Magnetic field sensor for measuring direct current

Abstract

The present invention relates to a magnetic field sensor for measuring direct current, comprising: a transducer (3) formed by coils in series having a superparamagnetic (SPM) core; an excitation module (2) that generates an excitation current Ie at an excitation frequency Fe via a midpoint coil (P11 / P12), an excitation voltage generator (VEXC), and an excitation impedance (Ze); and an analysis module (1) that includes an analysis impedance (Za) coupled to the midpoint coil (P11 / P12) and means for analyzing the current flowing through the analysis impedance (Za) to extract a component at an analysis frequency Fa that is an even multiple of the excitation frequency Fe; the excitation impedance together with the SPM coil forms a first series RLC circuit and a second series RLC-type circuit, the resonance frequency Fres_e of the first series RLC circuit is substantially equal to the excitation frequency Fe, and the analysis resonance frequency Fres_a of the second series RLC-type circuit is substantially equal to the analysis frequency Fa.

Description

Technical Field

[0001] The present invention relates to a device for non-contact measurement of the current flowing in a conductor by measuring the magnetic field induced by the current. More specifically, the present invention relates to a magnetic field sensor integrated with a transducer based on superparamagnetic material suitable for measuring direct current. Background Art

[0002] In order to measure current I, different physical principles can be used to generate a physical quantity representing the current I. For example, magnetic sensors implement transducers sensitive to magnetic quantities (such as the magnetic field induced by the current to be measured).

[0003] In particular, for example, in the document FR2891917, a current sensor implementing the so-called Néel effect is known. This technique is characterized by the use of a sensor consisting of coils whose magnetic cores are based on a charged composite material of nanoparticles with superparamagnetic properties. The superparamagnetic material used has the property of being non-hysteretic, so the advantage of this transducer is that there is no magnetic offset.

[0004] In a simplified way, a coil with a superparamagnetic core (or SPM coil) acts as a mixer. When an excitation current Fexc with a known frequency flows through the SPM core, in the absence of an external magnetic field Hext to be measured, the electromotive force (EMF) induced at the terminals of the coil will not have even harmonics. On the contrary, the presence of the external magnetic field Hext to be measured will cause the appearance of even harmonics based on the information of the magnetic field to be measured. In fact, through the amplitude and the phase of the even harmonics (especially the phase of the second harmonic), the amplitude and the polarity of the primary field can be accessed respectively. A feedback circuit can be coupled to the system to reduce the potential interference information due to the distortion caused by the system and to transmit a more useful measurement quantity, for example, proportional to the current to be measured.

[0005] For example, such a current sensor is briefly described in the document FR2980581, but the proposed configuration assumes the use of a specific signal generator capable of generating an ideal wave excitation signal (i.e., a signal with zero amplitude of even harmonics). However, the signals generated by a PWM (Pulse Width Modulation) type generator or an analogue-to-digital converter (ADC) necessarily contain harmonics (especially the second harmonic), the level of which can be high and which are present in the signal provided by the SPM coil, thus requiring the implementation of complex processing to extract the useful signal.

[0006] Document FR3038063 proposes a sensor that integrates a transducer formed by four SPM coils, which is coupled to two excitation generators (i.e., current I HF The high frequency generator F HF and current I BF The low frequency generator F BF In particular, the sensor is configured so that the excitation current flows through the coil, so that only the useful frequency F can be obtained at the output of the transformer. HF -F BF 、F HF +F BF Thus, even if the signal I HF ,I BF The useful signal at the output (especially the output of the transformer used for detection) will not be affected by the quality of the excitation signal. However, this configuration involves the selection of the ratio frequency F HF A much lower frequency F BF , so that the two frequencies in the useful output signal are relatively close and only 2.F away from each other. BF This low frequency spacing necessitates the use of relatively low cutoff frequency demodulation filters (especially below 2.F BF ), which results in a long response time and limited bandwidth. In addition, the inevitable imbalance of the SPM coil also leads to the HF -F BF 、F HF +F BF Frequency F HF In order to obtain a good signal-to-noise ratio, the amplification of the useful frequency must be high, so this component F HF The presence of creates an obstacle and is difficult to filter out due to its proximity to the useful frequency. Selective band-stop filters can be added to limit this problem, but besides the complexity and precision of the production of such selective filters, their integration also increases the response time of the system. In addition, the use of two excitation generators to inject two frequencies makes this solution very costly. Summary of the invention

[0007] Therefore, the object of the present invention is to propose an alternative DC current or DC magnetic field sensor, which integrates a coil with a superparamagnetic core. In particular, the present invention aims to propose a configuration that can improve the signal-to-noise ratio of the system, which configuration does not require the use of very expensive components, does not require the implementation of bulky circuits, and is not complicated to produce.

[0008] Accordingly, the present invention aims to provide a magnetic field sensor for measuring direct current, which is formed by at least one superparamagnetic (SPM) material transducer for undergoing an external magnetic field to be measured and electrically coupled to an excitation module and an analysis module.

[0009] According to the present invention, the transducer includes at least a pair of coils having superparamagnetic (SPM) cores, these SPM coils being substantially identical and connected in series between two end terminals of the transducer, and a common connection point of the SPM coils being connected to a reference potential.

[0010] In addition, the excitation module is configured to generate and inject an excitation current Ie into the transducer at a predetermined excitation frequency Fe, and the excitation module at least includes:

[0011] . A midpoint coil, which is mounted parallel to the terminals of the transducer;

[0012] . An excitation voltage generator, which is mounted between the reference potential and the midpoint, and the voltage generator preferably has an adjustable or tunable frequency; and

[0013] . An excitation impedance, which is configured to form a first series RLC type circuit together with the SPM coils, and a resonance frequency Fres_e of the first series RLC type circuit is substantially equal to the excitation frequency Fe.

