Article Contents

1. Introduction
2. FM: Sweep and Dual Sweep
3. Ultrasonic Power Into a Tank
4. FM: Upsweep
5. Multiple Frequencies (1)
6. Multiple Frequencies (2)
7. Cavitation
8. Transducer Impedence (1)
9. Transducer Impedence (2)
10. Transducer Impedence (3)
11. Universal Transducer
12. Applying the Technology (1)
13. Applying the Technology (2)
14. Applying the Technology (3)
15. Conclusion

Designer Waveforms: Ultrasonic Technologies to Improve Cleaning and Eliminate Damage
(p. 4)

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FM: Upsweep

If a delicate part has a resonant frequency at or near two times the sweep rate, the large amplitudes that the part goes through when it is resonated by the repetitive peak power pulses are likely to cause damage to the part.

With an understanding that resonance phenomena are caused by repeatedly pumping energy into a part at a resonant frequency of the part, it should now be obvious that a non-constant sweep rate will vary the spacing between the power pulses. Therefore, there is no fixed frequency at which the power pulses are supplied to the liquid and therefore, no repetitive single frequency to excite the part being cleaned into resonance. See figure 4A for a graph of power versus time when the generator sweeps the sweep rate.

Another designer waveform variation on conventional sweeping frequency is to monotonically sweep the frequency from high frequency to low frequency. This causes an ever-expanding wavelength in the tank. The ever-expanding ultrasonic wave puts an extra upward force on contamination in the liquid. For bottom-mounted transducers on a tank with overflow weirs, this extra upward force helps the system purge itself of the contamination. Figure 5 shows a frequency versus time graph of a monotonically sweeping system.

A practical ultrasonic generator for precision cleaning would combine the concepts in figure 2 and figure 5 (i.e., a monotonic sweep direction from high frequency to low frequency with the sweep rate constantly changing). This gives the advantages of sweeping frequency, quickly purging contamination from the tank and protecting the part from damaging resonance.

Although the situation is more difficult to visualize, it is worthwhile to consider the frequency of the amplitude modulation of an ultrasonic system (conventionally a single frequency, e.g., 120 Hz) and apply the non-constant frequency ideas and monotonic sweeping direction to this AM pattern. If, for example, the AM is a series of pulses and the spacing between these pulses is non-constant and always getting larger, you might expect results similar to those described for non-constant monotonically sweeping FM. Of course, the AM spacing cannot get larger forever. Typically, a degas time is inserted at the lowest AM frequency and the process starts over at the end of the degas time with the highest AM frequency.

Next, consider putting multiple frequencies into the ultrasonic tank.

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