WO2010034668A1

WO2010034668A1 - Inductive conductivity sensor

Info

Publication number: WO2010034668A1
Application number: PCT/EP2009/062081
Authority: WO
Inventors: Marco VÖLKER; Timo Kaufmann
Original assignee: Endress+Hauser Conducta Gesellschaft Für Mess- Und Regeltechnik Mbh+Co. Kg
Priority date: 2008-09-25
Filing date: 2009-09-18
Publication date: 2010-04-01
Also published as: DE102008048995A1

Abstract

An inductive conductivity sensor for detecting the electrical conductivity of a liquid medium comprises a first toroidal coil surrounding a first penetrating opening that can be subjected to the medium, said first toroidal coil inducing a current in the medium, a second toroidal coil surrounding a second penetrating opening that can be subjected to the medium, said second toroidal coil detecting a magnetic field generated by the current induced in the medium, characterized in that the first toroidal coil is disposed at an angle relative to the second toroidal coil.

Description

Inductive conductivity sensor

The invention relates to an inductive conductivity sensor for detecting the electrical conductivity of a liquid medium, comprising a first Ringpuie enclosing a first acted upon by the medium through opening, for inducing a current in the medium, and a second annular coil, which a second with the medium actable through opening encloses, for detecting a generated by the current induced in the medium magnetic field.

Such inductive conductivity sensors function in the manner of a double transformer, wherein the first toroidal coil serves as excitation coil and the second ring pump serves as EmpfängererspuJe. To measure the conductivity, this ring coil assembly, which forms a conductivity cell, is inserted into the medium so that a current path can form around the exciter coil and the receiver coil. When the excitation coil is applied with an AC signal, it generates a magnetic field which induces in the current path a current whose magnitude depends on the electrical conductivity of the medium. This current, which is also an alternating current, is measured inductively with the receiver coil. Therefore, the alternating current supplied by the receiver coil from the output signal or a corresponding alternating voltage supplied by the receiver coil is a function of the electrical conductivity of the medium to be examined.

For applying the excitation coil with such an alternating voltage signal, an inductive conductivity sensor comprises a transmitting device electrically connected to the excitation coil coil for supplying the coil with an alternating voltage and a receiving device electrically connected to the receiver coil Forwarding of the output signal of the receiver coil, so the measurement signal to the measuring electronics of the conductivity sensor, which optionally digitizes the measurement signal and determined by means of a microcontroller from the measurement signal, the conductivity measured value. The measuring signal and / or the conductivity measured value can then be forwarded to a higher-level unit and / or output via a display unit.

Inductive conductivity sensors of this type are known, for example, from US Pat. No. 3,603,873, DE 198 51 146 A1. DE 41 16 468 A1. DE 10 2006 025 194 A1 and DE 10 2006 056 174 A1.

Inductive conductivity sensors have a typical measuring range of about 20 μS / cm to 2 S / cm. The measurement range at low conductivities is limited by the fact that the signals of the receiver coil are very small and are superimposed by the residual coupling signal (output signal of the receiver coil in the absence of a medium). The Restkoppiungssignal is composed in particular of a capacitive coupling of the leads and toroidal coils and a parasitic inductive coupling of the Ringspuien.

The parasitic inductive coupling of the toroidal coils is due to an interference field, which is oriented perpendicular to the main component of the magnetic field of the individual toroidal coils, which extends along an annular closed path running within the toroidal coil.

From the prior art it is known to shield the interference field by means of high-permeability magnetic materials. For example, the toroidal coil can be encapsulated in a receptacle of post-annealed high permeability material, eg, Mumetal. This kind However, the shield has disadvantages. The high permeability of the Mumetalls decreases with cold deformation, so that the Mumetall has to be laboriously finished after processing in order to restore the high permeability. Obtaining such prefabricated shields for the production of shielded toroidal coils, for example for use in the manufacture of conductivity sensors, is correspondingly expensive. Care must also be taken with this type of production that further deformation of the prefabricated shield again leads to a reduction in the permeability.

