DE102006034884A1 - Fluid permittivity or permeability measurement unit has conducting resonator filled by dipping with holes below cutoff frequency - Google Patents

Abstract

A fluid permittivity or permeability measurement unit has a conducting resonator with space between coaxial inner (2) and outer (4) conductors completely filled by dipping in the medium, using openings (12) chosen to be below the cutoff of the electromagnetic waves used and a reactive termination at one end (10).

Description

The The invention relates to a measuring device for determining the complex permittivity and or permeability of a Fluids in the microwave range, in particular a dip sensor for standing and pouring Fluide, by the resonance behavior of as a microwave resonator executed Immersion sensor the electromagnetic properties of the fluid certainly. Because the permeability most of the known fluids corresponds to vacuum permeability the measurement of permittivity Main application. In principle, the explanations apply but analogously for the permeability.

The Measurement of the complex permittivity by means of microwave radiation is by transmission measurement through conduction structures associated with the medium under investigation Interaction be brought about by measuring the transmission between two transceivers, with the propagation of the signal at least partially takes place in the medium to be examined, or by resonant measurement means Cavity or stray field sensors possible. This basic Techniques are known in the art and are e.g. in [Montgomery; Technique of Microwave Measurements; McGraw-Hill; 1947].

Measurements of reflection and / or transmission may be made in line structures in which the signal propagates along the conduit filled with the substance, such as, for example U.S. 4,996,490 . US 5,157,339 . U.S. 4,423,623 and US 5,369,368 known. Typically, the real part of the permittivity is determined from the phase offset between the two ends of the line, and the imaginary part of the permittivity, which represents the losses in the high frequency range, is determined from the attenuation. All these solutions have in common that they are not suitable as immersion sensor, but the fluid must be pumped through a pipe-like structure, otherwise there is no sufficient mass transfer with the surrounding fluid is possible and the sample is not representative of the fluid to be examined. The low sensitivity to changes in permittivity due to the lack of a resonant measuring principle is another disadvantage of this approach.

The transmission / reflection measurement is in JP 59195145 A Simplified in that only the attenuation along the transmission path is measured.

This principle is in US 4,862,060 modified in that the detuning of a non-isolated oscillator is measured by the line impedance, wherein the line impedance in turn depends on the permittivity. Again, there is no resonant effect in the medium since the microwave signal propagates through the piping structure and only an external electronic oscillator is detuned by the line impedance.

Another way to measure reflection and transmission are antennas that send signals directly through a filled with the medium to be examined volume or through a microwave-transparent wall on the medium, as from US 4,764,718 or US 5,006,785 is known.

Measured variables are usually amplitude and / or phase of the reflected and / or transmitted signal; Alternatively, the term as out US 5,315,258 known. The measurement is usually carried out at a frequency of the electromagnetic signal, alternatively also at several frequencies, as in US 4,902,961 ,

Out US Pat. No. 6,249,129 An arrangement is known in which two waveguides for examining a substance in a tank through openings in the walls of the waveguides represent a transmitting and receiving antenna and build within the container a transmission path through the medium, which is a further modification of the principle of reflection - / Transmissionsmessung is.

A similar solution is out EP 0 462 803 A2 In this case, the transmission path is established between simple interruptions of the shielding of coaxial lines.

The resonant measurement offers compared to the transmission methods mentioned above due to the high sensitivity of the resonant frequency of a microwave resonator against changes the electromagnetic properties of its internal volume the advantage very high accuracy. In addition there are the easy measurability of the frequency with high accuracy and easy to produce such signals using PLL synthesizers.

The However, potential benefits have so far been found only in offline measurement, at which a sample of the medium, which fills the resonator completely, in the Resonator is introduced or in open resonators for solid samples well-defined geometry [Y. Kobayashi and M. Katoh. "Microwave Measurement of Dielectric Properties of Low-Loss Materials by the Dielectric Rod Resonator Method. "1985 Transactions on Microwave Theory and Techniques 33.7 (Jul [T-MTT]): 586-592.].

A possible modification for the Onli ne measurement is off JP 61040546 A known. In this case, the medium to be examined is guided through a metallic resonant cavity by means of a pipeline which is permeable to microwave signals. Its resonance behavior is measured by means of two coupling points and from this the electromagnetic properties of the medium are determined. Significant disadvantages here are the principle-based partial filling of the resonator and thus the reduced sensitivity of the sensor and the radiation through the cylindrical openings for supplying the medium, which leads to measurement errors due to environmental influence, poor EMC properties and mutual influence adjacent equal or similar sensors. This disadvantage can not be eliminated by introducing a helix close to the medium, since here too a considerable part of the field around the helix lies outside the medium.

