Patent Description:
The present invention relates generally to presses for use in ultrasonic welding or other systems for vibratory joining of plastic parts.

<CIT> discloses a caulking method and device for a metal plate of a tape reel. A plurality of bosses formed on one end surface of the tape reel are inserted in a plurality of boss holes formed on the metal plate. The bosses projected from the boss holes are caulked to fix the metal plate to the one end surface of the tape reel <NUM>. The front end surface of a welding horn is formed in a flat state, and is formed in a plane shape capable of simultaneously being brought into contact with all the bosses.

<CIT> and <CIT> describe an ultrasonic welding system including a motion control system that is coupled to and that causes controlled movement of an ultrasonic welding stack in accordance with control inputs that are based on one or more control signals that are received from one or more sensors. The motion control system initiates a welding operation, subsequent to which an initial motion delay occurs until a predetermined condition is satisfied. Subsequently, in response to the predetermined condition being satisfied, the ultrasonic welding stack is caused to move in accordance with a weld profile. Subsequently, in response to an occurrence of a predetermined delay initiating condition, the ultrasonic welding stack is caused to stop motion and to maintain a stationary position. Subsequently, in response to an occurrence of a predetermined delay terminating condition, motion of the ultrasonic welding stack is resumed in accordance with the weld profile.

In an ultrasonic welding cycle, the weld phase is defined as the period during which ultrasonic energy is being applied to the parts being joined. Various conditions have traditionally been used to end the weld phase, including: (<NUM>) elapsed time from the start of the weld, (<NUM>) reaching a predetermined press position, (<NUM>) traversing a predetermined distance from the start of the weld, (<NUM>) reaching a predetermined level of ultrasound energy from the start of the weld, and (<NUM>) reaching a predetermined level of ultrasound power.

Sometimes, the weld phase is followed by a hold phase, during which the molten material cools and solidifies while the ultrasonic stack continues to be pressed against the parts being joined. Various conditions have traditionally been used to define the end of the hold phase, including: (<NUM>) elapsed time from the end of the weld, (<NUM>) reaching a predetermined press position, and (<NUM>) traversing a specified distance from the end of the weld.

These traditional methods for ending the weld and hold phases of the joining process are not adequate on some applications, particularly those where there are physical variations in the parts being welded. For example, the energy directors of the parts to be welded can vary in height, width, volume, size, and shape, due to inconsistent molding processes. The parts to be welded can also be different materials, or have other variant geometrical properties. In a first, shorter energy director, the ultrasound weld process could end the weld phase once the first workpiece has moved a set distance. However, if a second, longer energy director is welded next, and the ultrasound weld process ends based on the distance of the first workpiece, the second workpiece will not be adequately welded due to the additional time and energy needed to weld the larger energy director. The remaining conditions listed above for ending the weld phase and the hold phase all fail to account for physically variant workpieces while maintaining accurate welding.

Additionally, when an ultrasound press is operating to repeatedly weld workpieces, physical variations from a first workpiece to a second workpiece can require the ultrasound press to be recalibrated frequently. This frequent recalibration can reduce efficiency and introduce further error to the welding process, especially when the calibration is based on a first set of workpieces but the second set have different physical characteristics.

Therefore, what is needed are systems and methods for accurately welding physically variant workpieces in an ultrasound weld.

In accordance with the present concepts, the present disclosure provides for an ultrasonic welding method for a pair of workpieces. The method comprises first pressing an ultrasonic welding stack against a first workpiece in the pair of workpieces. This causes the first workpiece to come into contact with a second workpiece in the pair of workpieces. The method then provides for initiating a weld phase by outputting energy from the ultrasonic welding stack to the first workpiece. The method then provides for monitoring, with at least one sensor, a weld force. The method then provides for determining whether the weld force has reached a predetermined level. Based on determining that the weld force has reached the predetermined level, the method provides for ending the weld phase.

In some examples of the first embodiment, the monitoring can occur after a time delay. The time delay can occur after the initiating of the weld phase.

In some examples, the time delay can be a length of time for the first workpiece to move a predetermined distance after the initiating of the weld phase.

