Ideal Ultrasonic Parameters for Delicate Parts Cleaning
(p. 4)
previous page next page
Sweeping Frequency (continued)
We have told two thirds of the story thus far. First we examined the way in which sweep effects the amount and uniformity of energy transferred to a fluid medium. Second, we treated the way in which this acoustic energy couples with a bubble distribution in a fluid. Lastly we must discuss those ways in which energy is transferred to a part.
All too often the designers of ultrasonic systems fail to address the question of how a part can be excited in an ultrasonics system. In many cases the knowledge that an ultrasonic system is composed of the sonicated fluid as well as the immersed part is critical. Methods by which energy is transferred to a part must be understood in order to prevent damage modes.
The potential damage modes associated with a fixed frequency sweep rate are eliminated by use of a non-constant sweep rate. This type of frequency modulation (FM) can be accomplished by making the sweep rate random or by changing the sweep rate as a function of time. This type of modulation, known as a "Designer Waveform" is often referred to as sweeping the sweep rate, or dual sweep. An example of a non-constant sweep rate is shown in figure 4.
Figure 4
Non-constant, or Dual, sweeping frequency
The main reason for a non-constant sweep rate is to eliminate any single frequency sweep rate from the system because a part being cleaned can be excited into resonance by two times the frequency of the single frequency sweep rate. Many delicate parts will fracture when excited into resonance.
When a typical Langevin type transducer is swept through a bandwidth of frequencies, the output power is not constant for each frequency. Generally, the output power of the generator peaks near the center of the bandwidth. When sweeping up in frequency, a peak pulse of power is put into the tank at the center of the sweep range. When sweeping down in frequency, another peak pulse of power is put into the tank at the center of this sweep range. This process continues producing equally spaced power pulses at a rate equal to two times the sweep rate. This is exactly the prescription required for the high amplitude oscillations associated with a resonant condition. Any time equally spaced periodic bursts of energy are injected into a system at or near it's resonant frequency, or that of any of its overtones, that system can undergo large amplitude oscillations that can lead to part damage. Introduction of a non-constant sweep rate will vary the spacing between the power pulses. Therefore, there is no fixed frequency at which the power pulses are supplied to the liquid and therefore, no repetitive single frequency to excite the part being cleaned into resonance. Designer Waveforms strive to eliminate all possible damage modes that can be introduced into that system
previous pagenext page
|
