The present invention relates generally to ultrasonic sensors, and more specifically, to systems and methods for providing protection and acoustic impedance matching in an ultrasonic transducer.
Ultrasonic transducers, particularly those used in industrial applications, typically have to meet certain minimum safety requirements and standards. Such requirements include protection from flying particles and inadvertent or accidental impacts, resistance to corrosion from liquids to be contacted, chemical inertness so as not to contaminate the liquid to be measured, a wide range of tolerance to static or dynamic pressure variations, and/or a wide range of operational or storage temperatures.
Current solutions for protecting ultrasonic transducers include the addition of a protective layer, such as a metallic protective layer, attached generally to the front of a piezoelectric element of the transducer. In these structures, acoustic wave reflections arise at two locations. A first reflection occurs generally at a boundary between the protection layer and a bonding layer. A second reflection also occurs at a boundary between the protection layer and radiation medium. The two distinct locations of these reflections result in a sharp resonant peak, reduced sensitivity and ringing of the received signal.
Moreover, ultrasonic transducers often utilize various forms of acoustic impedance matching solutions or acoustic impedance converters. These may include single or multiple layer structures of polymer and/or metal materials. In particular, acoustic impedance converters may utilize one or more one-quarter (¼, or “quarter”) wavelength thick layers for providing a desired acoustic impedance conversion. Moreover, acoustic impedance converters may utilize relatively thin metal and polymer layers on the order of, for example, one-thirty seventh ( 1/37) and one-sixteenth ( 1/16) of a wavelength in thickness, respectively, for achieving acoustic impedance conversion. These converters lack suitability as protective structures due to the minimal thickness of their metal layers. Moreover, as their metal and polymer layers are configured (e.g. sized) to function cohesively as an impedance converter having targeted characteristics, altering the thickness of the metallic layers for improving protection would lead to degraded intended performance.
Improved structures offering suitable protection, chemical inertness and resistance to thermal degradation and/or fatigue, as well as improved sensitivity and wideband operation of the transducer are desired.
In one embodiment of the present disclosure, an ultrasonic transducer is provided. The transducer includes a piezoelectric element, an acoustic matching layer and a front metal or protective layer. The acoustic matching layer is arranged on a surface of the piezoelectric element and has a thickness of at least one-quarter of a wavelength of a center resonant frequency of the transducer. The front metal layer is arranged on a surface of the acoustic matching layer opposite that of the piezoelectric element and has a thickness approximately equal to one-half (½, or “half”) of the wavelength of the center resonant frequency.
An ultrasonic transducer according to another embodiment of the present disclosure includes a piezoelectric element, an acoustic matching layer formed on a surface of the piezoelectric element, and a front protective structure. The acoustic matching layer comprises a thickness of at least one-quarter of a wavelength of a predetermined center resonant frequency of the transducer. The front protective structure is formed on a surface of the acoustic matching layer opposite that of the piezoelectric element and includes a first metal layer, a second metal layer, and a polymer layer arranged between the first and second metal layers.
A further embodiment of the present disclosure includes a method of forming an ultrasonic transducer. The method includes the steps of providing a piezoelectric element, a matching layer and a metal or protective layer. The matching layer is coupled to the piezoelectric element and has a thickness of at least one-quarter of a wavelength of a predetermined center resonant frequency of the transducer. The metal layer is arranged on a surface of the acoustic matching layer opposite that of the piezoelectric element, and has a thickness equal to one-half of the wavelength of the predetermined center resonant frequency.
It is to be understood that the figures and descriptions of this disclosure have been simplified to illustrate elements that are relevant for a clear understanding of the disclosed subject matter, while eliminating, for purposes of clarity, many other elements found in typical ultrasonic transducer-based measuring systems. However, because such elements may be well-known in the art, or because they do not facilitate a better understanding of the subject matter, a discussion of such elements is not provided herein. The disclosure herein is directed to all such variations and modifications known to those skilled in the art.
