Ultrasonic Transducer

Abstract

A method of optimising an ultrasonic transducer 100 with a bent blade 111 is disclosed. The method comprises a) defining a plurality of design parameters, wherein at least one design parameter is fixe

Description

The present invention relates to a method of optimising an ultrasonic transducer, particularly, although not necessarily exclusively, for surgical applications. Background to the Invention Ultrasonic transducers are known in the art for a variety of applications. One such application is in ultrasonic surgical devices (either for hard or soft tissue), which adopt ultrasonic vibrations to enhance cutting performance, employing a transducer mounted in a hand-held device. An ultrasonic transducer converts high-frequency electrical signals into mechanical vibrations. In ultrasonic surgical devices, the transducer is designed to operate at a predetermined frequency. The frequency of vibration is typically dependent on the tissue type encountered, such as skin, muscle, artery, or bone. Advantages of using ultrasonic surgical devices include their ability to coagulate vessels within a localised area, thereby minimising damage to surrounding tissue and reducing blood loss. The vibration of the ultrasonic transducer is typically achieved using piezoceramic rings which, when subjected to a high-frequency electrical signal, generate a force along the longitudinal axis of the ultrasonic transducer, inducing the desired vibrational motion. To optimise performance, the transducer typically comprises a front mass and back mass that are designed to resonate at the desired frequency. This resonance condition facilitates the efficient amplification of vibrations. A common requirement of ultrasonic surgical devices is for the surgical blade to bend, which improves visibility of the blade in operation. However, this intentional bending introduces asymmetry in the transducer's vibration pattern, which is detrimental to the performance of the ultrasonic transducer. Summary of the Invention There is generally a need for a method which addresses one or more of the problems identified above. Further aims and objects of the invention will become apparent from reading the following description. According to a first aspect of the invention, there is provided a method of optimising an ultrasonic transducer for surgical applications, the ultrasonic transducer comprising a blade having a bend, wherein the method comprises: a) defining a plurality of design parameters for the ultrasonic transducer, wherein at least one design parameter is fixed and at least one design parameter is variable; b) assigning an initial value to the at least one fixed design parameter and the at least one variable design parameter; c) measuring one or more output conditions of the ultrasonic transducer; d) adjusting at least one of the variable design parameters in response to the one or more measured output conditions; and e) repeating steps c) and d) to iteratively optimise the one or more output conditions to a predetermined value, wherein the one or more output conditions comprises blade shape of vibration, wherein the blade shape of vibration is defined by the tangency angle (0) between a tip vector (Vt) of the blade and a tip displacement vector (Vd) of the blade, and wherein the initial measured tangency angle is non-zero. The inventors of the present invention have surprisingly found that design parameters of the ultrasonic transducer can be iteratively adjusted to compensate for the asymmetry introduced in the transducer’s vibration pattern by the bend in the blade. By optimising the structure of the ultrasonic transducer, the operating performance is advantageously improved. Without wishing to be bound by theory, this is achieved by maximising the longitudinal motion of the blade and minimising the bending (transverse) motion. The blade may be described as an at least partially bent extrusion. The blade may preferably be tapered, in that that the cross-sectional area at a distal end is greater than the cross-sectional area at a proximal end. The shape of the cross-sectional area at the distal end may be the same as, or different to, the shape of the cross-sectional area at the proximal end. The shape of the cross-sectional area at the distal end may be straight circular. The shape of the cross-sectional area at the proximal end may be rectangular or square. The one or more output conditions are preferably representative of the mode shape of vibration; in other words, the shape in which the blade vibrates during operation. One or more of the at least one variable design parameters may be variable within a fixed range. The range may have an upper bound, a lower bound, or both. The one or more output conditions may comprise nodal plane position, nodal plane deflection angle, and / or blade shape of vibration. The predetermined value for one or more of the output conditions may be zero. The predetermined value for each output condition may be zero. Alternatively, some or all the output conditions may have a non-zero predetermined value. In some embodiments, the nodal plane position and nodal plane deflection angle have a predetermined value of zero, and the blade shape of vibration (represented by the tangency angle) may have a non-zero predetermined value. The ultrasonic transducer preferably comprises a back mass; a front mass; a fastening; an ultrasonic actuator arrangement (held between the back mass and the front mass); and an ultrasonic horn arrangement comprising the blade. The back mass, ultrasonic actuator arrangement, front mass and ultrasonic horn arrangement are arranged along a longitudinal axis of the transducer. Vibrations generated by the ultrasonic actuator arrangement are conducted into the front mass and into the ultrasonic horn arrangement along a vibrational energy transfer path and are amplitude amplified by the ultrasonic horn arrangement. The fastening may be a threaded fastening, optionally a prestressing bolt. The threaded fastening preferably extends through the back mass and ultrasonic actuator arrangement, and couples to the front mass or to the ultrasonic horn arrangement. The at least one fixed