Principles of ultrasonic technology for ultrasonic horns and transducer test and design

Ultrasonic transducers and horns and acoustic sets are devices which convert electrical energy into vibration. To understand the operation principles, it can be done an analogy between an ultrasonic welding set and an automobile.

The transducer performs the energy conversion (as the motor), the transformer adjusts the ratio between force and velocity (as the gearbox), and finally the sonotrode / ultrasonic horn directs and applies this energy to do the work desired (as the wheels).

In the automobile all the mechanical system parts must be well designed and harmonic in order to improve the energy transmission efficiency as much as possible. The same occurs with ultrasonic systems, however in that case the key parameter to the efficiency is the frequency of the parts that should be matched as close as possible (for instance 20 kHz +/- 50 Hz).

Operation

The transducer has two operating frequencies, which are easily identifiable in its electrical impedance curve. The impedance maximum corresponds to the anti-resonance frequency (maximum speed). Ultrasonic welding systems operate at anti-resonance frequency. The impedance minimum corresponds to the resonance frequency (maximum force). Ultrasonic cleaning systems operate at resonance frequency.

Sonotrode / horn tuning

Sonotrodes or horns must be tuned to work properly, to do not overheat and to have a long useful life. The frequency tolerance used to be around ±0.25% with reference the nominal value, for instance, 20 kHz ±50 Hz.

To increase the sonotrode / horn frequency:

Test of transducers

To work properly, the transducer frequency and impedance must be within tolerance ranges. For instance, for welding systems the frequency should be 2.5% higher than the nominal acoustic set frequency with a tolerance of +/- 0.25%.

The determinant factors for the frequency and impedance are the dimensional accuracy of the parts, the tightness applied, the ceramics quality and the tuning (in a similar was to the sonotrodes case).

Test of acoustic sets

The frequency and impedance of acoustic sets must be within acceptable ranges. In welding systems, the tolerance is ± 0.25% for the frequency, for example, 20khz ± 50Hz.

Performance depends on the frequency tuning and coherence between the parts. It may happen that a set has poor performance, even though operating at the right frequency, when combining transducers and converters uncoupled (one with low frequency and another with high frequency). This type of problem is detected by measuring the impedance.

Test of piezo ceramics

The piezoelectric ceramics are the transducers core and a critical element. For power applications, it is usually applied the PZT-8 and PZT-4 types.

Before re-assemblages, it is essential to prove the ceramics for micro cracks.

Predictive maintenance

Problems in ultrasonic systems can be easily avoided with predictive maintenance. In general, deviations in the frequency indicates wear, and in the impedance, coupling problems. These problems are solved by re-tightening and polishing the interfaces

References: Ultrasonic assembly of thermoplastic moldings and semi-finished products
- Recommendations on methods, construction and applications. ZVEI (German Electrical Manufactures Association).