A brief analysis of the principle of ultrasonic distance sensor

Ultrasonic sensors are sensors developed using the characteristics of ultrasonic waves. Ultrasound is a mechanical wave with a higher vibration frequency than sound waves. It is generated by the vibration of the transducer chip under the excitation of voltage. It has the characteristics of high frequency, short wavelength, small diffraction phenomenon, good directionality, and directional propagation. Ultrasound has strong penetrating power to liquids and solids, especially in solids, it can penetrate to a depth of tens of meters. When the ultrasonic wave encounters impurities or interfaces, it will produce a significant reflection to form a return wave, and when it encounters a moving object, it will produce the Doppler effect. Therefore, ultrasonic testing is widely used in industry, national defense, biomedicine, etc. as a detection method. A device capable of generating and receiving ultrasonic waves is an ultrasonic sensor, also commonly referred to as an ultrasonic transducer, or an ultrasonic probe. The ultrasonic probe is mainly composed of piezoelectric wafers, which can not only transmit ultrasonic waves, but also 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 (transverse wave), surface wave probe (surface wave), Lamb wave probe (Lamb wave), dual probe (one probe reflection, one probe reception) Wait. The core of an ultrasound probe is a piezoelectric wafer in its plastic or metal housing. The materials that make up the wafer can be of many kinds. Element sizes, such as diameter and thickness, also vary, so the performance of each probe is different. The main performance indicators of ultrasonic sensors include: (1) Working frequency. The operating frequency is the resonant frequency of the piezoelectric wafer. When the frequency of the AC voltage applied to it is equal to the resonant frequency of the wafer, the output energy is the largest and the sensitivity is also the highest. (2) Working temperature. Since the Curie point of piezoelectric materials is generally relatively high, especially the ultrasonic probe for diagnosis uses less power, so the working temperature is relatively low, and it can work for a long time without failure. Ultrasound probes for medical use have relatively high temperatures and require separate refrigeration equipment. (3) Sensitivity. Mainly depends on the manufacture of the wafer itself. The electromechanical coupling coefficient is large, and the sensitivity is high; otherwise, the sensitivity is low. Structure and working principle When a voltage is applied to a piezoelectric ceramic, mechanical deformation will occur with the change of voltage and frequency. On the other hand, when the piezoelectric ceramic is vibrated, an electric charge is generated. Using this principle, when an electrical 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. Ultrasonic sensors for outdoor use must be well sealed to prevent the intrusion of dew, rain and dust. Piezoceramics 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 accuracy is required to reach 1mm, and it has strong ultrasonic radiation. In high frequency detection, piezoelectric ceramics with vertical thickness vibration modes must be used. In this case, the matching of the acoustic impedance of the piezoelectric ceramic with 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. A difference of powers of 5 results in a large amount of losses on the piezoceramic vibration radiating surface. A special material is adhered to the piezoelectric ceramic as an acoustic matching layer that matches the acoustic impedance of air. This structure allows ultrasonic sensors to work well at frequencies up to hundreds of kHz. Technical principle and application of ultrasonic distance sensor Ultrasonic distance sensor can be widely used in material level, liquid level monitoring, robot collision avoidance, various ultrasonic proximity switches, and anti-theft alarm and other related fields.

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