[0014] In addition, the analysis module at least includes:

[0015] . An analysis impedance, which is connected to an external terminal of the midpoint coil, and the excitation impedance is configured to form a second series RLC type circuit together with the SPM coils, and an analysis resonance frequency Fres_a of the second series RLC type circuit is substantially equal to an analysis frequency Fa; and

[0016] . A device for analyzing a current flowing through the analysis impedance at the analysis frequency Fa to extract a component at the analysis frequency Fa that is an even multiple of the excitation frequency Fe.

[0017] Advantageously, the analysis frequency Fa is equal to 2.Fe.

[0018] According to one embodiment, the excitation impedance is connected between the excitation generator and the midpoint of the midpoint coil.

[0019] According to another embodiment, the excitation impedance is connected between the reference potential and the common connection point of the SPM coils.

[0020] According to a variant, the excitation impedance includes at least one excitation capacitor.

[0021] According to another variant, the excitation impedance includes at least one excitation capacitor and at least one excitation inductor, and the excitation inductor is connected between the excitation capacitor and the excitation voltage generator.

[0022] In practice, the midpoint coil may consist of two substantially identical windings wound around the same magnetic core.

[0023] Advantageously, the analysis impedance may be constituted by an analysis capacitor and an analysis resistor, and the analysis capacitor and the analysis resistor are serially mounted between two external terminals of the midpoint coil.

[0024] The analysis impedance may further include an analysis inductor, and the analysis inductor is serially mounted with the analysis capacitor and the analysis resistor between the external terminals of the midpoint coil.

[0025] According to another embodiment, the magnetic field sensor may further include a transformer, which is mounted between the midpoint coil and the analysis impedance. The transformer is constituted by a primary winding and a secondary winding. The primary winding is connected to the external terminals of the midpoint coil, and the secondary winding is connected to the terminals of the analysis impedance.

[0026] According to another embodiment, the magnetic field sensor may further include a transformer, which includes a primary winding and a secondary winding. The primary winding is formed by the midpoint coil, and the secondary winding is connected to the terminals of the analysis impedance.

[0027] According to another embodiment, the SPM coil of the transducer forms a first pair of SPM coils, and the sensor may further include a second pair of SPM coils, which is substantially the same as the first pair of SPM coils. In this another embodiment, the second pair of SPM coils are serially mounted, the end terminals of the first pair of SPM coils are connected to one coil of the second pair, and the end terminals of the first pair of SPM coils are connected to the other coil of the second pair.

[0028] Advantageously, the sensor may further include a feedback module, which is formed by at least one feedback voltage generator configured to generate a feedback current. In the case of a sensor with two SPM coils, the feedback voltage generator is preferably mounted to the end terminals of the transducer, and in the case of a sensor with four SPM coils, the feedback voltage generator is preferably mounted between the second pair of SPM coils and configured to generate a feedback current.

[0029] According to a variant, the feedback voltage generator may be formed by two voltage sources referenced to a reference potential.

[0030] According to another embodiment, the magnetic field sensor may further include a calibration module configured to find an optimal value Fopt of the excitation frequency Fe to be injected into the transducer.

[0031] In one variant, the calibration module may be configured to:

[0032] - vary the excitation frequency Fe of the excitation generator within a frequency range [Fe_min; Fe_max] and measure the corresponding excitation current Ie; and

[0033] - identify an optimal excitation frequency Fopt_e of the excitation frequency Fe corresponding to the maximum excitation current level Ie.

[0034] For example, the calibration module may be configured to measure the excitation current at the terminals of a resistor installed between the reference potential and the common connection point of the SPM coil of the transducer.

[0035] When the excitation impedance is installed between the reference potential and the common connection point of the two SPM coils of the transducer, the calibration module may also be configured to measure the excitation current at the terminals of the excitation capacitor.

[0036] In another variant, the calibration module may be configured to:

[0037] - vary the excitation frequency Fe of the excitation generator within a frequency range [Fe_min; Fe_max] and measure the sensitivity S of the excitation module; and

[0038] - identify an optimal excitation frequency Fopt_e of the excitation frequency Fe corresponding to the maximum sensitivity S. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Other features and advantages of the present invention will be more clearly understood by reading the following description with reference to the accompanying drawings, which are provided by way of non-limiting example, in which:

[0040] - Figure 1 is a simplified electrical circuit diagram of a sensor according to one embodiment, where the impedance Ze is formed by a capacitor Ce.

[0041] - Figure 2 is a simplified electrical circuit diagram of a sensor according to another embodiment, where an inductor Le is added to the excitation impedance Ze.

[0042] - Figure 3 is a simplified electrical circuit diagram of a sensor according to another embodiment, where an inductor La is added to the analysis impedance Za.

[0043] - Figure 4 is a simplified electrical circuit diagram of a sensor according to another embodiment, where an adjustment transformer T1 is added.

[0044] - Figure 5 is a simplified electrical circuit diagram of a sensor according to another embodiment, wherein Figure 4 the functions of the transformer T1 and the midpoint coil are combined in a single element T2.

[0045] - Figure 6 is a simplified electrical circuit diagram of a sensor according to another embodiment, wherein the excitation impedance Ze is installed between the common terminal of the transducer and the common potential.

[0046] - Figure 7 is a simplified electrical circuit diagram of a sensor according to another embodiment, including two additional SPM coils.

[0047] - Figure 8 is according to another embodiment of Figure 7 the sensor including a simplified electrical circuit diagram of a feedback circuit.

[0048] - Figure 9 is according to another embodiment of Figure 7 the sensor including a simplified electrical circuit diagram of a feedback circuit.