In US 3,806,798 an inductive conductivity sensor with two Ringspuien for measuring the conductivity of a fluid is described, wherein the toroidal coils each having a ferromagnetic ring core. To form each toroidal coil, an electrical conductor is in a first layer to form a first coil winding composed of a plurality of single turns and extending between a start and an end point, both on the toroidal core, about the toroidal core wound. Between the end point and the starting point, the same electrical conductor is further wound in a second layer to form a second coil winding around the toroidal core, the second coil winding being composed of the same number of individual turns having the same winding sense as the first coil winding however, their winding progress in the opposite direction is like the winding progress of the first coil winding. In this way, when the current flows through the electrical conductor, the interference field generated by the first coil winding should be substantially compensated by the corresponding interference field of the second coil winding.

In order to achieve a complete mutual compensation of the interference fields of the two coil windings, it comes in the Ringpuienanordnung of US 3,806,789 on the fact that the individual turns have the same slope and are oriented as exactly symmetrical to each other, so that occur at each point of the winding two opposite Magnetfeidkomponenten substantially equal strength. However, the production of such a coil winding with sufficient accuracy is extremely complex and implement only with great effort.

In US 4,740,755 it is stated that a coplanar and axially parallel coil arrangement of two toroidal coils in a conductivity sensor should reduce the residual coupling. However, this flexibility is lost in the arrangement of the toroidal coils and the structural design of the conductivity sensor. Furthermore, due to the design, the cell constant of such a coil arrangement is many times higher than in a Ringpuienanordnung in which the toroidal coils are arranged coaxially one behind the other, since the cell constant is proportional to the length of the current path. Such an increase in the cell constant leads to a substantial reduction in the dynamic range of the sensor.

It is therefore an object of the present invention to provide an inductive conductivity sensor which overcomes the disadvantages of the prior art. In particular, a conductivity sensor is to be specified, which has an acceptable ZeN constant with reduced parasitic inductive coupling.

This object is achieved by an inductive conductivity sensor for detecting the electrical conductivity of a liquid medium, comprising a first annular coil, which encloses a first through opening which can be acted upon by the medium, for inducing a flow in the medium, a second annular coil which encloses a second through-acting with the medium through opening, for detecting a magnetic field generated by the current induced in the medium, wherein the first annular coil is arranged inclined relative to the second annular coil.

The tendency of the toroidal coils against each other causes the Störfeider the toroidal coils overlap only slightly, resulting in a reduced inductive coupling of the coils. At the same time, as described in more detail below, the cell constant of such an arrangement is significantly lower than in a coplanar paraxial ring coil arrangement. The inclined arrangement of the coils thus represents a compromise between a conductivity sensor with koaxiai successively arranged toroidal coils, which has the advantage of a low cell constant, the interference fields of the toroidal coils maxmia! overlap, and a coplanar achsparailelen ring coil arrangement in which the parasitic inductive coupling of the toroidal coils is very low due to minimal overlap of the Störfeider, but on the other hand, has a high Zelikonstante.

The term "toroidal coil" here and in the following designates a coil with a closed magnetic path The coil may have a magnetic or magnetizable core or be designed as a coreless coil The magnetic path must be self-contained or at least bridged by air gaps The ring shape is the simplest form, but any other shapes are conceivable, such as ellipses, rectangles, or other polygons, such as a toroid having a central axis If the toroidal coil has no cylindric symmetry, but instead a cylindrical axis-symmetrical annulus coil is a rotational symmetry axis For example, designed as an ellipse or as a polygon, the central axis extends, for example, through the center of the polygon or through a central point located between the ellipse focal points within the ellipse.

If, therefore, a first annular coil is inclined relative to a second annular coil, this means that their central axes-in the case of cylindrically symmetrical coils-are rotational symmetry axes-intersect, i. an angle greater than 0 ° and less than 180 °.

The angle subtended by the central axes here and hereinafter means, in particular, the angle between a first half-line, which runs from the intersection of the central axes of the first and second toroidal coils along the central axis of the first toroidal coil through the first toroidal coil, and a second half-line which, starting from the point of intersection of the central axes of the first and second toroidal coils, runs along the central axis of the second toroidal coil through the second toroidal coil.

