Method for operating multi-frequency metal detector and multi-frequency metal detector

Abstract

To provide an improved method for operating a multi-frequency metal detector and an improved multi-frequency metal detector to be operated according to this method.SOLUTION: A metal detector (1) comprises: a drive coil (L61) for producing an electromagnetic field in a product; at least one detection coil (L62, L63) arranged to detect fluctuations in the magnetic field caused by metallic particles present in the product; and a multifrequency transmitter unit (10) comprising a converter (4) with a plurality of drive switches (S41, S42; S43, S44). The plurality of drive switches (S41, S42; S43, S44) is driven by a drive controller (2) according to operating instructions such that the drive switches (S41, S42; S43, S44) alternately conduct a drive current (iD) through the drive coil (L61) so that the generated electromagnetic field exhibits two or more different frequency components (fD1, fD2).SELECTED DRAWING: Figure 1a

Description

[Technical Field]

[0001] The present invention relates to a method for operating a multi-frequency metal detector, and to a multi-frequency metal detector operated in accordance with this method. [Background technology]

[0002]

[0002] As described in U.S. Patent Application Publication No. 20120206138(A1), metal detectors are used to detect and remove unwanted metal contamination. When properly installed and operated, metal detectors help reduce metal contamination and improve food safety. Modern metal detectors utilize a search head with a coil system having a drive coil that receives a drive signal and generates an electromagnetic field within the product, and at least one detection coil positioned to detect variations in the magnetic field caused by the presence of metal particles in the product, such as ferrous, non-ferrous, and stainless steel particles in a wide variety of products, including fresh and frozen foods.

[0003]

[0003] Metal detection systems that operate according to the "balanced coil" principle typically include three coils: a drive coil and two identical detector coils wound on a non-metallic frame, each precisely parallel to the other. The detector coil typically surrounds the drive coil, centered between them, and because the detector coils are identical, identical voltages are induced in each of them. To receive an output signal that is zero when the system is in a balanced state, a first receiver coil is connected in series with a second receiver coil having oppositely wound windings. Thus, the voltages induced in the receiver coils, which are of equal amplitude and opposite polarity, cancel each other out when the system is in a balanced state and no contamination is present in the observed product.

[0004]

[0004] However, as soon as a metal particle passes through the coil array, the electromagnetic field is disturbed first near one detector coil and then near the other. As the metal particle is transported through the detector coils, the voltage induced in each detector coil changes (by nanovolts). This change in equilibrium results in a signal at the detector coil output, which can be processed and amplified in a receiving device and then used to detect the presence of metal contamination in the product being observed.

[0005] In a typical metal detector, a signal processing channel provided in the receiver splits the received signal into two separate components 90° apart. The resultant vector has a magnitude and a phase angle that is typical of the product and the contaminants transmitted through the coil system. To identify metallic contaminants, the "product effect" must be removed or reduced. If the phase of the product is known, the corresponding signal vector can be reduced. Removing unwanted signals from the signal spectrum in this way results in higher sensitivity to signals arising from metallic contaminants.

[0006]

[0006] Therefore, methods applied to eliminate unwanted signals from the signal spectrum make use of the fact that metallic contaminants, products, and other disturbances have different effects on the magnetic field, and therefore the resulting signals have different phases. When passing through a coil system, the signal caused by a metal or product can be split into two components, specifically a resistive component and a reactive component, according to the conductivity and permeability of the measured object. The signal caused by ferrite is primarily reactive, while the signal from stainless steel is primarily resistive. Products that are conductive typically give rise to signals with a strong resistive component. By using a phase detector to distinguish the phases of signal components of different origin, it is possible to obtain information about products and contaminants. The signal components or phase and amplitude vary with the frequency of the applied drive signal, which is selected so that the signal components of the metallic contaminants are out of phase with the signal components of the observed product signal.

[0007]

[0007] U.S. Patent No. 8,473,235 discloses a metal detector having a drive circuit including a plurality of switches connected to a drive coil and driven by a drive controller, which alternately connects the drive coil across a potential difference to drive the drive coil at a predetermined operating frequency. The drive controller can be programmed to operate the plurality of switches to obtain any single operating frequency, which may be selectable within a range of 40 to 900 kHz in 1 Hz increments. However, driving the coil system with a square wave (or a trapezoidal wave due to the coil inductance) generates a large amount of relatively high-energy harmonics compared to conventional sinusoidal signals generated by tuned circuits. To avoid the adverse effects of these harmonics, the device includes a detection circuit for obtaining a signal from the coil system, which includes a phase-sensitive detector coupled to a low-pass filter that removes the above-mentioned interfering harmonics, which are generally considered the most undesirable.

[0008]

[0008] One operating frequency may be suitable for one particular metal contaminant, but the same frequency may not produce the desired results for other metal contaminants. By simultaneously using two or more transmission frequencies selected according to the product and potential contaminants, it is possible to obtain more accurate information about two or more different metal contaminants without switching operating frequencies.

[0009]

[0009] U.S. Patent No. 8,159,225 discloses a multi-frequency metal detector having a multi-frequency transmitter and a method for generating a multi-frequency drive signal by generating at least two square wave signals, each having a different fundamental frequency, and mixing selected square wave signals to create a switching signal containing different frequency components with relatively strong magnitudes at frequencies corresponding to the convolution of the fundamental frequencies of the two selected square wave signals. Again, in addition to the desired frequency components, there are other frequency components that need to be removed or suppressed.

[0010]

[0010] U.S. Patent No. 8,159,225 further discloses that a digital drive switching signal is applied to a full-bridge switching power stage connected to a drive coil of a metal detector. The full-bridge switching power stage consists of two half-bridges, each with two half-bridge switches, one half-bridge driven by a digital switching signal and the other half-bridge driven by an inverted digital switching signal. The current in the half-bridge switches of the full-bridge switching power stage corresponds to the current flowing in the drive coil. Therefore, with high currents in the drive coil, the half-bridge switches are required to be able to deliver this high coil current. Therefore, the multi-frequency transmitter must be dimensioned and constructed accordingly, which involves significant cost. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] U.S. Patent Application Publication No. 20120206138(A1) [Patent Document 2] U.S. Patent No. 8,473,235 [Patent Document 3] U.S. Patent No. 8,159,225 [Non-patent literature]

[0012] [Non-Patent Document 1] Sarbari Das and Manish Bharat, Implementation of IGBT series resonant inverters using pulse-density modulation, International Journal of Industrial Electronics and Electrical Engineering, Vol. 3, No. 2, February 2015 [Non-patent document 2] MJGrimble, MAJohnson, Jian Sun, Advances in industrial Control, Springer-Verlag London Limited2012, Chapter 2 Summary of the Invention [Problem to be solved by the invention]

[0013]

[0011] The present invention is therefore based on the object of providing an improved method for operating a multi-frequency metal detector, and an improved multi-frequency metal detector operating in accordance with this method. [Means for solving the problem]

[0014] The method and multi-frequency metal detector of the present invention advantageously make it possible to create a multi-frequency drive signal having at least two different frequency components, and filter circuits are avoided or at least reduced.

