DEVICE FOR DETECTING METALLIC OBJECTS IN AND ON OBJECTS MOVING AGAINST THE DEVICE

Description

TECHNICAL AREA OF INVENTION

[0001] The invention relates to a device for detecting metallic objects in and on objects moving relative to the device. More precisely, the invention relates to such a device with the features of the preamble of independent claim 1.

[0002] The non-invasive examination of objects to determine whether they contain metallic components is performed in various technical fields. One area is the inspection of products, for example in the food and other consumer goods industries, for metallic contamination. Another area is the protection of processing machinery from metallic objects that could damage it. Yet another area is the screening of individuals for weapons; that is, the objects being examined could also be people. The key is to determine whether the object in question, which may itself be electrically conductive due to its water and / or salt content, contains a metallic component that distinguishes it from a legitimate or harmless object. STATE OF THE ART

[0003] From EP 2 625 551 B1, a metal detection system is known, comprising a transmitting coil and two receiving coils arranged symmetrically to the transmitting coil and connected in series in a quadrupole configuration. The transmitting coil is connected to a transmitter unit, which outputs signals at selectable operating frequencies. The series-connected receiving coils are connected to an evaluation unit via a filter device. Without a metallic object in the metal detection system, the voltages induced in the two receiving coils equalize. When a metallic object is passed through the coils, received signals occur, which depend on the properties of the metallic object. The evaluation unit analyzes the received signals with respect to phase and amplitude.First, the phases and amplitudes of the corresponding received signals for the respective product containing a metal impurity of various particle sizes are determined at different operating frequencies of the transmitter unit. From this, several operating frequencies are identified at which the differences in the received signals are particularly large compared to the uncontaminated product. These identified operating frequencies are then applied simultaneously by the transmitter unit during operation of the metal detection system. The received signals are filtered accordingly in the filter unit and then evaluated separately in the evaluation unit.

[0004] From EP 2 562 565 A1, a metal detection device is known, comprising a transmitter with a transmitting coil and a receiver comprising two receiving coils connected in series in a quadrupole configuration, in which the transmitting coil is part of a parallel resonant circuit of the transmitter. The resonant frequency of the transmitting circuit is the operating frequency of the transmitting coil. The operating frequency is switchable by allowing a capacitor in the parallel resonant circuit to be switched between predetermined values. Thus, this known metal detection device can be operated at the most favorable operating frequency in a given case or successively at different operating frequencies.

[0005] From DE 10 2012 013 554 A1, a metal detection device is known in which a transmitting coil is part of a resonant circuit of a transmitting device and in which two receiving coils connected in series in a quadrupole configuration are part of a resonant circuit of a receiving device. A device for tuning the transmitting coil and the receiving coils includes a capacitor decade box for freely selectable adjustment of the resonant frequencies of both resonant circuits. The resonant circuits can be parallel or series resonant circuits. The respective resonant frequency of the transmitting device's resonant circuit is the operating frequency of the transmitting coil, and the resonant frequency of the receiving device's resonant circuit is tuned to this operating frequency. In this way, the response sensitivity of the metal detection device is increased.

[0006] From EP 1 760 494 B1, a metal detection device with the features of the preamble of independent claim 1 is known. The transmitting coil of the transmitting device is part of a multiply resonant oscillating circuit which has a resonant frequency at each of the several operating frequencies of the metal detection device.

[0007] From DE 30 23 446 A1, an identification system for detecting objects is known, consisting of a transmitting coil element, which is excited by a generator with a variable frequency, and a detector block to be attached to the object to be identified, with one or more resonant circuits, each tuned to a different frequency. The transmitting coil element consists of two identical transmitting coils connected in opposite phase, arranged such that the resultant of the magnetic field in a measuring coil located between them is zero when a resonant circuit is absent in the operating range and non-zero when one is present. The excited resonant circuit perturbs the magnetic field of the transmitting coils and induces a voltage in the measuring coil.By causing a resonant circuit within the detector block, tuned to a specific frequency, to oscillate using the signal emitted at that frequency, it is possible to precisely determine which detector block is causing the magnetic field disturbance and which resonant circuit is responsible. By using different resonant circuits tuned to different frequencies within the detector block, a large number of detector blocks can be distinguished from one another. Depending on the number of objects to be identified, more or fewer frequencies and more or fewer tuned circuits can be used.

[0008] A metal detection system with the features of the preamble of independent claim 1 is known from US patent 2012 / 0 086 455 A1. The filter device includes bandpass filters.

[0009] From US 2015 / 0 276 964 A1, a metal detection system with the features of the preamble of independent claim 1 is known, in which the evaluation device demodulates each of the partial signals at one of the operating frequencies with respect to contained phase information.

[0010] A combined electronic article surveillance and metal detection system is known from US Patent 8,659,428 B2. A transmitter periodically sends out interrogation signals with different first and second frequencies. A receiver receives a first signal at the first frequency and a second signal at the second frequency. An electronic article surveillance module detects an electronic article surveillance marker based on the received first signal at the first frequency when the transmitter is not transmitting. A metal detector module detects a metal object by receiving an output signal from the receiver when the transmitter is transmitting, filtering out signals received essentially at the first frequency, determining a first voltage of the second signal, comparing the first voltage to a second voltage, and determining that the first voltage differs from the second voltage by more than a predetermined value.The metal detector module is configured to filter out signals that are essentially received at the first frequency using a notch filter. TASK OF INVENTION

[0011] The invention is based on the objective of providing a device that can detect metallic objects of different sizes with high sensitivity at any point in or on objects moving relative to the device, even if the objects themselves are electrically conductive or if electrical or dielectric properties generally make the detection of the metallic objects more difficult. SOLUTION

[0012] The object of the invention is achieved by a device having the features of claim 1. The dependent claims relate to preferred embodiments of the device according to the invention. DESCRIPTION OF THE INVENTION

