Methods for level measurement

Abstract

Method for measuring the fill level (L) of a fill material (1) in a container (3) using an FMCW radar level gauge, - which, during measurement operation, periodically and linearly, in particular according to a sawtooth function, sends frequency-modulated microwave signals (S) in the direction of the filling material (1), - whose reflection signals (R) reflected from the surface of the contents are received after a signal transit time that depends on the fill level (L), and - determines the fill level (L) based on the signal travel time, at which - a reference function (F) beforehand R , F' R ) is derived, which represents an amplitude profile of a reflection signal (R) received by the level gauge when a known reference gas is located above the contents (1), - subsequently, auxiliary functions (F) are derived based on the reference signals (R) received during measurement operation. x , F' x) are derived, which represent the amplitude profile of the reflection signals (R) received by the level gauge while an unknown gas (X) is located above the contents (1), - the helper functions (F x , F' x ) with the reference function (F R , F' R ) can be compared, and - the level gauge diagnoses a composition of the unknown gas (X) located above the contents (1) that differs from the reference gas, if the comparison shows a significant deviation (Δ x ) between an auxiliary function derived in measurement operation (F x , F' x ) and the reference function (F R , F' R) results in - the level measuring device generates a measurement signal (M) by mixing the reflection signals (R) with the transmitted microwave signals (S) and subsequent filtering, which contains those frequencies (Δf) that correspond to the differences of the frequencies (f) of the signals simultaneously present at the mixer (51), - the reference function (F R , F' R ) is an envelope that represents the amplitude profile of a measurement signal (M) as a function of time (t) that arises when the reference gas is located above the filling material (1), and - the helper functions (F x , F' x ) Envelopes are those that represent the amplitude profile of measurement signals (M) as a function of time (t) that arise during measurement operation when the unknown gas (X) is located above the filling material (1).

Description

[0001] The invention relates to a method for measuring the fill level of a product in a container using a microwave-operated level gauge which, during measurement operation, sends microwave signals towards the product, receives the reflection signals reflected at the surface of the product after a signal transit time that depends on the fill level, and determines the fill level based on the signal transit time.

[0002] Corresponding level measuring devices are used in a variety of industries, e.g. in the manufacturing industry, in the chemical industry or in the food industry.

[0003] To determine signal propagation times, all known methods that allow relatively short distances to be measured using reflected microwaves can be applied. The best-known examples are pulse radar and frequency-modulated continuous-wave radar (FMCW radar).

[0004] Pulse radar periodically transmits short microwave pulses that are reflected by the surface of the material being transported and received again after a travel time that depends on the distance. An echo function is derived from the received reflection signal, representing the received signal amplitude as a function of the signal travel time. Each value of this echo function corresponds to the amplitude of an echo reflected at a specific distance from the antenna.

[0005] In the FMCW method, a microwave signal is continuously transmitted that is periodically linearly frequency-modulated, for example, according to a sawtooth function. The frequency of the received reflection signal therefore exhibits a frequency difference compared to the instantaneous frequency of the transmitted signal at the time of reception. This difference depends on the propagation time of the microwave signal and its reflection signal. The frequency difference between the transmitted and received signals, which can be obtained by mixing both signals and evaluating the Fourier spectrum of the mixed signal, thus corresponds to the distance of the reflecting surface from the antenna. Furthermore, the amplitudes of the spectral lines of the frequency spectrum obtained by Fourier transformation correspond to the echo amplitudes. This Fourier spectrum therefore represents the echo function in this case.

[0006] The echo function determines at least one useful echo, corresponding to the reflection of the transmitted signal at the surface of the contents. Given a known microwave propagation speed, the travel time of this useful echo directly yields the distance the microwaves travel from the measuring device to the surface of the contents and back. Using the installation height of the level gauge above the container, the desired fill level can then be calculated directly.

[0007] The propagation speed of the microwave signals on their way to the surface of the product and back is of central importance. In particular, the accuracy achievable with the measuring device depends crucially on how closely the propagation speed used to calculate the fill level corresponds to the actual propagation speed.

[0008] The propagation speed depends on the gas above the contents. It is either known or determined beforehand, e.g., based on reference measurements.

[0009] Depending on the processes occurring within the container, which vary depending on the specific application, the composition of the gas above the contents can change. This leads to a change in the propagation speed, which is generally neither detected nor taken into account by conventional level gauges. Consequently, such a change in the composition of the gas above the contents can significantly impair measurement accuracy, although this is not apparent to the user.

[0010] To ensure safe control of the process taking place in the container, it is essential to detect changes in the composition of the medium above the contents that affect measurement accuracy, in order to react appropriately at all times. This can be achieved, for example, by specifying appropriately adjusted safety margins for process control, e.g., for filling or emptying the container, which are determined based on the currently achievable measurement accuracy. A further objective is to quantitatively measure changes in the propagation speed, if possible, and even to compensate for them in the level measurement.

[0011] In principle, it is possible to detect changes in propagation speed using reference reflectors positioned at a predetermined distance from the antenna above the contents of the container. The measuring device measures the signal travel time – the time it takes for the microwave signals to travel to the reference reflector and back – and calculates the propagation speed from this measurement. However, there are many applications where reference reflectors are not used or are only used reluctantly because they take up space in the container and may potentially interfere with the process taking place within it.

