WO2007008428A2

WO2007008428A2 - Low-stress ultrasound transducer

Info

Publication number: WO2007008428A2
Application number: PCT/US2006/025253
Authority: WO
Inventors: Benjaman R. Johnson
Original assignee: Blackstone Ney Ultrasonics Inc
Current assignee: Blackstone Ney Ultrasonics Inc
Priority date: 2005-07-08
Filing date: 2006-06-28
Publication date: 2007-01-18
Anticipated expiration: 2008-01-08
Also published as: WO2007008428A3; TW200716266A

Abstract

A transducer includes a resonator assembly having a first surface and a second surface on opposite sides thereof, a front mass having a surface adjacent to the first surface of the resonator assembly, a back mass having a surface adjacent to the second surface of the resonator assembly, and a compression assembly mounted on the front mass and the back mass. The compression assembly is adapted to effect compression across the resonator assembly. The resonator assembly contains at least two driven active elements such that when at least one of the driven active elements is driven to increase in thickness, at least one of the other driven active elements is driven to decrease in thickness.

Description

LOW-STRESS ULTRASOUND TRANSDUCER

FIELD OF THE INVENTION

[0001] The present invention relates to ultrasound systems, and more particularly, to systems for generating high power ultrasonics energy and introducing the ultrasonics energy into fluid media for the purpose of cleaning and/or liquid processing.

BACKGROUND OF THE INVENTION

[0002] For years, ultrasonic energy has been used in manufacturing and processing plants to clean and/or otherwise process objects within liquids and to effect a process on a liquid. It is well known that objects may be efficiently cleaned by immersion in an aqueous solution and subsequent application of ultrasonic energy to the solution. Prior art ultrasound transducers include resonator components that are typically constructed of materials such as polarized piezoelectrics, ceramics, or magnetostrictives (aluminum and iron alloys or nickel and iron alloys). These resonator components are also referred to herein as active elements because they spatially oscillate at the frequency of an applied stimulating signal. The transducers are mechanically coupled to a tank containing a liquid that is formulated to clean or process the object of interest. The amount of liquid is adjusted to partially or completely cover the object in the tank, depending upon the particular application. When the transducers are stimulated to spatially oscillate, they transmit ultrasound into the liquid, and hence to the object. The interaction between the ultrasound-energized liquid and the object create the desired cleaning or processing action.

[0003] As shown in FIGS.1A-1D, one type of prior art ultrasound transducer includes one or more resonator components compressed between a front plate and a back plate, with the compression typically established by a nut and bolt assembly, with the bolt extending from the back plate, through the back plate, resonator components and the front plate and terminated by the nut adjacent to the front plate or terminated by a threaded bore in the front plate, which functions as a nut. FIG. IA shows a cross-sectional exploded view when the transducer is in a resting state, FIG. IB shows a cross-sectional view of the transducer when the resonator components are driven by the stimulating signal to increase in thickness, FIG.1C shows a cross- sectional view of the transducer when the resonator components are passing through the resting state position and FIG. ID shows a cross-sectional view of the transducer when the resonator components are driven by the stimulating signal to decrease in thickness. These four figures can be related to one cycle of the driver stimulating signal waveform as the zero degree, 90 degree, 180 degree and 270 degree positions, respectively. Observation of the transducer backmass shows significant movement at the 90 degree and 270 degree positions. The compression bolt is stretched and the resonator components are stressed at the 90 degree position. This movement, stretching and stress causes loss in the transducer and this loss is particularly significant at high frequency overtones of the fundamental frequency.

[0004] State of the art sandwich type transducers, also known as Langevin type transducers, operate in the manner shown in FIGS IA to ID. That is, the driven active elements, typically polarized piezoelectric ceramics, are driven to increase in thickness during one half cycle and driven to decrease in thickness during the next half cycle, resulting in the losses described above. These state of the art sandwich type transducers will be referred to herein as "resonant transducers" or "half wave resonant transducers" when used at their fundamental frequency and will be referred to as "harmonic transducers" when used at overtone frequencies.

SUMMARY OF THE INVENTION

[0005] In general, one or more ultrasound generators drive one or more ultrasound transducers or arrays of transducers, in accordance with the embodiments described herein, coupled to a liquid to clean and/or process a part or parts, or to produce a processing effect on the liquid. The liquid is preferably contained within a tank, and the one or more ultrasound transducers mount on or within the tank to impart ultrasound into the liquid. [0006] As defined in the technical literature, "ultrasound", "ultrasonic" and "ultrasonics" generally refer to acoustic disturbances in a frequency range above about eighteen kilohertz (khz) and which extend upwards to over five megahertz (Mhz). As is commonly used in the cleaning industry and as used herein, "ultrasonic" will generally refer to acoustic disturbances in a frequency range above about eighteen kilohertz and extending up to about 99khz. Ultrasound and ultrasonics will be used to mean the complete range of acoustic disturbances from about 18khz to 5Mhz, except when they are use with terms such as "lower frequency" ultrasound, "low frequency" ultrasound, "lower frequency" ultrasonics, or "low frequency" ultrasonics, then they will mean ultrasound between about 18khz and 99khz. "Megasonics" or "megasonic" refer to acoustic disturbances between about 351khz and 5Mhz. The prior art has manufactured "low frequency" and "megasonic" ultrasound systems. Typical prior art low frequency systems, for example, operate at 25khz, 40khz, and as high as 90khz. Typical prior art megasonic systems operate between 600khz and 2Mhz. Certain aspects of the invention apply to low frequency ultrasound and to megasonics. However, certain aspects of the invention apply to ultrasound in the 100khz to 350khz region, a frequency range which is sometimes denoted herein as "microsome" or "microsomes." The upper end of the microsome frequency range from about 300khz to 350khz is called herein "higher microsomes" or "higher frequency microsome". [0007] As used herein, "resonant transducer" means a transducer operated at a frequency or in a range of frequencies that correspond to a one-half wavelength (lambda) of sound in the transducer stack. "Harmonic transducer" means a transducer operated at a frequency or in a range of frequencies that correspond to 1 lambda, 1.5 lambda, 2 lambda or 2.5 lambda of sound, and so on, in the transducer stack. The harmonics of a practical physical structure are often not exact integer multiples of the fundamental frequency, the literature sometimes refer to these non- integer harmonics as overtones. Herein, harmonics will mean resonances higher in frequency than the fundamental resonant frequency. "Bandwidth" means the range of frequencies in a resonant or harmonic region of a transducer over which the acoustic power output of a transducer remains between 50% and 100% of the maximum value.

[0008] As used herein, "low-stress ultrasound transducer" means a transducer constructed or driven such that the active elements that are driven by a stimulating signal are not all driven to increase in thickness during one segment of time and are not all driven to decrease in thickness during some other segment of time. The active elements that are driven by a stimulating signal will hereinafter be called "driven active elements", note, a transducer may have active elements that are not driven during part or all of the time that a stimulating signal is applied to the transducer, these active elements during times that they are not driven will be called active elements or insulators. The term "polarized piezoelectric ceramic" will often be used herein interchangeably with driven active element, resonator, resonator component, resonator disc and disc resonators. This operation of the driven active elements within a low-stress ultrasound transducer typically causes a canceling or suppression effect at a frequency or in a range of frequencies that correspond to one-half wavelength of sound in the transducer stack. Therefore, when referring to a low-stress ultrasound transducer, fundamental frequency will mean the lowest frequency at which the assembly will resonate and harmonics, harmonic frequencies or overtones will mean resonances higher in frequency than the fundamental frequency. Bandwidth means the range of frequencies in the fundamental or a harmonic region of a low-stress ultrasound transducer over which the acoustic power output of the low-stress ultrasound transducer remains between 50% and 100% of the maximum value.

[0009] As used herein, "hz" refers to hertz which is cycles per second, "khz" refers to kilohertz and a frequency magnitude of one thousand hertz. "Mhz" refers to megahertz and a frequency magnitude of one million hertz.

[0010] As used herein, "stimulating signal" is generally an AC voltage at a frequency or in a range of frequencies that correspond to the bandwidth surrounding a fundamental or overtone frequency. A stimulating signal may be a fixed frequency, or it may sweep frequency. Sweeping frequency stimulating signals are typically obtained at the output of sweeping frequency generators. The simplest stimulating signal is single phase, that is, two wires or two connections supplying a single frequency voltage or sweeping frequency voltage. This invention uses single phase stimulating signals, but also uses multiple phase stimulating signals. For example, a split phase stimulating signal has two voltages that are 180 degrees out of phase and that are typically supplied on three wires or three connections. This split phase stimulating signal is often obtained at the output of a center tapped transformer. Three phase stimulating signals that are three voltages 120 degrees out of phase with each other are also used herein. In general, an n phase stimulating signal where n is an integer is applicable to this invention. This n phase stimulating signal has n voltages that are 360 divided by n degrees out of phase with each other.

