Planar inertial measurement units based on gyros and accelerometers with a common structure

[00002] This application claims priority of Provisional Application Ser. No. 60/341,312, filed on Dec. 17, 2001.

[00003] This invention was made with Government support under contract numbers N00014-00-M-0119 awarded by ONR and DASG60-99-M-0089 awarded by the Air Force. The Government has certain rights in the invention.

[00004] Inertial Measurement Units (IMUs) measure the motion of a body in six degrees of freedom using a set of gyroscopes and accelerometers with input axes aligned with three orthogonal axes. The instruments are typically self-contained devices with electronics and a package. Each instrument is tested prior to use in an IMU. The instruments are then assembled with the proper alignment onto a stable member of the system. With expensive high performance IMUs, the cost associated with this approach may be acceptable. However, with miniature, low cost instruments having correspondingly lower performance, the approach is not cost effective. The solution is to make use of integrating technologies such as Microelectromechanical Systems (MEMS) to fabricate all the instruments at the same time on one substrate to produce an IMU in one package. The testing will be done at the IMU level. In addition to cost and size reduction, integration provides benefits that improve performance. The practicality of integration, though, depends on the proper choice of instrument designs.

[00005] This invention relates to the design and fabrication of integrated, planar inertial measurement units (IMUs) based on planar gyroscopes and accelerometers having a common structure. The common structure is the key to a simpler structural design that is easier to build resulting in high fabrication yield without which integration is not possible. The common structure also simplifies the functional design resulting in improved performance. MEMS integration benefits performance because it prevents tolerance build-up that occurs with the assembly of separate parts to form the whole. With the absence of tolerance build-up, errors due to uncertainty are reduced and instrument stability improved.

[00006] This invention also relates to gyroscope and accelerometer designs based on the common structure. A set of gyroscopes and accelerometers results that becomes the basis from which various IMUs can be designed depending on the application and performance requirements.

[00007] Because of the common structure, instruments can also be structurally combined by sharing a common member to produce sets of instruments that are smaller, require less electronics and perform better. Two-instrument sets can be formed from which various IMUs can be designed. Three-instrument sets, four-instrument sets, etc. can also be formed from which IMU designs can be formed.

[00008] Generally an Inertial Measurement Unit (IMU) can contain any number of gyroscopes and accelerometers. The typical IMU is a six degree-of-freedom (DOF) design containing three single DOF gyroscopes and three single DOF accelerometers. However some applications do not require the measurement of six degrees of freedom and therefore a lower number of instruments will suffice. It is also true in the case of the six DOF IMU, that more than three gyroscopes and three accelerometers can be used. That is because integration allows more sophisticated designs to be configured whereby additional instruments are added to build-in redundancy or to differentially (with pairs of instruments) remove common mode errors, etc. Various combinations of gyroscopes and accelerometers will occur to those skilled in the art of IMU design.

[heading-00009] Several Considerations Apply that Drive the Integrated IMU Design

[none]

• [00010] Consideration1—MEMS integration is essential to achieving the best performance in a miniature IMU.
• [00011] Consideration2—MEMS gyroscopes and accelerometers based on a common structure reduce the requirements on fabrication processes thereby improving yield without which an Integrated IMU is not possible.
• [00012] Consideration3—MEMS gyroscopes and accelerometers based on the common structure simplify the IMU design.
• [00013] Consideration4—Standard gyroscope and accelerometer designs form the basis for designing various IMUs.
• [00014] Consideration5—An integrated IMU makes the most efficient use of space resulting in the smallest size.
• [00015] Consideration6—An integrated IMU requires one package that provides a common environment for all instruments.

[00033] A six degree-of-freedom (DOF) IMU traditionally refers to three gyros and three accelerometers combined on a common member. The instruments are aligned to measure rotations about three orthogonal axes and accelerations along three orthogonal axes. Other six DOF IMU configurations are possible with a different combination of gyroscopes and accelerometers. Additional instruments may be added to improve performance.

[heading-00034] Introduction

[00035] This invention applies to the design and fabrication of a planar integrated IMU using principally MicroElectroMechanical Systems (MEMS) technology. Other planar technologies may be applicable. To simplify fabrication and to enable all instruments to be fabricated on one substrate (chip), the gyroscope and accelerometer designs are based on a common structure. The common structure can be described with common analysis. An IMU based on common instruments will reduce development time and risks associated with the development of different instruments.

