METHOD FOR FMCW-BASED DISTANCE MEASUREMENT

Abstract

Disclosed is a method and a corresponding distance-measuring device for measuring a distance to an object using FMCW radar. The method includes the frequency-dependent determination of the amplitude of the radar signal, i.e. the frequency response in the output path and in the input path of the distance-measuring device. The standard windowing of the evaluation signal can be corrected using a correction factor dependent on the frequency responses. Thus the frequency dependence of the radar signal is compensated independently of device-internal or external interferences by adapting the window function. The result is more accurate and reliable distance measurement using FMCW radar. Because the distance can be determined by the disclosed method very accurately and without distortion, it is advantageous to use the distance-measuring device as a fill-level measuring device to measure the fill level of a filling material in a container.

Claims

1-8. (canceled)

9. A method for FMCW radar-based measurement of a distance to an object, the method comprising: generating a radio-frequency electrical signal frequency-modulated according to the FMCW principle; emitting the radio-frequency signal as radar signal in the direction of the object; receiving a reflected radar signal as an electrical received signal after reflection on the object; generating an evaluation signal by mixing the received signal with the radio-frequency signal; weighting frequencies of the evaluation signal by means of a defined window function; determining the distance from the weighted evaluation signal; determining a frequency dependence of an amplitude of the radio-frequency signal, of an amplitude of the received signal, of an amplitude of the evaluation signal, and of an amplitude of the radio-frequency signal in superposition with the received signal; and correcting the window function on the basis of the frequency-dependent amplitude of the radio-frequency electrical signal, the received signal, the evaluation signal and/or the amplitude of the radio-frequency signal in superposition with the received signal.

10. The method according to claim 9, wherein the frequency dependence of the amplitude of the received signal, of the radio-frequency signal, of the evaluation signal, and/or of the radio-frequency signal is/are measured in superposition with the received signal by generating the radio-frequency electrical signal with a constant frequency for one predefined timespan in stages within the FMCW frequency band.

11. The method according to claim 9, wherein the window function is corrected by means of one of the following correction functions: $K\left(f\right)={\left(1-\frac{A_{\mathrm{MP}2}\left(f\right)}{A_{\mathrm{MP}1}\left(f\right)}\right)}^{-1}$ $K\left(f\right)={\left(1-\frac{A_{\mathrm{MP}2}\left(f\right)}{A_{\mathrm{MP}3}\left(f\right)}\right)}^{-1}$ $K\left(f\right)={\left(1-\frac{A_{\mathrm{MP}4}\left(f\right)}{A_{\mathrm{MP}1}\left(f\right)}\right)}^{-1}$ $\mathrm{and}$ $K\left(f\right)={\left(1-\frac{A_{\mathrm{MP}4}\left(f\right)}{A_{\mathrm{MP}3}\left(f\right)}\right)}^{-1}$

12. The method according to claim 9, wherein the distance is determined by means of a Fast Fourier transform of the weighted evaluation signal.

13. The method according to claim 9, wherein a Hamming function, a Taylor function, or a Chebyshev function is used as the window function.

14. An FMCW radar-based distance-measuring device for measuring a distance to an object, comprising: a radio-frequency generation unit designed to generate a radio-frequency electrical signal frequency-modulated on the basis of the FMCW principle; a transmitting/receiving antenna which serves for emitting the radio-frequency signal as a radar signal and which serves for receiving a radar signal reflected on the object as an electrical received signal; a mixer by means of which the radio-frequency electrical signal can be mixed with the received signal such that an evaluation signal is generated; a weighting unit designed to weight the evaluation signal with respect to its frequencies by means of a defined window function; and an evaluation unit which is designed to determine the distance from the weighted evaluation signal, the evaluation unit including: a measuring unit configured to determine an amplitude of the radio-frequency signal, an amplitude of the received signal, an amplitude of the evaluation signal, and/or an amplitude of the radio-frequency signal in superposition with the received signal as a function of frequency, wherein the evaluation unit is configured to correct the window function on the basis of the frequency-dependent amplitude of the radio-frequency electrical signal, of the received signal, of the evaluation signal, and/or of the amplitude of the radio-frequency signal in superposition with the received signal.

