WO1997002720A1

WO1997002720A1 - Ultrasonic transducers method for fixing ultrasonic transducers and high output power ultrasonic transducers

Info

Publication number: WO1997002720A1
Application number: PCT/SE1996/000888
Authority: WO
Inventors: Bo Nilsson
Original assignee: Bo Nilsson
Priority date: 1995-07-06
Filing date: 1996-07-05
Publication date: 1997-01-23
Also published as: CN1194087A; CA2226276A1; AU6374196A; EP0890292A1; JPH11508750A

Abstract

Ultrasonic transducers comprising two metal parts, a core (1) and a bottom vessel (2). Between these piezoelectrical elements (3) are arranged as well as cable connection (4). The diameter of the core (1) and the inner diameter of the bottom vessel (2) are so adjusted that the core (1) can be shrunken fixedly in the bottom vessel (2) which rests on a support (6). A pressure on the upper part of the core (1) compresses the piezoelectrical elements (3) between the core (1) and the bottom vessel (2).

Description

Ultrasonic transducers method for fixing ultrasonic trans¬ ducers and high output power ultrasonic transducers

The present invention relates to ultrasonic transducers and more precisely to an ultrasonic transducer for such high power, as well as to a method for mounting such ultrasonic transducers.

The use of ultrasonics to solve various technical problems has very rapidly increased during the last decades. Among the applications are e.g. technologies for space research, aviation, communication, marine applications, applications in the auto¬ motive and other industries, laboratory and medical applications, gas lighters, nebulizers and alarm systems. The electromechanical transducers most commonly used in said connections are using piezoelectrical materials, which convert mechanical energy into electrical energy or vice versa. The material can in addition be polarized to change dimension merely horizontally, vertically or radially depending upon which the desired effect is. Piezoelec¬ trical properties exist naturally in certain crystalline materials and can be made to exist in certain other polycristal- line materials.

The most commonly used piezoelectrical materials for the manufacture of ultrasonic transducers are based upon piezoelec¬ trical ceramics manufactured from either leadzircontitanate (PZT) or leadtitanate (PT) . The ceramic material composition and the manufacturing process can be adapted to fit the application in order to achieve e.g. high power or high sensitivity. The ceramic materials are delivered in the shape one prefers to use them, e.g. in the form of circular discs or rings, square discs, tubes, spherical elements etc. They can also be delivered in various thicknesses depending upon what result one wishes to achieve.

The ceramic elements can either be glued directly onto the structure one wishes to transmit the ultrasound to, or be used for the manufacture of ultrasonic transducers, which in turn are applied onto this structure. For the transmission of high power ultrasonic transducers are used where the ceramic part has been precompressed by way of exposing it to a permanent compression caused by that two metal parts with the ceramic between them are compressed by means of one or several bolts which have been tightened using a torque so large that the desired pressure onto the ceramic part occurs. This design is generally referred to as a "sandwich transducer".

The invention as well as the background thereof are more closely described in the following with reference to the attached drawings, in which

Figs. 1 and 2 are examples of sandwich transducers, Fig. 3 is an example of a mounting of two ultrasonic transducers,

Fig. 4 is a partially broken longitudinal section through an ultrasonic transducer also showing the mounting thereof to a structure,

Fig. 5 is a cross-section through an alternative embodiment of an ultrasonic transducer , and

Fig. 6 is a longitudinal section through a further em¬ bodiment of an ultrasonic transducer.

