Method for detecting a fault state at an FMCW-based filling level measuring device

Abstract

The present disclosure relates to a method for detecting a fault state at an FMCW-based fill level measuring device, including performing two reference measurements successively in time, a first reference measurement signal and a second reference measurement signal, using the filling level measuring device under a predefined reference measurement condition. In each of the two reference measurement signals a characteristic parameter is determined, wherein a change in the characteristic parameter over time is determined by comparing the two reference measurement signals. A fault state is detected when the change in the characteristic parameter exceeds a predefined maximum characteristic parameter change.

Claims

1. A method for detecting an error state in a fill level measuring device configured to operate according to the frequency-modulated continuous wave (FMCW) measuring principle, the method comprising: measuring a first reference measurement signal under at least one predefined reference measurement condition using a FMCW-based fill level measuring device; determining at least one characteristic parameter of the first reference measurement signal; measuring at least one second reference measurement signal under the at least one predefined reference measurement condition using the FMCW-based fill level measuring device; determining a change in at least one characteristic value based on at least the first reference measurement signal and the at least second reference measurement signal; identifying an error state when the change in the at least one characteristic value exceeds a predefined maximum change in the characteristic value; generating a temporal change function at least based on the at least one characteristic value and the change in the at least one characteristic value; and when the change in the at least one characteristic value does not exceed the predefined maximum change in characteristic value, calculating a remaining operating duration before the predefined maximum change in characteristic value will be exceeded based on the temporal change function.

2. The method of claim 1, wherein the at least one characteristic parameter includes an amplitude, a frequency of a signal maximum of the respective reference measurement signal, an envelope of the amplitude, a phase position and/or a frequency of a low-frequency interference of a respective intermediate frequency reference measurement signal.

3. The method of claim 1, wherein the at least one predefined reference measurement condition is a fall below a minimum fill level.

4. The method of claim 1, wherein the temporal change function is generated from a regression analysis.

5. The method of claim 4, wherein the method of least squares is used to perform the regression analysis and/or to determine an appropriate type of regression.

6. The method of claim 1, further comprising, subsequent to determining the change in the at least one characteristic value based on at least the first reference measurement signal and the at least second reference measurement signal and when the at least one predefined reference measurement condition again occurs, determining a further reference measurement signal.

7. The method of claim 1, further comprising: generating a first correction curve using the first reference measurement signal; generating a second correction curve using the second reference measurement signal; and determining the change in the at least one characteristic value based on the first correction curve and the second correction curve.

8. A fill level measuring device, comprising: a signal generation unit configured to generate a radar transmission signal; a transmitting antenna and/or a receiving antenna adapted to transmit the transmission signal and/or to receive a radar received signal; a mixer configured to generate an intermediate frequency signal by mixing of the transmission signal with the received signal; and an evaluation unit configured to: determine a measurement signal and/or a reference measurement signal using the intermediate frequency signal; determine a fill level from the measuring signal; indicate an error state of the device by: measuring a first reference measurement signal under at least one predefined reference measurement condition; determining at least one characteristic parameter of the first reference measurement signal; measuring at least one second reference measurement signal under the at least one predefined reference measurement condition; determining a change in at least one characteristic value based on at least the first reference measurement signal and the at least second reference measurement signal; and indicating the error state when the change in the at least one characteristic value exceeds a predefined maximum change in the characteristic value; and when the change in the at least one characteristic value does not exceed the predefined maximum change in characteristic value, determine a remaining operating duration by: generating a temporal change function at least based on the at least one characteristic value and the change in the at least one characteristic value; and calculating the remaining operating duration before the predefined maximum change in characteristic value will be exceeded based on the temporal change function.

9. The device of claim 8, wherein the at least one characteristic parameter includes an amplitude, a frequency of a signal maximum of the respective reference measurement signal, an envelope of the amplitude, a phase position and/or a frequency of a low-frequency interference of a respective intermediate frequency reference measurement signal.

10. The device of claim 8, wherein the at least one predefined reference measurement condition is a fall below a minimum fill level.

11. The device of claim 8, wherein the temporal change function is generated from a regression analysis.

12. The device of claim 11, wherein the method of least squares is used to perform the regression analysis and/or to determine an appropriate type of regression.

