A MEMS resonant accelerometer includes two proof masses configured to resonate when driven with periodic signals. Each proof mass includes a resonator structure that vibrates relative to the proof mass and a dummy structure that does not resonate. When driven by a periodic drive signal, the resonator structures of the two proof masses may be used to determine the magnitude of acceleration in the direction perpendicular to the planes of the proof masses by sensing the frequency at which the resonators vibrate. For example, a differential oscillation frequency may be computed from the two sensed frequencies. The dummy structures are used to make the mass distribution of the two proof masses similar.

FIELD OF THE DISCLOSURE

The present application relates to microelectromechanical systems (MEMS) resonant accelerometers.

BACKGROUND

Some MEMS accelerometers include a resonator structure coupled to a proof mass. A driving electrode drives the resonator structure to vibrate out of the plane of the proof mass. The proof mass is configured to move in response to acceleration in the direction perpendicular to the plane of the proof mass. The oscillation frequency at which the resonator vibrates shifts based on the acceleration experienced by the MEMS accelerometer due to a shift in the distance between the resonator structure and the driving electrode.

Some MEMS accelerometers include two resonator structures coupled to the proof mass. Each resonator structure is separately driven and experiences a different shift in oscillation frequency because each resonator structure will be a different distance away from a driving electrode based on its location with respect to the proof mass. The difference in the oscillation frequencies of the two resonator structures may be used to determine the acceleration experienced by the MEMS accelerometer.

Some MEMS accelerometers use capacitive sensors to detect the amplitude of the motion of the resonators.

SUMMARY OF THE DISCLOSURE

In some embodiments, a microelectromechanical system (MEMS) resonant accelerometer includes two proof masses configured to resonate when driven with periodic signals. Each proof mass includes a resonator structure that vibrates relative to the proof mass and a dummy structure that does not resonate. When driven by a periodic drive signal, the resonator structures of the two proof masses may be used to determine the magnitude of acceleration in the direction perpendicular to the planes of the proof masses by sensing the frequency at which the resonators vibrate. For example, a differential oscillation frequency may be computed from the two sensed frequencies. The dummy structures are used to make the mass distribution of the two proof masses similar.

According to one aspect of the present application, a MEMS accelerometer is described. The MEMS resonant accelerometer may include a first teeter-totter structure and a second teeter-totter structure. The first teeter totter structure may include a first proof mass coupled to a substrate through a first anchor, the first proof mass having first and second mass portions having different masses and disposed at opposite sides of the first proof mass with respect to the first anchor; a first resonator structure pivotally attached to the first proof mass and configured to vibrate relative to the first proof mass; and a first dummy structure fixedly attached to the first proof mass. The second teeter-totter structure may include a second proof mass coupled to the substrate through a second anchor, the second proof mass having third and fourth mass portions having different masses and disposed at opposite sides of the second proof mass with respect to the second anchor; a second resonator structure pivotally attached to the second proof mass and configured to vibrate relative to the first proof mass; and a second dummy structure fixedly attached to the second proof mass.

According to another aspect of the present application, a MEMS resonant device is provided. The MEMS device may include an accelerometer. The accelerometer may include a first teeter-totter structure and a second teeter-totter structure. The first teeter totter structure may include a first proof mass coupled to a substrate through a first anchor, the first proof mass having first and second mass portions having different masses and disposed at opposite sides of the first proof mass with respect to the first anchor; a first resonator structure pivotally attached to the first proof mass and configured to vibrate relative to the first proof mass; and a first dummy structure fixedly attached to the first proof mass. The second teeter-totter structure may include a second proof mass coupled to the substrate through a second anchor, the second proof mass having third and fourth mass portions having different masses and disposed at opposite sides of the second proof mass with respect to the second anchor; a second resonator structure pivotally attached to the second proof mass and configured to vibrate relative to the first proof mass; and a second dummy structure fixedly attached to the second proof mass.

According to yet another aspect of the present application, a method for sensing accelerations using a MEMS accelerometer is provided. The method may causing a first resonator structure to vibrate out-of-plane by vibrating about a first axis, wherein the first resonator structure is coupled to a first proof mass that includes a first dummy structure that is the same mass as the first resonator structure; causing a second resonator structure to vibrate out-of-plane by vibrating about a second axis, wherein the second resonator structure is coupled to a second proof mass that includes a second dummy structure that is the same mass as the second resonator structure; sensing a first oscillation frequency of the first resonator structure and a second oscillation frequency of the second resonator structure; and computing a differential oscillation frequency from the first and second oscillation frequencies.

