Method for FMCW-based distance measurement in waveguides

Abstract

The present disclosure relates to a method for FMCW-based measurement of a distance of an object located in a waveguide, as well as a corresponding distance measurement device that, in particular, may be used for fill-level measurement in surge pipes or bypass pipes of containers. The method is based upon the fact that the transmission signal that is typical in FMCW is not ramp-like, and thus is emitted with constant frequency modulation. Rather, according to the present disclosure, a curvature of the frequency ramp is set to be at least approximately proportional to the frequency dependency of the propagation velocity of the transmission signal in the waveguide. The distortion effect is thus compensated for in that the propagation velocity of the transmission signal in waveguides is not constant, but, rather, decreases with falling transmission frequency.

Claims

1. A method for FMCW-based measurement of a distance of an object located in a waveguide, the method comprising: emitting a transmission signal along the waveguide in the direction of the object, wherein the transmission signal is emitted within a predetermined frequency band with a temporally-defined frequency modulation; receiving the reflected signal after reflection at an object; and determining the distance using a difference frequency between the reflected signal and the transmission signal, wherein the temporal frequency modulation of the transmission signal is proportionally approximated to a hyperbolic frequency dependency of a propagation velocity of the transmission signal in the waveguide, wherein a frequency dependency of the temporal frequency modulation is linearly approximated to the hyperbolic frequency dependency of the propagation velocity such that: the frequency band is subdivided into at least two sub-bands of equal size; a center frequency of each sub-band is determined; and a respective linear frequency modulation is set within each sub-band as a sub-band temporal frequency modulation, wherein the respective linear frequency modulations are set using a proportionality factor for each sub-band, the proportionality factor being proportional to the respective propagation velocity at the corresponding center frequency.

2. The method of claim 1, further comprising determining a fill-level of a bulk material in a container based on the distance, wherein the object is the bulk material located in the container.

3. The method of claim 2, wherein the fill-level is determined in a bypass pipe or surge pipe of the container, wherein the waveguide is the bypass pipe or surge pipe.

4. A distance measurement device for FMCW-based measurement of a distance of an object located in a waveguide, the device comprising: a high-frequency signal generator configured to generate an electrical high-frequency signal that, within a predetermined frequency band, exhibits a temporally-defined frequency modulation; a signal splitter for splitting the electrical high-frequency signal; at least one transmission/reception antenna for emitting the high-frequency signal as a transmission signal and/or for receiving the reception signal; a mixer adapted to mix the electrical high-frequency signal with the obtained reception signal; and an evaluation unit configured to determine the distance using a difference frequency of an intermediate frequency signal generated by the mixer, wherein the high-frequency signal generator is configured to proportionally approximate the temporal frequency modulation of the transmission signal to a hyperbolic frequency dependency of a propagation velocity of the transmission signal in the waveguide, wherein a frequency dependency of the temporal frequency modulation is linearly approximated to the hyperbolic frequency dependency of the propagation velocity such that: the frequency band is subdivided into at least two sub-bands of equal size; a center frequency of each sub-band is determined; and a respective linear frequency modulation is set within each sub-band as a sub-band temporal frequency modulation, wherein the respective linear frequency modulations are set using a proportionality factor for each sub-band, the proportionality factor being proportional to the respective propagation velocity at the corresponding center frequency.

5. The device of claim 4, wherein the evaluation unit is further configured to determine a fill-level of a bulk material in a container based on the distance, wherein the object is the bulk material located in the container.

6. The device of claim 5, wherein the fill-level is determined in a bypass pipe or surge pipe of the container, wherein the waveguide is the bypass pipe or surge pipe.

7. The device of claim 6, further comprising a process connection between the device and the container or bypass pipe or surge pipe of the container.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present disclosure is explained in more detail with reference to the following figures. Shown are:

(2) FIG. 1 shows a fill-level measurement device at a surge pipe of a container;

(3) FIG. 2 shows a principle schematic of an FMCW-based distance measurement device;

(4) FIG. 3 shows a sawtooth-shaped frequency modulation of the FMCW method;

(5) FIG. 4 shows a block diagram of a high-frequency generation unit of the distance measurement device;

(6) FIG. 5 shows the frequency dependency of the transmission signal in the waveguides;

(7) FIG. 6 shows an approximation of the frequency modulation, according to the present disclosure, to the frequency dependency of the transmission signal; and

(8) FIG. 7 shows a comparison of the frequency spectra between constant frequency modulation and frequency modulation according to the present disclosure.

