The design and manufacture of ultrasonic transducer

Transducer

An ultrasonic transducer is an energy conversion device. Its function is to convert the input electric power into mechanical power (i.e. ultrasonic) and then transmit it, while it consumes very little power.

The ultrasonic generator converts the 110VAC or 220VAC electrical supply current into high-frequency current and transmits it to piezoelectric ceramics. The piezoelectric ceramics resonate at the ultrasonic frequency. The material's piezoelectric effect converts the electrical signal into linear mechanical vibration. It then passes through the ultrasonic horn amplify (reduce) the amplitude, and finally transfer to the tool head to work. Ultrasonic transducers generally come in two forms: magnetostrictive and piezoelectric ceramics. Our company uses piezoelectric ceramics for all transducers.

Magnetostriction: A magnetic field's effect when a material is deformed or deformed under a magnetic field's action. These materials usually contain nickel that exhibits strong magnetostriction. Magnetostrictive materials (usually laminations) are used in magnetostrictive sensors. Due to eddy currents, magnetostrictive transducers usually have higher losses than piezoelectric transducers and therefore have higher cooling requirements.

Composition of the ultrasonic transducer

Almost all piezoelectric transducers for power ultrasonics are of the Langevin type — i.e., one or more piezoceramics that are mechanically compressed (prestressed) between a front driver and a back driver.

The ultrasonic transducer is mainly composed of a central piezoelectric ceramic element, front and rear metal covers, pre-stressed screws, electrode pads and insulating tubes. The specific composition is shown in the figure below.

transducer

  • Back (rear) driver:  The thick cylindrical element at the back of the piezoelectric transducer. The back driver abuts the rear-most ceramic and applies preload pressure, typically via a stacking bolt.
  • Electrodes: A thin electrically conductive disk placed between adjacent piezoelectric ceramics of a transducer. The transducer driving voltage is applied between these electrodes.
  • Front driver: Transmitting the transducer's ultrasonic energy to the horn or tool head, the front driver usually includes a mounting flange for connecting the housing.
  • Stack bolt: The bolt(s) used to sandwich the ceramics between the front and back drivers, thereby forming a resonant unit. When the bolt is tightened, it applies a compressive prestress to the ceramics.
  • Piezoelectric ceramic:Piezoelectric ceramics are the heart of the transducer. A component that converts the electrical signal of an ultrasonic generator into linear mechanical vibration.
Piezoelectric ceramic of ultrasonic transducer
Piezoelectric ceramic of ultrasonic transducer

Transducer core components

Piezoceramics are typically classified as "soft" or "hard". Hard piezoceramics are used for power applications. There are two basic types among hard piezoceramics, generally designated as PZT4 and PZT8. (PZT stands for lead-zirconate-titanate of which the piezoceramics are composed.) All transducers of our company use PZT-8 piezoceramics. PZT-8 piezoceramics have a higher quality factor Qm, a higher safe operating temperature (Curie temperature) and a lower dielectric loss (tanδ).

Performance of piezoelectric ceramics under low electric field
Performance of piezoelectric ceramics under the low electric field

The core of ultrasonic transducer production

Because piezoceramics are weak in tension, a static compressive prestress must be applied to prevent the piezoceramics from experiencing tensile stresses when the transducer vibrates ultrasonically. The prestress also assures that the piezoceramic interfaces make good contact so that the acoustic waves are transmitted well with minimum loss. The compressive prestress must be sufficient to prevent the piezoceramics from ever experiencing any ultrasonic tensile stress and prevent any lateral piezoceramic slippage. In the manufacturing process of ultrasonic transducers, the size of prestressing plays a decisive role in quality.

Tighten the transducer in the traditional way

After tightening the prestressed bolts, the piezoceramics can be prestressed. The traditional method uses a torque wrench to tighten the prestressed bolt after setting a specific torque, but this parameter is affected by many factors and often has deviations. When the same torque is used, the bolts' degree of lubrication and the front and rear drivers will affect the prestressing amount applied to the ceramic sheet.

Prestress vs torque Langevin type transducers
Prestress vs torque Langevin type transducers

Influence of prestress on the transducer

The piezoelectric ceramic impedance and frequency will stabilize with the prestress's increase. A larger prestress can reduce the impedance of the transducer. If the piezoelectric ceramic is not sufficiently clamped, the transducer impedance will become larger.

Is the prestress of the transducer bigger the better?

  • The properties of the piezoceramic are modified under compression-bias. In particular, the maximum safe temperature at which the material will function efficiently may be drastically reduced.
  • Excessive preload results in pronounced deploying, with unstable impedance and ageing in operation.
  • Excessive prestressing will reduce the transducer's service life, and in severe cases, it will also cause crystal cracking of the transducer.
  • The transducers prestress too large, it will show a small impedance, but the transducer's performance will decline over time, and high impedance products will accompany it.

Transducer production from HCSONIC

Get the best prestress of the transducer
Get the best prestress of the transducer.

We fix the transducer on the

We can customize various transducers with different frequencies according to customers' needs. The frequency range is from 15khz to 120Khz. They are used for welding, cutting, atomizing spraying, and sonochemical liquid treatment. HCSONIC ultrasonic transducer provides a one-year warranty service.

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