[00036] This invention also applies to the design and fabrication of the planar gyroscopes and accelerometers that are based on the common structure.

[00037] This invention also applies to the design and fabrication of instrument sets based on instruments that share a common member.

[heading-00038] Common Structure

[00039] The common structure 10 is illustrated in stick form in FIG. 1. It comprises an inner member 11 that is flexurally connected to an outer member 12 that is in turn flexurally connected to the case 14. Two sets of flexures define orthogonal axes of rotation for the inner and outer members, respectively. For the gyroscope and for the accelerometer, the outer member is driven into sinusoidal oscillation about the Drive Axis 15. For the gyroscope, the inner member is the sense member that responds to gyroscopic torque by oscillating about the Output Axis 16 at the same frequency as the outer drive member, but at an amplitude that is proportional to rotation rate. For the accelerometer, an unbalance mass 17 is added to the inner member converting it into a pendulum 18. The pendulum responds to acceleration by rotating about the Output Axis.

[00040] Three sets of co-ordinate frames (FIG. 1) are needed to describe instrument operation. The s,i,o co-ordinate frame rotates with the inner sense member about the Output Axis by angle . The x,y,z co-ordinate frame oscillates sinusoidally with the outer drive member about the Drive Axis with angle φ={tilde over (φ)} sinωt where {tilde over (φ)} and ω are the oscillation amplitude and frequency respectively. The a,b,c co-ordinate frame rotates in inertial space with the case and vehicle (strap-down implementation).

[heading-00041] Analysis of the Common Structure

[00042] The dynamics of the common structure are governed by the equation of motion for the inner sense member. $\begin{array}{cc}{I}_{\mathrm{So}}\ddot{\vartheta }+D_S\stackrel{.}{\vartheta }+\lfloor K_S+\left\{\left({\Omega }_b^2-{\Omega }_a^2\right)+\frac{1}{2}\left({\omega }^2-{\Omega }_c^2\right){\tilde{\varphi }}^2-2{\Omega }_a{\Omega }_c\tilde{\varphi }\sin \omega t+2{\Omega }_b\tilde{\varphi }\mathrm{\omega cos}\omega t\right\}\Delta I_S\rfloor \vartheta -\left({\Omega }_a{\Omega }_b+{\Omega }_a{\Omega }_c\tilde{\varphi }\sin \omega t+{\Omega }_a\tilde{\varphi }\mathrm{\omega cos}\omega t\right){\theta }^2=I_{\mathrm{So}}{\Omega }_a\tilde{\varphi }\mathrm{\omega cos}\omega t-\Delta I_S\left({\Omega }_a{\Omega }_b+{\Omega }_b{\Omega }_c\tilde{\varphi }\sin \omega t+{\Omega }_a\tilde{\varphi }\mathrm{\omega cos}\omega t\right)+{\tau }_p-{\tau }_{\mathrm{rebalance}}& \left(1\right)\end{array}$
where

• [00044] I_So inner sense member moment of inertia about the o-axis (Output Axis).
• [00045] D_S inner sense member damping.
• [00046] K_S inner sense member flexure stiffness.
• [00047] rotation angle of the inner sense member relative to the drive member
• [00048] {tilde over (φ)} rotation angle amplitude of the drive member relative to the case.
• [00049] Ω_a,Ω_b,Ω_c rotation rates of the case in inertial space about three orthogonal axes.
• [00050] ΔI_S=I_Si−I_Ss inertia difference of the inner sense member inertias about the i-axis and s-axis (tuning inertia).
  To the left of the equals sign in Equation (1) are included the usual torque terms dependent on inertia, damping and stiffness as well as a nonlinear term dependent on inner sense member angle of rotation. The stiffness term is dependent on vehicle rotation rates Ω_a,Ω_b,Ω_c, outer member oscillation frequency ω and a factor referred to as the tuning inertia, ΔI_S as shown in the following equation. $\begin{array}{cc}\lfloor K_S+\left\{\left({\Omega }_b^2-{\Omega }_a^2\right)+\frac{1}{2}\left({\omega }^2-{\Omega }_c^2\right){\tilde{\varphi }}^2-2{\Omega }_a{\Omega }_c\tilde{\varphi }\sin \omega t+2{\Omega }_b\tilde{\varphi }\mathrm{\omega cos}\omega t\right\}\Delta I_S\rfloor & \left(2\right)\end{array}$

