Method for determining an inner diameter of a sounding tube by a fill-level measuring device

Abstract

A method for determining an inner diameter of a sounding tube, which, for measuring the fill level of a fill substance located in a process space of a container, extends in the process space, or is placed alongside the container and connected with the process space. The method can be implemented in the case of a fill-level measuring device working according to the FMCW-principle. Besides the intermediate frequency of the difference signal, also its phase shift is ascertained, wherein the exact tube inner diameter can be determined based on the phase shift. An advantage of the method is that the fill-level measuring device with the help of the then exactly known tube diameter can be recalibrated and accordingly the fill level determined more exactly. The exact tube inner diameter does not have to have been previously known.

Claims

1. A method for determining an inner diameter of a sounding tube, wherein, for measuring a fill level of a fill substance located in a process space of a container, the sounding tube extends in the process space, or wherein the sounding tube is placed alongside the container and connected fluidically with the process space, the method comprises method steps as follows: by means of a periodic electrical signal, a microwave signal is produced, wherein the electrical signal has a periodic frequency change in the region of a center frequency; the microwave signal is transmitted along the sounding tube in a direction of a surface of the fill substance; an echo signal, which, after reflection of the microwave signal on the surface of the fill substance, is reflected along the sounding tube in opposite direction, is received and converted into an electrical, received signal; by mixing the received signal with the electrical signal, a difference signal with an difference frequency is produced; as a function of the fill level, a phase shift between the difference signal and the electrical signal is ascertained; a phase difference between the phase shift and a previously known, base phase shift, which is based on a reference inner diameter, is ascertained; and based on the phase difference, a tube inner diameter is determined; wherein the tube inner diameter (D) is calculated based on the formula $D=D_0*\frac{f_c}{f_r*\sqrt{1-{\left(\frac{}{}\right)}^2}}\mathrm{with}$ $=\sqrt{1-{\left(\frac{f_c}{f_r}\right)}^2}\mathrm{and}$ $=1+\frac{c}{8f_r}*\frac{d}{dL}$ and wherein the change $\left(\frac{d}{dL}\right)$ is ascertained by linear regression; wherein: c=speed of light; =Archimedes' constant; D.sub.0=reference inner diameter; f.sub.c=limit frequency in a tube with diameter D.sub.0; fr=reference frequency; and $\frac{d}{dL}$ =change of the phase difference as a function of the fill level.

2. The method as claimed in claim 1, wherein: the periodic frequency change is a sawtooth-shaped or triangular change of the electrical signal.

3. The method as claimed in claim 1, wherein: the fill level is measured based on the difference frequency and/or the base phase shift.

4. The method as claimed in claim 3, wherein: the measuring of the fill level is recalibrated based on the tube inner diameter.

5. The method as claimed in claim 1, wherein: the phase shift is ascertained by means of a fast Fourier transformation of the difference signal.

6. The method as claimed in claim 1, wherein: the tube inner diameter is determined based on the change $\left(\frac{d}{dL}\right)$ of the phase difference ((L)) as a function of the fill level.

7. The method as claimed in claim 1, wherein: the base phase shift is ascertained by a theoretical calculation and/or based on calibration data.

8. The method as claimed in claim 7, wherein: for the case, in which the base phase shift is ascertained based on calibration data, the calibration, on which the calibration data is based, is performed using a calibration tube, which has the reference inner diameter (D.sub.0).

9. The method as claimed in claim 8, wherein: the calibration is performed using a calibration tube, in the case of which the reference inner diameter (D.sub.0) is about equal to the tube inner diameter (D) of the sounding tube.

