Method for operating a metal detector and metal detector

Abstract

The present invention provides a method for operating a metal detector, which includes a balanced coil system having a driving coil connected to a transmitter unit, a first detection coil and a second detection coil that provide an output signal to a receiver unit, the transmitter unit providing a transmitter signal of an operating frequency, the receiver unit processing a related receiver signal including an unbalanced signal and providing a digital in-phase component and a digital quadrature component of the unbalanced signal, the method including: processing the digital in-phase component and the digital quadrature component, applying the digital in-phase component to a first control unit, which provides an in-phase control component, applying the digital quadrature component to a second control unit, which provides a quadrature control component; synthesizing a digital compensation signal having a frequency of the unbalanced signal and a phase and amplitude according to the in-phase control component and the quadrature control component; converting the digital compensation signal into an analog compensation signal, and applying the analog compensation signal to the balanced coil system or the receiver signal.

Description

Technical Field

[0001] The present invention relates to a method for operating a metal detector using one or more than two operating frequencies and a metal detector operated according to the method. Background Art

[0002] For example, industrial metal detectors as described in US8587301B2 are used to detect metal contaminants in products. If properly installed and operated, it will help reduce metal contamination and enhance food safety. Most modern metal detectors use probes that include a "balanced coil system". Detectors of this design are able to detect all types of metal contaminants, including ferrous, non-ferrous metals and stainless steel, in a variety of products, such as fresh and frozen products.

[0003] Metal detectors operating on the "balanced coil" principle usually consist of three coils wound on a non-metallic frame: a drive coil and two identical detection coils, each coil being exactly parallel to the others. Since the detection coils that usually surround the drive coil are identical, theoretically the same voltage is induced in the drive coils. In order to receive a zero output signal when the system is in balance, the first detection coil is connected in series with a second detection coil with an opposite winding. Therefore, under ideal conditions, provided that the system is in balance and there are no contaminants in the observed product, the voltages of equal amplitude and opposite polarity induced in the detection coils cancel each other.

[0004] However, once a metal particle passes through the coil arrangement, the electromagnetic field near one detection coil is disturbed first, then the electromagnetic field near the other detection coil is disturbed. As the metal particle travels through the detection coils, the voltage induced in each detection coil is changed (usually in nanovolts). This change in balance results in a signal at the output of the detection coils, which is processed, amplified, and subsequently used to detect the presence of metal contaminants in the product being observed in the receiver unit.

[0005] In the receiver unit, the input signal is usually separated into an in-phase component and a quadrature component. The vector composed of these components has an amplitude and a phase angle, which is typical for the product and contaminants transmitted through the coil system. In order to identify metal contaminants, the "product effect" needs to be removed or reduced. If the phase of the product is known, the corresponding signal vector can be reduced to produce a higher sensitivity for the detection of signals from metal contaminants.

[0006] The method for eliminating unwanted signals from the signal spectrum utilizes the fact that metal contaminants, products and other disturbances have different effects on the magnetic field so that the detected signals differ in phase. Materials with high electrical conductivity cause signals with a higher resistance signal component and a smaller reactance signal component. Materials with high magnetic permeability cause signals with a smaller resistance signal component and a higher reactance signal component. The signal caused by ferrite is mainly reactive, while the signal caused by stainless steel is mainly resistive. Conductive products generally cause signals with a strong resistance component. As a product or contaminant is transmitted through the metal detector, the phase angle between the resistance signal component and the reactance signal component of the signal vector generally remains constant.

[0007] As described above, under ideal conditions, a balanced coil system does not provide an output signal if no product or contaminant is present. However, as described in EP2812734B1, despite the fact that the same detection coil is set in a nearly completely balanced state at the factory site, there are still situations where the detection coil system cannot be in a balanced state in the absence of a test item or contaminant, resulting in, for example, the rejection of completely acceptable food. The balance of the detector may be disturbed by mechanical influences on the system, changes in environmental conditions, metal objects located near the detector, or relaxation or aging of components. In view of the high sensitivity of the metal detection system and the slight effect of contaminants on the output voltage of the coil system, imbalance may lead to saturation of the receiver channel, in particular the input amplifier and phase-sensitive detector that operate only under a limited voltage signal range. In order to eliminate this imbalance, an adjustable balance signal derived from the drive signal is combined with the output signal of the detector and changed until the combined output signal is compensated.

[0008] The output signal and the adjustable balance signal are filtered separately before being combined to remove one or more harmonics. In addition, the adjustable balance signal can be adjusted by means of an adjustable potentiometer, for example. This compensation circuit, which must be operated with the highest precision, usually requires expensive low-noise components. Nevertheless, the compensation circuit can not only counteract the imbalance, but can also introduce noise into the detection system, for example through the potentiometer. In addition, the compensation circuits applied so far are inflexible and limited to imbalances related to the operating frequency of the drive signal. Other interferences that are not directly related to the operating frequency cannot be corrected. In addition, the adjustment process, in particular when using a potentiometer, can take a considerable amount of time and may not completely eliminate the unbalanced signal.

[0009] In US5691640A, a compensation circuit for a metal detector is disclosed, which also uses signals derived from the drive signal. These signals are applied to a phase-sensitive detector and a multiplier, and a control signal is applied to the multiplier. The output signal of the multiplier is applied to the input of the receiver stage via a feedback isolation transformer. The disadvantages of this complex and expensive compensation circuit are the same as those described above. This circuit also requires expensive low-noise components and isolation transformers, which must be precisely designed to avoid introducing more imbalance and noise. In addition, the compensation circuit is also inflexible and limited to imbalances related to the operating frequency of the drive signal. Other interferences that are not directly related to the operating frequency cannot be corrected. In addition, the circuit can change its characteristics according to the selected operating frequency. Therefore, the complexity of the circuit and the additional imbalance that may be introduced will increase with a large increase in the operating frequency, especially if they are applied simultaneously. Summary of the invention

[0010] It is therefore an object of the present invention to provide an improved method for operating a metal detector, and an improved metal detector operated according to the method.

[0011] The method of the invention should allow imbalances occurring in any stage of the metal detector, in particular in the coil system, the receiver channels, and imbalances caused by external or internal influences of these stages to be precisely eliminated with little effort.

[0012] The compensation unit of the invention should be flexible and easily adaptable, not only to correct imbalances, but also to correct disturbances that adversely affect the performance of the metal detector. In particular, it should be able to eliminate various disturbances that occur independently of the operating frequency.

[0013] The metal detector of the present invention should avoid the need for expensive components and complex compensation circuits.

[0014] Furthermore, the compensation unit of the present invention should accurately eliminate the imbalance but not introduce its own noise or interference into the system.

