Visualization of magnetic fields using modulation screens and compressed sensing

Abstract

To provide systems and methods for mapping magnetic fields by visualizing the magnetic field.SOLUTION: The system is configured to utilize a high-sensitivity magnetic field sensor 6 (for example, a magnetometer 12 disposed inside a tube 16 made of magnetic shielding material) disposed on one side of a magnetic field modulation screen 2 to acquire measurement data representing an image of a magnetic field. The magnetic field modulation screen includes a plurality of magnetic field-generating pixel elements (for example, current-carrying loops made of a magnetic field generating material). As an arbitrary configuration, the system is configured to enable reduction in an amount of measurement data required to reconstruct an image of the original magnetic field by using compressive sensing techniques. Compressive sensing is enabled so that an electric current is not supplied to the different pixel element to be selected in consecutive sampling time of the magnetic field-generating pixel elements of the magnetic field modulation screen.SELECTED DRAWING: Figure 3B

Description

[Technical Field]

[0001] The technology disclosed herein relates, in general terms, to the visualization of magnetic fields. [Background technology]

[0002] Magnetic fields exist everywhere in the human environment. The frequency range of magnetic fields is wide and includes static magnetic fields from magnetic materials and the Earth's magnetic field at any given location, as well as fluctuating magnetic fields originating from various artificial and natural objects, and magnetic fields induced by the movement of a conductor surface through the Earth's magnetic field.

[0003] The lowest frequency band of electromagnetic waves commonly used in many applications is a category called extremely low frequency waves (ELF), which includes very low frequencies of a few hertz and is typically used for submarine communications. Currently, many techniques are used to visualize electromagnetic waves in the radio frequency and microwave frequency bands (e.g., magnetic resonance imaging and radar), but these are active imaging sources that require some kind of stimulus beforehand and are not suitable for visualizing very low frequency magnetic fields. An exception is the method of mapping static magnetic fields (e.g., magnetic fields generated from permanent magnets) using iron filings, but this is mainly used for educational purposes. Another exception is the method of statically imaging the arrangement of magnetic fields using an array of relatively low-sensitivity Hall effect sensors. However, the required needs include visualizing very low frequency (almost static) magnetic fields generated from the human body or equipment in a manner that can be converted into an electronic form. One method used in this framework is magnetoencephalography (MEG), which visualizes magnetic fields in the brain. In MEG, a three-dimensional image is generated by using multiple electrodes and synthesizing their outputs using computer processing. However, this technology is not suitable for generating images of moving objects and requires an array of multiple sensors rather than a single sensor.

[0004] The drawbacks of the aforementioned magnetic field mapping stem not from a lack of highly sensitive magnetic field sensors (magnetometers) capable of detecting weak magnetic fields from various sources, but from a lack of extremely sensitive, low-cost, and compact magnetic detector arrays. For example, a large array of coil-type magnetometers can be used to generate images using magnetic resonance imaging. However, such arrays are expensive and bulky, and only function in the high (radio) frequency range (RF). Furthermore, such systems typically generate images by detecting large-amplitude magnetic fields using a source that "irradiates" RF magnetic fields (similar to how radar works).

[0005] To improve upon current technology, there is a need to provide a compact system using a highly sensitive magnetometer that does not require an illumination source and can generate images using a single detector. [Overview of the Initiative]

[0006] The gist of this disclosure is a system and method for visualizing and mapping low levels of magnetic or static magnetic fields emitted from, for example, people indoors, submarines, or other sources, as will be described in detail later. According to some embodiments, the system acquires measurement data representing an image of the magnetic field using a highly sensitive magnetic field sensor (e.g., a magnetometer) positioned on one side of a magnetic field modulation screen. The magnetic field sensor is, for example, a shielded total-field sensor or a directional / vector sensor. The magnetic field modulation screen includes a plurality of magnetic field generating pixel elements (e.g., current carrier loops composed of magnetic field generating material).

[0007] According to one embodiment proposed in this disclosure, the system is, for example, an atomic magnetometer, with a frequency of 1 pT / Hz. 1 / 2A magnetometer with ultra-high sensitivity is used. Atomic magnetometers can detect magnetic fields ranging from constant (DC) fields to fields of several hundred Hz (amplitude of alternating fields). Typical atomic magnetometers are classified as either total-field magnetometers or scalar magnetometers, measuring only the total magnetic field at any measurement point in space and time (scalar measurement), and not directionality. However, the techniques proposed in this disclosure are not limited to scalar magnetometers and are readily applicable to direction / vector measuring magnetometers. The systems and methods proposed in this disclosure provide a combination of spatial resolution signals and temporal resolution signals. In some cases, it is possible to modify a total-field magnetometer by utilizing shielding techniques and the proximity of pixel arrays to restore false vector information. Furthermore, vector magnetometers can be used as an alternative to total-field magnetometers, and using a vector magnetometer reduces the required shielding. The technology described in detail below makes it possible to visualize low-level magnetic fields (e.g., less than 1 / 1,000 to 1 / 10,000 of the Earth's magnetic field) to visualize and identify moving objects. This is complicated when there are magnetic fields from various spurious sources, including living organisms. Identifying the characteristics of a specific source from a large background is difficult, especially with a single sensor of a reasonable level of form factor, resolution, and cost.

[0008] The aforementioned system employs compressed sensing technology in an optional configuration, and is configured to acquire and reproduce a signal that can be compressed at a rate significantly below the Nyquist frequency. This reduces the amount of measurement data that needs to be transmitted and stored. To enable compressed sensing, current is not supplied to different pixel elements selected for each sampling time among the magnetic field generating pixel elements in the magnetic field modulation screen.

[0009] Various embodiments of systems and methods for visualizing low-level magnetic fields will be described in detail below, but one or more of these embodiments can be characterized by one or more of the following aspects.

