DE102013108436B4 - Frequency modulated continuous wave radar level device and method of measurement therefor - Google Patents

Abstract

Measurement method for a frequency-modulated continuous wave radar level gauge, wherein a weight of a previous cycle and a weight of a current cycle, which is less than the weight of the previous cycle, are preset, and wherein after a measured result is obtained in the previous cycle, the measurement method comprising the steps of:constantly transmitting a frequency modulation signal as an FM signal, constantly varying a frequency of the FM signal and receiving a plurality of reflected signals of the FM signal;performing down-conversion mixing processing of the FM signal and the reflected signals, obtaining a frequency difference between the FM signal and each reflected signal and performing a Fourier transform to produce a discrete frequency spectrum corresponding to the frequency differences; selecting a characteristic frequency from the discrete frequency spectrum; andprocessing a measured result in the current cycle using a sum of a first product of a measured result in the previous cycle and the weight of the previous cycle, and a second product of a distance corresponding to the characteristic frequency and the weight of the current cycle, to process the result measured in the current cycle and taking the measured result in the current cycle as the measured result in the previous cycle for processing a measured result in a following cycle,wherein the step of selecting the characteristic frequency further comprises a step of selecting a characteristic interrogated point corresponding to a highest density of the discrete frequency spectrum, taking a frequency of the characteristic interrogated point as the characteristic frequency, the step of selecting s of the characteristic sampled point further includes:a characteristic sampled point reading step of sequentially reading sampled points in a direction from a highest frequency to lower frequencies, wherein three consecutive sampled points comprising a first point fn-1, a second fn, a third fn+1 with respective densities d n -1 , d n , d n +1 >0 are read out sequentially at a time from the highest frequency to lower frequencies; anda characteristic sampled point determining step of determining whether a sum read at a time of an intensity difference dn - dn-1 between the first sampled point fn-1 and the second sampled point fn of the three consecutive sampled points and an intensity difference dn - dn + 1 between the second sampled digit fn and the third sampled digit fn+1 of the three consecutive sampled digits is greater than a set limit value ds; check whether the sum (dn- dn-1)+(dn-dn+1) is greater than the set threshold ds or not, if greater, it is determined that a frequency corresponding to the second digit fn queried is the characteristic frequency and if not greater, the characteristic interrogated point reading step is continued to continue reading the interrogated points in the direction from the highest frequency to lower frequencies.

Description

1. Field of the Invention

The present invention relates to a frequency modulated continuous wave (FMCW) radar level gauge, and more particularly to an FMCW radar level gauge applicable to river level measurement with large fluctuations and suppressing the degree of variation from its measured results.

2. Description of the Prior Art

Radar level gauges are commonly used to measure the distance to solids or liquid levels. According to the types of measurement methods, radar level gauges can be broadly classified as time domain reflectometry (TDR time domain reflection) radar level gauge and Frequency Modulation Continuous Wave (FMCW) radar level gauge. The FMCW radar level gauges apply calculation methods that are more complicated than those of the TDR radar level gauges, while the calculation by the FMCW - radar level gauges is more precise than that by the TDR - radar level gauges.

• Schritt eins: Konstantes Übertragen eines Frequenzmodulation (FM) - Signals Ts, konstantes Erhöhen (oder Verringern) der Frequenz des FM- Signals Ts und Empfangen mehrerer reflektierter Signale, die erzeugt wurden, sobald die FM - Signale Ts an einem Festkörper und/oder einer flüssigen Oberfläche reflektiert wurden.
• Schritt zwei: Durchführen einer Abwärtskonvertierungsmischverarbeitung des übertragenen FM - Signals und der empfangenen reflektierten Signale, um Frequenzunterschiede zwischen dem FM- Signal und jedem der reflektierten Signale zu erhalten und Ausgeben von Überlagerungssignalen, wie in 7 gezeigt.
• Schritt drei: Durchführen einer Fourier- Transformation bei den Überlagerungssignalen in 7, um ein diskretes Frequenzspektrum, wie in 8 gezeigt, zu erzeugen und Auswählen einer charakteristischen Frequenz fp vom Frequenzspektrum mit der Peak-Intensität.
• Schritt vier: Berechnen eines gemessenen Abstands R mit der charakteristischen Frequenz fp der folgenden Gleichung $$\text{R=}\frac{fp\times C\times T}{2\text{F}}$$
  

mit
C: Lichtgeschwindigkeit;
T: Gesamtzeit die erforderlich ist, um das FM- Signal Ts zu übertragen (oder die reflektierten Signale Rs zu empfangen);
F: Gesamte Bandbreite des FM- Signals Ts (oder der reflektierten Signale Rs).With reference to 6 A conventional FMCW radar level gauge 70 regularly performs the following steps in a level measurement:

