GB2484702A

GB2484702A - Ultrasonic transducer with exterior impedance matching layer, for use in corrosive environments

Info

Publication number: GB2484702A
Application number: GB201017735A
Authority: GB
Inventors: Jingdong Zhang
Original assignee: Mobrey Ltd
Current assignee: Rosemount Measurement Ltd
Priority date: 2010-10-21
Filing date: 2010-10-21
Publication date: 2012-04-25
Also published as: GB201017735D0

Abstract

An ultrasonic transducer 20, suitable for use in corrosive environments, comprises one or more sensor elements 21 which are isolated from the surrounding environment by a Âcorrosion resistant housing 22 made from a material such as polyvinylidene fluoride (PVDF). An impedance matching layer 26, preferably comprising PVDF foam, is affixed to the non-corrosive housing 22 on the exterior side of a window 23 in the housing 22 which is remote from the sensor elements 21. A second impedance matching material 25 may be provided between the sensor elements 21 and the interior surface of the window 23. Ideally, the thickness of the first impedance matching layer 26 is equal to an odd multiple of a quarter wavelength of ultrasound at the working frequency of the transducer 20.

Description

IMPROVEMENTS IN OR RELA TING TO ULTRASONIC TRANSDUCERS

Field of the Invention

This invention relates to ultrasonic transducers.

Background to the Invention

An ultrasonic transducer is one of the key components in an ultrasound-based device used, for example, for the measurement of level. As is well known such devices transmit ultrasound waves to a target and, in turn, receive the waves reflected back from the target.

Referring to Figure 1, a typical prior art transducer comprises one or more sensor elements 10 within a tubular housing 11, one end 12 of the housing defining a window through which ultrasound waves generated by the sensor elements are transmitted, and echo signals are received. The sensor elements, which are typically piezoelectric elements, are surrounded within the housing by potting compound 13.

The potting compound not only physically locates the sensor elements but also provides a vibration damping function. In the known manner one or more matching layers 14 arc provided within the housing, between the sensor elements and the window, to provide a gradual change in impedance between the sensor elements and the air 15 surrounding the transducer.

To render the transducer suitable for use in hazardous, particularly corrosive, environments the window 12 is preferably defined by or covered with a thin plate formed from a non-corrosive material such as polyvinylidene fluoride (PVDF). The plate is typically formed integrally with the housing 11 and it will be appreciated that the plate also protects the matching layers 14 and sensor elements 10 from physical damage.

Whilst the transducer described above exhibits a robust resistance to corrosive effects, there is a significant impedance mis-match between the plate (window) and the surrounding air 15. This results in a significant loss in signal strength which, in turn, limits the range of the instrument.

In order to enhance the range of the transducer one could increase the power supplied to the sensor elements. However, in reality this is not an option because transducers of this type are commonly wired into an industiy-standard 4-2OmA power supply which imposes a limit on the power available. As an alternative one could increase the diameter of the window 12, and of the sensor clement, and then drive the sensor elements at a lower frequency to increase the range since the acoustic absorption is proportional to the square of the working frequency. Again this is not a realistic option as, in many instances, the aperture provided for insertion of the instrument will be limited to about 51mm.

It is an object of this invention to provide a method of providing or adapting an ultrasonic transducer; andlor an ultrasonic transducer so provided or adapted which will go at least some way in addressing the aforementioned problems; or which will at least provide a novel and useful choice.

Summary of the invention

Accordingly, in one aspect, the invention comprises a method of enhancing the range of an ultrasound transducer, said transducer having a one or more sensor elements operable to transmit and receive ultrasound waves; and a window formed of a corrosion-resistant material through which, in use, ultrasound waves are transmitted and received, said method being characterised in that it includes providing an impedance matching material on that side of said window remote from said one or more sensor elements.

Preferably said window is included in the wall of a corrosion resistant housing, said method comprising locating said one or more sensor elements within said housing and providing impedance matching material on an outer surface of that part of said housing defining said window.

Preferably said method further includes providing one or more second impedance matching materials between said one or more sensor elements and an inner surface of that part of said wall which defines said window.

Preferably impedance matching material comprises a foam-based material.

Preferably said method further includes forming a thin skin on that surface of said impedance matching material not in contact with said window.

Preferably said method fUrther includes selecting or configuring the thickness of said impedance matching material to achieve the optimum echo signal strength.

In a second aspect, the invention provides an ultrasound transducer having one or more sensor elements operable to transmit and receive ultrasound waves; and a window formed from a corrosion-resistant material through which, in use, ultrasound waves are transmitted and received, said transducer being characterised in that it includes an impedance matching material on that side of said window remote from said one or more sensor elements.

Preferably said transducer includes a housing formed from a corrosion-resistant material, said one or more sensor elements being positioned within said housing.

Preferably said window is integral with a wall surface of said housing, wherein said impedance matching material is fixed to an outer surface of said window.

Preferably said impedance matching material is adhered or otherwise bonded to said window.

Preferably said housing is formed from a solid material composition and said impedance matching material comprises a foamed form of said material composition.

Preferably said impedance matching material is formed or selected to have one or more of the following properties: closed cells, ageing and ultra violet resistance, resistance to solvents and chemicals.

