WO2008151597A1 - Inductive sensor arrangement and method for influencing the measuring behaviour of a measurement coil - Google Patents

Abstract

The invention relates to an inductive sensor arrangement comprising a measurement coil (2), wherein an oscillator (4) supplies the measurement coil (2) with an alternating voltage. In order to improve the measurement behaviour of the sensor arrangement and to reduce sensitivity in relation to lateral closeness, the invention is characterised in that an additional coil (3) is arranged about the measurement coil (2) and the additional coil (3) is connected to an amplifier (6) via which the additional coil (3) can be supplied with voltage (U2) derived from the alternating voltage of the oscillator (4, 8). The invention also relates to a corresponding method.

Description

 INDUCTIVE WORKING SENSOR ARRANGEMENT AND METHOD FOR INFLUENCING THE MEASUREMENT BEHAVIOR

A MEASURING SPOOL

The invention relates to an inductively operating sensor arrangement with a measuring coil, wherein an oscillator supplies the measuring coil with an alternating voltage. The invention further relates to a method for influencing the measurement behavior of a measuring coil, wherein the measuring coil is acted upon by an oscillator with an alternating voltage.

Inductive sensor arrangements play an important role in metrology. They can be used anywhere where electrically and / or magnetically conductive objects are to be detected. If such objects are in the measuring range of an inductive sensor, they have an effect on the sensor. The reaction can be used as a measuring signal. The most important fields of application are distance or proximity sensors as well as distance measuring sensors.

There are basically two different versions of inductive sensors known. In one embodiment, an electromagnetic field is generated by a coil, which induces eddy currents in a conductive object located in the measuring range of the sensor. These eddy currents affect the impedance of the coil (Lenz's rule). Thereby, both the real and the imaginary part of the impedance of the coil is changed as a function of the distance of the coil to the object. This change is used as a measurement signal and allows conclusions about the distance of the coil to the object.

Another measuring principle uses two separate coils, one coil exciting an electromagnetic field and the second coil detecting the field. A conductive measurement object located in the measurement field of this arrangement influences the coupling factor between the exciter coil and the detection coil. If the excitation and detection coils are located on the same side relative to the measurement object, the coupling between the coils increases. Such a system is known for example from DE 195 23 519 A1 or JP 57200803 A. On the other hand, if the measurement object is located between the excitation coil and the detection coil, this reduces

BESTATIGUNGSKOPIE DUT the coupling. The change in the coupling can be detected metrologically and used both for distance and distance measuring sensors.

A problem with the known sensor arrangements is that the sensors have a relatively high sensitivity to objects which approach the sensor laterally. This problem is addressed in practice with different approaches. According to one approach, the measuring coil is shielded to the rear and laterally by a metallic housing. By this measure, the measuring coil is influenced only by approaching an object from the front of the sensor front. The problem with this approach, however, is that the quality of the coil is reduced compared to an air coil, since the impedance is already changed by the shielding. Thus, the sensitivity is reduced to the approach of a DUT, which is reflected in a smaller usable measurement range and a lower resolution of the sensor compared to an air coil of the same size.

According to another approach known from practice, the coils are designed with a magnetic core. Due to the core, the coils have a higher attainable quality compared to air-core coils and thus greater sensitivity. Depending on the shape of the core, a bundling of the electromagnetic field is additionally achieved. A relative insensitivity to lateral approaches provides an embodiment of the core as a shell core.

A disadvantage of such coils is that they react sensitively to acting magnetic fields. Additional magnetic fields change the permeability of the core, which in turn affects the coil impedance. Therefore, these coils can not be used in environments in which magnetic fields are present (for example, in electric motors, lifting magnets or the like). In addition, the permeability of the core, especially at negative temperatures and temperatures above 100 ⁰ C, strongly dependent on the temperature of the core. The maximum achievable manufacturing tolerances of several percent in the permeability and the mechanical dimensions continue to have a negative impact. Coils with larger diameters (over about 30 mm) are difficult or impossible to realize because the production of the cores is difficult and extremely expensive. For detecting errors and noise, arrangements are known in which a plurality of coils are arranged one above the other and serve a coil for the detection of an error signal. Such a system is shown in DE 33 36 783 A1.

The present invention is therefore based on the object, a sensor arrangement and a method of the type mentioned in such a way and further, that the measurement behavior of inductive sensor assemblies is improved and in particular a reduction in sensitivity to lateral approaches is achieved.

