The sound above 20,000 Hz is generally referred to as ultrasound. The higher the frequency, the shorter the wavelength. Sound waves have a speed of sound of 1000-10000 meters per second in most solids. The frequency is as high as 108-1012 Hz, which is the microwave frequency band of the electromagnetic spectrum. Utilizing the propagation characteristics of ultrasound in solids, as well as the cavitation and radiation pressure generated by high-power ultrasound, ultrasound can produce many fascinating effects, such as ultrasonic cutting, ultrasonic grinding, and ultrasonic drilling.
1. Non-destructive testing <br> Solid parts, such as metal parts, will form some cracks or pores inside due to material or manufacturing process. Its strength will not be enough, or leakage will occur. Danger. How can I detect these defects without destroying the workpiece? In recent decades, many non-destructive testing methods have been invented, such as X-ray, ray, electromagnetic methods, ultrasonic methods, etc., each having a range of applications, and sometimes several methods are required.
Ultrasonic pulses are emitted on the surface of the workpiece. If there are holes or defects in the passing place, there will be a reflection signal. By comparing the reflected signal with the bottom of the workpiece, the depth of the injury can be known. By moving the ultrasound probe, you can know the level of injury. This method is called reflection detection, which is the earliest and most widely used. The method of transmitting an acoustic signal at one end of the workpiece and receiving the signal at the other end and displaying the intensity is called transmission flaw detection.
If the workpiece is bonded by two parts (or multiple parts), it is best to measure the acoustic impedance of the workpiece from a surface. That is, on the same probe, one piezoelectric crystal emits continuous sound waves, and the other piezoelectric crystal receives sound waves. If there is a poorly bonded structure, the structural reaction force will have a large change, thereby making a judgment on the workpiece. .
Advanced highways, airport runways, and railways are large, long, and demanding. These facilities are monitored not only during construction, but also during use to ensure traffic safety. Therefore, generally put a few probes on the train or car, take the method of on-going monitoring. There is also an ultrasonic instrument for measuring the level of the road surface.
2, ultrasonic hardness test <br> The tip of a longitudinally vibrating ultrasonic transducer is pressed against the surface of the object to be measured, resulting in indentation. The measured object will affect the vibration of the transducer, and the resonant frequency of the transducer will change. The smaller the hardness of the object to be measured, the deeper the indentation, and the greater the influence on the resonance frequency. In this way, the hardness of the surface of the object to be measured can be known by the change of the frequency of the transducer.
3. Thickness measurement <br> When ultrasonic is applied to a board, the board will resonate at a certain frequency, reaching a large amplitude. At this time, if the sound velocity of the sheet is known, the thickness of the sheet can be measured. Conversely, knowing the thickness of the board, you can calculate the speed of sound. The test results can be accurate to 0.1%-1%. This method is called resonance.
The pulse echo method is more suitable for thickness measurement and is the most widely used in industrial applications. The thickness of the plate is measured using known sound velocity values ​​using the time that the pulse travels back and forth between the two faces of the panel. In this way, steel sheets from a few tenths of a millimeter to several tens of millimeters thick can be measured.
4, liquid level and level measurement
The level or level of a large container can be measured by ultrasound, which is basically a measurement of the pulse round trip time. However, in the liquid level and level measurement, the acoustic pulse travels far away, and the sound absorption in the air or the measured liquid is relatively large, so the ultrasonic with lower frequency is used.
5, ultrasonic thermometer <br> Some medium temperature is too high, such as steam turbine intake, rocket exhaust and combustion gases in the cylinder and other temperatures up to 2000 ° C, thermonuclear plasma temperature reached 17000 ° C, the ordinary thermometer has long been. An ultrasonic thermometer is used at this time. It can not only measure the temperature of high temperature and some special cases, but also react quickly.
The principle of the ultrasonic thermometer is to measure the speed of sound and calculate the temperature by using the law that the sound velocity of the medium has a certain relationship with the temperature. The ultrasonic thermometer is actually a sound speed meter. Of course, when measuring high temperature, the ultrasonic transducer should also be equipped with a water jacket to protect it from cooling.
6. Measurement of flow rate and flow rate <br> In the production process of petroleum and chemical industry, the flow rate can be measured using ultrasound. The sound propagates in the flowing liquid. The speed of propagation is the speed of sound plus the flow rate in the forward direction, and the speed of sound is reduced in the countercurrent, so the flow rate can be measured by the time difference between the forward and reverse flow sounds. Two pairs of transmitting and receiving transducers are placed on the tube, and the transceiving is performed relatively. The time difference between the signals received by the two receiving transducers is measured, and the flow rate can be measured.
