Method for calibrating a sensor unit

Abstract

The invention relates to a method for calibrating a sensor unit for measuring magnetic fields, wherein the sensor unit comprises a nitrogen vacancy center magnetometer, which has a diamond crystal having nitrogen vacancy centers as a sensor medium, and a magnetic field generation device for generating a magnetic field in the area of ​​the sensor medium, wherein the method comprises the following steps: Generating a predetermined static magnetic field in the area of ​​the sensor medium by the magnetic field generation device, measuring a vectorial magnetic field by the magnetometer for at least three different orientations of the sensor medium while the predetermined static magnetic field is generated by the magnetic field generation device, determining an internal vectorial magnetic field based on the magnetic field strengths measured for the at least three different orientations and based on the at least three different orientations. The invention relates to a sensor unit for measuring magnetic fields, a computer program and a machine-readable storage medium.

Description

[0001] The invention relates to a method for calibrating a sensor unit for measuring magnetic fields, a sensor unit for measuring magnetic fields, a computer program and a machine-readable storage medium. State of the art

[0002] Various sensor technologies are known for measuring very small magnetic field strengths. One of the most sensitive sensor types is SQUID magnetometers (Superconducting Quantum Interference Devices), which are based on superconducting components and can resolve individual magnetic flux quanta or fields down to the range of a few femtotesla (fT). The disadvantage of SQUID sensors, however, is the need to cool them below the superconducting transition temperature, which for most superconducting materials is below 77 Kelvin, i.e., below the temperature of liquid nitrogen.

[0003] Quantum-based magnetic sensors are also known, for example, from DE 10 2018 220 234 A1 or DE 10 2018 214 617 A1.

[0004] Nitrogen-vacancy-based (NV-based) magnetometers can measure the vector magnetic field—the absolute amplitude and spatial orientation. To do so, these devices require a predetermined static magnetic field, also called a bias magnetic field (the offset field B0, typically generated using permanent magnets or coils), and very precise positioning of the diamond's lattice components and orientation relative to this field. Furthermore, precise knowledge of the B0 itself and precise knowledge of the diamond's orientation relative to B0 are required. These parameters are usually either unknown or known only to a limited extent and / or with insufficient accuracy.The reasons for these uncertainties include, but are not limited to, any type of assembly tolerances, the influence of the sensor housing and terminals on B0, the influence of magnetized parts of the sensor unit and soft magnetic materials on B0, tolerances in the assembly of the sensor unit components, the process technology of the diamond (growth and polishing affecting the final lattice orientation), strain of the diamond due to growth, polishing, temperature or assembly, temperature-related effects such as thermal drifts in the components, temperature influence on the assembly tolerances, temperature change of the magnetization of the B0 generating magnets, all tolerances related to optical pumping and optical sensing, tolerances in the electronics and the microwave resonator.

[0005] For these reasons, an initial calibration and subsequent recalibrations of the sensor unit may be necessary.

[0006] The patent US 8,494,799 B2 discloses a method for dynamically calibrating a compass of a portable device, for example a smartphone. Disclosure of the invention

[0007] The object underlying the invention is to provide a concept for calibrating a sensor unit for measuring magnetic fields.

[0008] This object is achieved by means of the respective subject matter of the independent claims. Advantageous embodiments of the invention are the subject matter of the respective dependent subclaims.

[0009] According to a first aspect, a method is provided for calibrating a sensor unit for measuring magnetic fields, wherein the sensor unit comprises a nitrogen vacancy center magnetometer having a diamond crystal containing nitrogen vacancy centers as the sensor medium, and a magnetic field generating device for generating a magnetic field in the region of the sensor medium, the method comprising the following steps: Generating a predetermined static magnetic field in the region of the sensor medium by the magnetic field generating device, measuring a vectorial magnetic field by the magnetometer for at least three different orientations of the sensor medium while the predetermined static magnetic field is being generated by the magnetic field generating device, determining an internal vectorial magnetic field based on the vectorial magnetic fields measured for the at least three different orientations and based on the at least three different orientations.

[0010] According to a second aspect, a sensor unit for measuring magnetic fields is provided, comprising: a nitrogen vacancy magnetometer having a diamond crystal containing nitrogen vacancy centers as the sensor medium, and a magnetic field generating device for generating a predetermined static magnetic field in the region of the sensor medium, wherein the sensor unit is configured to carry out all steps of the method according to the first aspect.

[0011] According to a third aspect, a computer program is provided which comprises instructions which, when the computer program is executed by a computer, for example by the sensor unit according to the second aspect, cause the computer to carry out a method according to the first aspect.

