Patent Description:
Inertial sensors are in wide use for a variety of motion sensing applications. Examples of these applications include, but are not limited to, independent navigation system for autonomous surface/subsurface navigation, in geo-referencing, mapping and surveying, and in high-end industrial, transportation, aerospace, and automotive applications.

Gyroscopic inertial sensors are considered a subclass of inertial sensors, which provide information about angular motion such as the rate or angle of rotation. With the development of modern manufacturing methods, it is becoming increasingly common for these gyroscopic inertial sensors to be MEMS-based. For example a MEMS-based gyroscopic inertial sensor is described in <CIT>. These sensors such as the sensor disclosed in <CIT>, typically comprise an axisymmetric structure is coupled to a substrate at one or more anchor point(s) via a flexure arrangement, all arranged in a plane. Perfectly axisymmetric structures can possess so-called degenerate modes of vibration whose natural frequencies are matched, one designated as the drive mode and the other designed as the sense mode in a Coriolis vibratory gyroscope implementation. Electrodes are then used to drive the ring portion in a driving mode of vibration within the plane. When rotation is applied to the sensor about an axis perpendicular to the plane, Coriolis forces couple energy into a sensing mode of vibration. A separate set of electrodes are then used for the capacitive sensing of the vibrational response of the ring portion in the sensing mode, allowing the detection and calculation of an angular velocity or angular acceleration.

However, in the described sensors, energy losses occur due to a number of factors, including due to energy dissipation in the sensor flexures and at the substrate anchors. These energy losses result in a lower quality factor of the vibrational modes of interest. A high quality factor translates to superior sensor performance.

Additionally, gyroscopic resonant sensors with vibrational modes with high quality factors can also be used in can be used in high-end resonant sensing and timing and frequency control applications.

It would therefore be desirable to produce an inertial sensor with a flexure arrangement that minimises energy dissipation, and results in an inertial sensor constructed from an axisymmetric structure possessing degenerate or near-degenerate vibrational modes with high quality factors.

<CIT> discloses an MEMS resonator comprising an anchor, springs and stiffener rings. The springs are connected to the anchor and to each stiffener ring including a stiffener ring on the outer perimeter. The stiffener rings are concentric with another and concentric with the anchor. Each pair of springs are in the form of a circle, which has its circumference not entirely closed because of the intersection of springs with the anchor. The resonator further comprises peripheral electrodes which are connected to metal contacts, and are not in physical contact with the resonator outer diameter or the outer stiffener ring.

The present invention is defined in the appended independent claims, to which reference should now be made.

In a first aspect, the invention comprises an inertial sensor comprising: a central anchor; a proof mass, wherein the proof mass surrounds the central anchor; a flexure, the flexure having a shape comprising a first set of spiral arms, the first set of spiral arms comprising a first plurality of N spiral arms and a second plurality of N spiral arms, where N is an integer greater than <NUM>, each of the arms connected between the central anchor and the proof mass and lying in a first plane, each of the arms of the first plurality of N spiral arms winding about the central anchor in a first sense and each of the arms of the second plurality of N spiral arms winding about the central anchor in a second sense, the second sense being opposite to the first sense; and a plurality of electrodes comprising at least one drive electrode for driving the proof mass in a first mode of vibration, and at least one sense electrode for sensing a response of the proof mass in a second mode of vibration.

This arrangement of the first and second pluralities of spiral arms is advantageous as it contributes to minimising anchor losses, resulting in an inertial sensor with modes of vibration with high quality factors. As used herein, the term 'spiral' means a shape generated by a point moving around a fixed point while continuously receding from or approaching the fixed point, such that the distance from the fixed point to each and every point on the spiral is of different length.

The flexure may have a shape such that the arms of the first plurality of N spiral arms are equally spaced at <NUM>/N degree intervals about the central anchor. The flexure may have a shape such that the arms of the second plurality of N spiral arms are equally spaced at <NUM>/N degree intervals about the central anchor. Either of these features are advantageous, as they allow for modes of vibration with high quality factors to be generated.

The flexure may have a shape such that each of the arms in the first plurality of N spiral arms meets or crosses all of the arms from the second plurality of N spiral arms at least once. This arrangement of the first and second pluralities of spiral arms enables a symmetric arrangement of flexures supporting the outer mass. This provides the desired degenerate or near-degenerate modes and provides robustness to both in-plane and out-of-plane shock and vibration.

The flexure may be formed from a single piece of a material such as single-crystal silicon. The points where the first plurality of N spiral arms meets or crosses with the arms from the second plurality of N spiral arms may be in the first plane. Advantageously, either of these features may simplify manufacturing of the inertial sensor and/or ensure robustness of the flexure. The flexure may have a uniform thickness. Thickness is defined in the direction perpendicular to the first plane.

