Views:4 Author:Site Editor Publish Time: 2019-03-31 Origin:Site
The piezoelectric ceramics vibrates under the action of an external alternating electric field to excite ultrasonic waves. The common vibration modes of piezoelectric ceramics are stretching vibration, bending vibration and shear vibration. The commonly used telescopic vibration modes of piezoelectric ceramic vibrators are further divided into a length vibration mode, a radial vibration mode, and a thickness vibration mode. The disc thickness vibration mode is shown. The vibrator is in the shape of a disk, and the polarization direction and the applied electric field are both parallel to the thickness direction, and the vibrator vibrates in the thickness direction. The fundamental resonance frequency of this vibration mode is inversely proportional to the thickness. The applicable frequency range is 30 to 100 kHz, and the relative bandwidth is within 1 to 4 9/5.
If two elongated Pzt ceramic hemisphere sheets of the same thickness and polarized are bonded together, bending vibration can be generated when an exciting electric field is applied to cause one of the sheets to be elongated and the other to be shortened. The two ceramic sheets that are bonded are polarized in opposite directions and connected to the power supply in series; Figure 3b shows the parallel connection of two ceramic sheets with the same polarization direction. In two ceramic adhesive sheets, the electric field only excites one of them to generate bending vibration. Similarly, bending two piezoelectric ceramic transducer sheets on a thin metal sheet or bonding a ceramic sheet to a thin metal sheet can also produce bending vibration. The resonant frequency of the thickness bending vibration mode is related to the relationship between the length z of the sheet and the total thickness t of the adhesive sheet. Among them, IV is a frequency constant. The thickness bending vibration mode applies to frequencies ranging from 500 Hz to 100 kHz. The size of such vibrators is generally (the width of the ceramic sheet) Z (6 ~ 10) w training ≥ 3.5 t. The thickness shear vibration mode is characterized by the electrode surface parallel to the polarization direction, In the alternating electric field, the piezoelectric cylinder component sheet generates shear vibration in the thickness direction. The thickness shear mode is easier to be excited and is mainly used in the high frequency range of 10 to 60 kHz, which will not be described in detail here. In an ultrasonic range finder, the piezoelectric transducer emits ultrasonic waves.
It is required that the amplitude of the vibrator vibration is large, so it is preferable to use the bending vibration mode. At the same time, because the acoustic impedance of piezo ceramic bowls is extremely low, it is impossible for a general piezoelectric material to achieve impedance matching with it, and therefore it must be realized by means of a transition layer. It is found that the piezoelectric ceramic piece is bonded to the thin metal piece, and the piezoelectric ceramic is used as the excitation source to generate a bending vibration mode, which has a large amplitude and a small acoustic impedance, and can achieve acoustic impedance matching with air. In the form of a ceramic sheet and a thin metal sheet bonding structure, the thin metal sheet can also serve as a protective film to protect the piezoelectric ceramics and electrodes from wear and damage. The material can be selected from high stability nickel-chromium-titanium alloy. Since the thinner is the metal piece, the higher is sound pressure reciprocating transmittance, the metal piece is designed to be thin, generally about 0.1 mm.
Vibrator shape and size
In an ultrasonic range finder, the transmitted ultrasonic field is required to be fan-shaped, and a rectangular vibrator is considered as a wave source. A rectangular wave source of length and width is used for piston vibration, and the longitudinal wave sound field radiated in the gas medium is similar to the disk source. The main beam of the piezoelectric ceramic disc is a quadrangular pyramid, which is a perspective view of the directional main lobe of the rectangular wave source.