[0015] Furthermore, the frequencies of the multi-frequency drive signal are preferably selectable as needed so that the metal detector can be flexibly used with a wide range of products and potential contaminants.

[0016]

[0014] The metal detector shall be equipped with an improved multi-frequency transmitter which has improved efficiency, is structurally simple, can be constructed at reduced cost, and allows for maximizing the coil current delivered to the drive coil.

[0017]

[0015] The method and metal detector of the present invention make it possible to produce a high drive current in the drive coil while keeping the current in the transmitter device relatively low, so that the transmitter circuitry can be sized accordingly, for example with reduced power capability and cost.

[0018]

[0016] In a first broad aspect of the present invention, there is provided a method for operating a metal detector comprising a drive coil for generating an electromagnetic field within a product, at least one detection coil arranged to detect variations in the magnetic field caused by metal particles present in the product, and a multi-frequency transmitter arrangement comprising a transducer 4 having a plurality of drive switches driven by a drive control device according to operating instructions such that the drive switches alternately conduct drive current to the drive coils so that the generated electromagnetic field exhibits two or more different frequency components.

[0019] An electromagnetic field having two or more distinct frequency components is - determining waveforms of the drive current for at least two different frequency components; - determining at least one pulse sequence signal (hereinafter referred to as PXM signal) corresponding to the defined waveforms of the drive current; - selecting at least one determined PXM signal, determined online or stored in a memory module, according to the provided operating instructions; - generating and applying the determined PXM signal to control a drive switch of a transmitter device, thereby generating a drive current having a prescribed waveform.

[0020]

[0018] At least one PXM signal, preferably a pulse width modulated or pulse density modulated signal, can be determined and then stored in memory for later use. However, preferably, the PXM signal is determined and generated online so that the metal detector can be tuned online to the product and contaminants or adapted online to changes in the product and contaminants. Thus, the user can adjust the metal detector according to their own personal requirements.

[0021]

[0019] Sarbari Das and Manish Bharat, Implementation of IGBT series resonant inverters "Using pulse-density modulation," International Journal of Industrial Electronics and Electrical Engineering, Volume 3, Issue 2, February 2015, explains that pulse density modulation is a form of modulation used to represent analog signals as digital data. In PDM, instead of a specific amplitude value, the relative density of pulses corresponds to the amplitude of the analog signal.

[0022]

[0020] Pulse width modulation techniques are described in Chapter 2 of MJ Grimble, MA Johnson, and Jian Sun, Advances in Industrial Control, Springer-Verlag London Limited 2012. In principle, a PWM signal can be created by comparing a reference signal with a carrier signal, such as a sawtooth carrier, an inverted sawtooth carrier, or a triangle carrier. Therefore, a preferably mathematically determined waveform of the drive current can be used as a reference signal to be compared with a carrier signal, such as a sawtooth or triangle carrier.

[0023]

[0021] The drive switches of the converter, preferably power FET devices, can be arranged, for example, as a bridge circuit or a half-bridge circuit. In a first embodiment, the drive switches are arranged as a full-wave bridge or H-bridge with first and second branches connected on one side to a first voltage potential, e.g., a drive voltage, and on the other side to a second voltage potential, e.g., ground, each having a first or second center tap connected to a first or second end of a drive coil, respectively, which serves as a load. A first pair of drive switches is arranged in the first branch and connected to each other at the first center tap, and a second pair of drive switches is arranged in the second branch and connected to each other at the second center tap. The drive switches are then controlled so that the first and second ends of the drive coil are alternately connected to the first and second voltage potentials. That is, the upper portion of the first branch and the lower portion of the second branch on one side, and the lower portion of the first branch and the upper portion of the second branch on the other side, are alternately activated, thus conducting an alternating current through the drive coil in a predetermined shape or waveform.

[0024]

[0022] In a second embodiment, the first and second drive switches form a half-bridge circuit connected to a first voltage potential, e.g., a first drive voltage, on one side and a second voltage potential, e.g., a second drive voltage, on the other side, and the first and second drive switches are connected at a center tap of the half-bridge circuit and are controllable so that a first end of a drive coil connected to the center tap is alternately connectable to the first voltage potential and the second voltage potential.

[0025] The drive switches are operated using a predetermined PXM signal or by two or more PXM signals. If only one PXM signal is provided, this PXM signal is applied to the drive switches directly or via an inverter so that each drive switch is accurately operated. If two PXM signals are provided, the drive circuit, which preferably includes an amplifier, can be simplified.

[0026] In a preferred embodiment, the step of determining the shape or waveform of the drive current for two or more different frequency components includes superposing current components individually for one of at least two different frequency components. Such frequency components may be sinusoidal and / or odd and / or even harmonics. Thus, the function of the drive current in the time domain can be mathematically determined, for example, by a corresponding program module. The PXM signal can be determined in advance or whenever a user selects a new operating frequency as a frequency component.

[0027]

[0025] Thus, the user of the metal detector can determine which frequencies or frequency components are suitable for detecting potential contaminants or metal particles. The signal function or drive current for each frequency component is then determined by superposition of each frequency component in the time domain. The fundamental angular frequency, i.e., the first frequency component, is ω. In the following equation, the remaining angular frequencies are selected as the 3rd, 7th, and 17th harmonics. The weights of each of the four frequency components are inversely proportional to the frequency. i(ωt)=Isin(ωt)+I / 3sin(3ωt)+I / 7sin(7ωt)+I / 17sin(17ωt)

[0026] A user of the metal detector can preferably determine any number of drive current waveforms for different sets of two or more different frequency components. Thus, for different products and / or different potential contaminants, the user can select a drive current waveform with a suitable set of frequency components. That is, the user can continuously optimize the detection process.

[0028] The PXM signal, which corresponds to the preferably mathematically determined waveform of the drive current, can be determined in various ways with the desired accuracy so that only the desired frequency components appear in the frequency domain. If any interfering frequency components remain, they can be suppressed by a filter located before or after the phase detector that receives the signal from the sensing coil and the reference signal from the transmitter unit.