Then there is the use of the Doppler effect, the sound waves and frequencies propagating in the fluid will change, and the degree of change is related to the flow rate and the original frequency. Continuous sound waves of the same frequency can be emitted upstream and downstream, and the frequency difference between the signals received by the two receiving transducers can be measured, and the flow rate can also be measured.
7. Ultrasonic viscometer <br> The viscosity of the liquid is different, and its shear impedance is different. Therefore, the transducer is placed in the object to be measured, so that the transducer performs shearing motion. Due to the difference in viscosity and resistance of the liquid, the transducer changes its relationship between the voltage and current at its input after being subjected to resistance, and the resistance change can be measured, and thus the viscosity is measured.
Diesel and kerosene have different sound velocities. Which part of kerosene is diesel in petroleum fractionation and transportation, which can be determined by the difference in sound velocity between kerosene and diesel. Ultrasonic oil separators have been used to transport several different oils in one pipeline, which cannot be solved with other hydrometers.
8, phacoemulsification <br> Everyone knows that oil and water can not be mixed together, but if you add high-powered ultrasound, after a while, the oil and water will be mixed together, and it will not be separated for a long time. This method is called phacoemulsification. In the cheese, milk, and pharmaceutical and cosmetic industries, various emulsion types are now widely used in ultrasound. Gasoline and kerosene can be emulsified by ultrasonic wave with a small amount of water, which has the effect of energy saving and pollution reduction in some combustion processes. Coal-water slurry and coal-water slurry can save fuel when burning, and after ultrasonic emulsification, coal powder is not easy to precipitate, and it is more convenient to use. This is due to the cavitation caused by strong ultrasound. The force of cavitation bubbles breaks up the liquid molecules and solid particles, and makes them into fine particles. The particles of different materials tend to stick together, such as oil and water. Water or oil-in-water condition.
9, ultrasonic cleaning <br> Some mechanical parts are small, but the shape is complex, there are holes, holes, tubes in the middle, it is not easy to wash with solvents, brushes, then you can ask for ultrasound. Put the parts in a jar with a little solvent, add ultrasound, and quickly clean the parts inside and out. This is also the cavitation bubble impact dirt, crushing the dirt film, peeling off the dirt from the surface of the object, and speeding up the dissolution of the dirt.
10. Ultrasonic pulverization <br> In order to obtain a very fine powder, it is usually milled for several hours using a ball mill. The particles are pulverized by ultrasonication, which is smaller and more uniform than the particles obtained by the general grinding method.
11, ultrasound tin
The surface of the aluminum has a very thin and strong oxide film, which causes great difficulty in soldering or tinting. The aluminum piece is placed in a bath with molten tin, and ultrasonic vibration is applied. Under the action of ultrasonic cavitation, the oxide film on the surface of the aluminum member can be removed, and the tin is firmly attached to the surface of the aluminum before the air is contacted. . However, in molten tin, the ultrasonic transducer is protected by a cooling water jacket.
12. Ultrasonic humidifier <br> When the air is too dry, it will cause various respiratory diseases, so it is often necessary to humidify the air in the winter in the north. Ultrasonic humidifiers currently on the market are actually ultrasonic atomizers that “break†water into water mist under the action of strong ultrasound. The water droplets of this water mist are very thin and only a few microns in diameter, so they can float in the air for a long time without falling down. Moreover, since there is no heating process, it does not affect the temperature in the room.
13. Ultrasonic welding <br> Ultrasonic welding can be used to weld metals or plastics. Some rare rare metals such as molybdenum, titanium, tantalum, etc. can be welded under ultrasonic action, so they are in the nuclear industry and space technology. More applications may be developed in other fields. Because it has the characteristics of basically no heat, no deformation, no residual stress, etc., it has great advantages in the lead wires in the soldering transistor and the integrated circuit chip.
14. Ultrasonic motor <br> The ultrasonic motor converts electrical energy into mechanical vibration, and then uses the friction between the stator and the rotor to rotate the rotor. Compared with electric motors, ultrasonic motors have the advantages of low speed, large torque, fast response speed, large positioning accuracy, small volume, high power density, no magnetic interference, and easy control. Due to these characteristics, it has great application prospects in robots, computers, instrumentation and other high-tech products.
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