Method for operating a metal detector and metal detector

Abstract

The method is used to operate a metal detector, which includes a balanced coil system (2) having a transmitter coil (21) connected to a transmitter unit (1) and first and second receiver coils (22A, 22B) connected to the input of a receiver unit (3). The transmitter unit (1) includes a transmitter signal path (tp) for which a transmitter signal (tx) having at least one fixed or selectable operating frequency and an associated quadrature signal (tx90°) is provided. The transmitter signal (tx) is applied to the input of a transmitter amplifier (12), which forwards the amplified transmitter signal (tx) directly or via a transmitter matching unit (13) to the transmitter coil (21). The receiver unit (3) includes at least one receiver signal path (rp) in which a modulated receiver signal received from the balanced coil system (2) is transmitted. The signal (rs) is applied directly or via a receiver matching unit (31) to a receiver amplifier (33), which applies the amplified and modulated receiver signal (rs) directly or indirectly to receiver phase detectors (34; 4534), which compare the modulated receiver signal (rs) with a reference signal corresponding to the transmitter signal (tx) and the quadrature signal (tx90°) to generate a demodulated complex receiver signal (rsc) having an in-phase receiver signal component (rs-I) and a quadrature receiver signal component (rs-Q), which are processed in a signal processing unit (45) including at least one signal processing path (sp), in which cargo or noise-related signal components of the complex receiver signal (rsc) are suppressed and signal components originating from metal contaminants are further processed.According to the application, at least one transmitter measuring channel (8) is provided, which receives a measurement signal (ms) taken from a transmitter signal path (tp) and comprises a measurement amplifier (83), which amplifies the measurement signal (ms) and forwards it directly or indirectly to a measuring phase-sensitive detector (84; 4584), which compares the measurement signal (ms) with reference signals corresponding to a transmitter signal (tx) and a quadrature signal (tx 90°) in order to produce a complex measurement signal (msc) with an in-phase measurement signal component (ms-I) and a quadrature measurement component (ms-Q), which complex measurement signal (msc) and the complex receiver signal (rsc) are applied to a first correction module (451), in which a signal component caused by an instability of the transmitter unit (1) is removed from the complex receiver signal (rsc).

Description

Technical Field

[0001] The present invention relates to a method for operating a metal detector using one or more operating frequencies, and to a metal detector operating according to said method. Background Technology

[0002] For example, the metal detector described in US8587301B2 is used to detect metal contamination in products. When properly adjusted and operated, it can help reduce metal contamination in products. Most modern metal detectors use probes with a balanced coil system. Detectors designed this way can detect all types of metal contaminants in a wide variety of products, including fresh and frozen products, including ferrous metals, non-ferrous metals, and stainless steel.

[0003] Metal detectors operating on the principle of balanced coils typically consist of three coils: a transmitter coil wound around a non-metallic frame and two identical receiver coils, each usually parallel to the other coils. Since the receiver coils (which typically surround the transmitter coil centered between them) are identical, the same voltage is induced in each of them. To receive a zero output signal when the system is balanced, the first receiver coil is connected in series with a second receiver coil having the opposite winding direction. Therefore, if the system is balanced and there are no contaminants in the observed product, the voltages induced in the receiver coils, with the same amplitude and opposite polarity, cancel each other out.

[0004] However, once the metal particle passes through the coil arrangement and is exposed to the magnetic field, eddy currents are forced to flow within the particle. These eddy currents generate a secondary magnetic field that first interferes with the primary electromagnetic field near one receiver coil and then near the other. As the metal particle is conveyed through the receiver coils, the voltage induced in each coil typically changes in nanovolts. This change in balance generates a signal at the output of the detection coil, which can be processed, amplified, and subsequently used to detect the presence of metal contaminants in the observed product.

[0005] Therefore, for optimal signal detection, a metal detector should be free from or unaffected by imbalances and interference that could impair the measurement. All modules and components of the metal detector should meet the highest standards.

[0006] In the receiver unit, the input signal received from the balanced coil system is typically divided into in-phase and quadrature components. The vector composed of these components has amplitude and phase angle, which are characteristics of the products and contaminants conveyed through the coil system. To identify metallic contaminants, it is necessary to remove or reduce the "product effect."

[0007] Methods for eliminating unwanted signals from the signal spectrum utilize the fact that metallic contaminants, products, and other interferences have different effects on magnetic fields, causing the detected signals to differ in phase and amplitude. Materials with high electrical conductivity produce signals with a high negative reactance signal component and a low resistivity signal component. Materials with high magnetic permeability produce signals with a low resistivity signal component and a high positive reactance signal component. Signals induced by ferrites are primarily reactive, while signals induced by stainless steel are primarily resistive. Conductive products typically produce signals with a strong resistivity component. When a product or contaminant is passed through a metal detector, the phase angle between the resistivity and reactance signal components of the signal vector typically remains constant. If the phase of the product vector is known, the corresponding signal vector can be suppressed, resulting in higher sensitivity for detecting signals originating from metallic contaminants.

[0008] As described in US8587301B2, distinguishing between the phases of signal components from different sources using a phase-sensitive detector allows information about products and contaminants to be obtained. The phase-sensitive detector (e.g., a mixer or analog multiplier circuit) allows demodulation of the modulated receiver signal and provides a baseband signal with amplitude and phase information of signal components related to the product, contaminants, and interferences affecting the metal detection process. If the phase of the signal originating 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, contaminant detection fails because the contaminant signal is suppressed along with the product signal. To separate the phase angle of the product signal from the phase angle of the contaminant, a suitable operating frequency is determined and applied.

[0009] As described above, imbalances in metal detectors should be avoided or compensated. US2020333498A1 discloses a method in which control data is provided to a compensation unit for compensating an unbalanced signal component extracted from a receiver signal. A digital in-phase component of the unbalanced signal is applied to a first control unit, which provides an in-phase control component for the unbalanced signal, and a digital quadrature component of the unbalanced signal is applied to a second control unit, which provides a quadrature control component for the unbalanced signal. In the compensation unit, a digital compensation signal having the frequency of the unbalanced signal is synthesized based on the phase and amplitude of the in-phase and quadrature control components provided for the unbalanced signal. The synthesized digital compensation signal is then converted into an analog compensation signal and applied to a balanced coil system or receiver signal to compensate for the unbalanced signal.

[0010] US20070296415A1 discloses an apparatus for detecting metal in the ground, and is therefore quite different from the metal detection device of the present invention. The coil current changes when the ground changes or when the distance between the search point and the ground changes. Such changes are corrected by monitoring and optimizing the coil current in the transmitter coil. Optimizing the coil current in the transmitter coil also corrects for slow drift in the receiver signal. However, only the coil current in the transmitter coil is corrected, not the receiver signal itself. This correction may even lead to rapid interference in the receiver signal, which is desirable in the metal detection device of the present invention but negligible in the metal detection device of US20070296415A1.

