Metal Detector

Abstract

The metal detection apparatus comprises a balanced coil system (2) having a transmitter coil (21) connected to a transmitter unit (1) using a first coil terminal (211) and a second coil terminal (212), and a first receiver coil (22A, 22B) connected to an input of a receiver unit (3), the output of which is connected to a signal processing unit (45). The transmitter unit (1) comprises a controllable frequency generator (11) providing an input signal having a selectable operating frequency to an input of an amplifier stage (12), the output of which is connected to an adaptation unit (14). 14) comprises a coupling transformer (143) having at least one primary coil and at least one secondary coil having a transformer coil terminal (143T) and a number of transformer tappings (143A, 143B, 143C), a number of tuning capacitors (144A, 144B, 144C) having first and second capacitor terminals, and switching devices (141A, 142A; 141B, 142B; 141C, 142C) using which at least one of the secondary coil and the tuning capacitors (144A, 144B, 144C) of the coupling transformer (143) can be connected to the transmitter coil (21). According to the invention, a plurality of individually selectable resonator channels are provided, each of which, when selected, comprises one of the transmitter coil (21), a tapped winding of the secondary coil of the coupling transformer (143), and tuning capacitors (144A, 144B, 144C) connected together by a first semiconductor switching device (141A; 141B; 141C) and by a second semiconductor switching device (142A; 142B; 142C), which are controllable by a channel selector (140) such that one of the plurality of resonator channels is necessarily selectable.

Description

[Technical Field]

[0001]

[0001] The present invention relates to metal detecting devices that use multiple operating frequencies. [Background technology]

[0002]

[0002] Industrial metal detection devices, such as those described in U.S. Pat. No. 8,587,301 B2, are used to detect metal contamination in products. When properly installed and operated, industrial metal detection devices help reduce metal contamination and improve food safety. Most modern metal detectors utilize a search head with a "balanced coil system." Detectors of this design are capable of detecting all metal contaminant types, including ferrous, non-ferrous, and stainless steel, in a wide variety of products, including fresh and frozen products.

[0003]

[0003] Metal detection devices operating according to the "balanced coil" principle include three coils: a transmitter coil and two identical receiver coils, typically parallel to each other, wound on a nonmetallic frame. These receiver coils, typically surrounding the transmitter coil, are identical, and identical voltages are induced in each of them. The first receiver coil is connected in series with the second receiver coil, which has opposite windings, to receive an output signal that is zero when the system is balanced. Thus, when the system is balanced and no contaminants are present in the product being observed, the voltages induced in the receiver coils, which are identical in amplitude but opposite in polarity, cancel each other out.

[0004]

[0004] However, as soon as a metal particle passes through the coil structure and is exposed to the magnetic field, eddy currents are forced to flow within the metal particle. The eddy currents first disturb the primary electromagnetic field near one receiver coil, and then generate a secondary magnetic field that disturbs the primary electromagnetic field near the other receiver coil. As the metal particle is transported through the receiver coils, the voltage induced in each receiver coil changes (by nanovolts). This change in balance allows the signal at the output of the detection coil present in the receiver unit to be processed, amplified, and possibly filtered, and then used to detect the presence of metal contaminants in the product being observed as it traverses the metal detection device on the conveyor system.

[0005]

[0005] In the receiver unit, the input signal is typically split into an in-phase component and a quadrature component. The vector formed by these components has a fixed magnitude and a fixed phase angle that is specific to the products and contaminants being carried through the coil system. To identify metallic contaminants, the "product effect" must be removed or reduced. Knowing the phase of the product allows the corresponding signal vector to be reduced, allowing for greater sensitivity in detecting signals emanating from metallic contaminants.

[0006]

[0006] Methods applied to remove undesired signals from the signal spectrum take advantage of the fact that metallic contaminants, products, and other disturbances have different effects on the magnetic field and therefore different phases of the detected signal. Materials with high electrical conductivity produce signals with a larger negative reactive signal component and a smaller resistive signal component. Materials with high magnetic permeability produce signals with a smaller resistive signal component and a larger positive reactive signal component. The signal produced by ferrite is primarily reactive, while the signal produced by stainless steel is primarily resistive. Products that are conductive typically produce signals with a strong resistive component. The phase angle of the signal vector between the resistive and reactive signal components usually remains constant as the product or contaminant is transported through the metal detection device.