DETAILED DESCRIPTION

(9) FIG. 1 shows a conventional arrangement of a fill-level measurement device 1 operating according to the FMCW principle at a surge pipe 2, which surge pipe 2 is arranged approximately vertically in a container 5. The surge pipe 2 normally has a standardized internal diameter D—for example, DN 100 according to the EN 10255 standard. Located in the container 5 is a fluid bulk material 3 whose fill-level L is to be determined by the fill-level measurement device 1. The fill-level measurement device 1 is mounted at the surge pipe 2 at a previously known installation height h, above the bulk material 3. Via measurement of the distance d from the surface of the bulk material 3, with knowledge of the installation height h, the fill-level L=h−d may be calculated by the fill-level measurement device 1. Depending upon the application, the container 5 may be more than 30 m high.

(10) The fill-level measurement device 1 is thus mounted at the upper end of the surge pipe 2 so that, to measure the distance d, it emits a microwave-based transmission signal S.sub.HF inside the surge pipe 2 in the direction of the bulk material 3 and, after reflection at the surface of said bulk material 3, receives the correspondingly reflected signal E.sub.HF. So that the bulk material 3 inside the pipe has a fill-level L that is identical to the remainder of the container 5, the surge pipe 2 has lateral compensation openings 6 that are distributed over the length of the pipe 2.

(11) As an alternative to a mounting of the pipe 2 within the container 5, an additional variant (not shown) consists in using a bypass pipe, mounted next to the container 5, for fill-level measurement, in that the fill-level measurement device 1 is mounted at the bypass pipe in a manner analogous to the surge pipe 2. In such a case, the bypass pipe is also hydrostatically connected to container 5, so that the same fill-level L as in the container 5 prevails in the bypass pipe.

(12) The fill-level measurement device 1 is normally connected to a superordinate unit 4, e.g., a process control system, via a bus system, for instance, “PROFIBUS,” “HART,” or “WirelessHART.” For one, information about the operating state of the fill-level measurement device 1 may be communicated via said bus system. Information about the fill-level L may also be communicated, in order to control possible inflows or outflows.

(13) A schematic of a fill-level measurement device 100 operating according to the FMCW method (or of a FMCW-based distance measurement device in general) is presented in FIG. 2. The core of the present disclosure is a high-frequency signal generation unit 10 for generation of an electrical high-frequency signal s.sub.HF. The frequency of the high-frequency signal s.sub.HF defines the frequency of the transmission signal S.sub.HF, which is within the microwave range. Therefore, the high-frequency signal generation unit 10 is configured as to generate the electrical high-frequency signal s.sub.HF with the ramp-like, i.e., constant, frequency modulation df/dt that is required in FMCW.

(14) Such a ramp-like frequency modulation according to the prior art is depicted in FIG. 3. In the diagram shown in FIG. 3, the frequency of the electrical high-frequency signal s.sub.HF or of the transmission signal S.sub.HF is plotted against time t. The frequency accordingly changes periodically, repeating within a predefined frequency band f.sub.start-f.sub.stop, with a constant modulation rate df/dt=const. 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 bands (f.sub.start-f.sub.stop) be set with consideration of is to regulatory requirements, which is why the ISM bands at 6 GHz, 26 GHz, or 80 GHz are preferably implemented as a frequency band f.sub.start-f.sub.stop. Depending upon the position of the frequency band f.sub.start-f.sub.stop, the bandwidth f.sub.start-f.sub.stop is, in particular, between 0.5 GHz and 10 GHz.

(15) For emitting the transmission signal S.sub.HF, according to FIG. 2, the electrical high-frequency signal s.sub.HF is supplied, via a signal splitter 11 (and, possibly, an output amplifier 12), to a transmission antenna 13. The transmission antenna 13 may be realized as a horn antenna or planar antenna (e.g., as a patch antenna or fractal antenna), for example.

(16) Via the reflection of the transmission signal S.sub.HF at the object whose distance d is to be determined (in fill-level measurement, the surface of the bulk material 3), a correspondingly reflected echo signal E.sub.HF is received at a reception antenna 14 of the transmission/reception unit 2. Analogously to the transmission antenna 13, the reception antenna 14 may thereby likewise be realized as a planar antenna. Also conceivable is the use of a combined transmission/reception antenna that is, accordingly, in contact with the signal splitter 11 via a transmission/reception diplexer.