[0015] In a first broad aspect of the invention, there is provided a method for operating a metal detector comprising a balanced coil system having a drive coil and first and second detection coils, the drive coil being connected to a transmitter unit providing a transmitter signal having at least one operating frequency, the first and second detection coils providing output signals to a receiver unit, the receiver unit processing a correlated receiver signal, typically comprising an unbalanced signal, and providing a digital in-phase component and a digital quadrature component of the demodulated unbalanced signal. The metal detector is designed for detecting contaminants, in particular metallic contaminants, in a product inspected by the metal detector, for which purpose the product is transported through the balanced coil system of the metal detector.

[0016] According to the present invention, the method comprises the following steps:

[0017] processing a digital in-phase component and a digital quadrature component of the unbalanced signal in a signal controller for providing control data to a compensation unit for compensating the unbalanced signal,

[0018] applying the digital in-phase component of the unbalanced signal to a first control unit that provides an in-phase control component for the unbalanced signal, and applying the digital quadrature component of the unbalanced signal to a second control unit that provides a quadrature control component for the unbalanced signal;

[0019] In the compensation unit, synthesizing a digital compensation signal having a frequency of the unbalanced signal corresponding to the at least one operating frequency and having a phase and an amplitude according to an in-phase control component and a quadrature control component provided for the unbalanced signal;

[0020] Convert the digital compensation signal into an analog compensation signal,

[0021] An analog compensation signal is applied to a balanced coil system or receiver signal to compensate for an unbalanced signal.

[0022] According to the method of the invention, the in-phase component and the quadrature component of the unbalanced signal are detected and the associated in-phase control component and the quadrature control component are determined or calculated in the control unit. Based on these in-phase control components and the quadrature control components, a compensation signal having a frequency, phase and amplitude suitable for compensating the unbalanced signal is synthesized. Thus, at least one control loop is formed to reduce the size and eventually eliminate any unbalanced signal by negative feedback of the control signal. Ideally, the compensation signal will be synthesized to be identical to the unbalanced signal and will be applied to the signal path 180° out of phase with respect to the unbalanced signal. In practice, preferably, a PID controller is used to fully compensate the unbalanced signal using a control term having a proportional, integral and differential influence on the digital in-phase control component and the digital quadrature control component provided to the compensation unit.

[0023] Therefore, according to the present invention, the digital in-phase component and the digital quadrature component of the unbalanced signal are processed to transmit the digital in-phase control component and the digital quadrature control component. The unbalanced signal and its components can be obtained in different ways. When the metal detector is preferably operated without any product transmission through the balanced coil system, an unbalanced signal may be observed. The transmission of the product can be delayed until the synthesized compensation signal is optimized. However, if the product signal is known or located in a different frequency range, the product can be measured and the product signal can be subtracted or separated from the receiver signal to obtain the unbalanced signal. Therefore, in a first control mode or calibration mode, the unbalanced signal is measured and a corresponding compensation signal is synthesized, and the unbalanced signal is completely or at least partially eliminated using the compensation signal.

[0024] After an initial calibration of the metal detector has been performed in the first control mode or calibration mode, the metal detector may be switched to a normal operating mode in which the product is inspected and the imbalance signal is eliminated with the aid of the stored synthetic compensation signal. In this operating mode, changes in the imbalance signal are ignored.

[0025] In a preferred embodiment, after the first control mode has been terminated, the metal detector is switched to a second control mode. In the second control mode, the product is inspected and the remaining unbalanced signal is further observed and further eliminated. For this purpose, for example, the unbalanced signal can be separated from the receiver signal by a filter stage that allows the unbalanced signal to pass.

[0026] In the normal operating mode or in the second control mode, the unbalanced signal is preferably monitored and if the unbalanced signal exceeds a predetermined threshold value, the metal detector returns to the first control mode. In the first control mode, the product transmission through the balanced coil system is preferably interrupted until the resultant compensation signal is optimized again. This optimization can usually be performed within a few seconds. Afterwards, the metal detector switches again to the normal operating mode or the second control mode.

[0027] The receiver signal can be demodulated in the analog or digital domain to recover the baseband signal of the product, contaminants and imbalance signal or any interference.

[0028] For example, the demodulation of signals in the digital domain is described in https: / / www.nutaq.com / blog / digital-iq-demodulator-high-speed-adc. In a digital I / Q demodulator, a receiver signal centered at the operating frequency is directly sampled by a single high-speed ADC device. The sampled receiver signal is mixed with digital cosine and sine wave samples of the operating frequency generated by a direct digital synthesizer (DDS) in a digital in-phase mixer and a digital quadrature mixer to convert the receiver signal to an in-phase signal and a quadrature signal in baseband. After filtering out high-frequency components in the output of the in-phase mixer and the digital quadrature mixer, the baseband signal is obtained. When the conversion from the operating frequency to baseband is performed in the analog domain, the use of processing the receiver signal in a high-speed ADC and a digital I / Q demodulator can generally reduce potential errors, such as gain imbalance and DC offset.

[0029] The conversion of the receiver signal to baseband can also be performed in the analog domain. In this embodiment, the analog receiver signal is compared with an analog in-phase reference signal in a first phase detector and with an analog quadrature reference signal in a second phase detector to produce analog in-phase and analog quadrature components of the baseband signal, which are converted to digital in-phase and digital quadrature components.

[0030] Thus, in a first embodiment of the metal detector of the invention or of the method of implementing the invention, the receiver signal is demodulated in the analog domain.In a second embodiment of the metal detector of the invention or of the method of implementing the invention, the receiver signal is demodulated in the digital domain.

[0031] The signal controller preferably operates in two or more modes, in which different sets of control items are applied to the control unit. After initialization of the metal detector, the first control mode is preferably set and maintained until a predetermined condition is met, such as reaching a limited time period, or the first control mode is maintained as long as the unbalanced signal exceeds a given threshold. As described above, during the first control mode or calibration mode, the product is preferably not checked. For example, the first control mode is maintained for a period of time, during which the unbalanced signal is generally reduced to a level at which the unbalanced signal has no significant effect on the measurement. Alternatively, the level at which the unbalanced signal has no significant effect on the measurement is predetermined and compared with the actual unbalanced signal. Once the predetermined condition is met, the first control mode is changed to the second control mode, in which the measurement of the product is performed while still observing and reducing the unbalanced signal in the control loop. In the first control mode, the unbalanced signal is rapidly reduced, while in the second control mode, the remaining unbalanced signal is slowly driven to a minimum value or completely eliminated. The control items for the second control mode are selected so that the signal correction in the control loop does not cause interference during the measurement of the product. In particular, the control items of the control unit are selected to avoid overshoot. Therefore, in the second control mode, the operating conditions can be continuously optimized while performing the measurement.

