METHOD FOR MEASURING A FILLING LEVEL

Abstract

A method for radar-based measurement of a filling level of a filling material in a container includes, in successive measurement cycles, generating an evaluation curve, and the relevant current evaluation curve is stored; a first difference curve is formed based on the evaluation curve of the current measurement cycle and a stored evaluation curve of a preceding measurement cycle; and the filling level is determined based on a maximum in the current first difference curve. The filling level is thus established from the difference curve rather than from the evaluation curve, and the filling level can be determined with greater certainty—in particular in the case of filling material having rippled surfaces.

Claims

1-14. (canceled)

15. A method for radar-based measurement of a filling level of a filling material in a container, the method comprising the following steps, which are repeated in successive measuring cycles: emitting a microwave signal in a direction of the filling material; receiving a receive signal generated after reflection of the microwave signal inside the container; generating an evaluation curve based on at least the receive signal; progressively storing the current evaluation curve in each measuring cycle; calculating a first difference curve based on the evaluation curve of a current measuring cycle and a stored evaluation curve of a preceding measuring cycle; and determining the filling level based on a maximum of the current, first difference curve.

16. The method of claim 15, wherein the evaluation curve is created with complex values.

17. The method of claim 16, wherein the first difference curve is created with complex values, and wherein the filling level is determined based on a maximum of the imaginary part or of the real part of the current, first difference curve.

18. The method of claim 15, wherein the measuring cycles in which the filling level is in each case re-determined are repeated at a defined measuring rate.

19. The method of claim 18, wherein a first distance of the current measuring cycle to the preceding measuring cycle, from which the stored evaluation curve is used to calculate the first difference curve, is set to be inversely proportionally as a function of the measuring rate.

20. The method of claim 15, wherein the first difference curve is calculated based on the current evaluation curve and of the evaluation curve created in the immediately preceding measuring cycle.

21. The method of claim 15, wherein, analogously to the first difference curve, a second difference curve is created, and wherein a second distance of the current measuring cycle to the preceding measuring cycle is selected, from which the stored evaluation curve is used to calculate the second difference curve, deviating from the first distance of the current measuring cycle to the preceding measuring cycle, from which the stored evaluation curve is used to calculate the first difference curve.

22. The method of claim 21, wherein, the filling level cannot be determined based on the first difference curve, the filling level is determined based on the second difference curve.

23. The method of claim 21, wherein the filling level is determined from the difference curve whose maximum has a greater amplitude.

24. The method of claim 15, wherein, when the filling level cannot be determined based on the first difference curve and/or based on the second difference curve, the filling level is determined based on the current evaluation curve.

25. A radar-based filling-level measurement device for determining a filling level of a filling material in a container, the device comprising: a signal-generation unit configured to emit a microwave signal in a direction of the filling material in each case in successive measuring cycles; a receiving unit configured to receive a corresponding receive signal generated after reflection of the microwave signal inside the container; an evaluation unit configured to: generate an evaluation curve in each measuring cycle based on at least the receive signal; progressively store the respective current evaluation curve; calculate a first difference curve by subtracting the evaluation curve of the current measuring cycle and the evaluation curve of a preceding measuring cycle; and determine the filling level based on the first difference curve.

26. The device of claim 25, wherein the signal-generation unit and the evaluation unit are configured to generate the evaluation curve and calculate the difference curve with complex values.

27. The device of claim 25, wherein the signal-generation unit, the receiving unit, and the evaluation unit are configured to determine the filling level based on the FMCW principle.

28. The device of claim 25, wherein the signal-generation unit is configured to emit the microwave signal in a frequency band of at least 60 GHz, and wherein the receiving unit and the evaluation unit are configured to process the corresponding receive signal.

Description

[0035] The invention is explained in more detail with reference to the following figures. Shown are:

[0036] FIG. 1: a typical arrangement of a radar-based filling-level measurement device; and

[0037] FIG. 2: a schematic representation of the method according to the invention.

[0038] For a basic understanding of the invention, FIG. 1 shows a typical arrangement of a freely-radiating, radar-based filling-level measurement device 1 on a container 2. In the container 2 is a filling material 3, whose filling level L is to be determined by the filling-level measurement device 1. For this purpose, the filling-level measurement device 1 is mounted on the container 2 above the maximum permissible filling level L. Depending upon the field of application, the installation height h of the filling-level measurement device 1 above the container bottom can be more than 100 m.

[0039] As a rule, the filling-level measurement device 1 is connected via a bus system, such as “Ethernet,” “PROFIBUS,” “HART,” or “Wireless HART,” to a higher-level unit 4, such as a process control system or a decentralized database. On the one hand, information about the operating status of the filling-level measurement device 1 can thus be communicated. However, information about the filling level L can also be transmitted via the bus system, in order to control any inflows or outflows that may be present at the container 2.

