Abstract:
A method of manufacturing high power sandwich type ultrasonic transducers and, more particularly, a new method of tuning high power sandwich type ultrasonic transducers without the need for a trimming process. A method in accordance with the present invention includes the steps of assembling a sandwich type ultrasonic transducer, measuring the resonant frequency of the ultrasonic transducer, and selecting from a plurality of tuning elements, whereby a dimension or material property of a selected tuning element alters the measured resonant frequency of the ultrasonic transducer to a desired resonant frequency after the tuning element is attached to the ultrasonic transducer.

Description:
This Application is a division of Ser. No. 09/292,134 filed Apr. 15, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates, in general, to apparatus and methods for manufacturing high power sandwich type ultrasonic transducers and, more particularly, to a new method of tuning high power sandwich type ultrasonic transducers. 
     BACKGROUND OF THE INVENTION 
     This application is related to the following copending patent applications: application Ser. No. 09/104,612 filed Jun. 25, 1998; application Ser. No. 09/104,789 filed Jun. 25, 1998; and application Ser. No. 09/104,648 filed Jun. 25, 1998, all assigned to the same assignee as the present invention and all of which are hereby incorporated herein by reference. 
     Ultrasonic instruments, including both hollow core and solid core instruments, are used for the safe and effective treatment of many medical conditions. Ultrasonic instruments, and particularly solid core ultrasonic instruments, are advantageous because they may be used to cut and/or coagulate organic tissue using energy in the form of mechanical vibrations transmitted to a surgical end-effector at ultrasonic frequencies. Ultrasonic vibrations, when transmitted to organic tissue at suitable energy levels and using a suitable end-effector, may be used to cut, dissect, or cauterize tissue. Ultrasonic instruments utilizing solid core technology are particularly advantageous because of the amount of ultrasonic energy that may be transmitted from the ultrasonic transducer through the waveguide to the surgical end-effector. Such instruments are particularly suited for use in minimally invasive procedures, such as endoscopic or laparoscopic procedures, wherein the end-effector is passed through a trocar to reach the surgical site. 
     Ultrasonic vibration is induced in the surgical end-effector by, for example, electrically exciting a transducer which may be constructed of one or more piezoelectric or magnetostrictive elements in the instrument handpiece. Vibrations generated by the transducer section are transmitted to the surgical end-effector via an ultrasonic waveguide extending from the transducer section to the surgical end-effector. 
     Sandwich type ultrasonic transducers, also called Langevin transducers, are well known and established for the production of high intensity ultrasonic motion. In United Kingdom Patent No. 145,691, issued in 1921, P. Langevin inventor, a sandwich of piezoelectric material positioned between metal plates is described to generate high intensity ultrasound. Sandwich transducers utilizing a bolted stack transducer tuned to a resonant frequency and designed to a half wavelength of the resonant frequency are described in United Kingdom Patent No. 868,784. 
     High-intensity ultrasonic transducers of the composite or sandwich type typically include front and rear mass members with alternating annular piezoelectric elements and electrodes stacked therebetween. Most such high-intensity transducers are of the pre-stressed type. They employ a compression bolt that extends axially through the stack to place a static bias of about one-half of the compressive force that the piezoelectric transducers can tolerate. When the transducers operate they are designed to always remain in compression, swinging from a minimum compression of nominally zero to a maximum peak of no greater than the maximum compressive strength of the material. 
     Other embodiments of the prior art utilize a stud that is threadedly engaged with both the first and second resonator to provide compressive forces to the transducer stack. Threaded studs are also known in the prior art for attaching and detaching transmission components to the transducer assembly. See, for example, U.S. Pat. Nos. 5,324,299 and 5,746,756. Such bolts and studs are utilized to maintain acoustic coupling between elements of the sandwich type transducer or any attached acoustic assembly. Coupling is important to maintain tuning of the assembly, allowing the assembly to be driven in resonance. 
     Sandwich type transducers are relatively high Q devices, and during operation are driven at resonance, and maintained within a relatively narrow frequency range by feedback control methods known in the art. See, for example, U.S. Pat. Nos. 5,630,420 and 5,026,387 which describe systems incorporating and controlling sandwich type transducers. 
     It is difficult to manufacture sandwich type transducers due to the high Q/narrow resonance range in which these devices operate. It is common to individually tune every transducer at least once during the manufacturing process. Even with the tight tolerances currently available with modern manufacturing processes, tolerance “stack-up” issues present challenges to designers of sandwich type transducers. “Stack-up” issues occur as normal variations due to combining multiple parts, each part having design tolerances, such that variations due to each part sum together to produce a significant variation. 