design parameter preferably comprises the length of the blade. The at least one fixed design parameter may comprise the operating frequency of the ultrasonic transducer. The fixed design parameters may comprise the length of the blade and the operating frequency of the ultrasonic transducer. The variable design parameters may comprise the bend angle of the blade. The blade may comprise a bent portion and a straight portion. In other words, the blade comprises a portion which is not bent. The variable design parameters may comprise the ratio of the length of the straight portion to the length of the bent portion. One or more of the back mass, front mass and ultrasonic horn arrangement may comprise a plurality of openings (i.e. apertures) opening towards the longitudinal axis and intersecting the vibrational energy transfer path and configured to provide an increased mechanical compliance in a direction along the vibrational energy transfer path. Preferably, the front mass comprises a plurality of openings. Suitable cross-sectional shapes for the openings include circular, oval, elliptical, round, triangular, quadrilateral, rectangular, square, rhombus, pentagonal, hexagonal, pentagonal, octagonal, etc. The openings are described in more detail in WO / 2023 / 007013, which is incorporated herein by reference. The variable design parameters may comprise the area or size of the plurality of openings (i.e. the radius if the openings have a circular cross-sectional shape, or other characteristic linear dimension(s) for other shapes). The front mass may comprise one or more (preferably a plurality of) openings in a first region (also termed the "top side"), and one or more (preferably a plurality of) openings in a second region (also termed the "bottom side"), wherein the first and second regions are preferably on opposing faces of the front mass. The terms “top” and “bottom” are relative and are used for the purpose of understanding the present invention, and should not be construed as limiting the orientation of the ultrasonic transducer. The variable design parameters may comprise the difference in area or size of the openings in the first ("top side") region and the openings in the second ("bottom side") region. In some embodiments, the variable design parameters may comprise the difference between the radius of the openings in the first ("top side") region and the radius of the openings in the second ("bottom side") region. Thus, while in some embodiments the openings may each have the same size, in other embodiments the openings may have differing size. The method is preferably computer-implemented. In some embodiments, a computer simulation is used to measure the one or more output conditions of the ultrasonic transducer, preferably using finite element analysis techniques. The method may further comprise step f): making an ultrasonic transducer in accordance with the optimised design parameters. In these embodiments, it may be that steps a) to e) are computer-implemented. The ultrasonic transducer may be for surgical, therapeutic, and / or diagnostic applications. For the avoidance of doubt, these applications include dentistry. The ultrasonic transducer may be for human and / or veterinary usage. According to a second aspect of the invention, there is provided a computer-implemented method of optimising an ultrasonic transducer for surgical applications, the ultrasonic transducer comprising a blade having a bend, wherein the method comprises: a) defining a plurality of design parameters for the ultrasonic transducer, wherein at least one design parameter is fixed and at least one design parameter is variable; b) assigning an initial value to the at least one fixed design parameter and the at least one variable design parameter; c) using a computer simulation to measure one or more output conditions of the ultrasonic transducer; d) adjusting at least one of the variable design parameters in response to the one or more measured output conditions; and e) repeating steps c) and d) to iteratively optimise the one or more output conditions to a predetermined value. Embodiments of the second aspect of the invention may include one or more features of the first aspect of the invention or its embodiments, or vice versa. According to a third aspect of the invention, there is provided a method of optimising an ultrasonic transducer for surgical applications, the ultrasonic transducer comprising a blade having a bent portion and a straight portion, wherein the method comprises: a) defining a plurality of design parameters for the ultrasonic transducer, wherein at least one design parameter is fixed and at least one design parameter is variable, wherein the at least one variable design parameter comprises the ratio of the length of the straight portion to the length of the bent portion; b) assigning an initial value to the at least one fixed design parameter and the at least one variable design parameter; c) measuring the tangency angle between a tip vector of the blade and a tip displacement vector of the blade; d) adjusting the ratio of the length of the straight portion to the length of the bent portion in response to the tangency angle; and e) repeating steps c) and d) to iteratively optimise the tangency angle to a predetermined value. The at least one fixed design parameter preferably comprises the length of the blade. Embodiments of the third aspect of the invention may include one or more features of the first or second aspects of the invention or their embodiments, or vice versa. According to a fourth aspect of the invention, there is provided a method of optimising an ultrasonic transducer for surgical applications, the ultrasonic transducer comprising a blade having a bend, wherein the method comprises: a) defining a plurality of design parameters for the ultrasonic transducer, wherein at least one design parameter is fixed and at least one design parameter is variable, wherein the at least one variable design parameter comprises the size of a plurality of openings in the ultrasonic transducer; b) assigning an initial value to the at least one fixed design parameter and the at least one variable design