13. The device of claim 8, wherein the evaluation unit is further configured to: generate a first correction curve using the first reference measurement signal; generate a second correction curve using the second reference measurement signal; and determine the change in the at least one characteristic value based on the first correction curve and the second correction curve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in more detail below with reference to the following figures. The following is shown:

(2) FIG. 1 shows a standard arrangement of an FMCW-based fill level measuring device on a container;

(3) FIG. 2 shows a typical circuit configuration of an FMCW-based fill level measuring device for carrying out the method according to the present disclosure;

(4) FIG. 3 shows schematic representations for determining characteristic parameters of received signals of an FMCW-based fill level measuring device; and

(5) FIG. 4 shows a regression of a characteristic parameter for determining the expected remaining operating duration of an FMCW-based fill level measuring device.

DETAILED DESCRIPTION

(6) To assist in understanding the method according to the invention, a typical arrangement of a fill level measuring device 1 on a container 2 and operating according to the FMCW measuring principle is shown first in FIG. 1. In the container 2 there is a filling material 3, whose level L is to be determined by the fill level measuring device 1. For this purpose, the fill level measuring device 1 is mounted on the container 2 above the filling material 3 at a known installation height h. Depending on the application, the container 2 can be up to more than 30 m high.

(7) The fill level measuring device 1 is arranged on the container 2 in such a way that in the direction of the surface of the filling material 3 it emits a radar transmission signal s.sub.HF typical of FMCW. After reflection of the radar transmission signal s.sub.HF at the filling material surface (or undesirably at a disruptive body inside the container 2, such as, for example, an inflow pipe 21 projecting into the container), the fill level measuring device 1 receives a radar received signal E.sub.HF. In this case, as is characteristic of FMCW, the frequency difference between the currently emitted radar transmission signal s.sub.HF and the radar received signal E.sub.HF is dependent on the distance d=h−L to the filling material surface.

(8) As a rule, the fill level measuring device 1 is connected via a bus system, such as “PROFIBUS”, “HART” or “Wireless HART” to a superordinate unit 4, such as a process control system. Information about a possible error state of the fill level measuring device can on the one hand be communicated via this. On the other hand information about the fill level L can also be transmitted in order to control any inflows 21 and/or outflows 22 that may be present on the container 2.

(9) FIG. 2 shows a suitable circuit configuration of an FMCW-based fill level measuring device 1 with which the method according to the invention can be implemented for detecting any error state: In order to generate a high frequency signal s.sub.HF typical of the FMCW measuring method, the fill level measuring device 1 comprises a corresponding signal generation unit 11. Here the high-frequency signal s.sub.HF is designed such that it has a frequency in the microwave range (as standard at 6 GHz, 26 GHz or 79 GHz, but also possible up to over 100 GHz). Here the frequency is not constant but varies periodically within a predetermined frequency difference (in the case of 79 GHz, the frequency difference could be 2 GHz, for example, so that a corresponding frequency would be set between 78 GHz and 80 GHz). In the FMCW method, a sawtooth-shaped (i.e. time-constant within this period) change in the frequency of the high-frequency signal s.sub.HF is usual in the case of the periodic change. However, any other form would also be conceivable, for example, a sinusoidal change in the frequency within the respective frequency difference.

(10) The periodicity of the (sawtooth-shaped) change can here be, as is typical of the FMCW method, in an order of up to several 100 MHz. The frequency difference of the high-frequency signal s.sub.HF is preferably to be dimensioned as large as possible in this case, since the resolution of the level measurement can be increased by increasing the bandwidth. A generally higher frequency of the high-frequency signal s.sub.HF is thus advantageous with regard to the resolution since at higher frequencies a greater—as seen in absolute terms—frequency difference can be implemented.

(11) Once it has been generated the high-frequency signal s.sub.HF is fed via a signal splitter 12 (and optionally a transmission amplifier 13) to a transmitting antenna 14. There, the high-frequency electrical signal s.sub.HF is converted into the actual radar transmission signal s.sub.HF and emitted accordingly.

