Metal detector

[0001] The invention relates to a sensor for determining the position of metal objects, in particular a metal detector operated in PI mode, with the features of the concept of independent claim 1 and an associated method with the features of the concept of overarching claim 9.

[0002] The metal detector operated in the mode of pulse induction (PI) method is known from DE 32 25 166 A1, it has several emitting coils and one receiving coil, with current flowing in the same direction in the emitting coils. The transmission coils are positioned so that they compensate for the transmission coil when the field is not disturbed. The current of both transmission coils can be adjusted individually. The coils are concentric.

[0003] A metal detector operated in the pulse induction method (PI) mode, which is the basis of the preamble to independent claim 1, is known from DE 103 01 951 A9. The interaction of primary and secondary coils is decoupled by the partial overlap of coplanar coil systems. The decoupling is done by mechanically shifting masses in the overlap area or by electric auxiliary compensation means e.g. in the form of additional compensation signals from the generator to the receiving circle. They compensate for the not fully decoupled part of the transmission energy in the receiving coil. The "feedback" between the detected signal of the receiving coil and the compensation action, i.e. closed control, does not occur.

[0004] From DE 103 18 350 B3 a comparable arrangement is known in which several coils are arranged in terms of their alternating magnetic field with overlapping, with an adjacent offset relative to each other. The rim of the coil arrangement is set by the largest coil, preferably the receiving coil.

[0005] From DE 36 19 308 C1, the inverse of the abovementioned principle is known, ie a peripheral transmitting coil with two receiving coils shaped as an "eight" in which the sent field is mutually suppressed.

[0006] DE 43 39 419 C2 discloses a metal detector with a transmitting coil and a receiving coil, which overlap in such a way that the variable induction coefficient is minimal. The coils are operated alternately as transmitting and receiving coils.

[0007] To reduce capacitive crosstalk from the transmitting coil to the receiving coil when the transmission and receiving coils are very close together, as in the case of printed coils, in DE 10 2004 047 189 A1 a shielding agent is proposed in the form of a shielding electrode between the transmission coil and receiving. Auxiliary coils are also provided for fine adjustment.

[0008] From EP 706 648 B1, amplitude control is generally known in which light signals are recorded while compensating for external influences such as external light influences, temperature or aging between the light transmitter and the light receiver. Light transmitters are driven periodically and alternately by a pulse generator. The light regulated in the amplitude of at least one light segment optionally works with the light of the next light transmitter such as, for example, a compensating light source on the light receiver, that a receiving signal is created without synchronous pulse signal parts. The receiving signal of the light receiver is fed to a synchronous demodulator, which in turn breaks down the receiving signal into two signal components corresponding to both light sources. They are compared in a comparator, whereby the state of the control value corresponding to the zero state is created. If there is no signal at the comparator output corresponding to this zero state, this control value is used to regulate the radiation power supplied to the light sources until this state is reached.

[0009] Starting from this prior art, the invention is based on the task of providing a simple and effective sensor and a suitable method.

[0010] The solution to this task is a sensor with the features of claim 1 and a method with the features of claim 9.

[0011] The sensor has at least one receiving coil and several transmission coils or parts of the transmission coil that divide the transmission coil in a specific manner, preferably as mirror halves. By interacting several transmission coils and at least one receiving coil, whereby the transmission coils are arranged overlapping partly on the receiving coil, a local point of optimal blanking of the fields in the receiving coils sent by the transmission coils is created. The coils are arranged in such a way that the parts of the transmission coils with the same current supply affect at least one receiving coil, at which a local point of optimal blanking of the sent fields arises in the receiving coils. This point moves or moves with the main or exclusive current supply to the first transmission coil or first portion of the transmission coil in the first direction, while with the main or exclusive current supply to the next transmission coil or the next portion of the transmission coil moves or moves in the next direction, preferably opposite to the first direction. This local point of optimal blanking is affected by the approach of the metal. The control system for regulating the currents of the part of the transmitting coil leads in the case of adjustment to a local shift of the optimal blanking point, causing the blanking of the reception signal. The regulatory value or change required for this is preferably used as a measure of the proximity of the metal.

[0012] The effect of this solution is a simple further decoupling between the transmitting and receiving systems also in constantly changing environmental conditions, such as when metal is approaching or mechanical changes in the coil body or the occurrence or changes of the earth's influence.