[0049] - Figure 10 is a schematic diagram of a calibration step for finding the optimal excitation frequency according to a method.

[0050] - Figure 11 is a schematic diagram of a calibration step for finding the optimal excitation frequency according to another method.

[0051] - Figure 12 is a graph showing the attenuation of the analysis frequency Fa of the interference signal present in the excitation voltage in the case where the inductor Le of Figure 2 is increased.

[0052] - Figure 13 is a graph showing the selectivity difference found between excitation and analysis according to an embodiment. DETAILED DESCRIPTION

[0053] The sensor of the present invention includes a transducer formed by at least a pair of coils having superparamagnetic cores (hereinafter referred to as SPM) and coupled to an excitation module and a regulator module or an analysis module. The transducer is used to receive the external magnetic field to be measured. In practice, the external magnetic field to be measured includes frequencies from direct frequencies to frequencies significantly lower than the excitation frequency (preferably at least 10 times lower). The excitation module is configured to generate and inject an excitation signal into the transducer, for example, in the form of a so-called "excitation" current with a frequency corresponding to a predetermined excitation frequency Fe. The regulator module is configured to recover and analyze the original SPM measurement signal (e.g., EMF) at the transducer terminals, which represents the time variation of magnetic induction in the SPM coils. The analysis of the original SPM signal particularly includes generating a useful signal that is easy to use and contains useful information representing the magnetic field or current to be measured. For example, the analysis includes removing the unwanted frequency components in the SPM signal and only retaining the useful frequency components in the SPM signal.

[0054] Therefore, contrary to the solution described in document FR3038063A1, a single excitation frequency Fe (especially a high excitation frequency) is adopted in this sensor, and thus the useful information related to the magnetic field or current to be measured is located in the even harmonics of the SPM measurement signal. Therefore, the analysis of the SPM signal can include separating or extracting the frequency components located at even multiples of the excitation frequency Fe. In other words, the analysis of the SPM signal must be carried out at an analysis frequency Fa equal to an even multiple of the excitation frequency Fe (preferably equal to the frequency 2.Fe at which the SPM signal level is maximum).

[0055] In practice, injecting a single excitation frequency Fe into the SPM coils involves using a high-quality excitation signal to minimize the interference signals that can be present in the original SPM measurement signal and can complicate its processing or the extraction of useful information.

[0056] In particular, the interference signals at the excitation frequency Fe will appear on the receiving chain of the regulator module. However, since the gap between the analysis frequency Fa and the interference frequency is equal to Fe, its level can be relatively simply reduced without overly affecting the system response time. For example, a low-selectivity filter or an average-selectivity filter that does not overly affect the system bandwidth can be adopted.

[0057] In addition, the SPM coil acts as a mixer, and the presence of harmonic components in the excitation current induces high-level interference components in the SPM measurement signal that can mask the useful components. For example, for a single-frequency excitation field of 500 A / m and a field to be measured of 1 A / m, the amplitude of the useful component (i.e., the second harmonic (order 2 harmonics) of the excitation frequency, i.e., at the frequency 2.Fe) for measuring the superparamagnetic effect is approximately 20 mA / m. If the excitation field has a second harmonic rate of 0.01% (i.e., 50 mA / m), the signal measured in the absence of the field to be measured corresponds to approximately 2.5 A / m, which is thus much larger than the useful component. However, it is difficult to obtain a second harmonic rate of 0.01%, and it is difficult to directly measure the component representing 0.004% of the total signal.

[0058] In practice, it is difficult to avoid these second harmonics in the excitation current.

[0059] For example, when using pulse-width modulation type techniques to generate the excitation current, second harmonics are necessarily present due to the increase and decrease of the time difference and the output resistance difference between the high state and the low state.

[0060] Using a signal synthesis-based excitation current generation technique using a digital-to-analog converter and a power amplifier, the source of the second harmonics lies in the non-linearity of the digital-to-analog converter and the amplifier. If the output current is low, a signal with a lower second harmonic rate can be obtained by actively filtering the signal from the digital-to-analog converter. However, the necessary excitation current can be between several tens of milliamperes and several hundred milliamperes. Commercially available amplifiers with a low distortion rate at these current levels and frequencies are rare and expensive, and producing a power stage with discrete components compatible with the required performance is both complex and burdensome.

[0061] To be able to use inexpensive amplifiers or pulse-width modulation systems on the market, a solution to improve the quality of the excitation signal injected into the SPM coil while ensuring a lower second harmonic rate can include inserting an excitation impedance Ze in the excitation circuit or module.

[0062] In an embodiment with 2 SPM coils, Ze = Ce

[0063] Figure 1 An electronic installation of a sensor according to a first embodiment is shown, in which an excitation impedance Ze including a capacitor Ce is added to attenuate the second harmonic components (i.e., at 2.Fe) in the excitation signal.

[0064] The sensor includes a transducer 3, which is composed of an inductor L N a pair of SPM coils (in Figure 1formed respectively by LN1 and LN2, and this pair of SPM coils are installed in series between the two end terminals 31 and 32 of the transducer. The common connection point 30 of the two coils LN1 and LN2 is connected to a reference potential (e.g., ground potential). The coils LN1 and LN2 are used to be subjected to an external magnetic field to be measured, such as induced by a primary current source IP.

[0065] The sensor further includes an excitation module 2, which is electrically coupled to the transducer 1 and configured to generate and inject an excitation current into the transducer 1 at a predetermined excitation frequency Fe. The excitation module 2 includes:

[0066] - A midpoint coil P11 - P12, which consists of two substantially identical windings wound around the same magnetic core;

[0067] - An excitation voltage generator VEXC, whose frequency is equal to the excitation frequency Fe; and

[0068] - An excitation impedance Ze, which includes a capacitor Ce.