• Step one: Constantly transmitting a frequency modulation (FM) signal Ts, constantly increasing (or decreasing) the frequency of the FM signal Ts, and receiving multiple reflected signals generated as soon as the FM signals Ts hit a solid and/or a liquid surface were reflected.
• Step two: performing down-conversion mixing processing of the transmitted FM signal and the received reflected signals to obtain frequency differences between the FM signal and each of the reflected signals, and outputting beat signals, as in 7 shown.
• Step three: Perform a Fourier transform on the beat signals in 7 , to get a discrete frequency spectrum, as in 8th shown to generate and select a characteristic frequency fp from the frequency spectrum with the peak intensity.
• Step Four: Calculate a measured distance R with the characteristic frequency fp from the following equation $$\text{R=}\frac{fp\times C\times T}{2\text{f}}$$
  

With
C: speed of light;
T: total time required to transmit the FM signal Ts (or to receive the reflected signals Rs);
F: Total bandwidth of FM signal Ts (or reflected signals Rs).

In an example with a period of time from t ₀ to t ₂ the total time is equal to (t ₂ - t ₀ ), the total bandwidth F is equal to (corresponding to the frequency of the FM signal Ts corresponding to b minus the frequency of the FM signal Ts to f ₀ ).

Conventional FMCW radar level gauges are often used to measure a level of an industrial tank. Since the level in an industrial vessel varies only slightly and slowly during liquid supply or liquid discharge, if the FMCW radar level gauge performs the above steps regularly, the distance R measured in each cycle will vary insignificantly. However, when the conventional FMCW radar level gauges are used to measure water levels of rivers, crest waves generated in rivers can cause the distances measured by the conventional FMCW radar level gauges in different cycles to vary dramatically. The calculated results are so diffuse that they are not suitable for architects or engineers to determine average water levels of rivers.

Furthermore, each FM signal Ts can result in multiple reflected signals Rs since industrial vessels have a semi-enclosed space. With reference to 8th a spectrum is obtained after the conventional FMCW radar level gauge has performed a Fourier transform. Picking a signal with the peak intensity in the step of selecting the characteristic frequency can be sufficiently precise to measure the level inside an industrial vessel. To measure the flux level in an open space, the way of selecting the characteristic frequency fails to get sufficient accuracy in determining the flux level.

US 2008/0282793 A1 (D1) only discloses a conventional radar level gauge and not the features of the present application. D1 discloses a power level equalization circuit that adjusts transmission power according to signals in a particular environment send an amplifier, an attenuator and a compensator. D1 only discloses a conventional power matching circuit that adjusts the equivalent wavelength match in the circuit. The method of measuring D1 thus differs from the method of measurement of the present application.

DE 43 08 373 C2 discloses the measurement in the ultrasonic range and the calculation of the moving average method, the moving average method also being a statistical method multiplying by weights.

JP S59-31 415 A discloses a calculation method placed on a vehicle fuel hose for inventory measurement that uses the moving average method for calculation.

An object of the invention is to provide an FMCW - radar level gauge according to claim 3 and a measuring method according to claim 1 therefor, which are suitable to avoid variations of measured results and to facilitate the statistical analysis of the average of the measured results.

• Konstantes Übertragen eines Frequenzmodulation (FM)- Signals, konstantes Variieren einer Frequenz des FM - Signals und Empfangen mehrerer reflektierter Signale des FM - Signals;
• Durchführen einer Abwärtskonvertierungsmischverarbeitung des FM - Signals und der reflektierten Signale, Erhalten eines Frequenzunterschieds zwischen dem FM - Signal und jedem reflektierten Signal und Durchführen einer Fourier-Transformation, um ein den Frequenzunterschieden entsprechendes diskretes Frequenzspektrum zu erzeugen;
• Auswählen einer charakteristischen Frequenz aus dem diskreten Frequenzspektrum; und

In order to achieve the above object, the measurement method for a frequency modulated continuous wave radar level gauge, in which a previous cycle weight and a current cycle weight smaller than the previous cycle weight are preset, after obtaining a measured result in a previous cycle, the steps on:

