Reduction of Orthogonal Bias Error of a Vibration-Type Gyroscope

Abstract

To make it difficult to be affected by charge transfer effect and to accurately match mode frequencies.SOLUTION: A vibration type angular velocity sensor includes: a mount 707; a planar vibration structure 703; and a plurality of compliant supports 705, 706a, 706b that extends between the mount and the planar vibration structure to support the vibration structure and allows the planar vibration structure to vibrate in its plane relative to the mount in response to electrical excitation. A first set 708 of transducers is disposed on the planar vibration structure in order to apply electrical excitation to the planar vibration structure 703 during use and to sense motion resulted from vibration of the planar vibration structure in its plane during use. A plurality of capacitive regions 702 are fixed at a fixed distance from the planar vibration structure in the plane. The capacitive region 702 forms a second set of transducers that are configured so as to apply electrostatic force to the planar vibration structure that induces change in frequencies of vibration of the planar vibration structure during use.SELECTED DRAWING: Figure 8

Description

Technical Field

[0001] The present disclosure generally relates to vibrating gyroscopes and angular velocity sensors, and more particularly to Coriolis angular velocity sensors having a vibrating structure such as a planar ring, and a method of forming a vibrating gyroscope to reduce orthogonal bias error without introducing a charge transfer effect.

Background Art

[0002] As the performance of microelectromechanical system (MEMS) gyroscopes improves, they are increasingly being used in more demanding applications to replace more expensive and larger devices such as optical fiber or rotating mass gyroscopes. However, the widespread adoption of MEMS gyroscopes remains limited because their performance is restricted, particularly from the perspective of the time and temperature stability of the bias. The bias of a gyroscope is the signal output without rotation, which then appears as an error in angular velocity measurement when the device is rotating.

[0003] Coriolis MEMS gyroscopes that utilize a vibrating structure in the form of a planar silicon resonator are one of the devices that exhibit the highest performance from the perspective of bias and overall stability. These devices typically utilize a pair of cos2θ resonance modes tuned to have exactly matching frequencies to achieve optimal performance. The accuracy of the mode frequency matching is very important in reducing the orthogonal bias error, which is a typical one of the major errors that limit the overall performance.

[0004] The orthogonal bias error is caused by measurement imperfections in the resonator structure. These imperfections cause vibrations in the response mode that are in quadrature (i.e., have a 90° phase relationship) to the motion induced by the applied rotation rate and can exist even when the device is not rotating. The magnitude of the orthogonal motion can also be large compared to the in-phase motion measured to provide rotation rate information.

[0005] In the presence of a large orthogonal signal, strict requirements are imposed on the phase accuracy of the detection system in order to recover the required rotational speed induced signal. Electronically devices that are accurately phase-aligned can enable substantial removal of the orthogonal signal. However, the practical implementation constraints of the accuracy with which this phase alignment can be set mean that a portion of the orthogonal signal typically remains and contaminates the in-phase signal induced by the true rotation.

[0006] A MEMS gyroscope utilizing a planar silicon ring resonator structure as described in US8555717 can achieve the highest performance within this class of Coriolis-type gyroscopes. Similar to other MEMS gyroscopes, the orthogonal bias is a significant error source contributing to the overall bias and error in the measurement of angular velocity, as described in US8555717.

[0007] The bias error is caused by a combination of the frequency mismatch ΔF between the primary and secondary cos2θ resonance modes and the angular alignment α of these modes with respect to the drive transducer. The magnitude of the orthogonal bias Ω Q is given by the following equation.

[0008] Ω Q =K×ΔF×sin4α K is a constant including terms for the mode coupling coefficient and the secondary drive and primary pickoff gains. As described in US8555717, the extent to which this error source degrades the rate bias performance depends on both the magnitude of the orthogonal bias Ω Q and the phase accuracy of the control electronics. When a small phase error φ E is present, the orthogonal bias error Ω Err is given by the following equation.

[0009] Ω Err =K×ΔF×sin4α×sinφ E Due to manufacturing tolerances in MEMS processing, these mode frequencies are typically separated by up to ±10 Hz immediately after manufacturing. This initial frequency offset can be adjusted using a laser trimming process, as described, for example, in US5739410 and US9677885. In this process, a laser is used to remove mass along the neutral axis of the ring, setting the frequency separation to approximately zero in the ambient temperature and pressure environment.

[0010] In principle, these mode frequencies exactly coincide, but in practice, the balancing accuracy of the finally assembled device is limited by several additional factors. These include the accuracy of both the frequency measurement and the laser balancing process itself, as well as the stress and strain effects induced by any subsequent packaging that varies the mode frequencies and alignment differently. Packaging stress and strain occur by using different materials for the MEMS device itself and the package to which it is bonded, resulting in a differential expansion effect. These effects are known to vary over the operating temperature range of the device and induce a corresponding shift in the orthogonal bias.

[0011] To achieve performance improvements, design changes to reduce the magnitude of the orthogonal bias error have been implemented in the design and manufacturing process of the device of US8555717. US9677885 describes a method for improving the resolution and accuracy of the laser balancing process. This makes it possible to significantly reduce the initial frequency separation ΔF, and thus reduce the offset value of Ω Q . US10422642 and US9709401 describe Ω over the operating temperature range of the device QDescribes design characteristics that limit the effect of the effects caused by these expansion differences to reduce fluctuations. US10422642 describes an attachment configuration that attaches magnetic circuit components onto a glass layer of a MEMS structure, which reduces stress and strain coupling to the resonator structure due to expansion differences. US9709401 describes the use of an octagonal glass spacer layer to which a silicon structure is bonded. The octagonal structure, unlike a typical square MEMS die structure, has the beneficial effect that stress and strain coupling to the silicon ring resonator structure is effectively symmetric with respect to the cos2θ mode pair through the support legs. This ensures that any resulting frequency fluctuations are substantially equal in the two modes and thus minimizes the shift in the orthogonal bias.

[0012] Using a planar silicon ring gyroscope that utilizes a capacitive transducer, Ω Q can be made very low (as described in, for example, US7637156 and US7958781). Such devices incorporate separate capacitor plates fixed around the inner and outer circumferences of the ring, and the ring forms a common capacitor plate therewith. As described in US7637156 and US7958781, a DC offset voltage is applied to the separate capacitor plates to locally reduce stiffness. These voltages can vary to accurately minimize ΔF and thus set Ω Q to approximately 0° / s. By adjusting these voltages in real time, this null value of the orthogonal bias can be maintained even in the presence of temperature-induced fluctuations in stress and strain that lead to changes in Ω Q .