In a presently preferred embodiment, the central axes of the toroidal coils enclose an angle of more than 0 ° and less than 100 °, in particular between 10 ° and 100 °, in particular between 30 ° and 100 °. Particularly preferred is an angle of 90 °.

Alternatively, the coils can also be arranged relative to one another such that, for example within the scope of the manufacturing tolerance, their central axes are slightly skewed relative to one another. In this case, the inclined arrangement of the first ring coil relative to the second ring coil means that at least one imaginary surface exists, in which both axes can be projected, and in which the projected axes intersect. In one embodiment, the coils are arranged in a common housing such that the central openings of the toroidal coils enclosed by the toroidal coils are connected to one another by at least one bore in the housing.

In a particularly advantageous development, the openings are connected to each other by a single cylinder bore in the housing. On the one hand, this is particularly easy to manufacture, on the other hand, a continuous cylinder bore can be flowed through particularly well by the measuring medium, so that no turbulence or air bubbles within the bore or within the through opening enclosed by the toroidal coils interfere with the measurement.

In an advantageous embodiment, the housing has additional holes, are guided by the conductor for electrical contacting of the toroidal coils.

In one embodiment, the housing consists of steel, in particular of steel with a permeability of more than 500, in particular more than 1500. Thus, the housing acts as an electrical and magnetic shielding of the toroidal coils, whereby the coupling of unwanted electrical and / or magnetic fields further reduced becomes. When the housing is used as an electrical shield, it is advantageously grounded.

In one embodiment, which is advantageous in particular for the case where an additional magnetic shielding of the toroidal coils is required, at least one of the toroidal coils is arranged in a housing mounted in a housing made of a material with high permeability, in particular more than 5000 , Similarly, an additional electrical shielding may be provided.

In a further embodiment, at least one of the toroidal coils is integrated in a multilayer printed circuit board, wherein the windings of the toroidal coil pass through a multiplicity of first conductor sections that run in a first plane of the printed circuit board, a multiplicity of second conductor sections that run in a second plane of the printed circuit board, and a variety of

Via contacts, which connect the first conductor sections with the second conductor sections, are formed.

A multilayer printed circuit board comprises a plurality of layers or layers stacked one above the other in a stacking direction, in which conductor tracks or conductor sections or other components can be arranged.

Such integrated in a printed circuit board Ringspuien can be prepared with conventional techniques of printed circuit board manufacturing, for example in the so-called PCB (printed circuit board) technology, such as in O. Dezuari, SE Gilbert, E. Belloy, MAM Gijs, "A new hybrid technology for pianar microtransformer fabrication ", Sensors and Actuators A 71, 1998, pp. 198-207, this type of fabrication is on the one hand easily automatable, on the other hand, the production of a toroidal coil with a PCB technique allows a higher precision in the production By making coil winding with higher precision, the turns can be made more uniform, resulting in more homogeneous and thus more predictable stray fields of the toroidal coils that are more easily compensated may be considered the more inhomogeneous truncation of a conventional ringp ule. The invention will now be explained with reference to the embodiments illustrated in the drawings. Show it:

Fig. 1 is a schematic longitudinal sectional view of a

Ring coil assembly in a conductivity sensor with a relative to the receiver coil inclined arranged transmitter coil;

FIG. 2 shows a schematic longitudinal section illustration of two annular coils of a conductivity sensor arranged in an inclined manner relative to one another in a housing according to a first embodiment; FIG.

3 shows a schematic longitudinal section illustration of two ring coils of a conductivity sensor arranged in an inclined manner relative to one another in a housing according to a second embodiment;

4 is a diagram of experimentally determined current signals of a

Receiver coil as a function of the resistance of a Leiterschleife passing through the Empfängererspuie and an exciter coil in various ring coil assemblies;

5 shows a diagram with curves of a normalized curve calculated for different ring coil arrangements

Leetfähigkeitssensor output signal as a function of normalized conductivity of a liquid medium.