[0013] In a device according to the invention for detecting metallic objects in and on objects moving relative to the device, a transmitting unit with a transmitting coil generates an alternating magnetic field having several operating frequencies. A receiving unit of the device detects the alternating field influenced by the moving objects with at least one receiving coil and outputs a received signal dependent on the detected alternating electric field. A filtering unit of the device filters out partial signals, each corresponding to one of the operating frequencies, from the received signal. An evaluation unit of the device evaluates the partial signals. The filtering unit of the device according to the invention bandstops each of the partial signals corresponding to one of the operating frequencies with respect to all other operating frequencies.A bandstop filter at the other operating frequencies, which strongly suppresses the received signal at these frequencies, exhibits a lower slope in the frequency-dependent phase response and thus lower sensitivity to small frequency shifts and temperature drifts than a narrowband bandpass filter of the transmitted partial signal at its assigned operating frequency. In particular, the effect on the phase of the partial signal at the respective operating frequency does not exhibit a strong frequency dependence. This is important because certain shifts between the operating frequencies and the filter frequencies, for example due to thermal influences, are practically unavoidable in the operation of industrial devices for detecting metallic objects.In contrast to narrowband bandpass filtering with a deep stopband, the strongest influence on the phase of the received signal in the bandstop filtering according to the invention occurs with respect to the other operating frequencies in the frequency domain far removed from the operating frequency assigned to the respective partial signal of interest. This applies without limitation to a large dynamic range of the partial signal achieved by the filtering and results in high overload resistance of the subsequent signal processing of the respective partial signal of interest at the associated operating frequency.

[0014] To implement band-stop filtering in the filter assembly of the device according to the invention, the device comprises a group of at least two notch filters for each of the partial signals and each of the other operating frequencies. The group of notch filters includes a first notch filter with a first mean filter frequency, which is preferably tuned to the respective other operating frequency. The group of notch filters further comprises a second and / or a third notch filter with a second and a third mean filter frequency, which are preferably 1% to 10% lower and higher, respectively, than the first mean filter frequency. The group of notch filters thus exhibits an overall transmission spectrum with a deeper and wider stopband than a single notch filter.

[0015] The filter arrangement of the device according to the invention can further comprise a bandpass filter, a low-pass filter, or a high-pass filter for each partial signal, wherein each edge of the respective filter maintains a distance of at least 10% of the operating frequency from the operating frequency to which the respective partial signal is assigned. Thus, even if the filter arrangement does include a bandpass filter for the respective operating frequency, this bandpass filter is very wide, with a large distance between its edges and the operating frequency and correspondingly little influence on the partial signal at the operating frequency. Preferably, the distance of each edge of the respective filter from the respective operating frequency is 20%, and even more preferably 30% of the operating frequency.

[0016] To achieve high sensitivity of the device according to the invention by obtaining as much information as possible about any metallic objects in the moving objects, the transmitting coil is preferably part of a multi-resonant resonant circuit of the transmitting device, which has a resonant frequency at each of the several operating frequencies, and / or the at least one receiving coil is part of a multi-resonant resonant circuit of the receiving device, which has a resonant frequency at each of the several operating frequencies. Particularly preferably, both the transmitting device and the receiving device have such a multi-resonant resonant circuit, wherein the resonant frequencies of the two resonant circuits are tuned to each other.

[0017] The filter device of the device according to the invention bandstops each of the partial signals that is assigned to one of the operating frequencies, and preferably also with respect to all other resonant frequencies of the multiply resonant oscillator circuit of the receiving device.

[0018] Due to the multiple resonant circuits, the device according to the invention exhibits high response sensitivity at several operating frequencies and thus a very high overall sensitivity to metallic objects. In the transmitter, excitation signals from the individual operating frequencies are added to form a sum signal. This addition can be performed, for example, electronically or in a signal transformer at the input of the multi-resonant circuit of the transmitter. It is sufficient to add the excitations at the individual operating frequencies. On the receiving side, the received signal from the receiving device is filtered in the filtering device in such a way that the partial signals assigned to each of the operating frequencies are separated from one another and, as far as possible, remain unchanged.When using the device according to the invention with multiple resonant oscillator circuits, the partial signals have a comparatively high signal-to-noise ratio and can therefore be filtered relatively easily by the filter device and evaluated easily by the evaluation device. Thus, even small metallic objects in and on the objects examined with the device according to the invention can be detected, even if the objects themselves have pronounced electrical and / or dielectric properties and therefore strongly influence the alternating magnetic field generated by the transmitting coil, i.e., themselves lead to a high received signal, the relative changes of which caused by a metallic object to be detected remain small.Due to the multiple operating frequencies used by the transmitter, these changes caused by the metallic object to be detected are captured simultaneously at these different operating frequencies, and with a high probability also at an operating frequency where they are clearly discernible, thus enabling reliable detection of the metallic object. Furthermore, the simultaneous changes in the received signal caused by the metallic object to be detected at the different operating frequencies can be correlated, providing a broader data basis for the reliable detection of the metallic object.

[0019] Each multi-resonant resonant circuit of the device according to the invention typically has interconnected sub-resonant circuits, and these interconnected sub-resonant circuits can each comprise a series resonant circuit and a parallel resonant circuit. It is generally known that a resonant circuit consisting of a series resonant circuit and a parallel resonant circuit interconnected with it has several, i.e., exactly three, resonances. Two of the three resonances are parallel resonances with a local maximum of the impedance. The parallel resonances arise from the coupled interaction of the inductances and capacitances of both interconnected sub-resonant circuits and can be used for the signals at the operating frequencies of the device according to the invention, near their respective resonant frequencies. The third resonance is a series resonance, in which the impedance for an excitation signal to excite the resonant circuit of the transmitting device is...The received signal for extraction from the receiver's resonant circuit exhibits a local minimum, and its resonant frequency lies between the two resonant frequencies of the parallel resonances. The resonant frequency of the third resonance is determined solely by the series resonance of the series resonant circuit, which is unaffected by the parallel resonant circuit. To realize three or more usable resonant frequencies, more than one series resonant circuit and one parallel resonant circuit must be interconnected in the respective multi-resonant circuit. Each additional operating frequency increases not only the complexity of the multi-resonant circuit but also the effort required to filter out and electronically process the partial signals, each corresponding to one of the operating frequencies, from the received signal in the filtering unit. It can therefore be practically advantageous to limit the number of resonant frequencies used as operating frequencies to two or three.