[0013] However, capacitive MEMS gyroscopes typically function by applying a fixed offset voltage between two transducer plates, making them particularly susceptible to the effects of charge movement (commonly referred to as charge trapping or dielectric charging). In the presence of a fixed voltage gradient, charge accumulates in the dielectric layer on the surface of the capacitor plate, which can be composed of a thin native oxide layer. This effectively shifts the effective voltage offset between the two plates over time, changing the transducer gain. If this effect occurs uniformly across all transducers, the scale factor of the device is shifted by non-uniform variations that cause a bias shift. While there are ways to address this variation in the scale factor (such as those described in US8347718), this is not effective in removing the bias shift. Thus, the charge movement effect represents a major performance limitation for capacitive MEMS gyroscopes. These effects do not exist in devices such as those described in US8555717 that use inductive transducers.

[0014] US8555717 describes a MEMS gyroscope that includes a vibrating ring structure having mechanically tuned vibration modes using a laser trimming process during the manufacture of the ring structure and that uses inductive drive and pickoff transducers. The use of inductive transducers is highly beneficial for several reasons, including their inherently linear behavior and their resistance to the charge movement effects that are known to adversely affect capacitive transducers. However, the limits in accuracy achievable using this mechanical mode balancing technique limit the overall performance of inductive transducer gyroscopes.

[0015] Using a MEMS vibration structure device with an electrostatic transducer as described in US7958781 can achieve higher accuracy. Such devices have the advantages that frequency matching can be performed electrically with high precision and cross-axis bias errors can be significantly eliminated. However, gyroscopes using electrostatic transducers are susceptible to the influence of the charge transfer effect, which has an adverse effect on the bias and scale factor stability of such devices.

[0016] Therefore, in order to achieve an improvement in bias performance, it is desirable to provide a vibrating gyroscope that is less susceptible to the influence of the charge transfer effect and can match the mode frequencies with high precision.

SUMMARY OF THE INVENTION

MEANS FOR SOLVING THE PROBLEM

[0017] According to a first aspect of the present disclosure, a mount, a planar vibration structure, and a plurality of compliant supports extending between the mount and the planar vibration structure to support the vibration structure, thereby enabling the planar vibration structure to vibrate in its plane relative to the mount in response to electrical excitation; a first set of transducers disposed on the planar vibration structure for applying an electrical excitation to the planar vibration structure during use and sensing a motion resulting from the vibration of the planar vibration structure in its plane during use; a plurality of capacitive regions fixed at a certain distance in the plane from the planar vibration structure, the capacitive regions forming a second set of transducers configured to apply an electrostatic force to the planar vibration structure during use to induce a change in the frequency of the vibration of the planar vibration structure, a vibrating angular velocity sensor is provided.

[0018] It will be appreciated that such a vibrating angular rate sensor includes a combination of two different types of transducers. A first set of transducers is disposed directly on the planar vibration structure to apply an electrical excitation and directly sense the motion resulting from the vibration of the planar vibration structure. Thus, the first set of transducers is linearly related to the vibration of the planar vibration structure. The first set of transducers can be used to measure the angular velocity. As will be further described below, examples of transducers that can be disposed directly on the planar vibration structure include inductive and piezoelectric transducers. The voltage applied and sensed using such linear transducers does not result in a charge transfer effect, but an orthogonal bias can occur during use, for example due to variations in ambient temperature.

[0019] A second set of transducers comprises a plurality of capacitive regions fixed at a certain distance from the planar vibration structure (rather than being disposed directly on the planar vibration structure). This second set of transducers can be used to apply an electrostatic force to the planar vibration structure in addition to the first set of transducers that are used for the normal function of driving the vibration and sensing the deformation for the purpose of measuring the angular velocity. By applying an electrostatic force during use, the balance of the vibration mode frequency can be adjusted and the orthogonal bias can be minimized. This is an essentially non-linear interaction with the planar vibration structure, but the second set of transducers is not intended to be used for the purpose of measuring the angular velocity.

[0020] Such a vibrating angular rate sensor can operate in any suitable manner that takes advantage of the benefits of the two types of transducers. The first set of transducers provides a linear interaction with the planar vibration structure, and the second set of transducers provides a non-linear interaction.

[0021] According to one or more examples of the present disclosure, a first subset of a first set of transducers is configured to vibrate a planar vibration structure in a primary mode relative to a mount, and a second subset of the first set of transducers is configured to sense motion resulting from vibration of the planar vibration structure relative to the mount in a secondary mode induced by a Coriolis force when an angular velocity is applied about an axis substantially perpendicular to the plane of the planar vibration structure, and a third subset of the first set of transducers is configured to sense motion resulting from vibration of the planar vibration structure in the primary mode. By detecting the vibration of the planar vibration structure in the primary mode using the third subset of the first set of transducers, the amplitude of the vibration in the primary mode is measured and compared to a reference level to adjust the magnitude of the vibration in the primary mode caused by the first subset of the first set of transducers, such that a constant amplitude of the vibration can be maintained. Further, in such examples, a second set of transducers is preferably configured to apply an electrostatic force to the planar vibration structure to induce a change in the frequency of vibration in the primary mode and / or the secondary mode to match the frequencies. The response of the planar vibration structure can be adjusted using the electrostatic force applied by the second set of transducers to match the frequency of vibration in the secondary mode to the frequency of vibration in the primary mode (or vice versa). Thereby, the cross-axis bias error appearing in the measurement of the angular velocity by the first set of transducers can be minimized.

[0022] According to one or more examples of the present disclosure, a fourth subset of the first set of transducers is configured to apply an electrical excitation to zero the vibration of the planar vibration structure in the secondary mode. The fourth subset of the first set of transducers can be used to zero the vibration of the planar vibration structure in the secondary mode induced by the Coriolis force. This can correspond to the closed-loop operation of a Coriolis-type gyroscope.

[0023] Various shapes have been proposed for Coriolis gyroscopes. The planar vibration structure can include a tuning fork structure. The planar vibration structure typically has a symmetric structure such as a disk or a ring. According to one or more examples of the present disclosure, the planar vibration structure is a ring resonator. The mount can be any suitable structure to which a plurality of compliant supports can be attached and can have various geometric shapes. The mount can be composed of a rigid frame or a central hub. According to one or more examples of the present disclosure, the mount is a rigid frame formed outside the planar vibration structure.

[0024] The plurality of capacitive regions can have a symmetric or asymmetric arrangement with respect to the planar vibration structure. In some examples, a single set of capacitive regions can be disposed on one side of the planar vibration structure and no capacitive regions are disposed on the other side. In some examples, the capacitive regions can be disposed around the entire perimeter of the planar vibration structure, but their arrangement is not symmetric and is non-uniform. The number of capacitive regions can be odd or even. However, according to one or more examples of the present disclosure, the plurality of capacitive regions are symmetrically arranged around the planar vibration structure on a circumference. This means that the electrostatic force can be evenly applied at various points around the planar vibration structure, maximizing the mode balancing effect. The symmetric arrangement of the capacitive regions can have a circumferential arrangement regardless of whether the planar vibration structure also has a circular shape. In an example where the planar vibration structure is a ring resonator, the circumferential arrangement can include several capacitor plates spaced equidistantly around the ring resonator. It is understood that the capacitive regions can be grouped together to assist in the electrostatic equilibrium regime regardless of whether the capacitor plates are spaced equidistantly or non-uniformly. In at least some examples, there is a group of four capacitive regions, and each of the capacitive regions in the group is arranged at 90° intervals in the circumferential arrangement. The circumferential arrangement can include two or more of these groups, for example, four groups of four capacitive regions, resulting in 16 equidistant capacitive regions.