     Currently it is known in the art to design the sandwich type transducer longer than desired for a given resonant frequency. During assembly the sandwich type transducer is tested for its resonant frequency, and then the assembly is trimmed shorter to bring it within the desired tuning range. This trimming process often occurs at attachment surfaces, where other acoustic assemblies such as end-effectors are to be attached. It is known that the surface finish quality at attachment surfaces is an important parameter for efficient acoustic assemblies, and the trimming process adds significant manufacturing issues and expense. See, for example, U.S. Pat. No. 5,798,599, which states that transducers require intimate surface contact between adjacent members, and that this intimacy requires surface finishes within 2 Newtonian rings per inch of flatness. 
     Thus there is a need for a transducer tuning method that does not require trimming at a contact surface. There is also a need for an acoustic assembly method that can account for variations of frequency resonance of individual acoustic assemblies. It would therefore be advantageous to eliminate the need for trimming of acoustic assemblies. It would further be advantageous to be able to design sandwich type transducer components to the desired length for resonance without adding length for tuning due to tolerance “stack-up” issues. It would also be advantageous to provide a method of tuning acoustic assemblies during manufacture that was capable of tuning high Q resonant devices from an existing resonant frequency to a desired resonant frequency. This invention addresses and solves these needs as described below. 
     SUMMARY OF THE INVENTION 
     The invention is a method along with the attendant apparatus for manufacturing high power sandwich type ultrasonic transducers and, more particularly, a new method of tuning high power sandwich type ultrasonic transducers without the need for a trimming process. A method in accordance with the present invention includes the steps of assembling a sandwich type ultrasonic transducer, measuring the resonant frequency of the ultrasonic transducer, and selecting from a plurality of tuning elements, whereby a specific tuning element alters the measured resonant frequency of the ultrasonic transducer to a desired resonant frequency after assembly with the ultrasonic transducer. In one embodiment of the present invention the tuning element is a connecting stud that is also used to connect an ultrasonic acoustic assembly to an end-effector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to organization and methods of operation, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings in which: 
     FIG. 1 illustrates a perspective view of an ultrasonic signal generator with a sectioned plan view of a sandwich type ultrasonic transducer and housing in accordance with the present invention; 
     FIG. 2 illustrates an exploded perspective view of a sandwich type ultrasonic transducer and housing in accordance with the present invention; 
     FIG. 3 illustrates a sectioned plan view of the distal-end of an acoustic assembly along with a plurality of attachment studs of differing lengths in accordance with the present invention; 
     FIG. 4 is a flow chart of an embodiment of an ultrasonic transducer assembly or tuning method in accordance with the present invention; 
     FIG. 5 is a flow chart of an embodiment of an ultrasonic transducer assembly or tuning method in accordance with the present invention; and 
     FIG. 6 is a perspective view of an alternate embodiment of a tuning stud in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a perspective view of an ultrasonic signal generator  15  with a sectioned plan view of a sandwich type ultrasonic transducer  82  and housing  20  in accordance with the present invention. The transducer  82 , which is known as a “Langevin stack”, generally includes a transduction portion  90 , a first resonator or end-bell  92 , and a second resonator or fore-bell  94 . The transducer  82  is preferably an integral number of one-half system wavelengths (nλ/2) in length as will be described in more detail later. An acoustic assembly  80  includes the transducer  82 , mount  36 , velocity transformer  64  and distal-end  95 . 
     The distal end of end-bell  92  is connected to the proximal end of transduction section  90 , and the proximal end of fore-bell  94  is connected to the distal end of transduction portion  90 . The first and second resonators  92  and  94  are preferably fabricated from titanium, aluminum, stainless steel, or any other suitable material. Fore-bell  94  and end-bell  92  have a length determined by a number of variables, including the thickness of the transduction section  90 , the density and modulus of elasticity of material used end-bell  92  and fore-bell  94 , and the resonant frequency of the transducer  82 . The fore-bell  94  may be tapered inwardly from its proximal end to its distal end to amplify the ultrasonic vibration amplitude as velocity transformer  64 , or alternately may have no amplification. 