[0012] According to a fourth aspect, a machine-readable storage medium is provided on which the computer program according to the fourth aspect is stored.

[0013] The invention is based on and incorporates the finding that the above-mentioned object is achieved by measuring a vectorial magnetic field for at least three different orientations using the nitrogen vacancy center magnetometer. Based on the respective measured vectorial magnetic fields and based on the three different orientations, an internal vectorial magnetic field is determined. The sensor unit can then be calibrated based on this internal vectorial magnetic field.

[0014] Whenever the description simply refers to "magnetic field," it should always be understood that this refers to the vector magnetic field. A magnetic field, as defined in this description, is therefore a vector magnetic field. This means that a measurement of a magnetic field is a measurement of a vector magnetic field. The magnetometer therefore measures a vector magnetic field.

[0015] A measurement of a magnetic field, as described above, is therefore a measurement of a vector magnetic field. The vector magnetic field is defined by the absolute value and direction of the magnetic field. Measuring a magnetic field therefore involves measuring both the absolute value and the direction of the magnetic field.

[0016] The magnetometer always measures the total magnetic field consisting of the external magnetic field and the internal magnetic field. Users are typically only interested in the external magnetic field. If the internal magnetic field were precisely known, the external magnetic field could be easily calculated as follows: external magnetic field = measured magnetic field - internal magnetic field. The calibration procedure specifically involves rotating or shifting the magnetometer in three different orientations. When the magnetometer shifts, the internal magnetic field shifts with it, while the external magnetic field does not shift. This property (shifting the internal magnetic field along with the magnetometer) allows the calculation of the actual external magnetic field, provided that the external vector magnetic field has been measured in at least three known orientations.

[0017] For each measurement in one orientation, all three components of the magnetic field are measured relative to a three-dimensional coordinate system.

[0018] At least three measurements are performed for three different orientations of the magnetometer.

[0019] Each measured magnetic field consists of the external magnetic field (if present) and the internal magnetic field of the magnetometer.

[0020] Since three components of the magnetic field are determined for at least three orientations, at least nine equations can be formulated, which will be explained in more detail below.

[0021] The internal magnetic field is generated by the sensor unit itself. The internal magnetic field is based on the static magnetic field generated by the magnetic field generating device and depends in particular on the intrinsic properties of the sensor unit, especially the properties of a sensor housing.

[0022] The internal magnetic field is only approximately known, since the influence of the sensor housing etc. is not known.

[0023] The aim of the method is to determine the exact internal field so that it can be subtracted from the external magnetic field measured by the magnetometer, so that ultimately the pure external magnetic field can be measured.

[0024] The concept described here thus encompasses a calibration procedure that enables the calibration of NV-based magnetometers. The procedure can be applied in magnetically shielded environments, but its main advantage is its applicability in noisy and unshielded environments as well. This eliminates the need for technically complex and expensive shielding measures.

[0025] The procedure requires, as a minimum, three vector magnetic field measurements taken by the magnetometer in three different orientations. The procedure requires no prior knowledge of the internal or intrinsic magnetic field, which can also be referred to as the offset field B0 or ​​bias magnetic field, the residual magnetization of the sensor unit components, the orientation of the diamond with respect to the bias field B0, or the ambient magnetic fields. Subsequent calibrations using the same procedure can also provide information about whether further recalibrations are necessary and how often they should be performed. The calibration procedure requires little or no additional hardware other than the sensor unit itself and is fast enough to be performed daily or even hourly. Temperature dependence and / or other drifts or changes in the environment are automatically taken into account.

[0026] A nitrogen vacancy center magnetometer as defined in the description comprises a diamond crystal having nitrogen vacancy centers as the sensor medium and is configured to detect a magnetic field strength at a measuring location by reading out a spin resonance in the sensor medium that is dependent on the magnetic field strength.

[0027] The magnetometer specifically reads multiple spin resonances using so-called ODMR and measures the strength and direction of the magnetic field—a vectorial magnetic field. ODMR stands for "Optically Detected Magnetic Resonance."

[0028] A nitrogen vacancy center magnetometer can also be referred to as an NV magnetometer, where "NV" stands for "nitrogen vacancy." When only the term "magnetometer" is used, the term "NV magnetometer" should always be used.

[0029] The internal magnetic field can also be referred to as an intrinsic magnetic field, bias magnetic field, offset magnetic field, or B0. Internal or intrinsic refers to the sensor unit.

[0030] The sensor unit comprises a magnetic field generating device for generating a, in particular substantially homogeneous, predetermined static magnetic field in the region of the sensor medium.

