How to operate a metal detector and metal detectors

Abstract

The method is useful for operating a metal detector including a balanced coil system (2) with a transmit coil (21) connected to a transmit unit (1) and first and second receive coils (22A, 22B) connected to an input of a receive unit (3). The transmit unit (1) includes a transmit signal path (tp) to which a transmit signal (tx) having at least one fixed or selectable operating frequency and an associated quadrature signal (tx90°) are provided, the transmit signal (tx) being applied to an input of a transmit amplifier (12) which transfers the amplified transmit signal (tx) directly or via a transmit matching unit (13) to the transmit coil (21), and the receive unit (3) includes at least one receive signal path (rp) to which a modulated receive signal (rs) received from the balanced coil system (2) is applied directly or via a receive matching unit (31) to a receive amplifier (33), which transfers the amplified modulated receive signal (rs) directly or indirectly to a receive amplifier (33). The modulated received signal (rs) is applied to a receive phase-sensitive detector (34; 4534), which compares the modulated received signal (rs) with reference signals corresponding to the transmitted signal (tx) and the quadrature signal (tx90°) to generate a demodulated complex received signal (rsc) having an in-phase received signal component (rs-I) and a quadrature received signal component (rs-Q), which are processed in a signal processing unit (45) including at least one signal processing path (sp), in which signal components of the complex received signal (rsc) related to the product or noise are suppressed and signal components originating from metal contamination are further processed.According to the invention, at least one transmit measurement channel (8) is provided, which includes a measurement amplifier (83) that receives a measurement signal (ms) taken from the transmit signal path (tp), amplifies the measurement signal (ms) and forwards it directly or indirectly to a measurement phase-sensitive detector (84; 4584), which compares the measurement signal (ms) with reference signals corresponding to the transmit signal (tx) and the quadrature signal (tx90°) and generates a complex measurement signal (msc) having an in-phase measurement signal component (ms-I) and a quadrature measurement component (ms-Q), and the complex measurement signal (msc) and the complex receive signal (rsc) are applied to a first correction module (451), and signal components due to instabilities of the transmit unit (1) are removed from the complex receive signal (rsc).

Description

[Technical Field]

[0001] The present invention relates to a method of operating a metal detector using one or more operating frequencies, and to a metal detector operating in accordance with this method. [Background technology]

[0002] Metal detectors, such as those described in U.S. Pat. No. 8,587,301 B2, are used to detect metallic contamination in products. When properly adjusted and operated, they can help reduce metallic contamination in products. Most modern metal detectors use a search head with a balanced coil system. Detectors of this design are capable of detecting all types of metallic contaminants, including ferrous, non-ferrous, and stainless steel, in a wide variety of products, both fresh and frozen.

[0003]

[0003] Metal detectors operating according to the balanced coil principle typically consist of three coils wound on a nonmetallic frame: a transmitter coil and two identical receiver coils, each typically parallel to the others. The receiver coils, typically positioned around the transmitter coil, are identical, so identical voltages are induced in each. To receive an output signal that is zero when the system is in a balanced state, the first receiver coil is connected in series with the oppositely wound second receiver coil. Therefore, when the system is in a balanced state and the product being observed is free of contaminants, the voltages induced in the receiver coils, which are identical in amplitude but opposite in polarity, cancel each other out.

[0004] However, when a metal particle passes through the coil and is exposed to a magnetic field, eddy currents flow in the metal particle. These eddy currents generate secondary magnetic fields that disrupt the primary electromagnetic field, first near one receiver coil and then near the other receiver coil. As the metal particle passes through the receiver coils, the voltage induced in each receiver coil changes, typically in the nanovolt range. This change in equilibrium produces a signal at the output of the detector coil, which is processed and amplified in the receiver unit and then used to detect the presence of metal contaminants in the observed product.

[0005]

[0005] Therefore, for optimal signal detection, a metal detector must be free from imbalances and disturbances that may impair the measurement. All modules and components of a metal detector must meet the highest standards.

[0006]

[0006] In the receiving unit, the input signal received from the balanced coil system is typically split into an in-phase component and a quadrature component. The vector formed by these components has an amplitude and a phase angle that is characteristic of the products and contaminants transported through the coil system. To identify metallic contaminants, it is necessary to remove or reduce the "product effect."

[0007]

[0007] Methods applied to remove unwanted signals from a signal spectrum take advantage of the fact that metallic contaminants, products, and other disturbances affect the magnetic field differently, so that the phase and magnitude of the detected signal differ. Materials with high electrical conductivity produce signals with a large negative reactive signal component and a small resistive signal component. Materials with high magnetic permeability produce signals with a small resistive signal component and a large positive reactive signal component. Ferrite signals are primarily reactive, while stainless steel signals are primarily resistive. Conductive products typically produce signals with a strong resistive component. The phase angle of the signal vector between the resistive and reactive signal components typically remains constant as the product or contaminant is transported through the metal detector. If the phase of the product vector is known, the corresponding signal vector can be suppressed, allowing signals originating from metallic contaminants to be detected with greater sensitivity.

[0008] As described in US8587301B2, a phase-sensitive detector can distinguish the phases of signal components of different origins to obtain information about the product and contaminants. A phase-sensitive detector, such as a frequency mixer or analog multiplier circuit, demodulates the modulated received signal, providing a baseband signal with amplitude and phase information about the signal components associated with the product, contaminants, and disturbances affecting the metal detection process. If the phase of the signal originating from the contaminant differs from that of the product signal, the product signal can be suppressed and the contaminant signal can be further processed. However, if the phase of the contaminant signal is close to that of the product signal, the contaminant signal will be suppressed along with the product signal, resulting in unsuccessful contaminant detection. To separate the phase angle of the product signal from that of the contaminant, an appropriate operating frequency is determined and applied.

[0009] As mentioned above, imbalance in a metal detector should be avoided or compensated for. US2020333498A1 discloses a method for providing control data for an imbalance signal component extracted from a received signal to a compensation unit used to compensate for the imbalance signal. The digital in-phase component of the imbalance signal is applied to a first control unit, which provides an in-phase control component of the imbalance signal, and the digital quadrature component of the imbalance signal is applied to a second control unit, which provides a quadrature control component of the imbalance signal. In the compensation unit, a digital compensation signal having the frequency of the imbalance signal is synthesized in phase and magnitude according to the in-phase and quadrature control components provided for the imbalance signal. The synthesized digital compensation signal is converted to an analog compensation signal and applied to a balanced coil system or a received signal to compensate for the imbalance signal.

[0010]

[0010] US20070296415A1 discloses an apparatus for detecting underground metals, which is significantly different from the metal detection apparatus of the present invention. Changes in coil current occur when the ground changes or when the distance between the search at and the ground changes. These changes are corrected by monitoring and optimizing the coil current of the transmitting coil. Optimizing the coil current of the transmitting coil also corrects for slow drifts in the received signal. However, only the coil current of the transmitting coil is corrected, not the received signal itself. Such corrections may even cause fast disturbances in the received signal, which are desirable in the metal detection apparatus of the present invention but are negligible in the metal detection apparatus of US20070296415A1.