The flexure may exhibit N-fold rotational symmetry about an axis perpendicular to the first plane. Preferably, the central anchor, flexure and proof mass each exhibit N-fold rotational symmetry about an axis perpendicular to the first plane. Advantageously, this allows for degenerate or near-degenerate modes of vibration with high quality factors to be generated.

Each arm may have a first end connected to the central anchor and a second end connected to the proof mass, wherein the first end connects to the central anchor at a point on the central anchor that is furthest from the second end of the arm.

Each arm may have the first end directly connected to the central anchor. Advantageously, this feature may result in higher quality factors for the modes of vibration. Alternatively each arm may have the first end connected to the central anchor via another component of the inertial sensor.

Each arm may have the second end directly connected to the proof mass. Alternatively each arm may have the second end connected to the proof mass via another component of the inertial sensor. Advantageously, this may result in higher quality factors for the modes of vibration.

The flexure may have a shape that comprises a second set of spiral arms nested concentrically around the first set of spiral arms. The second set of spiral arms advantageously comprises a first plurality of N spiral arms and a second plurality of N spiral arms, each of the arms of the first plurality of N spiral arms winding about the central anchor in a first sense and each of the arms of the second plurality of N spiral arms winding about the central anchor in a second sense, the second sense being opposite to the first sense. The arms of the second set of spiral arms may have a different curvature or shape to the arms of the first set of spiral arms.

The flexure may have a shape that comprises further sets of spiral arms, wherein the sets of spiral arms are concentrically nested.

The flexure may have a shape comprising a plurality of nested quatrefoils. Each quatrefoil may be rotated by <NUM> degrees relative to adjacent quatrefoils. A quatrefoil is defined as the outer perimeter of four partially overlapping identical shape. A quatrefoil therefore comprises four lobes, and has <NUM>-fold rotational symmetry about the centre of the quatrefoil. Examples of the identical shapes may include but are not limited to; circles, ellipses, Reuleaux triangles, and other Reuleaux polygons. The central anchor may be located at the centre of the quatrefoils. The proof mass may surround all of the quatrefoils. The proof mass may be connected to a quatrefoil at four points. Preferably, the proof mass is connected to an outermost quatrefoil. More preferably, the proof mass is connected to an outermost quatrefoil at the outermost points of each lobe of the outermost quatrefoil.

Preferably, the flexure has a shape comprising four nested quatrefoils, with each quatrefoil rotated by <NUM> degrees relative to adjacent quatrefoils. The flexure may have a shape comprising a plurality of nested quatrefoils, with each quatrefoil rotated by <NUM> degrees relative to adjacent quatrefoils as a result of the arrangement of the first plurality of N spiral arms and the second plurality of N spiral arms, as described above.

The proof mass may be ring shaped. The proof mass may have an inner diameter of between <NUM> millimetre and <NUM> millimetres. Preferably, the proof mass has an inner diameter between <NUM> millimetres and <NUM> millimetres.

The proof mass may have an outer diameter between <NUM> millimetre and <NUM> millimetres. Preferably, the proof mass has an outer diameter between <NUM> millimetre and <NUM> millimetres.

The width of the proof mass may be measured as between the inner diameter and the outer diameter. The width of the proof mass may be between <NUM> millimetres and <NUM> millimetres. The width of the proof mass may be between <NUM> millimetres and <NUM> millimetres. Preferably, the width of the proof mass is between <NUM> millimetres and <NUM> millimetres. More preferably, the width of the proof mass is between <NUM> millimetres and <NUM>. millimetre. Even more preferably, the width of the proof mass is between <NUM> millimetres and <NUM> millimetres. Advantageously, these widths of the proof mass, particularly the preferred widths of the proof mass, may achieve low thermo-elastic dissipation and high quality factors of the inertial sensor. This enables high mechanical sensitivity and excellent signal-to-noise ratio for the gyroscope.

The thickness of the proof mass may be greater than the thickness of the flexure. Advantageously, this improves the sensitivity and noise performances of the inertial sensor. The proof mass may have a thickness between <NUM> micrometres and <NUM> micrometres. The flexure may have a thickness between <NUM> micrometres and <NUM> micrometres.

The proof mass may have a first mass. The flexure may have a second mass. The proof mass and the flexure together may have a combined mass equal to the sum of the first mass and the second mass. The first mass may be between <NUM>% and <NUM>% of the combined mass. The first mass may be between <NUM>% and <NUM>% of the combined mass. Preferably, the first mass is between <NUM>% and <NUM>% of the combined mass. More preferably, the first mass is between <NUM>% and <NUM>% of the combined mass.