[0020] In the device according to the invention, the transmitting unit can be switched between several predetermined sets of operating frequencies in order to optimally adapt the operating frequencies to the objects being tested, i.e., their electrical and dielectric properties, and also to particularly relevant metallic objects. Each multi-resonant oscillator circuit of the device can then be switched to tune its resonant frequencies to the respective set of operating frequencies. Specifically, this can be achieved by making the capacitances of all interconnected sub-circuits in the respective multi-resonant oscillator circuit switchable between pre-defined, coordinated values. In principle, it is sufficient to switch only one capacitance in one of the sub-circuits of the respective oscillator circuit to switch to a different set of operating frequencies.By switching all capacitances in a coordinated manner, for example by quadrupling them, the value of all resonant frequencies of the respective resonant circuit can be changed by the same factor—in the chosen example, halved—so that their frequency ratio remains constant. Furthermore, it is possible to selectively vary the spacing of the resonant frequencies of each multi-resonant circuit in the device by switching an inductor in one of the sub-circuits of the respective resonant circuit. Ideally, this inductor should be that of a coil or coil assembly that is not simultaneously the transmitting coil or the at least one receiving coil or receiving coil assembly.

[0021] In the device according to the invention, it is advantageous if each multi-resonant oscillator circuit has a quality factor of at least 10 at each of the several operating frequencies, i.e., at its corresponding resonant frequency, in order to achieve the desired sensitivity. The lower the respective operating frequency, the more complex it is to achieve a high quality factor at that frequency. Thus, the quality factor preferably achieved at the lowest of the several operating frequencies is at least 15, and the quality factor preferably achieved at the highest of the several operating frequencies is at least 30. A very high quality factor of the respective multi-resonant oscillator circuit is not desirable in the device according to the invention.This is especially true for quality factors of 100 or more, because with increasing quality factor, the influence of the temperature-dependent resonance properties of the resonant circuit on the phase of the partial signals at the respective operating frequency can increasingly impair the quality of the signal evaluation.

[0022] The evaluation unit of the device according to the invention can be configured in a generally known manner to demodulate each of the partial signals with a reference signal of the operating frequency assigned to the respective partial signal with respect to contained phase information.

[0023] Furthermore, the receiving device can also have two identical receiving coils, preferably with the same geometry as the transmitting coil, arranged symmetrically in planes parallel to the transmitting coil, and connected in series in the receiving device, particularly in the resonant circuit of the receiving device, in a quadrupole configuration. A received signal only occurs in the receiving device if the symmetry of this setup is disturbed by an object that affects the alternating magnetic field emanating from the transmitting coil differently on one side of the transmitting coil than on the other. Such a disturbance of symmetry is caused by any object moving relative to the device, particularly through its coils. The nature of the disturbance depends on the specific object and its electrical and dielectric properties.An additional interference occurs when a metallic object is present in or attached to the object. The nature of this interference depends on the size, shape, and material of the metallic object, as well as its position within or attached to the object. Therefore, the course of the interference, which depends on the object's movement relative to the device, and the resulting received signal in the receiving device, allow conclusions to be drawn as to whether the object contains a metallic object. Furthermore, it may be possible to infer the position, type, and / or size of a metallic object within the object.

[0024] Even when essentially identical receiving coils are arranged symmetrically in planes parallel to the transmitting coil and connected in series in a quadrupole configuration within the receiving device, a non-zero received signal can occur in practice even before the alternating magnetic field generated by the transmitting coil is affected by objects moving relative to the device according to the invention. Therefore, it is generally known to provide an adjustment device designed to tune the received signal to zero at the respective operating frequency of the device when the alternating magnetic field has not yet been affected by the moving objects. In the device according to the invention, this tuning is performed simultaneously for all of the multiple operating frequencies. For this purpose, the device according to the invention preferably has a separate, i.e.,separate adjustment device with minimal cross-sectional effect on the signal components at the respective other operating frequency.

[0025] Specifically, one of the adjustment devices, in particular the adjustment device for the highest of the operating frequencies, can have at least one capacitor or resistor connected in parallel with one of the two receiving coils to directly adjust the characteristics of the two receiving coils at this operating frequency within the resonant circuit of the receiving device. To adjust the received signal at the operating frequency towards zero, the capacitor and / or resistor can be continuously adjustable. However, since the relevant characteristics of the two receiving coils, and thus their differences, generally remain constant, one of the adjustment devices can also be permanently configured.

[0026] The separate tuning device for each of the operating frequencies can include a transformer whose primary winding carries a portion of an excitation signal, primarily used to excite the transmitting device at the corresponding operating frequency. The secondary winding of this transformer has two winding halves and a midpoint between them. The midpoint of the secondary winding is connected to a signal output of the receiving device for the received signal. At least one capacitor or resistor is connected in parallel with one of the winding halves of the secondary winding to the signal output.The transformer adds a calibration signal to the signal output of the receiving device at the respective operating frequency. This calibration signal's phase and amplitude can be adjusted by dimensioning or, if possible, by changing the parallel-connected resistors and capacitors. This allows the received signal to be calibrated towards zero at each additional operating frequency. Since the relevant characteristics of the two receiving coils, and thus their differences, generally remain constant, as already mentioned, each separate calibration device for each additional operating frequency can also be permanently configured.