DETAILED DESCRIPTION

The inventor has recognized and appreciated that the sensitivity of MEMS resonant accelerometers to out-of-plane accelerations may be improved by using the difference between the vibration frequencies of two resonator structures coupled to a proof mass (referred to as a differential oscillation frequency). There are at least two advantages to using the differential oscillation frequency as compared to simply using a frequency shift of a single resonator structure. First, the magnitude of the differential oscillation frequency is twice that of the frequency shift of a single resonator structure. Second, using a differential oscillation frequency reduces inaccurate readings caused by shifts in the oscillation frequency of an individual resonator structure due to environmental causes. For example, the oscillation frequency of a resonator structure may shift due to a change in the temperature of the environment in which the MEMS accelerometer is located. In the case where a differential oscillation frequency is used to determine the acceleration, such environmental shifts in oscillation frequency effect each of the resonator structures in the same way such that when the difference in oscillation frequency is calculated, the environmental shift cancels out.

The inventors have further recognized and appreciated that using the differential oscillation frequency of two resonator structures coupled to a single proof mass may result in an inaccurate determinations of the acceleration experienced by the MEMS accelerometer due to the two resonator structures being mechanically coupled by the shared proof mass, resulting in mechanical energy being transferred between the two resonator structures. This mechanical coupling may result in, for example, locking and/or dead band in the response of a sensor. The inventors have additionally recognized and appreciated that the mechanical coupling of the two resonator structures may be reduced and practically eliminated by isolating the two resonator structures from one another.

Thus, some embodiments of the present application are directed to a MEMS resonant accelerometer in which two proof masses are used, each proof mass including only one resonator structure that vibrates relative to the proof mass. By using two separate proof masses, each coupled to only a single resonator structure, the mechanical coupling between the two resonator structures is effectively eliminated. At the same time, a differential oscillation frequency can still be used to calculate the acceleration experienced by the MEMS accelerometer by calculating the difference between a first resonator structure coupled to a first proof mass and a second resonators structures coupled to a second proof mass that is different from the first proof mass. For the differential oscillation frequency to be useful in determining the acceleration, the position of the resonator structure within one proof mass may be on the opposite side of an anchor axis than the position of the resonator structure within the other proof mass.

The inventors have further recognized and appreciated that to accurately determine an acceleration with a MEMS accelerometer that includes two proof masses, each with a single resonator structure, the combination of proof mass and resonator structure should be as similar as possible. Accordingly, a dummy structure is coupled to each proof mass to make the two proof masses similar in structure. Each dummy structure is located at a position within the respective proof mass that corresponds to the position of the resonator structure of the other proof mass. The dummy structures have substantially the same mass and shape as the resonator structures.

Some embodiments include a drive electrode for each proof mass, to drive the resonator structure of the corresponding proof mass at a particular frequency. The drive electrode is located on an underlying substrate in proximity to the associated resonator structure. No drive electrode is located in proximity to the dummy structure.

Some embodiments include a sense electrode for each proof mass, to sense a distance between the sense electrode and a portion of the resonator structure of the corresponding proof mass. The sense electrode is located on the underlying substrate in proximity to the associated resonator structure. No sense electrode is located in proximity to the dummy structure.

Such a single mass resonates when driven with a driving signal, and is allowed to move in response to accelerations. The frequency at which the single mass oscillates in response to acceleration may be detected using suitable detectors, thus providing a measure of the magnitude of the acceleration. Furthermore, some embodiments are directed to MEMS resonant accelerometers configured to operate differentially, in which a pair of masses is provided, with each of the masses being used as a resonator and a proof mass.

The aspects and embodiments described above, as well as additional aspects and embodiments, are described further below. These aspects and/or embodiments may be used individually, all together, or in any combination of two or more, as the application is not limited in this respect.

FIG. 1Ais a top view schematic diagram illustrating a MEMS accelerometer100, according to some non-limiting embodiments. The MEMS accelerometer100includes proof masses110and140, resonator structures112and144, and dummy structures114and142. The proof masses110and140are coupled to an underlying substrate101(seeFIG. 1C) via anchors116and146, respectively. The first resonator structure112is coupled to the proof mass110via tethers126and128, and the second resonator structure144is coupled to the proof mass140via tethers152and154. The first dummy structure114is coupled to the proof mass110via tethers122and124, and the second dummy structure142is coupled to the proof mass140via tethers156and158.

