Method for operating a metal detector and metal detector

Abstract

The present invention provides a method for operating a metal detector, which includes a coil system having two detection coils connected to a receiver unit and coupled to a drive coil, the drive coil and a capacitor forming a series resonant circuit connected to a transmitter unit, the transmitter unit having a first and a second drive switch driven as follows to form a half-bridge circuit: driven according to a first square wave signal, which is set to an operating frequency and applied to the first drive switch, and driven according to a second square wave signal, which is set to an operating frequency and applied to the second drive switch; the method includes: providing first and second voltage potentials with relatively different sizes; providing a first square wave signal with a variable first duty cycle, providing a second square wave signal with a variable second duty cycle, so that the first and second square wave signals are never activated at the same time; adjusting the first and second duty cycles to set the coil current to a value suitable for operating the metal detector and measuring contaminants.

Description

Technical Field

[0001] The present invention relates to a method for operating a metal detector using one or more operating frequencies and a metal detector operated according to the method. Background Art

[0002] For example, industrial metal detection systems such as those described in US8587301B2 are used to detect metal contaminants in products. If properly installed and operated, it will help reduce metal contamination and enhance food safety. Most modern metal detectors use probes that include a "balanced coil system". Detectors of this design are able to detect all types of metal contaminants, including ferrous, non-ferrous metals and stainless steel, in a variety of products, such as fresh and frozen products.

[0003] Metal detectors operating on the "balanced coil" principle usually consist of three coils wound on a non-metallic frame: a drive coil and two identical detection coils, each usually in parallel with the others. Since the detection coils, which usually surround the drive coil in the middle, are identical, the same voltage is induced in each detection coil. In order to receive an output signal of zero when the system is in balance, the first detection coil is connected in series with a second detection coil with an opposite winding. Therefore, when the system is in equilibrium and there are no contaminants in the observed product, the voltages of equal amplitude and opposite polarity induced in the detection coils cancel each other.

[0004] However, once a metal particle passes through the coil arrangement, the electromagnetic field near one detection coil is disturbed first, then the electromagnetic field near the other detection coil. As the metal particle travels through the detection coils, the voltage induced in each detection coil is changed (usually in nanovolts). This change in balance results in a signal at the output of the detection coils, which is processed, amplified, and subsequently used to detect the presence of metal contaminants in the product being observed in the receiver unit.

[0005] In the receiver unit, the input signal is usually separated into an in-phase component and a quadrature component. The vector composed of these components has a magnitude and a phase angle, which is typical for the product and contaminants transmitted through the coil system. In order to identify metal contaminants, the "product effect" needs to be removed or reduced. If the phase of the product is known, the corresponding signal vector can be reduced to produce a higher sensitivity for the detection of signals from metal contaminants.

[0006] The method for eliminating unwanted signals from the signal spectrum utilizes the fact that metal contaminants, products and other disturbances have different effects on the magnetic field so that the detected signals differ in phase. Materials with high electrical conductivity cause signals with a higher negative reactance signal component and a smaller resistive signal component. Materials with high magnetic permeability cause signals with a smaller resistive signal component and a higher positive reactance signal component. The signal caused by ferrite is mainly reactive, while the signal caused by stainless steel is mainly resistive. Conductive products generally cause signals with a strong negative reactance component. As the product or contaminant is transmitted through the metal detector, the phase angle between the resistive signal component and the reactive signal component of the signal vector generally remains constant.

[0007] US20150234075A1 discloses a metal detector with a drive circuit, which is used to establish an alternating magnetic field in a coil system to generate an output signal at a given frequency. The drive circuit includes a central processing unit, an electronic programmable logic device (EPLD), and a driver connected to four field effect transistors that form a full-wave bridge circuit through a selected potential difference, and the drive coil is connected to the output end of the bridge circuit. The potential difference, for example 24 volts, forms a drive current on the drive coil. US20150234075A1 also describes a half-wave bridge with only two FETs. When the drive coil is connected to both ends, the switch is arranged so that the drive current flows through one FET via the drive coil in one half cycle and flows through another FET in another half cycle.

