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. 5)

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Power Control

Power control typically comes in two flavors. The first is amplitude control and the second is duty cycle control. Amplitude control changes the integrated power into a fluid by scaling the amplitude of the pressure transmitted into a tank. Duty cycle control changes the power into a tank primarily by changing the period over which the ultrasonics are operated.

Amplitude control is important in applications where part damage due to peak cavitation intensity is a possibility. The magnitude of a pressure wave is directly effected by amplitude control. Though sound pressure cannot constitute a quantitative measurement of cavitation present in a tank, it is a parameter critical to the strength of cavitation. A certain minimum amount of ultrasonic energy must be present in a tank before a fluid will cavitate. That minimum amount of energy is known as the threshold of cavitation. The threshold of cavitation is a strongly rising function of the frequency of the ultrasonics (figure 5), especially in the high (>100kHz) frequency range.^6,7,8 At moderate powers, in a well established cavitation field, the implosion energy is roughly proportional to the square of the difference between the pressure amplitude and the ambient pressure. The reduction of the incident pressure wave's amplitude tends to reduce the mean value of the overall energy distribution of cavitation events, but it changes it slowly and only over a very narrow range. Because of this, amplitude control alone is an exceptionally rough and unreliable tool by which to eliminate part damage and cavitation erosion.

Duty cycle is a measure of the fraction of the time that a generator's ultrasonic output is turned on over a given period. At a duty cycle of 50% the ultrasonic output of a generator spends half of the time on and half of the time off. The primary effects of duty cycle are two fold. The first comes from an understanding that the mechanisms of contaminant removal as well as part damage are probabilistic in nature. Duty cycle serves to change the total number of cavitation events that a part is exposed to over a given period. With fewer implosions, and less opportunity to damage, a low duty cycle is kind to soft substrates. As with most variables care must be taken as there is a trade between damage and cleanliness level; less cavitation offers fewer opportunities to remove particles. The second positive effect of duty cycle power control is to reduce what is often referred to as the degas time of a fluid. Time averaged radii of oscillating bubbles tend to increase during sonication through rectified diffusion⁹. This means that large bubbles (compared to R0) continue to grow to a size where buoyancy forces drive them to the surface of the fluid. The off times introduced by a duty cycle help to give these bubbles an opportunity to travel to the surface. This is an active mechanism by which a bubble population purges itself of large bubbles as well as a pumping action that tends to degas a solution. Via the same mechanism bubble nuclei and small bubbles grow until such time as they are of resonant size and undergo transient collapse. The shattered bubble fragments from this collapse are then new nuclei that further seed the bubble population in a self-sustaining cycle. Members of a bubble population that fall just above the resonant size are allowed to dissolve to a smaller radius during the off times introduced by a duty cycle. These bubbles then have a large interaction cross-section and contribute to the cleaning process. These are two ways in which duty cycle, sometimes called pulse mode ultrasonics, can improve overall performance.

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