[0011] DE102017124407A1 discloses a metal detector in which a reference signal is derived from a transmitter, picked up at the connection point of a receiving coil, and optionally fed to a multiplier via an amplifier and / or a low-pass filter. This signal can then be used as a reference signal in subsequent processing to decompose the measured signal into real and imaginary parts, for which it must be converted to an intermediate frequency. The reference signal derived for decomposing the measured signal into real and imaginary parts differs from the complex measurement or correction signal derived for calibrating the receiver signal.

[0012] The correction loop implemented to compensate for imbalance has a relatively high time constant and a low baseband bandwidth so as not to affect the signal originating from the scanned object. Therefore, the correction loop ignores higher frequency instabilities that may occur in metal detectors, leaving the received signal susceptible to these instabilities. This limits the achievable signal-to-noise ratio and discrimination performance, and may lead to false alarms.

[0013] Therefore, the object upon which this invention is based is to provide an improved method for operating a metal detector using one or more operating frequencies, and an improved metal detector operating according to said method.

[0014] The method of this invention will allow for reliable compensation and / or elimination of imbalances in metal detectors, particularly those with higher frequencies. Imbalances in the received signal will be removed, enabling the detection of contaminant-related signals with a high signal-to-noise ratio. The method of this invention will allow for the correction or compensation of dynamic imbalances and interferences and / or substantially static imbalances. Therefore, undesirable modulation of the received signal in phase and / or amplitude by low-frequency or higher-frequency instabilities will be corrected or compensated. Furthermore, drift in the phase and / or amplitude of the receiver signal will be detectable and correctable.

[0015] The control loop used to control low-frequency imbalance will be improved so that higher-frequency imbalance will not interfere with the compensation of low-frequency imbalance. Summary of the Invention

[0016] The method is used to operate a metal detector, which includes a balanced coil system having a transmitter coil connected to a transmitter unit and first and second receiver coils connected to the input of a receiver unit. The transmitter unit includes a transmitter signal path for which a transmitter signal having at least one fixed or selectable operating frequency or transmitter frequency and an associated quadrature signal are provided via an internal or external frequency source. The transmitter signal is applied to the input of a transmitter amplifier, which forwards the amplified transmitter signal directly or via a transmitter matching unit to the transmitter coil. The receiver unit includes at least one receiver signal path in which a modulated receiver signal received from the balanced coil system is applied directly or via a receiver matching unit to a receiver amplifier, which forwards the amplified modulated receiver signal directly or indirectly to a receiver phase-sensitive detector. The receiver phase-sensitive detector demodulates the modulated receiver signal with a reference signal corresponding to the transmitter signal and the quadrature signal to generate a demodulated complex receiver signal having in-phase and quadrature receiver signal components. The in-phase receiver signal component and the quadrature receiver signal component are processed in a signal processing unit including at least one signal processing path, in which cargo or noise-related signal components of the complex receiver signals are suppressed and signal components originating from metal contaminants are further processed.

[0017] According to the present invention, at least one transmitter measurement channel is provided, the transmitter measurement channel receiving a measurement signal acquired from a transmitter signal path and including a measurement amplifier, the measurement amplifier amplifying the measurement signal and forwarding the measurement signal directly or indirectly to a measurement phase-sensitive detector, the measurement phase-sensitive detector demodulating the measurement signal with a reference signal corresponding to the transmitter signal and a quadrature signal to generate a complex measurement signal having an in-phase measurement signal component and a quadrature measurement component, the complex measurement signal and a complex receiver signal being applied to a first correction module, in which signal components caused by instability of the transmitter unit are removed from the complex receiver signal.

[0018] The transmitter signal path and receiver signal path may include additional electronic modules, such as additional amplifiers and filter units for processing the transmitter signal and the modulated or demodulated receiver signal. The signal processing path may also include additional modules, such as filter modules.

[0019] Furthermore, functional modules, such as phase-sensitive detectors, can be implemented in either the analog or digital domain. Therefore, the output signals of the receiver amplifier and the measurement amplifier can be applied to the inputs of the receiver phase-sensitive detector and the measurement phase-sensitive detector, both of which are connected to the signal processing unit at their outputs via analog-to-digital converters. Alternatively, the receiver phase-sensitive detector and the measurement phase-sensitive detector can be implemented as software modules within the signal processing unit or signal processing path.

[0020] Preferably, the measurement signal is picked up at the end of the transmitter signal path or at the transmitter coil of the balanced coil system, the measurement signal corresponding to the transmitter signal at a specific point in the transmitter signal path. In a preferred embodiment, the measurement signal can be passed from the output of the transmitter amplifier to the input of the measurement amplifier. In another preferred embodiment, the transmitter matching unit includes a coupling transformer having at least one primary coil and at least one secondary coil, the measurement signal from which can be passed from the at least one primary coil or at the at least one secondary coil to the input of the measurement amplifier. In another preferred embodiment, the measurement coil is coupled to the transmitter coil of the balanced coil system, the measurement signal from which can be passed to the input of the measurement amplifier. Since the measurement signal is compared in the measurement phase-sensitive detector with a reference signal, i.e., the transmitter signal provided by the frequency source and an associated quadrature signal, any instability or drift in phase or amplitude that has occurred along the observed transmitter signal path can be detected at the output of the measurement phase-sensitive detector. Therefore, information and signals related to the instability and drift of the transmitter channel that affect the modulated receiver signal can be advantageously applied to a correction module located in the signal processing path of the signal processing unit to remove at least a portion of the effects of transmitter instability and transmitter drift.

[0021] Compared to the imbalance in a balanced coil system, transmitter instability or interference typically occurs at higher frequencies, such as imbalances that may be caused by changes in thermal conditions. By eliminating or compensating for higher-frequency transmitter instability that is ignored in conventional metal detectors, signals related to cargo and contaminants can be detected with a higher signal-to-noise ratio. Furthermore, false alarms caused by transmitter instability are avoided.

[0022] Preferably, the measurement signal is continuously observed, and a related complex measurement signal, representing the effect of continuous instability of the transmitter unit on the modulated receiver signal, is continuously applied together with the complex receiver signal to a first correction module, in which the effect of continuous instability of the transmitter unit is removed. Signal components related to products and contaminants transported through the metal detector are preferably removed at a later stage in the metal detection module.

