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    Explanation of Noise of Micro Pump and How to Control it

    Author:河源市柏爾科技有限公司 Click: Time:2018-12-23 20:01:44

    Noise pollution is a kind of environmental pollution, and water pollution, air pollution and solid pollution are the most important environmental problems in the world. Physically speaking, noise is the sound produced by the body when it vibrates irregularly, which is transmitted to the human ear through air or other elastic media. Physiologically speaking, all voices that interfere with people's normal rest, study and work, and those that interfere with people's voices are noises. The audible frequencies of human ears range from 20Hz to 20000Hz. Noise can cause irritation, reduce work efficiency and endanger people's physical and mental health. Noise control has been paid more and more attention in many countries. Micropump is a common component of instruments and instruments, which is widely used in scientific research and medical field. Scientific research and medical institutions have strict requirements for noise control. For example, hospital management standards in China require operating room noise not higher than 38dB. Medical equipment is the main noise source in the operating room. Some ventilators and aspirators have noise as high as 70 dB. Reducing the noise pollution in the operating room is the necessity of the development of modern medicine. Micropumps are also common equipment in almost all laboratories. With the increasing attention of customers to the noise problem of pumps, reducing the noise of micro-pumps has become an important direction for manufacturers.

    Diaphragm pump is a kind of micro-pump. It uses motor to drive the film to and fro to produce vacuum, so as to inhale and press out the fluid. High efficiency of micro-pump; complete isolation of movement mechanism and conveying medium, no pollution to medium; non-dynamic seal, no leakage; good reliability; small size, light weight; no fear of idling; no need of lubrication maintenance; low self-noise. When miniature vacuum pump is used for pumping, it does not need irrigation pump. It absorbs water and drains water immediately after starting. It can pump air and water, and is not afraid of drying when there is no water. The advantages of micropump make it widely used in instrument industry.

    From the source point of view, the noise of micro-pump is mainly divided into three parts: hydrodynamic noise, mechanical noise of mechanical moving parts and motor noise.

    Hydrodynamic noise is the noise radiated from the vibration of pump body, valve and pipeline, which is caused by the continuous pulses of fluid pressure generated by the micro-pump when it works. Researchers from Aijiakang Company have measured that hydrodynamic noise is the main noise of micropump. The hydrodynamic noise of micropump belongs to medium and low frequency noise. When the miniature vacuum pump sucks, the intake valve opens, the outlet valve closes, and the air is sucked into the pump chamber. The pressure fluctuation in the intake and valve chamber produces violent pressure fluctuation. The pressure fluctuation of the gas radiates from the intake in the form of sound wave, forming the noise of the intake. The intake noise is a broadband continuous spectrum. Its fundamental frequency can be calculated by formula f=2n/60 (n is the speed per minute). In addition to the fundamental frequency, there are also high frequency harmonics, but the high frequency harmonics sound level is lower than the basic frequency sound level. The higher the rotating speed, the higher the vacuum degree, the larger the flow rate and the greater the noise of the intake. When exhausting, the intake valve closes, the outlet valve opens, the air expands, and the airflow flows rapidly through the outlet to produce sound waves forming exhaust noise. The exhaust noise is also a broadband continuous spectrum. The higher the speed, the greater the pressure, the greater the flow rate and the greater the noise of the exhaust port. The noise of the inlet and exhaust of the long plastic pipe will be reduced. The cause of noise of miniature vacuum pump is complex. Because of the periodic fluctuation of the pressure in the pump chamber, unstable flow of water produces a large number of eddies. The water flow produces noise to the pressure of film, pump chamber and valve, and the friction between water flow and pump body. Especially when the water temperature is high and the vacuum degree of the pump is low, the pressure in the suction stroke pump chamber may be lower than the saturated vapor pressure of water at that temperature. The water will vaporize and produce a large number of bubbles, forming a complex two-phase flow. During the compression stroke of the pump, under the action of high pressure, the bubbles burst rapidly, so that the water around the bubbles rushes to the center of the bubbles at a great speed, resulting in high-frequency, high-impact water hammer, continuously striking the internal parts of the pump. Pump will appear vibration and noise, this phenomenon is called cavitation. Cavitation occurs on the solid boundary, which is the result of the continuous action of the impact force produced by the rapid collapse of cavitation in the high pressure area on the solid boundary. Cavitation is harmful, which can damage parts, reduce efficiency, and produce vibration and noise. The noise produced by miniature vacuum pump belongs to low frequency noise. The hydrodynamic noise is determined by the working principle of vacuum pump. Improving the design of pump body can reduce the noise, but the effect of active noise reduction is far from people's expectation.