In the following detailed description, reference is made to the accompanying drawings that show, by way of illustration, embodiments in which the invention may be practiced. It is to be understood that the various embodiments of the invention, although different, are not necessarily mutually exclusive. Furthermore, a particular feature, structure, or characteristic described herein in connection with one embodiment may be implemented within other embodiments without departing from the scope of the invention. In addition, it is to be understood that the location or arrangement of individual elements within each disclosed embodiment may be modified without departing from the scope of the invention. The following detailed description is, therefore, not be taken in a limiting sense, and the scope of invention is defined only in the appended claims, appropriately interpreted, along with the full range of equivalents to which the claims are entitled. In the drawings, like numerals refer to the same or similar functionality through several views.
In general, when a protective layer (e.g. a metal layer) is arranged at the front of a transducer's piezoelectric element and the protective layer has direct contact to the radiation medium (e.g. for reasons of chemical inertness and cleanliness), the impedance of the protective layer is significantly higher than that of the transducer or piezoelectric layer. In order to maximize performance of the transducers utilizing a protective layer, improved design principles for creating suitable matching layers are needed. For example, one-half wavelength protective layers (i.e. metallic layers) may be utilized for achieving maximum acoustic wave propagation and resonance, as one-half wavelength thick layers do not convert impedance. These one-half wavelength thick metallic layers may be bonded directly to the piezoelectric body, or may be bonded thereto by a thin coupling gel or epoxy. When the thickness of this bonding layer is very thin, coupling efficiency becomes high, as does the sensitivity, thereby increasing the performance of the transducer.
However, thin bonding layers often experience thermal problems. For example, the thermal expansion coefficient(s) of stainless steel are about one order greater than that of a typical piezoelectric ceramic (PZT) material. Therefore, degradation of the sensor properties may occur due to internal strain, fatigue or delamination, while being operated or stored in an environment open to a wide temperature variation/range. Accordingly, bonding materials utilized between the transducer layer and a protective layer should be sufficiently thick so as to absorb any thermal expansion difference, however, they also need to be sized to maximize transducer performance.
An embodiment of the present disclosure relates to a highly-sensitive, wideband ultrasonic transducer utilizing a one-half wavelength thick protective metal layer mounted at the front end of a piezoelectric element. In another embodiment, a metal-polymer-metal structure may be implemented as an alternative to a one-half wavelength metal layer and may be featured by significantly broader bandwidth. A matching or coupling layer is provided between the protective layer (or protective structure) and the piezoelectric element. The matching layer is preferably embodied as an electrically conductive epoxy material configured to increase signal bandwidth as well as provide a means to form a connection to ground.
In operation, vibrations in the piezoelectric material of an ultrasonic transducer are transmitted through the matching or coupling layer and the protective layer, exciting an acoustic wave in an associated propagation medium (typically liquid) from the surface of the protective layer. The transmitted acoustic wave may be reflected by an object and subsequently received by the same transducer. Conventional circuitry for processing the received signal may also be provided. In another mode of operation, the excited acoustic wave is received by another discrete transducer having a similar structure and having circuitry for processing the received signal. In any event, based on the received signal and/or the determined propagation time of the signal, certain physical properties of the medium (e.g. liquid) may be measured and analyzed. In addition, based on the received signal and/or the determined propagation time of the signal, certain physical properties including the distance of an object from a given transducer or other structure may be measured and analyzed.
The transmission efficiency of an acoustic wave through a metal layer is generally very low when the metal layer is arranged between other dissimilar materials (e.g. polymer or water) as a result of the strong reflections at the material boundaries. For a metal layer contacting the radiation medium at an outer side, when the thickness of the metal layer is equal to one-half wavelength, the mechanical impedance of the metal layer seen from the transducer side becomes equal or substantially equal to that of the outside material (e.g. water or other liquid). The terms equal or substantially equal may be understood in the context of an idealized condition of a metal without loss, but actually a very small loss in the metal may make for a slight difference; however, such difference is negligible for determining mechanical impedance. Thus, a metal layer contacting the radiation medium at an outer side, when the thickness of the metal layer is equal or substantially equal to one-half wavelength, the mechanical impedance of the metal layer seen from the transducer side becomes equal or substantially equal to that of the outside material (e.g. water or other liquid).