[00052] To the right of Equation (1) are included torque terms that drive the inner sense member. They include:

• [00053] I_SoΩ_a{tilde over (φ)}ω cos ωt gyro torque for case rotation about the a-axis.
• [00054] ΔI_S(Ω_aΩ_b+Ω_bΩ_c{tilde over (φ)} sin ωt+Ω_a{tilde over (φ)}ω cos ωt) torque related to the inner sense member tuning inertia.
• [00055] τ_rebalance rebalance torque to maintain the inner sense member at null.
• [00056] τ_p=Pa pendulous torque applied to the inner sense member by acceleration, a, acting on pendulosity, P.
  Gyroscope Mechanization

[00058] Three single-degree-of-freedom (SDF) planar gyroscope configurations are possible to consider from the common structure described by the stick figure of FIG. 1. They are shown in FIGS. 2a, 2b, 2c. The gyroscope configurations are distinguished by the orientations of the Drive Axis (i), Output Axis (o) and Input Axis (s) and the motion of the inner and outer members relative to the plane.

[00059] For the first gyroscope configuration 20, FIG. 2a, the inner sense member 21 rotationally oscillates in and out of the plane about the Output Axis 22 by angle relative to the outer drive member 23. The outer drive member is rotationally oscillated in and out of the plane about the Drive Axis 24 by angle φ relative to the case 25. As the outer member is driven, the inner sense member is also driven relative to the case. The Drive and Output Axes are in the plane and orthogonal to each other. The Input Axis 26 is expected to be normal to the plane.

[00060] For the second gyroscope configuration 30, FIG. 2b, the inner sense member 31 rotationally oscillates about the Output Axis 32 that is normal to the plane by angle θ relative to the outer drive member 33. The outer drive member is rotationally oscillated in and out of the plane about the Drive Axis 34 by angle φ relative to the case 35. As the outer member is driven, the inner sense member is also driven relative to the case. The Drive and Input Axes are in the plane and orthogonal to each other. Rotation Rate is measured about the Input Axis 36.

[00061] For the third gyroscope configuration 40, FIG. 2c, the inner sense member 41 rotationally oscillates in and out of the plane about the Output Axis 42 by angle relative to the outer drive member 43. The outer drive member is rotationally oscillated about the Drive Axis 44 that is normal to the plane by angle φ relative to the case 45. As the outer member is driven, the inner sense member is also driven relative to the case. The Input and Output Axes are in the plane and orthogonal to each other. Rotation rate is measured about the Input Axis 46.

[00062] The gyroscope of the second configuration is appealing because zero tuning inertia, ΔI_S=0, can be realized by in-plane symmetry of the inner sense member. The benefit is the elimination of sensitivity to input rates about orthogonal axes as can be seen from the simplified equation of motion. The equation of motion becomes
I_So+D_S+K_S−(Ω_aΩ_b+Ω_aΩ_c{tilde over (φ)} sin ωt+Ω_a{tilde over (φ)}ω cos ωt)²=I_SoΩ_a{tilde over (φ)}ω cos ωt  (3)
where the non-linear term in ² is further ignored because the output angle is typically very small. The second configuration enables a good strap-down gyro. To obtain maximum gyroscope response and therefore maximum oscillation amplitude of the inner sense member, _Max, the inner and outer member resonance frequencies are matched. $\begin{array}{cc}{\tilde{\vartheta }}_{\mathrm{Max}}=\frac{I_{\mathrm{So}}}{D_{\mathrm{So}}}\tilde{\varphi }{\Omega }_a& \left(4\right)\end{array}$

[00065] A practical gyroscope however may be operated with a small offset between resonance frequencies. Typically the outer member is driven at resonance to minimize power consumption and the inner member responds off-resonance at the outer member frequency. The resonance frequencies are set by the stiffness of the flexures and inertia of the members. The outer member oscillation amplitude is held constant so that the output per given rotation rate is constant (constant scale factor).

[00066] A gyroscope embodiment based on the second configuration 50 is illustrated in FIG. 3. The inner sense member 51 is connected with four radial flexures 52 to the outer drive member 53 (ring shaped). The outer drive member is connected with a pair of torsional flexures 54 to the case 55. Drive comb actuators 56 oscillate the ring in and out of the plane. Ring comb pick-offs 57 sense the oscillation amplitude of the ring about the Drive Axis 58. Two sets of pick-offs are used that enable differential operation to eliminate common mode noise between them resulting in signals related to the motion only. Two sets of inner sense member comb pick-offs 59 measure the output oscillation of the inner sense member relative to the outer member ring. The pick-offs are connected differentially. The comb drive and pick-off designs are shown conceptually. In reality a large number of comb fingers are used along the circumference of the members.