[0069] The two external terminals 21 and 22 of the coil P11 - P12 are respectively connected to the two end terminals 31 and 32 of the transducer 3, and the capacitor Ce of the excitation impedance Ze is connected between the excitation generator VEXC and the midpoint 20 of the coil P11 - P12.

[0070] The coils LN1 and LN2 of the transducer 3 form a series resistor / inductor (RI) type charge. Inserting the capacitor Ce between the midpoint coil P11 - P12 and the excitation voltage generator VEXC means charging it with a series RLC type circuit, whose characteristic is its resonance frequency Fres_e (hereinafter referred to as "excitation resonance frequency"). The excitation resonance frequency Fres_e is defined by the following formula:

[0071]

[0072] Therefore, if the value of the capacitor Ce makes the resonance frequency Fres_e substantially equal to the excitation frequency (Fres_e = Fe), then this circuit will act as a band - pass filter for the excitation current and will attenuate the harmonic components of the excitation signal at the frequency 2.Fe. This attenuation depends on the quality factor Q of the circuit, which is given by the following formula:

[0073]

[0074] In addition, the gain at 2.Fres_e is given by the following formula:

[0075]

[0076] The sensor further includes an analysis or regulator module 1, which is configured to recover and analyze the original SPM measurement signal in order to remove the unwanted frequency components in the SPM signal and retain only the useful frequency components in the SPM signal. The analysis module 1 includes:

[0077] - An analysis impedance Za, which may be composed of an analysis capacitor Ca and an analysis resistor Ra. The analysis capacitor Ca and the analysis resistor Ra are installed in series between the two end terminals 21 and 22 of the midpoint coil P11 - P12;

[0078] - An analysis chain A, which is configured to supply the SPM signal containing useful information to 2.Fe. This SPM signal can be used by an external control unit or a control unit integrated into the sensor. The analysis chain includes, for example, an amplification circuit and a filtering circuit.

[0079] Therefore, the current flowing through the analysis impedance Za corresponds to the original SPM measurement signal. By analyzing the current flowing through the analysis impedance Za at an analysis frequency Fa that is an even multiple of the excitation frequency Fe (for example, at an analysis frequency Fa equal to 2.Fe), the useful signal containing information related to the field or current to be measured can be separated or extracted.

[0080] Regarding the excitation circuit, the combination of the coils LN1, LN2 and the impedance Za forms a series RLC - type circuit with a resonant frequency Fres_a (hereinafter referred to as the "analysis resonant frequency"). The analysis resonant frequency is equal to:

[0081]

[0082] In practice, the analysis frequency Fa is advantageously substantially equal to the analysis resonant frequency Fres_a.

[0083] An embodiment with 2 SPM coils, Ze = Ce, Le

[0084] Figure 2 Shows the electronic installation of a sensor according to another embodiment, where the excitation impedance Ze includes a capacitor Ce and an inductor Le.

[0085] As shown above, the attenuation of the second - harmonic level depends on the quality factor Q of the circuit. However, by construction, the SPM coils usually have a relatively mediocre quality factor (about 3 to 5) at the frequencies considered, which can result in an attenuation (opposite to the gain) of about 4 to 8. To increase this attenuation, a higher - quality - factor excitation inductor Le (for example, a coil) can be added in the Figure 1 installation. The inductance value of the excitation inductor Le is substantially greater than the inductance value of the SPM coil, and the resistance value of the excitation inductor is negligible compared to the resistance value of the SPM coil.

[0086] Therefore, Figure 2 the installation of Figure 1 corresponds to, in the installation of

[0087] the excitation impedance Ze also includes an excitation inductor Le connected between the excitation capacitor Ce and the excitation voltage generator VEXC so as to improve the quality factor of the excitation and thus reduce the distortion level to the frequency 2.Fe.

[0088]

[0089] In this configuration, the resonance frequency Fres_e of the excitation circuit is now equal to:

[0090] Optionally, when it is desired or considered to measure the excitation current, a measurement resistance Rsh of the excitation current can be added between the reference potential and the common terminal 30 of the transducer. The excitation current measurement is particularly useful when attempting to perform the previous calibration step to find the optimal value of the excitation frequency injected into the SPM coil (for which the excitation current level is maximum). The calibration for finding the optimal excitation frequency will be described in more detail below.

[0091] Figure 12 The filtering improvement achieved by adding a series inductor Le to the excitation circuit is shown. I(Ce) is the current flowing in the capacitor Ce and is thus also the excitation current. In this example, the excitation resonance frequency Fres_e is 250 kHz and is also chosen to be equal to the excitation frequency Fe. Therefore, the interference signal present in the excitation voltage at the analysis frequency Fa = 2.Fe (i.e., at 500 kHz) is attenuated by approximately 32 dB in the absence of the series coil Le, compared to nearly 50 dB in the presence of a 100 μH series coil Le, which almost represents a factor of 10.

[0092] Embodiment with 2 SPM coils, Ze = Ce, Le and Za = Ca, La

[0093] Figure 2 the installation of assumes that the SPM coils are almost perfectly paired such that only the useful frequency 2.Fe appears in the signal detected at the receiving circuit (i.e., the current flowing through the impedance Za). However, this perfect pairing may actually be difficult to achieve, and the imbalance between the SPM coils induces a component at the excitation frequency Fe in the output signal (i.e., the current flowing in the impedance Za), the amplitude of which may be much larger than the frequency of the useful signal, thus reducing the signal-to-noise ratio of the system.

