Method for checking the functional ability of a radar-based fill-level measuring device

Abstract

A method for checking the functional ability of an FMCW-based fill-level measuring device, which serves for measuring the fill level of a fill substance located in a container, as well as to a fill-level measuring device suitable for performing this method. For checking the functional ability, a microwave signal is produced, whose frequency change differs from the frequency change of the measurement signal used during regular measurement operation. By comparing the frequency of the difference signal resulting from the microwave signal with a predetermined reference frequency, it is ascertained, whether the fill-level measuring device is functionally able. Thus, the fill-level measuring device detects, independently, whether it is functionally able, or whether an error is present, caused principally by device-internal disturbance signals. This offers, especially, a clear advantage as regards meeting safety standards for the field device.

Claims

1. A method for checking the functional ability of a FMCW-based fill-level measuring device, which serves for measuring fill level of a fill substance located in a container, wherein the method comprises the steps as follows during measurement operation: 1) producing a first microwave signal by means of a first periodic electrical signal, wherein the first electrical signal has an approximately constant first frequency change solely across an entirety of a fixed frequency band, wherein the fixed frequency band is symmetrically positioned around a fundamental frequency; transmitting said first microwave signal in the direction of the surface of the fill substance; producing a first echo signal by reflection of said first microwave signal said first echo signal being received and converted into a first electrical received signal; producing a first difference signal by mixing said first received signal with said first electrical signal; ascertaining the frequency of said first difference signal; determining the fill level based on the frequency of said first difference signal, for checking the functional ability; and 2) producing a second microwave signal by means of a second periodic electrical signal, wherein said second electrical signal has, in the fixed frequency band around the fundamental frequency, a second approximately constant frequency change across the entirety of the fixed frequency band, which differs from the first frequency change, transmitting said second microwave signal in the direction of the surface of the fill substance; receiving a second echo signal, which is produced by reflection of said second microwave signal and converted it into a second electrical received signal; producing a second difference signal by mixing said second received signal with said second electrical signal; ascertaining a frequency of said second difference signal; checking whether the frequency of said second difference signal agrees with a reference frequency, wherein the reference frequency has a predetermined value in comparison with the frequency of said first difference signal; and for the case, in which the frequency of said second difference signal does not agree with said predetermined frequency, the measuring device is classified as functionally incapacitated.

2. The method as claimed in claim 1, wherein: said checking of the functional ability is performed in predefined intervals during measurement operation.

3. The method as claimed in claim 1, wherein: the first difference signal and/or the second difference signal are/is digitized by an analog/digital converter.

4. The method as claimed in claim 1, wherein: the frequency change of the first electrical signal corresponds to an approximate integer multiple of the second frequency change.

5. The method as claimed in claim 1, wherein: the frequency of the first difference signal is ascertained by a Fourier transformation of the first difference signal.

6. The method as claimed in claim 1, wherein: the frequency of the second difference signal is ascertained by a Fourier transformation of the second difference signal.

7. The method as claimed in claim 1, wherein: the frequency changes are produced by sawtooth-shaped excitation of the electrical signals.

8. The method as claimed in claim 1, wherein: the frequency changes are produced by triangular excitation of the electrical signals.

9. A fill-level measuring device adapted to perform a method for checking the functional ability of a FMCW-based fill-level measuring device, which serves for measuring fill level of a fill substance located in a container, wherein the method comprises steps of: 1) producing a first microwave signal by means of a first periodic electrical signal, wherein the first electrical signal has an approximately constant first frequency change solely across an entirety of a fixed frequency band, wherein the fixed frequency band is symmetrically positioned around a fundamental frequency; transmitting said first microwave signal in the direction of the surface of the fill substance; producing a first echo signal by reflection of said first microwave signal said first echo signal being received and converted into a first electrical received signal; producing a first difference signal by mixing said first received signal with said first electrical signal; ascertaining the frequency of said first difference signal; determining the fill level based on the frequency of said first difference signal, for checking the functional ability; and 2) producing a second microwave signal by means of a second periodic electrical signal, wherein said second electrical signal has, in the fixed frequency band around the fundamental frequency, a second approximately constant frequency change across the entirety of the fixed frequency band, which differs from the first frequency change, transmitting said second microwave signal in the direction of the surface of the fill substance; receiving a second echo signal, which is produced by reflection of said second microwave signal and converted it into a second electrical received signal; producing a second difference signal by mixing said second received signal with said second electrical signal; ascertaining a frequency of said second difference signal; checking whether the frequency of said second difference signal agrees with a reference frequency, wherein the reference frequency has a predetermined value in comparison with the frequency of said first difference signal; and for the case, in which the frequency of said second difference signal does not agree with said predetermined frequency, the measuring device is classified as functionally incapacitated, wherein the fill-level measuring device comprises: a signal producing unit for producing said electrical signals, an antenna unit for transmitting said microwave signals and/or for receiving said echo signals; a mixer for mixing said electrical signals; and an evaluating unit for determining the fill level and/or for checking the functional ability of the fill-level measuring device.