[0025] In an example where the planar vibration structure is a ring resonator having a circular shape, it is useful to fix the capacitive region on the circumference so as to keep the distance between the capacitive region and the ring resonator constant. The plurality of capacitive regions can be arranged and fixed in the circumferential direction on the outer or inner side in the radial direction of the ring resonator. As described above, the capacitive regions can be arranged in groups to assist the electrostatic equilibrium regime. Regardless of how the capacitive regions are grouped for the purpose of applying electrostatic force, the plurality of capacitive regions can have a physical arrangement due to the way the regions are formed during the manufacture of the sensor. According to one or more examples of the present disclosure, the plurality of capacitive regions include a plurality of pairs of capacitive plates arranged and fixed in the circumferential direction with respect to the ring resonator, and the pairs are separated by one of the compliant supports. As described above, the circumferential arrangement of the capacitive plates can be on the inner or outer side in the radial direction of the ring resonator.

[0026] As discussed above, the first set of transducers is disposed directly on the planar vibration structure to apply electrical excitation and sense the resulting motion. According to one or more examples of the present disclosure, the first set of transducers comprises or consists of inductive transducers. This means that the planar vibration structure is electrically excited to vibrate in the presence of a magnetic field and the transducer senses the change in voltage induced by the motion of the planar vibration structure.

[0027] According to one or more examples of the present disclosure, the sensor further comprises a magnetic circuit configured to generate a magnetic field perpendicular to the plane of the planar vibration structure. In such examples, the first set of transducers can comprise conductive tracking formed on the surface of the planar vibration structure.

[0028] According to one or more examples of the present disclosure, additionally or alternatively, the first set of transducers comprises or consists of piezoelectric transducers. This means that the planar vibration structure is electrically excited to vibrate and the piezoelectric transducer deforms together with the planar vibration structure to generate a corresponding change in current or voltage.

[0029] According to one or more examples of the present disclosure, the first set of transducers comprises a first set of piezoelectric electrodes formed on the surface of the planar vibration structure.

[0030] Regardless of the type of transducer in the first set, electrical excitation is required during use, which means that the sensor may include an electrical connection to the first set of transducers. In some examples, the electrical connection may be directly connected to the first set of transducers via, for example, wire bonding or electrical leads. According to one or more examples of the present disclosure, the compliant support comprises conductive tracking extending between the substrate and the first set of transducers. This means that the electrical connection can be conveniently made along the compliant support via the conductive tracking.

[0031] In order to enable a second set of transducers to apply an electrostatic force, an electrical connection is required during use. Since the capacitive regions are fixed at a certain distance from the planar vibration structure, it may be difficult for the second set of transducers to have the same type of electrical connection as the first set of transducers. Further, each electrical connection should enable the first and second sets of transducers to operate independently of each other. Thus, according to one or more examples of the present disclosure, the sensor comprises a first set of electrical connections to a first set of transducers and a second set of electrical connections to a second set of transducers, the first and second sets of electrical connections being independent of each other. In various examples, the second set of electrical connections is directly connected to the second set of transducers. According to one or more examples of the present disclosure, the sensor includes a direct electrical connection to the second set of transducers, for example via wire bonding or electrical leads. A direct electrical connection means a connection that does not rely on conductive tracking extending on a substrate and / or a compliant support. Thus, the direct electrical connection can be independent of any conductive tracking providing an electrical connection to the first set of transducers.

[0032] It will be appreciated that a plurality of capacitive regions fixed at a certain distance from the planar vibration structure means that there are gaps between them so that they are electrically insulated. The planar vibration structure is physically connected to the mount by a compliant support, and there is usually an electrical connection from the mount, through the compliant support, to the first set of transducers on the planar vibration structure. In such an example, an electrical insulating layer is required if the capacitive regions are fixed directly to the mount. Thus, in various examples, the plurality of capacitive regions are indirectly fixed to the mount so as to be electrically insulated from the planar vibration structure. For example, the plurality of capacitive regions can be indirectly fixed to the substrate via an intervening insulating layer (such as a glass pedestal layer).

[0033] According to one or more examples of the present disclosure, a plurality of capacitive regions are indirectly fixed to a mount such that they are electrically insulated from a compliant support as well as a planar vibration structure and the mount. To assist in the electrical insulation of the capacitive regions, the plurality of capacitive regions may also be fixed at a certain distance from the compliant support, i.e., having a gap therebetween. This distance may be substantially the same as the distance from the planar vibration structure. However, the inventors understand that the distance between the plurality of capacitive regions and the compliant support can have an undesirable effect on the rigidity of the compliant support. This effect can be reduced by increasing the distance between the plurality of capacitive regions and the compliant support. Thus, according to one or more examples of the present disclosure, one or more of the plurality of capacitive regions are fixed at a distance d1 from a planar vibration structure (in its plane) and at a distance d2 from a compliant support (in the same plane), where d2 > d1. Each of the plurality of capacitive regions can be fixed in this way.

[0034] In the vibratory angular rate sensor disclosed above, the mount, the planar vibration structure, and the compliant support can all be formed in the same material layer. This means that these components can be manufactured, for example, from a single silicon wafer. In some examples, the material layer is a silicon layer, but in some examples, it will be understood that other suitable material layers can be used provided that they have suitable mechanical properties and are somewhat conductive. In some examples, the capacitive regions can be formed separately and added to the device. However, for manufacturing efficiency, it is preferable that the plurality of capacitive regions are also formed in the same material layer as the mount, the planar vibration structure, and the compliant support. As described above, this material layer can be supported by an insulating layer. Thus, the insulating layer can physically support the capacitive regions disposed away from the planar vibration structure.

[0035] According to one or more examples of the present disclosure, the mount, the planar vibration structure, the compliant support, and the plurality of capacitive regions are formed in the same silicon material layer. In various examples, the vibrational angular rate sensor is a MEMS (e.g., silicon) device.

[0036] The inventors have also devised a suitable method for forming a vibrational angular rate sensor as disclosed herein. The following method may be particularly suitable for manufacturing MEMS (e.g., silicon) devices.