In some examples, determining whether the weld force has reached a predetermined level can include determining a weld force at the end of the time delay. This can yield a first weld force. The predetermined level can be based on the first weld force.

In some examples, the time delay can be a length of time for energy output of the weld phase to reach a predetermined level after the initiating of the weld phase. In these examples, determining whether the weld force has reached a predetermined level can include determining a weld force at the end of the time delay. This can yield a first weld force. The predetermined level can be based on the first weld force.

In some examples, the method can further include performing the steps of pressing, initiating, monitoring, determining, and ending for a plurality of pairs of workpieces.

In some examples, at least one workpiece in each of the plurality of pairs of workpieces can have at least one physical variation. The physical variation can be a different shape and/or size from at least one other workpiece in the plurality of pairs of workpieces.

In some examples, the predetermined level can be an identical level for each pair of workpieces in the plurality of pairs of workpieces.

In some examples, determining whether the weld force has reached a predetermined level can include determining a weld force rate of change at the end of the time delay. This can yield a first weld force rate of change. The predetermined level can be based on the first weld force rate of change.

In some examples, the time delay can be a length of time for energy output of the weld phase to reach a predetermined level after the initiating of the weld phase. In these examples, determining whether the weld force rate of change has reached a predetermined level can include determining a weld force rate of change at the end of the time delay. This can yield a first weld force rate of change. The predetermined level can be based on the first weld force rate of change.

The invention will be better understood from the following description of preferred embodiments together with reference to the accompanying drawings, in which:.

Although the invention will be described in connection with certain preferred embodiments, it will be understood that the invention is not limited to those particular embodiments. On the contrary, the invention is intended to cover all alternatives, modifications, and equivalent arrangements as may be included within the scope of the invention as defined by the appended claims.

The present disclosure provides new conditions for defining the weld and hold endpoints. These conditions are particularly useful in applications where the end goal of the joining process is to ensure that part features intended to abut come into full contact with each other. Additional applications where these new methods would be helpful include, but are not limited to, ultrasonic cut and seal, ultrasonic insertion, ultrasonic aided machining, ultrasonic staking, and other similar processes, as known in the art. The present disclosure can be applicable to ultrasonic welding systems, which include a traditional stand-alone ultrasonic welding press or any other apparatus containing a moveable ultrasonic welding stack which applies a controlled force, speed, or combination of force and speed to the parts being welded. These concepts are likewise applicable to systems on which the ultrasonic welding stack is stationary and the parts being joined move with respect to the stack with a controlled force, speed, or a combination of force and speed.

Turning now to the drawings and referring first to <FIG>, an illustrative ultrasonic welding machine is described, which can be used for the various embodiments of the present disclosure. For example, this illustrative ultrasonic welding machine can include an ultrasonic welding "stack" <NUM> that is mounted for controlled vertical movement by a bidirectional, electrically powered linear actuator <NUM> (<FIG>). The stack <NUM> will be described in more detail below in connection with <FIG>. The actuator <NUM> can be mounted within a main housing <NUM>, which also supports an auxiliary housing <NUM> that contains the power supply and electronic controls for the welding press. In a variation of this concept, the housing <NUM> and auxiliary housing <NUM> can be combined into one structure. The workpieces W1 and W2 (<FIG>) to be welded can be mounted in a stationary fixture below the ultrasonic stack <NUM>, and the actuator <NUM> can advance the stack <NUM> downwardly against the upper workpiece W1. The lower end of the stack <NUM> is pressed downwardly against the workpiece W1 to press the upper workpiece W1 against the lower workpiece W2 while applying mechanical vibrations to the workpiece W1 to effect the desired welding that joins the two workpieces W1 and W2 together.

The main housing <NUM> is mounted on a frame that includes a vertical column <NUM> extending upwardly from a base <NUM> that carries a fixture for receiving and supporting the workpieces to be welded. The housing <NUM> is typically adjustably mounted on the column <NUM> to allow the vertical position of the entire housing <NUM> to be adjusted for different workpieces. A control panel <NUM> is provided on the front of the base <NUM>.

