Metal detection apparatus

Abstract

To provide a metal detection apparatus configured to: reduce or avoid random disturbances that are caused by a transmitter unit and difficult to handle by an electronic compensation system; and reduce thermal losses that may cause imbalances.SOLUTION: According to the invention, multiple individually selectable excitation circuits are provided. When selected, each excitation circuit comprises a transmitter coil (21), tapped windings of an output winding of a coupling transformer (143), one of tuning capacitors (144A, 144B, 144C) connected to one another by a first semiconductor switching device (141A; 141B; 141C) designed as an electro-mechanical relay and by a second switching device (142A; 142B; 142C), which are controllable by a channel selector (140) such that one of the excitation circuits is always selectable.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001]

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

[0002]

[0002] Industrial metal detection devices, such as those described in US8587301B2, are used to detect metal contamination in products. When properly installed and operated, industrial metal detection devices will 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 comprise three coils, namely a transmitter coil and two identical receiver coils, typically parallel to each other, wound on a non-metallic frame. These receiver coils are identical, surrounding the transmitter coil, typically centrally located between them, and identical voltages are induced in each of them. The first receiver coil is connected in series with the second receiver coil, which has oppositely wound windings, to receive an output signal that is zero if the system is in a balanced state. Thus, when the system is in a balanced state 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 create a secondary magnetic field that first disturbs the primary electromagnetic field near one receiver coil and then 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 nano-volts). This change in balance allows the signal at the output of the detection coil present in the receiver unit to be processed, amplified, possibly filtered, and then used to detect the presence of metal contaminants in the observed product as it traverses the metal detection device on the conveyor system.

[0005]

[0005] At the receiver unit, the input signal is usually split into an in-phase component and a quadrature component. The vector constructed from these components has a certain magnitude and a certain phase angle that is specific to the products and contaminants carried 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, allowing greater sensitivity for 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 signals. Materials with high electrical conductivity result in signals with a larger negative reactive signal component and a smaller resistive signal component. Materials with high magnetic permeability result in signals with a smaller resistive signal component and a larger positive reactive signal component. The signal produced by ferrite is predominantly reactive, while the signal produced by stainless steel is predominantly resistive. Products that are conductive typically result in 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 conveyed through the metal detection device.

[0007]

[0007] By distinguishing between the phases of the signal components of different sources by means of a phase detector, information about the product and the pollutant can be obtained. A phase detector, for example 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 by a transmitter unit to the receiver unit. Thus, by selecting the phase of the reference signal to coincide with the phase of the product signal component, a phase difference and a corresponding product signal are obtained at the output of the phase detector, which is zero. If the phase of the signal emitted by the pollutant is different from that of the product signal, the product signal can be suppressed while the pollutant signal can be further processed. However, if the phase of the pollutant signal is close to that of the product signal, the detection of the pollutant fails because the pollutant signal is suppressed together with the product signal. In order to separate the phase angle of the product signal from that of the pollutant, an appropriate operating frequency is determined and applied.

[0008]

[0008] US8841903B2 discloses a metal detection device with a transmitter unit providing 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 providing an operating frequency to an 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 the selectable capacitor to be tuned to a selected operating frequency, independent of the rest of the transmitter unit. The amplifier stage includes a class A circuit that amplifies the signal with minimal distortion, but has 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 tapping at the same number of times counted from the first end. The amplifier stage has a first amplification wing having at least a first power transistor connected to at least one tapping of the first winding, and a second amplification wing having at least a second power transistor connected to at least one tapping of the second winding. The first amplification wing amplifies the first half wave of the input signal, and the second amplification wing amplifies the second half wave of the input signal. The secondary coil of the coupling transformer has multiple tappings. The first end of the transmitter coil is connected to one of these tappings, and the second end of the transmitter coil is selectively connected to another of these tappings via a switch. With this structure, the transmitter coil can be adapted to the amplifier stage over a wider range. The transmitter coil connected to the tuning capacitor forms a resonant circuit that can be tuned. The first end of the transmitter coil can be selectively connected to one side of one of the 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.