[0013] If more transmission coils and more receiving coils are provided, then at least one transmission coil and at least one receiving coil, respectively, may be arranged on different planes. Coils on one plane are rotated to coils on at least one further plane until the interaction of the coils is extinguished at a specific center angle, depending on the type of application. To enable a control action in which the control value is also a measured value, the coils are moved at least slightly preferably for example by a distance of 0.1 to two percent of the diameter of the coil parallel to each other or respectively rotates towards each other. In this way the transmitting and receiving coils are almost congruent in terms of circumference, which results in a very compact design. In practice, this arrangement of coils, which can advantageously be shaped as printed coils on opposite sides of the printed circuit board, leads to high detection sensitivity with the described electronics. With an overall diameter of 25 mm, the detection limit could be more than 500 mm in practice.

[0014] The signal induced in the reception coils by the fields sent from the transmission coils is fed to the amplifier. When using e.g. two receiver coils, the receiver coils can be connected in parallel or also in series. It is important that the signal induced in the receiving coils can be extinguished. Preferably, you can also choose an amplifier with a balanced input. A synchronous demodulator with a comparator behind it can be designed to compare the voltage signals assigned to the transmission coils to determine the control value. The control value provides the comparator output. In at least one regulated current source, the amplitude of the current supplied to the transmission coils is controlled, preferably continuously, so that the amplitudes of the voltage signals at the comparator inputs are of substantially the same magnitude. Then the blanking of the received signal in the coil or receiving coils corresponds to this. This blanking in turn corresponds to the complete decoupling between the transmitting coil and the receiving coils.

[0015] By synchronous demodulation of reception coil signals and comparison of phase-assignable output signals by means of a comparator, information (control value) is obtained that is used to control the currents of at least one of the transmission coils. This corresponds to a closed control circuit. With the correct mechanical arrangement of the transmission coils relative to the receiving coil or coils, the transmission coils are allocated by regulating the same amount of current, i.e. in this case the transmission signal is blanked in both or one receiving coil. Then the control value falls within e.g. the medium control range. When, for example, metal approaches, this control value changes accordingly, while the transmitting signal remains blank in the receiving coil or coils. However, due to possible production tolerances, the not quite correct mutual positioning of the transmission coils and the receiving coils only leads to a constant offset of the regulatory value relative to its ideal condition.

[0016] In general, especially in the case of printed coils, more coil planes than just two can be arranged on the circuit board.

[0017] Further advantages result from the dependent claims and the following description.

Brief description of the figures [0018] The invention is explained in more detail below based on the embodiments shown in the attached figures. The figures show:

Fig. 1 the sensor system according to the prior art PI method and the course of the respective amplitudes on the receiving coil,

Fig. 2 arrangement of two mirror halves in a first embodiment,

Fig. 3 mechanical arrangement of the coil halves according to Fig. 2 together with the receiving coil,

Fig. 4 sensor electronics with closed adjustment to stabilize the local optimal blanking point in the receiving coil,

Fig. 5, 6 shifts of the local blanking point with different current flow to the emitter coil system,

Fig. 7 graph of the control value of the sensor electronics over time,

Fig. 8 arrangement of the first half of the transmitting coil and the first half of the receiving coil in a further embodiment,

Fig. 9 arrangement of coils according to Fig. 8 with connection elements in electronics,

Fig. 10 arrangement of the coils according to Fig. 8 as a printed solution in cross section,

Fig. 11 sensor electronics according to Fig. 10 with closed adjustment of stabilization of the local optimal blanking point in the receiving coils.

Detailed Description of Preferred Embodiments [0019] The invention is explained, for example, in more detail by reference to the accompanying drawings. The presented embodiments are only examples and are not intended to limit the inventive idea to a particular system. Before describing the invention in detail, it should be noted that it is not limited to the related components of the device structure and to the respective method steps, since these components and method may be changed. The terms used herein are for the purpose of describing particular embodiments only and are not intended to be limiting. If, in addition, the singular or indefinite articles are used in the description or in the claims, this also applies to the plural of these elements, as long as the other context clearly does not indicate otherwise.

[0020] The term "local blanking point" as used in the application means a point formed on an overlap of at least two emitting coils with a fixed geometrical distribution on the imagined line between the centers of both transmission coils, in which, as the current flows through both coils, the resulting coils are extinguished magnetic field in the coil or receiving coils.