[0007]

[0007] Information about products and contaminants can be obtained by distinguishing between the phases of signal components from different sources using a phase detector. A phase detector, such as a frequency mixer or analog multiplier circuit, generates a voltage signal representing the phase difference between a signal input, such as the output signal of a receiver coil, and a reference signal provided to the receiver unit by a transmitter unit. Therefore, by selecting the phase of the reference signal to match the phase of the product signal component, the phase difference and corresponding product signal are obtained at the output of the phase detector, which is zero. If the phase of the signal emitted from the contaminant differs from the phase of the product signal, the product signal can be suppressed, while the contaminant signal can be further processed. However, if the phase of the contaminant signal is close to the phase of the product signal, the contaminant signal will be suppressed along with the product signal, and contaminant detection will fail. An appropriate operating frequency is determined and applied to separate the phase angle of the product signal from the phase angle of the contaminant.

[0008]

[0008] US8841903B2 discloses a metal detection device having a transmitter unit that provides a transmitter signal to a transmitter coil inductively coupled to a first receiver coil and a second receiver coil connected to a receiver unit connected to a signal processor. The transmitter unit includes a frequency generator that provides an operating frequency to the input of an amplifier stage, the output of which is connected to the transmitter coil via a coupling transformer. The output of the amplifier stage is connected to a first tapping via a first switch bank, and the transmitter coil is connected to a second tapping of the same transformer winding of the transformer via a second switch bank. This structure allows the resonant circuit consisting of the transmitter coil and a selectable capacitor to be tuned to a selected operating frequency independently of the rest of the transmitter unit. The amplifier stage amplifies the signal with minimal distortion, but is a class A circuit with low efficiency because the power transistors constantly consume current even in a quiescent state.

[0009]

[0009] US10184908B2 discloses a metal detection device having a coupling transformer with a primary coil having a first winding and a second winding connected to the output of an amplifier stage, and a secondary coil connected to a transmitter coil. The first winding and the second winding are connected to a supply voltage using a first end, and each has at least one tap counted from the first end. The amplifier stage has a first amplifier wing with at least a first power transistor connected to at least one tap of the first winding, and a second amplifier wing with at least a second power transistor connected to at least one tap of the second winding. The first amplifier wing amplifies the first half wave of the input signal, and the second amplifier wing amplifies the second half wave of the input signal. The secondary coil of the coupling transformer has multiple taps. The first end of the transmitter coil is connected to one of these taps, and the second end of the transmitter coil is selectively connected to another of these taps via a switch. This structure allows the transmitter coil to be matched to a wider range of amplifier stages. The transmitter coil, connected to the tuning capacitor, forms a tunable resonant circuit. The first end of the transmitter coil can be selectively connected to one side of one of multiple tuning capacitors via a switch, and the second end of the transmitter coil is directly connected to the other side of the tuning capacitor via multiple turns of the secondary winding. The connection of the transmitter coil to the secondary coil taps and tuning capacitors is established by an appropriate power relay, which can be selected from, for example, Power Relays, Catalogue 2020, Panasonic Industry.

[0010] A problem with such metal detection devices is that it takes a significant amount of time to change the system from a first tuned state to a second tuned state, making it nearly impossible to change the operating frequency back and forth and maintain optimum tuning while detecting contaminants in conveyed products.

[0011]

[0011] US20150234075A1 discloses a method for compensating for imbalances in a coil system and suppressing the effects of vibration and noise. A metal detector is calibrated to suppress signals caused by ferrite that resemble signals caused by noise. Then, by removing the signals caused by the ferrite, signals caused by vibration and noise are also automatically suppressed. According to this method, the output signal of the metal detector is measured with the ferrite present in the coil system and digitally adjusted to remove the resistive signal component of the ferrite. While the described method for compensating for imbalances and suppressing vibration and noise is highly effective, random disturbances caused by the transmitter unit itself can still be a source of problems. Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention is therefore based on the object of providing an improved metal detecting device that can be tuned to multiple operating frequencies.

[0013]

[0013] The metal detection device must be able to be tuned to different operating frequencies in the shortest possible time.

[0014] Furthermore, the metal detection device must be redesigned in a way that can reduce or avoid random disturbances introduced by the transmitter unit, which are difficult to handle with electronic compensation systems, and also reduce heat losses that can cause imbalances. [Means for solving the problem]

[0015]

[0015] In a first broad aspect of the present invention there is provided a metal detecting apparatus comprising a balanced coil system having a transmitter coil connected to a transmitter unit using first and second coil terminals, and first and second receiver coils connected to an input of a receiver unit, the output of which is connected to a signal processing unit; the transmitter unit comprising a controllable frequency generator which provides an input signal having a selectable operating frequency to an input of an amplifier stage, the output of the amplifier stage being connected to an adaptation unit; the adaptation unit comprising a coupling transformer having at least one primary coil, and at least one secondary coil having transformer coil terminals and a plurality of transformer tappings, a plurality of tuning capacitors having first and second capacitor terminals, and a switching device which can be used to connect at least one of the secondary coil of the coupling transformer and the plurality of tuning capacitors to the transmitter coil.