• constantly transmitting a frequency modulation (FM) signal, constantly varying a frequency of the FM signal and receiving a plurality of reflected signals of the FM signal;
• performing down-conversion mixing processing on the FM signal and the reflected signals, obtaining a frequency difference between the FM signal and each reflected signal, and performing a Fourier transform to generate a discrete frequency spectrum corresponding to the frequency differences;
• selecting a characteristic frequency from the discrete frequency spectrum; and

Processing a measured result in the current cycle, using a sum of a product of the measured result in the previous cycle and the weight of the previous cycle, and a product of a distance corresponding to the characteristic frequency and a weight of the current cycle, to obtain an im processing a result measured in the current cycle, and taking the result measured in the current cycle as a result measured in the previous cycle for processing a result measured in a following cycle.

To achieve the above object, the FMCW radar level gauge has a transceiver antenna and a processing unit.

The processing unit is connected to the transceiver antenna and is provided with a measurement method, a previous cycle weight and a current cycle weight. After a measured result is obtained in a previous cycle, the processing unit periodically executes the measuring process. Once the measurement procedure is carried out, the processing unit regularly transmits a frequency modulation (FM) signal, constantly varies a frequency of the FM signal, receives multiple reflected signals of the FM signal, performs down-conversion mixing processing of the FM signal and the reflected signals, obtains a Frequency difference between the FM signal and each reflected signal, generates a frequency spectrum corresponding to the frequency differences, selects a characteristic frequency from the frequency spectrum, processes a measured result in the current cycle with a sum of a product of the measured result in the previous cycle and the weight of the previous cycle , and a product of a distance corresponding to the characteristic frequency and a weight of the current cycle, and sets the measured result in the current cycle as the measured result in the previous cycle to Ve Processing of a measured result in the following cycle.

The FMCW radar level gauge and measurement method therefor takes into account the measured result (distance corresponding to the characteristic frequency) as soon as a measured result is processed in each cycle and includes the weighted effect on the measured result in the previous cycle and the distance corresponding to the characteristic frequency the previous cycle weight and the current cycle weight to obtain the measured result in the current cycle. Since the weight of the previous cycle is larger when there are large fluctuations in level, the measured result in each cycle will become the measured result in the previous cycle approximate, thereby suppressing the variations of the measured result in each cycle and facilitating the measurement of flowing levels such as river levels.

In addition, in the step of selecting the characteristic frequency, a characteristic sampled point corresponds to a highest frequency in the frequency spectrum because there are no multiple reflected signals during flow level measurement like in a closed space and the flow surface is mostly the lowest point in the environment ( the farthest point
to the FMCW radar level gauge). Accordingly, a highest characteristic frequency and a largest frequency difference between the FM signal and the reflected signal are selected, thereby ensuring a largest measured distance and highest accuracy in measurement, and facilitating measurement of a flux level in free space.

f

character list

• 1 Figure 12 is a functional block diagram of an FMCW radar level gauge according to the present invention;
• 2 Figure 12 is a flow chart of a measurement method implemented in a processor of the FMCW radar level gauge 1 is built in;
• 3 Fig. 12 is a flowchart of a characteristic frequency selection step of the measurement method 2 ;
• 4 Fig. 12 is a frequency-intensity plot of a discrete frequency spectrum obtained from the characteristic frequency selection step 3 is obtained;
• 5 Fig. 12 is a schematic view of a flow level measurement with the FMCW radar level gauge according to the present invention;
• 6 is a schematic view of a conventional radar level gauge for level measurement in a liquid container;
• 7 is a time-frequency plot of FM signals and reflected signals emitted by the conventional FMCW radar level gauge 6 to be obtained; and
• 8th . is a frequency - intensity plot of a discrete frequency spectrum emitted by the conventional FMCW radar level gauge 6 is obtained.

With reference to 1 an FMCW radar level gauge according to the present invention has a transceiver antenna 10 and a processing unit 20.

The transceiver antenna 10 serves to transmit a frequency modulation signal Ts and to receive a plurality of reflected signals Rs.

The processing unit 20 is connected to the transceiver antenna 10 and is provided with a measurement method, a previous cycle weight Pa and a current cycle weight Pb. The weight Pa of the previous cycle is larger than the weight Pb of the current cycle. For example, the weight Pa of the previous cycle is 0.9 and the weight Pb of the current cycle is 0.1. The processing unit 20 has a transmitter 21, a receiver 22, a processor 23, an operating interface 24, a power supply 25, a display and a communication port 27.