[0008] The metal detector may include a coil system that operates over a wide frequency band. Alternatively, a coil system tuned to a specific operating frequency may be used. The amplitude of the alternating current in the coil system is controlled by selecting the capacitance and operating frequency accordingly. The inductance can only be varied to a large extent due to the mechanical limitations of increasing or decreasing the number of individual turns of the drive coil. Capacitors are relatively expensive, difficult to install, and can only be used in sizes specified by the manufacturer. The transmitter frequency can also be adjusted relative to the resonant frequency, but significant movement is required to produce the required current change. Therefore, the procedure for optimizing the tuning of the coil system and its components to achieve an operating frequency and coil current within the desired range is cumbersome. Summary of the invention

[0009] The invention is therefore based on the object of providing an improved method for operating a metal detector and an improved metal detector operated according to the method.

[0010] The method of the invention should allow the metal detector to be tuned to the desired operating frequencies with little effort and to set the appropriate maximum coil current for each of these frequencies.

[0011] In particular, it should be possible to tune the metal detector to a desired operating frequency and set a suitable coil current without the need for non-standard components.Thus, the metal detector of the invention should be producible at reduced cost but with optimal performance.

[0012] The method and metal detector of the present invention should allow high coil currents to be generated in the drive coil, providing high sensitivity for the metal detector, while the current in the transmitter unit remains relatively low so that the transmitter circuit can be sized accordingly, for example with reduced power performance and cost.

[0013] Furthermore, it should be possible to operate the metal detector in a tuned mode dedicated to a specific frequency, for which each coil current is easily adjustable, and in a non-tuned mode, in which the operating frequency can be freely selected without the need to tune the coil system.

[0014] Switching between different operating modes should be easily and practically achievable without the need for additional components.

[0015] In a first broad aspect of the invention, there is provided a method for operating a metal detector, the metal detector comprising a balanced coil system having two detection coils connected to a receiver unit and inductively coupled to a drive coil, the drive coil together with at least one capacitor forming a series resonant circuit connected to a transmitter unit, the transmitter unit comprising a converter having a first drive switch and a second drive switch forming a half-bridge circuit, the first drive switch (S41) and the second drive switch (S42) being driven by a drive controller (2) as follows:

[0016] driven according to a first square wave signal, wherein the first square wave signal is set to a fixed or selectable operating frequency and is applied to a first drive switch, the first drive switch being connected on one side to the first voltage potential and on the other side to the center tap of the half-bridge circuit,

[0017] Driven according to a second square wave signal, the second square wave signal is set to a fixed or selectable operating frequency and is applied to a second drive switch, one side of the second drive switch is connected to the second voltage potential and the other side is connected to the center tap of the half-bridge circuit, and the drive current is provided to the drive coil via the second drive switch.

[0018] The method comprises the following steps:

[0019] providing a first voltage potential and a second voltage potential having different magnitudes relative to ground;

[0020] providing a first square wave signal having a first variable duty cycle and a second square wave signal having a second variable duty cycle, and causing the first square wave signal and the second square wave signal to never be activated at the same time;

[0021] The first duty cycle and the second duty cycle are adjusted to set the coil current in the drive coil to a value suitable for operating the metal detector and measuring contaminants.

[0022] In a preferred embodiment, the first voltage potential or the second voltage potential is set to ground or ground potential, respectively. Therefore, providing only one supply voltage in the converter is sufficient to generate a coil current in the drive coil, and the coil current can be adjusted to a suitable value by selecting corresponding values ​​for the first duty cycle and the second duty cycle.

[0023] If the center tap of the half-bridge circuit is connected to a voltage potential different from ground potential via one of the drive switches, the drive current is preferably driven to ground via the series resonant circuit.

[0024] According to the method of the present invention, the first voltage potential and the second voltage potential are applied to the series resonant circuit for respective durations determined by the first duty cycle and the second duty cycle. The coil current in the drive coil can be increased by increasing the duty cycle or duration of applying a voltage potential having a higher magnitude to the drive coil within each cycle of the operating signal, and by reducing the duty cycle or duration of applying a voltage potential having a lower magnitude or having a ground potential to the drive coil within each cycle of the operating signal.

