A SOLID-STATE, MULTI-AXIS GYROSCOPE

Abstract: A gyroscope (36) includes a gyro body-portion (48, 50, 52 or 54) which is driven into vibratory primary oscillation P and, if turned about on axis X, Y or Z experiences a Coriolis force induced vibration. Matching the primary oscillation mode and the secondary modes allow one to accurately detect motion about one, two or three axis.

Description: A SOLID-STATE. MULTI-AXIS GYROSCOPE This invention relates to a gyroscope and more specifically to a solid-state, multi-axis gyroscope. European Patent Application EP-A2-0,427,177 (Murata Manufacturing Co.) describes a vibrating column having an axial bore therethrough and on whose external surface there are piezoelectric drivers. By applying a signal to these drivers the column bends and vibrates. When the column is rotated a Coriolis force is produced between two external electrodes. This type of gyroscope is known as a solid-state gyroscope. Another example of a solid-state gyroscope, based on a vibrating ring, is described in a paper by M.W. Putty and K. Najafi, in conference proceedings 'A Micromachined Vibrating Ring Gyroscope', Proc. IEEE Solid-State Sensor and Actuator Workshop, Hilton Head, South Carolina, USA, June 13-16, 1994, pp. 213-220. Another example of a solid-state gyroscope is described in UK Patent Applications GB-A- 2,272,054 (GEC Marconi Ltd.) and GB-A-2,272,053 (GEC Marconi Ltd). The gyroscopes described include a resonator bell which is attached to a printed circuit board. One or more electrically conductive tracks on the printed circuit board cause the resonator bell to vibrate and act as a gyroscope. US Patent 5,450,751 (General Motors Corporation) discloses a micro-structure for a vibratory gyroscope having a ring portion supported so as to allow substantially undamped vibration. The ring portion is electrically conductive and comprises a charge plate for a plurality of radially disposed charge conductive sites. These sites, being disposed around the perimeter of the ring, sense radial displacements in the ring. These radial displacements are detected and used to provide an indication of the Coriolis force experienced by the gyroscope. All of the above mentioned gyroscopes have been found to be effective but only in detecting angular rotation about one axis. That is to say all are so called single axis gyroscopes. Optical gyroscopes are also single axis gyroscopes. Flywheel gyroscopes may be configured as single or dual axis gyroscopes, but these are expensive. US-A-5,490,420 discloses a gyroscopic device having a number of seismic masses supported on a system of beams. Between them, these seismic masses are used to detect motion about one or two distinct axes. Detection of rotation about a third distinct axis is disclosed but detection does not involve monitoring movement of the seismic mass itself. For many applications it is desirable to measure rate of turn about two, three or more axes simultaneously. For example in semi-active suspension systems on automobiles, it is desirable to detect rotation about all three axes, namely pitch, roll and yaw, simultaneously. In order to measure a rate of turn about all three axes it has previously been necessary to use three independent gyroscopes mounted so that their sensitive axis were orthogonal. Thus three single axis gyroscopes, special packaging and a process controller have been required. This proved expensive and complicated. The present invention arose in an attempt to overcome this and other problems, by providing a solid-state, multi-axis gyroscope, capable of detecting rotation about at least two and preferably three or more axes, two or more of which may be mutually perpendicular. According to the present invention, there is provided a gyroscope comprising: a gyro body- portion, having primary and secondary modes of vibration; driving means, for driving said gyro body into said primary mode of vibration; detecting means, for detecting coriolis force induced secondary vibration upon motion of said body about a given axis; characterised in that said body -portion has two or more independent secondary modes of vibration, each being excitable as a result of motion of said body portion about a separate associated axis and in that said detecting means includes means for detecting the vibration of said two or more secondary modes. Preferably, the gyroscope further includes two or more gyro-body portions, and in which the detector means includes means for detecting secondary vibrations in one or more body- portion. In a particular arrangement, the gyroscope comprises a pair of coupled body-portions in which the driving means drives one body-portion and the detection means detects secondary vibrations in the other body-portion. In an alternative arrangement, where the gyroscope comprises a pair of coupled body- portions, the detector means includes means for detecting secondary vibrations in both of said body-portions and further includes means for combining the output of said detectors in order to produce a signal indicative of rotation about the associated axis. In a preferred arrangement, the gyroscope comprises two or more pairs of said body- portions, each pair being angularly displaced relative to each other pair, thereby to facilitate detection of rotation about different axes. In a particular arrangement, the primary mode of vibration of each body-portion are in a common single plane. Conveniently, the independent secondary modes