For the same purpose, for example US Pat. No. 6,476,619 B1 a microwave cavity resonator known. This consists of a tubular cavity resonator, are passed through which to be examined yarn threads. In the cylindrical cavity of the resonator Einkoppelstrukturen for coupling a microwave signal and for detecting the resonance occurring are arranged. In addition to the already mentioned reduction of sensitivity, this expression has the further principle-related disadvantage that the measuring range is quite limited, since an unambiguous assignment of a measured resonance frequency to the permittivity of the substance under investigation is possible only in a very limited permittivity range. The reason for this is that in addition to the dominant fundamental mode, higher TE or TM eigenmodes occur very quickly as the frequency increases, and unambiguous assignment is no longer possible.

A modification to improve the sensitivity of partially filled resonators is out US 4,890,054 known. The physical properties of a thin layer are determined by means of a resonator, which is shaped so that the electric field in the region of the layer is as strong as possible. Due to the increase in the filling factor achieved thereby, however, the reduced sensitivity caused by the geometrically only partial filling of the resonator with the medium is only partially compensated.

Further versions of resonant microwave sensors are stray field sensors such as US 6,411,103 known. These act via an electromagnetic field which acts outside the actual resonator volume with the medium change. The effective fill factor is thus again similar to that of the partially filled sensors. These sensors act on simple interruptions of the shielding of the resonator or in addition via intermediate layers for adjustable coupling of the sensor to the substance to be measured as from DE 10102578 known.

It It is therefore an object of the invention to provide an improved measuring device for determination at least one physical property of a fluid over its to create electromagnetic properties, which is a simple Handling allows and a greater measuring accuracy having.

The invention and their benefits

These Task is accompanied by the resonant microwave immersion sensor solved the features specified in claim 1. Preferred embodiments emerge from the dependent claims.

The Measuring device according to the invention, which is used for Determining at least one physical property of a fluid serves to determine this property an electromagnetic auxiliary size. For this a line resonator is provided, which of a microwave generator be subjected to a tunable electromagnetic signal may be to put a standing wave inside the line resonator produce. According to the invention is the line resonator is designed so that it can be immersed in the fluid is. That the line resonator has openings through which the fluid to be analyzed into the cavity inside the resonator can penetrate and completely fill the volume of the resonator. For measurement, the entire line resonator is immersed in the fluid. This simplifies the application considerably, since the fluid does not pass through Lines in the line resonator must be routed. Furthermore, can the volume in the interior of the line resonator completely with filled the fluid to be analyzed so that a better measuring sensitivity and thus at the same Evaluation technique a more accurate result is achieved.

The line resonator according to the invention is further designed such that it has a substantially coaxial structure, ie forms a Koaxialleitungsresonator. This coaxially constructed line resonator has a tubular outer conductor and an inner conductor arranged in the interior. The two conductors form a section of a line, which is suitable in principle for the transmission of DC voltages. The inner conductor is disposed in the outer conductor so that the outer peripheral surface of the inner conductor is spaced from the inner peripheral surface, that is, the inner wall of the outer conductor. Here are the inner conductor and the outer conductor preferably arranged to each other so that their longitudinal axes parallel to each other. Preferably, both the outer conductor and the inner conductor are formed so that the outer peripheral wall of the inner conductor and the inner wall of the outer conductor extend parallel to these longitudinal axes. Between the inner conductor and the outer conductor, an annular cavity is formed, which receives the fluid to be analyzed. At least one end face of the outer conductor, ie the annular cavity between inner conductor and outer conductor, and / or the peripheral wall of the outer conductor have openings through which the fluid can enter the interior of the cavity, ie in the annular cavity. Thus, the line resonator can easily be filled by the fluid when immersed in the fluid.

Of the Coaxial construction of the line resonator allows the resonator in a very wide frequency range with purely transversal electromagnetic Modes (TEM modes) works. That it is inside the resonator only radial and circumferential extending field components, but not in the axial direction of the resonator, i. towards the end faces, extending field components. That in a big one Frequency range can no TE and no TM modes propagate. This is in particular the shape and are the cross-sectional dimensions of the line resonator, i.e. of the foreign and Inner conductor essentially irrelevant, so that the cross-sectional dimensions and shapes chosen arbitrarily can be without affecting the described function of the resonator.