[0094] The SPM effect in the SPM coil can be simulated by a voltage source, the level of which is proportional to the magnetic field in which the SPM coil is located, and the source impedance of which consists of the SPM coil itself. Thus, if the receiving circuit (impedance Za) consists of a resistor Ra and a capacitor Ca, the charging circuit of the SPM voltage source is a series RLC circuit. Thus, in addition, the presence of a bandwidth circuit is introduced, the resonant frequency of which will be at the analysis frequency 2.Fe. Thus, the interference components at the frequency Fe are attenuated, and the attenuation level is even higher, and the quality factor of this circuit is relatively high. If a capacitor is selected, the quality factor will be very low because it will necessarily be lower than the quality factor of the SPM coil due to the addition of the measured resistance Ra, which is usually greater than the resistance of the circuit formed by the SPM coil. Thus, the quality factor can be expected to be about 2 to 3.

[0095] Thus, in the same way as for the excitation circuit, a series inductor can be added to the analysis impedance Za of the regulation module to significantly increase the quality factor of the regulation circuit.

[0096] Thus, Figure 3 The installation of Figure 2 In the installation of , the analysis impedance Za also includes an analysis inductor La. Thus, the inductor La, the capacitor Ca, and the resistor Ra are installed in series between the external terminals 21, 22 of the midpoint coil P11 / P12.

[0097] In this configuration, the analysis resonant frequency Fres_a of the analysis circuit is equal to:

[0098]

[0099] Furthermore, if the analysis frequency Fa is fixed at 2.Fe and the excitation frequency Fe has been predefined, for example, through the above calibration steps, to maximize the excitation current level, the quality factor of the regulation circuit (or the receiving circuit) must preferably be limited in order to maintain the reasonableness of the attenuation for all frequency values that can be considered for the excitation frequency Fe by taking into account the tolerances of the components La, Ca.

[0100] For example, in order to ensure that the attenuation is less than 3 dB when the dispersion of the excitation frequency Fe is 3% and the dispersion of the resonant frequency Fres_a of the analysis circuit is also 3%, the quality factor can be considered to be limited to about 8.

[0101] Figure 13 An example related to the selective difference between excitation and analysis is shown. Due to the tolerances of the components (in this case, the tolerance of the excitation series coil value is + / -5%), the resonant frequency of the excitation will change. Thus, the analysis frequency, which is twice the excitation frequency, must be within the bandwidth of the receiving filter.

[0102] Embodiment with two SPM coils, Ze, Za, and transformer T1

[0103] In Figure 4 the installation shown, the transformer T1 is inserted between the midpoint coil P11 - P12 and the analysis impedance Za in order to adjust the impedance of the analysis circuit and increase the output level of the signal received by the impedance Za.

[0104] The transformer T1 consists of a primary winding P1 and a secondary winding S1. The primary winding P1 is connected to the external terminals 21, 22 of the midpoint coil P11 - P12, and the secondary winding S1 is connected to the terminals of the analysis impedance Za.

[0105] The operation of this circuit is similar to Figure 3 .

[0106] Embodiment with two SPM coils, Ze, Za, and transformer T2

[0107] In Figure 5 the installation shown, a single transformer T2 is used to perform the functions of the midpoint coil P11 - P12 and the transformer T1 of the Figure 4 circuit. Thus, the primary winding of the transformer T2 that is connected to the terminals 31, 32 of the transducer is configured to perform the function of the midpoint coil of the Figure 4 circuit, and the secondary winding that is connected to the terminals of the analysis impedance Za is configured to perform the adjustment function of the transformer T1 of the Figure 4 circuit.

[0108] Embodiment with two SPM coils, position of the impedance Ze

[0109] In Figure 6 the installation shown, contrary to the Figure 5 installation, the excitation impedance Ze is connected between the common terminal 30 of the transducer and the reference potential.

[0110] In particular, the advantage of this configuration is the option of measuring, for example, the excitation current at the terminals of the capacitor Ce, so that the measuring resistor Rsh, which tends to reduce the quality factor of the circuit, can be removed, and the measurement is usually referenced to the reference potential. Thus, the quality factor of the excitation is increased, and the common - mode voltage at the midpoint of the coil P11 - P12 and at the midpoint of the external terminals 31, 32 of the coils LN1, LN2 is reduced.

[0111] Especially when performing measurements in an electromagnetic environment that is slightly disturbed, Figures 1 to 6 the configuration shown for implementing two SPM coils excited by a single excitation frequency without a feedback circuit is particularly suitable for measurements in cases where the response linearity is not a very important parameter.

[0112] Embodiment with 2 SPM coils, with feedback VCR

[0113] The above can be improved by conventionally adding a feedback circuit or module VCR Figure 6 configuration. Traditionally, the feedback principle involves adjusting the feedback so as to obtain a zero SPM measurement signal, so the feedback value is proportional to the field to be measured. Therefore, this configuration provides good linearity and is particularly suitable for a disturbed electromagnetic environment, while maintaining to a certain extent the simplicity of its production.

[0114] Therefore, similar to Figure 8 installation, feedback can be carried out by adding a feedback voltage generator VCR at the end terminals 31, 32 of the transducer, adding a capacitor Cc between terminals 21, 31 and adding another capacitor Cd between terminals 22, 32 in the Figure 6 installation.

[0115] The feedback voltage generator VCR is configured to generate a feedback current, and the capacitors Cc, Cd with the same value C CR are configured to prevent the feedback current from flowing through the midpoint coil P11 - P12, so that the feedback current only flows through the SPM coil within the useful frequency range. In addition, it is also advisable to isolate the feedback voltage generator VCR from the reference potential by, for example, adding a power transformer and an isolation amplifier.

[0116] It can be well understood that the excitation resonance frequency Fres_e and the analysis resonance frequency Fres_a are modified due to the presence of these capacitors Cc, Cd forming the resonance circuit.