of vibration of each body-portion are orthogonal to each other, thereby to facilitate detection of rotation about two orthogonal axes. In a further preferred arrangement, one or more of said one or more body-portions has a third secondary mode of vibration excitable as a result of motion about a third axis and the detecting means includes means for detecting the vibration in said third mode, thereby to detect motion about said third axis. In a convenient arrangement of the above, the independent third secondary mode of vibration is orthogonal to one or other of the other two secondary modes of vibration, thereby to facilitate detection of rotation about orthogonal axes. Advantageously, the driving means drives the body-portion at resonance. Preferably, the secondary vibration is at resonance. Conveniently, the two or more secondary modes of vibration have the same natural frequency, multiple thereof or integer fraction thereof. Advantageously, the detector means further includes means for determining the magnitude of the or each secondary mode of vibration, thereby to facilitate determination of the rate of turn about each of said associated axis. In one specific arrangement, the gyroscope further includes support means for supporting said body-portion whilst allowing said primary and secondary modes of vibration to occur, said support means also acting to locate the body-portion relative to a support boss which, in operation, is subjected to the motion being detected. Preferably, when the gyroscope comprises two or more body-portions, the two or more body-portions vibrate radially outward of the boss and connected thereto, each by said support means. Conveniently, the two or more body-portions are connected to each other by flexible beams which form part of said support means. A preferred embodiment of the invention will now be described, by way of example only, and with specific reference to Figures 4-17, although general reference will be made to all the Figures in which: Figure 1 is a general view of an automobile and shows three mutually peφendicular axis, namely pitch, roll and yaw; Figure 2 is a graph showing three mutually peφendicular axis and illustrates the basic theory of how a single axis solid-state gyroscope operates; Figure 3 is a diagrammatical sectional view of a prior known single axis solid-state gyroscope; Figures 4 to 9 show different views of a solid-state, multi-axis gyroscope according to the present invention; Figures 10 to 15 show how the multi-axis gyroscope of Figures 4 to 9 behaves; Figure 16 shows an example, with dimensions in microns, of a multi-axis solid-state gyroscope having a thickness of 137 microns; Figure 17 shows diagrammatically possible fabrication steps of the gyroscope shown in Figure 16; and Figures 18, 19 illustrate an alternative form of the present invention. There is firstly a brief introduction with reference to Figures 1 to 3, which is intended to familiarise the reader with certain terms of the art. Figure 1 shows the three rotation axes of an automobile; roll, where a car with worn shock absorbers leans from side to side after, for instance, going down a deep pothole; pitch, where the front of the car may dive under heavy braking; and yaw, where the front of the car may change direction after suffering a heavy crosswind after passing, for example under a motorway bridge. All these movements can be corrected by appropriate steering, suspension and braking systems. In order to provide a correction signal, for example to an active suspension system for an automobile, control systems need an input signal as to the amount of unwanted motion. As all these motions are rotational about any given axis, what is needed is a rate of turn, or angular velocity measurement. Figure 2 shows when a body 10 of mass m has a linear velocity v along an x-axis, expressed as v sin (ωt), and a rate of turn Ω is applied about the z-axis, the body 10 experiences a force F along the y-axis, in the y-direction. The magnitude of the force F is proportional to the rate of turn Ω about the z-axis. Monitoring this force gives a direct measure of the angular velocity Ω. Figure 3 is an overall diagrammatical view of a single-axis, solid-state gyroscope 1 1. It is in form of a generally square structure and has portions 14, 16 and 18 which are composed of rectangular, solid beams. The beams are mounted on. for example, a glass substrate 20. Beam 16 has a free end 16a which acts as a cantilever. The velocity in the x-direction is provided by, for example, piezoelectric actuation by energising piezoelectric element 21 at the resonant frequency of the free standing part of beam 16. A rate of turn about the z- axis provides secondary motion in the y-direction. Beam 16a acts as a central electrode between two capacitor plates 14 and 18. As beam 16a moves, one capacitance increases whilst the other decreases. The interrogation of this capacitance change provides details of the rate of turn about the z-axis. Details of this device are described in a paper by K. Maenaka and T. Shiozawa entitled "A Study of Silicon Angular Rate Sensors using Anisotropic Etching Technology" Sensors and Actuators A 43, 1994, pp 72-77. Referring now to Figures 4 to 15, which show a multi-axis gyroscope 36 in isometric view. The central region 38 is connected to corner sections of a generally square gyro-body via four separate struts 40, 42, 44 and 46. The sixteen beams 100-115 are relatively thin compared