[0040] Since the filling-level measurement device 1 shown in FIG. 1 is designed as freely-radiating radar, it comprises a corresponding antenna. As indicated, the antenna can be designed as a horn antenna, for example. Especially in the case of radar frequencies above 100 GHz, the antenna can also be realized as a planar antenna. Regardless of the design, the antenna is oriented in such a way that corresponding microwave signals S.sub.HF are emitted in the direction of the filling material 3. In doing so, the microwave signals S.sub.HF are generated, depending upon the measuring method (pulse time-of-flight or FMCVV), in a corresponding signal-generation unit of the filling-level measurement device 1.

[0041] The microwave signals S.sub.HF are reflected at the surface of the filling material 3 and, after a corresponding signal propagation time, are received as receive signals E.sub.HF by the antenna or a downstream receiving unit of the filling-level measurement device 1. The filling level L can be determined from the receive signals E.sub.HF because the signal propagation time of the microwave signals S.sub.HF, E.sub.HF depends upon the distance d=h−L of the filling-level measurement device 1 to the filling-material surface.

[0042] To determine the filling level L, an evaluation unit of the filling-level measurement device 1 designed for this purpose creates an evaluation curve ZF.sub.n-1, ZF.sub.n for each measuring cycle n on the basis of the receive signal E.sub.HF. In this case, the evaluation curve ZF.sub.n-1, ZF.sub.n reproduces the amplitude A (or, in the case of complex-value recording, also, indirectly, the phase) of the reflected receive signal as a function of the measuring distance d or the signal propagation time of the transmit signal S.sub.HF/receive signal E.sub.HF.

[0043] When implementing the FMCW method, the evaluation unit of the filling-level measurement device 1 generates the evaluation curve ZF.sub.n-1, ZF.sub.n in principle by mixing the just received receive signal E.sub.HF with the currently emitted microwave signal S.sub.HF, wherein the microwave signal S.sub.HF is transmitted continuously for this purpose with a sawtooth-shaped frequency change.

[0044] In the case of the pulse time-of-flight method, the evaluation curve ZF.sub.n-1, ZF.sub.n is generated by undersampling the pulse-shaped receive signal E.sub.HF. The pulse frequency of the sampling signal in the sub-per mil range deviates from the pulse frequency of the microwave signal S.sub.HF or of the receive signal E.sub.HF.

[0045] In the case of both FMCW and the pulse time-of-flight method, the evaluation curve ZF.sub.n-1, ZF.sub.n represents the signal amplitude A of the receive signal E.sub.HF as a function of the measuring distance d. In the case of freely-radiating, radar-based filling level measurement, the corresponding measurement range h accordingly extends from the antenna of the filling-level measurement device 1 to the bottom of the container 2. Schematic evaluation curves ZF.sub.n-1, ZF.sub.n are shown in FIG. 2.

[0046] According to the prior art, the evaluation unit of the filling-level measurement device 1 determines the filling level L from the evaluation curve ZF.sub.n-1, ZF.sub.n. For this purpose, the location d of the maximum A.sub.m of the filling material surface is determined.

[0047] Due to the higher degree of focusing of the emitted microwave signal S.sub.HF and the potentially higher resolution of the measured filling level L, the highest possible radar frequencies at above 60 GHz are used, particularly when implementing the FMCW method. However, it is precisely in the case of sloshing filling materials 3 that a narrow beam cone can lead to the microwave signal S.sub.HF being deflected upon reflection at the filling material surface, so that the receive signal E.sub.HF is not received by the antenna of the filling-level measurement device 1. In such cases, the evaluation curve ZF.sub.n, as shown in FIG. 2, does not have a maximum A.sub.m in the corresponding measuring cycle n−1.

[0048] The method according to the invention, with which the filling level L can also be reliably determined under such circumstances, is therefore illustrated in more detail by FIG. 2. The method is based upon subtracting the evaluation curve ZF.sub.n of the current measuring cycle n from a stored evaluation curve ZF.sub.n-m of a preceding measuring cycle n-m (in the simplest case, the immediately preceding measuring cycle n−1) to create a first difference curve Diff.sub.n. Within the meaning of the invention, it is not of decisive importance which evaluation curve ZF.sub.n-m1, ZF.sub.n-1 is subtracted from the other curve. The filling-level measurement device 1 determines the filling level L on the basis of this first difference curve Diff.sub.n, according to the invention. To search for the maximum A.sub.m (or the minimum, depending upon how the difference is formed) in difference curve Diff.sub.n, any algorithm suitable for this can be implemented in the evaluation unit.