Preferably said housing is formed from solid polyvinylidene fluoride (PVDF) and said impedance matching material comprises a PVDF foam.

Preferably that surface of said impedance matching material not in contact with said window is provided with a thin skin.

Preferably the thickness of said impedance matching material is substantially equal to an odd multiple of a quarter wavelength of ultrasound in said impedance matching material at the working frequency of the transducer.

Preferably said impedance matching material comprises a disk having a periphery, said transducer further including a structure surrounding said periphery to the thickness of said impedance matching material.

Many variations in the way the present invention can be performed will present themselves to those skilled in the art. The description which follows is intended as an illustration only of one means of performing the invention and the lack of description of variants or equivalents should not be regarded as limiting. Wherever possible, a description of a specific element should be deemed to include any and all equivalents thereof whether in existence now or in the future. Any limitation in the scope of the invention should be established by reference to the appended claims alone.

S

Brief Descr iption of the Drawings The various aspects of the invention, in one preferred form, will now be described with reference to the accompanying drawings in which: Figure 1: shows a schematic cross-section of a typical prior art ultrasound transducer; Figure 2: shows a schematic cross-section of an ultrasound transducer according to the invention; Figures 3: illustrate the variation in echo size as a function of impedance to 5 matching material density; Figure 6: shows the variation of echo size with temperature for a given thickness of impedance matching material; and Figure 7: shows the variation of echo size with impedance matching material thickness.

Detailed Description of Working Embodiment

The typical prior art transducer shown in Figure 1 has been described above. As described above, the piezoelectric sensor assembly and/or any matching layer introduced to provide impedance matching, are exposed to the surrounding environment through window 12.

The present invention has been devised, in particular, to improve the range of ultrasonic transducers intended for use in corrosive environments. In such environments not only must the piezoelectric crystals and any matching layer be isolated from the corrosion-inducing materials, but the loss of signal strength, and thus range, arising from the introduction of the isolating material, must also be addressed. This loss of signal strength and operating range arises from the impedance mismatch between the isolating material and the environment surrounding the transducer.

When using such a transducer to measure fluid level and or flow rates in hazardous industrial applications, for example in environments containing corrosive fluids and/or gases, corrosion resistance can be introduced with relative ease by manufacturing the transducer housing from corrosion-resistant plastics materials such as, for example, polyvinylidene fluoride (PVDF). However, because of the surface rigidity of the PVDF housing, there is a significant mismatch in acoustic impedance between the PVDF housing (about 3.4x1 06 Rayl) and the air surrounding the transducer (3.3x102 Rayl). As described above, this mismatch leads to a significant loss of echo strength, and thus operating range.

Significantly we have found that impedance matching can be effected in such situations by applying one or more impedance matching materials to the exterior of the housing. For transmitters having a PVDF or like housing, a foam-based material can be affixed to that part of the exterior wall of the housing that defines the ultrasound window. i.e. that part of the housing through which ultrasound waves are transmitted and echo signals are received. Intuitively this is undesirable in corrosive environments as it would be expected that the matching layer would rapidly degrade.

However we have found that certain materials can provide both resistance to degradation as well as the desired impedance matching.

Referring to Figure 2, an ultrasound transducer 20 is shown comprising a piezoelectric sensor assembly 21 contained within a PVDF housing 22 such that, in use, ultrasound energy created by operation of the sensor assembly 21 is transmitted and received back through a window 23 in the housing 22. In this instance the window' 23 is integral with the housing and is defined in an end wall of the housing 22.

In the known manner a potting compound 24 surrounds the transducer assembly and an impedance matching layer 25 is provided between the piezoelectric sensor assembly 21 and the window 23 so as to provide a graduated change in impedance between the sensor assembly and the housing.

According to the invention, further impedance matching material 26 is provided on the outer surface of the window 23. This further impedance matching material 26 is preferably formed from one or more layers of a foam-based material. The outer surface 27 of the layer 26 is preferably formed or covered with a thin skin 27 to provide better mechanical strength and improve resistance to impacts. A protection ring 28, also formed of a plastics material such as PVDF may be provided about the periphery of the impedance matching 26, again to protect the edges of the impedance matching material.

The impedance matching 26 is conveniently formed as a disc of foam material which is adhered, welded or even spray coated on to the outer surface of window 13. The foam layer should be resistant to the corrosive environment to which, in use, it is exposed, should also be resistance to ultraviolet (UV) degradation and other aging effects and should provide a gradual reduction in impedance between that of the housing 22, and that of air. Whatever means is chosen to fix the matching material 26 to the window 13, should also be corrosion-resistant.

We have found that extremely beneficial effects can be achieved by forming the matching layer 26 from ZOTEK®F foams manufactured by Zotefoams plc (www.zotefoams.com). These foams are lightweight, closed cell PVDF foams that high UV and ageing resistance, high dielectric strength and high resistance to a wide range of solvents and aggressive chemicals. They can work effectively over a temperature range of -40°C to +80°C.

ZOTEK®F foams are available in sheet form in densities of 30, 40 and 75 kg/rn3.