[0027] In addition, the partial signals output by the filter device of the device according to the invention, each assigned to one of the operating frequencies, can be added in such a controlled manner with signals derived from the corresponding part of the excitation signal for the transmitter, such that the partial signals average out to zero over time. This adjustment can compensate for aging and temperature drift, but occurs so slowly that it does not significantly attenuate the changes in the partial signals caused by objects moving relative to the device.

[0028] Through all these measures, the filtering device and the evaluation device of the device according to the invention can be more closely adapted to the information-containing components of the received signal.

[0029] The evaluation unit of the device according to the invention is preferably configured to correlate information obtained at the multiple operating frequencies in order to detect the metallic objects in and on the objects moving relative to the device. For example, the progression of a ratio or difference of the partial signals or phase information obtained therefrom can be considered. The progression of these values ​​may depend more strongly on certain properties of the metallic objects than the individual partial signals considered or the information derived directly from them.

[0030] Specifically, the information obtained at the multiple operating frequencies can be related to one another depending on the corresponding times at which the underlying received signal was registered. Alternatively, the information obtained at the multiple operating frequencies can be related to one another depending on the corresponding positions of the respective object relative to the device. Often, however, these temporal and spatial considerations are equivalent because the objects being examined with the device according to the invention move, for example, on a conveyor belt at a constant speed relative to the device. The fact that the objects move relative to the device according to the invention can specifically mean that the objects are conveyed through the transmitting coil and all receiving coils of the device in a conveying direction perpendicular to the coil planes.However, it is also possible, for example, to convey the objects past the transmitting coil and all receiving coils of the device in a conveying direction parallel or even just along the coil planes.

[0031] Advantageous further developments of the invention result from the patent claims, the description and the drawings.

[0032] The advantages of features and combinations of features mentioned in the description are merely exemplary and can have an effect alternatively or cumulatively, without the advantages necessarily having to be achieved by embodiments according to the invention.

[0033] Regarding the disclosure content—not the scope of protection—of the original application documents and the patent, the following applies: Further features can be derived from the drawings—in particular, the geometries depicted and the relative dimensions of several components to one another, as well as their relative arrangement and functional connection. The combination of features from different embodiments of the invention or from features of different claims is also possible, deviating from the chosen cross-references of the claims, and is hereby encouraged. This also applies to features that are illustrated in separate drawings or mentioned in their description. These features can also be combined with features from different claims.Likewise, features listed in the patent claims may be omitted for further embodiments of the invention, but this does not apply to the independent patent claim of the granted patent.

[0034] The features mentioned in the claims and the description are to be understood, with regard to their number, as meaning that exactly that number or a greater number than the stated number is present, without the need for the explicit use of the adverb "at least". Thus, for example, if a series resonant circuit is mentioned, this is to be understood as meaning that exactly one series resonant circuit, two series resonant circuits, or more series resonant circuits are present. The features listed in the claims may be supplemented by further features or may be the only features that the respective device possesses.

[0035] The reference numerals contained in the patent claims do not constitute a limitation of the scope of the subject matter protected by the patent claims. They merely serve the purpose of making the patent claims easier to understand. BRIEF DESCRIPTION OF THE FIGURES

[0036] The invention will now be further explained and described with reference to preferred embodiments shown in the figures. Fig. 1 is a schematic view of an embodiment of the device according to the invention. Fig. 2 shows a first embodiment of a resonant circuit of a transmitting device of the apparatus according to Fig. 1 . Fig. 3 shows a first embodiment of an oscillating circuit of a receiving device of the apparatus according to Fig. 1 . Fig. 4 shows a second embodiment of the resonant circuit of the transmitting device of the apparatus according to Fig. 1 . Fig. 5 shows one embodiment of the resonant circuit according to Fig. 4 corresponding second embodiment of the resonant circuit of the receiving device of the apparatus according to Fig. 1 with switchable capacities. Fig. 6 shows a variant of the embodiment of the resonant circuit of the receiving device according to Fig. 5 with an additional switchable inductor. Fig. 7 shows an embodiment of the resonant circuit of the receiving device according to Fig. 3 further embodiment of the resonant circuit of the receiving device of the apparatus according to Fig. 1 with switchable capacities. Fig. 8 shows a variant of the embodiment of the resonant circuit of the receiving device according to Fig. 7 with an additional switchable inductor. Fig. 9 is a schematic representation of a first embodiment of a filter device of the apparatus according to Fig. 1 . Fig. 10 is a schematic representation of a second embodiment of the filter device of the apparatus according to Fig. 1 . Fig. 11 illustrates the formation of a bandstop filter of the filter device according to Fig. 9 or 10 from a group of three individual notched filters. Fig. 12 shows a resulting transmission spectrum of the group of notched filters according to Fig. 11 . Fig. 13 illustrates an embodiment of an evaluation device of the apparatus according to Fig. 1 ; and Fig. 14 Figure 1 is a schematic view of a further embodiment of the device according to the invention with additional adjustment devices. FIGURE DESCRIPTION