[0012] The operating principle of TDR, FMCW and pulse radar-based level measuring devices is generally described in “Radar level measurement - The user's guide, Peter Devine (2000)”.

[0013] Publication DE 102006019191 A1 describes a TDR-based level measuring device that can determine the measurement zero point more accurately using a probe reflection point.

[0014] Publication DE 10359534 A1 addresses a method by which the emissions of broadband transmission pulses of pulse transit time-based level measuring devices can be improved by randomly changing the polarity of the transmission pulses.

[0015] A method for detecting interference echoes in pulse transit time-based level measurement, which is based on determining the echo characteristics, is described in publication DE 04308373 A1.

[0016] It is an object of the invention to provide a method for measuring the fill level with which changes in the composition of the gas located above the filling material can be detected.

[0017] The invention comprises a method for measurement according to claim 1 or 2. In this method, the fill level of a product in a container is measured using a microwave-operated level gauge. - which sends microwave signals towards the contents during measurement operation, - whose reflection signals reflected from the surface of the contents are received after a signal transit time that depends on the fill level, and - determines the fill level based on the signal travel time, at which - a reference function is derived beforehand that represents the amplitude profile of a reflection signal received by the measuring device when a known reference gas is located above the contents, - Subsequently, auxiliary functions are derived from the reference signals received during measurement operation, which reproduce the amplitude profile of the reflection signals received by the measuring device while an unknown gas is located above the contents, - the auxiliary functions are compared with the reference function, and - the level gauge diagnoses a composition of the gas above the contents that differs from the reference gas if the comparison reveals a significant deviation between an auxiliary function derived during measurement operation and the reference function.

[0018] According to further training, the reference function and the auxiliary functions are frequency spectra derived from the reflection signals.

[0019] According to an alternative further education theory, the reference function and the auxiliary functions are energy spectra derived from the reflection signals.

[0020] According to further training - the reference function and the auxiliary functions are normalized functions using a normalization factor, and - The normalization factor normalizes an area enclosed under the reference function to a predetermined value.

[0021] A non-inventive variant comprises a method in which - the level gauge is a pulse radar device, -- whose microwave signals are short, periodically transmitted microwave pulses, -- which, by mixing the reflection signals with the transmitted microwave signals delayed by a variable delay time, in particular increasing according to a sawtooth function, and subsequent filtering, generates an intermediate frequency signal that reproduces the amplitudes of the reflection signals as a function of their signal propagation time, and - the reference function is an envelope that reproduces the amplitude profile of the intermediate frequency signal, which is generated when the reference gas is located above the filling material, as a function of frequency, and - the auxiliary functions are envelopes that reproduce the amplitude profile of the intermediate frequency signals that arise during measurement operation when the unknown gas is located above the filling material, as a function of frequency.

[0022] One variant comprises a method according to the invention in which - the level gauge is an FMCW radar device, -- whose microwave signals are periodically linear, especially according to a sawtooth function, frequency-modulated signals, -- which, by mixing the reflection signals with the transmitted microwave signals and subsequent filtering, generates a measurement signal that contains those frequencies corresponding to the differences in the frequencies of the signals simultaneously present at the mixer, - the reference function is an envelope that represents the amplitude profile of a measurement signal as a function of time, which arises when the reference gas is located above the fill material, and - the auxiliary functions are envelopes that represent the amplitude profile of measurement signals as a function of time, which arise during measurement operation when the unknown gas is located above the filling material.

[0023] According to a further development of the latter variant, the envelopes are generated by means of a Hilbert transformation of the respective measurement signals.

[0024] A second variant comprises a method according to the invention in which - the level gauge is an FMCW radar device, -- whose microwave signals are periodically linear, especially according to a sawtooth function, frequency-modulated signals, - the reflection signals are fed to a detector that records an amplitude profile of the reflection signals, and - the reference function is an envelope, -- which represents the amplitude profile of the reflection signal as a function of time, and -- which was derived from an amplitude profile of a reflection signal recorded by the detector, which is generated when the reference gas is located above the filling material, and - the auxiliary functions are envelopes that represent the amplitude profile of the reflection signals as a function of time, and -- which were derived from amplitude profiles of reflection signals recorded with the detector, which arise when the unknown gas is located above the filling material.

[0025] According to a preferred embodiment - determines a difference between an area enclosed under the reference curve and an area enclosed under the auxiliary curve, and - the difference is used as a measure of the deviation.

[0026] According to further training, a composition of the gas located above the filling material that deviates from the reference gas is diagnosed if the deviation exceeds a predetermined threshold.

[0027] According to further training, depending on the degree of deviation between the reference function and the auxiliary function, a diagnosis, a warning, an error message and / or an alarm is issued.

[0028] According to a further development, a measured signal transit time or the propagation speed of the microwave signals used for level measurement in the gas located above the contents is corrected by means of a correction factor that depends on the degree of the currently measured deviation between the reference function and the auxiliary function.

[0029] Furthermore, the invention comprises a further development of the inventive method, in which, prior to - in addition to the reference function, at least one reference function is derived which represents an amplitude profile of a reflection signal received by the measuring instrument when a known reference gas different from the reference gas is located above the contents, - a deviation between the reference function and the reference function is determined, - based on this deviation, the propagation speed of the microwave signals in the reference gas and the propagation speed of the microwave signals in the reference gas, a correction factor is derived which reflects the dependence of the propagation speed of microwave signals in an unknown gas located above the filling material as a function of the deviation between the reference curve and the auxiliary curve derived with this unknown gas.