[0023] In a preferred embodiment, during the calibration of the metal detector, complex measurement signals are processed to obtain complex or non-complex constant reference values, which represent the constant influence of the transmitter unit on the modulated receiver signal. This reference value reflects the state of the transmitter unit when the metal detector is calibrated and is used to normalize the complex receiver signal or complex measurement signal. For this purpose, as described below, the reference value can be applied to the complex receiver signal or complex measurement signal. The normalized measurement signal ms then no longer represents the absolute measurement of the transmitter channel, but only the change in the measurement of the transmitter channel during the time interval between the calibration performed and the actual measurement.

[0024] In one embodiment, a constant reference value and a complex receiver signal are applied to a second correction module, in which the complex receiver signal is normalized.

[0025] In another embodiment, a constant reference value and a complex measurement signal are applied to a normalization module, in which the complex measurement signal is normalized before being applied to a first correction module.

[0026] The metal detector of the present invention preferably includes a control loop in which low-frequency imbalances, for example, caused by a balanced coil system, are compensated. For this purpose, the control loop includes a loop control module that removes product- and contaminant-related signal components from a complex receiver signal to obtain a complex imbalance signal. Based on the determined complex imbalance signal and implemented control functions, such as a PID function, the loop control module provides a complex compensation signal. As described in US2020333498A1, the extracted imbalance signal components or extracted imbalance signal information can also be used to synthesize the compensation signal.

[0027] As described below, the compensation signal is preferably modified in one or more modification modules and then modulated using an operating frequency. The modulated digital compensation signal is then converted in a digital-to-analog converter (DAC) to provide a modulated analog compensation signal to a modulated receiver signal in a balanced coil system or receiver unit, for compensating for unbalanced signal components contained in the modulated receiver signal. Preferably, the compensation signal is applied to a compensation unit, such as a summing or subtraction unit, disposed in the receiver signal path, preferably before the receiver amplifier. The modulated analog compensation signal can be amplified in at least one amplifier stage as needed.

[0028] Most preferably, the compensation signal is modified from a complex measurement signal such that the compensation signal precisely corresponds to the imbalance occurring in the receiver channel, which is modified due to, for example, imbalance occurring in a balanced coil system or instability of the transmitter unit. In this way, it is ensured that low-frequency imbalances occurring in the receiver channel due to transmitter instability are completely compensated, canceled, or subtracted from the receiver signal. Therefore, the correction applied to the complex receiver signal from which the compensation signal is derived must be reversed by modifying the derived compensation signal. Thus, the compensation signal is derived from the extracted imbalance signal or imbalance signal information combined with the measurement signal or measurement signal information.

[0029] Therefore, by applying the complex compensation signal and the complex measurement signal to the first modification module, the correction applied to the complex receiver signal in the first correction module is removed from the complex compensation signal. Similarly, by applying the first-modified complex compensation signal and a constant reference value to the second modification module, the correction applied to the complex receiver signal in the second correction module is removed from the first-modified complex compensation signal. The first and second modification modules are arranged in series with each other in any order.

[0030] If the complex measurement signal has been normalized by a constant reference value and then applied to the first correction module, the corrections are removed from the compensation signal by applying the normalized complex measurement signal and the complex compensation signal to a single modification module, in which the normalization of the complex measurement signal by the constant reference value and the correction of the complex receiver signal by the normalized complex measurement signal are revoked.

[0031] Therefore, by removing the undesirable effects of the transmitter unit, the method of the present invention allows for obtaining a receiver signal with a high signal-to-noise ratio that is relevant to the product and contaminants, and removes low-frequency imbalances in the receiver unit with high accuracy.

[0032] The metal detector of the present invention can operate at one or more operating frequencies. If two or more operating frequencies are used, the receiver signal is preferably processed separately for each operating frequency in a dedicated receiver signal path and a dedicated signal processing path. For each operating frequency, the measurement signal is processed in a dedicated transmitter measurement channel. Thus, the receiver signal and the measurement signal are processed in parallel for each operating frequency. Attached Figure Description

[0033] Detailed aspects and examples of the invention are described below with reference to the accompanying drawings, in which...

[0034] Figure 1 A preferred embodiment of the metal detector of the present invention is shown, which includes a transmitter.

[0035] Unit 1, Balanced Coil System 2, Receiver Unit 3, and integrated in the control unit

[0036] The signal processing unit 45 in element 4, as well as the transmitter measurement channel 8 and compensation

[0037] Channel 9, the measurement signal ms obtained from the transmitter signal path tp along the transmission path...

[0038] The transmitter measurement channel 8 is forwarded to the signal processing unit 45, and the compensation signal cs

[0039] It is transmitted to receiver unit 3 along compensation channel 9;

[0040] Figure 2a An example of another preferred embodiment is shown. Figure 1 Metal detectors, which have

[0041] Measuring coil 23 and phase-sensitive detectors 34 and 84, measuring coil 23 is inductively coupled to ground.

[0042] The phase-sensitive detectors 34 and 84 are arranged in the analog domain and connected to the transmitter coil 21.

[0043] Figure 2b It shows Figure 2a Metal detectors that have phases implemented in the digital domain

[0044] Sensitive detectors 4534 and 4584;

[0045] Figure 3a An example of a preferred embodiment is shown. Figure 1 and Figure 2a Metal detection

[0046] The signal processing unit 45 of the detector;

[0047] Figure 3b An example of a preferred embodiment is shown. Figure 2b The signal processing unit 45 of the metal detector has a signal processing unit arranged in the digital domain.

[0048] Phase-sensitive detectors 4534 and 4584;

[0049] Figure 4 Another embodiment of the metal detector of the present invention is shown in one particular embodiment.

[0050] An example. Detailed Implementation

[0051] Figure 1 A block diagram of a metal detector of the present invention in a preferred embodiment is shown, comprising a transmitter unit 1, a balanced coil system 2 having a transmitter coil 21 and first and second receiver coils 22A, 22B, a receiver unit 3, and a control unit 4, the control unit 4 including a control program 40, a digital signal processing unit 45, and interface, input, and output devices. Symbolically, the metal detector may include a conveyor 6 on which products are conveyed through the balanced coil system 2.

[0052] Transmitter unit 1 includes a transmitter signal path tp, which has an internal or external frequency source 11 (e.g., a synthesizer), a transmitter amplifier 12, and preferably a transmitter matching unit 13. The transmitter signal path tp may include additional modules, such as filters. The frequency source 11 provides a transmitter signal tx having at least one fixed or selectable operating frequency. The frequency source 11 also provides a quadrature signal tx90°, which is 90° out of phase with respect to the transmitter signal tx. The transmitter signal tx is applied to the input of the transmitter amplifier 12, which may operate, for example, in Class A or Class B mode, or may be provided in an H-bridge embodiment.