In order to improve the transmission efficiency and to increase transducer bandwidth, a matching layer is inserted between the transducer material and the one-half wavelength thick protective metal layer. In an embodiment, the design of the matching layer includes: (1) thickness between one-quarter of a wavelength and four-tenths times the wavelength; in other words (1−1.7)×¼ of wavelength; (2) impedance between that of the transducer material and radiation material; (3) the material is an epoxy-based or like material such as an epoxy or powder loaded epoxy or conductive epoxy; (4) conductive epoxy or like material is used as an electrical connection between the transducer electrode and reference (ground) potential; (5) elastically soft conductive epoxy or like material designed as matching layer is provided with appropriate thickness which absorbs thermal expansion difference between the protection metal layer and transducer layer (e.g. piezoelectric ceramic material layer). To further improve bandwidth, the one-half wavelength metal protective layer may be replaced by a metal-polymer-metal structure, example of which is composed of stainless steel-polyimide-stainless steel layers each having a thickness of approximately 1/17 of the wavelength, and a polyimide layer having a thickness of approximately 1/20 of the wavelength. The matching layer is inserted between the transducer or piezoelectric element layer and protective layer to provide bandwidth several times broader than the design of a one-half wavelength protective metal layer.
Reference will now be made in detail to the present exemplary embodiments of the disclosure, examples of which are illustrated in the accompanying drawings.
Transducer 100 may be configured to be operated over a wide temperature range with a storage temperature also covering a wide range. As the thermal expansion of stainless steel (or other typical metals) is 15-17×10−6/deg. C. and piezoelectric ceramic has approximately 1/10 of this magnitude, matching layer 104 should be sufficiently thick so as to absorb resulting thermal expansion differences. If matching layer 104 is designed to be thin in order to obtain a relatively high sensitivity coming from a one-half wavelength design of a metal protective layer, internal strain at extreme temperatures would lead to a degradation of performance, due at least in part to the change of material performance, such as fatigue, delamination, internal cracking, and the like. Therefore, matching layer 104 should relatively soft or deformable, but not too thin.
Resulting distributions of stress and displacement in a one-half wavelength protective layer 401 (e.g. a metal layer) are shown in
Referring generally to
By way of example only, upon calculation of an exemplary embodiment, the outer stainless steel layers (shown in the embodiment as the same metal of uniform thickness) should be approximately 0.057 of the wavelength (i.e. 100 μm) for a 3.3 MHz design), and the internal polyimide layer comprises a thickness of 0.053 of the wavelength (i.e. 35 μm at 3.3 MHz or 25 μm polyimide and 5 μm epoxy at both sides for bonding). Thus, a single layer design of 900 μm thick metal layer stainless steel at 3.2 MHz-3.3 MHz may be replaced with a thinner structure of only 235 μm total thickness layer (i.e. a 35 μm polyimide layer sandwiched between two 100 μm thick stainless steel layers). At lower operating frequencies (e.g. at 1.0 MHz), the thicknesses of the layers should be designed to be thicker. For example, in such an exemplary embodiment, the outer layers of stainless steel comprise a thickness of 330 μm and the internal polyimide layer comprises a thickness of 115 μm (or 105 μm polyimide and 5 um epoxy at both sides). For other frequencies, the thickness can be designed with a relation of inverse proportionality.
In analogous fashion, materials sufficiently similar to the polymer layer 507 shown in
Thus, there is disclosed herein a system, device and method of transducer structure and functions wherein an impedance matching layer is inserted between a metallic protective layer and a transducer layer. The thickness of the matching layer is in the range of 0.25-0.4 of the wavelength in the material. The acoustic impedance of the matching layer is between those of the radiation medium and transducer material. In addition:
While the foregoing invention has been described with reference to the above-described embodiment, various additional modifications and changes can be made without departing from the spirit of the invention. Accordingly, all such modifications and changes are considered to be within the scope of the appended claims. Accordingly, the specification and the drawings are to be regarded in an illustrative rather than a restrictive sense. The accompanying drawings that form a part hereof, show by way of illustration, and not of limitation, specific embodiments in which the subject matter may be practiced. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments may be utilized and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. This Detailed Description, therefore, is not to be taken in a limiting sense, and the scope of various embodiments is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled.
Such embodiments of the inventive subject matter may be referred to herein, individually and/or collectively, by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed. Thus, although specific embodiments have been illustrated and described herein, it should be appreciated that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations of variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the above description.
This application claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 62/159,559 filed May 11, 2015, which is incorporated by reference herein in its entirety for all purposes.