The ultrasonic welding stack <NUM> includes the following three components (see <FIG>): an electromechanical transducer <NUM> which converts electrical energy into mechanical vibrations, a booster <NUM> to alter the gain (i.e., the output amplitude) of the mechanical vibrations produced by the transducer <NUM>, and a horn <NUM> to transfer the mechanical vibrations from the booster <NUM> to the parts to be welded.

As shown in <FIG>, the transducer <NUM> can include a connector <NUM> for attaching a high voltage coaxial cable <NUM> that delivers a high-frequency electrical signal for exciting the transducer <NUM>. This signal can be supplied by a separate ultrasonic signal generator (not shown). An alternative method of connection can also be utilized to permit easier removal and installation of the transducer. The transducer <NUM> can generate the ultrasonic vibrations as a Langevin piezoelectric converter that transforms electrical energy into mechanical movement. Power applied to the transducer <NUM> can range from less than <NUM> Watts up to <NUM> Watts at a typical frequency of <NUM>. Note that the same concepts will hold true for transducers of other frequencies and power levels which are regularly used in the welding processes of this invention.

The transducer <NUM> can be made from a number of standard piezoelectric ceramic elements separated by thin metal plates, clamped together under high pressure. When an alternating voltage is applied to the ceramic elements, a corresponding electric field is produced which results in a variation in thickness of the ceramic elements. This variation in thickness induces a pressure wave that propagates through the material and is reflected by the ends of the metal mass of the transducer. When the length of the assembly is tuned to its frequency of excitation, the assembly resonates and becomes a source of standing waves. The output amplitude from a <NUM>-kHz transducer is typically about <NUM> microns (<NUM> inches). This amplitude needs to be amplified by the booster <NUM> and the horn <NUM> to do useful work on the parts W1 and W2. The booster and horn act as an acoustic waveguide or transformer to amplify and focus the ultrasonic vibrations to the work piece.

The primary function of the booster <NUM> is to alter the gain (i.e., output amplitude) of the stack <NUM>. A booster is amplifying if its gain is greater than one and reducing if its gain is less than one. Gains at <NUM>-kHz typically range from less than one-half to about three.

The horn <NUM> cannot normally be clamped because it must be free to vibrate and thus only the transducer <NUM> and the booster <NUM> are secured. Thus, a secondary function (and sometimes the sole purpose) of the booster is to provide an additional mounting location without altering the amplification of the stack when secured in a press. The neutral or coupling booster is added between the transducer and horn and mounted in the press by a mounting ring which is placed at the nodal point (where the standing wave has minimal longitudinal amplitude).

The horn <NUM> has three primary functions. First, the horn <NUM> transfers the ultrasonic mechanical vibrational energy (originating at the transducer <NUM>) to the thermoplastic work piece (W1 and W2) through direct physical contact, and localizes the energy in the area where the melt is to occur. Second, the horn <NUM> amplifies the vibrational amplitude to provide the desired tip amplitude for the thermoplastic workpiece and welding process requirements. Third, the horn <NUM> applies the pressure necessary to force the weld when the joint surfaces are melted.

The horn is precision machined and is typically designed to vibrate at either <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>. The higher the frequency, the shorter the acoustic wavelength, and consequently the smaller the horn. The tuning of a horn is typically accomplished using electronic frequency measurement. Horns are usually manufactured from high-strength aluminum alloys or titanium, both of which have excellent acoustical properties to transmit the ultrasonic energy with little attenuation.

There are many different horn shapes and styles depending on the process requirements. Factors which influence the horn design are the materials to be welded and the method of assembly. The horn must amplify the mechanical vibration so that the amplitude is sufficient to melt the thermoplastic workpieces at their interface, and the gain of the horn is determined by its profile. The amplitude at the tip of the horn typically ranges from <NUM> to <NUM> microns peak to peak (<NUM> to <NUM> thousandths of an inch) at <NUM>. In an alternate variation, the horn can be designed so that it takes the form of a booster and combines the functions of stabilization and welding. In this variation, the booster is eliminated and the horn is secured in the press in the position of the booster mounting ring area.

