MEASURING DEVICE FOR DETERMINING A DIELECTRIC VALUE

Abstract

A measuring device for determining the dielectric value of a medium has at least the following components: a measuring sensor with at least one first electrically conductive electrode and a high-frequency unit which is designed to couple a high-frequency signal at least into the first electrode and determine the dielectric value of the medium using a corresponding reflection signal. The measuring device is characterized in that the first electrode is wound on a measuring rod which can be brought into contact with the medium. By winding the electrode, the measuring sensor can be advantageously configured to be significantly shorter without limiting the sensitivity of the measuring device. In this manner, the measuring device can also be used in the event of constricted installation conditions.

Claims

1-12. (canceled)

13. A measuring device for determining a dielectric value of a medium, comprising: a measuring sensor, including: a first electrically conductive electrode, a measuring rod, and a high-frequency unit designed to couple a high-frequency signal into the first electrode and to determine the dielectric value of the medium on the basis of a corresponding reflection signal, wherein the first electrode is wound on the measuring rod and can be brought into contact with the medium.

14. The measuring device according to claim 13, wherein the high-frequency unit is configured to generate the high-frequency signal in a pulsed manner such that the dielectric value can be determined using a Time Domain Reflectometry measuring principle.

15. The measuring device according to claim 13, wherein the measuring rod has a circular cross-section.

16. The measuring device according to claim 13, wherein the measuring rod includes an electrically insulating coating.

17. The measuring device according to claim 16, wherein the measuring rod is produced from a non-electrically conductive material having a defined relative dielectric constant of between 6 and 40.

18. The measuring device according to claim 17, wherein the electrically insulating coating has a smaller dielectric constant than the measuring rod and is produced from a glass, PTFE, ABS, or a ceramic.

19. The measuring device according to claim 13, wherein the first electrode has a winding density along the measuring rod of at least 2 windings for each wavelength that corresponds to the frequency of the high-frequency signal.

20. The measuring device according to claim 13, wherein the measuring sensor further includes a second electrode, wherein the second electrode is rod-like and disposed inside the measuring rod.

21. The measuring device according to claim 20, wherein the second electrode serves as ground potential for the high-frequency signal or the reflection signal.

22. The measuring device according to claim 20, wherein the measuring sensor further includes a third electrode wound congruently with the first electrode on the measuring rod, wherein the third electrode, in relation to the measuring rod, is arranged inside the first electrode and has a larger cross-sectional area than the first electrode.

23. The measuring device according to claim 22, wherein the high-frequency unit controls, with the high-frequency signal, the third electrode such that the phase positions of the high-frequency signal on the first electrode and the third electrode are the same.

24. The measuring device according to claim 13, wherein the high-frequency unit is designed to generate the high-frequency electric signal at a frequency between 0.1 GHz and 10 GHz.

Description

[0021] The invention is explained in more detail with reference to the following figures. The following are shown:

[0022] FIG. 1: a measuring device according to the invention for the dielectric value measurement of a medium in a container,

[0023] FIG. 2: a measuring sensor of the measuring device according to the invention, and

[0024] FIG. 3: a cross-sectional view of the measuring sensor according to the invention.

[0025] For a general understanding of the dielectric measuring device 1 according to the invention, a schematic arrangement of the measuring device 1 on a container 3 with a medium 2 is shown in FIG. 1: To determine the dielectric value of the medium 2, the measuring device 1 is arranged laterally on a connection of the container 2, e.g., a flange connection. For this purpose, the measuring device 1 is attached to the container inner wall approximately in a form-fitting manner. The medium 2 can be liquids, such as beverages, paints, cement, or propellants, like liquid gases or mineral oils. However, the use of the measuring device 1 for bulk-material-type media 2, such as grain for example, is also conceivable.

[0026] The measuring device 1 can be connected to a superordinate unit 4, such as, for example, a process control system. A “PROFIBUS,” “HART,” “wireless HART,” or “Ethernet” can, for example, be implemented as an interface. The dielectric value can be transmitted as an absolute value, or a complex value with real part and imaginary part. However, other information about the general operating state of the measuring device 1 can also be communicated.

[0027] As shown schematically in FIG. 1, the measuring device 1 according to the invention comprises a measuring sensor 11, which extends, after installation, into the inside of the container 3. In this way, the measuring sensor 11 is in contact with the medium 2 at a corresponding minimum fill-level of the medium 2 so that the measuring device 1 can determine the dielectric value of the medium 2 via the measuring sensor 11.