Figs. 1 and 2 show examples of sandwich transducers in different projections. The ceramic rings 3 are placed between the top metal part 1 and the bottom metal part 2, which have been tightened by means of the bolt 6, which in this case has been applied through a hole drilled through the bottom metal part and into a drilled and tapped hole in the top metal part whereafter it has been tightened by means of an applied torque which has been calculated so that the desired pressure is applied onto the ceramic rings. Between the rings is a contact shim with a solder tag 4 used for the connection to live and a corresponding shim with solder tag 5 between the ceramic ring and the metal part used for the connection to neutral . When mounting the finished ultrasonic transducers, these are glued onto the wall or bottom of e.g. a tank used for ultrasonic cleaning. The reason why the bottom metal part 2 in Fig. 2 has a conical shape is, that the contact surface is larger, and thus can transmit more ultrasonic energy into the tank. Fig. 3 shows a mounting of two ultrasonic transducers built up by that the bottom metal part 2 is an aluminium plate common to both transducers which in turn is then glued onto the bottom or walls of the tank as per above. In this connection the bolts used for applying pressure onto the ceramic rings are brought through two holes from the bottom side of the common bottom plate and through the ceramic rings into the drilled and tapped holes in the top metal part and tightened with a calculated torque to arrive at the desired pressure onto the ceramic rings.

The above mentioned design principle leaves us with three different problem areas:

1. The structure of the transducer means that in certain industrial applications the entire installation will have to be encapsulated, so that there is no risk that a short circuit occurs because of the presence of water or other liquids since the solder tags as well as the ceramic rings themselves are directly exposed to the ambient surroundings. This could in turn mean that one, due to that risk of condensation in closed chambers can occur, must install a continuous air purging using moisture free instrument air. In a large number of the industrial installations one has to expect that machine clean-ups are carried out using high pressure wash appliance which puts additional demand on the installation. The transducer design therefore does not meet the demands one has to put on a trans¬ ducer for industrial applications.

2. The way of mounting the transducers, where they are glued, most often using epoxy glue reinforced with an aluminium mesh onto the surface one wants to transfer the ultrasonic energy from, creates large mounting difficulties when the fixing is done in narrow areas and where the fixing has to be done from below. Since the transducers are glued onto the surface, a faulty transducer cannot easily be exchanged since the glue joint is very strong and can only be made to come off adding considerable heat to it which degrades the epoxy. The necessary water tight encapsulation as per 1. above is many times difficult to carry out on already existing equipment. The glue joint reduces the heat transfer from the transducers and in addition cuts down the efficiency of ultrasonic transmission into the liquid.

3. To dimension the transducers gets to be very complicated. In order to have the entire transducer to resonate at the desired frequency, one has to consider the influence of the lengths of the first metal section, the other metal section and the ceramic section as well as of speeds of sound, the cross section areas and the densities of these sections. The transmission of the ultrasonic wave and dissipation of heat from the first metal section to the other metal section and further on into the liquid is only done by means of the bolt which clamps the two sections together and then only by means of the pressure from the bolt head and the other metal section, which further reduces the efficiency and contributes to raise the transducer temperature.

The above mentioned three problems constitute major limitations for the possibilities to carry out an installation in a large number of industrial applications. It has therefore been regarded necessary to redesign the transducers and the way of mounting them in order to meet the following specifications:

1. The transducer must be designed so that it is submersible and is sealed so that it in itself is completely gas and liquid tight and is to be mounted in such a way that it can be subjected to high pressure cleaning without any risk of damage and breakdown.

2. The transducer must be designed in such a way that it consists of only one single metal housing with encapsulated piezoelectrical elements so that one gets only one resonating unit and thus avoids the necessity to fit the dimensions of each single resonating element to be in common resonans with all the others at the same time and without any phase displacement of the frequency between the different elements.

3. The efficiency for generating and transmitting of ultrasonic energy must be as high as possible and offer more ultrasonic transmission effect per contact surfae than does the present transducer technology.

4. The transducer must be designed and mounted in such a way that the cooling of the transducer is so good that the increase of transducer temperature is as little as possible.

5. The method of mounting the transducer must be based upon direct metal to metal contact electrically as well as accous- tically and offer possibilities to a service based upon modular exchange of transducers.

6. Transducers and method of mounting them must be adapted to one another in such a way that the distribution of ultrasonic energy into the liquid is as large as possible and so that no harmful concentrations of ultrasonic energy, so called "hot spots" will occur but that the ultrasonic energy will function in the same way and with the same concentration in the entire volume of the liquid.