    Mechanical noise is not the main source of noise in micro-pumps. It comes from the vibration of film inside the pump, bearing friction, eccentric motion of crankshaft and connecting rod, impact of one-way valve, etc. This kind of noise has randomness and broadband characteristics. In the aspect of active noise reduction, it is necessary to improve material selection, design, parts processing accuracy, balance and so on.

    Motor is also the main noise source of micro-pump. There are three kinds of motor noise: first, electromagnetic noise, second, mechanical noise, third, aerodynamic noise. Electromagnetic noise is mainly caused by periodically varying radial electromagnetic force or unbalanced magnetic tension in the motor, which causes magnetostriction and vibration of the core. The electromagnetic noise is also related to the vibration characteristics of the stator and the rotor itself. When the excitation force and natural frequency resonance, even if the electromagnetic force is very small, it will produce a lot of noise. In order to reduce electromagnetic noise, we should try our best to use sinusoidal windings to reduce the harmonic component, select appropriate air gap magnetic density, select appropriate slot matching, use rotor inclined slot, symmetrical and uniform magnetic circuit of stator and rotor, close overlapping pressure, pay attention to the roundness and coaxiality of stator and rotor, and avoid their resonance frequency. In order to avoid the resonance between the electromagnetic force and the casing, an appropriate elastic structure can be adopted. Mechanical noise is the most important part of motor noise, which comes from bearing noise, unbalanced rotor and assembly eccentricity. Carbon brushes also cause vibration and noise. In order to reduce mechanical noise, sealed bearings should be generally used to prevent impurities from entering. The processing of bearing balls, inner rings and outer rings must meet the design requirements, and strict demagnetization and cleaning procedures should be adopted. Grease must be clean. Bearing outer ring and bearing chamber, inner ring and shaft should not be too tight. In order to eliminate the axial clearance of the rotor, proper pressure must be applied to the bearing. For motors with strict noise requirements, low noise bearings should be selected. When the load is not too large, oil bearing can be used, which is sometimes about 10 dB lower than the noise of rolling bearings of the same size.  Adequate bearing oil can effectively reduce noise. The processing accuracy of the rotor must meet the design requirements. The rotor with higher speed must be balanced. Brushless motor can be used to eliminate brush noise. Aerodynamic noise is caused by the cooling fan of the motor belt. The thicker the motor iron shell is, the smaller the amplitude is. It can absorb vibration energy and shield sound transmission. Experiments show that if the thickness of iron shell is not much, the improvement of dB is not obvious.

    In addition to the noise sources of the above micro-pumps, the pipeline system connected in use may also produce larger noise. When the fluid flows rapidly in the pipeline, the periodic pressure fluctuation and the noise caused by the friction between the fluid and the pipe wall penetrate the pipe wall and propagate to the outside world. Especially the fluctuation of exhaust gas will stimulate the pipeline system and form multiple noise sources in the process of downstream discharge. The vibration of the pump body will also be transmitted directly to the pipeline. Hose connection at the inlet and outlet of the pump can reduce the vibration of the pump body propagating to the pipeline. Pipeline noise is mainly related to pressure fluctuation, frequency, fluid properties and flow pattern, pipe material and resistance components. Reducing the velocity in the pipeline is an effective way to control noise. Increasing wall thickness can also reduce noise, at a cost of course.

