VIBRATING RING GYROSCOPE

Inventors: Daniel S. Choi, Boohyun An, Jisung Lee, Waqas Amin Gill, Aveek Chatterjee, Hyun Kee Chang, Seungoh Han

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Application Serial No. 62/271,788, filed December 28, 2015, the entire disclosure of which is fully incorporated herein by reference.

TECHNICAL FIELD

[0001] The present application relates to vibrating ring gyroscopes, and more particularly, to vibrating ring gyroscopes on a (100) single crystalline silicon having an octagonal star- shaped anchor.

BACKGROUND

[0002] Microelectromechanical Systems (MEMS) vibrating ring gyroscope (VRG) is an attractive candidate of inertial sensors for measuring and maintaining the position of the device/ using of Coriolis forces to tune the properties of vibrating structures in the presence of rotation rates. Due to the symmetrical structure MEMS VRG provides lots of advantages than other vibrating gyroscopes, including excellent mode matching, high resolution, high thermal stability, and high performance in harsh environment. The ideal VRG should have perfect mode matching for its two identical flexural modes, however due to complex and exhaustive fabrication processes, there is always a mismatch between these two modal frequencies. A high aspect ratio poly and single crystal silicon technology adopted for ring gyroscope and a (111) oriented single crystal silicon VRG demonstrated by high gyroscopic performance. However, these technologies require more processing time for etching and packaging than (100) single crystal silicon-based technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0003] Reference should be made to the following detailed description which should be read in conjunction with the following figures, wherein like numerals represent like parts:

[0004] FIG. 1A illustrates an exemplary embodiment of a vibrating ring gyroscope consistent with at least one embodiment of the present disclosure;

[0005] FIG. IB illustrates an exemplary embodiment of a vibrating ring gyroscope including various dimension consistent with at least one embodiment of the present disclosure;

[0006] FIG. 2 shows the results from modal analyses in <110> direction consistent with at least one embodiment of the present disclosure;

[0007] FIG. 3A illustrates an exemplary embodiment of variation of modal frequencies with spring widths of <1 10> and <100> consistent with at least one embodiment of the present disclosure;;

[0008] FIG. 3B illustrates an exemplary embodiment of process sensitivities of the devices on vaiied widths of the support spring consistent with at least one embodiment of the present disclosure;

[0009] FIG. 3C illustrates an exemplary embodiment of fine tuning of modal frequency by controlling of radii of the support spring consistent with at least one embodiment of the present disclosure;

[0010] FIG. 4 illustrates one embodiment of the electrodes on a VRG consistent with at least one embodiment of the present disclosure;

[0011] FIG. 5 illustrates one embodiment of an interface electronics in Simulink model consistent with at least one embodiment of the present disclosure;

[0012] FIG. 6 illustrates one embodiment of the demodulated voltage output from sinusoidal angular velocity input waveform with 100 7s of amplitude (i.e., the demodulated voltage output due to sinusoidal angular velocity input, consistent with at least one embodiment of the present disclosure; and

[0013] FIG. 7 illustrates the demodulated output plot of simulated voltage vs. varied angular velocity input of 0.01, 0.1, 1, 10, 100, 1000 7s, consistent with at least one embodiment of the present disclosure. [0014] Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications and variations thereof will be apparent to those skilled in the art.

DETAILED DESCRIPTION

[0015] Global positioning system (GPS) navigation plays a major role in modern daily life. The navigation feature is now readily available in our electronic devices such as smartphones and tablets. GPS requires signals for its normal operation. However, GPS is equipped with inertial measurement unit (IMU) in case signals are not available such as working under the tunnel, inside buildings. A gyroscope is an integral part of IMU that tends to be used for measuring and controlling positions of moving objects when they are subjected to rotation. Micro-electromechanical systems (MEMS) vibrating ring gyroscope (VRG) is an attractive candidate for IMU. Due to the symmetrical structure, MEMS VRG can provide lots of advantages such as excellent mode matching, high resolution, and high thermal stability compared with other vibrating gyroscopes.

