A brief analysis of the working principle of ultrasonic sensors and their applications in flow measurement and other fields

Since the Italian scientist Spallanzani, who first discovered ultrasound in 1793, discovered the existence of ultrasound from bats, people have been more and more aware of and studied ultrasound. Because of the unique characteristics of ultrasound, ultrasonic sensors have increasingly reflected their importance in production and life. With the rapid advancement of science and technology today, ultrasonic sensors have been widely used in many fields of life and production and have played an important role. Through this article, readers can not only understand the difference between ultrasound and audible sound waves, understand the working principle of ultrasonic sensors, but also have a deeper understanding of the wide application of ultrasonic sensors (such as clamp-on ultrasonic flowmeters) in pipeline flow measurement, medical treatment, industrial production, liquid level measurement, ranging systems and other fields.
 
1. Overview of Ultrasonic Sensors
         Friends who have studied physics know that sound waves are the propagation form of the mechanical vibration state of an object. Ultrasonic waves refer to sound waves with a vibration frequency greater than 20,000 Hz. The number of vibrations per second is very high, exceeding the upper limit of human hearing. People call this inaudible sound wave ultrasonic waves. Ultrasonic waves are a mechanical oscillation in an elastic medium, which has two forms: transverse oscillation (transverse wave) and longitudinal oscillation (longitudinal wave). Longitudinal oscillation is mainly used in industrial applications. Ultrasonic waves can propagate in gases, liquids and solids at different propagation speeds. In addition, they also have refraction and reflection phenomena, and there is attenuation during the propagation process. The propagation laws of ultrasonic waves in the medium, such as reflection, refraction, diffraction and scattering, are not essentially different from the laws of audible sound waves. Compared with audible sound waves, ultrasonic waves have many unique characteristics: Propagation characteristics - ultrasonic waves have poor diffraction ability, and they can propagate in a directional and straight line in a uniform medium. The shorter the wavelength of ultrasonic waves, the more significant this characteristic is. Power characteristics - when sound propagates in the air, it pushes the particles in the air to vibrate back and forth and does work on the particles. At the same intensity, the higher the frequency of the sound wave, the greater its power.
         Because ultrasonic waves have a high flatness, their power is very large compared to general sound waves. Cavitation - when ultrasonic waves propagate in liquids, due to the violent vibration of the liquid power, small cavities will be generated inside the liquid. These small cavities will expand and close rapidly, causing violent collisions between liquid particles, thereby generating a pressure of thousands to tens of thousands of atmospheres. The pressure of thousands to tens of thousands of atmospheres is generated between particles. This violent interaction between particles causes the liquid to suddenly increase in temperature, thus emulsifying two immiscible liquids (such as water and oil), accelerating the dissolution of solutes, and accelerating chemical reactions. The various effects caused by ultrasound in liquids are called ultrasonic cavitation.
From the above, we can get the following characteristics of ultrasound: (1) Ultrasonic waves are highly directional when propagating and can be easily concentrated; (2) Ultrasonic waves can propagate in various media and can propagate over a sufficiently long distance; (3) The interaction between ultrasound and the sound-transmitting medium is moderate, and it is easy to carry information about the state of the sound-transmitting medium (diagnosis or effect on the sound-transmitting medium).
　　
2. Working principle and structural composition of ultrasonic sensors.
　　Ultrasonic sensors are sensors developed using the characteristics of ultrasound. Using ultrasound as a means, ultrasound must be generated and received. The device that performs this function is an ultrasonic sensor, which is usually called an ultrasonic transducer or ultrasonic probe.
Ultrasonic probes are mainly composed of piezoelectric chips, which can both emit and receive ultrasound. The core of an ultrasonic probe is a piezoelectric chip in its plastic or metal jacket. The chip can be made of many materials. The main materials of ultrasonic sensors are piezoelectric crystals (electrostrictive) and nickel-iron-aluminum alloys (magnetostrictive). Electrostrictive materials include lead zirconate titanate (PZT) and the like. The ultrasonic sensor composed of piezoelectric crystals is a reversible sensor that can convert electrical energy into mechanical vibrations to generate ultrasonic waves. At the same time, when it receives ultrasonic waves, it can also convert them into electrical energy, so it can be divided into a transmitter and a receiver.
       Some ultrasonic sensors are composed of a transmitting sensor (or wave transmitter), a receiving sensor (or wave receiver), a control part and a power supply. The transmitter sensor is composed of a transmitter and a ceramic vibrator with a diameter of about 15mm. The electrical vibration energy is converted into ultra-energy and radiated into the air; while the receiving sensor is composed of a ceramic vibrator transducer and an amplifier circuit. The transducer receives the wave to generate mechanical vibrations, converts it into electrical energy, and uses it as the output of the sensor receiver to detect the transmitted ultra-energy. The control part mainly controls the pulse chain frequency, duty cycle, sparse modulation, counting, and detection distance emitted by the transmitter.
　　
3. Practical application of ultrasonic sensors in production and life
 
