GB2118773A

GB2118773A - Thickness shear mode piezoelectric devices

Info

Publication number: GB2118773A
Application number: GB08306737A
Authority: GB
Inventors: John Birch; Peter Andrew Murkin; John Francis Werner
Original assignee: General Electric Company PLC
Current assignee: General Electric Company PLC
Priority date: 1982-03-24
Filing date: 1983-03-11
Publication date: 1983-11-02
Also published as: GB8306737D0; GB2118773B

Abstract

A thickness shear mode piezoelectric resonance device comprising a plate (7) of piezoelectric material having support means (9) around its edge and at least one pair of electrodes (13) at corresponding positions on the main faces of the plate and located outwardly from the centre region of an inflection of the curvature of the plate occurring due to externally applied forces, so that the effect of the forces on the resonance frequency of the device is minimised. <IMAGE>

Description

SPECIFICATION Thickness shear mode piezoelectric devices This invention relates to thickness shear mode piezoelectric devices.

Such devices, which find especial application as resonators in electric oscillators and filters, comprise a plate of piezoelectric material, e.g.

quartz, with an electrode on a corresponding region of each main face.

It has been recognized for many years that externally applied forces arising from acceleration will cause deformation of the piezoelectric plate of such a device introducing elastic non-linearities with consequent deviation of the resonance frequency of the device from its desired value.

To reduce such resonance frequency variation it has been proposed to support the piezoelectric plate of such a device around its periphery, thus reducing deformation of the plate, and hence the deviation of the device resonance frequency, due to externally applied forces normal to the plate.

It is an object of the present invention to provide a supported plate thickness shear mode piezoelectric device whose resonance frequency stability under externally applied forces is further improved.

According to the present invention in a thickness shear mode piezoelectric device comprising a plate of piezoelectric material with an electrode on a corresponding region of each main face and provided with support means surrounding the electrodes which serves to inhibit movement of the plate at the location of the support means under externally applied forces, the electrodes are located in the region of an inflection of the curvature of the plate in a direction outwardly from the centre of the plate occurring due to externally applied forces.

Preferably the electrodes are also located at a position with respect to the crystal axes of the plate such that the curvature of the plate normal to said direction occurring due to externally applied forces gives rise to a minimum deviation of the resonance frequency of the device.

The invention will now be further explained, and three devices in accordance with the invention will be described, with reference to the accompanying drawings in which: Figures 1 and 2 are plan and sectional views of a first known form of thickness shear mode piezoelectric device; Figures 3 and 4 are plan and sectional views of a second known form of thickness shear mode piezoelectric device; Figures 5 and 6 are plan and sectional views of a first device in accordance with the invention; and Figures 7 and 8 are plan views of a second and third device in accordance with the invention.

Referring to Figure ia a typical conventionai thickness shear mode piezoelectric device comprises a circular plate of quartz 1 provided centrally on each main face with a circular electrode 3 in the form of a layer of metal, e.g.

gold, from which a lead 5 extends radially, the electrodes being of identical area and in register and the leads extending in diametrically opposite directions.

The electrodes and the part of the plate sandwiched between them consistute a resonant structure whose resonance frequency is dependent on the geometry of the plate and electrodes. The device finds application in electrical circuits since to an electrical signal applied between its electrodes it appears to be a resonant circuit of extremely high Q and stable resonance frequency.

When a device as shown in Figure 1 is subjected to a force arising from acceleration in a direction normal to the plane of the plate 1, the plate 1 deforms as shown, exaggeratedly, in the sectional view of Figure 2, with consequent deviation of the resonance frequency of the device from its desired value.

Referring now to Figure 3, it has been proposed to reduce such deviation utilising a plate 7 supported around it periphery by means of a ring 9 of rigid material bonded to a main face of the plate. Due to the presence of the ring the plate when subjected to a force takes up a double curvature shape, as illustrated in Figure 4, rather than a single curvature shape, as illustrated in Figure 2, with consequent reduced deformation at the centre of the plate where the electrodes 11 are located.

In accordance with the present invention, deviation of resonance frequency is further reduced by positioning the device electrodes in the region of an inflection of the curvature of a ring supported plate, i.e. in a region where the curvature in a radial direction changes from one sense to the dther.

In one such a device shown in Figure 5, a pair of electrodes 1 3 are positioned between the centre of the plate 1 and the inner edge of the supporting ring 9.

Figure 6 shows an enlarged sectional view of part of the device of Figure 5 under externally applied force, the arrows indicating strain in the main faces of the plate. It will be seen that the part of the plate where the electrodes are positioned is virtually undeformed and strain free.

According to one analysis, for a ring-supported circular plate of radius a the curvature in a radial direction ut any radius r is inversely proportional to (1-3r2/a2) and in a tangential direction is inversely proportional to (1 -r2/a2). Thus on the basis of this analysis the electrodes in the device of Figure 5 will be positioned at a radial distance of a/ from the centre of the plate.

An alternative analysis indicates that the required radial distance a/2.

The electrodes are also positioned with respect to the crystal axes of the plate so as to reduce the effect of tangential curvature of the plate on the device resonance frequency to a minimum. In the case of an AT cut plate, the required position is where the tangential curvature is along an axis at 450 to the X and Z' crystal axes, as indicated in Figure 5.

Since the annular region of the plate where radial curvature is negligible is small, it will normally be preferable to utilise two or more resonator structures positioned at different positions around the annular region and electrically connected in parallel.

Figure 7 shows one such arrangement having two resonator structures 1 5 defined by the overlapping portions of metal layers 17, and Figure 8 shows another such arrangement having four resonator structures 1 9 defined by the overlapping portions of metal layers 21. The resonator structures 1 5 or 1 9 will normally be adjusted to the same resonance frequency to produce a single resonant response.

Claims

1. A thickness shear mode piezoelectric device comprising a plate of piezoelectric material with an electrode on a corresponding region of each main face and provided with support means surrounding the electrodes which serves to inhibit movement of the plate at the location of the support means under externally applied forces wherein the electrodes are located in the region of an inflection of the curvature of the plate in a direction outwardly from the centre of the plate occurring due to externally applied forces.

2. A device according to Claim 1 wherein the electrodes are also located at a position with respect to the crystal axes of the plate such that the curvature of the plate normal to said direction occurring due to externally applied forces gives rise to a minimum deviation of the resonance frequency of the device.

3. A device according to Claim 2 wherein the plate is an AT cut plate and the electrodes are located at a position where the curvature of the plate normal to said direction is along an axis at 450 to the X and Z' crystal axes.

4. A device according to any one of the preceding Claims wherein the plate is curcular and said electrodes are located substantially at a radial distance from the centre of the plate of 1/ of the radius of the plate.

5. A device according to any one of Claims 1 to 3 wherein the plate is circular and said electrodes are located substantially at a radial distance from the centre of the plate of half the radius of the plate.

6. A device according to any one of the preceding claims having two or more said electrodes on each main face.

7. A thickness shear mode piezoelectric device as hereinbefore described with reference to Figure 5, Figure 7 or Figure 8 of the accompanying drawings.