[0016] VRG utilizes a resonance vibration mode pair in the driving and sensing modes to maximize energy transfer between the two modes. A resonance pair of vibration modes refers to two modes, so-called wine-glass modes that have distinct mode shapes but identical natural frequencies. Ideally, these two identical modes should have the same value of a resonance frequency in VRG with symmetric shapes. However, there always exists a certain mode mismatch between these two frequencies because of inevitable process variation during the microfabrication process (such as, but not limited to, the effect of undercut during deep reactive ion etching, spatial variations in wafer, chip, and feature level). This frequency mismatch can be electrostatically tuned by applying optimum bias through tuning electrodes. High aspect ratio poly crystalline and (111) oriented single crystal silicon VRG may demonstrate high gyroscopic performance, however, the respective microfabrication requires additional processes for etching and packaging compared with other know (e.g., conventional) (100) single crystal silicon-based technology. However, fabrication of a symmetric structure gyroscope based on a (100) silicon wafer has more variations that should be controlled because of its inherent mechanical anisotropy which leads the frequency split to untunable values and makes etch-rate differences between crystalline directions of the wafer. Frequency tuning methods have been proposed to overcome the mechanical anisotropy by adjusting the width and position of spokes in the disk resonance gyroscope (DRG) structure designed on (100) silicon wafer.

[0017] A (100) single crystal silicon vibrating ring gyroscope (VRG) therefore has a significant mismatch between two flexural modes frequencies due to the mechanical anisotropy of the material. Previous VRGs have used a circular anchor for the attachment of the support springs.

[0018] According to at least one embodiment, a VRG consistent with the present disclosure includes a fine-tuned VRG on a (100) single crystalline silicon using an octagonal star-shaped anchor. For example, the circular anchors of previous VRG were replaced by octagonal star-shaped anchors with respective radii to accommodate the varied values of support spring at <110> and <100>. VRGs having octagonal star-shaped anchors achieved enhanced matching in between two identical flexural modes frequencies. The unique shape of the anchor of the present disclosure facilitates the independent controls of widths and radii of the support-springs that enable fine tuning of wine-glass modal frequencies between <110> direction and <100> direction which has 45° difference to each other.

[0019] By way of a general overview, a VRG consistent with at least one embodiment of the present disclosure comprises support-springs and a ring structure surrounded with one or more (e.g., a plurality) of electrodes. The electrodes are used for driving electrodes, sensing electrodes and tuning electrodes of the gyroscope. The operation of the ring gyroscope relies on two elliptically shaped vibration modes, <110> direction mode and <100> direction mode, which are also called the driving and sensing modes, respectively. Those flexural modes have identical natural frequencies due to the symmetry of the ring. Silicon (100) material has mechanical anisotropy that brings different resonant behaviors in different crystallographic directions of the wafer. Since circular shapes were commonly used for anchor structures that join the support springs with the substrate in previous works, there was a limitation on independent modification in two identical directions of modes.

[0020] The present disclosure features an octagonal star-shaped anchor that facilitates respective control of radii of the support-springs. FIG. 1(A) generally illustrates one embodiment of a vibrating ring gyroscope 10 with an octagonal star-shaped anchor 12, a plurality of support springs 16 (e.g., but not limited to, eight supported springs), and a plurality of electrodes 18 (e.g., but not limited to, sixteen electrodes). The octagonal star- shaped anchor 12 may include a plurality of tip regions 20 (e.g., but not limited to, eight tip regions). For example, each tip region 20 may correspond to a respective one of the eight sides of the octagon. As may be seen, the tip regions 20 each have a triangular shape, e.g., such that the anchor 12 has an overall octagonal star-shape. The triangular shape of the tip regions 20 may include a right triangle. Exemplary design parameters listed for the (100) single crystal silicon VRG 10 (e.g., generally illustrated in FIG. IB) are listed in Table 1 below. The widths for both the designs of the support spring beams 16 at <110> and <100> directions were varied from 4 μπι to 7 μπι (with an increment of 0.1 μπι) and radii for support spring beams 16 were also varied from 265 μπι to 310 μπι (with an increment of 1 μπι). The extensive parametric study by varying the design parameters was done by CoventorWare™ and MATLAB™.