　1. Application of ultrasonic wave in pipeline flow measurement field
       Ultrasonic flowmeter is the same as ultrasonic flowmeter, because there is no obstruction in the flow channel of the instrument, it is a kind of flowmeter suitable for solving the difficult problem of flow measurement, especially in large-caliber flow measurement, it has outstanding advantages, and it is one of the rapidly developing flowmeters.
         According to the principle of signal detection, ultrasonic flowmeter can be divided into propagation velocity difference method (direct time difference method, time difference method, phase difference method and frequency difference method), beam deviation method, Doppler method, cross-correlation method, spatial filter method and noise method. RZ-1158C time difference type clamp-on ultrasonic flowmeter produced by Runzhong Instrument Technology Co., Ltd. is a typical representative product. This kind of product shines in the market due to its convenient and fast characteristics.
The clamp-on ultrasonic flowmeter adopts the time difference measurement principle: a probe transmits a signal through the pipe wall, the medium, and the other side of the pipe wall, and is received by another probe. At the same time, the second probe also transmits a signal and is received by the first probe. Due to the influence of the medium flow rate, there is a time difference Δt between the two. According to the calculation, the conversion relationship between the flow rate V and the time difference Δt can be obtained, V=(C2/2L)×Δt, and then the flow value Q can be obtained.
 


2. Application of ultrasonic distance sensor technology.
　　The ultrasonic sensor consists of three parts: ultrasonic transducer, processing unit and output stage. First, the processing unit excites the ultrasonic transducer with voltage, and it emits ultrasonic waves in the form of pulses after being excited. Then the ultrasonic transducer enters the receiving state. The processing unit analyzes the received ultrasonic pulse to determine whether the received signal is the echo of the emitted ultrasonic wave. If it is, the travel time of the ultrasonic wave is measured, and the measured time is converted into travel. Divided by 2, it is the distance of the object reflecting the ultrasonic wave. Install the ultrasonic sensor in a suitable position, aim at the direction of change of the object to be measured and emit ultrasonic waves, and the distance between the surface of the object and the sensor can be measured. Ultrasonic sensors have transmitters and receivers, but an ultrasonic sensor can also have the dual functions of transmitting and receiving sound waves. Ultrasonic sensors use the principle of piezoelectric effect to convert electrical energy and ultrasonic waves into each other, that is, when transmitting ultrasonic waves, electrical energy is converted to transmit ultrasonic waves; and when receiving echoes, ultrasonic vibrations are converted into electrical signals. 3. Application
　　
of ultrasonic sensors in medicine.
　　The application of ultrasound in medicine is mainly to diagnose diseases, and it has become an indispensable diagnostic method in clinical medicine. The advantages of ultrasonic diagnosis are: no pain, no damage to the subject, simple method, clear imaging, high diagnostic accuracy, etc.
　　
4. Application of ultrasonic sensors in measuring liquid level.
　　The basic principle of ultrasonic measurement of liquid level is: the ultrasonic pulse signal emitted by the ultrasonic probe propagates in the gas, encounters the interface between air and liquid and is reflected. After receiving the echo signal, the round-trip propagation time of the ultrasonic wave is calculated, and the distance or liquid level height can be converted. The ultrasonic measurement method has many advantages that other methods cannot match: (1) It does not have any mechanical transmission parts and does not contact the measured liquid. It is a non-contact measurement, not afraid of electromagnetic interference, not afraid of strong corrosive liquids such as acids and alkalis, etc., so it has stable performance, high reliability and long life; (2) Its short response time can be conveniently practiced for real-time measurement without lag.
　
5. Application of ultrasonic sensors in ranging systems.
　　There are roughly the following methods for ultrasonic distance measurement: 1. Take the average voltage of the output pulse. This voltage (its amplitude is basically fixed) is proportional to the distance. The distance can be measured by measuring the voltage; 2. Measure the width of the output pulse, that is, the time interval t between transmitting and receiving the ultrasonic wave. The measured distance is s=1/2vt. If the measured accuracy is very high, it should be corrected by temperature compensation. Ultrasonic distance is suitable for high-precision medium and long distance measurements.
 
IV. Conclusion of the article
　　This article discusses the principles and characteristics of ultrasonic sensors, and summarizes the wide application of ultrasonic sensors in various aspects of production and life, such as external clamp-on ultrasonic flowmeters for pipeline flow measurement. However, ultrasonic sensors also have their own shortcomings, such as emission problems, noise problems, etc. These related technical difficulties still need to be further optimized, and the related basic research supporting ultrasonic products still has room for further expansion and extension. Therefore, further research and study of ultrasonic sensors is still of great value.