GB2272053A

GB2272053A - A solid state vibrational gyroscope

Info

Publication number: GB2272053A
Application number: GB9222979A
Authority: GB
Inventors: David Stuart Ansell
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1992-11-03
Filing date: 1992-11-03
Publication date: 1994-05-04
Anticipated expiration: 2012-11-03
Also published as: GB2272053B

Abstract

A solid state vibrational gyroscope comprises a resonator bell 21 having piezoelectric drive transducers 1a and 1b aligned with antinodes of a natural mode of vibration of the resonator bell. This is achieved by either constructing the resonator bell such that it is deliberately out of balance so that the antinodes are aligned with the transducers 1a, 1b, or alternatively by balancing the resonator bell such that the antinodes of the natural mode of vibrations always corresponds to the position of the drive transducer regardless of its location. The invention further provides for balancing the fully assembled resonator bell using the drivers 1a, 1b and sensors 2a, and 2b in the balancing process. <IMAGE>

Description

A SOLID STATE VIBRATIONAL GYROSCOPE This invention relates to a solid state vibrational gyroscope comprising a resonator in the form of a resonator bell, and in particular but not exclusively to such gyroscopes wherein the inputs and outputs from the resonator bell are provided by piezoelectric transducers upon a base portion of the resonator bell.

A vibrational gyroscope of the above type is disclosed in UK Patent No. 2164749. The gyroscope comprises a resonator bell, which term for the purposes of this specification, including the claims, means a member having a circular rim susceptible to resonance. If the resonator bell is substantially U or H shaped in a cross section along an axis about which rotation is to be sensed, then it can conveniently be supported by a central mounting point on the axis. However such resonator bells can also comprise cylinders supported by an outer planar flange located at some point along the cylinder.

A selection of possible resonator bell shapes is illustrated in Figures 1A, 2A, 3A and 4A of the accompanying drawings. Pairs of piezoelectric transducers are conventionally attached to opposite sides of the resonator bell such that they cause the resonator bell to resonate as illustrated in Figures 1B, 2B, 3B and 4B respectively. The forces applied to the resonator bell in each instance cause the rim or rims, to oscillate as shown in the plan view of Figure 5, (in the same manner in which a plastic beaker does when diametrically opposite edges of its upper lip are squeezed between thumb and forefinger). It can be seen from Figure 5 that if transducers are mounted on diameter A, then this will become an antinode, and a corresponding antinode will be along the diameter B perpendicular to diameter A.

Drive sense transducers are mounted along diameter B on the resonator bell, the output of which is fed back to the input signal for drive transducers on diameter A. This feedback loop maintains the energising of the resonator bell in phase with the resonant frequency of the resonator bell.

From Figure 5 it can be seen that nodes exist along diameters C and D. However if the resonator bell rotates about axis X then Coriolis forces rotate the antinodes around the axis X such that the amplitude of vibration along diameters C and D is proportional to the angular velocity of the gyroscope about the axis X. Rate sensor transducers mounted on axes C and D are used to provide a signal dependant on the angular velocity.

The principle behind solid state vibrational gyroscopes of the type described above is relatively straightforward. However in practice it is found that even when the resonator bells are manufactured as uniformly as present production techniques allow, the performance of each resonator bell varies greatly, and a large proportion do not have satisfactory characteristics.

According to a first aspect of the present invention there is provided a solid state vibrational gyroscope comprises a resonator bell, a drive transducer for causing the resonator bell to vibrate, and a sensor transducer for detecting vibration of the resonator bell, wherein the drive transducer is positioned at an antinode of a natural mode of vibration of the resonator bell.

