops  302   a ,  302   b  and  306   a ,  306   b , respectively, but suspended structures  301   a ,  301   b  continue to move (e.g., deflect) on suspension members  305   a ,  305   b , resulting in a stiffening of structures  301   a ,  301   b . This stiffening effect allows sensing of higher dynamics at full-scale, as described in reference to  FIGS. 1A and 1B . 
     Note that suspension members  305   a ,  305   b  are one example of soft springs that can be used to allow movement of the suspended structures in response to input acceleration. Although the design shown uses suspension members  305   a ,  305   b , any number or type of MEMS structures (e.g., torsion bars, anchors, deformable beams, hinges, membranes) can be used in any desired combination to allow movement of suspended structures  301   a ,  301   b ,  302   a ,  302   b . Also, the MEMS structures can have any desired geometric shape including rectangular, circular, shapes, as shown in  FIGS. 3A-3B  and  FIG. 6 . The suspended structure used for capacitive sensing need not be contained within an opening of another suspended structure as shown in  FIGS. 3A and 3B . For example, in other embodiments the suspended structures can be placed side-by-side and attached to a common anchor in a pendulous or “teeter-totter” configuration. 
       FIG. 4  is a conceptual drawing illustrating the dynamics of a multi-stage MEMS accelerometer for mixed g-level operations, according to an embodiment. The dynamics described in reference to  FIGS. 1A, 1B  can be extended to N stages, as shown in  FIG. 4 . System  400  includes sense mass  402   a  with electrodes  405   a - 405   d , masses  402   b - 402 N, soft springs  403   a - 403 N and mechanical stops  404   a - 404 N. 
     Each of masses  402   b - 402 N can make contact with their respective mechanical stops  404   a - 404 N at different input acceleration threshold levels. In the example shown, masses  402   b ,  402   c ,  402 N make contact with mechanical stops  404   a ,  404   b  and  404   c , respectively, at threshold values 96 g, 64 g and 32 g. Soft springs  403   a - 403 N having respective spring constants k 1 -k N  are disposed between masses  402   a - 402 N. The example multi-stage configuration shown in  FIG. 4  allows for more optimization of sensitivity for multiple full-scale ranges and allows for higher fidelity tuning of mechanical sensitivity with nonlinear capacitance. 
       FIG. 5  is a conceptual drawing illustrating the dynamics of a two-stage MEMS accelerometer for mixed g-level operations that reduces the rebound effect after a contact with a mechanical stop, according to an embodiment. System  500  includes sense mass  502   a  having electrodes  505   a - 505   d , mass  502   b , springs  503   a ,  503   b  (having respective spring constants k 1 , k 2 ) and compliant mechanical stops  504   a ,  504   b . Compliant mechanical stops  504   a ,  504   b  allow for gradual stopping of contact mass  502   b  instead of abrupt stopping when contact mass  502   b  makes contact with compliant mechanical stop  504   a  or  504   b , thus reducing the rebound effect after contact is made. 
       FIG. 6  is a conceptual drawing illustrating the dynamics of an angular MEMS accelerometer for mixed g-level operations, according to an embodiment. System  600  includes suspended structure  601 , suspended structure  602 , circular substrate  605 , torsional springs  603   a - 603   d  suspended structure  601  and torsional springs  604   a - 604   d  suspended structure  602 . Structure  602  includes mechanical stops  606   a - 606   d.    
     When the input angular acceleration {umlaut over (θ)} i  is less than a threshold value (e.g., &lt;32 g), suspended structures  601 ,  602  rotate in response at angular accelerations {umlaut over (θ)} 2 , {umlaut over (θ)} 1 , respectively. When the input acceleration is greater than the threshold value (e.g., &gt;32 g), structure  602  makes contact with one or more of mechanical stops  606   a - 606   d , but structure  601  continues to rotate, resulting in a stiffening of the MEMS sensor, which allows for sensing of higher dynamics at full-scale. 
     Example Signal Processing 
       FIG. 7  is a flow diagram of a readout circuit for processing differential capacitance of the MEMS accelerometer for mixed g-level operations, according to an embodiment. Readout circuit  700  implements a synchronous demodulation technique to reduce noise, increase linearity and dynamic range. In an embodiment, readout circuit  700  includes differential capacitors  701  comprising electrode pairs, reference signal generator  702 , amplifier  704 , synchronous demodulator  706 , low-pass filter  708  and analog-to-digital converter (ADC)  710 . The reference signal (PM) can be an AC voltage signal that can be a square-wave or sinusoidal signal. The reference signal PM is applied to suspended structures (e.g., suspended structures  301 ,  302 ,  601 ,  602 ) that form differential capacitive pairs with electrodes attached to, or formed in, substrate  308 . 
     Amplifier  704  is used to convert the differential sensing capacitance value to an amplified AC voltage and can be, for example, a trans-impedance amplifier. The amplified AC voltage is input into synchronous demodulator  706 , which operates at the excitation frequency f e  and is controlled by control signal X c (t). The output voltage of synchronous demodulator  706 , V out  is proportional to the product of the differential capacitance (C + −C − ) and feedback capacitor C f  of amplifier  704 : 
     
