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. 8)

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Transducer Impedence

The last method this article will consider as a way to produce a designer waveform is by tailoring the impedance versus frequency curve of the transducer array. Figures 7 and 8 show examples of impedance versus frequency curves of two different transducer arrays (Note: the curves were chosen as the best known example of extreme ends of the impedance spectrum and are technically plots of the logarithm to the base ten of the magnitude of the impedance versus frequency on a linear scale). The figure 7 curve has a very sharp and deep resonance at 132 kHz. It is well known to transducer designers that the way to form this type of curve is by using four design features: First, designing the transducer so all horizontal surfaces are perpendicular to the axis of the transducer; second, designing the piezoelectric ceramic so one of its radial resonances matches the desired longitudinal harmonic resonance of the transducer; third, adjusting the harmonic resonant frequency of each of the bonded transducers in the array to be equal at the desired harmonic; and fourth, compressing the stack with the proper force to form the harmonic structure seen in the curves. The figure 8 curve has a much flatter impedance versus frequency characteristic (for example, the figure 8 curve is about 50 times flatter at 104 kHz than the figure 7 curve is at 132 kHz). This was accomplished by design features that are exactly opposite to the first three design features used to make sharp and deep resonance characteristics. The transducers in the figure 8 array have a cone shaped back mass that has no part of its back surface perpendicular to the axis of the transducer; they are designed so no radial resonance of the piezoelectric ceramics matches any of the desired longitudinal resonances of the transducer, i.e., 40 kHz, 72 kHz, 104 kHz or 170 kHz. Lastly, each bonded transducer in the array is at a slightly different resonant frequency so the resonance of the array is spread over a bandwidth. Each of the transducer arrays represented in figures 7 and 8 has unique characteristics and form different AM patterns within the tank that have advantages for different applications.

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