Sensor system for detecting a magnetic field in an inertial coordinate system

Abstract

The invention relates to a sensor system (1) for detecting a magnetic field relative to an inertial coordinate system, comprising a magnetometer (2) movable relative to the inertial coordinate system, configured to detect the direction and magnitude of a magnetic field relative to a magnetometer coordinate system, and an evaluation device (4) connected to the magnetometer (2) and configured to receive or calculate a value for a rotation angle between the magnetometer coordinate system and the inertial coordinate system, and to determine the direction of the magnetic field in the inertial coordinate system from the rotation angle and the direction of the magnetic field detected by the magnetometer (2) in the magnetometer coordinate system. The invention also relates to a method for detecting the magnetic field relative to an inertial coordinate system with a magnetometer moved relative to the inertial coordinate system.

Description

[0001] The present invention relates to a sensor system for detecting a magnetic field in an inertial coordinate system and to a method for operating such a sensor system. State of the art

[0002] A variety of magnetometers can be used to detect a magnetic field, such as SQUID magnetometers, OPMs, or fluxgate magnetometers. The use of so-called NV magnetometers has proven particularly advantageous, as they are especially compact and simultaneously very sensitive. These magnetometers consist of a diamond whose crystal lattice has defects in the form of NV centers. In an NV center, a nitrogen atom occupies the lattice site of a carbon atom, with a defect located in the immediate vicinity of the nitrogen atom—again occupying the lattice site of a carbon atom. When such a crystal lattice is irradiated with excitation radiation with a wavelength between 490 nm and 575 nm, an electronic transition from a ground state occurs within the crystal lattice. 3 A2 into an excited state 3 E induced. From the excited state 3E relaxes the NV center back to its ground state by emitting fluorescence radiation in a wavelength range between 600 nm and 850 nm. 3 A2. The basic state 3 A2 has three magnetic substates with m s =0, m s =±1. The states with m s =0 and m s The values ​​=±1 differ by an energy difference of 2.87 GHz (zero field splitting). The excited state 3 E also has three magnetic substates with m s =0, m s =±1. Upon irradiation with the excitation radiation, the NV center is now partially removed from the m s =±1, 3 A2 ground state in the excited m s =±1, 3 It enters an E-state. From there, it relaxes back into the m predominantly without radiation and without maintaining spin. s =0, 3 A2 - Ground state. Simultaneously, the excited state relaxes. s =0, 3E also emits fluorescence radiation into the m s =0, 3 A2 - Ground state. Is the NV center now in the ground state? 3 When exposed to microwave radiation at a frequency of 2.87 GHz, the NV center oscillates between the m s =0, 3 A2 -ground state and the m s =±1, 3 A2 - Ground state. If the amplitude of the fluorescence radiation is measured as a function of the frequency of the microwave radiation during irradiation with the excitation radiation, a sudden drop in the amplitude of the fluorescence radiation (a so-called dip) occurs at a frequency of 2.87 GHz. The drop in the amplitude of the fluorescence radiation can be explained by the fact that - when irradiated with microwave radiation at a frequency of 2.87 GHz - the transition between the m s =0, 3 A2 ground state and the m s =±1, 3The A2 ground state is induced, which is excited by the excitation radiation in a spin-conserving manner, but is radiationless and not spin-conserving in the m s =0, 3 The A2 ground state can relax. Therefore, if microwave radiation of a precisely matching frequency is applied, the m s =0, 3 A2 ground state, during the m s =±1, 3 A2 ground state is filling up.

[0003] In a magnetic field, the m s =±1, 3 A2 ground state into two states with spin quantum number m s =1 and m s =-1 (Zeeman effect). If the amplitude of the fluorescence radiation is measured while changing the frequency of the microwave radiation, two dips are observed. The frequencies at which these dips occur depend on the size of the splitting of the m s =±1, 3A2 ground state and thus depends on the field strength and direction of the magnetic field, whereby the direction of the magnetic field can be determined from the projection of the magnetic field onto the respective NV axes.