[00067] The gyroscope is divided into electrical regions a-g using electrical isolation spacers to enable the independent operation of the pick-offs and drives. Spacers comprise an electrical insulator that is located in between sections that preserve mechanical functionality. Spacer 60 separates two halves of the inner sense member for differential operation of the inner member pick-off. Spacers 61 isolate the inner sense member from the outer member ring to enable operation of the pick-off. Spacers 62, 63 isolate the comb drive from the ring to enable operation of the drive combs. Spacers 63, 64 isolate the outer member pick-off from the comb drive actuators. Not all spacers are identified but they are obvious from symmetry.

[00068] A surface isolation film 65 is deposited over the torsional flexures to enable conductors to pass across them to separately connect to the two halves of the inner member.

[00069] A gyroscope embodiment based on the third configuration 70 is illustrated in FIG. 4. The inner sense member 71 is connected with two torsional flexures 72 to the outer drive member 73 (ring shaped). The outer drive member is connected with four radial flexures 74 to the case 75 (at least three flexures are needed). Comb actuators 76 drive the ring to oscillate about the normal to the plane. Ring comb pick-offs 77 sense the oscillation amplitude of the ring about the Drive Axis 78. Two sets of pick-offs enable differential operation to eliminate common mode noise between them, resulting in signals related to the motion only. Two sets of inner member comb pick-offs 79 measure the output oscillation of the inner sense member relative to the outer member ring. The pick-offs are connected differentially. The comb drive and pick-off designs are shown conceptually. In reality a large number of comb fingers are used along the circumference of the members.

[00070] The gyroscope is divided into electrical regions a-g using electrical isolation spacers to enable the independent operation of the pick-offs and drives. Spacer 80 separates two halves of the inner sense member for differential operation of the pick-offs. Spacers 81 isolate the inner member from the outer member ring to enable operation of the pick-off. Spacers 82, 83 isolate the comb drive from the ring to enable operation of the drive comb actuator. Spacers 83, 84 isolate the outer member pick-off from the comb drives.

[00071] A surface isolation film 85 is deposited over two radial flexures to enable conductors to pass across them to allow connection to the two halves of the inner member.

[00072] An embodiment of the gyroscope of the second configuration with opposing capacitive plates 90 instead of comb designs is illustrated in FIG. 5. Two layers are used: a device layer 91 and a substrate layer 92 (Pyrex in this case). The device layer is bonded to the substrate via a mesa 93 in the device layer. The mesa forms the capacitive gap 94 and allows motion of the outer member 95 in and out of the plane. The mesa and the Pyrex Layer form the case 96. One set of drive capacitive plates 97 oscillates the outer member relative to the case. A second set (not shown) is used to measure the outer member motion. A third set, pick-off plates 98 measures the motion of the inner sense member 99. In this embodiment, isolation spacers are not needed since the capacitive plates on the Pyrex Layer are isolated since Pyrex is an electrical insulator. A disadvantage of this design is that the sensed motion of the inner sense member pick-off contains the combined motions of the inner sense member relative to the outer member and relative to the case. A careful design of the inner sense member pick-off can reduce the sensed motion relative to the case, however. Surface isolation is not needed because connections are made on the Pyrex surface. An advantages of the two layer design is the flexibility to select materials and dimensions of one layer somewhat independently of the other.

[00073] An opposing capacitor plate design can be carried out where the substrate layer is of the same material as the device layer to reduce bimetallic stress. The substrate layer, however, can be made of a different thickness to add stability.

[00074] Combs and plates can be combined to actuate and sense motion of the instrument members. The combination used depends on the functionality desired.

[heading-00075] Dynamically-Tuned Gyroscope Mechanization

[00076] Because the two resonance frequencies of the fabricated gyro may not be matched, it may be necessary to include a mechanism in the design that will allow tuning after fabrication. The approach is to shape the inner member of the gyro of the second configuration so that it has a tuning inertia like the accelerometer. After the outer member is driven to resonance, the amplitude of the outer member oscillation is varied to tune the flexure stiffness of the inner member relative to the resonance frequency of the outer member.