[0010] No. 4,138,654A discloses the use of a relay switch to selectively couple a tuning capacitor in a preselector circuit of a radio receiver, thereby replacing tuning with a slow mechanical servo drive system.

[0011] A problem with such metal detection systems is that it takes a significant amount of time to change the system from a first tuned state to a second tuned state, and therefore it is nearly impossible to change the operating frequency back and forth and maintain optimum tuning while detecting contaminants in the transported product.

[0012]

[0012] US20150234075A1 discloses a method for compensating for imbalances in a coil system and suppressing the effects of vibrations and noise. A metal detector is calibrated to suppress signals caused by ferrite that resemble signals caused by noise. Then, signals caused by vibrations and noise are also automatically suppressed by removing the signals caused by ferrite. According to this method, the output signal of the metal detector is measured with the presence of ferrite in the coil system and digitally adjusted to remove the resistive signal component of the ferrite. Although the described method for compensating for imbalances and suppressing vibrations and noise is highly effective, random disturbances caused by the transmitter unit itself can still be a source of problems.

[0013]

[0013] EP 4033272 A1 relates to a metal detection apparatus comprising a balanced coil system including a transmitter coil, a first receiver coil and a second receiver coil, a coupling transformer including an input winding energized by an input signal having a selected operating frequency, and an output winding including a plurality of tappings, each pair of tappings selectable from among the plurality of tappings defining an associated partial winding consisting of turns of the output winding extending between the tappings of the selected pair, and a first plurality of individually selectable excitation circuits, each of the plurality of excitation circuits extracting a current from an associated one of the plurality of partial windings to the transformer. a first plurality of individually selectable excitation circuits configured to supply energy to the transmitter coil and including an associated first switching device connected to open or close an associated excitation circuit in response to an associated first control signal, each of the plurality of first switching devices being a semiconductor switching device; and a second plurality of individually selectable tuning capacitors, each of the plurality of tuning capacitors having an associated second switching device connected to open or close a resonant circuit including the transmitter coil and the associated tuning capacitor in response to an associated second control signal.

[0014]

[0014] In this prior art metal detection device, which can be tuned to multiple operating frequencies, the first and second switching devices used for tuning are both designed as semiconductor switching devices. The inventors have recognized, within the context of their research, that the tunability of this prior art device is not optimal for a particular operating frequency and / or for a particular circumference of the opening of the metal detection device through which the observed product traverses on the conveyor system. Summary of the Invention [Problem to be solved by the invention]

[0015]

[0015] The present invention is therefore based on the objective of optimizing the tunability of a metal detection device of the type referred to above. Furthermore, the metal detection device must be capable of being redesigned in such a way that it is possible to reduce or avoid the random disturbances brought about by the transmitter unit, which are difficult to handle with electronic compensation systems. Furthermore, it must be possible to reduce the heat losses that may be the cause of imbalance. [Means for solving the problem]

[0016] This problem is solved according to the present invention in that each of the plurality of second switching devices is an electromechanical relay switching device.

[0017]

[0017] The use of electromechanical relay switching devices, particularly subminiature high current electromechanical relay switches switching tuning capacitors, overcomes the problems discussed above. These problems are attributed to the channel capacitance and drain-source on-resistance of the semiconductor switching devices used in prior art devices as the second switching devices. On the other hand, the first switching devices, which are semiconductor switching devices, do not have the corresponding problems due to their lower drain-source voltage.

[0018]

[0018] The electromechanical relay switching device is preferably configured as a double pole with the contacts connected in parallel, which reduces vibration sensitivity.