parameter; c) measuring the nodal plane position; d) adjusting the size of the plurality of openings in response to the nodal plane position; and e) repeating steps c) and d) to iteratively optimise the nodal plane position to a predetermined value. The at least one fixed design parameter preferably comprises the length of the blade. Embodiments of the fourth aspect of the invention may include one or more features of the first to third aspects of the invention or their embodiments, or vice versa. According to a fifth aspect of the invention, there is provided a method of optimising an ultrasonic transducer for surgical applications, the ultrasonic transducer comprising a blade having a bend, wherein the method comprises: a) defining a plurality of design parameters for the ultrasonic transducer, wherein at least one design parameter is fixed and at least one design parameter is variable, wherein the at least one variable design parameter comprises the difference in size of a plurality of openings across at least two regions of the ultrasonic transducer; b) assigning an initial value to the at least one fixed design parameter and the at least one variable design parameter; c) measuring the nodal plane deflection angle; d) adjusting the difference in size, in response to the nodal plane deflection angle; and e) repeating steps c) and d) to iteratively optimise the nodal plane position to a predetermined value. By “difference in size” it is meant that the one or more openings in a first region have a first size, the one or more openings in a second region have a second size, and that the difference in size between the first size and the second size can be adjusted as a variable design parameter. The at least one fixed design parameter preferably comprises the length of the blade. Embodiments of the fifth aspect of the invention may include one or more features of the first to fourth aspects of the invention or their embodiments, or vice versa. According to a sixth aspect of the invention, there is provided a method of making an ultrasonic transducer in accordance with the parameters optimised by the method according to any one of the first to fifth aspects of the present invention. Embodiments of the sixth aspect of the invention may include one or more features of the first to fifth aspects of the invention or their embodiments, or vice versa. According to a seventh aspect of the invention, there is provided an ultrasonic transducer made in accordance with the parameters optimised by the method according to any one of the first to fifth aspects of the present invention. Embodiments of the seventh aspect of the invention may include one or more features of the first to sixth aspects of the invention or their embodiments, or vice versa. According to an eighth aspect of the invention, there is provided a kit of parts comprising parts operable to be assembled into an ultrasonic transducer according to the seventh aspect of the present invention. Embodiments of the eighth aspect of the invention may include one or more features of the first to seventh aspects of the invention or their embodiments, or vice versa. According to a ninth aspect of the invention, there is provided a surgical tool comprising an ultrasonic transducer according to the seventh aspect of the present invention. The surgical tool may further comprise one or more of a casing, a clamping jaw, and a mechanism for actuating the clamping jaw. The surgical tool may be operated by a human or by a programmable machine, such as a robot. Embodiments of the ninth aspect of the invention may include one or more features of the first to eighth aspects of the invention or their embodiments, or vice versa. According to a tenth aspect of the invention, there is provided a method of operation of an ultrasonic transducer according to the seventh aspect of the present invention. The method preferably comprises applying an electrical signal to the ultrasonic actuator arrangement to generate vibrations to be conducted into the front mass and into the ultrasonic horn arrangement along a vibrational energy transfer path and amplitude amplified by the ultrasonic horn arrangement. Embodiments of the tenth aspect of the invention may include one or more features of the first to ninth aspects of the invention or their embodiments, or vice versa. Brief Description of the Drawings There will now be described, by way of example only, various embodiments of the invention with reference to the drawings, of which: Figure 1 is a side perspective view of an ultrasonic transducer for use in an embodiment of the present invention; Figure 2 shows a blade for use in an embodiment of the present invention, in side view (Figure 2A), and in side perspective view (Figure 2B); Figure 3 is a side view of an ultrasonic transducer for use in an embodiment of the present invention; Figure 4 is a side view of a blade for use in an embodiment of the present invention, illustrating the tangency angle 0 between the tip vector and tip displacement vector; Figure 5 is a side view of an ultrasonic transducer for use in an embodiment of the present invention, illustrating the nodal plane deflection angle 5; Figure 6 is a graph showing the effect of adjusting / bent and a on the tangency angle 0; Figure 7 is a flowchart showing a method of optimising an ultrasonic transducer according to an embodiment of the present invention; and Figure 8 is a flowchart showing a method of optimising an ultrasonic transducer according to an embodiment of the present invention. Detailed Description of the Preferred Embodiments An explanation of the present invention will now be described with reference to Figures 1 to 8. Structure of Ultrasonic Transducer The ultrasonic transducer may have any suitable configuration known in the art, including those disclosed in WO / 2023 / 007013 and the applicant’s pending GB application number GB2308810.7, both of which are incorporated herein by reference. As shown in Figure 1, an exemplary transducer 100 has a back mass 101 and a front mass 102 with an ultrasonic horn arrangement 103 located forward of the front mass 102. Two piezoceramic rings 107 of opposing polarity sandwich an electrode 106 to