[0004] Such a magnetometer measures the direction of the magnetic field with respect to an internal coordinate system, the magnetometer coordinate system. If the magnetometer is rotated, a change in the direction of the magnetic field cannot be distinguished from a change in the direction of the magnetometer. In particular, if the magnetometer is moved along with a vehicle or aircraft, unforeseen rotations of the magnetometer can occur, leading to an unforeseen change in its direction, which may be indistinguishable from a change in the direction of the magnetic field.

[0005] It is an object of the invention to provide a sensor system that does not have the disadvantages of the prior art. Disclosure of the invention

[0006] The present invention relates to a sensor system for detecting a magnetic field relative to an inertial coordinate system according to claim 1, and to a method for detecting a magnetic field relative to the inertial coordinate system according to claim 8. Advantageous embodiments of the invention are the subject of the dependent claims and the description.

[0007] The sensor system according to the invention comprises a magnetometer movable relative to the inertial coordinate system, configured to detect the direction and, in particular, the field strength of the magnetic field relative to a magnetometer coordinate system, and an evaluation device connected to the magnetometer. The evaluation device is configured, firstly, to receive or calculate a value for a rotation angle between the magnetometer coordinate system and the inertial coordinate system. Secondly, the evaluation device is configured to determine the direction of the magnetic field in the inertial coordinate system from the rotation angle and the direction of the magnetic field detected by the magnetometer relative to the magnetometer coordinate system. The inertial coordinate system is, in particular, a stationary coordinate system, e.g.,The Earth's coordinate system, which for an observer moving with the Earth and within the scope of this application represents a stationary coordinate system. The magnetometer coordinate system is a coordinate system aligned with the principal axes of the magnetometer, for example, in the case of an NV magnetometer, with the edges of the NV diamond. The magnetometer coordinate system moves with the magnetometer.

[0008] With such a sensor system, the direction of the magnetic field in the inertial coordinate system can be determined – using a moving magnetometer – independently of the angle of rotation between the magnetometer coordinate system and the inertial coordinate system. This is particularly advantageous if, for example, unforeseen rotations of the magnetometer can occur during flight.

[0009] In a further development of the invention, the magnetometer is configured to detect the magnetic field with a first accuracy. The evaluation device is configured to receive data relating to a known magnetic field in the inertial coordinate system, which exhibits a second accuracy. The evaluation device is further configured to determine the angle of rotation between the magnetometer coordinate system and the inertial coordinate system from this data, which in particular includes the direction of the known magnetic field relative to the inertial coordinate system and the direction of the magnetic field detected by the magnetometer relative to the magnetometer coordinate system. The first accuracy is in particular 1 pT, i.e., the magnetometer is configured to detect the magnetic field with a maximum error of 1 pT. The data on which the evaluation device can access, however, can have a significantly lower second accuracy, e.g.,The data can be sent to the evaluation device from a database, such as an anomaly map, where each coordinate on the Earth's surface is linked to a value for the Earth's magnetic field at that location. The data can be stored there with second-order accuracy.

[0010] Such a sensor system is suitable for detecting even slight deviations between the actual direction of the Earth's magnetic field and the direction stored in a database and transmitted to the evaluation device. The direction of the Earth's magnetic field transmitted to the evaluation device is based, for example, on previous, less precise measurements or extrapolations of the magnetic fields in the vicinity of the magnetic field being measured. These deviations can indicate the presence of raw materials beneath the Earth's surface.

[0011] In a further development of the invention, the sensor system includes a rotation measuring device. The rotation measuring device is configured to detect the angle of rotation between the magnetometer coordinate system and the inertial coordinate system. In this case, the evaluation device is connected to the magnetometer and the rotation measuring device and is configured to determine the direction of the magnetic field in the inertial coordinate system from the angle of rotation detected by the rotation measuring device and the direction of the magnetic field detected by the magnetometer relative to the magnetometer coordinate system. This enables a reliable measurement of the angle of rotation.

