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Radar (Microwave) Level Meter -- Research on Echo Signal Processing

2022-11-09 08:38:00

The echo signal processing of the radar (microwave) level meter is derived from the ultrasonic measurement of the level because both are echo-ranging principles. The necessary condition for the work is that the echo of the measured material surface must be received, and then the distance must be calculated according to it and then converted into the corresponding electrical signal for output.

In practical application, the situation is complex (as with ultrasound), microwave propagation will be attenuated, and the echo amplitude at short distance is large, while that at long distance is small; The dielectric constant of the measured material also affects the echo amplitude, and the echo amplitude is large when the dielectric constant is high. The echo of the measured liquid is large, and the reflected signal is strong because the liquid surface is horizontal. The solid material surface is rough and has a repose angle, forming diffuse reflection, resulting in a weak signal. For various reasons, the echo amplitude fluctuates greatly, which can vary thousands of times.

In addition, the cross beam, various installation components, and welds in the silo will generate reflected echo, which will be mixed with the echo reflected by the material surface and accepted. In addition, the starting and stopping of large motors and the on and off of silicon-controlled rectifiers will make the power supply generate sharp pulses, which are also superimposed on the receiving wave. To measure the material level reliably, it is necessary to ensure that the required stock level echo can be received first, and also to identify the stock level echo we need from the above numerous echoes, so as to measure the distance correctly. This is the task to be solved by echo signal processing and the key to measurement.

The early ultrasonic level meter was gradually accepted and popularized after the above problems were solved and the measurement was reliable. The difference between radar (microwave) level meter and it is only that the energy wave used is different. After transmitting the microwave, receiving an echo, and converting it into an electrical signal, the following signal processing and ultrasonic measurement of level are identical. Therefore, the original manufacturers of ultrasonic level meters are developing radar (microwave) level timing, and it is natural to implant their ultrasonic signal processing technology into radar (microwave) level meters.

Radar (Microwave) Level Meter suppliers

Early echo processing (ultrasonic level measurement since the 1970s)

After the ultrasonic echo is converted into an electrical signal, it is very weak, in the range of several millivolts (mv) to several hundred millivolts (mv). In the early stage, one threshold curve was used to select the echo, and the first echo reaching the threshold was selected. As the echo signal weakens with the increase of the propagation distance, a time-varying threshold curve (TVT curve) is usually used. Its threshold value is higher at short distances than at long distances and can be adjusted according to the site conditions.

If multiple echoes reach the threshold value, but the first wave is not the stock level echo, the technician should use the oscilloscope to observe the echo curve to adjust the threshold curve, so that the stock level echo can be confirmed, which is troublesome.

Digital echo processing (since 1984)

In 1984, Milltronics Company first implanted the microprocessor into the ultrasonic level meter to digitize the measurement and signal processing. First, it digitized the analog echo curve, including the transmitted wave and all received waves and stored it in memory as the basis of digital signal processing. At the same time, the application experience of ultrasonic level meters accumulated by them in grain, cement, mining, and other industries over the years is integrated into the "acoustic intelligence" echo processing software, and the software algorithm is used to analyze many echoes in the echo curve to find the required material level echo.

Echo envelope

In actual processing, the echo curve is stored in the memory in the form of an envelope and processed. It represents what the microwave antenna "sees" from the silo and serves as the basis for signal processing. At first, the echo envelope line can only be observed after being connected with the electronic unit of the level meter through the oscilloscope and adjusted by the technician. With the development of science and technology, it can now be transmitted through HART, Profibus, or other communication protocols, and echo envelope lines can be generated on the computer screen running special software.

Radar (Microwave) Level Meter manufacturers

Other characteristics

Intelligent processing also supports the following implicit functions:

1) Averaging (combining multiple echo envelopes)

Each radar (microwave) level meter has its own random noise level. The single echo envelope seems to contain a lot of noise, which is caused by the random noise of the microwave level meter. Therefore, the signal processing software stores and averages multiple single echo envelopes, and the resultant averaged echo envelopes are used for signal processing. To reduce the impact of noise.
Echo envelop after averaging
The averaged noise baseline will be smoother and its amplitude will be lower than the peak noise. By using the averaged noise baseline, the quality of the composite signal will be improved immediately.

2) Dynamic TVT

The time-varying threshold curve (TVT) is automatically set and adjusted by intelligent processing software. The curvature of the TVT curve is derived from the echo envelope signal, and the height of the TVT curve above the echo envelope is derived from the strongest echo signal in the echo envelope. When there is a strong echo signal, the position of the TVT curve will be higher, so weak false signals will be lower than the TVT curve, and these false signals will not be included in echo processing. Therefore, it is not confirmed by echo processing. The higher TVT curve has lower sensitivity, which is conducive to suppressing weak false echoes.

When the echo signal is weak, the TVT curve will automatically reduce, and the degree of reduction is still based on the largest echo signal in the echo envelope at this time. At this time, the sensitivity is also improved. The consequent risk is that weak false signals may also be detected. At this time, the AUTO-FALSE-ECHOSUPPURESSION function should be started to prevent these FALSE echoes from being selected to affect the measurement.

3) Echo-shaping filter

Echo shaping is a time-based parameter, which is used to smooth the sawtooth echo. Especially in the application of solid materials, the material surface is often irregular. When the echo generated by the reflection of the material surface has multiple wave peaks, it will cause the reading of the level meter to jump, because the meter will lock the measured value at different points of the zigzag curve each time it measures, and it will get different readings, which will form a zigzag trend curve.

For example, during feeding, materials are continuously added to the silo. Generally, the material level should increase, but sometimes the material level reading will decrease intermittently. The instrument selects different points on the zigzag echo envelope. If some points are lower than the material level of the actual feeding point, the instrument will read the low material level value, and the trend seems unreasonable. The user can increase the echo reform setting to smooth the sawtooth shape. The multi-peak surface of the echo envelope will form a single, smooth shape, making the reading trend of the instrument consistent.
The filter is software, which is equivalent to flattening the surface of the measured material physically.

4) Automatic suppression of false echoes

If an obstacle generates a large echo in front of the stock level echo, the echo will exceed the default TVT curve, causing it to be incorrectly selected as a valid echo. The automatic false echo suppression function will correct the TVT curve so that the false echo is still below the TVT curve.

When automatic false echo suppression is used, the instrument first "learns" the echo envelope at this moment (including interference echo). The learned TVT curve follows the echo envelope and then rises above the false echo.
Set the automatic suppression range, so that the envelope formed by learning will replace the default TVT curve at the point where the obstacle is confirmed. In addition, the default TVT curve is still used. When the level echo rises above it, it is selected as the real echo. This can be achieved on the local display or remotely with a PC.

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