JP2008249583A - Magnetic object detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic object detector capable of detecting and announcing even a small magnetic object, using a magnetic sensor for detecting the change of magnetic flux density caused by the invasion of a magnetic object with no saturation even under a strong magnetic flux density environment, as an alarming means for the invasion of the magnetic object into a room where a strong magnet of a MRI diagnosis instrument and the like are placed. <P>SOLUTION: The magnetic object detector comprises: a magnetic sensor detecting magnetic flux density; a threshold; a comparing function comparing the output of the magnetic sensor with the threshold; and an announcing function announcing the result of the comparing function. With setting the solid angle between the detecting axis and the perpendicular axis or the horizontal axis in the magnetic sensor within 15°, direct current magnetic flux density applied to the detecting axis is reduced, and using a high resolution magnetic sensor having narrow input tolerance region, a small change in magnetic flux density caused by the invasion of a small magnetic object is detectable. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a magnetic material detector, and more particularly to a magnetic material detector that can detect a small magnetic material even in an environment with a strong magnetic flux density.

  As an apparatus indispensable for recent medical diagnosis, there is an MRI (Magnetic Resonance Imaging) diagnostic apparatus. This apparatus is a method of imaging information inside a living body using a nuclear magnetic resonance phenomenon, and requires a powerful magnet. In recent years, MRI diagnostic apparatuses using extremely powerful magnets such as 1.5T (Tesla) and 3T have been developed to improve resolution and processing speed. Such a strong magnet generally uses a superconducting magnet.

  As the magnet becomes stronger, the force that attracts the magnetic substance naturally becomes stronger. When a tool such as a driver made of a magnetic material such as iron or a medical instrument such as an oxygen cylinder is brought into the diagnostic room, these are sucked and stuck to the apparatus. Tools such as a screwdriver attached with a strong force cannot be removed with human power. To remove them, the superconducting magnet must be stopped once, but it takes several days to stop and restart, and during that time it cannot be diagnosed. In addition, a large amount of expenses are required. For this reason, once a superconducting magnet has been started, it is usually operated at all times and does not stop unless there is a special reason.

  In addition, if a magnetic tool or medical instrument is accidentally brought into the diagnostic room, there is a risk of injury due to the collision of the magnetic material with the examinee or inspection technician. Therefore, never bring a magnetic substance into the diagnostic room. When transporting the subject to the diagnosis room, it is necessary to use a dedicated stretcher made of a non-magnetic material such as aluminum.

  Laboratory technicians and related nurses are aware of the dangers of magnets and basically do not bring in magnetic materials. There is no problem with the subject because the laboratory technician or the like checks the presence or absence of a magnetic body in the body such as a pacemaker in advance. However, laboratory technicians do not always have complete knowledge. For example, since ferrite is not a metal, it does not appear to attach to a magnet, but is a magnetic material. There is a case where an object that is not considered to be such a magnetic material is brought in. During the examination, the subject may be in an emergency state, and the nurse may bring in an oxygen cylinder or the like. Some nurses are not familiar with MRI diagnostic equipment. Workers who don't know anything about magnets may bring stepladders and tools to maintain the examination room.

  In order to prevent these dangers, it is necessary to install a magnetic substance detector that detects and alerts the magnetic substance at the door of the diagnostic room of the MRI diagnostic apparatus.

  There is a metal detector as an apparatus similar to a magnetic substance detector. This device detects eddy currents flowing in metal, and also detects non-magnetic metal such as aluminum. Conversely, a magnetic material such as ferrite that does not pass current cannot be detected. Therefore, it does not make sense to use a metal detector at the diagnostic room door of the MRI diagnostic apparatus.

  As a prior art related to such magnetic body detection, Japanese Patent Laid-Open No. 5-52962 (Patent Document 1) uses a Helmholtz coil in two directions to alternately generate a uniform magnetic field, and the magnetic flux density B generated by the penetration of the magnetic body. There has been proposed an invention in which a change is detected by a detection coil in two directions and the electromotive force is amplified to detect the change.

JP-A-5-52962

  However, although the winding coil is used as the magnetic sensor in the invention described in Patent Document 1, the winding coil has a problem that the resolution is low and a small magnetic material cannot be detected.

  That is, the magnetic flux density B changes as a magnetic material enters a certain magnetic field. The change amount delta B of the magnetic flux density must be detected using a magnetic sensor, but in some cases, the value of the change amount delta B is extremely small. The amount of change Delta B in the magnetic flux density is basically proportional to the size of the magnetic material and inversely proportional to the square of the distance to the magnetic sensor. In addition, it is affected by the shape and orientation of the magnetic material. Accordingly, the change amount delta B cannot be simply obtained, but in some cases, only a very small change of about the change amount delta B = 10 nT may occur. A highly sensitive magnetic sensor is required to detect minute changes in magnetic flux density.