[0019]

[0019] In another preferred embodiment, the number of partial windings is greater than the number of excitation circuits, and a multi-way tap selector is arranged between the tappings and the excitation circuits to establish an individual connection between the excitation circuits and selected partial windings of the plurality of partial windings. The tap selector allows the selection of a number of partial windings available according to the total number of tappings from the total number of partial windings, the number of selected partial windings corresponding to the number of available excitation circuits. By doing so, a consistent high detector sensitivity can be achieved in all frequency modes.

[0020]

[0020] In accordance with the present invention, there is provided a plurality of individually selectable excitation circuits, preferably two, three or more, each of which, when selected, comprises a transmitter coil, one of a plurality of partial windings of the output winding of the coupling transformer, and one of a plurality of tuning capacitors connected to each other by a first semiconductor switching device and by a second switching device, which are controllable by a channel selector such that one of the plurality of excitation circuits is always selectable.

[0021]

[0021] The individually selectable excitation circuits are arranged in parallel so that they do not interact with each other. Thus, the selected excitation circuit is not interfered with by the circuitry of the other selectable excitation circuits. Thus, the correct tuning of the selected excitation circuit is not compromised by the remaining circuitry.

[0022]

[0022] In a preferred embodiment, for each excitation circuit, when selected, an associated partial winding of the output winding 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, using one of its coil terminals, to one of a number of tappings, in particular to a terminal tapping of the transformer output winding. In general, a turn of the transformer output winding between a pair of selected transformer tappings is defined as a "partial winding". For each excitation circuit, when selected, an associated tuning capacitor is preferably connected in series or parallel to the transmitter coil by an associated second switching device. By connecting the partial windings and the tuning capacitors in series or parallel, excitation circuits with different characteristics can be created.

[0023]

[0023] For each excitation circuit, when selected, an associated partial winding of the output winding of the coupling transformer and an associated tuning capacitor are most preferably connected in parallel to the transmitter coil by associated first and second switching devices, respectively.

[0024]

[0024] The metal detection apparatus according to the present invention is not only capable of switching between operating frequencies, but also maintains optimal tuning for each selected operating frequency when switched back and forth at short time intervals. Thus, the metal detection apparatus according to the present invention is able to detect a range of operating frequencies without, in effect, interfering with the measurement process. The operating program can be provided with an operating program that can be switched and tuned within short time intervals without being affected by the operating frequency. The operating program can therefore be designed such that a change in operating frequency is performed while a measurement of an object for detecting a contaminant is in progress. By changing the operating frequency, signals can be detected from different types of contaminants and products. By maintaining an optimal tuning, signals can be measured with the highest signal-to-noise ratio.

[0025]

[0025] The semiconductor switching device, preferably comprising at least one MOS-FET, can switch between intervals of approximately 20 ns and has a small channel resistance, typically less than 0.010 ohms, thus avoiding contact warming and corresponding losses due to undesirable shocks to the circuitry when encountering the power relay. Furthermore, a suitable semiconductor switching device can block voltages up to 150V and support currents up to 10A or more.

[0026]

[0026] However, power relays are considered to be more robust against high voltage oscillations, and therefore such power relays are integrated into the transmitter circuitry. The transmitter according to the 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 semiconductor switching devices, the input and output lines of the driver being galvanically isolated from each other. The control circuitry is therefore isolated from the power stage.

[0027]

[0027] The first semiconductor switching device is preferably a bidirectional MOS-FET unit. 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 comprise source terminals connected to each other on the one hand and to identical first terminals of two diodes on the other hand, the two diodes being connected using their second terminals to the drain terminal of the first MOS-FET or to the drain terminal of the second MOS-FET, respectively.

[0028]

[0028] 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 the excitation circuit, both of the two MOS-FETs of the associated first semiconductor switching device are switched on, and to deactivate the excitation circuit, both of the two MOS-FETs of the associated first semiconductor switching device are switched off. When switched on, current can flow in both directions through the circuit. In one direction, the current flows through the first MOS-FET and the associated first diode, and in the other direction, the 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 prevent current flow in either direction.