[0011]

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

[0012]

[0012] The correction loop implemented to compensate for the imbalance has a relatively high time constant and a low baseband bandwidth so as not to affect the signal emitted from the scanned object. As a result, the high-frequency instability that may occur in the metal detector is ignored by the correction loop, so the received signal is affected by the high-frequency instability, which limits the achievable signal-to-noise ratio and discrimination performance and may cause false alarms. Summary of the Invention [Problem to be solved by the invention]

[0013]

[0013] It is therefore an object of the present invention to provide an improved method for operating a metal detector using one or more operating frequencies, and an improved metal detector operating in accordance with this method.

[0014] The method of the present invention makes it possible to reliably compensate for and / or remove imbalances in a metal detector, particularly imbalances and disturbances with higher frequencies. The imbalances are removed from the received signal so that signals related to contaminants are detected with a higher signal-to-noise ratio. The method of the present invention makes it possible to correct or compensate for dynamic imbalances and disturbances, and / or essentially static imbalances. Therefore, undesired modulations of the phase and / or amplitude of the received signal due to low- or high-frequency instabilities must be corrected or compensated for. Furthermore, drifts in the phase and / or amplitude of the received signal must be detectable and correctable.

[0015] The control loop for controlling the low frequency imbalance must be improved so that the high frequency imbalance does not prevent the compensation of the low frequency imbalance. [Means for solving the problem]

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

[0017] According to the present invention, at least one transmit measurement channel is provided, including a measurement amplifier that receives a measurement signal tapped off the transmit signal path, amplifies the measurement signal, and forwards it directly or indirectly to a measurement phase-sensitive detector. The measurement phase-sensitive detector demodulates the measurement signal with a reference signal corresponding to the transmit signal and the quadrature signal, generating a complex measurement signal having an in-phase measurement signal component and a quadrature measurement component. The complex measurement signal and the complex receive signal are applied to a first correction module, which removes signal components resulting from instabilities in the transmit unit from the complex receive signal.

[0018]

[0018] The transmit and receive signal paths may include further electronic modules, such as additional amplifier and filter units, for processing the transmit signal and the modulated or demodulated receive signal. Similarly, the signal processing path may include further modules, such as filter modules.

[0019]

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

[0020]

[0020] A measurement signal corresponding to the transmit signal at a particular point in the transmit signal path is preferably picked up at the end of the transmit signal path or at a transmit coil of a balanced coil system. In a preferred embodiment, the measurement signal can be transferred from the output of the transmit amplifier to the input of the measurement amplifier. In another preferred embodiment, the transmit matching unit includes a coupling transformer having at least one primary coil and at least one secondary coil, and the measurement signal can be transferred from the at least one primary coil or the at least one secondary coil to the input of the measurement amplifier. In a further preferred embodiment, a measurement coil is coupled to the transmit coil of the balanced coil system, and the measurement signal can be transferred from the measurement coil to the input of the measurement amplifier. The measurement signal is compared in a measurement phase-sensitive detector with a reference signal, i.e., the transmit signal and associated quadrature signal provided by the frequency source, so that phase or amplitude instability or drift occurring along the observed transmit signal path can be detected at the output of the measurement phase-sensitive detector. Therefore, the obtained information and signals related to the instability and drift of the transmit channel affecting the modulated receive signal can be advantageously applied to a correction module provided in the signal processing path of the signal processing unit to remove at least part of the effects of the transmit instability and transmit drift.

[0021]

[0021] Transmission instabilities or interference generally occur at higher frequencies compared to imbalances in a balanced coil system, such as imbalances caused by changes in thermal conditions. By canceling or compensating for the higher frequency transmission instabilities that are ignored in conventional metal detectors, signals related to merchandise and contaminants can be detected with a higher signal-to-noise ratio. Furthermore, false alarms caused by transmission instabilities are also avoided.

[0022] The measurement signal is preferably continuously monitored and the associated complex measurement signal, which represents the effect of the continuous instability of the transmitting unit on the modulated received signal, is continuously applied together with the complex received signal to a first correction module, where the effect of the continuously occurring instability of the transmitting unit is removed. Signal components relating to products and contaminants transported through the metal detector are preferably removed at a later stage in the metal detection module.

[0023]

[0023] In a preferred embodiment, during calibration of the metal detector, the complex measurement signal is processed to obtain a constant reference value, complex or non-complex, that represents the constant influence of the transmitting unit on the modulated received signal. This reference value is a reference that reflects the state of the transmitting unit at the time the metal detector was calibrated and is used to normalize the complex received signal or the complex measurement signal. For this purpose, the reference value can be applied to the complex received signal or the complex measurement signal, as explained below. The normalized measurement signal m is no longer an absolute measurement value of the transmission channel, but represents only the change in the measurement value of the transmission channel between the time the calibration was performed and the time the actual measurement was performed.

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

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

[0026]

[0026] The metal detector of the present invention preferably includes a control loop in which low-frequency imbalances, such as those caused by a balanced coil system, are compensated for. To this end, the control loop includes a loop control module that removes signal components associated with the product and contaminants from the complex received signal to obtain a complex imbalance signal. Based on the determined complex imbalance signal and an implemented control function, 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 a compensation signal.

[0027]

[0027] As described below, the compensation signal is preferably modified in one or more correction modules and then modulated at the operating frequency. The modulated digital compensation signal is then converted in a digital-to-analog converter to provide a modulated analog compensation signal to the modulated receive signal in the balanced coil system or receive unit to compensate for the unbalanced signal component contained in the modulated receive signal. Preferably, the compensation signal is applied to a compensation unit, such as a summing unit or subtracting unit, provided in the receive signal path located before the receive amplifier. The modulated analog compensation signal may be amplified by at least one amplifier stage, if necessary.

[0028]

[0028] Most preferably, the compensation signal is modified by the complex measurement signal to accurately correspond to imbalances present in the receive channel, which imbalances are corrected due to, for example, imbalances occurring in a balanced coil system or instabilities in the transmit unit. In this way, it is ensured that low-frequency imbalances occurring in the receive channel affected by transmit instabilities are fully compensated for, cancelled out, or subtracted from the receive signal. Therefore, any corrections applied to the complex receive signal from which the compensation signal is derived must be undone by modifying the derived compensation signal. The compensation signal is therefore derived from a combination of the extracted imbalance signal or imbalance signal information and the measurement signal or measurement signal information.

[0029]

[0029] Thus, a correction applied to the complex receive signal in the first correction module is removed from the complex compensated signal by applying the complex compensation signal and the complex measurement signal to the first correction module. A correction applied to the complex receive signal in the second correction module is removed from the once-modified complex compensated signal by applying the once-modified complex compensation signal and a constant reference value to the second correction module. The first and second correction modules may be arranged in series with each other in any order.

[0030]

[0030] If the complex measurement signal is normalized by a fixed reference value and then applied to a first correction module, these corrections are removed from the compensation signal by applying the normalized complex measurement signal and the complex compensation signal to a single correction module, in which the normalization of the complex measurement signal by the fixed reference value and the correction of the complex received signal by the normalized complex measurement signal are reversed.

[0031]

[0031] By eliminating the undesirable effects of the transmitting unit, the method of the present invention can obtain received signals related to products and contaminants with a higher signal-to-noise ratio and can eliminate low-frequency imbalances of the receiving unit with greater accuracy.