The proof masses110and140may be made of a conductor and/or semiconductor material, such as single-crystal silicon or polycrystalline silicon. In some embodiments, proof masses110and140are connected to the respective anchors116and146via a plurality of tethers. One such configuration is illustrated inFIG. 1B, which is a schematic top view illustrating tethers that may be used with the MEMS accelerometer100, according to some non-limiting embodiments. As illustrated, tethers108and109may be formed by removing (for example, via etching) portions107from the first proof mass110. Portions107may be removed to form an anchored proof mass portion106and tethers130and131. The anchored proof mass portion106may be connected to anchor116(not shown inFIG. 1B), and may be connected to the body of the first proof mass110via tethers108and109. Tethers108and109may be configured to torque in the xz-plane in response to accelerations parallel to the z-axis, thereby allowing for rotations of the first proof mass110about the first anchor116. In this sense, the tethers108and109may be torsional beams. In the example illustrated inFIG. 1B, the tethers are configured to torque and they are separated along a direction parallel to the y-axis. The tethers108and109are substantially aligned to form an anchor axis about which the first proof mass110may rotate. The various aspects of the tethers108and109and anchored proof mass portion106described herein are not limited to any specific type or number of tethers. Though not illustrated, the second proof mass140may be connected to anchor146in a similar arrangement to that shown inFIG. 1B. Additionally, while the tethers108and109are illustrated as simple rectangular connections between the first proof mass110and the anchored proof mass portion106, more complicated tether structures may be used. For example, the tethers108and109may have a serpentine shape.

Referring back toFIG. 1A, the proof masses110and140are elongated in a direction parallel to the x-axis such that the proof masses110and140are rectangular. Embodiments are not limited to any particular size or shape, but, in some embodiments, the size and the shape of the first proof mass110is the same as the size, shape, and mass of the second proof mass140. In some embodiments, the first proof mass110is separated from the second proof mass140in a direction parallel to the y-axis. In some embodiments, the first resonator structure112and the first dummy structure114are arranged symmetrically about the first anchor116of the first proof mass110such that the distance between the first anchor116and the first resonator structure112is equal to the distance between the first anchor116and the first dummy structure114. Similarly, in some embodiments, the second resonator structure144and the first dummy structure142are arranged symmetrically about the second anchor146of the second proof mass140such that the distance between the second anchor146and the second resonator structure144is equal to the distance between the second anchor146and the second dummy structure142. The first anchor116and the second anchor146are substantially aligned along the x-direction. Thus, an imaginary line160in the x-y plane that connects the position of the first anchor116to the position of the second anchor146can be used to define two sides of the proof masses110and140. A first mass portion111of the proof mass110is on a first side of the line160and a second mass portion113of the proof mass110is on a second side of the line160. Similarly, a third mass portion141of the proof mass140is on a first side of the line160and a fourth mass portion143of the proof mass140is on a second side of the line160. The imaginary line160may be considered an anchor axis, about which the proof masses110and140may rotate. Sharing the same rotation axis may be desirable as undesired signals, generated for example due to stress in the substrate, may be equally sensed by the two proof masses and therefore may be rejected as common modes. Of course, embodiments are not limited in this respect as the anchors116and146may be partially offset from one another (e.g., by less than 100 nm, less than 250 nm, less than 500 nm, less than 1 μm, less than 1 μm, less than 5 μm, or less than 10 μm) along the x-axis.

While the respective dummy structures and resonator structures are arranged symmetrically about their respective anchors, the anchors116and146are not arranged to be positioned under the center of mass of the proof masses110and140. For example, the first anchor116is positioned towards a first side, in the x-direction, of the first proof mass110such that a first mass portion111has less mass than a second mass portion113. The first mass portion111and the second mass portion113are disposed at opposite sides of the first proof mass110with respect to the first anchor116. The second mass portion113is longer by a length L3in the x-direction than the length of the first mass portion111. The asymmetric arrangement of the first proof mass110relative to the first anchor116results in the first proof mass110rotating in the x-z plane when the first proof mass110experiences an acceleration in a direction parallel to the z-axis.

