Increase Ultrasonic Cleaner Efficiency With Sweep FrequencyIncrease Ultrasonic Cleaner Efficiency With Sweep Frequency

We use ultrasonic cleaners routinely for a broad set of applications today. They provide the user with high-efficiency, low-cycle time cleaning with simple push-button operation. Typical ultrasonic cleaners are integrated units. They have a stainless steel cleaning tank, with one or more ultrasonic transducer units fixed to the bottom of the tank. The objects for cleaning are suspended in a stainless steel wire mesh basket placed in the cleaning liquid bath. Once energized, the transducer generates ultrasonic waves throughout the bath that interact with waves reflected from the tank sides to form a 'stationary' or standing wave pattern. For a given cleaner and frequency the standing wave pattern is fixed. For enhanced cleaning efficiency, it is advantageous to vary the transducer frequency in some range. This method of operation is the sweep frequency mode, available in high quality ultrasonic cleaner designs.

The standing wave pattern formed in the liquid is a collection of alternating compression and rarefaction zones or bands. The distance between bands depends primarily on the frequency and tank geometry. The formation of millions of liquid vapor microbubbles occurs and they rapidly cool and condense back into the liquid, causing high-energy jets of liquid to fill the temporary void created by this condensation. These jets hammer the contaminated objects with a field of high velocity small liquid jets that cause the intense cleaning action. Ideally, for uniform cleaning, their distribution should lie more or less equally across the entire object.

The zones of maximal cavitation occur within the rarefaction bands. In case objects suspended in the mesh do not lie entirely within a rarefaction band, the cleaning action there and hence the cleaning efficiency reduces. The sweep frequency circuit continuously alters the transducer frequency in a restricted range. This creates a significant and continuous variation in the location of the compression and rarefaction bands, along with a variation in the distance between them. These band variations subject all parts of the objects to regions of maximal cavitation and cleaning.

An additional beneficial effect of these band variations, induced by the sweep frequency operation, is the increased uniformity of temperature distribution due to better mixing across the tank liquid volume and the suspended mesh. It also prevents the formation of 'hot spots'. This is desirable, particularly for temperature-sensitive materials of the objects for cleaning. Yet another benefit of the sweep frequency mode is the reduction in the likelihood of induced resonance of objects for cleaning. Any such resonance can be potentially damaging to that object. This is most important when the objects for cleaning are delicate in material or construction.

By design, the base frequency is also the resonance frequency of the combined transducer-generator system. If the sweep frequency circuit pushes the transducer too far off its natural resonance frequency, the deliverable power tends to decrease dramatically. Typical ultrasonic cleaners have sweep frequency ranges that span about ± 5% from the central frequency. For example, a 40 KHz cleaner has sweep frequency deviations from the base value of about ±2 KHz, an effective design limit.
by Dr. Pramit Sarma
References and Bibliography
Dr. Pramit Sarma is a consummate geek with an abiding love for technology innovation! At Tovatech he indulges in his passion for high precision engineering & scientific equipment. Tovatech is a leading North American supplier of ultrasonic cleaners. For more details on this article please visit the ultrasonic cleaner section of Tovatech's website.
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