DE102009021804A1 - Metal detector has at least one transmission coil and at least one reception coil working in a pulse induction mode - Google Patents

Abstract

The metal detector, especially working on pulse induction (PI), has at least one transmission coil (2.1,2.2) and at least one receiver coil (1.9). They are inductively coupled together and partially overlap for interactive decoupling, giving one point to extinguish the interaction. An electronic unit delivers current to the transmission coil(s) and evaluates a reception signal. On energizing the first coil the optimum extinguishing point is shifted in one direction and energizing the second coil shifts the point in the opposite direction. A control circuit regulates the current fed to the transmission coils to shift the optimum extinguishing point to extinguish the reception signal. The horizontal axis (3.2) of the transmission coils is tilted by an angle (W) against the horizontal reception coil axis (3.1).

Description

Field of the invention

The The invention relates to a sensor for locating metallic objects, In particular, operated in PI mode metal detector, with the Features of the preamble of claim 1 and an associated Method with the features of the preamble of claim 10.

State of the art

Such in Pulse Induction (PI) mode operated metal detector is from the DE 103 01 951 A9 known. The interaction of the primary and secondary coils is decoupled by partial overlap of the co-planar coil systems. An adjustment of the decoupling takes place by means of mechanically displaceable masses in the region of the overlap or by electrical compensation aids z. B. in the form of additional compensation signals from the generator in the receiving circuit. They compensate for the not completely decoupled portion of the transmission energy in the receiving coil. A "feedback" between detected signal of the receiving coil and compensation effect, so a closed loop control is not available.

From the DE 103 18 350 B3 a comparable arrangement is known in which a plurality of coils are overlapped with respect to their alternating magnetic field, adjacent to each other nested. A largest coil, preferably the receiving coil, determines the periphery of the coil arrangement.

From the DE 36 19 308 C1 is a reversal of the above principle, ie, a surrounding transmitting coil with two trained as "eight" receiving coils in which mutually canceled the field is extinguished.

From the DE 43 39 419 C2 For example, a metal detector with a transmitting coil and a receiving coil is known, which partly overlap so that the mutual induction coefficient is minimal. The coils are alternately operated as a transmitting and receiving coil.

In order to reduce a capacitive crosstalk from the transmitting coil into the receiving coil, when the transmitting and receiving coils are very close to each other, as is the case with a print coil, is in the DE 10 2004 047 189 A1 proposed a shielding measure in the form of a shielding electrode between transmitting and receiving coil. For fine adjustment further auxiliary windings are provided.

From the EP 706 648 B1 an amplitude control is known per se, are detected dynamically in the light signals with compensation of external influences such as extraneous light, temperature or aging influences between the light transmitter and light receiver. The light transmitters are operated periodically and alternately via a clock generator. The regulated in the amplitude of at least one light path optionally acts with the light of another light transmitter such as a compensation light source on the light receiver so that a received signal without isochronous signal components. The received signal of the light receiver is fed to a synchronous demodulator, which in turn decomposes the received signal into the signal components corresponding to the two light sources. These are compared with each other in a comparator, resulting in a signal corresponding to a zero state. If no signal corresponding to this zero state is present at the output of the comparator, this signal is used as a control value in order to regulate the radiant power supplied to the light sources until this state is reached.

Object of the invention

outgoing from this prior art is therefore the present invention The task is based on a simple and effective sensor and a to provide the associated method.

These The object is achieved by a sensor having the features of the claim 1 and solved by a method having the features of claim 10.

The sensor has at least one receiving coil and a plurality of transmitting coils or transmitting coil parts, which preferably divide a transmitting coil in a certain way as mirrored halves. Due to the interaction of a plurality of transmitting coils and at least one receiving coil, wherein the transmitting coils are arranged partially overlapping the receiving coil, a local point of optimal extinction results. The coils are arranged so that the Sendespulenteile exert an effect on the at least one receiving coil at the same current, in which a local point of optimal extinction. This point, however, shifts in a first direction when mainly or solely energizing a first transmitting coil or a first part of the transmitting coil, while it moves in a direction opposite to the first direction when mainly supplying power to another transmitting coil or to a further part of the transmitting coil. This shift is influenced by a metal approach. A control circuit for controlling the currents of the Sendespulenteile leads in the control case to a shift of the local point of the optimal cancellation, which causes an extinction of the received signal. The required control value or its change is preferably used as a measure of a metal approach.

