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 transmitting coil (21) connected to a transmitting unit (1) and first and second receiving coils (22A, 22B) connected to an input of a receiving unit (3) connected to a signal processing unit (45). The transmitting unit (1) includes a transmitting signal path (tp) for a transmitting signal (tx) having at least one fixed or selectable operating frequency, which is applied to the input of a transmitting amplifier (12) which transfers the amplified transmitting signal (tx) to the transmitting coil (21) either directly or via a transmitting matching unit (13). The receiving unit (3) includes at least one receiving signal path (rp) in which the receiving signal (rs) received from the balanced coil system (2) is applied directly or via a receiving matching unit (31) to a receiving amplifier (33), which directly or indirectly transfers the amplified receiving signal (rs) to a receiving demodulator (35; 4535) provided in the receiving unit (3) or implemented in a signal processing unit (45). The receiving demodulator (35; 4535) provides a demodulated complex receiving signal (rsc) having an in-phase receiving signal component (rs-I) and a quadrature receiving signal component (rs-Q). The in-phase receiving signal component (rs-I) and the quadrature receiving signal component (rs-Q) are processed in at least one signal processing path (sp) provided in the signal processing unit (45), in which signal components of the complex receiving signal (rsc) related to products or noise are suppressed and signal components originating from metal contamination are further processed. The invention comprises the steps of providing at least one measurement signal (ms) taken from the receiving signal path (rp) to a measurement channel (8), analysing the measurement signal (ms) in an evaluation module (88) or a receiving control module (488) and providing relevant measurement information based on the obtained measurement information, determining in the receiving control module (488) whether a critical state exists in which signal processing in the receiving signal path (rp) may be impaired, and providing a procedure for handling the critical state in the metal detector which is executed if a critical state exists.

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 installed and operated, they 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 contamination, including ferrous, non-ferrous, and stainless steel, in a wide variety of products, both fresh and frozen.

[0003]

[0003] Metal detectors that operate according to the balanced coil principle typically include a transmitter coil and two identical receiver coils, wound on a non-metallic frame, each typically parallel to the other coil. The receiver coils are typically arranged around the transmitter coil and are identical, so that identical voltages are induced in each. The first receiver coil is connected in series with the oppositely wound second receiver coil to receive an output signal that is zero when the system is in a balanced state. Thus, 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 array 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 can be processed and amplified in the receiver unit and then used to detect the presence of metal contaminants in the observed product.

[0005]

[0005] Therefore, under ideal conditions, a balanced coil system provides no output signal when no product or contaminants are present. However, as described in US 2020 / 333498 A1, even though identical detection coils are set to near-perfect balance on the factory floor, there are still cases where a metal detector fails to achieve balance in the absence of contaminants, resulting in, for example, the rejection of perfectly acceptable food products. A metal detector's balanced state can be disrupted by mechanical shock to the system, changes in environmental conditions, metal objects in the metal detector's vicinity, or loose or aging components. Given the high sensitivity of metal detection systems and the minimal effect of contaminants on the coil system's output voltage, imbalances can cause saturation of the receive channel, particularly input amplifiers and phase-sensitive detectors, which only operate over a limited signal voltage range. To eliminate such imbalances, an adjustable balance signal derived from the drive signal is combined with the detector's output signal and varied until the imbalance in the output signal is compensated.

[0006]

[0006] The correction loop implemented to compensate for the imbalance has a relatively high time constant and a low bandwidth so as not to affect the signal generated by the scanned object. Therefore, it takes a considerable amount of time to compensate for a strong imbalance signal. Such an imbalance can be caused by external influences, such as moving a metal object close to the metal detector, by a relatively large metal object contained in the product being observed, or by a malfunction of the metal detector itself, such as an oscillation occurring in the transmission channel.

[0007]

[0007] Therefore, different influences can have a strong influence on the output signal of the receiving unit, which can lead to ambiguities that can cause the output signal to indicate the presence of product contamination when it is not present, or to indicate the absence of contamination when it is present, which can lead to undesirable false positive or false negative reports.

[0008]

[0008] Metal detectors use a reference signal to decompose a measurement signal into real and imaginary parts. DE 10 2017 124 407 A1 discloses a metal detector in which such a reference signal is tapped at a connection point of a receiving coil.

[0009]

[0009] US20150276964A1 discloses a method for monitoring the operation of a metal detection system with a balanced coil system, for which a test signal or monitoring signal having a modulated monitoring frequency is applied to a monitoring coil inductively coupled to at least one of the receiving coils, the output signal of which is demodulated in a demodulation unit providing a demodulated monitoring signal for each operating frequency, the demodulated monitoring signal being compared with a reference in phase and / or amplitude to obtain information about the performance of the metal detector.

[0010]

[0010] US20120098667A1 discloses a further method for detecting faults in a metal detection system by introducing a monitoring signal. Introducing a monitoring signal in a first stage of a metal detection system, extracting the monitoring signal in a second stage, and analyzing the extracted monitoring signal in a third stage requires a great deal of effort but provides valuable information about the performance of the metal detection system. However, if a critical situation occurs, for example, when the output signal of the receiver stage is clipped at one or other end of the signal range, the desired information may not be obtained. Summary of the Invention [Problem to be solved by the invention]

[0011]

[0011] 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.

[0012] The method of the present invention avoids the undesirable false positive reports, ie, false alarms, and more importantly, false negative reports, caused by critical conditions occurring in metal detectors.

[0013]

[0013] Any critical condition occurring in a metal detector that could cause such a false report must be detected as quickly as possible so that necessary corrections can be made and the issuance of a false report can be avoided.

[0014] Furthermore, after the occurrence of a critical condition, the metal detector must be returned to normal operating conditions in the shortest possible time.

[0015] Furthermore, the critical condition must be detectable without introducing a test or monitoring signal into the metal detector. [Means for solving the problem]

[0016] The 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 inputs of a receive unit connected to a signal processing unit. The transmit unit includes a transmit signal path for a transmit signal having at least one fixed or selectable operating frequency. The transmit signal and a reference signal, such as a quadrature signal, are provided from an internal or external frequency source, for example by the signal processing unit.

