What Is an Ultrasonic Circuit?

Ultrasonic waves are part of sound waves, which are inaudible to the human ear and have a frequency higher than 20KHZ. They have in common with sound waves, that is, they are generated by the vibration of matter and can only propagate in the medium; at the same time, it is also widely used. Ground exists in nature, and many animals can transmit and receive ultrasonic waves. Among them, bats are the most prominent. It can use weak ultrasonic echoes to fly in the dark and capture food. But ultrasound also has its special properties, such as higher frequencies and shorter wavelengths, so it also has similarities with light waves with very short wavelengths. ^[1]

Basic principles of ultrasound

Ultrasonic waves are part of sound waves, which are inaudible to the human ear and have a frequency higher than 20KHZ. They have in common with sound waves, that is, they are generated by the vibration of matter and can only propagate in the medium; at the same time, it is also widely used. Ground exists in nature, and many animals can transmit and receive ultrasonic waves. Among them, bats are the most prominent. It can use weak ultrasonic echoes to fly in the dark and capture food. But ultrasound also has its special properties, such as higher frequencies and shorter wavelengths, so it also has similarities with light waves with very short wavelengths.

Ultrasonic waves are elastic mechanical vibration waves. Compared with audible sound, they also have some characteristics: the direction of propagation is strong, and they can be gathered into a narrowly oriented wire harness; the acceleration of the vibration of the particle in the propagation medium is very large; When the intensity reaches a certain value, cavitation occurs.

First, the beam characteristics

The sound waves emitted from the sound source are propagated in a certain direction (other directions are very weak), which is called beam emission. Because of its short wavelength, an ultrasonic wave will appear as a concentrated beam of light that advances in a certain direction when it passes through a small hole (a hole larger than the wavelength). Because of the strong directivity of ultrasound, it can collect information in a directional manner. Similarly, when an obstacle has a diameter larger than the wavelength in the direction of ultrasonic propagation, a "sound shadow" will be generated behind the obstacle. These are like light passing through small holes and obstacles, so ultrasound has similar beam emission characteristics as light waves.

The quality of ultrasonic beam emission is generally measured by the divergence angle (habitually

Represented by a semi-launched angle mortar). Taking a flat circular piston sound source as an example, its size is determined

When ultrasonic waves propagate in various media, as the propagation distance increases, the intensity of the ultrasound will gradually weaken and the energy will be gradually consumed. The characteristic of this energy being absorbed by the media is called acoustic absorption. In 1845, Stokes (G.G.) found that when sound waves passed through a liquid, the internal friction (ie, viscous effect) caused by the relative motion of liquid particles caused sound absorption, and thus derived the internal friction or Formula for sound absorption in liquids due to viscosity. Also, when a sound wave propagates in a liquid medium, the temperature in the compression zone will be higher than the average temperature; on the contrary, the temperature in the sparse zone will be lower than the average temperature. Therefore, due to heat conduction, heat is exchanged between the compressed and sparse part of the sound wave. This caused a reduction in the energy of sound waves. In 1868, Kirchhoff G. derived a formula for sound absorption caused by heat conduction.

It can be seen that the absorption coefficient a is proportional to the square of the frequency of the acoustic wave. When the frequency is increased by 10 times, the absorption coefficient is increased by 100 times. That is, the higher the frequency, the greater the absorption, and therefore the smaller the distance the sound waves travel. In gas, Einstein proposed in 1920 to determine the response rate of associated gas by acoustic dispersion, which promoted the study of the thermal relaxation absorption mechanism of gas molecules extended to liquids. It was concluded that the molecules in the medium interact with each other. Collisions cause molecular thermal relaxation absorption. So low-frequency sound waves can travel a long distance in the air, while high-frequency sound waves quickly decay in the air.

In solids, sound absorption depends largely on the actual structure of the solid.

