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(11) | EP 0 401 027 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | An acoustic transducer |
| (57) In order to reduce side lobes in the transmission characteristics of an acoustic
transducer the amplitude of transmission is tapered towards the edges of the aperture.
This is done by constructing the transducer from ceramic-epoxy composite elements
connected in a one-three configuration and distributed with varying volume fraction
in the width and length directions. |
[0002] It has previously been proposed to construct acoustic transducers using an array of ceramic pillars embedded in an epoxy. The effect of the epoxy is to improve acoustic matching to liquid-based loads and to increase bandwidth. Also, the use of a pillar-like transducer improves transduction efficiency.
[0003] The invention arose as a result of research into techniques for controlling the beam-shape (in particular for eliminating side lobes) in the radiation pattern of an acoustic transducer. Such control is normally achieved by applying a apodisation function to individual transducers across an array. Doing this requires complicated and expensive driving circuitry and also requires individual connections to be made to each element in the array. The inventors have now realised that this problem can be overcome using the technique described in the immediately preceding paragraph.
[0004] The invention provides an acoustic transducer comprising a layer of piezoelectric and non-piezoelectric material distributed across an aperture of the transducer and electrode means on each side of the layer for applying an input signal to it or receiving an output signal from it; the manner of distribution of the piezoelectric and non-piezoelectric being such that the amplitude of vibration caused by a given input signal (or the amplitude of an output signal caused by a given variation) is dependent on a control function defined by (a) the percentage and/or distribution and/or type of piezoelectric material and/or (b) the percentage and/or distribution and/or type of non-piezoelectric material, characterised in that the said function varies across the aperture.
[0005] By varying the aforementioned control function in this way, the required apodisation characteristics can be obtained from a single excitation source and the individual connections can be made using just two electrodes.
[0006] The control function can be varied in a number of different ways, for example by varying:-
(1) the proportion of the area of the aperture which is piezoelectric,
(2) the type of piezoelectric or non-piezoelectric used at different parts of the aperture, or
(3) the shape of piezoelectric or non-piezoelectric parts.
[0007] One possible construction comprises an array of individual peizoelectric elements seperated by a matrix of non-piezoelectric material. Alternative possibilities include using a single piezoelectric slab formed with holes into which non-piezoelectric is loaded. Another possibility would be to use a honeycomb or sponge-like structure of piezoelectric filled with non-piezoelectric or vice-versa. Another possibility would be to mix piezoelectric and non-piezoelectric, e.g. by powdering a ceramic piezoelectric material and mixing it with a suitable non-piezoelectric filler.
[0008] Another possible way of varying the aforementioned "function" would be to change the shape of the piezoelectric or non-piezoelectric parts across the aperture.
[0009] For ease of construction it is convenient to form the array from a number of blocks, the "function" being uniform for each block.
[0010] One way of performing the invention will now be described by way of example with reference to the accompanying drawings, in which:
Figure 1 shows a plan view of a transducer constructed in accordance with the invention;
Figure 2 is a graphical illustration of the amplitude of acoustic energy transmitted from the different parts of the transducer shown in Figure 1; and
Figure 3 is a cross-section through the line III-III of Figure 1.
[0011] The illustrated transducer comprises nine blocks, 1, 2A, 2B, 3A, 3B, 4A, 4B, 5A and 5B. Each block is individually made and the blocks are located together as shown.
[0012] Each block comprises a number of ceramic piezoelectric pillars, e.g. as shown at 5. These are made of a commonly used material, namely lead-zirconate-titanate type ceramic. The pillars are embedded in a hard-setting epoxy using established slice-and-fill techniques. The individual blocks are held together by adhesive and opposite sides are then coated with metallic paint to form electrodes 6 and 7.
[0013] It will be noted from Figure 1 that the pillars are approximately evenly distributed over the area of each block but that their spacing is greater in blocks towards the outside. For this reason, the amplitude of radiated energy as shown in Figure 2 is greatest from the centre block 1 and least from the outer blocks 4A and 4B. This variation extends in just one dimension in the illustrated arrangements but it will, of course, be understood that similar variations in two dimensions could be obtained in alternative constructions.
[0014] In operation as a transmitter, electric signals from circuitry 8 are applied to the pillars 5 via the electrodes 6 and 7 to transmit a desired acoustic signal in the direction shown by the arrow on Figure 3. The electrode 7 is mounted on a rigid substrate preventing substantial radiation in the opposite direction. In operation as a receiver, acoustic energy causes the pillar 5 to generate a potential difference across electrodes 6 and 7, and this is detected at 8. In both modes of operation, the tapering volume fraction (values given on Figure 1) across the width of the acoustic aperture gives the gain pattern of the transducer a pronounced main lobe and reduced side lobes. Of course, in other environments, a similar technique could be used to obtain given patterns having other characteristics, e.g. two main lobes or omnidirectional radiation and reception.
1. An acoustic transducer comprising a layer of piezoelectric and non-piezoelectric
material distributed across an aperture of the transducer, and electrode means on
each side of the layer for applying an input signal to it or receiving an output signal
from it; the manner of distribution of the piezoelectric and non-piezoelectric being
such that the amplitude of vibration caused by given input signals (or the amplitude
of an output signal caused by a given vibration) is dependent on a control function
defined by (a) the percentage and/or distribution and/or type of piezoelectric material
and/or (b) the percentage and/or distribution and/or type of non piezoelectric material,
characterised in that the said function varies across the aperture.
2. A transducer according to claim 1 comprising a number of blocks of elements, the
said function being uniform across each block but varying between adjacent blocks.
3. A transducer according to claim 1 or 2 comprising pillars of ceramic piezoelectric
material embedded in an epoxy material.
4. A transducer according to any preceding claim comprising two electrodes making
contact with the piezoelectric material across the aperture of the transduceri and
a single excitation source or receiver connected to the electrodes.