Abstract:
A multi-height energy-directing device, for facilitating staged welding together of workpieces. The device includes multiple elements extending from a body, wherein the elements do not all have a same height. Each element of a first group of the elements has a first height while each element of a second group of the elements has a second height differing from the first height. The elements are configured to channel welding energy through the device in a predetermined manner. The predetermine manner includes channeling the energy through taller, primary, elements, of said elements, in an initial stage of the welding process, while passing none through shorter, secondary, elements, of said elements, in the initial stage.

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to an improved technique for welding together multiple workpieces and, more particularly, to systems and methods for improved ultrasonic welding, using at least one multi-height energy-directing device between the pieces. 
     BACKGROUND 
     In automotive manufacturing, polymetric composites are being used increasingly due to their favorable characteristics, such as being lightweight, highly-conformable or shapeable, strong, and durable. Some composites are further colorable and can be finished to have most any desired texture. 
     The increased use in automobiles includes, for instance, in instrument and door panels, lamps, air ducts, steering wheels, upholstery, truck beds or other vehicle storage compartments, upholstery, external parts, and even engine components. Regarding engine components, and other under-the-hood (or, UTH) applications, for instance, polymers are configured, and being developed continuously, that can withstand a hot and/or chemically aggressive environment. Regarding external parts, such as fenders, polymers are being developed that are online paintability and have high heat and chemical resistance over longer periods of time. And many other potential usages in automotive applications are being considered continuously. 
     With the increased use of polymers and other low-mass materials, compression molding and post-mold joining techniques—e.g., ultrasonic welding —are also being used more commonly. 
     Because some materials being used increasingly, including polymer composites, have relatively low melting points, a challenge arises in efforts to melt the parts at an interface joining the parts quickly and with minimal melting of other portions of the workpieces. 
     Energy directors are sometimes used to expedite and control welding. Once challenge arising in using energy directors is that they require a relatively large amount of input energy to function properly. 
     More particularly, in order for input energy, such as high-frequency vibrations, in the case of ultrasonic welding, to pass effectively through conventional directors, the energy has to be high enough to reach the director, through the first workpiece, and further to overcome resistance also of the director, itself, at all places where the director will melt. 
     In addition to higher than desired energy requirement, conventional approaches also require an undesirably-high cycle time—i.e., the time required for the energy to negotiate the workpieces and director as needed to form the weld. 
     The increased time and energy requirements are cost prohibitive, especially when multiplied by repeated iterations processing in a manufacturing environment—e.g., automobile assembly plant. 
     SUMMARY 
     The present technology relates to systems and methods for improved ultrasonic welding using at least one multi-height energy-directing device positioned between the workpieces during application of high-frequency ultrasonic vibrations from a servo horn, for instance. 
     Other aspects of the present invention will be in part apparent and in part pointed out hereinafter. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a two-sided ultrasound system. 
         FIG. 2  illustrates an example multi-height energy director, according to an embodiment of the present technology. 
         FIG. 3  illustrates a leg, or protrusion, of the multi-height energy director of  FIG. 2 . 
         FIG. 4  illustrates a method for locating an energy director, such as the multi-height energy director of  FIG. 2 , positioned between workpieces to be welded together, and performing the welding to join the pieces. 
         FIG. 5  illustrates a side view of the multi-height energy director of  FIG. 2  positioned between the workpieces. 
         FIG. 6  illustrates welding-tool positions occasioned in locating the workpiece according to example scenarios. 
         FIG. 7  shows a graph comparing welding-tool displacement in an application direction (e.g., vertical) an orthogonal tool position (e.g., lateral location over the workpiece). 
         FIG. 8  illustrates the multi-height energy director of  FIG. 2  at an interim stage of welding, before a second level of the director contacts the second workpiece. 
         FIG. 9  illustrates the multi-height energy director of  FIG. 2  at a subsequent interim stage of welding, at which the second level of the director first contacts the second workpiece. 
         FIG. 10  illustrates an example weld formed using the energy director of  FIG. 2 . 
         FIG. 11  illustrates an example controller, for use in performing operations of the method of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present disclosure are disclosed herein. The disclosed embodiments are merely examples that may be embodied in various and alternative forms, and combinations thereof. As used herein, for example, exemplary, and similar terms, refer expansively to embodiments that serve as an illustration, specimen, model or pattern. 