[heading-00077] Dynamically-Tuned Accelerometer Mechanization

[00078] The accelerometer based on the common structure comprises a pendulum that is oscillated about the long axis of the pendulum through the use of the outer member. The oscillation dynamically tunes the stiffness of the flexures of the inner member so as to make them effectively weaker for motion of the pendulum about the Output Axis. The main benefits derived are increased accelerometer sensitivity and reduced bias instability related to the pick-off instability. The accelerometer is disclosed in U.S. Pat. No. 6,338,274, incorporated by reference herein.

[00079] The dynamically tuned accelerometer (DTA) is obtained conceptually from the common design by adding mass to the inner sense members of the gyroscope configurations to make them pendulous as shown in FIGS. 6a, 6b and 6c. The three DTA configurations are distinguished by the Drive Axis (i), the Output Axis (o) and the accelerometer Input Axis.

[00080] For the first DTA configuration 100, FIG. 6a, the inner member 101(pendulum) rotates out of the plane due to the action of acceleration on the pendulous mass 105. The outer member 102 is oscillated in and out of the plane about the Drive Axis 103. The accelerometer Input Axis 104 is normal to the plane. For the second DTA configuration 110, FIG. 6b, the inner member 101 rotates in the plane about the Output Axis 102 and the outer member 103 oscillates about an axis in the plane. The location of the pendulous mass 115 determines the direction of the Input Axis. The accelerometer Input Axis 104 and the Drive Axis are in the plane. For the third DTA configuration 120, FIG. 6c, the inner member 121 rotates about the Output Axis 122 in the plane and the outer member 123 oscillates about the Drive Axis 124 normal to the plane. The accelerometer Input Axis 125 is normal to the plane and aligned with the Drive Axis.

[00081] Dynamic tuning is better understood from the equation of motion after the rotational sensitivities are removed. The simpler equation of motion becomes

[heading-00082]  I_So+D_S+[K_S+φ^Y2 ΔI_S]=τ_p−τ_rebalance  (5)

[00083] The bracketed term contains the sum of the flexure stiffness and the dynamic stiffness. The dynamic stiffness is given by $\begin{array}{cc}{K}_D={\stackrel{.}{\varphi }}^2\Delta I_S=\frac{1}{2}\Delta I_S{\omega }^2{\tilde{\varphi }}^2& \left(6\right)\end{array}$

[00084] By oscillating the inner sense member by driving the outer member with an amplitude {tilde over (φ)} and frequency ω and by setting the inner member inertias to obtain ΔI_S=I_i−I_s0 (less than zero), a negative dynamic stiffness is obtained that can cancel or lessen the flexure stiffness. I_i,I_s are inertias about the i-axis and s-axis, respectively.

[00085] Not all three configurations are suitable for dynamic tuning, however, since the tuning inertia is not negative for all three. As shown in FIG. 6, only the first planar configuration satisfies the tuning inertia requirement. The second configuration can be modified to a tunable design, however, by elongating the structure along the i-axis and breaking the symmetry of the inner sense member as shown in FIG. 7. The third configuration cannot be made tunable in the planar form. It requires extension into the plane. A tunable second configuration accelerometer 130 comprises an inner sense member 131 that is not symmetric about the i-axis and s-axis. Radial flexures 132 enable the rotation of the pendulum in the plane.

[00086] It is necessary for the outer member oscillation amplitude to be held constant to maintain the accelerometer at the desired tuning level. The accelerometer is also operated closed loop otherwise an effectively weakened flexure will result in bottoming of the pendulum against stops.

[00087] Since the accelerometer is based on the same structure as the gyroscope it will have some sensitivity to rotation rate depending on how well it meets the conditions for operation of the gyroscope. Two conditions prevent the accelerometer from being a gyroscope however: the inner member resonance frequency is designed to be much lower than the outer drive member resonance frequency and the oscillation output of the inner sense member is filtered since it is sinusoidal.

[00088] Other sensitivities to rotation rates however can manifest themselves by changes in flexure stiffness, and bias torques coupled to ΔI_T as shown in Equation 1. These errors can be compensated by accelerometer measurements taken under rotation.