[0031] The NV center in the diamond has four possible arrangements, each of which is visible in the fluorescence spectrum as a separate frequency splitting, so that the magnetic field direction and strength can be extracted for each spatial direction.

[0032] For example, the magnetic field generating device comprises a Helmholtz coil arrangement, wherein at least the sensor medium of the magnetometer is arranged within the Helmholtz coil arrangement.

[0033] To generate such a predetermined static magnetic field, a Helmholtz coil arrangement is used, in particular, within which at least the sensor medium or the sensor head of the NV magnetometer is located. Helmholtz coils can generate a particularly homogeneous magnetic field.

[0034] For example, the magnetic field generating device comprises, for example, one or more permanent magnets and / or one or more arrays of permanent magnets, for example Halbach arrays, Aubert arrays, and / or one or more arrangements of several permanent magnets.

[0035] The sensor unit comprises, for example, at least one excitation light source for radiating light into the sensor medium of the magnetometer.

[0036] The sensor unit comprises, for example, at least one microwave source for generating a resonant field in the sensor medium.

[0037] The sensor unit comprises, for example, at least one photodetector for detecting resonance-dependent fluorescent light from the sensor medium.

[0038] The diamond, for example, is designed as a thin diamond plate.

[0039] Nitrogen vacancy magnetometers offer very high sensitivity, but are also easy to implement in a compact design, resulting in small sensor heads. Complex cooling, such as with SQUID sensors, is not necessary. Furthermore, NV vacancy magnetometers inherently allow for vector magnetic field measurements. This provides the technical advantage, for example, that a magnetic field can be measured efficiently by the sensor unit.

[0040] Furthermore, the sensor unit can comprise, for example, a microwave resonator tuned to the spin resonance of the sensor medium in the region of the sensor medium. A microwave resonator can generate a homogeneous microwave field at the relevant points of the sensor medium, while the microwave source can also be located remotely and fed into the microwave resonator via suitable connections.

[0041] The sensor unit described here is based in particular on a quantum-based or optically pumped NV magnetometer.

[0042] The magnetometers described here as examples use optically pumped and / or optically detected magnetic resonance (ODMR). This method exploits the fact that the energy levels of certain spin states of unpaired electrons split under the influence of an external magnetic field, the so-called Zeeman effect. The splitting of the energy levels results in altered transitions during relaxation from excited states, which can then be measured, for example, by optical excitation and microwave excitation and detection of the resulting fluorescence radiation, or by observing optical properties such as light absorption. The measured optical parameters can then be used to determine the magnetic field strength and direction.

[0043] Diamond NV magnetometers are based on the readout of magnetic resonances from specific defect centers in diamond, in particular nitrogen vacancies (NV), which occur as impurities in the carbon lattice of diamond and can also be deliberately introduced. If the NV center is optically excited in the normal state without an existing magnetic field, for example by irradiating a pump laser beam with a suitable wavelength (in this case in the green wavelength range, e.g., at 532 nm for off-resonance excitation), the electrons are lifted from the triplet ground state to the excited triplet state and relax with the emission of fluorescent light in the red wavelength range at, for example, 637 nm. Since the probability for non-spin-conserving transitions from the spin state is proportional to the spin quantum number m s =±1 is larger, excitation pumping ensures that the NV centers are mostly in the spin state m s=0 hyperpolarized. Stimulation can be achieved with laser pulses and / or continuous wave lasers.

[0044] Between the m s = 0 and m s =±1 spin states in the ground state, there is an energy difference corresponding to the frequency difference, which in this case is approximately 2.87 GHz. If, in addition to the optical excitation, microwave radiation is irradiated into the diamond, a dip in the red fluorescence occurs at this resonance frequency of 2.87 GHz (in the absence of the internal and external magnetic field), since the spin-polarized electrons are deflected by the microwave field from the m s = 0 in the m s =±1 ground state and from there by the pump light into the m s =±1 excited state. From there, however, mainly non-radiative transitions and weak infrared fluorescence transitions occur via the singlet state, while fluorescence in the red region disappears.

[0045] If an external magnetic field is present, the so-called Zeeman effect causes the splitting of the otherwise equally energetic m s=±1 triplet levels into energetically equidistant Zeeman levels. When the fluorescence intensity is plotted against the microwave frequency, two notches appear in the fluorescence spectrum, the frequency spacing of which is linearly proportional to the magnetic field strength of the external magnetic field. The magnetic field sensitivity is primarily defined by the minimum resolvable frequency shift and can reach up to 1 pT / √Hz. Since the NV center in single-crystal diamond has four possible arrangements in the crystal lattice, the presence of a directed magnetic field causes the NV centers present in the crystal to react with varying degrees to the external magnetic field depending on their position within the crystal. This results in four pairs of fluorescence minima appearing in the spectrum, from whose shape and relative positions both the magnitude of the magnetic field strength and the direction of the external magnetic field can be clearly determined.