(12) During measurement operation, a radar received signal E.sub.HF is generated by the reflection of the radar transmission signal s.sub.HF at the surface of the filling material 3 (and/or at a disruptive body 21 inside the container 2, such as an inflow pipe 21 projecting into the container; see FIG. 1). In the case of a calibration or reference measurement, the radar received signal E.sub.HF results from reflection of the radar transmission signal s.sub.HF by a predefined reference condition, for example, by a reference object arranged at a known distance d in a measuring path. A further reference condition could also be represented by a precisely known fill level L in the container 2 itself (for example, a known minimum fill level L.sub.min which, for example, cannot fall further due to a correspondingly arranged outflow 22; once again, see FIG. 1). In addition, a largely anechoic measuring environment (for example a corresponding absorption chamber) would also be conceivable as a reference condition. In this case, no radar received signal E.sub.HF at all is optimally produced.

(13) The radar received signal E.sub.HF is received at a receiving antenna 15 of the fill level measuring device 1 and converted back into an electrical signal (which in turn can be optionally amplified by a receiving amplifier 16). This is subsequently mixed with the radio-frequency signal s.sub.HF, by means of a receiving mixer 17, wherein the high-frequency signal s.sub.HF is for this purpose branched off from a signal splitter 12. As a result, an intermediate frequency signal s.sub.ZF1, s.sub.ZF2 typical of the FMCW method is generated in each case whose frequency F.sub.peak is dependent on the distance d and thus enables measurement of the fill level L. If a suitable transmitting/receiving switch is used, it would of course also alternatively be possible to use a single transmitting/receiving antenna instead of a separate transmitting antenna 14 and receiving antenna 15. This could be realized in a classic manner as a horn antenna. Towards higher frequencies, or if the transmitting and receiving antennas 14, 15 are realized separately, a design as a planar antenna, in particular as a patch antenna or fractal antenna, is however advantageous.

(14) In order to determine its frequency f.sub.peak (or, if the radar transmission signal is possibly also reflected at disruptive bodies, a plurality of frequencies f.sub.peak), the intermediate frequency signal s.sub.ZF1, s.sub.ZF2 will usually be subjected by a digitizing unit 18 to a (fast) Fourier transform and thus transferred into easily evaluable (reference) measurement signals s.sub.ref1, s.sub.ref2. At the same time an A/D conversion may also be carried out. The frequency spectra hereby resulting are shown schematically in FIG. 3a:

(15) The frequency spectra in each case represent the signal strength or the amplitude A of a corresponding (reference) measurement signal s.sub.ref1, s.sub.ref2 as a function of the frequency f. The two frequency spectra shown in FIG. 3a result from two reference measurements carried out one after the other under at least one and the same reference condition, for example, a precisely known minimum fill level L.sub.min in the container 2. Depending on the respective reference condition, different characteristic parameters that are contained in all two frequency spectra can be determined on the basis of the two spectra of the corresponding reference measurement signals s.sub.ref1 s.sub.ref2. As a characteristic value, for example, a signal maximum s.sub.peak or its amplitude A.sub.peak and/or its frequency f.sub.peak can be determined. The occurrence of the respective signal maximum s.sub.peak is dependent on the individual reference conditions and thus could, for example, result from the known minimum level L.sub.min of the reference measurement.

(16) It can be seen from the comparison of the two reference measurement signals s.sub.ref1, s.sub.ref2 in FIG. 3a that the characteristic values, i.e. the frequency f.sub.peak or the amplitude A.sub.peak of the signal maximum s.sub.peak does not necessarily remain constant over the period of time between the two reference measurements. For example, a frequency change Δf.sub.peak or an amplitude change ΔA.sub.peak of the signal maximum s.sub.peak may occur instead.

(17) One reason for an attenuation ΔA.sub.peak of the amplitude A.sub.peak over the time interval between the two reference measurements could be, for example, a gradual formation of a crust on the transmitting antenna 14 and/or the receiving antenna 15 due to dusty filling material 3. A frequency change Δf.sub.peak, on the other hand, could be attributed to an internal source of error of the fill level measuring device 1, for example, a detuning of the mixer 17.

(18) By carrying out a reference measurement at least twice at a temporally appropriate interval, according to the invention, therefore, not only the at least one characteristic parameter (e.g. the frequency f.sub.peak or the amplitude A.sub.peak of the signal maximum speak) itself but also its (their) change(s) ΔA.sub.peak,Δf.sub.peak over the time interval between the reference measurements are detected.