[0049] As can be seen from FIG. 2, the method has the advantage that the maximum A.sub.m of the filling-material surface appears in the first difference curve Diff.sub.n, even when the evaluation curve ZF.sub.n of the current measuring cycle n does not have a maximum A.sub.m, for example, due to sloshing of the filling material 3. The benefit is therefore that the maximum A.sub.m is contained in the evaluation curve ZF.sub.n-1 of the preceding measuring cycle n−1. Peripheral maxima in the evaluation curves ZF.sub.n, ZF.sub.n-m, which are caused by static internal components in the container 3 (for reasons of clarity, not shown in FIG. 2), are, however, masked by the method according to the invention, so that these maxima cannot lead to an erroneous determination of the filling level L.

[0050] With the method according to the invention, the filling level L can be determined very reliably, particularly when the filling-level measurement device 1 produces the evaluation curve ZF.sub.n and the difference curve Diff.sub.n with complex values.

[0051] Due to the fact that, in this case, even subtraction takes place with complex values (that is, the real parts or the imaginary parts of the evaluation curves ZF.sub.n and ZF.sub.n-m are subtracted separately from each other), any changes between the evaluation curves ZF.sub.n and ZF.sub.n-m can be recorded with even higher resolution. In the exemplary embodiment shown in FIG. 2, the graphs show the imaginary part of the evaluation curves ZF.sub.n, ZF.sub.n-1_ and, accordingly, the imaginary part of the difference curve Diff.sub.n. As a result of the complex-valued analysis of the evaluation curves ZF.sub.n, ZF.sub.n-1, the filling level can appear as a minimum, as is the case in the current evaluation curve ZF.sub.n.

[0052] In addition to the filling-level measurement device 1 having sufficient computing power and storage capacity, it is necessary for complex-valued recording of the difference curve Diff.sub.n that the signal-generation unit and the evaluation unit be clocked coherently.

[0053] Above all, if, in past measuring cycles n-m, the filling level maximum A.sub.m appears continuously at the same point or with the same amplitude in the evaluation curve ZF.sub.n, ZF.sub.n-m, it may happen that the filling level L cannot be determined from the difference curve Diff.sub.n. In this respect, it is advantageous if the filling-level measurement device 1 is designed to determine the filling level L from the current evaluation curve ZF.sub.n, at least in these cases.

[0054] For a reliable implementation of the method according to the invention, it may be advantageous, in addition to the current evaluation curve ZF.sub.n, to use in each case not only the evaluation curve ZF.sub.n-1 of the immediately preceding measuring cycle n−1, in order to form the difference curve Diff.sub.n of the current measuring cycle n. Depending upon the type of disturbance at the filling material surface, or its frequency of occurrence, it is advantageous to set a greater distance m of the current measuring cycle n to the measuring cycle n-m from which the stored evaluation curve ZF.sub.n-m is used, in order to form the respective current difference curve Diff.sub.n.

[0055] In this case, it is advantageous to adapt this distance m to the rate or frequency of the disturbance, taking into account the measuring rate r at which the filling-level measurement device 1 repeats the measuring cycles n. The sloshing of the filling material 3 can, in the process, be caused, for example, by an agitator which is operated at a defined frequency. If, then, the disturbance occurs at an approximate frequency of 1 Hz, and the measuring rate r is only 10 Hz, as is customary with a limited energy supply of the filling-level measurement device 1 by means of a 4-20 mA signal, the respective distance must be set to m=10 measuring cycles. Accordingly, if the filling-level measurement device 1 changes the measuring rate r during operation, it makes sense to adjust the distance m between the measuring cycles m, m-n to be inversely proportionally as a function of the measuring rate r.

[0056] Within the scope of the invention, the filling-level measurement device 1 can, for purposes of redundancy, be further developed in such a way that, in addition to the first difference curve Diff.sub.n, it also records a second difference curve Diff2.sub.n. The underlying distance l of the current measuring cycle n to the measuring cycle n-l, from which the stored evaluation curve ZF.sub.n-1 is used to form the difference curve, deviates from the corresponding first distance m of the first difference curve Diff.sub.n. The deviation of the two distances l, m can here be set, for example, with a fixed value of 10 measuring cycles. In this further development of the filling-level measurement device 1, the redundancy can, for example, be checked by determining the filling level L from both difference curves Diff.sub.n, Diff2.sub.n separately. There is redundancy if the filling level value L of both measurements matches.

LIST OF REFERENCE SIGNS

[0057] 1 Filling-level measurement device [0058] 2 Container [0059] 3 Filling material [0060] 4 Higher-level unit [0061] A.sub.m Maximum [0062] d Measuring distance [0063] Diff.sub.n First difference curve [0064] Diff2.sub.n Second difference curve [0065] E.sub.HF Receive signals [0066] h Installation height or measurement range [0067] L Filling level [0068] l Second distance [0069] n Measuring cycle [0070] m First distance [0071] S.sub.HF Microwave signals [0072] ZF.sub.n Evaluation curve