Patent Description:
This document pertains generally, but not by way of limitation, to sensors that are micro-electromechanical systems (MEMS), and more particularly, to MEMS gyroscope sensors.

Document <CIT> discloses a microelectromechanical systems (MEMS) device wherein one or more components of the MEMS device exhibit attenuated motion relative to one or more other moving components.

The solution is provided by the features of the independent claims. Variations are as described by the features of the dependent claims.

MEMS include small mechanical devices performing electrical and mechanical functions that are fabricated using photo-lithography techniques similar to techniques used to fabricate integrated circuits. Some MEMS devices are sensors that can detect motion such as an accelerometer or detect angular rate or rotation rate such as a gyroscope. A capacitive MEMS gyroscope undergoes a change in capacitance in response to a change in rotation rate. However, MEMS gyroscopes can be susceptible to errors such as quadrature error and offset error.

MEMS gyroscopes have a movable proof mass that moves in response to an electrical drive signal. The drive motion is along a drive axis (e.g., an X axis) of the proof mass. When the moving proof mass experiences rotation, a Coriolis force causes movement in a sense axis direction orthogonal to the drive axis (e.g., a Y axis). The drive axis and sense axis can be any axes that are mutually orthogonal. Movement of the proof mass in the sense axis direction causes a detectable change in capacitance representative of the rotation of the proof mass. The vibration pattern can be at any angle not necessarily the X axis or Y axis. For instance, by applying forces equally along X and Y axes, the vibration pattern angle will be at <NUM> degrees.

In general, MEMS gyroscopes are asymmetric between the drive axis direction and the sense axis direction because the amplitude of motion of the Coriolis mode vibration is thousands of times less than the amplitude of motion of the drive mode vibration. The axial symmetry, however, is beneficial for reduction of cross-axis damping errors, minimizing mass and momentum imbalance - all of which contribute to gyroscope output drift. The present inventors have recognized, among other things, that use of an MEMS gyroscope with axial symmetry provides advantages due to its intrinsic self-calibration properties.

In one approach, a gyroscope includes a substrate and a proof mass coupled to the substrate and configured to move in direction of a first axis (e.g., an X axis) and in direction of a second axis orthogonal to the first axis (e.g., a Y axis). The gyroscope includes a first axis shuttle structure or axis shuttle, to selectively drive the proof mass along the first axis as a drive axis, or to selectively sense movement of the proof mass under rotation (e.g., Coriolis sensing) along the first axis as a sense axis in response to the proof mass being driven along the second axis as the drive axis. The gyroscope also includes a second axis shuttle to selectively sense movement of the proof mass along the second axis as a sense axis in response to the proof mass driven along the first axis, or to selectively drive the proof mass along the second axis as the drive axis. The first axis shuttle is symmetric to the second axis shuttle along a diagonal axis that is diagonal to both the first axis and the second axis.

In another aspect of the present subject matter, a method of operating an MEMS gyroscope includes driving the MEMS gyroscope along a first axis of the MEMS gyroscope as a drive axis, sensing a response of the MEMS gyroscope along a second axis orthogonal to the first axis as a sense axis, and turning the MEMS gyroscope and changing the drive axis to the second axis changing the sense axis to the first axis.

This summary is intended to provide an overview of subject matter of the present patent application.

MEMS gyroscopes can be susceptible to errors such as quadrature error and common mode offset error. Quadrature error can be caused by the vibrating direction of the proof mass not being fully in line with the driving direction, resulting in an undesired vibration component perpendicular to, or in quadrature with, the driving direction. Common mode offset error can be caused by unequal damping of the proof mass in the two mutually orthogonal directions.

A MEMS gyroscope that is symmetric has intrinsic self-calibration properties that can improve performance by several orders of magnitude. Self-calibration of the gyroscope involves periodically switching an axis of the gyroscope from a Coriolis sensing mode to a resonator drive mode. This periodic switching requires axial symmetry of features of the gyroscope and equal electrode gaps in axis shuttle structures of the two mutually orthogonal axes. This symmetry allows the drive axis and the sense axis of the gyroscope to be interchanged when the orientation of the gyroscope is turned ninety degrees. This "mode turnability" between resonator mode and Coriolis mode of the axes cancels out many common mode offset error terms and quadrature error terms that change sign by the turning of the gyroscope. This causes the error term to average out over time.