[0117] Therefore:

[0118] And

[0119]

[0120] Where:

[0121] And

[0122]

[0123] In a similar way to Figure 9In a variant of the installation, the feedback voltage generator VCR can be formed by two voltage sources VCR1 and VCR2 referenced to a reference potential. The two voltage sources VCR1 and VCR2 are configured to generate voltages with the same value VCR / 2 but opposite polarities, and are connected to the SPM coils LN1 and LN2 through their respective amplifiers A1 and A2. In addition, in order to eliminate the common-mode noise caused by these two voltage sources VCR1 and VCR2, a common-mode inductor LMC can be added, and the value of the common-mode inductor LMC is much larger than the inductance values of the SPM coil and the excitation inductor Le. This configuration can avoid using isolated power supplies and isolated amplifiers, thereby reducing costs.

[0124] Embodiment with 4 SPM coils, without feedback

[0125] In order to better remove the signal at the analysis frequency Fa (this signal may exist in the SPM signal of the transducer with 2 SPM coils in the installation of the above figures, thus interfering with the measurement), additional SPM coils LN3 and LN4 can be added.

[0126] Therefore, as Figure 7 shown, for example, the installation of Figure 5 can be modified by coupling the first pair of coils LN1 and LN2 to a second pair of superparamagnetic coils LN3 and LN4 that are substantially the same as the first pair of coils.

[0127] In particular, the second pair of coils LN3 and LN4 are installed in series and the two pairs of coils are installed in parallel, such that the end terminal 31 of the first pair of coils is connected to coil LN3 and the end terminal 32 of the first pair of coils is connected to coil LN4.

[0128] In fact, the coils SPM, LN3, and LN4 are substantially subjected to the same magnetic field as the coils SPM, LN1, and LN2.

[0129] Therefore, at the analysis frequency Fa, the voltage e 12 induced in the first pair of coils LN1 and LN2 by the magnetic field variation (excluding the SPM effect) 34 is substantially the same as the voltage e Figure 7 induced in the second pair of coils LN3 and LN4. In the installation of Figure 9 the polarity points of each coil in the installation of 12 with respect to the primary field are such that the voltage between the end terminals 31 and 32 generated by the first pair of coils LN1 and LN2 is equal to the voltage e 34 and the voltage between the end coils 31 and 32 generated by the coil pair LN3 and LN4 is equal to –e 12 / 2 - e34 / 2, i.e., approximately 0. In addition, since the excitation current Ie does not flow through the coils LN3, LN4, they do not generate the SPM effect, so that the useful SPM voltage (VSPM) generated in the first pair of coils LN1, LN2 is located between the terminal ends 31, 32, but relative to Figure 6 the configuration of the pair of SPM coils is divided by 2.

[0130] Therefore, although the sensitivity of the circuit is divided by 2, this configuration with four identical SPM coils in the same magnetic field can remove the primary component existing at the frequency Fa.

[0131] Embodiment: 4 SPM coils, with feedback VCR

[0132] Just like the embodiment of 2 coils, a feedback circuit VCR can also be added in the Figure 7 installation to obtain a sensor more suitable for the disturbed electromagnetic environment.

[0133] Therefore, as Figure 8 shown, feedback can be achieved by adding a feedback voltage generator VCR between the second pair of SPM coils LN3, LN4 in the Figure 7 installation. Therefore, the generator VCR is installed between the terminal ends 33, 34 and configured to generate a feedback current.

[0134] Capacitors Cc, Cd with the same value C respectively located on the output lines (i.e., between the terminals 21, 31 and between the terminals 22, 32) are configured to prevent the feedback current from flowing through the midpoint coil P11 - P12, so that the feedback current only flows through the SPM coils within the useful frequency range. CR In fact, the feedback generator VCR has a low impedance at the analysis frequency Fa. In addition, in order to ensure that the excitation current only flows through the first pair of coils LN1, LN2, it is also advisable to isolate the feedback voltage generator VCR from the reference potential by, for example, adding a power transformer and an isolation amplifier.

[0135] It can be well understood that the resonance frequency Fres_e of the excitation circuit and the resonance frequency Fres_a of the analysis circuit are modified due to the presence of these capacitors Cc, Cd forming the resonance circuit.

[0136] Therefore:

[0137] Therefore:

[0138] And

[0139]

[0140] Where:

[0141] And

[0142]

[0143] Example: 4 SPM coils, with feedback VCR1 and VCR2

[0144] As Figure 9 shown, the feedback function can also be implemented by two voltage sources VCR1 and VCR2 referenced to the reference potential. The two voltage sources VCR1 and VCR2 are configured to generate voltages with the same value VCR / 2 but opposite polarities, and are connected to the SPM coils through their respective amplifiers A1 and A2.

[0145] Just as Figure 8 in the case, in order to avoid using expensive components associated with generating a feedback voltage source isolated from the reference potential, two voltage generators VCR1 and VCR2 are employed. These two voltage generators VCR1 and VCR2 are connected to the reference potential, and a common-mode inductor LMC is inserted between the two voltage generators VCR1 and VCR2 and the coils LN3 and LN4. The value of the common-mode inductor LMC is much larger than the inductance values of the SPM coils and the excitation inductance Le (preferably, ten times larger). Thus, the common-mode impedance of the branch formed by the coils LN3 and LN4 is significantly increased, forcing the excitation current to flow through the coils LN1 and LN2.

[0146] However, in order for the coils LN3 and LN4 to play their roles, a quasi-short circuit must exist between the external terminals of the transducer at the analysis frequency Fa. To obtain such a short circuit, a capacitor CMD can be placed between the external terminals of the transducer, which will enable the differential-mode current (i.e., the current flowing in the same direction in the coils LN1 to LN4) to flow correctly at the analysis frequency Fa.