The various aspects of the geometry of the first proof mass110described herein apply also to the second proof mass140such that the second proof mass140is also asymmetrically positioned above the second anchor146. For example, the second anchor146is positioned towards a first side, in the x-direction, of the second proof mass140such that a third mass portion141has less mass than a fourth mass portion143. The third mass portion141and the fourth mass portion143are disposed at opposite sides of the second proof mass140with respect to the second anchor146. The fourth mass portion143is longer by a length L3in the x-direction than the length of the third mass portion141. The asymmetric arrangement of the second proof mass140relative to the second anchor146results in the second proof mass140rotating in the x-z plane when the second proof mass140experiences an acceleration in a direction parallel to the z-axis.

The torquing and rotation of the proof masses110and140described above cause the structures to operate as teeter totters. That is, when one end of a proof mass moves in one direction out-of-plane (i.e., out of the x-y plane), the opposite end of the proof mass moves in the opposite direction by pivoting about the anchor, which acts as a fulcrum. In some embodiments, a first teeter totter structure includes at least the first proof mass110, the first anchor116, the first resonator structure112and the first dummy structure114, and a second teeter totter structure includes at least the second proof mass140, the second anchor146, second resonator structure144, and the second dummy structure142.

In some embodiments, the resonator structures112and144and the dummy structures114and142are formed by removing a portion of the proof mass material such that the resonator structures112and144and the dummy structures114and142are partially disconnected from and at least partially surrounded by one of the respective proof masses110or140and are substantially position in a plane defined by the respective proof masses110or140. A portion of the proof mass material may be left to form the tethers122,124,126,128,152,154,156, and158. These tethers may be similar to tethers108and109shown inFIG. 1B. The tethers122,124,126,128,152,154,156, and158are all oriented in a direction parallel to the y-axis—the same direction as the tethers108and109—and couple the resonator structures and the dummy structures to the respective proof mass. For the resonator structures112and144, the tethers126,128and152,154, respectively, may be the only physical connection to the respective proof masses110and140. In some embodiments tethers126and128are positioned directly opposite one another, separated in the y-direction but at the same position in the x- and z-direction such that the tethers126and128form a first axis about which the first resonator structure112may vibrate in a rotational manner when driven by a driving signal. In this way, the first resonator structure112is pivotally connected to the first proof mass110. Similarly, tethers152and154are positioned directly opposite one another, separated in the y-direction but at the same position in the x- and z-direction such that the tethers152and154form a second axis about which the second resonator structure144may vibrate in a rotational manner when driven by a driving signal. In this way, the second resonator structure144is pivotally connected to the second proof mass140. In some embodiments, the dummy structures114and142are connected to their respective proof masses110or140by additional tethers to prevent the dummy structures114and142from vibrating out of the plane of the proof mass.

In some embodiments, the resonator structures112and144and the dummy structures114and142have the same shape, mass, and dimensions. For example, the resonator structures112and144and the dummy structures114and142have a length L1in the x-direction, a width W1in the y direction, and a depth D1in the z-direction. The depth D1is the same as the depth of the proof masses110and140in the z-direction. In some embodiments, the length L1is greater than the distance from the resonator structures112and144and the dummy structures114and142to their respective anchors116and146. The total length L2in the x-direction of the proof masses110and140is greater than 2L1+L3.

In some embodiments, the first resonator structure112is not aligned with the second resonator structure144in the x-direction or the y-direction. For example, the first resonator structure112is disposed on the second side of the line160, surrounded by the second mass portion113of the first proof mass110and the second resonator structure144is disposed on the first side of the line160, surrounded by the third mass portion141of the second proof mass140.

Sense electrodes151and157, and drive electrodes153and155are shown in dashed lines inFIG. 1Ato indicate that they are disposed on a different xy-plane than proof masses110and140. As shown inFIG. 1C, which is a side view of the MEMS accelerometer100, the sense electrodes151and157, and drive electrodes153and155are disposed on a substrate101that underlies the proof masses110and140in the z-direction. In some embodiments, the sense electrodes151and157are positioned nearer to the anchors116and146, respectively, than the drive electrodes153and155. The drive electrodes153and155, which are in proximity to the resonator structures112and144, respectively, may form a pair of drive capacitors with resonator structures112and144, respectively. As illustrated inFIG. 1C, drive electrodes153and155may be coupled to drive circuitry150, which may be disposed on the same substrate as the MEMS accelerometer100, or a separate substrate. Drive circuitry150may be configured to excite the drive capacitors with alternating current (AC) signals (e.g., periodic signals), thereby causing the resonator structures112and144to pivot about the first axis and second axis, respectively, and as a result, to vibrate (via electrostatic attraction/repulsion) out-of-plane. The dummy structures114and142are not associated with sense electrodes or a drive electrodes because no measurement of the dummy structures114and142is needed. Thus, there are no sense electrodes or drive electrodes in proximity to the dummy structures114and142.