With This solution results in a simple tracking the decoupling between transmitting and receiving system also under constant changing environmental conditions, such. B. in mechanical changes of the bobbin or presence or changes of floor effects. The especially in a scheme according to the claims 8 and 16 possible high amplification of deviations from bar to bar leads to a large dynamic range from detection limit, d. H. highest range to more immediate Close range, in which metal rests on the sensor. This is z. B. important for a "linear" display of Distance from the detection limit range to the immediate one Close range in uniform display steps. The preferably continuous feedback of the received signal for current control in the at least two transmitting coils leads to the current complete decoupling, d. H. for optimal Erasing the emitted signal of the transmitting coil in the receiver coil. Among other things, the capacitive can be Compensate the influence of the transmitter coil on the receiver coil. By the complementary signal control and almost the same areal effect lifts the capacitive Influence of the transmitter coils on the receiver coil almost on. "Shielding measures" between Transmit and receive coils are not necessary. This is a cost-saving Construction with only a two-layer board possible.

Further Advantages emerge from the subclaims and the following Description of preferred embodiments.

Brief description of the figures

in the The invention will be described below with reference to the appended drawings Figures illustrated embodiment closer explained. Show it:

1 a sensor system according to the PI method according to the prior art and the associated amplitude characteristic at the receiving coil,

2 an arrangement of two mirrored coil halves,

3 a mechanical arrangement of the coil halves together with a receiving coil,

4 a sensor electronics with a closed control to stabilize the local point of the optimal cancellation in the receiving coil,

5 . 6 Displacements of the local point of the optimal extinction with different energization of the transmitting coil arrangement,

7 a diagram of the control value of the sensor electronics over time.

Description of preferred embodiments

Before the invention is described in detail, it should be noted that they do not affect the respective components of the device as well the respective process steps is limited, since these Components and methods may vary. The ones used here Terms are intended only for particular embodiments to describe and are not used restrictively. In addition, if in the description or in the claims the singular or indefinite articles are used, this also applies to the majority of these elements, as long as the overall context is unequivocal makes something else clear.

Of the term used in this application "local Point of extinction "refers to the case of an overlay resulting from at least two coils of fixed geometric arrangement Point on one between the centers of the two transmission coils imagined Line at which the flow of current through the two coils the thereby caused magnetic field in the receiving coil extinguished.

1 shows the amplitude curve in a sensor system according to the PI method in the prior art at the receiving coil 1.9 with displacement of the transmitting coil 1.10 or receiver coil to each other. The amplitude 1.7 , the receiver coil is over the shift in 1 applied below. The shift starts 1.1 and ends at 1.5 , Wherein the path traveled in the diagram of the displacement z. B. +/- 5 mm from the point 1.3 the optimal extinction is.

If z. B. the receiving coil in relation to the transmitting coil in the direction of the double arrow 1.6 shifted to the right, first takes the received signal 1.2 from. The signal has an isochronous phase position, in the assumed example 0 °. Upon reaching the local point of optimal extinction, so the decoupling point 1.3 the received signal is zero while the received signal 1.4 with further displacement with rotated by 180 ° phase rises again. The local point of optimum extinction is relatively stable only in laboratory operation. Manufacturing tolerances, temperature influences, mechanical deformation of the coil assembly or the presence of z. B. soil influences z. B. in the metal search in metal-containing Floors shift this point. Furthermore, the local point is also displaced by a metallic object brought into proximity. The possibility of mechanical position of a point of optimal extinction is in all the above influences z. In a region along the double arrow 1.6 to locate.