According to the invention the above object is solved by the features of claim 1. Thereafter, the sensor arrangement in question is characterized in that a further coil is arranged around the measuring coil and that the further coil is connected to an amplifier, via which the further coil can be fed with a voltage derived from the alternating voltage of the oscillator voltage.

In procedural terms, the above object is achieved by the features of claim 16. Thereafter, the method in question is characterized in that arranged around the measuring coil further coil is acted upon by means of an amplifier and / or phase shifter with a voltage which is derived from the output by the oscillator AC voltage, and that by the further coil the radiation characteristic the sensor arrangement is influenced.

According to the invention, it has first been recognized that the measurement behavior of an inductive sensor arrangement can also be positively influenced without encapsulation of the sensor coil and without the use of magnetic cores. Instead, comparatively simple measures which interfere less strongly with the behavior of the sensor arrangement are sufficient. For this purpose, a further coil is arranged around the actual measuring coil, which at least partially surrounds the measuring coil. The measuring behavior of this extended measuring arrangement can be improved in a surprisingly simple manner by feeding the further coil with a voltage which is derived from the alternating voltage supplying the measuring coil. For this purpose, the output from the oscillator AC voltage is tapped and fed via an amplifier of the other coil. By this measure, a change in the impedance of the other coil has a very small or no influence on the behavior of the measuring coil. The measuring coil, however, can be operated independently of the other coil. However, the fields of the measuring coil and the other coil are superimposed in such a way that an active shielding of the measuring coil against side influences is achieved. In addition, the use of the additional coil and its special feed means that the measuring field is emitted in a much more concentrated manner than conventional sensor arrangements. As a result, the local resolution of the coil arrangement is further increased.

To increase the flexibility of the application, a phase shifter could be placed before or after the amplifier. As a result, it is possible to influence the phase position between the alternating voltage supplying the measuring coil and the further coil, whereby the superimposition of the fields delivered by the two coils can be controlled. By selecting the phase position between the two voltages, therefore, the characteristic of the radiated measuring field can be influenced to a great extent.

Preferably, the measuring coil and the further coil are configured such that their winding axes are substantially parallel to each other. Preferably, the two winding axes substantially coincide, whereby the two coils are formed coaxially. It is essentially irrelevant how the two coils are constructed. Thus, the coils may be formed by a wire winding. On the other hand, a conductor track applied to a carrier could form a coil. For this purpose, a number of embodiments is known from practice.

The coils used could be designed as air coils, that is, there is no magnetic core disposed inside the coil. As a result, the sensor arrangement could also be used in environments with magnetic interference fields. An influence of the core by the interference fields is omitted. To ensure a coordinated superposition of the magnetic fields of the measuring coil and the further coil, the two coils could be arranged such that they terminate substantially flush in the measuring direction.

The amplifier, which provides the supply voltage for the further coil, could both amplify (gain greater than 1) and possibly attenuate the gain to be amplified (gain less than 1). Also, an impedance conversion (gain equal to 1) could be realized. Similarly, the phase shifter could be used to different degrees of phase shifts. Phase shifts of 0 ° can be realized as well as shifts of nearly 360 °.

Preferably, the amplifier is controllable in its gain, whereby the gain factor can be adjusted depending on the application situation. An optionally present in addition to the amplifier phase shifter could also be designed controllable. This configuration makes it possible to influence the individual magnetic fields and the total field output in a particularly flexible manner.

The measuring coil and the further coil could be designed such that the field of the further coil amplifies or reduces the field of the measuring coil. For this purpose, a suitable structural design of the coils can be used. On the other hand, by a suitable choice of the amplification factor and the phase position, the superposition of the individual fields can be changed. Fixed gain factors and phase shifts can be used as well as customizable ones.

The oscillator supplying the measuring coil could deliver a fixed-frequency voltage. In addition, the voltage could have a fixed amplitude. Corresponding oscillators are well known in practice.

Alternatively, the oscillator could be designed to be free-swinging. In this case, the oscillator could be formed by a resonant circuit, which contains, among other things, the measuring coil. This would reduce the frequency emitted by the oscillator to change the impedance of the measuring coil. This embodiment is used, for example, in a detection of impedance changes application.

The voltage that is used to derive the supply voltage for the other coil, could be tapped at various points. On the one hand, the tapping off of the voltage could take place after a coupling impedance, via which the measuring coil is connected to the oscillator. As a result, the supply voltage of the other coil follows the supply voltage with a defined phase position. However, the voltage used to derive the supply voltage of the further coil could also be tapped immediately after the oscillator.