[0010] One aspect of the summary described in detail below relates to a magnetic field sensing system. The magnetic field sensing system comprises a magnetometer, a magnetic field modulation screen, and a support frame that supports the magnetometer and the magnetic field modulation screen in a predetermined positional relationship. In one embodiment, the magnetic field modulation screen includes an array of magnetic field generating elements, each individually addressable and controllable, and an array of switches arranged to switchably connect a current source to each of the magnetic field generating elements. In one embodiment proposed, the magnetic field modulation screen includes a plurality of printed circuit boards, each printed with a set of magnetic field generating elements. The printed circuit boards are arranged in a superimposed state such that the sets of magnetic field generating elements are offset from each other.

[0011] In some embodiments, the magnetic field sensing system described in the preceding paragraph further includes a controller connected to the array of switches and configured to control the switches to supply current from the current source to all but one of the magnetic field generating elements. The magnetic field sensing system further includes a processing node connected to the controller and configured to send a control signal to the controller to control the magnetic field modulation screen so that, in a sequence of states in time, one of the magnetic field generating elements does not receive current from the current source.

[0012] In another embodiment, the system further includes a processing node connected to the array of switches and configured to control the switches such that, in a sequence of states in time, one of the magnetic field generating elements does not receive current from the current source.

[0013] Another aspect of the summary, detailed below, relates to a magnetic field visualization system. The system comprises a current source, a display device configured to display an image representing the distribution of a magnetic field, a magnetometer configured to detect a magnetic field over a given period and output a signal containing magnetic field measurement data, a magnetic field modulation screen, and a processing node. The magnetic field modulation screen is positioned in a fixed position relative to the magnetometer and includes an array of magnetic field generating elements and an array of switches connected to each of the magnetic field generating elements and the current source. The processing node is connected to the array of switches and is configured to control the switches so that, in sequentially consecutive states among the states at each instantaneous point over a given period, one of the magnetic field generating elements does not receive current from the current source. The processing node is further configured to process the magnetic field measurement data using a reconstruction algorithm, reconstruct an image representing the distribution of the magnetic field using compressed sensing technology, and then send the processed data to the display device to display the image representing the detected magnetic field distribution.

[0014] Another aspect of the gist described below in detail relates to a magnetic field modulation screen. The magnetic field modulation screen comprises a first printed circuit board on which spaced-apart first magnetic field generating elements are printed, a second printed circuit board on which spaced second magnetic field generating elements are printed, a third printed circuit board on which spaced third magnetic field generating elements are printed, and a fourth printed circuit board on which spaced fourth magnetic field generating elements are printed, wherein the first to fourth printed circuit boards are arranged in a superimposed state such that the first to fourth magnetic field generating elements are offset from each other. According to one embodiment, each magnetic field generating element includes a loop.

[0015] Another aspect of the summary, which is described in detail below, is a method for visualizing a magnetic field. The method includes: positioning a magnetic field modulation screen, which includes an array of individually addressable and controllable magnetic field generating elements, at a predetermined position relative to a magnetometer; positioning the magnetometer and the magnetic field modulation screen in a magnetic field; operating the magnetometer to continuously detect the magnetic field for a given period and to output a signal including magnetic field measurement data; operating the magnetic field modulation screen so that, at consecutive instantaneous intervals in chronological order during the given period, one of the magnetic field generating elements does not receive current from the current source; processing the magnetic field measurement data using an image reconstruction algorithm incorporating compressed sensing technology to reconstruct an image representing the distribution of the magnetic field; and displaying the image representing the detected magnetic field distribution.

[0016] Other aspects of systems and methods for visualizing low-level magnetic fields are disclosed below. [Brief explanation of the drawing]

[0017] The features, functions, and effects described in the above section can be realized individually in various embodiments and can also be combined with other embodiments. The above-mentioned aspects and other aspects will be described below with reference to the drawings, illustrating various embodiments. The accompanying drawings briefly described in this section are not drawn to scale.

[0018] [Figure 1] This is a three-dimensional diagram showing a magnetic field sensor placed inside a tube made of a magnetic shielding material, according to one embodiment. [Figure 2] This figure shows a pixel loop made of a magnetic field generating material according to one embodiment. By providing multiple such pixel loops on a printed circuit board, a magnetic field modulation screen is formed. [Figure 3A]A diagram showing a magnetic field sensor disposed in a tube made of a magnetic shielding material in three dimensions according to an embodiment. The sensor is disposed in a predetermined positional relationship with respect to a magnetic field modulation screen formed by laminating printed circuit boards (PCBs) having pixel loops of the type shown in FIG. 2. [Figure 3B] FIG. 3A is a partial cross-sectional side view showing a system configured by supporting a magnetic field sensor and a magnetic field modulation screen in a predetermined positional relationship by a magnetic shielding support frame. [Figure 4A] A diagram showing a loop pattern in which loops are spaced apart by a distance d to minimize interference between an energized loop and adjacent loops. [Figure 4B] A diagram showing a loop pattern in which loops are arranged at a desirable density to increase resolution. [Figure 4C] A diagram showing loops that are arranged on different PCBs among the laminated PCBs to achieve the desirable density shown in FIG. 4B using the loop pattern shown in FIG. 4A. [Figure 5] A block diagram showing members of a magnetic field visualization system according to an embodiment. [Figure 6] A block diagram showing members of a magnetic field visualization system according to another embodiment. [Figure 7] A block diagram showing members of a subsystem that controls which pixel loops receive current supply in the operation of the magnetic field modulation screen shown in FIG. 3A. [Figure 8A] A diagram showing an original image generated by projecting light onto a digital micro-mirror device including an array of N mirrors arranged in an array, where each mirror corresponds to one pixel of the image, and each mirror is individually controlled to perform a raster scan, and N measurements, i.e., one measurement for each mirror in the device, are measured. [Figure 8B] A diagram showing an image restored by projecting light onto a part of all N mirrors included in a digital micro-mirror device. By using compressive sensing, the original image shown in FIG. 8A is accurately approximated without measuring N measurements. [Figure 9] This is a block diagram showing the steps of a magnetic field visualization method according to one embodiment.