[0016]

[0016] In accordance with the present invention, a plurality of individually selectable resonator channels, preferably two, three or more, are provided, each of which, when selected, comprises a transmitter coil, a tapped winding of the secondary coil of a coupling transformer, and one of a plurality of tuning capacitors connected to each other by a first semiconductor switching device and by a second semiconductor switching device, which can be controlled by a channel selector so that one of the plurality of resonator channels is always selectable.

[0017]

[0017] The individually selectable resonator channels are arranged in parallel so that they do not interact with each other. Therefore, the selected resonator channel is not interfered with by the circuitry of the other selectable resonator channels. Therefore, the accurate tuning of the selected resonator channel is not compromised by the remaining circuitry.

[0018] In a preferred embodiment, for each resonator channel, when selected, an associated tapped winding of the secondary coil of the coupling transformer is connected in series or parallel to the transmitter coil by an associated first semiconductor switching device. The transmitter coil is switchably or rigidly connected to the transformer coil terminal using one of its multiple coil terminals. In general, the transformer winding between the transformer coil terminal and the selected transformer tapping is defined as the "tapped winding." For each resonator channel, when selected, an associated tuning capacitor is preferably connected in series or parallel to the transmitter coil by an associated second semiconductor switching device. By connecting the tapped winding and tuning capacitor in series or parallel, resonator channels with different characteristics can be created.

[0019]

[0019] For each resonator channel, when selected, it is most preferred that the associated tapped winding of the secondary coil of the coupling transformer and the associated tuning capacitor are connected in parallel to the transmitter coil by associated first and second semiconductor switching devices, respectively.

[0020]

[0020] The semiconductor switching device allows switching within a few nanoseconds, so that the metal detection device according to the present invention can not only switch operating frequencies but also maintain optimal tuning for each selected operating frequency when switched back and forth in short time intervals. Therefore, the metal detection device according to the present invention can be provided with an operating program that can switch and tune within short time intervals, virtually without interrupting the measurement process. Therefore, the operating program can be designed so that changes in operating frequency are implemented while measurements of the target to detect contaminants are in progress. By changing the operating frequency, signals can be detected from various types of contaminants and products. Maintaining optimal tuning allows signals to be measured with the highest signal-to-noise ratio.

[0021]

[0021] The semiconductor switching device, which preferably comprises at least one MOS-FET, can switch between intervals of approximately 20 ns and has a low channel resistance, typically less than 0.010 ohms, thus avoiding contact warming and corresponding losses due to unwanted shock to the circuitry when encountering the power relay. Furthermore, a suitable semiconductor switching device can block voltages up to 150 V and support currents up to 10 A or more.

[0022]

[0022] However, power relays are considered more robust against high voltage oscillations, and therefore such power relays are integrated into the transmitter circuitry. The transmitter according to the present invention is therefore designed accordingly. In a preferred embodiment, the channel selector comprises at least one isolated driver, such as a photovoltaic driver, for each of the first and second semiconductor switching devices, the input and output lines of the drivers being galvanically isolated from each other. The control circuitry is therefore isolated from the power stage.

[0023]

[0023] The first and second semiconductor switching devices are preferably bidirectional MOS-FET units. Each bidirectional MOS-FET unit preferably comprises a first MOS-FET and a second MOS-FET, each having a source terminal, a drain terminal, and a gate terminal. The two matched MOS-FETs preferably have source terminals connected to each other on the one hand and to identical first terminals of two diodes on the other hand, and the two diodes are each connected using their second terminals to the drain terminal of the first MOS-FET or the drain terminal of the second MOS-FET, respectively.

[0024]

[0024] Two MOS-FETs connected in series and facing in opposite directions have four possible states: on-on, on-off, off-on, and off-off. To activate a resonator channel, both of the two MOS-FETs of the associated first and second semiconductor switching devices are switched on, and to deactivate a resonator channel, both of the two MOS-FETs of the associated first and second semiconductor switching devices are switched off. When switched on, current can flow in both directions through the circuit. In one direction, current flows through the first MOS-FET and the associated first diode, and in the other direction, current flows through the second MOS-FET and the associated second diode. When switched off, current cannot flow in either direction because, with both FETs off, the two diodes block current flow in either direction.

[0025]

[0025] The input lines of the isolated drivers, each assigned to a first and second semiconductor switching device of an associated selectable resonator channel connected to a light-emitting diode, are preferably connected in series. The output lines of the isolated drivers are connected to the input terminals of the first and second semiconductor switching devices of the associated selectable resonator channel, respectively. A control voltage is applied to the interconnected gate terminals and, typically, the source terminals of the MOSFETs. Thus, the isolated drivers and the associated first and second semiconductor switching devices, together with the integrated MOSFET, are always switched simultaneously, without delay or time shift.