     The figures are not necessarily to scale and some features may be exaggerated or minimized, such as to show details of particular components. In some instances, well-known components, systems, materials or methods have not been described in detail in order to avoid obscuring the present disclosure. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the technology foci (e.g., claims), listed below, and as a representative basis for teaching one skilled in the art to variously employ the present disclosure. 
     While the description includes a general context of computer-executable instructions, the present disclosure can also be implemented in combination with other program modules and/or as a combination of hardware and software. The term application, or variants thereof, is used expansively herein to include routines, program modules, programs, components, data structures, algorithms, and the like. Applications can be implemented on various system configurations, including single-processor or multiprocessor systems, microprocessor-based electronics, combinations thereof, and the like. 
     I. General Overview Of The Disclosure 
     The present disclosure describes an ultrasonic welding technique for joining workpieces, such as polymeric composites. 
     One aspect of the disclosure relates to systems and methods for improved ultrasonic welding. The system includes at least one multi-height energy-directing device that, in use, is positioned between the workpieces during application of high-frequency ultrasonic vibrations from an ultrasonic energy application horn. 
     The system components, algorithm, and operations are described further below with reference to  FIGS. 1-9 . 
     II. Process, System Tooling, and Workpieces— FIGS. 1-11   
     The present technology is now described with reference to example systems, tooling, and workpieces. The figures are referenced to facilitate understanding of the technology, and not to limit scope thereof. 
     Reference to directions herein, such as upper, lower, up, down, and lateral, are provided to facilitate description of the present technology but does not limit scope of the technology. A description in which a servo horn is described as descending down upon a proximate workpiece is not limited, for example, to the horn moving vertically downward in the earth, or environment, frame. The horn in this case can be moving from left to right, for example, in the environment frame. 
     II.A. General Welding System— FIG. 1   
     Now turning to the figures, and more particularly, the first figure,  FIG. 1  shows an example welding system, indicated generally by reference numeral  100 . The system  100  is used to weld together two workpieces  101   1 ,  101   2 . 
     The system  100  includes a supporting, or under, structure  102 . The system also includes a welding arm  104  terminating in a welding energy application tip, or horn  106 . The horn can include, for instance, an ultrasonic servo horn, configured to apply energy, in the form of high-frequency vibrations, to the workpieces for welding them together. 
     The welding arm  104  extends from a second, or application-side, structure, or mass  108 . 
     In operation, an application-direction force  110  can be applied by and/or at the mass  108 . The force  110  pushes the arm  104  and horn  106  toward the workpieces  101  being welded together. A counterpart force  112  pushes the supporting structure  102  toward the workpieces. With the mass and application-direction force pushing toward the workpieces  101  from a first, application, direction, and the counter force  112  pushing toward the pieces  101  from an opposite direction, the workpieces  101  are kept at a desired compression during welding. 
     II.B. Multi-Height Energy-Directing Device— FIGS. 2 and 3   
       FIG. 2  shows an energy-directing device, or energy director  200 . 
     The energy director  200  can include any material described herein, including in connection with the workpieces. 
     In one embodiment, the energy director  200  is generally annular—e.g., has a generally annular, or ring-like, plan-view (e.g., top) profile. With reference to the figure, an upper portion of the annular configuration is identified by reference numeral  202 . 
     Importantly, it has been found that an annular weld can be as strong as continuous welds (i.e., welds not having a central void)—in one present finding, this is especially true when a ratio of an internal diameter to an external diameter is less than about 0.6. More specifically, under an applied tensile load, a predominant amount of the holding force created by a solid or continuous weld is provided by an outer annual portion of the weld, with a central portion of the weld contributing little holding force in comparison. A weld lacking the central portion, thus, can be formed with less energy than a continuous weld (one lacking a central void), and perhaps less time, without sacrificing joint strength. 
     While the energy director  206 , whether annular or other shape, can have other widths  208  without departing from the scope of the present technology, in one embodiment each director has a width  208  (e.g., diameter, or maximum width) between about 3 mm and about 20 mm. In one embodiment, the width  208  can be smaller, such as down to about 1 mm, and still possibly up to about 20 mm. 
     The upper portion  202  defines a central hole, or void  204 . While the void  204 , whether circular, oval, rectangular, or other, can have other internal widths  210  without departing from the scope of the present technology, in one embodiment each director  200  has one or more internal widths  210  between about 1.5 mm and about 12 mm. In one embodiment, the internal width  208  can be smaller, such as down to about 0.6 mm, and still possibly up to about 12 mm, for instance. 
