Application of Piezoelectric Ceramic Used in Ultrasonic Distance Transducer

In the past 20 years,piezoelectric crystal cost has developed rapidly at home and abroad. Due to their advantages of simple production, low cost and good stability, they have been widely used in the fields of electronics, light, heat and acoustics, and have a wide range. Ultrasonic transducers are made of piezoelectric ceramics which can produce ultrasonic waves with good directivity. They are ideal for measuring parameters such as speed and distance, and can work stably and reliably under the harsh environmental conditions.

About the ultrasonic ranging device, ultrasonic refers to sound waves with a frequency higher than 20 kHz, which is a mechanical wave. Because of its good directionality and tolerance to the environment, it is used in torque measurement technology. Ultrasonic ranging device is a non-contact ranging technique. The methods mainly include pulse method, phase method and frequency conversion method. Ultrasonic ranging device mainly uses the pulse method. The pulse method directly determines the distance value by measuring the time during which the carrier pulse signal travels back and forth over the distance to be measured. The formula of piezo plate energy generation is D=Vt2D/2, where D is the distance to be measured; V is the propagation speed of the carrier in air; t2D is the carrier round trip time. The accuracy of the pulse method is affected by the accuracy of time measurement, and the accuracy of time measurement is affected by the oscillation frequency. If the ultrasonic wave is used as the carrier and the distance measurement accuracy is D≤1cm, the time test accuracy is required to be t≤5.9×10-5s, that is, as long as the oscillation frequency reaches 1.7×104 Hz, this is very easy to implement.

Piezoelectric ultrasonic transducers are made by using the piezoelectric effect of piezoelectric materials. The polarized piezoelectric material undergoes mechanical deformation under the action of an applied electric field. This is called the inverse piezoelectric effect. Conversely, the mechanical deformation of the piezoelectric material also produces a voltage, which is called the positive piezoelectric effect. By using the inverse piezoelectric effect, the high-frequency voltage can be converted into high-frequency mechanical vibration to generate ultrasonic waves; the positive piezoelectric effect can also be used to convert the receive ultrasonic vibration into an electrical signal. This is how the ultrasonic transducer works. Piezoelectric ultrasonic transducers can be viewed as four-terminal networks with electrical and mechanical ends.

The choice of piezoelectric materials piezo electric plates is that piezoelectric materials for making ultrasonic transducers include piezoelectric single crystals, polycrystalline piezoelectric ceramics, piezoelectric high polymers, and piezoelectric composite materials. Among them, lead zirconate titanate piezoelectric ceramics have the advantages of high mechanical strength, temperature and humidity resistance, low cost and good electromechanical coupling effect, and have been widely used in ultrasonic transducers. The ultrasonic transducer in the ultrasonic range finder uses the lead zirconate titanate piezoelectric ceramic as the vibrator material.

If two elongated piezoelectric disc piezoelectric crystal of the same thickness and polarized are bonded together, bending vibration can be generated when an exciting electric field is applied to cause to be elongated and the other to be shortened. The two piezo ceramic sheets that are bonded are polarized in the opposite directions, and are connected in series to the power supply; the parallel connection modes of the two piezo ceramic sheets having the same polarization direction are shown. In two piezo ceramic adhesive sheets, the electric field only excites one of them to produce bending vibration. Similarly, bonding two piezoelectric ceramic sheets to a thin metal sheet, or bonding a ceramic sheet to a thin metal sheet, can also produce a bending vibration thickness. The resonant frequency fr of the bending vibration mode and the length of the sheet .The relationship between the total thickness t and the adhesive sheet is fr = Nlttl2, where Nlt is a frequency constant. The thickness bending vibration mode is applicable to a frequency range of 500 Hz to 100 kHz. The size of such a vibrator is generally the width of the ceramic sheet ,l = (6 ~ 10) w w ≥ 3.5t. The thickness shear vibration mode is characterized in that the electrode surface is parallel to the polarization direction, and the piezo ceramic sheet is subjected to shear vibration in the thickness direction under the action of an alternating electric field. The thickness shear mode is relatively easy to be excited, and is mainly used in the high frequency range of 10 to 60 kHz, which will not be described in detail. In the ultrasonic range finder, the piezoelectric transducer emits ultrasonic waves, and the amplitude of the vibrator vibration is large, so the bending vibration mode is preferable. At the same time, because the acoustic impedance of air 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 transducer ultrasound 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. . This article is in the form of a bonded structure of piezo ceramic sheets and thin metal sheets.

 The thin metal sheets can also act 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 metal is the higher,the sound pressure is reciprocating transmittance, the metal piece is designed to be thin, generally about 0.1 mm.The shape and size of the vibrator in the ultrasonic range finder requires that the transmitted ultrasonic field be fan-shaped, considering the use of a rectangular vibrator as the wave source. A rectangular wave source of length L and width W is used for piston vibration, and the longitudinal wave sound field is radiated in the gas medium is similar to the disk source. The main beam of the radiation is a quadrangular pyramid, is a perspective view of the directional main lobe of the rectangular wave source. The ultrasonic range finder emits a 36 kHz ultrasonic wave for single frequency ranging, that is, the resonant frequency of the piezoelectric ceramic oscillator is 36 kHz. This frequency not only satisfies the requirements of the system for detection range, accuracy and sensitivity, but also makes the transmitted ultrasonic waves have higher efficiency in air propagation. Through the formula calculation and experimental correction, the size of the piezoelectric ceramic rectangular vibrator is determined as: L=26.5mm, W=11.2mm, the piezoelectric transducer is designed as a close structure of fundamental frequency resonance, and the shell is made of engineering plastic. It can fix and protect metal and ceramic adhesive sheets. The two electrode pins are respectively connected to the electrode of the metal and the ceramic piece through the lead wires, and the connection manner can be soldered by solder or low temperature conductive glue.