[0021] Fig. 1 shows the waveform of the amplitude in the sensor system according to the PI method in the prior art on the receiving coil 1.9 when the transmission coil 1.10 or the receiving coil moves towards each other. The receiver coil amplitude 1.7 is plotted above the offset in Figure 1 at the bottom. The shift starts at 1.1 and ends at 1.5, where the distance traveled on the graph is e.g. +/- 5 mm from point 1.3 of optimal blanking.

[0022] If, for example, the receiving coil is shifted relative to the transmitting coil in the direction of the double right arrow 1.6, the received signal 1.2 first decreases. In this embodiment, the signal has a synchronous pulse phase position relative to the signal fed to the receiving coil of 0 °. When reaching the local blanking point, i.e. the deceleration point 1.3, the received signal is zero, while the received signal 1.4 increases again with a further phase shift by 180 °. The local point of optimal blanking is relatively stable only in laboratory conditions. Production tolerances, temperature influences, mechanical deformation of the coil system or occurrence of e.g. earth influences e.g. when searching for metals in metal-containing soils, they move this point. In addition, this local point is also moved by a nearby metal object. The possible geometrical position of the optimal blanking point should be located for all of the above-mentioned influences, e.g. in the area along the double arrow 1.6.

[0023] Despite all the above-mentioned influences, the optimal blanking point should always remain in the same place by simple means and closed regulation. This is achieved by the following action:

The prior art coil 1.10 is divided, preferably in halves, so that two substantially identical and mirror halves of the coil are formed. However, it is also possible to divide if it is possible with continuous or constant current supply and thus not abruptly moving the local blanking point. FIG. 2 shows the arrangement of these coil parts or coil halves 2.1 and 2.2 as transmission coils with terminals 2.3 of the first upper half of the coil 2.1 and 2.4 of the second lower half of the coil 2.2. the other two coil half connections are in this embodiment included together as 2.5. Complementary, complementary to voltage 2.6 and 2.7 on terminals 2.3 and 2.4 cause in both halves of the coil magnetic field of the same polarity. In this case, both coil halves function essentially as the only coil in the prior art. Half of the coil or parts of the coil are referred to below as transmitting coils

2.1 and 2.2.

[0024] Fig. 3 shows the mechanical arrangement of the first upper transmission coil 2.1 and the second lower transmission coil 2.2 together with the receiving coil 1.9. For better differentiation, the receiving coil 1.9 is marked with a dashed line. The diameter of the circular receiving coil 1.9 corresponds approximately to the diameter of the semi-circular parts of the transmitting coil.

[0025] The horizontal axis 3.2 of the transmission coil system, comprising both transmission coils 2.1 and 2.2, is inclined relative to the horizontal axis 3.1 of the receiving coil 1.9 at an angle W. Therefore, the transmission coil 2.2 covers the receiving coil 1.9 by a certain value more than the transmission coil 2.1. The angle W is in practice e.g. in the range of 1-10 °. The larger the expected tolerances, e.g. temperature influences, production tolerances, etc., the greater the angle W. Between the arrangement of the transmission coils 2.1, 2.2 and the receiving coil 1.9 is - measured in the exemplary embodiment from the respective center point - the distance A, which at the same high but complementary voltage on the transmission coils 2.1 and 2.2 more or less defines the range in which the local point 1.3 decoupling. Instead of angular rotation, other arrangements are also possible, such as moving the transmission coils towards each other, which causes a different coverage of the receiving coil 1.9. Thus, the receiving coil is preferably covered by two emitting coils 2.1, 2.2 with different surface areas. As long as the goal of achieving the offset of the local blanking point is achieved with a suitable current supply to the part of the emitting coil, it is irrelevant how the geometric arrangement of the emitting coils relative to the receiving coil is achieved or facilitated.

[0026] For the implementation of the above-described invention, the amplitude controlled method according to EP 706 648 B1 mentioned above is ideal for producing a sensitive metal detector. However, other ways are possible, as long as the current only reaches the first part of the transmitting coil, the optimal blanking point shifts in the first direction e.g. to the right to point 5.1, while with the current supply to the second part of the transmitting coil, the optimal blanking point moves in a further, preferably opposite to the first direction, second direction, e.g. to the left to point 6.1. Then the adjustment method ensures adjustment of the local offset of the optimal blanking point, and thus continuous blanking of the receiving signal 1.11.