[0032] The method of the invention allows to precisely eliminate with little effort imbalances occurring in any stage of the metal detector, in particular in the coil system, the receiver channel, and imbalances caused by external or internal influences of these stages. The compensation unit of the invention can be flexibly and easily adapted to correct any imbalances and further disturbances that have an adverse effect on the performance of the metal detector. In particular, disturbances that do not directly correspond to the operating frequency can be eliminated.

[0033] For any interference signal, the signal controller can provide a control signal, and the compensation unit can generate or synthesize a corresponding correction signal. The signal controller will provide information about the frequency, amplitude and phase of the interference signal, and the correction signal for it will be synthesized in the compensation unit. The compensation unit can generate or synthesize the correction signal in the baseband of the measured signal or in the offset of its frequency. For example, in the case of a frequency sideband being detected, such a frequency sideband can be eliminated by a correspondingly synthesized correction signal.

[0034] In a preferred embodiment, to eliminate any interfering signals:

[0035] In the signal controller, a digital in-phase component and a digital quadrature component and a frequency of the interference signal are determined;

[0036] determining or calculating a digital in-phase control component and a digital quadrature control component;

[0037] In the compensation unit, a digital correction signal is synthesized according to a digital in-phase control component and a digital quadrature control component of the interference signal;

[0038] Finally, the digital correction signal is added to the digital compensation signal.

[0039] The compensation unit of the present invention should avoid the need for expensive components and complex compensation circuits. The compensation unit preferably includes a field programmable gate array unit (FPGA), which can be easily adapted to future needs. Additionally or alternatively, the compensation unit may include one or more frequency synthesizers, preferably such as https: / / en.wikipedia.org / wiki / Direct_digital_synthesis A direct digital synthesizer described in the claims can be used to generate the frequencies required for one or more compensation signals. Alternatively, signal data of the waveforms of at least one operating frequency or its in-phase frequency component and orthogonal frequency component are sequentially read out from a memory unit or a lookup table. Data of one or more cycles of the frequency signal can be stored in a memory unit or a lookup table with a desired resolution, and can be sequentially read out using a system clock to generate the relevant frequencies. Using the system clock, the address of the data read out from the memory can be gradually increased. Therefore, using a constant system clock and a large number of waveforms stored in the memory, any relevant frequency can be generated with a desired resolution. However, the frequency can be changed by selecting a different system clock.

[0040] A particular advantage of the control loop of the present invention is that any unbalanced signal and any interfering signals are eliminated while not introducing interfering signals or noise into the metal detection system.

[0041] The method of the invention can be used for any type of metal detector, for example for metal detectors using only one operating frequency or using multiple frequencies which are independently selectable or can be applied simultaneously.For each imbalance signal occurring for the relevant operating frequency, a compensation signal is synthesized separately in the corresponding control channel.

[0042] Preferably, in the receiver unit or in the signal controller, the unbalanced signal is separated from other signal components by at least one low-pass filter. More preferably, the digital in-phase component of the unbalanced signal is applied to the first control unit via a first low-pass filter, and the digital quadrature component of the unbalanced signal is applied to the second control unit via a second low-pass filter. Demodulating the unbalanced signal usually produces a DC component. However, the signal from the product or contaminant passing through the metal detector causes a low-frequency modulation of the operating frequency. For example, when the receiver signal is demodulated using the reference frequency in the phase detector, this low frequency is regained. Therefore, the signals of the product and contaminants and the unbalanced signal can be separated from each other by a low-pass filter with a corresponding cut-off frequency (e.g. <0.5Hz). Therefore, the influence of the signals of the product and contaminants in the control channel can be excluded. Preferably, signal separation or filtering is performed by a digital signal processor.

[0043] Based on the digital in-phase control component and the digital quadrature control component, the digital compensation signal can be synthesized or generated in different ways. In a preferred embodiment:

[0044] Converting the digital in-phase control component and the digital quadrature control component into a phase control signal and an amplitude control signal;

[0045] forwarding the phase control signal and the amplitude control signal to the compensation unit;

[0046] controlling the synthesizer to generate an operating frequency having a phase according to a phase control signal applied to the synthesizer;

[0047] In the multiplier, the amplitude of the generated operating frequency is adjusted according to the provided amplitude control signal to provide a digital compensation signal.

[0048] For example, the CORDIC algorithm ( https: / / en.wikipedia.org / wiki / CORDIC ) is used to convert the digital in-phase control component and the digital quadrature control component into the phase control signal and the amplitude control signal.

[0049] In another preferred embodiment:

[0050] providing a digital in-phase control component and a digital quadrature control component to a compensation unit;

[0051] controlling the synthesizer to generate an in-phase frequency component and a quadrature frequency component corresponding to an operating frequency, i.e., a frequency of the unbalanced signal;

[0052] In the in-phase multiplier, the amplitude of the generated in-phase frequency component is adjusted according to the digital in-phase control component to provide a digital in-phase compensation component of the digital compensation signal; in the orthogonal multiplier, the amplitude of the orthogonal frequency component is adjusted according to the digital orthogonal control component to provide a digital orthogonal compensation component of the digital compensation signal;

[0053] The digital in-phase compensation component of the digital compensation signal and the digital quadrature compensation component of the digital compensation signal are combined to provide a digital compensation signal.

[0054] The digital compensation signals which have been synthesized for each operating frequency, ie for each unbalanced signal, are combined for providing a combined digital compensation signal, which is converted into an analog compensation signal.

[0055] As described, the compensation signal is fully synthesized in the compensation unit without using the signal generated by the transmitter unit. However, both the transmitter unit and the compensation unit preferably use a common master clock to synchronize them. By establishing phase coherence, interfering phase errors, phase drifts, phase noise, etc. are avoided in the system.

[0056] The compensation signal generated in the compensation unit can be applied to each point in the metal detector at which an unbalanced signal occurs. The compensation signal can preferably be induced into the coil system by means of a compensation coil inductively coupled to the detection coil. Alternatively, the unbalanced signal can be applied to the input of the receiver unit, for example to a compensation coil arranged on a balancing transformer, or preferably to the signal path of the receiver unit via a subtraction module. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Detailed aspects and examples of the present invention are described below with reference to the accompanying drawings, in which:

[0058] Figure 1 A preferred embodiment of the metal detector of the present invention is shown, which comprises a transmitter unit 1, a coil system 2 with a drive coil 21 and two detection coils 22, 23, a receiver unit 3, a control unit 9 and a signal controller 4 which provides data related to the unbalanced signal to a compensation unit 5, which synthesizes a compensation signal a introduced into the signal path of the coil system 2 or the receiver unit 3. COMP ;

[0059] Figure 2 Shows Figure 1The detection coils 22, 23 of the coil system 2 are connected to a metal detector of a balancing transformer 31, which is located at the input of the receiver unit 3 and comprises a balancing compensation coil 30, the compensation signal a COMP applied to the balance compensation coil 30;