[0037] According to a second aspect of the present disclosure, a first substrate is processed to define a planar vibration structure and a plurality of compliant supports, the compliant supports extending between a mount formed from the first substrate and the planar vibration structure to support the planar vibration structure, thereby enabling the planar vibration structure to vibrate relative to the mount in response to electrical excitation, the processing; forming a first set of transducers on the planar vibration structure for applying an electrical excitation to the vibration structure and sensing movement due to vibration of the planar vibration structure; and forming a plurality of capacitive regions fixed at a constant distance in its plane from the planar vibration structure, the capacitive regions forming a second set of transducers for applying an electrostatic force to the planar vibration structure to induce a change in the frequency of vibration of the planar vibration structure. A method of forming a vibrational angular rate sensor is provided that includes forming.

[0038] Thus, such a method results in a vibrational angular rate sensor that includes a combination of two different types of transducers. The first set of transducers is disposed directly on the planar vibration structure and can be used to measure angular velocity. The second set of transducers is formed by a plurality of capacitive regions fixed at a certain distance from the planar vibration structure and can be used to apply an electrostatic force to the planar vibration structure to balance the vibration mode frequency and minimize the cross-axis bias.

[0039] As described above, the plurality of capacitive regions can be formed independently of the first substrate. For example, they can be formed directly on a glass pedestal layer on which a substrate having a mount and a planar vibration structure is subsequently fixed. Alternatively, the first substrate having the mount and the planar vibration structure can be fixed to the glass pedestal layer, and then the capacitive regions can be formed using a suitable bottom-up, for example, deposition technique. Thus, the capacitive regions can be formed from the same material as or a different material from the first substrate. However, for manufacturing efficiency, it is desirable that the plurality of capacitive regions are also formed by processing the same first substrate (e.g., etching, micromachining, etc.). This can include additional processing steps implemented simultaneously or at different times. Thus, according to one or more examples of the present disclosure, forming a plurality of capacitive regions comprises processing the first substrate to define the plurality of capacitive regions in the same material layer.

[0040] The inventors recognize that when forming capacitive regions in the same material layer, the method should allow the capacitive regions to be indirectly fixed to the mount such that the capacitive regions are electrically insulated from the planar vibration structure. The first substrate can be fixed to a second substrate arranged to support the capacitive regions and provide electrical insulation therebetween. The second substrate can be an insulating layer such as a glass pedestal layer.

[0041] In at least some examples, before processing the first substrate to define a plurality of capacitive regions, the first substrate may be fixed to the second substrate. This means that the capacitive regions can be separated and electrically insulated from the planar vibration structure and the compliant support while being supported by the second substrate. For example, the first substrate may be anodically bonded to the second substrate before processing the first substrate. However, it may be desirable to process the first substrate (e.g., using deep reactive ion etching) before bonding the first substrate (such as a silicon wafer) to the second substrate. Thus, according to one or more examples of the present disclosure, forming a plurality of capacitive regions involves processing the first substrate to define a plurality of semi-insulating regions, then fixing the first substrate to the second substrate, where the second substrate comprises a support section configured to support the plurality of capacitive regions, fixing, and then separating each of the semi-insulating regions into a pair of electrically insulated capacitive regions. For example, the first substrate may be processed (e.g., etched) to define eight semi-insulating regions, and then the semi-insulating regions may be separated (e.g., laser cut) into pairs to form a total of sixteen electrically insulated capacitive regions.

[0042] In various examples disclosed herein, the first substrate is made of a conductive material (e.g., a silicon wafer). In various examples, the second substrate is made of an electrically insulating material (e.g., a glass layer).

[0043] In various examples disclosed herein, forming a first set of transducers comprises forming conductive tracking on the surface of the planar vibration structure. Thus, the first set of transducers may be formed as inductive. In at least some examples, the method further comprises fixing the first or second substrate to another substrate and attaching a magnetic circuit to this another substrate.

[0044] In various examples disclosed herein, alternatively or additionally, forming the first set of transducers comprises forming a set of piezoelectric electrodes on the surface of a planar vibration structure. Thus, the first set of transducers can be formed as piezoelectric type.

[0045] In various examples disclosed herein, alternatively or additionally, forming the second set of transducers comprises forming a set of electrodes on a capacitive region having an electrical connection independent of the first set of transducers.

[0046] Here, with reference to the accompanying drawings, specific exemplary embodiments of the present disclosure will be described.

Brief Description of the Drawings

[0047]

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DETAILED DESCRIPTION OF THE INVENTION

[0048] FIG. 1a shows exemplary data of the variation of the cross-axis bias Ω over the operating temperature range from -40°C to +85°C in a typical prior art inductive MEMS gyroscope such as the CRH02 and CRS39 devices manufactured by Silicon Sensing Systems Ltd. using a planar silicon ring-based MEMS sensor SGH03. The bias performance of these devices is mainly determined by the characteristics of the MEMS sensor, and the SGH03 is laser balanced as described above for the purpose of minimizing the cross-axis bias error Ω Q The bias performance of these devices is mainly determined by the characteristics of the MEMS sensor, and the SGH03 is laser balanced as described above for the purpose of minimizing the cross-axis bias error Ω Q to receive laser balancing as described above for the purpose of minimizing the cross-axis bias error Ω.

[0049] FIG. 1b shows Ω with the initial offset removed at 25°C Qshows the variation. The initial 25°C offset is typically less than ±20° / s and is limited by the accuracy of the laser balancing process described in US9677885. The temperature-dependent Ω Q variation is less than ±10° / s, which is mainly caused by changes in residual stress and strain. This error can change over time due to the influence of time and can also exhibit a certain degree of thermal hysteresis.

[0050] When targeting very low bias reproducibility, the Ω levels shown in FIGS. 1a and 1b, which are up to ±72,000 deg / hr, Q provide an orthogonal bias error Ω Err <±100 deg / hr requires a phase error accuracy of <0.1 degree. This represents a major limitation on the bias performance of the device. Therefore, to achieve a significant improvement in bias performance, the orthogonal bias Ω Q needs to be significantly reduced.

[0051] FIG. 2 shows a schematic plan view of a silicon layer 200 of a prior art planar ring MEMS structure 201, such as that utilized in the device of US8555717. The silicon layer 200 is composed of a planar ring resonator structure 203 that is flexibly attached by a number of compliant supports 205 to a mount 207, formed, for example, by etching in the silicon layer 200 (as will be further described below). Each of the compliant supports 205 comprises a pair of compliant legs 206a, 206b. The compliant legs 206a, 206b have one end attached to the periphery of the ring resonator structure 203 and the other end attached to the mount 207.

[0052] As shown in FIG. 3, eight transducers 208 formed by conductive tracking loops 210 on the surface of the MEMS structure 201 are schematically shown. Each of the compliant supports 205 is used to convey the conductive tracking loop 210 to the surface of the ring resonator structure 203, as shown in more detail in FIG. 3. Each tracking loop 210 passes from a first contact pad 209a disposed on the mount 207, along a first leg 206a of the compliant support 205, around eight segments on the surface of the ring resonator structure 203, and then returns to a second contact pad 209b on the mount 207 via a pair of adjacent legs 206b.