[0026] In a particularly preferred embodiment, the light-emitting diodes of the isolated driver are individually or collectively connected in series with a control resistor. Thus, one or more light-emitting diodes connected in series with the control resistor form a control loop connected to the output of a constant voltage supply device, preferably a low-dropout regulator. This circuit arrangement has the additional advantage of stabilizing the temperature of the MOSFET. The voltage across the light-emitting diode and the control resistor of the isolated driver is fixed. As the temperature increases, the voltage across the light-emitting diode drops proportionally, and the voltage across the control resistor and the current through it increase. The light-emitting diode emits more light toward the photosensitive diode in the isolated driver as the current increases. Therefore, as the control voltage applied to the MOSFET increases, the channel resistance of the MOSFET, which increases with temperature, decreases again. Thus, the channel resistance of the MOSFET remains constant during temperature changes.

[0028] Detailed aspects and examples of the present invention are described below with reference to the drawings. [Brief explanation of the drawings]

[0027] [Figure 1] 1 shows a preferred embodiment of a metal detection device according to the present invention, comprising a transmitter unit 1, a coil system 2, a receiver unit 3 and a control unit 4 with an operating program 40 and a signal processing unit 45, and further comprising an adaptation module 14 provided in the transmitter unit 1 with a plurality of selectable resonator channels 14A; 14B; 14C tuned to selected operating frequencies. [Figure 2] FIG. 1 shows a channel selector 140 comprising a portion of the adaptation unit 14 and a control circuitry for controlling a first semiconductor switching device 141A and a second semiconductor switching device 142A, wherein the channel selector 140 can be used to select a first resonator channel 14A. DETAILED DESCRIPTION OF THE INVENTION

[0028]

[0029] 1 shows a preferred embodiment of a metal detection apparatus according to the invention, comprising a transmitter unit 1, a balanced coil system 2 with a transmitter coil 21 and first and second receiver coils 22A, 22B, a receiver unit 3, and a control unit 4 with an operating program 40 implemented in a computing device, signal processing devices 45 such as a digital signal processor and input and output devices, by means of which the metal detection apparatus and the implemented measurement and calibration processes are monitored and controlled, for example by means of control signals 411, 414. A conveyor 6 is symbolically indicated, on which products can be transported through the balanced coil system 2.

[0029]

[0030] The transmitter unit 1 includes a controllable frequency generator 11 that provides an input signal r0° having a selectable operating frequency to the input of an amplifier stage 12, the output of which is connected to an adaptation unit 14. The adaptation unit 14 includes a plurality of tuning capacitors 144A, 144B, 144C and a coupling transformer 143. The coupling transformer 143 has at least one primary coil and at least one secondary coil with a transformer coil terminal 143T and a plurality of transformer tappings 143A, 143B, 143C. The output lines of the amplifier stage 12 are symmetrically connected to appropriate tappings M1, M1'; M2, M2'; M3, M3' of the primary coil of the coupling transformer 143. The configuration of the amplifier stage 12 can be designed to operate in any suitable mode, such as Class A or Class B. In this configuration, the amplifier operates in class B mode, with half the wave applied to tap M1 and the other half applied to the corresponding tap M1'.

[0030]

[0031] The coil terminal 143T is connected to a first terminal 211 of the transmitter coil 21, and the second terminal 212 of the transmitter coil 21 can be connected to one of the transformer tappings 143A, 143B, 143C. The winding of the secondary coil of the coupling transformer 143 between the transformer coil terminal 143T and the isolated transformer tappings 143A, 143B, 143C is defined as the "tapped winding."

[0031]

[0032] During operation of the metal detection system, a transmitter signal is applied to the transmitter coil 21 of the balanced coil system 2. Furthermore, the transmitter unit 1 provides an in-phase reference signal r0° and a quadrature reference signal r90° to the receiver unit 3. The transmitter signal induces signals in the identical receiver coils 22A and 22B that are of opposite polarity and equal magnitude as long as the system is balanced, i.e., as long as no product, specifically a product contaminated with metal, crosses the coil system 2. If a product, specifically a product contaminated with a conductive object, crosses the coil system 2, the magnitudes of the signals induced in the identical receiver coils 22A and 22B will change, and the oppositely polarized signals will no longer compensate for each other. Therefore, the transmitter signal induced in the receiver coils 22A and 22B is modulated with a baseband signal whose amplitude and frequency depend on the characteristics, dimensions, and speed of the conductive object.