[0046] For example, the total of 24 microwave resonances can be measured to measure the vector magnetic field.

[0047] The NV-center magnetometer offers a variety of advantages for this application. In addition to the aforementioned very high sensitivity, it can also cover a wide measurement range (> 1 Tesla). The underlying Zeeman effect is linearly dependent on the existing magnetic field and, moreover, exhibits no degradation because the measurement is based on quantum mechanical states. Furthermore, an NV-center magnetometer offers the possibility of vectorially determining external magnetic fields based on the various orientations present in the diamond lattice.

[0048] Additionally or alternatively, magnetic spin resonance in diamond can be read electrically. This involves detecting charge carriers that have been elevated into the diamond conduction band by two-photon ionization of the NV centers.

[0049] In one embodiment of the method, it is provided that for the measurements in the at least three different orientations, the sensor medium is arranged successively in at least three different predetermined holding positions of a calibration stand.

[0050] This provides the technical advantage, for example, that the orientations can be easily determined.

[0051] In one embodiment of the method, it is provided that the respective orientation is measured by at least one gyroscope.

[0052] This provides the technical advantage, for example, that any orientations can be used without the need to specify the orientations using a calibration stand, so that, for example, such a stand is not required for calibration.

[0053] In one embodiment of the method, it is provided that a three-dimensional x,y,z laboratory coordinate system is used to determine the at least three orientations.

[0054] For example, the axes of a coordinate system as described are not coplanar. For example, a coordinate system as described is a Cartesian coordinate system.

[0055] This provides, for example, the technical advantage that the orientations are defined in a comprehensible manner.

[0056] In the following, an L-coordinate system refers to the three-dimensional x, y, z laboratory coordinate system. "L" stands for "laboratory." The reference system is therefore the laboratory, i.e., the environment of the sensor unit.

[0057] In the following, the D coordinate system refers to a three-dimensional x, y, z diamond coordinate system. "D" stands for "diamond." The reference system is therefore the diamond.

[0058] In one embodiment of the method, it is provided that for the measurement in one of the at least three orientations, the sensor medium is rotated by a predetermined rotation angle, for example 45°, about the z-axis of the three-dimensional x,y,z laboratory coordinate system relative to another of the at least three orientations.

[0059] In one embodiment of the method, it is provided that for the measurement in one of the at least three orientations, the sensor medium is rotated about any axis in the xy-plane of the x,y,z laboratory coordinate system, which is not parallel, but in particular perpendicular to the z-axis of the x,y,z laboratory coordinate system.

[0060] This provides the technical advantage, for example, that one of the orientations can be easily determined.

[0061] A measurement of a magnetic field strength by the NV magnetometer, which is not located in a magnetically shielded environment, includes information about: B bias : internal / intrinsic magnetic field, which is generated, among other things, by the magnetic field generating device (e.g. generated by coils and / or permanent magnets), as well as the influence of the magnetized parts of the sensor unit and any magnetic field generated by the sensor unit itself, such as the field created by electrical currents within the sensor unit. B Erde : Earth's external magnetic field, including the magnetic field anomaly due to the presence of soft magnetic materials. B Rauschen : external magnetic field from nearby currents, magnets, etc.

[0062] The concept enables the measurement and differentiation between the internal and external magnetic field. The distinction between internal and external magnetic fields is important for the calibration of the sensor unit, especially the NV magnetometer, and for the correct determination of the bias magnetic field B bias essential.

[0063] To perform the calibration procedure, only three measurements in non-coplanar orientations are required, each after rotating the NV magnetometer by a known and defined angle. Increasing the number of measurements beyond the required minimum leads to a gradual improvement of the original calibration.

[0064] Furthermore, it is particularly advantageous for the calibration process if the active surface of the sensor medium (the diamond) is in the same position for all three measurements, which can be ensured, for example, using a flat surface and a ruler. In the case of highly inhomogeneous external magnetic fields that vary across the volume of the sensor medium, calibration can be supported by a simple mount, such as a tripod, with defined connections and orientations, which serves as a calibration stand.