The sense electrodes151and157, which are in proximity to the resonator structures112and144, respectively, may form a pair of sense capacitors with the resonator structures112and144, respectively. When the resonator structures112and144move out-of-plane, the sense capacitors experience a variation in capacitance, due to a change in the separation between the resonator structures and the sense electrodes. As such, the sense capacitors may detect motion of the resonator structures112and144(whether this motion is caused by accelerations experienced by the resonator structures, by drive signals, or by other reasons). In some embodiments, sense circuitry152, which is coupled to the sense electrodes151and157, may be configured to detect the frequenc(ies) with which resonator structures112and144vibrate, based on the signals obtained from the sense capacitors.

FIG. 1Dis a perspective view of a first proof mass110of the MEMS accelerometer100, according to some non-limiting embodiments. The first resonator structure112is shown vibrating out-of-plane. The first dummy structure114does not vibrate out of plane because of additional tethers that keep the first dummy structure114stationary.

FIG. 1Eis a perspective view of a second proof mass of the MEMS accelerometer100, according to some non-limiting embodiments. The second resonator structure144is shown vibrating out-of-plane. The second dummy structure142does not vibrate out of plane because of additional tethers that keep the first dummy structure114stationary.

FIGS. 2A-Care a side view of the MEMS accelerometer100under various conditions. First,FIG. 2Aillustrates a case in which no accelerations along the z-axis are present. If drive circuitry150drives the drive capacitors with a signal oscillating in time at a frequency f, the resonator structures may respond by vibrating out-of-plane with an oscillation period given by 1/f. As a result, the frequency detected by sense circuitry152may be equal to f. As illustrated, the average separation between a reference location X1of the second resonator structure144and sense electrode157is denoted by “s1” and the average separation between a reference location X2of the first resonator structure112and sense electrode151is denoted by “s2”. When the proof masses110and140are still and parallel to the xy-plane such that the separation between the first resonator structure112and the sense electrode151is equal to s2and the separation between the second resonator structure144and the sense electrode157is equal to s1, the proof masses110and140are said to be in “resting position”. The oscillating signal provided by drive circuitry150causes resonator structures112and144to vibrate out-of-plane, such that the separations oscillate from the resting positions s1and s2. Separations s1and s2may be less than 1 μm, less than 800 nm, less than 600 nm, less than 400 nm, less than 200 nm or less than 100 nm, as non-limiting example.

Second,FIG. 2Billustrates a case in which the MEMS resonant accelerometer100experiences an acceleration azdirected along the z-axis with no drive signals being applied to the drive capacitors. In this case, due to the proof masses110and140being anchored away from their centers of mass, the acceleration causes a net non-zero force to be applied to the center of the masses. As a result, the proof masses110and140rotate about their respective anchors116and146thus departing from their resting positions. For example, in cases in which azis directed in the negative z-direction, as shown by vector201, proof masses110and140rotate about the anchors such that the separation between the heavy mass portions (the second mass portion113and the fourth mass portion143) and the substrate101is reduced. Because the first resonator structure112is on the second side of the proof mass110, surrounded by the second mass portion113) and the second resonator structure144is on the first side of the proof mass140, surrounded by the third mass portion141, the first resonator structure112and the second resonator structure144move in opposite directions in the z-direction relative to the substrate101. In the illustrated case, the separation between reference location X1of second resonator structure144and sense electrode157is increased and denoted by “s1′” and the separation between reference location X2of first resonator structure112and sense electrode151is decreased and denoted by “s2”.