Number | Name | Date | Kind |
---|---|---|---|
4166967 | Benes | Sep 1979 | A |
4211948 | Smith et al. | Jul 1980 | A |
4383194 | Ohigashi et al. | May 1983 | A |
4446395 | Hadjicostis | May 1984 | A |
4507582 | Glenn | Mar 1985 | A |
4603276 | Coursant | Jul 1986 | A |
4672591 | Breimesser et al. | Jun 1987 | A |
4712037 | Verbeek et al. | Dec 1987 | A |
4771205 | Mequio | Sep 1988 | A |
4881212 | Takeuchi | Nov 1989 | A |
4976150 | Deka | Dec 1990 | A |
5511550 | Finsterwald | Apr 1996 | A |
6160340 | Guo et al. | Dec 2000 | A |
6307302 | Toda | Oct 2001 | B1 |
6548942 | Panasik | Apr 2003 | B1 |
6772490 | Toda | Aug 2004 | B2 |
6989625 | Suzuki et al. | Jan 2006 | B2 |
7956514 | Oakley et al. | Jun 2011 | B2 |
9050628 | Lautzenhiser | Jun 2015 | B2 |
20020027400 | Toda | Mar 2002 | A1 |
20040124746 | Suzuki et al. | Jul 2004 | A1 |
20040174095 | Bhardwaj | Sep 2004 | A1 |
20050001517 | Yogeswaren | Jan 2005 | A1 |
20050046311 | Baumgartner et al. | Mar 2005 | A1 |
20050061084 | Brun | Mar 2005 | A1 |
20050099097 | Baumgartner et al. | May 2005 | A1 |
20050127793 | Baumgartner et al. | Jun 2005 | A1 |
20050194865 | Angelsen et al. | Sep 2005 | A1 |
20050225211 | Oliver | Oct 2005 | A1 |
20050236930 | Hasegawa | Oct 2005 | A1 |
20070200460 | Scott | Aug 2007 | A1 |
20070222339 | Lukacs et al. | Sep 2007 | A1 |
20080007142 | Toda | Jan 2008 | A1 |
20090034370 | Guo | Feb 2009 | A1 |
20090062655 | Saito | Mar 2009 | A1 |
20090072668 | Gelly et al. | Mar 2009 | A1 |
20090147627 | Toda et al. | Jun 2009 | A1 |
20100066207 | Saito | Mar 2010 | A1 |
20100249670 | Smith et al. | Sep 2010 | A1 |
20100283355 | Shieh et al. | Nov 2010 | A1 |
20110050039 | Toda | Mar 2011 | A1 |
20110314933 | Mueller | Dec 2011 | A1 |
20140292941 | Kobayashi et al. | Oct 2014 | A1 |
20160296975 | Lukacs | Oct 2016 | A1 |
Number | Date | Country |
---|---|---|
06308104 | Nov 1994 | JP |
H10507600 | Jul 1998 | JP |
2008-85413 | Apr 2008 | JP |
200805611 | May 2008 | WO |
Entry |
---|
International Search Report and Written Opinion of the International Searching Authority, issued in related PCT/US2016/031931, dated Oct. 13, 2016. |
Toda, Minoru, “New type of matching layer for air-coupled ultrasonic transducers,” Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions on , vol. 49, No. 7, pp. 972,979, Jul. 2002. |
International Search Report dated Oct. 26, 2010 for related application No. PCT/US2010/046035. |
Kosinski, J. et al., “PZT-Based Stacked Acoustic Filters,” US Army Electronics Technology and Devices Laboratory, Proceeding of 8th IEEE International Symposium of Application of Ferroelectrics, pp. 281-284. |
Willatzen, Morten, “Ultrasound Transducer Modeling—Received Voltage Signals and the Use of Half-Wavelength Window Layers with Acoustic Coupling Layers,” IEEE Ultrasonics, Ferroelectrics and Frequency Control, vol. 46, No. 5, Sep. 1999, pp. 1164-1174. |
Supplementary Partial European Search Report, issued in related European Patent Application No. EP 16793476; dated Nov. 8, 2018. |
Number | Date | Country | |
---|---|---|---|
20160332198 A1 | Nov 2016 | US |
Number | Date | Country | |
---|---|---|---|
62159559 | May 2015 | US |