[0053] The output of transmitter amplifier 12 is connected to the input of transmitter matching unit 13, which preferably includes a coupling transformer having at least one primary coil and a secondary coil, and allows transmitter amplifier 12 to be adapted to transmitter coil 21. Impedance matching unit 13 preferably also includes a tuning capacitor, which can be selectively connected to the transmitter coil to create a resonant circuit tuned to a selected operating frequency, as shown below. Figure 4 Further description.

[0054] In the balancing coil system 2, when a product (potentially containing contaminants) is passed through the balancing coil system 2, the receiver signal rs is modulated by interference occurring in the magnetic field. However, even if the receiver coils 22A and 22B are identical and set in a balanced state at the factory site, there are still cases where the balancing coil system cannot maintain balance in the absence of product, which may lead to the rejection of perfectly acceptable food. The balance of the metal detector may be disturbed due to mechanical shock to the system, changes in environmental conditions, metal objects located near the detector, or loosening or aging of components. Given the high sensitivity of the metal detection system and the small effect of contaminants on the output voltage of the coil system, imbalance may also cause saturation of the receiver channels, particularly the input amplifier and phase-sensitive detector, as well as the ADC, which operate only within a limited voltage signal range. To remove such imbalance, an adjustable compensation signal is combined with and varied with the received signal until the undesired imbalance is compensated.

[0055] Receiver unit 3 includes at least one receiver signal path rp, preferably having a receiver matching unit 31, which, for example, includes a balancing transformer that adapts the impedance of the balancing coil system to the impedance of the receiver channel. A modulated receiver signal rs and a compensation signal CS provided by receiver matching unit 31 are applied to compensation unit 32, for example, a summation or subtraction unit. The compensation signal CS compensates for the imbalance contained in the modulated receiver signal rs, such that the compensated modulated receiver signal is forwarded to the input of receiver amplifier 33.

[0056] Receiver amplifier 33 delivers the amplified and compensated receiver signal rs to receiver phase detector 34, where the modulated receiver signal rs is compared or multiplied with an in-phase reference signal and a quadrature reference signal (i.e., the transmitter signal tx provided by frequency source 11 and the associated quadrature signal tx90°). At the output of receiver phase detector 34, a demodulated complex receiver signal rsc with an in-phase receiver signal component rs-I and a quadrature receiver signal component rs-Q is provided.

[0057] The complex receiver signal rsc is forwarded from the receiver phase-sensitive detector 34 via the receiver analog-to-digital converter 35-I; ​​35-Q to the digital signal processing unit 45, such as the digital signal processor disposed in the control unit 4. The control unit 4 also includes an operation or control program 40, which controls all processes of the metal detector during calibration and operation.

[0058] If the metal detector uses more than one operating frequency, the receiver unit 3 preferably includes a separate receiver signal path rp for each operating frequency. Therefore, the receiver signals will be processed in different receiver paths arranged in parallel.

[0059] In the signal processing unit 45, a signal processing path sp is implemented, in which the complex receiver signal rsc is processed. Preferably, the signal components of the complex receiver signal rsc related to cargo or noise are suppressed, and the signal components originating from metal contaminants are further processed and detected.

[0060] Furthermore, along the signal processing path sp, the complex receiver signal rsc is further processed to provide a compensation signal CS. Using the compensation signal CS, the aforementioned imbalance, which is constant or has a very low frequency, is removed from the modulated receiver signal rs. The digital compensation signal provided by the signal processing unit 45 is forwarded to the digital-to-analog converter 91, which provides the analog compensation signal, which is forwarded to the compensation unit 32 in the receiver unit 3.

[0061] As described above, the correction loop control signal has a relatively high time constant and low bandwidth before being modulated and applied to the compensation unit 32, so as not to affect the signal originating from the scanned object. Therefore, higher frequency instabilities that may occur in the metal detector are ignored in the compensation loop, causing the received signal to be affected by these higher frequency instabilities. This limits the achievable signal-to-noise ratio and discrimination performance, and may lead to false alarms. Furthermore, higher frequency instabilities can cause interference in the compensation loop, potentially impairing even the correction of low-frequency imbalances.

[0062] This higher frequency instability, which is ignored by the compensation circuit, is typically caused by transmitter unit 1. Furthermore, due to variations in transmitter unit 1, the transmitter coil signal st may also drift in phase and / or amplitude, which could also affect the measurement and detection of contaminants.

[0063] To eliminate such instability or anomalies occurring along the transmitter signal path tp, a transmitter measurement channel 8 is provided, which receives and processes the measurement signal ms obtained from the transmitter signal path tp. The measurement signal ms corresponds to the transmitter signal tx picked up at a specific point along the transmitter signal path tp. Preferably, the measurement signal ms is picked up near the output of transmitter unit 1, so that any undesirable modification of the transmitter signal tx is captured, and the related effects on the modulated receiver signal rs can be corrected or compensated. Figure 1In one embodiment, a measurement signal ms is picked up at the transmitter matching unit 13, so that any undesirable effects present at the output of the transmitter amplifier 12 can be identified, corrected, or compensated.

[0064] The transmitter measurement channel 8 includes a measurement amplifier 83, which receives, amplifies, and forwards the measurement signal ms to a measurement phase-sensitive detector 84. The measurement phase-sensitive detector 84 demodulates the measurement signal ms using an in-phase reference signal and a quadrature reference signal (i.e., the transmitter signal tx provided by the frequency source 11 and the associated quadrature signal tx90°) to generate a demodulated complex measurement signal msc with an in-phase measurement signal component ms-I and a quadrature measurement component ms-Q. The in-phase measurement signal component ms-I and the quadrature measurement component ms-Q are then forwarded to the signal processing unit 45 via analog-to-digital converters 85-I and 85-Q, respectively.

[0065] In the signal processing unit 45, as shown below... Figure 3a and Figure 3b As described, the complex measurement signal msc, and preferably a reference value msc-r derived from the complex measurement signal msc, are applied to at least one correction module 451, 452, 4510 disposed in the signal processing path sp. This removes instability and interference caused by the transmitter unit 1.

[0066] Preferably, in modification modules 4561 and 4562, such compensation or correction is revoked by applying a complex measurement signal msc or a component thereof to create modified complex compensation signals isc1 and isc2. These modified complex compensation signals isc1 and isc2 precisely correspond to the unbalanced signal components contained in the modulated receiver signal rs processed in the receiver signal path RP, which are also affected by interference and instability from transmitter unit 1. Therefore, using the compensation signal CS derived from the modified complex compensation signals isc1 and isc2, the corresponding unbalanced signal components in the modulated receiver signal rs can be completely eliminated.