Preferably, the supply of the amplified and possibly phase-shifted voltage in the further coil is low impedance. The feed impedance is preferably close to 0 Ω.

On the one hand, the sensor arrangement could be designed in such a way that the measuring coil, in conjunction with the further coil, both serves to generate an electromagnetic field and realizes a detection of the conductive materials. This is achieved in particular by measuring the impedance or its change as a function of conductive materials in the measuring range of the sensor arrangement. For this purpose, various methods are known from practice.

On the other hand, the sensor arrangement could be designed in such a way that the measuring coil generates an electromagnetic field in conjunction with the further coil and in addition a detection coil is provided. The detection coil would then serve to detect conductive materials. In this case, the coupling factor between the measuring coil and the detection coil would be determined.

To achieve a still further improved spatial resolution of the sensor arrangement, the detection coil could in turn have a further coil which is arranged around the detection coil. This would further increase the spatial resolution of the sensor array. When operating such a sensor arrangement, for example the one described above, the emission characteristic of the sensor arrangement is controlled by applying a further coil arranged around the measuring coil with a voltage derived from the supply voltage of the measuring coil. For this purpose, an amplifier and / or a phase shifter are used. It should be ensured that influencing the characteristics of the other coil as little or no influence on the supply voltage of the measuring coil takes.

For controlling the emission characteristic of the sensor arrangement, the supply voltage of the further coil is preferably influenced. The change in the supply voltage could be to influence the gain of the amplifier. Additionally or alternatively, the phase shift caused by the phase shifter could be influenced. As a result, the radiation characteristic of the measuring coil can be influenced in a wide range. However, the gain and / or the phase shift could also be fixed. Although the radiation characteristic can not be changed in this way, it can be set according to the specifications of the application case during the construction of the system.

There are now various possibilities for designing and developing the teaching of the present invention in an advantageous manner. For this purpose, on the one hand to the claims 1 and 16 subordinate claims and on the other hand to refer to the following explanation of preferred embodiments of the invention with reference to the drawings. In conjunction with the explanation of the preferred embodiments of the invention with reference to the drawings, also generally preferred embodiments and developments of the teaching are explained. In the drawing show

1 shows a section through a usable in the sensor assembly according to the invention measuring coil with another coil and a frontal view of the sensor assembly,

2 shows a sensor arrangement according to the invention, in which a tap of the voltage to be amplified takes place after a coupling impedance, 3 shows a diagram of a measurement of the distance of a measuring coil to a measuring object using a sensor arrangement known from practice,

4 is a diagram of a distance measurement corresponding to the measurement on which FIG. 3 is based, however, using a sensor arrangement according to the invention, FIG.

5 shows a sensor arrangement according to the invention, in which a tap of the voltage to be amplified takes place before a coupling impedance,

6 shows a sensor arrangement according to the invention, in which a tap of the voltage to be amplified takes place immediately after an oscillator,

FIG. 7 shows a sensor arrangement known from the prior art with a separate excitation and detection coil which detects a massive object to be measured, FIG.

8 shows the sensor arrangement according to FIG. 7, with which a wire grid is detected as the object to be measured, FIG.

9 is a distance diagram obtained with a sensor arrangement according to FIG. 7 or FIG. 8, FIG.

10 shows a sensor arrangement according to the invention, which has a separate exciter and detection coil,

11 is a distance diagram obtained with a sensor arrangement according to the invention shown in FIG. 10, FIG.

12 shows a sensor arrangement according to the invention, in which both the exciter and the detection coil have an additional coil. 1 shows a sensor arrangement 1, which consists of a measuring coil 2 and a further coil 3 arranged coaxially therewith. The measuring coil 2 and the further coil 3 are designed such that the measuring coil 2 can be accommodated in the interior of the further coil 3. In this case, the measuring coil 2 and the further coil 3 are arranged such that the two coils terminate flush in the measuring direction (directed to the right in FIG. 1). The two coils 2, 3 are configured as air coils, wherein the further coil 3 has more windings than the measuring coil 3. Such a sensor arrangement 1 is used in the described in connection with the further figures embodiments of the invention. 2 shows a first exemplary embodiment of the invention, in which the measuring coil 2 is used both for generating an electromagnetic field and for detecting conductive materials in the measuring range of the measuring coil 2. The measuring coil 2 is fed by an oscillator 4 via a coupling impedance 5 with an alternating voltage U ₁ . The oscillator 4 outputs to an AC voltage U ₁ with fixed frequency and amplitude. As a result, the measuring coil 2 generates an electromagnetic field F ₁ .