[0027] Fig. 4 shows the implementation of the closed-loop sensor electronics to stabilize the local optimal blanking point in the receiving coil 1.9 produced in the magnetic field 2.1 and 2.2 transmission coils. The pulse generator 4.8 supplies the first pulse signal 4.13 to the first regulated power source 4.10 and the second invert pulse signal 4.12 to the second controlled power source 4.9. The frequency of the pulse generator can be selected according to the inductance of the coils, in this embodiment it is about 120 kHz. The signal can be e.g. a rectangular or sinusoidal signal. The first regulated power source 4.10 powers the connection 2.4 of the lower transmission coil 2.2. Similarly, a second regulated power source 4.9 supplies connection 2.3 of the upper transmission coil 2.1. The signal applied to the receiving coil 1.9 is amplified by an 4.5 voltage amplifier - hereinafter referred to as the amplifier.

[0028] The output of the amplifier 4.5 is fed to the synchronous demodulator 4.6. It contains the first impulse signal from 4.18 and the second impulse signal 4.19 from the pulse generator 4.8 needed for demodulation. In the simplest case, a synchronous demodulator 4.6 supplies the output of the amplifier 4.5 during the entire pulse phase section synchronously to the respective inputs of the integrating 4.7 comparator. In this case, pulse signals 4.18 and 4.19 are as long as the transmit pulse phases.

[0029] With the same voltage of the first input signal 4.15 and the second input signal 4.17 of the integrating comparator 4.7, no synchronous pulse signal input on the receiving coil 1.9 is created. In this case, e.g. with external metal influences, the average value of the first pulse signal on the receiving coil 1.9 is compared with the average value of the second pulse signal. In the regulated state, the receiving signals already correspond to each other on the inputs of the 4.5 amplifier, and thus constitute zero at the output of the amplifier, so that the 4.5 amplifier only sees noise at the input. Therefore, it can be very amplifying or made as a high gain limiting amplifier. The same applies in the controlled state of the first input signal 4.15 and the second input signal 4.17. If no signal corresponding to this zero condition is applied to the output of the comparator 4.7, the control value 4.16 is thus continued and thus the current in the transmission coils 2.1 and 2.2 is controlled until this state is reached.

[0030] During the length of the pulse segment, the output signal of the receiving coil 1.9 exhibits small amplitude-dependent waveforms, depending on the type of metal. Therefore, for better analysis of metal properties, the scope of analysis of a synchronous demodulator can be selected in pulse cycles only in sections. For this, the first and second pulse signals 4.18 and 4.19 needed for demodulation are shortened accordingly and inserted into the pulse phase segment needed for metal analysis. The reading moments can be selected freely. They can e.g. be selected in small steps from a large number of nanoseconds and placed in any given or set points of the impulse segment or impulse signal to obtain specific information from the receiving signal.

[0031] Assigned by both synchronous demodulator 4.6 to both pulse signals 4.12 and 4.13, the output signals of synchronous demodulator 4.6 are tested by an integrating comparator 4.7 in terms of amplitude differences. The comparator can be made as a highly reinforcing connection system. Each, even the smallest 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 up to 240 dB have proven themselves. This can be done, for example, by two operational amplifiers with negative DC feedback in the entire control circuit, placed behind each other, i.e. when the coupling between the transmission coils and the receiving coil is switched on. Adjustable current sources 4.9 and 4.10 are inversely controlled relative to each other by means of the inverting step 4.11, the control value 4.16, to restore the state in which the comparator 4.7 will have the same magnitude of input signals amplitude, i.e. in which of both waveforms at comparator 4.7 inputs are not there will be no differences. If the current increases in one regulated power source, then it decreases accordingly in the other.

[0032] By shifting the current in both halves of the coil or the transmission coils, the local blanking point shifts infinitely over a wide range. The size of this range depends on the size of the coil used. It can be with a coil diameter of 50 mm, e.g. +/- 5 mm. Fig. 5 shows the offset of the local 5.1 point of optimal blanking to the right when the lower transmission coil

2.2 of the transmitting coil system receives a higher current than the upper transmitting coil 2.1. In the opposite case of transmission currents, point 6.1 of optimal blanking according to Fig. 6 goes to the left. The control system causes such a constant further adjustment of the determined value of the optimal blanking point that no differential signal adheres to the synchronous demodulator 4.6. This leads to the fact that time-changing or dynamic changes in the environment of the metal detector, such as the approach of metal is seen as a change in regulatory value 4.16.