[0025] Suitable first and second duty cycles that can be applied to the converter can be calculated or programmed in the drive controller.Providing different voltage potentials in the converter would require considerable work and the coil current would also be affected, which is advantageously avoided.

[0026] The first square wave signal and the second square wave signal are preferably generated with waveforms that are at least approximately complementary, so that the sum of the current first duty cycle and the current second duty cycle is approximately equal to "1". Therefore, one end of the series resonant circuit is preferably always connected to the first voltage potential or the second voltage potential, and the other end of the series resonant circuit is preferably always connected to a fixed potential, preferably a ground potential.

[0027] The duty cycle is the fraction of one cycle of the operating signal that the signal is activated. An activated signal may also be defined as a pulse having a corresponding duration or fraction of one cycle of the operating signal. Thus, the duty cycle of a periodic signal represents the ratio between the activated duration and the inactivated duration, which is scaled to 1, rather than 100% as the term "duty cycle" is used.

[0028] During the corresponding activation time of one cycle of the operating signal, one end of the series resonant circuit is set to ground by closing the corresponding drive switch. If the other end of the series resonant circuit is fixedly set to ground, the series resonant circuit is set to ground at both ends during this activation time, thus forming a closed loop. During this activation time, no energy is induced into the closed loop or series resonant circuit, and energy is lost due to heat loss and energy induced into the detection coil. By increasing this activation time or the related duty cycle, the coil current in the drive coil can be reduced. By reducing this activation time or the related duty cycle and by increasing the activation time or the related duty cycle during which a voltage potential with a higher magnitude is applied to the series resonant circuit, the coil current can be increased accordingly.

[0029] Preferably, the first duty cycle and the second duty cycle are adjusted so that the coil current in the drive coil is set to a value that keeps the voltage generated in the series resonant circuit below a predetermined maximum value. Thus, the coil current is maximized up to a value that brings the coil system to a permissible voltage but does not exceed the permissible voltage. In this way, the sensitivity of the metal detector is also optimized.

[0030] In a preferred embodiment, the voltage generated in the series resonant circuit or coil system is thus measured and compared with a reference value, for example a maximum permissible coil voltage. By adjusting the first duty cycle of the first square wave signal and the second duty cycle of the second square wave signal, the difference between the measured voltage and the reference value can be kept at a minimum. In this way, a closed-loop control can advantageously be formed which keeps the maximum coil current and the maximum voltage in the coil system constant.

[0031] In order to reduce the voltage generated in the series resonant circuit or increase the coil current while keeping the maximum voltage generated in the resonant circuit low, at least one first capacitor is arranged on one side of the drive coil and at least one second capacitor is arranged on the other side of the drive coil. The first capacitor and the second capacitor are preferably identical so that the voltage generated in the series resonant circuit is equal and opposite on both sides of the coil and is minimized relative to the ground potential. Therefore, the series resonant circuit is balanced relative to the ground.

[0032] In a preferred embodiment, the capacitance or inductance of the series resonant circuit and therefore the resonant frequency is adjustable. The capacitance can be changed by connecting or disconnecting a capacitor and the inductance can be changed by connecting or disconnecting a coil winding from the drive coil. Preferably, at least one third capacitor is provided which can be connected to at least one first capacitor via a first switch and / or at least one fourth capacitor is provided which can be connected to the second capacitor via a second switch. The first switch and / or the second switch are selectively opened or closed for changing the resonant frequency of the series resonant circuit to a desired value which is generally close to the operating frequency of the metal detector.

[0033] The fixed or selectable operating frequency is preferably arranged to be offset from the resonant frequency of the series resonant circuit in such a way that the coil current in the drive coil is adjustable within a current range in which the coil current in the drive coil and / or the voltage in the series resonant circuit remain below a predetermined maximum value. Alternatively, the resonant frequency of the series resonant circuit is offset relative to the operating frequency of the fixed or selected operating frequency of the metal detector. In this way, excessive coil currents can be avoided and the coil voltages can be kept within the range in which they can be controlled by closed loop.

