WO1990005358A1

WO1990005358A1 - Amplified transducer

Info

Publication number: WO1990005358A1
Application number: PCT/GB1989/001308
Authority: WO
Inventors: Ann Jennifer Cooper; Graham Wainwright
Original assignee: Meggitt (Uk) Limited
Priority date: 1988-11-02
Filing date: 1989-11-01
Publication date: 1990-05-17
Also published as: AU4623689A

Abstract

A piezoelectric transducer includes a piezoelectric crystal (2) having an electrical input and/or output (3) coupled to an intermdediate matching layer (4) with or without a diaphragm (5), the interface with the medium forming the signal transmission and/or -reception face. A perforate screen (6) is placed parallel to, but spaced substantially by zero or an integral number of half wavelengths of the ultrasonic signal from, the interface, for the amplification of emitted and/or incoming ultrasonic signals.

Description

PESCRIPTIQN

AMPLIFIED TRANSDUCER

The invention is concerned with piezoelectric transducers of the kind which are used, for example, in distance measuring systems, such as level sensors for containers of liquid. In the case of a level sensor, ultrasonic pulses are sent out by the transducer, and the echo from the liquid surface is received by the same or similar transducer. The distance is derived from the time of flight.

In the case of existing transducers, there is a limit to the sensitivity, as determined by the signal to noise ratio of the echo pulse. This is at least partly the result of divergence of the ultrasonic wave front, so that the greater the distance which the ultrasonic energy has to travel, the lower its amplitude at any point on the wave front. In a typical transducer, the piezoelectric crystal is coupled to an intermediate matching layer or layers. The other face of this intermediate matching layer(s) may form the signal transmission/and or reception face which is exposed to the air or other medium through which the ultrasonic pulses travel. More commonly, the interface with the medium is formed by the face of a diaphragm of different or similar material, depending on the environment in which it is desired to operate the transducer, which is coupled to the face of the matching layer remote from the piezoelectric crystal.

This intermediate matching is important because one of the chief difficulties of ultrasonic transmission is in transferring the energy from the piezoelectric crystal to the transmission medium. Since the acoustic impedances differ, energy is reflected at the interface rather than being transmitted. This problem is particularly sever with transmission into air which has a very lo acoustic impedance of 0.0004 x 10* Pa.sec/m compare with 30 x 10' Pa.sec/m for a typical crystal. single matching layer should have an acousti impedance between that of the crystal and that of th medium and, for optimum transmission, the matchin layer impedance should be the geometric mean of that of the crystal and medium, i.e. about 0.1 x 10' Pa.sec/m when the medium is air. However, solid substances such as plastics material only go down to about 2 x 10' Pa.sec/m.

In the case of multiple matching layers the acoustic impedances should be graded from high adjacent to the crystal to low adjacent to the interface.

We have now surprisingly found that the amplitude of the pulses transmitted from and/or received by, the transducer can be appreciabl increased by providing a perforate screen parallel to the interface but spaced from the interface by less than 5% of a wavelength or by substantially a integral number of half-wavelengths of the ultrasonic signal. By substantially an integral number of half wavelengths is meant within 5% of a wavelength of exactly an integral number of half wavelengths, i.e. 45 to 55%, or 95 to 105%, or 145 to 155%, etc., of a wavelength.

It is not yet known exactly how this phenomeno occurs. Initially it was believed that with a spacing of substantially an integral number of half wavelengths, the ultrasonic signal was repeatedl reflected between the face of the interface and solid portions of the screen, prior to, or after, passin through the openings in the screen. A resonant effect was thus believed to occur, as the gap betwee screen and interface acted as a half wave transformer. analogous to electrical transmission theory, causing the interface to vibrate at a greater amplitude in phase with the signal.

Thus, in accordance with one aspect of the invention, a piezoelectric transducer comprises a piezoelectric crystal which has an electrical input and/or output, the crystal being coupled to an intermediate layer or layers through which pulsed ultrasonic energy is, in use, transmitted from and/or to the crystal, and, to the side of the layer(s) remote from the crystal, an ultrasonic signal-transmission and/or - reception interface, which is parallel to, and spaced by substantially an integral number of half-wavelengths of the ultrasonic signal from, a perforate screen.

However, after further experimentation, it is also now believed that the air or other medium in the holes through the screen is a zone of acoustic impedance above that of the free air or other medium into which the transducer is radiating, or from which the transducer is receiving, but below that of the matching layer in the body of the transducer. In this way the zone acts as an additional matching layer and improves the efficiency of transfer of energy between the transducer and medium. This improvement in efficiency works on both transmit, i.e. transducer to medium, and on receive, i.e. from medium to transducer.