A resonator bell only functions as described with reference to Figure 5 in an ideal case where it has perfect symmetry about the axis about which rotation is to be sensed. However, resonator bells which can be produced by present techniques have small imperfections in geometry, mass distribution and stiffness distribution which result in the resonator bell having a preferred, or natural, mode of vibration, with antinodal points in fixed positions, which may not necessarily correspond to the position at which the resonator bell is driven. There are two such natural modes which are always at 450 to each other. These are termed the principal modes of vibration, and they are the only two-diameter modes in which the resonator bell will naturally vibrate. They are independent and can both exist simultaneously with any amplitude and phase.However they have fractionally different frequencies.

If the resonator bell is excited at a point other than an antinode of one of the principal modes, then both principal modes are excited, and the interaction of the two modes determines the position of the nodes. If the principal modes are 900 out of phase the vibration takes the form of a rotating wave so the nodal lines continuously rotate in the body so that there are no apparent nodes.

By employing the present invention the drive transducer is aligned with an antinode of a natural mode of vibration of the resonator bell and therefore only this mode is excited, resulting in nodes occurring at 450 to the antinodal points. Only when the gyroscope is rotated will energy be coupled by Coriolis forces to the second natural mode.

In one embodiment of the invention the gyroscope is manufactured such that the resonator bell has a deliberate imbalance such that the antinode of a natural mode of vibration is at a predetermined position. Having a known imbalance at a set position means that it is known where the antinodal point of that natural mode will be and the drive and sensor transducers can be mounted accordingly.

In an alternative embodiment the gyroscope is manufactured such that the resonator bell is balanced and has an antinode of a natural mode of vibration dependant only on the position of the drive transducer. In this case the preferred natural mode is governed by the position at which it is driven, and not by an inherent assymetry. When the resonator bell is balanced both of the natural modes of vibration have the same frequency and the resonator bell has perfect symmetry. Wherever such a resonator bell is driven will be an antinode of natural mode of vibration. The advantage of such a resonator bell is that it is particularly sensitive as there is no problem of rotation of the resonator bell causing energy to be coupled to modes of different frequencies.

The balance of the resonator bell may be altered during manufacture of the gyroscope by altering the stiffness of the resonator bell at one or more positions on the resonator bell. This can be achieved by adding or removing material from high stressed areas. Alternatively the mass of the resonator bell at one or more positions on the resonant bell can be altered.

Preferably the stiffness or mass is altered at four positions, one on each of four radii spaced equiangularly about the axis about which vibration is to be sensed. This minimises the amount of material that needs to be added or subtracted from the resonator bell.

The resonator bell may conveniently be balanced by laser trimming. Alternatively the resonator bell could be balanced by adding mass, and this may conveniently be achieved by adding self-adhesive material, for example small spots of ink fired from a jet.

Preferably the drive and sensor transducers comprise piezoelectric elements which may be mounted in contact with a surface of the resonator bell. Preferably piezoelectric material is deposited over a base portion of the resonator bell remote from the rim, and a plurality of drive and sensor transducers are defined by the area of respective electrodes in contact with the piezoelectric material. This enables the piezoelectric material to be very thin.

Transverse extension or contraction of the piezoelectric material causing the base portion to flex, similar to the way in which a bimetallic strip flexes when one side expands or contracts more than the other. Reducing the thickness of the piezoelectric material reduces the damping effect on the resonator bell. Advantageously the piezoelectric material is deposited by sputtering.

Preferably the balance of the resonator bell, incorporating drive and sensor transducers is altered with the resonator bell on its mounting.

A solid state vibrational gyroscope in accordance with the invention can advantageously comprise drive circuitry, output circuitry and the resonator bell all on a printed circuit board. Such an assembly can easily be incorporated into other circuits.

Preferably a gyroscope in accordance with the invention has appropriate signals applied to the drive circuitry during manufacture to drive the resonator bell, enabling the resonator bell to be balanced in dependence upon the output from output circuitry.

According to a second aspect of the invention there is provided a method of manufacturing a solid state gyroscope comprising aligning a drive transducer with an antinode of a natural mode of vibration of a resonator bell of the gyroscope.