As the frequency increases, the vibration amplitude decreases. Higher frequencies are used for seaming of thin ma terials and delicate parts that do not require a lot of amplitude. Since the horn becomes smaller at higher frequencies, closer spacing can also be achieved.

Plastic welding is the most common application of ultrasonic assembly. To perform ultrasonic plastic welding, the tip of the horn is brought into contact with the upper workpiece W1, as shown in <FIG>. Pressure is applied and ultrasonic energy travels through the upper workpiece, increasing the kinetic energy (or heat) at the contact point of the two workpieces. The heat melts a molded ridge of plastic on one of the workpieces, and the molten material flows between the two surfaces. When the vibration stops, the material solidifies forming a permanent bond.

Although an illustrative ultrasonic weld is discussed above for purposes of the present application, any ultrasound weld can be used for the systems and methods of the present disclosure. Additional discussions of the physical components and control systems of weld processes can be found, for example, in Klinstein, et al.

<FIG> demonstrate a small subset of the exemplary physical variations that can occur on one or more of the parts to be welded.

<FIG> is a pictorial representation of a set of parts to be welded, consisting of a cap <NUM> and body <NUM>. The cap contains an energy director 102a, a cross-sectional closeup of which is shown in <FIG>. Prior to welding, the parts are placed against each other as shown in the cross-sectional closeup view of <FIG>. The goal of the joining process is for surface <NUM> on the cap <NUM> to abut against surface <NUM> on the body <NUM> as shown in <FIG> when the energy director is fully consumed, or melted, during the weld phase. In practice, the geometry of the energy director can vary from one part to another as shown in <FIG>, where <FIG> shows a fully formed energy director with a nominal height X, and <FIG> shows a partially formed energy director with a smaller height of Y, which can occur as a result of inconsistencies in the molding process.

None of the conventional techniques for ending the weld can consistently achieve the weld goal of the surfaces <NUM> and <NUM> being in contact with each other for every set of parts. For example, if the weld were to end using the condition of traversing a predetermined distance from the start of the weld, and if this distance were set to the nominal energy director height, on parts where the energy director was shorter, there would be a gap between surfaces <NUM> and <NUM> at the end of the weld. If the welding process were to use the condition of waiting an elapsed time from the start of the weld, there would similarly be a gap between surfaces <NUM> and <NUM>. Even ending the weld when a predetermined level of ultrasound power has been reached or a predetermined level of energy output from the start of the weld can be inaccurate. Ultrasound power can drift over time, for reasons such as temperature changes of the ultrasonic welding stack during use, leading to inconsistent weld results, where the parts in the welded assembly would be fully seated in some instances, and having a gap in other instances.

In another example, if the weld were to end using the condition of reaching a predetermined press position, an energy director which was wider at the base, along surface <NUM>, than other energy directors would not be evenly welded such that surface <NUM> directly abuts surface <NUM>.

<FIG> show exemplary situations in the prior art where the traditional methods for ending the weld and hold phases of the joining processes would be inadequate. Although a small subset of physical variations are described above with respect to <FIG>, other variations can include variations in height, width, volume, size, and shape. The parts to be welded can also be different materials, or have other variant geometrical properties. Although <FIG> primarily discuss variations in an energy director, variations can occur in any height of a plastic part, diameter of a hole, density of either workpiece, amount of debris in a hole, and other similar variations, as known by a person skilled in the art of ultrasound welding. These variations can occur, for example, due to variations in a molding process or damage during shipping or handling of the parts. Additional variations in the weld process can be any other geometric or material property variations in the parts to be welded, as known by a person skilled in art. All these variations can cause inconsistencies in the weld process when the conventional conditions for ending the weld phase and hold phase are used.

The present disclosure provides for an automated weld process which can consistently weld parts with different physical characteristics such that a first surface of a first workpiece smoothly abuts a second surface of a second workpiece. <FIG> provides an exemplary methodology <NUM> for an ultrasonic welding method, according to an embodiment of the present disclosure.