[0030] The invention and its advantages will now be explained in more detail with reference to the figures in the drawing, in which two embodiments are shown; identical parts are provided with the same reference numerals in the figures. Fig. Figure 1 shows: a pulse radar level measuring device according to the invention; Fig. Figure 2 shows: an amplitude profile of a reference function and an auxiliary function as a function of frequency; Fig. Figure 3 shows: an FMCW level measuring device according to the invention; Fig. Figure 4 shows: the frequencies of transmit and reflect signals of an FMCW radar level gauge as a function of time; Fig. Figure 5 shows: an amplitude profile of a measurement signal recorded with a reference gas, as well as a reference function representing this amplitude profile as a function of time; and Fig. Figure 6 shows: an amplitude profile of a measurement signal recorded with a different gas, as well as an auxiliary function representing this amplitude profile as a function of time.

[0031] Fig. Figure 1 shows a first embodiment of a microwave-based level measuring device operating on the time-of-flight principle for measuring the fill level of a substance 1 in a container 3, with which the method according to the invention can be carried out. The level measuring device is a pulse radar level measuring device. It comprises measuring electronics 5 and an antenna 7 connected to the measuring electronics 5 and arranged above the substance 1.

[0032] The measuring electronics 5 include a circuit 9 for generating microwave signals. The microwave signals are short, periodically emitted microwave pulses, which are fed to the antenna 7, for example, via a transmit-receive switch 11, such as a directional coupler. The antenna 7 transmits the microwave signals S into the container 3 in the direction of the contents 1 and receives the reflection signals R from the contents of the container 3 back towards the antenna 7. This is in Fig. 1 is symbolically represented by arrows. The reflection signals R are fed to the measuring electronics 5, which uses these signals to determine a signal transit time, dependent on the fill level, required for the path from the level measuring device to the surface of the contents and back, and uses this signal transit time to determine the fill level L.

[0033] In the illustrated embodiment, the circuit 9 comprises a pulse repetition frequency f Roscillating oscillator 13 to which a first control circuit 15 is connected, which transmits at a frequency f S The oscillating microwave generator 17 is controlled by the control circuit 15. The control circuit 15 starts the microwave generator 17 for a very short time interval, corresponding to the desired pulse duration of the microwave signals to be transmitted, and then stops it again. This process is repeated at the pulse repetition frequency f applied to the control circuit 15. R This frequency is, for example, a few megahertz. The microwave generator 17 is connected to the antenna 7 via the transmit-receive switch 11, e.g., a directional coupler.

[0034] Signals S transmitted via antenna 7 are reflected at the surface of the contents, and their reflection signals R are received back by antenna 7 after a transit time that depends on the fill level. The received reflection signals R are then fed via the transmit / receive switch 11 to a first input of a mixer 19.

[0035] The one with the pulse repetition rate f R The oscillating oscillator 13 is connected to a second microwave generator 25 via a time delay stage 21 and a second control circuit 23, which operates identically to the first control circuit 15. The second microwave generator 25 is constructed identically to the first microwave generator 17. The second control circuit 23 causes the second microwave generator 25 to operate at the pulse repetition frequency f RRecurring microwave signals are generated. These are present at a second input of the mixer 19. The time delay stage 21 delays the incoming signals by a variable delay time, e.g., increasing according to a sawtooth function of finite width. Thus, in the mixer 19, a reflection signal R, delayed by a level-dependent propagation time, is superimposed on a microwave signal that, under ideal conditions, has the same shape but is delayed by a known variable delay time. The mixed signal available at the output of the mixer 19 corresponds to the correlation of the microwave signals arriving at its two inputs. It contains a high-frequency component, which includes frequencies essentially determined by the sum of the frequencies present at the inputs, and a low-frequency component, which includes frequencies essentially determined by the difference between the frequencies present at the inputs.A low-pass filter 27 is used to filter out the low-frequency component and feed it into further processing and / or evaluation. This low-frequency component is generally referred to as the intermediate frequency (IF) signal and is a low-frequency representation of the reflection signal R.

[0036] The intermediate frequency (IF) signal is acquired using a sampling and holding circuit 29 and subsequently digitized in an analog-to-digital converter (ADC) 31. An output of the ADC 31 is connected to a signal processing unit (SPU), e.g., a microprocessor, which records the respective signal amplitudes A(t) together with the corresponding delay time (t). The delay time t corresponds to the propagation time of the corresponding segment of the reflected signal R. The SPU 33 thus contains an echo function that reproduces the amplitudes A(t) of the reflected signal R as a function of their propagation time t.

[0037] To determine the fill level L, it is preferably not the direct recording of the actual intermediate frequency signal ZF as an echo function that is used, but rather its envelope. A typical curve of this is shown in Fig. The signal processing module 33 is represented in Figure 1. This echo function exhibits two distinct maxima, M1 and M2. The first maximum, M1, corresponds to the short microwave pulse of the transmitted signal S, which is recorded directly via the transmit / receive switch 11. The second maximum, M2, is due to a reflection of the transmitted signal S at the surface of the contents. The time between the occurrence of the two maxima, M1 and M2, corresponds to the desired signal propagation time. The signal processing module 33 determines this signal propagation time and, based on the propagation speed of the microwave signals, calculates the current fill level L.