[0065] As previously described, the minimal calibration protocol is based on three measurements, numbered 1, 2, and 3, with different orientations. When formulating corresponding equations relative to a diamond coordinate system (denoted by a superscript "D"), these rotations in the diamond coordinate system only affect the contribution of the external magnetic field. The first rotation can, for example, be 0°, meaning no rotation is performed for the first measurement. Therefore, the magnetic field vectors in the diamond coordinate system can be calculated as follows: Beff,1D=BbiasD+Bext,1D Beff,2D=BbiasD+Bext,2D Beff,3D=BbiasD+Bext,3D

[0066] A rotation of the sensor medium can be described as a rotation in the x, y, and z coordinates of the laboratory coordinate system (denoted by a superscript "L"). The rotation in the laboratory coordinate system "L" does not correspond to a rotation in the x, y, and z coordinates of the diamond coordinate system. In particular, BbiasD invariant to rotations in the D-coordinate system, while Bext,jL=BextL is invariant under rotations in the L-coordinate system. This can be summarized as follows, using transformation matrices from the L-coordinate system to the D-coordinate system and vice versa: Bext,jD=RD→Lt⋅Bext,jL where R D→L is the transformation matrix from the D-coordinate system to the L-coordinate system, where the inverse transformation RD→Lt the unity condition RD→Lt⋅RD→L=1 fulfilled.

[0067] Since the calibration rotations of the sensor medium are assumed to be known in the L-coordinate system, they can be represented as known rotation matrices corresponding to the rotation RD→Lt in the above equation. As an example, three non-coplanar rotations are chosen as follows.

[0068] For the first measurement, it is assumed that for the measurement of the initial external field BextL no rotation (unit rotation) is required. The second measurement is performed after a rotation of δ (for example, δ = 45°) around the z-axis of the L-coordinate system. The third measurement must ensure the non-coplanarity condition and can be performed around any axis in the xy-plane of the L-coordinate system that is not parallel to the z-axis of the L-coordinate system, but, for example, perpendicular to the z-axis. For example, a rotation around an axis in the xy-plane by an amount of η° (e.g., η = 20°).

[0069] Accordingly, the above equations (1) to (3) can be rewritten as: Beff,1D=BbiasD+RD→Lt⋅BextL Beff,2D=BbiasD+Rz(δ)⋅RD→Lt⋅BextL Beff,3D=BbiasD+R(η)⋅Rz(δ)⋅RD→Lt⋅BextL where R z describes a rotation around the z-axis of the L-coordinate system and R(η) describes a rotation around a vector in the xy-plane of the L-coordinate system by an angle of η.

[0070] To simplify the calculations, for example, the L-coordinate system can be aligned with the position of the sensor medium after the first rotation and the axis of the second rotation can be chosen as the x-axis of the L-coordinate system, ie BextL is unknown, it can be assumed, for example, that BextL itself as Bext,newL If the "new" index is omitted, this results in: Beff,1D=BbiasD+Rz(δ)⋅RD→Lt⋅BextL Beff,2D=BbiasD+RD→Lt⋅BextL Beff,3D=BbiasD+Rx(η)⋅RD→Lt⋅BextL

[0071] This system of 9 equations includes 9 variables:

[0072] The intrinsic (constant) magnetic field components of the sensor Bbias,xD, Bbias,yD and Bbias,zD.

[0073] The external magnetic field components Bext,xL, Bext,yL, Bext,zL.

[0074] The transformation angles of the transformation between the two coordinate systems α, β and γ.

[0075] The transformation between the two coordinate systems can be represented as three individual rotations around the three axes of the coordinate system: RD→L=Rx(α)⋅Ry(β)⋅Rz(γ).

[0076] While the inverse transformation is given by the transpose: RL→D=RD→Lt=(Rx(α)⋅Ry(β)⋅Rz(γ))t=Rzt(γ)⋅Ryt(β)⋅Rxt(α).

[0077] The use of one or more gyroscopes allows calibration of the sensor unit without the need for additional elements (such as a ruler or a calibration stand).

[0078] In addition, the use of one or more gyroscopes allows to increase the number of measurements for calibration, resulting in higher calibration accuracy.

[0079] Furthermore, active angular position determination enables additional real-time calibration checks and recalibrations required for specific applications. The method can therefore be applied anywhere and at any time, provided it is physically possible to rotate the sensor medium around multiple axes during the active calibration time.

[0080] According to one embodiment, gyroscopic sensors (including, but not limited to, microelectromechanical systems (MEMS) gyroscopes or gyrometers) are used to measure the angular variations during rotations and movements of the sensing medium. These angular measurements, along with the measurements of the vector magnetic field itself by the magnetometer, can be used to construct the necessary orientation changes for the calibration process.

