ULTRASONIC STACK

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No.

62/542,864 filed August 9, 2017. The entire disclosure of the above application is incorporated herein by reference.

FIELD

[0002] The present disclosure relates to the control of phase between an output voltage of an ultrasonic power supply exciting an ultrasonic stack and the ultrasonic stack.

BACKGROUND

[0003] This section provides background information related to the present disclosure which is not necessarily prior art.

[0004] An ultrasonic stack is used in ultrasonic devices such as ultrasonic welders. The ultrasonic stack includes one or more ultrasonic converters typically attached to one or more acoustically driven passive components. It should however be understood that the ultrasonic stack can have only the ultrasonic converter (or converters) and not acoustically driven passive components. Typically, an ultrasonic stack has an ultrasonic converter attached to a booster and an ultrasonic horn attached to the booster. The booster and ultrasonic horn are acoustically driven passive components. An ultrasonic power supply provides the electrical excitation that drives the ultrasonic converter.

[0005] In ultrasonic systems having ultrasonic stacks excited by ultrasonic power supplies, the output voltage and output current of the ultrasonic power supply exciting the ultrasonic stack need to be in phase with each other to keep the ultrasonic stack in resonance. That is, a phase difference between the output voltage and the output current needs to be zero. As can be seen in Fig. 1 , the parallel and series resonance points of an ultrasonic stack are approximately at where the phase difference between the output voltage and the output current is zero.

[0006] In the prior art, phase difference is controlled by changing the frequency of the stack. The frequency of the output voltage is used as a baseline and the phase difference between the output voltage and output current is measured. A feedback controller modifies the measured phase difference and a measured period of the output voltage is adjusted by the modified phase difference. The frequency of the stack is then changed to move it toward the desired resonance point of operation (be it series or parallel). As can be seen in Figure 1 , moving the frequency of the stack toward the resonance point will make the phase difference zero.

[0007] Fig. 2 shows a simplified model of an ultrasonic system 100 having an ultrasonic stack 102 powered by an ultrasonic power supply 104 in which phase is controlled in the above described manner. Ultrasonic stack 102 includes an ultrasonic converter 106 attached to a booster 108 which in turn is attached to an ultrasonic horn 1 10. Ultrasonic converts are also commonly referred to in the art as ultrasonic transducers, and are often piezoelectric ultrasonic transducers. Ultrasonic power supply 104 is electrically coupled to ultrasonic converter 106 and provides the electrical excitation that drives ultrasonic converter 106. A voltage sensor 1 12 senses the output voltage of ultrasonic power supply 104 and a frequency of the output voltage of ultrasonic power supply 104 is detected by frequency detector 1 14 which generates an output T(V) which is the period of the output voltage, this period being 1/ where f is the frequency of the output voltage. A current sensor 1 16 senses the output current of ultrasonic power supply 104. A phase detector 1 18 detects a phase difference between the output voltage and output current of ultrasonic power supply 104 and generates an output which is the phase difference φ between the output voltage and the output current of the ultrasonic power supply 104. A feedback controller 120 modifies the phase difference with feedback control (which may be proportional-integral-derivative control, commonly referred to as PID control) to generate a modified phase difference φ which is then subtracted by summer 122 from the period of the output voltage T(V) to modify this period and generates an output Tmodified(V). Tmodified(V) is the period of the output voltage T(V) modified by the subtraction of the compensated phase difference φ between the output voltage and the output current of the ultrasonic power supply 104. The output Tmodified(V) is provided as an input to a digital dynamic synthesizer 124 that determines a desired frequency ftmodified(V) for the output voltage of ultrasonic power supply 104. The desired frequency ftmodified(V) is provided as an input to a PWM module 126 that generates a pulse-width-modulated signal, referred to herein as a PWM signal, that controls ultrasonic power supply 104 including controlling the frequency of the output voltage of ultrasonic power supply 104 so that it is at the desired frequency ftmodified(V). Feedback controller 120, summer 122, digital dynamic synthesizer 124 and PWM module 126 are illustratively included in a controller 128 which may for example be a single device, such as a microcomputer or programmable controller. Feedback controller 120, summer 122, digital dynamic synthesizer 124 and PWM module may also be separate devices that collectively make up controller 128. In an aspect, controller 128 is part of ultrasonic power supply 104 but it should be understood that controller 128 could be separate from ultrasonic power supply 104.

[0008] The above described method is limited in speed of convergence by the inertia of the ultrasonic stack. Typical ultrasonic stacks have Q's (quality factors) of between 100 and 1000. This means that the ultrasonic stack, if perturbated 90 degrees out of phase, is not able to correct its phase in under 100 to 1000 cycles if relying on changing the frequency of the ultrasonic stack to correct phase. While out of phase, power is reflected back into the power supply that could damage components and cause overloads.

SUMMARY

[0009] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0010] In accordance with an aspect of the present disclosure, an ultrasonic system has an ultrasonic stack excited by an ultrasonic power supply. A phase of an output voltage of the ultrasonic power supply is controlled to match a phase of the ultrasonic stack.

[0011] In an aspect, a phase difference between the output voltage and the output current is modified and subtracted from a period of the output current to generate a modified period of the output current. A desired frequency of the output current is determined based on the modified period of the output current and the ultrasonic power supply controller so that the output voltage of the ultrasonic power supply has the desired frequency.

[0012] In an aspect, the phase difference is modified with any of integral, integral-derivative, proportional, proportional-integral, proportional-derivative, or proportional-integral derivative control. In an aspect, the phase difference is modified with proportional, proportional-integral or proportional-integral-derivative control.