The proof mass may have a first volume. The flexure may have a second volume. The proof mass and the flexure together may have a combined volume equal to the sum of the first volume and the second volume. The first volume may be between <NUM>% and <NUM>% of the combined volume. The first volume may be between <NUM>% and <NUM>% of the combined volume. Preferably, the first volume is between <NUM>% and <NUM>% of the combined volume. More preferably, the first volume is between <NUM>% and <NUM>% of the combined volume.

Advantageously, the first mass being a significant proportion of the combined mass, or the first volume being a significant proportion of the combined volume, may result in an inertial sensor with low thermo-mechanical noise, and may provide good immunity to fabrication tolerances, and so may achieve a low as-fabricated frequency split between degenerate and near-degenerate modes of vibration. This may enable relative ease of mode-matching during device operation. This enables high mechanical sensitivity and excellent signal-to-noise ratio for the gyroscope.

The proof mass may have an aspect ratio of between <NUM> and <NUM>. The aspect ratio may be defined as the ratio of the width of the proof mass to the thickness of the proof mass. Preferably, the proof mass has an aspect ratio of between <NUM> and <NUM>. More preferably, the proof mass has an aspect ratio of between <NUM> and <NUM>. Advantageously, such an aspect ratio allows for the first mass to be significant proportion of the combined mass, which may result in an inertial sensor with a high quality factor, whilst also allowing the inertial sensor to retain a relatively low thickness.

Advantageously, these dimensions allow for the inertial sensor to correspond to dimensions used in popular MEMS manufacturing techniques, allowing ease of manufacturing.

The flexure may have a shape further comprising a plurality of N radial spokes, wherein each of the radial spokes are connected to the proof mass. Advantageously, a plurality of N radial spokes contributes to minimising anchor losses, resulting in an inertial sensor with modes of vibration with high quality factors.

Each of the radial spokes may be connected to at least one arm from the first plurality of N spiral arms or the second plurality of N spiral arms. Preferably, each of the radial spokes are connected to at least one arm from the first plurality of N spiral arms and at least one arm from the second plurality of N spiral arms. The N radial spokes may be connected to the proof mass and spaced at <NUM>/N degree intervals about the central anchor. Each of the N radial spokes may be connected to an outermost quatrefoil. Preferably, each of the N radial spokes are connected to an outermost quatrefoil at the outermost points of each lobe of the outermost quatrefoil.

The width of each of the radial spokes of the plurality of N radial spokes may be between <NUM> micrometres and <NUM> micrometres. Preferably, the width of each of the radial spokes of the plurality of N radial spokes is between <NUM> micrometres and <NUM> micrometres. The width of each of the arms of the first plurality of N arms and of the second plurality of N arms may be between <NUM> micrometres and <NUM> micrometres. Preferably, the width of each of the arms of the first plurality of N arms and of the second plurality of N arms is between <NUM> micrometres and <NUM> micrometres. Width is defined in a direction parallel to the first plane. The plurality of N radial spokes contributes to minimising anchor losses, resulting in an inertial sensor with vibrational modes with high quality factors.

N may be an integer multiple of <NUM>. Preferably, N is equal to <NUM>.

Advantageously, the flexure and the proof mass may be integrally formed. Also advantageously, the flexure and the proof mass may be formed from a single piece of a material. Either, or both, of these two features allow for simplification of manufacturing and reduce the likelihood of manufacturing defects, minimising the splitting of vibrational modes. Preferably, the material is silicon. Silicon may be selected due to the ease of manufacturing and etching. The material may also comprise a buried silicon dioxide layer integrated between two distinct layers of single-crystal silicon if the starting substrate is a silicon-on-insulator (SOI) wafer.

The plurality of electrodes may comprise at least one electrode positioned outside of the proof mass. The plurality of electrodes may comprise X electrodes positioned outside of the proof mass, wherein X is an integer multiple of <NUM>. X may be equal to <NUM>. In this context, outside of the proof mass is defined by a component being located outside a region defined by an outer perimeter of the proof mass, when viewing the inertial sensor perpendicular to the first plane.

The plurality of electrodes comprising an integer multiple of <NUM> electrodes positioned outside of the proof mass allows for independent driving of the first mode of vibration and measurement of the response from the second mode of vibration. Additionally, the plurality of electrodes comprising <NUM> electrodes positioned outside of the proof mass enables selective tuning of the frequency of one mode of vibration with respect to the other mode of vibration, to match the frequencies of the two modes of vibration. This is referred to as mode-matching. Mode-matching enhances the sensitivity of the inertial sensor.

The plurality of electrodes may comprise at least one electrode positioned inside of the proof mass. The plurality of electrodes may comprise Y electrodes positioned inside the proof mass, wherein Y is an integer multiple of <NUM>. Y may be equal to <NUM>. In this context, inside of the proof mass is defined by a component being located within a region defined by an inner perimeter of the proof mass, when viewing the inertial sensor perpendicular to the first plane. Advantageously, having at least one electrode positioned inside of the proof mass, in addition to electrodes outside the proof mass, provides additional tunability of the inertial sensor and an increased transduction area, and may cancel capacitive feedthrough effects.