[0012] The evaluation device can be configured to determine an initial rotation angle between the magnetometer coordinate system and the inertial coordinate system before the measurement begins, for example, by comparing the direction of the magnetic field transmitted to it (known relative to the inertial coordinate system) with the direction of the magnetic field that the magnetometer measures relative to the magnetometer coordinate system for that magnetic field. This can be particularly important if the rotation measuring device measures an angle by which the magnetometer rotates during a rotation and sums the angles measured for the respective rotations to obtain the rotation angle. In this case, the rotation angle can only be determined if the initial rotation angle is known.

[0013] In a further development of the invention, the evaluation device is configured to continuously record the magnetic field detected by the magnetometer relative to the magnetometer coordinate system, to distinguish short-term fluctuations from long-term changes, and to filter out the short-term fluctuations from the recorded magnetic field signal. In this way, short-term fluctuations, which occur within a time window of, for example, a few milliseconds, such as those caused by turbulence during a flight, can be distinguished from and filtered out by long-term changes in the direction of the magnetic field, which occur, for example, due to a change in the aircraft's flight path.

[0014] The magnetometer can include at least one vector magnetometer, e.g., an NV magnetometer. NV magnetometers are very compact, yet highly sensitive and easy to use.

[0015] The magnetometer can also be configured as a gradiometer with at least two magnetometer units. In this case, the sensor system can be designed to measure the magnetic field strength at different locations and determine the direction of the magnetic field from this measurement. Furthermore, such an arrangement can distinguish noise and / or interference artifacts from the signal being detected.

[0016] The magnetometer can also be configured as a gradiometer with, for example, five magnetometer units arranged at the corners of a pyramid with a square base. Each magnetometer unit is designed to determine the magnetic field strength along one of the directions of the magnetometer coordinate system (e.g., H). x , H y , H zFrom the measured field strengths and the respective distances between the magnetometer units, the derivatives of the magnetic field strengths with respect to one of the coordinates of the magnetometer coordinate system can then be calculated (e.g., δH). x / δx, δH y / δx, δH y / δy, δH z / δx, δH z / δy) Such an arrangement makes it possible to determine the direction of the magnetic field very precisely.

[0017] The evaluation unit of the sensor system can include at least one noise suppression filter. The noise can be traced back to a local source. The sensor system can be configured to determine the direction of the magnetic field causing the noise. In this case, the filter can be configured to selectively remove noise from the signal that can be assigned to a specific direction. The noise can also or alternatively occur periodically. The filter can, for example, be configured as a low-pass or band-pass filter and be suitable for removing interference signals in a specific frequency band from the signal. Alternatively or additionally, a notch filter can be provided, which is configured to remove predetermined frequencies from the signal.The filter can also be designed as an integrator, which integrates the signal over a predetermined time interval, thus removing noise and time-varying signal components from the signal.

[0018] By evaluating directional information, it is also possible, for example in the context of magnetotelluric measurements, to distinguish between an induced magnetic field and the magnetic field of the source causing the induced magnetic field.

[0019] The rotation measuring device can be designed as a rotation rate sensor, e.g., as a MEMS rotation rate sensor or as a gyroscope. These are compact and can therefore be easily integrated into the sensor system.

[0020] The evaluation device can be configured to detect a deviation of the rotation angle detected by the rotation measuring device from the rotation angle that the evaluation device can determine based on the direction of the magnetic field determined by the magnetometer in the magnetometer coordinate system. Many rotation measuring devices are designed such that measurement errors that may occur with each individual measurement can accumulate into a total measurement error that increases with each measurement, so that the rotation angle detected by the rotation measuring device deviates more and more from the actual rotation angle with an increasing number of measurements. The evaluation device can then be configured to compare the two rotation angles detected in different ways.If the measured values ​​deviate from each other, the evaluation device can be configured to correct the rotation angle detected by the rotation measuring device and transmitted to the evaluation device, thereby recalibrating the rotation measuring device. The evaluation device can be configured to perform this recalibration of the rotation measuring device at fixed time intervals. This enables a very precise measurement of the direction and / or field strength of the magnetic field.