15. The distance-measuring device according to claim 14, wherein the measuring unit used to determine the frequency-dependent amplitude includes: a second signal divider which is used for branching off the radio-frequency signal of the received signal or of the evaluation signal; a detector arranged downstream of the second signal splitter; a low-pass filter connected downstream of the detector; and second analog-to-digital converter arranged downstream of said low-pass filter.

16. A use of the FMCW radar-based distance-measuring device as claimed in claim 14, wherein the use includes measuring a fill level of a filling material in a container.

Description

[0049] The invention is explained in more detail with reference to the following figures. The following is shown:

[0050] FIG. 1: A typical arrangement of a fill-level measuring device on a container,

[0051] FIG. 2: a basic circuit configuration of an FMCW-based distance-measuring device,

[0052] FIG. 3: diagrams for clarification of the signal processing in the distance-measuring device,

[0053] FIG. 4: a circuit configuration of the distance-measuring device according to the invention,

[0054] FIG. 5: a measuring unit for determining the frequency-dependent amplitude, and

[0055] FIG. 6: a possible modulation of the frequency of the radio-frequency signal.

[0056] For a basic understanding of the invention, FIG. 1 shows a typical arrangement of a freely radiating, radar-based fill level measurement device 1 on a container 2. In the container 2 is a filling material 3, whose fill level L is to be determined by the fill level measurement device 1. For this purpose, the fill level measurement device 1 is mounted on the container 2 above the maximum permissible fill level L. Depending on the field of application, the height h of the container 2 can be up to 125 m.

[0057] As a rule, the fill 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 fill level measurement device 1 can thus be communicated. On the other hand, information about the fill 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.

[0058] Since the fill-level measurement device 1 shown in FIG. 1 is designed as freely radiating radar measuring device, it comprises a corresponding transmitting/receiving antenna 14. As indicated, the transmitting/receiving antenna 14 can be designed as a horn antenna, for example. Regardless of the design, the transmitting/receiving antenna 14 is oriented in such a way that a corresponding radar signal S.sub.HF is emitted in the direction of the filling material 3 according to the FMCW principle.

[0059] The radar signal E.sub.HF is reflected at the surface of the filler 3 and, after a corresponding signal time-of-flight, is correspondingly received as an electrical received signal e.sub.HF at the transmitting/receiving antenna 14. The signal time-of-flight of the radar signal S.sub.HF, E.sub.HF depends on the distance d=h−L of the fill-level measuring device 1 from the filling material surface.

[0060] In contrast to the shown embodiment variant, and electrically conductive probe, such as a hollow conductor or coaxial cable which extends toward the base of the container, can be used in place of the transmitting/receiving antenna 14. This embodiment variant is known as TDR (“Time Domain Reflectometry”).

[0061] The basic circuit design of a fill-level measuring device 1 operating according to the FMCW method is illustrated in FIG. 2: The core of the structure is a radio-frequency signal generation unit 11, 12 for generating a radio-frequency electric signal s.sub.HF. The frequency of the radio-frequency signal s.sub.HF defines the frequency of the radar signal S.sub.HF lying in the microwave range. Therefore, the radio-frequency signal-generating unit 11, 12 must be designed to generate the radio-frequency electric signal s.sub.HF having the ramp-shaped frequency change required in FMCW.

[0062] In the case of a ramp-shaped frequency change in accordance with the FMCW principle, the frequency f periodically increases repetitively within a predefined frequency band Δf with a constant rate of change (cf. also FIG. 6). The periodicity of the individual frequency ramps may thereby be within a range of a few 100 ms. The duration of the individual ramp may be in a range between 100 μs and 100 ms. The position of the frequency band Δf is to be set taking into account regulatory requirements, for which reason the ISM bands at 6 GHz, 26 GHz or 80 GHz are preferably implemented as frequency band Δf. The bandwidth lies in particular between 0.5 GHz and 10 GHz, depending on the position of the frequency band Δf.

[0063] In order to generate the radio-frequency signal s.sub.HF, the radio-frequency generation unit 11, 12 shown in FIG. 2 comprises a radio-frequency oscillator 12 which is controlled by means of a ramp-generating unit 11. The regulation takes place in the form of a phase control (known as “phase locked loop, PLL”). Thus, the frequency f of the radio-frequency oscillator 12 is stabilized on the one hand against fluctuations in the ambient temperature. On the other hand, the ramp-shaped frequency change of the radio-frequency signal s.sub.HF is set.