[0060] Figure 3 It shows a signal controller 4 and a compensation unit 5 that are provided with two control channels in a preferred embodiment;

[0061] Figure 4 It shows that in another preferred embodiment, a signal controller 4 and a compensation unit 5 are provided for two control channels;

[0062] Figure 5 shows the generation of compensation signal a with different operation modes COMP Flowchart of the process. DETAILED DESCRIPTION

[0063] Figure 1 The block diagram of the metal detector of the present invention in a preferred embodiment is shown, which comprises a transmitter unit 1, a balanced coil system 2 with a drive coil 21 and a first detection coil 22 and a second detection coil 23, a receiver unit 3, a signal controller 4, a compensation unit 5 and a control unit 9, such as a personal computer, preferably including a signal processing circuit, such as at least one signal processor, which is designed to process the signals dI, dI and dI related to the product and the contaminants provided by the receiver unit 3. Q The signal controller 4 and the compensation unit 5 for compensating for the imbalance in the metal detector are preferably also implemented in the control unit 9. The signal controller 4 is preferably implemented in a digital signal processor and the compensation unit 5 is preferably implemented in a field programmable gate array (FPGA) receiving data from the signal controller 4.

[0064] The transmitter unit 1 provides the drive coil 21 of the balanced coil system 2 with a signal having at least one operating frequency f TX The transmitter signal s1 provides the receiver unit 3 with an operating frequency f TX The reference signal s R The transmitter signal s1 induces signals in the same detection coils 22, 23, which are of equal amplitude but opposite polarity as long as the system is in equilibrium, ie as long as no product or contaminant is transported through the metal detector.

[0065] Operating frequency f TX Can be fixed or can be selected from multiple frequencies f TX-A 、f TX-B Alternatively, multiple frequencies f TX-A 、f TX-BThe frequency synthesizer or generator 10 may be generated simultaneously in a transmitter unit 1 provided with at least one frequency synthesizer or generator 10 .

[0066] Once the product or contaminant passes through the balancing coil system 2, the signal induced in the detection coils 22, 23 will change. As a result, the operating frequency f induced in the detection coils 22, 23 TX Modulated by a baseband signal whose amplitude and frequency depend on the electromagnetic properties, size and speed of movement of the object. Depending on the speed of movement, the frequency of the baseband signal will change.

[0067] An unbalanced signal results in a constant imbalance or constant signal offset, which can cause interference and saturation of the receiver channel. Such an unbalanced signal can be caused by mechanical shock to the system, changes in environmental conditions, metal objects located near the detected component, or loosening or aging of the component.

[0068] exist Figure 1 In the preferred embodiment shown, the output signals of the detection coils 22, 23 are applied to an input stage 32 of the receiver unit 3. The input stage 32 generally comprises at least one amplifier and at least one filter unit. The input stage 32 transfers the receiver signal s3 to the in-phase channel I-CH and the quadrature channel Q-CH via an optional subtraction unit 33. The in-phase channel I-CH and the quadrature channel Q-CH, which are designed identically, comprise a phase detector 34I, 34Q, respectively, which is connected to a filter stage 35I, 35Q, followed by a gain stage 36I, 36Q and an analog-to-digital converter 37I, 37Q.

[0069] It receives at its input the receiver signal s3 and the in-phase reference signal s provided by the transmitter unit 1. RI Or quadrature reference signal s RQ The phase detectors 34I, 34Q act as demodulators and provide at their output the in-phase component a of the baseband signal derived from the transmission products and contaminants or imbalances. I Or the orthogonal component a Q . In-phase reference signal s R The signal from transmitter unit 1 is sent directly to the first phase detector 34I and via the phase shifter 38 to the second phase detector 34Q.

[0070] The analog in-phase signal a provided at the output of the phase detectors 34I, 34Q I and orthogonal signal a Q The signals are forwarded via filter units 35I, 35Q to gain units 36I, 36Q which preferably allow the amplitude of the processed signal to be set to a desired value. The filtered and calibrated signals are then converted from analog in-phase and quadrature signals to digital in-phase and quadrature signals in analog-to-digital converters 37I, 37Q.I , d Q The output terminals of the analog-to-digital converters 37I and 37Q provide a digital in-phase signal and a digital quadrature signal. I , d Q is forwarded to the control unit 9, where the digital in-phase signal and the digital quadrature signal d I , d Q is analyzed, and if a signal associated with contamination is detected, an alarm is sounded.

[0071] The control unit 9 generally comprises a processing unit and an operating program 90 designed to implement the functions of a single frequency or multi-simultaneous frequency metal detector of the present invention. The operating program 90 is enhanced to implement the method of the present invention. The signal controller 4 and the compensation unit 5 are preferably also implemented in a signal processing or signal generating circuit contained in the control unit 9.

[0072] The digital in-phase signal and the digital quadrature signal d provided at the output of the analog-to-digital converters 37I and 37Q I , d Q It is also forwarded to the signal controller 4. In the signal controller 4, the digital in-phase signal d including the in-phase component of the unbalanced signal is I is applied to the first control unit 41I, which provides an in-phase control component d for the unbalanced signal. CI The quadrature signal d includes the quadrature component of the unbalanced signal Q is applied to the second control unit 41Q, which provides an orthogonal control component d for the unbalanced signal. CQ .

[0073] In the first control unit 41I and the second control unit 41Q, preferably the same control terms Kp, Ki, Kd are applied, which have an effect on the provided digital in-phase control component d CI and digital quadrature control component d CQ has a proportional and / or integral and / or differential influence. Therefore, a PID controller, as is known in the art, is preferably implemented. The PID controller continuously calculates an error value as the difference between the desired setpoint and the measured process variable, and applies corrections based on proportional, integral and differential terms, denoted P, I and D respectively. These values ​​can be interpreted in terms of time: P depends on the current error, I on the accumulation of past errors and D is a prediction of future errors based on the current rate of change. The weighted sum of these three actions is used to adjust the process via a control element. In the metal detector of the present invention, the measured process variable is the digital in-phase component d of the imbalance signal I and digital quadrature component d QThe goal is to completely eliminate the unbalanced signal, the setting value is zero, and the correction value is the calculated in-phase control component d CI and the orthogonal control component d CQ And the analog compensation signal a derived from it COMP The control element can be viewed as applying a compensation signal a COMP injection point.

[0074] In-phase control component d CI and the orthogonal control component d CQ is forwarded to the converter 42 which preferably uses a CORDIC algorithm to convert the digital in-phase control component d CI and digital quadrature control component d CQ Converted into phase control signal c PH and amplitude control signal c M . Figure 1 shows a digital in-phase control component d CI and digital quadrature control component d CQ The corresponding amplitude and phase angle α of the control vector d C The control vector d corresponding to the unbalanced signal C The amplitude and phase angle α are usually constant. The signals associated with the product and contaminants have a constant phase angle but have an amplitude that varies at a frequency corresponding to the speed at which the product and contaminants are transmitted through the metal detector.