[0013] In an aspect, the desired frequency of the output voltage is determined by a digital dynamic synthesizer that determines the desired frequency based on the modified period of the output current. DRAWINGS

[0014] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

[0015] Fig. 1 is graph showing a typical impedance and phase response of a prior art ultrasonic stack showing parallel and series resonances;

[0016] Fig. 2 is a simplified diagrammatic view of a prior art ultrasonic system in which phase is controlled by altering a frequency of an ultrasonic stack of the ultrasonic system;

[0017] Fig. 3 is a diagrammatic view of an ultrasonic system in which phase is controlled by changing a phase of an output voltage to match a phase of an ultrasonic stack of the ultrasonic system in accordance with an aspect of the present disclosure; and

[0018] Fig. 4 is a graph comparing phase convergence of the prior art ultrasonic system of Fig. 2 and phase convergence of the ultrasonic system of Fig. 3 in accordance with an aspect of the present disclosure.

[0019] Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. DETAILED DESCRIPTION

[0020] Example embodiments will now be described more fully with reference to the accompanying drawings.

[0021] Fig. 3 is a simplified model showing ultrasonic system 300 having an ultrasonic stack 102 powered by an ultrasonic power supply 104 in which phase is controlled by changing the phase of the output voltage of the ultrasonic power supply to match the phase of the ultrasonic stack 102 instead of changing the phase of the ultrasonic stack to match the phase of the output voltage of the ultrasonic power supply. In an aspect, this is accomplished by changing the phase of the output voltage of the ultrasonic power supply to bring the phase difference between the output voltage and output current of the ultrasonic power supply 104 to zero. It should be understood that with the exception of this phase control, ultrasonic system 300 is the same as ultrasonic system 100 shown in Fig. 1 and the following discussion will focus on this phase control. [0022] In ultrasonic system 300, frequency detector 1 14' detects a frequency of the output current of ultrasonic power supply 104 and generates an output l(V) which is the period of the output current, this period being 1/ where f is the period of the output current. The modified phase difference φ generated by feedback controller 120 is subtracted by summer 122 of controller 128' from the period of the output current l(V) to modify this period and generates an output Tmodified(l). Tmodified(l) is the period of the output current T(l) modified by the subtraction of the compensated phase difference φ between the output voltage and the output current of the ultrasonic power supply 104. The output Tmodified(l) is provided as an input to digital dynamic synthesizer 124 that determines a desired frequency ftmodified(V) for the output voltage of ultrasonic power supply 104. The desired frequency ftmodified(V) is provided as an input to PWM module 126 that generates a pulse-width-modulated signal, referred to herein as a PWM signal, that controls ultrasonic power supply 104 including controlling the frequency of the output voltage of ultrasonic power supply 104 so that it is at the desired frequency ftmodified(V).

[0023] In an aspect, feedback controller 120 can modify the phase difference φ by applying any of integral, integral-derivative, proportional, proportional- integral, proportional-derivative, proportional-integral derivative control, or other methods of phase feedback including but not limited to adaptive feedback. In an aspect, feedback compensator modifies the phase difference φ by applying proportional, proportional-integral or proportional-integral-derivative control. In this regard, proportional, proportional-integral or proportional-integral-derivative control responds faster than pure integral control.

[0024] As can be seen in Fig. 4, in both the prior art method used in ultrasonic system 100 and a method in accordance with an aspect of the present disclosure such as used in ultrasonic system 300, the frequency of the voltage and current are the same, but phase between them is not zero just after a perturbation. The frequency of the stack is relatively constant in the short term. In order to have the power supply match phase, the frequency of the voltage has to be first slowed if the phase difference is positive, or increased if the phase difference is negative. Note that at the point the phase initially matches, the frequency of the voltage is not equal to that of the current.

[0025] When the phase matches, the PID gives no correction. If the voltage period is used as the baseline for the DDS frequency for the next half cycle as in the prior art shown in Fig. 2, the voltage frequency would not change and therefore the phase would overshoot as shown in the line in Fig. 4 for the prior-art labeled Voltage - Prior Art. This means that the phase would not be able to converge in a short time and would be unstable for anything but small PID correction constants.

[0026] If the current period is used as the baseline for the DDS frequency in accordance with an aspect of the present disclosure as used in ultrasonic system 300 shown in Fig. 3, during the next half cycle, the voltage frequency would change to the current frequency and therefore there would be no phase overshoot as shown by the dotted line in Fig. 4 labeled Voltage - Agile Feedback. Frequency matches and phase matches in a short time.

[0027] Fig. 4 shows phase convergence of the method in accordance with an aspect of the present disclosure, such as utilized in ultrasonic system 300, in half a cycle. But because the inductance of the power supply is typically around 10 times less than the inductance of the stack, and having the stack Q of between 100 and 1000, the power supply takes approximately 10 to 100 cycles to converge. This is still approximately 10 times faster than the prior art method such as utilized in ultrasonic system 100.

[0028] As used herein, the term controller, control module, control system, or the like may refer to, be part of, or include an Application Specific Integrated Circuit (ASIC); an electronic circuit; a combinational logic circuit; a field programmable gate array (FPGA); a processor (shared, dedicated, or group) that executes code; a programmable logic controller, programmable control system such as a processor based control system including a computer based control system, a signal processor, or other suitable hardware components that provide the described functionality or provide the above functionality when programmed with software as described herein; or a combination of some or all of the above, such as in a system-on-chip. The term module may include memory (shared, dedicated, or group) that stores code executed by the processor. When it is stated that such a device performs a function, it should be understood that the device is configured to perform the function by appropriate logic, such as software, hardware, or a combination thereof.

[0029] Spatially relative terms, such as "inner," "outer," "beneath," "below," "lower," "above," "upper," and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the example term "below" can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[0030] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.