The inertial sensor may be a micro-electro-mechanical system or MEMS device. The inertial sensor may be a gyroscopic sensor.

The first mode of vibration and the second mode of vibration may both be cos(nθ) modes, where n is an integer greater than or equal to <NUM>. Preferably, the first mode of vibration and the second mode of vibration are cos(3θ) modes. Advantageously, cos(3θ) modes of the described inertial sensor display high quality factors.

In a second aspect, the invention comprises an inertial sensor comprising: a central anchor; a proof mass, wherein the proof mass surrounds the central anchor; and a flexure connected between the proof mass and the central anchor, wherein the proof mass is suspended from the central anchor by the flexure, the flexure having a shape comprising a plurality of nested quatrefoils.

Each quatrefoil may be rotated by <NUM> degrees relative to adjacent quatrefoils. A quatrefoil is defined as the outer perimeter of four partially overlapping identical shape. A quatrefoil therefore comprises four lobes, and has <NUM>-fold rotational symmetry about the centre of the quatrefoil. Examples of the identical shapes may include but are not limited to; circles, ellipses, Reuleaux triangles, and other Reuleaux polygons. The central anchor may be located at the centre of the quatrefoils. The proof mass may surround all of the quatrefoils. The proof mass may be connected to a quatrefoil at four points. Preferably, the proof mass is connected to an outermost quatrefoil. Preferably still, the proof mass is connected to an outermost quatrefoil at the outermost points of each lobe of the outermost quatrefoil.

Preferably, the flexure has a shape comprising four nested quatrefoils, with each quatrefoil rotated by <NUM> degrees relative to adjacent quatrefoils. The flexure may have a shape comprising a plurality of nested quatrefoils, with each quatrefoil rotated by <NUM> degrees relative to adjacent quatrefoils as a result of the arrangement of the first plurality of N spiral arms and the second plurality of N spiral arms as described above.

In a third aspect, the invention comprises a navigation system comprising an inertial sensor as described in any embodiment according to the first or second aspect of the invention.

In a fourth aspect, the invention comprises a method of inertial sensing using an inertial sensor as described in any embodiment according to the first or second aspect of the invention, the method comprising the steps of; driving the proof mass in the first mode of vibration using at least one drive electrode; sensing the response of the proof mass in a second mode of vibration using at least one sense electrode; and tuning the frequency of the first mode of vibration with respect to the second mode of vibration or tuning the frequency of the second mode of vibration with respect to the first mode of vibration to match the frequencies of the first mode of vibration and the second mode of vibration; and calculating the value of an input measurand based on the response of the proof mass in the second mode of vibration.

The value of the input measurand may be calculated based on the difference between the resonant frequency of the first mode and the resonant frequency of the second mode. Advantageously, this enables high dynamic range measurements and offers the potential for a reduced temperature dependence of scale factor.

Features described with reference to one aspect may be applied to any other aspect of the invention.

<FIG> are schematic illustrations of a plan view and a perspective view respectively of an inertial sensor <NUM> in accordance with a particular embodiment of the invention. The inertial sensor <NUM> comprises a central anchor <NUM>, a ring shaped proof mass <NUM>, and a flexure <NUM>. The central anchor <NUM> is attached to a substrate (not shown). Flexure <NUM> is connected to both the central anchor and the proof mass. Flexure <NUM> further comprises a first plurality of four spiral arms <NUM>, a second plurality of four spiral arms <NUM>, and four radial spokes <NUM>. The first plurality of four spiral arms <NUM>, the second plurality of four spiral arms <NUM>, and the four radial spokes <NUM> all lie in a first plane, the first plane being the plane of the paper in <FIG>.

Each of the arms of the first plurality of four spiral arms <NUM> wind about the central anchor <NUM> in a first clockwise sense, when viewed from the perspective of <FIG>. Each of the arms of the second plurality of four spiral arms <NUM> wind about the central anchor <NUM> in a second anticlockwise sense, when viewed from the perspective of <FIG>. The second sense is therefore opposite to the first sense.

Each of the arms from the first plurality of four spiral arms <NUM> and the second plurality of four spiral arms <NUM> are directly connected at a first end to the central anchor <NUM>.

Each of the arms of the first plurality of four spiral arms <NUM> and the second plurality of four spiral arms <NUM> are also indirectly connected at a second end to the proof mass <NUM>. This indirect connection is via the four radial spokes <NUM>. Each of the four radial spokes <NUM> are directly connected to the proof mass <NUM>, one arm from the first plurality of four spiral arms <NUM>, and one arm from the second plurality of four spiral arms <NUM>. The four radial spokes <NUM> are equally spaced at <NUM> degree intervals about the central anchor <NUM>.

