Measuring device for determining dielectric values

Abstract

The invention relates to a measuring device (1) for determining a dielectric value (DK) of a medium (2), the measuring device comprising at least the following components: a measuring sensor (11) having at least one first conductive electrode (111); and a high-frequency unit designed to convert a high-frequency signal (S HF ) is coupled at least into a first electrode (111) and a corresponding reflection signal (THF) is used to determine the dielectric value of the medium (2). According to the invention, the measuring device (1) is characterized in that the first electrode (111) is wound around a measuring rod (110) which can be in contact with the medium (2). By winding the electrode, the measuring sensor (11) can advantageously be configured to be significantly shorter without limiting the sensitivity of the measuring device (1). In this way, the measuring device (1) can also be used under restricted installation conditions.

Description

Technical Field

[0001] The invention relates to a measuring device for determining the dielectric value of a filling material. Background Art

[0002] In automation technology, in particular in process automation technology, field devices are frequently used for detecting and / or modifying process variables. The measured variable to be determined can be, for example, the level, flow, pressure, temperature, pH value, redox potential, conductivity or dielectric value of a medium. In order to detect the corresponding measured value, the field devices each include a suitable sensor or are based on a suitable measuring principle. Various such field devices are manufactured and sold by Endress+Hauser.

[0003] Determining the dielectric value (also called "dielectric constant" or "relative permittivity") of the filling material in a container is particularly important for solid, liquid and gaseous fillings such as propellants, wastewater or chemicals, since this value can constitute a reliable indicator of impurities, moisture content or composition of the substance. Within the scope of the present invention, the term "container" also refers to non-closed containers such as basins, lakes or flowing bodies of water.

[0004] To determine the dielectric value, the capacitance measurement principle can be used according to the prior art, especially in the case of liquid bulk materials. In this case, the following effect is used: the capacitance of a capacitor changes proportionally to the dielectric value of the medium located between the two electrodes of the capacitor.

[0005] TDR sensors (TDR is the abbreviation for “time domain reflectometry”) are used to measure dielectric values. In this measuring principle, the sensor transmits a pulsed high-frequency signal with a frequency between 0.1 GHz and 150 GHz along a conductive measuring sensor and measures the transmission time of the pulse until the reflected high-frequency signal is received. In this case, the following effect is used: the pulse transmission time depends on the dielectric value of the substance surrounding the measuring sensor. The functional principle of TDR sensors is described, for example, in the published patent application EP 0622 628A2. TDR sensors are sold in many designs, for example by the company IMKO Mikromodultechnik GmbH. TDR sensors also have the advantage that, in addition to determining the dielectric value, the dielectric loss factor of the substance can potentially be determined.

[0006] Furthermore, the TDR principle can also be used to determine the dielectric value of the (liquid) medium inside the container in a nearly parasitic manner during its radar-based level measurement. Here, too, the corresponding high-frequency signal is guided into the medium via an electrically conductive measuring sensor. Such a combined level and dielectric measurement is described in the application document DE 10 2015 117 205 A1. In the case of a level measurement based on TDR, the measuring sensor extends in principle over the entire height of the container.

[0007] Even if only dielectric values ​​are to be measured, a TDR-based measuring sensor cannot optionally be designed to be significantly more compact in order to ensure a certain minimum signal transmission time and thus achieve a sufficiently high measurement accuracy. In case of doubt, there is therefore the risk that a TDR-based dielectric value measuring device is too bulky for certain installation situations. Summary of the invention

[0008] The present invention is therefore based on the following object: to provide a compact dielectric value measuring device.

[0009] The invention achieves this object by a measuring device for determining the dielectric value of a medium, the measuring device comprising the following components:

[0010] - a measuring sensor having

[0011] o at least one first conductive electrode,

[0012] A high-frequency unit is designed to couple a high-frequency signal into at least the first electrode and to determine a dielectric value of the medium based on a corresponding reflection signal.

[0013] In the context of the present invention, the term "high frequency" generally defines signals between 0.1 GHz and 200 GHz. However, depending on the measurement range, it is advantageous that the high frequency unit is designed to generate high frequency electrical signals between 0.01 GHz and 10 GHz, in particular between 0.433 GHz and 6 GHz.

[0014] According to the invention, the measuring device is characterized in that the first electrode is wound on a measuring rod which can be in contact with the medium. This has the advantage that the measuring sensor can thus be significantly shorter, without the sensitivity of the measuring device being restricted. The measuring device can thus also be used under restricted installation conditions. In this case, the geometry of the measuring rod is not fixedly predefined. The measuring rod can, for example, have a circular or rectangular cross section. It is also conceivable that the measuring rod does not have a straight axis, but that, in fact, the axis is appropriately bent depending on the place of use.

