Segmented ring transducers

Details Segmented ring transducers Segmented ring transducers

1996-10-28

1998-04-14

Dougherty, Thomas M.

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

H01L 4108, B06B 106

Patent

active

057396256

DESCRIPTION:

BRIEF SUMMARY
The invention relates to transducers employing segmented rings of piezoelectric ceramic blocks as used for sound projectors in underwater applications and in particular to arrangements for applying a pre-stress to such piezoelectric blocks.
A transducer commonly used for low frequency, high output operation is the flextensional transducer as described in UK patents number 2211693 and 2209645. One disadvantage of these transducers is that depth compensation arrangements need to be provided for deep water operation otherwise there is a loss of linearity of performance. Free flooding ring transducers do not require depth compensation however.
Conventional ring transducers incorporate a number of linear stacks of rectangular shaped blocks of piezoelectric ceramic material separated by tapered wedges to form a ring arrangement. The segmented ring requires pre-stressing as an active transducer otherwise the mechanical couplings between the ceramic blocks and between the blocks and the wedges will fail when a certain level of ac voltage is applied to the piezoelectric elements. Thus the usable ac voltage will be relatively low and limit the acoustic output of the transducer. Known transducers use a compression band around the outer circumference of the segmented ring to keep the ceramic and the wedges under compression. The piezoelectric ceramic is poled and driven with an electrical ac voltage signal in its thickness mode which is perpendicular to the force applied by the pre-stress band.
The conventional pre-stress arrangement is non-ideal in that the ceramic is not pre-stressed in direction of its thickness mode. High power acoustic measurements on such known segmented rings have shown that these devices are susceptible to distortion. This is apparently brought about by mechanical joint failures due to lack of pre-stress exerted on the segmented ring by the pre-stress band. The conventional pre-stress band is formed around the segmented ring by means of a filament winding process. With these processes it is difficult to measure and control accurately the amount of pre-stress exerted on to the segmented ring. Furthermore, it is found that there is an uncertain reduction in the initial amount of pre-stress due to fibre relaxation.
U.S. Pat. No. 3,043,967 discloses a ring transducer comprising a number of arcuate ring sections, each section comprising a number of rectangular piezoelectric ceramic blocks with several tapered wedges spaced within the section. However, the piezoelectric ceramic blocks are prestressed using pre-stress bands and therefore suffers from the problems previously outlined.
The object of the invention is to provide a segmented ring transducer which overcomes the pre-stress difficulties of the known transducers.
The invention provides:
a segmented ring transducer comprising a plurality of arcuate ring sections coupled together, each arcuate ring section comprising a plurality of rectangular piezoelectric ceramic blocks arranged into a stack with one or more tapered wedges spaced in the stack characterised in that the piezoelectric stack being assembled between opposed end couplings, the opposed end couplings being connected together by pre-stress bolts in a ring section to hold together the ring section assembly.
Ideally, the arcuate ring sections in a ring transducer are identical. The adjacent arcuate ring sections can be connected together by further bolts.
The ring transducer may be formed into a complete ring or a split ring with an arcuate portion of the ring missing. The split ring may be formed by omitting one or more identical arcuate ring sections or by omitting an arcuate portion of the ring which is not equivalent to an integral number of arcuate ring sections.
Preferably, each arcuate portion of the ring or split ring is identical and the wedges are spaced in each arcuate section such that in the assembled ring the ceramic blocks form a regular polygon.
The invention will now be described by way of example only with reference to the accompanying Drawings of which:
FIG. 1 illustr

REFERENCES:
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patent: 3230505 (1966-01-01), Parker et al.
patent: 4814660 (1989-03-01), Yamada et al.
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patent: 5132582 (1992-07-01), Hayashi et al.
patent: 5172344 (1992-12-01), Ehrlich
Soviet Physics Acoustics, vol. 37, No. 2, 1 Mar. 1991, pp. 142-144, XP 000234437, Glazanov V E et al "Input Impedance of a Radially Excited Incomplete Cylindrical Layer".

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