The connection point of each of the arms to the central anchor <NUM> is at a point on the central anchor <NUM> that is furthest from the second end of the arm in question. This may be described as the arms completing <NUM> degrees of winding, as the length of each arm in both the first and second pluralities of four spiral arms <NUM>, <NUM> traces an arc of <NUM> degrees about a point centred on the central anchor.

In this particular embodiment of the invention, the first plurality of four spiral arms <NUM>, the second plurality of four spiral arms <NUM>, and the four radial spokes <NUM> are all integrally formed from a single piece of material. As these components of the flexure all lie in the first plane, and the flexure has a consistent thickness, the points at which arms from the first plurality of four spiral arms <NUM> and the second plurality of four spiral arms <NUM> meet or cross also lie in the first plane. This may be referred to as the superposition of the first plurality of four spiral arms <NUM> and the second plurality of four spiral arms <NUM>.

In the embodiment of <FIG> the proof mass is also integrally formed with the flexure. The proof mass and flexure may be formed from silicon. The dimensions of the flexure and proof mass are such that over <NUM>% of the total combined mass of the flexure and proof mass is in the proof mass.

An alternative way to describe the shape of a portion of the flexure <NUM> in <FIG> is that it is a plurality of concentrically nested quatrefoils. The arms of the first plurality of four spiral arms <NUM> and the second plurality of four spiral arms <NUM> intersect in such a way that the flexure may be viewed to comprise four concentrically nested quatrefoils, with concentrically adjacent quatrefoils rotated by <NUM> degrees relative to each other. The points at which arms from the first plurality of four spiral arms <NUM> and the second plurality of four spiral arms <NUM> meet or cross are the points at which concentrically adjacent quatrefoils meet. It would be appreciated by a person skilled in the art that a similar flexure shape may be formed from a different integer number of concentrically nested quatrefoils, and provide the same advantage of a high quality factor.

The nested quatrefoils do not need to be shaped to provide continuous spiral arms extending from one quatrefoil to the next, as shown in <FIG>. Instead, the quatrefoils may be shaped to give rise to concentrically nested arrangements of sets of spiral arms, the spiral arms in each set of spiral arms having a different shape or curvature to the spiral arms in an adjacent set of spiral arms. Each set of spiral arms comprises a first plurality of N spiral arms and a second plurality of N spiral arms, where N is an integer greater than <NUM>, each of the arms of the first plurality of N spiral arms winding about the central anchor in a first sense and each of the arms of the second plurality of N spiral arms winding about the central anchor in a second sense, the second sense being opposite to the first sense. Each set of spiral arms may define a quatrefoil. Each spiral arm in each set of spiral arms connects, at least at one end, to two spiral arms in another set of spiral arms.

<FIG> is a schematic illustration of a plan view of an inertial sensor <NUM> in accordance with an alternative embodiment of the invention. The inertial sensor <NUM> of this alternative embodiment also comprises a central anchor <NUM>, a ring shaped proof mass <NUM>, and a flexure <NUM>. The proof mass <NUM> and central anchor <NUM> are identical to that of the embodiment shown in <FIG>. The flexure <NUM> further comprises a first plurality of four spiral arms <NUM>, a second plurality of four spiral arms <NUM>, and four radial spokes <NUM>, similar to that of the embodiment shown in <FIG>.

The embodiment in <FIG> differs from the embodiment shown in <FIG> in the degrees of winding of each of the arms about the central anchor. In the embodiment shown in <FIG> each of the arms complete <NUM> degrees of winding, as the length of each arm in both the first and second pluralities of four spiral arms <NUM>, <NUM> traces an arc of <NUM> degrees about a point centred on the central anchor <NUM>.

The arms of the first plurality of four spiral arms <NUM> and the second plurality of four spiral arms <NUM> intersect in such a way that the flexure comprises <NUM> concentrically nested quatrefoils, with concentrically adjacent quatrefoils rotated by <NUM> degrees relative to each other. As in the embodiment shown in <FIG>, the points at which arms from the first plurality of four spiral arms <NUM> and the second plurality of four spiral arms <NUM> meet or cross are the points at which concentrically adjacent quatrefoils meet.

<FIG> is a schematic illustration of a plan view of an inertial sensor <NUM> in accordance with an alternative embodiment of the invention. The inertial sensor <NUM> of this alternative embodiment also comprises a central anchor <NUM>, a ring shaped proof mass <NUM>, and a flexure <NUM>. The proof mass <NUM> is identical to that of the embodiment shown in <FIG> and <FIG>.