[0015] In the context of the present invention, the term "unit" is understood to mean in principle any electronic circuit that is suitable for the intended purpose proposed. Thus, depending on the requirements, it can be an analog circuit for generating or processing the corresponding analog signal. However, it can also be a digital circuit that interacts with a program, such as an FPGA or a storage medium. In this case, the program is designed to perform the corresponding method steps or to apply the necessary computing operations of the corresponding unit. In this context, in the sense of the present invention, the various electronic units of the level measuring device can also potentially access a common physical memory or be operated via the same physical digital circuit.

[0016] In principle, within the scope of the invention, it is not provided that the measuring principle is based on the determination of the dielectric value by the reflected high-frequency signal. However, it is particularly advantageous to determine the dielectric value by one of the TDR measuring principles. In these cases, the high-frequency unit is designed to generate the high-frequency signal in a pulsed manner. The measuring sensor in the measuring rod can include a second electrode as ground potential with respect to the high-frequency signal. In this case, the second electrode can, for example, have the same geometry as the measuring rod, but its dimensions are correspondingly reduced, i.e., for example, rod-shaped with a circular cross section.

[0017] In order that the electromagnetic near field of the high-frequency signal can penetrate into the medium with minimal losses, the first electrode must be electrically insulated from the measuring rod. For this purpose, for example, the first electrode can be designed with a corresponding insulating sheath. However, for this purpose, the measuring rod can also be made of a non-conductive material. In order to amplify the electromagnetic field of the high-frequency signal in the medium, it is advantageous here that the insulating material of the measuring rod has a relative dielectric constant between 6 and 40.

[0018] In order to protect the measuring rod or the first electrode from environmental influences such as abrasive media, the measuring rod according to the invention can be provided with a non-conductive coating. If the measuring rod is made of an insulating material, then in this case the electrically insulating coating can have a smaller dielectric constant than the measuring rod, so that it is ensured, so to speak, that a gradient of the dielectric constant is generated from the interior of the measuring sensor to the outside. As a result, the penetration of the electromagnetic field of the high-frequency signal into the medium is amplified. Thus, the coating can consist, for example, of glass, PTFE, ABS or ceramic.

[0019] Since the sensitivity of the dielectric value measurement is potentially higher the higher the winding density of the first electrode around the measuring rod, it is advantageous if the first electrode has a winding density of at least 2 windings along the measuring rod for each wavelength corresponding to the frequency of the high-frequency signal.

[0020] The measuring sensor of the measuring device according to the invention can be extended in such a way that the third electrode is wound on the measuring rod so as to approximately coincide with the first electrode. In this case, the third electrode is arranged inside the first electrode relative to the measuring rod. If the high-frequency unit controls the third electrode with the first high-frequency signal so that the phase position of the high-frequency signal at the first electrode and the third electrode is approximately the same, this has the following advantages: the electromagnetic near field of the high-frequency signal along the first electrode is amplified in the medium, and the sensitivity of the dielectric value measurement is thereby increased. This effect can be used in particular when the third electrode has a larger cross-sectional area than the first electrode. In this context, the cross-sectional area should be at least 10% larger, but a cross-sectional area of ​​up to 400% larger is also conceivable. In this context, an elongated cross-section is also advantageous. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will be explained in more detail with reference to the following drawings. As shown below:

[0022] Figure 1 A measuring device for measuring the dielectric value of a medium in a container according to the present invention is shown;

[0023] Figure 2 A measuring sensor of a measuring device according to the invention is shown, and

[0024] Figure 3 A cross-sectional view of a measuring sensor according to the invention is shown. DETAILED DESCRIPTION

[0025] In order to have an overall understanding of the dielectric measuring device 1 according to the present invention, Figure 1 : A schematic arrangement of a measuring device 1 on a container 3 with a medium 2 is shown in FIG: In order to determine the dielectric value of the medium 2, the measuring device 1 is arranged laterally on a connection (e.g., a flange connection) of the container 3. For this purpose, the measuring device 1 is attached to the inner wall of the container substantially in a form-fitting manner. The medium 2 can be a liquid, such as a beverage, paint, cement or a propellant (e.g., liquefied gas or mineral oil). However, it is also conceivable to use the measuring device 1 for a bulk material type medium 2, such as, for example, grain.

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

[0027] like Figure 1Schematically shown, the measuring device 1 according to the invention comprises a measuring sensor 11 which, after installation, extends into the interior 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 operating mode of the measuring device 1 is based on the measurement sensor 11, the high-frequency signal s HF is applied to the measuring sensor so that the high frequency signal s HF The electromagnetic near field penetrates the medium 2. For this purpose, the measuring sensor 11 comprises a first conductive electrode 111. The high-frequency unit of the measuring device 1 can transmit the high-frequency signal s HF Based on the signal r correspondingly reflected at the end of the first electrode 111 HF , the dielectric value can be determined by the high frequency unit. High frequency signal s HF The frequency f HF The frequency f between 0.433 GHz and 6 GHz will be adapted to the specific type of medium 2 or the specific value range of the dielectric value to be measured. HF Suitable for media with high water content2.

