Ultrasonic Measurement

Last Updated on September 19, 2022 by You Ling

Ultrasonic transducer

Ultrasonic transducer

1、Piezoelectric effect

When pressure is applied to a certain type of crystal, electricity is generated. For this kind of crystal, the external deformation makes the internal electron distribution present a local non-uniform electric field distribution, thus generating electricity. In medical ultrasonic transducers, the inverse piezoelectric effect is used for ultrasonic emission, while the positive piezoelectric effect is used for ultrasonic reception.

When the piezoelectric body is changed by an external force, its internal polarization phenomenon occurs, and at the same time, positive and negative charges are generated on the two surfaces. When the external force is removed, it will return to the uncharged state. As the direction of force changes, the polarity of the charge will also change.

2、Piezoelectric vibrator

Piezoelectric materials have electromechanical coupling effect. There are two kinds of materials: inorganic piezoelectric materials (piezoelectric crystals and piezoelectric ceramics) and organic piezoelectric materials (piezoelectric polymers). Piezoelectric ceramics commonly used in medicine have strong piezoelectricity, high dielectric constant, and can be processed into any shape, but their mechanical quality is low, electrical loss is large, and their stability is poor.

Piezoelectric vibrator refers to the application of an alternating electric field on the piezoelectric material, which makes the piezoelectric material produce some form of vibration. The vibration modes are divided into several types.
① When the polarization direction is parallel to the electric field direction, the vibrator will produce stretching movement, and its vibration direction is consistent with the wave propagation direction, which belongs to longitudinal wave stretching vibration.
② When the polarization direction is parallel to the electric field direction, the vibrator generates shear vibration. The vibration direction of shear vibration is perpendicular to the propagation direction of wave, which belongs to shear vibration of shear wave.
③ When the polarization direction is parallel to the electric field direction, but the two are false, the vibrator will generate bending vibration. Its vibration direction is also perpendicular to the wave propagation direction, which belongs to shear wave bending vibration.

3、Specific application

Ultrasonic transducer is a kind of energy converter. Its function is to convert the input electrical power into mechanical power (i.e. ultrasonic) and then transmit it. However, it only consumes a small amount of power.

The ultrasonic generator (also called ultrasonic power supply) converts 220V mains power into high-frequency current and transmits it to the piezoelectric ceramics, which resonates with the ultrasonic frequency. The piezoelectric effect of the material converts the electrical signal into linear mechanical vibration, and then amplifies (reduces) the amplitude through the ultrasonic horn, and finally transmits it to the tool head for work.

The ultrasonic transducer is mainly composed of central piezoelectric ceramic elements, front and rear metal covers, pre-stressed screws, electrode pieces and insulating tubes. See the following figure for specific composition.

Rear driver: a cylindrical element on the back of the piezoelectric transducer. The rear driver is close to the last piece of piezoelectric ceramic, and usually applies pre tightening pressure through laminated bolts.
Electrode plate: a thin conductive disk between piezoelectric ceramics adjacent to the transducer. The transducer drive voltage is applied between these electrodes.
Front end driver: transmits the ultrasonic energy of the transducer to the horn or tool head. The front driver usually includes a mounting flange for connecting the housing.
Prestressed screw: a single stacked bolt passes through the center hole of the ceramic. After tightening the bolt, the ceramic will be compressed between the rear drive and the front drive to apply the required prestress to the ceramic.
Piezoelectric ceramic: Piezoelectric ceramic is the heart of the transducer, and it is a component that converts the electrical signal of the ultrasonic generator into linear mechanical vibration.

Piezoelectric ceramics are generally classified as “soft” or “hard”. Hard piezoelectric ceramics are used for power applications. There are two basic types of hard piezoelectric ceramics, commonly known as PZT4 and PZT8 (PZT represents lead zirconate titanate composed of piezoelectric ceramics). All transducers of our company use PZT-8 piezoelectric ceramics. PZT-8 piezoelectric ceramics have higher quality factor Qm, higher safe working temperature (Curie temperature) and lower dielectric loss (tan δ）。

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