Methodology <NUM> begins at step <NUM> pressing an ultrasonic welding stack against a first workpiece. The first workpiece thereby comes into contact with a second workpiece.

After contact between the first and second workpiece, methodology <NUM> proceeds to step <NUM> and initiates a weld phase. The weld phase is initiated by outputting energy from the ultrasonic welding stack to the first workpiece. After initiating the weld phase, method <NUM> provides for monitoring a sensed parameter at step <NUM>. The monitoring can be done by at least one sensor. The at least one sensor can be positioned on or near the weld stack or on or near at least one of the pair of workpieces. The at least one sensor can be configured to measure a variety of parameters, including at least one of weld force, weld force rate of change, ultrasound power, ultrasound power rate of change, ultrasound energy, ultrasound energy rate of change, speed, or speed rate of change.

In some examples of step <NUM>, the monitoring can occur after a time delay. The time delay can be a length of time sufficient so that a condition is completed. For example, a length of the time delay can be a length of time for the first workpiece to move a predetermined distance after the initiating of the weld phase. In other examples, the time delay can be a predetermined elapsed time from the start of the weld. In other examples, the length of the time delay can be a length of time for the ultrasonic welding stack to move to a set press position. In other examples, a length of the time delay comprises a length of time for energy output of the weld phase to reach a predetermined energy level after the initiating of the weld phase.

Monitoring any of the sensed parameters can be delayed until the end of the time delay.

After completing the monitoring in step <NUM>, the method provides for determining whether the sensed parameter has reached a predetermined level at step <NUM>.

In some examples of step <NUM>, determining whether the sensed parameter has reached a predetermined level can include determining the sensed parameter at the end of the time delay. This can yield a "first" value of the sensed parameter. The predetermined level can be based on the "first" value of the sensed parameter.

After determining that the sensed parameter has reached a predetermined level, the method provides for ending the weld phase at step <NUM>.

In some examples of method <NUM>, the method further comprises performing the steps of pressing, applying, initiating, monitoring, determining, and ending for a plurality of workpieces. Each workpiece in the plurality of workpieces can have physical variations in a shape and a size from at least one other workpiece in the plurality of workpieces. The predetermined level can be an identical level for each workpiece in the plurality of workpieces. This means that the ultrasonic press does not need to be recalibrated between pairs of workpieces.

Therefore, an exemplary methodology, according to an embodiment of the present disclosure can weld pieces with a finer tolerance, higher accuracy, and lower error rate than conventional methods.

Additional embodiments of method <NUM> can provide for determinations of when to end a hold phase of the weld process. For example, in some applications, it is desirable to end the weld phase before reaching the fully joint depth, and instead achieve the full depth during the hold phase, where the horn continues to press the parts together after the ultrasound energy has been discontinued. For any of these applications, the concepts described with reference to ending of the weld phase (above with respect to steps <NUM>-<NUM>) are equally applicable to ending the hold phase. For welding systems on which the press force is being controlled, conditions for ending the hold based on speed or speed rate of change can be used analogously to those based on force or force rate of change for systems where the press speed is being controlled.

In some examples, the hold phase can end when a predetermined level of force has been reached, when a predetermined level of force rate of change has been reached, when a predetermined level of speed has been reached, and/or when a predetermined level of speed rate of change has been reached.

In some examples, the hold phase can end when a predetermined relative level of a sensed parameter has been reached. The relative level can be referenced to the level of the sensed parameter as sensed at the end of the weld phase. In these examples, the sensed parameter can be force, force rate of change, speed, and/or speed rate of change.

Various additional embodiments of the steps of methodology <NUM> are discussed further with respect to <FIG> below.

<FIG> provides an illustrative example of force during a weld process over a period of time. <FIG> shows the Force applied on the parts being welded versus Time for a typical weld, which begins at Time tt and ends at twe. The condition for ending the weld in this example is the Force reaching a predetermined level Fwe. This method for ending the weld is beneficial in achieving consistent results in the welding of the parts described with reference to <FIG>. In applications where the goal of the joining process is for the surfaces <NUM> and <NUM> to abut, there is a relatively abrupt increase in the force when these surfaces come into contact with each other. By identifying an appropriate predetermined level of force which ensures that surfaces <NUM> and <NUM> are fully seated against each other and ending the weld when this level of force is reached, the goal of the joining process is consistently achieved from one assembly to another, even when there are variations in the geometry of the energy director, particularly its height.