[0034] The metal detector of the present invention can be operated at one or more operating frequencies tuned using a coil system in a tuning mode. The operating values ​​for the first duty cycle and the second duty cycle are preferably predetermined and stored or calculated online, and the coil current in the drive coil can be set to an appropriate value for each optional operating frequency using the first duty cycle and the second duty cycle. Then, one of the predetermined operating frequencies can be selected together with the associated operating values ​​of the first duty cycle and the second duty cycle so that for each selected operating frequency, the desired maximum coil current and / or maximum coil voltage occurs.

[0035] In addition, the metal detector can selectively operate in a tuned mode or a non-tuned mode as described above, wherein in the non-tuned mode, the drive coil is not part of the resonant circuit. In the tuned mode, the drive coil forms a series resonant circuit with at least one capacitor and is connected to a half-bridge circuit arranged in the converter. In the non-tuned mode, the drive coil is not part of the series resonant circuit and is connected to a full-bridge circuit arranged in the converter.

[0036] In a preferred embodiment, the full-bridge circuit can be converted into a half-bridge circuit and vice versa. In the tuned mode, only the first half-bridge circuit of the full-bridge circuit is switched, while the second half-bridge circuit grounds the non-switched side of the coil. In this way, the conversion can be easily performed without the need for additional components. In the non-tuned mode, the center taps of the first half-bridge circuit and the second half-bridge circuit of the full-bridge circuit are preferably directly connected to the two ends of the drive coil. In the tuned mode, the center tap of the first half-bridge circuit of the full-bridge circuit is directly or indirectly connected to one of the two ends of the drive coil, while the other end of the drive coil is preferably directly or indirectly set to ground. In this way, the converter can easily switch from operating in the tuned mode to operating in the non-tuned mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Detailed aspects and examples of the present invention are described below with reference to the accompanying drawings, in which:

[0038] Figure 1The metal detector 1 of the present invention is shown, which includes a converter 4 with two drive switches S41, S42, which form a half bridge and are controlled by means of drive signals d1, d2 provided by a drive controller 2 and provide a drive current i to a drive coil L61 of a coil system 6 of the metal detector 1. D , the drive coil L61 and the capacitors C1 and C2 and optionally also the capacitors C3 and C4 form a series resonant circuit;

[0039] Figure 2 Shows the application to Figure 1 An example of drive signals d1 and d2 for driving switches S41 and S42 in the converter 4;

[0040] Figure 3 A converter 4 including a full bridge circuit is shown. Figure 1 The metal detector 1 drives switches S41 and S42 to form a first half-bridge circuit, drives switches S43 and S44 to form a second half-bridge circuit, the second half-bridge circuit is activated in the non-tuned mode and deactivated in the tuned mode, and in the tuned mode the first half-bridge circuit is as shown in the reference Figure 1 Operate as described. DETAILED DESCRIPTION

[0041] Figure 1 A preferred embodiment of a metal detector 1 of the present invention is shown, the metal detector 1 comprising a transmitter unit 10 and a receiver unit 11 and a balanced coil system 6 having a drive coil L61 connected to the output of the transmitter unit 10 and two detection coils L62, L63 connected at one end to ground potential and at the other end to an input stage 7 of the receiver unit 11. In the input stage 7, the input signal is typically amplified and filtered and then forwarded to a phase detector module 8 which typically comprises two phase detector units. An in-phase reference signal I is received from a frequency generator or synthesizer 29. REF and the quadrature reference signal Q REF The phase detector module 8 provides the in-phase and quadrature components of the baseband signals of the product and the contaminant. Using the in-phase reference signal I REF and the quadrature reference signal Q REF The demodulation of provides phase information and allows to distinguish the phases of signals of different origin and to obtain information about the observed product and contaminants (if present). The phase detector module 8 provides in-phase and quadrature signals to a control unit 9, which is preferably equipped with a signal processor, input-output devices, a keyboard and a display. With the help of the control unit 9, the operation of the metal detector 1 can be controlled. In particular, the operating conditions of the metal detector 1, in particular the drive current I D and operating frequency.