[0032]

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

[0033]

[0034] Detailed aspects and examples of the present invention will be described below with reference to the drawings. [Brief explanation of the drawings]

[0034] [Figure 1]1 shows a preferred embodiment of a metal detector of the present invention, comprising a transmitting unit 1, a balanced coil system 2, a receiving unit 3, a signal processing unit 45 integrated in a control unit 4, a transmitting measurement channel 8 through which a measurement signal ms taken from the transmitting signal path tp is forwarded to the signal processing unit 45, and a compensation channel 9 through which a compensation signal cs is forwarded to the receiving unit 3. [Figure 2a] 1 in a further preferred embodiment with a measuring coil 23 inductively coupled to the transmitting coil 21 and with phase-sensitive detectors 34, 84 arranged in the analog domain. [Figure 2b] 45 shows the metal detector of FIG. 2a with phase-sensitive detectors 4534, 4584 implemented in the digital domain. [Figure 3a] FIG. 4 shows the signal processing unit 45 of the example metal detector of FIGS. 1 and 2a in a preferred embodiment. [Figure 3b] FIG. 4 shows the signal processing unit 45 of the example metal detector of FIG. 2b in a preferred embodiment with phase-sensitive detectors 4534, 4584 arranged in the digital domain. [Figure 4] 10A-10C show further examples of metal detectors of the present invention in certain embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0035] 1 shows a block diagram of a metal detector of the present invention in a preferred embodiment, which includes a transmitting unit 1, a balanced coil system 2 with a transmitting coil 21 and first and second receiving coils 22A, 22B, a receiving unit 3, a control unit 4 including a control program 40 and a digital signal processing unit 45, as well as interfaces and input / output devices. The metal detector is symbolically shown to include a conveyor 6 on which products are transported past the balanced coil system 2.

[0036] The transmitter unit 1 includes a transmitter signal path tp, which includes an internal or external frequency source 11, such as a synthesizer, a transmitter amplifier 12, and preferably a transmitter matching unit 13. The transmitter signal path tp may include further 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 further provides a quadrature signal tx90°, which is offset in phase by 90° relative to the transmitter signal tx. The transmitter signal tx is applied to the input of the transmitter amplifier 12, which may operate in class A or class B mode, for example, or may be implemented in an H-bridge configuration.

[0037] The output of the transmit amplifier 12 is connected to the input of a transmit matching unit 13, which preferably includes a coupling transformer having at least one primary coil and a secondary coil, to match the transmit amplifier 12 to the transmit coil 21. The impedance matching unit 13 also preferably includes a tuning capacitor selectively connectable to the transmit coil to form a resonant circuit tuned to a selected operating frequency, as will be further described below with reference to FIG.

[0038] In a balanced coil system 2, when a product (possibly containing contaminants) is transported through the balanced coil system 2, the received signal rs is modulated by disturbances generated in the magnetic field. However, even if the receive coils 22A and 22B are identical and balanced on the factory floor, the balanced coil system may not reach equilibrium in the absence of product, potentially resulting in the rejection of perfectly acceptable food products. The metal detector's equilibrium state can be disrupted by mechanical shock to the system, changes in ambient conditions, metal objects in the detector's vicinity, or loose or aging components. Given the high sensitivity of metal detection systems and the minimal effect that contaminants have on the coil system's output voltage, imbalances can also cause saturation of the receive channel, particularly input amplifiers and phase-sensitive detectors and ADCs, which operate only over a limited voltage signal range. To eliminate such imbalances, an adjustable compensation signal is combined with the receive signal and varied until the undesired imbalance is compensated.

[0039] The receiving unit 3 includes at least one receiving signal path rp, preferably equipped with a receiving matching unit 31, which includes, for example, a balancing transformer, for matching the balanced coil system impedance to the receiving channel impedance. The modulated receiving signal rs and the compensation signal cs provided by the receiving matching unit 31 are applied to a compensation unit 32, such as an addition unit or a subtraction unit. The compensation signal cs compensates for any imbalance contained in the modulated receiving signal rs, and the compensated modulated receiving signal is forwarded to the input of the receiving amplifier 33.

[0040] The receive amplifier 33 outputs the amplified and compensated receive signal rs to a receive phase-sensitive detector 34, where the modulated receive signal rs is compared or multiplied with in-phase and quadrature reference signals, i.e., the transmit signal tx and associated quadrature signal tx90° provided by the frequency source 11. The output of the receive phase-sensitive detector 34 provides a demodulated complex receive signal rsc having an in-phase receive signal component rs-I and a quadrature receive signal component rs-Q.

[0041]

[0041] The complex received signal rsc is transferred from the receive phase sensitive detector 34 via receive analogue to digital converters 35-I; ​​35-Q to a digital signal processing unit 45, for example a digital signal processor provided in the control unit 4. The control unit 4 further comprises an operating or control program 40, which controls all processes of the metal detector during calibration and operation.

[0042]

[0042] If the metal detector uses multiple operating frequencies, the receiving unit 3 preferably includes a separate receiving signal path rp for each operating frequency, so that the received signal is processed in different receiving paths arranged in parallel.

[0043]

[0043] In the signal processing unit 45, a signal processing path sp is implemented, in which the complex received signal rsc is processed, in which signal components related to the product or noise are preferably suppressed, and signal components originating from metal contamination are further processed and detected.

[0044]

[0044] Further, along the signal processing path sp, the complex received signal rsc is further processed to provide a compensation signal cs in which the above-mentioned imbalances having constant or very low frequencies have been removed from the modulated received signal rs. The digital compensation signal provided by the signal processing unit 45 is forwarded to a digital-to-analog converter 91 which provides an analog compensation signal which is forwarded to the compensation unit 32 of the receiving unit 3.

[0045] As mentioned 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 generated by the scanned object. As a result, high-frequency instabilities that may occur in the metal detector are ignored by the compensation loop, and the received signal is affected by high-frequency instabilities, limiting the achievable signal-to-noise ratio and discrimination performance and potentially causing false alarms. Furthermore, higher-frequency instabilities may cause disturbances in the compensation loop, potentially impairing the correction of even low-frequency imbalances.

[0046]

[0046] Such high frequency instabilities, which are ignored by the compensation loop, are usually caused by the transmitting unit 1. Furthermore, changes in the transmitting unit 1 may also cause the transmitting coil signal st to drift in phase and / or amplitude, which may also affect the measurement and detection of contaminants.

[0047] To eliminate such instabilities or anomalies occurring along the transmit signal path tp, a transmit measurement channel 8 is provided, which receives and processes a measurement signal ms tapped off from the transmit signal path tp. The measurement signal ms corresponds to the transmit signal tx picked up at a particular point along the transmit signal path tp. Preferably, the measurement signal ms is picked up near the output of the transmit unit 1 so that undesired modifications of the transmit signal tx can be captured and their associated effects on the modulated receive signal rs corrected or compensated for. In the embodiment of FIG. 1, the measurement signal ms is picked up at the transmit matching unit 13 so that undesired effects present at the output of the transmit amplifier 12 can be determined and corrected or compensated for.