The transmitter 21 is connected to the transceiver antenna 10 and outputs the FM signal Ts to the transceiver antenna 10 so that the transceiver antenna 10 transmits the FM signal Ts.

The receiver 22 is connected to the transceiver antenna 10 and the transmitter 21 to receive the reflected signals Rs from the transceiver antenna 10 and to receive the FM signal Ts from the transmitter 21, and has a down-conversion mixer 221. The down-conversion mixer 221 performs down-conversion mixing processing of the transmitted FM signal and the received reflected signals to obtain the frequency differences between the FM signals and each of the reflected signals, and outputs beat signals corresponding to the frequency differences.

The processor 23 is connected to the transmitter 21 and the down-conversion mixer 221 of the receiver 22, and is provided with the measurement method, the previous cycle weight Pa and the current cycle weight Pb. A detailed description of the measurement method will be discussed later.

The operator interface 24 is connected to the processor 23 and serves to set the weight Pa of the previous cycle and the weight Pb of the current cycle.

The power supply 25 is connected to the processor 23 so that the processor 23 regulates current drawn from the power supply 25, and has a power detection terminal 251 connected to a remote host 100 for the remote host 100 to detect the power consumed by the power supply 25 .

The display 26 is connected to the processor 23 and serves to display the weight Pa of the previous cycle and the weight Pb of the current cycle.

The communication port 27 is connected to the processor 23 and the remote host 100 for the remote host 100 to set up in the processor 23 the weight Pa of the previous cycle and the weight Pb of the current cycle.

With reference to 2 , the processor 23 in the processing unit 20 periodically executes the measurement process after acquiring a measured result Rn-1 in a previous cycle. Since measurement methods for measured results in previous cycles are not exclusive, the measurement method includes the following steps.

Step S10: The processor 23 controls the transmitter 21 to constantly output the FM signal Ts to the transceiver antenna 10, so that the transceiver antenna 10 transmits the FM signal Ts, constantly increases (or decreases) the frequency of the FM signal Ts ) and several reflected signals Rs of the FM signal via the transceiver antenna 10 receives.

Step S20: After the down-conversion mixer 221 of the receiver 22 performs down-conversion mixing processing of the FM signal Ts output from the transmitter 21 and the reflected signals Rs received by the receiver 22, the processor 23 obtains a frequency difference between the FM signal and each reflected signal and performs a Fourier transform to generate a discrete frequency spectrum corresponding to the frequency differences. In the present embodiment, the processor 23 first performs a fast Fourier transform on the frequency differences and obtains the discrete frequency spectrum associated with the frequency differences after performing a chirp Z -transform.

Step S30: The processor 23 selects a characteristic frequency from the discrete frequency spectrum. A detailed description of the step will be given later.

mit
Rn-1 das gemessene Ergebnis im vorangegangenen Zyklus
Rn das gemessene Ergebnis im gegenwärtigen Zyklus
Pa die Gewichtung des vorangegangenen Zyklus
Pb die Gewichtung des gegenwärtigen Zyklus
R ein der charakteristischen Frequenz entsprechender Abstand.Step S40: The processor 23 processes a measured value in the current cycle. The measured result in the current cycle is expressed by the following equation. $$\text{Rn=}\left(\text{father}\times \text{Rn-1}\right)+\left(\text{pb}\times \text{R}\right)$$

With
Rn-1 the measured result in the previous cycle
Rn the measured result in the current cycle
Pa is the weight of the previous cycle
Pb the weight of the current cycle
R a distance corresponding to the characteristic frequency.

The obtained measured result Rn in the current cycle is taken as the measured result Rn-1 in the previous cycle during the following cycle of the measurement process. In the present embodiment, the processor 23 controls the current consumed by the power supply 25 after processing the measured result Rn in the current cycle, so that the remote host 100 can detect the current consumed by the power supply 25 via the current detection terminal 251 and the measured result Rn in the current cycle, which is processed in each cycle.

With reference to 3 and 4 , a characteristic sampled point corresponding to a highest frequency is selected from the discrete frequency spectrum, and the frequency of the characteristic sampled point is set as a characteristic frequency. The step S30 further has the following steps.

A characteristic sampled point reading step S31: sequentially reading sampled points in a direction from the highest frequency to lower frequencies. In the present embodiment, the highest frequency is predetermined. Three consecutive sampled digits fn-1, fn, fn+1 with respective sampled digit densities dn-1, dn, dn+1 are read out sequentially at a time in the direction from the highest frequency to lower frequencies.