form a piezoelectric stack 108 (also termed an ultrasonic actuator arrangement). The back mass 101, piezoelectric stack 108, front mass 102 and ultrasonic horn arrangement 103 are arranged along a longitudinal axis A of the transducer 100. The piezoelectric stack 108 is held between the back mass 101 and the front mass 102 by a threaded fastening which, in this particular example, is a prestressing bolt. The head of the bolt 104 is positioned adjacent to the back mass 101. The threaded portion of the prestressing bolt (not shown) extends through the back mass 101 and piezoelectric stack 108, and couples to the front mass 102. The front mass 102 has a plurality of openings (i.e. holes or apertures) 109,110 formed through it. The openings open towards the longitudinal axis A and intersect the vibrational energy transfer path, providing an increased mechanical compliance in an axial direction parallel to the longitudinal axis A and along the vibrational energy path. The openings 109,110 have a circular cross-sectional shape, but it will be appreciated that the openings 109,110 may have any suitable shape. The front mass 102 has a proximal portion 102a, intermediate portion 102b and distal portion 102c. The proximal portion 102a is in contact with the piezoelectric stack 108. The distal portion 102c is in contact with the ultrasonic horn arrangement 103. There is a plurality of openings 109 formed in the proximal portion 102a (also called flange apertures), and there is a plurality of openings 110 formed in the distal portion 102c (also called main apertures). It will be appreciated that, while preferred, it is not essential for the ultrasonic transducer to have an arrangement of openings. In operation, a driving signal is applied to electrode 106 and the front 102 and back 101 masses are earthed, causing oscillation of the piezoelectric rings 107. The back mass 101, piezoceramic rings 107, electrodes 106, the front mass 102 and ultrasonic horn arrangement 103 are arranged along a longitudinal axis A of the transducer. Vibrations generated by the piezoceramic rings 107 are conducted into the front mass 102 and into the ultrasonic horn arrangement 103 along a vibrational energy transfer path. The vibrations are then amplitude amplified by the ultrasonic horn arrangement 103. Referring to Figures 1 and 2, the ultrasonic horn arrangement 103 comprises a blade 111. The blade 111 has a curved (or bent) profile. The blade 111 can also be considered to have a tapered profile, in that that the cross-sectional area at the distal end (i.e. distal to the front mass 102) is greater than the cross-sectional area at the proximal end. The shape of the cross-sectional area at the distal end is preferably straight circular, to provide continuity with the front mass. The shape of the cross-sectional area at the proximal end is preferably rectangular, to provide a flat surface for a clamping mechanism (e.g. a clamping jaw). As previously noted, an ultrasonic transducer blade is typically bent to improve visibility during operation. However, the blade 111 is usually not bent along its entire length. Instead, the blade 111 comprises a straight portion 111a and a bent portion 111b. The straight portion 111 a is at the proximal end, and the bent portion 111 b is at the distal end. In operation, the bent blade 111 vibrates between a resting state and a fully extended (i.e. tensioned) state. It will be appreciated that various modifications can be made to the structure of the ultrasonic transducer. For example, different fastening means can be used. Design Parameters The present invention requires a plurality of design parameters for the ultrasonic transducer, wherein at least one design parameter is fixed and at least one design parameter is variable. Design parameters are parameters which define the structure and shape of the ultrasonic transducer. Typically, the fixed design parameter is the length of the blade 111. The blade length I may be assigned any suitable value (and will be dependent on the specific application), but typically is between 10 mm and 25 mm. There are many variable design parameters, some or all of which may be adjusted during the method of the present invention. Blade Parameters As noted above, the blade 111 comprises a straight portion 111a and a bent portion 111b. Thus, a blade parameter is the ratio of the length of the straight portion 111a ( / straight) to the length of the bent portion 111b ( / bent). For the purpose of the present application, / ratio is defined as / straight divided by / bent. While / ratio is a variable parameter, it is preferred that it is variable within a fixed range. For example, it is preferred that / ratio is at least 0.2 and no greater than 3.5. A further blade parameter is the bend angle (a) of the bent portion 111b. While the bend angle is a variable parameter, it is preferred that it is variable within a fixed range. For example, it is preferred that the bent angle is at least 5° and no greater than 35°. Aperture Parameters As it is not essential in the present invention for the ultrasonic transducer to have at least one arrangement of openings, clearly these parameters are only relevant for ultrasonic transducers having at least one arrangement of openings (i.e. apertures). As shown in Figure 1 and discussed above, there is a plurality of openings 109 formed in the proximal portion 102a of the front mass 102 (also called flange apertures), and there is a plurality of openings 110 formed in the distal portion 102c of the front mass 102 (also called main apertures). While for the purpose of this detailed description it is the main apertures 110 that will be considered, the size and shape of the flange apertures 109 may also be relevant design parameters. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 A variable aperture parameter is the area or size (i.e. radius) of the main apertures 110, defined as r. However, it is possible to have further control over the aperture size by varying the relative area or size of different groups of the main apertures 110. As shown in Figure 3, a plurality of the main apertures 110 in a first region can be termed the top side main apertures 110a, and a plurality of the main apertures 110 in a second region can be termed the bottom side main apertures 110b. The top side main apertures 110a have a radius of rtop and the bottom side main apertures 110b have a radius of / bottom. Thus, rtop and / bottom are also variable aperture parameters. The difference in radius between the top and bottom apertures, rdiff, is / bottom - rtop. Other Parameters Aside from blade and aperture parameters, there are other design parameters of the ultrasonic transducer, of which none, some, or all may be adjusted during the method of the present invention. These design parameters can either be fixed or variable. 20 21 22 Table 1: Other design parameters Design Parameter Typical Values / Ranges Length of the ultrasonic transducer* Typically not more than 50 mm. However, in some applications, may be at least 100 mm. Maximum diameter of the ultrasonic transducer Not more than 15 mm Outer diameter of front mass About 3 mm to about 13 mm Inner diameter of front mass About 1 mm to about 8 mm Outer diameter of back mass About 3 mm to about 13 mm Inner diameter of back mass About 1 mm to about 8 mm Thread diameter of prestressing bolt About 3 mm to about 6 mm, or about 3 mm, or about 6 mm Length of prestressing bolt About 10 mm to about 20 mm, or about 10 mm, or about 20 mm ★measured from the proximal end of the back mass to the distal end of the ultrasonic horn arrangement, along the vibrational energy transfer path. Output Conditions The inventors of the present invention have identified at least three output conditions of the ultrasonic transducer. These output conditions are representative of the mode shape of vibration, and thus are an indicator as to the performance of the ultrasonic transducer. The output conditions can be calculated using a finite element analysis (FEA) software package. In the following examples, Abaqus™ was used as the FEA software. However, it will be appreciated that there are alternative software packages that can be used. The measurements are made by using the software to find the resonant modes of vibration. The software calculates the shapes and frequencies of vibration. The output conditions are measured from the shapes of vibration provided by the software. The simulation results have been experimentally verified using 3D laser vibrometry techniques (which are known in the art) from physical (i.e. real world) ultrasonic transducers. Blade Shape of Vibration The inventors have found that the resonant mode shape is affected by some or all of the above-mentioned design parameters of the ultrasonic transducer. To quantify this, tip vectors can be used (see Figure 4): • Vt is the tip vector and is tangential to the (shape of the) blade tip. • Vd is the tip displacement vector and is reflective of how the blade vibrates. Vd is inline with vibration path of the tip. In other words, Vd is the vector between the blade tip in the undeformed state of the blade, and the blade tip in the fully extended (i.e. tensioned) state of the blade. • 0 is the tangency angle between Vt and Vd. If 0 is equal to zero, the displacement of the blade's tip is fully tangentially with the tip profile, indicating a purely longitudinal vibration mode (see right-hand-side of Figure 4). If 0 is non-zero, the vibration mode exhibits flexural (or bending) characteristics (see left-hand-side of Figure 4). In most applications, it is desirable for 0 to be equal, or as close as possible, to zero. This ensures minimal energy dissipation into a bending mode. However, there may be some applications where a non-zero bending mode is required, such that a value of 0 of greater or less than zero is desired. Nodal Plane Position &Deflection Angle Each resonant mode has locations of minimal amplitude, known as nodes, and locations of maximum amplitude, known as anti-nodes, which constitute a nodal plane. To maximise vibration amplitude along the longitudinal axis A, the nodal plane should be perpendicular to the longitudinal axis A. The nodal plane deflection angle (5) is the angle between the nodal plane (denoted by 501 in Figure 5) and the axis perpendicular to the longitudinal axis A (denoted by 502 in Figure 5). A lower value of 5 (i.e. close to zero) indicates a more efficient conversion of the force generated by the piezoelectric stack 108 into longitudinal vibrations. It is also desirable (and, in some applications, necessary) for the position of the nodal plane to be aligned with the fixing point on the ultrasonic transducer. The fixing point is defined as the position along the longitudinal axis A where the fastening (e.g. the prestressing bolt) couples to the front mass or to the ultrasonic horn arrangement (depending on the specific configuration of the ultrasonic transducer). The nodal plane position (n) is defined as the distance of the nodal plane relative to the fixing point along the longitudinal axis A. A positive value of n indicates the nodal plane is positioned forward of the fixing point in the direction of the blade 111, and a negative value of n indicates the nodal plane is positioned backward of the fixing point in the direction of the back mass 101. Effect of Adjusting Variable Design Parameters on Output Conditions The following non-limiting examples show how changing one or more of the variable design parameters affects one or more of the output conditions. Nodal Plane Position The inventors of the present invention have surprisingly found that by changing the size of all the main apertures 110 equally, the centre of the nodal plane can be displaced along 2 3 4 5 the longitudinal axis A. Thus, the position of the nodal plane can be adjusted towards the fixing point by changing the size of the main apertures 110. Table 2: An example showing the effect of aperture size on nodal plane position. Radius of main apertures (r) 1 mm Nodal plane position (n) 1 mm 0.4 -0.96 0.5 -0.63 0.6 -0.45 0.7 -0.34 0.8 0.31 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Nodal Plane Deflection Angle The inventors of the present invention have surprisingly found that the nodal plane deflection angle (5) can be controlled by changing the asymmetry in the main apertures 110. That is, by changing the