     While the illustrated director  200  has a generally annular plan, or top, profile shape, the director can have other plan profile shapes. Other example shapes include oval, square, or other rectangular shapes, with a central void. 
     The energy director  200  includes a plurality of energy-director (ED) elements  206 . The elements may be referred to by other names such as a height-control ED element, protrusion, or ridge, or an elevation-control element, protrusion, or ridge. 
     The ED elements  206  extend, or protrude (e.g., protrude downward), from the upper portion  202  of the director  200 , such as shown in  FIG. 2 . In one embodiment, the ED  200  is formed during compression molding of one of the workpieces (e.g., proximate workpiece  101   1 ), and so is a contiguous part of that workpiece. 
     While the ED element  206  can have other shapes, in the illustrated embodiment, each director has a generally triangular side profile. Other example shapes include square, otherwise rectangular, or rounded—e.g., semi-circle or ovular. 
     In the illustrated embodiment, each ED element  206  includes an upper, or first, side, or base, connecting to the upper portion  202  of the element  206 . In the embodiment in which the ED  200  is formed during compression molding of one of the workpieces (e.g., proximate workpiece  101   1 ), and so is a contiguous part of that workpiece, the upper portion  202  of the element  206  includes the workpiece  101   1 . The sides extend from the base to a point opposite the upper portion  202 . 
     Importantly, the ED elements  206  do not all have the same characteristics. In one embodiment, at least one characteristic differing amongst at least some of the ED elements  206  is a height  212  of the elements. Benefits of this feature are described further below in connection with the welding sub-process of the method  400  of  FIG. 4 . 
     Generally, the benefits relate to an advantageous channeling of welding energy—e.g., ultrasonic vibrations—through primary ED elements initially, while passing less or not at all through secondary ED elements, in an early stage of welding, and through the secondary elements, while passing less or not at all through the primary ED elements in a subsequent stage of the welding. 
       FIG. 3  shows a side view of any of the ED elements  206  of  FIG. 2 . Along with the height  212  indicated in  FIG. 2 ,  FIG. 3  shows that the ED elements  206  can be defined by other features, such as width  302 . 
     While the ED elements  206  can have other widths  302  without departing from the scope of the present technology, in one embodiment each ED element  206  has a width  302  between about 1.0 mm and about 4.0 mm. In one embodiment, the width  302  can be smaller, such as down to about 0.2 mm, and still possibly up to about 4.0 mm. 
     Continuing with the triangular embodiment of  FIGS. 2 and 3 ,  FIG. 3  shows a vertical side length  304  as another size characteristic of the ED director. 
     In one embodiment, a ratio of the height  212  to the width  302  (H/W) is between about 0.3 and about 1.0. 
     In one embodiment, each primary element  206   1  of the elements  206  has a height of between about 0.5 mm and about 6.0 mm, and each secondary element  206   2  has a height between about 0.4 mm and about 4.0 mm. 
     The ED elements  206  can have any appropriate thickness, and, related, any desired three-dimensional shape, and each element can have any desired size—e.g., thickness or thicknesses. The elements  206  can have a generally pyramid shape. For ED elements having rounded sides, the three-dimensional shape can be prismatic (e.g., rectangular or triangular prism), cylindrical, conical, frustoconical, pyramid (e.g., triangle pyramid, or tetrahedron), partial sphere (e.g., semi-sphere, demi-sphere, or hemisphere), etc. ED elements  206  can have straight and/or curved sides. 
     As mentioned, the ED elements  206  do not all have the same characteristics. In a contemplated embodiment, along with or instead of varying heights, not every one of the ED elements  206  on a single energy director  200  has the same shape. Again, as with varying heights, benefits of varying the shape amongst the ED elements  206  are described further below in connection with the welding sub-process of the method  400  of  FIG. 4 . And again, generally, the benefits relate to an advantageous channeling of welding energy—e.g., ultrasonic vibrations—through primary ED elements initially, while passing less or not at all through secondary ED elements, in an early stage of welding, and through the secondary elements, while passing less or not at all through the primary ED elements in a subsequent stage of the welding. 
     II.C. Algorithm and Method of Operation— FIGS. 4-11   
     Now turning to the fourth figure,  FIG. 4  shows an exemplary algorithm, by way of a flow chart  400 , defining a method for (a) locating an energy director, such as the energy director  200  of  FIG. 2 , and (b) welding workpieces together by applying welding energy to a proximate workpiece at the identified location so that it channels through, and melts, the novel energy director as desired. The result is effective and efficient welding, and a more accurate and robust weld formed with less overall cycle time, energy, and energy-director material as compared to traditional techniques. 