From the above, it can be seen that there are many reasons for the sound absorption of different media, but the main reasons are the viscosity of the media, heat conduction, the actual structure of the media, and the relaxation effect caused by the microdynamic process of the media. The absorption varies with the frequency of the sound. Ultrasonic waves are high-frequency sound waves. When propagating in the same medium, as the frequency increases, the energy absorbed by the medium becomes larger. E.g. frequency

The frequency of the energy absorbed by the ultrasonic waves in Hz in the air is

The sound wave of Hz is 100 times larger; the ultrasonic waves of the same frequency are different due to the propagation medium. For example, when it is transmitted in gas, liquid and solid, its absorption is the strongest, weakest and smallest. Therefore, ultrasonic waves travel the shortest distance in the air.

When an ultrasonic wave propagates in a homogeneous medium, due to the absorption of the medium, the influence of the sound intensity decreases with increasing distance, which is the sound wave attenuation.

When the initial intensity of the ultrasonic wave is J0, after a distance of x meters, its intensity is

Jx = Joe-2ax "'

Where a is the absorption coefficient (attenuation coefficient).

From the above, the absorption coefficients of sound waves in various media can be obtained.

It can be seen that the ultrasonic intensity decays exponentially. For example, after leaving the sound source, an ultrasonic wave with a frequency of 106 Hz passes through a distance of 0.5 m in the air, and its intensity will be reduced by half; when it travels in water, it will be reduced by half after a distance of 500 m

It can be seen that the distance traveled in water is equivalent to 1000 times the distance traveled in air. As the frequency increases, the attenuation becomes faster. If the ultrasonic wave with a frequency of 1011Hz propagates in the air, it will disappear without a trace when it leaves the sound source. In highly viscous liquids, ultrasound is absorbed faster. For example, at 200C, to reduce the intensity of an ultrasonic wave with a frequency of 300kHz to half, only 0.4m of air is sufficient.

In the water, it will pass through 440m, in the transformer oil, it will propagate about 100m, and in paraffin, it only needs to propagate about 3m. Therefore, extremely granular materials (rubber, bakelite, asphalt) are good insulators for ultrasonic waves.

The energy transmitted by ultrasonic waves is much greater than the audible sound. Because when the sound wave reaches a certain substance, the molecules in the substance also vibrate due to the action of the sound wave. The frequency of the vibration is the same as the frequency of the sound wave. Therefore, the frequency of the molecular vibration determines the speed of the molecular vibration. As a result, the molecules of the substance obtain energy from vibrations. In addition to the mass of the molecules, the energy is proportional to the square of the vibration speed of the molecules, and the vibration speed is related to the frequency of the molecular vibrations. That is, the higher the energy obtained by the substance molecules. The frequency of ultrasonic waves can be much higher than the frequency of sound waves, so ultrasonic waves can obtain greater energy from material molecules. This shows that ultrasound itself can

To supply enough energy to the material.

The sound waves that we usually hear in human ears have low frequency and low energy. For example, a loud conversation sound is approximately equal to an intensity of 50uW / cm2. But ultrasound has much more energy than sound waves. E.g. frequency

The energy of ultrasonic vibration in Hz has the same specific amplitude and the frequency is

The energy of a sound wave vibration in Hz is 1 million times greater, because the energy of a sound wave is proportional to the square of the frequency. It can be seen that the huge mechanical energy of ultrasound is mainly

The material particle has a great acceleration.

In normal operation, the loudness of a loudness speaker with normal loudness is

W / cm2; the sound of the gun's firing sound is

W / cm2; The sound of medium loudness makes the acceleration of the water mass point only a few percent of the acceleration of gravity (980cm / s2), so it will not affect the water. However, if ultrasound is applied to water, the acceleration of the water mass point may be hundreds of thousands or even millions of times greater than the acceleration of gravity.

Water particles produce rapid movements. It plays an extremely important role in ultrasonic extraction.

Cavitation is a common physical phenomenon in liquids. Due to physical effects such as eddy currents or ultrasonic waves in the liquid, local negative pressure zones are formed in some parts of the liquid, which causes the liquid or liquid-solid interface to break and form tiny cavities or air bubbles. These cavities or air bubbles generated in the liquid are in an unstable state, and they have a process of primary, development, and subsequent rapid closure. When they rapidly close and burst, a micro-shock wave is generated, which causes a large pressure in the local area. This phenomenon of cavitation or bubble formation in a liquid and subsequent rapid closure is called cavitation.

