Everyone knows that in the detection and control of industrial production equipment, it is very helpful to know the working principle of the required instruments and meters, and it is very helpful to select the appropriate measurement and adjustment instruments. In the current level measurement control of industrial and mining enterprises, in addition to the selection
,那么它是如何工作的呢?一般来说我们把声波频率超过20kHz的声波称 Ultrasonic level gauges are often used outside of measuring instruments such as various float level gauges, pressure transmitters and differential pressure transmitters, so how does it work? Generally speaking, we have a sound wave with a sound frequency exceeding 20kHz. Weigh
It is an ultrasonic wave. Ultrasonic wave is a kind of mechanical wave, that is, a kind of propagation process of mechanical vibration in elastic medium. It is characterized by high frequency, short wavelength, small diffraction phenomenon, and good directivity, which can be determined by rays.
To spread. Ultrasonic waves have very low attenuation in liquids and solids, and therefore have strong penetrability. Especially in solids that are opaque to light, ultrasonic waves can penetrate tens of meters in length, and there will be significant reflection when they encounter impurities or interfaces.
The ultrasonic level meter works by using this principle. In the ultrasonic detection technology, no matter what kind of ultrasonic instrument, it is necessary to transmit the ultrasonic wave converted from electrical energy, and then receive it back and convert it into electrical signals.
The device that accomplishes this is called an ultrasonic transducer and is also called a probe. Place the ultrasonic transducer above the liquid or level to be measured, and transmit the ultrasonic wave downward. The ultrasonic wave passes through the air medium and encounters the water surface or object.
The interface is reflected back, then received by the transducer and converted into an electrical signal. The electronic detection part detects this signal and turns it into a level signal for display and outputs a standard signal for other instruments or control devices.
Use. According to the principle of ultrasonic wave propagation in a medium, if the medium pressure, temperature, density, humidity and other conditions are constant, the propagation speed of the ultrasonic wave in the medium is a constant. So when it is measured that the ultrasound is emitted by
When the time required to receive the reflection of the object or liquid surface is received, the distance of ultrasonic wave can be converted to measure the level or liquid level data indirectly.
The first invention of ultrasound
From the end of the 19th century to the beginning of the 20th century, after the piezoelectric effect and the anti-piezoelectric effect were discovered in physics, people have solved the method of using electronics to generate ultrasonic waves, and since then, the development and promotion of ultrasonic technology have been quickly opened
In 1922, the first invention patent for ultrasound treatment appeared in Germany.
A literature report on the clinical effects of ultrasound treatment was published in 1939.
In the late 1940s, the rise of ultrasound treatment in Europe and the United States, until the first international conference on medical ultrasound held in 1949, did not have the exchange of ultrasound treatment papers, laying the foundation for the development of ultrasound therapy.
Many papers have been published at the 2nd International Conference on Ultrasound Medicine in 1956, and ultrasound treatment has entered a practical mature stage.
工作原理 At this time in 1993, American physicists discovered how the ultrasonic level gauge works
The working principle of the ultrasonic level gauge is that a high-frequency ultrasonic pulse sent by a transducer (probe) meets the surface of the measured medium and is reflected back. Part of the reflected echo is received by the same transducer and converted into an electrical signal. Ultrasonic pulses
The speed of sound waves propagates. The time interval from transmitting to receiving the ultrasonic pulse is proportional to the distance from the transducer to the surface of the measured medium. The relationship between this distance value S and the speed of sound C and the transmission time T can be expressed by the formula
Means: S = CxT / 2.
Because the transmitted ultrasonic pulse has a certain width, the reflected wave and the transmitted wave in a small area close to the transducer overlap and cannot be identified, and the distance value cannot be measured. This area is called the measurement dead zone. Dead zone
The size is related to the model of the ultrasonic level gauge.
Ultrasonic ranging principle
The principle of ultrasonic ranging is to use the propagation velocity of ultrasonic waves in the air to be known. Measure the time when sound waves reflect back after encountering obstacles. Calculate the actual transmission point to obstacles based on the time difference between transmitting and receiving
Distance. It can be seen that the principle of ultrasonic ranging is the same as that of radar.