[0053] A schematic cross-sectional view of an inductive gyroscope 250 including a fully assembled MEMS structure 201 is shown in FIG. 4. The mount 207 of the silicon layer 200 is coupled to a glass pedestal layer 209, which is in turn coupled to a glass support 211. A magnetic circuit 213, consisting of an annular disk lower pole piece 215, a disk-shaped permanent magnet 217, and an annular upper pole piece 219, is also coupled to the glass support 211. The magnetic circuit 213 is arranged to provide a magnetic field B that converges perpendicular to the plane of the ring around the planar ring resonator structure 203.

[0054] As shown in FIG. 5, the MEMS structure 201 is typically mounted on a base 8 of a hermetic metal can package, and electrical connections are made from the contact pads 209a, 209b of each transducer 208a - 208h of a set of inductive transducers via wire bonds 9 to insulated pin connections 11 of the package base 8. The package is mounted on a printed circuit board (PCB) via contact pins 11 of the base 8 that connect to an electrical circuit (not shown in FIG. 5).

[0055] The pin connections 11 labeled 301 - 318 in FIG. 6 are arranged on the PCB as shown, with an electrical connection between pins 301 - 318. The primary drive (PD) current is applied to pin 312 via the circuitry on the PCB at the PD input 50. The PD current flows around the first tracking loop to pin 311. Thus, pins 311, 312 correspond to the first primary drive transducer 208c. Pin 311 is connected to pin 303 via tracking 501 on the PCB, and the PD current then flows around the second tracking loop to pin 302. Pins 302, 303 correspond to the second primary drive transducer 208g, which is diametrically opposed to the first primary drive transducer 208c, forming a symmetry pair of primary drive transducers. Similarly, the first primary sense or pick-off (PPO) tracking loop connects from pin 316 around the eight segments of the ring to pin 317, and pin 317 is electrically connected to pin 307 via tracking 521 on the PCB. The second tracking loop extends from pin 307 through the second MEMS ring segment to pin 308, and pin 308 is then connected to the PPO output 52 on the PCB. Pins 316, 317 and 307, 308 correspond to a symmetry pair of radially opposed primary pick-off transducers 208a, 208e.

[0056] Two secondary drive (SD) loop segments (the first connected to pins 310 and 309, the second connected to pins 301 and 318) are similarly connected in series by tracking 511 on the PCB between pins 309 and 310, and the SD current is applied from SD input 51 via pin 310. Pins 309, 310 and 301, 318 correspond to a symmetric pair of radially opposed secondary drive transducers 208d, 208h. The secondary sense or pick-off (SPO) loop between pins 313 and 315 is connected by tracking 531 on the PCB to a second loop between pins 304 and 306. Pin 306 is connected to SPO output 53 on the PCB. Pins 313, 315 and 304, 306 correspond to a symmetric pair of radially opposed secondary pick-off transducers 208b, 208f. Pins 305 and 314 connect the silicon layer of the MEMS structure 201 and the can package base to the PCB ground.

[0057] As described in US8555717, an alternating current signal is applied via PD input 50 and SD input 51 to transducers 208c, 208d, 208g, 208h on a particular segment of the ring to apply a Lorentz force that controls the oscillatory motion of the ring resonator structure 203. When a rotation at an angular velocity Ω is applied about an axis perpendicular to the plane of the ring, a Coriolis force couples energy into the oscillation of the secondary mode and the amplitude of the oscillation is proportional to the applied angular velocity. The resulting motion of the ring resonator structure 203 in the magnetic field induces a voltage in the transducers 208e, 208f, 208a, 208b of the remaining ring segments, which are used to provide a signal indicative of the ring motion and which can be detected by the PPO and SPO outputs 52, 53.

[0058] FIG. 7 shows a typical manufacturing process for forming the planar ring MEMS structure 201 shown in FIGS. 2 through 6. The manufacturing process begins, in step 601, with a silicon substrate 200 having a typical thickness of about 100 μm. In step 602, a thin insulating oxide layer 220 is formed on the upper surface of the silicon wafer 200, with holes provided at specific locations to allow subsequent grounding of the underlying conductive silicon layer 200. Next, a thin metal layer 230 is deposited over the oxide layer and the holes (in step 603) and patterned to form insulated conductive tracks (not shown in FIG. 7) for electrical connections. Next, in step 604, a photoresist layer is deposited over the metal tracking and patterned to define the areas that will subsequently be etched by a deep reactive ion etching (DRIE) process. In this step, the silicon layer 200 is first temporarily bonded to a support wafer (not shown in FIG. 7), and then narrow (10 μm to 30 μm), high aspect ratio trenches (typically 10:1) are etched throughout the thickness of the wafer to define the ring resonator structure 203, compliant support 205, and mount 207. Thereafter, when the silicon layer 200 is removed from the support wafer, the silicon sections between the legs 206a, 206b of the compliant support 205 and the sections within the ring resonator structure 203 are not supported and are thus removed. Next, the photoresist layer is removed, and the silicon layer 200 is anodically bonded to a glass pedestal layer 209 that has been processed to include cavities under the positions of the ring resonator 203 and compliant support 205, as well as through holes for placing the components of the magnetic circuit 213 (in step 605). Next, a lower glass support 211 having a disk-shaped metal lower pole 215 attached to its upper surface is adhesively bonded to the pre-anodically bonded silicon wafer 200 and glass pedestal layer 209 such that the pole piece 215 is centered in the hole in the glass pedestal layer 209 under the silicon ring structure 203 (in step 606).In step 607, the remaining components of the magnetic circuit 213 including the disk-shaped permanent magnet 217 and the upper pole piece 219 are subsequently adhesively bonded to complete the device assembly and complete the induction gyroscope 250 as shown in the assembly in FIG. 4.

[0059] The applicant recognizes that the MEMS structure 201 shown in FIGS. 2 to 4 can be advantageously processed so that the mode frequency can be electrostatically balanced, for example, by the addition of a plurality of separate capacitor plates arranged around the outer periphery of the ring. FIG. 8 shows a schematic plan view of an exemplary MEMS structure 701 fabricated in accordance with the present disclosure, including a plurality of separate capacitive regions 702, herein referred to as "capacitor plates" 702, that enable electrostatic balancing of the MEMS vibration structure 703 in an induction gyroscope of the type described above.

[0060] In FIG. 8, the silicon layer 700 comprises a ring resonator structure 703 that is flexibly attached to the mount 707 by a number of compliant supports 705. Comparing with FIG. 2, it can be grasped that there are further 16 equally angularly spaced capacitor plates 702 symmetrically arranged around the circumference of the ring resonator structure 703. In this example, the capacitor plates 702 are arranged radially outside the planar vibration structure 703, between the radial portion of the compliant support 705 and the outer periphery of the ring resonator structure 703. The capacitor plates 702 form a second set of transducers that are fixed in the same plane as the planar vibration structure 703 and are set at a distance d1 from the planar vibration structure 703. When the ring resonator structure 703 vibrates in the plane, any unbalanced motion can be micro-adjusted by applying a voltage to one or more of the capacitor plates 702 to generate an electrostatic force across the gap d1.