[0029] The output lines of the isolated drivers are respectively connected to the input terminals of the first semiconductor switching devices of the associated selectable excitation circuits. A control voltage is applied to the interconnected gate terminals and to the source terminals, typically of a MOS-FET.

[0030] In a particularly preferred embodiment, the light emitting diodes of the isolated drivers are individually or collectively connected in series with a controlled resistor. Thus, one or more light emitting diodes connected in series with a controlled resistor form a control loop that is connected to the output of a constant voltage supply device, preferably a low dropout regulator. This circuit arrangement has the further advantage that , the temperature of the MOS-FET can be stabilized. The voltage across the light-emitting diode and the control resistor of the isolated driver is fixed. An increase in temperature causes a proportional drop in the voltage across the light-emitting diode and an increase in the voltage across the control resistor and the current through the control resistor. The light-emitting diode emits more light towards the light-sensitive diode provided in the isolated driver as the current increases. Thus, when the control voltage applied to the MOS-FET increases, the channel resistance of the MOS-FET, which increased with temperature, decreases again. Thus, the channel resistance of the MOS-FET remains constant during temperature changes.

[0032] Detailed aspects and examples of the present invention are described below with reference to the drawings. [Brief description of the drawings]

[0031] [Figure 1] FIG. 1 shows essential parts of a preferred embodiment of a metal detection device according to the present invention. [Diagram 2] 2 is a schematic diagram illustrating the operating principle of a multi-way tap selector used in the metal detection device of FIG. 1; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032]

[0033] A preferred embodiment of the metal detection apparatus according to the present invention generally comprises a transmitter unit, a balanced coil system having a transmitter coil and a first receiver coil and a second receiver coil, a receiver unit and a control unit comprising an operating program implemented in a computing device, a signal processing device such as a digital signal processor and input devices and output devices, using the control unit the metal detection apparatus and the implemented measurement and calibration process are monitored and controlled, and products can be transported on a conveyor through the balanced coil system.

[0033]

[0034] The transmitter unit comprises a controllable frequency generator providing an input signal r0° having a selectable operating frequency to the input of the amplifier stage, the output of which is connected to an excitation unit 14 shown in FIG. 1. The excitation unit 14 comprises a number of tuning capacitors 144A, 144B, 144C and a coupling transformer 143 having at least one input winding and at least one output winding with tappings 143T at both ends of the winding and a number of intermediate tappings 143A, 143B, 143C between the ends. The output lines of the amplifier stage are symmetrically connected to the appropriate tappings of the primary winding of the coupling transformer 143. The configuration of the amplifier stage can be designed to operate in any suitable mode, such as class A mode or class B mode. In this configuration, the amplifier operates in class B mode, with half the wave applied to one of the tappings and the other half wave applied to the other of the tappings.

[0034]

[0035] The tapping 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 tappings 143A, 143B, 143C. The turns of the output winding of the coupling transformer 143 extending between any selected pair of tappings 143A, 143B, 143C, 143T are defined as "partial windings."

[0035]

[0036] During operation of the metal detection device, an excitation signal is applied to the transmitter coil 21 of the balanced coil system. In addition, the transmitter unit provides an in-phase reference signal r0° and a quadrature reference signal r90° to the receiver unit. The excitation signal induces a signal in one and the same receiver coil (not shown in FIG. 1) which is of opposite polarity and of the same magnitude as long as the system is balanced, i.e. as long as no product, in particular a product contaminated with metal, crosses the coil system. When a dyed product is moving across the coil system, the magnitude of the signal induced in the one and the same receiver coil will change, and the oppositely polarized signals will no longer compensate each other. The signal induced in the receiver coil is therefore modulated with a baseband signal whose amplitude and frequency depend on the properties, dimensions and speed of movement of the conductive object.