[0033] Therefore, without the metal influence in the area of ​​sensor activity, such a balance of transmission currents arises that on the receiving coil 1.9 there is no synchronous pulse parts and thus the optimal blanking point is maintained. Thus, the control value 4.16 at the control output of the control system in Figure 4 assumes according to Figure 7 a certain electrical value corresponding to the local position of the optimal blanking place. The approach of metal 7.4 changes the place of optimal blanking. Thus, in the receiving coil 1.9, a signal is created with synchronous pulse parts, detected by synchronous detection and immediately regulated by continuous current control of the transmission currents in 4.9 and 4.10, until the synchronous pulse parts in the receiving coil are extinguished. Fig. 7 shows the resting state of the control value 4.16 and the change in the area of ​​approach of the metal 7.4. For example, the difference between the resting state can be used to detect the approach of a metal

4.16 and the amended regulatory value 7.3.

[0034] In this closed regulation system, therefore, as in the state of the art, the size of the signal generated on the receiving coil is not measured when the signal metal approaches and does not show the user on the appropriate indicator, but the regulatory value or rather the change in the regulatory value occurring for the local shift of the optimal blanking point when approaching metal.

[0035] The current regulation takes place in the μs range, so that also when the metal is pulled quickly through the metal, the output signal of the receiving coil is always kept on the synchronous demodulator without the synchronous pulse parts. In principle, it is sufficient here to regulate the current in only one transmission coil or in the middle of the transmission coil, but the dynamic range is therefore limited.

[0036] The control value 4.16 (offset) changes in the case of production tolerance, temperature or earth influence, but the optimal quenching of the signal 1.11 on the receiving coil 1.9 remains in any case maintained.

[0037] The function of the split coil system:

[0038] Ideally, both transmitting coils 2.1 and 2.2 receive the same current or are supplied with the same voltage and behave as one coil. Suppose the optimal blanking point is located as in Figure 1 in the middle.

[0039] If the current flows "only" to the transmission coil 2.2 relative to the transmission coil 2.1, point 5.1 of optimal blanking changes and goes according to Fig. 5 "to the right". This means that the emitting coil arrangement would have to be shifted to the right to reach the optimal blanking point. Conversely, the supply of current only to the transmission coil 2.1 causes the point of optimal blanking 6.1 according to Fig. 6 to "left". Therefore, the emitting coil arrangement would have to be shifted to the left to reach the optimal blanking point. However, since all current relations are possible due to the closed adjustment in Fig. 4, with a constant arrangement of the transmission coils, one can always find the optimal blanking point. This is especially important when you have to reconcile with relatively large manufacturing tolerances of the coils. "Switching on" auxiliary windings as in DE 10 2004 047 189 A1 or mechanically movable masses is not required.

[0040] A further advantage lies in the high dynamics without often merely "limiting" the measured values ​​for large metal parts if the distance is smaller than the specified one.

[0041] In the representation of Fig. 2, the transmission coil is shown as round, but of course other shapes are also possible, e.g. the prior art "double D" arrangement, or the asymmetrical arrangement of the transmission coils moved above and below the receiving coil. This system can also be used analogously to the method of operation described above, also with differential measurement methods such as using two receiving coils inside the transmission coils (patent DE 36 19 308 C1).

[0042] It is important that the transmission coil or at least a substantial part thereof is divided and, with the same current supply to both parts of the coil, exerts an effect on the coil or receiving coils, at which a local optimal blanking point 1.3 is created, and with the current supply only to the first half or first part of the transmission coil, the optimal blanking point was shifted in the first direction, e.g. to the right to point 5.1, while with the power supply to the second half or second part of the transmitting coil, the optimal blanking point moved in the opposite direction to the second direction, e.g. to the left to point 6.1. In addition, there is a continuous regulation of the currents of both transmission coils, which causes the shift of the local optimal blanking point and thus the continuous blanking of the receiving signal 1.11. For the analysis of the presence of metal, the control value of the differential current control of both halves of the transmitting coil is included.

[0043] Figs. 8 to 11 show further embodiments of the invention. As in the first version, the optimal blanking point always remains in the same place with straight centers and closed adjustment. This is achieved by using according to FIGS. 8 and 9 several transmission coils 2.1, 8.3, preferably two substantially identical and mirror coil halves. Also, several take-up coils 8.1, 8.2 are also used as the take-up coil, also formed in this embodiment by two essentially identical and mirror halves of the coil. However, a different division is possible if, by this, a constant or constant and, thus, not abrupt displacement of the local blanking point is achieved with the appropriate current supply.