[0067] Figure 2a An example of another preferred embodiment is shown. Figure 1 A metal detector. Frequency source 11 is shown, including a digital frequency source module 411, which is preferably implemented in control unit 4 or directly in signal processing unit 45. Digital frequency source module 411 provides digital in-phase and quadrature reference signals, which are converted into an in-phase transmitter signal tx and an associated quadrature signal tx90° in digital-to-analog converters 11-I and 11-Q.

[0068] Figure 2aIt is also shown that the measurement signal ms can also be picked up by the measurement coil 23, which is inductively coupled to the transmitter coil 21.

[0069] Figure 2b It shows Figure 2a The metal detector has phase-sensitive detectors 4534 and 4584 implemented in the digital domain. Therefore, a reference signal is not required in the analog domain. All signal information for demodulating the modulated receiver signal rs and the measurement signal ms can be obtained in the signal processing unit 45. Analog-to-digital conversion can also be incorporated into the signal processing unit 45.

[0070] Module 39, which has been added to Figure 2b In this context, it is indicated that additional electronic modules such as amplifier units and filter units can exist in the receiver signal path rp to process the receiver signal rs.

[0071] Figure 3a A preferred embodiment is shown. Figure 1 and Figure 2a The signal processing unit 45 is for a metal detector. The signal processing unit 45 is designed to process digital in-phase and quadrature signals along the signal processing path sp. Also in this embodiment, a phase-sensitive detector can be implemented in the signal processing unit 45.

[0072] At the input side of the signal processing unit 45, the complex receiver signal rsc and the complex measurement signal msc are applied to the input stages 450 and 458, in which known interference is preferably removed by digital filters.

[0073] In a preferred embodiment, during the calibration of the metal detector, the complex measurement signal MSc is processed to provide a complex or non-complex constant reference value MSc-r, which represents the constant influence of transmitter unit 1 on the modulated receiver signal RS. The complex measurement signal MSc is forwarded, for example, to calibration module 400, which determines the phase and / or amplitude of the constant reference value MSc-r during the calibration process. Calibration module 400 is preferably part of control program 40 or signal processing unit 45. The constant reference value MSc-r is stored in module 459 and used for normalizing the measurement signal MS, such that the normalized measurement signal MS no longer represents the absolute measurement of the transmitter channel, but only the change in the measurement of the transmitter channel during the time interval between the calibration performed and the actual measurement. The reference value MSc-r is typically recaptured only if the conditions of transmitter unit 1 have changed significantly, such as component changes or changes in the applied signal voltage. As described below, the measurement signal MS can be normalized before or after it has been applied to the first calibration module.

[0074] When a constant reference value msc-r is determined during calibration, the complex measurement signal msc is continuously observed, allowing for continuous correction of all instabilities or imbalances caused by transmitter unit 1. According to the method of the invention, some or all instabilities, drifts, and interferences caused in the transmitter channel or transmitter signal path tp can be corrected or compensated.

[0075] To correct for the recurring receiver imbalance caused by transmitter instability, a complex receiver signal RSC, still containing signal components related to the product and contaminants transported through the metal detector, and a continuously acquired complex measurement signal MSC are applied to a first correction module 451. In the first correction module 451, the effect of the recurring instability of transmitter unit 1 on the complex receiver signal RSC is removed, for example, through a complex division function or a Kalman filter implemented in the first correction module. After this correction, the signal components related to the product and contaminants can be detected with a higher signal-to-noise ratio. Furthermore, false alarms that might be caused by transmitter instability are avoided.

[0076] Optionally, a first-corrected complex receiver signal rsc1, which still contains signal components related to the product and contaminants transmitted through the metal detector, and a constant reference value msc-r are applied to a second correction module 452, in which the corrected receiver signal rsc1 is preferably normalized by applying a complex multiplication function.

[0077] The metal detector of the present invention may include only the first calibration module 451, but preferably includes the first calibration module 451 and the second calibration module 452 arranged in series in any order.

[0078] The complex receiver signal rsc2 at the output of the second correction module 452 is forwarded to both the signal detection module 454 and the compensation signal section 456, where a compensation signal CS is created as described below. In the signal detection module 454, the product component still present in the complex receiver signal rsc is suppressed, and the signal component related to contaminants is further processed and preferably compared with a threshold. If the threshold is exceeded, a concurrent signal is sent to indicate the presence of contaminants.

[0079] If you have already referred to Figure 1As described, the metal detector includes a compensation loop. To determine the imbalance requiring correction, signal components related to the product and contaminants are removed from the complex receiver signal RSC in the loop control module 4560, which provides the complex compensation signal ISC. Since the imbalance signal components contained in the modulated receiver signal RSC processed in the receiving unit 3 are affected by the drift and instability of the transmitter unit 1, the corrections performed in the correction modules 451 and 452 are revoked. Therefore, the complex compensation signal ISC provided by the loop control module 4560 is modified accordingly in at least one modification module 4561, 4562.

[0080] In order to cancel the correction applied to the complex receiver signal rsc in the first correction module 451, the complex compensation signal isc and the complex measurement signal msc are applied to the first modification module 4561 provided in the compensation signal section 456. In the first modification module 4561, the complex compensation signal isc is preferably modified by applying a complex multiplication function using the continuously obtained complex measurement signal msc.

[0081] In order to cancel the correction applied to the first-corrected complex receiver signal rsc1 in the second correction module 452, the first-corrected complex compensation signal isc1 provided by the first modification module 4561 and the reference value msc-r stored in module 459 are applied to the second modification module 4562 provided in the compensation signal section 456. In the second modification module 4562, the first-corrected complex compensation signal isc1 is modified by the reference value msc-r, preferably by applying a complex division function, or alternatively by applying a Kalman filter.

[0082] Then, the complex compensation signal isc2, which has been modified twice, delivered by the second modification module 4562, corresponds to the unbalanced signal component of the modulated receiver signal rs processed in receiver unit 3. Due to the symmetry between the unbalanced signal component in the modulated receiver signal rs processed in receiver unit 3 and the complex compensation signal isc2, the cancellation of imbalance and interference caused by transmitter instability in the modulated receiver signal rs is optimized.

[0083] If the second correction module 452 is not implemented, then the corresponding second modification module 4562 is also not implemented. As mentioned, the arrangement order of the correction modules 451, 452 and the modification modules 4561, 4562 can be changed. The functions implemented in the corresponding modules 451 and 4561, and, if present, in modules 452 and 4562, are preferably opposite to each other.