This embodiment differs from that shown in <FIG> in that the embodiment shown in <FIG> comprises a flexure <NUM> further comprising a first plurality of eight spiral arms <NUM>, a second plurality of eight spiral arms <NUM>, and eight radial spokes <NUM>. The eight radial spokes <NUM> are equally spaced at <NUM> degree intervals about the central anchor <NUM>. Additionally, the central anchor <NUM> has a different shape to that of the central anchors in the embodiments shown in <FIG> and <FIG> in order to accommodate the different number of arms.

In the embodiment shown in <FIG> each of the arms complete <NUM> degrees of winding, as the length of each arm in both the first and second pluralities of <NUM> spiral arms <NUM>, <NUM> traces an arc of <NUM> degrees about a point centred on the central anchor <NUM>.

An alternative way to describe the shape of the flexure <NUM> in <FIG> is that it comprises a plurality of concentrically nested, similar shapes, each comprising eight lobes. Each of the concentrically nested shapes have <NUM>-fold rotational symmetry. Each of the concentrically nested shapes comprising eight lobes may be considered to be formed by the outer perimeter of the overlap of two identical quatrefoils, offset by a <NUM> degree rotation from one another. The arms of the first plurality of eight spiral arms <NUM> and the second plurality of eight spiral arms <NUM> intersect in such a way that the flexure is viewed to comprise four concentrically nested shapes comprising eight lobes, with concentrically adjacent shapes comprising eight lobes rotated by <NUM> degrees relative to each other. The points at which arms from the first plurality of eight spiral arms <NUM> and the second plurality of eight spiral arms <NUM> meet or cross are equivalent to the points at which concentrically adjacent shapes comprising eight lobes meet.

<FIG> is a schematic illustration of an electrode arrangement of an inertial sensor <NUM> in accordance with a particular embodiment of the invention. The arrangement of the proof mass <NUM>, flexure <NUM> and central anchor <NUM> is identical to that shown in <FIG>.

In this particular embodiment, the plurality of electrodes consists of a set of <NUM> electrodes <NUM> that are arranged inside of the proof mass <NUM>, and a set of <NUM> electrodes <NUM> that are outside of the proof mass <NUM>. Each set of <NUM> and <NUM> electrodes are arranged in one of two concentric circles centred on the central anchor <NUM>. Electrodes in each set are evenly spaced around each of the circles. Each of the electrodes in the set of <NUM> electrodes <NUM> positioned inside of the proof mass <NUM> are substantially identical to one another. Also, each of the electrodes in the set of <NUM> electrodes <NUM> positioned outside of the proof mass <NUM> are substantially identical to one another.

Each electrode in the plurality of electrodes may be used to perform a function of the inertial sensor in use. These functions include, but are not limited to driving the proof mass <NUM> in the first or second mode of vibration, sensing variations in capacitance when the proof mass <NUM> vibrates in the first or second mode of vibration, or applying a bias voltage to aid in matching the resonant frequencies of the first and second modes of vibration. The electrodes that are used to perform these functions may be referred to as drive electrodes, sense electrodes and mode-matching electrodes respectively.

Each drive electrode is positioned adjacent to the proof mass <NUM>. Each drive electrode is configured to generate an electrostatic force which acts upon the proof mass <NUM>. Each sense electrode is positioned adjacent to the proof mass <NUM>. Each sense electrode is configured to detect variations in capacitance when the proof mass <NUM> oscillates in a mode of vibration.

In use, a DC bias voltage is applied to the inertial sensor <NUM>, such that the central anchor <NUM>, flexure <NUM>, and proof mass <NUM> are electrostatically biased. The DC bias voltage is applied to the inertial sensor at bias points <NUM>. A DC bias voltage enables electrostatic drive and capacitive sense for both vibrational modes using signals at both frequencies of the vibrational modes. The method of operation of the inertial sensor <NUM> is described in detail in <FIG> and <FIG> and their respective descriptions.

In the embodiment of <FIG>, the first set of drive electrodes <NUM> and the first set of sense electrodes <NUM> are arranged adjacent to antinodes of the driving mode of the proof mass <NUM>. The second set of drive electrodes <NUM> and the second set of sense electrodes <NUM> are arranged adjacent to the antinodes of the sensing mode of the proof mass <NUM>. However, the arrangement of which electrodes perform which functions may vary.

The embodiment of <FIG> includes mode-matching electrodes <NUM>. Mode-matching electrodes <NUM> may be located either inside of outside of the proof mass <NUM>, but in this particular embodiment shown in <FIG> they are located outside of proof mass <NUM>. Each mode-matching electrode <NUM> is positioned adjacent to the proof mass <NUM>. Each mode-matching electrode <NUM> is configured to generate an electrostatic force which acts upon the proof mass <NUM> to locally adjust the stiffness of the proof mass <NUM>.