[0081] The calibration procedure therefore comprises, for example, a rotation of the magnetometer and a continuous recording of its angular position and / or its three-dimensional orientation using one or more gyroscopic sensors, i.e., the gyroscopes. The rotations comprise at least three non-colinear or non-coplanar orientations, as described above, but may additionally include, for example, a random rotation and roll and / or pitch and / or yaw of the magnetometer, which also provide sufficient information for calibration. The recording or measurement of the angular orientation of the NV magnetometer during calibration can be performed, for example, by a 3D gyroscope and / or by three independent 1-axis gyroscopes, with the specific implementation depending on the actual use, the required precision, bandwidth, cost, etc.

[0082] The use of one or more gyroscopes described here has the following advantages in particular: The need for additional external calibration equipment (such as a ruler or calibration stand) can be avoided. Specific, pre-determined rotation requirements can be avoided, which facilitates the implementation of the procedure. On-the-fly calibration is possible, e.g., during commissioning of the sensor unit.

[0083] Real-time verification of the sensor unit's calibration and recalibrations, as well as checking whether recalibration is necessary and, if so, how often, can be enabled.

[0084] The sensor unit is enabled to track its calibration status by introducing self-awareness of the calibration status.

[0085] More than three measurements can be made for calibration, resulting in higher calibration accuracy.

[0086] Continuous temporal tracking of magnetometer orientation using gyroscopes allows the application of known methods to improve calibration, control, and prediction, such as Kalman filtering for processing the gyroscopic data, further improving the accuracy of the data used for magnetometer calibration.

[0087] Rotations such as pitch, roll, and rotation allow the acquisition of multiple data points at closely spaced positions at different times, which can reduce the influence of dynamic time-dependent magnetic noise during calibration.

[0088] Since the sampling rates of miniature gyroscopes can exceed 1000 Hz, a standard calibration of 3 seconds (about 1 second for each rotation axis) can allow over 3000 measurements.

[0089] To ensure the overall accuracy of the calibration, the measurement error of the gyroscopes can be taken into account. The drift of commercially available (MEMS) gyroscopes, for example, is on the order of 60 mrad / h (3.5° / h). Furthermore, the point-to-point error at a sampling rate of 100 Hz to 1000 Hz is on the order of 40 µrad / s (0.002° / s). These point-to-point error values ​​enable particularly precise calibration. For example, when using a calibration stand, with a manufacturing tolerance of 50µm of the calibration stand and a magnetometer package of about 10cm, an angular error in positioning the sensor medium on the stand of about 0.0057° (~50µm / 10cm) is expected, which corresponds to a cumulative error of 2.87s of measurements with a gyroscope (0.0057° / [0.002° / s]).During this time, a gyroscopic calibration performs at least 287 measurements at a sampling rate of 100 Hz (compared to a measurement with a calibration stand), which further reduces the error through statistical averaging in the calibration algorithm and Kalman filtering of the gyroscopic data.

[0090] Thus, real-time calibration, real-time recalibration, continuous calibration, continuous calibration improvement, and self-awareness of the calibration status can be implemented for embodiments in which the NV magnetometer itself or the system implementing the NV magnetometer has a non-static position over time. In this case, optimal control methods for dynamic systems can be used to incorporate continuously acquired angular position and magnetic field data to improve calibration.

[0091] The general definition and equations for the measurements and reconstruction of the bias magnetic field are based on the above, but omit the simplifications related to only three measurement orientations and introduce a reconstruction procedure as follows: B bias : internal / own or intrinsic magnetic field (e.g., generated by Helmholtz coils and / or permanent magnets), as well as the influence of the magnetized parts of the sensor unit and any magnetic field generated by the sensor unit itself, e.g., the magnetic field resulting from electrical currents within the sensor unit. This magnetic field is invariant to rotations and translations from the perspective of the reference frame attached to the diamond or to the sensor unit itself (D-coordinate system). B ext: external magnetic field, consisting of the Earth's magnetic field, including the magnetic field anomaly due to the presence of soft magnetic materials, as well as contributions from the external magnetic field of nearby currents, magnets, etc.

[0092] The equations that describe the rotation of the sensor medium in the diamond coordinate system are: Beff,1D=BbiasD+RD→Lt⋅BextL Beff,1D=BbiasD+R(δj)⋅RD→Lt⋅BextL,j>1 where the first equation (I.) refers to the initial orientation of the sensor medium at the beginning of the calibration procedure without any rotation, while the second equation (II.) additionally contains the general rotation matrix R(δ). This general rotation can be represented as successive rotations around the three principal axes: R(δ)=Rx(α)⋅Ry(β)⋅Rz(γ), where the angles α, β, and γ are measured by the one or more gyroscopes.