(17) After reception, in the reception antenna 14, the echo signal E.sub.HF is converted back into a purely electrical signal e.sub.HF (which may, in turn, optionally be amplified by a reception amplifier 15). This is subsequently mixed with the high-frequency signal s.sub.HF of the high-frequency signal generation unit by means of a receiving mixer 16, wherein, for this, the high-frequency signal s.sub.HF is branched off from the signal splitter 11. An intermediate frequency signal e.sub.LF that is typical in an FMCW method is thereby generated that forms the basis for the determination of the distance d. According to the FMCW principle, the frequency of the intermediate frequency signal e.sub.LF is thereby proportional to the distance d of the object, such that a suitable evaluation unit (e.g., within the measurement device 100 or superordinate unit 4) may determine the distance d via a measurement of the frequency of the intermediate frequency signal e.sub.LF. To determine the frequency of the intermediate frequency signal e.sub.LF, this may be subjected to a (fast) Fourier transformation, for example.

(18) To understand how the ramp-like, electrical high-frequency signal s.sub.HF is generated by the high-frequency generation unit 10, in FIG. 4, a standard embodiment variant implemented according to the prior art is presented. The functional principle of the high-frequency generation unit 10 shown there is known by the term, “phase locked loop” (PLL). The high-frequency generation unit 10 is respectively based upon a controllable, electrical high-frequency oscillator 105 (realized according to the prior art as a VCO) that generates the electrical high-frequency signal s.sub.HF. Corresponding to the application in FMCW radar, the frequency of the high-frequency signal s.sub.HF is in the GHz range, from 1 GHz up to more than 100 GHz.

(19) In the shown high-frequency generation unit 10, the frequency of the high-frequency signal s.sub.HF is regulated via feedback, and thus is, on the one hand, stabilized against fluctuations of the ambient temperature; on the other hand, the sawtooth-shaped frequency modulation df/dt is here set. The feedback is realized in that a control signal s.sub.c is branched off the high-frequency signal s.sub.HF of the high-frequency oscillator 105 and supplied to a phase comparator 102. The phase comparator 102 compares the current phase shift of the control signal s.sub.c with a reference signal s.sub.clock of constant frequency. The reference signal s.sub.clock here has an exactly pre-adjustable reference frequency, with negligible temperature drift.

(20) For example, a quartz oscillator may be used as a source of the reference signal s.sub.clock, which quartz oscillator typically generates the reference signal s.sub.clock with a frequency in a range between 10 MHz and 100 MHz.

(21) Depending upon the phase difference between the control signal s.sub.c and the reference signal s.sub.clock, the phase comparator 102 generates a control signal s.sub.DC which is supplied to a corresponding control input of the high-frequency oscillator 105.

(22) Given use of a VCO as a high-frequency oscillator 105, this requires a direct voltage for controlling the frequency of the high-frequency signal s.sub.HF. Therefore, a charge pump 103 is connected downstream of the digital phase comparator 105 in the embodiment variant shown in FIG. 4. A corresponding digital/analog transformation of the control signal six is thus performed by the charge pump 103. Depending upon the conversion of the high-frequency signal generation unit 10, it is, possibly, additionally necessary to implement a level adaptation by means of a level converter (not shown), before or after the charge pump 103. The control signal six may additionally have interference components—in particular, high-frequency interference components—corrected by means of a filter 104 that is arranged between the phase comparator 102 and the high-frequency oscillator 105. The filter 104 in FIG. 4 is accordingly designed as a low-pass filter.

(23) In the control loop of the high-frequency generation unit 10 as depicted in FIG. 4, the ramp-like frequency modulation df/dt of the high-frequency signal s.sub.HF, which is typical with FMCW radar, is set at the frequency divider 107. For this, a frequency divider 107, known as a “fractional-N divider” according to the prior art, is controlled so that its division factor N is temporally constant, and thus, in effect, varies in a ramp shape. With fractional-N dividers, the smallest resolvable frequency resolution depends upon the word width N; it is 20-32 bit.