[0029] In a preferred embodiment, the pulse sequence or PXM signal is obtained by approximating a triangular or trapezoidal signal, preferably mathematically determined, to the waveform of the drive current so that the maximum and minimum values of the determined waveform of the drive current and the maximum and minimum values of the triangular or trapezoidal signal correspond and / or coincide with each other. The switching angles of the falling and rising edges of the PXM signal are then sequentially defined at the maximum and minimum values of the triangular or trapezoidal signal. The approximation of the triangular or trapezoidal signal is preferably done in such a way that the minimum or maximum values of the determined waveform of the drive current and the maximum or minimum values of the triangular or trapezoidal signal overlap at the same positions. The basic concept of this morphological approximation is that if two signals have similar time-domain waveforms, they will share similar amplitude spectra. Alternatively, one of the PWM or PDM methods can be used to obtain the pulse sequence signal.

[0030]

[0029] In a further preferred embodiment, the described process for preferably mathematically determining the waveform of the drive current for selected frequency components and for determining the corresponding PXM signal is automatically performed by a processor and corresponding program provided within the metal detector. Thus, a user of the metal detector can freely select two or more preferred frequency components, and the processor and program then determine the PXM signal or associated switching angles of the PXM signal. Alternatively, switching angles for a set of standard operating frequencies can be securely programmed and stored within the drive control unit and selected according to the industrial or non-industrial process being performed.

[0031]

[0030] The waveform of the drive current of the embossing frequency component is preferably determined over the duration of one cycle of the frequency component having the lowest frequency present in the waveform of the drive current. This time segment of the drive current contains all the information of the continuous drive current. Therefore, the actual drive current can be generated by sequentially repeating the preferably mathematically determined waveform of the drive current over the period of the lowest frequency contained in the drive current by repeated application of a corresponding sequence of PXM signals.

[0032]

[0031] The drive current can be supplied directly to the drive coil so that the drive current provided by the transducer and the coil current delivered to the drive coil are identical. However, in a preferred embodiment, the drive current is supplied to the drive coil through an admittance device, which, together with the drive coil, forms a resonant circuit active within two or more different frequency components of the drive current. By tuning the resonant circuit of the admittance device and drive coil to the frequency components of the drive current, or vice versa, the current appearing in the drive coil becomes significantly larger than the drive current flowing through the drive switch or power FET. A multi-frequency transmitter device with a power FET can be sized for a current smaller than the actual current required in the drive coil. Furthermore, when the output impedance of the transducer is high by appropriately tuning the resonant circuit, the level of frequency components desired for detecting contaminants in a product is increased, while other undesirable frequency components are not increased or even reduced, thus improving the signal-to-noise ratio of the resulting signal and the sensitivity of the metal detector.

[0033] In a preferred embodiment, the admittance device comprises at least a first branch having a first capacitor and a first inductor that form a first resonant circuit together with the drive coil, and a second branch having a second capacitor and a second inductor that form a second resonant circuit together with the drive coil. Preferably, the branches and / or individual capacitors and inductors can be connected individually or in groups to the drive coil to establish individual resonant circuits corresponding to frequency components embossed in the drive current.

[0034]

[0033] Most preferably, one of the stored PXM signals having a particular set of frequency components and the corresponding resonant circuit in the admittance device are jointly selectable by a user of the metal detector. In this manner, the metal detector can be instantly optimized for any combination of product and contamination.

[0035] The implementations of the method and device for creating a multi-frequency drive current on one side and using an admittance device on the other side are particularly advantageous when implemented in combination, but also offer significant improvements when implemented independently of each other. That is, the admittance devices of the present invention, which together with the drive coil form two or more individually tuned resonant circuits, can also be advantageously applied in metal detectors in which the multi-frequency drive current is obtained according to another method.

[0036]

[0035] Detailed aspects and examples of the present invention are described below with reference to the drawings. [Brief explanation of the drawings]

[0037] [Figure 1a] 1 shows a metal detector 1 of the present invention comprising a transducer 4 having four drive switches S41, S42, S43, S44 for a bridge and controlled using a selectable pulse width or pulse density modulated signal sPXM, hereinafter referred to as the PXM signal, provided by a drive control device 2, and providing a drive current iD to a drive coil L61 via an admittance device 5. [Figure 1b] 1a shows the metal detector 1 with a converter 4 having two drive switches S41, S42 forming a half bridge and controlled using a selectable PXM signal sPXM provided by a drive control device 2 and providing a drive current iD to a drive coil L61 via an admittance device 5. [Figure 2] 1a shows the metal detector 1 of FIG. 1a equipped with a drive controller 2 that provides a first PXM signal sPXM1 used to control drive switches S41, S42 and a corresponding second PXM signal sPXM2 used to control drive switches S43, S44. [Figure 3] 1a shows a metal detector 1 equipped with a drive control device 2 that enables selection of one of a plurality of stored PXM signals sPXM, each having a particular set of frequency components, and having an admittance device 5 that enables selective connection of at least one of a plurality of branches, each comprising at least one capacitor C51, C52, C5n and at least one inductor L51, L52, L5n, to a drive coil L61 to create a resonant circuit tuned to the frequency components of the selected PXM signal sPXM. [Figure 4] A diagram showing the mathematically determined waveform of the drive current iD or i(ωt) including four frequency components ω, 3ω, 7ω, and 17ω and the associated PXM signal sPXM determined by approximation of the triangular signal iDA to the determined waveform of the drive current i(ωt). [Figure 5] 1a, 1b, 2 and 3 show the coil current iL61 in the drive coil L61, the currents iL51, iL52 in the branches of the admittance device 5, and the drive current iD delivered by the drive switches S41, S42, S43, S44 in the transducer device 4 for the complete frequency spectrum of the metal detector, and show that when the frequency components fD1, fD2 of the drive current iD are set to the resonant frequencies fRES1, fRES2 of the admittance circuit 5, the drive current iD is significantly lower than the coil current iL61. DETAILED DESCRIPTION OF THE INVENTION

[0038]

[0036] Figure 1a shows a first embodiment of the inventive metal detector 1, comprising a transmitter unit 10, a receiver unit 11, a drive coil L61 connected to the output of the transmitter unit 10, and a balanced coil system 6 having two detector coils L62 and L63, one connected to ground potential and the other connected to the input stage of the receiver unit 11. In the input stage 7, the input signal is typically amplified and filtered before being forwarded to a phase detector 8. The phase detector 8 makes it possible to distinguish the phases of signal components of different origins and obtain information about the observed product and, if present, contaminants. A typical phase detector, such as a frequency mixer or analog multiplier circuit, generates two independent voltage signals representing the in-phase and quadrature components provided by the input stage 7, as well as a reference signal fm provided by the transmitter unit 10. The output signal of the phase detector 8 is further processed in a control unit 9, preferably equipped with a signal processor, input / output devices, a keyboard, and a display. Using the control unit 9, a user can control the operation of the metal detector 1. In particular, the user can select the operating conditions of the metal detector, specifically the applied drive current and operating frequency as described below. The receiver device 11 may further include features commonly known from conventional metal detectors.