10. The fill-level measuring device as claimed in claim 9, wherein: said evaluating unit includes a bandpass filter, which is transmissive for the frequencies of the difference signals.

11. The fill-level measuring device as claimed in claim 9, wherein: said evaluating unit includes an amplifier for amplifying the difference signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be explained based on the appended drawing, the figures of which show as follows:

(2) FIG. 1 is a fill-level measuring device for performing the method of the invention;

(3) FIG. 2a is a sawtooth-shaped excitation of the electrical signals s.sub.1 and s.sub.2;

(4) FIG. 2b is a triangular excitation of the electrical signals s.sub.1 and s.sub.2;

(5) FIG. 3a is a frequency spectrum of the difference signals IF.sub.1, and IF.sub.2, in the case of which the frequency f.sub.2 of the second difference signal IF.sub.2 agrees with the reference frequency f.sub.p; and

(6) FIG. 3b is a frequency spectrum of the difference signals IF.sub.1, and IF.sub.2, in the case of which the frequency f.sub.2 of the second difference signal IF.sub.2 does not agree with the reference frequency f.sub.p.

DETAILED DISCUSSION IN CONJUNCTION WITH THE DRAWINGS

(7) FIG. 1 shows a fill-level measuring device, which is suitable for embodiment of the method of the invention and accordingly is able to check the functional ability of the fill-level measuring device. The illustrated fill-level measuring device is based on a construction usual for FMCW-based fill level measuring devices. Responsible for producing the microwave signals S.sub.1 and S.sub.2 is a signal producing unit 3. As typical for FMCW, signal producing unit 3 produces electrical signals s.sub.1 and s.sub.2, which lie in the region of a fundamental frequency f.sub.0 in the GHz-region and have constant frequency changes f′.sub.1, f′.sub.2. In measurement operation, the signal producing unit 3 produces exclusively a first electrical signal s.sub.1 with a first constant frequency change f′.sub.1. For checking the functional ability of the fill-level measuring device, instead of the first signal s.sub.1, a second electrical signal s.sub.2 is produced, which has a second frequency change f′.sub.2 deviating from the first frequency change f′.sub.1. In such case, it depends on the control of the signal producing unit 3, in which intervals the checking is performed.

(8) The signal producing unit 3 can be, for example, a voltage controlled oscillator, which includes a suitable quartz crystal. According to the invention, the signal producing unit 3 produces signals s.sub.1 and s.sub.2 with differing frequency changes f′.sub.1, f′.sub.2. Possible characteristic forms of excitation, with which the signal producing unit 3 produces the electrical signals s.sub.1 and s.sub.2, are shown in FIG. 2a and FIG. 2b.

(9) FIG. 2a shows a sawtooth-shaped excitation of the electrical signals s.sub.1 and s.sub.2. As shown, the sawtooth-shaped excitation of the frequency changes f′.sub.1 and f′.sub.2 is, on the one hand, linear, and, on the other hand, has a frequency increasing with time. A sawtooth-shaped excitation with frequency decreasing with time is, however, also implementable for the invention. The periodicities of the electrical signals s.sub.1 and s.sub.2 deviate slightly from one another in FIG. 2a. An agreement of the periodicities is not necessary according to the invention. Advantageously, however, the periodicities at least lie in the same order of magnitude.

(10) A possible alternative excitation form for the electrical signals s.sub.1 and s.sub.2 is shown in FIG. 2b. Here, the excitation occurs with triangular frequency changes f′.sub.1 and f′.sub.2. In such case, the magnitude |f′.sub.1| or |f′.sub.2| of the frequency change f′.sub.1 or f′.sub.2 in the part of the period, in which the frequency increases, and in the part of the period, in which the frequency decreases, are equal. Also, the triangular excitations of the electrical signals s.sub.1 and s.sub.2 shown in FIG. 2b do not have exactly equal periodicity. As in the case of the sawtooth shaped excitation, it is, however advantageous, also in the case of triangular excitation that the periodicities of the electrical signals s.sub.1 and s.sub.2 lie in the same order of magnitude or are even equal.