<FIG> is an example of a mode match MEMS gyroscope <NUM>. The gyroscope <NUM> includes four identical quadrants each including a moveable proof mass 106A - <NUM> D coupled to a substrate. Each proof mass is movable in the direction of two mutually orthogonal axes. The axes in the example are a horizontal axis and a vertical axis labeled the X axis and the Y axis respectively, as an example, but the axes can be any mutually orthogonal axes. The first axis can be aligned to any in-plane or out-of-plane angle in three-dimensional (3D) space and the second axis is orthogonal to the first axis. Multiple gyroscopes can be assembled on surfaces with different angles. In an example intended to be illustrative and non-limiting, the multiple gyroscopes can be mounted on a foursided pyramid with <NUM>-degree surfaces to realize <NUM>-axis sensing. In another example, multiple gyroscopes can be mounted on vertical or horizontal boards for <NUM>-axis sensing.

<FIG> is an example of a mode match MEMS gyroscope <NUM> that is one quadrant or one-fourth of the MEMS gyroscope of <FIG>. The gyroscope <NUM> includes one proof mass 106B coupled to the substrate. The gyroscope <NUM> also includes box linkage <NUM>, outer couplers <NUM>, and stress relieved anchors <NUM>. The gyroscope <NUM> also includes shuttle structures or shuttle frames that can be referred to as "shuttles. " The MEMS gyroscope <NUM> includes two shuttles <NUM> for the X axis and two shuttles <NUM> for the Y axis. The shuttles <NUM>, <NUM> are coupled to the substrate by stress relieved frame anchors <NUM> and to the proof mass 106B with the box linkage <NUM> and a lever having a pivot point around frame anchors <NUM>.

A shuttle <NUM>, <NUM> is both a drive structure that causes the proof mass 106B to move in response to an electrical signal, and a sense structure that produces an electrical signal representative of movement of the proof mass 106B. The Y axis shuttles <NUM> include Y Force electrodes <NUM>, Y sense or Y pickoff electrodes <NUM>, and one or more Y frequency tuning electrodes <NUM>. The X axis shuttles <NUM> include X Force electrodes <NUM>, X pickoff electrodes <NUM>, and one or more X frequency tuning electrodes <NUM>. The gyroscope <NUM> also includes quadrature electrodes <NUM> used for trimming to reduce quadrature error.

An X axis shuttle <NUM> and a Y axis shuttle <NUM> can be used interchangeably in either a drive mode to drive the proof mass 106B or a sense mode to sense a Coriolis effect on the proof mass 106B. As an example, if the Y axis is the sense axis and is in a Coriolis sense mode and the X axis is the drive axis and is in a drive mode, the Y force electrodes <NUM> are Coriolis force electrodes, the pickoff electrodes <NUM> are Coriolis pickoff electrodes, and the tuning electrodes <NUM> are Coriolis tuning electrodes. The X force electrodes <NUM> are drive electrodes, the X pickoff electrodes <NUM> are velocity pickoff electrodes, and the X tuning electrodes <NUM> are resonant frequency tuning electrodes. Similarly, if the Y axis is the drive axis and the X axis is the sense axis, the Y force electrodes <NUM> are drive electrodes, the pickoff electrodes <NUM> are velocity pickoff electrodes, and the tuning electrodes <NUM> are resonant frequency tuning electrodes. The X force electrodes <NUM> are Coriolis force electrodes, the X pickoff electrodes <NUM> are Coriolis pickoff electrodes, and the X tuning electrodes <NUM> are Coriolis tuning electrodes. To facilitate this mode match of the axes, several structures of the gyroscope <NUM> are symmetric.

<FIG> shows the upper right one-fourth (<NUM>/<NUM>) portion of the gyroscope <NUM> of <FIG>, or the upper right one-sixteenth (<NUM>/<NUM>) portion of the gyroscope <NUM> of <FIG>. A diagonal axis <NUM> is drawn through the portion of the gyroscope and shows that box linkage <NUM>, outer couplers <NUM>, and springs <NUM> are symmetric about the diagonal axis. <FIG> shows a portion of a Y axis shuttle <NUM> and an X axis shuttle <NUM>. Many features of the Y axis shuttle <NUM> are symmetric to the X axis shuttle <NUM> about the diagonal axis <NUM>. The Y force electrodes <NUM> are symmetric with the X force electrodes <NUM>, the Y pickoff electrodes <NUM> are symmetric with the X pickoff electrodes <NUM>, and the Y tuning electrodes <NUM> are symmetric with the X tuning electrodes <NUM>. Y axis shuttle springs <NUM> may or may not be symmetric with X axis shuttle springs <NUM>.