The basic process of cavitation and the difference between cavitation and boiling are briefly described as follows: When the liquid is heated under constant pressure or decompressed by static or dynamic methods at a constant temperature, it can reach vapour bubbles or gas-filled gas The vacuoles (or cavities) start to appear and develop, and then they close. This state is called "boiling" if it is caused by an increase in temperature; it is called "cavitation" if the temperature is basically unchanged and it is caused by a local pressure drop.

From the above basic cavitation process, it can be seen that cavitation has the following characteristics: cavitation is a phenomenon that occurs in liquids, and solids or gases will not cavitation under any normal environment; cavitation is the result of decompression of liquids. Therefore, cavitation can be controlled by controlling the degree of decompression; cavitation is a dynamic phenomenon that involves the development and closure of cavitation.

Ultrasonic cavitation is a unique physical phenomenon caused by strong ultrasound when it propagates in a liquid, and it is also a unique physical process that causes the rapid repetitive movement of the cavity in the liquid, which grows, compresses, closes, and rebounds. Local high pressure and high temperature are generated when the cavities collapse and close. Due to the frequency, sound intensity and surface tension, viscosity of the liquid, and the surrounding temperature and pressure in the sound field, the tiny gas nuclei in the liquid may respond under the effect of the sound field. It's easing, or it can be strong. Therefore, people divide acoustic cavitation into two types: steady state and transient state.

Steady-state cavitation mainly refers to the dynamic behavior of cavitation bubbles containing gas and vapor, which is a kind of bubble vibration with a longer life. This cavitation process generally occurs when the sound intensity is less than 1 W / cm2. The cavitation bubble vibrates for a long time and lasts several sound wave periods. In the sound field, since the surface area of the bubble during expansion is larger than that during compression, the gas diffused into the bubble during expansion is larger than that diffused out of the bubble during compression, so that the bubble increases in the vibration process. When the vibration amplitude is large enough, the bubble will change from steady state to transient cavitation, and then collapse.

Transient cavitation generally refers to cavitation bubbles generated at sound intensities greater than 1 W / cm2, and vibration is completed in only one sound cycle. This kind of bubble vibrating in the sound field, when the sound intensity is high enough and the sound pressure is negative half a cycle, the liquid receives a large tensile force, and the bubble nucleus swells rapidly, which can reach several times its original size. Then, when the sound pressure is positive half a cycle, The bubbles are compressed and burst into many small bubbles due to sudden collapse to form new cavitation nuclei. When the bubble rapidly shrinks, the gas or vapor inside the bubble is compressed, and within a very short time when the cavitation bubble collapses, a high temperature of about 5000K is generated in the bubble, similar to the temperature on the surface of the sun; a high pressure of about 500 atmospheres is locally generated, which is equivalent to Pressure on the deep ocean floor; temperature change rate is as high as 109K / s; accompanied by the generation of strong shock waves and jets and luminous speeds up to 400km per hour, small popping sounds can also be heard. It can be seen that the energy provided by cavitation causes local high pressure, high temperature, and high gradient flow, which provides a new extraction method for the difficult-to-extract components in medicinal materials.

The research on ultrasonic cavitation began in the 1930s. After the discovery of acoustic luminescence (SL) by Monnesco and Frenzel et al., The research on the motion of ultrasonic cavitation bubbles caused by the pursuit of luminescence and the measurement of its basic effects. They used the measurement of ultrasonic cavitation group bubbles in liquids to study the "multi-bubble cavitation"; by the 1960s, Wang Chengzhang and Zhang Dejun of the Chinese Academy of Sciences, under the guidance of Academician Ying Chongfu, studied the single unit generated by the dynamic method. The complete movement process of cavitation bubbles, and experimentally proved that both the optical and electromagnetic radiation of cavitation occur at the moment of bubble closure, they also studied the cavitation

Emulsification and mechanical effects. In the 1980s, the American Gaitan and Crum et al. Used acoustic suspension technology to "entrain" a single bubble at the antinode of the standing wave field of the container, and synchronized it with the external ultrasonic field to generate a periodic cavitation process, and measured it. These results provide a theoretical basis for the application of ultrasound in industry, agriculture, and medicine, and also provide conditions for the measurement of ultrasonic cavitation.