     In some embodiments, the algorithm controls only some aspects of the method, such as the sub-process associated in  FIG. 4  with reference numeral  406 . In another, it controls operations  406  and  408 , and in another operations  404 ,  406 ,  408 , and  410 , for example. The operations are described further below, in turn. 
     While joining two workpieces is described primarily herein, the number is presented as an example, and more than two pieces may be joined according to the teachings of the present disclosure. 
     It should be understood that the steps of the method  400  are not necessarily presented in any particular order and that performance of some or all the steps in an alternative order is possible and is contemplated. The steps have been presented in the demonstrated order for ease of description and illustration. Steps can be added, omitted and/or performed simultaneously without departing from the scope of the appended technology foci (e.g., claims). And it should also be understood that the illustrated method  400  can be ended at any time. 
     In certain embodiments, some or all steps of this process, and/or substantially equivalent steps are performed by, or at least initiated by a computing device, such as a processor executing computer-executable instructions stored or included at a computer-readable medium. And any one or more steps of the process can be performed, initiated, or otherwise facilitated by automated machinery, such as robotics. 
     The method  400  outlined by the flow chart of  FIG. 4  is described now with additional reference to the tools and components of  FIGS. 5-10 . Characteristics of the elements shown, e.g., shape, size, and number, are presented to facilitate the present description and not to limit scope of the present technology. 
     The method  400  begins  401  and flow proceeds to block  402 , whereat an energy director, such as the director  206  shown in  FIG. 2 , is positioned between the workpieces.  FIG. 5  shows an example positioning of the energy director between adjacent workpieces. 
     In a contemplated embodiment, the energy director is formed in a sub-process of molding at least one of the workpieces. For instance, a mold in which the first workpiece is compression molded can include recesses and/or protrusions configured (e.g., sized and shaped) to form the energy director at a desired location of the workpiece. 
     As provided, the workpieces being welded together can be similar or dissimilar. Regarding dissimilar workpiece materials, one workpiece can be a plastic or other polymer, for instance, and the other can be steel, aluminum, an alloy, or other metal, etc. Thus, the teachings of the present disclosure can be used to join a polymer (e.g., polymer composite) to another polymer, or to join a polymer to a metal, for instance. 
     In one embodiment, the material includes polyethylene. In one embodiment, the material includes polyethylene terephthalate (PET), high density polyethylene (HDPE) and/or ethylene vinyl alcohol (EVOH). 
     In one embodiment, at least one of the workpieces being joined includes a polymer. At least one of the workpieces can include synthetic, or inorganic, molecules. While use of so-called biopolymers (or, green polymers) is increasing, petroleum based polymers are still much more common. 
     Material of one or both workpieces may also include recycled material, such as a polybutylene terephthalate (PBT) polymer, which is about eighty-five percent post-consumer polyethylene terephthalate (PET). 
     In one embodiment one or both of the workpieces includes some sort of plastic. In one embodiment, the material includes a thermo-plastic. 
     In one embodiment one or both of the workpieces includes a composite. For example, in one embodiment one or both of the workpieces includes a fiber-reinforced polymer (FRP) composite, such as a carbon-fiber-reinforced polymer (CFRP), or a glass-fiber-reinforced polymer (GFRP). The composite may be a fiberglass composite, for instance. In one embodiment, the FRP composite is a hybrid plastic-metal composite. 
     The material in some implementations includes a polyamide-grade polymer, which can be referred to generally as a polyamide. 
     Material of one or both workpieces may also include includes polyvinyl chloride (PVC). 
     In one embodiment, the material includes acrylonitrile-butadiene-styrene (ABS). 
     In one embodiment, the material includes a polycarbonate (PC). 
     Material of one or both workpieces may also comprise a type of resin. Example resins include a fiberglass polypropylene (PP) resin, a PC/PBT resin, and a PC/ABS resin. 
     The workpieces may be pre-processed, such as heated and compression molded prior to the welding. 
     In most manufacturing processes, more than one weld will made to connect two adjacent workpieces. The positioning of step  402  can thus include positioning multiple energy devices between the workpieces. 