size of the “top side” main apertures 110a compared to the “bottom side” main apertures 110b (or vice versa). The nodal plane deflection angle can be controlled by increasing the size of the apertures on one side, decreasing the size of the apertures on the other, or both at the same time. Typically, the size of the top side main apertures 110a is reduced to move 5 towards zero. Without wishing to be bound by theory, it is believed that decreasing the size of an aperture rotates the nodal plane in one direction, and increasing the size of an aperture rotates the nodal plane in the other direction. Table 3: An example showing the effect of relative difference in main aperture size on the nodal plane deflection angle. A positive value of raff indicates that the top side apertures are smaller than the bottom side apertures. Difference in radius between bottom and top apertures (rditf) 1 mm Nodal plane deflection angle (5) 0.000 11.3° 0.025 7.7° 0.050 3.1° 0.075 -3.0° 0.100 -5.9° Blade Shape of Vibration The inventors of the present invention have surprisingly found that the tangency angle 0 between Vd and Vt can be controlled by adjusting the ratio of the length of the straight portion 111a ( / straight) to the length of the bent portion 111b ( / bent), as well as by adjusting the bend angle (a) of the bent portion 111b. The inventors have also surprisingly found that if 0 is equal to zero, adjusting the bend angle (a) has minimal effect on 0. On the other hand, if 0 is non-zero, then increasing the bent angle (a) amplifies the tangency angle 0. Figure 6 is a graph which illustrates the effect of changing / bent (for a fixed value of / of 18 mm) and a on the tangency angle 0. Flowcharts Figure 7 is a flowchart showing a method of optimising an ultrasonic transducer according to an embodiment of the present invention. In this embodiment, the ultrasonic transducer is permitted to have one or more arrangements of openings (i.e. apertures). As a first step (represented by box 701), the one or more fixed design parameters of the ultrasonic transducer are defined. In this example, the blade length I is defined, and assigned a fixed value of 18 mm. The next step (represented by box 702) is to define the one or more variable design parameters, to assign an initial value to each of the one or more variable parameters, and to define whether any of the variable design parameters are variable within a fixed range. In this example: • / straight is defined and assigned an initial value of 6 mm, and is variable between 4 mm and 14 mm; • a is defined and assigned an initial value of 5°, and is variable between 5° and 35° (typically a low initial value is assigned to a); • r is defined and assigned an initial value of 0.5 mm, and is variable between 0.4 mm and 1 mm; and 1 • Tdiff is defined and assigned an initial value of 0 mm, and is variable between -0.1 2 mm and +0.1 mm. 3 4 It will be appreciated that other initial values can be used as appropriate. 5 6 At this step, the output conditions are also defined, and are each assigned a 7 predetermined (i.e. target) value. In other words, the optimised output conditions are 8 defined. In this example: 9 • the nodal plane position (n) should be sufficiently close, or equal, to zero (i.e. 10 having a predetermined value of zero). 11 • the nodal plane deflection angle (5) should be sufficiently close, or equal, to zero 12 (i.e. having a predetermined value of zero); and 13 • the tangency angle (0) should be sufficiently close, or equal, to zero (i.e. having a 14 predetermined value of zero). 15 16 By “sufficiently close to” it is meant that the output condition need not be equal to zero, but 17 close enough that the output conditions meet the requirements of the particular application 18 of the ultrasonic transducer. In other words, the output condition is equal to the 19 predetermined value within a margin of error, or within a margin of acceptable tolerance. 20 An assessment of whether the measured output condition is “sufficiently close to” the 21 optimised output condition would be within the abilities of the skilled person. For the 22 purpose of the following examples, the measured output conditions will be assessed as to 23 whether they are equal to zero. 24 25 It will be appreciated that the optimised output conditions need not be sufficiently close, or 26 equal, to zero. In some embodiments, it is desirable for the optimised tangency angle (0) 27 in particular to be non-zero. 28 29 Once the design parameters and initial values are established, the nodal plane position n 30 is measured (represented by box 703). At decision step 704, it is asked whether n is equal 31 to zero (i.e. the nodal plane is positioned at the fixing point). If the answer is no (“N”), the 32 size of the main apertures 110 (r) is adjusted (represented by box 705), and the cycle 33 repeated. If the answer is yes (“Y”), the method proceeds to step 706, where the nodal 34 plane deflection angle 5 is measured. At decision step 707, it is asked whether 5 is equal 35 to zero (i.e. the nodal plane is perpendicular to the longitudinal axis A). If the answer is no, the size of the top side main apertures 110a (rtop) and / or bottom side main apertures 110b (rbottom) are adjusted (represented by box 708). If 5 is greater than zero, rtop is decreased and / or rbottom is increased, such that rditf has a value of greater than zero. If 5 is less than zero, rtop is increased and / or rbottom is decreased, such that rdjff has a value of less than zero. The cycle (through steps 706 and 707) is then repeated. If the answer at decision step 707 is yes, the method proceeds to step 709. At step 709, the tangency angle 0 is measured. At decision step 710, it is asked whether 0 is equal to zero. If the answer is no, the value of / ratio is adjusted (represented by box 711). This is achieved by changing the lengths of / straight and / bent (noting that I is fixed at 18 mm). The cycle (through steps 709 and 710) is then repeated. If the answer at decision step 710 is yes, the method proceeds to step 712. At step 712, the bend angle a is adjusted (i.e. increased) to a desired value. The tangency angle 0 is then measured (represented by