[heading-00089] Multi-Sensor Mechanization

[00090] The common structure can be mechanized to form the gyro or the accelerometer as discussed above. For the proper set of conditions, one instrument can be made to sense both rotation rate and acceleration. The multi-sensor embodiment 220 is shown in FIG. 8. It combines the gyroscope second configuration with the tunable accelerometer second configuration. The key feature is an inner member 221 with the appropriate tuning inertia. Other conditions include the proper choice of flexure stiffness and inertias for the inner and outer members 222. The gyroscope function operates as the gyroscope of the second configuration but with tunability. The accelerometer operates as the accelerometer of the second configuration. The gyroscope and accelerometer outputs are separable because the gyroscope output is oscillatory and the accelerometer output is DC level.

[heading-00091] Integrated IMU, Level 1

[00092] The integrated IMUs are planar embodiments comprising various distributions of planar gyroscopes and accelerometers having the common structure, as described above. That is, each gyroscope and accelerometer is composed of an outer member that is driven and an inner sense member that responds to either rotation rate or acceleration.

[heading-00093] First Integrated IMU Based on the Common Structure

[00094] A first six DOF, integrated IMU embodiment 140 is based on gyroscopes of the second configuration and accelerometers of the first and second configurations as shown in FIG. 9a. These configurations are further distinguished by outer drive members that oscillate in and out of the plane. The x, y, z axes form the co-ordinate frame for the IMU. The x-axis and y-axis are in the plane and the z-axis is normal to the plane. Two gyroscopes of the second configuration are used to sense rotation rates about the x-axis and y-axis by aligning the Input Axis of the first with the x-axis and aligning the Input Axis of the second with the y-axis. These are identified as G_x and G_y. The accelerometer of the first configuration is used to sense acceleration along the z-axis since its Input Axis is normal to the plane. It is identified as A_z. Two accelerometers of the second configuration are used to sense acceleration input along the x-axis and y-axis. These are identified as A_x and A_y. To simplify the organization of the gyroscopes and accelerometers in the IMU design, each instrument is designed to occupy a square space (cell) of the same size. This allows any orientation or location of instruments in the IMU design.

[00095] The first IMU comprises nine unit cells arranged in a three by three matrix. In the first row, from left to right, are located gyro G_y 141, accelerometer A_y 142 and gyro G_x 143. In the second row are located accelerometer A_x 144, a space left for test devices and a second accelerometer A_x 145. In the third row are located gyro G_x 146, accelerometer A_y 147 and accelerometer A_z 148. Each instrument is attached to the substrate 149 by mesa structures 150 or similar structures that support the devices from the substrate so the devices are free to move. The purpose of this arrangement of instruments is to place four accelerometers, two A_x and two A_y in a cross configuration 151, with the accelerometers displaced an equal distance R from the IMU center. The cross configuration of accelerometers responds to the sum of linear and centrifugal accelerations. By the appropriate summing and differencing of the signals from the four accelerometers, rotation rate and linear accelerations along the two axes in the plane can be separated. The separation of signals is possible because rotation rate causes all the pendulums to rotate outwards while acceleration causes one pendulum to rotate outwards and the second to rotate inwards for each set of two accelerometers along each axis. Rotation rate Ω is related to the centrifugal acceleration by the relation
a_centrifugal=Ω²R  (7)

[00097] A second benefit of the cross configuration of accelerometers is that the measurement of acceleration along the x-axis and along the y-axis can be done differentially by each set of two accelerometers. Differential operation is a means to cancel non-acceleration, common mode signals. In the first embodiment, the two y-gyroscopes can also be operated differentially.

[00098] A first variation on the first embodiment can be obtained by adding a second gyro G_y in the location of accelerometer A_z and moving the accelerometer into the center cell position. The result is that all instruments are placed symmetrically about the center of the IMU and all except for the z accelerometer can be operated differentially.

[00099] A second variation on the first embodiment is to use any other planar accelerometer design regardless of whether it is dynamically tunable or not.

[heading-00100] Second Integrated IMU Based on the Common Structure

[00101] A second, six DOF, integrated IMU embodiment is based on gyroscopes of the second configuration and accelerometers of the first and second configurations as shown in FIG. 9b. It varies from the first embodiment by the orientation of the accelerometers in the cross configuration. The Drive Axis is oriented radially with the IMU center. In this case the accelerometers are sensitive to linear acceleration plus angular acceleration. Rotation rate is obtained by the appropriate summing and differencing of the signals from the four accelerometers. In this case, however, the rotation rate is obtained by integrating the sensed acceleration signal.