[0029] In principle, there is no provision in the context of the present invention for a signal based on the reflection signal r HF The measuring principle for determining the dielectric value. This is because, depending on the design, the dielectric value can influence the transmitted high-frequency signal. HF transmission time, amplitude attenuation and phase shift.

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

[0031] In the case of FMCW, it is used to generate a high frequency signal s HF The high-frequency unit can accordingly include a "phase-locked loop (PLL)". This is based on a controllable, electrically high-frequency oscillator (typically implemented as a VCO) which generates a high-frequency electrical signal s HF In this case, the high frequency signal s HF The frequency of is regulated via feedback and is therefore stabilized against fluctuations in the ambient temperature on the one hand; on the other hand, the high-frequency signal s, which is typical for FMCW, is HF The sawtooth frequency change is set via feedback: the feedback is realized in that the control signal is derived from the high frequency signal s of the high frequency oscillator. HF The phase comparator, in turn, converts the control signal s cThe current phase shift is compared with a constant frequency reference signal. The reference signal here has a reference frequency that can be precisely preset and whose temperature drift is negligible.

[0032] In the case of FMCW, the high-frequency unit can, for example, convert the transmitted high-frequency signal s HF and the signal r reflected at the end of the first electrode 111 HF The mixing determines the transit time and therefore the dielectric value, since the frequency of the mixed signal varies linearly with the transit time.

[0033] With the implementation of TDR method, for high frequency signals HF The high-frequency unit for the cyclic, pulsed generation of the reflected signal r can, for example, include a corresponding cyclically controlled oscillator, for example a voltage-controlled oscillator or simply a quartz oscillator. In order to determine the transit time or the dielectric value, the high-frequency unit can process the reflected signal r by undersampling according to the pulse transit time method. HF Due to undersampling, the reflected signal r HF It is stretched along the time axis so that the signal maximum corresponding to the signal transmission time can be determined in a simplified manner in terms of circuits.

[0034] Despite the possible time stretching, the first electrode 111 must have a defined minimum length so that the high-frequency unit can determine the (reflected) high-frequency signal s with sufficient resolution. HF , r HF The signal transmission time of the measuring device 11 is changed according to the transmission time of various media 2 with various dielectric values. In practice, this leads to the need for the measuring sensor 11 to have a corresponding minimum length. However, depending on the installation situation or the container size, the measuring device 1 may be too large for the corresponding application.

[0035] Based on the measuring sensor 11 according to the present invention, Figure 2 As shown, the measuring device 1 can have a significantly more compact design: As shown, for this purpose, the first electrode 111 is wound around the 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 a sheathed Cu or Au cable, or the measuring rod 110 can also be made of a non-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 is potentially. min The first electrode 111 is designed to correspond to the high frequency signal s according to the following formula: HF The frequency f HF Each wavelength λ HFThe winding density is at least 2 times along the measuring rod 110. min :

[0037]

[0038] (where c is the propagation speed of electromagnetic waves, about 3*10 8 m / s). For minimum winding density # min , which results in:

[0039]

[0040] Therefore, in the high frequency signal s HF The frequency f of 1 GHz HF According to the minimum winding density of this formula, min This corresponds to about 20 wraps per meter.

[0041] exist Figure 2 In the embodiment variant shown, the measuring rod 110 has a straight axis and a circular cross section. However, generally, within the scope of the invention, the cross-sectional shape is not specified as fixed 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 the location of use requires it.

[0042] as relative to the (reflected) high frequency signal s HF , r HF The ground potential at Figure 2 In the embodiment variant shown, the measuring electrode 11 comprises a rod-shaped second electrode 112 having a cross section which is likewise circular relative to the cross section in the center of the measuring rod 110, wherein the second electrode 112 is also 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 the region of the measuring rod 110 where the first electrode 111 is wound around it.

[0043] In measuring sensor 11 Figure 2 In the embodiment variant shown, 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 remain sensitive to the high-frequency signal s along the first electrode 111, the dielectric value 113 of the medium 2 to be examined is preferably 110. HF In order to have a sufficient influence on the electromagnetic field, the coating 113 advantageously has a lower dielectric constant than the likewise electrically insulating measuring rod 110. If the measuring rod 110 is thus made of a relative dielectric constant between 6 and 40 F*m -1If the electrically insulating coating 113 is made of a material between 100 and 110 , the electrically insulating coating 113 can be made of a material such as glass, PTFE, ABS or ceramic, for example, which has a correspondingly smaller relative dielectric constant than the measuring rod 110 .

