GB2235533A

GB2235533A - Piezoelectric sensor device

Info

Publication number: GB2235533A
Application number: GB8918387A
Authority: GB
Inventors: Roger James Williamson; Peter William Graves
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1989-08-11
Filing date: 1989-08-11
Publication date: 1991-03-06
Anticipated expiration: 2009-08-11
Also published as: GB8918387D0; GB2235533B

Abstract

A piezoelectric sensor, e.g. for pressure acceleration or displacement measurement, is formed by plasma etching of a quartz crystal blank. The sensor comprises a flexible diaphragm (11) supported around its periphery by an integral rigid annulus (12). Conductive electrodes (13) applied to one surface of the diaphragm define a surface acoustic wave device whose characteristic frequency is determined by displacement of the diaphragm. Use of the sensor in a radio microphone arrangement, e.g. for a cordless telephone, and as an accelerometer, in an inertial guidance system, are mentioned. <IMAGE>

Description

PIEZOELECTRIC SENSOR DEVICE This invention relates to piezoelectric sensors or transducers e.g. for the measurement of pressure, acceleration or displacement. The invention further relates to a radio microphone incorporating a piezoelectric sensor.

A number of piezoelectric sensors for the measurement of e.g. acceleration have been described.

These devices incorporate a surface acoustic wave device formed on the surface of a body of piezoelectric material, such as quartz, and have a characteristic frequency determined by the properties of the piezoelectric material and the spacing between the electrodes. Such a device is described for example in our published specifications No. 2 143 326 and 2 144 546. These devices comprise a relatively thick cantilever or beam of piezoelectric material supported at one end. Displacement of the cantilever e.g. in response to an accelerating force causing bending, alters the dimensions of the surface acoustic wave device and introduces strain to the piezoelectric material thus producing a corresponding change in its characteristic frequency. This frequency change provides a measure of the displacement.

A disadvantage of conventional devices is that they are formed from a relatively thick piezoelectric blank. Because of the high tensile strength of the commonly used piezoelectric material, i.e. quartz, the magnitude of the displacement is very small and the sensitivity or response of the device is therefore limited.

The object of the present invention is to minimize or to overcome this disadvantage.

According to one aspect of the invention there is provided a piezoelectric sensor device, including an integral piezoelectric structure comprising a flexible diaphragm supported round its periphery by a rigid annulus, and conductive electrodes so disposed on one surface of the diaphragm to define a surface acoustic wave device, the characteristic frequency of the surface acoustic wave device being dependent upon the degree of flexure of the diaphragm.

According to another aspect of the invention there is provided a radio microphone, including a surface acoustic wave piezoelectric transducer for converting audio signals to corresponding electrical signals, wherein said transducer further provides a radio frequency detecting element for said microphone.

According to another aspect of the invention, there is provided a radio microphone, including a surface acoustic wave piezoelectric transducer, and drive/feedback circuit for maintaining the device in an oscillating state at its characteristic frequency, wherein the transducer comprises an integral piezoelectric structure of a flexural diaphragm responsive to sound waves and supported around its periphery by a rigid annulus, there being conductive electrodes so disposed on one surface of the diaphragm as to define a surface acoustic wave device, and wherein the arrangement is such that flexible displacement of the diaphragm responsive to sound waves incident thereon causes a corresponding frequency modulation of the oscillator formed by the surface acoustic wave device and the drive circuit.

When used in a radio microphone arrangement the sensor device performs the dual function of converting sound waves to electrical signals and of providing a radio frequency determining element, the radio frequency being the characteristic frequency of the sensor. This allows the provision of a single circuit with very few components. This is of particular advantage in the construction of a cordless telephone where cost and physical size are primary considerations.

In a further application a loading means may be provided on the sensor diaphragm to permit use as a sensitive accelerometer.

Embodiments of the invention will now be described with reference to the accompanying drawings in which:- Fig. 1 is a sectional view of a piezoelectric surface acoustic wave sensor; Fig. 2 is a plan view of the sensor of Fig. 1; Fig. 3 shows in schematic form a radio microphone construction employing the sensor of Figs. 1 and 2; and Fig. 4 shows an alternative radio microphone arrangement.

Referring to Figs. 1 and 2, the sensor is formed as an integral structure from a body of piezoelectric material, e.g. quartz crystal, and comprises a flexible diaphragm 11 supported around its periphery by a rigid annular frame member 12. The structure is of generally circular symmetry and may be formed by selective plasma etching from a quartz crystal blank. Typically the crystal blank is lapped and polished to provide smooth parallel major surfaces and is then plasma etched via a mask with a radio frequency plasma of a fluorinated material such as CF4, C2F6, C3F8, SF6 or mixtures thereof.

Generally the blank from which the structure is etched is 70 to 90 microns in thickness, and the diaphragm is etched to a thickness of 10 to 20 microns. The diaphragm diameter may be 2 to 4 mm.

One surface of the diaphragm 11 supports an array of conductive electrodes 13 defining a surface acoustic wave (SAW) device whose characteristic frequency is determined by the interelectrode spacing and by the physical properties, including the particular crystal cut, of the piezoelectric material.

Advantageously the structure is mounted on a similarly etched piezoelectric blank 14 so that the SAW device is disposed in a sealed cavity 15 and is thus protected from possible contamination or damage. In some applications the cavity 15 may be evacuated.