[0093] For example, calibration using gyroscopes involves multiple rotations. In this case, the orientation of the sensor medium at each calibration iteration j = 1, 2,...n, with a total of n rotations, can be represented as a matrix multiplication of the corresponding rotations. n can be three.

[0094] R(δ1) = I, where I is the identity matrix and where R(δ1) is the initial rotation according to step j=1.

[0095] R j (δ j ) = R x (α j ) · R y (β j ) · R z (γ j ),j > 1, where R j (δ j ) is the individual rotation in step j > 1 with respect to the current (last detected) position.

[0096] R(δ j ) = R j (δ j ) · R(δ j-1 ),j > 1, where R(δ j) is the final orientation after the j>1 rotations, defined as the application of the j-step rotation R j (δ j ), which refers to the previous position R(δ j-1 ), which in turn is a result of the multiplication of j-1 rotation matrices.

[0097] Since B is a vector 3D quantity, a magnetic field measurement at three different orientations contains nine equations and nine unknown variables, thus forming a well-defined system. If more than three measurements are performed, such as when measuring the orientation of the sensor medium using gyroscopes, more than nine equations result, resulting in the following system: Beff,1D=BbiasD+R(δj)⋅RD→Lt⋅BextL,j=1,2,3…n,n>3 is overdetermined and can be solved, for example, using the method of least squares as follows minBbiasD,RD→Lt,BextL(∑j=1n>3|Beff,jD−BbiasD−R(δj)⋅RD→Lt⋅BextL|p),p≥1, or min minBbiasD,RD→Lt,BextL(maxj|Beff,jD−BbiasD−R(δj)⋅RD→Lt⋅BextL|) |·| - Euclidean standard norm and any other norm

[0098] This over-specification and least squares fitting (where, for example, p = 2) is statistically advantageous for error reduction compared to measuring only three different orientations.

[0099] The system of equations can be solved using other methods, for example: gradient method, machine learning methods, other optimization methods, linear and nonlinear optimization methods, etc.

[0100] After applying one of the described calibrations, either with the help of a calibration stand and / or with the help of one or more gyroscopes, the values ​​of BbiasD,RD→Lt,RD→L In this case, any additional measurement of the total magnetic field BeffD, as measured in the diamond coordinate system, can be directly related to the external magnetic field in the laboratory coordinate system as follows: BextL=RD→L(BeffD−BbiasD)

[0101] In the case where the measurement is performed after the magnetometer has been rotated with respect to the laboratory coordinate system, which may be the case, for example, if the magnetometer was not returned to its original position after calibration but remained rotated with respect to the original position, which can be described by a known rotation matrix R(δ), the equation for BextL be defined as follows: BextL=RD→LRt(δ)(BeffD−BbiasD)

[0102] For example, the sensor unit is programmed to execute the computer program.

[0103] The invention is explained in more detail below using preferred embodiments. These show: Fig. 1 a flowchart of a method for calibrating a sensor unit for measuring magnetic fields, Fig. 2 a sensor unit for measuring magnetic fields, Fig. 3 a machine-readable storage medium, Fig. 4 a perspective top view of a calibration stand and Fig. 5 a top view of the calibration stand according to Fig. 4.

[0104] Fig. 1 shows a flow diagram of a method for calibrating a sensor unit for measuring magnetic fields, wherein the sensor unit comprises a nitrogen vacancy center magnetometer having a diamond crystal having nitrogen vacancy centers as the sensor medium, and a magnetic field generating device for generating a magnetic field in the region of the sensor medium.

[0105] According to a step 101, a predetermined static magnetic field is generated in the region of the sensor medium by the magnetic field generating device. In a step 103, a first magnetic field strength is measured by the magnetometer while the predetermined static magnetic field is generated by the magnetic field generating device. Here, the NV magnetometer, i.e., the sensor medium, is in a first orientation.

[0106] Subsequently, the NV magnetometer is brought or displaced into a second orientation, which is different from the first orientation. According to step 105, a second measurement of a second magnetic field strength is performed by the NV magnetometer for the second orientation while the predetermined static magnetic field is generated by the magnetic field generating device.

[0107] Subsequently, the sensor medium, i.e., the NV magnetometer, is brought or shifted into a third orientation, which is different from the first orientation and different from the second orientation. According to a step 107, a third measurement of a third magnetic field strength is performed by the magnetometer for the third orientation while the predetermined static magnetic field is generated by the magnetic field generating device.