One embodiment of the invention will now be described by way of example only with reference to the accompanying drawings of which: Figures 1A, 2A, 3A and 4A illustrate various resonator bell configurations suitable for use with the present invention; Figures 1B, 2B, 3B and 4B illustrate the deformation on resonance experienced by the resonator bells illustrated in Figures 1A to 4A respectively; Figure 5 is a plan view illustrating the deformations of the peripheral edge of any one of the cylindrical portions of the resonator bells illustrated in Figures 1A to 4A respectively; Figure 6 illustrates a gyroscope in accordance with the invention wherein the resonator bell is mounted on a printed circuit board; Figure 7 is a schematic circuit diagram for a gyroscope in accordance with the invention; and Figure 8 is a graph illustrating the effect of adding mass to the resonator bell.

Referring to Figure 6 there is illustrated a solid state vibrational gyroscope in accordance with the present invention. This comprises a metallic resonator bell 21 which is mounted via dielectric support 22 on printed circuit board 23. On the lower surface of the resonator bell 21 there is a layer of piezoelectric material 24 which has been deposited by sputtering. Electrodes 25 are deposited on this layer, which electrodes define transducers. Electrodes 25 are connected via gold wire 26 to the printed circuit board 23 which comprises the electronic circuitry necessary for driving the resonator bell 21. The resonator bell 21 is balanced by the addition of masses 26 by the method described below with reference to Figures 8 and 9.

Referring now to Figure 7, there is illustrated the resonator bell 21 which is shown inverted. The resonator bell 21 has nodal lines 30 and 31 on which masses 26 and 33 are deposited These are nodal lines of a natural mode of vibration of the resonator bell with the electrodes, piezoelectric material and masses added thereto.

Figure 7 schematically illustrates the electrical circuitry that is connected to the electrodes of the resonator bell, which in turn define the piezoelectric transducers. The terminals of this circuitry have been labelled with the reference numbers of the electrodes on the resonator bell to which they respectively contact. Drive electrodes 1A, 1B and 2A and 2B are located on the antinodes of the natural mode. These electrodes and piezoelectric material beneath them define transducer elements which are energised respectively by amplifier 34 and inverse amplifier 35.

The expansion and contraction of the piezoelectric material beneath the electrodes causes the resonator bell to deform as illustrated in Figures 2B and 5. This is due to lateral expansion contraction of the piezoelectric material causing the base to which it is attached to bend in the same way that a bi-metallic strip bends when heated.

Electrodes 7, 8, 9 and lO define the piezoelectric sensors which detect the output. Signals from the sensors are fed to differential amplifier 36, the output of which is fed to phase locked loop 37 which is amplitude stabilised and causes the phase locked loop to drive the amplifiers 34 and 35 at the resonant frequency of the resonator bell. The phase locked loop is preset to the approximate resonant frequency of the resonator bell, and the loop defined by amplifiers 34, 35, resonator bell 21, differential amplifier 36, and phase locked loop 37, maintains resonance of the resonator bell at a fixed amplitude.

Differential amplifier 38 receives an input from electrodes 3 and 4, which again define sensors. These electrodes lie on nodes 30 and 33, and in the absence of rotation of the gyroscope each provides a zero output. This is because each sensing element defined by the electrode has an equal area to either side of the nodal line, and therefore any voltage induced in one half will be cancelled by the opposite voltage being induced in the other half.

On rotation, the nodal lines of figure 7 shift such that the voltage generated in one half of each sensor 3 and 4 will be greater than that generated in the other half, due to the area of the sensor to one side of the nodal line being greater than that to the other. The signals from electrodes 3 and 4 are in opposite sense so the differential amplifier 38 provides an output at the resonant frequency of the resonator bell when the resonator bell is rotated. The amplitude of the output is dependent on the rate of rotation.

Part of the output from differential amplifier 38 is applied to a phase shifter 39 which introduces a 900 phase shift, and is used to drive amplifier 40 and inverse amplifier 41. These are connected to electrodes 5 and 6, which causes transducers defined by the piezoelectric material below electrodes 5 and 6 to damp the vibration at the natural nodes of the resonator bell.