[0037] The in Fig. 1 In one embodiment, the device 1 serves to detect metallic objects 2 in or on objects 3 that are moving relative to the device. Fig. 1The object 3 is a product 5 packaged in a box 4, which may contain a metallic object 2. The object 3 is moved in a conveying direction 6, for example on a conveyor belt (not shown here), through coils 7 to 9 of the device 1. The coils 7 to 9, each shown here with only one turn, can also have several turns. The middle coil is a transmitting coil 7, which is part of a transmitting device 10. The outer coils are two receiving coils 8 and 9. The receiving coils 8 and 9 are arranged symmetrically to the transmitting coil 7 in planes parallel to the transmitting coil 7, typically closer to the transmitting coil 7 than this. Fig. 1The receiving coils 8 and 9 can have the same geometry, i.e., geometric shape and size, as the transmitting coil 7. The receiving coils 8 and 9 are parts of a receiving device 11 and are connected in series in a quadrupole configuration. The transmitting device 10 is configured to generate an alternating magnetic field with the transmitting coil 7. The receiving device 11 detects the alternating magnetic field with the receiving coils 8 and 9. Due to the symmetrical arrangement and the connection of the receiving coils 8 and 9 in a quadrupole configuration, no received signal is generated as long as the magnetic configuration in the device 1 is also symmetrical with respect to the transmitting coil 7. When the object 3 moves through the coils 7 to 9, the symmetry of the magnetic configuration is disturbed, and the received signal propagates along the path of the object 3 through the device 1 in a manner characteristic of that object.When the metallic object 2 is present in or on the object 3, the received signal exhibits a significantly different characteristic curve than without the metallic object 2. The specific curve of the received signal also depends on the operating frequency at which the alternating magnetic field is generated by the transmitting coil 7. The device 1 is designed such that the transmitting unit 10 generates the alternating magnetic field with several operating frequencies simultaneously with the transmitting coil 7, and that the received signal is initially evaluated separately by the receiving coils 8 and 9 at each of these operating frequencies. For this purpose, a filtering unit 14 is connected between the receiving unit 11 and an evaluation unit 13. This filtering unit extracts partial signals from the received signal, each corresponding to one of the operating frequencies. These partial signals are then evaluated by the evaluation unit 13.

[0038] Fig. 2shows a multi-resonant oscillating circuit 15, which is part of an embodiment of the transmitting device 10 according to Fig. 1 is and which includes the transmitting coil 7. The transmitting coil 7, together with a first capacitor 16, is part of a series resonant circuit 17, which is connected to a parallel resonant circuit 18 consisting of a second coil 19 and a second capacitor 20. The resonant circuit 15 has several resonant frequencies that can be used as operating frequencies. The resonant circuit 15 can be excited to oscillations via terminals 21 and 22, and thus to generate alternating magnetic fields via the transmitting coil 7. Instead of using the coil of the series resonant circuit 17 as the transmitting coil 7, the coil 19 of the parallel resonant circuit 18 could also be used as the transmitting coil.

[0039] A multi-resonant oscillating circuit of the receiving device 11 can, in principle, be constructed in the same way as the oscillating circuit 15 according to Fig. 2, wherein the two receiving coils 8 and 9 are connected in series in a quadrupole configuration at the location of the transmitting coil 7.

[0040] Fig. 3 Figure 1 shows a multi-resonant resonant circuit 23 of the receiving device 11, in which the receiving coils 8 and 9 together with a capacitor 24 form a partial resonant circuit configured as a parallel resonant circuit 25. Another coil 26 and another capacitor 27 form a series resonant circuit 28 connected to it. Fig. 3 further indicates how the initially symmetrical received signal of the receiving device 11 is transmitted via a transformer 38 with matched impedance and asymmetrically at connections 29 and 30 to the filter device 14 according to Fig. 1is available. The primary and secondary sides of the transformer 38 have a fixed ground reference. The transformer 38 is designed so that its influence on the multi-resonant resonant circuit 23 remains small. The transformer 38 can be configured to increase the voltage of the received signal at terminals 29 and 30. A transformer corresponding to the transformer 38 can also be provided at the transmitter 10 to connect a signal generator with a matching impedance and symmetry of an excitation signal it provides to the resonant circuit 15. Thus, the transformer 38 is an option in all the Figs. 2 to 8 shown oscillating circuits.

[0041] According to Fig. 4 ,The figure again shows the resonant circuit 15 of the transmitter 10. The capacitor 16 of the series resonant circuit 17 is divided into two equal sub-capacitors 31. This makes the series resonant circuit 17, and also the entire resonant circuit 15, symmetrical about a center point of the transmitter coil 7. Furthermore, galvanic isolation is achieved, which makes it easier to provide the transmitter coil 7 with a fixed potential reference. According to Fig. 4 The center point of the transmitting coil 7 is connected to ground via a center tap 12. This results in a symmetrical drive of the transmitting coil 7. Instead of the direct connection of the center point of the transmitting coil 7 to ground shown, a connection via a resistor with a high resistance relative to the impedance of the transmitting coil 7 at the operating frequencies is also possible. Furthermore, the ends of the transmitting coil 7 could be connected to ground via identical high-resistance resistors to establish the potential reference.

[0042] Fig. 5 shows a resonant circuit 15 of the transmitting device 10 according to Fig. 4 corresponding design of the resonant circuit 23 of the receiving device 11. The receiving coils 8 and 9 are part of the series resonant circuit 28, in which the capacitor 27 is made of Fig. 3 The power is divided into subcapacitors 32, and another coil 33, together with capacitor 24, forms the parallel resonant circuit 25. The midpoint between the receiving coils 8 and 9 is connected to ground to provide a reference potential. Instead of the direct connection of the midpoint to ground shown, a connection via a resistor with a high resistance relative to the impedance of the receiving coils 8 and 9 at the operating frequencies is also possible. Furthermore, the opposite outer ends of the two receiving coils 8 and 9 could be connected to ground via identical high-resistance resistors to establish the potential reference.

[0043] Further circuit configurations are possible to create multi-resonant resonant circuits 15 and 23 for the transmitter 10 and the receiver 11. Crucially, several resonant frequencies are available as operating frequencies for the transmitter 10 or, in the receiver 11, for tuning to the operating frequencies of the transmitter. The precise position of the resonant frequencies is determined by the capacitances of capacitors 16, 20, 24, and 27 and the inductances of coils 7 to 9, 19, 26, and 33 of the sub-resonant circuits of resonant circuits 15 and 23. By appropriately selecting these capacitances and inductances, the resonant frequencies of resonant circuits 15 and 23 can be tuned to desired values.

[0044] Fig. 5Figure 34 also shows how, with the aid of switches 34 and 35, the capacitances in the parallel resonant circuit 25 and the series resonant circuit 28 can be increased by connecting further capacitors 36 and 37 in parallel. In this way, the resonant frequencies of the resonant circuit 23 are shifted to lower frequencies, while maintaining a frequency ratio of the resonant frequencies.