[0032]

[0033] The receiver coil output signal is applied to a matching unit 31, which may comprise, for example, a balanced transformer with a center-tapped primary winding that mirrors the receiver coil and two identical center-tapped secondary windings, each with its opposite tail connected to an amplifier 32. The output of the amplifier 32 is connected to a demodulation unit 33, which provides at its output the in-phase and quadrature components of the demodulated receiver signal, i.e., the in-phase and quadrature components of the baseband signal originating from the conveyed product. The in-phase and quadrature components of the baseband signal provided at the output of the demodulation unit 33 are converted from analog to digital form by analog-to-digital converters 34-I and 34-Q. The output signals of the analog-to-digital converters 34-I and 34-Q are forwarded to a signal processing unit 45, such as a known digital signal processor, provided in the control unit 4. The signal processing unit 45 suppresses the signal components originating from the product and processes the signal components originating from the contaminants. The receiver unit may further comprise a filter unit and a gain adjustment unit.

[0033]

[0034] The measurement process is controlled by an operating program provided in the control unit 4. Using the control signal 411, the operating frequency is selected according to the planned measurement process. Using the control signal 414, the adaptation unit is configured according to the selected operating frequency.

[0034]

[0035] The adaptation unit 14 includes first semiconductor switching devices 141A, 141B, 141C and second semiconductor switching devices 142A, 142B, 142C that can be controlled by a channel selector 140 to select one of the resonator channels 14A, 14B, 14C at any time. The channel selector 140, which receives a command 414 from the control unit 4, is designed to activate the first semiconductor switching devices 141A, 141B, 141C and second semiconductor switching devices 142A, 142B, 142C of only one of the resonator channels 14A, 14B, 14C. For example, the top resonator channel 14A is activated by activating the first semiconductor switching device 141A and the second semiconductor switching device 142A, while the remaining first semiconductor switching devices 141B, 141C and second semiconductor switching devices 142B, 142C are switched off. When the first semiconductor switching device 141B and the second semiconductor switching device 142B of the second resonator channel 14B are activated, the first semiconductor switching devices 141A, 141C and the second semiconductor switching devices 142A, 142C are switched off. When the first semiconductor switching device 141C and the second semiconductor switching device 142C of the third resonator channel 14C are activated, the first semiconductor switching devices 141A, 141B and the second semiconductor switching devices 142A, 142B are switched off. Thus, in the illustrated embodiment, the metal detecting apparatus comprises three resonator channels 14A; 14B; 14C, each comprising a pair of first and second semiconductor switching devices 141A, 142A; 141B, 142B; 141C, 142C, respectively. In other embodiments, the metal detecting device may comprise two, four or more resonator channels 14A, 14B, 14C, . . . instead of three resonator channels 14A, 14B, 14C.

[0035]

[0036] When selected, an individual resonator channel of the plurality of resonator channels 14A, 14B, 14C comprises a transmitter coil 21, one of tuning capacitors 144A, 144B, 144C, and a tapped winding of the secondary coil of the coupling transformer 143. In this embodiment, the associated tuning capacitor 144A; 144B or 144C and the tapped winding of the secondary coil of the coupling transformer 143 are connected in parallel to the transmitter coil 21 when the associated pair of first and second semiconductor switching devices 141A, 142A; 141B, 142B; 141C, 142C are activated.

[0036]

[0037] A first coil terminal 211 of the transmitter coil 21 is fixedly connected to the coil terminal 143T of the secondary winding of the coupling transformer 143 and to the first terminals of the tuning capacitors 144A, 144B, and 144C. A second coil terminal 212 of the transmitter coil 21 is fixedly connected to the common first terminals of each pair of first and second semiconductor switching devices 141A, 142A; 141B, 142B; 141C, 142C. The second terminal 212 of the transmitter coil 21 can be connected to one of the transformer taps 143A, 143B, and 143C by the first semiconductor switching devices 141A, 141B, and 141C. Thus, the tapped winding of the secondary coil of the coupling transformer 143 between the selected tapping 143A, 143B, 143C and the coil terminal 143T can be connected in parallel to the transmitter coil 21. The second terminal 212 of the transmitter coil 21 can be connected to the second terminals of the tuning capacitors 144A, 144B, 144C by the second semiconductor switching devices 142A, 142B, 142C. Thus, the tuning capacitors 144A, 144B, 144C can be connected in parallel to the transmitter coil 21. However, the channel selector 140 is preferably designed so that only one pair of the first and second semiconductor switching devices 141A, 142A; 141B, 142B; 141C, 142C is activated at a time. A control signal 414 applied from the control unit 4 to the channel selector 140 can be used to select one of the resonator channels 14A, 14B, 14C to tune the balanced coil system 2 to an operating frequency selected by the control unit 4.