[0011] As used herein, successive frequencies are said to "sweep" when the period or the half period of two or more of the waveforms are unequal to each other. [0012] Sweeping frequency generators change their output frequency through successive frequencies in a bandwidth, e.g., sweeping from the lowest frequency in a chosen bandwidth through the bandwidth to the highest frequency in the chosen bandwidth, then sweeping from this highest frequency through the bandwidth back to the lowest frequency. The function of time for these frequency changes is typically linear, but other functions of time, such as part of an exponential, are possible. As used herein, "sweep frequency" refers to the reciprocal of the time that it takes for successive frequencies to make a round trip, for example, change from one frequency through the other frequencies and back to the original frequency. Although sweep rate might technically be interpreted as the rate of change from one successive frequency to the next, the more common usage for sweep rate will be used herein; that is, "sweep rate" means the same as sweep frequency. It is generally undesirable to operate an ultrasound transducer at a fixed, single frequency because of the resonances created at that frequency. Therefore, an ultrasound generator can sweep the operational frequency through some or all of the available frequencies within the transducer's bandwidth at a "sweep rate." Accordingly, particular frequencies have only short duration during the sweep cycle (i.e., the time period for sweeping the ultrasound frequency up and down through a range of frequencies within the bandwidth). "Sweep the sweep rate" or "double sweeping" or "dual sweep" refer to an operation of changing the sweep rate as a function of time so that the sweep rate is non constant. "Random sweep rate" or "chaotic sweep rate" refer to sweep rates where the successive sweep rates are numbers that are described by no well defined function, i.e., random or chaotic numbers. [0013] The "resonator or resonator assembly" refers to the central components compressed between the front mass and the back mass in a sandwich type transducer. These central components typically include two or more driven active elements and one or more electrodes. In some embodiments the central components may also include one or more insulators (some of which may be non driven active elements), one or more masses, one or more heatsinking elements or electrodes that also can be used as heatsinking elements or masses. [0014] The present invention concerns the applied uses of ultrasound energy, and in particular the application and control of ultrasonics to clean and/or process parts within a liquid, or to impart a processing effect on the liquid. Generally, in accord with the invention, one or more ultrasound generators drive one or more ultrasound transducers, or arrays of transducers, coupled to a liquid to clean and/or process the part. The liquid is preferably held within a tank; and the transducers mount on or within the tank to impart ultrasound into the liquid. In this context, the invention is particularly directed to one or more of the following aspects and advantages: [0015] According to one aspect of the present invention, the transducer includes a resonator assembly having a first surface and a second surface on opposite sides thereof, a front mass having a surface adjacent to the first surface of the resonator assembly, a back mass having a surface adjacent to the second surface of the resonator assembly, and a compression assembly mounted on the front mass and the back mass. The compression assembly is adapted to fasten the front and back masses, biasing those masses toward each other and thereby effecting compression across the resonator assembly, hi one preferred form, at least one of the components of the resonator assembly that is a driven active element is reversed in polarity compared to known sandwich type transducers such that at least one of the components that is a driven active element is driven to decrease in thickness while other of the components that are driven active elements are driven to increase in thickness.

[0016] hi an alternative form, the resonator assembly components are wired to a switching device such as a relay which allows them to act as in a known sandwich type half wave resonant transducer during one state of the relay, i.e., the driven active elements all are driven to increase in thickness during one half cycle and all are driven to decrease in thickness during the next half cycle, and to act in the inventive way in the other state of the relay, i.e., at least one of the components that is a driven active element is driven to decrease in thickness while other of the components that are driven active elements are driven to increase in thickness during one half cycle and at least one of the components that is a driven active element is driven to increase in thickness while at least one other of the components that are driven active elements are driven to decrease in thickness during the next half cycle.

[0017] It is known to a person skilled in the art of ultrasound transducer design that electrodes are needed in the aspects of the invention described above and in aspects of the invention described later in this specification. In a known sandwich type resonant transducer or in the various forms of the inventive low-stress ultrasound transducer, there are many ways and positions in which to realize the needed electrodes. For example, in one of the simplest sandwich transducer designs containing two driven active elements, a back mass, a front mass, a compression assembly and driven by a single stimulating signal supplied by two wires, there are at least four different configurations of electrodes known to one skilled in the art and any of these might be employed in the design of a given transducer. Configuration one has one center electrode between the two driven active elements. Electrical contact is made to this electrode from the first wire from the stimulating signal and the second wire from the stimulating signal is connected to either the front mass or the back mass, or in the case that the radiating diaphragm is in electrical contact with the front mass, this second wire from the stimulating signal can be connected to the radiating diaphragm. Configuration two has a center electrode between the two driven active elements and a second electrode between the front mass and the surface of the driven active element that is adjacent to the front mass. The stimulating signal wires are connected to these two electrodes. Contact to the driven active element surface adjacent to the back mass is made by the electrical contact from the front mass, through the compression assembly, to the back mass which contacts the driven active element surface adjacent to the back mass. Configuration three has a center electrode between the two driven active elements and a second electrode between the back mass and the surface of the driven active element that is adjacent to the back mass. The stimulating signal wires are connected to these two electrodes. Contact to the driven active element surface adjacent to the front mass is made by the electrical contact from the back mass, through the compression assembly, to the front mass which contacts the driven active element surface adjacent to the front mass. Configuration four has a center electrode between the two driven active elements, a second electrode between the back mass and the surface of the driven active element that is adjacent to the back mass, and a third electrode between the front mass and the surface of the driven active element that is adjacent to the front mass. Electrical contact is made to the center electrode from the first wire from the stimulating signal and the second wire from the stimulating signal is connected to electrodes two and three. Since electrode configurations are well known to those skilled in the art and since there are many variations of electrode configurations that accomplish the same function, in this specification and in the claims of this invention the electrode configuration will sometimes be eliminated or partially defined where clarity results. It should be understood that the needed electrodes that are not detailed could be any of the many configurations known to those skilled in the art. It should also be understood that when adjacent surfaces are described, for example, "wherein said two polarized piezoelectric ceramics are arranged with said positive polarity surface of said first polarized piezoelectric ceramic adjacent to said negative polarity surface of said second polarized piezoelectric ceramic", there may be an electrode between these adjacent surfaces. For purposes of clarity herein, the described surfaces are still considered to be adjacent when there is an electrode between them, even when this electrode is not detailed and simply assumed as one included as part of the many possible electrode configurations. In the specific example given, there would be an electrode between the two polarized piezoelectric ceramics, i.e., the center electrode, and any of the four electrode configurations described above would be applicable for this example.

[0018] Various aspects of the low-stress ultrasound transducer can be implemented without the use of a center electrode. One example of this aspect of the invention has two driven active elements, one stacked on top of the other, with electrodes at the opposite ends of the stack. An insulator is typically needed at one end of this electrode and active element stack to prevent the back mass, compression assembly and front mass from shorting out the electrodes. The driven active elements are oriented such that when the electrodes are driven by a stimulating signal, one of the driven active elements is driven to increase in thickness while the other driven active element is driven to decrease in thickness. With this configuration of electrodes, the capacitance of driven active elements is in series, making the total transducer capacitance substantially one quarter the value of the value that results with one of the other four electrode configurations. This has the advantage of reducing current necessary from the stimulating signal source, a feature particularly valuable at frequencies in the higher microsomes or megasonic frequency ranges.

[0019] According to another aspect of the present invention, the tank bottom or other radiating surface is also used as the front plate or front mass of the transducer assembly, therefore, the compression assembly is mounted on the radiating surface and the back mass. The two or more driven active elements within the resonator assembly operate in the inventive way, i.e., at least one of the driven active elements are driven to decrease in thickness while the other driven active elements are driven to increase in thickness.