       
         
           
             
               
                 
                   
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                     out 
                   
                   ∝ 
                   
                     
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                         C 
                         f 
                       
                     
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                   17 
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     The output voltage, Vow, of synchronous demodulator  706  is input into low-pass filter  708 , which outputs a DC output signal with an amplitude and phase corresponding to the sensed capacitance change. Low-pass filter  708  (e.g., a Bessel filter) limits the bandwidth, and thus increases the resolution of the voltage signal. ADC  710  converts the filtered DC output signal into a digital value, which can be used by various applications, as described in reference to  FIG. 8 . ADC converter  710  can be implemented using, for example, a delta-sigma ADC. In an embodiment, feedback can be included to increase the dynamic range of circuit  700 . 
     Example System Architecture 
       FIG. 8  is architecture for an electronic system that uses a multi-stage, MEMS accelerometer for mixed g-level operations, according to an embodiment. Architecture  800  can be included in any electronic device that uses motion sensors, including but not limited to: smart phones, tablet computers, wearable devices (e.g., a smart watch) and automotive systems. 
     Architecture  800  includes processor(s), memory interface  802 , peripherals interface  803 , motion sensors  804   a  . . .  804   n , display device  805  (e.g., touch screen, LCD display, LED display), I/O interface  806  and input devices  807  (e.g., touch surface/screen, hardware buttons/switches/wheels, virtual or hardware keyboard, mouse). Memory  812  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  812  stores operating system instructions  808 , sensor processing instructions  809  and application instructions  812 . Operating system instructions  808  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  808  may include instructions for handling basic system services and for performing hardware dependent tasks. Sensor-processing instructions  809  perform post-processing on motion sensor data (e.g., averaging) and provide control signals to motion sensors. Application instructions  810  implement software programs that use data from one or more motion sensors  804   a  . . .  804   n , such as navigation, digital pedometer, tracking or map applications. At least one motion sensor  804   a  is the multi-stage, capacitive based MEMS accelerometer, described in reference to  FIGS. 3A, 3B, 4 and 6 . 
     For example, in a navigation application executed on a smart phone, acceleration data is provided by the capacitive MEMS accelerometer to processor(s)  801  through peripheral interface  803 . Processor(s)  801  execute sensor-processing instructions  809 , to perform further processing of the acceleration data (e.g., averaging). Processor(s)  801  execute instructions for the navigation application, which draws a map on display device  805  including a location marker that shows the location of the smartphone on the map. The acceleration data is used to determine the speed and direction of the smart phone on the map. If a user is walking with the smartphone, the acceleration data can be used to count steps using known digital pedometer techniques. The step count can be multiplied by the user&#39;s stride length to determine a distance traveled by the user. Accordingly, the navigation application benefits from the multi-stage, capacitive based MEMS accelerometer embodiments disclosed herein by obtaining more accurate measurements of acceleration from which a more accurate speed, direction and distance traveled can be determined. 
     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: 20171220
Publication Date: 20200804
Grant Date: 20200804
Priority Date: 20171220
Inventors: PAINTER, CHRISTOPHER C.
TSANG, SEE-HO
Assignee: APPLE INC
CPC Classifications: [{"code": "G01P2015/0831", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01P2015/0871", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01P2015/0871", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01P15/0802", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01P2015/0831", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01P15/125", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01P15/125", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01P15/0802", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01P2015/0862", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01P15/125", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01P15/0802", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01P2015/0862", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01P2015/0871", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01P2015/0831", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 66814371