[0028] The mode of operation of the measuring device 1 is based upon a measuring sensor 11, into which a high-frequency signal s.sub.HF is applied so that the electromagnetic near-field of the high-frequency signal s.sub.HF penetrates the medium 2. For this purpose, the measuring sensor 11 comprises a first, electrically-conductive electrode 111 into which a high-frequency unit of the measuring device 1 can couple the high-frequency signal s.sub.HF. Based upon a signal r.sub.HF reflected accordingly at the end of the first electrode 111, the dielectric value can be determined by the high-frequency unit. The frequency f.sub.HF of the high-frequency signal s.sub.HF is to be adapted to the specific type of medium 2 or to the specific value range of the dielectric value to be measured. Accordingly, a frequency f.sub.HF between 0.433 GHz and 6 GHz is suitable for media 2 having a high water content.

[0029] In principle, the measuring principle according to which the dielectric value is determined on the basis of the reflected signal r.sub.HF is not specified in the context of the invention. This is because, depending upon the design, the dielectric value can influence the transit time, the amplitude damping, and the phase shift to the transmitted high-frequency signal s.sub.HF.

[0030] In the case of the signal transit time, the FMCW method (“Frequency-Modulated Continuous Wave”) or a variant of the TDR method (“Time Domain Reflectometry”), for example, can be implemented analogously to the guided radar.

[0031] In the case of FMCW, the high-frequency unit for generating the high-frequency signal s.sub.HF can accordingly comprise a “phase-locked loop, PLL.” This is based upon a controllable, electrical, high-frequency oscillator (implemented as VCO as standard), which generates the high-frequency electrical signal s.sub.HF. The frequency of the high-frequency signal s.sub.HF in this case is regulated via feedback and is therefore stabilized, on the one hand, against fluctuations in the ambient temperature; on the other hand, the sawtooth frequency change to the high-frequency signal s.sub.HF, which is typical for FMCW, is set via the feedback: The feedback is realized in that a control signal branches off from the high-frequency signal s.sub.HF of the high-frequency oscillator and is fed to a phase comparator. In turn, the phase comparator compares the current phase shift of the control signal sc with a constant-frequency reference signal. The reference signal here has a precisely pre-settable reference frequency with negligible temperature drift.

[0032] In the case of FMCW, the high-frequency unit can determine the transit time and thus the dielectric value, e.g., by mixing the transmitted high-frequency signal s.sub.HF with the signal r.sub.HF reflected at the end of the first electrode 111, since the frequency of the mixed signal varies linearly with the transit time.

[0033] With implementation of the TDR method, the high-frequency unit for cyclical, pulsed generation of the high-frequency signal s.sub.HF can, for example, comprise a correspondingly cyclically-controlled oscillator, e.g., in turn, a voltage-controlled oscillator or only a quartz oscillator. To determine the transit time or the dielectric value, the high-frequency unit can process the reflected signal r.sub.HF in accordance with the pulse-transit time method by undersampling. As a result of the undersampling, the reflected signal r.sub.HF is stretched along the time axis so that the signal maximum corresponding to the signal transit time can be determined in a simplified manner in terms of circuitry.

[0034] Despite possible time-stretching, the first electrode 111 must have a defined minimum length so that the high-frequency unit can determine with sufficient resolution the signal transit time of the (reflected) high-frequency signal s.sub.HF, r.sub.HF, or the transit time change as a function of various media 2 having various dielectric values. In practice, this results in the measuring sensor 11 needing to have a corresponding minimum length. As a result, however, the measuring device 1 can be too bulky for the respective application, depending upon the installation situation or container size.

[0035] On the basis of the measuring sensor 11 according to the invention, as shown in FIG. 2, the measuring device 1 can have a significantly more compact design: As shown, the first electrode 111 is wound for this purpose on a measuring rod 110, wherein the measuring rod 110 is electrically insulated from the first electrode 111. For insulation purposes, the first electrode 111 can be designed, for example, as an encased Cu or Au cable, or the measuring rod 110 can also be made of a non-electrically-conductive material for this purpose.

