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The ultrasonic transducer system
The ultrasonic transducer system (figure 18) consists of its components converter, booster and sonotrode.  Figure 18: Top: Transducer system of an ultrasonic spot-welding machine
The converter The use of piezoelectric effects
Figure 19: Converter Highly efficient ring-shaped piezoceramics are used to transform electric energy into mechanical energy with an efficiency of approximately 95 percent. The ceramics are sintered from crystalline powder similar to the one used for the manufacture of china. The crystalline powder consists mainly of small crystallites with many so-called elementary cells which exist in the form of dipoles below the Curie-temperature. In order to obtain the required piezo effect, piezo ceramics with their tied-in crystallites need to be polarized after pressing and sintering. The term polarization is used to describe the linear alignment of the dipoles of the elementary cells inside the crystallite when applying a polarization voltage and a temperature below the Curie-temperature to the burned shaped part (figure 20). When the aligned and polarized shaped parts are cooled down to ambient temperature, ant the polarization voltage is still being applied, the dipoles inside the crystallites retain their position. The shaped part has become a piezo-ceramic component.
The behavior of a ceramic piezoelectric body under different conditions can be taken from figure 21 (dimensional changes exaggerated). Figure 21 (a), depicts a piezoelectrical cylinder with no load. If an external force, either by pressure or by traction, acts on the material, the resulting deformation of the cylinder produces a voltage at the electrodes. If the mechanical force is aligned in such a way that the piezo-ceramic electrical cylinder partly assumes its former shape (b), the voltage has the same polarity as that used for the proper polarization process. On the other hand, i.e. when there is mechanical traction in the same direction, the polarity of the electrical voltage will be in the opposite direction (c).
 Figure 21: The piezoelectrical effect inside a cylindrical body made from piezoelectrical ceramics. For a better understanding, only one dipole is featured in this case. If a direct voltage with a polarity which is opposite to that of the voltage used for the polarization is used and is applied to the electrode, the cylinder contracts (d). For the opposite case, there will be an extension of the cylinder (e). If an alternating voltage is applied to the electrode, the ceramic piezoelectrical cylinder will alternate between contracting and expanding in time with the cycle of the voltage (f). Figure 22 shows the mechanical principle of an ultrasonic converter of a type usually used in the frequency range of 20 to 40 kHz. In general, the required ultrasonic power is so high that special design measures must be used to take up the load on the ceramic material and the coupled mechanical construction element. This is why converters for metal welding are made from high-grade materials i.e. titanium alloys. To make sure the converter parts will be able to absorb the oscillating forces, they are pre-stressed for high strength by arranging the piezoceramic rings in pairs in axial direction. The resonance frequency of the converter made in this way has to be designed to suit the working frequency of the ultrasonic generator. 
Via a threaded bolt, the axial and mechanical coupling is made, and thus the sonic and energy transfer to the booster and the sonotrode.
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Today, the reverse piezoelectric effect is almost exclusively used to produce oscillations. Piezoelectric materials periodically change under the effect of an electric alternating voltage.