Measurement of cavitation intensity

According to current reports, there is no absolute measurement method for ultrasonic cavitation intensity, but the application effect of ultrasound in some aspects is directly related to the cavitation intensity, so trying to measure the cavitation intensity in practical applications has Significance. The cavitation intensity is not only related to the pressure generated when the cavitation bubbles are closed, the number of cavitation bubbles per unit volume, but also to various types of cavitation bubbles, so only relative strength can be measured. At present, it is mainly studied from the perspective of ultrasonic cleaning to directly measure the effect of ultrasonic cleaning. The methods are:

Corrosion method: Aluminium, tin or lead foil with a thickness of about 20um is subject to cavitation corrosion at a certain distance in the sound field, taken out within a certain period of time, and the weight of the corrosion sample is weighed to measure the relative cavitation strength. This method is called erosion method. This method measures the relative cavitation intensity from the liquid surface to different depths. The measurement method requires that the surface finish of the metal sample is consistent, and multiple measurements are performed to find the average value.

Chemical method: Put sodium iodide in carbon tetrachloride, and measure the relative cavitation intensity by the amount of iodine released under the effect of acoustic cavitation. This method is called chemical method. This method uses a spectrophotometer or a radiotracer method for the quantitative determination of iodine release. Because the ultrasonic intensity is 5 -30 W / cm2, the amount of iodine released increases with the increase of sound intensity for 1 min. Therefore, the cavitation intensity is measured by the amount of the released amount.

Decontamination method: Use a workpiece with radioactive contamination as a cleaning sample. After ultrasonic cleaning, quantitatively measure the amount of dirt removed to measure the effect of ultrasonic cleaning or the relative cavitation intensity. This method is called Decontamination Act. There are also methods for measuring cavitation noise in practical applications, which are not described here.

Negative effect and application of ultrasonic cavitation

Due to the non-linear vibration of bubbles generated by acoustic cavitation and the burst pressure when they burst, many physical and chemical effects can occur with cavitation. These effects have a negative effect, but they also have applications in engineering. For example, the surface of high-speed rotating propeller blades used by ships is often affected by the pressure of cavitation and "corroded" into some marks. When cavitation is severe, the appearance of a large number of bubbles will affect the thrust of the propeller. In the civil industry, cavitation "corrosion" can damage pipes and components. However, the use of the shock wave generated by cavitation, or the local high temperature of bubble closure can be beneficially used in industry. Such as ultrasonic cleaning, it is the use of acoustic waves to construct special-shaped channels, and ultrasonic cavitation can be used to clean the micro-machine parts placed in the detergent; ultrasonic descaling and waterproof scale deposition can also be performed in the boiler; Emulsification of pharmaceutical production process, industrial preparation of emulsions of mixed solutions such as oil-water; ultrasonic welding (destroying metal surface oxide layer, promoting metal welding); using ultrasonic cavitation to promote certain chemical reaction processes; breaking plants Thin wall, to promote the dissolution of chemical components into solvents, and improve the application rate of chemical components. ^[2]