[0048] The transmit 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 with in-phase and quadrature reference signals, i.e., the transmit signal tX and associated quadrature signal tX90° provided by the frequency source 11, to generate a demodulated complex measurement signal mS with an in-phase measurement signal component mS-I and a quadrature measurement signal component mS-Q, which are forwarded to the signal processing unit 45 via analog-to-digital converters 85-I; 85-Q.

[0049] In the signal processing unit 45, 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 provided in the signal processing path sp, as will be explained below with reference to Figures 3a and 3b, thereby eliminating instabilities and disturbances caused by the transmitting unit 1.

[0050] Preferably, such compensation or correction is undone in the correction modules 4561, 4562 by application of the complex measurement signal msc or components thereof to create modified complex compensation signals isc1, isc2 that exactly correspond to the imbalance signal components contained in the modulated received signal rs processed in the receive signal path rp, which is also affected by the disturbances and instabilities of the transmitting unit 1. As a result, the compensation signal cs obtained from the modified complex compensation signals isc1, isc2 allows the corresponding imbalance signal components in the modulated received signal rs to be completely cancelled.

[0051]

[0051] Figure 2a shows the metal detector of Figure 1 in a further preferred embodiment. It is shown that the frequency source 11 includes a digital frequency source module 411, which is preferably implemented in the control unit 4 or directly in the signal processing unit 45. The digital frequency source module 411 provides digital in-phase and quadrature reference signals, which are converted in the digital-to-analog converters 11-I, 11-Q into an in-phase transmit signal tx and an associated quadrature signal tx 90°.

[0052] FIG. 2 a further shows that the measurement signal ms can also be picked up by a measurement coil 23 inductively coupled to the transmission coil 21 .

[0053]

[0053] Figure 2b shows the metal detector of Figure 2a with phase-sensitive detectors 4534, 4584 implemented in the digital domain. Therefore, no reference signal is required in the analog domain. All signal information for demodulation of the modulated received signal rs and the measurement signal ms is available in the signal processing unit 45. Analog-to-digital conversion can also be incorporated in the signal processing unit 45.

[0054]

[0054] The added module 39 in Figure 2b indicates that additional electronic modules such as amplifier and filter units may be present in the receive signal path rp to process the receive signal rs.

[0055]

[0055] Figure 3a shows the signal processing unit 45 of the metal detector of Figures 1 and 2a in a preferred embodiment. 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.

[0056] At the input of the signal processing unit 45, the complex received signal rsc and the complex measurement signal msc are applied to input stages 450, 458 in which known interference is preferably removed by digital filters.

[0057] In a preferred embodiment, during calibration of the metal detector, the complex measurement signal msc is processed to provide a complex or non-complex constant reference value msc-r representing the constant influence of the transmitting unit 1 on the modulated received signal rs. The complex measurement signal msc is, for example, transferred to a calibration module 400, which determines the constant reference value msc-r in phase and / or amplitude during the calibration process. The calibration module 400 is preferably part of the control program 40 or the signal processing unit 45. The constant reference value msc-r is stored in module 459 and used to normalize the measurement signal msc. The normalized measurement signal msc is no longer an absolute measurement value of the transmitting channel, but represents only the change in the measurement value of the transmitting channel during the time interval between the calibration and the actual measurement. The reference value msc-r is typically only recaptured if the state of the transmitting unit 1 changes significantly, for example, due to changes in components or applied signal voltages. As will be described below, the measurement signal ms can be normalized before being applied to the first correction module or after being applied to the first correction module.

[0058]

[0058] While a constant reference value msc-r is determined during calibration, the complex measurement signal msc is continuously monitored so as to be able to continuously correct any instabilities or imbalances caused by the transmitting unit 1. The method of the present invention makes it possible to correct or compensate for some or all of the instabilities, drifts and disturbances occurring in the transmission channel or in the transmission signal path tp.

[0059] To correct the receiver imbalance that continuously appears due to transmitter instabilities, the complex received signal rsc, which still contains signal components related to the product and contaminants transported through the metal detector, and the continuously obtained complex measurement signal msc are applied to a first correction module 451, in which the influence of continuously occurring instabilities of the transmitter unit 1 on the complex received signal rsc is removed, for example, by a complex division function or a Kalman filter, respectively, 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 due to transmitter instabilities are avoided.

[0060]

[0060] Optionally, the once corrected complex received signal rsc1, which still contains signal components related to products and contaminants transported through the metal detector, and a constant reference value msc-r are applied to a second correction module 452 in which the corrected received signal rsc1 is preferably normalized by application of a complex multiplication function.

[0061]

[0061] The metal detector of the present invention may include only the first correction module 451, but preferably includes the first correction module 451 and the second correction module 452 arranged in series in any order.

[0062] The complex received signal rsc2 at the output of the second correction module 452 is forwarded on the one hand to a signal detection module 454 and on the other hand to a compensation signal section 456, which generates a compensation signal cs as described below. In the signal detection module 454, product components still present in the complex received signal rsc are suppressed and the signal components related to the contaminants are further processed and preferably compared with a threshold value indicative of the presence of a contaminant, which, if exceeded, signals the presence of a contaminant.

[0063] As already explained with reference to Figure 1, the metal detector includes a compensation loop. To determine the imbalance that needs to be corrected, signal components related to the product and contaminants are removed from the complex received signal rsc in a loop control module 4560, which provides a complex compensation signal isc. Since the imbalance signal components contained in the modulated received signal rs processed by the receiving unit 3 are subject to drift and instability in the transmitting unit 1, the corrections made by the correction modules 451 and 452 are undone. Therefore, the complex compensation signal isc provided by the loop control module 4560 is corrected accordingly in at least one correction module 4561, 4562.

[0064]

[0064] In order to cancel the correction applied to the complex received signal rsc in the first correction module 451, the complex compensation signal isc and the complex measurement signal msc are applied to a first correction module 4561 provided in the compensation signal section 456, in which the complex compensation signal isc is corrected by the successively obtained complex measurement signal msc, preferably by application of a complex multiplication function.

[0065]

[0065] In order to cancel the correction applied to the complex received signal rsc1 once corrected in the second correction module 452, the once corrected complex compensation signal isc1 provided by the first correction module 4561 and the reference value msc-r stored in module 459 are applied to a second correction module 4562 provided in the compensation signal section 456, in which the once corrected complex compensation signal isc1 is corrected by the reference value msc-r, preferably by applying a complex division function, alternatively by applying a Kalman filter.

[0066]

[0066] The twice-corrected complex compensation signal isc2 delivered by the second correction module 4562 corresponds to the imbalance signal component of the modulated received signal rs processed in the receiving unit 3. The symmetry between the imbalance signal component of the modulated received signal rs processed in the receiving unit 3 and the twice-corrected complex compensation signal isc2 optimizes the cancellation of imbalances and disturbances due to transmitter instabilities in the modulated received signal rs.

[0067]

[0067] If the second correction module 452 is not implemented, the corresponding second modification module 4562 is also not implemented. As mentioned above, 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, 4561, and, if present, modules 452, 4562, are preferably reversed.

[0068]

[0068] The twice-modified complex compensation signal isc2 is applied to a modulation module 4563, which modulates the imbalance signal isc2 at a carrier frequency corresponding to the operating frequency tx. The resulting digitally modulated compensation signal cs is sent from the output of the modulation module 4563 to a digital-to-analog converter 91, which applies the analog modulated compensation signal cs to a compensation unit 32 (such as a summation unit or a subtraction unit) in the receiving unit 3 via an amplifier 92.