Finally,FIG. 2Cillustrates a case in which the MEMS resonant accelerometer100experiences an acceleration azdirected along the z-axis and drive signals are applied to the drive capacitors. As in the case illustrated inFIG. 2A, the drive signals, which oscillate at frequency f, cause the resonator structures112and144to oscillate out-of-plane. However, due to the presence of the z-axis acceleration az, the average separations between reference locations X1and X2 andthe sense electrodes157and151, respectively, are s1′ rather than s1and s2′ rather than s2. As a result, the resonant frequencies of the resonator structures, which depend on the separation, shift in opposite directions. The extent to which the resonant frequencies shifts from frequency f may depend on the magnitude of acceleration az. Therefore, in some embodiments, sense circuitry152may infer the magnitude of the acceleration based on the shift in the resonant frequencies.

According to one aspect of the present application, differential signals may be generated in response to accelerations parallel to the z-axis. Compared to single-ended signals, differential signals may be more immune to common mode signals, such as undesired signals caused by deformations of the substrate due to stress. To generate differential signals, in some embodiments, sense electrode151may be positioned in proximity to the first resonator structure112(e.g., such that the first resonator structure112and sense electrode151spatially overlap, at least in part, in the xy-plane while being separated along the z-axis), and the sense electrode157may be positioned in proximity to the second resonator structure144(e.g., such that the second resonator structure144and the sense electrode157spatially overlap, at least in part, in the xy-plane while being separated along the z-axis). This configuration is illustrated inFIG. 1AandFIG. 1C. Because the first resonator structure112and the second resonator structure144are on opposite sides of the anchor axis defined by line160, when the separation between sense electrode151and the first resonator structure112increases, the separation between sense electrode157and the second resonator structure144decreases (and vice versa). As a result, when the frequency of the signal sensed by sense electrode151increases, with respect to the frequency of the driving signal, the frequency of the signal sensed by sense electrode157decreases, and differential signals may be generated. In some embodiments, resonator masses112and144may be driven to vibrate out-of-phase with respect to each other (e.g., with a phase difference that is 180°, or between 170° and 190°).

In one example, assuming that the frequency of the driving signal is f, the frequency of the signal sensed by sense electrode151in the presence of a z-axis acceleration may be f1=f+Δf1and the frequency of the signal sensed by sense electrode157may be f2=f−Δf2. Sense circuitry152may be configured to compute f1−f2, thus obtaining Δf1+Δf2. The magnitude of the acceleration may be inferred from Δf1+Δf2. Being a differential detection, common mode signals captured by both sense electrodes151and157(such as signals caused by deformations of the substrate due to stress) may be rejected (or at least limited). In some embodiments, the sense electrodes may be positioned such that, in the presence of z-axis accelerations, Δf1=Δf2=Δf. In these embodiments, f1−f2=2Δf.

In some embodiments, the sense electrodes are positioned such that one sense electrode is proximate the heavy mass portion of a proof mass and the other sense electrode is proximate the light mass portion of the other proof mass. For example,FIG. 1AandFIG. 1Cillustrate a case in which sense electrode151is proximate the second mass portion113(which is heavier than the first mass portion111) of the first proof mass110, and sense electrode157is proximate the third mass portion141(which is the lighter than the fourth mass portion143) of the second proof mass140. This configuration may cause the separation between one resonator structure, embedded within a particular mass portion, and its corresponding sense electrode to increase and the separation between the other resonator structure, embedded within a different mass portion, and its corresponding sense electrode to increase. As a result, differential signals may be generated in response to accelerations, and undesired signals (e.g., signal offsets caused by stress in the substrate or sense signals arising from accelerations that are not parallel to the z-axis) may be rejected or at least limited.

As described above, drive circuitry150may be configured to provide drive signals to the drive capacitors and sense circuitry152may be configured to detect signals provided by the sense capacitors, and to detect variations in resonant frequency with respect to the frequency of the drive signals. A non-limiting implementation of drive circuitry150and sense circuitry152is shown inFIG. 3. System300may comprise proof masses110and104(arranged in any one of the configurations described above) sense circuits302and304, excitation feedback circuits312and314, drive circuits322and324, differential frequency circuit306and processing unit308.