[0044] Figure 3 A cross-sectional view of a measuring sensor 11 according to the invention is shown: In this figure, an expanded embodiment variant is shown, in which a third electrode 114 is wound around the measuring rod 110 approximately coinciding with the first electrode 111. In this case, the third electrode 114 is arranged within the first electrode 111 relative to the measuring rod 110.

[0045] In the illustration shown, both the first electrode 111 and the third electrode 114 have a cross section that is elongated relative to the cross section of the measuring rod 10. In this case, the length and therefore the cross-sectional area of ​​the third electrode 114 is approximately 150% greater than the cross-sectional area of ​​the first electrode 111. This embodiment variant of the measuring sensor 11 enables the high-frequency signal s HF The electromagnetic field can be applied to the medium 2 to be examined along the first electrode 111 with increased intensity. As a result, the sensitivity of the measurement can in turn be increased. A prerequisite is that the high-frequency unit of the measuring device 1 also transmits the high-frequency signal s HF is applied to the third electrode 114 and is substantially in phase with respect to the first electrode 111. Thus, the size of the measurement sensor 11 can be further reduced by the third electrode 114 without reducing the sensitivity. The sensitivity or range of the measurement area around the measurement sensor 11 can in turn be reduced by the high frequency signal s HF The intensity of the signal applied to the third electrode 114 can be variably adjusted.

[0046] Reference Mark List

[0047] 1. Measuring equipment

[0048] 2 Medium

[0049] 3 Container

[0050] 4 Parent unit

[0051] 11 Measuring sensor

[0052] 110 Measuring rod

[0053] 111 First electrode

[0054] 112 second electrode

[0055] 113 Coating

[0056] 114 Third electrode

[0057] f HF The frequency of the high-frequency signal

[0058] r HF Reflected signal

[0059] s HF High frequency signal

[0060] # min Minimum winding density

[0061] λ HF The corresponding wavelength of the high-frequency signal.

Claims

1. A measuring device for determining the dielectric value of a medium (2), comprising: A measuring sensor (11) having at least one first electrode (111), The high frequency unit is designed to convert the high frequency signal (s HF ) is coupled to at least the first electrode (111), and based on the corresponding reflection signal (r HF ) to determine the dielectric value (DK) of the medium (2), It is characterized in that The first electrode (111) is wound on a measuring rod (110) capable of contacting the medium (2). The measuring sensor (11) in the measuring rod (110) comprises a second electrode (112), The second electrode (112) acts as the high frequency signal (s HF ) or the reflected signal (r HF ) of the ground potential, The second electrode (112) extends at least over the region of the measuring rod (110) where the first electrode (111) is wound around the measuring rod (110).

2. The measuring device according to claim 1, wherein the high-frequency unit is configured to generate the high-frequency signal (s) in a pulsed manner. HF ) so that the dielectric value can be determined using the TDR measurement principle.

3. The measuring device according to claim 1 or 2, wherein the measuring rod (110) has a circular cross section.

4. The measuring device according to claim 1 or 2, wherein the measuring rod (110) comprises an electrically insulating coating (113).

5. The measuring device according to claim 4, wherein the measuring rod (110) is made of a non-conductive material. 6 . The measuring device according to claim 5 , wherein the non-conductive material has a defined relative permittivity between 6 and 40.

7. The measuring device according to claim 6, wherein the electrically insulating coating (113) has a smaller dielectric constant than the measuring rod (110).

8. The measuring device according to claim 7, wherein the electrically insulating coating (113) is made of glass, PTFE, ABS or ceramic.

9. The measuring device according to claim 1 or 2, wherein for the high frequency signal (s HF ) frequency (f HF ) for each wavelength (λ HF ), the first electrode (111) has a winding density of at least 2 times along the measuring rod (110).

10. The measuring device according to claim 1, wherein the second electrode (112) is rod-shaped.

11. A measuring device according to claim 1 or 2, wherein a third electrode (114) is wound on the measuring rod (110) substantially overlapping the first electrode (111), and the third electrode is arranged inside the first electrode (111) relative to the measuring rod (110).

12. The measuring device according to claim 11, wherein the third electrode has a larger cross-sectional area than the first electrode (111).

13. The measuring device according to claim 11, wherein the high frequency unit utilizes the high frequency signal (s HF ) controls the third electrode (114) so ​​that the high-frequency signal (s) on the first electrode (111) and the third electrode (114) HF ) are roughly the same phase.

14. The measuring device according to claim 1 or 2, wherein the high frequency unit is designed to generate a high frequency signal (s) with a frequency between 0.1 GHz and 10 GHz. HF ).

15. The measuring device according to claim 14, wherein the high frequency unit is designed to generate a high frequency signal (s) with a frequency between 0.433 GHz and 6 GHz. HF ).

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