[0084] The complex compensation signal isc2, modified twice, is applied to the modulation module 4563, which modulates the unbalanced signal isc2 onto a carrier frequency corresponding to the operating frequency tx. The resulting digitally modulated compensation signal cs is delivered from the output of the modulation module 4563 to the digital-to-analog converter 91, which applies the analog modulated compensation signal cs to the compensation unit 32, such as a summation or subtraction unit, in the receiver unit 3 via an amplifier 92.

[0085] Figure 3b An example of a preferred embodiment is shown. Figure 2b The signal processing unit 45 of the metal detector has phase-sensitive detectors 4534 and 4584 arranged in the digital domain. Of course, the phase-sensitive detectors 4534 and 4584 can also be used in other applications. Figure 1 This is implemented in the simulation domain shown. Therefore, Figure 3a and Figure 3b The signal processing unit 45 in the embodiments can be used in any embodiment of the metal detector of the present invention. Functional entities can be moved from the analog domain to the digital domain or from the digital domain to the analog domain as needed. Phase-sensitive detectors 4534 and 4584 provide complex receiver signals RSC and complex measurement signals MSC to the corresponding interference suppression modules 450 and 458.

[0086] exist Figure 3b In the embodiment of the signal processing unit 45, the correction of the complex receiver signal rsc and the corresponding modification of the complex compensation signal isc are performed in different ways. The complex measurement signal msc is applied (preferably normalized) to the first correction module 451 and the corresponding modification module 4561.

[0087] Optionally, a constant reference value msc-r is provided in module 459 and applied together with the complex measurement signal msc to normalization module 4510, where the complex measurement signal msc is normalized using the constant reference value msc-r. Since only the normalized measurement signal msc obtained in only one correction module 451 is applied to the complex receiver signal rsc, only one corresponding modification module 4561 is needed to undo the correction to the compensation signal isc.

[0088] Figure 4 Another example of a metal detector is shown, which includes a transmitter unit 1, a balanced coil system 2, a receiver unit 3, a transmitter measurement channel 8, and a control unit 4 having a control program 40 and a signal processing unit 45. The operating principle of this metal detector corresponds to... Figure 1The operating principle of the metal detector shown is illustrated. The circuitry of this metal detector, without the transmitter measurement channel 8 and the associated signal processing module shown in Figure 3, essentially corresponds to the metal detector shown in US10184908B2. Therefore, known and already operating metal detectors can be advantageously enhanced using the solution of this invention.

[0089] Transmitter unit 1 includes a frequency generator 11 that provides a transmitter signal tx to upper and lower amplifier wings 12A and 12B disposed in transmitter amplifier 12. Each amplifier wing 12A and 12B includes a preamplifier that amplifies half a wave of the transmitter signal tx, as is the case in the embodiments of the first and second operational amplifiers OA and OA'. The transmitter signal tx is applied to the inverting input of the first operational amplifier OA via resistor R1 and to the non-inverting input of the second operational amplifier OA' via resistor R2'. The non-inverting input of the first operational amplifier OA and the inverting input of the second operational amplifier OA' are respectively connected to each other via resistors R2 and R1' and connected to a voltage potential corresponding to half of the first power supply voltage -Ub. The outputs of the first and second operational amplifiers OA and OA' are connected to their inverting inputs via resistors R3 and R3' and to the base of the corresponding first or second power transistors T and T' via resistors R4 and R4'.

[0090] Since the input transmitter signal tx is applied to the inverting input of the first operational amplifier OA, the positive half-wave of the transmitter signal tx is inverted and then amplified in the first amplifier wing 12A. That is, both the first and second operational amplifiers OA and OA' deliver a negative half-wave to the base of the corresponding first or second power transistor T, T'. The first or second power transistor T, T' is connected to the zero potential 0V via resistors R5, R5' and to their emitters via resistors R6, R6'. The emitters are correspondingly connected to the first power supply voltage -Ub via resistors R7 or R7'. The collectors of transistors T, T' are connected to the corresponding first or second switches S11, S12 located at the input of the transmitter matching unit 13. The transmitter matching unit 13 includes a coupling transformer 131 and a plurality of tuning capacitors 132, 133. The coupling transformer 131 has two primary coils 1311A, 1311B and one secondary coil 1312.

[0091] The collector of the first power transistor T is connected via a first switch S11 to one of a plurality of taps A, B, C, D of the first primary coil 1311A of the coupling transformer 131. The collector of the second power transistor T' is connected via a second switch S12 to one of a plurality of taps A', B', C', D' of the second primary coil 1311B of the coupling transformer 131. The first primary coil 1311A and the second primary coil 1311B are identically designed but wound in opposite directions, and they are connected to a common tap of the second power supply voltage +Ub. Taps A, B, C, D and A', B', C', D' are located at the same number of turns counting from the common tap. The first and second switches S11, S12 are controlled such that corresponding taps are always selected, thereby applying the same load to the power transistors T, T' and maintaining symmetry. Therefore, in this preferred embodiment, the power stages with power transistors T, T' in the amplifier wings 12A, 12B are identical.

[0092] Using switch S2, transmitter coil 21 can be connected to suitable taps E, F, G of secondary coil 1312. Using another switch S3, one of tuning capacitors 132 and 133 can be connected in parallel to transmitter coil 21. Switches S2 and S3 are selected to configure the resonant circuit tuned to the selected operating frequency.

[0093] Figure 4 It is also shown that the measurement signal ms is picked up at the output of transmitter amplifier 12, at switches S11 and / or S12 for differential signals on the primary side of coupling transformer 131. Alternatively, the measurement signal ms can be picked up on the secondary side of coupling transformer 131, for example at switches S2 and S3 for differential signals, with switch S2 connecting transmitter coil 21 to one of taps E, F, or G. As another alternative, the measurement signal ms can be picked up at measurement coil 23, which is inductively coupled to transmitter coil 21. The measurement signal ms is transmitted to... Figure 1 The transmitter measurement channel 8 is shown, and as referenced Figure 1 The process is carried out as shown in Figure 3.