[0021] The sensor system can be configured to switch between a first operating mode, in which the angle of rotation is detected using the rotation measuring device, and a second operating mode, in which the angle of rotation is determined based on the direction of the magnetic field detected by the magnetometer. The sensor system can also operate in a third mode, in which the angle of rotation is detected using both methods. In this case, the sensor system can be switched between the first, second, and third operating modes. This allows the sensor system, for example, to operate in the first operating mode at and near the geographic poles. In a magnetic field where the deviations of the field strength from the mean field strength are small, the sensor system can operate in the second operating mode or—if a particularly accurate measurement is required—in the third operating mode.

[0022] The invention also relates to a method for detecting a magnetic field relative to an inertial coordinate system comprising the following steps: a. Determining the direction of the magnetic field relative to a magnetometer coordinate system moving with the magnetometer at a specific time, b. Determining an angle of rotation that the inertial coordinate system assumes relative to the magnetometer coordinate system at the specified time, c. Calculating the direction of the magnetic field in the inertial coordinate system.

[0023] The determination of the magnetic field can include determining its strength and direction. The magnetic field can be measured using a magnetometer, for example, a vector magnetometer, particularly an NV magnetometer. Alternatively, the magnetic field can also be determined using a gradiometer with at least two, and preferably five, magnetometer units. This method allows the direction of the magnetic field to be determined independently of the magnetometer's current orientation.

[0024] The angle of rotation can be determined, for example, by a rotation measuring device such as a gyroscope or a (MEMS) rotation rate sensor. This enables very precise measurement of, for example, the Earth's magnetic field.

[0025] In addition, the following steps can be performed: d. Transmitting a direction of the magnetic field known relative to the inertial coordinate system to an evaluation device of the magnetometer, e. Determining a direction of the magnetic field known in the inertial coordinate system relative to the magnetometer coordinate system, f. Comparing the direction of the magnetic field determined relative to the magnetometer coordinate system with the known direction of the magnetic field in the inertial coordinate system by the evaluation device.

[0026] In this way, for example, an initial rotation angle, i.e. the rotation angle between the magnetometer coordinate system and the inertial coordinate system before the first measurement, can be determined.

[0027] The procedure may include the following additional steps: g. Comparing the rotation angle detected by the rotation measuring device with the rotation angle determined by the evaluation device by comparison, h. if applicable. Correction of the rotation angle detected by the rotation measuring device and determination of the direction of the magnetic field relative to the inertial coordinate system using the corrected rotation angle.

[0028] The angle of rotation between the inertial coordinate system and the magnetometer coordinate system can be determined using the rotation measuring device. Alternatively, the direction of the magnetic field in the magnetometer coordinate system can be determined continuously, i.e., at predefined time intervals. From the directions of the magnetic field thus determined in the magnetometer coordinate system and the initial angle of rotation, the angle of rotation between the two coordinate systems can then be calculated. The angles of rotation determined in different ways can be compared. If they differ, the angle of rotation determined by the rotation measuring device can be corrected, and the rotation measuring device can be recalibrated in this way.This may be necessary because small measurement errors that can occur with each measurement of the rotation angle by the rotation measuring device can accumulate over time. Brief description of the drawings Fig. Figure 1 shows a first schematic embodiment of the invention. Fig. Figure 2 shows a second embodiment of the invention. Fig. Figure 3 shows the method according to the invention in a block diagram. Embodiments of the invention