[0039] The transmitter device 10 is a multi-frequency transmitter designed to provide a drive signal having multiple frequencies, e.g., 2 to 8 frequencies, providing good sensitivity to a wide range of products and pollutants. The transmitter device 10 includes a drive control device 2, a drive device 3, a converter 4, and preferably a drive current i provided by the converter 4. D to the drive coil L61.

[0040] In this embodiment of the invention, the drive controller 2 receives data relating to the states of the drive switches S41, S42, S43, S44 for all clock cycles over a long period of time at the lowest operating frequency, e.g., pulse width modulated signals or PXM signals s PXM Nosui The memory device 23 has one memory module 231 in which the timing angles α1, α2, ... are stored at associated addresses. PXM The determination of is discussed below with respect to Figure 4. As outlined above, any pulse sequence PXM signal s that corresponds to a drive current when applied to drive switches S41, S42, S43, S44 PXM can be used. Preferably, a pulse width modulated or pulse density modulated signal or sequence is applied. Therefore, instead of using the acronyms PWM for pulse width modulated signals and PDM for pulse density modulated signals, the acronym PXM is used, which stands for a modulated pulse sequence corresponding to the drive current.

[0041] After the system is reset by the reset signal rs issued by the controller 9, the memory module 231 is reset by the PXM signal s PXM The data are sequentially read out from the memory module 231 and sequentially addressed by the address counter 22 using the address signal ad to be applied to the driving switches S41, S42, S43, S44 via the driver 3. PXMis routed to the input of drive switch S41 via drive elements 31 and 311, to the input of drive switch S42 via drive elements 32 and 321, to the input of drive switch S44 via drive elements 31' and 312, and to the input of drive switch S43 via drive elements 32' and 322. Drive elements 32 and 32' are inverters that ensure that drive switches S42 and S43 are always open when drive switches S41 and S44 are closed, and that drive switches S42 and S43 are always closed when drive switches S41 and S44 are open. In this manner, AC current flows through drive coil L61 while avoiding short circuits. For ease of illustration, elements 31, 31' and 32, 32' are duplicated. However, the output of element 31 may be connected to the inputs of elements 311 and 312, and the output of element 32 may be connected to the inputs of elements 321 and 322 without the need for elements 31' and 32'.

[0042] To obtain phase-synchronous operation of the metal detector, a clock device 21 is provided, which delivers a reference signal fm to an address counter 22, a memory device 23 and the phase detector 8.

[0043]

[0041] PXM signal PXM Since the data is preferably stored for only one period of the lowest operating frequency, the data is repeatedly read out from the memory module 231. Therefore, the drive current i shown in FIG. D The segments are produced sequentially and repeatedly until the user terminates the operation or changes the setting. Thus, the address counter 22 counts from the lowest address number to the highest address number and restarts at the lowest address number.

[0044]

[0042] The driving switches S41, S42, S43, and S44 are connected to a driving voltage V DThe PXM signal s is arranged in a full-wave bridge circuit or H-bridge with a first branch and a second branch, the opposite ends of which are connected to ground. The first branch has a first center tap connected to a first end of the drive coil L61. The second branch has a second center tap connected to a second end of the drive coil L61. A first pair of drive switches S41, S42 are arranged in the first branch of the bridge and connected to each other at the first center tap. A second pair of drive switches S43, S44 are arranged in the second branch and connected to each other at the second center tap. As explained above, the PXM signal s PXM to the drive switches S41, S42; S43, S44, the first and second ends of the drive coil L61 are supplied with a drive voltage V D and ground potential, respectively.

[0045] The converter 4 converts, for example, the PXM signal s PXM The formula used to determine: i(ωt)=Isin(ωt)+I / 3sin(3ωt)+I / 7sin(7ωt)+I / 17sin(17ωt) According to the PXM signal PXM is the drive current i containing the desired frequency components, preferably the lowest frequency harmonics. D Convert to.

[0046] As will be explained below with reference to Figure 4, the PXM signal s PXM is preferably converted into a driving current i after conversion in the converter 4, for example, having four frequency components of this formula or their approximate values. D The desired harmonic is broadened to increase sensitivity. Furthermore, a relatively small drive current i D At high coil current i L61 These objectives are to generate a drive current i D is achieved by inducing a current through the admittance device 5 into the drive coil L61.

[0047] In the embodiment shown, the admittance device 5 comprises several branches, each comprising a capacitor C51, C52, C5n and an inductor L51, L52, L5n. The number of branches n is determined by the drive current i D Each of the branches C51, L51; C52, L52; C5n, L5n, together with the drive coil L61, supplies a drive current i D The coil current i in the driving coil L61 at resonance forms a resonant circuit tuned to the corresponding frequency components ω, 3ω, 7ω, and 17ω. L61 is the drive current i D Therefore, on the one hand, the drive current i flowing through the drive switches S41, S42; S43, S44 is significantly larger than D can be reduced while the high coil current i L61 is achieved. The converter 4 can therefore be dimensioned for lower currents and can be constructed at reduced cost.

[0048] The metal detector of FIG. 1a receives the PXM signal s stored in the drive control unit 2. PXM The admittance device 5 having branches C51, L51; C52, L52; C5n, L5n is fixed to resonate with the drive coil L61 at this set of frequency components ω, 3ω, 7ω, 17ω.

[0049] FIG. 1b shows a half-bridge and a selectable PXM signal s such as a pulse width or pulse density modulated signal provided by the drive controller 2. PXM 1a in an embodiment with a transducer 4 having two drive switches S41, S42 controlled using a drive current i D to the drive coil L61 via the admittance device 5. The drive switches S41, S42 have a first voltage potential V D , e.g., to a first drive voltage and, on the opposite side, to a second voltage potential V S, for example, a second drive voltage. The drive switches S41 and S42 are connected at a center tap of the half-bridge circuit, and a first end of the drive coil L61 connected to the center tap is connected to a first voltage potential V D and a second voltage potential V S are controlled so that they are alternately connected to

[0050] Furthermore, as explained above, in the preferred embodiment, the PXM signal s PXM can be generated online and transferred to the converter 4. In Fig. 1b, a selector switch S2 is provided, which is controlled by the controller 9 using a control signal ctrl. The selector switch S2 is connected to the PXM signal s provided by the memory module 23. PXM-STORED or the PXM signal s provided online by a processor unit 25, e.g. a digital signal processor DSP, controlled by the control unit 9 using a control signal ctrl. PXM-ONLINE The processor unit 25 may be configured to receive a program that can generate suitable pulse width modulated and / or pulse density modulated signals. The processor unit 25, which is preferably integrated into the control unit 9 along with other circuits, may also be configured to receive a signal for later use. A PXM signal may be generated that is stored in memory device 23 .