[0042] The transmitter unit 10 comprises a drive controller 2, a drive unit 3 and a converter 4. The drive controller 2 comprises a frequency generator or synthesizer 29 which is controlled by the main control unit 9 to generate a desired operating frequency f TX and a corresponding in-phase reference signal I applied to the phase detector module 8 REF and the quadrature reference signal Q REF The operating frequency is applied to a drive control unit 23 which provides drive signals d1, d2, for example pulse width modulated signals, which are arranged to generate a desired maximum coil current i L61 In the drive unit 3 , the drive signals d1 , d2 are amplified by amplifiers 31 , 32 and forwarded to the converter 4 .

[0043] The converter 4 comprises two drive switches S41, S42 forming a half bridge and controlled by means of drive signals d1, d2. The drive switches S41, S42 provide a drive current i to the drive coil L61 of the coil system 6. D The drive coil L61 forms a series resonant circuit together with the capacitors C1 and C2. The resonant frequency of the series resonant circuit and the operating frequency of the metal detector 1 preferably cancel each other. The resonant circuit composed of the capacitors C1 and C2 and the drive coil L61 is grounded at one end and connected to the center tap of the half-bridge circuit S41, S42 of the converter 4 at the other end.

[0044] The first capacitor C1 is connected to one terminal of the drive coil L61, and the second capacitor C2 is connected to the other terminal of the drive coil L61. Therefore, considering the capacitors C1, C2 connected to the terminals of the drive coil L61, the resulting series resonant circuit is balanced. The third capacitor C3 can be connected in parallel to the first capacitor C1 using switch u1, and the fourth capacitor C4 can be connected in parallel to the second capacitor C2 using switch u2, so that the resonant frequency of the series resonant circuit can be set to a corresponding value. By adding or reducing capacitors or coil windings, the resonant frequency can be set to any desired value.

[0045] The upper end of the first drive switch S41 of the half-bridge circuit is connected to the voltage potential V D The lower end is connected to the center tap of the half-bridge circuit. The upper end of the second drive switch S42 is connected to the center tap of the half-bridge circuit, and the lower end is grounded. During the activation of the first drive signal d1, the first drive switch S41 connects the capacitor C2 or the relevant terminal of the series resonant circuit to the voltage potential V D. In this part of the relevant period of the operating frequency, the second drive switch S42 is open. In the next part of the relevant period of the operating frequency, the second drive switch S42 is closed. In this part, the capacitor C2 or the relevant terminal of the series resonant circuit is grounded. Therefore, in this part of the relevant period of the operating frequency, the resonant circuit is grounded via the capacitor C1 and to ground via the capacitor C2 and the second drive switch S42, thus forming a closed loop. In the first part of each period of the operating frequency, energy is injected into the series resonant circuit, while in the second part of each period of the operating frequency, no energy is added, but energy may be lost due to resistance and due to induction of the magnetic field in the detection coils L62, L63. By adjusting the ratio of the first part and the second part or the relevant duty cycle of the first drive signal d1 and the second drive signal d2, the energy present in the resonant circuit and thus the maximum coil current i L61 The voltages appearing at capacitors C1 and C2 can be set to desired values.

[0046] In the drive control unit 23, the duty cycle of the drive signals d1, d2 is calculated by means of the control program 231 or retrieved from a storage module in which data of the duty cycle for each selectable operating frequency and each coil current i for each operating frequency are stored. L61 The duty cycle data can be recursively read out from the memory by addressing the address counter 22 by transmitting the address information "ad" to the drive control unit 23. Alternatively, a timer can be set to indicate the length of the activation time period within the cycle of the operating signal for each drive signal d1, d2, which continuously calculates and adjusts the duty cycle as needed.