In Fig. 1 is a circuit according to the invention Ringpuienanordnung 1 for an inductive conductivity sensor with the corresponding Interference fields shown. The excitation coil 3, which is shown greatly simplified in longitudinal section as a rectangle, is configured as an annular coil with a coil winding, not shown, which encloses a through opening 4 which can be acted upon by the measuring medium. The exciter coil 3 has a central axis Z3. If the excitation coil 3, for example, designed cylindrically symmetric, the central axis Z3 forms a rotational symmetry axis. The associated receiver coil 5 is also an annular coil shown in longitudinal section as a rectangle with a coil winding, not shown, which encloses a through opening 6, and has a central axis Z5. In the case that the receiver coil 5 is designed cylindrically metric, the central axis Z5 forms a rotational symmetry axis of the receiver coil fifth

FIG. 1 also shows the magnetic interference field 7 generated by current flow from the exciter coil 3 as well as the magnetic interference field 9 generated accordingly by the current flowing from the receiver coil 5. The interference fields 7 and 9 cause substantial overlap with the initially described parasitic inductive coupling of the exciter coil 3 with the receiver coil. 5

The receiver coil 5 is arranged inclined relative to the exciter coil 3. The central axes Z3 and Z5 intersect as a result at the intersection S. The angle between a first half-line H1, which extends from the intersection S along the central axis Z3 through the exciter coil 3, and a second half-line H2, which starting from the intersection point S along the central axis Z5 passes through the receiver coil 5, corresponds to the angle α included by the central axes Z3 and Z5. In the example of FIG. 1, this angle α = 90Λ This leads, as seen in Fig. 1, to a relatively small overlap of interference fields 7 and 9, which in particular is significantly lower, ais this would be the case with a coaxial arrangement of the exciter coil 3 and the receiver coil 5 in succession. The lower overlap of the interference fields leads to a reduced parasitic inductive coupling of the two toroidal coils.

FIG. 2 shows an example of a toroidal coil assembly 201 housed in a carrier according to the principle described in connection with FIG. 1 for use in a conductivity sensor for measuring the conductivity of a liquid medium.

The exciter coil 203 and the receiver coil 205 are again designed as ring coils. In Fig. 2, the receiver coil 205 and the exciter coil 203 are shown schematically in longitudinal section as rectangles. They are fixed in a housing 211, which has a bore 215, which can be flowed through by the liquid medium, so that the through openings 204 and 206 of both annular coils 203 and 205 are acted upon simultaneously with the medium. In this way, in the sensor operation, a current path 213 can form through the medium whose conductivity is to be measured, which extends in part through the bore 215. The bore 215 comprises two cylindrical sections, the cylindrical axes of symmetry each extending along one of the central axes of the ring coils 203 and 205 or parallel thereto.

An advantageous variant for a ring coil arrangement 301 housed in a housing according to the principle described in connection with FIG. 1 is shown in FIG. The excitation coil 303 and the receiver coil 305 are formed in a similar manner in this arrangement as in the Ringpuienanordnungen shown in Fig. 1 and Fig. 2. Their central axes in turn include an angle α of 90 °. The toroidal coils 303 and 305 are fixed in a housing 311, wherein for the electrical contacting of the ring coils 303 and 305 additional holes 319 are provided in the housing 311 through which conductors for electrical contacting of the toroidal coils 303 and 305 can be guided. The housing 311 is grounded and thus simultaneously serves as an electrical shield for the toroidal coils 303 and 305. The housing 311 is provided with a bore 315 through which a liquid medium can flow, so that the through openings 304, 306 of both annular coils 303 and 305 are simultaneously acted upon by the medium and can form in operation a current path 313, which passes through both Rängspulen 303 and 305 and then closes around the coils.

In contrast to the exemplary embodiment shown in FIG. 2, however, a single through-cylinder bore 317 is provided here, its cyan axis of symmetry with the central axes of the annular coils

303 and 305 includes an angle of about 45 °. Modifications of this geometry are conceivable, but it is crucial that the

Bore 317 is designed as a single continuous cylinder bore, so that no kinking of the bore 317 required estst to the

Actuation of both through openings of the Ringspuien with the

To ensure medium. This allows a comparison with the embodiment shown in FIG. 2, a simplified manufacturing, since the

Drill tool for producing the bore 317 in the housing 311 is only once to set.