[0094] List of reference numerals

[0095] 1. Transmitter Unit

[0096] 11. Frequency source, synthesizer

[0097] 11-I, 11-Q Digital-to-Analog Converters

[0098] 12 Transmitter Amplifier

[0099] 12A and 12B amplifier sections

[0100] 13 Transmitter Matching Unit

[0101] 131 Coupling Transformer

[0102] 1311A, 1311B primary coils

[0103] 1312 secondary coil

[0104] 132, 133 Tuning capacitors

[0105] 2. Coil System

[0106] 21. Transmitter Coil

[0107] Receiver coils for 22A and 22B

[0108] 23 Measuring coil

[0109] 3 Receiver Unit

[0110] 31 Receiver Matching Unit

[0111] 32 Compensation Units

[0112] 33 Receiver Amplifier

[0113] 34 Receiver Phase-Sensitive Detector / Demodulator

[0114] 35I and 35Q receiver analog-to-digital converters

[0115] 4 Control Unit

[0116] 40 Control Procedure

[0117] 400 / CAL Calibration Module

[0118] 411 Digital Frequency Source Module

[0119] 45 Signal Processing Units / Modules

[0120] 450, 458 Interference Suppression Modules

[0121] 451 / DIV Complex Division Correction Module

[0122] 4510 / DIV Normalization Module

[0123] 452 / MUL Complex Multiplication Correction Module

[0124] 454 / MD Metal Detection Module

[0125] 456 Compensation Signal Section

[0126] 4560 Loop Control Module

[0127] 4561 / MUL Complex Division Modification Module

[0128] 4562 / MUL Complex Multiplication Modification Module

[0129] 4563 / MOD Modulation Module

[0130] 458 / KIR Interference Suppression Module

[0131] 459 / TRV Drive Reference Value

[0132] 6 Product Conveyor

[0133] 8 Transmitter Measurement Channels

[0134] 83 Measurement Amplifier

[0135] 84 Measurement Phase-Sensitive Detector / Demodulator

[0136] 85I, 85Q Measurement Analog-to-Digital Converters

[0137] 9. Compensation Channel

[0138] 91 Digital-to-Analog Converter

[0139] 92 Amplifier in the compensation channel

[0140] cs Modulated compensation signal

[0141] isc complex compensation signal

[0142] isc1 is a modified complex compensation signal

[0143] isc2 complex compensation signal modified twice

[0144] ms measurement signal

[0145] MSC complex measurement signals

[0146] MSC-N normalized complex measurement signal

[0147] MSC-R transmitter reference values

[0148] ms-I measurement signal in-phase component

[0149] The quadrature components of the ms-Q measurement signal

[0150] rs Modulated receiver signal

[0151] rsc complex receiver signal

[0152] rs-I: In-phase component of the complex receiver signal

[0153] The quadrature components of the rs-Q complex receiver signal

[0154] rsc1 is a complex receiver signal that has undergone one-time calibration.

[0155] rsc2 is a complex receiver signal that has undergone two corrections.

[0156] tx; tx90° Transmitter signal, quadrature signal

[0157] OPT Optional Modules

Claims

1. A method for operating a metal detector, the metal detector comprising a balanced coil system (2) having a transmitter coil (21) connected to a transmitter unit (1) and first and second receiver coils (22A, 22B) connected to an input terminal of a receiver unit (3), the receiver unit (3) being connected to a signal processing unit (45). The transmitter unit (1) includes a transmitter signal path (tp), for which a transmitter signal (tx) with at least one fixed or selectable operating frequency and an associated quadrature signal (tx90°) are provided. The transmitter signal (tx) is applied to the input of a transmitter amplifier (12), which forwards the amplified transmitter signal (tx) directly or via a transmitter matching unit (13) to the transmitter coil (21). The receiver unit (3) includes at least one receiver signal path (rp) in which a modulated receiver signal (rs) received from the balanced coil system (2) is applied directly or via a receiver matching unit (31) to a receiver amplifier (33), which forwards the amplified modulated receiver signal (rs) directly or indirectly to a receiver phase detector (34; 4534). The receiver phase-sensitive detectors (34, 4534) compare the modulated receiver signal (rs) with a reference signal corresponding to the transmitter signal (tx) and the quadrature signal (tx90°) to generate a demodulated complex receiver signal (rsc) with an in-phase receiver signal component (rs-I) and a quadrature receiver signal component (rs-Q). The in-phase receiver signal component (rs-I) and quadrature receiver signal component (rs-Q) are processed in a signal processing unit (45) including at least one signal processing path (sp), in which cargo- or noise-related signal components of the complex receiver signal (rsc) are suppressed and signal components originating from metal contaminants are further processed, characterized in that, At least one transmitter measurement channel (8) is provided, the transmitter measurement channel (8) receiving a measurement signal (ms) obtained from the transmitter signal path (tp) and including a measurement amplifier (83) amplifying the measurement signal (ms) and forwarding the measurement signal (ms) directly or indirectly to a measurement phase-sensitive detector (84; 4584). The measurement phase-sensitive detector (84; 4584) compares the measurement signal (ms) with a reference signal corresponding to the transmitter signal (tx) and the quadrature signal (tx90°) to generate a complex measurement signal (mssc) with an in-phase measurement signal component (ms-I) and a quadrature measurement component (ms-Q). The complex measurement signal (MSC) and the complex receiver signal (RSC) are applied to a first correction module (451), in which the signal components caused by the instability of the transmitter unit (1) are removed from the complex receiver signal (RSC), and The method further includes the following steps: A control loop is provided to remove unbalanced signal components contained in the modulated receiver signal (rs) in the following manner: In the loop control module (4560), the signal components related to the product and contaminants are removed from the complex receiver signal (rsc) to obtain the complex compensation signal (isc). The correction applied to the complex receiver signal (rsc1) in the first correction module (451) is canceled by applying the complex compensation signal (isc) and the complex measurement signal (msc) to the first modification module (4561) that provides the modified compensation signal (isc1); In the modulation module (4563) for providing the modulated compensation signal (cs), the modified compensation signal (isc1) is modulated on a carrier frequency corresponding to the operating frequency (tx). The modulated compensation signal (cs) is converted into an analog modulated compensation signal (cs) in the digital-to-analog converter (91); and A simulated modulated compensation signal (cs) is applied to a compensation unit (32) located in the receiver signal path (rp) to compensate for unbalanced signal components contained in the modulated receiver signal (rs).

2. The method for operating a metal detector according to claim 1, wherein, The method includes the following steps: processing the complex measurement signal (msc) during calibration of the metal detector in the calibration module (400) to obtain a complex or non-complex constant reference value (msc-r), the complex or non-complex constant reference value (msc-r) representing the constant influence of the transmitter unit (1) on the modulated receiver signal (rs) and used to directly or indirectly normalize the complex measurement signal (msc).

3. The method for operating a metal detector according to claim 2, wherein, The method includes the following steps: applying the constant reference value (msc-r) and the complex receiver signal (rsc1) to a second correction module (452) to provide a normalized complex receiver signal (rsc2); and The correction applied to the complex receiver signal (rsc1) in the second correction module (452) is canceled by applying the complex compensation signal (isc1) and the constant reference value (msc-r) to the second modification module (4562) located in the control loop.