The Formation of the microwave immersion resonator according to the invention as a TEM line resonator with purely transversal electromagnetic Moden has the advantage that the resonances in a wide range the operating frequency depends only on the length of the resonator. this leads to to that second-order resonance at twice the resonant frequency the fundamental frequency is, so here the uniqueness range of the sensor is considerably increased. First a change the permittivity by a factor of 4 when using a single sensor with fixed length no longer be assigned uniquely.

Preferably can the meter but also have several of the described resonators, which can be used simultaneously whereby the measuring range further increases and with sufficient Number of line resonators can be extended as desired.

To the Operation of the measuring device is expediently a tunable oscillator is provided, which is a tunable Microwave signal generated, which in the line resonator via a suitable Einkoppelstruktur is coupled. The oscillator is tuned that a predetermined frequency range is traversed, then determine at which frequency a resonance occurs. These then represents the electromagnetic auxiliary variable, which is characteristic for one certain physical property of the fluid is which located inside the line resonator. Preferably, the outer conductor and the inner conductor at one end or both ends of the line resonator connected to each other. Through this connection becomes a defined geometric positioning of the inner conductor across from the outer conductor reached, i. the inner conductor is spaced from the inner wall of the supervisor fixed in this. The connection can be made by an outer conductor on a front side final End plate be created. It can at this end of the free space between inner and outer conductor Completely to be introverted. Alternatively, the connection between outdoor and Inner conductor be formed so that openings to the space between the outside and Inner conductor remain at this 5 end face.

The Connection between foreign and inner conductor at one end face or both end faces expediently designed to form a reactive termination of the resonator becomes. This means that the connection between outer and inner conductor is so designed such that in this area essentially no resistance, i.e. no ohmic resistance is formed. This can be the connection for example, by a metallic compound, e.g. an end plate be created. Alternatively, it is also possible, the front side substantially open, in which case the size of the openings is preferably defined so is that no radiation from the resonator to the outside through this end is done through. This can be done by that the electromagnetic occurring during operation of the resonator Field is below the cutoff frequency of these openings. The reactive one Termination at the end faces of the line resonator leads to that the line resonator has no transmission characteristics and generates a standing wave inside the line resonator can be.

Especially Preferably, the inner conductor is coaxially arranged to the outer conductor, i.e. the axis of symmetry in the longitudinal direction of the inner conductor lies on the axis of symmetry in the longitudinal direction of the supervisor. This allows a compact and more preferably symmetrical Structure of the resonator can be achieved. More preferably, the outer conductor a circular one Cross-section on. So can the outer conductor be formed as a pipe section of a metallic tube.

Further also preferably the inner conductor has a circular cross-section on. Thus, the inner conductor of a metallic tube or rod material be made very easy.

If both inner conductor and outer conductor a circular one Have cross-section and inner conductor and outer conductor coaxial with each other are arranged, it is further achieved that the annular gap between the inner circumference of the outer conductor and the outer circumference of the inner conductor in the radial direction a constant width over the entire circumference. Furthermore, the outer conductor and / or the inner conductor preferably over her entire length a constant cross section, i. they are circular cylindrical educated. Thus, a constant gap width of the annular gap between inner and outer conductor over the whole length of the line resonator can be achieved.

Of the outer conductor preferably has openings in its outer wall on, which are dimensioned such that at the operating frequency of the line resonator an electromagnetic field in the openings is below the limit wave number. The openings allow a fast and good flow through of the resonator with the fluid to be measured when the resonator in this is dipped. This ensures that the cavity in the Interior of the outer conductor fast completely is filled with the fluid to be analyzed. The openings However, they are preferably dimensioned so small in coordination the chosen ones Operating frequencies of the resonator, that no radiation of the electromagnetic Radiation through the openings outwards. This is achieved by dimensioning the openings so that the operating frequencies occurring below the threshold number of openings lie. Particularly preferred is a plurality of openings formed in the peripheral wall to the entry and exit of To favor fluid in the interior of the line resonator.