[0064] For emitting the radio-frequency signal s.sub.HF, the radio-frequency electrical signal s.sub.HF according to FIG. 2 is supplied to the antenna 14 via a first signal splitter 19 (and possibly a connected output amplifier, not shown) and a transmitting/receiving switch 13. The radio-frequency signal s.sub.HF is thus emitted as radar signal S.sub.HF in the direction of the filling material 3. In this case, the antenna 14 can be realized, for example, as a horn or planar antenna (for example as a patch antenna or a fractal antenna).

[0065] A correspondingly reflected radar signal E.sub.HF is received at the antenna 14 by the reflection of the radar signal S.sub.HF on the object, the distance d of which is to be determined (with fill-level measurement, the surface of the filling material 3).

[0066] After reflection, the radar signal E.sub.HF in the transmitting/receiving antenna 14 is converted back into a purely electrical received signal e.sub.HF (which in turn may optionally be amplified by a receiving amplifier). The received signal e.sub.HF is subsequently mixed by means of a mixer 15 with the radio-frequency signal s.sub.HF of the radio-frequency signal-generation unit 11, 12, wherein the radio-frequency signal s.sub.HF is branched off from the first signal divider 19 for this purpose. In this way, an evaluation signal ZF typical of the FMCW method is generated, which forms the basis for determining the distance d. According to the FMCW principle, the frequency of the evaluation signal ZF is thereby proportional to the distanced of the object, such that a suitable evaluation unit 18 may determine the distance d via a measurement of the frequency f of the evaluation signal ZF. To determine the frequency of the evaluation signal ZF, this may be subjected to a (Fast) Fourier transform, for example, FFT for short.

[0067] The time curve of the evaluation signal ZF is shown in FIG. 3. In the ideal case, therefore, when radar signal S.sub.HF is reflected on the planar surface of the filling material 3, the evaluation signal ZF has only a discrete frequency f. After the Fourier transform by the evaluation unit 18, this frequency f in the evaluation curve FFT, FFT′ corresponds to the (single) maximum which corresponds to the distance d to the filling material surface. The resulting evaluation curve FFT, FFT′ reflects the amplitude A as a function of the frequency f or as a function of the distance d. The evaluation unit 18 determines the fill level L from the evaluation curve FFT, FFT′ accordingly by detecting and locally assigning this global maximum which is caused by the filling material surface.

[0068] As can be seen from FIG. 3, however, in addition to this maximum in the evaluation signal ZF there are still further frequency components in practice. In addition, the Fourier transform causes parasitic secondary maxima. A separation of such secondary maxima can be suppressed by a so-called windowing of the evaluation signal ZF. The frequency spectrum of the evaluation signal ZF* is weighted, i.e. multiplied, with a corresponding window function F(f). The window function F(f) shown in FIG. 3 is a Taylor function. In order to achieve this function, the middle frequencies f are weighted more heavily than the frequencies at the edge regions of the measuring range h. However, depending on the general reflection ratios, any other function, such as a Hamming function or a Chebyshev function, can also be implemented as a window function F(f) in the weighting unit 17. The comparison between the evaluation curve FFT′ without windowing and the evaluation curve FFT after the windowing of the evaluation signal ZF* makes it clear from FIG. 3 that a large part of the secondary maxima can thereby be masked out, so that these can no longer be incorrectly used for determining the fill level L. In the circuit shown in FIG. 2, a weighting unit 17 is correspondingly connected upstream of the evaluation unit 18. Since the weighting takes place on a digital basis, an analog/digital converter 16 is in turn connected upstream of the weighting unit 17.

[0069] In addition to the strict linearity of the frequency ramp, the error-free determination of the distance d or of the fill level L based on the evaluation signal ZF* also presupposes that the further components 13, 14, 15, 16 of the fill-level measuring device 1 have no parasitic frequency dependence. However, this can occur above all with progressive operation of the fill-level measuring device 1. As a result, this in turn makes it possible to produce secondary maxima which cannot be masked out by means of the windowing described above.