4. The method for operating a metal detector according to claim 2, wherein, The method includes the following steps: applying the constant reference value (msc-r) and the complex measurement signal (msc) to a normalization module (4510), the normalization module (4510) providing a normalized measurement signal (msc-n) to the first correction module (451); and The correction applied to the complex receiver signal (rsc1) in the first correction module (451) is canceled by applying the complex compensation signal (isc) and the normalized measurement signal (msc-n) to the first modification module (4561) located in the control loop.

5. A method for operating a metal detector according to any one of claims 1-4, wherein, The method includes the following steps: a) Pick up the measurement signal (ms) at the output of the transmitter amplifier (12), or b) The transmitter signal (tx) is transformed in the transmitter matching unit (13) by means of a coupling transformer (131) having at least one primary coil (1311A, 1311B) and at least one secondary coil (1312), and the measurement signal (ms) is picked up at the at least one primary coil (1311A, 1311B) or at the at least one secondary coil (1312), or c) Couple the measurement coil (23) to the transmitter coil (21) and pick up the measurement signal (ms) at the measurement coil (23).

6. The method for operating a metal detector according to any one of claims 1-4, wherein, The method includes the following steps: providing a dedicated receiver signal path (rp) and a dedicated signal processing path (sp) for each operating frequency, as well as a dedicated transmitter measurement channel (8).

7. A metal detector operated by the method defined in any one of claims 1-6.

8. The metal detector according to claim 7, wherein, The metal detector includes a balanced coil system (2) having a transmitter coil (21) connected to a transmitter unit (1) and first and second receiver coils (22A, 22B) connected to the input of a receiver unit (3), the receiver unit (3) being connected to a signal processing unit (45), and the metal detector includes a frequency source (11) providing a transmitter signal (tx) and an associated quadrature signal (tx90°) having at least one fixed or selectable operating frequency. The transmitter unit (1) includes at least one transmitter signal path (tp) with a transmitter amplifier (12) connected directly or via a transmitter matching unit (13) to the transmitter coil (21). The receiver unit (3) includes at least one receiver signal path (rp) in which the receiver coils (22A, 22B) of the balanced coil system (2) are connected directly or via a receiver matching unit (31) to the input of the receiver amplifier (33), and an amplified and modulated receiver signal (rs) from the receiver amplifier (33) is applied directly or indirectly to the receiver phase detectors (34, 4534). The receiver phase-sensitive detectors (34, 4534) are designed to compare the modulated receiver signal (rs) with a reference signal corresponding to the transmitter signal (tx) and the quadrature signal (tx90°) to generate a demodulated complex receiver signal (rsc) with an in-phase receiver signal component (rs-I) and a quadrature receiver signal component (rs-Q). The signal processing unit (45) includes at least one signal processing path (sp), in which cargo or noise-related signal components of the complex receiver signal (rsc) are suppressed and signal components originating from metal contaminants are further processed; in At least one transmitter measurement channel (8) is provided, the transmitter measurement channel (8) including a measurement amplifier (83), the measurement amplifier (83) having an input connected to the transmitter signal path (tp) to receive a measurement signal (ms) and an output directly or indirectly connected to a measurement phase-sensitive detector (84; 4584). The measurement phase-sensitive detector (84; 4584) is designed to compare the measurement signal (ms) with a reference signal corresponding to the transmitter signal (tx) and the quadrature signal (tx90°) to generate a demodulated complex measurement signal (mssc) with an in-phase measurement signal component (ms-I) and a quadrature measurement component (ms-Q). The measurement phase-sensitive detector (84; 4584) is connected to a first correction module (451) implemented in the signal processing path (sp), in which the signal components from the complex receiver signal (rsc) caused by the instability of the transmitter unit (1) are removable.

9. The metal detector according to claim 7 or 8, in, The measurement signal (ms) can be transmitted from the output of the transmitter amplifier (12) to the input of the measurement amplifier (83), or The transmitter matching unit (13) includes a coupling transformer (38) having at least one primary coil (381A, 381B) and at least one secondary coil (382), wherein the measurement signal (ms) can be transmitted from the at least one primary coil (381A, 381B) or the at least one secondary coil (382) to the input of the measurement amplifier (83), or The measuring coil (23) is inductively coupled to the transmitter coil (21) and connected to the input of the measuring amplifier (83) to provide the measuring signal (ms).

10. The metal detector according to claim 7 or 8, wherein, The signal processing unit (45) includes: A control loop is used to remove unbalanced signal components contained in the modulated receiver signal (rs); The loop control module (4560) provides a complex compensation signal (isc) that does not contain signal components related to the product and contaminants. In the first modification module (4561), the complex compensation signal (isc) and the complex measurement signal (msc) are applied to the first modification module (4561) to provide a modified compensation signal (isc1), in which the correction applied to the complex receiver signal (rsc1) in the first correction module (451) is canceled; The modulation module (4536) provides a modulated compensation signal (cs). A digital-to-analog converter (91) converts a digitally modulated compensation signal (isc) into an analog modulated compensation signal (cs); and A compensation unit (32) is provided in the receiver signal path (rp), in which a modulated analog compensation signal (cs) can be applied to the modulated receiver signal (rs) to remove imbalance.

11. The metal detector according to claim 7 or 8, wherein, A calibration module (400) is provided, which is designed to process the complex measurement signal (MSC) during the calibration of the metal detector to obtain a complex or non-complex constant reference value (MSC-R) representing the constant influence of the transmitter unit (1) on the modulated receiver signal (RS).

12. The metal detector according to claim 11, wherein, A second correction module (452) is provided, to which the constant reference value (msc-r) and the complex receiver signal (rsc1) can be applied, and the second correction module (452) is designed to provide a normalized complex receiver signal (rsc2); and The control loop includes a second modification module (4562) to which the complex compensation signal (isc1) and the constant reference value (msc-r) can be applied. Furthermore, in the second modification module (4562), the correction applied to the complex receiver signal (rsc1) in the second correction module (452) is canceled.

13. The metal detector according to claim 11, wherein, A normalization module (4510) is provided, to which the constant reference value (msc-r) and the complex measurement signal (msc) can be applied, and the normalization module (4510) provides a normalized measurement signal (msc-n) to the first correction module (451); and The complex compensation signal (isc) and the normalized measurement signal (msc-n) can be applied to the first modification module (4561), in which the correction applied to the complex receiver signal (rsc1) in the first correction module (451) is canceled.

14. The metal detector according to any one of claims 8, 12-13, wherein, The output of the receiver phase-sensitive detector (34) and the output of the measurement phase-sensitive detector (84) are connected to the signal processing unit (45) via analog-to-digital converters (35-I; ​​35-Q), or the receiver phase-sensitive detector (4534) and the measurement phase-sensitive detector (4584) are implemented in the software domain of the signal processing unit (45).

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