[0020] A person skilled in the art will realize that many of the descriptions of the preferred embodiments within this specification and many of the claims state or describe transducer operation over one half cycle of the stimulating signal. For example, a statement like, "at least one of said at least two driven active elements is driven to decrease in thickness while at least one of the other of said at least two driven active elements is driven to increase in thickness when said ultrasound transducer is driven by said at least one stimulating signal", is for one half cycle of the stimulating signal. It is clear to one skilled in the art that during the next half cycle of the stimulating signal the drive voltage reverses sign and the driven active elements that were driven to increase in thickness during the prior half cycle are now driven to decrease in thickness, and vice versa. This half cycle presentation of many of the embodiments and in many of the claims was adapted to simplify the wording for clarity of understanding of the invention. [0021] This new arrangement of the components that are driven active elements was not previously envisioned by those skilled in the state of the art because it causes a canceling of the half wave resonator effect at the fundamental half wave frequency of a sandwich type transducer. This effect was previously thought to be necessary for the operation of this type of transducer. However, the inventor of this new arrangement was able to visualize low stress and therefore low loss operation at overtone frequencies when one or more of the components that are driven active elements was arranged so it increased in thickness while the other components that are driven active elements decreased in thickness. He theorized that this effect would reduce motion of the bolt and other components within the transducer assembly and reduce stress within the driven active elements within the resonator assembly; therefore, reducing friction and other losses associated with these motions. This reduced loss would allow higher overtone frequencies that were over damped in prior state of the art transducers to become under damped and usable in this new low-stress ultrasound transducer. The inventor did experimentation to verify his theory and found that operation into the higher microsomes and the megasonic frequencies was possible with this new sandwich transducer construction.

[0022] According to one preferred embodiment, the resonator assembly includes at least two driven active elements. According to another preferred embodiment, the resonator assembly includes three or more driven active elements placed one on top of the other. [0023] According to another preferred embodiment, the compression assembly is mounted on central regions of the front and back masses. Ih one preferred form, the compression assembly includes a bias bolt and nut assembly, and the front mass, the resonator assembly, and the back mass define a bore extending along a central axis of the front mass, the resonator assembly, and the back mass for receiving the bolt. In some embodiments, the bolt is adapted to pass through the central bore of the stacked back mass, resonator assembly and front mass, and the nut is a discrete element, which is screwed onto a lead end of the bias bolt. In other embodiments, the bore defined in the front mass includes threads on an inner surface of said bores, and the lead end of the bias bolt is screwed into the threaded bore in the front mass, so that the front mass acts as the "nut". The bolt can extend from the front mass to a nut in or adjacent to the back mass or it can extend from the back mass to a nut adjacent to or within the front mass. The bias bolt and nut adjustably engage the back mass and the front mass so as to compress the two or more polarized piezoelectric ceramics between the back mass and the front mass. [0024] According to another aspect of the present invention, the compression assembly is mounted on peripheral regions of the front mass and the back mass. In one preferred form, the compression assembly includes at least two bolt and nut assemblies, and the front mass and the back mass define at least two bores at peripheral regions extending along axes parallel to a central axis of the front mass and the back mass for receiving the at least two bolts. The nut can be a discrete element that is adapted to be screwed on a lead end of a corresponding bolt, or alternatively, the bore in the front mass includes threads on an inner surface for engaging the threads on the lead end of the bolt, such that the front mass functions as the "nut". The bias bolts adjustably engage the back mass and the front mass so as to compress the two or more polarized piezoelectric ceramics between the back mass and the front mass.

[0025] The transducer may further include insulators disposed between the bolt and the resonator assembly, and electrodes connected to the resonator assembly.

[0026] According to a further aspect of the present invention, the sandwich type ultrasonic transducer preferably has a low-density back mass (i.e., aluminum, magnesium, etc.) and is used to produce a device with an especially wide bandwidth. This large bandwidth allows effective sweeping over a dramatically larger range of frequencies. This sweeping can be conventional linear sweep or one of the non constant sweeps known as double sweeping, dual sweep, random sweep, or chaotic sweep. A low-density back mass provides a larger surface area compared to that of a prior art steel back mass of the same acoustic length. This increased surface area also allows higher heat dissipation per transducer that in turn allows a higher overall power output at the overtone frequencies.

[0027] According yet another aspect of the present invention, the resonators are made from ceramic, preferably non-silvered polarized piezoelectric ceramic. Elimination of the oft-applied silver to the faces of the polarized piezoelectric ceramic is accomplished through a lapping process that ensures extreme flatness of the polarized piezoelectric ceramics. These flat non- silvered surfaces optimize utilization for high power applications. A transducer characterized by an especially high bandwidth may or may not contain non-silvered polarized piezoelectric ceramics. An example of another improvement is the incorporation of multiple concentric ceramic polarized piezoelectric elements in place of the solid ceramic polarized piezoelectric discs often used. In one application the size and geometry of these concentric cylindrical shells are tailored to ensure that the radial resonant frequencies of the polarized piezoelectric ceramics do coincide with that of the transducer assembly for maximized output at that frequency. In another application these concentric rings are tailored to ensure that the radial resonant frequencies of the polarized piezoelectric ceramics do not coincide with that of the transducer assembly to minimize strain at those frequencies. These polarized piezoelectric ceramics can be silvered or lapped free of silver. [0028] According to another aspect of the present invention, another improvement of the transducer is a deviation from cylindrical symmetry on any of the components for the reason of • yielding a device of extreme bandwidth as well as the manipulation/elimination of radial resonant frequencies. An example of this deviation from cylindrical symmetry includes slots on the sides of the front mass or elliptical masses. If properly implemented, deviation from cylindrical symmetry, including the addition of flats or slots on the sides of the high power ultrasonic transducer front mass, can result in a device with exceptionally large bandwidth. In a similar way to concentric ceramics, it can also result in a transducer having radial resonance frequencies that are tailored with respect to the rest of the frequency spectrum, specifically the longitudinal resonance. Large bandwidth allows effective sweeping over a dramatically larger range of frequencies. This transducer is designed specifically to have as flat an impedance verses frequency curve as possible in the region of said transducer's resonance at its overtones. This design feature is intended to maximize the benefits obtained from the sweeping of frequencies within some bandwidth about some center frequency.

[0029] According to another aspect of the present invention, another configuration of the transducer is a resonator assembly where there is a mass between two driven active elements. While one driven active element is driven to decrease in thickness, the other is driven to increase in thickness. This moves the mass toward the driven active element that is driven to decrease in thickness during one half cycle. During the next half cycle, the driven active elements undergo the opposite change and the mass is driven in the other direction. This produces a sound wave that is propagated from the center portion of the transducer without causing a gross movement of the front mass, back mass or bias bolt.

[0030] According to another aspect of the present invention, another configuration of the transducer is a resonator assembly where there is a mass between four driven active elements. While one pair of driven active elements is driven to decrease in thickness, the other pair is driven to increase in thickness. This moves the mass toward the driven element pair that is driven to decrease in thickness during one half cycle. During the next half cycle, the other driven active element pair undergoes a decrease in thickness and the mass is driven in their direction. This produces a sound wave that is propagated from the center portion of the transducer without causing a gross movement of the front mass, back mass or bias bolt. One skilled in the art will recognize that there are two ways to realize this aspect of the present invention. One way is to construct the transducer such that one pair of driven active elements on one side of the mass have their positive polarity surfaces next to each other and the second pair of driven active elements on the other side of the mass have their negative polarity surfaces next to each other. With this construction, a single phase stimulating drive signal can be used to get the inventive operation. The other way to realize the inventive operation, is to construct the transducer such that like polarity surfaces are next to each other in both pairs of driven active elements, and to use out of phase stimulating signals to drive the driven active elements so they operate in the inventive way, i.e., when one pair of driven active elements is driven to decrease in thickness, the other pair is driven to increase in thickness.

[0031] According to another aspect of the present invention, another configuration of the transducer is a resonator assembly where there is a mass between four driven active elements. The electrodes of the four driven active elements are wired to a switching device such as a relay which allows them to act as in a known sandwich type half wave resonant transducer during one state of the relay, i.e., the driven active elements all increase in thickness during one half cycle and all decrease in thickness during the next half cycle, and to act in the inventive way in the other state of the relay, i.e., a first pair of driven active elements are driven to decrease in thickness while the second pair of driven active elements are driven to increase in thickness during one half cycle and the first pair of driven active elements are driven to increase in thickness while second pair of driven active elements are driven to decrease in thickness during the next half cycle.

[0032] According to another aspect of the present invention, another configuration of the transducer is a resonator assembly where there is an insulating mass between two driven active elements. These driven active elements have an orientation typical of prior state of the art sandwich type transducers, however, the driven active elements are independently driven by signals that are out of phase such that while one driven active element is driven to decrease in thickness, the other is driven to increase in thickness. This moves the insulating mass toward the driven element that is driven to decrease in thickness during one half cycle. During the next half cycle, the driven active elements undergo the opposite change and the insulating mass is driven in the other direction. This produces a sound wave that is propagated from the center portion of the transducer without causing a gross movement of the front mass, back mass or bias bolt. [0033] A preferred way to produce the out of phase signals is with a center-tapped transformer. [0034] According to another aspect of the present invention, another configuration of the transducer is a resonator assembly containing three driven active elements that are properly insulated and each is driven by one of three stimulating signals that are each 120 degrees out of phase with each other. During any point in time, while at least one of said three driven active elements are driven to increase in thickness, at least one of the other driven active elements is driven to decrease in thickness. The net displacement of the resonator assembly is zero for any time period, however, a sound wave at the frequency of the stimulating signal is generated. [0035] After studying the above described aspects of the invention, a person skilled in the art will realize that a low-stress ultrasound transducer of the sandwich type construction driven by a stimulating signal(s) in a way that net motion of the resonator assembly is reduced or substantially eliminated is the main application of the invention. The most basic aspect of the invention is the resonator assembly and how it is driven. Therefore, aspects of the invention related to a resonator, resonator assembly construction and stimulating signals for resonator assemblies will now be summarized.