[0147] Therefore, in the Figure 9 installation:

[0148] - The common-mode coil LMC can prevent the excitation current from flowing through the coil pair LN3 and LN4 while ensuring a high bandwidth for the feedback loop; and

[0149] - The capacitor CMD can ensure a short circuit of the coil pair LN3 and LN4 at the analysis frequency Fa.

[0150] Calibration for seeking the optimal excitation frequency

[0151] As described above, the excitation impedance Ze increased to attenuate the second harmonic component can bring the quality factor to approximately 30 to 50.

[0152] However, choosing a fixed excitation frequency Fe has proven to be incompatible with a high quality factor. When the excitation frequency Fe deviates from the excitation resonance frequency Fres_e of the excitation circuit, the excitation current can be significantly reduced while improving the quality factor.

[0153] Therefore, the solution includes adjusting the frequency of the excitation generator VEXC to correspond to the optimal excitation frequency Fopt_e, which results in a higher excitation current level. In other words, by adjusting the value of the excitation frequency Fe to the value of the optimal excitation frequency Fopt_e, the resulting excitation current remains at an acceptable level.

[0154] In practice, the control unit can be coupled to the excitation module and configured to:

[0155] - During a previous calibration phase (steps 10 and 10’): find the optimal excitation frequency Fopt_e to be injected into the SPM coil; and

[0156] - During a usage phase (step 11) for measuring an external magnetic field: control the excitation voltage generator VEXC to generate a voltage at an excitation frequency substantially equal to the optimal excitation frequency Fopt_e.

[0157] The control unit can be located inside or outside the sensor and means for storing the value of the optimal excitation frequency can be provided in the sensor.

[0158] The optimal excitation frequency Fopt_e can be obtained by finding the excitation frequency Fe at which the excitation current level Ie is maximum. Thus, the optimal excitation frequency Fopt_e can be found by measuring the excitation current Ie during the calibration phase by means of Figures 2 to 5 、 Figure 7 and Figure 8 the measurement resistor Rsh provided in the installation of Figure 6 and Figure 9 the excitation capacitor Ce in the installation of

[0159] Figure 10 A search step according to a method is generally illustrated, which includes: scanning the excitation frequency Fe within a frequency range (Fe_min; Fe_max); and finding the excitation frequency F_opt at which the excitation current Ie measured, for example, via the resistor Rsh or via the capacitor ce, is maximum within this range.

[0160] Another solution for the calibration phase includes: finding the optimal excitation frequency Fopt_e to be injected into the SPM coil at which the circuit sensitivity is maximum.

[0161] Figure 11Generally shows the search steps according to another method, which includes: scanning the excitation frequency Fe within the frequency range (Fe_min; Fe_max); and searching for the excitation frequency F_opt at which the circuit sensitivity S is the maximum within this range.

[0162] The circuit sensitivity S can be measured by measuring the SPM signal level difference (step 12), which is obtained by changing the feedback voltage VCR between two predetermined levels VCRmin and VCRmax.

[0163] The excitation frequency Fe and the analysis frequency Fa can be adjusted by the control unit within the ranges [Fe_min; Fe_max] and [Fa_min; Fa_max] respectively. In practice, the excitation frequency Fe varies by a few percentage points around the nominal frequency Fe_nom between several tens of kHz and several hundreds of kHz. Similarly, the analysis frequency Fa varies by a few percentage points around the nominal value Fa_nom between several tens of kHz and several hundreds of kHz (even several MHz).

[0164] In the calibration phase, the optimal excitation frequency Fopt stored in the memory is searched for; then, in the usage phase, the excitation module is controlled to generate an excitation signal at this optimal excitation frequency.

[0165] The excitation voltage generator VEXC can be a square wave signal generator (for example, a PWM (Pulse Width Modulation) type generator). Since the configuration of the present invention allows the use of a quasi-sine excitation signal, the excitation voltage generator VEXC can also be a generator based on an analog-to-digital converter coupled to filtering and power amplification.

[0166] The present invention is of course not limited to the examples of the described embodiments, but extends to any modifications and variations that are obvious to those skilled in the art within the scope of the appended claims. In addition, the technical features of the above various embodiments and variations can be combined together in whole or only in part.

Claims

1. A magnetic field sensor for measuring direct current, formed by at least one superparamagnetic (SPM) material transducer (3), the transducer (3) being adapted to be subjected to an external magnetic field to be measured and being electrically coupled to an excitation module (2) and an analysis module (1). Characterized in that: - The transducer (3) includes at least a pair of SPM coils (LN1, LN2), the SPM coils (LN1, LN2) in the at least a pair of SPM coils (LN1, LN2) being coils with a superparamagnetic (SPM) core, the SPM coils (LN1, LN2) in the at least a pair of SPM coils (LN1, LN2) being identical and connected in series between two end terminals (31, 32) of the transducer (3), and a common connection point (30) of the SPM coils (LN1, LN2) in the at least a pair of SPM coils (LN1, LN2) being connected to a reference potential; - The excitation module (2) is configured to generate and inject an excitation current Ie into the transducer (3) at a predetermined excitation frequency Fe, and at least includes: Midpoint coils (P11 - P12), which are mounted parallel to the terminals of the transducer (3); An excitation voltage generator (VEXC), which is mounted between the reference potential and the midpoint (20) of the midpoint coils (P11 - P12); and An excitation impedance (Ze), which is configured to form a first series RLC type circuit together with the at least a pair of SPM coils (LN1, LN2), and a resonant frequency Fres_e of the first series RLC type circuit is equal to the excitation frequency Fe; - The analysis module (1) at least includes: An analysis impedance (Za), which is connected to external terminals (21, 22) of the midpoint coils (P11 - P12), the excitation impedance being configured to form a second series RLC type circuit together with the at least a pair of SPM coils (LN1, LN2), and an analysis resonant frequency Fres_a of the second series RLC type circuit is equal to an analysis frequency Fa; and A device for analyzing a current flowing through the analysis impedance (Za) at the analysis frequency Fa to extract a component at the analysis frequency Fa that is an even multiple of the excitation frequency Fe.

