GB2201064A - Piezoelectric tranaducer - Google Patents

Abstract

A transducer for generating an ultrasonic output comprises a piezo-electric disc 10 (connected to means for applying an excitation voltage to the component and means for detecting voltages generated by the component); a cylindrical member 11 of a material capable of resonance secured to one face of the component; and an annular ring 17 of material capable of resonance affixed concentrically to the member; the device being constructed so that, at a particular frequency of excitation voltage, the component causes the member 11 to resonate in a longitudinal mode; and the annular member 17 to resonate in a radial mode. Preferably the phasing of longitudinal vibrations transmitted by the member and vibrations transmitted by the annular ring are matched in air or other medium into which the output is to propagate vibrations by securing the annular ring at a predetermined axial position relative to the member. A further cylindrical member 14 can be used to sandwich the piezoelectric disc 10 between itself and the first cylindrical member 11 to provide resonance in a longitudinal mode at the particular frequency. <IMAGE>

Description

TRANSDUCER DEVICE This invention relates to a transducer device. It is particularly concerned with a transducer making use of the piezo-electric effect. This refers to the behaviour of a crystal which on the one hand develops voltages when the crystal is subject to mechanical strain and on the other hand produces expansion and contraction along different axes of the crystal when the crystal is subject to an electric field.

According to the present invention there is provided a transducer device comprising: 1) a piezo-electric component in the form of a disc; 2) means for applying an excitation voltage to the component and means for detecting voltages generated by the component; 3) a cylindrical member being of a material capable of resonance secured to one face of the component 4) an annular ring of material capable of resonance affixed concentrically to the member; the device being constructed so that, at a particular frequency of excitation voltage, the component causes the member to resonate in a longitudinal mode; and the annular ring to resonate in a radial mode.

In a first preferred form of the present invention phasing of longitudinal vibrations transmitted by the member and vibrations transmitted by the annular ring are matched in air or other medium into which vibrations are to be propagated by securing the annular ring at a predetermined axial position relative to the member.

In a second preferred form of the present invention a further cylindrical member is is secured to a face of the component opposite to that by means of which the member is secured to the component and the combination provides for both the member and the further member to be capable of resonance in a longitudinal mode at a particular frequency.

In a third preferred form of the present invention the annular ring has a surface of revolution inclined at an angle to the face between the component and the member.

In a fourth preferred form of the present invention the annular ring is fabricated from an elastomeric material capable of absorbing vibration transmitted from the component and from the cylindrical member or cylindrical members. Typically the use of an elastomeric material serves to increase the natural damping of the device so that short pulses of ultra sound can be generated by brief excitation of the component.

An exemplary embodiment of the invention will now be described with reference to the accompanying drawing which is a longitudinal section of a transducer device.

A piezo-electric component 10 of polarized PZT (RTM') ceramic has a first face 11 bonded to a first cylindrical member 12 and a second face 13 bonded to a second cylindrical member 14. The members 12 14 are of plastics PVC material. The component 10, and the members 12, 14 are mounted coaxially on axis A. Leads 15 enable excitation to be applied to the component 10 and an output voltage from the component 10 caused by its physical distortion to be detected. .An annular ring 16 made from an elastomeric material 'Neoprene' ('RTM') is secured adjacent to the outer periphery of the disc shaped component 10. The ring 16 has a face 17 in the form of a surface of revolution which is disposed concentrically with axis A. The face 17 is inclined at an angle R to the axis A.

When the component 10 has an excitation voltage pulse (or a sequence of high frequency pulses) applied to it by way of leads 15 a resonance is generated by the component and in the first and second members is transmitted through the first member 12 and thereafter through a narrow cone of air in the direction of arrow 18. Ultrasonic vibration is also transmitted into the annular member where the radial progression of the signal also contribute to the transmission of n ultrasonic tranmission into the surrounding air. Once the transmitted ultrasonic signal strikes a target in the direction of arrow 18 a proportion of the outgoing signal is reflected back to the transducer and impinges on first cylindrical member 12 and the annular ring 17.This serves to transmit the reflected signal to the first face 11 of the component 10 and to the outer periphery 19 causing the component to generate a measurable.output voltage. The time between transmission of the outgoing wave and reception of the reflected one by the component are indicative of the distance of the target from the transducer.

The resonant frequency of a transducer device according to the present invention is dependent upon the velocity of sound in the materials of its construction. A typical device would have a piezo-electric component in the form of a thin disc of 50 mm diameter bonded concentrically to cylindrical members of 54 mm diameter fabricated from PVC. To match the radial and longitudinal resonant frequencies of the assembly the optimum length of the cylindrical members would be 19 mm on either side of the disc. The resonant frequency of such a device will be 40 KHz. An annular ring of internal diameter 54 mm and outer diameter 90 mm and with its outer periphery fixed rigidly and fabricated from an elastomeric material would also resonate at 40 KHz. Such a device has an effective range through air in excess of 7 metres.

Claims (6)

1 A transducer device comprising: 1) a -piezo-electric component of substantially disc form; 2) means for applying an excitation voltage to the component and means for detecting voltages generated by the component; 3) a cylindrical member being of a material capable of resonance secured to one face of the component 4) an annular ring of material capable of resonance affixed concentrically to the member; the device being constructed so that, at a particular frequency of excitation voltage, the component causes the member to resonate in a longitudinal mode; and the annular member to resonate in a radial mode.

2 A transducer device as claimed in Claim 1 wherein the phasing of longitudinal vibrations transmitted by the member and vibrations transmitted by the annular ring are matched in air or other medium into which vibrations are to be propagated by securing the annular ring at a predetermined axial position relative to the member.

3 A transducer device as claimed in Claim 1 or Claim 2 wherein a further cylindrical member is is secured to the piezo-electric component on a face of the component opposite to that by means of which the member is secured to the component and the combination provides for both the member nd the further member to be capable of resonance in a longitudinal mode at the particular frequency.

4 A transducer device as claimed in any preceding claim wherein the annular member has a surface of revolution inclined at an angle to the face between the component and the member.

5 A transducer device as claimed in any preceding claim wherein the annular ring is fabricated from an elastomeric material capable of absorbing radial vibration transmitted from the component and from the cylindrical member or cylindrical members.

6 A transducer device as hereinbefore described with reference to the accompanying drawing.