(19) The core of the invention is that the change in characteristic value, for example, that of the amplitude ΔA.sub.peak, is compared with at least one predefined maximum change in characteristic value ΔA.sub.peak,max, Δf.sub.peak,max, which is assigned to the respective characteristic value. In this case, the maximum change in characteristic value ΔA.sub.peak,max, Δf.sub.peak,max represents a threshold value, after which a reliable level measurement is no longer possible and thus an error state of the fill level measuring device 1 has occurred.

(20) In the case of a maximum amplitude change ΔA.sub.peak,max, this could be that amplitude value above which the amplitude A.sub.peak of the signal maximum s.sub.peak in a (reference) measurement signal has dropped down to a minimum amplitude A.sub.peak,min, from which the signal maximum s.sub.peak can no longer be unequivocally recognized on account of the signal-to-noise ratio. If, however, a maximum change in frequency Δf.sub.peak,max is defined as the maximum change in characteristic value, this could be a maximum permitted change in frequency, up to which a defined minimum resolution of fill level measurement is guaranteed and accordingly no error state yet triggered.

(21) If, on the other hand, the maximum change in characteristic value ΔA.sub.peak,max, Δf.sub.peak,max is exceeded, this will be detected by a corresponding evaluation unit 19 (see FIG. 2) and, if appropriate, forwarded to the superordinate unit 4.

(22) That, within the meaning of the invention, not only the intermediate frequency signals s.sub.ZF1 s.sub.ZF2, transformed into frequency spectra can be used for the determination of the characteristic parameter A.sub.peak, f.sub.peak in reference measurement but also “raw” intermediate frequency signals s.sub.ZF1 s.sub.ZF2 themselves, can be seen from FIG. 3b. It also appears that the characteristic parameter here can be an envelope A.sub.Hüll (i.e. the amplitude characteristic), a low-frequency interference f.sub.mean or a phase ϕ of the intermediate frequency signal s.sub.ZF1, s.sub.ZF2 (in relation to the high-frequency signal s.sub.HF). These can change their values due to aging of the components. Thus, for example, aging affects the attenuation at different frequencies and thus changes the envelope A.sub.Hüll.

(23) FIG. 4 illustrates a development of the method according to the invention. This development is in other words based on the idea of approximating a remaining time period Δt.sub.r by determining the change in at least one specific characteristic parameter via at least two or more reference measurements, up to the duration at which the respective maximum change in characteristic value is likely to be exceeded and the error state of the fill level measuring device 1 will thus occur. The precondition for this is that the change in the corresponding characteristic parameter at the time of currently the last reference measurement in each case has not yet exceeded the maximum change in characteristic value.

(24) In FIG. 4 this development is illustrated by way of example on the basis of the amplitude change ΔA.sub.peak of the signal maximum s.sub.peak. On the basis of the amplitude change ΔA.sub.peak, which was detected over the period between at least two reference measurements on the basis of the corresponding reference measurement signals s.sub.ref1, s.sub.ref2, . . . , s.sub.refn, a change function dA.sub.peak/dt is created. For this purpose, a regression of the amplitude change ΔA.sub.peak can be carried out. In the exemplary embodiment shown, a linear regression is used for this, since here the amplitude decrease ΔA.sub.peak is approximately constant overtime. An amplitude A.sub.peak that is constantly decreasing over time can be caused, for example, by a continuous increase in the crusting on the transmitting antenna 14 and/or the receiving antenna 15.

(25) In general, however, the choice of a suitable regression type (i.e. exponential, logarithmic, etc. as well) within the meaning of the invention is not limited to linear regression, but is rather made to depend on the individual course of the change in a particular characteristic parameter (to find a suitable regression type and/or to perform the actual regression, for example, the least square method could be used).

(26) Following the creation of the change function dA.sub.peak/dt, the expected remaining operating duration Δt.sub.r is thereby approximated (on the basis of the amplitude A.sub.peak at the time of the last reference measurement) until the amplitude change ΔA.sub.peak becomes so great that the amplitude A.sub.peak will have fallen below the minimum amplitude A.sub.peak,min. By means of this development of the invention, therefore, an error state according to the principle of “predictive maintenance” can already be detected in advance.