[0047] In this preferred embodiment, the voltage appearing in the series resonant circuit is measured in the A / D converter 21 and converted into a digital value which is provided to the drive control unit 23. The measured voltage can be compared with a reference value to determine a difference, which is minimized by changing the duty cycle of the drive signals d1, d2 accordingly. The drive control unit 23 can be used as a PID controller which, based on the measured difference, applies a correction based on a proportional term, an integral term and a differential term.

[0048] Figure 2 Shows the application to Figure 14 . From time t1 to t2, the duty cycle of the first drive signal d1 is 0.25 or π / 2, and the duty cycle of the second drive signal d2 is 0.75 or 3π / 2. The first drive signal d1 is activated in the first part of the cycle of the drive signal d1. The second drive signal is activated in the second part of the cycle of the drive signal d2. The length of the first part and the length of the second part of the first drive signal and the second drive signal correspond to the length of the full cycle 2π of the drive signals d1 and d2. Since the activation part of the first drive signal d1 is relatively short and the series resonant circuit is connected to the drive voltage V for only a short period of time, the activation part of the first drive signal d1 is relatively short and the series resonant circuit is connected to the drive voltage V for only a short period of time. D , so the coil current i L61 Relatively small.

[0049] From time t3 to t4, the duty cycle of the first and second drive signals d1, d2 is set to 0.5 or π. Therefore, for half of the period 2π, the series resonant circuit is connected to the drive voltage V D , for the other half of the period 2π, the series resonant circuit is grounded. During this time interval, the energy in the series resonant circuit and the maximum coil current i generated L61 Increase accordingly.

[0050] Figure 3 A converter 4 including a full bridge circuit is shown. Figure 1 The metal detector 1 of the present invention has a full-bridge circuit having four drive switches S41, S42, S43, and S44 that receive drive signals d1, d2, d3, and d4 from the drive unit 3, respectively. The full-bridge circuit includes a first half-bridge circuit having drive switches S41 and S42 and a second half-bridge circuit having drive switches S43 and S44. In the non-tuned mode, the complete full-bridge circuit having the first half-bridge circuit and the second half-bridge circuit is activated. In the tuned mode, only the first half-bridge circuit of the full-bridge circuit is activated, while the drive switch S43 of the second half-bridge circuit remains open and S44 remains closed.

[0051] In the non-tuned mode, capacitors C1 and C2 are short-circuited by switches u3 and u4. One terminal of the drive coil L61 is connected to the center tap of the first half-bridge circuit, and the other terminal of the drive coil L61 is connected to the center tap of the second half-bridge circuit. In this configuration, the full-bridge circuit can be operated using drive signals d1, d2, d3, d4 to generate any frequency at one time or multiple frequencies at one time.

[0052] By changing the switch settings of switches u3 and u4, and by opening the drive switch S43 of the second half-bridge circuit and closing S44, the converter 4 can be reconfigured as follows Figure 1As shown. In this state, only the first half-bridge circuit with drive switches S41 and S42 is activated. One terminal of the drive coil L61 is connected to the center tap of the first half-bridge circuit, and the other terminal of the drive coil L61 is grounded via S44. In this configuration, the half-bridge circuit is as shown in reference Figure 1 Operate as described.