[0019] Refer to the drawings below. In these drawings, similar elements are indicated by the same reference numerals in different drawings. [Modes for carrying out the invention]

[0020] The following describes in detail exemplary embodiments of systems and methods for visualizing magnetic fields. However, this specification does not describe all features of practical embodiments. Those skilled in the art will understand that in the process of developing practical embodiments, there will be many items to be decided to achieve the developer's objectives, which will differ from embodiment to embodiment, such as satisfying system-related and commercial constraints. Furthermore, even if such development efforts are complex and time-consuming, they will be considered routine efforts for those skilled in the art based on the interests of this disclosure.

[0021] According to the embodiment described in detail below, the magnetic field visualization system uses a highly sensitive magnetic field sensor (e.g., an atomic magnetometer) positioned on one side of the magnetic field modulation screen to acquire measurement data representing an image of the magnetic field radiated from a source located on the opposite side of the magnetic field modulation screen. The magnetic field modulation screen includes a plurality of magnetic field generating pixel elements (e.g., a current-carrying loop made of a magnetic field generating material).

[0022] The system is configured, in any configuration, to acquire and reproduce a compressible signal at a rate significantly below the Nyquist frequency using compressed sensing technology, thereby reducing the amount of measurement data to be transmitted and stored. To enable compressed sensing, current is selectively withheld from individual magnetic field generating pixel elements in the magnetic field modulation screen.

[0023] Compressed sensing (also known as "compressed sampling") is a conventional technique that generates images using a single pixel detector across a frequency range from terahertz waves to the visible spectrum. The principle of compressed sensing is based on the mathematical property that holds true for many images: any image can be considered "sparse" in N-dimensional space, given any appropriate basis set. This means that an image can be represented using a smaller set of basis vectors (most of the remaining components are zero or very small), and therefore, an image can be reconstructed from a significantly smaller number of measurements than the number of pixels that make up the original image. Compressed sensing is a technique that converts an analog image into an N-dimensional vector by recording multiple images of a single scene using a single pixel detector. The pixel detector records images captured through a magnetic field modulation screen, which acts as a pseudo-random filter generating 0s and 1s. If such a signal is recorded by a single pixel detector, the signal can be decoded (decompressed) to reconstruct the original image, since the algorithm used to construct the aforementioned 0 and 1 filter is known.

[0024] The system proposed in this disclosure combines the principle of such compressed sensing with the extremely high sensitivity of an atomic magnetometer, thereby enabling the generation of a magnetic field image that represents changes in the magnetic field within the observation field of the magnetic field sensor. An example of the system is described in the following paragraphs. This system includes a magnetic field sensor, a pseudo-random image generation (pixel generation) algorithm, a magnetic field modulation screen that functions as a physical imaging filter, and an image reconstruction algorithm. The pseudo-random image generation algorithm and the image reconstruction algorithm are configured to apply compressed sensing techniques that reduce the number of measurement data points required for visualization of the magnetic field.

[0025] A magnetic field sensor with sufficient sensitivity contains isotopes 85There is an atomic magnetometer that measures the total magnetic field (scalar) at a given position by utilizing optical transitions between the energy levels of Rb. Such magnetometers can be implemented using magnetometers with various configurations and elements for alkali vapor. In addition, the systems and concepts described herein 85 do not require the use of Rb magnetometers, and any magnetometer can be used as long as it can accurately measure the magnetic field with very high sensitivity. 85 The configuration using an Rb magnetometer is an example for explaining the concept of the technology proposed by this disclosure.

[0026] 85 An Rb magnetometer pumps the atoms in the ground state by circularly polarized light to make them into excited-state atoms having a large Zeeman magnetic moment m F . This excitation is optically detected by a highly sensitive photodetector. When the atoms that have transitioned to the excited state re-emit light and return to the ground state, m F changes by ±1 or 0 according to the normal rules. Here, if the light polarization is adjusted so as to cause a transition where m F = +1, then by these processes, after the re-emission of light, the distribution of the atoms in the lower state having a large m F value increases, 85 and the intensity of the light passing through the Rb vapor decreases. Next, if the atoms in the lower state having a large m F value are depolarized using an RF magnetic field, then the RF frequency ωL (Larmor frequency) at this time changes according to the intensity of the ambient magnetic field as a whole. This is 85 optically observed by the increase in the light passing through the Rb vapor. The above is a description briefly explaining part of the operating principle of the sensor.

[0027] To implement the concept of compressed sensing, this specification proposes the following method: (a) limiting the sensor's field of view to a planar aperture; and (b) observing an "image" through a modulated screen with the sensor. The modulated screen generates values ​​of 0 and 1 in the sensor's field of view using a pseudo-random 0 and 1 pixel generation algorithm, which divides the planar aperture into pairs of pixels of each value. These values ​​are transmitted to a processor and recorded in a computer-readable non-temporary tangible storage medium. In one embodiment, rather than detecting each pixel individually, the entire screen is measured as a single image. Assuming a sufficiently fast sampling rate or that the image does not change rapidly, the value of each pixel can be derived by comparing each sample value and utilizing the fact that the pattern is known, thereby allowing the image to be reconstructed.

[0028] In one proposed embodiment, the sensor element is a small (e.g., a few millimeters in size) sealed cavity provided in an atomic magnetometer. Sensor elements in atomic magnetometers are often housed in components several centimeters in size, which are larger than the element itself, and these components often also house several electronic components necessary for driving the sensor element. To achieve the above objective (a), the sensor element is placed inside a tube made of magnetic shielding material. The field of view of the magnetometer is defined by the geometry of the magnetic shielding tube, such as the size and shape of the opening at one end of the tube.