Despite all the above-mentioned influences, the point of optimal extinction should always remain at exactly the same local location with simple means and a closed control. This is achieved by the following measure:
The transmitting coil 1.10 according to the prior art is divided and preferably halved so that form two substantially identical and mirror-image coil halves. However, another division is possible if, with appropriate energization, a continuous or steady and thus not jerky displacement of the local point of extinction can be achieved. 2 shows the arrangement of these coil parts or coil halves 2.1 and 2.2 as transmitting coils with the terminals 2.3 for the first upper bobbin half 2.1 and 2.4 for the second lower coil half 2.2 , The two remaining terminals of the coil halves are in the embodiment to 2.5 summarized. Complementary tensions that add to a tension 2.6 and 2.7 at the connections 2.3 and 2.4 cause a magnetic field of the same polarity in both coil halves. In this case, the two coil halves behave essentially like a single coil in the prior art. The coil halves or better coil parts are referred to below as transmitting coils 2.1 . 2.2 designated

3 shows a mechanical arrangement of the first upper transmitter coil 2.1 and the second lower transmitting coil 2.2 together with the receiver coil 1.9 , For better distinctness is the receiving coil 1.9 drawn dashed. The diameter of the circular receiving coil in this embodiment 1.9 corresponds approximately to the diameter of the semicircular Sendespulenteile.

The horizontal axis 3.2 the transmitting coil assembly, which the two transmitting coils 2.1 and 2.2 is opposite to the horizontal axis 3.1 the receiver coil 1.9 tilted by the angle W. Therefore, the transmitting coil covers 2.2 the receiver coil 1.9 by a certain amount more than the transmitting coil 2.1 , The angle W is in practice z. B. in the range of 1-10 °. The larger the expected tolerances, eg. As temperature influences, manufacturing tolerances, etc., the greater the angle W should be selected. Between the arrangement of the transmitting coils 2.1 . 2.2 and the receiving coil 1.9 consists - measured in the embodiment of the respective center of - the distance A, the same size, but complementary mental voltage at the transmitting coil 2.1 and 2.2 roughly determines the area in which the local point 1.3 the decoupling comes to rest. Instead of an angular rotation and other arrangements such. B. a displacement of the transmitting coils to each other conceivable, so that a different coverage of the receiving coil 1.9 results. Thus, the receiving coil of the preferably two transmitting coils 2.1 . 2.2 covered with different surface dimensions. As long as the goal is achieved that a shift of the local point of cancellation can be achieved with appropriate energization of Sendespulenteile, it does not matter which geometric arrangement of the transmitting coil relative to the receiving coil this is achieved or facilitated.

Ideal for the realization of the invention described above, a method has been mentioned with an amplitude control according to the input EP 706 648 B1 exposed for the creation of a sensitive metal detector. However, other methods are conceivable as long as the point of optimal extinction in energizing only a first part of the transmitting coil in a first direction z. B. right to the point 5.1 shifts while energizing a second part of the transmitting coil, the point of optimal cancellation in a direction opposite to the first direction of the second direction z. B. to the left to the point 6.1 shifts. A control method then ensures that a shift of the local point of the optimal cancellation is compensated and thus a continuous cancellation of the received signal 1.11 he follows.

4 shows here as an exemplary embodiment of a sensor electronics with a closed control to stabilize the local point of the optimal cancellation in the receiving coil 1.9 one in the transmission coils 2.1 and 2.2 generated magnetic field. A clock generator 4.8 provides a first clock signal 4.13 to a first regulated power source 4.10 and a second inverted clock signal 4.12 to a second regulated power source 4.9 , The frequency of the clock generator can be selected depending on the inductance of the coils, in the embodiment, it is about 120 kHz. The signal can z. B. be a square or sine wave signal. The first regulated power source 4.10 feeds the connection 2.4 the lower transmitter coil 2.2 , Similarly, the second regulated power source feeds 4.9 the connection 2.3 the upper transmitter coil 2.1 , The at the receiving coil 1.9 applied signal is connected to the AC amplifier 4.5 - hereinafter referred to as amplifiers - amplified.