[0036] The higher the winding density of the first electrode 111 around the measuring rod 110, the higher the sensitivity of the dielectric value measurement also potentially is. However, at least the winding density #.sub.min is to be designed such that the first electrode 111 has a winding density #.sub.min along the measuring rod 110 of at least 2 windings for each wavelength λ.sub.HF that corresponds to the frequency f.sub.HF of the high-frequency signal s.sub.HF according to:

[00001]${\lambda }_{HF}=\frac{c}{f_{HF}}$

[0037] (where c is the propagation velocity of electromagnetic waves at approx. 3*10.sup.8 m/s). This results, for the minimum winding density #.sub.min, in:

[00002]${\#}_{min}\sim 2\star \frac{f_{HF}}{c}$

[0038] At a frequency f.sub.HF of the high-frequency signal s.sub.HF of 1 GHz, the minimum winding density #.sub.min according to this formula thus corresponds to approximately 20 windings per meter.

[0039] In the embodiment variant shown in FIG. 2, the measuring rod 110 has a straight axis and a circular cross-section. In general, however, the cross-sectional shape within the scope of the invention is not prescribed fixedly and can also be rectangular, for example. In contrast to the embodiment variant shown, the axis of the measuring rod 110 can also be curved, if this is required by the place of use.

[0040] As ground potential with respect to the (reflected) high-frequency signal s.sub.HF, r.sub.HF, the measuring electrode 11 in the embodiment variant shown in FIG. 2 comprises a rod-shaped, second electrode 112 with a likewise round cross-section in relation to the cross-section in the circle center of the measuring rod 110, wherein the second electrode 112 is again electrically insulated from the measuring rod 110. In this case, the second electrode 112 is designed to be long enough to extend at least over that region of the measuring rod 110 in which the first electrode 111 is wound on the measuring rod 110.

[0041] In the embodiment variant shown in FIG. 2 of the measuring sensor 11, an electrically-insulating coating 113 is applied to the measuring rod 110 or around the first electrode 111. The coating 113 serves primarily to mechanically or chemically protect the first electrode 111 from external influences. In order for the dielectric value of the medium 2 to be examined to still have sufficient influence on the electromagnetic field of the high-frequency signal s.sub.HF along the first electrode 111, it is advantageous if the coating 113 has a smaller dielectric constant than the also electrically-insulating measuring rod 110. If the measuring rod 110 is thus made of a material having a relative dielectric constant between 6 and 40 F*rm.sup.−1, the electrically-insulating coating 113 can be produced, for example, from a material, such as glass, PTFE, ABS, or a ceramic, which has a correspondingly smaller relative dielectric constant than the measuring rod 110.

[0042] FIG. 3 shows a cross-sectional view of the measuring sensor 11 according to the invention: In this view, an expanded embodiment variant is shown in which a third electrode 114 is wound on the measuring rod 110 approximately congruently with the first electrode 111. In this case, the third electrode 114 is arranged, in relation to the measuring rod 110, within the first electrode 111.

[0043] In the illustration shown, both the first electrode 111 and the third electrode 114 have an elongate cross-section in relation to the cross-section of the measuring rod 10. In this case, the length and thus the cross-sectional area of the third electrode 114 are approximately 150% greater than the cross-sectional area of the first electrode 111. This embodiment variant of the measuring sensor 11 makes it possible for the electromagnetic field of the high-frequency signal s.sub.HF to be applied with increased intensity, along the first electrode 111, to the medium 2 to be examined. As a result, the sensitivity of the measurement can, in turn, be increased. The prerequisite is that the high-frequency unit of the measuring device 1 also apply the high-frequency signal s.sub.HF to the third electrode 114, and in fact approximately in-phase with respect to the first electrode 111. The size of the measuring sensor 11 can thus potentially be further reduced by the third electrode 114, without reducing the sensitivity. The sensitivity, or the extent of the measuring region around the measuring sensor 11, can in turn be variably adjusted via the signal strength with which the high-frequency signal s.sub.HF is applied into the third electrode 114.

LIST OF REFERENCE SIGNS

[0044] 1 Measuring device [0045] 2 Medium [0046] 3 Container [0047] 4 Superordinate unit [0048] 11 Measuring sensor [0049] 110 Measuring rod [0050] 111 First electrode [0051] 112 Second electrode [0052] 113 Coating [0053] 114 Third electrode [0054] f.sub.HF Frequency of the high-frequency signal [0055] r.sub.HF Reflected signal [0056] s.sub.HF High-frequency signal [0057] #.sub.min Minimum winding density [0058] λ.sub.HF Corresponding wavelength of the high-frequency signal