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While transducer is the more technically correct term, for purposes of plastic and metal welding machines, converter is possibly the more common term, and is completely synonymous in this context.
The sandwich-style ultrasonic transducer, or converter, was developed by Paul Langevin in 1917. Langevin’s transducer used quartz elements, the most efficient piezoelectric material known at the time, but it would be immediately recognizable as a high power transducer even today.
In electrical engineering terms, a transducer is a device that converts energy from one form to another, usually one of them being electrical energy. A transducer can be used to change motion, force, or pressure into electical energy, or, as in the case of high-power ultrasonic transducers, electrical energy into motion, force, or pressure. It is common to construct such devices of wire coils wrapped around a magnetostrictive core, or to pass electric current directly through stacks of piezoelectric material, as Langevin did.
Because these devices convert energy from one form to another, a synonymous term, converter, has come into common use. The name of the device is of little consequence, and quite often nearly identical devices made by competing companies will bear different names.
The high-power ultrasonic converters shown below are of the Langevin sandwich type. The active elements are piezoelectric. In this case, a precisely made ceramic material called lead zirconate titanate (PZT).
The PZT material is formed into discs, each having a center hole. The discs are polarized such that electric current flowing from one flat face to the other in one direction causes the discs to get slightly thicker, and current flow the opposite direction causes the discs to get slightly thinner. By stacking these discs with electrodes between the faces and clamping them between two bodies of resonant metal using a central bolt which passes through the holes in the PZT discs, a converter capable of converting electrical signals into a resonant mechanical motion of considerable power is created. PZT transducers are relatively efficient at this conversion, resulting in less energy being lost as waste heat, while magnetostrictive transducers are capable of higher overall power output levels, depsite being less efficient.
When coupled with tuned components such as boosters and sonotrodes (horns), the energy can be transmitted to thermoplastic work pieces to be welded, or to materials that require this unique form of energy to undergo some chemical or physical change..