GB2272056A - Solid-state resonator device - Google Patents

Abstract

The device comprises a piezoelectric layer 30 disposed between a resonator 21 and one or more electrodes 32, 34 and an insulating layer deposited either between the electrodes and the piezoelectric layer or between the piezoelectric layer and the resonator, the insulating layer enabling the piezoelectric layer to be made very thin without the substantial risk of short-circuits being formed between the electrodes and the resonator. The resonator is particularly suitable for use as a solid state gyroscope. <IMAGE>

Description

SOLID-STATE RESONATOR DEVICE The invention relates to a solid-state resonator device comprising a resonator and a piezoelectric element disposed thereon, in which electrical signals applied to the piezoelectric element cause the resonator device to vibrate.

In particular, but not exclusively, the resonator device forms part of a vibrational gyroscope.

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

A typical resonator bell shape is illustrated in Figure 1A 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 FIgure 1B.

The forces applied to the resonator bell cause the rim or rims to oscillate as shown in Figure 2 (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 2 that if drive transducers are mounted on diameter A, this will become an antinode, and a corresponding antinode will be along the diameter B perpendicular to diameter A. The output from sensor transducers mounted along diameter B provide feed back to the input signal for transducers on diameter A, and this feedback maintains the energising of the resonator bell in phase with the resonant frequency of the resonator bell.

From Figure 2 it can be seen that nodes exist along diameters C and D. However, if the resonator bell is rotated about axis X (see also Figures 1A and IB), 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 about axis X. Sensors mounted on axis C and D are used to provide a signal dependent on the angular velocity.

It is an aim of the present invention to provide a solid-state resonator device with improved sensitivity.

According to a first aspect of the invention there is provided a solid-state resonator device comprising a resonator having a conductive surface, an electrode and a piezoelectric element disposed between the conductive surface and the electrode such that said conductive surface, electrode and piezoelectric element define a transducer, wherein the transducer further comprises an insulating layer between the conductive surface and the electrode.

The advantage of forming the transducer in this manner is that the piezoelectric layer may be made very thin without short-circuits between the electrode and the resonator being caused by imperfections in the piezoelectric layer whereby at various points on the piezoelectric surface the piezoelectric layer may be thinner than normal.

The inventor has found that by employing the present invention in, for example, a gyroscopic device, the sensitivity of the device is enhanced. This is because it enables a thinner layer of piezoelectric material to be applied, reducing the mass of the layer and therefore reducing its damping effect on the resonator. The invention is particularly advantageous when the piezoelectric material is applied to the resonator by sputtering.

The presence of the insulator does not prevent operation of the transducer as it is dependant on the electric field applied across the piezoelectric material.

However, it is advisable to maximise the value of the electric field by making the insulating layer thin.

The insulating layer may be applied either between the resonator conductive surface and the piezoelectric layer or between the piezoelectric layer and the electrode.

Normally, however, it will be found more convenient to apply the insulating layer between the piezoelectric layer and the electrode, enabling the piezoelectric material to be attached directly to the resonator.

Advantageously, the insulating layer is a conformal coating, following the contours of the underlying surface and being of a substantially uniform thickness, giving rise to a uniform electric field across the piezoelectric material. Use of a conformal coating also enables the overall mass of the insulating layer to be minimised, again reducing the damping on the resonator.

The insulating layer is preferably applied by a process of vapour deposition and is advantageously composed of a polymer such as "Parylene" (a trade mark of the Union Carbide Corporation).

The resonator may include a cylindrical portion and a substantially planar base portion, remote from a rim, on which the transducer is disposed. The resonator may comprise a plurality of transducers, some of which may be sensors for picking up vibrations and converting them into electrical signals, and others drive transducers for setting the resonator in vibration in response to electrical drive signals.

In accordance with a second aspect of the invention there is provided a gyroscope comprising a resonator device in accordance with the invention, providing an output derived from signals generated by sensor transducers located on the resonating device, which signals are an indication of the speed of rotation of the resonator device about an axis.

The invention will now be described, by way of example only, with reference to the drawings, of which: Figure 1A illustrates a typical resonator; Figure 1B illustrates the deformation on resonance experienced by the resonator illustrated in Figure lA; Figure 2 is a plan view illustrating the deformations of the peripheral edge of the cylindrical portion of the resonator illustrated in Figure lA; Figure 3 is a partial cross-section of a transducer constructed in accordance with the invention, and Figure 4 shows an embodiment of a solid-state resonator used as part of a gyroscope in accordance with the present invention; Referring now to Figure 3, a metallic resonator 21, similar to the type shown in Figure 1A and inverted, comprises a cylindrical portion 22, a planar portion 23 and a boss (not shown) in the centre of the planar portion. The resonator 21 has a piezoelectric layer 30 deposited on it by a sputtering process. The piezoelectric layer 30 may be composed of any one of a number of suitable materials, among them zinc oxide, lithium niobate and aluminium nitride. On top of the piezoelectric layer 30 is vapour-deposited an insulating layer 38, composes of the insulating material Parylene and on top of that is deposited, also by sputtering, a gold electrode layer 37. The electrode layer 37 is applied through a mask which defines the electrode configuration of the resonator device, each electrode defining a discrete transducer such as those shown as 32 and 34 in Figure 3. A suitable material for the resonator would be steel or aluminium.