[0036] A preferred aspect of the present invention is a resonator assembly for use in a sandwich type transducer driven by at least one stimulating signal has at least two driven active elements and proper electrodes to couple the at least one stimulating signal to the at least two driven active elements, wherein at a point in time, at least one of the at least two driven active elements is driven to decrease in thickness while at least one of the other of the at least two driven active elements is driven to increase in thickness when the resonator assembly is installed in a sandwich type transducer and is driven by at least one stimulating signal.

[0037] In another preferred aspect of the present invention a resonator assembly for use in a sandwich type transducer driven by at least one stimulating signal has a first polarized piezoelectric ceramic and a second polarized piezoelectric ceramic. Wherein the first polarized piezoelectric ceramic has a positive polarity surface and a negative polarity surface on opposite sides and the second polarized piezoelectric ceramic has a positive polarity surface and a negative polarity surface on opposite sides. The two polarized piezoelectric ceramics are arranged with the positive polarity surface of the first polarized piezoelectric ceramic adjacent to the negative polarity surface of the second polarized piezoelectric ceramic. [0038] In yet another preferred aspect of the present invention a resonator assembly has two modes of operation for use in a sandwich type transducer assembly. The resonator assembly consists of a first polarized piezoelectric ceramic with a positive polarity on one side and a negative polarity on the other side and a second polarized piezoelectric ceramic with a positive polarity on one side and a negative polarity on the other side; and it has a center electrode with one side adjacent to the positive polarity side of the first polarized piezoelectric ceramic and with the other side of the center electrode adjacent to the positive polarity side of the second polarized piezoelectric ceramic. A second electrode adjacent to the negative polarity side of the first polarized piezoelectric ceramic and a third electrode adjacent to the negative polarity side of the second polarized piezoelectric ceramic plus an insulating means to prevent the sandwich type transducer assembly from electrically shorting the second electrode to the third electrode completes the resonator assembly. A series mode of the resonator assembly is formed when a first electrical connection is made to the second electrode and when a second electrical connection is made to the third electrode to realize one set of frequency ranges; and, a parallel mode of the resonator assembly is formed when a first electrical connection is made to the center electrode and a second electrical connection is made to both the second and third electrodes to realize a second set of frequency ranges.

[0039] In another preferred aspect of the present invention a resonator assembly with two modes of operation for use in a sandwich type transducer assembly and a two state switching system driven by a stimulating signal consists of a stimulating signal delivered between lead one and lead two; a first polarized piezoelectric ceramic with a positive polarity on one side and a negative polarity on the other side; a second polarized piezoelectric ceramic with a positive polarity on one side and a negative polarity on the other side; a center electrode with one side adjacent to the positive polarity side of the first polarized piezoelectric ceramic and with the other side of the center electrode adjacent to the positive polarity side of the second polarized piezoelectric ceramic; a second electrode adjacent to the negative polarity side of the first polarized piezoelectric ceramic; a third electrode adjacent to the negative polarity side of the second polarized piezoelectric ceramic; an insulating means to prevent the sandwich type transducer assembly from electrically shorting the second electrode to the third electrode; a series mode of the resonator assembly where lead one of the stimulating signal is connected to the second electrode and lead two of the stimulating signal is connected to the third electrode; a parallel mode of the resonator assembly where lead one of the stimulating signal is connected to the center electrode and lead two of the stimulating signal is connected to both of the second and third electrodes; and a switching system with a first state and a second state. When the switching system is in the first state, the stimulating signal is connected to the series mode of the resonator assembly, and when the switching system is in the second state, the stimulating signal is connected to the parallel mode of the resonator assembly.

[0040] In yet another preferred aspect of the present invention a resonator assembly with two modes of operation for use in a sandwich type transducer and driven by at least one stimulating signal consists of at least two driven active elements; and electrodes to couple the at least one stimulating signal to the at least two driven active elements. Wherein the resonator assembly in the first mode has at least one of the at least two driven active elements driven to decrease in thickness while at least one of the other of the at least two driven active elements is driven to increase in thickness when the resonator assembly is installed in the sandwich type transducer and is driven by the at least one stimulating signal, and wherein the resonator assembly in the second mode has all of the at least two driven active elements driven to decrease in thickness simultaneously or driven to increase in thickness simultaneously when the resonator assembly is installed in the sandwich type transducer and is driven by the at least one stimulating signal. [0041] In another preferred aspect of the present invention a series resonator assembly for use in a sandwich type transducer driven by a stimulating signal consists of at least two driven active elements, each stacked on top of one another; and electrodes at the opposite ends of the driven active elements stack to couple the stimulating signal to the driven active elements stack. Wherein at a point in time, at least one of the at least two driven active elements is driven to decrease in thickness while at least one of the other of the at least two driven active elements is driven to increase in thickness when the resonator assembly is installed in the sandwich type transducer and is driven by the at least one stimulating signal.

[0042] In yet another preferred aspect of the present invention a series resonator assembly for use in a sandwich type transducer driven by a stimulating signal consists of a first polarized piezoelectric ceramic and a second polarized piezoelectric ceramic stacked one on top of the other; and two electrodes at the opposite ends of the polarized piezoelectric stack to couple the stimulating signal to the polarized piezoelectric stack. Wherein the first polarized piezoelectric ceramic has a positive polarity surface and a negative polarity surface on opposite sides thereof, and wherein the second polarized piezoelectric ceramic has a positive polarity surface and a negative polarity surface on opposite sides thereof, and wherein the two polarized piezoelectric ceramics are arranged with the positive polarity surface of the first polarized piezoelectric ceramic adjacent to and in electrical contact with the positive polarity surface of the second polarized piezoelectric ceramic forming a series connection of the two polarized piezoelectric ceramics.

[0043] In yet another preferred aspect of the present invention a series resonator assembly for use in a sandwich type transducer driven by a stimulating signal consisting of a first polarized piezoelectric ceramic and a second polarized piezoelectric ceramic stacked one on top of the other; and two electrodes at the opposite ends of the polarized piezoelectric stack to couple the stimulating signal to the polarized piezoelectric stack. Wherein the first polarized piezoelectric ceramic has a positive polarity surface and a negative polarity surface on opposite sides thereof, and wherein the second polarized piezoelectric ceramic has a positive polarity surface and a negative polarity surface on opposite sides thereof, and wherein the two polarized piezoelectric ceramics are arranged with the negative polarity surface of the first polarized piezoelectric ceramic adjacent to and in electrical contact with the negative polarity surface of the second polarized piezoelectric ceramic forming a series connection of the two polarized piezoelectric ceramics.

[0044] In yet another preferred aspect of the present invention a resonator assembly with two modes of operation for use in a sandwich type transducer assembly and a two state switching system driven by a stimulating signal consisting of a stimulating signal delivered between lead one and lead two; a first polarized piezoelectric ceramic with a positive polarity on one side and a negative polarity on the other side; a second polarized piezoelectric ceramic with a positive polarity on one side and a negative polarity on the other side; a center electrode with one side adjacent to the negative polarity side of the first polarized piezoelectric ceramic and with the other side of the center electrode adjacent to the negative polarity side of the second polarized piezoelectric ceramic; a second electrode adjacent to the positive polarity side of the first polarized piezoelectric ceramic; a third electrode adjacent to the positive polarity side of the second polarized piezoelectric ceramic; an insulating means to prevent the sandwich type transducer assembly from electrically shorting the second electrode to the third electrode; a series mode of the resonator assembly where lead one of the stimulating signal is connected to the second electrode and lead two of the stimulating signal is connected to the third electrode; a parallel mode of the resonator assembly where lead one of the stimulating signal is connected to the center electrode and lead two of the stimulating signal is connected to both of the second and third electrodes; and a switching system with a first state and a second state. Wherein, when the switching system is in the first state, the stimulating signal is connected to the series mode of the resonator assembly, and wherein, when the switching system is in the second state, the stimulating signal is connected to the parallel mode of the resonator assembly. [0045] m yet another preferred aspect of the present invention a resonator assembly with two modes of operation for use in a sandwich type transducer assembly consisting of a first polarized piezoelectric ceramic with a positive polarity on one side and a negative polarity on the other side; a second polarized piezoelectric ceramic with a positive polarity on one side and a negative polarity on the other side; a center electrode with one side adjacent to the negative polarity side of the first polarized piezoelectric ceramic and with the other side of the center electrode adjacent to the negative polarity side of the second polarized piezoelectric ceramic; a second electrode adjacent to the positive polarity side of the first polarized piezoelectric ceramic; a third electrode adjacent to the positive polarity side of the second polarized piezoelectric ceramic; and an insulating means to prevent the sandwich type transducer assembly from electrically shorting the second electrode to the third electrode. Wherein a series mode of the resonator assembly is formed when a first electrical connection is made to the second electrode and when a second electrical connection is made to the third electrode to realize one set of frequency ranges; and, wherein a parallel mode of the resonator assembly is formed when a first electrical connection is made to the center electrode and a second electrical connection is made to both of the second and third electrodes to realize a second set of frequency ranges.