(24) A ramp-like frequency modulation df/dt of the transmission signal S.sub.HF leads to correct measurement results, insofar as its propagation velocity is constant. In principle, this is the case, as long as the nearly constant speed of light can be used as a basis for the propagation velocity. This is the case with free radiation of the transmission signal S.sub.HF. However, this is no longer the case for radiation in a waveguide such as a pipe, for example. There, the effective propagation velocity c.sub.G is reduced according to the formula,

(25) $c_G\left(f\right)=c_0*\frac{\sqrt{{\left(\frac{f}{f_c}\right)}^2-1}}{f/f_c}$
with decreasing frequency. This relationship is presented in FIG. 5 for various modes (TE 11, TE 31, TE 02) of the transmission signal S.sub.HF. That limit frequency f.sub.c at which the propagation velocity c.sub.G converges at low frequencies (not explicitly shown in FIG. 5) is referred to as a “cut-off frequency.” As is apparent from FIG. 5, it is dependent upon the respective prevailing mode of the transmission signal S.sub.HF (and upon the diameter D of the waveguide).

(26) According to the present disclosure, a distortion of the distance measurement is counteracted in that the frequency modulation df/dt of the electrical high-frequency signal s.sub.HF or of the transmission signal S.sub.HF is not constant; rather, the frequency dependency of the temporal frequency modulation (df/dt) is set to be as proportional as possible to the frequency dependency of the propagation velocity (c.sub.G(f)) of the transmission signal (S.sub.HF) in the waveguide 2:

(27) $\frac{\mathrm{df}}{\mathrm{dt}}~c_G\left(f\right)$

(28) With the high-frequency generation unit 10 depicted in FIG. 4, this idea can be implemented by corresponding control of the fractional-N divider 107, for example. Depending upon the type of fractional-N divider 107 that is used, such a proportionality may be at least approximately established in that the frequency dependency of the frequency modulation df/dt is linearly approximated to the frequency dependency of the propagation velocity c.sub.G(f). This is illustrated in FIG. 6.

(29) There, the frequency band f.sub.start-f.sub.stop is subdivided into four, equally large sub-bands s.sub.1, s.sub.2, s.sub.3, s.sub.4, wherein, accordingly, the center frequency f.sub.1, f.sub.2, f.sub.3, f.sub.4 of each sub-band s.sub.1, s.sub.2, s.sub.3, s.sub.4 is known. Corresponding to this, the high-frequency generation unit 10 is set so that the electrical high-frequency signal s.sub.HF respectively has a linear frequency modulation Δf.sub.1, Δf.sub.2, Δf.sub.3, Δf.sub.4 within the sub-bands s.sub.1, s.sub.2, s.sub.3, s.sub.4. The linear frequency modulations Δf.sub.1, Δf.sub.2, Δf.sub.3, Δf.sub.4 are thereby set using a common proportionality factor proportional to the respective propagation velocity c.sub.G(f.sub.1, 2, 3, 4) at the corresponding center frequency f.sub.1, f.sub.2, f.sub.3, f.sub.4.

(30) An ideal linear approximation of the frequency modulation df/dt to the propagation velocity with regard to its frequency dependencies would involve subdividing the frequency band f.sub.start-f.sub.stop into infinitely many sub-bands s.sub.1, s.sub.2, s.sub.3, s.sub.4. For the purpose of improved approximation, it is therefore within the sense of the present disclosure to maximize the number of sub-bands s.sub.1, s.sub.2, s.sub.3, s.sub.4 to the greatest extent technically possible. The control of the fractional-N dividers 105 in seven sub-bands s.sub.1, . . . , s.sub.7 is already technically feasible. As an alternative to the depiction in FIG. 4, it would, naturally, additionally be conceivable to dimension the sub-bands s.sub.1, s.sub.2, s.sub.3, s.sub.4 in different sizes, in order, for example, to, in turn, achieve an improved approximation.

(31) FIG. 7 illustrates the effect that results due to the curvature, according to the present disclosure, of the frequency ramp of the transmission signal S.sub.HF (linearly approximated with seven sub-bands s.sub.1, . . . , s.sub.7) in comparison to constant, ramp-like excitation (constant frequency modulation df/dt). With logarithmic presentation of the amplitudes, it shows a section of the frequency spectrum of the two, associated, intermediate frequency signals e.sub.LF, in the case of measurement of the previously-known distance d (e.g., 30 m) of an object in a DN150 surge pipe. The antenna that is used thereby predominantly excites the desired TE 01 mode. The propagation velocity c.sub.G(f) of this mode is, accordingly, used for approximation. The two measurement curves clarify that the frequency maximum (at approximately 137 kHz), corresponding to the object or its distance d, on the one hand, markedly increases the sharpness of the maximum in the case of the approximation according to the present disclosure. On the other hand, the maximum exhibits a greater amplitude. According to the present disclosure, the distance d may thus be determined more precisely, and also with greater certainty, on the basis of the intermediate frequency signal e.sub.LF.