TRANSDUCER DESIGN AND METHOD OF TRANSDUCER MOUNTING The transducer and the method of mounting the same must be individually designed in a way that they together function as one single unit in order to meet the above mentioned specifications. The transducer must have metal to metal contact between the different transducer elements and for external mounting they are fixed together with metal to metal contact into a fixing ring which in turn has been welded onto the surface which constitutes the base of the transducer installation.

Fig. 4 shows an example of a cross section side-view of such a structure. The transducer consists of a core 1 located inside a bottom vessel 2 with two circular piezoelectric ceramic discs 3 with a contact shim with a solder tag 4 between them for cable connection to the generator live connector via the milled and drilled hole 5 through the core. By securing total metal-ceramic contact when pressure is applied to the ceramic discs which are silver coated, the connection to the generator common can be accomplished via the metal parts of the transducer. The bottom vessel 2 is threaded in the bottom part in order to allow for screwing same into the threaded mounting ring 6 when fixing the transducer during the mounting. The bottom vessel 2 is also threaded at the top end to allow for screwing same into a threaded connector part 7 to allow for connecting several transducers together and for securing protected cable connections of the entire transducer assembly. A hole 5 has been drilled through the transducer core from the location of the solder tag of the contacts shim to the top center of the transducer core to allow for cable connection to the generator and thereafter one arranges total electrical insulation between the cable core and the transducer metal parts.

After cabeling and insulation the drilled hole is filled with epoxy or a similar type of sealing material in such a way that a complete sealing is achieved. The dimensions of the core 1 outside and the bottom vessel 2 inside diameters are selected such to each other that they can be regarded as one single metal part after one has been fixing them together by shrinking, welding or by another suitable fixing method.

After one has fixed the piezoelectric discs 3 with the contact shim with cable connection between them to the bottom part of the core 1, the cable is brought through the drilled hole 5 in the core 1 with insulated cable connection and cable, the core 1 is chilled by exposing it to e.g. liquid nitrogene. The core 1 is then positioned inside the bottom vessel 2 in a hydraulic press, where the two parts are pressed together with a pressure so high, that the desired precompression of the ceramic will occur. This can be controlled by means of a load cell mounted in the hydraulic press. As a safety precausion, the electrical voltage emitted by the piezoelectric ceramics 3 when exposed to a pressure is measured. Should it divert substantially from what is normal, it is probably due to that the ceramic discs 3 do not meet specifications and the manufacture of this transducer is abandoned. When the core 1 and the bottom vessel 2 have been shrunk together, they will be fixed together maintaining the same pressure by means of welding, pins, screws or the like, so that the desired pressure against the ceramic discs 3 will be maintained after that the transducer has been removed from the hydraulic press. After the transducer has been removed from the hydraulic press, the drilled hole 5 is filled from the bottom of the hole with a suitable sealing material e.g. epoxy. Since the transducer parts have been fixed together under a predetermined pressure, constant and repeatable transducer properties per tranducer type is secured.

When mounting the transducer, the mounting ring 6 is first welded onto the plate wall through which one desires to transmit the ultrasound into the liquid. Into the bottom of the cup formed that way, one applies an adhesive with a high content of a metal, e.g. colloidal silver. This is done to secure a very good metal to metal contact between the transducer and the plate despite the uneven surface of a product such as a welded stainless steel container. After that, the transducer is screwed into the mounting ring using a torque which secures that this contact is achieved and the connector part screwed onto the top part of the bottom vessel, the transducer cable will be connected to the high tension cable. All cable parts, both supply cable and transducer cables are enclosed within protecting pipes with sealed pipe connections and the transducer installation is completed. After that, all transducers for the installation are installed in the same way and the entire system installation is ready with completely sealed transducers and all cable connections protected by pipes dimensioned so that the entire installation can be exposed to high pressure cleaning without risk of damage. If one wishes to manufacture a submersible unit made up by several transducers, they are connected in the same way via a connector pipe and with cables protected by pipes as per above. The connector pipe 7 showed in Fig. 4, has been shown in a T- shaped form for connection of one single transducer but can as well be manufactured for connecting several transducers to it. If e.g. one has four transducer connections per connector pipe, one can have a submersible unit which emits ultrasonic energy in all four directions, which can be a great advantage e.g. in an installation into a tank with large diameter and height. Submersible transducers will preferably be manufactured out of acid-proof materials, whereas transducers for external mounting generally will be manufactured out of dural, since this material transfers heat away from the transducer much better than does acid-proof materials. Transducers for external mounting in corrosive environment will of course also be manufactured out of acid-proof materials.

