WO1997008917A1

WO1997008917A1 - Ultrasonic transducer

Info

Publication number: WO1997008917A1
Application number: PCT/DK1996/000360
Authority: WO
Inventors: Søren LOLK
Original assignee: Danfoss A/S
Priority date: 1995-08-31
Filing date: 1996-08-29
Publication date: 1997-03-06
Also published as: DK96795A; DK172322B1; AU6785196A; PL325195A1; RU2159517C2

Abstract

An ultrasonic transducer with a piezo-electric element (7) for use with liquids under pressure is well suited for use as receiver, if an efficient electric shield has been made. In an embodiment this shield is made exclusively as an untwining of the screen conductor (18) of the connected coaxial cable, which is led radially into an adhesive layer (T) between the piezo-electric element and its support (13).

Description

Ultrasonic transducer

The invention concerns an ultrasonic transducer for radiation into fluid media including a housing terminated with a tight diaphragm, which is set-up in an outer frame and has a piezo-electric disc as oscillating element supported in the centre of the housing and electrically connected by means of electrodes on both sides.

Numerous constructions of this type are known, the basic form of which was developed for localising objects under the water at the end of World War 1. DK 151 676 describes such a transducer with modern components for use among others in heating plant for measuring the flow speed in pipes. It describes a piezo-electric ceramic disc provided with electrodes on front and rear, which is only supported near the edge. On the front it is provided with a protective film. This construction has excellent qualities as ultrasound transmitter, if it is certified that the static pressure on the outside is not too high, and that there is always an overpressure on that side. However, the construction described, which is not fit for versatile applications, in which, for instance, the pressure difference between front and rear becomes negative, or in which large positive pressure differences occur. In che construction described, these will cause either an exhaustion or a crushing of the piezo-electric element, the consequence of both will be damaging of the transducer. Further, the electric shield of the transducer unit is not sufficient to enable its use as receiver. This has turned out to be a large disadvantage ^n the constructions, in which the potential of the tran. .αcer housing is not lying on device earth. It is the purpose of the invention to produce an ultrasonic transmitter of the type mentioned in the introduction, which permits a versatile application, both as transmitter and receiver, at any static pressure appearing in practice, and with an efficient shield against electrical noise interference.

This is reached by means of a construction according to the invention, which is characterised by the fact that the centering media make out a share of the electric shield, which is placed between the support and the piezo-electric element.

The piezo-electric element is supported efficiently by forming a close contact to a supporting body on a large share of the surface according to claim 2.

By letting the front electrode continue on the edge of the disc, the fingers of the screen/centering medium are at the same time enabled to form contact.

The shield becomes complete, as it is only broken by a hole, through which the internal conductor can be led, and the surroundings of the hole can be connected to the screen of the coaxial cable used.

The fact that the shield consists of an untwisted end of the screen of the connecting coaxial cable, in which each conductor is mainly led along a radius along the rear of the piezo-electric disc, makes it possible to reach a cheap and simple construction.

An advantageous centering without use of an additional centering element inside the supporting bowl is reached by letting the ends of the conductors of the coaxial cable fold around the edge of the disc.

By embedding the conductors in an adhesive layer between the support and the piezo-electric disc, a fixing of same is obtained, and making the adhesive layer sufficiently thick will provide security against local tensions in the piezo-electric disc, where two conductors are crossing each other.

In an advantageous embodiment the conductors are led between an isolating cone and a cone-shaped groove in the support in a way that isolates and protects them.

In another advantageous embodiment, a spring, one end of which is leaning against the inside of the isolating cone, is fixing the centre conductor of the coaxial cable to the rear electrode of the piezo-electric disc.

In yet another advantageous embodiment the pull release is simplified in a way that it consists of a knot placed non-turnably in a well in the supporting element.

The construction is extremely fit for production, as all assemblings are made axially, as a prefabricated adhesive layer is used between all surfaces. In addition this layer acts as electric isolation. A union nut is not used, however, a spring ring in a track in the frame near the connection side of the transducer defines the axial position, and .an O-ring at the opposite end of the housing provides a plant pressure.

In the following the invention is described by means of drawings, in which: Fig. 1 shows a longitudinal section through an ultrasonic transducer in an outer frame.

Fig. 2 shows a different embodiment of the ultrasonic transducer according to the invention.

An outer frame 1, which could for instance be a wall in a pipe or in a container, or part of a flow speed meter, has a well-shaped groove 2, and two O-rings 3 and 4 provide the sealing against the transducer housing 5. The housing is finished by a diaphragm 6, against which the O-ring 4 is fixed. The piezo-electric element 7 is covered with metal, partly on the front forming the front electrode 8, partly on the rear forming the rear electrode 9, as there is a wide, non-covered ring 10 forming the separation between these two electrodes. Around the piezo-electric disc a ring of fingers 11 is placed, which continues in a shield plate 12 having a tin-shaped platform 13 on the inside. The disc 7 and the shield are supported by a bowl-shaped support 14 having a skirt-shaped side 15 surrounding the ring of fingers 11 and centering the construction in the housing 5. The rear electrode 9 is connected to the centre conductor 16 in a coaxial cable 17, the screen 18 of which is connected to the shield 13. The coaxial cable 17 is led through a sealing 19 in the housing 5.