with their depth so that rectangular gyro-body portions 48, 50, 52 and 54 are able to move or vibrate radially as shown diagrammatically by arrows P in Figure 4, while remaining rigid to flexing out of the plane of the gyroscope. This radial motion is excited, for example, by Lorentz force actuation, and is known as the primary mode. For best performance, the primary mode of vibration is a resonant one. Under the influence of a rate of turn this primary mode becomes coupled to one or more distinct secondary resonances. Each one of these secondary resonances is excited independently of the other by rotation about an associated axis which may or may not be peφendicular to one or more other axis. It is this coupling of the primary with multiple (e.g. x, y and z) secondary modes which has been utilised in the present invention. Figure 5 shows the effect of a rate of turn about the x-axis. The masses of body portions labelled 50 and 54 experience no Coriolis force. Masses of body portions 48 and 52 experience equal and opposite Coriolis force induced motion normal to the plane of the gyroscope. This Coriolis force excites a rocking motion in the direction of arrows Xs about the x-axis and is hereinafter referred to as the X secondary mode. Due to the 90° symmetry of the structure the same is true of a rate of turn about the y-axis, but this time gyro-body portions 50 and 54 experience the Coriolis force such that they rock in the direction of arrows Ys. Figure 6 depicts this and this is hereinafter referred to as the Y secondary mode. If a rate of turn is applied about the z-axis, normal to the gyro-body, then all four of the gyro-body portions 48, 50, 52 and 54 experience a Coriolis force induced motion in the plane of the structure and this is shown in Figure 7. These forces all act together to produce a third rotational motion about the z-axis, this is called the Z secondary mode Zs. Thus gyroscope 36 has one primary and three secondary modes, any two of which may be exploited to provide a multi-axes gyroscope. Two of the secondary modes (X S ,Y S ) are degenerate by virtue of the symmetry of the structure. As a result of the design symmetry the four diagonal support struts 40, 42, 44 and 46 in the centre of the gyroscope 36 experience only tensile and compressive forces along their length during motion in the primary mode P. In the primary mode all the struts are relatively stiff and as a result little energy is dissipated. For this reason it is believed that the primary motion has a high Q value. The primary motion of gyroscope 36 is particularly suited to Lorentz actuation, although other types of actuation may be employed. If Lorentz actuation is employed, a conductive track 56 may be deposited on the surface of gyroscope 36 as shown in Figure 8. A sinusoidal current (I sin ωt) in the presence of magnetic field (not shown), normal to the plane of the gyro-body, is passed along track 56, such that the radial motion of the primary mode P is generated. Strut 46 carrying the drive current (top left) has zero net current flowing along it. Current carrying wires 56a and 56b therefore do not cause any forces to be developed on the strut. Secondary sensing may be performed by positioning one or more motion sensors 61-68 beneath respective gyro-body portions 48, 50, 52 and 54 on the substrate. Additional sensors 70-77 may be provided at the corners. The substrate itself may be a glass disc or a second silicon wafer. Figure 9 shows diagrammatical ly one possible arrangement of electrodes 61 to 77 which can sense the amplitudes of the primary and three secondary modes. Electrodes 61 to 68 are positioned beneath the moving masses of gyro-body portions 48 to 54 and sense the primary motion and the X and Y secondary motions. Electrodes 70-77 beneath the corners 69a-69d of the gyro-body and can sense all three of the secondary motions. As with previous devices, the modelling of the design of this specific example of the present invention has been carried out using the properties of single crystal silicon. The gyroscope is substantially square symmetric and is manufactured on { 100} silicon with one of the sides of the structure parallel to the { 1 10} direction. Orientation at 45 degrees to the { 1 10} direction also preserves elastic symmetries of gyroscope 36. With the multi-axis gyroscope shown in Figures 4 to 9 there are four different modes which have to be matched by tuning the overall dimensions: the X and Y secondary modes are automatically matched by the symmetry of the multi-axis gyroscope leaving only three independent modes to match. The gyroscope is uniform throughout its thickness (in the Z direction). This means that any resonance whose motion is purely in the plane of the multi- axis gyroscope body 38. that is whose motion is confined to the X and Y directions only, will be independent of the thickness of the structure. Both the primary P and the Z secondary modes Zs meet this criterion and therefore have resonant frequencies which are independent of the gyroscope thickness. This is confirmed with finite element (FE) analysis, as shown in Figures 10 to 15. The X and Y secondary modes Xs. Ys do move out of the plane. That is they possess a Z component, and therefore their resonant frequencies are dependent upon the thickness of the gyroscope. This ability to separate the modes into those dependent upon the thickness of the gyroscope and those which are not, is extremely useful in matching the