[0075] The phase control signal c forwarded to the compensation unit 5 PHa and amplitude control signal c M With the frequency control signal f C-A Together they include the components used to synthesize and control vector d C The corresponding digital compensation signal d COMP In this example, for the first operating frequency f TX-A Generate or synthesize a compensation signal. The corresponding frequency control signal f C-A With the phase control signal c PHa are applied together to the frequency generator 51 in the compensation unit 5. Therefore, the frequency generator 51 generates a frequency signal f G-Aα , which has a frequency corresponding to the control signal f C-A The frequency and phase control signal c PHα The generated frequency signal f G-Aa is supplied to the multiplier 52, generating a frequency signal f G-Aa The amplitude of the multiplier 52 is adjusted according to the amplitude control signal c provided. M is adjusted to provide a digital compensation signal d COMP .

[0076] The generated or synthesized digital compensation signal d COMP is forwarded to a digital-to-analog converter 54 which provides a corresponding analog compensation signal a which can be applied to various points within the metal detector. COMP For example, the analog compensation signal a COMP The receiver signals s2, s3 are applied to the balanced coil system or the input of the receiver unit 3 or in the signal path of the receiver unit 3. In general, in all embodiments, the analog compensation signal a COMP Before being applied, it is amplified in an amplifier 6 , for example a buffer amplifier.

[0077] In a first preferred embodiment, the analog compensation signal a COMP is applied to a compensation coil 24 that is inductively coupled to at least one of the drive coils. In a second embodiment, the analog compensation signal a COMP is applied to the subtraction unit 33, in which the unbalanced signal present in the system is converted into an analog compensation signal a COMP Reduce, preferably to zero.

[0078] For any other interference signal present in the metal detector, the digital in-phase component and the digital quadrature component and the frequency can be determined, and the digital in-phase control component and the digital quadrature control component can be calculated. In the compensation unit 5, a digital correction signal d can be synthesized according to the digital in-phase component and the digital quadrature component and the frequency of the interference signal. CORR and add it to the digital compensation signal d COMP (See Figure 3 and Figure 4 ).

[0079] Thus, using the method of the invention, any detected interference can be eliminated by analyzing the interference signal and synthesizing a corresponding correction signal. The operating conditions and operating parameters of the metal detector are selected by means of a control program 90. The control unit 9 preferably communicates with the transmitter unit 1 via a first control bus 91 and with the signal controller 4 and the compensation unit 5 via a second control bus 945. In a preferred embodiment, all processing performed in the metal detector, conventional measurement processing and all processing related to the method of the invention are preferably controlled by the control unit 9.

[0080] For example, if two operating frequencies f TX-A 、f TX-B , it indicates that the transmitter unit 1 generates these two operating frequencies f TX-A 、f TX-B In addition, the two control channels 4A, 4B; 5A, 5B are opened to compensate for the two operating frequencies f TX-A 、f TX-B Related unbalanced signals. Figure 1 The first receiver module, the first signal controller 4A and the first compensation unit 5A shown in FIG. Figures 2 to 4 ) is part of a first control loop for a first operating frequency f TX-A The unbalanced signal is eliminated in the first control loop. Figure 1 The second receiver module, the second signal controller 4B and the second compensation unit 5B shown in FIG. Figure 3 , Figure 4 ) is part of a second control loop for a second operating frequency f TX-B The unbalanced signal is eliminated in the second control loop. In the receiver module, for example, Figure 1 The in-phase channels and quadrature channels I-CH, Q-CH are arranged as shown. Thus, each signal controller 4A, 4B is provided with a corresponding operating frequency f TX-A 、f TX-B The demodulated digital in-phase component of the unbalanced signal d I and digital quadrature component d Q .

[0081] In order to ensure phase coherence of all processing performed in the metal detector, the at least one frequency signal f generated in the compensation unit 5 is G-A 、f G-B or its in-phase frequency component f GI and the orthogonal frequency components f GQ (See Figure 4 ) and the corresponding operating frequency f generated in the transmitter unit 1 TX-A 、f TX-B Regarding phase and frequency synchronization. Figure 1 The master clock mcl is shown being applied to the frequency generator 10 in the transmitter unit 1 , the analog-to-digital converters 37I and 37Q in the receiver unit 3 , and the frequency generator 51 , the multiplier 52 and the digital-to-analog converter 54 in the compensation unit 5 .

[0082] Figure 2 Shows Figure 1 The detection coils 22, 23 of the coil system 2 are connected to a metal detector of a balanced transformer 31 located at the input end of the receiver unit 3. The output signal of the detection coils 22, 23 is applied to the center-tapped primary winding of the balanced transformer 31 which is mirrored with the detection coils 22, 23. In addition, the balanced transformer 31 includes two identical center-tapped secondary windings, whose opposite tails are connected to the input stage 32 of the receiver unit 3. The balanced transformer 31 also includes a balanced compensation coil 30, which compensates the signal a COMPThe voltage is applied to the balanced compensation coil 30 via the amplifier 6 and the single-ended to differential converter 60 (hereinafter: SETD converter). The SETD converter 60 provides a pair of differential signals at its output.

[0083] Figure 3 shows a frequency dedicated to the selected operating frequency f TX-A 、f TX-B The signal controller 4 and the compensation unit 5 of the two control channels of FIG. Each control channel comprises a signal controller 4A, 4B and a compensation unit 5A, 5B, respectively. The signal controller 4A, 4B comprises a low pass filter 40I, 40Q for separating the unbalanced signal from other signals, for example related to the product or contaminants. Thus, the signals related to the product and contaminants are inhibited and do not affect the processing in the control loop. The digital in-phase component d of the unbalanced signal I The digital quadrature component d CQ The signal is applied to the second control unit 41Q via the second low-pass filter 40Q.

[0084] As reference Figure 1 The digital in-phase signal and the digital quadrature signal d provided at the output of the relevant low-pass filters 40I and 40Q are I-A , d Q-A ;d I-B , d Q-B is forwarded to the control units 41I, 41Q. In each control channel, the digital in-phase signal d I-A ,d I-B is applied to the first control unit 41I, which provides the in-phase control component d of the unbalanced signal CI-A ,d CI-B . Orthogonal signal d QA ,d QB is applied to the second control unit 41Q, which provides a quadrature control component d of the unbalanced signal CQ-A ,d CQ -B.