[0044] Fig. 3 shows the overlap of the receiving coil and their transmission coil halves in a first embodiment. To clearly reduce the space required for this arrangement, in the second embodiment in Figs. 8 to 11 more emitting coils 2.1, 8.3 and more receiving coils 8.1, 8.2 are used. For a better understanding, Fig. 8 shows only the first half of the transmitting coil 2.1 and the first half, drawn in dashed line, of the receiving coil 8.1. The respective successive halves complement the semicircular coils, respectively, in the shape of a circle, which gives the image of Figure 9.

[0045] The transmission coils 2.1, 8.3 and the receiving coils 8.1, 8.2 are coupled as in the first embodiment inductively and overlap the mutual interaction. The optimal blanking point can generally be reached by turning / shifting, which will be discussed below. Through the sensor electronics shown in Fig. 11, the current is supplied to the transmission coils and the reception signal analysis of 4.20 receiving coils. Transmission coils 2.1, 8.3 act with the same current supply on the receiving coils, at which a local point 1.3 of optimal quenching arises, i.e. in which the overlapping of coils with a fixed geometric arrangement leads to a point on the imaginary line between the two transmission coils, in which at flow of the current caused by both coils, the magnetic field induced in this case is suppressed in the receiving coils. With the power supply to the first transmission coil 2.1, the optimal blanking point moves in the first direction, while with the power supply to the next transmission coil 8.3 it moves in the second direction, preferably opposite to the first direction. While in the first embodiment the point of optimal blanking at the same current supply determined the distance A, in the second embodiment it is determined by the rotation of the upper halves of the coils, comprising at least one transmitting coil and the receiving coil relative to the lower halves of the coils, which also consist of at least one transmitting coil and receiving coil.

[0046] Through the control system for regulating the amplitude of the current of the transmission coils, it is possible to shift the local blanking point so that the reception signal is blanked. In turn, the local point of optimal blanking is dependent on external influences on the magnetic field, so that, for example, the approach of metal can be detected by appropriate current regulation. The control value 4.16 determined here is also the measured value.

[0047] According to figures 8 to 10, at least one transmission coil 2.1, respectively; 8.3 and at least one receiving coil 8.2; 8.1 is placed on the first plane

10.2 and at least the next plane 10.3. The coils located on the first plane are rotated to the coils located on at least the next plane by a central angle, which can be seen especially in Fig. 8, relative to the adopted common central axis M1, M2. If the coils actually lie congruently circumferentially over each other, then depending on the type of application, at least one central angle is created at which blanking takes place. To return to the state in which the control system can regulate and produce the control value, starting from this "blanked state", the planes on which the coils are placed are shifted to each other in parallel. In other words, their central axes M1, M2 are brought together to a distance B in Figures 8 or 10. This distance can be very small, in principle from 0.1 to two percent of the diameter of the coil.

[0048] Fig. 10 shows that the opposite coil 2.1 on plane 10.3 is reflected in the receiving coil 8.2, while the first receiving coil 8.1 is completed by the second transmitting coil 8.3. Transmission coil 2.1 and receiver coil 8.2 lie on the next plane 10.3, and transmit coil 8.3 and receiver coil 8.1 lie on the first plane 10.2.

[0049] Figure 10 shows the arrangement of the coils as a printed solution in cross section, with only two planes 10.2 and 10.3 being shown. It goes without saying that it is in the printed solution that other surfaces and subsequent coils are also possible. The carrier material 10.1, i.e. the plate, is the carrier of planar coils located at the top and bottom. The transmitting coil 8.3 and the receiving coil 8.1 are located together at the top together, while the transmitting coil 2.1 and receiving coil 8.2 are located at the bottom. With the appropriate position of the upper coils relative to the lower coils, the signal sent by the transmission coils is completely blanked in the receiving coils 8.1, 8.2. While in the first embodiment the angle of inclination leads to a shift in the center point of the coil system, in the second embodiment an offset is made by a distance B, which can be, for example, 0.5 mm. In principle, however, the slope and offset can also be combined with each other if desired. It is important that the optimal blanking point can be maintained following a closed adjustment as shown in Fig. 11, always in the exact same place possible for the control value to be used as a measured value. The receiving coils can be connected in series or in parallel as long as blanking is possible.