[0036]

[0037] The output signal of the receiver coil is applied to a matching unit, for example comprising a balanced transformer with a center-tapped primary winding that is a mirror image of the receiver coil, and two identical center-tapped secondary windings, the tails of which are connected to the amplifier. The output of the amplifier is connected to a demodulation unit, which provides at its output part 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 part of the demodulation unit are converted from analog form to digital form in an analog-to-digital converter. The output signal of the analog-to-digital converter is forwarded to a signal processing unit, such as a known digital signal processor, provided to the control unit. The signal processing unit suppresses the signal components originating from the product and processes the signal components originating from the pollutant. The receiver unit may further comprise a filter unit and a gain adjustment unit.

[0037]

[0038] The measurement process is controlled by an operating program provided in the control unit, so that an operating frequency is selected according to the planned measurement process and the excitation unit 14 is configured according to the selected operating frequency.

[0038]

[0039] The excitation unit 14 comprises first semiconductor switching devices 141A, 141B, 141C and second switching devices 142A, 142B, 142C that can be controlled by a channel selector 140 so that one of the excitation circuits 14A; 14B; 14C can always be selected. The channel selector 140, which receives a command from the control unit, is designed so that the first semiconductor switching devices 141A, 141B, 141C and the second switching devices 142A, 142B, 142C of only one of the excitation circuits 14A; 14B; 14C are activated. For example, the top excitation circuit 14A is activated by activating the first semiconductor switching device 141A and the second switching device 142A, while the remaining first semiconductor switching devices 141B, 141C and the second switching devices 142B, 142C are switched off. When the first semiconductor switching device 141B and the second switching device 142B of the second excitation circuit 14B are activated, the first semiconductor switching devices 141A, 141C and the second switching devices 142A, 142C are switched off. When the first semiconductor switching device 141C and the second switching device 142C of the third excitation circuit 14C are activated, the first semiconductor switching devices 141A, 141B and the second switching devices 142A, 142B are switched off. Thus, the metal detection device in the illustrated embodiment comprises three excitation circuits 14A; 14B; 14C, each of which comprises a pair of first and second switching devices 141A, 142A; 141B, 142B; 141C, 142C. In other embodiments, the metal detection device may comprise two, four or more excitation circuits 14A, 14B, 14C, . . . instead of the three excitation circuits 14A, 14B, 14C.

[0039]

[0040] When selected, each of the multiple excitation circuits 14A, 14B, 14C includes a transmitter coil 21, one of the tuning capacitors 144A, 144B, 144C, and one of multiple partial windings of the output winding of the coupling transformer 143. In this embodiment, the associated tuning capacitor 144A; 144B or 144C and the partial winding of the output winding of the coupling transformer 143 are coupled to the associated pair of first and second switching devices. When the switching devices 141A, 142A; 141B, 142B; 141C, 142C are activated, they are connected in parallel to the transmitter coil 21.

[0040]

[0041] A first coil terminal 211 of the transmitter coil 21 is fixedly connected to the tapping 143T of the output winding of the coupling transformer 143 and to the first terminals of the tuning capacitors 144A, 144B, 144C. A second coil terminal 212 of the transmitter coil 21 is fixedly connected to the common first terminals of the respective pairs of the first and second 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 tappings 143A, 143B, 143C by the first semiconductor switching devices 141A, 141B, 141C. Thus, a partial winding of the output winding of the coupling transformer 143 between the selected tapping 143A, 143B, 143C and the terminal tapping 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 switching devices 142A, 142B, 142C. The tuning capacitors 144A, 144B, 144C can thus be connected in parallel to the transmitter coil 21. However, the channel selector 140 is preferably designed such that only one pair of the first and second switching devices 141A, 142A; 141B, 142B; 141C, 142C is activated at a time. A control signal applied from the control unit to the channel selector 140 can be used to select one of the excitation circuits 14A, 14B, 14C in order to tune the balanced coil system to an operating frequency selected by the control unit.