[0017]

[0017] A transmit signal is applied to the input of a transmit amplifier, which transfers the amplified transmit signal to a transmit coil directly or via a transmit matching unit. The receive unit includes at least one receive signal path, through which a receive signal received from the balanced coil system is applied to the receive amplifier directly or via a receive matching unit, and the receive amplifier transfers the amplified receive signal directly or indirectly to a receive demodulator provided in the receive unit or implemented in a signal processing unit. The receive demodulator provides a demodulated complex receive signal having an in-phase receive signal component and a quadrature receive signal component, which are processed in at least one signal processing path provided in the signal processing unit, where signal components of the complex receive signal related to products or noise are suppressed and signal components derived from metal contaminants are further processed.

[0018]

[0018] The present invention includes the steps of providing at least one measurement signal extracted from the receive signal path and provided to a measurement channel, analyzing the measurement signal in an evaluation module or receive control module and providing related measurement information, and determining in the receive control module based on the obtained measurement information whether a critical condition exists in which signal processing in the receive signal path is impaired or may be impaired. Furthermore, if a critical condition exists, a procedure for handling critical conditions in the metal detector is executed, which procedure defines at least one appropriate action or multiple appropriate actions that can be initiated individually or in combination.

[0019]

[0019] Thus, a critical condition can be reliably detected without introducing test or monitoring signals into the metal detector which may be adversely affected by the critical condition.

[0020]

[0020] A detected critical condition relates to a condition in which signal processing in the receive signal path is impaired or potentially impaired, where at least one of the receive stages exceeds or is expected to exceed specified operating conditions, potentially causing erroneous measurement results.

[0021] During a test or calibration procedure, a critical condition can be intentionally created to observe the operation of the receiver unit and the corresponding changes in the measurement signal in one or more receiver stages. For example, a large metal piece can be introduced into the coil system, increasing the measurement signal and the received signal of the receiver stage until the received signal becomes distorted, clipped, or no longer linearly amplified. Alternatively, a critical condition can be simulated by bringing a metal object (possibly a magnetic metal object) into the vicinity of the coil system. Thus, by observing the measurement signal, it is possible to determine when a critical condition occurs and signal processing in the receive signal path is impaired or potentially impaired. For example, according to the above embodiment, the input signal to the receiver unit can be increased until saturation of one of the receiver stages is detected. The measurement signal, possibly reduced by a margin, can then serve as an indicator of a critical condition in the receiver unit.

[0022]

[0022] In a preferred embodiment, the measurement information is evaluated in a receiving control module, preferably implemented in a control unit or signal processing unit, to classify the measurement information, assign the measurement information to at least one, preferably at least a first or second, class of criticality condition, and determine how to process the complex received signal in accordance with the classification of the criticality condition and how to apply corrective measures.

[0023] According to the measurement information, the critical state is preferably - the first class related to critical conditions caused by products or contaminants, or - the second class, related to critical states caused by external influences, or - A third class related to critical states caused by irregular conditions of the metal detector, such as saturation, or -Assigned to the fourth class, which relates to critical states caused by drifts occurring in metal detectors. For each of these classes, the critical state procedure defines the appropriate action to terminate the critical state.

[0024]

[0024] The execution of the criticality procedure preferably includes: a) providing and applying control information or control signals to at least one functional module provided in the receive signal path in order to reset the receive unit or to return the receive unit or part thereof to a normal operating state or to hold the receive unit or part thereof in a stable state; and / or b) providing and applying control information to at least one functional module in the signal processing path to reset the signal processing unit or to return the signal processing unit or part thereof to a normal operating state or to keep the signal processing unit or part thereof in a stable state; and / or c) providing information to a control program implemented in the control unit, which program initiates an acoustic or optical alarm signal emitted by the metal detector; and / or d) providing information to a control program implemented in the control unit, which process the measurement data according to a protocol established for the occurrence of a critical situation; and / or e) using a balance control loop to eliminate imbalances occurring in the receive signal path and providing and applying control information to at least one functional module provided in the balance control loop to reset the balance control loop or return the balance control loop to an operating state in which the imbalance is at least roughly corrected or held at a stable value; and / or f) using a balance control loop for removing imbalances occurring in the receive signal path, the balance control loop deriving an imbalance signal from the receive signal or, if a critical condition exists, from the measurement signal; Contains at least one of the following:

[0025]

[0025] Furthermore, the metal detector can be provided with specific operating modes for one or more classes of critical conditions, so that after detection of a critical condition, the operating mode of the metal detector can be changed to the appropriate mode.

[0026]

[0026] If the metal detector uses two or more operating frequencies, preferably a dedicated receive signal path and a dedicated signal processing path are provided for each operating frequency, and a dedicated measurement channel is provided if the signal processing in the measurement channel depends on the operating frequency. Thus, signal processing is performed in parallel in different signal channels for each operating frequency.

[0027]

[0027] The measurement channel includes a measurement signal path through which the measurement signal is processed. In a preferred embodiment, the measurement signal is applied to a measurement demodulator provided in the measurement channel or implemented in the signal processing unit. The measurement demodulator provides a demodulated complex measurement signal having an in-phase measurement signal component and a quadrature measurement signal component. The in-phase and quadrature measurement signal components are analyzed in the signal processing unit or the receiver control module to provide relevant measurement information. Thus, the measurement signal can be analyzed not only before demodulation, for example by comparing the signal amplitude or signal energy with at least one reference value, but also after demodulation to detect anomalies in the baseband signal.

[0028]

[0028] At least one measurement signal corresponding to the received signal at a particular point in the received signal path is preferably picked up at the input or output of an active module, such as an amplifier, provided in the receiving unit. Preferably, two or more measurement signals are picked up at different points in the received signal path so as to ensure that the occurrence of a critical condition can be detected together with the exact location of its occurrence.

[0029]

[0029] The receiver demodulator and / or measurement demodulator are preferably provided in the embodiment of a phase-sensitive detector, which compares the applied receive signal or measurement signal with a reference transmit signal and an associated reference quadrature signal to provide a demodulated complex receive signal or a demodulated complex measurement signal. In all embodiments described below, the receiver demodulator or measurement demodulator is preferably a phase-sensitive detector and will be described accordingly. However, in each embodiment, the demodulator can be provided in any other embodiment known to those skilled in the art.

