EP1921600A1

EP1921600A1 - Ultrasonic transducer

Info

Publication number: EP1921600A1
Application number: EP06076994A
Authority: EP
Inventors: designation of the inventor has not yet been filed The
Original assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Current assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Priority date: 2006-11-08
Filing date: 2006-11-08
Publication date: 2008-05-14

Abstract

Ultrasonic transducer, comprising an electro/acoustical conversion element (1) for transmitting and/or receiving an ultrasonic signal, a housing (3) and a cover layer (4). The cover layer is made of a PEI material. An ultrasonic systemmay comprise such an ultrasonic transducer, as well as signal processing means for processing the ultrasonic signal, which signal processing means are arranged to discriminate internal reflections inside the cover layer and to process those internal reflections.

Description

Field of the invention

The invention concerns a transducer, e.g. a piezoelectric type transducer, which can be applied in a liquid environment, hereinafter called "underwater transducer"

Background of the invention

The (piezoelectric) element of a state of the art transducer that is applied to generate ultrasound at high frequencies in e.g. water, may become damaged within a short time by the water penetrating the thin matching layer, e.g. made of an epoxy material, that is applied to adapt the acoustic impedance of the piezoelectric element to the acoustic impedance of the water.
In a prior art transducer the active electro/acoustical element may be a 100 µm thick piezoelectric composite element. The element has a heavy backing at the non-radiating side. An epoxy layer serves as the matching layer between acoustical impedances of the piezoelectric composite element and the water, to ensure efficient ultrasound radiation. The layer thickness of e.g. 40 µm equals a quarter wavelength at the nominal radiation frequency.
The transducer's composite material is not easy to machine. That is a serious problem for shaping the radiating surface with the desired precision, in order to define radiation characteristics, like the directivity and focus.
An even more important problem is that the transducer's active element gets degraded by water that penetrates the thin epoxy layer when the transducer is immersed in water. The lifetime in water is only a few days. The epoxy layer thus gives insufficient protection against the surrounding water. Making the epoxy layer thicker would introduce the problem of too much acoustical damping, because the specific damping of 0.3 dB/(MHz·mm) of the material is relatively high in the frequency range around 15 MHz. Making the layer thick enough to be effective for protection against water, would diminish the radiation efficiency too much.

Summary of the invention

Aim of the invention is to provide a cover layer that:

has an specific acoustic impedance which is - like the prior art epoxy material - well fit for acoustic coupling with the surrounding water, without introducing an extra, reflecting acoustic transition;
shows a specific damping that is an order of magnitude smaller than prior art materials, allowing to make a thicker coupling layer, offering an effective protection against water, however keeping the effective damping acceptable;
can be shaped, for example with equipment that is used to manufacture lenses.

According to the invention it is preferred that the cover layer is made by the polymer polyethyleneimine (PEI), is made by a derivative or mixture which is substantially based on that polymer polyethyleneimine or on its monomer, ethyleneimine, or is made by a copolymer of the monomer ethyleneimine and one or more other monomers. All of those PEI based material are considered to be fit for covering ultrasonic underwater transducers and are called PEI material hereinafter or indicated as PEI cover or PEI layer.
Some data about PEI: the PEI monomer consists of a three-membered ring. Two corners of the molecule consist of -CH2-linkages. The third corner is a secondary amine group, =NH. In the presence of a catalyst this monomer is converted into a highly branched polymer with about 25% primary amine groups, 50% secondary amine groups, and 25% tertiary amine groups. This product is sometimes called "pure polyethyleneimine" in order to differentiate it from certain copolymers of ethyleneimine and acrylamide. The latter mixture is copolymerized to produce so-called "modified PEI," that has a molecular mass up to about 2 million grams per mole. Each of these types of products is delivered to the mill in the form of viscous solutions. Solids contents are the range of about 10 to 50%, depending on molecular mass.
The new PEI layer gives an effective protection against water, mainly because it can be much thicker than the prior art cover layer, however, without unacceptable damping. It is true that the relative thickness may introduce internal reflections, however, the layer is also thick enough to allow internal reflections to be discriminated and subsequently eliminated well by signal processing means. The thickness of the PEI cover layer may be e.g. 2 mm (instead of 40 µm in the prior art), which is about 12 times the ultrasound wavelength.
Summarizing, applying PEI material enables a more robust (thicker) cover layer without unacceptable damping, having the advantage that it is well water-resistant and, besides, offering the opportunity that acoustic reflections can be discriminated well and thus eliminated by rather simple signal processing.
Moreover, the PEI layer's outer surface can be given the desired shape with proper precision, for example flat or spherical; e.g. the outer surface can be shaped as an acoustic lens.
Finally, it is noted that EP1073312 discloses the application of PEI as a cover layer for a non-ultrasonic transducer for non-underwater applications. WO/2000/032013 discloses a similar application of PEI, viz. used as a transducer cover.

Exemplary Embodiment

Figure 1 shows a lateral cross-section of an exemplary embodiment of an ultrasonic transducer according to the invention.
The ultrasonic transducer shown in figure 1 comprising a piëzoëlectric electro/acoustical conversion element 1, called PEE hereinafter, which PEE 1 is fit for transmitting or receiving (or both) an ultrasonic signal of e.g. 15 MHz. The PEE 1 is mounted on a heavy backing element 2 at the non-radiating side. Both are mounted within a housing 3. A PEI cover layer 4, having e.g. a thickness of about 2 mm covers the PEE 1 and protects it from the liquid (e.g. water) during operation. The housing provides a fastening edge 5. The PEE 1 may be connected with a processor (not shown) via electical connections wires 6.
The ultrasonic transducer may be connected to a signal processor which is arranged to process ultrasonic signals which are received by the PEE 1 from other underwater transducers or are received back due to reflections of the transmitted signal from e.g. underwater objects.
Due to the its thickness, the cover layer 4 may cause internal reflections when the PEE 1 transmits or receives signals. The signal processor is arranged to discriminate those internal reflections inside the cover layer and to process those internal reflections in a proper way, e.g. by eliminating those signal reflections from the "normal" signals received from or transmitted to other ultrasonic transducers.

Claims

Ultrasonic transducer, comprising an electro/acoustical conversion element (1) for transmitting and/or receiving an ultrasonic signal, a housing (3) and a cover layer (4), wherein said cover layer is made of a PEI material.
Ultrasonic system, comprising an ultrasonic transducer according to claim 1, as well as signal processing means for processing the ultrasonic signal, which signal processing means are arranged to discriminate internal reflections inside the cover layer and to process those internal reflections.