In recent years, various high-sensitivity magnetic sensors have been developed, and there is also a sensor called a superconducting quantum interference device having a sensitivity of 10 fT. If such a high sensitivity magnetic sensor is used, it is very easy to detect the change amount delta B = 10 nT.
However, not limited to magnetic sensors, all sensors have a proportional relationship between resolution and dynamic range, which means an allowable input range that can be measured. An object under resolution cannot be measured. In addition, the object to be measured exceeding the dynamic range is saturated and cannot be measured.

  It is recommended that the scalar quantity of the DC magnetic flux density B leaking outside the MRI diagnosis room is 500 μT or less. This is a magnetic flux density that is ten times or more stronger than 30 μT of the average geomagnetic DC magnetic flux density B.

  It is extremely difficult to detect the variation delta B = 10 nT in an environment where the DC magnetic flux density B = 500 μT. For example, it is like detecting a 0.1 mm change in a 5 m long bar. If a tape measure with a large dynamic range is used, a length of 5 m can be measured, but it cannot be measured in units of 0.1 mm. If calipers with high resolution are used, measurement can be performed in units of 0.1 mm, but the length of 5 m is saturated and cannot be measured.

  A magnetic sensor using a winding coil having a large dynamic range does not saturate, but has a low resolution and cannot detect a minute change amount of change amount delta B = 10 nT. A high-sensitivity superconducting quantum interference device can detect a minute amount of change, but is saturated due to a strong DC magnetic flux density of B = 500 μT, and cannot perform a detection operation.

  This problem can be easily solved with a magnetic sensor having a large dynamic range and a high resolution. However, it is extremely difficult to develop practically, and even if it is developed, it is extremely expensive and cannot be applied. There are only two methods: a method of detecting a small magnetic flux density change amount Delta B using a magnetic sensor with a large dynamic range and a method of using a high resolution magnetic sensor so as not to saturate. When a magnetic sensor with a low resolution is used, signals below the resolution cannot be detected. Even if a high-performance amplifier is used, the signal and noise only increase at the same amplification factor, so there is no effect at all. Therefore, there is a need for a technique for using a high-sensitivity magnetic sensor with a low dynamic range so as not to saturate in an environment of strong DC magnetic flux density.

  In the invention described in Patent Document 1, magnetic flux density is generated alternately by two-direction Helmholtz coils. This change in magnetic flux density affects the MRI diagnostic apparatus, and correct diagnosis cannot be performed. Further, if the magnetic field is 30 μT, the magnetic field generated by itself can be controlled. However, it is difficult to control the magnetic field within a strong magnetic field of 500 μT, and the magnetic substance itself cannot be detected.

  In order to prevent magnetic mutual interference, there is no problem if the magnetic substance detector is installed at a position more than a dozen meters away from the diagnostic room. However, it cannot be installed unless it has a particularly large waiting room.

  Furthermore, in the invention described in Patent Document 1, since it is necessary to pass the coil through the floor surface, a step is generated. Some subjects are physically handicapped, which not only obstructs the passage of canes and wheelchairs, but also creates the risk of falling.

  In order to construct a Helmholtz coil and a detection coil, it is necessary to wind an enameled wire around a bobbin that is large enough for a person to pass through, and costs and assembly steps are required. The entire device is also large, and installation work is costly. In addition, a great amount of power is consumed to generate a magnetic field in the Helmholtz coil.

  Noise enters the detection coil. It is essentially impossible to distinguish whether the signal generated in the detection coil is due to magnetic material or noise. This noise causes malfunction. If the threshold for determining the presence or absence of a magnetic material is set high, the frequency of malfunctions decreases. However, since the detection sensitivity decreases, it becomes impossible to detect a small magnetic material. Moreover, the magnitude of noise varies depending on the installation environment. Even if the system operates without any problems under certain installation environments, malfunctions may occur frequently under other installation environments.

In view of the above problems, an object of the present invention is to detect a minute change in magnetic flux density by using a high-sensitivity magnetic sensor without saturation in an environment of a strong DC magnetic flux density B in the vicinity of an examination room. To do.
It is another object of the present invention to realize a device that can withstand noise at low cost, low power consumption, without generating a magnetic field that affects MRI diagnosis from the device, and without providing a step on the floor.