[0037]

[0038] It is therefore also preferred that the associated resonator channel 14A, 14B or 14C is automatically selected using a respective change in the operating frequency selected using the control signal 411. Thus, not only the change in operating frequency but also the tuning of the balanced coil system 2 can be performed within the shortest possible time, typically within a fraction of a microsecond, depending on the switching speed of the first and second semiconductor switching devices 141A, 142A; 141B, 142B; 141C, 142C. Thus, during the measurement process, it is possible to change the operating frequency and tuning of the balanced coil system back and forth in short time intervals.

[0038]

[0039] The resonator channels 14A, 14B, and 14C are independently selected and deselected, so that the selected resonator channel 14A, 14B, or 14C is not corrupted by the components of the deselected resonator channel 14A, 14B, or 14C. The resonator channels 14A, 14B, and 14C are effectively arranged in parallel and share only the transmitter coil 21 and a portion of the secondary coil of the coupling transformer 143. This design allows for reliable suppression of interference of the activated resonator channel 14A, 14B, or 14C by the components of the deactivated resonator channel 14A, 14B; 14A, 14C; or 14B, 14C.

[0039]

[0040] The first and second semiconductor switching devices 141A, 142A; 141B, 142B; 141C, 142C are preferably MOS-FET circuits, which, together with the channel selector 40, are described in more detail below with reference to FIG. 2.

[0040]

[0041] 2 illustrates a portion of the channel selector 140 provided within the adaptation module 14, as well as a pair of first and second semiconductor switching devices 141A, 142A of the first resonator channel 14A. The first and second semiconductor switching devices 141A, 142A are preferably provided in the form of bidirectional MOSFET units. Each of the bidirectional MOSFET units 141A, 142A; 141B, 142B; 141C, 142C includes a first MOSFET T1 and a second MOSFET T2, each having a source terminal S, a drain terminal D, and a gate terminal G. The source terminals S of the MOSFETs T1 and T2 are connected to one another on the one hand and to identical first terminals, i.e. anodes, of two diodes D1, D2 on the other hand, which are connected using their second terminals, i.e. cathodes, respectively, to the drain terminal D of the first MOSFET T1 or the second MOSFET T2. The two MOSFETs T1 and T2, connected in series and facing oppositely to one another, have four possible states: on-on, on-off, off-on and off-off, but are always switched on and off in pairs. To activate the first resonator channel 14A, both MOSFETs T1 and T2 of the associated first and second semiconductor switching devices 141A, 142A; 141B, 142B; 141C, 142C are switched on, and to deactivate the first resonator channel 14A, both MOSFETs T1 and T2 of the associated first and second semiconductor switching devices 141A, 142A; 141B, 142B; 141C, 142C are switched off. When switched on, current can flow in both directions through the MOSFET circuit. In one direction, current flows, for example, through the first MOSFET T1 and the associated first diode D1, and in the other direction, current flows through the second MOSFET T2 and the associated second diode D2.When switched off, current cannot flow in either direction because with both MOSFETs T1 and T2 off, the two diodes D1 and D2, with their cathodes facing opposite directions, block the flow of current in either direction. The two MOSFETs T1 and T2 and the two diodes D1 and D2 are matched to each other so that the required current, typically in the range of 10 A or more, can flow in both directions.

[0041]

[0042] The channel selector 140 includes an identical channel selector module for each of the selectable resonator channels 14A, 14B, 14C. The illustrated channel selector module 140A includes isolated drivers 145A, 146A for the first and second semiconductor switching devices 141A, 142A, which are used to galvanically isolate the control lines of the channel selector 140 from the control inputs of the semiconductor switching devices 141A, 142A. The isolated drivers 145A, 146A include light-emitting diodes 1453A, 1463A and photosensitive diodes 1454A, 1464A, and are preferably optically isolated MOS-FET drivers with internal or external turn-off circuits 1455A, 1465A used to shorten turn-off times and thereby increase overall switching speed. The current required to drive the internal circuitry is preferably drawn from the low voltage primary side of the isolation barrier, i.e., the LED current on input lines 1451A, 1461A of isolated drivers 145A, 146A. Output lines 1452A, 1462A on the secondary side of the isolation barrier of isolated drivers 145A, 146A are connected to input terminals G, S of associated semiconductor switching devices 141A, 142A.

[0042]

[0043] The first input terminal G is connected to the gate terminals of the two MOSFETs T1 and T2. The second input terminal S is connected to the source terminals of the two MOSFETs T1 and T2. Therefore, the MOSFETs T1 and T2 of each semiconductor switching device 141A; 142A are both switched on and off simultaneously.

[0043]

[0044] The light emitting diodes 1453A, 1463A of the isolated drivers 145A, 146A assigned to the first semiconductor switching device 141A and the second semiconductor switching device 142A are connected in series so that they always carry the same current and always switch on and off at the same time.