box 713) because, as previously discussed, the bend angle a may have an effect on the tangency angle 0. At decision step 714, it is asked whether 0 is still equal to zero. If the answer is no, the method returns to step 711 and the value of / ratio is adjusted. If the answer is yes, the method proceeds to step 715. At step 715, each of the nodal plane position n, the nodal plane deflection angle 5 and the tangency angle 0 are remeasured. If all three values are equal to zero, then the method is complete and the ultrasonic transducer has been optimised (represented by box 717). If any of the three values are non-zero, the method returns to step 703 and the optimisation method is repeated. The inventors have found the method to be iterative (as some or all of the variable design parameters have an effect on some or all of the output conditions), but that it converges on optimised output conditions. As described above, Figure 7 shows at least three “iterative optimisation cycles” - the first being 703,704,705, the second being 706,707,708 and the third being 709,710,711. It will be appreciated that it is not essential for all three cycles to be present in the method of the present invention, and may instead comprise one or two of the above-described cycles. Alternatively, it will be appreciated that none of the above-described cycles are present in an embodiment, and instead comprises cycles which use other design parameters and / or output conditions. It is within the abilities of the skilled person to decide which cycles are appropriate, depending on the specific application of the ultrasonic transducer and / or any specific design requirements or restrictions. As an example of a design restriction, Figure 8 is a flowchart showing a method of optimising an ultrasonic transducer according to an embodiment of the present invention. In this embodiment, the ultrasonic transducer is absent of any openings or apertures. As a first step (represented by box 801), the one or more fixed design parameters of the ultrasonic transducer are defined. In this example, the blade length I is defined, and assigned a fixed value of 18 mm. The next step (represented by box 802) is to define the one or more variable design parameters, to assign an initial value to each of the one or more variable parameters, and to define whether any of the variable design parameters are variable within a fixed range. In this example: • / straight is defined and assigned an initial value of 6 mm, and is variable between 4 mm and 14 mm; and • a is defined and assigned an initial value of 5°, and is variable between 5° and 35°; At this step, the output conditions are also defined, and are each assigned a predetermined (i.e. target) value. In this example: • the tangency angle (0) should be sufficiently close, or equal, to zero (i.e. having a predetermined value of zero). Once the design parameters and initial values are established, the tangency angle 0 is measured (represented by box 809). At decision step 810, it is asked whether 0 is equal to zero. If the answer is no (“N”), the value of / ratio is adjusted (represented by box 811). This is achieved by changing the lengths of / straight and / bent (noting that I is fixed at 18 mm). The cycle (through steps 809 and 810) is then repeated. If the answer at decision step 810 is yes, the method proceeds to step 812. At step 812, the bend angle a is adjusted (i.e. increased) to a desired value. The tangency angle 0 is then measured (represented by box 813). At decision step 814, it is asked whether 0 is still equal to zero. If the answer is no, the method returns to step 811 and the value of / ratio is adjusted. If the answer is yes, then the method is complete and the ultrasonic transducer has been optimised (represented by box 817). A method of optimising an ultrasonic transducer with a bent blade is disclosed. The method comprises a) defining a plurality of design parameters, wherein at least one design parameter is fixed and at least one design parameter is variable; b) assigning an initial value to the at least one fixed design parameter and the at least one variable design parameter; c) measuring one or more output conditions; d) adjusting at least one of the variable design parameters in response to the one or more measured output conditions; and e) repeating steps c) and d) to iteratively optimise the one or more output conditions to a predetermined value. The design parameters of the ultrasonic transducer can be iteratively adjusted to compensate for the asymmetry introduced in the transducer’s vibration pattern by the bend in the blade. By optimising the structure of the ultrasonic transducer, the operating performance is advantageously improved. Throughout the specification, unless the context demands otherwise, the terms “comprise” or “include”, or variations such as “comprises” or “comprising”, “includes” or “including” will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers. Furthermore, unless the context clearly demands otherwise, the term “or” will be interpreted as being inclusive not exclusive. The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method of optimising an ultrasonic transducer for surgical applications, the ultrasonic transducer comprising a blade having a bend, wherein the method comprises:a) defining a plurality of design parameters for the ultrasonic transducer, wherein at least one design parameter is fixed and at least one design parameter is variable;b) assigning an initial value to the at least one fixed design parameter and the at least one variable design parameter;c) measuring one or more output conditions of the ultrasonic transducer;d) adjusting at least one of the variable design parameters in response to the one or more measured output conditions; ande) repeating steps c) and d) to iteratively optimise the one or more output conditions to a predetermined value, wherein the one or more output conditions comprises blade shape of vibration, wherein the blade shape of vibration is defined by the tangency angle (0) between a tip vector (Vt) of the blade and a tip displacement vector (Vd) of the blade, and wherein the initial measured tangency angle is non-zero.