[0038] If air is present above the product, the microwave signals S and their reflection signals R can propagate to and from the product surface virtually unimpeded. However, if another gas X, such as water vapor or ammonia, is present above the product 1, significant interactions with the gas molecules occur, depending on the type and composition of the gas X. These interactions are frequency-dependent and have a marked, gas-specific effect on the amplitude of the received reflection signals R, depending on the frequencies contained in the microwave signals. In particular, polar gas molecules are set into rotational oscillations by the microwave signals. These interactions lead to increased absorption and thus to a change in electrical susceptibility, which in turn causes a change in the propagation speed of the microwave signals.The rotation lines of various polar media are characteristic of the gas and fall within the frequency range of the microwave signals of modern level gauges. They are typically strongly pressure- and Doppler-widened. The pressure broadening of these rotation lines, in particular, causes these absorptions to noticeably affect the amplitude profile of the reflection signals R in a frequency-dependent manner. According to the invention, this effect is used to detect changes in the composition of the gas X located above the material 1.

[0039] This is achieved according to the invention by first defining a reference function F, e.g. within the framework of a calibration procedure. RThe reference function F is derived and represents the amplitude profile of the reflection signals R received by the measuring device when a known reference gas is present above the material 1. The reference gas is preferably a gas with no or very low interactions with the microwave signals, e.g., air. R the amplitude profile of the associated reflection signal R as a function of time t or frequency f over a predefined time or frequency range.

[0040] Subsequently, auxiliary functions F are continuously, periodically, sporadically, or as required, based on the reference signals R received during measurement operation, in the same manner. x derived, which represent the amplitude profile of the reflection signals R received by the measuring device while an unknown gas X, which may be significantly different from the reference gas, is located above the filling material 1.

[0041] The reference function F R and the auxiliary functions F subsequently recorded during measurement operation x The amplitude profile of the corresponding reflection signals R, which is characteristic of gas X, could in principle be derived directly from the received reflection signals R, which are available, for example, at the output of the transmit / receive switch 11. For this purpose, an envelope of the amplitude profile of the reflection signals R would be recorded and evaluated as a function of time t over the duration of a reflection pulse R, or as a function of frequency f over the bandwidth of the frequencies contained in the transmitted signal S. However, since these are very high-frequency signals, high-quality spectrum analyzers would be required, which are currently too large and too expensive to be used in measuring instruments.

[0042] The information about the frequency-dependent absorption characteristic of the gas X currently located above the material 1 is not only directly contained in the reflection signal R, but also in a multitude of derived auxiliary signals. These auxiliary signals are generated exclusively using transformations that affect the amplitudes of the reflection signals equally across all frequencies f contained in the reflection signal R. Accordingly, a change in the composition of the gas above the material 1 can also be determined using auxiliary signals derived from the reflection signals R in this way.

[0043] In conjunction with the pulse radar level gauge shown, the intermediate frequency (IF) signal, which is already available at the output of the analog-to-digital converter 31, is particularly suitable for this purpose. The amplitude profile of the IF signal reflects the characteristic amplitude profile of the reference signal R for the gas. For this purpose, the digitized IF signal is fed, for example, to a signal processing unit 35.

[0044] To derive the reference function F R and the auxiliary functions F X Only the portion of the intermediate frequency (IF) signal that reproduces the reflection signal R is used. This is done, for example, by defining a time window that selects the range in which the reflection pulse lies. This corresponds to the time window in which the second maximum M2 of the echo function lies. This is in Fig. 1 symbolically represented by function block 37.

[0045] As a reference function F R or as an auxiliary function F X Preferably, an envelope is now determined that represents the amplitude profile of this area of ​​the intermediate frequency signal IF lying in the time window.

[0046] Since the absorption is gas-specific and depends on the frequency f of the microwave signals, it is particularly advantageous here to use reference and auxiliary functions F. R , F X to use signals that reproduce the amplitude response of the associated reflection signals R as a function of frequency f within a specified frequency range. Preferably, the frequency range used is that of the intermediate frequency (IF) signal, into which the frequencies contained in the transmitted signal S are transferred during the derivation of the IF signal. The derivative of the reference function F R and the auxiliary functions F XThis occurs when the signal processing 35 subjects the selected time segment of the digitized intermediate frequency signal IF to a Fourier transform FT and derives from it the frequency spectra characteristic of the gas located above the filling material 1.

[0047] The amplitudes A of the reflection signals R, and thus also the amplitudes of the intermediate frequency signals IF, additionally depend on the reflection properties of the material 1, in particular on its dielectric constant. However, this dependence is generally constant over the frequency range of the microwave signals used and is preferably normalized by normalizing the reference function F. R and the auxiliary functions F X eliminated. This is done using the reference function F. R a normalization factor is determined by which the values ​​under the reference function F RThe enclosed area is normalized to a predetermined value. The same normalization factor is then used to normalize the auxiliary functions F recorded during measurement operation. X used.