[0045] In a specific embodiment of the resonant circuit 23 of the receiving device 11 according to Fig. 5For a device 1 for detecting metallic objects 2 with higher operating frequencies, when only the capacitances of the partial capacitors 32 and the capacitor 24 are active, the lower of the two operating frequencies is 150 kHz and the higher of the two operating frequencies is 600 kHz. If the capacitances in the parallel resonant circuit 25 and the series resonant circuit 28 are quadrupled by connecting the additional capacitors 36 and 37 in parallel, the lower of the two operating frequencies is halved to 75 kHz and the higher of the two operating frequencies to 300 kHz. The frequency ratio of the two operating frequencies of 1:4 remains unchanged.

[0046] In a specific embodiment of the resonant circuit 23 of the receiving device 11 according to Fig. 5For a device 1 for detecting metallic objects 2 with lower operating frequencies, when only the capacitances of the partial capacitors 32 and the capacitor 24 are active, the lower of the two operating frequencies is 75 kHz and the higher of the two operating frequencies is 300 kHz. If the capacitances in the parallel resonant circuit 25 and the series resonant circuit 28 are quadrupled by connecting the additional capacitors 36 and 37 in parallel, the lower of the two operating frequencies is halved to 37.5 kHz and the higher of the two operating frequencies to 150 kHz. The frequency ratio of the two operating frequencies, which is still 1:4, remains unchanged.

[0047] Fig. 6shows how the frequency ratio of the resonant frequencies can be changed by increasing the inductance of the parallel resonant circuit 25 by opening a switch 59 and thereby connecting a further coil 60, which is initially short-circuited, in series with the coil 33.

[0048] In contrast to the specific embodiment of the resonant circuit 23 of the receiving device 11 given above, according to Fig. 5 For a device 1 with additional capacitors 36 and 37 connected and the resulting operating frequencies of 75 kHz and 300 kHz, increasing the inductance of the parallel resonant circuit 25 by the additional coil 60, while simultaneously reducing the capacitance of the parallel resonant circuit 25 by opening the switch 34, selectively doubles the higher of the two operating frequencies to 600 kHz. The frequency ratio of the two operating frequencies doubles accordingly from 1:4 to 1:8.

[0049] Fig. 7 shows how, in the case of the multiply resonant oscillating circuit 23 in the basic embodiment according to Fig. 3 In this circuit, where receiving coils 8 and 9 are part of the parallel resonant circuit, the capacitances in all sub-resonant circuits can be increased by capacitors 36 and 37, which can be switched on and off using switches 34 and 35. Switching on capacitors 36 and 37 lowers all resonant frequencies of the resonant circuit 23, while switching them off raises them.

[0050] Fig. 8 shows how the frequency ratio of the resonant frequencies can be additionally changed by opening the switch 59 and thereby connecting the initially short-circuited further coil 60 in series with the coil 26, thus increasing the inductance of the series resonant circuit 28.

[0051] Fig. 9 illustrates an embodiment of the filter device 14 according to Fig. 1In the event that the device 1 is operated simultaneously at two operating frequencies, an input amplifier 40 is connected to a signal input 39. Two different signal processing paths lead to two signal outputs 41 and 42. In one signal processing path, a low-pass filter 43 is connected in series with a band-stop filter 44, effective at the higher of the two operating frequencies, and an output amplifier 45, so that a partial signal with the lower of the two operating frequencies is present at output terminal 41. In the other signal processing path, a high-pass filter 46 is connected in series with a band-stop filter 47, effective at the lower of the two operating frequencies, and an output amplifier 48, so that a partial signal with the higher of the two operating frequencies is present at output terminal 42. The two output terminals 41 and 42 are connected to the evaluation unit 13 according to Fig. 1connected.

[0052] Fig. 10 Figure 1 shows an embodiment of the filter device 14 for three operating frequencies of the device 1. Accordingly, there is an additional signal output 49 for a third partial signal. In the signal path to the additional signal output 49, a broadband bandpass filter 50 is connected in series with copies of the bandstop filter 44 for the highest of the three operating frequencies and the bandstop filter 47 for the lowest of the three operating frequencies, as well as an output amplifier 51. Thus, the third partial signal with the middle of the three operating frequencies is output at the additional signal output 49. In the signal paths to the signal outputs 41 and 42, additional bandstop filters 52 for the middle of the three operating frequencies are arranged in order to also cancel this partial signal out from the outputs 41 and 42 with the lowest and highest of the two operating frequencies, respectively.

[0053] The bandstop filters 44, 47, 52 according to the Fig. 9 and 10 are preferably bandstop filters of larger width and each consists of a group of three individual notched filters, as is the case for the bandstop filter 47 in Fig. 11 The following is shown: A notch filter 53 with a mean filter frequency 61 is connected in series with a notch filter 54 with a slightly lower filter frequency and a notch filter 55 with a slightly higher filter frequency. The sum of these combinations results in a transmission spectrum or filter function of the bandstop filter 47 as shown in Fig. 12This is represented by a stopband 62 with high attenuation 63 and width 64 around the mean filter frequency 61, in the region of which the bandstop filter 47 strongly suppresses the received signal. Specifically, the attenuation 63 of the stopband can be in the range of -70 dBc to -60 dBc, and the width 64 can simultaneously be 10% to 20% of the mean filter frequency 61.