[0041]

[0042] It is thus also preferred that the associated excitation circuit 14A, 14B or 14C is automatically selected using the respective change in the operating frequency selected using the control signal of the control unit. Thus, depending on the change in operating frequency as well as the switching speed of the first and second switching devices 141A, 142A; 141B, 142B; 141C, 142C, the tuning of the balanced coil system can also be performed within the shortest possible time. Thus, during the measurement process, it is possible to change the operating frequency and the tuning of the balanced coil system back and forth in short time intervals.

[0042]

[0043] The excitation circuits 14A, 14B, 14C are individually selected and deselected, so that the selected excitation circuits 14A, 14B or 14C are not impaired by the elements of the deselected excitation circuits 14A, 14B or 14C. The excitation circuits 14A, 14B, 14C are practically arranged in parallel and have in common only the transmitter coil 21 and a part of its output windings of the coupling transformer 143. This design makes it possible to reliably suppress interference of the activated excitation circuits 14A or 14B or 14C by the elements of the deactivated excitation circuits 14A, 14B; 14A, 14C; 14B, 14C.

[0043]

[0044] The first switching devices 141A, 141B, 141C are preferably MOS-FET circuits, which, along with the channel selector 140, are described in further detail below.

[0044]

[0045] The channel selector 140 is shown diagrammatically at the bottom of Fig. 1. The channel selector 140 with the excitation unit 14 comprises an identical channel selector module for each one of the selectable excitation circuits 14A, 14B, 14C. The first semiconductor switching device 141A is preferably provided in the form of a bidirectional MOS-FET unit. The individual bidirectional MOS-FET units 141A, 141B, 141C are , 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 of the MOS-FETs T1 and T2 are connected to each other on the one hand and to identical first terminals, i.e. anodes, of a first diode and a second diode (not shown in FIG. 1), on the other hand, which are connected using their second terminals, i.e. cathodes, to the drain terminal D of the first MOS-FET T1 or the second MOS-FET T2, respectively. The two MOS-FETs T1 and T2, connected in series and facing in opposite directions, 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 excitation circuit 14A, both MOS-FETs T1 and T2 of the associated first semiconductor switching device 141A, 141B, 141C are switched on, and to deactivate the first excitation circuit 14A, both associated two MOS-FETs T1 and T2 of the associated first semiconductor switching device 141A, 141B, 141C are switched off. When switched on, current can flow in both directions through the MOS-FET circuit. In one direction, current flows, for example, through the first MOS-FET T1 and through the associated first diode, and in the other direction through the second MOS-FET T2 and the associated second diode. When switched off, current cannot flow in either direction, since with both MOS-FETs T1 and T2 off, the two diodes with their cathodes facing in opposite directions prevent current flow in either direction. The two MOS-FETs T1 and T2 and the two diodes are matched to each other so that the required current, typically in the range of 10 A or more, can flow in both directions.

[0045]

[0046] Each channel selector module, for example module 140A, comprises isolated drivers for the first semiconductor switching device 141A, by means of which the control lines of the channel selector 140 are galvanically isolated from the control inputs of the semiconductor switching device 141A. The isolated drivers comprise light-emitting and photosensitive diodes and are preferably optically isolated MOS-FET drivers, each with an internal or external turn-off circuit used to shorten the turn-off time and thereby increase the 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 of the input line of the isolated driver. The output lines of the isolated drivers, provided on the secondary side of the isolation barrier, are connected to the input terminals G, S of the associated semiconductor switching device 141A. Furthermore, individual control signals are applied to the control input terminals C of each of the associated second switching devices 142A, 142B, 142C for switching these devices between the on and off states.

[0046]

[0047] The first input terminal G is connected to the gate terminals of the two MOS-FETs T1 and T2. The second input terminal S is connected to the source terminals of the two MOS-FETs T1 and T2. Thus, the MOS-FETs T1 and T2 of each semiconductor switching device 141A are both switched on and off simultaneously.