     The transduction portion  90  of the transducer  82  preferably comprises a piezoelectric section of alternating positive electrodes  96  and negative electrodes  98  (see FIG.  2 ), with piezoelectric elements  100  alternating between the electrodes  96  and  98 . The piezoelectric elements  100  may be fabricated from any suitable material, such as, for example, lead zirconate-titanate, lead meta-niobate, lead titanate, or other piezoelectric crystal material. Each of the positive electrodes  96 , negative electrodes  98 , and piezoelectric elements  100  have a bore extending through the center. The positive and negative electrodes  96  and  98  are electrically coupled to wires  102  and  104 , respectfully. Wires  102  and  104  are encased within cable  25  and electrically connectable to generator  15  of ultrasonic system  10 . 
     Referring to FIG. 1, the transducer  82  of the acoustic assembly  80  converts the electrical signal from generator  15  into mechanical energy that results in longitudinal vibratory motion of the ultrasonic transducer  82  and any attached end-effector at ultrasonic frequencies. When the acoustic assembly  80  is energized, a vibratory motion standing wave is generated through the acoustic assembly  80 . The amplitude of the vibratory motion at any point along the acoustic assembly  80  depends on the location along the acoustic assembly  80  at which the vibratory motion is measured. A minimum or zero crossing in the vibratory motion standing wave is generally referred to as a node (i.e., where motion is usually minimal), and an absolute value maximum or peak in the standing wave is generally referred to as an anti-node. The distance between an anti- node and its nearest node is one-quarter wavelength (λ/4). FIG. 2 illustrates an exploded perspective view of a handpiece assembly  70  including the ultrasonic transducer  82  and housing  20  in accordance with the present invention. Handpiece assembly  70  includes cable  25 , housing  20 , acoustic assembly  80 , and a selected stud  50 . housing  20  includes proximal portion  22 , distal portion  24 , nose-cone  34 , and O-rings  21 ,  23 , and  32 . Acoustic assembly  80  includes transducer  82  described above, and ancillary components including acoustic isolator  26 , electrode assembly  30 , bolt  106 , positive electrodes  96 , negative electrodes  98 , and insulator  28 . 
     Referring to FIGS. 1 and 2, wires  102  and  104  transmit the electrical signal from the generator  15  to electrodes  96  and  98 . The piezoelectric elements  100  are energized by an electrical signal supplied from the generator  15  in response to a foot switch  118  to produce an acoustic standing wave in the acoustic assembly  80 . The electrical signal causes disturbances in the piezoelectric elements  100  in the form of repeated small displacements resulting in large compression forces within the material. The repeated small displacements cause the piezoelectric elements  100  to expand and contract in a continuous manner along the axis of the voltage gradient, producing longitudinal waves of ultrasonic energy. The ultrasonic energy is transmitted through the acoustic assembly  80  to the end-effector. 
     The piezoelectric elements  100  are conventionally held in compression between end-bell  92  and fore-bell  94  by a bolt  106 . The bolt  106  preferably has a head, a shank, and a threaded distal end. The bolt  106  is inserted from the proximal end of end-bell  92  through the bores of end-bell  92 , the electrodes  96  and  98 , and piezoelectric elements  100 . The threaded distal end of the bolt  106  is screwed into a threaded bore in the proximal end of fore-bell  94 . 
     In order for the acoustic assembly  80  to deliver energy all components of acoustic assembly  80  must be acoustically coupled. The distal end of the transducer  82  may be acoustically coupled to the proximal end of an ultrasonic end-effector by a threaded connection such as stud  50 . 
     The components of the acoustic assembly  80  are preferably acoustically tuned such that the length of any assembly is an integral number of one-half wavelengths (nλ/2), where the wavelength λ is the wavelength of a pre-selected or operating longitudinal vibration drive frequency f d  of the acoustic assembly  80 , and where n is any positive integer. It is also contemplated that the acoustic assembly  80  may incorporate any suitable arrangement of acoustic elements. 
     FIG. 3 illustrates a sectioned plan view of the distal-end  95  of acoustic assembly  80  along with the plurality of attachment studs  50  of differing lengths in accordance with the present invention. Distal-end  95  includes bore  110 , threaded portion  111 , and terminal face  112 . Studs  50  may be sorted by size or mass such as, for example, P 1  through P 5  as described below in Table 1. 
     A method of manufacture and method of tuning have been developed to eliminate the need to trim acoustic assembly  80  at, for example, terminal face  112  during the manufacturing process. Utilizing the methods of the present invention, acoustic assembly  80  may be designed to have an acoustic length (nλ/2). “Stack-up” resonant frequency discrepancies of acoustic assembly  80  may be corrected by proper selection of a tuning element such as, for example, stud  50 , when the relationship between stud size or mass and the frequency effect on acoustic assembly  80  of insertion of stud  50  into threaded portion  111  of bore  110  is understood. It can be appreciated that other tuning elements may be utilized to correct for resonant frequency variations, such as, for example, selection from a plurality of fore-bells  94 , end bells  92 , or other ancillary components, each of which having varying masses. 
     
