Many ultrasonic generators are configured to operate at one power level - full on. There are occasions, however, when controlling the ultrasonic power is critical to a successful ultrasonic cleaning process. This is especially true when cleaning parts made of relatively soft materials including brass, copper and, in some cases, even aluminum. Other cases requiring precise power control are those involving parts with delicate geometries. Many of the parts in the head assembly of a computer hard drive fall into this category. Controlling the power output of an ultrasonic generator would, at first, seem like a very simple thing. It is natural to assume that one would vary the amplitude of the ultrasonic generator's electrical output in much the same way as one varies the loudness of a radio by using a volume control. This concept was discussed in the blog Ultrasonics - Sound - Amplitude. In practice, however, many ultrasonic generators do not actually vary the amplitude of the output signal but, instead, use other schemes to deliver reduced overall power. In some applications, the difference between true amplitude control and these other options can be significant. Power of an ultrasonic generator is most commonly expressed in units called watts. We are all familiar with watts from ratings on light bulbs, microwave ovens, hair dryers and other electrical appliances. You may have also noticed, however, that some items, audio amplifiers as one example, have two ratings. One, "average watts," expresses the average power capability of the unit in continuous use while the other, "peak watts," expresses the unit's capability to produce an impulse of power for a very short period of time. A power control control on an ultrasonic generator varies the average power delivered to the transducers but not always by reducing the amplitude of either the electrical signal it delivers or the amplitude of the vibration of the ultrasonic transducers. Consider the following examples.
In all of the above cases, power is reduced by going from A to B. The blue line represents the waveform delivered to the ultrasonic transducers by the ultrasonic generator. The dashed red line shows the part of the waveform that has been removed to reduce power.In all of the above cases, the average power output of the ultrasonic generator is reduced by going from waveform A to waveform B. In the first case, the amplitude of the electrical output of the ultrasonic generator is reduced to accomplish the reduction in power. The amplitude of vibration of the ultrasonic transducers is, in turn, reduced proportionately. In the second case, the average power output is reduced by, in essence, controlling the output if the generator on a time-proportioning or "duty cycle" basis. If the unit is operating at 100% power for 1/2 of the time and at 0% power for the other 1/2 of the time, the "average" is 50%. When the power is on, the ultrasonic transducers are vibrating at an amplitude that is 100% of their capacity. Finally, in the third example, the output waveform is "clipped" to prevent the delivery of power impulses that exceed the desired power intensity. The illustration is typical but the reader should be aware that the same reduced average power result can be accomplished using several different schemes. In summary, just because the dial on an ultrasonic generator is adjusted to 50% does not necessarily mean that the part being cleaned does not see higher (and potentially damaging) power levels on an intermittent basis. In the case of a time-proportioning or "duty cycle" power control, the same result might (although not always) be achieved by reducing ultrasonic exposure time - ie. going from 60 seconds to 30 seconds. Likewise, if a part shows no damage when exposed to full ultrasonic power for 30 seconds but does show damage after 60 seconds exposure, a time-proportioning power control will likely allow the part to be exposed for 60 seconds at 50% power with no damage. If, on the other hand, part damage is noted immediately when the part is exposed to 100% ultrasonic power, a power control that truly varies amplitude is the only way to prevent potential damage.
- FJF -