I. Overview of ultrasonic principle The principle of ultrasonic cleaning is the high-frequency oscillating electrical signal produced by the generator. The transducer is converted into high-frequency mechanical vibration, which is transmitted to the cleaning liquid, and the workpiece is efficiently cleaned. Its working mechanism is to use the cavitation effect to double or more than ten times the sale of land to improve the cleaning effect. When the liquid is put into the washing machine and the ultrasonic wave is applied, the ultrasonic wave is a dense and dense phase in the cleaning liquid, which radiates high-frequency waves that propagate, so that the liquid vibrates back and forth at high speed. In the negative pressure area of the vibration, the surrounding liquid is too late to replenish, forming countless tiny vacuum bubbles. In the positive pressure area, the small bubbles suddenly close under pressure. During the closing process, strong shock waves are formed due to collisions between liquids. A momentary high pressure of several thousand atmospheres acts on the workpiece being cleaned. The greasy and impurities adsorbed on the work piece are quickly separated from the work piece under the action of continuous instantaneous high pressure. So as to achieve the purpose of cleaning. Two main parameters of ultrasound Two main parameters of ultrasound: Frequency: F20KHz; Power density: p = transmission power (W) / emission area (cm2); usually p0.3w / cm2; Ultrasonic waves propagating in liquid It can clean the dirt on the surface of the object. The principle can be explained by the phenomenon of "cavitation": when the pressure of the sound wave propagating in the liquid reaches an atmospheric pressure, the power density is 0.35w / cm2. The peak value can reach vacuum or negative pressure, but in fact there is no negative pressure, so a great pressure is generated in the liquid, which pulls the liquid molecules into cavities and cavitation nuclei. This cavity is very close to a vacuum. It ruptures when the ultrasonic pressure reverses to the maximum. The strong impact caused by the rupture knocks the dirt on the surface of the object down. This phenomenon of shock waves caused by the collapse of countless tiny cavitation bubbles is called the "cavitation" phenomenon. Too little sound intensity does not produce cavitation effects. The ultrasonic cleaning machine is composed of three main parts: (1) stainless steel cleaning tank filled with cleaning liquid (2) ultrasonic generator (3) ultrasonic transducer ultrasonic cleaning machine has the advantages of high cleanliness, low machine noise, long equipment life, etc. . And can be more complex geometric shapes, such as various blind holes, micro holes, deep holes and other parts difficult to clean with other cleaning methods for efficient cleaning. Due to the above unique properties, people are more and more recognized and accepted. Second, the equipment characteristics When the ultrasonic cleaning machine is filled with water and connected to the power supply, the circuit converts 50 Hz AC power to ultrasonic frequency AC power and generates oscillations. The formation of this oscillation is to form a resonant circuit through the inductor and the capacitor of the transducer, and The oscillation signal is continuously carried out through feedback. After being amplified by the transistor, it is sent to the series resonance circuit. This resonance frequency is precisely adjusted to the natural resonance frequency of the transducer before the machine leaves the factory to exert the best effect of the transducer. The transducer is bonded to the bottom surface of the stainless steel cleaning tank through a stud and a strong adhesive. The transducer transmits ultrasonic mechanical energy to the liquid in the tank through the bottom of the tank, and then acts on the workpiece being cleaned in the liquid, thereby achieving It has the function of ultrasonic cleaning. The high-power transistor works in the saturation state of the switch, so its output waveform is square. When the square wave enters the resonance circuit, it becomes a sine wave after filtering by the inductor and capacitor, so the current waveform acting on the transducer actually becomes a sine wave. There are two types of ultrasonic power generators for ultrasonic cleaners, one is a self-excited circuit, and the other is a separately excited circuit. The self-excited circuit has a simple structure, practicality and good economy; the other-excited circuit has high power and has multiple protections such as frequency tracking, current limiting, and heating. The two circuits are suitable for different levels of enterprises and a wider range of customer needs. Third, the use method 1. Connect the generator and the cleaning tank connection cable. 2. Fill the tank with the selected cleaning solution. 3. Connect the generator to 220V ± 10% 50Hz AC power. 4. Turn on the power switch of the generator, and the power indicator light is on (at this time, the liquid in the tank starts to vibrate and cavitation). Fourth, matters needing attention 1. In order to extend the service life, it is recommended to place the equipment in a ventilated and dry area, and the fan holes on the rear side of the generator should be cleaned regularly. There are vents on all sides of the generator to make the air flow unobstructed. 2. (1) The cleaning tank must be filled with liquid before it can be turned on. The minimum water level height is> 100mm (bottom vibration type) and placed horizontally. When the transducer is on the side, the cleaning tank groove is 100mm along the side. Will damage the machine. (2) When the temperature of the cleaning cylinder is normal temperature, do not inject high-temperature liquid directly into the cylinder, so as to prevent the transducer from loosening and affecting the normal use of the machine. (3) When the cleaning liquid needs to be replaced due to pollution, do not inject low-temperature liquid directly into the high-temperature cylinder, which may also cause the transducer to fall off. At the same time, the heater switch should be turned off to prevent the heater from being filled with liquid damage. (4) Regularly check the transducer, and do not make it wet or hit, so as not to cause unnecessary losses. 3. After use, turn off the main power. 4. Do not restart the machine immediately after shutdown. The gap time should be more than 1 minute. ^[3]