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Stud 50 sorted into lengths and associated tuning ranges 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Transducer 
                 55375 
                 55440 
                 55540 
                 55650 
                 55750 
               
               
                 Frequency 
                 +/−25 
                 +/−40 
                 +/−60 
                 +/−50 
                 +/−50 
               
               
                 P Level 
                 P1 
                 P2 
                 P3 
                 P4 
                 P5 
               
               
                 Stud Length 
                 .400 
                 .420 
                 .445 
                 .475 
                 .515 
               
               
                 (inches) 
                 stud 
                 stud 
                 stud 
                 stud 
                 stud 
               
               
                   
               
             
          
         
       
     
     Table 1 is provided as an example of frequency ranges and stud  50  lengths for an embodiment of the present invention. A measured transducer  82  frequency is shown in the first row, along with a frequency deviation range correctable by the stud  50  disclosed in each column. It may be appreciated that a transducer  82  may be designed to have a resonant frequency of 55,540 Hertz, corresponding to the column containing P Level (P 3 ). If the measured resonant frequency during assembly is within +/−60 Hertz of 55,540 Hertz, then a 0.445 inch stud may be inserted to keep transducer  82  within its design limits for frequency. As the measured resonant frequency of transducer  82  deviates above or below the designed frequency, an appropriate stud  50  length may be selected from Table 1 to compensate for the deviation and bring transducer  82  within the desired resonant frequency range. 
     FIG. 4 is a flow chart of an embodiment of ultrasonic transducer  82  assembly or tuning method in accordance with the present invention. Acoustic assembly  80  may be designed to have a resonant frequency f d , and an effective acoustic length of (nλ/2). However, tolerance “stack-up” variations in material properties of components, or other aspects of the assembly may cause acoustic assembly  80  to deviate from its designed resonant frequency as shown above in Table 1. During the assembly process ultrasonic transducer  82  or the entire acoustic assembly  80  may be measured for resonant frequency. Deviations from the desired resonant frequency may be corrected by proper selection and insertion of a tuning stud  50 . 
     The flow chart of FIG. 4 includes the steps of: 
     A) assembling a sandwich type ultrasonic transducer  82 , designated as process  115 ; 
     B) measuring the resonant frequency of the ultrasonic transducer  82 , designated as process  116 ; and 
     C) selecting a stud  50  from a plurality of Studs P 1  through P 5 , whereby the length of a selected stud can alter the measured resonant frequency of the ultrasonic transducer to a desired resonant frequency, designated as process  117 . 
     In another embodiment of the present invention, studs  50  of equal size but varying densities may be used. For example studs P 1  through P 3  may be of equal length, but stud P 1  may be manufactured from Aluminum, stud P 2  may be manufactured from stainless steel, and stud P 3  may be manufactured from Tungsten. The assembly or tuning process may select from one of the three studs of different densities to compensate for differences in resonant frequency. 
     A further embodiment of the present invention may be appreciated when considering a simple resonator model. The ability of a mass located around an anti-node of vibration to alter resonant frequency may be envisioned as analogous to a mass hanging at the end of a spring. If the mass is displaced and released, the mass spring system will vibrate at a resonant frequency. If the mass is increased, the resonant frequency will decrease. If the mass is decreased, the resonant frequency will increase. 