[0070] In order to overcome this, the idea according to the invention is to determine the amplitude response A.sub.MP1,2,3,4(f) of the radio-frequency signal s.sub.HF, the reception signal e.sub.HF and/or the evaluation signal ZF and to take this into account by means of a corresponding correction function K(f)) in the window function F(f). In this case, the term “amplitude response” in the context of the invention is defined as a function of the amplitude A.sub.MP1,2,3,4(f) of the respective signal s.sub.HF, e.sub.HF, ZF as a function of the frequency f. To implement this idea, the circuit shown in FIG. 2 can be expanded in the way illustrated in FIG. 4:

[0071] Individual measuring points MP1,2,3,4 can be defined at different points of the circuit in order to determine the amplitude response A.sub.MP1,2,3,4(f) of the signals s.sub.HF, e.sub.HF, ZF. The determination of the amplitude response A.sub.MP3(f) of the radio-frequency signal s.sub.HF is possible at a measuring point MP3 between the radio-frequency oscillator 12 and the transmitting/receiving switch 13. The amplitude response A.sub.MP2(f) of the received signal e.sub.HF can in turn be determined at a measuring point MP2 between the transmitting/receiving switch 13 and the mixer 15. The amplitude response A.sub.MP4(f) of the evaluation signal ZF can be detected at a corresponding measuring point MP4 between the mixer 15 and the weighting unit 17. Since both the radio-frequency signal s.sub.HF and the incoming received signal e.sub.HF are guided between the transmitting/receiving switch 13 and the transmitting/receiving antenna 14, the amplitude response A.sub.MP1(f) of the received signal (e.sub.HF) superimposed by the radio-frequency signal (s.sub.HF) can also be tapped off at this measuring point MP1.

[0072] At the individual measuring points MP1, 2, 3, 4 the respective amplitude response A.sub.MP1,2,3,4(f) can be recorded by means of a measuring unit shown in FIG. 5. For this purpose, a second signal divider 181 is arranged in the signal path at the respective measuring point MP1, 2, 3, 4 from which the respective analog signal s.sub.HF, e.sub.HF, ZF is branched off into a detector 182. The detector 182 provides an electrical signal whose amplitude is proportional to the power at the respective measuring point MP1, 2, 3, 4. Accordingly, a diode, for example, can be used as detector 182. Thus, a voltage proportional to the power or amplitude A.sub.MP1,2,3,4(f) is generated. The amplitude A.sub.MP1,2,3,4(f) is thus present in digital form by analog/digital conversion of this voltage by means of a second analog/digital converter 184.

[0073] In the embodiment shown in FIG. 4, the respective digitized amplitude A.sub.MP1,2,3,4(f) is recorded by the evaluation unit 18. It goes without saying that the respective signal s.sub.HF, e.sub.HF, ZF can also be supplied to the evaluation unit 18 in analog form, provided that the evaluation unit 18 comprises corresponding inputs for ascertaining the analog value.

[0074] In order that the amplitude response A.sub.MP1,2,3,4(f) can be recorded by the evaluation unit 18 over the complete frequency band, the evaluation unit 18 must control the ramp generation unit 11 or the frequency of the radio-frequency signal s.sub.HF accordingly.

[0075] In order to determine the amplitude response A.sub.MP1,2,3,4(f), it is appropriate for the respective amplitude A.sub.MP1,2,3,4 to be recorded within the FMCW frequency band Δf at several discrete frequency interpolation points, so that the amplitude response A.sub.MP1,2,3,4(f) can be obtained as a rising function by means of suitable regression from the individual amplitude values A.sub.MP1,2,3,4 at the respective frequency interpolation point. For finding a suitable regression type (e.g. linear, quadratic or polynomial), the evaluation unit 18 can use, for example, the least squares method (known in the art as “least square fit”).

[0076] A possible activation of the ramp generation unit 11 by the evaluation unit 11 in which the frequency response of the radio-frequency signal s.sub.HF has four discrete frequency interpolation points, is illustrated in FIG. 6: The four frequency interpolation points are implemented in this representation in each case after the end of a frequency ramp required for the FMCW method. In this case, the frequency interpolation points are equally distributed with respect to their respective frequency f rising above the frequency band Δf. The interval length Δf of the individual frequency reference points is to be dimensioned at least as long as the evaluation unit 18 requires for determining the respective amplitude A.sub.MP1,2,3,4 with the desired resolution or bit length. In this type of generation of the amplitude response A.sub.MP1,2,3,4(f), the evaluation unit 18 should be designed such that it records the respective amplitude A.sub.MP1,2,3,4 during the time interval Δt of the corresponding frequency reference point.