[0050] Transmission coils 2.1, 8.3 and receiver coils 8.1, 8.2 are formed by several parts of the coils shaping by association the shape of a preferably circular arrangement, preferably the transmission coils and receiver coils, as in this embodiment, are formed by substantially two halves of the coils of the same size.

[0051] Fig. 11 shows as an embodiment the closed-loop sensor electronics for stabilizing the local optimal blanking point in the receiving coils 8.1,

8.2 magnetic field generated in the transmitting coils 2.1 and 8.3. The pulse generator 4.8 supplies the first pulse signal 4.13 to the first regulated power source 4.10 and the second invert pulse signal 4.12 to the second controlled power source 4.9. The frequency of the pulse generator can be selected depending on the inductance of the coils, in this implementation it is about 120 kHz. The signal can be e.g. a rectangular or sine signal. The first regulated power source 4.10 supplies one coil connection. Similarly, a second regulated power source 4.9 supplies the second coil connection. The signal applied to the receiving coils is amplified by an 4.5 voltage amplifier - hereinafter the amplifier. The output of the 4.5 amplifier is fed to the synchronous demodulator 4.6. It contains the first impulse signal from 4.18 and the second impulse signal 4.19 from the pulse generator 4.8 needed for demodulation. In the simplest case, the synchronous demodulator 4.6 supplies the output signal of the amplifier 4.5 during the entire pulse phase segment synchronously to the respective inputs of the integrating 4.7 comparator. In this case, pulse signals 4.18 and 4.19 are as long as the transmit pulse phases.

[0052] With the same voltage of the first input signal 4.15 and the second input signal 4.17 of the integrating comparator 4.7, no synchronous pulse signal input on the receiving coils 8.1, 8.2 is created. In this case, e.g. with external metal influences, the average value of the first pulse signal on the receiving coils 8.1, 8.2 is compared with the average value of the second pulse signal. In the regulated state, the reception signals at the amplifier's inputs

4.5 correspond to the zero state, so that the 4.5 amplifier only sees noise at the input. Therefore, it can be amplified or made as a high gain limiting amplifier. The same applies in the controlled state of the first input signal 4.15 and the second input signal 4.17. J f the output of the comparator 4.7 is not applied no signal corresponding to a zero-limiting ago, the value adjustment 4.16 is far away and led by a current in the coils broadcasting 2.1

8.3 is regulated until this state is reached.

[0053] The control system causes such a constant further adjustment of the determined value of the optimum blanking point that no differential signal adheres to the synchronous demodulator 4.6. This leads to the fact that time-varying or dynamic changes in the environment of the metal detector such as the approach of a metal are perceived as changes in the control value 4.16.

List of determinations [0054]

1.1 shift starting point

1.2 signal with 0 ° phase position

1.3 decoupling point

1.4 signal with 180 ° phase position

1.5 shift end point

1.6 double arrow moving the receiving coil to the transmitting coil

1.7 receiver coil amplitude

1.9 receiving coil

1.10 transmitting coil (prior art)

1.11, 4.20 receiving signal

2.1 first upper half of the coil

2.2 second lower half of the coil

2.3 connection of the first upper half of the coil

2.4 connection of the second lower half of the coil

2.5 jointed connectors for the first and second half of the coil

2.6 make-up voltage to 2.7

2.7 make-up voltage up to 2.6

3.1 horizontal axis of the receiving coil

3.2 horizontal axis of the transmission coils system

In the angle of inclination

A, B space

4.5 AC power amplifier

4.6 synchronous demodulator

4.7 integrating comparator

4.8 pulse generator

4.9 second regulated power source

4.10 first regulated power source

4.11 inverting degree

4.12 second impulse signal

4.13 first impulse signal

4.15 the first input signal of the integrating comparator

4.16 regulatory value

4.17 second input signal of the integration comparator

4.18 the first impulse signal needed for demodulation

4.19 second impulse signal needed for demodulation

5.1 right blanking point shifted to the right

6.1 left blank point for optimal blanking

7.3 adjustment value when approaching metal

7.4 metal approach area

8.1 first half of the receiver coil

8.2 second half of receiving coil

8.3 second half of the transmitting coil

10.1 support material (plate)

10.2 first plane

10.3 the next plane

M1, M2 central axis