A characteristic sampled point determination step S32: determining whether a sum of an intensity difference between the first sampled point and the second sampled point of the three consecutive sampled points read out at a time and an intensity difference between the second sampled point and of the third scanned digit of the three consecutive scanned digits is greater than a set limit value d ₅ . If positive, proceed to the following step, if negative, return to step S31 to continue reading the sampled digits in the direction from highest frequency to lower frequency.

A characteristic frequency determining step S33: selecting the frequency of the second sampled point fn as a characteristic frequency.

The set limit value ds is user configurable or can be configured as a fixed fraction of the intensity of the second sampled point dn, for example half the intensity of the second sampled point dn. Given the set limit, it indicates determining if ((dn - dn-1 ) + (dn -dn-1)) is greater than 0.5dn. With further reference to 4 , in the characteristic sampled point determination step S32, a sampled point f ₄ is selected as a characteristic frequency, which satisfies the condition statement in step S32 and has the highest frequency and a relatively high intensity. It will be appreciated that a conventional FMCW radar level gauge differs from the present invention in that the conventional FMCW radar level gauge will select f ₈ for distance calculation, which corresponds to the peak intensity. With reference to 5 , as far as measuring a river gauge in an open space is concerned, the measured distance from the FMCW radar level gauge to the river should be the smallest compared to the neighboring soil, since the height of a river gauge is lower than that of all neighboring soils. Therefore, it is preferable to choose a characteristic frequency in favor of a longer measured distance than to choose one with higher intensity. Because the measured distance is proportional to the characteristic frequency, a distance measured with the characteristic frequency f ₄ is far more precise than that measured with the frequency f ₈ with the peak intensity.

Whenever the FMCW radar level gauge is used to measure river levels and significant fluctuations such as crest waves occurring on the river, one measure to cope with the fluctuations is to set in the processor 23 a larger value of the weight Rn-1 des previous cycle and a lower value of the weight Rn of the current cycle. As an example, 0.9 and 0.1 are given for Rn _-1 and Rn, respectively, with the result being equal to 0.9Rn-1 + 0.1Rn. Even if there is a large difference between the measured results in the current cycle and the previous cycle, the fluctuation between the measured results in the current cycle and the previous cycle can be reduced after being weighted so that the measured result in each cycle converges and therefore good for statistical analysis of a user.

In summary, the FMCW radar level gauge and measurement method according to the present invention makes the measured result converge in each cycle and has higher accuracy when used to measure a liquid level in open space such as a river.

Although numerous features and advantages of the present invention have been set forth in the foregoing specification, along with details of structure and functions, the disclosure is illustrative only. Changes can be made in details, particularly in the range of shape, size and arrangement of parts, fully within the principles of the invention, which are characterized by the broad general meaning of the terms expressed in the appended claims are.

Claims (5)