In the example of <FIG>, the quadrature electrodes <NUM> are not symmetric about the diagonal axis <NUM> but can be placed symmetrically along the diagonal axis <NUM>. <FIG> shows that the quadrature electrodes <NUM> are symmetric about either of the X axis or Y axis. <FIG> is another example of a <NUM>/<NUM> portion of a gyroscope <NUM> where the quadrature electrodes <NUM> are symmetric about a diagonal axis <NUM> of the gyroscope <NUM>.

Returning to <FIG>, the symmetry of the MEMS gyroscope <NUM> allows mode switching each of the two mutually orthogonal axes between drive mode and sense mode. As part of the mode switching, the gyroscope <NUM> can be turned ninety degrees (<NUM>°). If the drive axis and the sense axis are the X-axis and Y-axis respectively, this turning or flipping changes sensitivity of the Coriolis effect from the positive Z-axis to the negative Z-axis. The turning together with the mode switching between the axes, cancels out common mode errors due to coupling between the X-axis and Y-axis. The error canceling can be viewed as self-calibration to reduce the offset and quadrature error sources, such as cross-axis damping and other errors common between X and Y axes. The error reduction reduces the offset and quadrature error of the gyroscopes that results in much smaller as-born error signals, drift in error over time, and environmental stresses than is possible without the mode switching.

The movement of the shuttles <NUM>, <NUM> is substantially the same as the movement of the proof mass 106B. For example, if the proof mass 106B moves two micrometers (<NUM>), the frame of the shuttle moves <NUM>. As explained previously herein, the shuttles <NUM>, <NUM> include parallel plate pickoff electrodes <NUM>, <NUM>, that are used to sense the Coriolis effect or velocity depending on the mode of the corresponding axis of the shuttles. The movement of a shuttle changes the size of the capacitive gap space between the pickoff electrodes. It is desired to have the change in gap size to be large (e.g., thirty-percent (<NUM>%) or more of the movement of the proof mass) for mode turnability. However, this much of a change in the gap size can lead to nonlinearities in the movement of the shuttle springs <NUM>, <NUM>, when it is desired for the shuttle springs <NUM>, <NUM>, to move linearly. Previous approaches addressed the non-linearity by using an asymmetric gyroscope design in which the displacement of the sense axis was kept small and turnability of the axes was not possible.

To reduce or eliminate the nonlinearity due to the large displacement of the shuttle springs <NUM>, <NUM>, the box linkage <NUM> and stress relieved anchors <NUM> have minimal positive nonlinear stiffness. In addition, negative nonlinear stiffness (that can be referred to as spring softening) is generated in the pickoff electrodes <NUM>, <NUM>, to tune the gaps produced between the electrodes. Gaps are designed for a particular DC voltage to generate spring softening (negative cubic nonlinearity) that is equal to spring hardening (positive cubic nonlinearity) due to shuttle displacement. This spring softening using voltage cancels the positive nonlinear stiffness (or spring hardening). The overall effect is that the motion frequency of the gyroscope is linear over the full displacement of the gaps. Further tuning of frequencies (over process variations) is done by applying a DC voltage to the frequency tuning electrodes <NUM>, <NUM> to adjust the voltage difference across the gaps. Moreover, a control loop for tuning electrodes maintains the mode match condition over time, temperature, and environmental stress effects.

Multiple shuttle structures contribute to the total negative nonlinear stiffness K(-ve) including the pickoff electrodes <NUM>, <NUM>, the tuning electrodes and the quad electrodes or <MAT> The spring softness can be tuned so that the frequency of the drive mode for the X axis is the same as the frequency of the drive mode for the Y axis.