[0046] The higher frequency ranges, the additional frequency ranges and the broad bandwidths at high frequencies made possible by the present invention results in improved processes. As a result, methods using the present invention have unique commercial value. [0047] In a preferred aspect of the present invention a method for producing sound energy with a sandwich type transducer consists of a stimulating signal for driving the driven active elements of the transducer, whereby the stimulating signal causes at least one of the driven active elements to decrease in thickness while the other driven active elements are driven to increase in thickness, thereby producing sound energy at frequencies above those of half wave resonant transducers and with lower energy loss.

[0048] In another preferred aspect of the present invention a method for producing sound energy with a sandwich type transducer and switching system that can be driven in two states. At least one stimulating signal is supplied for driving the driven active elements of the transducer. Whereby the at least one stimulating signal causes at least one of the driven active elements to decrease in thickness while the other driven active elements are driven to increase in thickness during state one of the switching system, and whereby the at least one stimulating signal causes all of the driven active elements to decrease in thickness simultaneously or to increase in thickness simultaneously during state two of the switching system, thereby producing sound energy at a larger number of frequency bands than those of half wave resonant transducers. [0049] In order to teach another way to understand certain aspects of the present invention, that is, those aspects where there is an even number of driven active elements in each transducer, the operation of low-stress ultrasound transducers at half wave resonant frequencies of sandwich type transducers will be described. In a resonant half wave sandwich transducer the driven active elements are oriented and driven to pump energy into the structure at the half wave resonant frequency of the sandwich structure. One way to describe the present invention with an even number of driven active elements is to start with a resonant half wave sandwich transducer and reverse the polarity of half of the driven active elements. This will be referred to herein as a "reversed polarity structure". With this reversed polarity structure, the action of half of the driven active elements cancels the action of the other half of the driven active elements at the half wave resonant frequency of the sandwich structure, and this frequency is suppressed. This suppression of what is normally the desired operational frequency is the reason that this reversed polarity structure was previously unknown. A second way to describe the present invention with an even number of driven active elements is to again start with a resonant half wave sandwich transducer, but this time to reverse the stimulating signal drive voltage to half of the driven active elements. This will be referred to herein as a "reversed drive voltage configuration". With this reversed drive voltage configuration, the action of half of the driven active elements cancels the action of the other half of the driven active elements at the half wave resonant frequency of the sandwich structure, and this frequency is suppressed. Either the reverse polarity structure or the reversed drive voltage configuration can be used to construct a low-stress ultrasound transducer. This results in the advantages of lower power loss and higher frequency operation because the higher frequencies become under damped.

[0050] In another aspect of the present invention the number of driven active elements can be odd and the effects of a reversed drive voltage configuration realized with phase shifting of the stimulating voltages. For example, when there are three driven active elements in the low-stress ultrasound transducer and each is driven by one of three stimulating signals where each is shifted 120 degrees from each other, the suppression of the half wave resonant frequency of the sandwich structure is accomplished because (sin ( wt + 0 )) + (sin ( wt + 2*PI/3 )) + (sin ( wt + 4*PI/3 )) = 0 for any specified angular frequency w or any time t. This aspect of the present invention is applicable to any number of phase shifted stimulating signals greater than one and a like number times any integer of driven active elements. This condition of the displacement of the driven active elements adding up to zero for all times with phase shifted stimulating signals will be referred to herein as a "zero displacement phase shifted drive voltages configuration". This zero displacement phase shifted drive voltages configuration suppresses the half wave resonant frequency of the sandwich structure and results in low loss and frequencies above the half wave resonant frequency of the sandwich structure.

[0051] In another preferred aspect of the present invention a sandwich type ultrasound transducer is driven by n stimulating signals where n is any integer greater than 3, consisting of a resonator assembly having a first surface and a second surface on opposite sides thereof and containing n driven active elements and n-1 insulators, the insulators between alternate driven active elements such that no driven active elements are adjacent to each other; a front mass having a surface adjacent to the first surface of the resonator assembly; a back mass having a surface adjacent to the second surface of the resonator assembly; electrodes to couple the n stimulating signals to the n driven active elements; a compression assembly coupling the front mass and the back mass, and adapted to effect a compression across the resonator assembly, wherein each of the n stimulating signals are substantially equal in frequency and magnitude, but substantially 360 divided by n degrees phase shifted from each other, wherein at a point in time, at least one of the n driven active elements is driven to decrease in thickness while at least one of the other of the n driven active elements is driven to increase in thickness when the ultrasound transducer is driven by the n stimulating signals.

[0052] In another embodiment of the invention, a sandwich type ultrasound transducer driven by at least one stimulating signal comprises a resonator assembly having a first surface and a second surface on opposite sides thereof and containing at least two driven active elements; a front mass having a surface adjacent to the first surface of the resonator assembly; a back mass having a surface adjacent to the second surface of the resonator assembly; electrodes to couple the at least one stimulating signal to the at least two driven active elements; a compression assembly coupling the front mass and the back mass, and adapted to effect a compression across the resonator assembly. Each of the driven active elements has a positive polarity on one surface, wherein at a point in time, at least one of the at least two driven active elements is driven with a positive voltage on the surface with a positive polarity while at least one of the other of the at least two driven active elements is driven with a negative voltage on the surface with a positive polarity when the ultrasound transducer is driven by the at least one stimulating signal. [0053] By applying the forgoing teachings, it will be clear to one skilled in the art that there are many ways to implement the inventive transducer operation, that is, during any point in time, while at least one of the driven active elements in a resonator assembly is driven to increase in thickness, at least one of the other driven active elements is driven to decrease in thickness. As previously described, this new form of transducer operation can be realized by polarity placement of the driven active elements or by phasing of the stimulating signals. Two stimulating signals 180 degrees out of phase, three stimulating signals 120 degrees out of phase and four stimulating signals 90 degrees out of phase are preferred aspects of the invention, however, the invention is not limited to these numbers or ideal phases. The invention encompasses any configuration of at least two driven active elements where the net motion in the transducer assembly or the resonator assembly is reduced by causing at least one of the driven active elements to increase in thickness while at that point in time at least one of the other driven active elements is driven to decrease in thickness. This reduction of net motion reduces losses in the transducer assembly allowing operation at high frequency overtones due to the emergence of an under damped condition at these frequencies and it also provides more efficient and reliable operation at the lower overtone frequencies.

BRIEF DESCRIPTION OF DRAWINGS

[0054] The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which:

[0055] FIG. IA shows a cross-sectional view of a prior art transducer in a resting state;

[0056] FIG. IB shows a cross-sectional view of the prior art transducer in FIG. IA when the transducer is driven to 90 degrees of the stimulating signal;

[0057] FIG.1C shows a cross-sectional view of the prior art transducer in FIG. IA when the transducer is driven to 180 degrees of the stimulating signal;

[0058] FIG. ID shows a cross-sectional view of the prior art transducer in FIG. IA when the transducer is driven to 270 degrees of the stimulating signal;

[0059] FIG.2A shows a cross-sectional view of a transducer according to one preferred embodiment of the present invention.

[0060] FIG.2B shows a cross-sectional view of the transducer in FIG.2A when the transducer is driven to 90 degrees of the stimulating signal;

[0061] FIG.2C shows a cross-sectional view of the transducer in FIG.2A when the transducer is driven to 180 degrees of the stimulating signal;

[0062] FIG.2D shows a cross-sectional view of the transducer in FIG.2A when the transducer is driven to 270 degrees of the stimulating signal;

[0063] FIG.3A shows a cross-sectional view of a transducer with two driven active elements and a center mass according to a preferred embodiment of the present invention.