[0069]

[0069] Figure 3b shows an example of the signal processing unit 45 of the metal detector of Figure 2b in a preferred embodiment with phase-sensitive detectors 4534, 4584 arranged in the digital domain. Of course, the phase-sensitive detectors 4534, 4584 can also be implemented in the analog domain as shown in Figure 1. Therefore, the signal processing unit 45 in the embodiments of Figures 3a and 3b can be used in any embodiment of the metal detector of the present invention. Functional entities are moved from the analog domain to the digital domain or from the digital domain to the analog domain as necessary. The phase-sensitive detectors 4534, 4584 supply complex received signals rsc and complex measured signals msc to the corresponding interference cancellation modules 450, 458.

[0070]

[0070] In the embodiment of the signal processing unit 45 of Fig. 3b, the correction of the complex received signal rsc and the corresponding modification of the complex compensation signal isc are performed in a different way: the complex measurement signal msc, preferably normalized, is applied to a first correction module 451 and to a corresponding correction module 4561.

[0071] Optionally, a constant reference value msc-r is supplied to module 459 and applied together with the complex measurement signal msc to a normalization module 4510, which normalizes the complex measurement signal msc by the constant reference value msc-r. Since only the resulting normalized measurement signal msc is applied to the complex received signal rsc in a single correction module 451, only a single corresponding correction module 4561 is needed to undo this correction for the compensated signal isc.

[0072]

[0072] Figure 4 shows a further embodiment of a metal detector, which comprises a transmitting unit 1, a balanced coil system 2, a receiving unit 3, a transmitting measurement channel 8, and a control unit 4 with a control program 40 and a signal processing unit 45. The operating principle of this metal detector corresponds to that of the metal detector shown in Figure 1. The circuit of this metal detector, with the exception of the transmitting measurement channel 8 and the associated signal processing module shown in Figure 3, basically corresponds to the metal detector shown in US 10184908 B2. Known and already operating metal detectors can therefore be advantageously enhanced by the solution of the present invention.

[0073] The transmitter unit 1 includes a frequency generator 11, which supplies a transmit signal tx to upper and lower amplifier wings 12A, 12B of a transmitter amplifier 12. Each amplifier wing 12A, 12B includes a preamplifier, in the form of a first and second operational amplifier OA, OA', respectively, amplifying a half-wave of the transmit signal tx, which is applied to the inverting input of the first operational amplifier OA via a resistor R1 and to the non-inverting input of the second operational amplifier OA' via a resistor R2'. The non-inverting input of the first operational amplifier OA and the inverting input of the second operational amplifier OA' are connected to each other via resistors R2 and R1' and to a potential corresponding to half the first power supply voltage -Ub. The outputs of the first and second operational amplifiers OA, OA' are connected to their inverting inputs via resistors R3, R3' and to the bases of the respective first and second power transistors T, T' via resistors R4, R4'.

[0074] The input transmit signal tx is applied to the inverting input of the first operational amplifier OA, so that the positive half-wave of the transmit signal tx is inverted and amplified by the first amplifier 12A. That is, both the first operational amplifier OA and the second operational amplifier OA' supply the negative half-wave to the base of the respective first or second power transistor T, T'. The base is connected to zero potential 0V via resistors R5, R5', and its emitter is connected to the first power supply voltage -Ub via resistors R6, R6', respectively. The collectors of the transistors T, T' are connected to the corresponding first or second switches S11, S12 provided at the input of the transmit matching unit 13. The transmit matching unit 13 includes a coupling transformer 131 having two primary coils 1311A, 1311B and one secondary coil 1312, and a plurality of tuning capacitors 132, 133.

[0075] The collector of the first power transistor T is connected via a first switch S11 to one of multiple taps A, B, C, D of a 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 multiple taps A', B', C', D' of a second primary coil 1311B of the coupling transformer 131. The first and second primary coils 1311A and 1311B, which are identically designed but wound in opposite directions, are connected to a common tap of a second supply voltage +Ub. The taps A, B, C, D and A', B', C', D' are arranged at the same number of turns from the common tap. The first and second switches S11 and S12 are controlled to always select corresponding taps so that the same load is applied to the power transistors T and T' and symmetry is maintained. Therefore, in this preferred embodiment, the power stages with the power transistors T, T' of the amplifier wings 12A, 12B are completely identical.

[0076]

[0076] A switch S2 allows the transmit coil 21 to be connected to the appropriate tappings E, F, G of the secondary coil 1312. A further switch S3 allows one of the tuning capacitors 132, 133 to be connected in parallel with the transmit coil 21. Switches S2 and S3 are selected to form a resonant circuit tuned to the selected operating frequency.