  In order to solve the above problems, the present invention has a magnetic sensor for detecting a magnetic flux density, a threshold value, a comparison function for comparing the output of the magnetic sensor with the threshold value, and a function for notifying a result of the comparison function. In the magnetic detector, a configuration is adopted in which the solid angle between the detection axis of the magnetic sensor and the vertical or horizontal axis is within 15 °.

  Moreover, this invention employ | adopts the structure which installs each magnetic sensor in a different position while providing the said magnetic sensor in multiple in the said magnetic body detector.

  The magnetic flux density B is a vector. There is a magnetic flux density B of scalar quantity in the vector axis direction, but the magnetic flux density B in the direction orthogonal to the vector axis is zero. On the other hand, the magnetic sensor also has a vector axis called a detection axis. Although it has the maximum sensitivity in the detection axis direction, the magnetic flux density B in the direction orthogonal to the detection axis is not detected.

  Therefore, if the detection axis of the magnetic sensor is set in a direction perpendicular to the vector axis of the magnetic flux density B generated by the superconducting magnet, the DC magnetic flux density B applied to the detection axis of the magnetic sensor is zero. Therefore, even if a highly sensitive magnetic sensor having only a small dynamic range is used, it is not saturated. When a magnetic material enters the magnetic field, the vector of the magnetic flux density B changes, and the magnetic flux density changes in the direction of the detection axis of the magnetic sensor. If this change amount delta B is detected, it is possible to detect the intrusion of the magnetic material.

  However, the direction of the magnetic flux density B generated by the superconducting magnet differs depending on the location and is not a fixed direction. The size of the diagnostic room and the position of the door to which the magnetic sensor is attached vary. The vector axis direction of the magnetic flux density B generated at the position of the magnetic sensor is not only the positional relationship between the superconducting magnet and the magnetic sensor, the distance, and the strength of the generated magnetic field, but also magnetic materials such as geomagnetism and reinforcing bars and equipment inside the building. It is determined by. This can be dealt with by examining the vector axis of the DC magnetic flux density B using a measuring instrument and adjusting the detection axis of the magnetic sensor to be orthogonal. However, it requires specialized knowledge and advanced technology, and cannot be installed by ordinary workers.

  Here, the environment where the MRI diagnostic apparatus is installed will be considered. In a general MRI diagnostic apparatus, the central axis of a cylindrical superconducting magnet is installed in the horizontal direction. The central axis of the magnet is about 1 m from the floor, and there is not much difference between manufacturers and models. There is a certain distance between the horizontal direction and the depth direction of the diagnostic room in order to reduce the leakage magnetic flux density B. It is 6-15m depending on the installation location. On the other hand, the size of the door is not much different and is about 2m in height and about 1m in width. That is, the center axis of the magnet is at the approximate center of the height of the door.

  When a cylindrical magnet is placed horizontally, a DC magnetic flux density B is generated in the horizontal plane direction of the central axis, but the vertical DC magnetic flux density Bz is zero. Therefore, if a magnetic sensor is installed on the same horizontal plane as the central axis of the magnet and the detection axis is set in the vertical direction, the DC magnetic flux density B applied in the detection axis direction of the magnetic sensor is 0 and does not saturate.

  In the case where the position is away from the same horizontal plane as the central axis of the magnet, the DC magnetic flux density Bz is also generated in the vertical axis direction. However, the DC magnetic flux density Bz in the vertical direction is characterized by being smaller than the DC magnetic flux densities Bx and By in the horizontal direction. Even in a strong DC magnetic flux density environment where the scalar quantity B is 500 μT, the DC magnetic flux density applied in the direction of the detection axis of the magnetic sensor can be reduced. If the DC magnetic flux density Bz applied to the magnetic sensor is small, it is possible to use a high-sensitivity magnetic sensor having a high resolution but a low dynamic range.

  This utilizes the feature that the dimension of the diagnostic room is larger in the horizontal direction than in the vertical direction. In the case of the length of the above-mentioned bar, if a 5m long bar is viewed from an oblique direction, it can be viewed short, and even without using a caliper with high resolution, measurement can be performed without saturation. is there.

  Considering simply, the amount of change Delta B in the magnetic flux density is also reduced at the same ratio, and even if a high-resolution magnetic sensor is used, it cannot be detected in the end. However, the change amount delta B also has a direction, and the direction is not always the same as the DC magnetic flux density B. The scalar amount of the change amount delta B is determined by various conditions. If the direction of the change amount delta B is close to the direction of the detection axis of the magnetic sensor, it can be sufficiently detected.