[0029] Figure 1 is a three-dimensional diagram showing a magnetic field sensor 10 in one embodiment. The magnetic field sensor 10 includes a scalar magnetometer 12 (hereinafter referred to as "magnetometer 12") placed inside a tube 16 made of a magnetic shielding material. The tube 16 has a longitudinal axis and openings at both ends. According to one embodiment, the geometry of the tube 16 is cylindrical and has circular openings at both ends. When the magnetometer 12 is placed inside the tube 16, the observation field of the magnetometer 12 is defined by the geometry of the tube 16, for example, by the size and shape of the opening at the end of the tube 16 located between the magnetic field source being observed and the magnetometer 12.

[0030] According to one embodiment, the magnetometer 12 uses alkali metal vapor (for example, 85 This is an atomic magnetometer having a cell (within an outer housing) containing alkali metal vapor. This cell is made of glass or other transparent material. By heating the cell, the density of the alkali metal vapor can be brought to a predetermined density. A pump light beam is generated by a laser (not shown in Figure 1), and this pump light beam is passed through a linear polarizer to become linearly polarized. The pump light beam is usually expanded and collimated by one or more lenses, and then, by passing through a waveplate, the pump light beam is converted from linearly polarized to circularly polarized. This circularly polarized light is then guided to a cell filled with alkali metal vapor. The light beam that has passed through the vapor cell is incident on a photodetector (not shown in Figure 1), and the incident photons are converted to obtain an electrical signal indicating a measurement of the amount of light that has passed through the cell. This electrical signal is output to a signal processor via an electrical cable 14. The amount of light collected by the photodetector is proportional to the spin polarization of the alkali atoms in a direction parallel to the optical path of the pump light that has passed through the vapor cell. Furthermore, this spin polarization is a function of the magnetic field near the vapor cell.

[0031] As mentioned earlier, the magnetometer 12 is placed inside a tube 16 made of a magnetic shielding material to limit the observation field. In one proposed embodiment, the sensor's magnetic shielding material is a high-permeability ferromagnetic metallic alloy, an example being Mu-METAL®, commercially available from Magnetic Shield Corp. in Bensenville, Illinois. For example, a nickel-iron soft ferromagnetic alloy is largely magnetized to a high level by an external magnetic field, but its remanent magnetization is very small; that is, when the magnetic field is removed, the magnetization field is also substantially demagnetized. Due to this property, high-permeability magnetic metallic alloys are often used for shielding magnetic fields. However, for shielding higher frequency magnetic fields, combinations of different materials such as ferrite are also used. There are several compositions of high-permeability magnetic metallic alloys. One example is a composition containing approximately 77% nickel, 16% iron, 5% copper, and 2% chromium or molybdenum. Another example is a composition containing approximately 80% nickel and 5% molybdenum, along with small amounts of various other elements such as silicon, with the remaining 12% to 15% being iron. In addition to these, various alloys with unique formulations are sold under their respective trade names. High-permeability magnetic metal alloys typically have a relative permeability of 80,000 to 100,000, compared to the permeability of ordinary steel, which is several thousand.

[0032] According to one embodiment, the sensor observes an "image" (i.e., a distribution of magnetic fields) through the screen, and transmits the instantaneously sampled data to a processor. The sensor observes the image through a set of pixels, which are either kept transparent or opaque according to an algorithm. Here, it may be difficult to set some pixels to a valid value sufficiently lower than the signal flowing through them, i.e., to generate zero-value (opaque) pixels. In compressed sensing for optical applications, such processing is easy, as the target image can be projected onto a digital micromirror array prior to being projected onto the sensor. The digital micromirror chip has a very large number of aluminum mirrors (millions) on a silicon substrate. Each mirror (tens of micrometers in size) can be individually addressed. Each mirror has two states (corresponding to 0 and 1). In this disclosure, the state in which the micromirror projects light (pixels) onto the sensor is defined as "1", and the state in which the micromirror does not project light onto the sensor is defined as "0".

[0033] In the case of magnetic fields, there are no methods equivalent to those used in optical applications, and this is particularly difficult when dealing with low-frequency magnetic fields, as the wavelengths of such electromagnetic waves can extend for miles. The technology of this disclosure solves this problem by providing a circuit board with magnetic field generating loops, each representing a pixel and measuring in millimeters. The size of these pixels can be changed according to the resolution and amplitude of the magnetic field targeted in each application, as well as the integrity of the image. In this example, the pixels are composed of metal loops that generate a magnetic field when an electric current flows through them.

[0034] Figure 2 shows a pixel loop 20 made of a magnetic field generating material. In one embodiment, a magnetic field modulation screen is formed by providing a plurality of pixel loops 20 on a printed circuit board. In this example, the pixel loop 20 is circular with radius r. In other embodiments, the pixel loop may have a shape other than circular (e.g., square or circular). Current I is supplied to the pixel loop 20 via current supply lines 22 and 24. Since the current supply lines 22 and 24 are provided parallel to and in contact with each other, the magnetic fields generated by the currents flowing in opposite directions through each supply line cancel each other out due to the proximity of the supply lines. This is a similar principle to how the magnetic fields of two wires that are conductors are canceled out in a twisted configuration. As mentioned earlier, in order to generate pixels that represent the essence of an image or signal (which is equivalent to projecting the pixels of a micromirror onto a sensor), it is necessary to prevent current from flowing through these particular loops. To generate pixels that correspond to a value of 0 or a value close to 0, a signal of a given current that generates a magnetic field at the pixel is supplied to each individually addressable pixel loop 20. This current signal is a type of "white noise," meaning it is essentially zero when averaged over time. It should be noted that the characteristic frequency of the white noise current injected into the pixel loop should be significantly higher than the sensor's frequency response, so that the sensor detects only the time-averaged white noise. The magnitude of the injected current is adjusted so that the magnitude of the magnetic field induced in the pixel loop 20 is equal to the magnitude of the magnetic field the sensor is trying to detect. This current magnitude can be set if the signal to be detected is known in advance, or, if the target magnetic field fluctuates over time, it can be set by measuring the total magnetic field detected by the sensor. Furthermore, if it is necessary to minimize the heat generated in the control circuit, it is possible to reduce the duty cycle of the white noise generated in the pixel loop while ensuring satisfactory results. On the other hand, the refresh rate (compressed sensing) should be set to a frequency detectable by the sensor element.