[0029] Fig. Figure 1 shows a sensor system 1 comprising a magnetometer 2, an evaluation device 4, and a rotation measuring device 3. The magnetometer 2 is configured to detect the field strength and direction of a magnetic field in a magnetometer coordinate system. The rotation measuring device 3 is configured to determine an angle of rotation between the magnetometer coordinate system and an inertial coordinate system. The inertial coordinate system can be a stationary coordinate system, such as the Earth's coordinate system. The rotation measuring device 3 can be configured, for example, as a gyroscope or as a MEMS rotation rate sensor. The rotation measuring device can be spatially connected to the magnetometer 2, for example, arranged on a common base plate, so that the rotation measuring device 3 and the magnetometer 2 can be moved together, in particular rotated together.The rotation measuring device 3 can be configured to detect the angle by which the magnetometer 2 has been rotated. If the rotation measuring device 3 and the magnetometer 2 are aligned in the inertial coordinate system at the beginning of the measurement, the angle measured by the rotation measuring device 2 corresponds to the angle of rotation between the inertial coordinate system and the magnetometer coordinate system. Alternatively, an initial angle of rotation between the magnetometer coordinate system and the inertial coordinate system can be known, so that the angle of rotation between the two coordinate systems can be determined from the angle detected by the rotation measuring device and the initial angle of rotation. For this purpose, the evaluation device 4 can be connected to the magnetometer 2 and the rotation measuring device 3 via a data line. The evaluation device 4 can be configured to process data such as...The evaluation device 4 is designed to receive and, in particular, store the rotation angle detected by the rotation measuring device 3 and the direction of the magnetic field detected by the magnetometer 2. Furthermore, the evaluation device 4 can be configured to determine, from the received data and the initial rotation angle, the rotation angle between the two coordinate systems and, from this, the direction of the magnetic field in the inertial coordinate system.

[0030] The sensor system 1 can include one or more filters 5 for noise reduction, e.g., a low-pass filter, a band-pass filter, a notch filter, or an integrator. These filters can be integrated into the evaluation device 4. With the aid of the filter(s), a signal detected by the magnetometer can be freed from noise and / or interference artifacts.

[0031] The sensor system 1 can have an output interface 6 configured to output signals determined by the evaluation device 4 and / or acquired by the magnetometer 2 and / or the rotation measuring device 3, such as the direction of the magnetic field in the inertial coordinate system and possibly the direction of the magnetic field in the magnetometer coordinate system. Further parameters, such as temperature or voltage values ​​characteristic of the sensor system 1, can also be output. The signals can be combined in a single data line, e.g., a bus (e.g., I2C), or aggregated via an Ethernet or USB interface. Input and output signals can also be combined. The evaluation device 4 can comprise a CPU or be integrated into an ASIC.

[0032] The sensor system 1 can also be configured without the rotation measuring device 3. In this case, the evaluation device 4 is configured to evaluate the direction of the magnetic field determined by the magnetometer 2 in the magnetometer coordinate system. The evaluation device 4 can be configured to determine the angle of rotation between the inertial coordinate system and the magnetometer coordinate system from the initial rotation angle and the direction of the magnetic field determined by the magnetometer 2 in the magnetometer coordinate system.

[0033] The evaluation device 4 can also be configured to compare the rotation angle resulting from the angles detected by the rotation measuring device 3 with the rotation angle resulting from the directions of the magnetic field detected by the magnetometer. If the rotation angles differ, the rotation angle based on the angles detected by the rotation measuring device 3 can be corrected to the value of the rotation angle resulting from the directions of the magnetic field detected by the magnetometer. The rotation measuring device 3 can then be recalibrated accordingly.

[0034] The magnetometer 2 can, as in Fig. Figure 2 shows that the magnetometer can be configured as a gradiometer. In this case, the magnetometer 2 has several magnetometer units 7. These can, for example, be arranged at the corners of a pyramid with a square base. Another magnetometer unit 7 can be arranged inside the pyramid. Other arrangements of the magnetometer units 7 are also possible. The direction of the magnetic field can be determined very precisely from the measurements taken at the respective positions of the magnetometer units 7.

[0035] Fig.Figure 3 shows a method for detecting a magnetic field in an inertial coordinate system, comprising steps S1 to S3. In step S1, the direction of the magnetic field is determined in a magnetometer coordinate system. A magnetometer, in particular a vector magnetometer, e.g., an NV magnetometer, can be used for this purpose. In step S2, a rotation angle between the inertial coordinate system and the magnetometer coordinate system is determined. This can be done, for example, using the rotation measuring device 3. Alternatively or additionally, the rotation angle between the two coordinate systems can be determined from a direction of the magnetic field in the magnetometer coordinate system determined by the magnetometer 2. Both methods assume that the initial rotation angle between the two coordinate systems is known. The initial rotation angle can be determined, for example, by...This can be determined by the magnetometer detecting a direction of the magnetic field known in the inertial coordinate system within the magnetometer coordinate system.