     With continued reference to  FIG. 4 , with the energy director(s) positioned between the workpieces, flow proceeds to step  404  whereat the arrangement is positioned adjacent the weld system. This operation can include moving the workpiece/ED arrangement toward the welding system, and or moving aspects or an entirety of the welding system toward the arrangement. 
     The initial, coarse, positioning of step  404  can include positioning an ultrasonic horn of the system close to an estimated or believed location of the energy director to be used in the first weld. 
     Flow proceeds to the fine energy-director locating sub-process, or routine  406 . As shown in  FIG. 4 , the locating routine  406  includes multiple sub-steps, distinguished by superscripts—i.e.,  406   1-5 . From step  404 , the method turns particularly to the first routine step  406   1  whereat the welding head, or horn (e.g., ultrasonic servo horn, like the sonotrode tip of the example of  FIG. 1 ) is lowered. The horn is lowered toward the proximate workpiece—i.e., the workpiece closest to the horn, such as in  FIGS. 1 and 5 . 
     The descending is illustrated in  FIG. 6 . The coarse positioning of step  404  does not usually position the horn directly over the energy director. Instead the horn usually ends up initially positioned only partially over the energy director, as indicated by path  604  in  FIG. 6 , or not over the director at all, as indicated by path  602  in  FIG. 6 . The target path is the third  606 , which is reached by one or more iterations of the routine  406   1-5 , as further described below. 
     The descending operation  406   1  is performed under the operation of a controller connected directly or indirectly to the welding horn. Features of an example controller is shown in  FIG. 11 , and described further below. The controller controls, e.g., a rate at which the horn is lowered toward the proximate workpiece. The controller can, for instance, control or be a part of a robotic apparatus, or robot, controlling movement of the welding horn. 
     At the next step  406   2  of the routine  406 , the controller determines whether a push-back force being received at the weld horn from the workpiece, indicates that the horn has been lowered to a local terminal point. The controller determines this based on feedback (e.g., from a load cell) indicating a force, exerted by the workpiece  101   1 , on the welding horn. The control receives the force indications from a sensor (not shown in detail) that may be part of, or connected to, the welding system, or part of, or connected to, automated robotic apparatus controlling movement of the welding horn. 
     If it is determined at step  406   2  that the horn has not reached its local terminal point, then flow of the algorithm returns back to the first routine step  406   1 , as shown in  FIG. 4 . This will occur, for instance, while the horn is being lowered and had not yet contacted the workpiece. It will also occur when the horn has contacted the workpiece but not been lowered enough to receive a sufficient amount of push-back force from the workpiece. 
     When it is determined at step  406   2  that the horn has reached its local terminal point, then flow of the algorithm proceeds to step  406   3 , whereat the controller determines a displacement that the horn traveled in order to reach the point, or otherwise determines a location of the terminal point—e.g., a vertical distance from any reference frame. The displacement can be determined by, e.g., an encoder connected directly or indirectly to the horn. In one embodiment, the system is configured to take horn displacement measurements continually, at short intervals, or otherwise quickly as the horn descends. The system is further configured to compare the regular displacement values determined with a target displacement value continuously or at short regular intervals or otherwise quickly as the horn descends. 
     At step  406   4 , the controller determines whether the displacement, or vertical position, of the horn corresponding to the local terminal point is indicative of the horn having been lowered to a target location of the workpiece arrangement—i.e., the location of the arrangement having the energy director between the workpieces and directly, fully, below the welding horn. 
     The controller is programmed, or calibrated, with data identifying values, or ranges, of horn displacements, or positions, corresponding to expected, or likely, positions of the horn with respect to the target location of the workpiece arrangement. The data indicates, for instance, that the horn will be at a predetermined vertical position, within an error window, or range, when the horn has contacted the target position, because the horn will be opposed by the threshold force earlier. 
     This is because the workpiece arrangement is thicker where the energy director is, or at least the top workpiece will not give as much to the horn when the energy director is there. When the horn pushes on a location of the workpiece that is not over the energy director, the horn is able to push down farther on the workpiece before the horn finally experiences the threshold push-back force. The data indicates, based on the horn displacement to the threshold force, where the horn is—e.g., over or not over the energy director, can be generated in lab testing, for instance. The data can also provide an indication, based on the horn displacement to the threshold force, of how far the horn is from the energy director. 
     This concept is described further with reference to  FIGS. 6 and 7 . 