[0085] In each control channel, the in-phase control component d CI-A ,d CI-B and the orthogonal control component d CQ-A , dC Q-B is forwarded to the converter 42 which preferably uses a CORDIC algorithm to convert the digital in-phase control component d CI-A ,d CI-B and digital quadrature control component d CQ-A ,d CQ-B Converted into phase control signal c PH-α 、c PH-β and amplitude control signal cM-A 、c M-B .

[0086] In each control channel, the phase control signal c forwarded to the compensation unit 5 PH-α 、c PH-β and amplitude control signal c M-A 、c M-B With the frequency control signal f C-A 、f C-B together include the digital compensation signal d COMP-A d COMP-B The corresponding frequency control signal f C-A 、f C-B With the phase control signal c M-A 、c M-B are applied to the relevant frequency generator 51. The frequency generator 51 generates a signal having a frequency corresponding to the frequency control signal f C-A 、f C-B The frequency and the corresponding phase control signal c M-A 、c M-B The corresponding frequency signal f of the phase angle α, β G-Aα 、f G-Bβ The generated frequency signal f G-Aα 、f G-Bβ is supplied to the multiplier 52, generating a frequency signal f G-Aa 、f G-Bβ The amplitude of the multiplier 52 is adjusted according to the amplitude control signal c provided. M-A 、c M-B is adjusted to provide a digital compensation signal d COMP-A d COMP-B .

[0087] Digital compensation signal d COMP-A d COMP-B are combined in adder 53, which preferably also combines the correction signal d as described above. CORR At the output of the adder 53 a combined digital compensation signal d is provided. COMP , which is converted into an analog compensation signal a in the subsequent digital-to-analog converter 54 COMP As reference Figure 1 or Figure 2 The analog compensation signal a COMP Receiver signals s2 , s3 which are applied to the balanced coil system 2 or to the input of the receiver unit 3 or to the signal path of the receiver unit 3 .

[0088] Figure 4FIG. 4 shows a signal controller 4 and a compensation unit 5 in another preferred embodiment. Each control channel includes a signal controller 4A, 4B and a compensation unit 5A, 5B. In each control channel, the in-phase control component d CI-A ,d CI-B and the orthogonal control component d CQ-A ,d CQ-B is directly forwarded to the relevant compensation unit 5A, 5B. The frequency control signal f C-A 、f C-B is applied to the relevant frequency synthesizer 51 / 51L in the compensation unit 5A, 5B, and the frequency synthesizer 51 / 51L generates a frequency control signal corresponding to f C-A or C-B The in-phase frequency component f GI-A 、f GI-B and the orthogonal frequency components f GQ-A 、f GQ-B .

[0089] In each control channel, the in-phase multiplier 52I is used to calculate the digital in-phase control component d CI-A ,d CI-B Adjust the generated in-phase frequency component f GI-A 、f GI-B The amplitude is used to provide the digital in-phase compensation component d COMP-I-A ,d COMP-I-B In the quadrature multiplier 52Q, the digital quadrature control component d CQ-A ,d CQ-B Adjust the orthogonal frequency component f GQ The amplitude is used to provide the digital quadrature compensation component d COMP-Q-A ,d COMP-Q-B . Digital in-phase compensation component d COMP-I-A ,d COMP-I-B and digital quadrature compensation component d COMP-Q-A ,d COMP-Q-B They are then combined in adder 53, which provides a combined digital compensation signal d COMP , which is converted into an analog compensation signal a in the subsequent digital-to-analog converter 54 COMP .

[0090] At least one frequency signal f G-A 、f G-B or their in-phase frequency components f GI and the orthogonal frequency components f GQ At least one frequency signal f is obtained by direct digital synthesis or by sequentially reading out from a memory unit or a look-up table 51-L. G-A 、f G-B or their in-phase frequency components f GI and the orthogonal frequency components fGQ US5892692A describes a digital waveform oscillator that uses a lookup table to generate a distortion-free digitized waveform.

[0091] US2007067123A1 discloses an arbitrary waveform generator that can be used to generate any desired waveform in a compensation unit. The control channels can use the waveform generator alone, or can share the waveform generator. It is described that the arbitrary waveform generator can generate arbitrary signal waveforms specific to its application. These arbitrary signal waveforms can be used to simulate "real world" signals, which include flicker, drift, noise and other anomalies on the arbitrary signal waveform that components, such as the device under test, will encounter when leaving the laboratory or manufacturing workshop.

[0092] In a preferred embodiment, the method of the present invention uses such a waveform generator to generate the compensation signal d as described above. COMP Or correction signal d CORR In relatively simple embodiments, a waveform generator or arbitrary waveform generator will usually be sufficient to compensate for unbalanced signals or interfering signals that occur recursively in metal detectors. Since the signal waveform is defined point by point, the resolution of the output signal can be improved by using more update points that define the generated signal waveform.

[0093] exist Figure 3 and Figure 4 In the example, two control channels or two operating frequencies f are used. TX-A 、f TX-B The present invention has been described. However, in a metal detector of the present invention operating according to the method of the present invention, any number of control channels may be implemented.

[0094] The signal controller 4, 4A, 4B preferably operates in two or more modes, wherein different sets of control items are applied to the control unit. Figure 5 shows the generation of compensation signal a with different operation modes COMP Flowchart of the process.

[0095] According to the flowchart, perform the following steps:

[0096] Initialize the metal detector (system startup);

[0097] Then set the metal detector to the first control mode (control mode 1);

[0098] In control mode 1, a first set of control terms Kp, Ki, Kd is applied to the first control unit 41I and the second control unit 41Q, wherein the control terms Kp, Ki, Kd are selected in such a way that the digital in-phase control component d CI and digital quadrature control component d CQ being provided with a first correction factor;

[0099] Maintaining control mode 1 for a defined period of time or as long as the imbalance signal exceeds a given threshold;

[0100] If the defined time period has elapsed or the imbalance signal has fallen below a given threshold, the second control mode (control mode 2) is entered and the metal detector is changed to a state where a measurement process can be performed;

[0101] In control mode 2, a second set of control terms Kp, Ki, Kd is applied to the first control unit 41I and the second control unit 41Q, wherein the control terms Kp, Ki, Kd are selected in such a way that the digital in-phase control component d CI and digital quadrature control component d CQ being provided with a second correction factor which is lower than the first correction factor and which results in a reduced correction;

[0102] Control mode 2 is maintained as long as the imbalance signal does not exceed a given threshold.