All surfaces are provided with a prefabricated adhesive layer T, so that the assembling can be made by adding a ring of adhesive to the support 14, which is then fixed in the housing 5. A ring of adhesive is added to the underside of the shield 12 which is then fixed in the support 14, centered by the fingers 11 and the skirt 15. A ring of adhesive is added to the rear of the piezo-electric disc 7 which is then fixed in the shield 11, 12, centered by the fingers 11 against the edge of the disc. A roundel of adhesive is added to the piezo-electric disc 7 in a way that it protrudes the edge of the housing 5, after which the diaphragm 6 is fixed. The O-rings 3 and 4 are placed in their respective tracks, and the housing 5 is mounted in the frame 1. A locking ring 20 is placed in a track in the frame 1 at the rear of the housing 5. The connection of the coaxial cable 17 is not described, as this is known to a person skilled in the art. A certain form of pull release can be obtained by crimping the connection of the screen 18 of the coaxial cable to the screen 13.

From fig. 2 it appears how the practical construction of the ultrasonic transducer can be even more simplified under preservation of the full advantages of the invention. The reference numbers for unchanged elements are the same as in fig. 1. Here the support 14 is continued to the rear to form a cone-shaped bowl 14a. The shield 11, 12, 13 is here completely replaced by an untwisting and spreading of the plaited screen 18 of the coaxial cable 17, which is fixed between the adhesive layer T and the piezo-electric disc and forms the electric contact to the metal cover 8 of the front. Thus a special production of a screen with tin-shaped platform and flaps for contact is avoided. The centre conductor 16 in the coaxial cable 17 is fixed to the rear electrode 9 by means of a spring 20. The other end of the spring rests against the inside of a cone 21 made of PTFE, which is placed around the isolation of the centre conductor and prevents the conductors 18 of the screen from touching the spring 20 or the electrode 9. Thus the threads become a safe course in the intermediary between the cone-shaped bowl 14a of the support and the outside of the PTFE cone 21. In practice it is not necessary to use all the threads of the screen, provided that a maximum distance between neighbouring threads of 3 mm is safeguarded along the periphery of the piezo-electric disc 7. The adhesive layer between the piezo-electric disc 7 and the support 14 is thicker than the remaining layers, in order to permit the crossing of two threads without causing damaging local tensions in the piezo-electric disc due to pressure influence.

In the present embodiment the pull release is formed by a knot 22 on the coaxial cable. In order to avoid a rotation of the end of the coaxial cable, the knot is placed in a cylindrical well 14b made in one piece with the support 14. The generator curve for the cylinder is mainly oval or figure-eight-shaped, as this shape catches a knot efficiently. The knot 22 is held down in the well by an isolating bushing 23 placed from the inside in a hole in the housing 2. Especially out of regard for sealing the knot can be surrounded by an elastic, maybe adhesive mass.

Claims

Patent Claims

1. Ultrasonic transducer for radiation into fluid media including a housing (2) terminated with a tight diaphragm, which is set-up in an outer frame (1) and has a piezo-electric disc (7) as oscillating element supported in the centre of the housing and electrically connected by means of electrodes (8, 9) on both sides, c h a r a c t e r i s e d i n t h a t the centering media are part of the electric shield (13), which is led between the support (14) and the piezo-electric element.

2. Ultrasonic transducer according to claim 1, c h a r a c t e r i s e d i n t h a t the primarily thickness-oscillating piezo-electric disc (7) is supported on its rear side on the largest share of its surface around the rear electrode (9) .

3. Ultrasonic transducer according to claim 1, c h a r a c t e r i s e d i n t h a t the electric shield includes edge media (11) building contact with the front electrode (8) of the piezo-electric disc.

4. Ultrasonic transducer according to claim 1, c h a r a c t e r i s e d i n t h a t the electric shield has a hole, the edge of which is connected to the screen (18) of a connecting coaxial cable (17) .

5. Ultrasonic transducer according to claim 1, c h a r a c t e r i s e d i n t h a t the shield consists of an untwisted end of the screen (18) of the connecting coaxial cable (17), in which a number of conductors are mainly led along a radius along the rear of the piezo-electric disc (7) .

6. Ultrasonic transducer according to claim 5, c h a r a c t e r i s e d i n t h a t the conductors are embedded in an adhesive layer between the support (14) and the piezo-electric disc (7) .

7. Ultrasonic transducer according to claims 3 and 5, c h a r a c t e r i s e d i n t h a t after the disc parallel course, the conductors are led around the edge of the disc (7) inside the side (15) of the support to build centering media for the disc.

8. Ultrasonic transducer according to claim 5 or 6, c h a r a c t e r i s e d i n t h a t in their course between the cable and the fixing in the adhesive layer the conductors are mainly led cone-shaped between a groove in the support and an isolating cone (21) .

9. Ultrasonic transducer according to claim 8, c h a r a c t e r i s e d i n t h a t the centre conductor (16) in the coaxial cable forms contact through fixing against the electrode (9) through spring pressure between the inside of the isolating cone (21) and the centre conductor (16) .

10. Ultrasonic transducer according to any of the preceding claims, c h a r a c t e r i s e d i n t h a t the pull release for the coaxial cable is formed by a knot (22) on same, placed in a cylindrical well closing tightly around the knot along its surface.

11. Ultrasonic transducer according to claim 10, c h a r a c t e r i s e d i n t h a t the knot is surrounded by an elastic, maybe adhesive mass when being placed in the well.

12. Method for manufacturing ultrasonic transducer according to any of the preceding claims, c h a r a c t e r i s e d i n t h a t all elements are assembled by means of a pressure in the axial direction and by means of prefabricated adhesive layers (T) between all surfaces.

13. Method according to claim 11, c h a r a c t e r i s e d i n t h a t the set-up in the outer frame is secured by means of a locking ring (20) combined with an O-ring (4) .