<FIG> are COMSOL Multiphysics ® simulations of two of the degenerate modes of vibration that may be used in the driving and sensing of the inertial sensor <NUM> when in use. These two vibration modes may be referred to as cos3θ modes. In use, one of these degenerate modes of vibration may be driven by the first set of drive electrodes <NUM>, not shown in <FIG>. This first mode of vibration is referred to as the driving mode as previously. As a result of a Coriolis force generated by an angular velocity of the inertial sensor, energy will be coupled into the other of the two degenerate modes of vibration, then referred to as the sensing mode as previously. The outline of the stationary inertial sensor <NUM> is also included for reference. It can be seen that significant localised strain of the proof mass <NUM> occurs during the vibration in these two modes. Additionally, the simulation shows significant strain on the four radial spokes <NUM>. Advantageously, there is relatively low strain in both of the degenerate modes of vibration on the central portion of the flexure <NUM>, near the second ends of the arms, and the central anchor <NUM>. This localised relatively low strain in both of the degenerate modes of vibration reduces energy loss through anchor losses, resulting in an increased quality factor for these modes of vibration. The actual strain experienced by the inertial sensor in use may be less than or more than that represented in <FIG>.

<FIG> are data plots showing ring-down responses of the driving and sensing modes of vibration respectively. The driving and sensing modes of vibration measured are identical to those shown in <FIG>.

The output voltage <NUM> is directly proportional to the amplitude of vibration of the driving and sensing modes of vibration. The output voltage <NUM> is measured using a lock-in amplifier. Quality factors for the driving and sensing modes of vibration are calculated from the ring-down response using equation (<NUM>).

The decay time <NUM>, τ, is the time taken for the output voltage to decay to <NUM>/e of the output voltage <NUM> at time <NUM> equals zero. The frequency of the mode of vibration is denoted by f. Both the driving and sensing modes of vibration show measured quality factors in excess of <NUM> million.

<FIG> is an Allan deviation plot, displaying data collected from an inertial sensor as in <FIG> and <FIG> and described previously. The data is measured at zero input rotation about an axis perpendicular to the first plane. ARW is shown to be <NUM> °/√ h, and BI is shown to be <NUM> °/h.

The angle random walk (ARW) and bias instability (BI) are key metrics to evaluate and compare the performances of gyroscopic inertial sensors. Bias may be defined as the average over a specified time of gyroscopic output measured at specified operating conditions that has no correlation with input rotation or acceleration. Bias is typically expressed in degrees per hour (°/h). Bias instability may be defined as the random variation in bias as computed over specified finite sample time and averaging time intervals. Bias instability is also typically expressed in degrees per hour (°/h). Angle random walk may be defined as the angular error build-up with time that is due to white noise in angular rate. Angle random walk is typically measured in degrees per square root of hour (°/√h).

<FIG> and <FIG> are COMSOL Multiphysics ® simulations of two alternative vibrational modes of the inertial sensor of <FIG> and <FIG>, described previously. <FIG> illustrates a vibrational mode that may be referred to as the <NUM> vibrational mode. <FIG> illustrates a vibrational mode that may be referred to as the primary wine glass vibrational mode, or may instead be referred to as the cos(2θ) vibrational mode. Both the <NUM> vibrational mode and primary wine glass vibrational mode also display lower strain on the central portion of the flexure <NUM>, near the second ends of the arms, and the central anchor <NUM> relative to the strain on the proof mass <NUM> and radial spokes <NUM>. This is advantageous in a similar way to the modes of vibration shown in <FIG>. The effect of reduced strain close to the central anchor <NUM> results in reduced energy loss through anchor losses, resulting in an increased quality factor for both modes of vibration.

As both the <NUM> and the primary wine glass vibrational modes have been shown to demonstrate high quality factors, the inertial sensor utilised with either the <NUM> or the primary wine glass vibrational mode may be implemented in high-end resonant sensor and timing and frequency control applications.

The inertial sensors described previously may be used in a navigation system. <FIG> is a schematic of an example navigation system <NUM> according to the third aspect of the invention, comprising three inertial sensors <NUM> according to the first or second aspect of the invention. The navigation system <NUM> further comprises a navigation computer <NUM> comprising a memory. The signals from the three inertial sensor are inputted into the navigation computer. The navigation computer processes the signals from the three inertial sensors, and calculates the acceleration, velocity and position of the navigation system <NUM> based on the signals. The navigation system may be used in autonomous surface or subsurface navigation, geo-referencing, mapping and surveying, transportation, aerospace, and automotive applications.

<FIG> is a schematic of a method of operation according to the fourth aspect of the invention of an inertial sensor according to the first or second aspect of the invention. The inertial sensor is first driven in a first mode of vibration <NUM>, referred to as the driving mode. This is achieved through a combination alternating and DC voltages being applied to a first set of drive electrodes relative to the proof mass.