[0035] Figure 3A is a three-dimensional diagram showing a magnetic field sensor 10 of the type shown in Figure 1 positioned relative to a magnetic field modulation screen 2. The magnetic field modulation screen 2 includes a stack of multiple printed circuit boards (PCBs), and this stack is positioned within the observation field of view of the sensor 10. In the example shown in Figure 3A, the magnetic field modulation screen 2 includes four PCBs 4a to 4d, which are superimposed on each other to form a planar aperture. Each PCB 4a to 4d has an array of pixel loops printed on the substrate (not shown in Figure 3A), and these arrays are offset from each other, as will be explained in detail with reference to Figures 4A to 4C. The rectangle formed by the overlapping of the four substrates is used as the imaging plane (pixel array) for the sensor.

[0036] Figure 3B is a side view showing a magnetic field sensing system 6 including a magnetic field sensor 10, with a portion of it shown in cross-section. The sensor is supported by a shielding support frame 8 in a predetermined positional relationship with respect to the magnetic field modulation screen 2. The support frame 8 can be configured according to the application, as long as it can appropriately position the pixel plane in front of the sensor on the measurement direction side. In the configuration illustrated in Figure 3B, the support frame 8 has a flange 18a that holds a tube 16 made of magnetic shielding material, and a flange 18b to which the magnetic field modulation screen 2 is attached. The support frame 8 is configured to be located in the observation field of the magnetic field sensor 10 with the array of pixel loops of the magnetic field modulation screen 2 overlapping and offset from each other. The support frame 8 is made of a rigid or semi-rigid material. The surface of the support frame 8 (the surface that partially covers the magnetometer 12) may be coated with magnetic shielding material.

[0037] By placing the sensor inside a tube made of a high-permeability magnetic metal alloy, the noise floor of the sensor can be significantly reduced (by about two orders of magnitude), and the observation field of view can be somewhat limited. However, a high-permeability magnetic metal alloy alone is not sufficient to achieve the objective of creating a detectable configuration that divides the imaging plane into individual pixels to generate an image representing the distribution of the magnetic field. To achieve this objective, the magnetic field sensor 10 is inserted into a support frame 8, with one surface of the support frame facing the sensor and all other surfaces covered with a mu-metal layer.

[0038] Next, a method for generating pixels on the imaging plane for the purpose of visualizing magnetic fields will be described in detail. To convert the aperture into a set of pixels, the system proposed in this disclosure does not detect each pixel individually, but measures the entire screen as a single image at a given sampling rate, and then compares each measured sample to detect a signal unique to any pixel. If the algorithm and loop cycle pattern are known, the value of each pixel can be derived, making it possible to reconstruct the image. According to the method proposed in this disclosure, scalability and cost reduction can be expected. In this method, individual pixels can be reconstructed using a single sensor by associating them with a known pseudo-random pattern determined by a compressed sensing algorithm. Each pixel constitutes a part of the whole image. In this system, the magnetic field is "nulled" (effectively generating a digital value of 0) at various pixel positions, allowing the signal to pass through this pixel (effectively corresponding to a digital value of 1). This makes it possible to rasterize the pixel array and generate an image.

[0039] As explained earlier with reference to Figure 2, each pixel loop 20 is composed of a loop-shaped wire, and these are arranged to form an array. When current flows through a pixel loop 20, a magnetic field is generated depending on the size of the loop, the number of turns, and the magnitude of the current. In this case, no current flows through the pixel loops that represent the essence of the image (signal). If the magnetic field changes rapidly, a secondary magnetic field may be generated in the uncurrented loops (Lenz's Law). However, such signals are expected to be small. This is because the object of imaging is a slow-changing magnetic field, and the magnitude of the secondary signal will depend on the derivative of the magnetic flux in a given loop. Nevertheless, this provides motivation to provide a method for nulling the pixel loops 20 to a state where the amplitude is almost zero.

[0040] The system proposed in this disclosure is configured to generate a magnetic field in individually addressable pixel loops 20 by supplying current to each loop. The current supplied to a given loop is a type of "white noise" whose time average is essentially zero. The idea is that if the amplitude of the signal input to the pixel loop (from the observed source) is of the same order as the amplitude of the white noise present in the pixel loop, the observed signal will be masked by the white noise. This can be experimentally confirmed by measuring the total magnetic field through an aperture when no power is supplied to any loop. Another point to note is that the characteristic frequency of the white noise current injected into the pixel loop should be sufficiently higher than the frequency response of the sensor itself. This allows the sensor to detect only the time average of the white noise (null) at the location of the energized pixel loop. This process makes it possible to generate a composite image by nulling the signals in all loops and then rasterizing the individual pixels in the array to collect the signals from each loop. Given that the object being imaged is a very weak magnetic field, an embodiment is possible in which multiple measurements are taken for a single loop and then averaged.

[0041] Figures 4A to 4C schematically show the loop pattern of a modulation screen according to one embodiment. In the loop pattern shown in Figure 4A, the pixel loops 20 provided on PCB 4 are spaced apart from each other by a distance d >> 2r (where d is the distance from center to center of the pixel loop and r is the radius of the loop), and are arranged to minimize interference between energized loops and neighboring loops. In the loop pattern shown in Figure 4B, the pixel loops 20 are arranged at a specific density set to improve resolution. By miniaturizing the pixel loops, it is possible to increase the density and thus the resolution. Figure 4C shows multiple arrays of pixel loops arranged on each of the stacked PCBs 4a to 4d. In this figure, pixel loops arranged in the type of pattern shown in Figure 4A are overlapped with a staggered arrangement, thereby achieving the desired density shown in Figure 4B. In Figure 4C, pixel loop 20a (shown as a solid circle) is printed on PCB 4a, pixel loop 20b (shown as a short dashed circle) is printed on PCB 4b, pixel loop 20c (shown as a long dashed circle) is printed on PCB 4c, and pixel loop 20d (shown as a dotted circle) is printed on PCB 4d. PCBs 4a to 4d constitute the stack shown in Figures 3A and 3B.