[0051] In all the embodiments discussed, the PXM signal s PXM can be selected from the memory device 23 and / or from the processor device 25 in the presence of any configuration of drive switches S41, ..., S44 and with any configuration of admittance devices 5, if present. Thus, the features of the individual embodiments can be freely combined. In particular, the processor device 25 can select the PXM signal s having any set of operating frequencies. PXMAt the same time, the admittance device 5 can be automatically tuned to the same set of operating frequencies. The processor device 25 can replace the memory device 23 in all disclosed circuits or can be used to generate the PXM signal s PXM can be used as an alternative source of

[0052] FIG. 2 shows the first PXM signal s used to control the drive switches S41, S42. PXM1 and a corresponding second PXM signal s used to control the drive switches S43, S44. PXM2 1a shows a metal detector 1 equipped with a drive and control device 2 providing a first PXM signal s PXM1 is stored in memory module 23A and the second PXM signal s PXM2 is preferably stored in memory module 23B at the corresponding address. Thus, address counter 22 receives the first PXM signal s PXM1 and the second PXM signal s PXM2 Both memory modules 23A and 23B can be addressed synchronously to simultaneously read PXM signals s. PXM1 and s PXM2 By having two PXM signals, it is possible to store the voltage V when both PXM signals are at ground potential or when both are at a drive voltage V D This allows the converter 4 to have a 0 volt differential at its output when the voltage is at a potential of 0. This allows for the generation of trapezoidal waves and better current control.

[0053] FIG. 3 shows a number of stored PXM signals s, each with a particular set of frequency components. PXM and a drive control device 2 that allows the driver to select one of the selected PXM signals s PXM1a shows the metal detector 1 having an admittance device 5 that allows selective connection of at least one of a plurality of branches, each preferably comprising at least one capacitor C51, C52, C5n and at least one inductor L51, L52, L5n, to the drive coil L61 to create a resonant circuit tuned to the frequency component of each PXM signal s PXM are stored individually in the corresponding memory modules 231, 232, 23n. The branches of the admittance device 5 can be individually activated using switches S51, S52, S5n, which are operated using the selector 50.

[0054] A particular PXM signal s having a desired set of operating frequencies ω1, ω2, ω3, ω4 PXM and to select the corresponding resonant circuits or branches C51, L51; C52, L52; C5n, L5n in the admittance device 5, the control device 9 provides a frequency selection signal sf, for example to the address counter 22, optionally to the memory device 23, and to the selector 50. The address counter 22 then addresses the selected memory module 231, 232, or 23n, and the selector 50, the corresponding switches S51, S52, S5n.

[0055]

[0052] Thus, the metal detector 1 of Figure 3 can be selectively tuned to any set of frequencies selected for a particular product and potential contaminant. The resonant circuit can be tuned by adding capacitors and inductors, for example, with switches, such as electronic switches. The values of these items can also be changed electronically.

[0056] FIG. 4 shows a block diagram of a signal containing four frequency components ω, 3ω, 7ω, and 17ω, but without interference. , which shows the mathematically determined waveform of the drive current i(ωt). i(ωt)=Isin(ωt)+I / 3sin(3ωt)+I / 7sin(7ωt)+I / 17sin(17ωt) Further shown is the mathematically determined waveform of the triangular signal i(ωt) DA The associated PXM signal s determined by approximation of PXM The driving current i D and the coil current i L61 1 and 3, and three potential slopes, including zero, in FIG. 2, the mathematically determined waveform of the drive current i(ωt) is resembled or approximated using triangular or trapezoidal segments. The mathematically determined waveform of the drive current i(ωt) is shown by the dashed line. The approximated triangular signal i DA The waveform of the triangular signal i closely follows the waveform of the mathematically determined drive current i(ωt). In the first half of the period or positive half-wave, DA The maximum value of is set to the mathematically determined maximum value of the drive current i(ωt). In the second half of the period or negative half-wave, the triangular signal i DA The minimum value of is set to the mathematically determined minimum value of the drive current i(ωt). The approximated triangular signal i DA is the actual drive current i D The approximated triangular signal i DA is the PXM signal s PXM which is then converted into the actual drive current i D , the approximated triangular signal i DA In Figure 4, the brackets indicate that the actual drive current i D Also, at least approximately, the approximated triangular signal i DA However, if the higher frequencies are suppressed, the virtual driving current i D will more closely resemble the mathematically determined drive current i(ωt).

[0057]

[0054] Approximation by triangular or trapezoidal segments has the advantage that undesired signals occur far away from the selected frequency components ω1, ω2, ω3, ω4 and therefore do not significantly affect the measurement. Furthermore, the typical location of such interfering signals in the Fourier spectrum is known, and they can be easily suppressed at the input stage 7 of the receiver device 11 by appropriately selected filters. The above-mentioned U.S. Pat. No. 8,473,235 discloses a circuit in which a filter stage follows the phase detector. In the present invention, the filtering effort is smaller. However, any known filtering technique can also be applied to the signals delivered by the sensing coils L62, L63 before or after demodulation, i.e., before and / or after the phase detector 8.

[0058]

[0055] The determined triangular signal i DA Using the PXM signal s PXM can be determined, which are required to control the drive switches S41, S42, S43, S44 in the converter 4. These switching angles α1, α2, . . . are determined by the determined triangular signal i DA The relative maximum and minimum values of the PXM signal s PXM The falling edge of the determined triangular signal i DA is set to occur at the maximum value of PXM signal s PXM The rising edge of the determined triangular signal i DA The resulting PXM signal s is set to occur at the minimum value of PXM or PXM signals PXM1 , s PXM2 , are then stored in the memory device 23, ie in one of the memory modules 231, 232, 23n; 23A, 23B.

