A brief analysis of the working principle of ultrasonic/ultrasonic sensor/ultrasonic sensor

Definition of Ultrasound A sound wave is a mechanical wave that can propagate in gases, liquids, and solids. Sound waves can be divided into infrasound waves, sonic waves and ultrasonic waves by frequency. The frequency of sound waves is between 16Hz-20kHz, which is a mechanical wave that the human ear can hear; infrasound waves are mechanical waves with frequencies lower than 16 Hz, and ultrasonic waves are mechanical waves with frequencies higher than 20kHz. The characteristics of ultrasonic waves are high frequency, short wavelength and small diffraction phenomenon. Its most notable features are good directionality, small attenuation in liquids and solids, and strong penetrating ability. It will produce obvious reflection and refraction when it encounters the interface of the medium, so it is widely used in industrial testing. Ultrasonic propagation speed Ultrasonic waves usually include longitudinal waves, transverse waves and surface waves. Their propagation speed depends on the elastic constant of the medium and the density of the medium. Only longitudinal waves can propagate in gas and liquid. The speed of sound in gas is 344m/s, and the speed of sound in liquid is 900-1900m/s. In a solid, there is a certain relationship between the speed of sound of longitudinal waves, transverse waves and surface waves. It is generally considered that the sound speed of shear waves is half of the speed of longitudinal waves, and the speed of surface waves is about 90% of the speed of shear waves. When the ultrasonic wave propagates in the medium, the energy gradually attenuates as the propagation distance increases. The attenuation of energy is determined by the diffusion, scattering and absorption of ultrasonic waves. Ultrasonic waves are used as detection means to generate and receive ultrasonic waves. The device that accomplishes this function is the ultrasonic sensor. Ultrasonic sensor performance indicators The main performance indicators of ultrasonic sensors include; (1) operating 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. Because the Curie point of piezoelectric materials is generally high, especially when the ultrasonic probe for diagnosis uses low power, 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. Working principle of ultrasonic sensor Ultrasonic sensor can be divided into piezoelectric type, magnetostrictive type, electromagnetic type, etc. according to its working principle, and piezoelectric type is the most commonly used. Piezoelectric ultrasonic generators use the principle of inverse piezoelectric effect to convert high-frequency electrical vibrations into high-frequency mechanical vibrations to generate ultrasonic waves. When the frequency of the applied alternating voltage is equal to the natural frequency of the piezoelectric material, resonance will occur, and the ultrasonic waves generated at this time are the strongest. Piezoelectric ultrasonic sensors can generate high-frequency ultrasonic waves ranging from tens of kilohertz to tens of megahertz, and their sound intensity can reach tens of watts per square centimeter. The typical structure of piezoelectric ultrasonic sensor is mainly composed of piezoelectric wafer, absorption block (damping block), protective film and so on. Piezoelectric wafers are mostly disc-shaped, and the ultrasonic frequency is inversely proportional to its thickness. The two sides of the piezoelectric wafer are plated with silver layers, which are used as conductive plates, the bottom surface is grounded, and the top surface is connected to the lead wire. In order to avoid the wear of the piezoelectric wafer due to direct contact between the sensor and the DUT, a protective film is bonded under the piezoelectric wafer. The function of the absorption block is to reduce the mechanical quality of the piezoelectric wafer and absorb the energy of ultrasonic waves. Selection requirements When selecting and installing ultrasonic sensors, it is necessary to clarify some basic conditions, otherwise it will directly affect the measurement results of the sensor. 1. Detection range and size The size of the object to be detected directly affects the detection range of the ultrasonic sensor. The sensor must detect a certain sound level before it can output. Large parts reflect most of the sound to the ultrasonic sensor so that the sensor can detect the part at its maximum sensing distance. Small parts reflect only a small fraction of the sound, resulting in a greatly reduced sensing range. 2. Characteristics of the detected object The ideal object detected by the ultrasonic sensor should be large, flat and high in density, and be perpendicular to the front of the transducer. The most difficult objects to detect are those that are small and made of sound-absorbing material, or that are at an angle to the transducer. Liquid detection is easy if the liquid level is stationary and perpendicular to the sensor surface. If the liquid level fluctuates greatly, the response time of the sensor can be extended to average the fluctuations for a more consistent reading. However, ultrasonic sensors cannot yet accurately detect liquids that have a foamy surface, because the foam deflects the direction of sound propagation. At this time, the reverse ultrasonic mode of the ultrasonic sensor can be used to detect irregularly shaped objects. In reverse ultrasonic mode, the ultrasonic sensor detects a flat background, such as a wall. Anything that passes between the sensor and the wall blocks the sound waves. The sensor can then detect the presence of the object by detecting this disturbance. 3. Temperature-induced attenuation The sensor is also designed with temperature compensation to adjust for slow changes in ambient temperature. but,It cannot accommodate temperature gradients or rapid changes in ambient temperature. 4. Whether there is vibration around, whether it is the vibration of the sensor itself or the vibration of nearby machines, it may affect the accuracy of distance measurement. These problems can be reduced by using a rubber anti-vibration device when installing the sensor. Rails are also sometimes used to eliminate or reduce component vibration. 5. False detection caused by the environment The ultrasonic sensor performs non-contact and wear-free detection of the detected object. It can detect transparent or colored objects, metal or non-metal objects, solid, liquid and powdery substances. Its detection performance is virtually unaffected by any environmental conditions, including smoke and rain.
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