10. A fill-level measuring device for performing a method comprising the following steps: a method for determining an inner diameter of a sounding tube, wherein, for measuring the fill level of a fill substance located in a process space of a container, the sounding tube extends in the process space, or wherein the sounding tube is placed alongside the container and connected fluidically with the process space, the method comprises method steps as follows: by means of a periodic electrical signal (s), a microwave signal is produced, wherein the electrical signal (s) has a periodic frequency change in the region of a center frequency; the microwave signal is transmitted along the sounding tube in a direction of a surface of the fill substance; an echo signal, which, after reflection of the microwave signal on the surface of the fill substance, is reflected along the sounding tube in opposite direction, is received and converted into an electrical, received signal; by mixing the received signal with the electrical signal, a difference signal with an difference frequency is produced; as a function of the fill level, a phase shift between the difference signal and the electrical signal is ascertained; a phase difference between the phase shift and a previously known, base phase shift, which is based on a reference inner diameter, is ascertained; and based on the phase difference, the tube inner diameter is determined, wherein the tube inner diameter (D) is calculated based on the formula $D=D_0*\frac{f_c}{f_r*\sqrt{1-{\left(\frac{}{}\right)}^2}}\mathrm{with}$ $=\sqrt{1-{\left(\frac{f_c}{f_r}\right)}^2}\mathrm{and}$ $=1+\frac{c}{8f_r}*\frac{d}{dL}$ and wherein the change $\left(\frac{d}{dL}\right)$ is ascertained by linear regression, and wherein: c=speed of light; =Archimedes' constant; D.sub.0=reference inner diameter; f.sub.c=limit frequency in a tube with diameter D.sub.0; fr=reference frequency; and $\frac{d}{dL}$ =change of the phase difference as a function of the fill level; the fill-level measuring device, comprising: a signal production unit for producing an electrical signal; an antenna unit for transmitting the microwave signal and/or for receiving the echo signal and/or for changing the reflected echo signal into an electrical, received signal; a mixer for mixing the electrical signal with the received signal; and an evaluating unit for determining the tube inner diameter based on the phase difference and/or for determining the fill level based on the frequency of the difference signal.

11. The fill-level measuring device as claimed in claim 10, wherein: said evaluating unit includes a further processing unit for digitizing and/or for filtering and/or for amplifying the difference signal.

12. The fill-level measuring device as claimed in claim 11, wherein: said further processing unit includes a bandpass filter, which is especially transmissive for the difference frequency of the difference signal.

13. The fill-level measuring device as claimed in claim 10, further comprising: a display unit for display of the tube inner diameter.

14. The fill-level measuring device as claimed in claim 10, wherein: the fill-level measuring device recalibrates itself based on the tube inner diameter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 is a typical arrangement of an FMCW fill level measuring device on a sounding tube;

(3) FIG. 2 is a block diagram of a fill level measuring device for performing the method of the invention;

(4) FIG. 3 is a sawtooth-shaped excitation of the microwave signal; and

(5) FIG. 4 is a phase difference between the phase shift of the received signal and a previously known, base phase shift.

DETAILED DISCUSSION IN CONJUNCTION WITH THE DRAWINGS

(6) FIG. 1 shows a typical arrangement on a sounding tube 11 of a fill level measuring device 1 working according to the FMCW-principle. Sounding tube 11 is arranged approximately vertically in a process space 4 of a container 2. Located in the process space 4 is a fill substance 3, whose fill level L is to be determined by the fill-level measuring device 1. Fill-level measuring device 1 is mounted above the fill substance 3 on the sounding tube 11 at a previously known, installed height h. In such case, the container 2, and the process space 4, can, depending on field of application, have a height of greater than 30 m. Fill-level measuring device 1 is placed in such a manner on the upper end of the sounding tube 11 that it transmits, i.e. sends, a microwave signal S along the sounding tube 11 toward the fill substance 3 and after reflection on the surface of the fill substance 3 receives an echo signal E.

(7) Alternatively to insertion of the sounding tube 11 within the container 2, another option is that the sounding tube 11 is placed alongside the container 2. Also in this case, the sounding tube 11 would be connected in such a manner with the process space 4 that the fill level L there reigns likewise in the sounding tube 11.

(8) As a rule, the fill-level measuring device 1 is connected via a bus system, for instance, a PROFIBUS, HART or Wireless HART bus system, with a superordinated unit 5, for example, a process control system. In this way, on the one hand, information concerning the operating state of the fill level measuring device 1 can be communicated. Also information concerning the fill level L can be transmitted, in order, in given cases, to control inlets 21 and/or outlets 22 present on the container 2.

(9) Sounding tube 11 has in practice an inner diameter D, which, as a rule, is not exactly known, or deviates from its nominal value. Moreover, the fill-level measuring device 1 is usually calibrated on a calibration tube 16 having a reference inner diameter D.sub.0, which does not exactly correspond to the tube inner diameter D. A reason for this can be, for example, deposits in the sounding tube or differing manufacturing methods. The result is that the fill-level measuring device 1 cannot determine the fill level L exactly. Thanks to the method of the invention, it is, however, possible, based on the reference inner diameter D.sub.0, to ascertain the tube inner diameter D exactly. By means of the exactly ascertained tube inner diameter D, it is then possible to recalibrate the fill-level measuring device 1 based on the tube inner diameter D, so that an exact measurement of the fill level L can be performed.

(10) If the fill-level measuring device 1 has a display unit or has access to a display unit, it is additionally possible to display the exactly ascertained tube inner diameter D on such display unit. In this connection, an embodiment provides that the fill-level measuring device 1 does not perform the recalibration automatically, but, instead, only after consent of service- or service personnel, to the extent that it is considered necessary due to the indicated tube inner diameter D.