This effect seems to work best when the spacing between the interface and the screen is very small, of the order of less than 5% of the wavelength of the ultrasonic signal. This is probably because, as the ultrasound beam will inevitably diverge, minimum energy will be lost from the gap. Thus, in accordance with a second aspect of the invention, a piezoelectric transducer comprises a piezoelectric crystal which has an electrical input and/or output, the crystal being coupled to an intermediate layer or layers through which pulsed ultrasonic energy is, in use, transmitted from and/or to the crystal, and, to the side of the layer(s) remote from the crystal, an ultrasonic signal-transmission and/or - reception interface, which is parallel to, and spaced by less than 5% of a wavelength of the ultrasonic signal from, a perforate screen. The absolute minimum size of the gap between the interface and screen is probably dependent upon the boundary conditions at the ends of the holes through the screen. However the spacing between the screen and interface is so small in comparison to the wavelength of the signal, that in practice insignificant destructive interference, between energy reflected and passing to and fro between the diaphragm and screen, occurs.

The wavelength of the signal will depend not only upon the piezoelectric crystal, but also upon the velocity of sound in the working medium to which the interface is exposed. In the normal case, the medium will be air at ambient temperature. In other systems, e.g. in which the medium may be contaminated with volatile gases, or in high or low temperature systems, the maximum spacing will depend on the expected wavelength.

Although the size, shape, or distribution of the holes in the screen are not thought to be of ultimate importance, it is believed that there may be advantages if the holes taper, e.g. frustoconically in a direction towards the interface. Also, for convenience the screen is provided with a regular pattern of holes. At present it is considered preferable if the ratio of the total open area to the total solid area of the screen is between 1:2 and 2:1. The material from which the screen is made should have a high specific acoustic impedance, e.g. above a value of 2 x 10* Pa.sec/m, to provide the necessary reflection. Most metals and some hard

- . plastics materials fall within this category. 5 The invention is illustrated by way of example, in the accompanying drawings, in which:-

Figure 1 is a part-sectional side view of the piezoelectric transducer according to the first aspect of the invention; and, 10 Figure 2 is a part sectional side view of a piezoelectric transducer according to the second aspect of the invention.

Figure 1 shows a piezoelectric transducer including a housing 1, a piezoelectric crystal 2, 15 which is connected to a complementary electrical circuit by a lead 3, a matching layer 4, a diaphragm 5, which provides an interface, and a perforate steel grating screen 6 having holes 7. The housing 1 has an opening 8, through which ultrasonic signals may 20 pass between the diaphragm and the external environment. Placed across the opening on a flange 9 of the housing is the perforate screen 6, such that ultrasonic signals passing between the diaphragm and the external environment must pass through the 25 screen. The screen and the diaphragm are positioned parallel to one another, but spaced by a resilient annular elastomeric sealing ring 10. The sealing ring can be so dimensioned as to separate the diaphragm and screen by substantially an integral 30 number of half-wavelengths of the ultrasonic signal. The screen, sealing ring and diaphragm may be fixed in position with glue 11 between the housing and screen, between the screen and sealing ring, and between the sealing ring and diaphragm. A flexible 35 potting compound 12 surrounds the crystal, lead and matching layer.

Figure 2 shows a piezoelectric transducer. where like parts are given the same reference numerals as in Figure 1.

In this example, the diaphragm 5 and screen 6 are spaced only by the width of the layer of glue between them to hold them in a relative fixed position. The width of the layer of glue is determined so that the diaphragm and screen are separated by less than 5% of the wavelength of the ultrasonic signal. In this case the sealing ring 10 is provided between the screen 6 and flange 9.

The illustrated transducer typically may have an operating frequency of 50 kHz, with a corresponding wavelength in ambient air of about 6.6mm. In the second example, the screen and diaphragm may then be spaced by about 0.3mm.

Claims

1. A piezoelectric transducer, comprising a piezoelectric crystal (2) which has an electrical input and/or output (3), the crystal being coupled to an intermediate layer or layers (4) through which pulsed ultrasonic energy is, in use, transmitted from and/or to the crystal, and to the side of the layer(s) remote from the crystal, an ultrasonic signal-transmission and/or - reception interface, which is parallel to, and spaced by substantially an integral number of half-wavelengths of the ultrasonic signal from, a perforate screen (6) (Fig. 1) .

2. A piezoelectric transducer, comprising a piezoelectric crystal (2) which has an electrical input and/or output (3) , the crystal being coupled to an intermediate layer or layers (4) through which pulsed ultrasonic energy is, in use, transmitted from and/or to the crystal, and, to the side of the layer(s) remote from the crystal, an ultrasonic signal-transmission and/or - reception interface, which is parallel to, and spaced by less than 5% of a wavelength of the ultrasonic signal from, a perforate screen (6) (Fig. 2).