Sense circuit302may be coupled to sense electrode151and sense circuit304may be coupled to sense electrode157. Sense circuits302and304may collectively serve as sense circuitry152. Drive circuit322may be coupled to drive electrode153and drive circuit324may be coupled to drive electrode155. Drive circuits322and324may collectively serve as drive circuitry150. Sense circuits302and304may be configured to receive signals generated in response to motion of the proof masses, and to obtain the frequencies at which the signals resonate. As described above, the frequencies at which these signals resonate may be different, depending on the magnitude of the acceleration experienced by the proof masses, from the resonant frequency of the drive signals. In some embodiments, sense circuits302and304may each comprise a phase-locked loop (PLL). The PLLs may be configured to lock to the frequencies at which the received signals resonate, and to output values representative of these frequencies.

Differential frequency circuit306may be configured to combine the frequencies obtained with sense circuits306. This may be performed in the analog and/or the digital domain. As such, sense circuits302and304may comprise analog-to-digital converters in some embodiments. In some embodiments, differential frequency circuit306subtracts the frequency obtained with sense circuit302from the frequency obtained with sense circuit304(or vice versa), thus obtaining a differential representation of the acceleration experienced by the MEMS resonant accelerometer. The result of this operation may be, for example, Δf1−Δf2or 2Δf. Processing unit308may infer the magnitude of the acceleration based on such a differential representation. For example, processing unit308may include a memory loaded with a look-up-table (LUT) mapping acceleration magnitude to Δf1−Δf2(or 2Δf). The LUT may be generated, for example, using a calibration procedure.

In some embodiments, the resonator structures may be driven based on the frequencies sensed by the sense circuits. As such, feedback loop circuits may be provided. In the example ofFIG. 3, excitation feedback circuit312couples sense circuit302to drive circuit322and excitation feedback circuit314couples sense circuit304to drive circuit324. The excitation feedback circuits may be configured to cause the drive circuits to select the driving frequencies based on the sensed frequencies. This may be done, for example, to ensure that the proof masses do not oscillate outside a motion range deemed safe. Drive circuits322and324may each comprise an oscillator configured to output a periodic signal (e.g., a sinusoidal signal). In some embodiments, the signals provided by drive circuits322and324may be out-of-phase with respect to one another (e.g., with a phase difference of 180° or between 170° and 190°).

MEMS resonant accelerometers of the type described herein may be used in connection with other electrical components to form MEMS devices. An example of such a MEMS device400is depicted inFIG. 4. The MEMS device400may be deployed in various settings to detect acceleration, including sports, healthcare, military, and industrial applications, among others. For example, the MEMS device400may be a wearable sensor deployed in monitoring sports-related physical activity and performance, patient health, military personnel activity, or other applications of interest of a user. In another example, the MEMS device400may be used in seismic applications, such as to sense and/or predict earthquakes.

The MEMS device400may comprise MEMS resonant accelerometer402, drive circuitry150, sense circuitry152, I/O interface408, and power unit404. MEMS resonant accelerometer402may be implemented using any one of the embodiments described above. Drive circuitry150and sense circuitry152have been described above.

The MEMS device400may periodically transmit, via wired connections or wirelessly, signals representative of sensed accelerations to an external monitoring system, such as a computer, a smartphone, a tablet, a smartwatch, smartglasses, or any other suitable receiving device. I/O interface408may be configured to transmit and/or receive data via Wi-Fi, Bluetooth, Bluetooth Low Energy (BLE), Zigbee, Thread, ANT, ANT+, IEEE 802.15.4, IEEE 802.11.ah, or any other suitable wireless communication protocol. Alternatively, or additionally, I/O interface408may be configured to transmit and/or receive data using proprietary connectivity protocols. I/O interface408may comprise one or more antennas, such as a microstrip antenna. In some embodiments, I/O interface408may be connected to a cable, and may be configured to transmit and/or receive signals through the cable.

The MEMS device400may be powered using power unit404. Power unit may be configured to power drive circuitry150, sense circuitry152, and I/O interface408, or just a subset of these. In some embodiments, power unit404may comprise one or more batteries. The MEMS device400may, in at least some embodiments, consume sufficiently little power to allow for its operation for extended periods based solely on battery power. The battery or batteries may be rechargeable in some embodiments. Power unit404may comprise one or more lithium-ion batteries, lithium polymer (LiPo) batteries, super-capacitor-based batteries, alkaline batteries, aluminum-ion batteries, mercury batteries, dry-cell batteries, zinc-carbon batteries, nickel-cadmium batteries, graphene batteries or any other suitable type of battery. In some embodiments, power unit404may comprise circuitry to convert AC power to DC power. For example, power unit404may receive AC power from a power source external to the MEMS device400, such as via I/O interface408, and may provide DC power to some or all the components of the MEMS device400. In such instances, power unit404may comprise a rectifier, a voltage regulator, a DC-DC converter, or any other suitable apparatus for power conversion.