[0021] TABLE 1

DESIGN PARAMETERS OF VIBRATING RING GYROSCOPE

Design parameters Device A Device B

(μΐη) (μιη)

1 Ring radius 750

2 Anchor radius 200

3 Ring width 5

4 Gap between ring and

1.4

electrode

5 Spring radius at 273 297

<110>

6 Spring radius at 278 295

<100>

7 Spring width at 5 5

<110>

8 Spring width at 5.6 5.4

<100>

[0022] Calculation of widths of the support-springs 16 in <110> and <100> directions for rough tuning of the frequencies were performed after fixing the radius of the ring as 750 μπι, which offers a range of resonance frequency from approximately 20 kHz to approximately 30 kHz. FIG. 2 shows the results from modal analyses in <110> direction using CoventorWare & MEMS+ . The widths for both designs of the support-spring beams 16 at <110> and <100> directions varied from 4 μπι to 7 μπι with an increment of 0.1 μπι and radii for support-spring beams 16 were also varied from 265 μπι to 310 μπι with an increment of 1 μπι.

[0023] While the parametric study showed the several combinations of lowest frequency splits, two combinations were selected to proceed with further calculation and fabrication from the results (Table 1). The modal frequencies of two modes were sensitively changed by adjusting the widths of the support springs 16 as expected and the frequency splits of the VRG structure 10 could be tuned to 11.5 Hz (Device A) and 12.4 Hz (Device B) on the structures by controlling of the radii of the support springs 16 (FIGS. 3A-3C) from hundreds of Hz on untuned structures. The modal frequencies of Device A and Device B are around 24.5 kHz and 21.9 kHz, respectively. In particular, FIG. 3A generally illustrates variation of modal frequencies with spring widths of <110> and <100>, FIG. 3B generally illustrates process sensitivities of the devices on vaiied widths of the support spring, and FIG. 3C generally illustrates fine tuning of modal frequency by controlling of radii of the support spring 16.

[0024] The design features for Device A were selected to reduce the mismatch using an octagonal star-shaped anchor to facilitate the spring radius and width to minimize the mode mismatch. Device A has a support spring with the radius of 273 μπι in <110>, and 278 μπι at <100> and the width of the support spring in <110> is 5.0 μπι and at <100> is 5.6 μπι. Device B has the support spring with the radius of 297 μπι in <110> and 295 μπι in <100> and the width of the support spring in <110> is 5.0 μπι and in <100> is 5.4 μπι. It should be appreciated that the instant application is not limited to these dimensions unless specifically claimed as such.

[0025] Since the VRG 10 is well mode-matched in the static state by structure design, the frequency mismatches between the driving and the sensing modes should occur when a DC bias voltage is applied to the anchor. Further mode-matching calculations were performed prior to simulation of interface electronics. The configuration of the electrodes is shown in FIG. 4. In particular, FIG. 4 is a representation of the electrodes 18 on the VRG 10, in which the electrodes for 'Driving' are represented as electrodes Dl _a -b and D2 _a -b, the electrodes for 'Sensing' are represented as electrodes Sl _a -b and Sl _a -b, and the electrodes for tuning are represented as electrodes Tl _a _b, T2 _a _b, T3 _a _b, and T4 _a _b. A DC bias voltage of 1 V was applied to center of the ring structure in order to decrease stiffness of the ring structure and a DC tuning bias voltage of 0.27 V was applied to a couple of tuning electrodes, which compensates frequency mismatching by DC bias for Device A. AC driving voltage with amplitude of 48 mV calculated by 80 % of maximum displacement without pull-in occurrence and frequency of 21829.6 Hz which is the resonance frequency from calculation.

[0026] In order to simulate VRG with interface electronics, the MEMS+ ^® model was imported into Simulink with Simscape tools of transimpedance amplifier and demodulator as shown in FIG. 5. The differential capacitance changes between the sensing electrodes in different directions (45° and 135°) according to angular velocity input is measured by voltage meter after the amplifier. Then the amplified voltage signal is passed through a filter and demodulated. The parameters of interface electronics are assumed as C _p = 6.4 pF and C _f = 64 fF where C _p is the shunting junction capacitance and C _f is the total feedback network shunt capacitance of the amplifier. Sinusoidal angular velocity inputs with 2 Hz of frequency and varied amplitude (0.01, 0.1, 1, 10, 100, and 1000 7s) are simulated. FIG. 6 shows one embodiment of the demodulated voltage output from sinusoidal angular velocity input waveform with 100 7s of amplitude (i.e., the demodulated voltage output 60 due to sinusoidal angular velocity input 62). The output corresponds well with input waveform without delays after the initial stage of driving with the resonance frequency. After simulation of demodulated output signal per each varied amplitude, the results were plotted in FIG. 7 (which generally illustrates the demodulated output plot of simulated voltage vs. varied angular velocity input of 0.01, 0.1, 1, 10, 100, 1000 7s). The demodulated voltage output proportionally changes as angular velocity input increases in the range of ±1000 7s.