[0064] FIG.3B shows a cross-sectional view of the transducer in FIG.3A when the transducer is driven to 90 degrees of the stimulating signal;

[0065] FIG.3C shows a cross-sectional view of the transducer in FIG.3A when the transducer is driven to 180 degrees of the stimulating signal;

[0066] FIG.3D shows a cross-sectional view of the transducer in FIG.3A when the transducer is driven to 270 degrees of the stimulating signal; [0067] FIG.4A shows a cross-sectional view of a transducer with four driven active elements and a center mass according to a preferred embodiment of the present invention.

[0068] FIG.4B shows a cross-sectional view of the transducer in FIG.4A when the transducer is driven to 90 degrees of the stimulating signal;

[0069] FIG.4C shows a cross-sectional view of the transducer in FIG.4A when the transducer is driven to 180 degrees of the stimulating signal;

[0070] FIG.4D shows a cross-sectional view of the transducer in FIG.4A when the transducer is driven to 270 degrees of the stimulating signal;

[0071] FIG.5 schematically shows a series and parallel resonator assembly with a switching device to obtain two modes of operation;

[0072] FIG.6 schematically shows a conventional resonator assembly driven by out of phase stimulating signals to suppress a half wave fundamental resonant frequency;

[0073] FIG.7 schematically shows a resonator assembly with three driven active elements and a three phase stimulating signal with each voltage 120 degrees out of phase with each other.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0074] FIG. 2A shows a cross-sectional view of a transducer 500 according to one preferred embodiment of the present invention. FIG. 2B shows a cross-sectional view of transducer 500 when transducer 500 is driven to 90 degrees of the stimulating signal. FIG. 2C shows a cross- sectional view of transducer 500 when transducer 500 is driven to 180 degrees of the stimulating signal. FIG. 2D shows a cross-sectional view of transducer 500 when transducer 500 is driven to 270 degrees of the stimulating signal. The transducer 500 employs a Langevin architecture, also known in the art as a sandwich transducer. According to one embodiment of the invention, the transducer 500 as shown in FIG. 2A includes a back mass 502, a front mass 504, a resonator assembly including a first polarized piezoelectric ceramic 506 with a positive polarity indicated by a dot 536 near its bottom surface and a second polarized piezoelectric ceramic 508 with a positive polarity indicated by a dot 538 near its bottom surface, and a compression assembly including a central bias bolt 516. The transducer may further include an insulator, which is not shown in the drawings, disposed between the bolt 516 and the polarized piezoelectric ceramics 506 and 508, and electrodes (not shown) in any of four known configurations and connected to the polarized piezoelectric ceramics 506 and 508. In the embodiment of FIGS.2A to 2D, spaces are drawn between individual parts, for example, between 502 and 506, between 506 and 508, between 508 and 504 and between the head of bolt 516 and its mating surface in the back mass 502. These spaces are drawn for clarity, in the actual compressed sandwich transducer assembly the respective surfaces are pressed together by the force of the compression assembly. This force is typically in the range of 500 to 10,000 pounds, insuring contact of the surfaces and compression of the polarized piezoelectric ceramics. The back mass 502, front mass 504, first polarized piezoelectric ceramic 506 and second polarized piezoelectric ceramic 508 are each characterized by a substantially annular shape extending about a central axis AY with the positive polarity surfaces indicated by dots 536 and 538 each oriented toward the front mass 504. A two wire stimulating signal is applied to transducer 500 with the first wire connected to the center junction where the positive polarity surface of polarized piezoelectric ceramic 536 is adjacent to the negative polarity surface of polarized piezoelectric ceramic 538. The second wire supplying the stimulating signal is connected to the two remaining surfaces of polarized piezoelectric ceramics 506 and 508. When the stimulating signal is a sine waveform on the first wire, a maximum positive value of voltage occurs at the 90 degree position of the sine wave, this drives transducer 500 to the condition shown in FIG. 2B, where polarized piezoelectric ceramic 506 has increased in thickness and polarized piezoelectric ceramic 508 has decreased in thickness. This action of the polarized piezoelectric ceramics 506 and 508 results in substantially zero net displacement which reduces losses in the transducer 500 and allows the under damped condition at higher overtone frequencies. These higher overtone frequencies in the higher microsomes and megasonic frequency ranges become usable. [0075] In the embodiment shown in FIGS. 2A to 2D, the bore of the front mass 504, does not extend completely through the front mass 504 along the axis AY. Other embodiments may include an inner bore of the front mass 504 that extends completely through the front mass 504. The front mass 504 further includes threads on the walls of the inner bore. In one preferred embodiment, the threads are machined into the inner bore, although other techniques known in the art may also be used to create threads in the inner bore.

[0076] The back mass 502, front mass 504, first polarized piezoelectric ceramic 506 and second polarized piezoelectric ceramic 508 are stacked so as to be adjacent and disposed along the common central axis AY, as shown in FIGS. 2A to 2D. The first polarized piezoelectric ceramic 506 and the second polarized piezoelectric ceramic 508 are "sandwiched" between the back mass 502 and the front mass 504. The bias bolt 516 is preferably symmetrically disposed about the common axis AY, and includes a first end 526 and a second end 528. The outer radius near first end 526 is characterized by an abrupt change, forming a shelf 530. The second end 528 includes threads along the outer surface for mating with the threads on the walls of the inner bore of the front mass 504. The transducer 500 is assembled by passing the bias bolt 516 through the bore of the back mass 502, the bore of the first polarized piezoelectric ceramic 506, the bore of the second polarized piezoelectric ceramic 508, and into the bore of the front mass 504. The threads on the bias bolt 516 engage the threads in the bore of the front mass 504. As the bias bolt 516 is tightened, the bias bolt 516 is drawn into the bore of the front mass 504, and the shelf 530 on the bias bolt 516 contacts the shelf 524 on the back mass 502, thereby applying a force to the back mass 102 along the axis AY toward the front mass 504. Further tightening the bias bolt 516 compresses the first polarized piezoelectric ceramic 506 and the second polarized piezoelectric ceramic 508 between the front mass 504 and the back mass 502. The bias bolt 516 can be tightened or loosened to adjust the amount of compression on the polarized piezoelectric ceramics 506 and 508.

[0077] Electrodes connected to the polarized piezoelectric ceramics 506 and 508 provide input ports to the polarized piezoelectric ceramics for a stimulating signal from an ultrasonic signal generator. In some embodiments of the transducer 500, the polarized piezoelectric ceramics may receive the stimulating signal via an electrically conducting front mass and/or an electrically conducting back mass, instead of or in addition to the electrodes. The polarized piezoelectric ceramic components within the transducer 500 spatially oscillate in one or more modes associated with the frequency of the applied stimulating signal. The transducer 500 transmits the spatial oscillations via the front mass as ultrasound, to (for example) a tank that contains a cleaning solution and an object to be cleaned.

[0078] FIG. 3 A shows another embodiment of the invention where a sandwich type transducer 600 is similar to the construction and operation of transducer 500 in FIG. 2 A, except that transducer 600 has a central mass 601 between the polarized piezoelectric ceramics 506 and 508. During the first half cycle of operation when driven by a stimulating signal, polarized piezoelectric ceramic 506 is driven to increase in thickness and polarized piezoelectric ceramic 508 is driven to decrease in thickness causing the central mass 601, to move toward the front mass 504. This is clearly seen in FIG. 3B which shows the transducer 600 at the middle of this first half cycle, i.e., at 90 degrees. FIG. 3C shows transducer 600 at the end of the first half cycle or at 180 degrees where the condition is the same as in FIG. 3 A. During the second half cycle, the opposite effect occurs where the central mass 601 is moved toward the backmass 502 as shown in FIG. 3D at 270 degrees. In FIGs. 3A to 3D electrodes and the compression bolt are not shown and spaces between the components were inserted for clarity.

[0079] FIGs. 4A to 4D show another aspect of the invention where a transducer 650 containing central mass 601 has the mass driven by two pairs of polarized piezoelectric ceramics. [0080] FIG. 5 shows another embodiment of the invention where a resonator assembly 907 and a relay 910 are configured for two modes of operation. The resonator assembly 907 is constructed with a stack of five components in the following order and orientation, bottom electrode 901 is adjacent to the negative polarized surface of polarized piezoelectric ceramic 902 which has its positive polarized surface adjacent to the center electrode 903, which is adjacent to the positive polarized surface of polarized piezoelectric ceramic 904, which has its negative polarized surface adjacent to the top electrode 905. This resonator assembly 907 is configured for use in a sandwich type transducer by use of a relay 910. When the relay 910 is in the deenergized condition, it is said to be in a first state. When relay 910 is energized, it is said to be in a second state. This embodiment of the invention provides two modes of operation. In a first mode, where the relay is in the first state, a sandwich type transduόer using this resonator assembly can be operated at the low loss high frequencies made possible by the present invention. In a second mode, where the relay is in the second state, a sandwich type transducer using this resonator assembly can be operated at the half wave fundamental resonant frequency of a conventional transducer.