[0044]

[0045] The light-emitting diodes 1453A and 1463A of the isolated drivers 145A and 146A are further connected in series with a control resistor 147A, thus forming a control loop 148A connected to the output of a constant voltage supply device 149A. This circuit arrangement stabilizes the temperatures of the MOSFETs T1 and T2 of the semiconductor switching devices 141A and 142A. The voltages across the light-emitting diodes 1453A and 1463A and the control resistor 147A of the isolated drivers 145A and 146A are fixed by the constant voltage supply device 149A. As the temperature increases, the voltages across the light-emitting diodes 1453A and 1463A decrease, and the voltage across and current through the control resistor 147A increase. As the current through the light-emitting diodes 1453A, 1463A increases, they emit more light toward the photosensitive diodes 1455A, 1465A of the isolated drivers 145A, 146A. Therefore, when the control voltages applied to the MOSFETs T1 and T2 of the semiconductor switching devices 141A, 142A increase accordingly, the channel resistances of the MOSFETs T1 and T2, which increased with temperature, decrease again. Therefore, the channel resistances of the MOSFETs T1 and T2 remain constant during temperature changes. Therefore, temperature changes do not affect the flowing current and the activated resonator channels 14A, 14B, 14C. Because the channel resistances of the MOSFETs T1 and T2 are extremely low when switched on, the temperature losses and adverse effects of this circuitry in the metal detection device are low, thereby reducing calibration requirements.

[0045]

[0046] The constant voltage supply device 149A has an input for a supply voltage VCC and a control input EN to which a control signal 414 is applied, which varies, for example, between the potential of the supply voltage VCC and ground potential. When ground potential is applied, current can flow through the control loop 148A to ground or to the ground potential applied to the control input EN. The voltage applied to the control loop 148 by the constant voltage supply device 149A is maintained constant.

[0046]

[0047] The constant voltage supply device 149A is preferably a low dropout regulator that is able to regulate this output voltage even when the supply voltage VCC is very close to the output voltage supplied to the control loop 148A. In Figure 2, a small diagram shows that at time t1, the control input EN is set to ground potential, so that current is flowing through the control loop 148A, i.e., the control resistor 147A and the light-emitting diodes 1453A, 1463A. Therefore, both semiconductor switching devices 141A; 142A, and therefore the first resonator channel 14A, are correctly activated at time t1. [Explanation of symbols]

[0047] 1 transmitter unit 11 Controllable Frequency Generator 12 Amplifier Stage 14 Compatible units 14A, 14B, 14C selectable resonator channels 140 Channel Selector 140A Channel selector module for resonator channel 14A 141A, 141B, 141C First semiconductor switching device 142A, 142B, 142C Second semiconductor switching device 143 Coupling Transformer 143A, 143B, 143C Transformer Tapping of Secondary Coils 143T Secondary coil terminal 144A, 144B, 144C tuning capacitors 145A, 146 Isolated driver of resonator channel 14A 1451A, 1461A Isolated driver 145A, 146A input line 1452A, 1462A Isolated driver 145A, 146A input line 1453A, 1463A Isolated driver 145A, 146A light emitting diodes 1454A, 1464A Isolated Driver 145A, 146A Light Sensitive Diode 1455A, 1465A Turn-off circuitry for isolated drivers 145A, 146A 147A control resistor 148A Control Loop 149A Voltage Supply Device / Low Dropout Regulator 2 Coil System 21 Transmitter coil 211, 212 Transmitter coil terminals 22A, 22B receiver coils 3 Receiver Unit 31 Matching Unit 32 Amplifier 33 Phase Detector 34I, 34Q analog-to-digital converters 4. Control Unit 40 Operation Program 411 Control signal applied to frequency generator 414 Control signals applied to the adapter module 45 Signal Processing Unit / DSP 6 Conveyor T1, T2 MOS-FET semiconductor switching device D1, D2 Semiconductor switching device diode