Each electrode 32 and associated portion of the insulating layer 38, piezoelectric layer 30 and resonator planar portion 23, constitutes a piezoelectric transducer, whereby signals may either be fed to the transducer to cause it to mechanically deform, or be taken from the transducer in response to the mechanical deformation of it. Such signals appear across the electrode 32 and the resonator surface 23, which, being metallic, is a conductive surface and is earthed.

The insulating layer 38 is conformally coated, so as to follow the contours of the piezoelectric layer 30, and serves to protect the transducer from operating failure due to short-circuits forming between the electrode layer 37 and the piezoelectric layer 30 at flaw sites situated in the piezoelectric layer. Such a flaw is shown as a crevice 36 in Figure 3 and it can be seen that the insulating layer 38 both prevents short-circuits and also allows a uniform electric field to be produced across the piezoelectric material.

The exact transducer configuration used depends on the particular application envisaged for the resonator device.

Figure 4 shows the resonator device configured for use as a vibrational gyroscope, in which the resonator bell 21 has disposed on its surface a piezoelectric layer (not shown), which covers most of the planar surface 23 of the bell except the central hub 20. An insulating layer (likewise not shown) covers the piezoelectric layer. Masked sputtering of an electrode layer on top of the insulating layer results in a number of discrete electrodes, and hence transducers, 1 to 10.Transducers la, Ib, 2a and 2b are used as drivers for setting the bell in vibration, transducers 3, 4, 5 and 6 are used as drive sensors for sensing the results of the vibrations of the drivers 1 and 2 and maintaining a feedback loop in the drive circuit so as to keep the bell oscillations in resonance, and finally transducers 7, 8, 9 and 10 are used as node sensors for sensing vibrations resulting from movement of the nodes due to rotation of the bell 21.

The bell 21 is conductive, being made of either steel or aluminium, Beryllium Copper or other such material having the appropriate mechanical properties and forms a common return path for all the signals going to and coming from the various transducer electrodes.

Circuitry similar to that shown in British patent specification GB 2164749, the contents of which are hereby incorporated by way of reference, can be employed to provide the drive signals to the driver transducers, couple the signals from the drive sensors to the drive circuit to provide the required resonance-maintaining feedback, and process the rotation-induced signals present at the rotation-sensing transducers. Such processing includes display or recording of the speed of rotation sensed.

As an alternative to incorporating the insulating layer between the electrode layer and the piezoelectric layer, it is possible to apply the insulating layer to the planar surface 22 of the bell before the piezoelectric layer is applied, the insulating layer then being disposed between the resonator and the piezoelectric layer. The effect of this positioning of the insulating layer is the same as that of the preferred positioning between the electrode and the piezoelectric layer.

Although the insulation coating has been described above as being conformal, it may also be non-conformal without seriously impairing the performance of the resonator device. Further, deposition of both the piezoelectric and electrode layers may be by means other than sputtering.

Claims (13)

1. A solid-state resonator device comprising a resonator having a conductive surface, an electrode, and a piezoelectric element disposed between the conductive surface and the electrode such that said conductive surface, electrode and piezoelectric element define a transducer, wherein the transducer further comprises an insulating layer between the conductive surface and the electrode.

2. A solid-state resonator device as claimed in claim 1, in which the insulating layer is disposed between the electrode and the piezoelectric element.

3. A solid-state resonator device as claimed in claim 1, in which the insulating layer is disposed between the resonator and the piezoelectric element.

4. A solid-state resonator device as claimed in any preceding claim, in which the insulating layer is a conformal coating.

5. A solid-state resonator device as claimed in any preceding claim, in which the insulating layer is vapour-deposited on the piezoelectric element.

6. A solid-state resonator device as claimed in any preceding claim, in which the insulating layer is composed of Parylene.

7. A solid-state resonator device as claimed in any preceding claim, in which the piezoelectric element is applied to the resonator by sputtering.

8. A solid-state resonator device as claimed in any preceding claim, in which the resonator includes a cylindrical portion and a substantially planar base portion on which the transducer is disposed.

9. A solid-state resonator device as claimed in any preceding claim, in which the resonator has a planar base portion on which is deposited a layer of piezoelectric material, a layer of insulating material and a plurality of electrodes defining a corresponding plurality of transducer elements.

10. A solid-state resonator device as claimed in claim 9, in which some of the transducer elements are sensor elements and some are driver elements for causing the resonator to vibrate.

11. A solid-state resonator device substantially as shown in, or as hereinbefore described, with reference to Figure 3 of the drawings.

12. A gyroscope comprising a resonator device as claimed in any preceding claim.

13. A gyroscope substantially as shown in, or as hereinbefore described, with reference to Figure 4 of the drawings.