The output signal of the amplifier 4.5 becomes the synchronous demodulator 4.6 fed. This receives a necessary for demodulation first clock signal 4.18 and second clock signal 4.19 from the clock generator 4.8 , In the simplest case, the synchronous demodulator 4.6 the output signal of the amplifier 4.5 during the entire portion of a clock phase synchronously with the corresponding inputs of the integrating comparator 4.7 respectively. In this case, the clock signal 4.18 and 4.19 as long as the transmission clock phases.

At the same voltage of the first input signal 4.15 and the second input signal 4.17 of integrating comparator 4.7 Thus, no isochronous signal component at the receiving coil 1.9 , This z. B. in the case of metal inflow from the outside, the average value of a first clock signal at the receiver coil 1.9 compared with the average value of the second clock signal. In the regulated state already correspond to those at the inputs of the amplifier 4.5 present receive signals a zero state, so that the amplifier 4.5 sees only noise at the entrance. Therefore it can amplify very high, or be executed as a high-gain limiter amplifier. The same also applies in the regulated state for the first input signal 4.15 and the second input signal 4.17 , Located at the output of the comparator 4.7 no signal corresponding to this zero state becomes the control value 4.16 as long tracked and thereby the current in the transmitter coil 2.1 . 2.2 as long as regulated, until this state is reached.

During the length of the clock section, the output signal of the receiving coil 1.9 small, determined by the type of metal amplitude curves. For better analysis of the metal properties, the sampling range of the synchronous demodulator can therefore only be selected in sections in the clock cycles. For this purpose, the first and second clock signal required for demodulation is 4.18 and 4.19 shortened accordingly and placed in the required for the metal analysis section of the clock phase. The sampling times are freely selectable. You can z. B. in small steps of z. B. a few tens of nanoseconds and be at any predetermined or predetermined positions of the clock section or clock signal to win certain information from the received signal.

The by the synchronous demodulator the two clock signals 4.12 and 4.13 assignable output signals of the synchronous demodulator 4.6 be from the integrating comparator 4.7 examined for amplitude differences. The comparator can be designed as a high-gain comparator circuit. Every little deviation of the input voltages or input signals 4.15 and 4.17 leads to a corresponding deviation of the control value 4.16 from the current value. In practice, "open loop" gains of up to 240 dB have been proven. This can be z. B. by two mutually behind, AC moderately attenuated operational amplifier with a DC negative feedback over the entire control loop, so including the coupling between the transmitting coil and the receiving coil exist. The regulated power sources 4.9 and 4.10 are by means of Invertierstufe 4.11 against each other with the control value 4.16 controlled inverted to restore the state at the same amplitude of the input signals to the comparator 4.7 are pending, ie in the case of the two waveforms no differences at the inputs of the comparator 4.7 occur. If the current of one of the regulated power sources increases, it drops accordingly in the other one.

Due to the displacement of the current in both coil halves or transmitting coils, the local point of optimum extinction is steplessly shifted in a wide range. The size of the area depends on the size of the coil used. He can z. B. at a coil diameter of 50 mm z. B. +/- 5 mm. 5 shows the displacement of the local point 5.1 the optimal extinction to the right when the lower transmitter coil 2.2 the transmission coil arrangement receives a higher current than the upper transmission coil 2.1 , In the opposite case of the transmission currents, the point moves 6.1 the optimal extinction according to 6 to the left. The control circuit now ensures that the determined value for the point of optimal cancellation is constantly adjusted so that no difference signal at the synchronous demodulator 4.6 is applied. This leads to time-variable or dynamic changes in the environment of the metal detector such. B. a metal approach as changes in the control value 4.16 be perceived.