[0061] FIG. 9 shows an exemplary layout of an inductive transducer 708 suitable for use with an additional electrostatic equilibrium plate of the present disclosure. The inductive transducer 708 is disposed on a segment of the ring resonator structure 703, and the ring resonator structure 703 is supported by eight compliant supports 705 as shown in FIG. 8. The compliant supports 705 are connected to mounts 707 formed in the same silicon layer as the ring resonator structure 703, and each compliant support 705 comprises a symmetric pair of compliant legs 706a, 706b. Each inductive transducer 708 comprises a conductive tracking loop 710. Each conductive tracking loop 710 passes from a first contact pad 709a disposed on the mount 707, along the first leg 706a of the compliant support 705, around eight segments on the surface of the ring resonator structure 703, and then back to a second contact pad 709b on the mount 707 via the adjacent legs 706b of the pair. The electrostatic equilibrium plate 702 is disposed radially outside the ring resonator structure 703, between the radial portion of the compliant support 705 and the outer periphery of the ring resonator structure 703. The electrostatic equilibrium plate 702 is fixed at a first distance d1 from the ring resonator structure 703 and at a second distance d2 from the adjacent compliant support legs 706a, 706b.

[0062] As shown in FIG. 10, the capacitor plate 702 is firmly bonded to a glass pedestal layer 709 that is machined to include an additional support section 712 that extends beneath the capacitor plate 702. Each of the compliant supports 705 shown in FIG. 8 includes a pair of symmetric compliant legs 706a, 706b. The compliant legs 706a, 706b have one end attached to the periphery of the ring resonator structure 703 and the other end attached to the mount 707. In this example, the capacitor plates 702 are arranged in eight pairs 702a - 702h, and each pair 702a - 702h is fixed between the respective compliant legs 706a, 706b of the support 705. In order to achieve electrostatic equilibrium of the ring resonator structure 703 in the above - type inductive gyroscope, conductive pads are applied to the surface of the capacitor plate 702, as further described below, to enable a DC voltage to be selectively applied. The ring resonator structure 703 is electrically excited and its motion is sensed by applying conductive tracking to the surface of the MEMS structure 701 in a manner equivalent to that described with respect to FIG. 3 and as shown in FIG. 10. Therefore, as schematically shown in FIG. 8, eight inductive transducers 708 are arranged on the ring resonator structure 703.

[0063] FIG. 10 shows a cross-sectional view of a inductive gyroscope 750 comprising a MEMS structure 701 fully assembled in accordance with the present disclosure and including capacitor plates 702. Conductive pads 704a are formed on the surface of each capacitor plate 702 and have electrical leads (such as wire bonds) 704b that function as direct electrical connections enabling the application of voltage to the conductive transducer 702. Similar to the prior art inductive gyroscope 250 shown in FIG. 3, the mount 707 of the silicon layer 700 of the MEMS structure 701 is bonded to a glass pedestal layer 709, which in turn is bonded to a glass support 711. However, in FIG. 10, the glass pedestal layer 709 is machined to include an additional support section 712 that extends under the capacitor plates 702. A magnetic circuit 713 composed of an annular disk lower pole piece 715, a disk-shaped permanent magnet 717, and an annular upper pole piece 719 is also bonded to the glass support 711 in the same manner as the magnetic circuit of the prior art gyroscope 250. The magnetic circuit 713 is arranged to provide a magnetic field B that converges perpendicular to the plane of the ring around the ring resonator structure 703.

[0064] A plan view of the glass layer 709 is shown in FIG. 11, which shows a raised mounting area including 16 additional support sections 712 to which the capacitor plates 702 are attached in FIG. 10. The areas under the ring resonator structure 703 and the compliant support 705 remain hollowed out as described above, allowing the ring resonator structure 703 to move freely.

[0065] Using the MEMS structure 701, signals indicating the movement of the ring resonator structure 703 can be detected in the same manner as in the prior art MEMS structure 201. On the other hand, by adding the capacitor plates 702, it becomes possible to apply electrostatic balance to better match the mode frequencies.

[0066] The mode frequency ω, according to the following equation, is related to the negative spring K ElecIt can be adjusted by varying the voltage on the capacitor plate 702, which functions as and locally reduces the effective stiffness and thus the frequency ω of the ring resonator structure 703.

[0067]

Number

[0068] m is the modal mass of the ring resonator structure 703, and K is the combined spring constant of the ring resonator structure 703 and the compliant support 705. The electrical spring constant K Elec is given by the following equation.

[0069]

Number

[0070] ε0 is the permittivity of free space, A is the area of the capacitor plate 702, V is the differential voltage between the ring resonator structure 703 and the capacitor plate 702, and d is the capacitor gap (i.e., the minimum distance between the capacitor plate 702 and the ring resonator structure 703).

[0071] The ring resonator structure 703 and the capacitor plate structure 702 can be fabricated from bulk silicon using a DRIE process, and trenches with a high aspect ratio can be created. The capacitor gap d1 between the surfaces of the capacitor plate 702 and the ring resonator structure 703 is preferably on the order of 10 μm to 30 μm in order to provide a large capacitance and thus a wide tuning range.

[0072] As can be seen in Fig. 8, there is a constant gap d1 between the capacitor plate 702 and the ring resonator structure 703, and a constant gap d2 between the capacitor plate 702 and the adjacent compliant support legs 706A, 706b, which are shown as equal in Fig. 8, i.e., d1 = d2. Thus, the electrostatic tuning for each individual capacitor plate 702 adjusts the electrical spring stiffness of both the ring resonator structure 703 and the adjacent compliant support 705. The effective axis for ring stiffness adjustment lies at the midpoint of the capacitor plate 702, but the change in the stiffness of the compliant support 705 acts on the attachment point of the compliant support 705 to the ring resonator structure 703. Thus, the resulting stiffness tuning axis is shifted somewhat from the center of the capacitor plate 702 towards the compliant support 705. Thus, the electrostatic tuning axis of the capacitor plate 702 is no longer at equal angular intervals. Assuming that the magnitude of the shift is known, this can be taken into account when determining the required tuning voltage. However, the influence on the stiffness of the compliant support 705 can be significantly reduced by making the gap d2 between the capacitor plate 702 and the compliant support 705 larger than the gap d1 between the capacitor plate 702 and the ring resonator structure 703. This example can be seen in Fig. 9, where it can be grasped that the gap d2 between the capacitor plate 702 and the compliant support 705 is larger than the gap d1 between the capacitor plate 702 and the ring resonator structure 703. When the gap d2 is made twice as large as the ring gap d1, the effect is reduced to one-eighth due to the gap dependence of the stiffness tuning.