To feed the other coil 3 with a derived from the supply voltage U _{1 of} the measuring coil 2 voltage U ₂ , the voltage U _{1 is} tapped after the coupling impedance 5 and fed to an amplifier 6 with adjustable gain. A phase shifter 7 arranged downstream of the amplifier 6 can influence the phase position. This can cause an adjustable phase shift (under certain circumstances also equal to zero). The voltage U ₂ generated in this way is fed to the further coil 3 at low impedance (feed impedance as close as possible to 0 Ω). As a result, this generates an electromagnetic field F ₂ , which is superimposed on the field F _{1 of} the measuring coil 2. Depending on the amplitude and phase position, gains or reductions in the field strength result in the field area. The fields F ₁ and F ₂ will automatically have the same frequency.

The amplitude and phase ratio of the voltages U ₁ and U ₂ at the two coils 2, 3 remains constant when approaching a DUT, since the voltage U ₂ at the other coil 3 is derived directly from the voltage U ₁ at the measuring coil 2. As soon as the field F _{1 is changed} by a conductive object in its measuring range, the field F ₂ changes accordingly. A conductive object, which is largely only in the field area F ₂ , but has no or only a small influence on the impedance - and thus on the measurement signal - the measuring coil 2. The field of the other coil 3 shields the measuring coil 2 thus by superimposing against lateral influences. The shielding effect, and the field focusing of the field F ₁ can be influenced U ₂ for the further coil 3 by adjusting the amplitude and the phase position of the voltage. The quality of the measuring coil 2 is not reduced, as for example in the case of a metal-shielded sensor, and thus, with the same coil diameter of the measuring coil 2, the same detection distance results as with an unshielded standard measuring coil. With suitable values of the coils and suitable settings of the amplitude and phase relationships, a significantly better spatial resolution of the measurement can be achieved.

As an example of the effectiveness of the field focussing achievable by the sensor arrangement 1 according to the invention, FIGS. 3 and 4 are considered. Both figures each show diagrams of distance measurements in which an inductive sensor has been moved past a metal lid located on a container. The lid has a deflection due to a negative pressure in the container. FIGS. 3 and 4 each show a series of measurements made at different pitches. The ordinate of the diagrams shows the distance of the measuring coil to the cover, while the abscissa represents the displacement of the sensor.

3 shows a distance diagram of a measurement taken with a sensor arrangement known from practice with only a simple measuring coil. The deflection of the lid is not visible even at a close basic distance. The signal is much wider than the cover diameter due to the wide field of view.

In contrast, the distance diagrams in FIG. 4 are obtained with a sensor arrangement 1 according to the invention. It can be clearly seen that a considerable improvement of the spatial resolution can be achieved even with a relatively large measuring distance. In addition, the width in the diagram corresponds much better to the actual diameter of the lid. Another embodiment is shown in FIG. The circuit corresponds essentially to that of FIG. 2, but the tap is located for the derivation of the supply voltage U ₂ for the further coil 3 immediately after the oscillator 4 before the coupling impedance. 5

The measuring coil 2 is in turn fed by an oscillator 4 via a coupling impedance 5 with a fixed frequency and amplitude. The output by the oscillator 4 AC voltage is tapped via an amplifier 6 with adjustable gain and with (or without) phase shift via a phase shifter 7 low impedance (feed impedance as close as possible 0 Ω) fed into the further coil 3. As a result, a second electromagnetic field F _{2 is} generated, which is superimposed on the field F ₁ automatically with the same frequency. However, the amplitude and phase relationship of the voltages at the two coils does not remain constant when approaching a measuring object to the measuring coil 2-in contrast to the first exemplary embodiment. The voltage U ₂ at the other coil 3 is constant by the tap directly after the oscillator 4 and does not change with changing the impedance of the measuring coil.