[0103] For example, control mode 1 is maintained for a period of time, during which the unbalance signal generally decreases to a level at which the unbalance signal has no significant effect on the measurement. Alternatively, a level at which the unbalance signal has no significant effect on the measurement is predetermined and compared with the actual unbalance signal. As soon as the unbalance signal decreases below a corresponding threshold, control mode 1 is changed to control mode 2, in which the measurement of the product is performed while the unbalance signal is still observed and reduced in the control loop. In control mode 1, the unbalance signal decreases rapidly, while in control mode 2, the remaining unbalance signal is slowly driven to a minimum value. The control terms Kp, Ki, Kd for the second control mode are selected so that the signal correction in the control loop does not cause interference during the product measurement. In particular, the control terms Kp, Ki, Kd for the control unit are selected to avoid overshoot. Therefore, in the second control mode, the operating conditions are continuously optimized while the measurement can be performed.

[0104] Reference numerals list

[0105] 1 Transmitter unit

[0106] 10 Frequency generator in transmitter unit 1

[0107] 2 Coil system

[0108] 21. Driving coil

[0109] 22, 23 Detection coil

[0110] 24 Compensation coil

[0111] 3 Receiver unit

[0112] 31 Balance transformer

[0113] 32 Input stage with amplifier and / or filter

[0114] 33 Subtraction Unit

[0115] 34I, 34Q Phase Detector

[0116] 35I, 35Q filter unit

[0117] 36I, 36Q Gain Units; Controllable Gain Units

[0118] 37I, 37Q Analog-to-Digital Converters

[0119] 38 Phase Shifter

[0120] 4, 4A, 4B Signal Controller

[0121] 40I, 40Q low pass filter

[0122] 41I, 41Q control unit

[0123] 42 Converter (CORDIC)

[0124] 5, 5A, 5B Compensation Unit

[0125] 51, 51L frequency synthesizer, lookup table

[0126] 52, 52I, 52Q multipliers

[0127] 53 Adder

[0128] 54 A / D converter

[0129] 6 Amplifier

[0130] 60 Single-ended to differential converter

[0131] 9. A control unit, preferably comprising:

[0132] -Signal Controller 4

[0133] - Compensation unit 5

[0134] - Digital signal processor to process signals related to product contamination and / or imbalance

[0135] a I , a Q Analog in-phase signal and analog quadrature signal

[0136] a COMP Analog compensation signal

[0137] c M Amplitude control signal

[0138] c PH Phase control signal

[0139] d I , d Q Digital in-phase signal and digital quadrature signal

[0140] d CI , d CQ Digital in-phase control signal and digital quadrature control signal

[0141] d COMP Digital compensation signal

[0142] d CORR Digital correction signal

[0143] f C Frequency control signal

[0144] f G Generated frequency signal

[0145] f TX , f TX-A , f TX-B Operating frequency

[0146] Kp, Ki, Kd Control Items

[0147] mcl Master clock

[0148] s1 transmitter signal

[0149] s2 Input signal at receiver unit 3

[0150] s3 Signal in the receiver path

[0151] s RI ,s RQ Reference signal

Claims

1. A method for operating a metal detector, the metal detector comprising a balanced coil system (2) having a drive coil (21) and a first detection coil and a second detection coil (22, 23), the drive coil (21) being connected to a transmitter unit (1), the transmitter unit (1) providing a signal having at least one operating frequency (f TX ), the first detection coil and the second detection coil (22, 23) provide an output signal (s2) to a receiver unit (3), the receiver unit (3) processes the associated receiver signal (s3) including the unbalanced signal and provides a demodulated digital in-phase component and a digital quadrature component (d I , d Q ), the digital in-phase component and the digital quadrature component (d I , d Q ) is analyzed in a control unit (9) in order to detect metal contaminants in a product inspected by a metal detector, the method comprising the following steps: The demodulated digital in-phase component and the digital quadrature component (d) of the unbalanced signal in the signal controller (4) are filtered using at least one low pass filter (40I, 40Q). I , d Q ) and the demodulated digital in-phase component and digital quadrature component (d I , d Q ) separation; The digital in-phase component and the digital quadrature component (d I , d Q ), for separating the unbalanced signal from signals related to the product and contaminants, and providing control data to a compensation unit (5) for compensating the unbalanced signal, wherein the step of processing the unbalanced signal comprises: The digital in-phase component (d I ) is applied to the first control unit (41I), and the first control unit (41I) provides a digital in-phase control component (d CI ), the digital quadrature component of the unbalanced signal (d Q ) is applied to the second control unit (41Q), and the second control unit (41Q) provides a digital quadrature control component (d CQ ); In the compensation unit (5), a frequency having a frequency which is equal to the at least one operating frequency (f TX ) corresponding to the frequency of the unbalanced signal and having a digital in-phase control component and a digital quadrature control component (d CI ,d CQ ) of the phase and amplitude digital compensation signal (d COMP ); The digital compensation signal (d COMP ) is converted into an analog compensation signal (a COMP ); The simulated compensation signal (a COMP ) is applied to the balanced coil system (2) or the receiver signal (s2, s3) to compensate for the unbalanced signal.

2. A method for operating a metal detector according to claim 1, wherein: The method comprises the following steps: generating a signal for different operating frequencies (f TX-A 、f TX-B ) is a simulated compensation signal of multiple unbalanced signals (a COMP ).

3. A method for operating a metal detector according to claim 1, wherein: The method comprises the following steps: using control items (Kp, Ki, Kd) in a first control unit (41I) and a second control unit (41Q), wherein the control items (Kp, Ki, Kd) are applied to the provided digital in-phase control component (d CI ) and digital quadrature control components (d CQ ) has proportional, integral, and derivative effects.

4. A method for operating a metal detector according to claim 1, wherein: The method comprises the following steps: In the signal controller (4), the digital in-phase component and the digital quadrature component and the frequency of the interference signal are determined; Determine or calculate a digital in-phase control component and a digital quadrature control component for the interference signal based on the digital in-phase component and the digital quadrature component of the interference signal; In the compensation unit (5), a digital correction signal (d CORR ); The digital correction signal (d CORR ) is added to the digital compensation signal (d COMP )middle.

5. A method for operating a metal detector according to claim 1, wherein: The method comprises the following steps: The digital in-phase control component (d CI ) and digital quadrature control components (d CQ ) is converted into a phase control signal (c PH ) and amplitude control signal (c M ); A phase control signal (c) is provided to a synthesizer (51) provided in the compensation unit (5). PH ), amplitude control signal (c M ) and the frequency control signal (f C ); The control synthesizer (51) generates a frequency control signal (f C ) and has a frequency according to the phase control signal (c PH ) of the phase frequency signal (f G ); In the multiplier (52), according to the provided amplitude control signal (c M ) to adjust the generated frequency signal (f G ) amplitude, used to provide a digital compensation signal (d COMP ).