A first set of sense electrodes sense the response of the proof mass to being driven in the driving mode. A first feedback loop <NUM> utilises the first set of sense electrodes and the first set of drive electrodes. The first feedback loop <NUM> is configured to regulate the frequency, amplitude, phase, or other characteristics of the driving mode.

Variations in capacitance when the proof mass oscillates in the sensing mode may be detected <NUM> by a second set of sense electrodes. The amplitude of the vibration of the sensing mode may then be calculated from the variations in capacitance when the proof mass oscillates in the sensing mode. To achieve this, the amplitude of the vibration of the sensing mode may have to first be separated from the amplitude of the vibration of the driving mode. The angular rotation may then be calculated <NUM> from the amplitude of the vibration of the sensing mode.

In order to enhance the sensitivity of the gyroscope, mode-matching <NUM> may be used. This process comprises detecting the difference in frequencies of the driving mode and the sensing mode, and applying voltages to mode-matching electrodes. In use, when a voltage difference is present between a mode-matching electrode and the proof mass, an electrostatic force is generated. This allows the stiffness of the proof mass to be locally adjusted. Therefore, the driving mode and sensing modes frequencies of the inertial sensor can be relatively adjusted using the mode-matching electrodes, to ensure that the driving mode and sensing mode frequencies are accurately matched.

This method of detecting angular rotation is referred to as open loop sensing.

<FIG> is a schematic of an alternative method of operation according to the fourth aspect of the invention, of an inertial sensor according to the first or second aspect of the invention. As in open loop sensing, the inertial sensor is first driven in a first mode of vibration <NUM>, and a first feedback loop <NUM> is configured to regulate the frequency, amplitude, phase, or other characteristics of the driving mode, and the variations in capacitance when the proof mass oscillates in the sensing mode may be detected <NUM> by a second set of sense electrodes. Additionally, as in open loop sensing mode control, the process of mode-matching <NUM> may be used.

Where the method of <FIG> differs from that of <FIG>, is that a second set of drive electrodes are used to generate an electrostatic force which acts upon the proof mass. A second feedback loop <NUM> utilises the second set of sense electrodes, and second alternating voltages are applied to the second set of drive electrodes. These second alternating voltages from the second set of drive electrodes exert forces on the proof mass, and are configured to reduce the amplitude of the response of the sensing mode to zero using the second feedback loop <NUM>. The angular rotation is then calculated in step <NUM>. The amplitude of the second alternating voltages required to reduce the amplitude of the vibration of the sensing mode to zero is employed to calculate the angular rotation experienced by the inertial sensor.

This method of detecting angular rotation is referred to as closed loop sensing mode control or force-to-rebalance sense mode control.

<FIG> is a schematic of a further alternative method of operation according to the fourth aspect of the invention, of an inertial sensor according to the first or second aspect of the invention. As in open and closed loop sensing mode control, the inertial sensor is first driven in a first mode of vibration <NUM>, and a first resonance tracking feedback loop <NUM> is configured to regulate the amplitude, phase, or other characteristics of the driving mode.

Similar to the operation of drive mode in <FIG>, a second set of drive electrodes are used to generate an electrostatic force which acts upon the proof mass. A second feedback loop <NUM> utilises the second set of sense electrodes, and second alternating voltages are applied to the second set of drive electrodes. The second resonance tracking feedback loop <NUM> is configured to regulate the amplitude, phase, or other characteristics of the sensing mode <NUM>.

Additionally, as in open and closed loop sensing mode control, the process of mode-matching <NUM> may be used.

Claim 1:
An inertial sensor (<NUM>; <NUM>; <NUM>) comprising:
a central anchor (<NUM>; <NUM>; <NUM>);
a proof mass (<NUM>; <NUM>; <NUM>),
wherein the proof mass surrounds the central anchor;
a flexure (<NUM>; <NUM>; <NUM>),
the flexure having a shape comprising a first set of spiral arms, the first set of spiral arms comprising a first plurality of N spiral arms (<NUM>; <NUM>; <NUM>) and a second plurality of N spiral arms (<NUM>; <NUM>; <NUM>), where N is an integer greater than <NUM>, each of the arms connected between the central anchor and the proof mass and lying in a first plane, each of the arms of the first plurality of N spiral arms winding about the central anchor in a first sense and each of the arms of the second plurality of N spiral arms winding about the central anchor in a second sense, the second sense being opposite to the first sense; and
a plurality of electrodes comprising,
at least one drive electrode (<NUM>, <NUM>) for driving the proof mass in a first mode of vibration, and
at least one sense electrode (<NUM>, <NUM>) for sensing a response of the proof mass in a second mode of vibration.