[0073] The electrostatic equilibrium provided by the capacitor plate 702 can be used in addition to or instead of the conventional laser balancing process to match the mode frequencies. Thus, the required frequency tuning range is ±10 Hz, which drives the requirements for a small capacitor gap and a high maximum differential voltage range to achieve the required electric spring stiffness adjustment range. However, these requirements are not as stringent when providing the fine tuning function using the standard laser balancing process. In the embodiment shown in FIG. 8, the required tuning range is limited to less than ±0.5 Hz, allowing for a voltage drop and a larger gap. Using a larger gap, as shown in FIG. 9, is particularly advantageous as it allows for a larger amplitude of motion of the ring structure, resulting in improved noise performance. Thus, the addition of the capacitor plate 702 enables the application of the electrostatic equilibrium function to existing inductively actuated planar MEMS ring resonator devices. This has obvious advantages from the perspective of bias performance characteristics.

[0074] However, in practice, there are significant challenges from the perspective of manufacturing and packaging the MEMS structure 701. Ideally, the manufacturing process should be substantially compatible with existing manufacturing processes and equipment.

[0075] Typical prior art processes for manufacturing MEMS sensors, such as those described with respect to FIG. 7 for MEMS sensor 201, include a DRIE step 604 in which the silicon sections between legs 206a, 206b of compliant support 205, and the sections within ring resonator structure 203 are not supported and are separated from the wafer when the support wafer is removed. Thus, it is only possible to add a capacitor plate as part of a typical manufacturing process without modification by changing the design of the prior art MEMS structure. This is because the capacitor plate is similarly not supported and is separated from the wafer when the support handle is removed. Therefore, design changes and additional process steps are required such that it is possible to form capacitor plates at the necessary positions around the periphery of the ring resonator structure of an existing MEMS structure such as MEMS structure 201.

[0076] To achieve this, the DRIE trench etching pattern can be processed to provide pairs of capacitor plates 702 attached to silicon layer 700 at each of compliant supports 705. Specifically, the trench pattern is etched to provide a pair of capacitor plates 702 at each compliant support 705 between compliant legs 706a, 706b.

[0077] FIG. 12a shows an exemplary embodiment in which a silicon arc belonging to ring 703 is formed with a semi-insulating region 702' supported by a rigid support beam structure 801. The semi-insulating region 702' is partially separated by an etching trench, leaving a short non-etched silicon section 803 between adjacent sides of two portions of the semi-insulating region 702'. The semi-insulating region 702' is firmly attached to support glass region 712 shown in FIG. 11 during an anodic bonding process that integrally bonds silicon layer 700 and glass layer 709.

[0078] The non-etched silicon section 803 between pairs of the semi-insulating regions 702' of the arc can conveniently be removed using a laser cutting process, removing the silicon between the support beam structures 801 to create a laser cut region 805 as shown in FIG. 12b. Next, each semi-insulating region 702' is divided into capacitively isolated regions 702, i.e., pairs of capacitor plates 702. A femtosecond laser is suitable for this purpose as it is known to produce high-quality sidewall profiles with minimal heat-affected zones. However, since the requirement is simply to electrically isolate the resulting capacitor plates 702, a lower-cost laser cutting solution can also be used when precise measurement of the cut lines is not critical from the perspective of the operation of the gyroscope and alternatives. The laser cutting process can be applied as an additional step without interfering with subsequent manufacturing process flows.

[0079] When 16 electrostatically balanced capacitor plates 702 are formed, they are electrically connected to a DC voltage (or ground potential). Thus, a differential voltage can be applied between the capacitor plates 702 and the ring resonator structure 703 fixed at ground potential to implement electrostatic balance. To balance the cos2θ mode, the capacitor plates 702 are used in sets of four with an angular separation of 90° relative to each other. Four separate DC voltages need to be applied to the MEMS device, and each voltage must be applied to all four plates within each set, requiring 16 separate connections to the MEMS. The DC voltages can be applied using wire bonds 704b (as shown in FIG. 10) formed between pins and metal bonding pads 704a on the surface of the capacitor plates 702 by additional pins within a package similar to those shown in FIGS. 4 and 5. This approach may increase the number of required pin connections and the overall size of the package, but the addition of the capacitor plates 702 enables the application of the electrostatic balance function to an existing inductively actuated planar MEMS ring resonator device, reducing the impact of orthogonal bias errors.

[0080] In addition to the application to a gyroscope equipped with an inductive transducer, such as the inductive gyroscope 750 shown in FIG. 10, the present invention can also be applied to other types of MEMS gyroscopes that employ an alternative transducer mechanism and require frequency adjustment or matching of the relative frequencies of two operating modes. For example, the capacitor plates for electrostatic balance can be applied to MEMS gyroscopes that employ a piezoelectric (PZT) transducer, such as the type of MEMS gyroscope described in US8381590 where the PZT transducer is formed on the upper surface of a ring resonator structure.

[0081] In a PZT gyroscope, the construction of the PZT transducer on the surface of the resonator includes forming an insulating oxide layer on the upper surface of silicon on which a metal electrode layer is deposited. Next, a thin film PZT layer is formed on top of the metal layer, and a second metal top electrode layer is formed on the PZT layer. Next, the PZT layer structure is patterned such that the transducer is provided only in specific regions of the ring and leg structures. FIG. 14 of US8381590 shows an exemplary PZT MEMS gyro structure and the layout of the transducer with respect to a ring structure internally mounted to a rigid central hub via a number of compliant supports. It will be understood that alternative configurations are possible, including configurations where the ring is attached via external legs that connect to a rigid outer frame.

[0082] FIG. 13 shows an exemplary layout of a PZT transducer 908 suitable for use with an additional electrostatic equilibrium plate of the present disclosure. The PZT transducer 908 is disposed on a segment of the ring resonator structure 903, which is supported by eight compliant supports 905, similar to that seen in FIG. 8. The compliant supports 905 are connected to the mount 907 in the same silicon layer as the ring resonator structure 903, and each compliant support 905 includes a pair of compliant legs 906a, 906b as described above. The PZT transducer 908 includes a pair of piezoelectric electrodes 919a, 919b mounted on the surface of the resonator structure 903, which are connected to contact pads 913a, 913b on the mount 907 by respective metal trackings 910a, 910b. The overall layout of the PZT transducer 908 around the ring resonator structure 903 is similar to that shown in FIGS. 8 and 9 for the induction transducer 708 of the induction gyroscope. An electrostatic equilibrium plate 902 similar to the capacitor plate 702 is added to the MEMS structure to enable adjustment of the mode frequency as described above with respect to the induction gyroscope 750. The electrostatic equilibrium plate 902 is fixed at a first distance d1 from the ring resonator structure 903 and at a second distance d2 from the adjacent compliant support legs 706a, 706b.