The axis shuttles <NUM>, <NUM>, in <FIG> include primary frequency tuning electrodes <NUM>, <NUM>. The axis shuttles <NUM>, <NUM>, also include minor tuning electrodes. <FIG> shows axis shuttle <NUM> with primary frequency tuning electrode <NUM> and minor tuning electrodes <NUM>. The minor tuning electrodes <NUM> enable higher resolution tuning of the frequency for the same voltage difference after the tuning with the primary frequency tuning electrodes <NUM>, <NUM> is completed. In some examples, tuning is applied to the shuttles of the axis that has the highest frequency. Primary tuning may be performed first by trimming a DC voltage applied to the primary frequency tuning electrodes <NUM>, <NUM>. After the primary trim, a continuous AC closed mode matching loop uses the minor tuning electrodes <NUM> to match the drive and sense frequencies of the axes while gyroscope is running.

Returning to <FIG>, the mode match MEMS gyroscope <NUM> includes four proof masses 106A-D and four quadrants - one quadrant four each proof mass. The movement of the proof masses is cross quadrant. For example, proof masses 106A and 106C may move with similar phase and proof masses 106B and 106D with similar phase <NUM> degrees out of phase with proof masses 106A and 106C. The arrows <NUM> in <FIG> show an example of driving to move the proof masses in the X axis direction. The arrows <NUM> show that proof masses 106A and 106C move together in the X axis direction, and proof masses 106B and 106D move together in the X axis direction. The arrows <NUM> show an example of the Coriolis force sense direction in the Y axis direction. The arrows <NUM> show that proof masses 106A and 106C move together in the Y axis direction, and proof masses 106B and 106D move together in the Y axis direction.

The primary tuning and the minor tuning may be performed cross quadrant. The primary tuning electrodes of the upper left quadrant and the lower right quadrant may be electrically connected together. A cross diagonal tuning signal is provided to those cross quadrants and the cross quadrants are tuned together. Similarly, the primary tuning electrodes of the upper right quadrant and the lower left quadrant may be electrically connected together, and a cross diagonal tuning signal is provided to tune those cross quadrants together. <FIG> is an example of routing in the MEM gyroscope <NUM> to provide separate tuning signals to the upper left and lower right quadrants, and to the upper right and lower left quadrants.

Driving and sensing may be performed cross quadrant. For example, if the X axis is the drive axis for the mode match MEMS gyroscope <NUM>, and the Y axis is the sense axis for the mode match MEMS gyroscope <NUM>, the X axis pickoff electrodes for the upper left quadrant and the lower right quadrant may be electrically connected together, and those X axis pickoff electrodes provide a combined signal representing displacement of the proof masses 106A and 106C. The X axis pickoff electrodes for the upper right quadrant and the lower left quadrant may be electrically connected together, and those X axis pickoff electrodes provide a combined signal representing displacement of the proof masses 106B and 106D. <FIG> is an example of routing for the X axis pickoff electrodes.

Similarly, the Y axis pickoff electrodes for the upper left quadrant and the lower right quadrant may be electrically connected together, and those Y axis pickoff electrodes provide a combined signal for Coriolis force on the proof masses 106A and 106C. The Y axis pickoff electrodes for the upper right quadrant and the lower left quadrant may be electrically connected together and those Y-axis pickoff electrodes provide a combined signal for Coriolis force on the proof masses 106B and 106D. <FIG> is an example of routing for the Y axis pickoff electrodes.

The symmetry of the axis shuttles and the tunability of the axis shuttles <NUM>, <NUM> allow for the turning and mode switching of the mode match MEMS gyroscope <NUM>. The turning and mode switching periodically changes the axis of vibration from one axis (e.g., the X-axis) to the orthogonal axis (e.g., the Y-axis), which flips the sign of the scale factor of the sensed Coriolis signal so that errors (e.g., from cross-axis damping) are canceled. The turning and mode switching is performed periodically to change the principal axis of vibration (e.g., from the X-axis to the Y-axis) and back (from the Y-axis to the X-axis). The period of the turning should be faster than the time constant of bias instability of Allan Variance.

The output voltage of the MEMS gyroscope is converted to a rotation rate signal. During the flipping of the MEMS gyroscope <NUM>, the gyroscope is turned off and the MEMS gyroscope <NUM> stops sensing. After flipping, the MEMS gyroscope <NUM> is turned on and the drive signal builds up the vibrations of the resonant movement to the desired displacement (e.g., <NUM>). During the flipping time, rate information is not available and would normally be lost and information from the gyroscope would be interrupted.

