FMCW is an abbreviation of the prefix of English Frequency Modulated Continuous Wave. FMCW technology is the earliest technology used in radar level measurement equipment.
The FMCW microwave level meter uses a linearly modulated high-frequency signal and generally uses 10 GHz or 24 GHz microwave signals. It is an indirect measurement method based on a complex mathematical formula that calculates the level distance from the spectrum. The antenna emits a continuously modulated high-frequency microwave signal that is linearly modulated and scanned while receiving the return signal. The frequency difference between the transmitted microwave signal and the returned microwave signal is proportional to the distance to the surface of the medium.
If we think that the slope of the transmitted microwave signal that is linearly modulated is K, the frequency of the transmitted signal and the reflected signal is rf, the delay time difference is rt, the distance from the transmitting antenna to the surface of the medium is R, and C is the speed of light.
Then we can get: rt = 2R/C
Because of the frequency-modulated microwave signal, we have: rf = K×rt;
After the two types of merger, we get the formula:
R = C× rf/2K (Formula 2)
According to formula 2, we can see that the distance R from the antenna to the surface of the medium is proportional to the difference between the transmit frequency and the reflected frequency difference rf.
The signal processing section mixes the transmitted signal and the echo signal to obtain the spectrum of the mixed signal, and distinguishes the different frequency signals through independent Fast Fourier Transform (FFT) changes. Finally, an accurate digital echo signal is obtained and the antenna is calculated to the medium. The distance of the surface.
In fact, the FMCW signal is cycled between two different frequencies. Currently, FMCW microwave level meters on the market are mainly based on two frequencies: 9 to 10 GHz and 24.5 to 25.5 GHz.
Microwave level meters using the FMCW principle all have a continuous self-calibration processing function. The processed signal is compared with an internal reference signal representing a known fixed distance. Any difference is automatically compensated, which eliminates possible measurement drift caused by temperature fluctuations or aging of the electronics inside the transmitter.
2.2 Pulse
Pulse radar level gauges, similar to ultrasonic technology, use the time difference principle to calculate the distance to the surface of the medium. The device transmits pulses of a fixed frequency, and then receives and builds echo patterns. The propagation time of the signal is directly proportional to the distance to the medium. However, unlike sound waves using ultrasonic waves, radar uses electromagnetic waves. It uses tens of thousands of pulses to "scan" the container and get a complete echo image.
In general, the accuracy and reliability of the pulsed microwave level meter are not as good as those of the FMCW microwave meter. However, the pulse level meter is much lower in price than the FMCW, so it is currently the most widely used microwave level meter in the market. Of course, many manufacturers have greatly improved the reliability of pulse radars by enhancing echo processing functions.
2.3. Guided wave radar
Also known as Time Domain Reflective Radar, TDR. Guided wave radar is a combination of non-contact radar and guided wave antenna. It transmits microwave signals to the probe or cable so that the microwave energy is concentrated around the probe or cable without "spreading". It usually uses pulse wave, but it also uses continuous wave.
The working principle is measured in such a way that it has the advantage of being able to measure media with a low dielectric constant, effectively avoiding the influence of interfering substances in the vessel, being unaffected by water vapor, and measuring solids. At the same time, like all contact level measuring devices, it has the disadvantages of easy adhesion, wear, and even cable breakage, which is greatly affected by dust.
2.4. Technical Review
I once heard from users that a pulsed technique is much better than a continuous frequency modulation radar. I think this argument is not scientific. Of course, the radar of FMCW technology has the disadvantages of relatively high cost and large power consumption, but its working mode ensures that its reliability is better and the signal distortion will be reduced to a minimum. Therefore, in some applications with more complex working conditions, it can still reflect its advantages. Of course, after years of vigorous development, pulsed radar technology has also made tremendous leaps, overcoming many technical deficiencies, and greatly improving reliability.
Some time ago, we have a home agent to tell me that there is a radar supplier sales with customers mentioned that pulse radar can be implemented in the same installation of more than one radar, and the signal will not affect each other; and FMCW radar technology will have problem. But did not explain why. After I heard it, I felt very surprised that the market has reached this level of competition? In fact, the radar signal installed in the same warehouse interacts with each other, which is precisely a disadvantage of the pulse radar. Because its working state is equivalent to the “single-work†mode of communication, it is easy to receive signals from other radars of the same specifications. The wrong measurement signal is generated. Therefore, it is necessary to correct the problems by setting the "synchronization" function. Most pulse radars have this function, which is equivalent to the coordination of the working conditions of multiple radars, and they can be sent simultaneously or simultaneously. The continuous frequency modulation radar does not have this problem due to the use of "full-duplex" operation in similar communications.
The existence of various technologies has value, otherwise the market will eliminate it. Therefore, my opinion is that all kinds of technologies that can survive in the market, brands, and brands have their value. There is no good or bad in the simple sense. It depends on where and how use. The best fit is the best. Just like a mahjong tile, any card may be a good card or a bad card, depending on how you play.