[0054] Fig. 13 Figure 1 illustrates a part of the evaluation unit 13 of the device 1, insofar as it operates at two operating frequencies. The signal outputs 41 and 42 are shown in the diagram. Fig. 9The received partial signals are demodulated by demodulators 56 and 57 using a reference signal from the respective operating frequency. The result is a time course of quadrature components with the values ​​xL and yL at the lower operating frequency and xH and yH at the higher operating frequency, from which the phase and amplitude of the respective partial signal at specific times can be reconstructed, thus determining the specific relative positions of the object 3 with respect to the device 1 according to Fig. 1correspond. In unit 58 of the evaluation unit 13, the individual values ​​are related to each other depending on time and / or position. This provides an extended database for the detection of metallic objects 2 in and on the objects 3, which allows the detection of the metallic objects 2 even if the objects themselves are electrically conductive or have other pronounced electrical or dielectric properties and therefore strongly influence the alternating magnetic field generated by the transmitting coil 7. Not indicated in Fig. 13 , that alternatively or in addition to the values ​​calculated directly from the partial signals, values ​​of difference signals between the partial signals as they occur with an actual object 3 and partial signals as they occur with a reference object without a metallic object 2 can also be evaluated.

[0055] The in Fig. 14The illustrated embodiment of the device 1 has the receiving device 11 with the resonant circuit 23 and the transformer 38 according to Fig. 3 and the transmitting device 10 in a corresponding configuration with the transmitting coil 7 together with the capacitor 20 in the parallel resonant circuit 18 and another coil 65 together with the capacitor 16 in the series resonant circuit 17. A transformer 66 serves to couple in an excitation signal 70. The excitation signal 70 comes partly 69 from a first electronics unit 67 and partly 71 from a second electronics unit 68. The electronics unit 67 includes, among other things, the demodulator 57, while the second electronics unit 68 includes the demodulator 56 according to Figure 13The electronics 67 and 68 thus form the evaluation unit 13. Furthermore, they include AC voltage sources for part 69 of the excitation signal 70 for the transmitter 10, which has the higher of the two operating frequencies, and for part 71 of the excitation signal 70, which has the lower of the two operating frequencies. Furthermore, it shows Fig. 14Two adjustment devices 72 and 73. The adjustment devices 72 and 73 are part of the receiving device 11 and serve to adjust the received signal at a signal output 74 to zero at both operating frequencies before the alternating field generated by the transmitting coil 7 is influenced by an object being examined for a metallic component. In the present embodiment, the adjustment device 72 for the higher of the two operating frequencies comprises at least one capacitor or an ohmic resistor connected in parallel with one of the two receiving coils 8 or 9. This is shown in Fig. 14A capacitor 76 is connected in parallel with the receiving coil 8, and an ohmic resistor 77 is connected in parallel with the receiving coil 9. The capacitor 76 and the ohmic resistor 77 need not both be present. The positions of the capacitor 76 and the ohmic resistor 77 can be reversed, and / or at least one of the receiving coils 8 and 9 can have both a capacitor and an ohmic resistor connected in parallel. The parallel-connected capacitor and / or ohmic resistor allows the tuning device 72 to compensate for differences between the two receiving coils 8 and 9 at the higher of the two operating frequencies directly within the resonant circuit 23 of the receiving device 11. The tuning device 73 for the lower of the two operating frequencies includes a transformer 80, to whose primary winding 81 the part 71 of the excitation signal 70 for exciting the transmitting device 10 at the lower operating frequency is applied.The secondary winding 82 of the transformer 80 has a center point 83 and two identical winding halves 84 and 85 on either side of the center point 83. The center point 83 is connected to the reference ground of the signal output 74 for the received signal. At least one capacitor or resistor is connected in parallel with one of the two winding halves 84 and 85 to the signal output 74. To illustrate all options, see below. Fig. 14It is shown that a resistor 86 and a capacitor 88 are connected in parallel to the signal output 74 of winding half 84, and a resistor 87 and a capacitor 89 are connected in parallel to the signal output 74 of winding half 85. The resistors 86 and / or 87 and / or the capacitors 88 and / or 89 are dimensioned such that a calibration signal is added to the signal output 74 by the calibration device 73. This calibration signal is directed opposite to the received signal at the lower of the two operating frequencies, so that it is adjusted towards zero as long as no object under test influences the alternating magnetic field of the transmitting coil 7. Both calibration devices 72 and 73 compensate for practical shortcomings of the measuring arrangement with the transmitting coil 7 and the receiving coils 8 and 9. In an ideal measuring arrangement in every respect, the calibration devices 72 and 73 would not be necessary; in every real measuring arrangement, they are advantageous. REFERENCE MARK LIST

[0056] 1 Device 2 Object 3 Item 4 Box 5 Product 6 Arrow 7 Transmitting coil 8 Receiving coil 9 Receiving coil 10 Transmitting device 11 Receiving device 12 Center tap 13 Evaluation device 14 Filter device 15 Resonant circuit 16 Capacitor 17 Series resonant circuit 18 Parallel resonant circuit 19 Coil 20 Capacitor 21 Connection 22 Connection 23 Resonant circuit 24 Capacitor 25 Parallel resonant circuit 26 Coil 27 Capacitor 28 Series resonant circuit 29 Connection 30 Connection 31 Partial capacitor 32 Partial capacitor 33 Coil 34 Switch 35 Switch 36 Capacitor 37 Capacitor 38 Transformer 39 Signal input 40 Input amplifier 41 Signal output 42 Signal output 43 Low-pass filter 44 Band-stop filter 45 Output amplifier 46 High-pass filter 47 Band-stop filter 48 Output amplifier 49 Signal output 50 Band-pass filter 51 Output amplifier 52 Band-stop filter 53 Notch filter 54 Notch filter 55 Notch filter 56 Demodulator 57 Demodulator 58 Evaluation unit 13 59 Switch 60 Coil 61 Average filter frequency 62 Stopband 63 Attenuation 64 Width 65 Coil66 Transformer 67 First electronics 68 Second electronics 69 Part of the excitation signal 70 Excitation signal 71 Part of the excitation signal 72 Adjustment device 73 Adjustment device 74 Signal output 76 Capacitor 77 Ohmic resistor 80 Transformer 81 Primary winding 82 Secondary winding 83 Center of the secondary winding 82 84 Half of the secondary winding 82 85 Half of the secondary winding 82 86 Ohmic resistor 87 Ohmic resistor 88 Capacitor 89 Capacitor