6. A method for operating a metal detector according to claim 1, wherein: The method comprises the following steps: Provides a digital in-phase control component (d CI ) and digital quadrature control components (d CQ ) and the frequency control signal (f C ); The synthesizer (51) is controlled to generate a frequency control signal (f C ) of the in-phase frequency component (f GI ) and the orthogonal frequency components (f GQ ); In the in-phase multiplier (52I), the digital in-phase control component (d CI ) adjusts the generated in-phase frequency component (f GI ) amplitude, used to provide a digital compensation signal (d COMP ) of the digital in-phase compensation component (d COMP-I ), in the quadrature multiplier (52Q), according to the digital quadrature control component (d CQ ) adjusts the orthogonal frequency components (f GQ ) amplitude, used to provide a digital compensation signal (d COMP ) of the digital quadrature compensation component (d COMP-Q ); The digital compensation signal (d COMP ) of the digital in-phase compensation component (d COMP-I ) and the digital compensation signal (d COMP ) of the digital quadrature compensation component (d COMP-Q ) combination, used to provide a digital compensation signal (d COMP ).

7. A method for operating a metal detector according to claim 2, wherein: The method comprises the following steps: COMP-A ,d COMP-B ) combination, used to provide a combined digital compensation signal (d COMP ), and the combined digital compensation signal (d COMP ) is converted into an analog compensation signal (a COMP ).

8. A method for operating a metal detector according to claim 5 or 6, wherein: The method comprises the following steps: at least one frequency signal (f) is sequentially read out by direct digital synthesis or from a memory unit or a look-up table (51L) or an arbitrary waveform generator. G-A 、f G-B ) or the at least one frequency signal (f G-A 、f G-B ) of the in-phase frequency component (f GI ) and the orthogonal frequency components (f GQ ) waveform data to generate the at least one frequency signal (f G-A 、f G-B ) or the at least one frequency signal (f G-A 、f G-B ) of the in-phase frequency component (f GI ) and the orthogonal frequency components (f GQ ).

9. A method for operating a metal detector according to claim 8, wherein: The method comprises the following steps: at least one frequency signal (f G-A 、f TXG-B ) or the at least one frequency signal (f G-A 、f TXG-B ) of the in-phase frequency component (f GI ) and the orthogonal frequency components (f GQ ) and the corresponding operating frequency (f TX-A 、f TX-B ) about phase and frequency synchronization.

10. A method for operating a metal detector according to any one of claims 1 to 7, wherein: The method comprises the following steps: COMP ) is applied to the compensation coil (24) of the balanced coil system (2) or the analog compensation signal (a COMP ) is applied to an input signal (s2) of the receiver unit (3), or is applied to a receiver signal (s3) in a signal path of the receiver unit (3) via a summing module or a subtraction module (33).

11. A method for operating a metal detector according to any one of claims 1 to 7, wherein: The method comprises the following steps: Initialize the metal detector; The metal detector is set to a first control mode, in which no product is measured; In the first control mode, a first set of control items (Kp, Ki, Kd) is applied to the first control unit (41I) and the second control unit (41Q), and the first set of control items (Kp, Ki, Kd) is selected in such a way that the digital in-phase control component (d CI ) and digital quadrature control components (d CQ ) is provided with a first correction factor; maintaining the first control mode for a defined period of time or as long as the imbalance signal exceeds a given threshold, and entering an operating mode of the measuring product after the defined period of time has elapsed or the imbalance signal has fallen below a given threshold; or The first control mode is maintained for a defined period of time or as long as the unbalanced signal exceeds a given threshold, and after the defined period of time has elapsed or the unbalanced signal has dropped below a given threshold, a second control mode is entered, in which the product is measured and the unbalanced signal is further observed and reused.

12. A method for operating a metal detector according to claim 11, wherein: The method comprises the following steps: in the first control mode, also applying the determined and stored digital in-phase control component (d CI ) and digital quadrature control components (d CQ ), or digital compensation signal (d COMP ) or analog compensation signal (a COMP ).

13. A method for operating a metal detector according to claim 11, wherein: The method comprises the following steps: in a second control mode, applying a second set of control items (Kp, Ki, Kd) to a first control unit (41I) and a second control unit (41Q), the second set of control items (Kp, Ki, Kd) being selected in such a way that the digital in-phase control component (d CI ) and digital quadrature control components (d CQ ) is provided with a second correction factor which is lower than the first correction factor and results in a reduced correction.

14. A method for operating a metal detector according to claim 11, wherein: The method comprises the steps of monitoring the imbalance signal and returning to the first control mode if the imbalance signal exceeds a predetermined threshold.

15. A metal detector operated according to the method defined in any one of claims 1 to 14.

16. A metal detector according to claim 15, comprising a balanced coil system (2) having a drive coil (21) and a first and a second detection coil (22, 23), the drive coil (21) being connected to a transmitter unit (1) providing a signal having at least one operating frequency (f TX ), the first detection coil and the second detection coil (22, 23) provide an output signal (s2) to the receiver unit (3), the receiver unit (3) is designed to process the receiver signal (s3) including the unbalanced signal and provide the demodulated digital in-phase component and digital quadrature component (d3) of the unbalanced signal to the control unit (9). I , d Q ) to detect metal contaminants in products inspected by the metal detector, and to provide a digital in-phase component and a digital quadrature component (d) of the demodulated unbalanced signal to a signal controller (4) I , d Q ), the signal controller (4) comprising at least one low-pass filter (40I, 40Q) for demodulating the digital in-phase component and the digital quadrature component (d I , d Q ) and the demodulated digital in-phase component and digital quadrature component (d I , d Q ) separation; in, The signal controller (4) is also designed to provide control data to a compensation unit (5), which is designed to compensate for an unbalanced signal, wherein: a) In the signal controller (4), the digital in-phase component (d I ) is suitable for being designed to provide the digital in-phase control component (d CI ) of the first control unit (41I), and the digital quadrature component (d Q ) is suitable for being designed to provide digital quadrature control components for unbalanced signals (d CQ ) a second control unit (41Q); b) The compensation unit (5) is designed as follows: The synthesized frequency has the same frequency as the at least one operating frequency (f TX ) corresponding to the frequency of the unbalanced signal and having a digital in-phase control component and a digital quadrature control component (d CI ,d CQ ) of the phase and amplitude digital compensation signal (d COMP ); The digital compensation signal (d COMP ) is converted into an analog compensation signal (a COMP ), The simulated compensation signal (a COMP ) is applied to a compensation coil (24) of a balanced coil system (2) or an input signal (s2) of a receiver unit (3) or a receiver signal (s3) in a signal path of a receiver unit (3) via a summing module or a subtraction module (33) for compensating an unbalanced signal.

17. The metal detector according to claim 15 or 16, wherein: The signal controller (4) and the compensation unit (5) include a circuit for generating a signal for different operating frequencies (f TX-A 、f TX-B ) appears under the digital compensation signal of the unbalanced signal (d COMP ) multiple control channels.

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