Article Contents

1. Introduction
2. FM: Sweep and Dual Sweep
3. Ultrasonic Power Into a Tank
4. FM: Upsweep
5. Multiple Frequencies (1)
6. Multiple Frequencies (2)
7. Cavitation
8. Transducer Impedence (1)
9. Transducer Impedence (2)
10. Transducer Impedence (3)
11. Universal Transducer
12. Applying the Technology (1)
13. Applying the Technology (2)
14. Applying the Technology (3)
15. Conclusion

Designer Waveforms: Ultrasonic Technologies to Improve Cleaning and Eliminate Damage
(p. 10)

previous pagenext page

Transducer Impedence (continued)

When combined with a constant sweep rate and a wide generator frequency sweep range, this figure 9 characteristic produces a unique amplitude modulation pattern in the tank (equally spaced, repetitive, high power spikes, the highest power point always at the same frequency). Figure 10 shows a plot of this characteristic versus time. It should be noted that the typical ultrasonic generator available today has an average output power of about 40 watts per transducer and, although the peak power in figures 9 and 10 is many times this 40 watt value, the very low power at the other frequencies in the range results in the average output power being typical of other modern systems.

It is well known in the ultrasonic cleaning industry that if you want to cavitate liquids that are difficult to cavitate (e.g., semiaquous solvents), it is best to use a sweep rate (FM) that is at the upper response limit of the transducers and a very high peak power to average power ratio for the AM. This very high peak power to average power ratio is what you get with transducer arrays of the figure 7 type when driven by a constant voltage generator. The trade-off for being able to more easily cavitate in difficult to cavitate situations is potential damage to precision parts by one or more of four mechanisms. One, the evenly spaced high power spikes can excite the precision part into resonance and cause fractures. Two, the shock wave from the high power spikes can cause mechanical damage to microstructures. Three, the surface cavitation zone extends further from the radiating surface into the liquid at higher peak powers, since the energy in each cavitation implosion is higher in surface cavitation than in the bulk liquid, cavitation pitting or craters can occur on regions of the precision parts that are reached by this extended surface cavitation zone. Four, although the system is sweeping frequency, most of the power is at essentially one frequency or a very narrow range of frequencies that simulate a single frequency. This can exhibit the problems of the prior generation of single frequency ultrasonic systems. There is a fifth trade-off for designs of the type shown in figure 7; it has to do with performance. It is well known that each different range of frequencies has a unique cleaning effect and the only practical way to deliver many different frequency ranges and permutations of these ranges to a part is to drive the transducer array at each different frequency range with a multiple frequency generator. The transducer array of figure 7 has such extremely different impedance characteristics between frequency ranges (for example, the resonance at 66 kHz has approximately a 450 ohm to 2,500 ohm spread from resonance to anti resonance while the resonance at 132 kHz has approximately a 50 ohm to 10,000 ohm spread) that it is impractical to build a multiple frequency generator to drive these diverse characteristics. Therefore, the performance advantage of multiple frequency ranges is sacrificed for the easier cavitation caused by the high power spikes.

previous pagenext page