[0081] Another advantage to the embodiment of the invention shown in FIG. 5 is the series and parallel connections of the polarized piezoelectric ceramics. In the first state of relay 910 where the resonator assembly 907 is operating at higher frequencies, e.g., higher microsomes and megasonic frequencies, the polarized piezoelectric ceramics are in series which causes the capacitance of the resonator assembly to be one-half of that of a single polarized piezoelectric ceramic. This low capacitance has an advantage at high frequencies because corresponding inductive components within a generator producing a high frequency stimulating signal 911 are of a practical size and value. Another advantage is that the currents needed to drive the resonator assembly are reduced by the lower capacitance. In the second state of relay 910 where the resonator assembly 907 is operating at the half wave resonant fundamental frequency or possibly at other resonant harmonic or overtone frequencies, the polarized piezoelectric ceramics are in parallel which provides a lower impedance and lower drive voltages at these lower frequencies. [0082] It will be clear to a person skilled in the art that other switching devices can be used in place of relay 910 in FIG. 5. For example, triacs can used to switch the stimulating signals to the proper electrodes in resonator assembly 907.

[0083] FIG. 6 shows yet another embodiment of the invention where resonator assembly 907 from FIG. 5 is now driven by two stimulating signals that are 180 degrees out of phase. As in FIG. 5, the resonator assembly in FIG. 6 is constructed with three electrodes 901, 903, and 905 and with two polarized piezoelectric ceramics 902 and 904, but this time the stimulating signal 911 is transformed with transformer 912 so that it is split into two stimulating signals 913 and 914 that are 180 degrees out of phase with each other. The first stimulating signal 913 is connected to electrodes 903 and 907 and the second stimulating signal 913 is connected to electrodes 901 and 903. The polarity of the polarized piezoelectric ceramics 902 and 904 and the orientation of the stimulating signals 913 and 914 cause one polarized piezoelectric ceramic to increase in thickness, say polarized piezoelectric ceramic 914, while the other polarized piezoelectric ceramic 913 is driven to decrease in thickness. When this driven apparatus 900 in FIG. 6 is used in conjunction with a sandwich type transducer assembly, the half wave fundamental resonant frequency is suppressed and the resultant low loss provides for the high frequencies of the present invention.

[0084] FIG. 7 shows another embodiment of the invention where a resonator assembly 730 consisting of three driven active elements 702, 706 and 710 are each driven by stimulating signals 742, 746 and 750 respectively that are 120 degrees out of phase with each other. Insulators 704 and 706 separate the driven active elements and electrodes 701, 703, 705, 707, 709 and 711 supply the stimulating signals 742, 746 and 750 to the driven active elements 702, 706, and 710. For purposes of illustration, assume that the first stimulating signal 742 has a waveform given by the equation Vl = sin ( wt + 0 ). This first stimulating signal 742 is connected to the first driven active element 702 at electrodes 701 and 703 with Vl at the positive polarity side of the first driven active element 702. Now assume that the second stimulating signal 746 has a waveform given by the equation V2 = sin ( wt + 2*PI/3 ). This second stimulating signal 746 is connected to the second driven active element 706 at electrodes 705 and 707 with V2 at the positive polarity side of the second driven active element 706. And now assume that the third stimulating signal 750 has a waveform given by the equation V3 = sin ( wt + 4*PI/3 ). This third stimulating signal 750 is connected to the third driven active element 710 at electrodes 709 and 711 with V3 at the positive polarity side of the third driven active element 710. This connection results in the driven active elements seeing a voltage equal to the sum of Vl, V2, and V3 at any point in time. Since this sum is always zero, the net displacement of this resonator assembly 730 is substantially zero when driven by stimulating sources 740 and therefore the half wave fundamental frequency of any sandwich type transducer in which it is installed will be suppressed. These are the conditions for the present invention and they result in implementations of the low-stress ultrasound transducer.

[0085] In the above-illustrated exemplary embodiments, the lead end of the bias bolt is screwed into a threaded bore in the front mass for tightening the transducer assembly, so that the front mass acts as a "nut". In alternative forms, the bore may extend through the front mass and a nut, which is a discrete element, is screwed onto the lead end of the bias bolt to tighten the transducer assembly. A person skilled in the art should also appreciate that other compression assemblies for tightening and compressing the front and back masses can be used to replace the bolt and nut assembly.

[0086] According to a further embodiment of the present invention, the back mass 502 is fabricated from a low-density material (with respect to prior art back mass components) such as aluminum, magnesium, beryllium, titanium, or other similar materials known in the art, including alloys and other mixed composition materials. As used herein, the term "low density material" describes a material with a density of less than 6.0 grams per cubic centimeter (g/cc). In one preferred embodiment, the back mass 502 is made of type 7075-T651 aluminum, although other similar materials may also be used. In a preferred embodiment, the front mass 504 is made of type 2024 aluminum, although other similar materials may also be used. The back mass 502 and front mass 504 being made from different materials contributes to the ultrabroad bandwidth of the transducer 500. A low density back mass 504 results in a physically longer back mass, or a larger surface area as compared to a higher density back mass of the same acoustic length. The increased length (or larger surface area) further contributes to the multiple center frequencies of operation, and the ultrabroad bandwidth at each of the center frequencies. The polarized piezoelectric ceramics 506 and 508 are fabricated from a ceramic material that has been polarized via techniques well know in the art to imbue a piezoelectric effect. In other embodiments, the resonators may include other piezoelectric materials known in the art, such as natural piezoelectrics (e.g., quartz) or magnetostrictives. Further, although the embodiment of FIGS. 2 A to 7 includes two or four polarized piezoelectric ceramics, other embodiments of the transducer may include multiple resonators including odd numbers.

[0087] The transducer 500 can be operated at a dedicated single frequency, or it can be excited at multiple frequencies, i.e., at any of its higher frequency overtones.

[0088] The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalency of the claims are therefore intended to be embraced therein.

Claims

What is claimed is:

1. A sandwich type ultrasound transducer driven by at least one stimulating signal comprising: a resonator assembly having a first surface and a second surface on opposite sides thereof and containing at least two driven active elements; a front mass having a surface adjacent to said first surface of said resonator assembly; a back mass having a surface adjacent to said second surface of said resonator assembly; electrodes to couple said at least one stimulating signal to said at least two driven active elements; a compression assembly coupling said front mass and said back mass, and adapted to effect a compression across said resonator assembly, wherein at a point in time, at least one of said at least two driven active elements is driven to decrease in thickness while at least one of the other of said at least two driven active elements is driven to increase in thickness when said ultrasound transducer is driven by said at least one stimulating signal.

2. An ultrasound transducer according to claim 1 , wherein said at least one stimulating signal supplies a frequency or frequencies in the higher microsomes or megasonic range of frequencies.

3. An ultrasound transducer according to claim 1, wherein said at least two driven active elements are polarized piezoelectric ceramics.

4. An ultrasound transducer according to claim 1, wherein said at least one stimulating signal supplies a sweeping frequency signal.

5. An ultrasound transducer according to claim 1 , wherein said at least one stimulating signal has a first half cycle between zero degrees and 180 degrees, wherein said at least one of said at least two driven active elements are driven to decrease in thickness while the other of said at least two driven active elements are driven to increase in thickness when said at least one stimulating signal is in said first half cycle, wherein said at least one stimulating signal has a second half cycle between 180 degrees and 360 degrees, wherein said at least one of said at least two driven active elements is driven to increase in thickness while the other of said at least two driven active elements are driven to decrease in thickness when said stimulating signal is in said second half cycle.

6. A sandwich type ultrasound transducer array and switching device system driven by a stimulating signal comprising: an array of at least two ultrasound transducers coupled to a switching device with two states, and, at least two driven active elements in each of said at least two ultrasound transducers, wherein, state one of said switching device couples said array of at least two ultrasound transducers such that all of said driven active elements are driven to increase in thickness at the same time when driven by said stimulating signal, and state two of said switching device couples said array of at least two ultrasound transducers such that when at least one of said two driven active elements is driven to increase in thickness, at least one other of said at least two driven active elements are driven to decrease in thickness when driven by said stimulating signal.

7. A method for producing sound energy with a sandwich type transducer comprising: a stimulating signal for driving the driven active elements of the transducer, whereby the stimulating signal causes at least one of the driven active elements to decrease in thickness while the other driven active elements are driven to increase in thickness, thereby producing sound energy at frequencies above those of half wave resonant transducers and with lower energy loss.

8. A method of claim 7, wherein said stimulating signal has a frequency or frequencies in the higher microsomes or megasonic frequency ranges.