(11) An example of a circuit of the fill level measuring device 1 for performing the method of the invention is shown in FIG. 2. For reasons of perspicuity, only signal paths and no control paths are shown in FIG. 2. Pivotal for producing the microwave signal S is a signal production unit 12. It produces a typical FMCW electrical signal s, which is located in the region of a center frequency f.sub.0 in the GHz region and exhibits a constant frequency change f.sub.0.

(12) As shown in FIG. 3, it can be a sawtooth-shaped excitation having a center frequency of 79 GHz currently usual for FMCW and a bandwidth of 2 GHz. In contrast to this sawtooth-shaped excitation with frequency linearly increasing with time, another option is a sawtooth-shaped excitation with frequency decreasing with time.

(13) Signal production unit 12 can be, for example, a voltage controlled oscillator, which includes a quartz oscillator suitable for such purpose.

(14) In the fill-level measuring device, which is shown in FIG. 2, the microwave signal S is produced in an antenna unit 13 by means of the electrical signal s. Additionally evident from FIG. 2 is that the antenna unit 13, besides producing the microwave signal S, also receives the echo signals E, which arise by reflection of the microwave signal S on the surface of the fill substance 3. Alternatively to the illustrated situation, this could also happen according to the invention via a separate receiving antenna.

(15) The echo signal E is converted by the antenna unit 13 into an electrical, received signal e. In measurement operation, then the received signal e is mixed in a mixer 14 with the transmitted signal s. By mixing the received signal e with the transmitted signal s, a difference signal IF is formed, wherein the intermediate frequency f.sub.IF of the difference signal IF is derived from the difference between the instantaneous frequency of the transmitted signal s and the instantaneous frequency of the received signal e.

(16) For ascertaining the intermediate frequency f.sub.IF of the difference signal IF as well as the phase shift .sub.actual(L) between the difference signal IF and the electrical signal s, the fill-level measuring device includes an evaluating unit 15. The ascertaining of these two values f.sub.IF, .sub.actual(L) is performed, in such case, per fast Fourier transformation by a calculational unit 152 provided for this. As usual in the case of processing this data, this happens based on digitized signals. Therefore, in the case of the evaluating unit 15 illustrated in FIG. 2, a further processing unit 151 is placed in front of the fast Fourier transformation. This could likewise be designed in such a manner that the difference signal IF is subjected to an amplification or bandpass filtering, which is transmissive especially for the intermediate frequency f.sub.IF of the difference signal IF.

(17) Based on the intermediate frequency f.sub.IF as well as the phase shift .sub.actual(L), such as is usual in the case of FMCW-based fill-level measuring devices, the fill level L is ascertained by a microcontroller 153.

(18) According to the invention, based on a difference forming, the phase shift .sub.actual(L) is compared with a base phase shift .sub.base(L). In the case of the fill-level measuring device illustrated in FIG. 2, this is done by the microcontroller 153. In such case, the base phase shift .sub.base(L) is stored in the microcontroller 153. It can be obtained, for example, by a calibration measurement on a calibration tube 16 having a reference inner diameter D.sub.0.

(19) Forming the difference leads to a phase difference (L). A characteristic phase difference (L) as a function of fill level L is shown in FIG. 4 (curve a). As characteristic in the case of tube inner diameters D, which deviate from the reference inner diameter D.sub.0 of the calibration measurement, the illustrated curve has an approximately linear rise

(20) $\frac{d}{dL}$
with increasing distance h-L.

(21) Based on the slope

(22) $\frac{d}{dL},$
the exact tune inner diameter D can be determined. This is calculated via the relationship:

(23) $D=D_0*\frac{f_c}{f_r*\sqrt{1-{\left(\frac{}{}\right)}^2}}\mathrm{with}$$=\sqrt{1-{\left(\frac{f_c}{f_r}\right)}^2}\mathrm{and}$$=1+\frac{c}{8f_r}*\frac{d}{dL}.$

(24) The calculating occurs in the case of the fill-level measuring device 1 illustrated in FIG. 2, again, by the microcontroller 153. In such case, the slope

(25) $\frac{d}{dL},$
such as shown in FIG. 4, is determined, for example, by linear regression of the phase difference (L).

(26) By exactly determining the tube inner diameter D, the fill-level measuring device 1 can correspondingly recalibrate itself and then determine the fill level L exactly. With regard to the phase difference (L), the recalibration has the result that the phase shift .sub.actual(L) approximately matches the base phase shift .sub.actual(L) (see curve b in FIG. 4).