(11) In the fill-level measuring device shown in FIG. 1, microwave signals S.sub.1 and S.sub.2 are produced in an antenna unit 4 by means of the electrical signals s.sub.1 and s.sub.2, wherein the microwave signals S.sub.1 and S.sub.2 are transmitted in the direction of a fill substance 2 located in a container 1. Corresponding to the electrical signals s.sub.1 and s.sub.2, also the microwave signals S.sub.1 and S.sub.2 have the frequency characteristics shown in FIG. 2a or FIG. 2b. FIG. 1 shows that the antenna unit 4, besides producing the microwave signals S.sub.1 and S.sub.2, also receives echo signals E.sub.1 and E.sub.2, which arise from reflection of the microwave signals S.sub.1 and S.sub.2 on the surface of the fill substance 2. Alternatively to the illustration, there could according to the invention also be a separate receiving antenna present.

(12) The echo signals E.sub.1 and E.sub.2 are converted by the antenna unit 4 into electrical received signals e.sub.1 and e.sub.2. In measurement operation, then the received signal e.sub.1 is mixed in a mixer 5 with the transmitted signal s.sub.1. Likewise, in the case of checking the functional ability, the received signal e.sub.2 is mixed with the transmitted signal s.sub.2.

(13) Difference signals IF.sub.1 and IF.sub.2, arise from the mixing of the received signal e.sub.1 or e.sub.2 with the transmitted signal s.sub.1 or s.sub.2, wherein the particular characteristic frequencies f.sub.1, f.sub.2 of the difference signals IF.sub.1 and IF.sub.2 result from the frequency difference between the instantaneous frequency of the transmitted signal s.sub.1, or s.sub.2, and the instantaneous frequency of the received signal e.sub.1, or e.sub.2.

(14) For ascertaining the frequencies f.sub.1, f.sub.2 of the difference signals IF.sub.1 and IF.sub.2, the fill-level measuring device includes an evaluating unit 6. The ascertaining is performed, in such case, by fast Fourier transformation by a computing unit provided for such. As usual in the processing of such data, this happens based on digital data. Therefore, in the case of the evaluating unit 6 illustrated in FIG. 1, the fast Fourier transformation is placed after an analog/digital converter. Likewise, as usual in the case of FMCW-based fill-level measuring devices, the evaluating unit 6 includes a signal amplifier as well as a bandpass filter, wherein the bandpass filter is transmissive for the frequencies f.sub.1 and f.sub.2 of the difference signals IF.sub.1 and IF.sub.2. In this way, the frequencies f.sub.1, f.sub.2 stand out in the frequency spectrum obtained from the fast Fourier transformation as much as possible. From the frequency spectra obtained for the difference signals IF.sub.1 and IF.sub.2, it can be derived, whether the fill-level measuring device is functionally able.

(15) Two schematic frequency spectra, which result after performance of fast Fourier transformation, are shown in FIGS. 3a and 3b, wherein the fill-level measuring device is functionally incapacitated in the case of the frequency spectra illustrated in FIG. 3b.

(16) In both figures (FIGS. 3a and 3b), the frequency spectra contain both a spectrum of the difference signal IF.sub.1 obtained in the case of measurement operation, as well as also a spectrum of the difference signal IF.sub.2 obtained while checking the functional ability. Each of the difference signals IF.sub.1, IF.sub.2 includes a characterizing maximum associated with the relevant difference frequency f.sub.1, f.sub.2. In the frequency spectra illustrated in FIG. 3a, the frequency f.sub.2 of the difference signal IF.sub.2 agrees with the reference frequency f.sub.p, while this is not the case in FIG. 3b. From this, it is derived according to the invention that the fill-level measuring device is functionally incapacitated.

(17) The determining of f.sub.1 and f.sub.2 based on the frequency spectra and the checking, whether the frequency f.sub.2 agrees with the reference frequency f.sub.p, are performed in the case of the fill-level measuring device shown in FIG. 1 by a microcontroller μC, wherein the microcontroller μC is a component of the evaluating unit 6. Thus, the fill-level measuring device can check for functional ability. As a result of this, it can be displayed, for example, on a display of the fill-level measuring device, when the device is functionally incapacitated, or this information can be transmitted to a superordinated unit, so that safety-critical fill levels in the container 1 caused by defective fill level measured values are avoided.