[0077] It goes without saying that the design of the frequency response of the radio-frequency signal s.sub.HF shown in FIG. 6 is for illustrative purposes only. In order to achieve a more precise mapping of the individual amplitude response A.sub.MP1,2,3,4(f), the number of frequency interpolation points can of course be arbitrarily increased. It is also not fixed in accordance with the invention to implement a frequency interpolation point after each individual frequency ramp. For example, a single frequency interpolation point can also be implemented only after every tenth frequency ramp.

[0078] The evaluation unit 18 can generate the correction function K(f) on the basis of the detected amplitude transitions A.sub.MP1,2,3,4(f). In this case, the secondary maxima which are caused by any parasitic frequency dependence of the electrical components 13, 14, 15 are optimally suppressed when the correction function K(f) sets the amplitude response A.sub.MP2,4(f) in the reception path (i.e., e.sub.HF or ZF) in relation to the overall generated frequency-dependent transmission power.

[0079] The correction function K(f) is then obtained on the basis of

[0080] or

[00002]$K\left(f\right)={\left(1-\frac{A_{\mathrm{MP}4}\left(f\right)}{A_{\mathrm{MP}1}\left(f\right)}\right)}^{-1}$

[0081] or is corrected on the basis of

[00003]$K\left(f\right)={\left(1-\frac{A_{\mathrm{MP}4}\left(f\right)}{A_{\mathrm{MP}3}\left(f\right)}\right)}^{-1}$

[0082] Instead of normalizing to the pure transmit signal s.sub.HF, the normalization can also be effected to the transmit signal s.sub.HF superimposed by the receive signal e.sub.HF at the measuring point MP1 between the transmitting/receiving switch 13 and the transmitting/receiving antenna 14. In this case, the correction function K(f) yields:

[00004]$K\left(f\right)={\left(1-\frac{A_{\mathrm{MP}4}\left(f\right)}{A_{\mathrm{MP}1}\left(f\right)}\right)}^{-1}$$\mathrm{or}$$K\left(f\right)={\left(1-\frac{A_{\mathrm{MP}2}\left(f\right)}{A_{\mathrm{MP}1}\left(f\right)}\right)}^{-1}$

[0083] The correction function K(f) obtained on the basis of one of these formulas is transmitted to the weighting unit 17 by the evaluation unit 18 according to the invention. The weighting unit 17 can in turn adjust the window function F(f) by multiplying with the correction function K(f). Thus, in addition to the windowing, aging-related deviations, mismatches, parasitic frequency responses of the components 11, 12, 13, 15 or also deposits on the transmitting/receiving antenna 14 in the evaluation signal ZF* are compensated because the correction function K(f) adapts to the aging via the changing amplitude ratio. This correspondingly increases the accuracy and the security of the distance measurement or fill-level measurement, in particular with increasing operating time of the measuring device.

LIST OF REFERENCE SIGNS

[0084] 1 Fill level measuring device [0085] 2 Container [0086] 3 Bulk material [0087] 4 Superordinate unit [0088] 10 Radio-frequency generation unit [0089] 11 Ramp generation unit [0090] 12 Radio-frequency oscillator [0091] 13 Transmitting/receiving switch [0092] 14 Antenna [0093] 15 Mixer [0094] 16 First analog-to-digital converter [0095] 17 Weighting unit [0096] 18 Evaluation unit [0097] 19 First signal divider [0098] 181 Second signal divider [0099] 182 Detector [0100] 183 Low-pass filter [0101] 184 Second analog-to-digital converter [0102] A.sub.MP(f) Frequency response [0103] d Distance [0104] E.sub.HF Reflected radar signal [0105] e.sub.HF Received signal [0106] F(f) Window function [0107] f Frequency [0108] h Installation height [0109] K(f) Correction function [0110] L Fill level [0111] MP.sub.1-4- Measurement point [0112] S.sub.HF Radar signal [0113] S.sub.HF Electrical radio-frequency signal [0114] ZF Evaluation signal [0115] ZF* Evaluation signal after windowing [0116] Δf Frequency band [0117] Δf Time period