[00102] The second IMU embodiment 160 comprises nine unit cells arranged in a three by three matrix. In the first row, from left to right, are located gyro G_y 161, accelerometer A_x 162 and gyro G_x 163. In the second row are located accelerometer A_y 164, a space left for test devices and a second accelerometer A_y 165. In the third row are located gyro G_x 166, accelerometer A_x 167 and accelerometer A_z 168.

[00103] A variation on this embodiment places a second gyro G_y at the location of accelerometer A_z and moves the accelerometer to the central cell position.

[heading-00104] Third Integrated IMU Based on the Common Structure

[00105] A third, six DOF, integrated IMU embodiment is based on gyroscopes of the third configuration and accelerometers of the first and second configurations as shown in FIG. 9c. This configuration mixes gyroscopes having outer drive members that oscillate about the normal to the plane and accelerometers with outer drive members that oscillate in and out of the plane. The x, y, z axes form the co-ordinate frame for the IMU. The x-axis and y-axis are in the plane and the z-axis is normal to the plane. Two gyroscopes of the third configuration are used to sense rotation rates about the x-axis and y-axis by aligning the Input Axis of the first with the x-axis and aligning the Input Axis of the second with the y-axis. These gyroscopes are identified as G_x and G_y. The accelerometer of the first configuration is used to sense acceleration along the z-axis since its Input Axis is normal to the plane. It is identified as A_z. Two accelerometers of the second configuration are used to sense acceleration input along the x-axis and y-axis. These are identified as A_x and A_y . To simplify the organization of the gyroscopes and accelerometers in the IMU design, each instrument can be designed to occupy a square space (cell) of the same size. This allows any orientation or location of instruments in the IMU design.

[00106] The cross configuration of the accelerometers is the same as it was for the first IMU embodiment. It senses the sum of the linear and centrifugal accelerations. The signals are separated as described for the first embodiment.

[00107] The third IMU embodiment 170 comprises nine unit cells arranged in a three by three matrix. In the first row, from left to right, are located gyro G_y 171, accelerometer A_y 172 and gyro G_x 173. In the second row are located accelerometer A_x 174, a space left for test devices and a second accelerometer A_x 175. In the third row are located gyro G_x 176, accelerometer A_y 177 and accelerometer A_z 178.

[00108] A variation on this embodiment places a second gyro G_y at the location of accelerometer A_z and moves the accelerometer to the central cell position.

[heading-00109] Fourth Integrated IMU

[00110] A fourth, six DOF IMU embodiment comprises the addition of a z-gyro without the common structure. In this case all the other instruments would be common. The cross configuration would not be necessary but it would be useful for the differential operation of the accelerometers.

[heading-00111] Instrument Sets Based on the Common Structure

[00112] Instrument sets comprise gyroscopes and accelerometers based on the common structure that are combined by sharing the outer member. Two sets are possible based on the gyroscope of the second configuration and accelerometers of the first and second configurations. All share an outer member that oscillates in and out of the plane. Instrument sets are not multisensors as one instrument does not sense both acceleration and rotation rate. The sets comprise separate instruments by virtue of the separate inner sense members that can be optimized separately.

[00113] The first embodiment of an instrument set 180 combines gyro G_x 181 with accelerometer A_x 182 or equivalently gyro G_y with accelerometer A_y as shown in FIG. 10. A capacitive comb drive 183 is used to oscillate the outer member 184 in and out of the plane. Separate pick-offs (not shown) are used to sense the motion of the inner members. The outer member is connected with torsional flexures 185 to the case mesa 186.

[00114] The second embodiment of an instrument set 190 combines accelerometer A_x 191 with accelerometer A_z 192 or equivalently accelerometer A_y with accelerometer A_z as shown in FIG. 11. A capacitive comb drive 193 is used to oscillate the outer member 194 in and out of the plane. The outer member is connected with torsional flexures 195 to the case mesa 196.

[00115] A third embodiment of an instrument set combines three instruments with a common outer member. Gyroscope G_x is combined with accelerometer A_x and accelerometer A_z or equivalently gyro G_y is combined with accelerometer A_y and accelerometer A_z.

[00116] The significance of the gyro/ accelerometer set is that the gyro sensitivity of the accelerometer to rotation is about the same axis as the gyroscope, therefore the gyro signal can be used to compensate the accelerometer for its gyro-related error. A consideration for the design of the gyro/accelerometer instrument set is that the requirements for the gyroscope and accelerometer need to be met separately.