[0036] Finally, in step S3, the direction of the magnetic field is calculated in the inertial coordinate system.

Claims

[1] Sensor system (1) for detecting a magnetic field relative to an inertial coordinate system with • a magnetometer (2) movable relative to the inertial coordinate system, which is configured to detect a direction and magnitude of a magnetic field relative to a magnetometer coordinate system, • an evaluation device (4) which is connected to and set up with the magnetometer (2), i. to receive or calculate a value for a rotation angle between the magnetometer coordinate system and the inertial coordinate system and ii. to determine the direction of the magnetic field in the inertial coordinate system from the angle of rotation and the direction of the magnetic field detected by the magnetometer (2) in the magnetometer coordinate system. [2] Sensor system (1) according to claim 1, • wherein the magnetometer (2) is configured to detect the magnetic field with a first accuracy and wherein • the evaluation device (4) is set up to receive data relating to a known magnetic field in the inertial coordinate system and which have a second accuracy, and to determine from this data and the direction of the magnetic field detected by the magnetometer (2) in the magnetometer coordinate system an angle of rotation between the magnetometer coordinate system and the inertial coordinate system. • where the first accuracy is higher than the second accuracy. [3] Sensor system (1) according to claim 1 or 2 with a rotation measuring device (3) for detecting the angle of rotation between the magnetometer coordinate system and the inertial coordinate system, [4] Sensor system (1) according to one of the preceding claims, wherein the evaluation device (4) is configured to continuously record the magnetic field detected by the magnetometer (2) relative to the magnetometer coordinate system and to distinguish its short-term fluctuations from its long-term changes and to filter out the short-term fluctuations from the recorded magnetic field signal. [5] Sensor system (1) according to one of the preceding claims, wherein the magnetometer (2) is designed as a vector magnetometer, in particular as an NV magnetometer. [6] Sensor system (1) according to one of the preceding claims, wherein the magnetometer (2) is designed as a gradiometer with at least two, in particular five, magnetometer units (7). [7] Sensor system (1) according to one of the preceding claims, wherein the evaluation device (4) is configured to detect a deviation of the rotation angle detected by the rotation measuring device (3) from the rotation angle determined by the evaluation device on the basis of the direction of the magnetic field determined by the magnetometer (2) in the magnetometer coordinate system. [8] Method for determining the direction of a magnetic field relative to an inertial coordinate system using a magnetometer (2) moving relative to the inertial coordinate system comprising the steps: • Determining a direction of the magnetic field relative to a magnetometer coordinate system moving with the magnetometer (2) at a specific time, • Determining a rotation angle that the inertial coordinate system assumes relative to the magnetometer coordinate system at the specified time, • Calculating the direction of the magnetic field relative to the inertial coordinate system. [9] Method according to claim 8, wherein the determination of the angle of rotation is carried out by a rotation measuring device (3). [10] Method of claim 8 or 9, wherein the determination of the angle of rotation comprises the following steps: • Transmitting a direction of the magnetic field known relative to the inertial coordinate system to an evaluation device (4) of the magnetometer (2), • Determining the direction of the magnetic field relative to the inertial coordinate system in the magnetometer coordinate system, • Compare the direction of the magnetic field determined relative to the magnetometer coordinate system with the known direction of the magnetic field in the inertial coordinate system and determine the angle of rotation between the magnetometer and the inertial coordinate system. [11] Method according to claim 9 or 10 with the additional steps • Comparing the rotation angle detected by the rotation measuring device with the rotation angle determined from the comparison between the direction known in the inertial coordinate system and the direction of the magnetic field measured in the magnetometer coordinate system, • If necessary, correction of the rotation angle detected by the rotation measuring device (3) and determination of the direction of the magnetic field in the inertial coordinate system using the corrected rotation angle.

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