[0053] Reference numerals list

[0054] 1 Metal Detector

[0055] 10 Transmitter Units

[0056] 11 Receiver Unit

[0057] 2 Drive controller

[0058] 21 A / D converter

[0059] 22 Address counter

[0060] 23 Drive control unit

[0061] 231 Control Program

[0062] 29 Frequency Generator

[0063] 3 Drivers

[0064] 31, 32, 33, 34 Amplifier

[0065] 4 Converters

[0066] S41, S42, S43, S44 drive switch

[0067] 6 (balanced) coil system

[0068] L61 drive coil

[0069] L62, L63 detection coil

[0070] 7 Input Stage

[0071] 8 Phase detector unit

[0072] 9 Control unit / computer system

[0073] ad addressing signal

[0074] C1, C2, C3, C4 Capacitors connected / connectable to the drive coil

[0075] u1, u2 are switches for changing the resonant frequency

[0076] u3, u4 Switches for changing the operation mode

[0077] d1, d2 driving signal

[0078] f TX Operating frequency

[0079] f D1 , f D2 Frequency Component

[0080] i D Drive current

[0081] i L61 Coil current

[0082] V D first voltage potential, first driving voltage

[0083] V S The second voltage potential, the second voltage or ground

Claims

1. A method for operating a metal detector (1), the metal detector (1) comprising a balanced coil system (6) having two detection coils (L62, L63) connected to a receiver unit (11) and inductively coupled to a drive coil (L61), the drive coil (L61) together with at least one capacitor (C1, C2) forming a series resonant circuit connected to a transmitter unit (10), the transmitter unit (10) comprising a converter (4) having a first drive switch (S41) and a second drive switch (S42) forming a half-bridge circuit, the first drive switch (S41) and the second drive switch (S42) being driven by a drive controller (2) as follows: The first square wave signal (d1) is driven by a first square wave signal (d1) which is set to a fixed or selectable operating frequency (f TX ) and is applied to the first drive switch (S41), one side of which is connected to the first voltage potential (V D ) and the other side is connected to the center tap of the half-bridge circuit, The second square wave signal (d2) is driven by a second square wave signal (d2) which is set to a fixed or selectable operating frequency (f TX ) and is applied to the second drive switch (S42), one side of which is connected to the second voltage potential (V S ) and the other side is connected to the same center tap of the half-bridge circuit, driving current (i D ) is provided to the driving coil (L61) via the second driving switch (S42); The method comprises the following steps: Provide a first voltage potential (V D ) and the second voltage potential (V S ); providing a first square wave signal (d1) having a first variable duty cycle, providing a second square wave signal (d2) having a second variable duty cycle, and causing the first square wave signal (d1) and the second square wave signal (d2) to never be activated at the same time; The first duty ratio and the second duty ratio are adjusted by the following operation to change the coil current (i L61 ) are set to values ​​appropriate for operating the metal detector and measuring contaminants: predetermining and storing or calculating online operating values ​​for the first duty cycle and the second duty cycle, using the first duty cycle and the second duty cycle to set the coil current in the drive coil to an appropriate value for each selectable operating frequency, and One of the predetermined operating frequencies is selected together with associated operating values ​​of the first duty cycle and the second duty cycle.

2. A method for operating a metal detector (1) according to claim 1, wherein: The first voltage potential (V D ) or the second voltage potential (V S ) is arranged to be grounded and / or connected to a series resonant circuit.

3. A method for operating a metal detector (1) according to claim 1 or 2, wherein: The method comprises the following steps: generating a first square wave signal (d1) and a second square wave signal (d2) having waveforms that are at least approximately complementary, so that the sum of a current first duty cycle and a current second duty cycle is approximately equal to 1.

4. A method for operating a metal detector (1) according to claim 1 or 2, wherein: The method comprises the following steps: adjusting a first duty cycle and a second duty cycle to change a coil current (i L61 ) is set to a value that keeps the voltage generated in the series resonant circuit below a predetermined maximum value.

5. A method for operating a metal detector (1) according to claim 1 or 2, wherein: The method comprises the following steps: D ) is driven from the center tap of the half-bridge circuit through a series resonant circuit to ground.

6. A method for operating a metal detector (1) according to claim 1 or 2, wherein: The method comprises the steps of arranging the at least one first capacitor (C1) on one side of the driving coil (L61) and arranging at least one second capacitor (C2) on the other side of the driving coil (L61).

7. A method for operating a metal detector (1) according to claim 6, wherein: The first capacitor (C1) and the second capacitor (C2) are identical.

8. A method for operating a metal detector (1) according to claim 1 or 2, wherein: The method comprises the following steps: providing at least one third capacitor (C3) connectable to at least one first capacitor (C1) via a first switch (u1) and / or providing at least one fourth capacitor (C4) connectable to at least one second capacitor (C2) via a second switch (u2); closing or opening the first switch (u1) and / or the second switch (u2) to change the resonant frequency of the series resonant circuit.