[0048] Fig. Figure 2 shows the reference function F derived in this way from the intermediate frequency signal IF. R and an example of an auxiliary function F derived with respect to a gas X X The reference function F R Here, is the envelope of the normalized frequency spectrum of the intermediate frequency signal ZF, derived from the reflection signal R and recorded with the reference gas. The auxiliary function F x is the envelope of the normalized frequency spectrum of the intermediate frequency signal IF derived from the reflection signal R, which was recorded with the gas X.

[0049] The reference function F Ressentially corresponds to the frequency spectrum of the transmitted signal, i.e., it exhibits a pronounced maximum at the frequency f corresponding to the transmission frequency. S corresponding frequency f S' The intermediate frequency (IF) signal rises and falls off symmetrically and steeply on both sides of the maximum. This shape is caused by the oscillation behavior of the transmitting oscillator 17 and the switching on and off processes caused by the control circuit 15.

[0050] In contrast, the normalized frequency spectrum F recorded with the other gas X shows X Depending on the frequency f, the amplitudes A are sometimes significantly lower. x (f). The amplitude differences at a given frequency are a measure of how strongly the gas X interacts with the microwaves at that frequency f. The frequency-dependent amplitude differences are characteristic of the gas X.

[0051] Alternatively, corresponding normalized energy spectra can of course be used as reference and auxiliary functions, which are proportional to the square of the amplitudes of the corresponding sections of the intermediate frequency signals ZF derived from the reflection signals R as a function of the frequency f.

[0052] According to the invention, the auxiliary function F x with the reference function F R compared. If the comparison reveals a significant deviation Δ x Between the two functions, the level gauge diagnoses a composition of gas X located above the contents that differs from the reference gas.

[0053] For the quantitative determination of the deviation Δ x between the reference function F R and the helper function F X Preferably, a difference is calculated between a value under the normalized reference function F. Renclosed area and one under the normalized auxiliary function F X enclosed area determined, and as a measure of Δ x Deviation used.

[0054] If the deviation exceeds Δ x If a predetermined threshold value is reached, the level gauge diagnoses a composition of gas X above the contents that deviates from the reference gas. Additionally or instead of diagnosis, depending on the degree of deviation Δ X between the current helper function F X and the reference function F R A warning, an error message and / or an alarm may be issued via an output 37 connected to the signal processing unit 35.

[0055] As described above, the absorption, which depends on the composition of gas X, affects the propagation speed of the microwave signals. As a first approximation, the propagation speed is lower the higher the absorption. Since the deviation Δ X between the auxiliary function F derived with gas X X and the reference function F R A measure of absorption can be determined by the measured deviation Δ X a deviation Δ X A dependent correction factor for the propagation speed can be determined.

[0056] The deviation Δ X The dependent correction factor is preferably determined in a calibration procedure. The reference function F is used in this process. R and at least one further auxiliary function F B recorded. While for the reference function F RIf a reference gas is used that has as little effect as possible on the propagation of the microwave signals, the reference functions F are defined. B Reference gases B are used, in which interactions occur, so that the propagation velocities v B the microwave signals in the reference gases B differ significantly from the propagation speed v R The microwave signals must differ in the reference gas. Additionally, the propagation velocities v must differ for multiple reference gases. B to distinguish the microwave signals in the reference gases from one another. The propagation speeds v R ,v B are either known in advance or are determined, for example, experimentally.

[0057] As a first approximation, we assume that the propagation speed v x The microwave signals in a gas X are lower the stronger the associated auxiliary function F is. X from the reference function FR If it deviates, a deviation Δ from the currently measured deviation can be directly derived from this. x dependent correction factor for determining the propagation speed v x to drain into the unknown gas X.

[0058] Using only one reference gas B, the deviation Δ is calculated. x corrected propagation speed v x for example according to: vX=[1+Δx(vB−vR)ΔB vR]vR where v x the desired propagation speed in the unknown gas X; v R the propagation speed in the reference gas; v B the propagation speed in the reference gas; Δ x the deviation between the auxiliary function F x of the unknown gas X and the reference function F R ; and Δ B the deviation between the auxiliary function F B of the reference gas and the reference function FR mean

[0059] The correction factor for the propagation speed v x or the corrected propagation speed v X For example, in signal processing 35, the value is calculated and fed to signal processing 33, which then determines the fill level L based on the corrected propagation speed v. x determined. Alternatively, the current deviation Δ can of course also be determined based on the reflection signals R. x determined and fed to signal processing 33, which is then used based on this deviation Δ x the corrected propagation speed v x determined and used to calculate the fill level L.

[0060] Instead of a correction factor for the propagation speed, a correction factor for the measured signal propagation time can of course be determined in a completely analogous manner, which takes the changed propagation speed into account. In this case, the fill level L is then determined based on the signal propagation time corrected by the corresponding correction factor.

[0061] The invention can also be applied analogously in conjunction with FMCW radar level measuring devices. Fig. Figure 3 shows an embodiment of this. Here too, the measuring device has measuring electronics 39 and an antenna 7 connected to the measuring electronics 39 and arranged above the filling material 1.

[0062] The measuring electronics 39 include a circuit 41 for generating microwave signals. The circuit 41 includes, for example, a modulator 43 and a voltage-controlled oscillator 45. The modulator 43 controls the oscillator 45 such that it periodically delivers frequency-modulated transmission signals S in a linear, sawtooth pattern. The transmission signals S are fed to the antenna 7 via a splitter 47 and a transmit-receive switch 49, e.g., an isolator. The antenna 7 transmits the signals S into the container 3 and receives their reflected signals R. The reflected signal R is present at a first input of a mixer 51 via the transmit-receive switch 49. A second input of the mixer 51 is connected to the splitter 47 and is fed with the transmitted signal S via the splitter.