[heading-00117] IMU Integration, Second Level

[00118] A close inspection of FIG. 9a indicates that a second level of integration is possible if instrument sets are used. The arrangement of the instrument sets in the IMU embodiment 200 is shown in FIG. 12. Three gyro/accelerometer sets 201 and one accelerometer/accelerometer set 202 are used. Four accelerometers are arranged in a cross configuration 203 as before to enable the sense of rotation rate in addition to linear acceleration. The centrifugal acceleration mode is used.

[00119] A z-gyro can be added to the center cell to form a variation on the IMU embodiment.

[00120] A variation on the IMU embodiment uses the instrument sets so that the drive axes of all four sets are arranged radially along the x-axis and y-axis. In this case the four accelerometer configuration measures angular acceleration plus linear acceleration.

[00121] The benefits of these embodiments is the reduction of drive electronics and size.

[heading-00122] IMU Integration, Third Level

[00123] An additional level of integration or perhaps simplification can be made to the embodiment described in FIG. 12 by using common drive electrodes 207, 208 to drive all the outer members 209 to form the embodiment 210 shown in FIG. 13. A consideration for this design is the drive of the four outer members at the same resonance frequency. Resonance drive is used to minimize the power needed. Tuning mechanisms may be needed for the outer members.

[00124] A benefit of this design includes one set of drive electronics for the IMU.

[heading-00125] Fabrication Approach

[00126] The fabrication approach is described for one instrument but applies equally as well to a set of instruments fabricated at the same time on one substrate. The Dissolved Wafer Process (DWP) is representative of a typical process. The process is a good match to the requirements for the integrated IMU design that include the use of one structural material for the devices to reduce stress and warpage on the parts and to allow the fabrication of instruments on a common substrate that are separated structurally at the device level. The DWP process is representative of a transfer process in that the devices are fabricated on one substrate and transferred onto a second substrate. The advantage of the DWP is the flexibility in the choice of materials and thickness of the device and substrate layers. Low stress devices can be fabricated onto a stable rigid base layer for stability.

[00127] The process flow is described using five masks in FIG. 14.

• [00128] Step a: The starting material is a 4″ diameter silicon wafer 230 with a thickness of p++ boron diffused Epitaxial layer 231 grown on top.
• [00129] Step b: Apply Mask 1 to pattern mesas 232, posts 233 and wall structures 234; plasma etch into the epitaxial layer to form them with some height. The mesa is the point of contact between the silicon wafer and the Pyrex substrate after bonding. The mesa height is selected to form the capacitive gap that allows movement of the inner and outer members. The post electrically grounds the metallizations during anodic bonding so that the voltage applied does not destroy the metallizations. The wall is formed around the full IMU. Its function is to keep the gap volume clean during cutting of the bonded wafers.
• [00130] Step c: Apply Mask 2 to pattern the accelerometer structure 235: plasma etch into the epitaxial layer to form the sidewalls of the device. Etch through the full thickness of the epitaxial layer and partially into the silicon substrate.
• [00131] Step d: Start the Pyrex wafer 236. Apply Mask 3 to pattern a well 237 into the Pyrex. Plasma etch to form the well. The thickness of the mesa plus the well depth make up the gap. A gap dimension is selected that prevents sticking of the device members to the Pyrex that gives good capacitive sensing and actuation.
• [00132] Step e: Apply Mask 4 to pattern recessed trenches 238 in the Pyrex in preparation for metallization. Etch trenches into the glass to a suitable depth.
• [00133] Step f: Deposit chromium/platinum metal film over the full wafer surface. This will deposit metal into the etched trenches formed in the previous step, extending slightly above the top of the trench. Remove the remaining resist and other metal using lift off, leaving only metal in the trenches 239.
• [00134] Step g: Anodically bond the Pyrex layer to the epitaxial side of the silicon wafer 240. The devices are ready for post processing.
  Post Processing

[00136] The resulting two-wafer lamination is cut to separate the IMUs. The individual IMUs are placed into chambers of a Teflon container and all immersed into a chemical EDP etch to dissolve the silicon substrate 241 leaving untouched the epitaxial structures 242 bonded to the Pyrex 243. The epitaxial structure comprises the set of devices. The IMU chips are then placed into packages and wire bonded.