9. A method for operating a metal detector (1) according to claim 1 or 2, wherein: The method comprises the following steps: setting a fixed or selectable operating frequency (f TX ) and the resonant frequency of the series resonant circuit so that the coil current (i L61 ) is adjustable within the current range, wherein the coil current (i L61 ) and / or the voltage generated in the series resonant circuit is maintained below a predetermined maximum value.

10. A method for operating a metal detector (1) according to claim 1 or 2, wherein: The method comprises the steps of measuring a voltage at a capacitor (C1, C2) of a series resonant circuit, comparing the measured voltage with a reference value, and adjusting a first duty cycle of a first square wave signal (d1) and a second duty cycle of a second square wave signal (d2) according to a difference between the measured voltage and the reference value, wherein the difference corresponds to a coil current (i L61 ) of the predetermined value.

11. A method for operating a metal detector (1) according to claim 1 or 2, wherein: The method comprises the following steps: Providing a second half-bridge circuit composed of drive switches (S43, S44) for the converter (4), and forming a full-bridge circuit with the first half-bridge circuit composed of the first drive switch (S41) and the second drive switch (S42), wherein the full-bridge circuit is configured to be activated in a non-tuned mode and deactivated in a tuned mode; selectively operating a balanced coil system (6) in a tuning mode, wherein a drive coil (L61) forms a series resonant circuit with at least one capacitor (C1, C2) and is connected to a half-bridge circuit, or The balanced coil system (6) is operated in a non-tuned mode, wherein the drive coil (L61) is not part of the series resonant circuit and is connected to a full bridge circuit.

12. A metal detector (1) operated according to the method defined in any one of claims 1 to 11, wherein: The metal detector (1) comprises a balanced coil system (6) having two detection coils (L62, L63) connected to a receiver unit (11) and inductively coupled to a drive coil (L61), the drive coil (L61) together with at least one capacitor (C1, C2) forming a series resonant circuit connected to a transmitter unit (10), the transmitter unit (10) comprising a converter (4) having a first drive switch (S41) and a second drive switch (S42) forming a half-bridge circuit, the first drive switch (S41) and the second drive switch (S42) being driven by a drive controller (2) as follows: The first square wave signal (d1) is driven by a first square wave signal (d1) which is set to a fixed or selectable operating frequency (f TX ) and is applied to the first drive switch (S41), one side of which is connected to the first voltage potential (V D ) and the other side is connected to the center tap of the half-bridge circuit, The second square wave signal (d2) is driven by a second square wave signal (d2) which is set to a fixed or selectable operating frequency (f TX ) and is applied to the second drive switch (S42), one side of which is connected to the second voltage potential (V S ) and the other side is connected to the center tap of the half-bridge circuit, driving the current (i D ) is provided to the driving coil (L61) via the second driving switch (S42); in, The first voltage potential (V D ) and the second voltage potential (V S ) are provided to have different sizes relative to the ground; The first square wave signal (d1) is provided to have a variable first duty cycle, and the second square wave signal (d2) is provided to have a variable second duty cycle, so that the first square wave signal (d1) and the second square wave signal (d2) are never activated at the same time; wherein, The first duty cycle and the second duty cycle are adjustable to adjust the coil current (i L61 ) is set to a value suitable for operating the metal detector and measuring contaminants, wherein operating values ​​for the first duty cycle and the second duty cycle are predetermined and stored or calculated online, the coil current in the drive coil is set to an appropriate value for each selectable operating frequency using the first duty cycle and the second duty cycle, and One of the predetermined operating frequencies is selected along with associated operating values ​​of the first duty cycle and the second duty cycle.

13. The metal detector (1) according to claim 12, wherein: The first voltage potential (V D ) or the second voltage potential (V S ) is set to ground.

14. The metal detector (1) according to claim 12 or 13, wherein: The converter (4) can be converted from a half-bridge circuit including a first drive switch (S41) and a second drive switch (S42) to a full-bridge circuit including a first half-bridge circuit having the first drive switch (S41) and the second drive switch (S42) and a second half-bridge circuit having a third drive switch (S43) and a fourth drive switch (S44).

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