[0077] 4 further shows that the measurement signal ms is picked up at the output of the transmitting amplifier 12 at the switches S11 or S12, or at the switches S11 and S12, for a differential signal on the primary side of the coupling transformer 131. Alternatively, the measurement signal ms may be picked up on the secondary side of the coupling transformer 131, for example at the switches S2 and S3, which connect the transmitting coil 21 to one of the taps E, F or G for a differential signal. Further alternatively, the measurement signal ms may be picked up at a measuring coil 23 inductively coupled to the transmitting coil 21. The measurement signal ms is forwarded to the transmitting measurement channel 8 shown in FIG. 1 and processed as described with reference to FIGS. 1 and 3. <Additional Notes> [Form 1] A method of operating a metal detector including a balanced coil system (2) having a transmitting coil (21) connected to a transmitting unit (1) and first and second receiving coils (22A, 22B) connected to inputs of a receiving unit (3) connected to a signal processing unit (45), comprising: The transmitting unit (1) comprises a transmitting signal path (tp) to which a transmitting signal (tx) having at least one fixed or selectable operating frequency and an associated quadrature signal (tx90°) are supplied, the transmitting signal (tx) being applied to the input of a transmitting amplifier (12) which transfers the amplified transmitting signal (tx) to the transmitting coil (21) directly or via a transmitting matching unit (13); The receiving unit (3) includes at least one receiving signal path (rp) through which the modulated receiving signal (rs) received from the balanced coil system (2) is applied to a receiving amplifier (33) directly or via a receiving matching unit (31), and the receiving amplifier (33) transfers the amplified modulated receiving signal (rs) directly or indirectly to a receiving phase-sensitive detector (34; 4534); The receive phase-sensitive detector (34, 4534) compares the modulated receive signal (rs) with reference signals corresponding to the transmit signal (tx) and the quadrature signal (tx90°) to generate a demodulated complex receive signal (rsc) having an in-phase receive signal component (rs-I) and a quadrature receive signal component (rs-Q); The in-phase received signal component (rs-I) and the quadrature received signal component (rs-Q) are processed in the signal processing unit (45) including at least one signal processing path (sp), in which signal components of the complex received signal (rsc) related to the product or noise are suppressed and signal components originating from metal contaminants are further processed, At least one transmitting measurement channel (8) is provided, which includes a measurement amplifier (83) for receiving a measurement signal (ms) taken from the transmitting signal path (tp), amplifying the measurement signal (ms) and forwarding it directly or indirectly to a measurement phase-sensitive detector (84; 4584); The measurement phase-sensitive detector (84; 4584) compares the measurement signal (ms) with the reference signals corresponding to the transmit signal (tx) and the quadrature signal (tx90°) to generate a complex measurement signal (msc) having an in-phase measurement signal component (ms-I) and a quadrature measurement component (ms-Q); the complex measurement signal (msc) and the complex received signal (rsc) are applied to a first correction module (451), in which signal components due to instabilities of the transmitting unit (1) are removed from the complex received signal (rsc); A method of operating a metal detector, comprising: [Form 2] In the method for operating a metal detector according to aspect 1, providing a control loop for removing imbalanced signal components contained in the modulated received signal (rs); removing, in a loop control module (4560), signal components related to the product and the contaminant from said complex received signal (rsc) to obtain a complex compensated signal (isc); - undoing the correction applied to the complex received signal (rscl) in the first correction module (451) by applying the complex compensation signal (isc) and the complex measurement signal (msc) to a first correction module (4561) which provides a corrected compensation signal (isc1); modulating the modified compensation signal (isc1) with a carrier frequency corresponding to the operating frequency (tx) in a modulation module (4563) for providing a modulated compensation signal (cs); converting the modulated compensation signal (cs) into a modulated analog compensation signal (cs) in a digital-to-analog converter (91); applying the modulated analog compensation signal (cs) to a compensation unit (32) in the receive signal path (rp) to compensate for the unbalanced signal component contained in the modulated receive signal (rs); A method of operating a metal detector, including: [Form 3] In the method for operating a metal detector according to aspect 1 or 2, processing said complex measurement signal (msc) during calibration of said metal detector in a calibration module (400) to obtain a complex or non-complex constant reference value (msc-r) representing the constant influence of said transmitting unit (1) on said modulated received signal (rs) and used to directly or indirectly normalize said complex measurement signal (msc), How a metal detector works. [Form 4] In the method for operating a metal detector according to aspect 3, applying said constant reference value (msc-r) and said complex received signal (rsc1) to a second correction module (452) to provide a normalized complex received signal (rsc2); applying said complex compensation signal (isc1) and said constant reference value (msc-r) to a second correction module (4562) provided in said control loop, thereby canceling the correction applied to said complex received signal (rsc1) in said second correction module (452); A method of operating a metal detector, including: [Form 5] In the method for operating a metal detector according to aspect 3, applying the constant reference value (msc-r) and the complex measurement signal (msc) to a normalization module (4510) and providing a normalized measurement signal (msc-n) to the first correction module (451); applying the complex compensation signal (isc) and the normalized measurement signal (msc-n) to the first correction module (4561) in the control loop, thereby canceling the correction applied to the complex received signal (rsc1) in the first correction module (451); A method of operating a metal detector, including: [Form 6] In the method for operating a metal detector according to any one of aspects 1 to 5, a) picking up the measurement signal (ms) at the output of the transmission amplifier (12), or b) transforming the transmission signal (tx) in the transmission matching unit (13) by a coupling transformer (131) having at least one primary coil (1311A, 1311B) and at least one secondary coil (1312), and picking up the measurement signal (ms) in the at least one primary coil (1311A, 1311B) or the at least one secondary coil (1312); or c) coupling a measurement coil (23) to the transmitting coil (21) and picking up the measurement signal (ms) with the measurement coil (23); A method of operating a metal detector, including: [Form 7] In the method for operating a metal detector according to any one of aspects 1 to 6, providing, for each operating frequency, a dedicated receive signal path (rp), a dedicated signal processing path (sp), and a dedicated transmit measurement channel (8); How a metal detector works. [Form 8] A metal detector operating in accordance with the method of any one of aspects 1 to 7. [Form 9] In the metal detector according to aspect 8, The present invention includes a balanced coil system (2) having a transmitting coil (21) connected to a transmitting unit (1) and first and second receiving coils (22A, 22B) connected to the input of a receiving unit (3) connected to a signal processing unit (45), and a frequency source (11) for supplying a transmitting signal (tx) having at least one fixed or selectable operating frequency and an associated quadrature signal (tx90°), The transmitting unit (1) includes at least one transmitting signal path (tp) having a transmitting amplifier (12) connected to the transmitting coil (21) directly or via a transmitting matching unit (13); The receiving unit (3) includes at least one receiving signal path (rp) in which the receiving coils (22A, 22B) of the balanced coil system (2) are connected to an input of a receiving amplifier (33) directly or via a receiving matching unit (31), and the amplified modulated receiving signal (rs) from the receiving amplifier (33) is applied directly or indirectly to a receiving phase-sensitive detector (34, 4534); The receive phase-sensitive detector (34, 4534) is designed to compare the modulated receive signal (rs) with reference signals corresponding to the transmit signal (tx) and the quadrature signal (tx90°) to generate a demodulated complex receive signal (rsc) having an in-phase receive signal component (rs-I) and a quadrature receive signal component (rs-Q); the signal processing unit (45) comprises at least one signal processing path (sp), in which signal components of the complex received signal (rsc) related to the product or noise are suppressed and signal components originating from metal contaminants are further processed; At least one transmit measurement channel (8) is provided, said transmit measurement channel having an input connected to the transmit signal path (tp) for receiving a measurement signal (ms), said transmit measurement channel including a measurement amplifier (83) having an output connected directly or indirectly to a measurement phase-sensitive detector (84; 4584); the measurement phase-sensitive detector (84; 4584) is designed to compare the measurement signal (ms) with the reference signal corresponding to the transmission signal (tx) and the quadrature signal (tx90°) to generate a demodulated complex measurement signal (msc) having 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) capable of removing signal components from the complex received signal (rsc) due to instabilities of the transmitting unit (1); metal detector. [Form 10] In the metal detector according to aspect 8 or 9, The measurement signal (ms) can be transmitted from the output of the transmission amplifier (12) to the input of the measurement amplifier (83), or the transmitting matching unit (13) comprises a coupling transformer (38) having at least one primary coil (381A, 381B) and at least one secondary coil (382), and 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 an input of the measurement amplifier (83), or a measuring coil (23) is inductively coupled to the transmitting coil (21) and connected to the input of the measuring amplifier (83) to provide the measuring signal (ms); metal detector. [Form 11] In the metal detector according to any one of aspects 8 to 10, The signal processing unit (45) a control loop for removing imbalanced signal components contained in the modulated received signal (rs); a loop control module (4560) for providing a complex compensation signal (isc) that does not include signal components related to the product and the contaminants; a first correction module (4561) to which the complex compensation signal (isc) and the complex measurement signal (msc) are applied to provide a modified compensation signal (isc1) that cancels the correction applied to the complex received signal (rsc1) in the first correction module (451); a modulation module (4536) for providing a modulated compensation signal (cs); a digital-to-analog converter (91) for converting the digitally modulated compensation signal (isc) into an analog-modulated compensation signal (cs); a compensation unit (32) in the receive signal path (rp) capable of applying the modulated analog compensation signal (cs) to the modulated receive signal (rs) to remove imbalance; Including metal detectors. [Form 12] In the metal detector according to any one of aspects 8 to 11, a calibration module (400) is provided which is designed to process the complex measurement signal (msc) during calibration of the metal detector to obtain a constant reference value (msc-r), complex or non-complex, which represents the constant influence of the transmitting unit (1) on the modulated received signal (rs); metal detector. [Form 13] In the metal detector according to aspect 12, a second correction module (452) is provided to which the constant reference value (msc-r) and the complex received signal (rsc1) can be applied, the second correction module (452) being designed to provide a normalized complex received signal (rsc2); a second correction module (4562) is provided in the control loop to which the complex compensation signal (isc1) and the constant reference value (msc-r) are applied; the correction applied to the complex received signal (rsc1) in the second correction module (452) is cancelled; metal detector. [Form 14] In the metal detector according to aspect 12, a normalization module (4510) is provided, to which the constant reference value (msc-r) and the complex measurement signal (msc) can be applied, and which supplies a normalized measurement signal (msc-n) to the first correction module (451); the complex compensation signal (isc) and the normalized measurement signal (msc-n) can be applied to the first correction module (4561), in which the correction applied to the complex received signal (rsc1) in the first correction module (451) is cancelled; metal detector. [Form 15] In the metal detector according to any one of aspects 9 to 14, the output of the receiving phase-sensitive detector (34) and the output of the measuring phase-sensitive detector (84) are connected to the signal processing unit (45) via analog-to-digital converters (35-I; ​​35-Q), or the receiving phase-sensitive detector (4534) and the measuring phase-sensitive detector (4584) are implemented in the software domain of the signal processing unit (45); metal detector. [Explanation of symbols]