Displacement of the diaphragm, e.g. in response to an applied pressure, induces a corresponding strain in the piezoelectric material thus causing a corresponding change in the characteristic frequency of the SAW device. The magnitude of this frequency change corresponds to the displacement of the diaphragm. As the diaphragm 11 is very thin, it is flexible and thus has a high sensitivity to pressure changes. Also, the very small intertia of the diaphragm provides a good high frequency response that permits use of the sensor e.g. as a microphone for detecting acoustic signals.

In some applications the surface of the diaphragm opposite the SAW device may be provided with a coating whereby to modify the vibrational modes of the device e.g. to improve or optimise its bandwidth.

In a further embodiment, a loading mass (not shown) may be provided on the diaphragm. Typically this loading means is integral with the diaphragm and is defined by a suitable mask prior to plasma etching of the diaphragm. Such an arrangement may be used as a sensitive accelerator e.g. in an inertial guidance system.

Fig. 3 illustrates an application of the sensor of Figs. 1 and 2 in the construction of a radio microphone e.g. for use in a cordless telephone. In this arrangement the SAW device of the sensor 31 is coupled to a drive/feedback amplifier 32 to form an oscillator operating at a radio frequency. Displacement of the sensor diaphragm responsive to sound waves incident thereon causes corresponding changes in the frequency of the SAW device. This produces a frequency modulated signal which is transmitted to a remote station (not shown) via antenna 33. ~ It will be appreciated that, when employed in a radio microphone arrangement, the piezoelectric sensor performs the dual function of converting incident sound waves to corresponding electric signals, and of providing a radio frequency determining element for the microphone.

An alternative radio microphone construction is illustrated in Fig. 4. This arrangement functions as a passive transmitter that is activated by an applied radio signal thereby re-radiating a modulated signal.

In the construction of Fig. 4, the sensor 41 is coupled to a resonant circuit 42, e.g. a cavity resonator, tuned to the characteristic frequency of the SAW device in the rest position of the diaphragm. In this condition a radio signal at the characteristic frequency transmitted to the radio microphone via antennae 43 causes resonance of the SAW device and resonant circuit. Displacement of the diaphragm in response to an incident sound wave causes corresponding changes in the SAW device frequency. These frequency changes cause corresponding amplitude changes as the SAW device frequency moves away from and towards the resonant circuit frequency. This produces an amplitude modulation of the signal that is re-radiated from the antenna 43 to a remote station (not shown).

Whilst the sensor device has been described above with particular reference to its use as a radio microphone e.g. for a telephone instrument it will be appreciated that it is in no way limited to that particular application.

Claims

Claims.

1. A piezoelectric sensor device, including an integral piezoelectric structure comprising a flexible diaphragm supported round its periphery by a rigid annulus, and conductive electrodes so disposed on one surface of the diaphragm to define a surface acoustic wave device, the characteristic frequency of the surface acoustic wave device being dependent upon the degree of flexure of the diaphragm.

2. A sensor device as claimed in claim 1 comprising an integral structure formed by plasma etching from a quartz crystal blank.

3. A sensor device as claimed in claim 1 or 2, wherein said diaphragm is 10 to 20 microns in thickness and 2 to 4 mm in diameter.

4. A sensor device as claimed in claim 1, 2 or 3, wherein said diaphragm is of non-uniform thickness.

5. A sensor device as claimed in any one of claims 1 to 4, wherein said diaphragm is provided with a coating whereby to modify its vibrational modes.

6. A sensor device substantially as described herein with reference to and as shown in Figs. 1 and 2 of the accompanying drawings.

7. A radio microphone incorporating a sensor device as claimed in any one of claims 1 to 6.

8. A radio microphone, including a surface acoustic wave piezoelectric transducer for converting audio signals to corresponding electrical signals, wherein said transducer further provides a radio frequency deterring element for said microphone.

9. A radio microphone, including a surface acoustic wave piezoelectric transducer, and drive/feedback circuit for maintaining the machine in an oscillating state of its characteristic frequency, wherein the transducer comprises an integral piezoelectric structure of a flexible diaphragm responsive to sound waves and supported around its periphery by a rigid annulus, there being conductive electrodes so disposed on one surface of the diaphragm as to define a surface acoustic wave device, and wherein the arrangement is such that flexural displacement of the diaphragm responsive to sound waves incident thereon causes a,corresponding frequency modulation of the oscillator formed by the surface acoustic wave device and the drive circuit.

10. A passive radio microphone, including a surface acoustic wave piezoelectric transducer, and a resonant circuit tuned to the characteristic frequency of the transducer, the arrangement, in use, being activated by an external oscillating electromagnetic signal at said characteristic frequency, wherein the transducer comprises an integral piezoelectric structure of a flexible diaphragm responsive to sound waves and supported around its periphery by a rigid annulus, there being conductive electrodes so disposed on one surface of the diaphragm as to define a surface acoustic wave device, and wherein the arrangement is such that displacement of the diaphragm responsive to sound waves incident thereon causes corresponding changes in the characteristic frequency of the surface acoustic wave device thereby modulating the oscillating signal, said modulated signal being re-radiated from the resonant circuit.

11. A radio microphone substantially as described with reference to and as shown in Fig. 3 and Fig. 4 of the accompanying drawings.

12. A cordless telephone provided with a radio microphone as claimed in any one of claims 7 to 11.