  However, the direction of the change amount delta B and the direction of the detection axis of the magnetic sensor do not always coincide with each other. If they are orthogonal, they cannot be detected at all. Therefore, a plurality of magnetic sensors are provided and detected at different positions. The relative difference between the direction of the change amount delta B and the direction of the detection axis varies depending on the position of the magnetic sensor. Even if a certain magnetic sensor does not detect orthogonally at all, it is possible to detect the variation delta B with another magnetic sensor.

  As described above, the magnetic flux density variation delta B is inversely proportional to the square of the distance between the magnetic body and the magnetic sensor. Therefore, it is impossible to detect the entire door, which is a space through which the magnetic material passes, with a single sensor. By providing magnetic sensors on both the left and right sides of the door, a space of about 50 cm, which is half the width of the door of about 1 m, may be detected. In addition, it is uncertain which position of the magnetic body passes through the upper and lower positions of the door. Therefore, the space of the whole door can be detected by providing the magnetic sensor in the vertical direction.

  When magnetic sensors are provided on both upper and lower sides of the door, the vertical DC magnetic flux density Bz is applied more strongly at the upper and lower magnetic sensors away from the center axis plane of the magnet than the magnetic sensors near the center axis. Is done. Even when the detection axis of the magnetic sensor is strictly installed in the vertical direction, it is impossible to prevent a certain amount of DC magnetic flux density Bz from being applied.

It is also technically difficult to make the direction of the detection axis of the magnetic sensor strictly vertical. A certain amount of error will inevitably occur. When installed to the inclination of the solid angle theta _D rather than the detection axis of the magnetic sensor in the vertical direction, theta _D is greater if the detection axis direction of the magnetic flux density Bs increased, thereby approaching saturation levels.

Considering the above, it is not particularly significant to set the detection axis of the magnetic sensor strictly in the vertical direction. No problem be given some degree of tolerance to the solid angle theta _D.

If the DC magnetic flux density Bs in the detection axis direction is set to 30% or less of the DC magnetic flux density B, the solid angle θ _D between the detection axis and the vertical axis of the magnetic sensor is set to 15 ° or less as shown in the following formula 1. Just do it. Within this range, it becomes difficult to saturate.

In some open type MRI diagnostic apparatuses, the central axis of the magnet faces the vertical direction. In the case of such an apparatus, the same result can be obtained if the detection axis of the sensor is installed in the horizontal direction. In this case, the solid angle θ _D between the detection axis and the horizontal axis of the magnetic sensor may be set to 15 ° or less.

  In addition to superconducting quantum interference elements, various types of high-sensitivity magnetic sensors have been developed. There are various types such as an MR (magnetoresistive element) sensor, an MI (magnetic impedance element) sensor, and a fluxgate sensor. In the present invention, the type of the magnetic sensor is not specified, and the type is not specified if the sensor has a small dynamic range but a high sensitivity. None of the magnetic sensors described above require a coil for generating a magnetic field, does not affect the MRI diagnosis, and consumes little power. Moreover, since it is not necessary to wind the detection coil around the entire door, there is no need to provide a step on the floor. Since it is not necessary to create a large coil, not only can the number of assembly steps be reduced, but it can also be installed without using special measuring instruments or skills, thereby reducing the overall cost.

  Further, according to the present invention, in the magnetic substance detector, a conversion circuit that converts an analog output amount of the magnetic sensor into a digital amount is installed in the immediate vicinity of each magnetic sensor, and an arithmetic circuit that calculates the digital amount of the conversion circuit; In addition, it is also possible to adopt a configuration provided with a function of comparing the calculation result of the calculation circuit with the threshold value.

  Noise and power supply fluctuations are causes for misjudging the presence / absence of a magnetic material. Noise enters due to the mutual inductance M formed by the cable and the noise source. When the distance between the magnetic sensor and the determination unit is long, it must be connected by a long cable. However, since the long cable has a large mutual inductance M, noise is superimposed on the detection output of the magnetic sensor. As a result, the presence / absence of the magnetic substance is erroneously determined.

  If the magnetic sensor and the determination unit are brought close to each other, the cable is shortened, so that the mutual inductance M is reduced and noise can be reduced. However, it is impossible to shorten the cable, assuming that it is attached to the door. Therefore, it is effective to provide a conversion circuit for converting the detection output of the magnetic sensor, which is an analog value, into a digital value in the immediate vicinity of the magnetic sensor. If it is a digital value, even if the cable is long, it is not affected by noise if the transmission quality is ensured.

  Furthermore, the present invention can employ a configuration in which the same number of power supply circuits as the magnetic sensors are provided in the magnetic body detector, and the power supply circuits are installed in the immediate vicinity of each magnetic sensor and the conversion circuit.