Another solution to the transducer design is shown in Fig. 5, where one as the piezoelectric element has used a piezoelectric ceramic pipe 1 coated in- and outside with silver. This pipe has been precompressed by positioning it between an inside pipe 2 and an outside pipe 3, where the diameters 4 and 5 have been dimensioned in such a way that one by cooling the inner pipe 3 and heating the outer pipe 2 arrives at a desired precompression onto the ceramic pipe 1, after that the temperatures of the elements have arrived at ambient or operation temperature. In such a way one can, by mounting several ceramic segments between long inner and outer pipes before the shrink compression is done, build together long transducer units where one can arrive at very high output power per transducer unit.

Fig. 6 shows a length section of such a structure where a number of ceramic rings 1 with radial polarization have been positioned between an outer pipe 2 and an inner pipe 3. Each transducer end wall 4 which preferably is manufactured out of stainless steel, can be manufactured to make sure that the end connections are completely water and gas tight by means of welding, O-ring seals or the like. This manufacturing method thus allows for the manufacture of completely gas and water tight transducers very well suited as submersible ultrasonic units. Only two electrical connection points with connectors 6 and 7 are used on the inner pipe 3 and the outer pipe 2 and the emission will be radially outwards for submersible units. If the polarity is shifted, the ultrasonic power will be transmitted radially inwards to be used for e.g. deaeration, sterilization, homogenization and improved reactivity between different substances in a liquid pumped through the open area 8 inside the inner pipe. If the transducer unit is intended to be used for radiation outwards, the open area 8 inside the inner pipe will have to be used for the circulation of a cooling medium since the energy added to the inner pipe via the ceramic pipe will cause a temperature increase of pipe and ceramic and will have to be transferred away since it should otherwise increase the temperature of these parts to an unaccep¬ table level .

Since the transducer unit is manufactured in the form of a cylindrical bar, the ultrasonic energy for submersible units will be emitted radially outwards, which means that the ultrasonic energy will be evenly distributed within the surrounding liquid. Point 6 of the transducer specification where this need has been identified has therefore been met in an ideal way. It offers possibilities to work with very high output power. If one uses ceramic rings with an outside diameter of 76 mm and with a wall thickness of 6,35 mm, a submersible unit could emit 10 kW and maybe up to 20 kW per meter transducer unit. For transducer units with transmission radially inwards, one reaches very high ultrasonic effect into the liquid pumped through the pipe even at very high rate of flow. This opens up completely new possibi¬ lities for the use of ultrasonic energy in industrial processes. These levels of output power are completely impossible to achieve using the present ultrasonic transducers as per Fig. 1-3, which are normally limited to max 200 W and it therefore constitutes a completely new thinking within the ultrasonic field of technology.

The above mentioned transducer structure is not limited to com¬ pletely round profiles but can of course be used for all shapes of profiles, elliptical, quadrangular, hexagonal etc. where the ceramic part has a hole inside it so that by shrinking can achieve a precompression of the ceramic, so that it can be used for high power output installations. Several areas of application have been mentioned on page 1 of this document but the technology which has been described in this patent will make many other application areas possible. The expert in the matter can find many other areas of application for this high power technology but these are intended to be within the scope of this invention.

