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Technical principle and application of ultrasonic range transducer

Views: 0     Author: Site Editor     Publish Time: 2023-03-24      Origin: Site


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         Technical principle and application of ultrasonic range transducer

Ultrasonic transducers are sensors developed using the characteristics of ultrasonic waves. Ultrasound is a mechanical wave with a vibration frequency higher than that of sound waves, which is generated by the vibration of the transducer chip under the excitation of voltage. It has high frequency, short wavelength, small diffraction phenomenon, especially good directionality, and can be oriented as rays dissemination characteristics. Ultrasound has a great penetrating ability to liquids and solids, especially in solids that are opaque to sunlight, it can penetrate to a depth of tens of meters. Ultrasonic waves will produce significant reflections when they encounter impurities or interfaces to form echoes, and when they encounter moving objects, they will produce Doppler effects. Therefore, ultrasonic detection is widely used in industry, national defense, biomedicine, etc. Ultrasound transducer for distance is used as a detection method, and ultrasonic waves must be generated and received. The device that completes this function is an ultrasonic sensor, which is customarily called a medical device, or an ultrasonic probe.

The ultrasonic transducer probe is mainly composed of piezoelectric ceramics , which can both transmit and receive ultrasonic waves. Low-power ultrasonic probes are mostly used for detection. It has many different structures, which can be divided into straight probe (longitudinal wave), oblique probe (shear wave), surface wave probe (surface wave), Lamb wave probe (Lamb wave), double probe (one probe reflection, one probe reception) wait.

The core of an ultrasound level transducer is a piezoelectric ceramics in its plastic or metal jacket. There are many kinds of materials that make up the wafer. The size of the chip, such as diameter and thickness, is also different, so the performance of each probe is different, and we must know its performance in advance before using it. The main performance indicators of ultrasonic sensors include:

(1) Working frequency. The operating frequency of ultrasonic distance transducer is the resonance frequency of the piezoelectric ceramics crystal. When the frequency of the AC voltage applied to its two ends is equal to the resonant frequency of the chip, the output energy is the largest and the sensitivity is the highest.

(2) Working temperature. Since the curie point of the piezoelectric material is generally relatively high, especially when the power of the piezoelectric ultrasonic probe for diagnosis is relatively small, the working temperature is relatively low, and it can work for a long time without failure. The temperature of the ultrasonic probe used in the medical treatment is relatively high, and a separate cooling device is required.

(3) Sensitivity. Much depends on making the wafer itself. The greater the electromechanical coupling coefficient, the higher the sensitivity; otherwise, the lower the sensitivity.

Structure and working principle

When voltage is applied to piezoelectric ceramics transducer, mechanical deformation will occur with changes in voltage and frequency. On the other hand, when the piezoelectric ceramic is vibrated, an electric charge is generated. Utilizing this principle, when an electric signal is applied to a vibrator composed of two piezoelectric ceramics or a piezoelectric ceramic and a metal sheet, the so-called bimorph element, ultrasonic waves will be emitted due to bending vibration. Instead, when ultrasonic vibrations are applied to the bimorph element, an electrical signal is generated. Based on the above effects, piezoelectric ceramics can be used as ultrasonic sensors.

Like an ultrasonic transducer, a composite vibrator is flexibly fixed on the base. The composite vibrator is a combination of a resonator and a bimorph element vibrator consisting of a metal plate and a piezoelectric ceramic plate. The resonator is in the shape of a trumpet, the purpose is to effectively radiate the ultrasonic waves generated by vibration, and to effectively focus the ultrasonic waves on the most important part of the vibrator.

Ultrasonic transducer for outdoor use must have good sealing to prevent the intrusion of dew, rain and dust. Piezoelectric ceramics are fixed on the inside of the top of the metal box. The base is fixed to the open end of the box and covered with resin. For ultrasonic sensors used in industrial robots, the thickness is required to reach 1mm, and it has strong ultrasonic radiation.

It is impossible to achieve this at frequencies higher than 70kHz using the bending vibration of a conventional bimorph element vibrator. Therefore, in high-frequency detection, piezoelectric ceramics with vertical thickness vibration mode must be used. In this case, the matching of the acoustic impedance of piezoelectric ceramics and air becomes very important. The acoustic impedance of piezoelectric ceramics is 2.6×107kg/m2s, while the acoustic impedance of air is 4.3×102kg/m2s. Differences of powers of 5 lead to substantial losses at the piezoceramic vibrating radiating surface. A special material adhered to the piezoelectric ceramic acts as an acoustic matching layer, matching the acoustic impedance of air. This structure can enable the ultrasonic sensor to work normally even when the frequency is as high as hundreds of kHz.

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