2. The method according to claim 1, wherein the one or more output conditions are representative of a mode shape of vibration.

3. The method according to claim 1 or claim 2, wherein one or more of the at least one variable design parameters are variable within a fixed range.

4. The method according to any one of claims 1 to 3, wherein the one or more output conditions comprise nodal plane position and / or nodal plane deflection angle.

5. The method according to any one of claims 1 to 4, wherein the at least one fixed design parameter comprises the length of the blade.

6. The method according to any one of claims 1 to 5, wherein the variable design parameters comprise the bend angle of the blade.

7. The method according to any one of claims 1 to 6, wherein the blade comprises a bent portion and a straight portion.

8. The method according to claim 7, wherein the variable design parameters comprise the ratio of the length of the straight portion to the length of the bent portion.

9. The method according to any one of claims 1 to 8, wherein the ultrasonic transducer comprises a back mass; a front mass; a fastening; an ultrasonic actuator arrangement held between the back mass and the front mass; and an ultrasonic horn arrangement comprising the blade.

10. The method according to claim 9, wherein one or more of the back mass, front mass and ultrasonic horn arrangement comprises a plurality of openings.

11. The method according to claim 10, wherein the variable design parameters comprise the size of the plurality of openings.

12. The method according to any one of claims 9 to 11, wherein the front mass comprises one or more openings in a first region and one or more openings in a second region.

13. The method according to claim 12, wherein the variable design parameters comprise the difference in size of the openings in the first region and the openings in the second region.

14. The method according to any one of claims 1 to 13, wherein the method is at least partially computer-implemented, optionally wherein a computer simulation is used to measure the one or more output conditions of the ultrasonic transducer.

15. The method according to any one of claims 1 to 14, wherein the method further comprises step f): making an ultrasonic transducer in accordance with the optimised design parameters.

Images