[0059] FIG. 4 shows the mathematically determined waveform of the drive current i(ωt), the approximated triangular signal i, over the duration of one period of the lowest frequency ω. DA , and the determined PXM signal sPXM Therefore, the PXM signal s PXM By repeatedly reading the data from the associated memory modules 231, 232, 23n; 23A, 23B, the PXM signal s PXM This allows for the establishment of a continuous stream of

[0060] FIG. 5 shows the relationship between the coil current i in the drive coil L61 for the complete frequency spectrum of the metal detector of FIGS. 1a, 1b, 2, and 3. L61 , the brand of the admittance device 5 Current in the switch i L51 , i L52 (see FIG. 3), and the drive current i delivered by the drive switches S41, S42, S43, S44 in the converter device 4. D The coil current i L61 The gradient of the driving current i D The curves of each resonant frequency f RES1 , f RES2 As a result, the driving current i D At these spectral positions, the coil current i L61 Therefore, the resonant frequency f of the admittance device 5 and the driving coil L61 is RES1 , f RES2 The driving frequency f is set to D1 , f D2 or vice versa, with a relatively small drive current i D This results in a large coil current i L61 can be achieved. The explanation for this advantageous approach is that the current i appearing in the branch of the admittance device 5 L51 , i L52 and the resonant frequency f of the admittance device 5 RES1 , f RES2 The current i L51 , i L52 is the coil current i in the driving coil L61 L61The admittance device 5 includes in its branches passive elements, such as inductors L51; L52, L5n and capacitors C51, C52, C5n, which, when in resonance with the drive coil L61, generate a current circulating between the drive coil L61 and the branches of the admittance device 5. Advantageously, the resonant frequency f RES1 , f RES2 The default drive frequency f is set to D1 , f D2 The power circulation at drive frequency f is restricted within the loop formed by admittance device 5 and drive coil L61, forcing the drive point admittance to zero on an ideal lossless system. As a result, D1 , f D2 The drive current i flows through the drive switches S41, S42, S43, and S44, typically MOSFETs. D is the coil current i flowing through the driving coil L61 and the branch of the admittance device L61 Among other advantages, this allows for extending the spectrum towards lower frequencies and for driving a low impedance drive coil L61.

[0061] In the drawings, a preferred embodiment of the admittance device 5 is shown. However, at a defined frequency f RES1 , f RES2 Any other circuit that preferably makes it possible to selectively reach a resonant circuit operating at .gtoreq. ... <Additional Notes> [Form 1] A method for operating a metal detector (1), the metal detector (1) comprising a drive coil (L61) for generating an electromagnetic field within a product, at least one detection coil (L62, L63) arranged to detect variations in the magnetic field caused by metal particles present in the product, and a multi-frequency transmitter device (10) comprising a transducer (4) having a plurality of drive switches (S41, S42; S43, S44), the plurality of drive switches (S41, S42; S43, S44) configured to drive a drive current (i D ) to the driving coil (L61) so that the generated electromagnetic field has two or more different frequency components (f D1 , f D2 ) is driven by a drive control device (2) according to an operation command to exhibit, and the method includes: - At least two different frequency components (f D1 , f D2 ) for the driving current (i D ) determining the waveform of - a modulated pulse sequence, such as a pulse width modulated or pulse density modulated signal, and said driving current (i D ) corresponding to the determined waveform of at least one PXM signal (s PXM ) and - determining the at least one determined PXM signal (s) online or stored in a memory module (231, 232) according to the provided operating instructions; PXM ), - said at least one determined PXM signal (s PXM ) to control the driving switches (S41, S42; S43, S44); A method for operating a metal detector (1), comprising: [Form 2] In the method for operating a metal detector (1) according to aspect 1, the driving current (i D) is a sinusoidal frequency component (f D1 , f D2 At least two different frequency components (f D1 , f D2 2. A method for operating a metal detector (1), comprising superimposing current components related to: [Form 3] In the method for operating a metal detector (1) according to aspect 1 or 2, the driving current (i D The frequency component (f D1 ) for at least the cycle duration of the driving current (i D ) and determining the waveform of the driving current (i D ) is determined by repeating the waveform of the driving current (i D 2. A method for operating a metal detector (1), comprising the step of generating a signal. [Form 4] In the method for operating a metal detector (1) according to the first, second or third aspect, a triangular or trapezoidal signal is applied to the driving current (i D and the drive current (i) is adjusted so that the determined maximum and minimum values of the waveform of the triangular signal correspond to and / or coincide with each other. D ) is determined by approximating the waveform of the PXM signal (s PXM ) of the PXM signal (s) at the maximum and minimum values of the triangular or trapezoidal signal. PXM and sequentially defining switching angles (α1, α2, ...) for the falling and rising edges of the signal (α1, α2, ...). [Form 5] In the method for operating the metal detector (1) according to any one of the first to fourth aspects, D1 , f D2 Two or more PXM signals (s) with different sets of PXM ) and determining the two or more PXM signals (s PXM) to the stored PXM signal (s PXM ) in a memory module (231, 232) that is selectable for generating and applying. [Form 6] In the method for operating the metal detector (1) according to any one of the first to fifth aspects, the PXM signal (s PXM )of, - One side is connected to a first voltage potential (V D ) and on the other side a second voltage potential (V S ), and a center tap of the half-bridge circuit is connected to the first voltage potential (V D ) and the second voltage potential (V S ), or a step of applying a voltage to first and second drive switches (S41, S42) of the drive switches (S41, S42; S43, S44) that are controlled to be alternately connected to the first and second drive switches (S41, S42); - One side is connected to a first voltage potential (V D ) at one end and a second voltage potential such as ground potential at the other end, and each having a first or second center tap, the first or second center tap being connected to a first and second end of the drive coil (L61) by means of a first pair of the drive switches (S41, S42) disposed in the first branch and connected to each other at the first center tap, and by means of a second pair of the drive switches (S43, S44) disposed in the second branch and connected to each other at the second center tap, respectively, and the first and second ends of the drive coil (L61) are connected to the first voltage potential (V D ) and the second voltage potential to the drive switches (S41, S42; S43, S44) controlled to be alternately connected to the second voltage potential. A method for operating a metal detector (1), comprising: [Form 7] In the method for operating the metal detector (1) according to any one of the first to fifth aspects, a signal drive is provided to control the input of the drive switches (S41, S42; S43, S44). The PXM signal (s) is transmitted via a driver (3) having individual drive elements and / or inverters (31, 31', 32, 32', 311, 312, 321, 322). PXM 2. A method for operating a metal detector (1), comprising the step of applying a voltage to the metal detector (1). [Form 8] In the method for operating the metal detector (1) according to any one of the first to seventh aspects, D ) to the driving coil (L61) directly or via an admittance device (5), wherein the admittance device (5) together with the driving coil (L61) induces the two or more different frequency components (f D1 , f D2 ) forms an active resonant circuit in which the coil current (i L61 ) is the driving current (i D ) is greater than 1. A method for operating a metal detector (1), comprising the step of: [Form 9] A method for operating a metal detector (1) according to claim 8, comprising using at least a first branch having a first capacitor (C51) and a first inductor (L51) in the admittance device (5) to form a first resonant circuit together with the drive coil (L61), and a second branch having a second capacitor (C52) and a second inductor (L52) in the admittance device (5) to form a second resonant circuit together with the drive coil (L61). [Form 10] In the method for operating a metal detector (1) according to aspect 8 or 9, a set of frequency components (f D1 , f D2 PXM signal (s PXM ) and the set of frequency components (fD1 , f D2 and activating a resonant circuit in said admittance device (5) corresponding to said metal detector (1). [Form 11] A metal detector (1) operating according to the method as defined in any one of aspects 1 to 10. [Form 12] In a metal detector (1) according to an eleventh aspect, the metal detector (1) comprises a drive coil (L61) for generating an electromagnetic field within a product, at least one detection coil (L62, L63) arranged to detect variations in the magnetic field caused by metal particles present in the product, and a multi-frequency transmitter device (10) comprising a converter (4) having a plurality of drive switches (S41, S42; S43, S44), the plurality of drive switches (S41, S42; S43, S44) configured to drive a drive current (i D ) to the driving coil (L61), so that the generated electromagnetic field has two or more different frequency components (f D1 , f D2 ) according to an operating command to exhibit the two or more frequency components (f D1 , f D2 ) determined for the drive current (i D At least one PXM signal (s) corresponding to the waveform of PXM ) data is stored, and said at least one PXM signal (s PXM The metal detector (1) is driven by a drive control device (2) using a memory device (23) provided within the drive control device (2), in which a memory device (23) is selectable to control the plurality of drive switches (S41, S42; S43, S44). [Form 13] In the metal detector (1) according to the eleventh or twelfth aspect, the method for operating the metal detector (1) according to any one of the first to fifth aspects comprises: PXM )of, - One side is connected to a first voltage potential (V D ) and on the other side a second voltage potential (VS ), and a center tap of the half-bridge circuit is connected to the first voltage potential (V D ) and the second voltage potential (V S ) can be controlled to be alternately connected to the applying a voltage to first and second drive switches (S41, S42) of the drive switches (S41, S42; S43, S44); or - One side is connected to a first voltage potential (V D ) at one end and a second voltage potential such as ground potential at the other end, and each having a first or second center tap, the first or second center tap being connected to a first and second end of the drive coil (L61) by means of a first pair of the drive switches (S41, S42) disposed in the first branch and connected to each other at the first center tap, and by means of a second pair of the drive switches (S43, S44) disposed in the second branch and connected to each other at the second center tap, respectively, and the first and second ends of the drive coil (L61) are connected to the first voltage potential (V D ) and the second voltage potential to the drive switches (S41, S42; S43, S44), which are controllable to be alternately connected to the first voltage potential. A metal detector (1). [Form 14] In the metal detector (1) according to the 11th, 12th or 13th aspect, the converter (4) is connected to the driving coil (L61) directly or via an admittance device (5), and the admittance device (5) together with the driving coil (L61) detects the two or more different frequency components (f D1 , f D2), and the admittance device (5) preferably comprises at least a first branch having a first capacitor (C51) and a first inductor (L51) in the admittance device (5) that form the first resonant circuit together with the drive coil (L61), and a second branch having a second capacitor (C52) and a second inductor (L52) in the admittance device (5) that form the second resonant circuit together with the drive coil (L61). [Form 15] 15. The metal detector (1) according to claim 14, wherein switching means (50, S51, S52) are provided which allow the resonant circuits in the admittance device (5) to be individually activated and deactivated according to a selection. [Explanation of symbols]