    In production practice, noise is generally controlled by three methods: sound absorption, sound insulation and noise elimination. Sound absorption is a technique in which part of the energy of sound wave is absorbed by objects and converted into other energy to reduce noise. This technique is mainly used in indoor space. Sound insulation is to enclose noise-producing machinery and equipment in a small space, or to change the connection between noise source and support from rigid connection to elastic connection, so that noise is greatly attenuated in the process of transmission or the vibration of the object is reduced. Sound barrier and noise shield are two main sound insulation designs. Muffler is a kind of equipment which can not only make air flow through but also effectively reduce noise. Mufflers are mainly divided into six categories: resistance muffler, resistance muffler, impedance composite muffler, microperforated plate muffler, small hole muffler and active muffler. Resistive mufflers use porous sound-absorbing materials to reduce noise, fixing sound-absorbing materials on the inner wall of airflow passage or arranging them in a certain way in the pipeline. Resistant muffler is the reflection and interference of sound energy caused by the change of impedance caused by the sudden change of the cross-section of the pipeline or the adjacent resonator, so as to reduce the sound energy radiated from the muffler. Impedance compound muffler is composed of Resistance Muffler and resistance muffler through proper structure, and has both muffling characteristics. The aperture of the microperforated plate muffler is less than 1 mm, and the acoustic resistance of the microperforated plate is much higher than that of the ordinary perforated plate. The low porosity can increase the width of the sound absorption band, and the appropriate depth behind the plate can improve the position of the resonance absorption peak. Small-hole muffler is to make air flow through small holes, the spectrum of jet noise will move to high frequency, so that the audible components of the spectrum will be reduced, thereby reducing the interference and harm of noise to human beings. Active muffler is a kind of interference sound wave which is controlled by computer to produce a kind of frequency and intensity which is consistent with the need of noise elimination, but whose phase is opposite, to eliminate noise. Mufflers are often used to reduce the noise of the inlet and outlet of various aerodynamic equipment or the transmission along the pipeline.

    AJK pump, the leader of domestic micro-pump industry, has many years of experience in developing micro-pump, relying on the technical strength of national aerodynamic research institutions and universities, continuously optimizes the pump body structure design, chooses high-quality materials, strictly controls the processing accuracy and reduces the matching clearance in the manufacturing process, and effectively reduces the noise of micro-pump. Especially the newly developed long-life and adjustable-pressure micro-pump has many models, such as pressure-regulated vacuum pump, pressure-regulated air pump (air pumping and air pumping), speed-regulated water pump (water pumping and air pumping). It takes the lead in adopting advanced imported brushless motor. It not only has the advantages of long life, no clutter interference, easy and reliable flow regulation, but also can output speed feedback signal, and eliminates brush noise. Voice. The noise of high quality imported brushless motor is significantly lower than that of the original motor. Brushless motor micro pump is equipped with flexible vibration absorber which can rotate to isolate the vibration of the pump body and facilitate the installation of users. "AJK" company also designed a special muffler for micro-pump, using advanced multi-muffler principle, low pressure loss, good noise reduction effect. It is better to install special mufflers at the inlet and outlet of the pump. When users use micro-pumps, the noise of the whole system is also related to all the components on the air path. Noise can be effectively reduced by reducing the flow velocity in the pipeline under allowable conditions. The use of "AJK" speed control pump can flexibly adjust the pump speed and flow rate according to the need to reduce noise. Buffer tanks can also be installed in the pipeline to eliminate pressure and flow pulsation in the pipeline, reduce pipeline vibration and reduce system noise. In addition to reducing the intensity of the noise, changing the frequency of the noise can also make the noise sound less uncomfortable. Users'pipeline systems are different in actual use, so it is necessary to debug the whole gas system to get the best noise reduction effect.


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