Power unit404may comprise energy harvesting components and/or energy storage components, in some embodiments. Energy may be harvested from the surrounding environment and stored for powering the MEMS device400when needed, which may include periodic, random, or continuous powering. The type of energy harvesting components implemented may be selected based on the anticipated environment of the MEMS device400, for example based on the expected magnitude and frequency of motion the MEMS device400is likely to experience, the amount of stress the system is likely to experience, the amount of light exposure the system is likely to experience, and/or the temperature(s) to which the system is likely to be exposed, among other possible considerations. Examples of suitable energy harvesting technologies include thermoelectric energy harvesting, magnetic vibrational harvesting, electrical overstress harvesting, photovoltaic harvesting, radio frequency harvesting, and kinetic energy harvesting. The energy storage components may comprise supercapacitors in some embodiments.

In some embodiments, the MEMS device400may comprise one or more other MEMS components, such as gyroscopes, resonators, and/or other types of accelerometers. The MEMS components may be used collectively to analyze the overall behavior of a person or an object, on which the MEMS device400is disposed.

One representative application of the MEMS device400is in health monitoring devices. In such application, the MEMS device400may be configured to be attached, tied or clipped to the body of a user. For example, the MEMS device400may be attached to a user's head, chest, arm or leg. In some embodiments, the MEMS device400may be configured to detect accelerations caused by the user's cardiovascular activity and/or pulmonary activity. Additionally, or alternatively, the MEMS device400may be configured to monitor a user's physical activity, for example by counting the number of steps, by measuring stride length, and/or by measuring a limb's motion range.

FIG. 5is a flowchart of a method500of for sensing accelerations using a MEMS accelerometer, according to some non-limiting embodiments. At act502, the method500includes causing a first resonator structure to vibrate out-of-plane by vibrating about a first axis, wherein the first resonator structure is coupled to a first proof mass that includes a first dummy structure that is the same mass as the first resonator structure. For example, this may be achieved by driving the first resonator structure112via drive electrode153using drive circuitry152, as described above.

At act504, the method500includes causing a second resonator structure to vibrate out-of-plane by vibrating about a second axis, wherein the second resonator structure is coupled to a second proof mass that includes a second dummy structure that is the same mass as the second resonator structure. For example, this may be achieved by driving the second resonator structure144via drive electrode153using drive circuitry152, as described above. In some embodiments, causing the first resonator structure to resonate may be performed via electrostatic attraction and repulsion.

At act504, the method500includes causing a second resonator structure to vibrate out-of-plane by vibrating about a second axis, wherein the second resonator structure is coupled to a second proof mass that includes a second dummy structure that is the same mass as the second resonator structure. For example, this may be achieved by driving the second resonator structure144via drive electrode155using drive circuitry152, as described above. In some embodiments, causing the second resonator structure to resonate may be performed via electrostatic attraction and repulsion.

At act506, the method500includes sensing a first oscillation frequency of the first resonator structure and a second oscillation frequency of the second resonator. For example, this may be achieved by capacitive sensing the separation between the first resonant structure112and the sensing electrode151using sense circuitry152and capacitive sensing the separation between the second resonant structure144and the sensing electrode157using sense circuitry152, as described above.

At act508, the method500includes computing a differential oscillation frequency from the first and second oscillation frequencies. In some embodiments, computing the differential oscillation frequency includes computing a difference between the first and second oscillation frequencies.

At act510, the method500includes obtaining information indicative of an acceleration based on the differential oscillation frequency. For example, this may be achieved using processing unit308to look-up an acceleration magnitude in a table mapping acceleration magnitude to the differential frequency, as described above.

Having thus described several aspects and embodiments of the technology of this application, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those of ordinary skill in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the technology described in the application. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described. In addition, any combination of two or more features, systems, articles, materials, and/or methods described herein, if such features, systems, articles, materials, and/or methods are not mutually inconsistent, is included within the scope of the present disclosure.

Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. Use of such ordinal terms in the claims does not necessarily have the same meaning or refer to the same component as components the specification that use the same ordinal terms.