[0063] The instantaneous frequency of the reflection signal R applied to the first input of the mixer 51 has a frequency difference Δf compared to the instantaneous frequency of the transmit signal applied in parallel to the second input of the mixer 51 at the time of reception, which depends on the signal propagation time that the microwaves need for the path to the surface of the product and back.

[0064] The mixing signal available at the output of mixer 51 corresponds to the correlation of the microwave signals received at its two inputs. It contains a high-frequency component, which includes frequencies essentially determined by the sum of the frequencies present at the inputs, and a low-frequency component, which includes frequencies essentially determined by the difference between the frequencies present at the inputs. The output signal of mixer 51 is applied to a filter 53, which filters out the low-frequency component and feeds it to an analog-to-digital converter 55. The filtered, digitized signal is fed as a measurement signal M to a signal processor 57, which uses this measurement signal M to determine the fill level L.

[0065] This is usually done by measuring the signal – as in Fig. 3 is represented by the FT function block in signal processing 57 – it is Fourier-transformed, and its frequency spectrum is evaluated. The frequencies of the Fourier-transformed measurement signal correspond to the frequency differences Δf between the respective transmitted signal S and the associated reflected signal R, and thus to the distance of the reflecting surface from the transmitting and receiving device or the associated signal propagation time. Furthermore, the amplitudes A(Δf) of the spectral lines of the frequency spectrum obtained by Fourier transformation correspond to the amplitudes of the reflected signal R.

[0066] To determine the fill level, it is preferably not the direct recording of the actual measurement signal as an echo function that is used, but rather its envelope. A typical curve of this is shown in Fig. 3 in the building block symbolizing the signal processing 57. The echo function also exhibits two distinct maxima here, the first of which, M1, corresponds to the transmitted signal S, which is recorded directly via the transmit / receive switch 11, and the second of which, M2, is due to a reflection of the transmitted signal S at the surface of the contents. The difference between the two frequency differences Δf at which the two maxima M1 and M2 occur corresponds to the desired signal propagation time. The signal processing 57 determines this signal propagation time and calculates the current fill level L from it using the propagation speed of the microwave signals.

[0067] Analogous to the previously described embodiment, an auxiliary function F, dependent on the gas X located above the filling material 1, is also calculated here based on the reflection signals R. Xderived, which reproduces the amplitude profile of the reflection signals R and with a corresponding reference function F derived with a reference gas R compared. According to the invention, the level measuring device also diagnoses a change in the composition of the gas X above the filling material 1 compared to the reference gas, when the comparison between the auxiliary function F X and the reference function F R a significant deviation Δ X results.

[0068] Here too, normalized auxiliary functions F are preferably used. X and a normalized reference function F R used, and the deviation Δ X determined based on the difference in the areas enclosed by the two normalized functions. Just as in the previous embodiment, the diagnosis can also be additionally or instead of the degree of deviation Δ X between the current helper function F Xand the reference function F R A warning, an error message and / or an alarm may be issued, and a correction factor for the propagation speed or the signal travel time can be determined in the manner described above in connection with the pulse radar device.

[0069] The derivative of the auxiliary or reference function F R , F xThis is achieved, for example, by feeding the reflection signal R available via the transmit / receive switch 49, for instance, via a splitter 59 to both the mixer 51 and a separate signal processing branch, in which the reflection signal R is detected by a suitable detector 61, e.g., a rectifier diode, digitized by an analog-to-digital converter 63, and then fed to a signal processing unit 65, e.g., a microprocessor. In contrast to pulse radar, it is already possible with today's detectors 61 to derive the amplitude profile of the reflection signal R directly from the reflection signal R. The reason for this is that FMCW level sensors already contain a frequency-tunable source. In the illustrated embodiment, this is provided by the circuit 41 for generating the microwave signals with the modulator 43 and the voltage-controlled oscillator 45.The frequency difference between the transmitted signal S and the reference signal R, due to the signal propagation delay, is very small compared to the transmitted frequencies and can therefore be neglected when recording the amplitude profile of the reflected signal R using the detector 61. For recording the amplitude profile, it is assumed that the instantaneous frequency of the reflected signal R is equal to the known instantaneous frequency of the transmitted signal S. Based on this frequency information, the amplitude profile of the reflected signals R can be recorded directly using currently available detectors 61. These are very inexpensive compared to spectrum analyzers and can therefore be readily used in level measuring devices. The signal processing unit 65 derives the reference function F from this. R or the auxiliary functions F Xfrom, for example by determining a normalized envelope that represents the respective amplitude profile A(t) of the corresponding reflection signal R as a function of time.

[0070] Fig. Figure 4 shows the time dependence of the frequencies f of the transmitted signals S and the reflected signals R, characteristic of the FMCW method, under ideal conditions, i.e., without absorption. The frequencies f of both signals increase linearly over the duration T of each transmission period, with a time shift Δt between the two signals that depends on the fill level L. Ideally, the amplitude of the transmitted signals S generated by circuit 39 is constant over the entire frequency range. Accordingly, the amplitudes A of the transmitted signals S and the reflected signals R are constant over the duration of their transmission and reception periods, respectively, under ideal conditions, i.e., without absorption. For the derivation of the auxiliary functions F x and the reference function F RPreferably, a time window is chosen that begins at time t0 + Δt and ends at time t0 + T, where t0 denotes the start of transmission of the transmit signal S.