A conductive object, which is largely only in the field region F ₂ , has no or only a small influence on the impedance of the coil and thus on the measurement signal. The field of the other coil 3 largely shields the measuring coil 2 by superposition of the fields against lateral influences. The shielding effect can be influenced by the adjustment of the amplitude and the phase position and thus the strength of the field F ₂ . Thus, a bundling of the field F ₁ results as in the first embodiment, however, with different effects on the phase and the amplitude of the measuring coil. 2

Fig. 6 shows a third embodiment of the invention. Again, the circuit diagram largely corresponds to the circuit diagram of FIG. 2. However, in this exemplary embodiment, the oscillator is designed as a free-running oscillator 8. This can be formed for example by a resonant circuit consisting of the measuring coil 2 and a capacitor, not shown, connected in parallel. As a result, not only the impedance of the measuring coil 2, but also the frequency of the exciting AC voltage changes when approaching a conductive object. The alternating voltage at the measuring coil 2, which generates the electromagnetic field F ₁ , is tapped via an amplifier 6 with adjustable gain, and with (or without) phase shift via a phase shifter 7 in the further coil 3 low impedance (feed impedance as close as possible 0 Ω) fed. Thus, a second electromagnetic field F _{2 is} generated, which is superimposed on the field F ₁ automatically with the same frequency. The amplitude and phase relationship of the voltages at the two coils 2, 3 remains constant, since the voltage U ₂ at the further coil 3 is derived directly from the voltage U ₁ at the measuring coil 2. As soon as the field F _{1 is changed} by a conductive object in its measuring range, the field F ₂ changes accordingly. However, a conductive object, which is largely only in the field region F ₂ , has no or only a small influence on the impedance of the measuring coil 2 and thus on the measuring signal. The field of the other coil 3 shields the measuring coil 2 by superimposing against lateral influences. The shielding effect and the field concentration of the field F ₁ can be influenced by adjusting the amplitude and the phase position at the further coil 3 and thus the field F ₂ .

FIGS. 7 to 12 relate to a system with a separate excitation and detection coil. Here, FIGS. 7 to 9 show the conditions which are shown in a system known from practice. A free-running oscillator 8, which contains, inter alia, a measuring coil 9, feeds the measuring coil 9 with an alternating voltage U ₁ . 9 thereby generates the measuring coil, an electromagnetic field F _3, which is radiated toward a detection coil 10 degrees. The distance between the exciter and the detection coil can be a multiple of the coil diameter, wherein the diameter of the two coils 9, 10 are equal. However, the coils may also have different diameters.

A measuring object 11 is moved between the two coils 9, 10. Depending on the position x of the measuring object 11, the coupling between the exciter and the detection coil is changed. Therefore, the voltage U _Θ generated by the detection coil 10 is dependent on the position x. In another application, this effect can be used, for example, to make a statement about the dimensions of a small or a narrow object (for example, a wire). FIG. 8 essentially corresponds to FIG. 7. However, FIG. 8 does not show a solid measurement object but a measurement object 12 consisting of individual wires lying close to one another.

9 shows a diagram which reproduces the voltage U _e in the detection coil 10 as a function of the position x of the measurement object 11, 12. In this case, the same diagrams arise both in the case of the massive measurement object 11 and in the case of the measurement object 12 consisting of wires. The individual wires can not be detected separately because of the very wide measuring field in the arrangement known from practice. For this purpose, the Ortsauf solution of the system is too low.

FIGS. 10 to 12 relate to a sensor arrangement according to the invention. FIG. 10 shows a fourth exemplary embodiment of the invention, in which the excitation coil has, in addition to a measuring coil 2, a further coil 3. The measuring coil 2 is fed by an oscillator 4 with an alternating voltage of fixed frequency and amplitude. The measuring coil 2 thereby emits an electromagnetic field F ₃ . The further coil is supplied with a voltage which is derived by means of an amplifier 6 with adjustable gain and a phase shifter 7 with adjustable phase shift from the supply voltage of the measuring coil 2. The other coil 3 thereby generates an electromagnetic field F _4, the field F ₃ superimposed. These superimposed fields are emitted in the direction of an opposite simple detection coil 10, which generates a voltage U _Θ as a result of the alternating field. The distance between the excitation coil arrangement (formed by the measuring coil 2 and the further coil 3) and the detection coil 10 can be a multiple of the coil diameter of the measuring coil 2 or of the detection coil 10.

Between the excitation coil arrangement and the detection coil 10, in turn, a measuring object 12 consisting of individual wires lying close to one another is moved past. The measurement object 12 influences the coupling between the excitation coil arrangement and the detection coil 10. By suitable choice of the coil dimensions and the amplitude and phase relationships of the voltages on the excitation coil arrangement, the voltage U _{0 can} be adjusted so that the spatial resolution of the arrangement is substantially better than just with an excitation coil. FIG. 11 shows the profile of the voltage U _θ which occurs when the measuring object 12 is moved in the x-direction, ie substantially perpendicular to the coil axes. It is clear that the individual wires of the test object 12 are well distinguishable from each other and thus even their distance to each other can be determined. The spatial resolution is thus significantly improved in comparison to the arrangement known from practice.