frequencies of the various modes. In order to match the modes the shape of the design is first adjusted so as to match the primary P and secondary Z mode Zs. That is the two modes that are entirely in the plane of the gyroscope body 38. This was done in a model having a constant thickness. The dimensions of the structure are then varied. Once the "in plane modes" are matched the thickness of the gyroscope is adjusted, while keeping all other dimensions fixed, so as to bring the secondary X and Y modes Xs, Ys into line with the primary P and secondary Z mode Zs. The dynamics of the present invention are quite complex and are best explained with reference to the mode shape plots produced by the finite element analysis of the structure. Figure 10 shows the structure of the gyroscope. Figure 11 shows the restraints that were imposed on the structure. An ideal mechanical earth was assumed on one face of the central square gyro-body region 38, the restrained freedoms are marked with arrows. Many modes of resonance occur with this device, but only the ones of relevance to this structure are shown. Figure 12 shows the first mode of interest, the primary mode. The plot shows the masses 50-54 at their furthest point from the central region of gyro-body 36. The motion is entirely in the X-Y plane. Figure 13 shows the X secondary motion. The square ring is depicted as rocking about the X-axis: this motion would be excited by a rate of turn about the X- axis. This motion is also showing in Figure 14 viewed along the Y-axis. Similar plots can be obtained for the Y secondary' motion. The X and Y secondary motions are the same, just rotated 90° with respect to one another, about an axis which passes through the centroid of the gyroscope. Figure 15 shows the last mode of interest, the Z secondary motion. This mode is a torsional resonance of the structure about the Z-axis and is excited by a rate of turn about the Z-axis. The dimensions of a preferred embodiment of a multi-axis gyroscope, with the four matched modes are shown in Figure 16. In order to maximise the sensitivity as an accelerometer it is possible that the dimensions may need to be adjusted to minimise any stiffness of the gyroscope. That is by making the masses larger and the beams and/or the struts thinner. It will be appreciated that the invention has been described by way of example only and variation to the above embodiment may be made without departing from the scope of the invention. For example, the device may be manufactured from a material other than silicon such as, for example, metal. It will also be appreciated that other types of displacement actuators (drives) and displacement sensors (detectors) may be used. For example; piezoelectric drive and/or detector devices; magneto-strictive drive and/ detection; capacitive drive and/or detection devices. Turning now briefly to figures 18 and 19, it will be appreciated that the present invention may be configured in any one of a number of ways. For example, the triangular arrangement of body portions facilitates detection about non-orthogonal X. Y axes and axis Z which is orthogonal to each of X and Y. In this specific arrangement, the body portions 150, 152, 154 are each supported by flexible beams 202-213 similar to beams 100-115 of figure 4. Each pair of beams extends towards a corner portion 220-224 which is in turn connected to a central boss 138 by one or other of beams 140-144. Operation of this specific arrangement is very similar to that shown and described above save for the fact that rotation about each of the marked axis will produce movement in different pairs of body portion depending upon which axis rotation is about. For example, rotation about axis X would produce secondary motion of portions 150 and 152, and, hence detection of this secondary motion Xs would provide an indication of motion and/or rate of turnabout axis X. Similarly, rotation about axis Y. will produce secondary oscillation Ys in portions 150 and 154 but not 152. Rotation about axis Z will produce secondary oscillations of all three portions as shown by arrow Zs. This specific arrangement therefore provides a practical example of the present invention when designed to operate and detect motion about non- orthogonal axis. Indeed, the axis marked on the drawings need not be the only axis being monitored - one might, for example, monitor rotation about orthogonal axes A, B shown in figure 19. In this arrangement, rotation about axis A would produce secondary oscillation in 150 and 154 but not 152. Rotation about axis B would produce secondary oscillation in portion 152 but not portion 150 or 154. Whilst these oscillations will not have maximum values and are therefore somewhat more difficult to detect and evaluate, they still offer a beneficial detection option. In view of the above, it will be appreciated that the present invention may employ one, two or more body-portions to facilitate detection and/or measurement of rotation about one, two or more orthogonal or non-orthogonal axis. In order to process the output of the motion detectors one would employ sensors 161 to 166 similar to those discussed with reference to figure 9 and some form of signal processing means shown schematically at 250 in figure 19. Such a device would be configured in a manner well known to those skilled in the art to process the signals from each detector and provide an output indicative of rotation about any one or more specific axis.