The Coupling of a microwave signal in the line resonator can done in different ways, for example via a Probe coupling, Loop coupling (loop-couplig) or aperture coupling (aperture-coupling). About such Coupling points can over it In addition, the resonance measurement done. This can be an additional Coupling point or antenna be provided. Alternatively, the Resonance measurement also over the coupling point for the coupling of the microwave signal take place. The resonance measurement can be done as a transmission measurement or as a reflection measurement.

There the line resonator according to the invention As a TEM resonator, preferably a length is selected which by the wavelength of the used microwave signal is fixed. The resonance frequency is independent of the transverse dimensions. Around as far as possible To ensure measuring range should be the dimension in the longitudinal direction of the line resonator preferably an integer multiple half the wavelength be particularly preferred, a low integer multiple. Particularly preferably, the length of the line resonator corresponds half the wavelength. this makes possible a particularly compact design of the line resonator. alternative can the length be any integer multiple of half the wavelength, for example, be equal to the wavelength or 1.5 times wavelength be. However, they are also larger multiples half the wavelength as length for the line resonator selectable.

The Operating frequencies of the line resonator are preferably in a frequency range of 300 kHz to 1 GHz and / or in a frequency range from 1 GHz to 30 GHz and / or in a frequency range of 30 GHz up to 1 THz. In these frequency ranges can be particularly good results be achieved.

drawing

One embodiment the subject of the invention is shown in the drawing.

It shows:

1 a view of the measuring device according to the invention.

description of the embodiment

following the invention will be described by way of example with reference to the accompanying figure described. This figure shows a schematic, perspective View of the line resonator according to the invention, which forms a microwave immersion resonator.

The Measuring device according to the invention is used analysis of fluids for permittivity or permeability. For this For example, the invention uses one or more line resonators over which the properties to be determined via a resonance change an electromagnetic microwave signal acting on the fluid be recorded. For example, you can this way in many cases indirectly the density or certain mixing ratios with another fluid be determined.

In the case of the measuring device according to the invention, at least one line resonator is used for this purpose, which has a substantially coaxial construction. It can be used to expand several such line resonators, which have, for example, the structure described below.

This structure is characterized by a rod-shaped electrically conductive inner conductor 2 determined, which is inside a tubular electrically conductive outer conductor 4 is arranged. Here is the inner conductor 2 arranged so that its outer peripheral surface of the inner peripheral surface or inner wall of the outer conductor 4 is spaced, so that an annular cavity or gap 6 between outer and inner conductor 2 is formed. In the example shown in the figure, the outer conductor 4 as well as the inner conductor 2 each circular cylindrical and arranged concentrically with each other, so that the annular gap 6 has a constant width in the radial direction over the circumference and over the length of the line resonator. However, alternative arrangements are possible in which the outer conductor 4 and the inner conductor 2 are not concentric with each other and / or have no circular cylindrical configuration. Important in the arrangement is only that always an annular gap 6 is created, ie the peripheral surface of the inner conductor 2 everywhere from the inner peripheral surface of the outer conductor 4 is spaced.

The connection of the outer conductor 4 and the inner conductor 2 takes place in the example shown via a cover plate 8th , which is designed as a reactive closure, ie the outer conductor 4 with the inner conductor 2 connects via the lowest possible ohmic resistance. This is the end plate 8th preferably formed as a metal plate. In the example shown, the end plate closes 8th the annular gap 8th at the front 10 of the line resonator, so that this on the front side 10 is formed closed. Alternatively, however, it is possible in the end plate 8th Also provide openings so that a fluid from the front side 10 forth in the gap 6 can penetrate.

To the penetration of the fluid 11 into which the line resonator is immersed, in the gap 6 between inner conductors 2 and outer conductor 4 to allow are in the wall of the outer conductor 4 numerous openings 12 educated.

The openings 12 are sized so that their size is tuned to the operating frequencies of the line resonator, so that the operating frequency is always below the cutoff frequency of the openings 12 lies, so that no radiation of the electromagnetic waves through the outer wall 4 the resonator is outwards.

In the example shown, that is the closed end 10 opposite end face 14 open. In this case, this front side forms a reactive closure, so that the line section consisting of inner conductor 2 and outer conductor 4 forms a resonator, inside which a standing wave can be generated. In the example shown are the end faces 10 and 14 designed differently, but it is also possible to form both end faces identical. Thus, both faces open as the end face shown 14 be formed or it can both end faces as the end face shown 10 be closed with a cover plate.