[0062]

[0059] 1 metal detector 10 Transmitter device 11 Receiver equipment 2. Drive control device S2 Selector Switch 21 Clock Device 22 Address Counter 23 Memory Device 23A, 23B memory modules 231, 232, 233 memory modules 3. Drive unit 31, 31' drive element 32, 32' Inverting drive element 311, 312, 321, 322 Amplifiers 4 Converter S41, S42, S43, S44 drive switches 5 Admittance Device 50 Selector C51, C52, C53 admittance capacitors L51, L52, L53 Admittance inductors S51, S52, S53 Admittance switches 6 (balanced) coil system L61 drive coil L62, L63 detection coil 7 Input Stage 8 Phase Detector 9. Control Device / Computer System ad Address signal fm clock signal f D1 , f D2 Frequency Components i D Drive Current i(ωt) mathematically calculated driving current i DA Approximated Triangular Signal i L51 Admittance current in inductor L51 i L52 Admittance current in inductor L52 i L61 Coil Current rs Reset signal sf Frequency Selective Signal s PXM Pulse-width or pulse-density modulated signals V D First voltage potential, first drive voltage V S Second voltage potential, second drive voltage

Claims

1. A method for operating a metal detector (1), the metal detector (1) comprising a drive coil (L61) for generating an electromagnetic field in a product, at least one detection coil (L62, L63) arranged to detect variations in the magnetic field caused by metal particles present in the product, and a multi-frequency transmitter device (10) comprising a transducer (4) having a plurality of drive switches (S41, S42; S43, S44), the plurality of drive switches (S41, S42; S43, S44) configured to drive a drive current (i Ｄ ) to the driving coil (L61), so that the generated electromagnetic field has two or more different frequency components (f Ｄ１ , f Ｄ２ ) is driven by a drive control device (2) according to an operation command to exhibit, and the method includes: at least two different frequency components (f Ｄ１ , f Ｄ２ ) for the driving current (i Ｄ ) determining the waveform of a modulated pulse sequence, which is a pulse-width modulated or pulse-density modulated signal, and said drive current (i Ｄ ) at least one PXM signal (s) corresponding to the determined waveform of ＰＸＭ ) - said at least one determined PXM signal (s) being determined online or stored in a memory module (231, 232) according to said operating instructions provided; ＰＸＭ ) and said at least one determined PXM signal (s ＰＸＭ ) to control the driving switches (S41, S42; S43, S44); 1. A method for operating a metal detector (1), comprising:

2. 2. A method for operating a metal detector (1) according to claim 1, wherein the driving current (i Ｄ ) the step of determining the waveform of at least two different frequency components (f Ｄ１ , f Ｄ２ 2. A method for operating a metal detector (1), comprising superimposing current components related to a voltage applied to a metal detector (1).

3. 3. A method for operating a metal detector (1) according to claim 1 or 2, wherein the driving current (i Ｄ The frequency component (f) having the lowest frequency present in Ｄ１ ) for at least the cycle duration of the driving current (i Ｄ ) and determining the waveform of the driving current (i Ｄ ) is determined by repeating the waveform of the driving current (i Ｄ 2. A method for operating a metal detector (1), comprising the step of generating a signal.