Claims: CLAIMS 1. A gyroscope comprising". a gyro body-portion, having primary and secondary modes of vibration; driving means, for driving said gyro body into said primary mode of vibration; detecting means, for detecting coriolis force induced secondary vibration upon motion of said body about a given axis; characterised in that said body-portion has two or more independent secondary modes of vibration, each being excitable as a result of motion of said body portion about separate associated axis and in that said detecting means includes means for detecting the vibration of said two or more secondary modes. 2. A gyroscope as claimed in Claim 1 and having two or more gyro-body portions, and in which the detector means includes means for detecting secondary λ'ibrations in one or more body-portion. 3. A gyroscope as claimed in Claim 2 and comprising a pair of coupled body-portions in which the driving means drives one body-portion and the detection means detects secondary vibrations in the other body-portion. 4. A gyroscope as claimed in Claim 2 and comprising a pair of coupled body-portions in which the detector means includes means for detecting secondary vibrations in both of said body-portions and further includes means for combining the output of said detectors in order to produce a signal indicative of rotation about the associated axis. 5. A gyroscope as claimed in Claim 4 and comprising two or more pairs of said body- portions, each pair being angularly displaced relative to each other pair, thereby to facilitate detection of rotation about different axis. 6. A gyroscope as claimed in Claim 4 or 5 in which the primary mode of vibration of each body-portion are in a common single plane. 7. A gyroscope as claimed in any one of Claims 4 to 6 in which the independent secondary modes of vibration of each body-portion are orthogonal to each other, thereby to facilitate detection of rotation about two orthogonal axes. 8. A gyroscope as claimed in any one of Claims 1 to 7 in which one or more of said one or more body-portions has a third secondary mode of vibration excitable as a result of motion about a third axis and the detecting means includes means for detecting the vibration in said third mode, thereby to detect motion about said third axis. 8a. A gyroscope as claimed in Claim 8 in which the independent third secondary mode of vibration is orthagonal to the other two secondary modes of vibration, thereby to facilitate detection of rotation about three orthogonal axis. 9. A gyroscope as claimed in any one of Claims 1 to 8 in which the driving means drives the body-portion at resonance. 10. A gyroscope as claimed in any one of Claims 1 to 9 in which the or each secondary vibration is at resonance. 1 1. A gyroscope as claimed in any one of Claims 1 to 10 in which the two or more secondary modes of vibration have the same natural frequency, multiple thereof or integer fraction thereof. 12. A gyroscope as claimed in any one of Claims 1 to 11 in which the detector means further includes means for determining the magnitude of the or each secondary mode of vibration, thereby to facilitate determination of the rate of turn about each of said associated axis. 13. A gyroscope as claimed in any one of Claims 1 to 12 and further including support means for supporting said body-portion whilst allowing said primary and secondary modes of vibration to occur, said support means also acting to locate the body-portion relative to a support boss which, in operation, is subjected to the motion being detected. 14. A gyroscope as claimed in Claim 13 and in which the gyroscope comprises two or more body-portions in which the two or more body-portions vibrate radially outward of the boss and connected thereto, each by said support means. 15. A gyroscope as claimed in Claim 14 in which the two or more body-portions are connected to each other by flexible beams which form part of said support means.

Inventors: COOPER GEORGE, COOPER, GEORGE
Applicants: BRITISH TECH GROUP, COOPER GEORGE, BRITISH TECHNOLOGY GROUP LIMITED, COOPER, GEORGE
Application year: 1997
Main classification: G01C19
IPC codes: G01C19, G01P9