[0047]

[0048] The light-emitting diode of the isolated driver is further connected in series with the control resistor, thus forming a control loop connected to the output of the constant voltage supply device. This circuit arrangement allows the temperature of the MOS-FETs T1 and T2 of the semiconductor switching devices 141A, 141B, 141C to be stabilized. The voltage across the light-emitting diode and the control resistor of the isolated driver is fixed by the constant voltage supply device. When the temperature increases, the voltage across the light-emitting diode drops accordingly, and the voltage across the control resistor and the current through the control resistor increase accordingly. The light-emitting diode is connected to the output of the constant voltage supply device. When the current increases, the voltage across the light-emitting diode drops accordingly. The larger the resistance of the MOSFETs T1 and T2 of the semiconductor switching devices 141A, 141B, 141C, the more light they emit towards the light-sensitive diodes in the isolated drivers. Thus, when the control voltage applied to the MOSFETs T1 and T2 of the semiconductor switching devices 141A, 141B, 141C is correspondingly increased, the channel resistance of the MOSFETs T1 and T2, which increased with temperature, is reduced again. Thus, the channel resistance of the MOSFETs T1 and T2 remains constant during temperature changes. Thus, the temperature changes do not affect the current flowing in and the activated excitation circuits 14A, 14B, 14C. Due to the extremely small channel resistance of the MOSFETs T1 and T2 when they are switched on, the temperature losses and adverse effects of this circuitry of the metal detection device are small, thus reducing the correction requirements.

[0048]

[0049] The constant voltage supply device has an input for a supply voltage VCC and a control input EN to which a control signal from the signal processing device is applied, the control signal varying, for example, between the potential of the supply voltage VCC and a ground potential. When a ground potential is applied, a current can flow through the control loop to ground or to the ground potential applied to the control input EN. The voltage applied to the control loop by the constant voltage supply device is kept constant.

[0049]

[0050] The constant voltage supply device is preferably a low dropout regulator that is able to regulate the supply voltage VCC even when this output voltage is very close to the output voltage supplied to the control loop.

[0050]

[0051] Depending on the number of tappings of the output winding of the coupling transformer 143, the number of available partial windings can be greater than the number of excitation circuits, e.g. the three excitation circuits 14A, 14B and 14C in FIG. 1. In this case, as symbolically shown in FIG. 1, a multi-way tap selector 150 can be arranged between the tappings 143A, 143B, 143C... for selectively establishing a connection between the available excitation circuits 14A, 14B, 14C and a corresponding selection of the available partial windings. The multi-way tap selector can thereby be used to feed a variable selection of the partial windings to the excitation circuits 144A, 144B, 144C. This is shown diagrammatically in FIG. 2, which shows a three-way tap selector with three tap selector links A, B, C for selectively coupling to a corresponding subset of the tappings of the output winding of the coupling transformer 143. The designations "TH," "TM," and "TL" shown in Figures 1 and 2 indicate that the selected excitation circuit 14A, 14B, 14C is tuned to a high frequency, a medium frequency, and a low frequency, respectively. [Explanation of symbols]

[0051] 14 Excitation Unit 14A, 14B, 14C Selectable Excitation Circuit 140 Channel Selector 140A Channel selector module for excitation circuit 14A 141A, 141B, 141C First semiconductor switching device 142A, 142B, 142C Second switching device 143 Coupling Transformer 143A, 143B, 143C Transformer tapping of output windings 143T Output winding terminal tapping 144A, 144B, 144C Tuning Capacitors 21 Transmitter coil 211, 212 Transmitter coil terminals T1, T2 MOS-FET semiconductor switching device 150 Multi-way Tap Selector D Drain terminal S Source terminal G Gate terminal C Control input terminal