[0042] As shown in Figure 4A, the pixel loops 20 on each PCB 4 are spaced a predetermined distance apart from one another. Even if "white noise" is input to a given loop and a magnetic field is induced, the magnitude of such a magnetic field is not sufficient to generate spurious signals in neighboring loops. That is, the magnitude of the induced magnetic field is sufficiently small to be smaller than the amplitude of the "signal" in the "image" of the unenergized loop. Since the pixel loops on one plane are physically spaced apart, a very sparse pixel array is formed, resulting in a very low-resolution image. However, by stacking multiple circuit boards on top of each other, the pixel loops on these circuit boards are arranged to be adjacent to each other. Therefore, even though the actual distance between pixel loops is increased, a high-density array can be formed when projected onto the sensor's aperture plane. Note that Figures 4A and 4B are merely conceptual examples. In the future, more complex modulation screens can be realized, and more electronic controls can be incorporated.

[0043] The magnetic field induced in a pixel loop by the current supplied to it is a dipole magnetic field symmetric with respect to the axis along the normal at the center of the loop. This magnetic field is prone to coupling with the magnetic field of an adjacent loop if the distance d (shown in Figure 4A) between the selected loop and the adjacent loop is insufficient. The dipole magnetic field in the loop is 1 / d 3 The magnetic field attenuates. In the applications proposed in this disclosure, the distance d is operationally determined such that the magnitude of the magnetic field induced in an adjacent unenergized loop is approximately 1% of the magnetic field induced in an energized loop. In effect, if the radius of each loop is r, the distance is 4.2r. This operational determination can be modified in other embodiments.

[0044] In the proposed embodiment, the pixel loop 20 is formed using copper traces on the PCB. To satisfy the requirement of reducing the magnetic field buffer to about 1%, the loop arrangement shown in Figure 4A is necessary, but such a loop arrangement severely limits the image resolution. Ideally, a loop configuration in which the loops are closely adjacent to each other is desirable, as shown in Figure 4B. Therefore, to satisfy both objectives, four PCBs with pixel loops 4a to 4b are superimposed, as shown in Figure 4C, and these pixel loops are arranged offset from each other.

[0045] Furthermore, since the wires supplying current to the loop also generate a magnetic field, the current supply wires 22 and 24 (shown in Figure 2) must be positioned so that they substantially overlap (on different PCB layers). This arrangement cancels out the magnetic fields induced by the input and output currents, respectively.

[0046] Figure 5 is a block diagram showing the components of a magnetic field visualization system 30 for visualizing a magnetic field according to one embodiment. The magnetic field visualization system 30 includes a processing node 32 that has the function of bidirectional communication with a magnetic field sensor 10. The processing node 32 sends control signals to the magnetic field sensor 10 and receives sensor data from the magnetic field sensor 10. If the magnetic field sensor 10 includes an atomic magnetometer, the control signal is a signal that controls the driving of the laser, and the sensor data is passed to the processor. The magnetic field sensor 10 measures the magnetic field 28 to be imaged along axis A, which is shown by a dashed line in Figure 5. More specifically, the magnetic field sensor 10 individually measures the magnetic field of the pixel loops selected by the processing node 32 from among the pixel loops of the magnetic field modulation screen. The processing node 32 is configured to directly inject current as noise into the selected pixel loops 20, which is done by directly controlling the on / off state of switches (not shown in Figure 5, see switch array 46 in Figure 7). When the processing node 32 receives sensor data from the magnetic field sensor 10, it processes this sensor data and sends it to the display device 26 in a format suitable for visualizing the detected magnetic field. More specifically, the pixels on the display device 26 screen perform multi-color display, gray shading, symbol display, and other displays, all of which are functions of the magnetic flux associated with each sequentially de-energized pixel loop in the pixel loop of the magnetic field modulation screen. The processing node 32 may also be a computer with multiple processors to perform different tasks such as sensor control, data processing, display control, and switch control. Specifically, for example, one processor is configured to run a magnetic field visualization application recorded in executable computer code on a computer-readable non-temporary tangible storage medium. This application includes an image reconstruction algorithm configured to reconstruct an image representing the distribution of the detected magnetic field using compressed sensing techniques.

[0047] Figure 6 is a block diagram showing the components of a magnetic field visualization system 40 for visualizing a magnetic field according to an alternative embodiment. The magnetic field visualization system 40 includes a programmable control unit (PCU) 44 that controls a switch array 46 (see Figure 7) according to control signals received from a processing node 32. The PCU 44 is, for example, a microcontroller board. Based on which pixel loops are not nulled, a current source 42 supplies current (e.g., white noise) to the other pixel loops. The noise current supplied to these pixel loops functions to substantially "block" the magnetic field 28 when the control signals activate the switches in the switch array 46 and current flows to the corresponding pixel loops. The processing node 32 (e.g., a personal computer) processes the sensor data received from the magnetic field sensor 10 and sends this processed data to the display device 26 in a format suitable for visualizing the detected magnetic field.

[0048] Note that the individually addressable and controllable magnetic field generating elements included in the magnetic field modulation screen 2 do not necessarily have to be loops. For example, the individually addressable and controllable magnetic field generating elements included in the magnetic field modulation screen 2 may be wires that are not loop-shaped, or other members that function as inductors capable of inducing a magnetic field. Figure 7 is a block diagram showing the members of the aperture subsystem. This subsystem controls which magnetic field generating elements 34 in the magnetic field modulation screen 2 are supplied with noise current during magnetic field modulation. The magnetic field generating elements 34 (e.g., pixel loops) are connected to their respective switches in the switch array 46. The state of these switches is controlled by the PCU 44 according to control signals sent from the processing node 32.