2. The magnetic field sensor according to claim 1, Characterized in that, The analysis frequency Fa is equal to twice the excitation frequency Fe.

3. The magnetic field sensor according to claim 1, Characterized in that, The excitation impedance (Ze) is connected between the excitation voltage generator (VEXC) and the midpoint (20) of the midpoint coils (P11 - P12).

4. The magnetic field sensor according to claim 1, Characterized in that, The excitation impedance (Ze) is connected between the reference potential and the common connection point (30) of the at least a pair of SPM coils (LN1, LN2).

5. The magnetic field sensor according to claim 1, Characterized in that, The excitation impedance (Ze) includes at least one excitation capacitor (Ce).

6. The magnetic field sensor according to claim 3, Characterized in that, The excitation impedance (Ze) includes at least one excitation capacitor (Ce) and at least one excitation inductor (Le), and the at least one excitation inductor (Le) is connected between the at least one excitation capacitor (Ce) and the excitation voltage generator (VEXC).

7. The magnetic field sensor according to claim 1, wherein, the midpoint coil (P11 - P12) is composed of two identical windings wound around the same magnetic core.

8. The magnetic field sensor according to claim 1, wherein, the analysis impedance (Za) is constituted by an analysis capacitor (Ca) and an analysis resistor (Ra), and the analysis capacitor (Ca) and the analysis resistor (Ra) are installed in series between two external terminals (21, 22) of the midpoint coil (P11 - P12).

9. The magnetic field sensor according to claim 8, wherein, the analysis impedance (Za) further includes an analysis inductor (La), and the analysis inductor (La) is installed in series with the analysis capacitor (Ca) and the analysis resistor (Ra) between the external terminals (21, 22) of the midpoint coil (P11 - P12).

10. The magnetic field sensor according to any one of claims 1 to 9, wherein, it further includes a transformer (T1), the transformer (T1) is installed between the midpoint coil (P11 - P12) and the analysis impedance (Za), the transformer (T1) is constituted by a primary winding (P1) and a secondary winding (S1), the primary winding (P1) is connected to the external terminals (21, 22) of the midpoint coil (P11 - P12), and the secondary winding (S1) is connected to the terminals of the analysis impedance (Za).

11. The magnetic field sensor according to any one of claims 1 to 9, wherein, it further includes a transformer (T2), the transformer (T2) includes a primary winding and a secondary winding, the primary winding is formed by the midpoint coil (P11 - P12), and the secondary winding is connected to the terminals of the analysis impedance (Za).

12. The magnetic field sensor according to any one of claims 1 to 9, wherein, the at least one pair of SPM coils (LN1, LN2) of the transducer (3) forms a first pair of SPM coils (LN1, LN2), and the sensor further includes a second pair of SPM coils (LN3, LN4) identical to the first pair of SPM coils (LN1, LN2), the second pair of SPM coils (LN3, LN4) is installed in series, the end terminals (31) of the first pair of SPM coils (LN1, LN2) are connected to one coil (LN3) of the second pair of SPM coils (LN3, LN4), and the end terminals (32) of the first pair of SPM coils (LN1, LN2) are connected to the other coil (LN4) of the second pair of SPM coils (LN3, LN4).

13. The magnetic field sensor according to any one of claims 1 to 9, wherein, It further includes a feedback module, and the feedback module includes at least one feedback voltage generator (VCR). The at least one feedback voltage generator (VCR) is mounted to the end terminals (31, 32) of the transducer (3) and configured to generate a feedback current.

14. The magnetic field sensor according to claim 12, wherein, it further includes a feedback module, and the feedback module includes at least one feedback voltage generator (VCR). The at least one feedback voltage generator (VCR) is mounted between the second pair of SPM coils (LN3, LN4) and configured to generate a feedback current.

15. The magnetic field sensor according to claim 13, wherein, the at least one feedback voltage generator (VCR) is formed by two voltage sources (VCR1, VCR2) referenced to the reference potential.

16. The magnetic field sensor according to any one of claims 1 to 9, wherein, it further includes a calibration module, and the calibration module is configured to find the optimal value Fopt of the excitation frequency Fe to be injected into the transducer (3).

17. The magnetic field sensor according to claim 16, wherein, the calibration module is configured to: - change the excitation frequency Fe of the excitation voltage generator (VEXC) within the frequency range [Fe_min; Fe_max] and measure the corresponding excitation current Ie; and - identify the optimal excitation frequency Fopt_e of the excitation frequency Fe corresponding to the maximum level of the excitation current Ie.

18. The magnetic field sensor according to claim 17 in combination with claim 3, wherein, the calibration module is configured to measure the excitation current at the terminals of a resistor mounted between the reference potential and the common connection point of the at least one pair of SPM coils (LN1, LN2) of the transducer (3).

19. The magnetic field sensor according to claim 17 in combination with claim 5 or 6, wherein, the calibration module is configured to measure the excitation current at the terminals of the at least one excitation capacitor (Ce).

20. The magnetic field sensor according to claim 16, wherein, the calibration module is configured to: - change the excitation frequency Fe of the excitation voltage generator (VEXC) within the frequency range [Fe_min; Fe_max] and measure the sensitivity S of the excitation module (2); and - identify the optimal excitation frequency Fopt_e of the excitation frequency Fe corresponding to the maximum sensitivity S.

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