Recovering the rotation rate information would allow uninterrupted operation of the MEMS gyroscope <NUM>. The rotation rate information can be determined from the electrostatic force (FVIRT) used to turn the MEM gyroscope <NUM> from <NUM>° to <NUM>°. The flipping of the MEMS gyroscope <NUM> can be performed under open loop or closed loop control.

In open loop control, the electrostatic force used to turn the MEMS gyroscope <NUM> is constant, and a different value of force turns the MEMS gyroscope at a different rate (e.g., in the range from <NUM> degrees per second (<NUM>°/sec) to <NUM>°/sec). The electrostatic force FVIRT acts as a virtual rotation rate ΩVIRT, <MAT> where ω is the frequency of oscillation and E is energy. The turn time of the MEMS gyroscope is determined from the virtual rate. As an example, for a virtual rate of <NUM>°/sec, the turn time from <NUM>° to <NUM>° is <NUM>/<NUM> or <NUM> milliseconds (<NUM>), which corresponds to the gyroscope bandwidth of <NUM> Hertz. The physical rotation rate is obtained by subtracting the applied virtual slewing rate (in this example <NUM>°/sec).

In closed loop control, the electrostatic force FVIRT varies. For example, the control loop may be a Proportional-Integral-Derivative (PID) loop that changes the angle from <NUM>° to <NUM>° using a varying FVIRT. For example, the PID angle control loop may change FVIRT to start at a fast rate at first (e.g., <NUM>,<NUM>°/sec) and then a slow rate as it nears <NUM>° for accuracy. The profile of the electrostatic force and the virtual rate are known. The virtual rate can be subtracted from the gyroscope output to get the physical rotation rate information.

<FIG> is a block diagram of a control circuit for open loop control. The frequency of oscillations ω is controlled by a phased-locked loop (PLL) loop that tracks the gyro motion at any arbitrary pattern angle. With the PLL loop enabled, the Gyro Model extracts the slow-varying variables: energy (E), quadrature error (Q), and a pattern angle (θ). A set of PID controllers regulate the following: energy E (amplitude E) by the command voltage EPID, quadrature null action QPID, and a feedforward control FVIRT that continuously rotates the pattern angle (θ) at the rate ΩVIRT. The control forces fx, fy are applied along the pattern angle θ, which is accomplished by the coordinate transform followed by modulation of the command voltages at PLL frequency ω.

The following forces were applied to X and Y electrodes: EPID to control amplitude, QPID to null quadrature, and FVIRT to set virtual rate ΩVIRT:<MAT><MAT>.

The forces for applying virtual rates are the same as the forces to operate the gyroscope in a rate mode if the virtual rate is controlled to operate the gyroscope at a fixed angle (either <NUM>° or <NUM> °).

Claim 1:
A gyroscope (<NUM>), comprising:
a substrate;
a proof mass (106B) coupled to the substrate and configured to move in direction of an X axis and in direction of a Y axis orthogonal to the X axis;
an X axis shuttle (<NUM>) to selectively drive the proof mass along the X axis as a drive axis or sense movement of the proof mass along the X axis as a sense axis in response to the proof mass driven along the Y axis as the drive axis;
a Y axis shuttle (<NUM>) to selectively sense movement of the proof mass along the Y axis as a sense axis in response to the proof mass driven along the X axis or drive the proof mass along the Y axis as the drive axis; and
wherein the X axis shuttle is symmetric to the Y axis shuttle along a diagonal axis (<NUM>) that is diagonal to both the X axis and the Y axis,
wherein the X axis shuttle includes drive-or-sense (drive/sense) electrodes (<NUM>, <NUM>) symmetrical about the diagonal axis to drive/sense electrodes (<NUM>, <NUM>) of the Y axis shuttle, and wherein the gyroscope further comprises:
springs (<NUM>) coupled to the proof mass;
a gap space between the drive/sense electrodes of the X axis shuttle and Y axis shuttle, wherein displacement of the X and Y axis shuttles changes the gap space between the drive/sense electrodes; and
characterized in that
a predetermined DC voltage applied to the gap space causes motion frequency of the proof mass to be linear over full displacement of the X and Y axis shuttles and over a full change of the gap space due to the displacement of the X and Y axis shuttles.