Without the influence of metal in the sensor-active area, therefore, a balance of the transmission currents in the way that at the receiving coil 1.9 no isochronous shares arise and thus the point of optimal extinction is always observed. The control value 4.16 at the control output of the control circuit in 4 takes accordingly 7 a certain electrical value corresponding to the local position of the location of the optimum extinction. A metal approach 7.4 changes the location of the optimal extinction. Thus arises in the receiving coil 1.9 a signal with isochronous components, which is detected by the synchronous rectification and immediately by the continuous readjustment of the transmission currents in 4.9 and 4.10 is readjusted until the isochronous shares in the receiving coil are extinguished. 7 shows the idle state of the control value 4.16 and the change in the area of a metal approach 7.4 , To detect a metal approach can now z. For example, the difference between hibernation 4.16 and changed control value 7.3 be evaluated.

In This system of closed regulation will therefore not be the same as in Prior art, the size of the receiver coil resulting signal measured at metal approach and in a corresponding display made visible to the user, but the control value or even better the change of the control value, that for the local shift of the point of the optimal Extinction at metal approach occurs.

The Readjustment takes place in the μs range, so too faster Metal sweep the output of the receiver coil always to a state without isochronous portions of the synchronous demodulator is held. In principle, it is also sufficient if the electricity regulated in only one transmission coil or transmitting coil half but this limits the dynamic range.

In the case of a manufacturing tolerance, a temperature influence or the influence of the soil, the control value changes 4.16 (Offset), but the optimum extinction of the signal remains 1.11 at the receiver coil 1.9 in each case received.

Function of the divided transmitting coil arrangement:

Ideally, both transmit coils 2.1 and 2.2 immediately energized, or supplied with the same voltage and behave like a coil. Suppose that the point of optimal extinction as in 1 lies in the middle.

Will "only" the transmission coil 2.2 in relation to the transmitting coil 2.1 energized, the point changes 5.1 the optimal extinction and migrate according to 5 "to the right". That is, the transmit coil assembly would have to be shifted to the right of the receive coil to reach the point of optimal cancellation. Conversely, allows an exclusive energization of the transmitter coil 2.1 the point 6.1 the optimal extinction according to 6 Hike "to the left". The transmit coil assembly would therefore have to be shifted to the left to reach the point of optimal cancellation. However, since all possible current value ratios due to the closed control in 4 are possible, so the point of optimal extinction in fixed transmitting coil arrangement can always be taken with certainty. This is particularly important when relatively large production tolerances of the coils must be accepted. A "turn on" of auxiliary windings as in the DE 10 2004 047 189 A1 or mechanically displaceable masses is eliminated.

One Another advantage lies in the high dynamics without the usual "limitation" of Measured values for large metal parts, if a certain Distance is fallen below.

In the prior art usually disturbs a capacitive crosstalk from the transmitting to the receiving coil. This is very noticeable when the transmitter and receiver coils are very close to each other, as is the case with print coils. In order to reduce the crosstalk, therefore, a shielding measure between transmitting and receiving coil is proposed. ( DE 10 2004 047 189 A1 )

In The described invention can be dispensed with this shield be, because the capacitive effects of the two transmitting coil halves due to the complementary voltages and the mirrored Arrangement of the transmitting coil halves in the receiving coil almost completely cancel. Therefore, a metal detector can be used very easy to realize with only a two-sided printed circuit board.

In the illustration according to 2 the transmitter coil was shown around, but of course other forms are possible, for. B. the known from the prior art "double D" arrangement or an asymmetrical arrangement of the two staggered transmitting coils above and below the receiving coil. Also, this arrangement can be analogous to the above-described mode of action in differential measurement methods such. B. the use of two receiving coils within the transmitting coils use (patent DE 36 19 308 C1 ).

It is essential that the transmitting coil, or at least a substantial part of it, is shared and, with the same energization of the two coil parts, exerts an effect on the receiving coil or coils, at which point a local point of optimal extinction 1.3 arises, and when energized only a first half or a first part of the transmitting coil, the point of optimal extinction in a first direction z. B. right to the point 5.1 shifts while energizing a second half or a second part of the transmitting coil, the point of optimal cancellation in a direction opposite to the first direction of the second direction z. B. to the left to the point 6.1 shifts. Furthermore, there is a continuous control of the currents of the two transmitting coils, which leads to a shift of the local point of the optimal cancellation, and thus a continuous cancellation of the received signal 1.11 causes. To evaluate the presence of metal, the control value of the differential current regulation of the two transmitting coil halves is used.