[0030] Observing the receiving signal path has the advantage that analysis of the measurement channel and the measurement signal provides information that allows for the detection of critical conditions and supports the interpretation of the receiving unit's output signal. The control information provided by the receiving control unit allows the metal detector, including entities such as the transmitting unit and / or the receiving unit and / or the signal processing unit, and / or the balancing control loop or functional modules of these stages, to be controlled in response to a detected critical condition. In this way, the metal detector can be returned to normal operation as quickly as possible. Furthermore, the control information allows for control of the measurement process and the handling of the measurement data. It is possible to determine whether the measurement data is valid or invalid. As a result, false positive and false negative reports can be avoided when a critical condition occurs. Furthermore, the critical condition can be notified to the operator, and countermeasures, such as resetting the receiving unit or parts thereof, can be automatically initiated, preferably before the measurement results are compromised. Critical conditions can be corrected periodically without interrupting the measurement process. However, if the critical condition cannot be corrected in time or if the critical condition could corrupt the measurement information, interrupting the measurement process becomes the final option.

[0031]

[0031] The measurement signals can be analyzed in the hardware domain or in the software domain. The evaluation module and / or the reception control module can therefore be provided in the hardware domain or implemented in a control unit or a signal processing unit. The evaluation module can also be integrated into the reception control module, such that the reception control module includes an evaluation sub-module and a control sub-module.

[0032]

[0032] In the hardware domain, at least one reference signal, reference value or threshold value is preferably provided, which is compared in at least one comparator unit with the measurement signal or a measurement value derived from the measurement signal. By comparing the measurement signal or measurement value with the reference signal or reference value signal, a saturation state can be detected. By comparing the measurement signal or measurement value with two or more reference signals or reference values, saturation with signal clipping at both ends of the signal range can be detected. Furthermore, by appropriately setting the reference value, it can be detected whether the received signal corresponding to the measurement signal has reached a critical range while the measurement process is not yet disturbed.

[0033]

[0033] When saturation occurs in the receive signal path, the output voltage may go to one extreme or the other. However, saturation often causes undefined or nonlinear behavior of the receiving unit, making it impossible to properly interpret. In the receive signal path, the first amplifier may saturate, while the behavior of subsequent amplifiers may become undefined. For example, if the first amplifier is saturated, there is no guarantee that the second amplifier in a back-to-back amplifier arrangement will output a saturated, amplified copy of the input signal. As a result, the metal detector may appear to be in a normal state, but may actually be in a critical state. Therefore, the method of the present invention can detect critical states that would otherwise result in false positive or false negative reactions.

[0034]

[0034] It is important that when saturation conditions exist in the receive signal path, the measurement channel does not also saturate and can still process the measurement signal. For this purpose, an attenuator, preferably a controllable attenuator, is provided that can attenuate the measurement signal as needed. Alternatively or additionally, the measurement signal may be processed by a phase-sensitive detector, which has a larger range than the receive phase-sensitive detector to be able to process high-amplitude signals, etc., that would cause saturation in the receive signal path.

[0035]

[0035] The measurement channel can also be advantageously used as part of an auxiliary balance control loop that can compensate for any imbalance that may have caused a critical condition, at least to the extent that the receiving unit can return to normal operating conditions. In a metal detector, the received signal is typically analyzed or processed to determine an imbalance signal component related to the imbalance occurring in the metal detector. A compensation signal is derived or synthesized based on the determined imbalance signal component, and this compensation signal is applied to a compensation unit in the received signal path to compensate for the determined imbalance. In the event of a critical condition in which the imbalance signal component cannot be derived from the received signal, the measurement signal is analyzed or processed to determine an imbalance signal component related to the imbalance occurring in the metal detector and to provide a compensation signal. The compensation signal derived from the measurement signal is applied to a compensation unit in the received signal path to compensate for the determined imbalance signal component. Thus, the measurement channel becomes part of an auxiliary correction loop, allowing for rough but rapid correction of any imbalance condition occurring in the metal detector. Thus, after detecting a critical situation, the control unit can change the operating mode and temporarily use the auxiliary balance control loop until the receiving unit returns to normal operation, preferably until the measurement signal indicates that the received signal has returned to an acceptable range.

[0036]

[0036] Preferably, the history of the measurement signals and / or data obtained from the analysis of the measurement signals and / or metal detector status data relating to critical situations are recorded so that future critical situations can be predicted and preventative measures can be taken quickly, or when a critical situation occurs, additional data is available for interpreting a current critical situation.

[0037]

[0037] When the receiving unit provides an output signal indicating the presence or absence of a contaminant, the signal information is verified by the information obtained by analyzing the measured signal, so that false positive and false negative reports can be avoided. If an imbalance causes the received signal to have extreme values, maximum or minimum signal values, the received signal can be correctly interpreted and corrective measures can be taken immediately.

[0038] In a preferred embodiment, the measurement signal or a measurement derived therefrom is compared to at least one reference signal or value to determine a saturation condition in the receive signal path. A saturation condition indicates that the input of an active module, such as an operational amplifier in a receive stage, such as a filter or gain stage, has risen above a maximum input voltage, causing the active module or associated receive stage to exhibit nonlinear or undefined behavior, provide a minimum or maximum output voltage, signal clipping, or the like. In the worst case, a saturation condition can cause the receive unit to output a signal indicating the absence of a contaminant, even though a contaminant is actually present.

[0039]

[0039] Furthermore, the status of the metal detector and the measurement process is preferably continuously monitored, and relevant status information is provided to the receiving control module, which classifies the measurement information taking into account the status information related to the measurement information. If the status information indicates that no product is present in the balanced coil system, assignment of the critical state to the first class is excluded. If the status indicates that vibrations have been detected, the critical state is assigned to the second class. If the correction loop indicates that a high correction signal is being applied to compensate for the imbalance, the critical state is assigned to the fourth class.

[0040]

[0040] For the interpretation of critical states, the state information is preferably observed and recorded over a period of time so that the occurrence of critical states can be tracked and the critical states can be avoided by countermeasures. Thus, the historical information can be advantageously used to correctly assign critical states to the relevant classes.