     As referenced,  FIG. 6  shows three example paths  602 ,  604 ,  606 . At a first lateral position over the proximate workpiece  101   1 , the horn descends along the first example path  602 . Because the energy director  200  is nowhere near a line of the path  602 , when the horn contacts the workpiece  101   1 , the workpiece, not being restricted by any energy director, there, will give, or displace more than it would if the director were there. The horn is thus able to move farther downward before the predetermined threshold force, from the workpiece  101   1 , opposes the horn&#39;s downward movement. 
       FIG. 7  is a graphical representation corresponding to the three paths shown in  FIG. 6 . More particularly,  FIG. 7  shows a graph  700  having an y-axis  702  representing welding horn displacement and an x-axis  704  indicating horn lateral, or orthogonal, position. The first bar  706  corresponds to the first path  602  of  FIG. 6 . Accordingly, the displacement is very high because the path  602  is not over, and not relatively near to, the energy director  200  in  FIG. 6 . 
     The second bar  708  in  FIG. 7  corresponds to the second path  604  of  FIG. 6 . Accordingly, the displacement is lower, but still not as low as it should be because the path  606  is still not directly and completely over the energy director  200 . In some embodiments, the energy director is not rigid, and rather has some flexibility. The horn thus is opposed by less force when lowered on a portion of the workpiece  101   1  that is not completely over the energy director (e.g., the second path  604 ), because less of the director is acting to resist the downward movement of the horn. When the horn is lowered directly over the horn (e.g., the third path  606 ), more (i.e., all) of the energy director is beneath the workpiece where the horn is lowered, and so more of the director opposes the downward movement of the horn, and the workpiece thus displaces less before experiencing the threshold feed-back force. 
     The third bar  710  in  FIG. 7  corresponds to the third path  606  of  FIG. 6 . Accordingly, the displacement is relatively low because the path  606  is directly over the energy director  200 , which limits the horn from descending further. 
     With continued reference to  FIG. 4 , assuming the welding horn is, in a first iteration of the routine  406 , at a first lateral position corresponding to the first path  602 , then at step  406   3 , the controller would determine that the horn has displaced a relatively-large amount to reach the termination point—e.g., the first relatively-large displacement  706 . 
     At the next step  406   4 , the controller determines whether the displacement (e.g., displacement  706  corresponding to the first path  602 ) indicates that that horn is directly over the energy director. Because the displacement is relatively high in this first iteration (e.g., displacement  706 ), the controller, based on the pre-programmed data (e.g., from previous lab testing) concludes that the horn is not directly over the director. Thus, from the decision  406   4 , flow of the algorithm continues to step  406   5  whereat the controller determines a next lateral location to move the horn to for a next descent and measuring. 
     Determining, in step  406   5 , where the horn should be moved for the next horn drop, in one embodiment includes consideration of the displacement determined in the last step  406   4 . For instance, if the last displacement (e.g., displacement  706 ) is very high, then the lateral distance to move the horn for the next drop would be greater. If the last displacement is low—e.g., very close to what it would be if the horn was directly over the energy director, then the later distance, to move the horn for the net drop, would be much less. 
     Following repositioning of the horn at step  406   5 , steps  406   1  to  406   5  are repeated. 
     Once the iteration results at step  406   4  with a horn displacement at or below a threshold, or target displacement, then the controller concludes that the horn has been lowered directly over the energy director. With reference to FIGS.  6  and  7 , for instance, when the horn is lowered along the third path  606  of  FIG. 6 , the horn will only travel a minimal displacement  710 , being below a threshold displacement  712  also indicated in  FIG. 7 . The displacement values at or below the threshold displacement  712  can be referred to as a displacement tolerance range. 
     In response to determining, at  406   4  that the horn moved only a target displacement (e.g.,  710 ) to reach the threshold push-back force, and so that the horn was lowered onto the workpiece  101   1  directly over the workpiece, then flow of the algorithm proceeds from the energy-director-locating routine  406  to welding step  408 . 
     At step  408 , welding energy is applied from the welding horn to the proximate work piece  101   1  at the determined location, directly above the energy director. For ultrasonic welding, the energy includes high-frequency ultrasonic vibrations excited and passing from the welding horn. 
     As described above, the energy director is designed so that the welding energy passes initially more or completely through some of the energy-director (ED) elements ( 206 ) than others. For instance, in the multi-height embodiments, the energy would pass through the taller ED elements  206   1  initially, and not through the shorter elements  206   2 , because the taller elements contact the distal workpiece  101   2  creating a path between the workpieces  101   1 ,  101   2 . The energy would not flow freely through the shorter ED elements at this point because the shorter elements do not touch the distal piece  101   2 , and so there is no path through the shorter elements to the distal piece  101   2  for the energy. 