9. A sandwich type ultrasound transducer comprising: a resonator assembly having a first surface and a second surface on opposite sides thereof and containing a first polarized piezoelectric ceramic and a second polarized piezoelectric ceramic; a front mass having a surface adjacent to said first surface of said resonator assembly; a back mass having a surface adjacent to said second surface of said resonator assembly; a compression assembly coupling said front mass and said back mass, and adapted to effect a compression across said resonator assembly; wherein said first polarized piezoelectric ceramic has a positive polarity surface and a negative polarity surface on opposite sides thereof, wherein said second polarized piezoelectric ceramic has a positive polarity surface and a negative polarity surface on opposite sides thereof, wherein said two polarized piezoelectric ceramics are arranged with said positive polarity surface of said first polarized piezoelectric ceramic adjacent to said negative polarity surface of said second polarized piezoelectric ceramic.

10. A resonator assembly for use in a sandwich type transducer driven by at least one stimulating signal comprising: at least two driven active elements; electrodes to couple said at least one stimulating signal to said at least two driven active elements; wherein at a point in time, at least one of said at least two driven active elements is driven to decrease in thickness while at least one of the other of said at least two driven active elements is driven to increase in thickness when said resonator assembly is installed in said sandwich type transducer and is driven by said at least one stimulating signal.

11. A resonator assembly for use in a sandwich type transducer driven by at least one stimulating signal comprising: a first polarized piezoelectric ceramic and a second polarized piezoelectric ceramic; wherein said first polarized piezoelectric ceramic has a positive polarity surface and a negative polarity surface on opposite sides thereof, wherein said second polarized piezoelectric ceramic has a positive polarity surface and a negative polarity surface on opposite sides thereof, wherein said two polarized piezoelectric ceramics are arranged with said positive polarity surface of said first polarized piezoelectric ceramic adjacent to said negative polarity surface of said second polarized piezoelectric ceramic.

12. A method for producing sound energy with a sandwich type transducer and switching system that can be driven in two states comprising: at least one stimulating signal with at least a first frequency and a second frequency for driving the driven active elements of the transducer, whereby said at least one stimulating signal operating at said first frequency causes at least one of the driven active elements to decrease in thickness while the other driven active elements are driven to increase in thickness during state one of the switching system, whereby the at least one stimulating signal operating at said second frequency causes all of the driven active elements to decrease in thickness simultaneously or to increase in thickness simultaneously during state two of the switching system, thereby producing sound energy at a larger number of frequency bands than those of half wave resonant transducers.

13. A resonator assembly with two modes of operation for use in a sandwich type transducer assembly comprising: a first polarized piezoelectric ceramic with a positive polarity on one side and a negative polarity on the other side; a second polarized piezoelectric ceramic with a positive polarity on one side and a negative polarity on the other side; a center electrode with one side adjacent to said positive polarity side of said first polarized piezoelectric ceramic and with the other side of said center electrode adjacent to said positive polarity side of said second polarized piezoelectric ceramic; a second electrode adjacent to said negative polarity side of said first polarized piezoelectric ceramic; a third electrode adjacent to said negative polarity side of said second polarized piezoelectric ceramic; an insulating means to prevent said sandwich type transducer assembly from electrically shorting said second electrode to said third electrode; wherein a series mode of said resonator assembly is formed when a first electrical connection is made to said second electrode and when a second electrical connection is made to said third electrode to realize one set of frequency ranges; and, wherein a parallel mode of said resonator assembly is formed when a first electrical connection is made to said center electrode and a second electrical connection is made to both of said second and third electrodes to realize a second set of frequency ranges.

14. A resonator assembly with two modes of operation for use in a sandwich type transducer assembly and a two state switching system driven by a stimulating signal comprising: a stimulating signal delivered between lead one and lead two; a first polarized piezoelectric ceramic with a positive polarity on one side and a negative polarity on the other side; a second polarized piezoelectric ceramic with a positive polarity on one side and a negative polarity on the other side; a center electrode with one side adjacent to said positive polarity side of said first polarized piezoelectric ceramic and with the other side of said center electrode adjacent to said positive polarity side of said second polarized piezoelectric ceramic; a second electrode adjacent to said negative polarity side of said first polarized piezoelectric ceramic; a third electrode adjacent to said negative polarity side of said second polarized piezoelectric ceramic; an insulating means to prevent said sandwich type transducer assembly from electrically shorting said second electrode to said third electrode; a series mode of said resonator assembly where lead one of said stimulating signal is connected to said second electrode and lead two of said stimulating signal is connected to said third electrode; a parallel mode of said resonator assembly where lead one of said stimulating signal is connected to said center electrode and lead two of said stimulating signal is connected to both of said second and third electrodes; a switching system with a first state and a second state; wherein, when said switching system is in said first state, said stimulating signal is connected to said series mode of said resonator assembly, and wherein, when said switching system is in said second state, said stimulating signal is connected to said parallel mode of said resonator assembly.

15. A resonator assembly with two modes of operation for use in a sandwich type transducer and driven by at least one stimulating signal comprising: at least two driven active elements; electrodes to couple said at least one stimulating signal to said at least two driven active elements; wherein said resonator assembly in the first mode has at least one of said at least two driven active elements decreasing in thickness while at least one of the other of said at least two driven active elements is driven to increase in thickness when said resonator assembly is installed in said sandwich type transducer and is driven by said at least one stimulating signal, and wherein said resonator assembly in the second mode has all of said at least two driven active elements decreasing in thickness simultaneously or increasing in thickness simultaneously when said resonator assembly is installed in said sandwich type transducer and is driven by said at least one stimulating signal.

16. A sandwich type ultrasound transducer driven by at least one stimulating signal comprising: a resonator assembly having a first surface and a second surface on opposite sides thereof and containing an even number driven active elements; a front mass having a surface adjacent to said first surface of said resonator assembly; a back mass having a surface adjacent to said second surface of said resonator assembly; electrodes to couple said at least one stimulating signal to said even number of driven active elements; a compression assembly coupling said front mass and said back mass, and adapted to effect a compression across said resonator assembly, wherein said even number of driven active elements are arranged in a reverse polarity structure to suppress the half wave resonant frequency and to reduce loss and enhance operation at higher frequencies above said half wave resonant frequency when said ultrasound transducer is driven by said at least one stimulating signal.

17. A sandwich type ultrasound transducer driven by an even number of stimulating signals comprising: a resonator assembly having a first surface and a second surface on opposite sides thereof and containing an even number driven active elements; a front mass having a surface adjacent to said first surface of said resonator assembly; a back mass having a surface adjacent to said second surface of said resonator assembly; electrodes to couple said even number of stimulating signals to said even number of driven active elements; a compression assembly coupling said front mass and said back mass, and adapted to effect a compression across said resonator assembly, wherein said even number of driven active elements are driven in a reversed drive voltage configuration by said even number of stimulating signals to suppress the half wave resonant frequency and to reduce loss and enhance operation at higher frequencies above said half wave resonant frequency.

18. A sandwich type ultrasound transducer driven by an integer number n greater than one of stimulating signals comprising: a resonator assembly having a first surface and a second surface on opposite sides thereof and containing an integer number p of driven active elements where p divided by n is an integer; a front mass having a surface adjacent to said first surface of said resonator assembly; a back mass having a surface adjacent to said second surface of said resonator assembly; electrodes to couple said n stimulating signals to said p number of driven active elements; a compression assembly coupling said front mass and said back mass, and adapted to effect a compression across said resonator assembly, wherein said p number of driven active elements are driven in a zero displacement phase shifted drive voltages configuration by said n number of stimulating signals to suppress the half wave resonant frequency and to reduce loss and enhance operation at higher frequencies above said half wave resonant frequency.

19. A sandwich type ultrasound transducer driven by at least one stimulating signal comprising: a resonator assembly having a first surface and a second surface on opposite sides thereof and containing at least two driven active elements; a front mass having a surface adjacent to said first surface of said resonator assembly; a back mass having a surface adjacent to said second surface of said resonator assembly; electrodes to couple said at least one stimulating signal to said at least two driven active elements; a compression assembly coupling said front mass and said back mass, and adapted to effect a compression across said resonator assembly, each of said driven active elements having a positive polarity on one surface, wherein at a point in time, at least one of said at least two driven active elements is driven with a positive voltage on said surface with a positive polarity while at least one of the other of said at least two driven active elements is driven with a negative voltage on said surface with a positive polarity when said ultrasound transducer is driven by said at least one stimulating signal.

20. An ultrasound transducer according to claim 19, wherein said at least one stimulating signal supplies a frequency or frequencies in the higher microsomes or megasonic range of frequencies.