[0041]

[0041] When saturation of the receive signal path is detected, at least one active element or module in the signal receive path is preferably reset so that the receive unit is returned to a normal state. For example, at least one capacitor associated with the saturation condition is discharged or replaced with a discharged capacitor. Alternatively, the amplifier and filter stages are reset and returned to a normal operating state. Alternatively, based on an assessment of the critical condition, the receive unit or its elements can be maintained in that state, for example, by temporarily blocking the signal path or temporarily retaining the charge on a capacitor.

[0042] Preferably, an operating value and a reset value of a gain parameter are provided for at least one controllable amplifier stage disposed in the receive signal path. In the event of a critical condition associated with saturation of the receiving unit, the at least one amplifier stage is reset by setting the gain parameter to a reset value. The gain parameter is then continuously changed from the reset value to the operating value within a gain reset period. Additionally or alternatively, an operating value and a reset value of a filter parameter are provided for at least one controllable filter stage disposed in the receive signal path. In the event of a critical condition associated with saturation of the receiving unit, the at least one filter stage is reset by setting the filter parameter to a reset value, and the filter parameter is then changed from the reset value to the operating value within a filter reset period. The gain reset period and the filter reset period are selected to be as short as possible and to reliably eliminate any saturation condition. The change of the gain parameter or filter parameter from the reset value to the operating value can be linear or follow a curve.

[0043] The method of the invention therefore makes it possible to detect critical conditions such as the saturation of an active module in the receive signal path and to initiate appropriate corrective action in the shortest possible time.

[0044] Detailed aspects and embodiments of the present invention are described below with reference to the drawings. [Brief explanation of the drawings]

[0044] [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 and a signal processing unit 45 integrated into a control unit 4 having a measurement channel 8 in which at least one measurement signal ms, msx taken from the receiving signal path rp is analysed in order to detect critical conditions, such as saturation conditions, occurring in the receiving unit 3. [Figure 2]1 in a further preferred embodiment, in which the measurement signal ms demodulated by the phase-sensitive detector 85 is analyzed in the control unit 4. FIG. [Figure 3] 3 shows the metal detector of FIG. 2 in a preferred embodiment in which the operating frequency tx is provided by a signal processing unit 45 and the phase sensitive detectors 435, 485 of the receive channel 3 and the measurement channel 8 are implemented in the signal processing unit 45. DETAILED DESCRIPTION OF THE INVENTION

[0045] 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.

[0046] The transmitter unit 1 includes a transmit signal path tp having an internal frequency source 11, such as a synthesizer, or an external frequency source 11, for example located in the control unit 4, a transmit amplifier 12, and preferably a transmit matching unit 13. The transmit signal path tp may include further modules, such as a filter device. The frequency source 11 provides a transmit 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 transmit signal tx. The transmit signal tx is applied to the input of the transmit amplifier 12, which may operate in class A or class B mode, for example, or may be implemented as an H-bridge.

[0047] 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 a transmit coil 21. The impedance matching unit 13 also preferably includes a tuning capacitor selectively connectable to the transmit coil 21 to form a resonant circuit tuned to a selected operating frequency.

[0048] In a balanced coil system 2, when a contaminant-laden product 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 be balanced in the absence of product, potentially resulting in the rejection of perfectly acceptable product. The metal detector's balance can be disturbed 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 channels, particularly the input amplifiers and phase-sensitive detectors and analog-to-digital converters, which operate only over a limited voltage signal range. To eliminate such imbalances, a balance control loop is provided with an adjustable compensation signal that is combined with the receive signal and varied until the imbalance is compensated.

[0049] The receiving unit 3 includes at least one receiving signal path rp, preferably equipped with a receiving matching unit 31, e.g., a balancing transformer, that matches the balanced coil system impedance to the receiving channel impedance. The receiving signal rs and a compensation signal cs provided by the receiving matching unit 31 are applied to a compensation unit 32, such as an addition or subtraction unit. The compensation signal cs compensates for imbalances in the receiving signal rs, and the compensated receiving signal is forwarded to the input of a receiving amplifier 34, possibly via a filter stage 33. The filter stage 33 and the receiving amplifier 34 are preferably controllable by control signals gc and fc. The control signal gc can set the gain of the receiving amplifier 34. The control signal fc can set at least one filter parameter of the filter stage 33. These exemplary control signals gc and fc and the controllable functional elements 33 and 34 can be used to reset the receiving unit 3 if a critical condition is detected.

[0050] The receive amplifier 34 delivers the amplified and compensated receive signal rs to a receive demodulator 35 (DEM) in the embodiment of a receive phase-sensitive detector 35 (PSD), where the receive signal rs is compared with in-phase and quadrature reference signals, i.e., the transmit signal tx and associated quadrature signal tx90° provided by the frequency source 11. At the output of the receive phase-sensitive detector 35, a demodulated complex receive signal rsc having an in-phase receive signal component rs-I and a quadrature receive signal component rs-Q is provided.

[0051]

[0051] The complex received signal rsc is transferred from the receive phase sensitive detector 35 via receive analogue to digital converters 36-I; 36-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.

[0052]

[0052] 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 signals are processed for the different operating frequencies in the different receiving paths rp arranged in parallel with each other.

[0053]

[0053] In the signal processing unit 45, a signal processing path sp is implemented in which the complex received signal rsc is processed, whereby signal components of the complex received signal rsc related to the product or noise are preferably suppressed, and signal components originating from metal contamination are further processed and detected.

[0054]

[0054] Further along the signal processing path sp, the complex received signal rsc is processed to determine imbalance components present in the received signal and to provide a compensation signal cs in which the determined imbalances, which are constant or have very low frequencies, have been removed from the receiver signal rs. The digital compensation signal provided by the signal processing unit 45 is forwarded to a digital-to-analog converter 91 which, after modulation, provides an analog compensation signal cs which is forwarded to the compensation unit 32 of the receiving unit 3.