The coupling of the microwave signal required for the operation of the line resonator takes place through the supply line 16 , The exact coupling can be done in various ways, for example as probe-coupling, loop-coupling or aperture-coupling. It is possible for a reflective measurement of the resonance to provide only one coupling point, via which the coupling of the microwave signal and the decoupling of the resonance signal takes place. At resonance then results in a reflection minimum. For a transmission measurement of the resonance, a second coupling point is required in order to decouple or detect the resonance signal. At resonance then results in a transmission maximum.

The Microwave signal is from a tunable oscillator, not shown generated, with which predetermined frequency ranges during the measurement can be passed through to determine at what frequency in the fluid-filled line resonator a resonance occurs. This resonance frequency represents a characteristic Size for that too sensing physical property of the fluid, e.g. the permittivity. The position of the resonance determines the real part of the permittivity (the dielectric constant), the goodness the resonance is directly linked to the losses in the medium, the loss angle. About these Sizes can then by means of further information indirectly, for example the Density of the fluid or the concentration of certain fractions of the fluid Fluids are determined. Ie. at the intended resonant measurement the tunable oscillator always generates certain predetermined frequencies, there is no detuning of the oscillator due to interaction with the line resonator.

As stated, the line resonator shown, which has the described coaxial design, is a pure TEM line resonator which operates over a very wide frequency range with purely transversal electromagnetic modes (TEM). This allows more accurate measurement results, since the unique keitsbereich of the sensor over known sensors, which work with TE and TM modes, ie electromagnetic fields with field components, which are directed in all directions, have, is significantly increased. In this case, this property is largely independent of the geometric dimensions of the cavity resonator, so that the line resonator can be constructed very compact. This favors the possibility of the entire line resonator in the fluid 11 immerse as shown in the figure. It is therefore not necessary to supply the fluid through lines to the meter. Instead, a probe with a described line resonator is inserted directly into the fluid 11 immersed. In order to further increase the measuring range, ie the uniqueness range of the resonances, a plurality of the described line resonators can be used simultaneously, which in particular can also be integrated in a measuring probe.

2

inner conductor

4

outer conductor

6

gap

8th

End plate

10

faceplate

11

fluid

12

openings

14

front

16

leads

Claims (11)

Measuring device for determining the permittivity or permeability of a fluid with at least one line resonator immersible in the fluid, which is fed from a frequency-stable tunable oscillator, which is designed such that it has an external conductor ( 4 ) and one in its interior spaced from the outer conductor ( 4 ) arranged inner conductor ( 2 ), wherein at least one end face ( 10 . 14 ) of the Leader ( 4 ) and / or a peripheral wall of the outer conductor ( 4 ) Inlet openings ( 12 ) for the fluid, characterized in that the space between outer conductor ( 4 ) and inner conductor ( 2 ) is completely fillable when immersed in the medium and all openings of the resonator are designed so that the standing in the interior of the resonator standing electromagnetic wave below the cutoff wavenumber of the inlet openings ( 12 ) and thus can not leave the resonator. Measuring device according to Claim 1, in which the outer conductor ( 4 ) and the inner conductor ( 2 ) on a front side ( 10 ) or both end faces ( 10 . 14 ) are interconnected. Measuring device according to Claim 2, in which the connection ( 8th ) between external ( 4 ) and inner conductor ( 2 ) on the front side ( 10 ) or both end faces ( 10 . 14 ) forms a reactive termination of the resonator. Measuring device according to one of the preceding claims, in which the inner conductor ( 2 ) to the outer conductor ( 4 ) is arranged coaxially. Measuring device according to one of the preceding claims, in which the outer conductor ( 4 ) has a circular cross-section. Measuring device according to one of the preceding claims, in which the inner conductor ( 2 ) has a circular cross-section. Measuring device according to one of the preceding claims, in which the outer conductor ( 4 ) is tubular. gauge according to one of the preceding claims, in which the coupling a microwave signal into the line resonator via a probe coupling, Loop coupling or aperture coupling takes place. gauge according to one of the preceding claims, wherein the line resonator is operated in a frequency range of 300 kHz to 1 GHz. gauge according to one of the preceding claims, wherein the line resonator is operated in a frequency range of 1 GHz to 30 GHz. gauge according to one of the preceding claims, wherein the line resonator is operated in a frequency range of 30 GHz to 1 THz.