     Using the above analogy, as the stud size or mass increases, the overall resonant frequency of acoustic assembly  80  may be decreased from a measured resonant frequency to a desired resonant frequency within a range useable to correct for manufacturing variations. Likewise as the mass selected is decreased, the resonant frequency would be increased. 
     The ability of an added mass to alter the frequency of an acoustic assembly  80  changes as the mass deviates from an anti-node of acoustic vibration. If a mass is added at a node of vibration its effect on, the resonant frequency is due primarily to any stiffness it adds near the node. In the spring/mass analogy, the added mass at a node is analogous to increasing the spring rate. Alternately, if that same mass is located at an anti-node of vibration, its effect on the resonant frequency is due to the increased mass in the mass/spring analogy. The effect of the mass&#39; exact location about (within λ/4) an anti-node is much less pronounced than increasing or decreasing mass near an anti-node, but is still sufficient to tune within a limited frequency range. Changing location of a single mass with respect to its location about an anti-node of vibration has a similar effect as changing the mass located at the anti-node. This is due to the effectiveness (of the mass to alter frequency) being highest exactly at an anti-node, and its effectiveness decreasing as a cosine function as it is displaced from the anti-node. 
     Thus it is also possible to correct for resonant frequency variation by proper location of the center of mass of a tuning clement such as stud  50 . As illustrated in FIG. 3, studs P 1  through P 5  have centers of mass C 1  through C 5  respectively. If stud  50  is inserted into threaded portion  111  of bore  110  such that stud  50  extends from terminal face  112  at a consistent length regardless of which stud P 1  through P 5  is selected, then the location of the center of mass of stud  50  within distal-end  95  will vary as the length of stud  50  varies, as illustrated in FIG.  3 . 
     FIG. 5 is a flow chart of an embodiment of an ultrasonic transducer assembly or tuning method in accordance with the present invention. The flow chart of FIG. 5 includes the steps of selecting at least one piezoelectric clement, wherein the piezoelcctric element includes a central opening, designated as process  121 ; selecting an end-bell, wherein the end-bell includes a central opening, designated as process  120 ; selecting a fore-bell, the fore-bell including: a proximal surface; a distal surface; and a body separating the proximal surface and the distal surface; wherein the proximal surface includes a first threaded bore, and wherein the distal surface includes a second threaded bore, designated as process  119 ; selecting ancillary pieces, designated as process  123 ; assembling a transducer sandwich, designated as process  124 ; measuring the resonant frequency, designated as process  125 ; determining the appropriate range, designated as decisions  126  through  130 ; selecting from a plurality of studs according to the measured frequency rangy, designated processes  132  through  136  respectively to decisions  126  through  130 ; inserting the appropriate stud into the second threaded bore of the distal surface of the fore-bell, designated as process  137 ; and checking that the proper correction was accomplished by re-measuring resonance, designated as process  142 . Assemblies falling outside acceptable resonant frequency ranges are removed from the build as illustrated in process  131 . 
     FIG. 6 is a perspective view of an alternate embodiment of a tuning stud  120  in accordance with the present invention. Tuning stud  120  comprises a proximal threaded portion  122 , a central non-threaded portion  126 , and a distal threaded portion  124 . Central non-threaded portion  126  may be altered in length or diameter to vary the amount of resonant frequency shift desired when tuning stud  120  is inserted into threaded portion  111  of bore  110  illustrated in FIG.  3 . Central non-threaded portion  126  may also comprise materials of differing density, thereby altering the mass of tuning stud  120 . It can be appreciated that central non-threaded portion  126  may also be a washer placed onto stud  50  to perform as tuning stud  120 . 
     While preferred embodiments of the present invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.