[0071] The normalized reference function F derived with the reference gas R and a normalized auxiliary function F derived with another gas X X, which each represent the amplitude profile of the associated reflection signal R as a function of time t over a period, are in Fig. 3 in a functional block within signal processing 65 is shown in comparison. The reference function F derived with the reference gas. R The function exhibits a constant amplitude A(t) within the selected time window. This is because practically no absorption occurs in the reference gas. In contrast, the auxiliary function F derived with respect to gas X exhibits a constant amplitude A(t). x areas emerge where the amplitude is significantly lower than the corresponding amplitude of the reference function F.R These amplitude differences are due to the frequency-dependent absorption characteristic of gas X.

[0072] Just like with the previously described pulse radar level gauge, the reference function F R and the auxiliary functions F X Not only can the composition of the gas above the fill material 1 be derived directly from the corresponding reflection signals R, but also from signals derived from them, the generation of which uses exclusively transformations that affect all frequencies f contained in the reflection signal R equally. Accordingly, a change in the composition of the gas above the fill material 1 can alternatively also be determined using these auxiliary signals derived from the reflection signal R.

[0073] In conjunction with the FMCW radar level gauge shown, the digitized measurement signal M present at the output of the analog-to-digital converter 55 is particularly suitable for this purpose. For this, the digitized measurement signal M is fed, for example, to a signal processing unit 65'. Since this is an alternative embodiment of the invention, the signal processing unit 65' is in Fig. 3 as an alternative to the previously described signal processing branch, which includes the splitter 59, the detector 61, the analog-to-digital converter 63 and the signal processing 65, is shown with a dashed line.

[0074] In this variant, only that part of the digitized measurement signal M that reproduces the reflection signal R may be used. This is done, for example, by defining a suitable time window in which the measurement signal M is used to derive the auxiliary functions F'. X and the reference function F' R is used.

[0075] As in Fig. As shown in section 4, there is a time shift Δt between the transmitted signal S and the reflected signal R, which depends on the fill level L. At the minimum fill level L = L min the maximum possible time shift Δt = t max . Accordingly, for the derivative of the auxiliary functions F' x and the reference function F' R preferably a time window chosen that at time t0 + t max begins, and ends at time t0 + T, where t0 denotes the start of the transmission of the transmit signal S.

[0076] Fig. Figure 5 shows a reference function F' derived from the measurement signal M in this time window. R The reference function F' R is here a preferably standardized one, e.g. by a - in Fig. 3. Envelope represented by a corresponding function block - Hilbert transformation HT obtained, which defines the amplitude profile A R(t) of the measurement signal in this time window when the reference gas is located above the fill material 1. The measurement signal M was derived from the reflection signal R as described above, and thus reflects the amplitude profile of the reflection signal R. Provided that the amplitudes of the transmitted signals S are constant for all transmitted frequencies, and the reference gas causes practically no frequency-dependent amplitude change in the microwave signals, the reference function F' R This remained constant throughout the entire time window.

[0077] Fig. Figure 6 shows an example of an auxiliary function F' derived in the same way from the measurement signal M in this time window. x. The auxiliary function F' x Therefore, the normalized envelope obtained, for example, through a Hilbert transformation, which defines the amplitude profile A, is shown here. x(t) of the measurement signal M in this time window is represented when gas X is located above the fill material 1. The gas X exerts a significant interaction with the microwave signals, which is reflected in the value relative to the reference function F'. R significantly changed amplitude profile of the auxiliary function F' X This is reflected again. Both functions are shown in comparison in signal processing 65'. 1 Filling material 3 containers 5 Measuring instrument electronics 7 Antenna 9 Circuit for generating microwave signals 11 Transmit / receive switch 13 Oscillator 15 Control circuit 17 Microwave generator 19 mixers 21 Time delay level 23 Control circuit 25 microwave generator 27 Low-pass filter 29 Sampling-hold circuit 31 Analog-to-Digital Converters 33 Signal processing 35 Signal processing 37 Exit 39 Measuring instrument electronics 41 Circuit for generating microwave signals 43 Modulator 45 Oscillator 47 splinters 49 Transmit / receive switch 51 mixers 53 filters 55 Analog-to-Digital Converters 57 Signal processing 59 splinters 61 Detector 63 Analog-to-Digital Converters 65 Signal processing