[0083] FIG. 14 shows a cross-sectional view of a piezoelectric gyroscope 950 comprising a MEMS structure 901 including a ring resonator 903 and an electrostatic equilibrium plate 902 for electrostatic equilibrium. In this cross-sectional view, it can be seen that conductive pads 904a are formed on the surface of each electrostatic equilibrium plate 902 and have electrical leads (such as wire bonds) 904b that function as a direct electrical connection enabling a voltage to be applied to the conductive transducer 904a.

[0084] Similar to the MEMS structures 201 and 701 shown in FIGS. 4 and 10 respectively, the mount 907 of the silicon layer 900 of the MEMS structure 901 is bonded to the glass pedestal layer 909, which is then bonded to the glass support 911. The glass pedestal layer 909 is machined to include an additional support section 912 that fixes the capacitor plate 902 at a distance d1 from the ring resonator 903. In this way, the capacitor plate 902 can be added to an existing piezoelectric MEMS gyroscope to enable an electrostatic balancing function and reduce cross-axis bias errors.

[0085] Thus, according to the present disclosure, it will be appreciated that an electrostatic balancing function can be applied to an existing MEMS gyroscope, resulting in an improvement in the bias performance characteristics of the device that is not substantially affected by the charge trapping effect.

Claims

1. A mount (707; 907), a planar vibration structure (703; 903), and a plurality of compliant supports (705, 706a, 706b; 905, 906a, 906b) extending between the mount (707; 907) and the planar vibration structure (703; 903) to support the vibration structure (703; 903), thereby enabling the planar vibration structure (703; 903) to vibrate in its plane with respect to the mount (707; 907) in response to electrical excitation. The planar vibration structure (703; 903) is a ring resonator. A first set of transducers (708; 908) disposed on the planar vibration structure (703; 903) for applying electrical excitation to the planar vibration structure (703; 903) during use and sensing motion resulting from vibration of the planar vibration structure (703; 903) in its plane during use. A plurality of capacitive regions (702; 902) fixed at a certain distance in the plane of the planar vibration structure (703; 903) from the planar vibration structure (703; 903), the capacitive regions (702; 902) forming a second set of transducers (704a; 904a) configured to apply an electrostatic force to the planar vibration structure (703; 903) during use to induce a change in the frequency of vibration of the planar vibration structure (703; 903). A vibration type angular velocity sensor (750; 950) further comprising the above.

2. A first subset of the first set of transducers (708; 908) is configured to vibrate the planar vibration structure (703; 903) with respect to the mount (707; 907) in a primary mode. A second subset of the first set of transducers (708; 908) is configured to sense motion resulting from vibration of the planar vibration structure (703; 903) with respect to the mount (707; 907) in a secondary mode induced by a Coriolis force when an angular velocity is applied about an axis substantially perpendicular to the plane of the planar vibration structure (703; 903). A third subset of the first set of transducers (708; 908) is configured to sense motion resulting from vibration of the planar vibration structure (703; 903) in the primary mode. The second set (702; 902) of the transducers is configured to apply an electrostatic force that induces a change in the frequency of the vibration in the primary mode and / or the secondary mode to the planar vibration structure (703; 903) so as to match the frequencies. The vibrating angular velocity sensor (750; 950) according to claim 1. **Claim 3** A fourth subset of the first set (708; 908) of the transducers is configured to apply an electrical excitation to zero the vibration of the planar vibration structure (703; 903) in the secondary mode. The vibrating angular velocity sensor (750; 950) according to claim 2. **Claim 4** The vibrating angular velocity sensor (750; 950) according to any one of claims 1 to 3, wherein the plurality of capacitive regions (702; 902) are symmetrically arranged around the planar vibration structure (703; 903) on a circumference. **Claim 5** Further comprising a magnetic circuit (713) configured to generate a magnetic field perpendicular to the plane of the planar vibration structure (703). The vibrating angular velocity sensor (750) according to any one of claims 1 to 4, wherein the first set (708) of the transducers comprises a conductive tracking (710) formed on the surface of the planar vibration structure (703). **Claim 6** The vibrating angular velocity sensor (950) according to any one of claims 1 to 3, wherein the first set (908) of the transducers comprises a first set of piezoelectric electrodes formed on the surface of the planar vibration structure (903). **Claim 7** Comprising a first set (710; 910a, 910b) of electrical connections to the first set (708; 908) of the transducers and a second set (704b; 904b) of electrical connections to the second set (704a; 904a) of the transducers. The vibrating angular velocity sensor (750; 950) according to any one of claims 1 to 6, wherein the first set of the electrical connections and the second set of the electrical connections are independent of each other. **Claim 8** The compliant support (705; 905) comprises a conductive tracking (710; 910a, 910b) extending between the mount (707; 907) and the first set (708; 908) of the transducers. The vibrating angular velocity sensor (750; 950) according to any one of claims 1 to 7, further comprising a direct electrical connection (704b; 904b) to a second set (704a; 904a) of the transducer.

9. One or more of the plurality of capacitive regions (702; 902) are at a distance d from the planar vibration structure (703; 903) １ and at a distance d from the compliant support (705; 905), ２ where d ２ > d １ The vibration type angular velocity sensor (750; 950) according to any one of claims 1 to 8.

10. A method of forming a vibrating angular velocity sensor (750; 950), comprising: processing a first substrate (700; 900) to define a planar vibration structure (703; 903) and a plurality of compliant supports (705; 905), wherein the compliant supports (705; 905) extend between a mount (707; 907) formed from the first substrate (700; 900) and the planar vibration structure (703; 903) to support the planar vibration structure (703; 903), thereby enabling the planar vibration structure (703; 903) to vibrate relative to the mount (707; 907) in response to electrical excitation; wherein the planar vibration structure (703; 903) is a ring resonator; forming a first set (708; 908) of transducers on the planar vibration structure (703; 903) for applying electrical excitation to the vibration structure (703; 903) and sensing motion due to vibration of the planar vibration structure (703; 903); forming a plurality of capacitive regions (702; 902) fixed at a constant distance in the plane of the planar vibration structure (703; 903), wherein the capacitive regions form a second set (704a; 904a) of transducers for applying an electrostatic force to the planar vibration structure (703; 903) to induce a change in the frequency of vibration of the planar vibration structure (703; 903); The method further comprising.

11. The method according to claim 10, wherein forming the plurality of capacitive regions (702; 902) comprises processing the first substrate (700; 900) to define the plurality of capacitive regions (702; 902) in the same material layer.

12. The method according to claim 10 or 11, wherein forming the first set (708) of transducers comprises forming conductive tracking on the surface of the planar vibration structure (703).

13. The method according to any one of claims 10 to 12, wherein forming the first set (908) of the transducers comprises forming a set of piezoelectric electrodes on the surface of the planar vibration structure (903).

14. The method according to any one of claims 10 to 13, wherein forming the second set (704a; 904a) of the transducers comprises forming a set of electrodes on the capacitive region (702; 902) having an electrical connection independent of the first set (708; 908) of the transducers.

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