Measurement method for a frequency-modulated continuous wave radar level gauge, wherein a weight of a previous cycle and a weight of a current cycle, which is less than the weight of the previous cycle, are preset, and wherein after a measured result is obtained in the previous cycle, the measurement method comprising the steps of: constantly transmitting a frequency modulation signal as an FM signal, constantly varying a frequency of the FM signal and receiving a plurality of reflected signals of the FM signal; performing down-conversion mixing processing on the FM signal and the reflected signals, obtaining a frequency difference between the FM signal and each reflected signal, and performing a Fourier transform to generate a discrete frequency spectrum corresponding to the frequency differences; selecting a characteristic frequency from the discrete frequency spectrum; and processing a measured result in the current cycle using a sum of a first product of a measured result in the previous cycle and the weight of the previous cycle, and a second product of a distance corresponding to the characteristic frequency and the weight of the current cycle to process the result measured in the current cycle and where the measured Result in the current cycle is taken as the measured result in the previous cycle for processing a measured result in a following cycle, wherein the step of selecting the characteristic frequency further includes a step of selecting a characteristic sampled location corresponding to a highest density of the discrete frequency spectrum, wherein a frequency of the characteristic interrogated point is taken as the characteristic frequency, wherein the step of selecting the characteristic interrogated point further includes: a characteristic interrogated point reading step comprising sequentially reading interrogated points in a direction from a highest frequency to lower frequencies, wherein three successive sampled locations comprising a first location fn-1, a second fn, a third fn+1 with respective densities dn -1 , dn, dn+1 >0, sequentially at a time from the highest Fre frequency to be read out at lower frequencies; and a characteristic sampled point determining step of determining whether a sum read out at a time of an intensity difference dn - dn-1 between the first sampled point fn-1 and the second sampled point fn of the three consecutive sampled points and an intensity difference dn - dn+1 between the second sampled digit fn and the third sampled digit fn+1 of the three consecutive sampled digits is greater than a set limit value ds; check whether the sum (dn- dn-1)+(dn-dn+1) is greater than the set threshold ds or not, if greater, it is determined that a frequency corresponding to the second digit fn queried is the characteristic frequency and if not greater, the characteristic interrogated point reading step is continued to continue reading the interrogated points in the direction from the highest frequency to lower frequencies. measurement method claim 1 , wherein in the step of selecting the characteristic sampled point, a sampled point with the highest frequency is predetermined. Frequency Modulated Continuous Wave (FMCW) radar level gauge comprising a transceiver antenna (10); a processing unit (20) connected to the transceiver antenna (10) and with a measurement method claim 1 or 2 , provided with a previous cycle weight and a current cycle weight, wherein after a measured result is obtained in the previous cycle, the processing unit (20) regularly executes the measuring method, wherein when the measuring method is executed, the processing unit ( 20) regularly transmits a frequency modulation signal as an FM signal, constantly varies a frequency of the FM signal, receives multiple reflected signals of the FM signal, performs down-conversion mixing processing of the FM signal and the reflected signals, a frequency difference between the FM signal and each reflected signal, generates a frequency spectrum corresponding to the frequency differences, selects a characteristic frequency from the frequency spectrum, a measured result in the current cycle with a sum of a product of the measured result in the previous cycle and the weight de s previous cycle and a product of a distance corresponding to the characteristic frequency and a weight of the current cycle, and sets the measured result in the current cycle as the measured result in the previous cycle for processing a measured result in the following cycle. FMCW radar level gauge claim 3 wherein the processing unit (20) comprises: a transmitter (21) which is connected to the transceiver antenna (10) and the FM signal to the transceiver antenna (10) outputs, so that the transceiver antenna (10) the FM -Signal transmits; a receiver (22) connected to the transceiver antenna (10) and the transmitter (21) to receive the reflected signals from the transceiver antenna (10) and to receive the FM signal from the transmitter (21). received, and a down-conversion mixer (221), said down-conversion mixer (221) performing down-conversion mixing processing of the FM signal and the reflected signals to obtain the frequency differences between the FM signal and each of the reflected signals and outputting beat signals corresponding to the frequency differences; and a processor (23) connected to the transmitter (21) and the downconversion mixer (221) of the receiver (22) and provided with the measurement method, the previous cycle weight and the current cycle weight; wherein a characteristic sampled point corresponding to a frequency is selected from the frequency spectrum and the frequency of the characteristic sampled point is adopted as the characteristic frequency; wherein selecting the characteristic sampled point from the frequency spectrum includes: a characteristic sampled point reading step comprising reading sampled points sequentially in one direction from one highest frequency to lower frequencies, reading three consecutive sampled digits fn-1, fn, fn+1 with respective sampled digit densities dn-1, dn, dn+1 sequentially at a time from the highest frequency to lower frequencies; and a characteristic sampled point determining step by determining whether a sum of an intensity difference dn-1-dn between a first sampled point fn-1 and a second sampled point fn of the three consecutive sampled points read out at a time, and an intensity difference dn-dn+1 between the second sampled point fn and the third sampled point fn+1 of the three consecutive sampled points is greater than a set limit value ds if the sum ((dn-dn-1)+(dn- dn+1)) is greater than the set limit value ds, it is determined that a frequency corresponding to the second sampled digit fn is the characteristic frequency, and if not larger, the characteristic sampled digit reading step is continued to read the sampled digits to continue in the direction from the highest frequency to lower frequencies. FMCW radar level gauge claim 4 wherein the processing unit (20) further comprises: an operator interface (24) connected to the processor (23) and operative to establish the weight of the previous cycle and the weight of the current cycle; a power supply (25) connected to the processor (23) for the processor (23) to regulate power consumed by the power supply (25), and a power sensing terminal (251) connected to a remote host (100). is for the remote host (100) to detect the current consumed by the power supply (25); a display (26) connected to the processor (23) for displaying the weight of the previous cycle and the weight of the current cycle; and a communications port (27) connected to the processor (23) for the remote host (100) to set up the previous cycle weight and the current cycle weight in the processor (23).

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