1.1

starting point the shift

1.2

signal with phase angle 0 °

1.3

decoupling point

1.4

signal with phase angle 180 °

1.5

endpoint the shift

1.6

double arrow Shift of the receiver coil to the transmitter coil

1.7

amplitude the receiver coil

1.9

receiving coil

1.10

transmitting coil (State of the art)

2.1

first upper bobbin half

2.2

second lower bobbin half

2.3

connection the first upper bobbin half

2.4

connection the second lower coil half

2.5

summarized Connections of the first and second coil half

2.6

To 2.7 complementary tension

2.7

To 2.6 Complementary tension

3.1

horizontal Axis of the receiver coil

3.2

horizontal Axis of the transmitting coil arrangement

W

angle tilting

A

distance the receiving and transmitting coil assembly

4.5

AC amplifier

4.6

synchronous

4.7

integrating comparator

4.8

clock generator

4.9

Second regulated power source

4.10

First regulated power source

4.11

inverting

4.12

second clock signal

4.13

first clock signal

4.15

first Input signal of the integrating comparator

4.16

control value

4.17

second Input signal of the integrating comparator

4.18

For Demodulation required first clock signal

4.19

For Demodulation needed second clock signal

5.1

To right-shifted point of optimal extinction

6.1

To left shifted point of optimal extinction

7.3

control value at metal approach

7.4

Area a metal approach

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - DE 10301951 A9 [0002]
• - DE 10318350 B3 [0003]
• - DE 3619308 C1 [0004, 0043]
• - DE 4339419 C2 [0005]
• - DE 102004047189 A1 [0006, 0039, 0041]
• - EP 706648 B1 [0007, 0027]

Claims (18)