[0049] The experimental system has an aperture containing 384 pixel loops arranged with an element pitch of 0.248 inches, for example. In the experiment, the aperture subsystem receives control signals from the PCU44 and executes the received control signals to arbitrarily set the state of a set of pixels in the pixel array. A single-winding current loop with a radius of 3 mm was selected as the element from which the magnetic field is induced. In other embodiments, this element may include multiple windings. In these loops (actually located on separate PCBs), the effective distance between adjacent loops is 0.3 mm. This selection is reasonable because the strength of the magnetic field induced in the loop by a drive current of 1 mA is a maximum of 5.2 × 10⁻¹⁶. -7 This has been confirmed by numerical simulation results indicating that it is a Tesla.

[0050] The pixel loop array proposed in this disclosure is similar to a digital micromirror device. Figure 8A shows an image generated by projecting light onto a digital micromirror device containing an array of N mirrors. Each mirror corresponds to one pixel in the image, and by individually controlling each mirror and performing a raster scan, N measurements are obtained, i.e., one measurement for each mirror in the device. Figure 8B shows an image reconstructed using compressed sensing by randomly projecting light (filtering) onto only some, but not all, of the N mirrors in the digital micromirror device. The original image shown in Figure 8A is accurately approximated without taking N measurements. The original image consists of 56 × 56 pixels, i.e., a total of 3,136 pixels. In contrast, the image on the right was generated using only 800 measurements. Depending on the detail of the image to be visualized, i.e., depending on the degree of sparsity in a particular basis, the number of measurements required can easily be reduced by several orders of magnitude.

[0051] Compressed sensing is based on the property that most images are sparse in some domain. This means that an image can be represented by a set of K basis vectors, where K is a number much smaller than the number of pixels in the image. In compressed sensing, it is possible to directly obtain the compressed image, without the overhead of first acquiring a large number of measurements or encoding the positions of the sparse coefficients, which is common in other image compression techniques. The key steps are: (a) designing a stable measurement matrix such that when an N-dimensional compressible image is reduced to K dimensions (where K < N), important information in the original image is not lost; and (b) executing a restoration algorithm to restore the image from the K measurements. One promising application of this technology is to use compressed sensing in a method for imaging the distribution of a magnetic field, which is a technique for reducing the number of measurements required to obtain an image.

[0052] As mentioned in the above example, one method for designing the K×N measurement matrix Θ involves generating a pseudo-random sequence of 0s and 1s to determine each row of the matrix. The column components in a given row of this matrix correspond to one pixel of the image. Thus, each measurement can be regarded as a linear combination of the pixels of the original image. The K-dimensional measurement vector y is given by ΘΨx = Θs = y. Here, s represents the image that is sparse under the basis Ψ, and the image is regarded as sparse.

[0053] The restoration algorithm uses y, Θ, and Ψ, and restores the N-dimensional image x by solving the above linear system. Since this linear system is underdetermined, there are infinitely many s′ that satisfy Θs = y.

[0054] According to classical methods, an underdetermined linear system can be solved by finding the solution vector with the minimum l2 norm. Here, the l norm of the vector pIt is given by the following formula.

number

[0055] However, it is rare to obtain a sparse solution by minimizing the norm l2. It has been proven that by minimizing the norm l1, it is possible to accurately reconstruct a sparse signal and accurately approximate a compressible signal with high probability by using only M≧cK log(N / K) independent and identically distributed Gaussian random measurements.

[0056] The components described earlier with reference to Figures 3A, 4C, and 5-8 can be used in the magnetic field visualization method described herein. Figure 9 is a block diagram showing the steps of a magnetic field visualization method 100 according to one embodiment. First, a magnetic field modulation screen including an array of individually addressable and controllable magnetic field generating elements is positioned relative to a magnetometer in a predetermined position (step 102). Next, the magnetometer and the magnetic field modulation screen are positioned in the magnetic field (step 104). Once other system settings are complete, the magnetometer is operated to continuously detect the magnetic field for a certain period of time and output a signal including magnetic field measurement data (step 106). In addition, the magnetic field modulation screen is operated so that no different magnetic field generating element receives current from the current source at consecutive instantaneous intervals in chronological order during that period (step 108). The magnetic field measurement data is processed using an image reconstruction algorithm incorporating compressed sensing to reconstruct an image representing the distribution of the magnetic field (step 110). The image representing the detected magnetic field distribution is displayed on the screen of the display device (step 112).

[0057] While systems and methods for visualizing low-level magnetic fields have been described with reference to various embodiments, those skilled in the art will understand that various modifications are possible without departing the scope of this disclosure, and that their components can be replaced with equivalents. In addition, various modifications are possible without departing the scope of this disclosure to adapt the teachings herein to individual circumstances. Therefore, the claims of this disclosure should not be limited to the specific embodiments described.

[0058] The embodiments described above use one or more computer systems. The term “computer system” as used in the claims may include a single processor or computing device, or it may include multiple processors or computing devices that can communicate wirelessly or via wired connections. A processor or computing device may include, for example, any one of the following: a processor, a computer, a controller, a central processing unit, a microcontroller, a reduced instruction set computer processor, an application-specific integrated circuit, a programmable logic circuit, a field-programmable gate array, a digital signal processor, and / or any other circuit or processing unit capable of executing the instructions described herein.

[0059] The methods described herein may be coded into executable instructions and embodied in, for example, computer-readable non-temporary tangible storage media such as storage devices and / or memory devices. When such instructions are executed by a processor or computer device, they cause the device to perform at least a portion of the methods described herein.

[0060] In interpreting claims relating to methods described below, unless there is wording that specifies or indicates that all or some of the steps described in the claim are described in a particular order, it is not required that these steps be performed in alphabetical order or in the order they are described (if the claims are alphabetically listed, these are simply for reference to the steps described earlier). Furthermore, in interpreting claims relating to processes, unless there is wording that explicitly indicates that any part of two or more steps is performed simultaneously or alternately, such interpretation is not excluded.

