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