An even further improvement is provided by an arrangement in which a coil 14 is also assigned to a detection coil 13. Such a circuit is shown in FIG. 12. The circuit corresponds (apart from the further coil 14 to the detection coil 13) of the circuit shown in Fig. 10. This results in the detection coil 13 and the other coil 14 different receiving voltages, which allow by suitable combination of the summation with the aid of an adjustable amplifier and a phase shifter an even better spatial resolution than the circuit of FIG.

Finally, it should be emphasized that the previously purely arbitrarily chosen embodiments serve only to discuss the teaching of the invention, but this does not limit to the embodiments.

Claims

Patent claims 1. inductively operating sensor arrangement with a measuring coil (2), wherein an oscillator (4, 8) supplies the measuring coil (2) with an AC voltage, characterized in that around the measuring coil (2) a further coil (3) is arranged and that the another coil (3) with an amplifier (6) is connected, via which the further coil (3) with one of the AC voltage of the oscillator (4, 8) derived voltage (U ₂ ) is fed. 2. Sensor arrangement according to claim 1, characterized in that before or after the amplifier (6) a phase shifter (7) is provided. 3. Sensor arrangement according to claim 1 or 2, characterized in that the measuring coil (2) and the further coil (3) are formed substantially coaxially. 4. Sensor arrangement according to one of claims 1 to 3, characterized in that the coils (2, 3) are designed as air coils. 5. Sensor arrangement according to one of claims 1 to 4, characterized in that the measuring coil (2) and the further coil (3) terminate substantially flush in the measuring direction. 6. Sensor arrangement according to one of claims 1 to 5, characterized in that the amplifier (6) and / or the phase shifter (7) are controllable. 7. Sensor arrangement according to one of claims 1 to 6, characterized in that the field (F ₂ , F ₄ ) of the further coil (3), the field (F ₁ , F ₃ ) of the measuring coil (2) amplified or reduced. 8. Sensor arrangement according to one of claims 1 to 7, characterized in that the oscillator (4) emits a voltage at a fixed frequency. 9. Sensor arrangement according to one of claims 1 to 7, characterized in that the measuring coil (2) is part of a freely oscillating oscillator (8), the frequency of which changes as a function of the impedance of the measuring coil (2). 10. Sensor arrangement according to one of claims 1 to 9, characterized in that the tap of the voltage used to derive the supply voltage (U ₂ ) for the further coil (3) to a coupling impedance (5), via which the measuring coil (2) connected to the oscillator (4). 11. Sensor arrangement according to one of claims 1 to 9, characterized in that the tap of the voltage used to derive the supply voltage (U ₂ ) for the further coil (3) immediately after the oscillator (4, 8). 12. Sensor arrangement according to one of claims 1 to 11, characterized in that the feeding of the amplified and / or phase-shifted voltage (U ₂ ) in the further coil (3) is low impedance. 13. Sensor arrangement according to one of claims 1 to 12, characterized in that the measuring coil (2) serves both for generating an electromagnetic field (F ₂ ) and for the detection of conductive materials. 14. The sensor arrangement according to one of claims 1 to 13, characterized in that the measuring coil (2), an electromagnetic field (F ₃₎ is generated and a detection coil (10, 13) for the detection of conductive materials (11, 12). 15. Sensor arrangement according to claim 14, characterized in that a further coil (14) is arranged around the detection coil (13) as well. 16. A method for influencing the measurement behavior of a measuring coil (2), in particular using a sensor arrangement according to one of claims 1 to 15, wherein the measuring coil (2) by an oscillator (4, 8) is acted upon by an AC voltage, characterized in that a Another coil (3) arranged around the measuring coil (2) is acted on by means of an amplifier (6) and / or phase shifter (7), which voltage is supplied by the oscillator (4, 8). Been AC voltage is derived, and that by the further coil (3), the emission characteristics of the sensor arrangement (1) is affected. 17. The method according to claim 16, characterized in that for controlling the emission characteristic of the sensor arrangement (1), the supply voltage of the further coil (3) is changed. 18. The method according to claim 16 or 17, characterized in that the change in the supply voltage by changing the amplification factor of the amplifier (6) and / or the phase difference of the phase shifter (7) is performed.