Therefore, <FIG> shows how a weld phase can end based on when a predetermined level of force (e.g. Fwe) has been reached.

<FIG> show exemplary graphs with data on force and force rate of change. In some examples of the present disclosure, as discussed above with respect to <FIG>, the present disclosure can provide for ending the weld phase when a predetermined level of force rate of change has been reached. <FIG> shows the Force applied on the parts being welded versus Time for a typical weld, which begins at Time tt and ends at twe. <FIG> shows the corresponding Force Rate of Change versus Time, which is the slope of <FIG>. The condition for ending the weld in this example is the Force Rate of Change reaching a predetermined level F'we.

One application where this concept is beneficial in obtaining consistent weld results is in ultrasonic staking. <FIG> shows a cross-sectional close-up view of two parts designed to be joined by means of ultrasonic staking. The lower part <NUM> contains a post 704a, and the upper part <NUM> contains a hole 702a through which the post is inserted prior to staking as shown in <FIG>. During the staking operation, the vibrating ultrasonic horn is pressed against the top of the post, imparting energy to melt the post at the tip. At the end of the welding operation, the post is fully formed, trapping the upper part as shown in <FIG>. The goal of the staking process is to fully form the post, firmly trap the upper part, and provide good joint aesthetic appearance, which includes preserving the area on the upper part surrounding the formed post from being deformed by the joining process.

Part variations are common in applications of this type, including in the thickness <NUM> of the upper part <NUM>, the height <NUM> and diameter <NUM> of the post of the lower part <NUM> as indicated in <FIG>, as well as the material properties of both parts. As a result, the force during the weld cycle, when the parts are joined using an electrically actuated press on which the press speed is being controlled, may vary from one set of parts to another as illustrated in <FIG>.

<FIG> shows Force versus Time for two different weld cycles which resulted in optimal weld joints, where there are geometric variations in the set of parts joined in cycle <NUM> compared to those joined in cycle <NUM>. <FIG> shows the resulting Force Rate of Change versus Time. The weld starts at Time tt and ends at t1we for cycle <NUM> and t2we for cycle <NUM>. The force curves start at the same initial value Ft for both cycles, then diverge due to differences in the parts, ending at different levels at the completion of the welds. However, the general shape of the force curves is similar, particularly during the latter half of the weld. Near the end of both welds, when the formed posts come into contact with the tops of the upper parts, the forces rise abruptly. The resulting curves of the Force Rate of Change are similar. Near the end of both welds, these curves are virtually identical but slightly shifted in time, allowing for the same predetermined level of Force Rate of Change, F'we, to be used to end both welds at the optimal points. Therefore, using the condition of reaching a predetermined Force Rate of Change to end the weld allows for consistently achieving the goals of an ultrasonic stacking operation, even when there are variations in the parts being joined.

<FIG> show exemplary graphs with data on force and distance versus time. In some examples of the present disclosure, as discussed above with respect to <FIG>, the present disclosure can provide for evaluating whether the weld process should complete after a delay. For example, as discussed above with respect to methodology <NUM>, the weld process can delay evaluating whether a sensed parameter has reached a predetermined level until some portion of the weld phase has been completed. <FIG> shows an example of where the delay can end when the distance traversed from the start of the weld has reached a predetermined value D<NUM>. <FIG> shows the Distance traversed from the start of the weld versus Time for a typical weld, which begins at Time tt and ends at twe. <FIG> shows an example of where the delay can be a set period of time and the weld process can end when Force reaches a predetermined level Fwe after t<NUM>. <FIG> provides data corresponding to <FIG>. In some examples of the present disclosure, the weld process can wait until the distance traversed from the start of the weld reaches the predetermined value D<NUM> at tt and then end the weld when force reaches the predetermined level Fwe.