Claims

1. A metal detection device, comprising: a balanced coil system including a transmitter coil (21), a first receiver coil and a second receiver coil; a coupling transformer (143) including an input winding energized by an input signal having a selected operating frequency, and an output winding including a plurality of tappings (143A, 143B, 143C, 143T), each pair of tappings selectable from among the plurality of tappings (143A, 143B, 143C, 143T) defining an associated partial winding consisting of turns of the output winding extending between the tappings of the selected pair; a first plurality of individually selectable excitation circuits, each of the plurality of excitation circuits configured to supply energy from an associated one of the plurality of partial windings to the transmitter coil (21) and including an associated first switching device connected to open or close the associated excitation circuit in response to an associated first control signal, each of the plurality of first switching devices being a semiconductor switching device; a second plurality of individually selectable tuning capacitors, each of said plurality of tuning capacitors having an associated second switching device connected to open or close a resonant circuit including said transmitter coil (21) and the associated tuning capacitor in response to an associated second control signal; Equipped with 10. A metal detecting apparatus, wherein each of said plurality of second switching devices is an electromechanical relay switching device.

2. 2. The metal detecting device of claim 1, wherein the number of partial windings is greater than the number of excitation circuits, and a multi-way tap selector is disposed between the tappings (143A, 143B, 143C, 143T) and the excitation circuits to individually establish a connection between the excitation circuits and selected partial windings of the plurality of partial windings.

3. 2. The metal detecting apparatus of claim 1, wherein the electromechanical relay switching device is configured as a dual pole with contacts connected in parallel.

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

5. 2. The metal detecting apparatus of claim 1, wherein for each excitation circuit (14A; 14B; 14C), when selected, the associated tuning capacitor (144A; 144B; 144C) is connected in series or in parallel to the transmitter coil (21) by the associated second switching device (142A; 142B; 142C).

6. 2. The metal detecting apparatus according to claim 1, wherein, when each excitation circuit (14A; 14B; 14C) is selected, the associated partial winding of the secondary coil of the coupling transformer (143) is connected in parallel to the transmitter coil (21) by the associated first switching device (141A; 141B; 141C) and the associated tuning capacitor (144A; 144B; 144C) is connected in parallel to the transmitter coil (21) by the associated second switching device (142A; 142B; 142C).

7. 2. The metal detecting device according to claim 1, wherein said plurality of individually selectable excitation circuits (14A; 14B; 14C) are arranged in parallel.

8. 8. The metal detection apparatus of claim 1, wherein the first and second control signals are generated by a channel selector (140) comprising at least one isolated driver (145A, 146A), such as a photovoltaic driver, for each of the first and second switching devices (141A; 141B; 141C), and wherein the at least one isolated driver (145A, 146A) is used to galvanically isolate control lines from the control inputs of the first and second switching devices (141A; 141B; 141C).

9. Metal detecting apparatus according to any one of the preceding claims, characterized in that the first switching device (141A; 141B; 141C) is a bidirectional MOS-FET unit.

10. 10. The metal detecting device according to claim 9, 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.

11. 10. The metal detecting apparatus of claim 9, wherein input lines of isolated drivers respectively assigned to the first switching device (141A; 141B; 141C) and the second switching device (142A; 142B; 142C) of an associated selectable excitation circuit (14A; 14B; 14C), each connected to a light-emitting diode, are connected in series, and output lines of the isolated drivers are connected to the input terminals (G, S) of the first switching device (141A; 141B; 141C) and the second switching device (142A; 142B; 142C) of the associated selectable excitation circuit (14A; 14B; 14C), respectively.

12. 12. The metal detecting apparatus of claim 11, wherein the light emitting diode of the isolated driver is connected in series with a control resistor, thus forming a control loop connected to the output of a constant voltage supply device.

13. 13. The metal detecting apparatus of claim 12, wherein the constant voltage supply device is a low dropout regulator.

14. 9. A metal detecting device according to claim 8, characterized in that the device, in particular the frequency generator providing the input signal and the channel selector (140), can be controlled by a control program implemented in a computer system, and the control program can be used to select the operating frequency and the associated excitation circuit (14A, 14B, 14C).

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

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