Using the method described in Roos et al. Modeling of Inductive Conductivity Sensors, Proceedings Simulations with the Finite Element Method in Precision Engineering and Microtechnology, 1996 model for inductive conductivity sensors with coaxially arranged ring coils became the cell constant of the arrangement according to FIG. 3 at 10 to 15 cm ^-1 estimated. Although this value is even greater than the cell constant of a coaxial ring coil arrangement in which the toroidal coils are arranged axially one behind the other (this is in the order of 5 cm ^"1 ), but significantly less typical cell constants of coplanar axis-parallel ring coil arrangements (order of magnitude of 50 cm ^"1 ). The cell constant of the toroidal coil arrangement according to FIG. 3 is still somewhat below the cell constant of the toroidal coil arrangement according to FIG. 2.

In FIG. 4, measurement signals of the receiver coil are shown in three different ring coil arrangements, in which

Measuring structure instead of a liquid medium a Leiterschieife is provided such that sow the field coil and the receiver coil as the

Current path interspersed in a liquid medium and surrounds. The

Conductor loop thus replaces the current path that would form in a liquid medium when the excitation coil is exposed to an alternating voltage.

If the exciter coil is subjected to an alternating voltage, then a current is induced in the conductor loop whose current strength depends on the resistance of the conductor loop. This current is measured inductively with the receiver coil. The measuring current plotted on the ordinate of the diagram in FIG. 4 corresponds to the alternating current signal output by the receiver coil. The abscissa shows the conductor loop resistance. Both abscissa and ordinate have a logarithmic scale. In the experiment, the conductor loop resistance was varied by switching in resistor elements with different ohmic resistance. The so-called "characteristic curve" of a toroidal coil arrangement thus reproduces the strength of the measuring signal as a function of the ohmic resistance of the measuring medium (which is reciprocal to the conductivity), in the present case the conductor loop. only to determine the residual coupling, without the cell constant of the toroidal coil assembly enters into the output from the receiver coil current signal.

In Fig. 4, three characteristics are plotted for three different ring coil assemblies. In the first ring coil arrangement (crosses) exciter and receiver coils are arranged axially one behind the other. In the second Ringpuienanordnung (squares) exciter and receiver are coplanar achsparaliel arranged in the third Ringpuienanordnung (circles) exciter and receiver coil are arranged such that their central axes enclose an angle of 90.

As can be seen from the diagram shown in FIG. 4, the three arrangements in a Wäderstandsbereich between 10 ² and 5-10 ⁴ Ω a comparable, linear behavior. With higher resistances and correspondingly lower conductivities, the residual coupling of the ring coil arrangements becomes noticeable. Thus, the measured characteristic is yet to a resistivity of 10 ⁶ Ω approximately linearly, for example, the coplanar Ringspuienanordnung, while the corresponding characteristic for the first Ringspuienanordnung, wherein the coils are arranged axially one behind the other, already at a resistance of approximately 5-10 ⁴ Ω kinks and runs at higher resistance values substantially parallel to the abscissa. This is an indication of the influence of the residual coupling on the output signal of the receiver coil. As stated at the outset, the residual coupling is essentially caused by the undesired parasitic inductive coupling between the excitation coil and the receiver coil.

The characteristic curve of the third ring coil arrangement according to the invention deviates from a linear resistance only at a resistance of approximately 5 10 ⁵ Ω. The influence of the residual coupling is therefore significantly lower here than in the coaxial first ring coil arrangement.

As mentioned previously, the cell constant of the respective toroidal coil assembly does not matter in the experiment underlying the characteristics shown in the graph of FIG. FIG. 5 shows simulated traces of a normalized lightness sensor output signal in the three considered ring coil assemblies as a function of normalized conductivity in a liquid medium. In the calculations, only the cell constant of the respective ring coil arrangement was taken into account, while all other proportionalities, for example the proportionality to the input voltage, the square of the number of turns, etc., were neglected. Therefore, the characteristics shown in FIG. 5 only represent a relative comparison of the individual ring coil arrangements with one another. Absolute readings can not be taken from the chart.