Claims

1. Ultrasonic transducer comprising a core (1) , a vessel (2), and at least one piezoelectric element (3) pressed therebetween, one or more cable entries (5) being extended through the core (1) , characterized by that the vessel (2) is a bottom vessel, that the core (1) and the bottom vessel (2) are made from a rigid material, and that the piezoelectric elements (3) are air and water proof compressed with an evenly distributed permanent static pressure, the pressure being maintained by a stable connection between the core (1) and the bottom vessel (2) such as crimping, welding, glueing, screwing.

2. Method for mounting ultrasonic transducers comprising a core

(1) a bottom vessel (2) and at least one piezoelectric element

(3) , characterized by that the piezoelectric elements (3) are located against the bottom of the bottom vessel (2) , that the core (1) is entered into the bottom vessel (2) and that in this connection electrical connections to the piezoelectric elements

(3) are guided up through one or more cable entries (5) in the core (1) , that the core (1) and the bottom vessel (2) are so compressed that the piezoelectric elements (3) are subjected to a predetermined pressure, and that when the predetermined pressure has been reach, the core (1) and the bottom vessel (2) are firmly locked to each other by crimping, welding, glueing, screwing or similar.

3. Method according to claim 2, characterized by that the pressure build-up is accomplished prior to the time the shrink connection has occurred, and that it is maintained until this pressure has been made permanent by locating of core (1) and bottom vessel (2) by means of welding, glueing, screw connection or the like between these two parts.

4. Method according to claim 1 or 2, characterized by that the desired pressure is achieved by such a threading of the core (1) and bottom vessel (2) that screwing of the parts (1,2) together with a torque which results in that the desired pressure onto the piezoelectric elements (3) between core (1) and bottom vessel (2) is achieved, which is equal to the desired pressure.

5. Method according to any of claims 2-4, characterized by that the cable entry or entries (5) is or are completely filled with a sealing material preferably epoxy based so that the ultrasonic transducer will become completely liquid and gas proof and therefore suitable for submersible installations.

6. Method according to any of claims 2-5, characterized by that the cable connections of the ultrasonic transducer to the supply cable are drawn via a connector part (7) which is screwed water proof onto the bottom vessel (2) , which is extended to a protecting pipe which furthermore has been screwed water proof onto the next connector part so that one gets an installation, which is submersible and can be exposed to high pressure cleaning.

7. Method according to any of claims 2-6 intended for external mounting onto tanks for liquids, characterized by that the ultrasonic transducer is screwed into a mounting ring (6) , which in turn has been welded onto the tank, a layer of colloidal metal carried by an adhesive, preferably silver, and that the transducer is screwed against the liquid tank with a torque, that gives a good metal to metal contact and where the colloidal metal compensates for the unevennesses between ultrasonic transducer and metal wall so that the entire unit forms one single metal body with enclosed compressed piezoelectric elements.

8. Ultrasonic transducer according to claim 1, characterized by that the vessel is an outer cylindrical pipe (2) , that the core is an internal cylindrical element (3) , the outer pipe (2) and the core (3) being mutually concentric, and that at least one tubular piezoelectric element (1) is compressed between the cylindrical pipe (2) and the core.

9. Ultrasonic transducer according to claim 8, characterized by that the core (3) is an internal tubular element.

10. Method for mounting an ultrasonic transducer comprising an outer pipe (2) , and internal pipe (3) and one or more tubular piezoelectrical elements (1) located between the outer pipe (2) and the core (3) , characterized by that all diameters are so combined that the outer pipe (2) and the core (3) are heated to such a desired temperature that when mounting, they are possible to be shrinked fixedly around the piezoelectrical element or elements (1) and subject this or these for a pressure giving rise to the desired compression thereof.

11. Method according to claim 10, characterized by that the outer pipe (2) is made from two pipe segments of such dimensions that they, after compression, by applying a desired compression thereupon, thereafter are welded together.