Article Contents

1. Introduction
2. Sweeping Frequency (1)
3. Sweeping Frequency (2)
4. Sweeping Frequency (3)
5. Power Control
6. Center Frequency Control (1)
7. Center Frequency Control (2)
8. Center Frequency Control (3)
9. Center Frequency Control (4)
10. Conclusion

Ideal Ultrasonic Parameters for Delicate Parts Cleaning
(p. 2)

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Sweeping Frequency

Sweeping frequency, the most primitive type of frequency modulation (FM), has had a major impact on the ultrasonic cleaning industry since the late 1980s. When done correctly, it improves the performance of an ultrasonic cleaner and generally reduces the damage to delicate parts caused by constant frequency ultrasonics. For example, figure 1 shows a graph of frequency versus time for a typical sweeping frequency 104 kHz ultrasonic generator with a four kilohertz bandwidth and a 500 Hz sweep rate.

Introducing a change in the frequency, as a function of time, of an ultrasonic array can effect what happens in a tank in a number of ways. This includes how energy is transferred to the fluid, how efficiently that sound energy is converted into cavitational energy, and how that energy is transferred to the part. All parts absorb energy to a greater or lesser degree and unless extreme precaution is taken in the generation of the FM, this energy can be a significant source of damage. In spite of this fact, it is an effect often neglected.

Sound energy is transferred to a tank via an electromechanical device called a transducer, vibrating at resonance or one of its overtones. Resonance is defined as that frequency at which a mass, in this case a transducer, oscillates with maximum speed amplitude.⁵ A transducer's resonant frequency is determined solely by its geometry and composition. An electrical signal, supplied by an ultrasonic generator to the transducers, is converted into an acoustic signal, which is then transmitted into the bath. Natural variations, Δf, exist in the resonant frequency, f, of one transducer compared to another. These random variations occur as a result of variables as mundane as dimensional tolerance build up and transducer to tank bond variations. These resonant frequency variations occur with equal probability on either side of the average frequency of the array, f₀=104 kHz in our example, and can be on the order of hundreds of hertz. Thus the resonant frequency distribution of the composite transducer array has an average frequency of f₀ with a width of Δf, figure 2. With this in mind, any generator supplying a constant frequency, f₀, to a transducer array will be exciting only a fraction of the transducers at their actual resonant frequency. The rest of the transducers will radiate less efficiently, by virtue of being driven off resonance, and portions of the tank will appear acoustically dim. In contrast, if the frequency is swept, say plus or minus 2 kHz, the transducers will all spend an equal amount of time at their resonant frequency. If the transducers are swept quickly (with respect to the typical lifetime of a sound wave propagating through water) the entire array will be excited equally and the sound field in the tank will be very uniform in time. This is a way in which the act of sweeping the frequency of the signal to the transducers results in a more uniform and effective energy transfer into the tank.

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