In FIG. 5, analogous to FIG. 4, three curves of the normalized output signals for three different respective ränge coil arrangements in a conductivity sensor in a liquid are plotted. In the first ring coil arrangement (crosses) exciter and receiver coils are arranged axially one behind the other. In the second ring coil arrangement (squares) exciter and receiver coil are arranged coplanar axis-parallel. In the third ring coil arrangement (circles), the exciter and receiver coils are arranged such that their central axes enclose an angle of 90 °. On the ordinate, a normalized output signal of the conductivity sensor corresponding to a current signal of the receiver coil is plotted, on the abscissa a normalized conductivity of the liquid is plotted. The abscissa and ordinate each have a logarithmic scale. As can be seen from FIG. 5, the third ring coil arrangement, in which the central axes of the receiver and exciter coil enclose an angle of 90 °, has an equally large linear measuring range as the second toroidal coil arrangement with coplanar axis-parallel toroidal coils, namely from unit-less normalized conductivity values of more than a value of 10 ^"4. in the area between the unitless normalized conductivity values ^{5-10" 6} and l O ^"4, the third annular coil assembly shows less deviation from the ideal linear behavior than the second annular coil arrangement. For use in an inductive conductivity sensor Thus, to measure the conductivity of a liquid medium, the third toroidal coil assembly, in which the central axes of the excitation and receiver coils enclose an angle of 90 °, is particularly well suited because it compromises between a toroidal coil assembly having coaxially successively disposed toroidal coils that support the Advantageous having a low cell constant, but the interference fields of the Ringspuien overlap maxmial, and a coplanar paraxial ring coil arrangement in which the parasitic inductive coupling of the toroidal coils is very low due to minimal overlap of the interference fields, but on the other hand has a high cell constant represents.

Claims

claims

An inductive conductivity sensor (1, 201, 301) for detecting the electrical conductivity of a liquid medium, comprising a first annular coil (3, 203, 303) which encloses a first through-acting with the medium through opening, for inducing a current in the medium a second annular coil (5, 205, 305), which encloses a second through-acting with the medium through opening, for detecting a generated by the current induced in the medium magnetic field, characterized in that the first annular coil (3, 203, 303) opposite the second Rängspule {5, 205, 305) is arranged inclined.

2. conductivity sensor (1, 201, 301) according to claim 1, wherein the annular coils (3, 5, 203, 205, 303, 305) has a central axis (Z3, Z5), in particular a Rotationsssymmetheachse, and wherein the central axis (Z3) of the first annular coil (3, 203, 303) with the central axis (Z5) of the second annular coil (5, 205, 305) an angle (α) of more than 0 ° and less than 180 °, in particular of 90 ° , includes.

3. conductivity sensor (1, 201, 301) according to claim 1 or 2, wherein the toroidal coils (203, 205, 303, 305) in a housing (211, 311) are arranged such that the of the toroidal coils (203, 205, 303, 305) enclosed central openings of the toroidal coils (203, 205, 303, 305) by at least one bore (215, 315) in the housing (211, 311) are interconnected.

4. Conductivity sensor according to claim 3, wherein the central openings are interconnected by a single cylindrical bore (315) in the housing (311).

5. Conductivity sensor according to one of claims 3 or 4, wherein the housing (211, 311) has additional bores (319), are guided by the conductor for electrically contacting the Ringspuien (203, 303, 205, 305).

6. Conductivity sensor according to one of claims 3 to 5, wherein the housing (211 ₃ 311) made of steel, in particular of steel! having a permeability of more than 500, in particular more than 1500 exists.

7. Conductivity sensor according to one of claims 3 to 6, wherein at least one of the toroidal coils (203, 205, 303, 305) arranged in a housing (211, 311) receptacle made of a material having a high permeability, in particular more than 5000, respectively is.

8. Conductivity sensor according to one of claims 1 to 7, wherein at least one of the toroidal coil (3, 5, 203, 205, 303, 305) is integrated in a multilayer printed circuit board, wherein the windings of the toroidal coil by a plurality of first conductor sections, which in a first level of the circuit board, a plurality of second conductor sections extending in a second plane of the circuit board, and a plurality of vias, which connect the first conductor sections with the second conductor sections are formed.