Claims

1. Apparatus (1) for detecting metal objects (2) in and on articles (3) moving relative to the apparatus (1), comprising - a transmitter device (10) including a transmitter coil (7) and configured for generating an alternating magnetic field at multiple operating frequencies using the transmitter coil, - a receiver device (11) including at least one receiver coil (8, 9) and configured for detecting the alternating magnetic field influenced by the moving articles (3) and for outputting a receiver signal as a function of the detected alternating magnetic field, - a filter device (14) configured for filtering partial signals which are each associated with one of the operating frequencies out of the receiver signal, and - an analysis device (3) configured for analyzing the partial signals, characterized in that the filter device (14) is configured for band-elimination filtering each of the partial signals which is associated with one of the operating frequencies with regard to all others of the operating frequencies, wherein the filter device (14) comprises a group of notch filters (53, 54, 55) for each of the partial signals and for each of the others of the operating frequencies to be band-elimination filtered, of which - a first notch filter (53) has a first central filter frequency, and - a second notch filter (54) has a second central filter frequency which is lower than the first central filter frequency, and / or a third notch filter (55) has a third central filter frequency which is higher than the first central filter frequency.

2. Apparatus (1) of claim 1, characterized in that, optionally, the second central filter frequency of the second notch filter (54) is by 1 % to 10% lower than the first central filter frequency of the first notch filter (53), and the third central filter frequency of the third notch filter (55) is by 1 % to 10 % higher than the first central filter frequency of the first notch filter (53).

3. Apparatus of claim 1 or 2, characterized in that the first central filter frequency of the first notch filter (53) is tuned to the respective operating frequency of the others of the operating frequencies to be band-elimination filtered.

4. Apparatus (1) of any of the preceding claims, characterized in that the filter device comprises a band pass filter (50) or a low pass filter (43) or a high pass filter (46) for each partial signal, wherein each edge of the respective filter keeps a distance of at least 10 %, preferably of at least 20 % and most preferably of at least 30 %, of the operating frequency to the operating frequency with which the respective partial signal is associated.

5. Apparatus (1) of any of the preceding claims, characterized in that - the transmitter coil (7) is part of a multi-resonant oscillator circuit (15) of the transmitter device (10) which has a resonance frequency at each of the multiple operating frequencies, and / or - the at least one receiver coil (8, 9) is part of a multi-resonant oscillator circuit (23) of the receiver device (11) which has a resonance frequency at each of the multiple operating frequencies.

6. Apparatus (1) of claim 5, characterized in that each multi-resonant oscillator circuit (15, 23) comprises interconnected partial oscillator circuits, wherein, optionally, the interconnected partial oscillator circuits include a series oscillator circuit (17, 28) and a parallel oscillator circuit (18, 25).

7. Apparatus (1) of any of the preceding claims, characterized in that the transmitter device (10) is switchable between a plurality of predetermined sets of operating frequencies.

8. Apparatus (1) of claim 7, so far as dependent on any of the claims 5 and 6, characterized in that each multi-resonant oscillator circuit (15, 23) is switchable to adapt its resonance frequencies to the respective set of operating frequencies in that the capacitances in all of the partial oscillator circuits interconnected in the respective multi-resonant oscillator circuit (15, 23) are switchable between predetermined values.

9. Apparatus (1) of any of the claims 5, 6 and 7, and 8 so far as dependent on any of the claims 5 and 6, characterized in that each multi-resonant oscillator (15, 23), at each of the multiple operating frequencies, has a quality factor of at least 10, wherein preferably the quality factor at the lowest of the multiple operating frequencies is at least 15 and the quality factor at the highest of the multiple operating frequencies is at least 30.

10. Apparatus (1) of any of the preceding claims, characterized in that the analysis device (13) is configured for demodulating each of the partial signals with respect to contained phase information using a reference signal of that one of the multiple operating frequencies which is associated with the respective partial signal.

11. Apparatus (1) of any of the preceding claims, characterized in that the receiver device (11) has two equal receiver coils (8, 9) which are symmetrically arranged in planes which are parallel to the transmitter coil (7) and which are connected in series in quadrupole configuration in the receiver device, wherein, optionally, the two equal receiver coils (8, 9) of the receiver device (11) have the same geometry as the transmitter coil (7).

12. Apparatus (1) of claim 11, characterized in that the two equal receiver coils (8, 9) of the receiver device (11) are connected in series in the oscillator circuit (23) of the receiver device (11).

13. Apparatus (1) of any of claims 11 and 12, characterized in that the receiver device (11), for each of the operating frequencies, comprises a separate balancer device (72, 73) which is configured for balancing the receiver signal at the respective operating frequency towards zero when the alternating magnetic field is not influenced by the moving articles (3).

14. Apparatus (1) of claim 13, characterized in - that one of the balancer devices (72) comprises at one least one capacitor (76) or ohmic resistor (77) which is connected in parallel to one of the two receiver coils (8, 9), and / or - that another one of the balancer devices (73) comprises a transformer (80), a part (71) of an excitation signal (70) for exciting the transmitter device (10) at the associated operating frequency being present at a primary winding (81) of the transformer (80), and a secondary winding (82) of the transformer (80) having two winding halves (84, 85) and a center point (83) between the two winding halves (84, 85), wherein the center point (83) is connected to a signal output (74) of the receiver device (11) for the receiver signal, and wherein at least one capacitor (88, 89) or ohmic resistor (86, 87) is connected in parallel to one of the winding halves (84, 85) of the secondary winding (82) towards the signal output (74).

15. Apparatus (1) of any of the preceding claims, characterized in that the analysis device (13) is configured for interrelating the information obtained at the multiple operating frequencies in order to detect the metal object (2) in the articles (3) moving with relative to the apparatus (1), wherein, optionally, the analysis device (13) is configured for interrelating the information obtained at the multiple operating frequencies depending on a position of the respective article (3) with relative to the apparatus (1) or depending on the point in time of the information.