Claims

1. A magnetic field sensing system comprising a magnetometer, a magnetic field modulation screen, and a support frame that supports the magnetometer and the magnetic field modulation screen in a predetermined positional relationship, wherein the support frame holds the magnetometer at one end and is attached to the magnetic field modulation screen at the other end opposite to the one end, thereby supporting the magnetic field modulation screen at a predetermined distance in front of the magnetometer.

2. The magnetic field sensing system according to claim 1, further comprising a magnetic shield.

3. The magnetic field sensing system according to claim 1 or 2, wherein the magnetic field modulation screen includes an array of magnetic field generating elements, each of which can be individually addressed and controlled.

4. The magnetic field sensing system according to claim 3, further comprising a current source, wherein the magnetic field modulation screen further comprises an array of switches arranged to switchably connect each of the magnetic field generating elements to the current source.

5. The magnetic field sensing system according to claim 4, wherein each magnetic field generating element includes a loop.

6. The magnetic field sensing system according to claim 5, wherein each magnetic field generating element further includes first and second current supply lines, the first and second current supply lines being electrically connected to the loop and arranged parallel to each other.

7. The magnetic field sensing system according to claim 1 or 2, wherein the magnetic field modulation screen includes a plurality of printed circuit boards, each of which a set of magnetic field generating elements is printed, and the printed circuit boards are arranged in a superimposed state such that the sets of magnetic field generating elements are offset from each other.

8. The magnetic field sensing system according to claim 4, further comprising a controller connected to the array of switches and configured to control the switches to supply current from the current source to all but one of the magnetic field generating elements.

9. The magnetic field sensing system according to claim 8, further comprising a processing node connected to the controller and configured to send a control signal to the controller to control the magnetic field modulation screen such that one of the magnetic field generating elements does not receive current from the current source in consecutive states in time.

10. The magnetic field sensing system according to any one of claims 4 to 6, 8, or 9, further comprising a processing node connected to the array of switches and configured to control the switches such that one of the magnetic field generating elements does not receive current from the current source in consecutive states in time.

11. A magnetic field sensing system according to any one of claims 1 to 10, further comprising a tube made of a magnetic shielding material and having an opening at an axis and one end, wherein the magnetometer is disposed inside the tube, and a portion of the observation field of the magnetic field modulation screen is defined by the opening in the tube that is closest to the magnetic field modulation screen.

12. A current source and A display device configured to display an image representing the distribution of a magnetic field, A magnetometer configured to detect a magnetic field over a given period and output a signal containing magnetic field measurement data, The array of magnetic field generating elements is positioned in a fixed position relative to the magnetometer. and a magnetic field modulation screen including an array of switches connected to each of the magnetic field generating elements and the current source, A magnetic field visualization system comprising: a processing node connected to the array of switches and configured to control the switches such that one of the magnetic field generating elements does not receive current from the current source in consecutive states in time among the states at each instant during a given period, wherein the processing node is further configured to process the magnetic field measurement data using a restoration algorithm, restore an image representing the distribution of the magnetic field using compressed sensing technology, and then send the processed magnetic field measurement data to the display device to display the image representing the detected magnetic field distribution.

13. The magnetic field modulation screen includes a predetermined number of magnetic field generating elements, The magnetic field visualization system according to claim 12, wherein the processing node is further configured to control the array of switches such that, in a sequence of states in chronological order, the number of magnetic field generating elements that do not receive current is less than the predetermined number of magnetic field generating elements.

14. The magnetic field visualization system according to claim 12 or 13, further comprising a tube made of a magnetic shielding material and having an opening at an axis and one end, wherein the magnetometer is disposed inside the tube, and a portion of the observation field of the magnetic field modulation screen is defined by the opening in the tube that is closest to the magnetic field modulation screen.

15. The magnetic field visualization system according to claim 14, wherein the magnetometer is an atomic magnetometer.

16. The magnetic field visualization system according to claim 12, wherein the magnetic field modulation screen includes a plurality of printed circuit boards, each of which a set of magnetic field generating elements is printed, and the printed circuit boards are arranged in a superimposed state such that the sets of magnetic field generating elements are offset from each other.

17. A magnetic field modulation screen used in combination with a magnetometer, wherein the magnetic field modulation screen is supported at a predetermined distance in front of the magnetometer via a support frame, the support frame holds the magnetometer at one end and is attached to the magnetic field modulation screen at the other end opposite to the one end, and the magnetic field modulation screen is, A first printed circuit board on which a set of first magnetic field generating elements arranged at intervals is printed, A second printed circuit board on which a set of second magnetic field generating elements arranged at intervals is printed, A magnetic field modulation screen in which the first and second printed circuit boards are arranged in a superimposed state such that the pairs of the first and second magnetic field generating elements are offset from each other.

18. A third printed circuit board on which a set of third magnetic field generating elements arranged at intervals is printed, The present invention further comprises a fourth printed circuit board on which a set of fourth magnetic field generating elements arranged at intervals is printed, The magnetic field modulation screen according to claim 17, wherein the first to fourth printed circuit boards are arranged in a superimposed state such that the sets of the first to fourth magnetic field generating elements are offset from each other.

19. Each magnetic field generating element includes a loop, as described in claim 18, for the magnetic field modulation screen.

20. A method for visualizing magnetic fields, A magnetic field modulation screen, which includes an array of individually addressable and controllable magnetic field generating elements, is positioned at a predetermined location relative to the magnetometer. The magnetometer and the magnetic field modulation screen are positioned within the magnetic field. The magnetometer is operated to continuously detect the magnetic field for a given period of time and to output a signal including magnetic field measurement data. The magnetic field modulation screen is operated so that, at consecutive instants in chronological order during the given period, one of the magnetic field generating elements does not receive current from the current source. and, The magnetic field measurement data is processed using an image reconstruction algorithm incorporating compressed sensing technology to reconstruct an image representing the distribution of the magnetic field. A method comprising displaying the image representing the distribution of the detected magnetic field.

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