[0027] Table 1 (above) illustrates a parametric study for a VRG 10 consistent with at least one embodiment of the present disclosure. The modal frequencies of two modes are sensitively changed by controlling the widths of the support springs 16 and the frequency splits of the VRG structure 10 could be tuned as 11.5 Hz (Device A) and 12.4 Hz (Device B) on the calculated structures by controlling of the radii of the support springs 16 (FIGS. 3A- 3C) from hundreds of Hz on un-tuned structures. The modal frequencies of Device A and Device B are around 24.5 kHz and 21.9 kHz, respectively. The orthogonal star-shaped anchor 12 is shown in FIG. 1 which was used to facilitate the spring beam radii to decrease the split as much as with the widths of the support springs.

[0028] VRG structure 10 comprises octagonal star-shaped anchor 12 [0029] The octagonal star-shaped anchor 12 allows respective control of spring radii 16 on driving and sensing axes

[0030] The octagonal star-shaped anchor 12 allows for frequency tuning on anisotropic materials

[0031] The inventors have discovered a MEMS based method to fine-tune the operating frequency of a vibrating ring gyroscope 10 on a (100) single crystalline silicon using an octagonal star-shaped anchor 12.

[0032] The unique shape of the anchor 12 facilitates the respective control of radii of the support springs 16 that enables fine tuning of wine-glass modal frequencies between two axes on the gyroscope.

[0033] Modal frequency tuning of Vibration Ring Gyroscope (VRG) 10 in anisotropic material.

[0034] Frequency matching during fabrication of VRG 10 on (100) Si wafer.

[0035] Respective control of thicknesses and radiuses of spring damper on driving and sensing axes.

[0036] The VRG 10 platform provides a design feature to match the resonant frequency of drive and sense mode. This ensures low power operation as electrostatic spring softening can be avoided.

[0037] Exemplary uses for VRG 10:

[0038] Sensor fusion is a trend that more complex applications require more sensors, integrity is a key index for system.

[0039] The VRG 10 consistent with the present disclosure may be used for build up internal platform.

[0040] The VRG 10 consistent with the present disclosure may be used with MEMS customer to co-develop next generation device.

[0041] According, at least one embodiment of the present disclosure features a highly symmetric structure of a (100) silicon vibrating ring gyroscope with an octagonal star-shaped anchor to minimize the mode mismatch in operational resonance frequencies. Due to the mechanical anisotropy of a (100) silicon, Young's modulus varies in different directions and affects the stiffness of the support springs. The octagonal star-shaped anchor in the vibrating ring gyroscope enables to adjust the radius and width of the support springs in the crystallographic directions of <110> and <100> to compensate the anisotropy of a (100) silicon. The extensive parametric study of various designing parameters was done with MEMS+ ^® and MATLAB ^® . A significant decrease in the mode mismatch from 1.56 kHz to 11.6 Hz (<0.05 % of 24.5 kHz resonant frequency) was achieved.

[0042] Circular anchors were replaced by octagonal star-shaped anchors with respective radii to accommodate the varied values of support spring in the crystallographic directions of <110> and <100>. An optimum design of a VRG was achieved that has mode-mismatching under 13 Hz between two identical flexural modes frequencies by this structural modification. Furthermore, interface electronics of the designed VRG were simulated with angular velocity input in the Simulink.

[0043] As used in any embodiment herein, a "circuit" or "circuitry" may comprise, for example, singly or in any combination, hardwired circuitry, programmable circuitry, state machine circuitry, and/or firmware that stores instructions executed by programmable circuitry.

[0044] The term "coupled" as used herein refers to any connection, coupling, link or the like by which signals carried by one system element are imparted to the "coupled" element. Such "coupled" devices, or signals and devices, are not necessarily directly connected to one another and may be separated by intermediate components or devices that may manipulate or modify such signals. Likewise, the terms "connected" or "coupled" as used herein in regard to mechanical or physical connections or couplings is a relative term and does not require a direct physical connection.

[0045] While the principles of the invention have been described herein, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation as to the scope of the invention. Other embodiments are contemplated within the scope of the present invention in addition to the exemplary embodiments shown and described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention, which is not to be limited except by the following claims.