This concept is very useful in applications where the desired criteria for ending the weld is reaching a predetermined level of force, but the force during the early part of the weld is higher than this predetermined force, particularly on electrically actuated presses on which the press speed is being controlled. In the example cited with reference to <FIG>, the force in the early part of the weld phase exceeds the predetermined level of force for ending the weld (Fwe). If the criteria used for ending the weld in this application consisted only of the force reaching the predetermined level Fwe, the weld would end prematurely at Time tp, causing an incomplete joint between the parts. However, by delaying the process of evaluating whether the force has reached the predetermined value Fwe until after some part of the weld has already been completed, in this case until the Distance traversed from the start of the weld reaches D<NUM>, the weld ends at the desired point, resulting in a complete joint.

Although exemplary embodiments of the present disclosure are discussed with respect to force for <FIG>, the sensed parameter can also be force rate of change. Similar to delaying the monitoring of the weld phase when measuring force, the present disclosure can provide for ending the weld phase when a predetermined relative level of the force rate of change has been reached, where the process of evaluating whether the force rate of change has reached the predetermined relative level is delayed until some portion of the weld phase has already completed, where the relative level is referenced to the level sensed at the end of the delay. In some examples, the delay can end when the distance traversed from the start of the weld has reached a predetermined value.

Although exemplary embodiments of the present disclosure are discussed with respect to force and force rate of change for <FIG>, the present disclosure also contemplates that the sensed parameter can be ultrasound power rate of change, ultrasound energy rate of change, speed, or speed rate of change.

<FIG> show another example of delaying the process of evaluating whether the sensed parameter has reached the predetermined level. <FIG> shows the Distance traversed from the start of the weld versus Time for a typical weld, which begins at Time tt and ends at twe. <FIG> shows the corresponding Force versus Time. During the weld, the Distance traversed from the start of the weld reaches a value of D<NUM> at Time t<NUM>, at which point the Force is F<NUM>. The condition for ending the weld in this example is the increase in the Force by ΔFwe above the level of Force sampled at the end of a delay (F<NUM>), which occurs when the Distance traversed from the start of the weld reaches the predetermined value D<NUM>.

By using this concept, consistent weld results can be obtained on applications where variations in the geometry and material properties of the parts being joined cause the forces during the weld process to vary from one set of parts to another, particularly on electrically actuated presses on which the press speed is being controlled.

<FIG> show graphs of Distance traversed from the start of the weld and Force versus Time for the weld phases of two different joining cycles which resulted in optimal weld joints. The force curves start at the same initial value Ft for both cycles, then diverge, ending at different levels at the completion of the welds. However, the general shape of the force curves is similar, particularly near the end of the weld. The forces in each cycle are sampled at the end of a delay, in this case consisting of the Distance traversed from the start of the weld reaching D<NUM> (at t<NUM>). For cycle <NUM>, the force at this point is F<NUM>, and for cycle <NUM>, it is F<NUM>. Owing to the fact that the relative change in the force from this point to the optimal end of the weld is essentially the same for both cycles, the same condition for ending the weld based on a relative change in force can be used to end both welds. The condition in this example is reaching the predetermined relative level of force ΔFwe referenced to the level sampled at the end of the delay. Therefore, the weld for cycle <NUM> ends when the force increases by ΔFwe from F<NUM>, and the weld for cycle <NUM> ends when the force increases by the same amount ΔFwe from F<NUM>. Both cycles produced optimal results even though the forces during the weld, in particular at the end of the weld, were different.

Claim 1:
An ultrasonic welding method comprising:
pressing an ultrasonic welding stack (<NUM>) against a first workpiece (W1) of a first pair of workpieces such that the first workpiece comes into contact with a second workpiece (W2) of the first pair of workpieces in response to the pressing;
initiating a weld phase by outputting energy from the ultrasonic welding stack to the first workpiece of the first pair of workpieces;
monitoring, with at least one sensor, a weld force rate of change;
determining whether the weld force rate of change has reached a predetermined level; and
based on determining that the weld force rate of change has reached the predetermined level, ending the weld phase.