When applied to a transmitter having an PVDF housing, the relative acoustic impedances are: Air 3.3xlO2Rayl PDVF Housing 3.4 x 106 Rayl ZOTEK®F 30 -8 x i0 Rayl ZOTEK®F 40 -2 x io Rayl ZOTEK®F 75 -5 x io Rayl Referring now to Figures 2 to 4, these show test results obtained from transducers mounted in a im anechoic test box and provided with external matching layers 16 of ZOTEK®F 30, ZOTEK®F 40 and ZOTEK®F 75 respectively. In each case tests were undertaken with different thicknesses of foam.

It will be noted that the optimum thickness of foam depends on the density of the foam. For ZOTEK®F 30 density foam the optimum thickness is about 1.5mm, for ZOTEK®F 40 density foam the optimum thickness is about 3mm whilst for ZOTEK®F 75 density foam the optimum thickness is about 3.5mm. At the optimum thicknesses, the provision of matching layers made from all grades of ZOTEK®F foam considerably enhance echo strength when compared with the performance obtained when no external matching layer is present.

Figures 3 to 5 also show plots of the resonant frequency of the ultrasound transducer, varying with thickness, in the various grades of ZOTEK®F foam. It can be seen that the echo size is highest when the thickness of the foam approximates an odd multiple of a quarter wavelength of the ultrasound in that particular material.

Turning now to Figure 6, it will be noted that, as expected, echo size varies with temperature and that, with most of the grades and thicknesses tested, there is a marked improvement in echo size, using an external matching layer as described, at temperatures over 0°C. The optimum effect of the external matching layer arises at temperatures of around 3 0°C, after which the advantageous effects decline.

Referring now to Figure 7, this shows an attempt to find the optimum foam thickness, for a given foam grade, over a range of temperatures. The tests shown here were conducted using ZOTEK®F foam of 75km1m3 density. Three different tests were carried out using layers of thickness 3.5mm, 3.8mm and 4,1mm.

As can be seen, the optimum thickness over the greatest temperature range is 3.5mm.

Between -10°C and +70°C echo size declines with increasing thickness.

It can thus be seen that the invention, at least in the case of the embodiment described, provides a method, and an ultrasound transmitter when formed using the method, which results in enhanced echo amplitudes being realised in those instances where the transmitter must be configured or adapted to operate in corrosive environments.

Claims

Claims 1. A method of enhancing the range of an ultrasound transducer, said transducer having one or more sensor elements operable to transmit and receive ultrasound waves; and a window formed of a corrosion-resistant material through which, in use, ultrasound waves are transmitted and received, said method being characterised in that it includes providing an impedance matching material on that side of said window remote from said one or more sensor elements.
2. A method as claimed in claim 1 wherein said window is included in the wait of a corrosion-resistant housing, said method comprising locating said one or more sensor elements within said housing and providing impedance matching material on an outer surface of that part of said housing defining said window.
3. A method as claimed in claim 2 further including providing one or more second impedance matching materials between said one or more sensor elements and an inner surface of that part of said wall which defines said window.
4. A method as claimed in any of the preceding claims wherein said impedance matching material comprises a foam-based material.
5. A method as claimed in any one of the preceding claims further including forming a thin skin on that surface of said impedance matching material not in contact with said window.
6. A method as claimed in any one of the preceding claims further including selecting or configuring the thickness of said impedance matching material to achieve the optimum echo signal strength.
7. An ultrasound transducer having one or more sensor elements operable to transmit and receive ultrasound waves; and a window formed from a corrosion-resistant material through which, in use, ultrasound waves are transmitted and received, said transducer being characterised in that it includes an impedance matching material on that side of said window remote from said one or more sensor elements.
8. A transducer as claimed in claim 7 including a housing formed from a corrosion resistant material, said one or more sensors being positioned within said housing.
9. A transducer as claimed in claim 8 wherein said window is integral with a wall surface of said housing, wherein said impedance matching material is fixed to an outer surface of said window.
10. A transducer as claimed in any one of claims 7 to 9 wherein said impedance matching material is adhered or otherwise bonded to said window.
11. A transducer as claimed in any one of claims 8 to 10 wherein said housing is formed from a solid material composition and wherein the impedance matching material comprises a foamed form of said material composition.
12. A transducer as claimed in any one of claims 7 to 11 wherein said impedance matching material is formed or selected to have one or more of the following properties: closed cells, ageing and ultra violet resistance, resistance to solvents and chemicals.
13. A transducer as claimed in claim 11 wherein said housing is formed from solid polyvinylidene fluoride (PVDF) and said impedance matching material comprises a PVDF foam.
14. A transducer as claimed in any one of claims 7 to 13 wherein that surface of said impedance matching material not in contact with said window is provided with a thin skin.
15. A transducer as claimed in any one of claims 7 to 14 wherein the thickness of said impedance matching material is substantially equal to an odd multiple of a quarter wavelength of ultrasound in said impedance matching material at the working frequency of the transducer.
16. A transducer as claimed in any one of claims 7 to 15 wherein said impedance matching material comprises a disk having a periphery, said transducer further including a structure surrounding said periphery to the thickness of said impedance matching material.