The output of differential amplifier 38 also passes through band pass filter 42, which is tuned to the resonant frequency of the resonator bell to prevent spurious signals arising from other vibrations induced into the resonator bell from influencing the output signal of the gyroscope The signal is then fed to phase sensitive detector 43 which also receives a signal from the drive loop, comprising differential amplifier 36, phase lock loop 37, amplifiers 34 and 35, and the resonator bell 21. The phase sensitive detector receives two in-phase sine wave signals plus any other signals due to spurious noise. The two sine wave signals combine to give a full wave rectified output which passes through low pass filter 44 to output 45. The amplitude of this output is dependent upon the rate of rotation of the gyroscope.Any spurious signals received by the phase sensitive detector have a higher frequency component and will be rejected by low pass filter 44.

Figure 8 graphically illustrates the effect of adding mass to the resonator bell at the nodal points of a first natural mode of vibration, which are the antinodal points of a second mode of vibration. Adding mass at the antinodal points of the higher frequency mode reduces the frequency of that mode until the frequencies of the two natural modes are the same, at which point the resonator bell is perfectly balanced. It will be appreciated that the same effect can be achieved by removing mass at the antinodal points of the low frequency mode such that the frequency of that mode increases to that of the other mode. Once the resonator bell is perfectly balanced, the position of the electrodes is immaterial relative to the position of the resonator bell for the resonator bell will have a natural mode dependent only upon the point at which it is driven.

Figure 9 illustrates apparatus for producing a resonator bell in accordance with the present invention.

The resonator bell 50 is mounted via support 51 to a printed circuit board 52 on which the circuitry 53, 54 of Figure 7 is mounted. This circuitry is connected to electrodes deposited on the base portion of the resonator bell 50 and is also connected via ribbon cable 55, to controller 56.

The controller 56 controls four lasers 57, and motor 58, which drives a rotating table 59 on which the gyroscope is mounted.

Controller 56 applies an appropriate signal to the printed circuit board 52 such that the circuitry on the board energises the resonator bell. The output from the printed circuit board is then monitored by the controller 56 which both rotates the turntable 59 and energises the lasers 57 such that mass is laser trimmed off the resonator bell such as to bring the antinodes of a natural mode of vibration into alignment with the drive electrodes of the resonator bell. This can either be achieved by perfectly balancing the bell so that the natural antinodes of vibration occur where the resonator bell is driven, or alternatively by producing a deliberate imbalance such that the antinodes of a natural mode of vibration occur at the drive transducers.

Although Figure 9 shows an arrangement where the resonator bell is laser trimmed, the lasers could be replaced by various means for either depositing or removing mass from selected points, one example being inkjets.

Claims

1. A solid state vibrational gyroscope comprising a resonator bell, a drive transducer for causing the resonator bell to vibrate, and a sensor transducer for detecting vibration of the resonator bell, wherein the drive transducer is positioned at an antinode of a natural mode of vibration of the resonator bell.

2. A gyroscope as claimed in claim 1 manufactured such that the resonator bell has a deliberate imbalance whereby the antinode of a natural mode of vibration is at a pre-determined position.

3. A gyroscope as claimed in claim 1 wherein the gyroscope is manufactured such that the resonator bell is balanced and has an antinode of a natural mode of vibration dependent only on the position of the drive transducer.

4. A gyroscope as claimed in any preceding claim wherein the balance of the resonator bell is altered during manufacture of the gyroscope by altering the stiffness of the resonator bell at one or more positions on the resonator bell.

5. A gyroscope as claimed in claim 4 wherein the stiffness is altered on four radii spaced equiangularly about the axis about which rotation is to be sensed.

6. A gyroscope as claimed in any preceding claim wherein the balance of the resonator bell is altered during manufacture of the gyroscope by altering the mass of the resonator bell at one or more positions on the resonator bell.