[0055]

[0055] The metal detector further comprises a measurement channel 8 with a conditioning module 81 which receives at its input the measurement signal ms taken from the reception signal path rp. In this embodiment, the measurement signal ms is taken from the output of the reception amplifier 34. Symbolically, it is shown that a plurality of measurement signals ms, msx can be picked up along the reception signal path rp and processed in the measurement channels 8 along the measurement signal path mp. The picked up measurement signals ms, msx are time-multiplexed in the conditioning module 81 or are processed in dedicated measurement channels 8 arranged in parallel to each other. Each measurement channel 8 can be adapted to the measurement signal ms, msx applied to it.

[0056]

[0056] The conditioning module 81 processes the measurement signal ms to obtain a signal representative of the received signal at the output of the receiving amplifier 34 and comparable to at least one reference signal or reference value tha, thb provided by the control unit 4, preferably a receiving control module 488 implemented in the control unit 4. In this embodiment, two reference voltages or thresholds tha, thb are provided, which define the upper and lower limits of the signal range in which the measurement signal ms should be during normal operation.

[0057]

[0057] The reference signals tha, thb and the output signal of the adjustment module 81 are applied respectively to a first comparator module 82A and a second comparator module 82B, which provide associated output signals isa, isb indicating the occurrence of a critical condition in which the measurement signal ms exceeds a signal range or threshold tha, thb.

[0058] The adjustment module 81 can convert the measurement signal ms to a DC voltage using a rectifier, which is then compared by the comparator modules 82A, 82B with at least one reference voltage or threshold value tha, thb. The reference voltages or threshold values ​​tha, thb can be set to different values. The first threshold value tha can be set to a value corresponding to a signal range that is not normally exceeded during normal operation of the metal detector, but that does not yet indicate a critical level of the received signal rs. The second threshold value thb can be set to a value that indicates possible signal clipping or receiver saturation. Thus, the measurement signal ms can be analyzed by the comparators 82A, 82B and can output measurement information. These information or output signals issa, isb are forwarded to the receiver control module 488, which determines whether a critical state has occurred and initiates appropriate countermeasures.

[0059] To facilitate this task and to initiate appropriate corrective action when a critical condition occurs, the receiver control module 488 preferably receives supplemental information regarding the current state of the metal detector. The presence of a product in the balanced coil system 2 is sensed by the sensor unit 51 and reported to the receiver control module 488 by a signal s51. External influences such as vibration and / or temperature conditions are measured by the sensor or sensor group 52 and reported to the receiver control module 488 by a signal s52. Additionally, a timer may be provided to report the duration that either of the thresholds tha, thb has been exceeded or that an external influence has occurred.

[0060] Based on the received measurement information and status information, the receiver control module 488 preferably classifies the critical condition into, for example, a first class associated with a critical condition caused by a product or contaminant, a second class associated with a critical condition caused by an external influence such as vibration, a third class associated with a critical condition caused by an irregular state of the metal detector such as a saturated state, or a fourth class associated with a critical condition caused by a drift occurring in the metal detector. Based on the classification of the critical condition, the receiver control module 488 can initiate a predetermined correction procedure for the assigned class.

[0061] The receiver control module 488 may, for example, observe that a product enters the balanced coil system 2 and that the measurement signal m simultaneously exceeds the first and / or second thresholds t a and t b. The receiver control module 488 may therefore assign this critical condition to a first class. Depending on the time the thresholds t a and t b are exceeded, the receiver control module 488 may continue normal operation or may initiate corrective measures.

[0062]

[0062] If the thresholds tha and thb are exceeded while an external influence such as vibration is detected, the critical state is assigned to the second class. As a corrective measure, the metal detection process is stopped and the operator is notified of the critical state. This procedure has the advantage that appropriate measures can be selectively taken for each class of critical state. For example, measurement data can be discarded only if the critical state indicates that the measurement results may be invalid.

[0063]

[0063] If the thresholds tha and thb are exceeded without the presence of a product in the balanced coil system 2 and without the detection of any external influences, the state of the metal detector may be irregular and the critical state is assigned to the third class, in which case corrective measures are usually always initiated.

[0064]

[0064] If an exceedance of the thresholds tha, thb is detected together with a large compensation signal cs, a drift or imbalance according to the fourth class may be present. This condition is again notified to the operator, for example by sound or light or by a message on the display of the control unit 4.

[0065]

[0065] The classification or interpretation of the measurement information may further be aided by historical data. For this purpose, data on the critical state, the associated state data and classification data, as well as preferably data on external influences and the course of the compensation signal cs, are preferably registered. Preferably, the periods during which the level of the measurement signal fell within the critical range between two appropriately set thresholds tha and thb can also be registered. Such historical data can also be used to analyze future critical states or for preventive measures. If the current data pattern corresponds to the registered data pattern, it is highly likely that the critical state will be classified in the same way.

[0066] The method of the present invention can provide an explanation for previously uninterpretable critical conditions. By knowing the cause of the critical condition, however, appropriate corrective action can be selectively taken, thereby restoring the metal detector to normal operation in the shortest possible time.

[0067]

[0067] If the threshold thb is exceeded and a persistent saturation state in the receiving unit 3 is detected, the receiving channel or its active modules (such as at least one filter or amplifier) ​​are reset and returned to their normal operating state, so that the detected saturation state is ended as quickly as possible and the measurement process can continue without interruption or after a short time.

[0068] Typically, saturation is caused and maintained by a charged capacitor, such as a capacitor in a filter, amplifier, or integrator. Discharging the capacitor in the associated receive signal path rp can resolve the saturation. For example, a resistor can be connected in parallel with the capacitor using a switch, such as a field-effect transistor, to allow the capacitor to be discharged when needed. Because discharging a capacitor takes time, capacitors can be paired and alternately connected to the electronic circuit. This arrangement allows a charged capacitor to be instantly replaced with a discharged one, quickly restoring the receiver unit 3 to normal operation. As an example, an RC filter with a charged first capacitor C can be replaced with an identical RC* filter with a discharged second capacitor C*. Figure 1 shows that a switch S, operated by a control signal fc, can connect either the first capacitor C or the second capacitor C* to the filter stage 33. By changing the setting of the switch S, the first capacitor C can be replaced by the second capacitor C* in the electronic circuit, and vice versa.