[0078]

[0078] 1 transmitting unit 11 Frequency Sources, Synthesizers 11-I, 11-Q digital-to-analog converter 12 Transmitting amplifier 12A, 12B amplifier section 13 Transmission Matching Unit 131 Coupling transformer 1311A, 1311B primary coil 1312 Secondary coil 132, 133 Tuning capacitors 2 Coil System 21 Transmitting coil 22A, 22B receiving coils 23 Measuring coil 3 receiving unit 31 Receiving matching unit 32 compensation units 33 Receiving amplifier 34 Receiver Phase Sensitive Detector / Demodulator 35I, 35Q receiving analog-to-digital converter 4. Control Unit 40 Control Program 400 / CAL Calibration Module 411 Digital Frequency Source Module 45 Signal Processing Units / Modules 450, 458 Interference Rejection Module 451 / DIV Complex Division Correction Module 4510 / DIV Normalization Module 452 / MUL Complex Multiplication Correction Module 454 / MD Metal Detection Module 456 Compensation signal section 4560 Loop Control Module 4561 / MUL Complex Division Correction Module 4562 / MUL Complex Multiplication Correction Module 4563 / MOD Modulation Module 458 / KIR Interference Rejection Module 459 / TRV drive reference value 6 Product conveyor 8 transmit measurement channels 83 Measuring Amplifier 84 Measurement Phase Sensitive Detector / Demodulator 85I, 85Q Measurement Analog / Digital Converter 9 Compensation Channels 91 Digital-to-analog converter 92 Amplifier in compensation channel cs modulated compensation signal isc Complex compensation signal isc1 complex compensation signal modified once isc2 twice-corrected complex compensation signal ms measurement signal msc complex measurement signal msc-n normalized complex measurement signal msc-r transmitter reference value ms-I In-phase component of the measurement signal ms-Q Quadrature components of the measurement signal rs modulated received signal rsc Complex received signal rs-I In-phase component of the complex received signal rs-Q Quadrature components of the complex received signal rsc1 complex received signal corrected once rsc2 twice corrected complex received signal tx;tx90° Transmit signal, quadrature signal OPT Option module

Claims

1. A method of operating a metal detector including a balanced coil system (2) with a transmitting coil (21) connected to a transmitting unit (1) and first and second receiving coils (22A, 22B) connected to the inputs of a receiving unit (3) connected to a signal processing unit (45), comprising: The transmitting unit (1) includes a transmitting signal path (tp) to which a transmitting signal (tx) having at least one fixed or selectable operating frequency and a quadrature signal (tx 90°) to the transmitting signal (tx) are supplied, the transmitting signal (tx) being applied to the input of a transmitting amplifier (12) which transfers the amplified transmitting signal (tx) to the transmitting coil (21) directly or via a transmitting matching unit (13); The receiving unit (3) includes at least one receiving signal path (rp) through which a receiving signal (rs) modulated by a commodity received from the first and second receiving coils (22A, 22B) and conveyed through the balanced coil system (2) is applied directly or via a receiving matching unit (31) to a receiving amplifier (33), and the receiving amplifier (33) transfers the amplified modulated receiving signal (rs) directly or indirectly to a receiving phase-sensitive detector (34; 4534); The receive phase-sensitive detector (34, 4534) compares the modulated receive signal (rs) with reference signals corresponding to the transmit signal (tx) and the quadrature signal (tx90°) to generate a demodulated complex receive signal (rsc) having an in-phase receive signal component (rs-I) and a quadrature receive signal component (rs-Q); The in-phase received signal component (rs-I) and the quadrature received signal component (rs-Q) are processed in the signal processing unit (45) including at least one signal processing path (sp), in which signal components of the complex received signal (rsc) related to goods or noise are suppressed and signal components originating from metal contaminants are further processed, At least one transmitting measurement channel (8) is provided, which includes a measurement amplifier (83) for receiving a measurement signal (ms) taken from the transmitting signal path (tp), amplifying the measurement signal (ms) and forwarding it directly or indirectly to a measurement phase-sensitive detector (84; 4584), the measurement phase-sensitive detector (84; 4584) compares the measurement signal (ms) with the reference signals corresponding to the transmit signal (tx) and the quadrature signal (tx90°) to generate a complex measurement signal (msc) having an in-phase measurement signal component (ms-I) and a quadrature measurement component (ms-Q); The complex measurement signal (msc) and the complex received signal (rsc) are applied to a first correction module (451), and signal components due to instabilities of the transmitting unit (1) are removed from the complex received signal (rsc), and a complex received signal (rsc1) is output from the first correction module (451). A method of operating a metal detector, comprising:

2. 2. The method of operating a metal detector according to claim 1, providing a control loop for removing imbalanced signal components contained in the modulated received signal (rs); - in a loop control module (4560), removing signal components relating to the product and the contaminant from said complex received signals (rscl, rsc2) to obtain a complex compensated signal (isc); - undoing the correction applied to the complex received signal (rsc) in the first correction module (451) by applying the complex compensation signal (isc) and the complex measurement signal (msc) to a first correction module (4561) which provides a corrected compensation signal (isc1); modulating said modified compensation signal (isc1) with a carrier frequency corresponding to said operating frequency (tx) in a modulation module (4563) for providing a modulated compensation signal (cs); converting said modulated compensation signal (cs) into a modulated analog compensation signal (cs) in a digital-to-analog converter (91); applying the modulated analog compensation signal (cs) to a compensation unit (32) provided in the receive signal path (rp) to compensate for the unbalanced signal component contained in the modulated receive signal (rs); A method of operating a metal detector, including:

3. 3. The method for operating a metal detector according to claim 1 or 2, processing said complex measurement signal (msc) during calibration of said metal detector in a calibration module (400) to obtain a complex or non-complex constant reference value (msc-r) representing the constant influence of said transmitting unit (1) on said modulated received signal (rs) and used to directly or indirectly normalize said complex measurement signal (msc), How a metal detector works.