     With the welding energy passing through the taller ED elements  101   1 , the taller elements are melted first, as well as the workpieces adjacent the taller elements. This stage is shown in  FIG. 8 . 
     Regarding the welding operation, more particularly, for ultrasonic welding, heat is generated from intermolecular friction at and between the energy directors and the workpieces where the welding energy (e.g., HF vibrations) are passing. The heat causes the director and workpieces to melt, creating the joining weld. 
     The arrangement is under some compression, at least due to the weight of the proximate workpiece  101   1 , and by downward force of the horn. In some embodiments, the horn is configured (e.g., spring loaded) and/or controlled to apply a downward force on the proximate piece  101   1  during welding. Thus, as the ED elements melt, the top workpiece  101   1  lowers. 
     After the taller elements are melted further, a subsequent stage, shown in  FIG. 9 , is reached whereby the taller ED elements  206   1  have melted sufficiently for the shorter ED elements  206   2  to contact the distal workpiece  101   2 . 
     At this point, because the taller ED elements  206   1  have been at least partially melted, and the shorter ED elements  206   2  have not yet been melted and not contact the lower workpiece  101   2 , the shorter ED elements  206   2  now present a lower-resistance path for the welding energy (e.g., HF vibrations) than the taller ED elements  206   1 . 
     Thus, from the stage shown in  FIG. 9  of the welding sub-process  408 , the welding energy channels mostly, or at least more, through the shorter elements, melting them and the workpieces  101   1 ,  101   2  adjacent the shorter elements. 
     Upon solidification, the melted portions form weld nuggets between the workpieces, and these welds will hold the workpieces  101   1 ,  101   2  together. For embodiments in which a generally annular energy director is used (e.g., the director  206  of  FIG. 2 ), the resulting weld can be generally annular, likewise. An example weld is shown in  FIG. 10  (the weld is shown without the workpieces  1011 ,  101   2  that the weld holds together). 
     As provided, it has been found that an annular weld can be as strong as continuous welds (i.e., welds not having a central void). More specifically, a predominant amount of the holding force created by a solid or continuous weld is provided by an outer annular portion of the weld, with a central portion of the weld contributing little holding force in comparison. A weld lacking the central portion, thus, can be formed with less energy than a continuous weld (one lacking a central void), and perhaps less time, without sacrificing joint strength. 
     After a pre-set amount of time, application of welding energy is ceased, and the horn retrieved from the proximate workpiece  101   1 . The system is pre-programmed with the amount of time to apply the welding energy. The timing can be determined in lab testing, for instance. 
     With final reference to  FIG. 4 , at step  410 , the controller determines whether there are any other welds to make. If so, then flow returns to step  404  whereat the horn is repositioned for locating a next energy director in the locating routine  406 . Once the next energy director is located, flow proceeds again to the welding operation  408 , and so on. 
     While two ED element heights are disclosed, in a contemplated embodiment, the energy director includes more than two heights, and so a corresponding number of welding stages greater than two. 
     As referenced above, instead of or along with height difference between ED elements  206 , the elements can have shape difference controlling where and when the welding energy is channeled, thereby controlling what parts of the energy director melt in a first stage and which in a second stage. While two ED shapes are presented as a primarily example, here, more than two ED shapes is possible, and so a corresponding number of welding stages. 
     While two primary welding stages are described—e.g., a first stage during which the taller ED elements  206   1  channel the weld energy and melt, and a second stage during which the shorter ED elements  206   2  channel the weld energy and melt. As referenced, while the energy transfers through the shorter element more in the second stage, energy may still transfer, to a lesser degree, through the taller elements since they are still intact between the workpieces  101   1 ,  101   2 . 
     The present welding technique  408  results in the ED elements, tall and then short, melting progressively, at a desired time interval. 