[0069] Alternatively, when detecting a critical state, for example caused by a large metal contamination, the receiving unit 3 is preferably temporarily held in its current state, for example by interrupting the receiving signal path or by holding constant the charge of a capacitor, which may be connected to an active module such as an operational amplifier. This has the advantage that the metal detector can automatically return to its normal operating state without the need for further action.

[0070]

[0070] In a further preferred embodiment, at least one controllable filter stage 33 installed in the receive signal path rp is provided with operating and reset values ​​of filter parameters. When a critical condition is detected, the at least one filter stage 33 is preferably reset by setting the filter parameters to reset values, and then the filter parameters are changed from the reset values ​​to the operating values ​​within a filter reset period. The resetting of the filter stage 33 can be performed in the analog domain and in the digital domain. The filter parameters can be set and changed by a filter control signal fc applied to the filter stage 33.

[0071]

[0071] In another preferred embodiment, an operating value and a reset value of a gain parameter are provided for at least one controllable amplifier stage 34 installed in the receive signal path rp. When a critical condition is detected, the at least one amplifier stage 34 is reset by setting the gain parameter to a reset value, and then the gain parameter is continuously changed from the reset value to the operating value within a gain reset period. The gain parameter can be set and changed by a control signal gc applied to the receive amplifier 34.

[0072]

[0072] A filter stage 33 and an amplifier stage 34 are preferably provided on the input side of the receive phase-sensitive detector 35 for amplifying and filtering the modulated receive signal rs, and / or on the output side of the receive phase-sensitive detector 35 for amplifying and filtering the demodulated receive signal or baseband signal rs.

[0073]

[0073] Figure 2 shows the metal detector of Figure 1 in a further preferred embodiment. In this embodiment, the measurement signal mS is applied in the measurement channel 8 to a measurement demodulator 85 (DEM), preferably provided in the embodiment of a phase-sensitive detector 85 (PSD). The measurement phase-sensitive detector 85 compares the measurement signal mS, taken from the output of the receiving amplifier 34, with an in-phase reference signal and a quadrature reference signal, i.e., a transmission signal tX and an associated quadrature signal tX90°, preferably provided by the frequency source 11, to generate a demodulated complex measurement signal mSC having an in-phase measurement signal component mS-I and a quadrature measurement component mS-Q, which are transferred to the signal processing unit 45 via analog-to-digital converters 86-I; 86-Q.

[0074] As mentioned above, it is important that when saturation conditions exist in the receive signal path rp, the measurement channel 8 does not also saturate and can still process the measurement signal ms. For this purpose, the measurement channel 8 optionally includes an attenuator 80 that attenuates the measurement signal ms. Alternatively, the measurement phase-sensitive detector 85 optionally has a larger range compared to the receive phase-sensitive detector 35, which operates with higher precision. Thus, the measurement phase-sensitive detector 85 can process signals with high amplitudes that would saturate the receive phase-sensitive detector 35. The attenuator 80 preferably has a control input to which a control signal ac can be applied to set the desired amount of attenuation. The attenuation is preferably set by the control unit 4, preferably the signal control unit 45 or the receive control unit 488.

[0075]

[0075] The measurement signal ms or msx may be picked up at another point along the receive signal path rp and amplified and / or attenuated along the measurement signal path mp, as required, before being applied to the input of the measurement demodulator 85 (DEM) or measurement phase-sensitive detector 85 (PSD).

[0076] In the signal processing unit 45 or the receiving control module 488, the complex measurement signal msc is analyzed and relevant measurement information is provided. Based on the obtained measurement information, the receiving control module 488 determines whether a critical condition exists. As described above, the critical condition can be classified and corrective measures provided for the relevant class can be applied. This embodiment has the advantage that the measurement signals ms, msx can be more easily analyzed in the digital domain by the signal processing unit 45 or the receiving control module 488. However, it is recommended that the measurement signals ms, msx are analyzed continuously and that the measurement information is preferably collected before saturation occurs in the receiving signal path rp.

[0077] As explained with reference to Figure 1, a balance control loop is provided to cancel imbalances occurring in a metal detector. The complex received signal rsc is processed along a signal processing path sp in a signal processing unit 45 to determine the imbalance component in the complex received signal rsc and to provide a corresponding compensation signal cs, which is forwarded to the compensation unit 32 of the receiving unit 3 via a digital-to-analog converter 91.

[0078]

[0078] Since the measurement channel 8 can operate even when a critical condition disrupts the proper functioning of the receiving unit 3, the measurement signal ms is preferably analyzed after the detection of the critical condition to determine an imbalance signal component related to the imbalance occurring in the metal detector and to provide a compensation signal csm corresponding to the determined imbalance signal component. The determined imbalance is compensated for by applying the compensation signal csm derived from the measurement signal ms to a compensation unit 32 provided in the receiving signal path rp. As a result, the measurement channel 8 forms part of an auxiliary balance control loop that is activated when a critical condition occurs. The auxiliary balance control loop thus returns the receiving unit 3 to a state in which the normal balance control loop can be reactivated in order to optimize the compensation of the imbalance.

[0079]

[0079] After detecting a critical condition, the receiver control module 488 can initiate corrective measures. Process control commands pc cause the measurement process or operating mode of the metal detector to be changed as needed. Control signals gc, fc can change the operating parameters of the amplifier unit 34 and the filter unit 33. The control signal pc can activate or deactivate the auxiliary balance control loop.