The formula for ranging is expressed as: L = C × T
Where L is the measured distance length; C is the propagation speed of the ultrasonic wave in the air; T is the time difference between the measured distance propagation (T is half of the value of the time from transmission to reception).
Ultrasonic ranging is mainly used for distance measurement in back-up reminders, construction sites, industrial sites, etc. Although the current ranging range can reach 100 meters, the accuracy of the measurement is often only in the order of centimeters.
Due to the advantages of easy directional emission, good directivity, easy to control intensity, and no direct contact with the measured object, it is an ideal method for measuring liquid height. Need to reach millimeters in precise level measurement
Level of measurement accuracy, but at present, domestic ultrasonic ranging ASICs have only centimeter-level measurement accuracy. By analyzing the causes of ultrasonic ranging errors, increasing the measurement time difference to the microsecond level, and using
After the LM92 temperature sensor compensates the speed of sound wave propagation, our designed high-precision ultrasonic rangefinder can achieve millimeter-level measurement accuracy.
Because of adopting the advanced microprocessor and unique EchoDiscovery echo processing technology, the ultrasonic level gauge can be applied to various complicated working conditions. The built-in temperature sensor of the transducer can achieve temperature compensation of the measured value.
The ultrasonic transducer adopts the patented technology of the best acoustic matching, so that its transmitting power can be radiated more effectively, and the signal strength can be improved, so as to achieve correct measurement.
安装要求。 Ultrasonic level gauge / level gauge installation requirements.
When the transducer emits ultrasonic pulses, it has a certain emission opening angle. From the lower edge of the transducer to the surface of the measured medium, there must be no obstacles in the area radiated by the transmitted ultrasonic beam, so it should be installed as much as possible
Avoid measures inside the tank, such as: people ladders, limit switches, heating equipment, brackets, etc. Also note that the ultrasonic beam must not intersect the feed stream.
Also pay attention when installing the meter: the highest material level must not enter the measurement blind zone; the meter must be kept at a certain distance from the tank wall; the installation of the meter should make the emission direction of the transducer perpendicular to the liquid surface as much as possible.
According to the ultrasonic ranging formula L = C × T, it can be known that the ranging error is caused by the propagation speed error of the ultrasonic wave and the time error of the measuring distance propagation.
When the distance measurement error is required to be less than 1mm, it is assumed that the known ultrasonic speed C = 344m / s (room temperature at 20 ° C), and the propagation error of the sound speed is ignored. The ranging error s △ t <(0.001 / 344) ≈0.000002907s is 2.907ms.
Under the premise that the propagation speed of the ultrasonic wave is accurate, the accuracy of the difference in the propagation time of the measurement distance can be guaranteed to be less than 1 mm as long as the accuracy of the difference in the measurement time is in the microsecond range. 12MHz crystal used as clock reference
The 89C51 microcontroller timer can easily count to 1 μs accuracy, so the system uses the 89C51 timer to ensure that the time error is within the 1mm measurement range.
Ultrasonic propagation velocity error
The propagation speed of ultrasound is affected by the density of air. The higher the density of air, the faster the propagation speed of ultrasound, and the density of air is closely related to the temperature.
The relationship between known ultrasonic velocity and temperature is as follows:
In the formula: r — the ratio of the constant pressure heat capacity of the gas to the constant capacity heat capacity, for air is 1.40,
R — universal gas constant, 8.314kg · mol-1 · K-1,
M—gas molecular weight, air is 28.8 × 10-3kg · mol-1,
T — Absolute temperature, 273K + T ℃.
The approximate formula is: C = C0 + 0.607 × T ℃
In the formula: the sound wave velocity is 332m / s when C0 is zero degree;
T is the actual temperature (° C).
When the accuracy of ultrasonic ranging is required to reach 1mm, the ambient temperature of ultrasonic propagation must be considered. For example, when the temperature is 0 ℃, the ultrasonic speed is 332m / s, and the temperature is 350m / s at 30 ℃.
The change in ultrasonic speed was 18 m / s. If an ultrasonic wave measures a distance of 100m at a sound velocity of 0 ° C at a temperature of 30 ° C, the measurement error will reach 5m, and the measurement error of 1m will reach 5mm.