7. A gyroscope as claimed in claim 6 wherein the mass is altered on four radii spaced equiangularly about the axis about which rotation is to be sensed.

8. A gyroscope as claimed in any preceding claim, wherein the balance of the resonator bell is altered by laser trimming.

9. A gyroscope as claimed in claim 6 or 7 wherein mass is added to the resonator bell.

10. A gyroscope as claimed in any preceding claim wherein the drive and sensor transducers comprise piezoelectric material.

11. A gyroscope as claimed in claim 10 wherein the piezoelectric material is deposited over a base portion of the resonator bell remote from its rim, or rims, and a plurality of drive and sensor transducers are defined in the piezoelectric material by the area of respective electrodes in contact with the piezoelectric material.

12. A gyroscope as claimed in claim 10 or 11, wherein the piezoelectric material is deposited by sputtering.

13. A gyroscope as claimed in any preceding claim wherein the balance of the resonator bell, incorporating drive and sensor transducers, is altered with the resonator bell assembled on its mounting.

14. A gyroscope as claimed in any preceding claim in which the drive circuitry, output circuitry, and the resonator bell are on a printed circuit board.

15. A gyroscope as claimed in any preceding claim wherein during manufacture appropriate signals are applied to drive circuitry to cause the bell to resonate, and wherein the balance of the resonator bell is altered in dependence on the output from the sensor transducer.

16. A gyroscope substantially as illustrated in, or described with reference to, Figures 6, 7, 8 or 9 of accompanying drawings.

17. A method of manufacturing a solid state gyroscope comprising aligning a drive transducer with an antinode of a natural mode of vibration of a resonator bell of the gyroscope.

18. A method as claimed in claim 17 comprising determining the position of the antinode of a natural mode of vibration of the gyroscope by manufacturing the gyroscope such that the resonator bell has a deliberate imbalance.

19. A method as claimed in claim 17 comprising manufacturing the gyroscope such that the resonator bell is balanced.

20. A method as claimed in any one of claims 17 to 19, comprising changing the balance of the resonator bell by altering the stiffness of the resonator bell at one or more positions on the resonator bell.

21. A method as claimed in claim 20 comprising altering the stiffness of the resonator bell on four radii spaced equiangularly about the axis about which rotation is to be sensed.

22. A method as claimed in any one of claims 17 to 19 comprising changing the balance of the resonator bell by altering the mass of at least one or more positions on the resonator bell.

23. A method as claimed in claim 22 comprising altering the mass on four radii equiangularly spaced about the axis about which rotation is to be sensed.

24. A method as claimed in any one of claims 17 to 23 comprising altering the mass of the resonator bell by laser trimming.

25. A method as claimed in any one of claims 17 to 23 comprising depositing mass on the resonator bell to alter the balance of the resonator bell.

26. A method as claimed in any one of claims 17 to 26 comprising forming piezoelectric drive and sensor transducers on the resonator bell.

27. A method as claimed in claim 26 comprising depositing a layer of piezoelectric material over a base portion of the resonator bell remote from its rim, or rims, and defining drive and sensor transducers by depositing respective electrodes in contact with the piezoelectric material.

28. A method as claimed in claim 26 or claim 27 wherein piezoelectric material is deposited on the surface of the resonator bell by sputtering.

29. A method as claimed in any one of claims 17 to 28 comprising altering the balance of the resonator bell after the drive and sensor transducers are located thereon.

30. A method as claimed in any one of claims 17 to 29 wherein the drive circuitry for the drive transducer, output circuitry, and the resonator bell are mounted on a printed circuit board.

31. A method as claimed in any one of claims 17 to 30 comprising applying appropriate signals to drive circuitry for the resonator bell and altering the balance of the resonator bell in dependence upon the output from sensor transducers on the resonator bell.

32. A method substantially as hereinbefore described with reference to, or as illustrated in, Figures 6,7,8 or 9 of the accompanying drawings.