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. 5)

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Multiple Frequencies

There is a 40-year history of multiple frequency cleaning and processing systems. These systems can be organized into several classes of equipment. The first class of equipment consists of a tank holding liquid with two or more transducer arrays that couple sound energy into the tank. Each of these arrays is driven by a different frequency generator. Typically all the generators are operated at the same time or there is an overlap in the operating times of the generators so that two or more frequencies are simultaneously put into the tank for at least part of the cleaning cycle. The history of this first class of equipment starts in 1959 when Norman Branson constructed three transducer arrays on a tank driven by three generators where the operation periods of these generators overlapped. In 1974 a tank was designed and built at Branson Cleaning Equipment Company that had an array of 25 kHz transducers on the bottom and a second array of 40 kHz transducers on one side; each of these arrays was simultaneously driven by the appropriate frequency generator. Similar systems were designed and built by others in the 1970's, e.g., Blackstone, but no useful application was found for the technology. In the mid 1990's Amerimade Technology sold systems consisting of tanks with angled walls and two arrays of transducers on different walls. Each array was driven by a different frequency generator: one sweeping around 72 kHz and the other sweeping around 104 kHz. At about the same time, Zenith sold a two-frequency system where the different frequency sound waves intersected at 90 degrees. Unlike the earlier 25 kHz and 40 kHz systems that found no useful application, the personal computer industry now existed and these Amerimade and Zenith systems were sold in large volume to the hard disk drive industry. In a 1997 patent, Honda describes high and low frequency transducers on a tank, where the high frequency transducers are driven normally and the low frequency transducers are driven for short periods of time to intermittently destroy the high frequency bubbles. In a 1999 patent, Pedziwatr shows two arrays of transducers interspersed on a tank and driven by two different frequency generators. In another 1999 patent, Ferrell describes two arrays of transducers on different angled walls of a plastic container and driven by different frequency generators.

A second class of multiple frequency cleaning equipment has one array of multiple frequency transducers that couple sound into the liquid in the tank. This array is driven by a pulse or square wave generator or some other form of shock excitation where the generator output is rich in harmonic frequencies. Multiple resonances in the multiple frequency transducer array are excited by the appropriate harmonics in the generator's output. Therefore, multiple frequencies are simultaneously coupled into the tank from a single transducer array and a single generator. Most of the history for this class of equipment is found in the patent literature; we are not aware of commercial systems employing these concepts prospering in the marketplace. In U.S. Pat. No. 3,315,102, Quint describes driving a tank with simultaneous multiple frequencies through shock excitation from a spark gap generator. In U.S. Pat. No. 3,371,233, Cook teaches shock excitation of a non-symmetrical transducer to simultaneously produce many frequencies in a tank. U.K. Pat. No. 1,331,100 shows a non-symmetrical transducer that can simultaneously vibrate at a number of different frequencies and harmonics of these frequencies. When driven by a generator with a harmonic-rich output, this transducer produces simultaneous multiple frequencies. Other transducers capable of multiple frequencies are disclosed by Thompson, Goodson and Puskas in U.S. Pat. Nos. 4,633,119, 5,748,566 and 6,002,195 respectively. In U.S. Pat. No. 5,076,854, Honda teaches that rapidly switching to different frequencies shocks the transducer into producing multi-frequencies in between the drive frequencies. In U.S. Pat. No. 5,462,604, Shibano gives a variation on this theme by saying he produces square wave drive characteristics in the liquid by driving the transducer with odd integer multiples of the natural resonant frequency of the transducer (i.e., the Fourier components of the square wave).

The third class of multiple frequency equipment is a new class of liquid cleaning and processing equipment where there is one transducer array and one generator that produces continuously changing frequencies in a bandwidth of the transducer array for a period of time and then discontinuously jumps to a different bandwidth where changing frequencies are continuously produced for another period of time before another discontinuous jump to a third bandwidth and so on.

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