Claims

Method for measuring the fill level (L) of a substance (1) in a container (3) using an FMCW radar level gauge, which, during measurement operation, periodically and linearly transmits frequency-modulated microwave signals (S), in particular according to a sawtooth function, towards the substance (1), receives the reflection signals (R) reflected from the surface of the substance after a signal propagation time dependent on the fill level (L), and determines the fill level (L) based on the signal propagation time, whereby: a reference function (FR, F'R) is derived beforehand, which represents an amplitude profile of a reflection signal (R) that the level gauge receives when a known reference gas is present above the substance (1); and subsequently, auxiliary functions (Fx, F'x) are derived from the reference signals (R) received during measurement operation, which represent the amplitude profile of the reflection signals (R) received by the level gauge.while an unknown gas (X) is located above the contents (1),- the auxiliary functions (Fx, F'x) are compared with the reference function (FR, F'R), and- the level gauge diagnoses a composition of the unknown gas (X) located above the contents (1) that differs from the reference gas if the comparison reveals a significant deviation (Δx) between an auxiliary function (Fx, F'x) derived during measurement and the reference function (FR, F'R), - the level gauge generates a measurement signal (M) by mixing the reflection signals (R) with the transmitted microwave signals (S) and subsequent filtering, which contains those frequencies (Δf) that correspond to the differences in the frequencies (f) of the signals simultaneously present at the mixer (51),- the reference function (FR, F'R) is an envelope that represents the amplitude profile of a measurement signal (M) as a function of time (t), which is generated,when the reference gas is located above the filling material (1), and the auxiliary functions (Fx, F'x) are envelopes that represent the amplitude profile of measurement signals (M) as a function of time (t) that arise during measurement operation when the unknown gas (X) is located above the filling material (1). Method for measuring the fill level (L) of a substance (1) in a container (3) using an FMCW radar level gauge, which, during measurement operation, periodically and linearly transmits frequency-modulated microwave signals (S), in particular according to a sawtooth function, towards the substance (1), receives the reflection signals (R) reflected from the surface of the substance after a signal propagation time dependent on the fill level (L), and determines the fill level (L) based on the signal propagation time, whereby: a reference function (FR, F'R) is derived beforehand, which represents an amplitude profile of a reflection signal (R) that the level gauge receives when a known reference gas is present above the substance (1); and subsequently, auxiliary functions (Fx, F'x) are derived from the reference signals (R) received during measurement operation, which represent the amplitude profile of the reflection signals (R) received by the level gauge.while an unknown gas (X) is located above the contents (1),- the auxiliary functions (Fx, F'x) are compared with the reference function (FR, F'R), and- the level gauge diagnoses a composition of the unknown gas (X) located above the contents (1) that differs from the reference gas if the comparison reveals a significant deviation (Δx) between an auxiliary function (Fx, F'x) derived during measurement and the reference function (FR, F'R),- the reflection signals (R) are fed to a detector (61) which records an amplitude profile of the reflection signals (R), and- the reference function (FR) is an envelope,-- which represents the amplitude profile of the reflection signal (R) as a function of time (t), and-- which was derived from an amplitude profile of a reflection signal (R) recorded with the detector (61), which arises when the unknown gas (X) is located above the contents (1). Reference gas is locatedand- the auxiliary functions (FX) are envelopes that represent the amplitude profile of the reflection signals (R) as a function of time (t), and-- which were derived from amplitude profiles of reflection signals (R) recorded with the detector (61) that arise when the unknown gas (X) is located above the filling material (1). Method according to claim 1 or 2, wherein the reference function (FR, F'R) and the auxiliary functions (Fx, F'x) are frequency spectra derived from the reflection signals (R). Method according to claim 1 or 2, wherein the reference function (FR, F'R) and the auxiliary functions (Fx, F'x) are energy spectra derived from the reflection signals (R). Method according to claim 1 or 2, wherein the reference function (FR, F'R) and the auxiliary functions (Fx, F'x) are normalized functions by means of a normalization factor, and the normalization factor normalizes an area enclosed under the reference function (FR, F'R) to a predetermined value. Method according to claim 1, wherein the envelopes are generated by means of a Hilbert transformation of the respective measurement signals (M). Method according to claim 1 or 2, wherein a difference between an area enclosed under the reference curve (FR, F'R) and an area enclosed under the auxiliary curve (Fx, F'x) is determined, and the difference is used as a measure of the deviation (Δx). Method according to claim 1 or 2, wherein a composition of the unknown gas (X) located above the filling material (1) that deviates from the reference gas is diagnosed when the deviation (Δx) exceeds a predetermined threshold value. Method according to claim 1 or 2, wherein, depending on the degree of deviation (Δx) between the reference function (FR, F'R) and the auxiliary function (Fx, F'x), a diagnosis, a warning, an error message and / or an alarm is issued. Method according to claim 1 or 2, wherein a measured signal transit time or propagation speed of the microwave signals (S) used for level measurement in the unknown gas (X) located above the filling material (1) is corrected by means of a correction factor which depends on the degree of the currently measured deviation (Δx) between the reference function (FR, F'R) and the auxiliary function (Fx, F'x). A method according to claim 1 or 2, wherein, in addition to the reference function (FR, F'R), at least one reference function (FB) is derived which represents an amplitude profile of a reflection signal (R) received by the level measuring device when a known reference gas (B) different from the reference gas is located above the contents (1), a deviation (ΔB) between the reference function (FR, F'R) and the reference function (FB) is determined, and, based on this deviation (ΔB), the propagation speed (vR) of the microwave signals (S) in the reference gas and the propagation speed (vB) of the microwave signals (S) in the reference gas (B), a correction factor is derived which determines the dependence of the propagation speed (vx) of microwave signals (S) in an unknown gas (X) located above the contents (1) as a function of the deviation (Δx) between the reference curve (FR,F'R) and the auxiliary curve (Fx, F'x) derived with this unknown gas (X).

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