Claims

1. A metal detection device, comprising: A balanced coil system (2), comprising: a transmitter coil (21) connected to the transmitter unit (1) using a first coil terminal (211) and a second coil terminal (212); a first receiver coil and a second receiver coil (22A, 22B) connected to an input of a receiver unit (3), the output of which is connected to a signal processing unit (45); a balanced coil system (2) having The transmitter unit (1) a controllable frequency generator (11) for providing an input signal having a selectable operating frequency to an input of an amplifier stage (12), the output of which is connected to an adaptation unit (14); The adaptation unit (14) a coupling transformer (143) having at least one primary coil and at least one secondary coil with transformer coil terminals (143T) and a plurality of transformer tappings (143A, 143B, 143C); a plurality of tuning capacitors (144A, 144B, 144C) having first and second capacitor terminals; and switching devices (141A, 142A; 141B, 142B; 141C, 142C) capable of connecting at least one of the secondary coil of the coupling transformer (143) and the tuning capacitors (144A, 144B, 144C) to the transmitter coil (21), the metal detecting device is provided with a plurality of individually selectable resonator channels (14A, 14B, 14C), each of which, when selected, comprises the transmitter coil (21), a tapped winding of the secondary coil of the coupling transformer (143), and one of the tuning capacitors (144A, 144B, 144C) connected together by a first semiconductor switching device (141A; 141B; 141C) and by a second semiconductor switching device (142A; 142B; 142C), which are controllable by a channel selector (140) so that one of the plurality of resonator channels (14A; 14B; 14C) is always selectable; the channel selector (140) comprises at least one isolated driver (145A, 146A), such as a photovoltaic driver, for each of the first semiconductor switching devices (141A; 141B; 141C) and the second semiconductor switching devices (142A; 142B; 142C), and the control lines are galvanically isolated from the control inputs of the first semiconductor switching devices (141A; 141B; 141C) and the second semiconductor switching devices (142A; 142B; 142C) using the at least one isolated driver (145A, 146A).

2. 2. The metal detecting apparatus of claim 1, wherein for each resonator channel (14A; 14B; 14C), when selected, an associated tapped winding of the secondary coil of the coupling transformer (143) is connected in series or in parallel to the transmitter coil (21) by an associated first semiconductor switching device (141A; 141B; 141C).

3. 3. The metal detecting apparatus according to claim 1, wherein for each resonator channel (14A; 14B; 14C), when selected, an associated tuning capacitor (144A; 144B; 144C) is connected in series or in parallel to the transmitter coil (21) by an associated second semiconductor switching device (142A; 142B; 142C).

4. 2. The metal detecting apparatus of claim 1, wherein, for each resonator channel (14A; 14B; 14C), when selected, an associated tapped winding of the secondary coil of the coupling transformer (143) is connected in parallel to the transmitter coil (21) by an associated first semiconductor switching device (141A; 141B; 141C) and an associated tuning capacitor (144A; 144B; 144C) is connected in parallel to the transmitter coil (21) by an associated second semiconductor switching device (142A; 142B; 142C).

5. 5. A metal detection device according to claim 1, wherein the plurality of individually selectable resonator channels (14A; 14B; 14C) are arranged in parallel.

6. 6. The metal detecting apparatus according to claim 1, wherein the first semiconductor switching device (141A; 141B; 141C) and the second semiconductor switching device (142A; 142B; 142C) are bidirectional MOS-FET units.

7. 7. The metal detecting device according to claim 6, wherein each bidirectional MOS-FET unit comprises a first MOS-FET (T1) and a second MOS-FET (T2) each having a source terminal (S), a drain terminal (D) and a gate terminal (G), the source terminals (S) being connected to each other on the one hand and to identical first terminals of two diodes (D1, D2) on the other hand, the two diodes being connected using their second terminals to the drain terminal (D) of the first MOS-FET (T1) or the drain terminal (D) of the second MOS-FET (T2), respectively.

8. 8. The metal detection apparatus according to claim 6 or 7, characterized in that the input lines (1451A, 1461A) of isolated drivers (145A, 146A) assigned to the first semiconductor switching device (141A; 141B; 141C) and the second semiconductor switching device (142A; 142B; 142C) of the associated selectable resonator channel (14A; 14B; 14C), respectively connected to a light emitting diode (1453A, 1463A), are connected in series, and the output lines (1452A, 1462A) of the isolated drivers (145A, 146A) are connected to the input terminals (G, S) of the first semiconductor switching device (141A; 141B; 141C) and the second semiconductor switching device (142A; 142B; 142C) of the associated selectable resonator channel (14A; 14B; 14C), respectively.

9. 9. The metal detecting device of claim 8, wherein the light emitting diodes (1453A, 1463A) of the isolated drivers (145A, 146A) are connected in series with a controlled resistor (147A) and thus form a control loop (148A) connected to the output of a constant voltage supply device (149A).

10. 10. The metal detecting apparatus of claim 9, wherein the constant voltage supply device (149A) is a low dropout regulator.

11. 11. The metal detection device according to claim 1, wherein the transmitter (1), in particular the frequency generator (11) and the channel selector (140), can be controlled by a control program (40) implemented in a computer system (4), and the control program (40) can be used to select the operating frequency and the associated resonator channel (14A, 14B, 14C).

12. 12. The metal detecting device of claim 11, wherein the control program (40) is designed such that, within the process of measuring the conveyed goods, the operating frequency and the associated resonator channels (14A, 14B, 14C) are alternately changed or adjusted between at least two settings or operating frequencies.

13. 13. Metal detection device according to one of the preceding claims, characterized in that the adaptation unit (14) comprises at least three selectable resonator channels (14A, 14B, 14C).

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