4. 4. A method for operating a metal detector (1) according to claim 1, 2 or 3, wherein a triangular or trapezoidal signal is applied to the driving current (i Ｄ ) and the maximum and minimum values of the determined waveform of the triangular signal correspond and / or coincide with each other. Ｄ ) to determine the waveform of the PXM signal (s ＰＸＭ ) at the maximum and minimum values of the triangle or trapezoid signal; ＰＸＭ and sequentially defining switching angles (α1, α2, . . . ) for the falling and rising edges of the signal (α1, α2, . . . ).

5. A method for operating a metal detector (1) according to any one of claims 1 to 4, wherein each frequency component (f Ｄ１ , f Ｄ２ Two or more PXM signals (s) with different sets of ＰＸＭ ) and determining the two or more PXM signals (s ＰＸＭ ) to the stored PXM signal (s ＰＸＭ ) in a memory module (231, 232) selectable for generating and applying one of the signals.

6. A method for operating a metal detector (1) according to any one of claims 1 to 5, wherein the PXM signal (s ＰＸＭ )of, - one side is at a first voltage potential (V Ｄ ) and on the other side to a second voltage potential (V Ｓ ), and a center tap of the half bridge circuit is connected to the first voltage potential (V Ｄ ) and the second voltage potential (V Ｓ a step of applying a voltage to first and second drive switches (S41, S42) of the drive switches (S41, S42; S43, S44) that are controlled to be alternately connected to the first and second drive switches (S41, S42); - one side is at a first voltage potential (V Ｄ ) at one end and a second voltage potential such as ground potential at the other end, and each having a first or second center tap, the first or second center tap being connected to a first and second end of the drive coil (L61) by means of a first pair of the drive switches (S41, S42) disposed in the first branch and connected to each other at the first center tap, and by means of a second pair of the drive switches (S43, S44) disposed in the second branch and connected to each other at the second center tap, Ｄ ) and the second voltage potential to the drive switches (S41, S42; S43, S44) controlled to be alternately connected to the second voltage potential.

1. A method for operating a metal detector (1), comprising:

7. A method for operating a metal detector (1) according to any one of claims 1 to 5, characterized in that the PXM signal (s) is transmitted via a driver (3) each comprising a signal driver element and / or inverter (31, 31', 32, 32', 311, 312, 321, 322) for controlling the input of the drive switches (S41, S42; S43, S44). ＰＸＭ 2. A method for operating a metal detector (1), comprising the step of applying a voltage to the metal detector (1).

8. A method for operating a metal detector (1) according to any one of claims 1 to 7, wherein the drive current (i Ｄ ) to the driving coil (L61) directly or via an admittance device (5), wherein the admittance device (5) together with the driving coil (L61) induces the two or more different frequency components (f Ｄ１ , f Ｄ２ ) forms an active resonant circuit in which the coil current (i Ｌ６１ ) is the driving current (i Ｄ 2. A method for operating a metal detector (1), comprising the step of:

9. 9. A method for operating a metal detector (1) according to claim 8, comprising the step of using at least a first branch having a first capacitor (C51) and a first inductor (L51) in the admittance device (5) forming a first resonant circuit together with the drive coil (L61), and a second branch having a second capacitor (C52) and a second inductor (L52) in the admittance device (5) forming a second resonant circuit together with the drive coil (L61).

10. A method for operating a metal detector (1) according to claim 8 or 9, comprising the step of: Ｄ１ , f Ｄ２ PXM signal (s ＰＸＭ ) and the set of frequency components (f) of the selected PXM signal (sPXM). Ｄ１ , f Ｄ２ and activating a resonant circuit in said admittance device (5) corresponding to said metal detector (1).

11. A metal detector operating according to a method as defined in any one of claims 1 to 10 １）。

12. 12. A metal detector (1) according to claim 11, comprising a drive coil (L61) for generating an electromagnetic field in a product, at least one detection coil (L62, L63) arranged to detect variations in the magnetic field caused by metal particles present in the product, and a multi-frequency transmitter device (10) comprising a converter (4) having a plurality of drive switches (S41, S42; S43, S44), the plurality of drive switches (S41, S42; S43, S44) being arranged to drive a drive current (i Ｄ ) to the driving coil (L61), so that the generated electromagnetic field has two or more different frequency components (f Ｄ１ , f Ｄ２ ) and according to an operating command to exhibit the two or more frequency components (f Ｄ１ , f Ｄ２ ) determined for the drive current (i Ｄ At least one PXM signal (s) corresponding to the waveform of ＰＸＭ ) data is stored, and said at least one PXM signal (s ＰＸＭ a memory device (23) provided within the drive control device (2), in which a plurality of drive switches (S41, S42; S43, S44) are selectable to control the plurality of drive switches (S41, S42; S43, S44); and a metal detector (1) driven by the drive control device (2).

13. A method for operating a metal detector (1) according to any one of claims 1 to 5, comprising the steps of: ＰＸＭ )of, - one side is at a first voltage potential (V Ｄ ) and on the other side to a second voltage potential (V Ｓ ), and a center tap of the half bridge circuit is connected to the first voltage potential (V Ｄ ) and the second voltage potential (V Ｓ ), or applying a voltage to first and second drive switches (S41, S42) of the drive switches (S41, S42; S43, S44), which are controllable to be alternately connected to the first and second drive switches (S41, S42); - one side is at a first voltage potential (V Ｄ ) at one end and a second voltage potential such as ground potential at the other end, and each having a first or second center tap, the first or second center tap being connected to a first and second end of the drive coil (L61) by means of a first pair of the drive switches (S41, S42) disposed in the first branch and connected to each other at the first center tap, and by means of a second pair of the drive switches (S43, S44) disposed in the second branch and connected to each other at the second center tap, Ｄ ) and the second voltage potential to the drive switches (S41, S42; S43, S44), which are controllable to be alternately connected to the first voltage potential. A metal detector (1).

14. 14. The metal detector (1) according to claim 11, 12 or 13, wherein the transducer (4) is connected to the driving coil (L61) either directly or via an admittance device (5), and the admittance device (5) together with the driving coil (L61) detects the two or more different frequency components (f Ｄ１ , f Ｄ２ ), and the admittance device (5) preferably comprises at least a first branch having a first capacitor (C51) and a first inductor (L51) in the admittance device (5) which form the first resonant circuit together with the drive coil (L61), and a second branch having a second capacitor (C52) and a second inductor (L52) in the admittance device (5) which form the second resonant circuit together with the drive coil (L61).

15. 15. A metal detector (1) according to claim 14, wherein switching means (50, S51, S52) are provided which make it possible to individually activate and deactivate the resonant circuits in the admittance device (5) according to a selection.

16. 3. A method for operating a metal detector (1) according to claim 2, wherein said at least two different frequency components (f Ｄ１ , f Ｄ２ ) is a sinusoidal frequency component (f Ｄ１ , f Ｄ２ ) a method for operating a metal detector (1).

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