[0108] According to a step 109, a magnetic field strength of an internal magnetic field is determined based on the measured first magnetic field strength, the measured second magnetic field strength and the measured third magnetic field strength as well as based on the three different orientations.

[0109] Fig. Figure 2 shows a sensor unit 201 for measuring magnetic fields, comprising: a nitrogen vacancy center magnetometer 203, which has a diamond crystal 205 having nitrogen vacancy centers 207 as the sensor medium, and a magnetic field generating device 209 for generating a predetermined static magnetic field in the region of the sensor medium, wherein the sensor unit is configured to carry out all steps of the method according to the first aspect.

[0110] The sensor unit 201 comprises a 3D gyroscope 211, which is configured to measure corresponding angles relative to all three axes x, y, z of a three-dimensional x, y, z coordinate system. For example, instead of or in addition to the 3D gyroscope 211, the sensor unit 201 comprises a separate gyroscope for each axis, i.e., a single-channel gyroscope.

[0111] Fig. 3 shows a machine-readable storage medium 301 on which a computer program 303 is stored. The computer program 303 includes instructions that, when executed by a computer, cause the computer to perform a method according to the first aspect.

[0112] Fig. 4 shows a calibration stand 401 in a perspective top view.

[0113] The calibration stand 401 comprises several stop surfaces against which a sensor unit according to the second aspect can rest in different orientations. For example, such stop surfaces are provided with the reference numerals 403, 405, 407, 409, 411, 413, 415, 417, 419, 421, 423, 425.

[0114] Fig.Figure 5 shows a top view of the calibration stand 401. Three frames 501, 503, and 505 are shown as sections, resting against different stop surfaces. This defines three orientations in which the sensor unit can be successively moved, allowing a vector magnetic field to be measured in each of these orientations.

[0115] Reference numeral 501 denotes a first orientation. Reference numeral 503 denotes a second orientation, and reference numeral 505 denotes a third orientation. Here, the third orientation 505 is outside the plane of the paper.

[0116] These stop surfaces 403 to 425 allow the sensor unit to be brought into different orientations or positions. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2018 220 234 A1

[0003] DE 10 2018 214 617 A1

[0003] US 8,494,799 B2

[0006]

Claims

[1] Method for calibrating a sensor unit (201) for measuring magnetic fields, wherein the sensor unit (201) comprises a nitrogen vacancy center magnetometer (203) having a diamond crystal (205) having nitrogen vacancy centers (207) as the sensor medium, and a magnetic field generating device (209) for generating a magnetic field in the region of the sensor medium, the method comprising the following steps: generating (101) a predetermined static magnetic field in the region of the sensor medium by the magnetic field generating device (209), measuring (103, 105, 107) a vector magnetic field by the magnetometer (203) for at least three different orientations of the sensor medium, while the predetermined static magnetic field is generated by the magnetic field generating device (209), Determining (109) an internal vectorial magnetic field based on the vectorial magnetic fields measured for the at least three different orientations and based on the at least three different orientations. [2] Method according to claim 1, wherein for the measurements in the at least three different orientations, the sensor medium is arranged successively in at least three different predetermined holding positions of a calibration stand (401). [3] Method according to claim 1 or 2, wherein the respective orientation is measured by at least one gyroscope (211). [4] Method according to one of the preceding claims, wherein a three-dimensional x,y,z laboratory coordinate system is used to determine the at least three orientations. [5] Method according to claim 4, wherein for the measurement in one of the at least three orientations the sensor medium is rotated by a predetermined rotation angle about the z-axis of the three-dimensional x,y,z laboratory coordinate system relative to another of the at least three orientations. [6] Method according to claim 4 or 5, wherein for the measurement in one of the at least three orientations, the sensor medium is rotated about any axis in the xy-plane of the x,y,z laboratory coordinate system, which is not parallel, but in particular perpendicular to the z-axis of the x,y,z laboratory coordinate system. [7] Sensor unit (201) for measuring magnetic fields, comprising: a nitrogen vacancy center magnetometer (203) having a diamond crystal (205) having nitrogen vacancy centers (207) as the sensor medium, and a magnetic field generating device (209) for generating a predetermined static magnetic field in the region of the sensor medium, wherein the sensor unit (201) is configured to carry out all steps of the method according to any one of the preceding claims. [8] Sensor unit (201) according to claim 7, comprising at least one gyroscope which is arranged to measure the respective orientation. [9] Computer program (303) comprising instructions which, when the computer program (303) is executed by a computer, cause the computer to carry out a method according to one of claims 1 to 6. [10] Machine-readable storage medium (301) on which the computer program (303) according to claim 9 is stored.

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