[0080]

[0080] Figure 3 shows the metal detector of Figure 2 in a preferred embodiment, in which the transmission signal tx or the operating frequency is provided by a signal processing unit 45. The receiving unit 3 and the phase-sensitive detectors 435, 485 of the measurement channel 8 are implemented in the signal processing unit 45. Furthermore, a filter module 4533 for processing the demodulated in-phase and quadrature signal components of the received signal rs and a filter module 4583 for processing the demodulated in-phase and quadrature signal components of the measurement signal ms are implemented in the signal processing unit 45. Since the unbalanced signal components exist in the low frequency range, the filter module 4533 can filter the unbalanced signal components contained in the received signal rs, and the filter module 4583 can filter the unbalanced signal components contained in the measurement signal ms. Therefore, a compensation signal can be derived from the received signal rs for applications in a normal balanced control loop, or from the measurement signal ms for applications in an auxiliary balanced control loop, as needed. [Explanation of symbols]

[0081]

[0082] 1 transmitting unit 11 Frequency Sources, Synthesizers 12 Transmitting amplifier 13 Transmission Matching Unit 2 Coil System 21 Transmitting coil 22A, 22B receiving coils 3 receiving unit 31 Receiving matching unit 32 compensation units 33 Filter unit 34 Receiving amplifier 35, 4335 Receiver Phase Sensitive Detector / Demodulator 36I, 36Q Receiver Analog-to-Digital Converter 4. Control Unit 40 Control Program 45 Signal Processing Units / Modules 435, 485 Phase-sensitive detector 488 Receiving Control Module 4533 Filter module for processing the ms-I signal component of the rs signal 4583 Filter module for processing the ms-Q signal component of the ms signal 51 Product Sensors 52 External influence measurement sensor / vibration sensor 6 Product conveyor 8 measurement channels 80 Attenuator 81 Adjustment Unit 82A, 82B comparator 85, 4585 Measuring Phase Sensitive Detector / Demodulator 86I, 86Q Measurement Analog-to-Digital Converter 88 Evaluation Module 9 Compensation Channels 91 Compensation Channel Digital-to-Analog Converter 92 Compensation Channel Amplifier ac control signal to attenuator 80 cs compensation signal csm Compensation signal corresponding to the determined unbalanced signal component gc is the control signal for setting the gain or receive amplifier 34 fc a control signal for setting at least one filter parameter of the filter stage 33 isa, isb output signal mp measurement signal path ms, msx measurement signal msc complex measurement signal ms-I In-phase measurement signal component ms-Q orthogonal measurement components pc process control command rp Receive signal path rs received signal rsc Complex received signal rs-I In-phase received signal component rs-Q Quadrature received signal components sp signal processing path s51 Sensor 51 signal s52 Sensor 52 signal tha, thb Reference signal threshold / reference value tp Transmit signal path tx Transmit signal tx90° quadrature signal (90° phase offset from the transmit signal) C First capacitor C* Second Capacitor S Switch

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) comprises a transmitting signal path (tp) for a transmitting signal (tx) having at least one fixed or selectable operating frequency, the transmitting signal (tx) being applied to the input of a transmitting amplifier (12) which transfers the amplified transmitting signal (tx) directly or via a transmitting matching unit (13) to the transmitting coil (21); The transmitting unit (1) provides a reference transmitting signal (tx) and a reference quadrature signal (tx90°) to the receiving unit (3) or the signal processing unit (45); The receiving unit (3) includes at least one receiving signal path (rp) through which a 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) directly or indirectly transfers the amplified receiving signal (rs) to a receiving demodulator (35; 4535) provided in the receiving unit (3) or implemented in a signal processing unit (45); The receiver demodulator (35; 4535) provides a demodulated complex receive signal (rsc) having an in-phase receive signal component (rs-I) and a quadrature receive signal component (rs-Q) based on the reference transmit signal (tx) and the reference quadrature signal (tx90°) received from the transmitter unit (1); The in-phase received signal component (rs-I) and the quadrature received signal component (rs-Q) are processed in at least one signal processing path (sp) provided in the signal processing unit (45), in which signal components of the complex received signal (rsc) related to products or noise are suppressed and signal components resulting from metal contaminants are further processed; providing at least one measurement signal (ms) taken from said receiving signal path (rp) to a measurement channel (8); - analyzing said measurement signals (ms) in an evaluation module (88) or a reception control module (488) and providing relevant measurement information; Based on the measurement information obtained, determining in the receive control module (488) whether a critical condition exists that may impair signal processing in the receive signal path (rp); providing a procedure for processing a critical condition in the metal detector that is executed if a critical condition exists; A method of operating a metal detector characterized by:

2. 10. The method of claim 1, evaluating said measurement information in said receiving control module (488), classifying said measurement information, assigning said measurement information to at least one, preferably at least a first or second class of criticality condition, and providing an indication of each class of criticality condition to a criticality condition processing procedure in said metal detector; A method comprising:

3. 3. The method of claim 2, determining the critical state according to the measurement information; - the first class relating to critical states caused by products or contaminants, or - a second class related to critical states caused by external influences such as vibrations, or a third class related to critical states caused by irregular conditions of the metal detector, such as a malfunction of the electronics, or a fourth class related to critical states caused by drifts occurring in the metal detector; a step of assigning A method comprising:

4. 4. The method according to claim 1, wherein The execution of criticality procedures is a) providing and applying control information or control signals (gc; fc) to at least one functional module (33, 34) provided in said receiving signal path (rp) in order to reset said receiving unit (3) or to return said receiving unit (3) or a part thereof to a normal operating state or to keep said receiving unit (3) or a part thereof in a stable state; b) providing and applying control information (pc) to at least one functional module (4533) in said signal processing path (sp) in order to reset said signal processing unit (45) or to return said signal processing unit (45) or a part thereof to a normal operating state or to keep said signal processing unit (45) or a part thereof in a stable state; c) providing information (pc) to a control program (40) implemented in said control unit (4), which program (40) initiates an acoustic or optical alarm signal emitted by said metal detector; d) providing information (pc) to the control program (40) implemented in the control unit (4), the control program (40) processing the measurement data according to a protocol established for the occurrence of a critical state; e) using a balance control loop to eliminate imbalances occurring in the receiving signal path (rp) and providing and applying control information to at least one functional module (32) provided in said balance control loop to reset said balance control loop or to return said balance control loop to an operating state in which the imbalance is at least roughly corrected or held at a stable value; f) using a balance control loop for eliminating imbalances occurring in the receive signal path (rp), the balance control loop deriving an imbalance signal from the receive signal (rs) or, if a critical condition exists, from the measurement signal (ms); The method includes at least one of the following:

5. 5. The method according to claim 1, wherein applying the measurement signal (ms) to a measurement demodulator (85; 4585) provided in the measurement channel (8) or implemented in the signal processing unit (45), which provides a demodulated complex measurement signal (msc) having an in-phase measurement signal component (ms-I) and a quadrature measurement signal component (ms-Q), which are analyzed in the signal processing unit (45) or in the receiver control module (488) to provide relevant measurement information; A method comprising:

6. 6. The method according to any one of claims 1 to 5, providing said receiver demodulator (35; 4535) or said measurement demodulator (85; 4585), or said receiver demodulator (35; 4535) and said measurement demodulator (85; 4585), in an embodiment of a phase-sensitive detector, for comparing said applied receive signal (rs) or measurement signal (ms) with said reference transmit signal (tx) and an associated reference quadrature signal (tx90°) to provide said demodulated complex receive signal (rsc) or said demodulated complex measurement signal (msc), A method comprising:

7. 7. The method according to claim 1, wherein providing at least one measurement reference signal or measurement reference value (tha, thb) and comparing said measurement signal (ms) or a measurement derived from said measurement signal (ms) with the at least one measurement reference signal or measurement reference value (tha, thb) to determine whether a critical condition exists; A method comprising:

8. 8. The method according to any one of claims 1 to 7, providing an attenuator (80) for attenuating the measurement signal (ms) taken from the receive signal path (rp) or providing a measurement phase-sensitive detector (85, 4585) having a range greater than that of the receive phase-sensitive detector (35, 4535); or providing an attenuator (80) for attenuating the measurement signal (ms) taken from the receive signal path (rp), and providing a measurement phase-sensitive detector (85, 4585) having a range greater than that of the receive-sensitive detector (35, 4535); A method comprising:

9. 9. The method according to any one of claims 1 to 8, analyzing the received signal (rs) to determine an imbalance signal component related to an imbalance generated in the metal detector, providing a compensation signal (cs) corresponding to the determined imbalance signal component, and applying the compensation signal (cs) to a compensation unit (32) disposed in the received signal path (rp) to compensate for the detected imbalance signal component; If a critical state is detected, analyzing the measurement signal (ms) to determine an imbalance signal component related to an imbalance generated in the metal detector, providing a compensation signal (csm) corresponding to the determined imbalance signal component, and applying the compensation signal (csm) to the compensation unit (32) provided in the receive signal path (rp) to compensate for the determined imbalance signal component; A method comprising:

10. 10. The method according to any one of claims 1 to 9, - comparing said measurement signal (ms) or measurement value with said at least one reference signal or reference value (tha, thb) to determine a saturation state in said receive signal path (rp); A method comprising:

11. 11. The method according to any one of claims 1 to 10, capturing or recording the status of the metal detector and the measurement process and providing relevant status information, such as current or historical status information, to the receiving control module (488), which classifies the measurement information taking into account the status information associated with the measurement information; A method comprising:

12. 12. The method according to any one of claims 1 to 11, - the presence of a product in said balanced coil system (2), - the presence of vibrations, - the presence of external electromagnetic interference at the metal detector; detecting at least one of: providing relevant status information to the receiving control module (488) which evaluates the measurement information taking into account the obtained status information; A method comprising:

13. 13. The method according to any one of claims 1 to 12, and / or resetting said at least one controllable active module, such as an amplifier stage (34) located in said receive signal path (rp), by providing an operating value and a reset value of an operating parameter, such as a gain parameter, to said at least one controllable active module, such as an amplifier stage (34), located in said receive signal path (rp), setting said operating parameter to a reset value, and continuously varying said operating parameter from said reset value to said operating value within a gain reset period; providing operating and reset values ​​for filter parameters of at least one controllable filter stage (33) located in said receive signal path (rp); resetting the at least one filter stage (33) when a critical condition is detected by setting the filter parameter to a reset value and changing the filter parameter from the reset value to the operating value within a filter reset period; A method comprising:

14. 14. The method according to any one of claims 1 to 13, Depending on the detected criticality state, Controlling or resetting said receiving unit (3) by discharging or replacing at least one capacitor (C, C*) in the electronic circuit of said receiving unit (3), or Depending on the criticality conditions detected, especially those caused by contaminants, keeping the receiving unit (3) in its current state; A method comprising:

15. 15. The method according to any one of claims 1 to 14, For each operating frequency, a dedicated receive signal path (rp), and A dedicated signal processing path (sp) is provided, If the signal processing in the measurement channel (8) depends on the operating frequency, providing a dedicated measurement channel (8); A method comprising:

16. A metal detector operating in accordance with the method of any one of claims 1 to 15.

17. 17. A metal detector according to claim 16, comprising 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), The transmitter unit (1) comprises a transmitter signal path (tp) to which a transmitter signal (tx) having at least one fixed or selectable operating frequency and an associated quadrature signal (tx 90°) are provided, the transmitter signal (tx) being applicable to the input of a transmitter amplifier (12) which transfers the amplified transmitter signal (tx) directly or via a transmitter matching unit (13) to the transmitter coil (21); The receiving unit (3) includes at least one receiving signal path (rp) through which a receiving signal (rs) received from the balanced coil system (2) can be applied directly or via a receiving matching unit (31) to a receiving amplifier (33), and the receiving amplifier (33) transfers the amplified receiving signal (rs) directly or indirectly to a receiving phase-sensitive detector (35; 4535) provided in the receiving unit (3) or implemented in the signal processing unit (45); a receiving phase-sensitive detector (35, 4535) designed to compare the received signal (rs) with the reference transmitted signal (tx) and the reference 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); the in-phase received signal component (rs-I) and the quadrature received signal component (rs-Q) are processed in at least one signal processing path (sp) provided in the signal processing unit (45), in which signal components of the complex received signal (rsc) related to products or noise are suppressed and signal components resulting from metal contaminants are further processed; At least one measurement channel (8) is provided for receiving a measurement signal (ms) extracted from the reception signal path (rp), at least one evaluation module (88, 488) is provided, designed to analyze said measurement signals (ms) and to provide relevant measurement information; a receive control module (488) is provided which is designed to determine whether a critical condition exists and, if a critical condition exists, to provide control information (gc, fc, pc) for controlling the measurement process or for controlling at least one functional module (33, 34, 4534) provided in the receive signal path (rp), or for controlling the measurement process and for controlling at least one functional module (33, 34, 4534) provided in the receive signal path (rp), A metal detector characterized by:

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