     The technique  408  also allows use of less energy to perform the welding than would be required if the energy director was solid with no ED elements, or if every ED element was the same height and shape. For instance, if the energy director had ten (10) equal ED elements, energy sufficient to channel the energy through all ten elements simultaneously would be needed throughout one long, single stage. If the energy director, though, included five taller ED elements and five shorter ED elements, then in the first stage, only energy sufficient to channel the energy through the five taller elements is needed, that energy level being less than the energy level of the previous example in which the energy had to be channeled through all ten equal ED elements. In the second stage, generally, only energy sufficient to channel the energy through the five smaller elements is mostly needed, that energy level also being less than the energy level of the previous example in which the energy had to be channeled through all ten equal ED elements. In theory, further, a sum of the first-stage and second-stage energy application is less than the total energy that would be required for the arrangement having the ten identical ED elements. 
     II.D. Example Controller— FIG. 11   
       FIG. 11  illustrates schematically features of an example controller, such as computing device. The controller is indicated in  FIG. 11  by reference numeral  1100 . As provided, the controller  1100  can control or be part of a robotic apparatus  1102 . 
     As shown, the controller  1100  includes a memory, or computer-readable medium  1104 , such as volatile medium, non-volatile medium, removable medium, and non-removable medium. The term computer-readable media and variants thereof, as used in the specification and technology foci (e.g., claims), refer to tangible, non-transitory, storage media. 
     In some embodiments, storage media includes volatile and/or non-volatile, removable, and/or non-removable media, such as, for example, random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), solid state memory or other memory technology, CD ROM, DVD, BLU-RAY, or other optical disk storage, magnetic tape, magnetic disk storage or other magnetic storage devices. 
     The controller  1100  also includes a computer processor  1106  connected or connectable to the computer-readable medium  1104  by way of a communication link  1108 , such as a computer bus. 
     The computer-readable medium  1104  includes computer-executable instructions  1110 . The computer-executable instructions  1110  are executable by the processor  1106  to cause the processor, and thus the controller  1100 , to perform any combination of the functions described in the present disclosure. These functions are described, in part, above in connection with  FIG. 4 , and supporting illustrations of  FIGS. 1-3 and 5-10 . 
     In a contemplated embodiment, the controller is in communication with one or more remote devices  1112 . For instance, a central computer or service in the manufacturing plant can communicate with the controller  1100 , such as to provide instructions to and/or receive feedback (e.g., operations reports) from the controller  1100 . 
     The computer processor  1106  is also connected or connectable to at least one interface  1112  for facilitating communications, between the controller  1100  and any other local components  1114 , such as, for instance, sensor devices like the force sensors referenced above. 
     The interface  1112  can also be configured to facilitated communications with any remote device  1116 . 
     For communicating with the local components  1114 , the interface  1112  can include one or both of wired connections and wireless components—e.g., transceiver, transmitter, and/or receiver. 
     For communicating with the remote components  1116 , the interface  1112  includes one or both of a short-range transceiver (or transmitter and/or receiver) and a long-range transceiver (or transmitter and/or receiver). 
     The remote components  1116  can include databases, servers, other processors, other storage mediums, and/or other computing devices, such as other systems in a manufacturing plant communicating instructions to and/or receiving data from (e.g., performance reports) the controller  1100 . 
     Although shown as being a part of the controller  1100 , completely, the interface  1112 , or any aspect(s) thereof, can be partially or completely a part of the controller  1100 . The interface  1112 , or any aspect(s) thereof, can be partially or completely external to and connected or connectable to the controller  1100 . 
     III. Select Features Of Implementation 
     Only select features of the implementation are summarized here by way of example. 
     The techniques taught herein result in stronger ultrasonic welds. The strength is improved, for instance, through use of a generally annular, or ring-shaped, energy director. 
     The techniques can also result in savings in energy director material, as material is not needed or included in the central region of the energy director. 
     Another benefit of the present technology is energy savings, as less energy is needed to initiate the initial-stage and subsequent-stage welding, as described above in connection with the welding sup-process  408  of  FIG. 4 . 
     The technology can also result in reduced welding cycle times, as also referenced above. 
     Such efficient, effective, and robust joining solutions support increased use of polymeric components to be joined to similar materials (e.g., polymeric composite/polymeric composite connection) or dissimilar materials (e.g., a polymeric/metal connection, etc.). Related benefits including weight reduction, performance enhancements, and corrosion resistance follow. 
     IV. Conclusion 
     The above-described embodiments are merely exemplary illustrations of implementations set forth for a clear understanding of the principles of the disclosure. Variations, modifications, and combinations may be made to the above-described embodiments without departing from the scope of the technology foci (e.g., claims). All such variations, modifications, and combinations are included herein by the scope of this disclosure and the following technology foci (e.g., claims).