4. In the method of operating a metal detector according to claim 3, which cites claim 2, applying said constant reference value (msc-r) and said complex received signal (rsc1) to a second correction module (452) to provide a normalized complex received signal (rsc2); applying said compensation signal (isc1) and said constant reference value (msc-r) to a second correction module (4562) provided in said control loop, thereby canceling the correction applied to said complex received signal (rsc1) in said second correction module (452); A method of operating a metal detector, including:

5. A method for operating a metal detector according to claim 3, which cites claim 2, applying said constant reference value (msc-r) and said complex measurement signal (msc) to a normalization module (4510) and providing a normalized measurement signal (msc-n) to said first correction module (451); applying said complex compensation signal (isc) and said normalized measurement signal (msc-n) to said first correction module (4561) in said control loop to cancel the correction applied to said complex received signal (rsc) in said first correction module (451); A method of operating a metal detector, including:

6. 6. A method for operating a metal detector according to any one of claims 1 to 5, a) picking up the measurement signal (ms) at the output of the transmission amplifier (12), or b) transforming the transmission signal (tx) in the transmission matching unit (13) by a coupling transformer (131) having at least one primary coil (1311A, 1311B) and at least one secondary coil (1312), and picking up the measurement signal (ms) in the at least one primary coil (1311A, 1311B) or the at least one secondary coil (1312); or c) coupling a measurement coil (23) to the transmitting coil (21) and picking up the measurement signal (ms) with the measurement coil (23); A method of operating a metal detector, including:

7. 7. A method for operating a metal detector according to any one of claims 1 to 6, comprising: providing, for each operating frequency, a dedicated receive signal path (rp), a dedicated signal processing path (sp) and a dedicated transmit measurement channel (8); How a metal detector works.

8. A metal detector operating in accordance with the method of any one of claims 1 to 7.

9. 9. The metal detector according to claim 8, The present invention includes a balanced coil system (2) having a transmitting coil (21) connected to a transmitting unit (1) and first and second receiving coils (22A, 22B) connected to the input of a receiving unit (3) connected to a signal processing unit (45), and a frequency source (11) for supplying a transmitting signal (tx) having at least one fixed or selectable operating frequency and an associated quadrature signal (tx 90°), The transmitting unit (1) includes at least one transmitting signal path (tp) having a transmitting amplifier (12) connected to the transmitting coil (21) directly or via a transmitting matching unit (13); The receiving unit (3) includes at least one receiving signal path (rp) in which the receiving coils (22A, 22B) of the balanced coil system (2) are connected directly or via a receiving matching unit (31) to an input of a receiving amplifier (33), and the amplified modulated receiving signal (rs) from the receiving amplifier (33) is applied directly or indirectly to a receiving phase-sensitive detector (34, 4534); The receiving phase-sensitive detector (34, 4534) is designed to compare the modulated receiving signal (rs) with reference signals corresponding to the transmitting signal (tx) and the quadrature signal (tx90°) to generate a demodulated complex receiving signal (rsc) having an in-phase receiving signal component (rs-I) and a quadrature receiving signal component (rs-Q); said signal processing unit (45) comprising at least one signal processing path (sp), in which signal components of said complex received signal (rsc) related to the product or noise are suppressed and signal components originating from metal contaminants are further processed; At least one transmit measurement channel (8) is provided, said transmit measurement channel having an input connected to the transmit signal path (tp) for receiving a measurement signal (ms), said transmit measurement channel including a measurement amplifier (83) having an output connected directly or indirectly to a measurement phase-sensitive detector (84; 4584), the measurement phase-sensitive detector (84; 4584) is designed to compare the measurement signal (ms) with the reference signals corresponding to the transmission signal (tx) and the quadrature signal (tx90°) to generate a demodulated complex measurement signal (msc) having 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) capable of removing signal components from the complex received signal (rsc) that are due to instabilities of the transmitting unit (1) and outputting a complex received signal (rsc1); metal detector.

10. 10. The metal detector according to claim 8 or 9, The measurement signal (ms) can be transmitted from the output of the transmission amplifier (12) to the input of the measurement amplifier (83), or The transmitting matching unit (13) includes a coupling transformer (131) having at least one primary coil (1311A, 1311B) and at least one secondary coil (1312), and the measurement signal (ms) can be transmitted from the at least one primary coil (1311A, 1311B) or the at least one secondary coil (1312) to an input of the measurement amplifier (83), or a measuring coil (23) is inductively coupled to the transmitting coil (21) and connected to the input of the measuring amplifier (83) to provide the measuring signal (ms); metal detector.

11. The metal detector according to any one of claims 8 to 10, The signal processing unit (45) a control loop for removing imbalanced signal components contained in the modulated received signal (rs); a loop control module (4560) for providing a complex compensation signal (isc) that does not include signal components related to the product and contaminants; a first correction module (4561) to which the complex compensation signal (isc) and the complex measurement signal (msc) are applied to provide a modified compensation signal (isc1) in which the correction applied to the complex received signal (rsc) in the first correction module (451) is cancelled; a modulation module (4563) for modulating the modified compensation signal (isc1) with a carrier frequency corresponding to the operating frequency (tx) to provide a modulated compensation signal (cs); a digital-to-analog converter (91) for converting the modulated compensation signal (cs) into a modulated analog compensation signal (cs); a compensation unit (32) in the receive signal path (rp) capable of applying the modulated analog compensation signal (cs) to the modulated receive signal (rs) to remove imbalances; Including metal detectors.

12. The metal detector according to any one of claims 8 to 11, a calibration module (400) is provided which is designed to process the complex measurement signal (msc) during calibration of the metal detector to obtain a constant reference value (msc-r), complex or non-complex, which represents the constant influence of the transmitting unit (1) on the modulated received signal (rs); metal detector.

13. In the metal detector according to claim 12, which cites claim 11, a second correction module (452) is provided to which the constant reference value (msc-r) and the complex received signal (rsc1) can be applied, the second correction module (452) being designed to provide a normalized complex received signal (rsc2); a second correction module (4562) is provided in the control loop to which the compensation signal (isc1) and the constant reference value (msc-r) are applied; the correction applied to the complex received signal (rsc1) in the second correction module (452) is cancelled; metal detector.

14. In the metal detector according to claim 12, which cites claim 11, a normalization module (4510) is provided, to which the constant reference value (msc-r) and the complex measurement signal (msc) can be applied, and which normalization module (4510) supplies a normalized measurement signal (msc-n) to the first correction module (451); said complex compensation signal (isc) and said normalized measurement signal (msc-n) can be applied to said first correction module (4561), in which the correction applied to said complex received signal (rsc) in said first correction module (451) is cancelled; metal detector.

15. 15. The metal detector according to claim 9, The output of the receiving phase-sensitive detector (34) and the output of the measuring phase-sensitive detector (84) are connected to the signal processing unit (45) via analog / digital converters (35-I; ​​35-Q), or the receiving phase-sensitive detector (4534) and the measuring phase-sensitive detector (4584) are implemented in the software domain of the signal processing unit (45). metal detector.

Images