Sensor for locating metallic objects, in particular in PI mode operated metal detector, with at least one transmitting coil ( 2.1 . 2.2 ) and at least one receiving coil ( 1.9 ) which are inductively coupled together and partially overlapped for interaction decoupling, one point ( 1.3 ) can be achieved optimal extinction of the interaction, and with a sensor electronics for energizing the transmitter coil and for evaluating a received signal ( 1.11 ) of the receiving coil, characterized in that - several, powered by the sensor electronics transmitting coils ( 2.1 . 2.2 ), - that the transmitting coils ( 2.1 . 2.2 ) at the same current effect on the at least one receiving coil ( 1.9 ), where a local point ( 1.3 ) of the optimal extinction, while during energization of a first transmitting coil ( 2.1 ) the point of optimum extinction shifts in a first direction, while when energizing another transmitting coil ( 2.2 ) the point of optimal cancellation shifts in a direction opposite to the first direction, and - that a control circuit is provided for controlling the currents of the transmitter coil parts, which leads to a displacement of the local point of the optimal cancellation, which causes an extinction of the received signal ( 1.11 ) causes. Sensor according to claim 1, characterized in that the transmitting coils are formed by a plurality of transmitting coil parts, which together form approximately the shape of a preferably the receiving coil ( 1.9 ) have corresponding transmitting coil. Sensor according to claim 1 or 2, characterized in that the transmitting coils ( 2.1 . 2.2 ) or the Sendespulenteile are formed by two substantially equal sized Sendespulenhälften. Sensor according to one of the preceding claims, characterized in that the transmitting coils ( 2.1 . 2.2 ) and the receiving coil ( 1.9 ) are arranged co-planar. Sensor according to one of the preceding claims, characterized in that preferably two transmitting coils ( 2.1 . 2.2 ) the receiving coil ( 1.9 ) cover with different surface dimensions. Sensor according to one of the preceding claims, characterized in that an axis ( 3.2 ), preferably an axis of symmetry of the transmitting coil arrangement, which transmits the transmitting coils ( 2.1 . 2.2 ), with respect to an axis ( 3.1 ), preferably an axis of symmetry, of the receiving coil ( 1.9 ) is tilted by an angle (W). Sensor according to one of the preceding claims, characterized in that the centers of an arrangement of the transmitting coils ( 2.1 . 2.2 ) and the receiving coil ( 1.9 ) are separated by a distance (A) from each other, which at approximately equal but complementary voltage at the transmitting coil approximately determines the area in which the local point ( 1.3 ) comes to rest. Sensor according to one of the preceding claims, characterized in that a comparator ( 4.7 ) for comparing the transmitting coils ( 2.1 . 2.2 ) is provided for determining a control value, and that at least one regulated current source ( 4.9 . 4.10 ) is provided, in which the control value for regulating the amplitude of the current supplied to the transmitting coil, the amplitude preferably continuously controls so that the amplitudes of the voltage signals at the inputs of the comparator ( 4.7 ) are substantially the same size. Sensor according to one of the preceding claims, characterized in that the change in the control value, for the local displacement of the point ( 1.3 ) performs the optimal extinction at metal approach, which is measured value. Method for locating metallic objects with a sensor, in particular with a metal detector operated in PI mode, with at least one transmitting coil ( 2.1 . 2.2 ) and at least one receiving coil ( 1.9 ) which are inductively coupled together and partially overlapped for interaction decoupling, one point ( 1.3 ) optimal extinction of the interaction can be achieved, wherein by means of a sensor electronics, the at least one transmitting coil is energized and a received signal ( 1.11 ) of the receiving coil, characterized in that - several transmitting coils ( 2.1 . 2.2 ) are energized by the sensor electronics, - that the transmitting coils with the same current effect on the at least one receiving coil ( 1.9 ), where a local point ( 1.3 ) of the optimal cancellation, while energizing a transmitting coil ( 2.1 ) the point of optimum extinction shifts in a first direction, while when energizing another transmitting coil ( 2.2 ) the point of optimal cancellation shifts in a direction opposite to the first direction, and - that the currents of the transmitting coils are controlled such that there is a displacement of the local point of optimum cancellation, which causes cancellation of the received signal ( 1.11 ) causes. A method according to claim 10, characterized in that a plurality of transmitting coil parts are used as transmitting coils, which together form approximately the shape of a preferably the receiving coil ( 1.9 ) have corresponding transmitting coil. Method according to claim 10 or 11, characterized that as transmitting coils or transmitting coil parts two substantially equal sized transmitting coil halves are used. Method according to one of claims 10 to 12, characterized in that the transmitting coil parts and the receiving coil be arranged co-planar. Method according to one of claims 10 to 13, characterized in that the receiving coil of the preferably two transmitting coils ( 2.1 . 2.2 ) is covered with different surface dimensions. Method according to one of claims 10 to 14, characterized in that an axis ( 3.2 ), preferably an axis of symmetry, of the transmitting coil arrangement, which transmits the transmitting coils ( 2.1 . 2.2 ), with respect to an axis ( 3.1 ), preferably an axis of symmetry, of the receiving coil ( 1.9 ) is tilted by an angle (W). Method according to one of claims 10 to 15, characterized in that the parts of the transmitting coil ( 2.1 . 2.2 ) associated voltage signals for determining a control value are preferably continuously compared, and that the control value by means of at least one regulated current source ( 4.9 . 4.10 ) controls the amplitude of the current supplied to the parts of the transmitting coil such that the amplitudes of the voltage signals at the inputs of the comparator ( 4.7 ) are substantially the same size. Method according to one of Claims 10 to 16, characterized in that the change in the control value which is responsible for the local displacement of the point ( 1.3 ) leads the optimal extinction at metal approach, as measured value z. B. is used for a metal approach. Method according to one of claims 10 to 17, characterized in that the transmitting coils ( 2.1 . 2.2 ) in time with a clock generator ( 4.8 ) and that the received signals of the receiving coil ( 1.9 ) are sampled in the clock, wherein the sampling times are freely selectable, are preferably adjusted in small steps of a few tens of nanoseconds and at any predetermined or predetermined positions of the clock section are to win certain information from the received signal.