  If there is a power supply fluctuation in the power supply supplied to the magnetic sensor, the output signal is affected by the fluctuation. When the cable is long, since there is a resistance component R, when the load current fluctuates, the power supply voltage fluctuates. Therefore, it is effective to provide a power supply circuit in the immediate vicinity of the magnetic sensor, and it is possible to stably supply power to the magnetic sensor even if the load current fluctuates.

  The present invention can also employ a configuration having a function of making the threshold variable in the magnetic substance detector.

  According to the magnetic substance detector according to the present invention, a highly sensitive magnetic sensor can be used even in an environment with a strong DC magnetic flux density B. It is possible to detect.

  It is also possible to realize a low noise, low power consumption, high noise resistance detector without generating a magnetic field that affects the MRI diagnosis from the apparatus and without providing a step on the floor.

  The magnetic substance detector according to the present invention is characterized by detecting a minute change Delta B in magnetic flux density without saturation by setting the detection axis of the magnetic sensor in the vertical direction or the horizontal direction. Embodiments of a magnetic substance detector according to the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic explanatory view showing a configuration aspect of a magnetic substance detector according to the present invention. FIG. 1 is applied when the central axis 5 of the magnet 4 is horizontal. Solid angle theta _D between the detection axis 2 and the vertical axis 3 of the magnetic sensor 1 is shown that is within 15 °. The central axis 5 of the magnet 4 is installed horizontally. In addition, the shape of the magnetic sensor 1 and the magnet 4 does not show a specific shape, but shows a function typically.

FIG. 2 is another schematic explanatory view showing the configuration of the magnetic substance detector according to the present invention. In FIG. 2, it is applied when the central axis 5 of the magnet 4 is vertical. It shows that the solid angle θ _D between the detection axis 2 of the magnetic sensor 1 and the horizontal axis 6 is within 15 °.

Next, step by step, the effectiveness of the configuration shown in FIGS. 1 and 2 will be described.
First, the principle of detecting the magnetic body 7 by the magnetic sensor 1 will be described. FIG. 3 shows a state where only the magnetic sensor 1 is installed in the magnetic field. A vector of the DC magnetic flux density B is schematically represented by a quadrangular pyramid. A DC magnetic flux density 8 having a scalar quantity Bt1 is applied to the magnetic sensor 1. The direction of the detection axis 2 and the DC magnetic flux density 8 has a solid angle θ1. What the magnetic sensor 1 detects is only the component in the direction of the detection axis 2, and this scalar quantity is Bs1. Bt1 and Bs1 are expressed by the following formula 2.

  Next, FIG. 4 shows a state in which the magnetic body 7 has entered the magnetic field. The DC magnetic flux density 8 changes to the DC magnetic flux density 9 by the magnetic body 7. The scalar quantity changes from Bt1 to Bt2, and the solid angle also changes from θ1 to θ2. As a result, the scalar quantity Bs2 of the direction component of the detection axis 2 of the magnetic sensor 1 changes as shown in the following equation (3).

  Therefore, the amount of change delta Bs of the scalar quantity of the direction component of the detection axis 2 of the magnetic sensor 1 by the magnetic body 7 can be expressed by the following equation (4).

  The change amount delta Vo of the detection output of the magnetic sensor is expressed by the following formula 5, where the conversion coefficient is α.

  By determining the value of Delta Vo with a certain threshold value, the presence or absence of a magnetic material can be determined.

  Looking at Equation 2, even if a DC magnetic flux density 8 having the same scalar quantity B1t is applied, the scalar quantity Bs1 of the magnetic flux density in the direction of the detection axis 2 is determined by the DC magnetic flux density 8 and the solid angle θ1 of the detection axis 2. You can see it changing. If the direction of the DC magnetic flux density 8 coincides with the direction of the detection axis 2, cos 0 ° = 1, and the entire scalar quantity Bt 1 is applied to the magnetic sensor 1. When the scalar quantity Bt1 exceeds the dynamic range that is the allowable input range of the magnetic sensor 1, it is saturated and cannot be detected. In addition, when the direction of the DC magnetic flux density 8 and the direction of the detection axis 2 are opposite directions, cos 180 ° = −1, which is similarly saturated.

  On the other hand, if the direction of the DC magnetic flux density 8 and the direction of the detection axis 2 are orthogonal, cos 90 ° = 0, and the scalar quantity Bt1 is not applied to the magnetic sensor 1. Therefore, the magnetic sensor 1 does not saturate no matter what large value the scalar quantity Bt1 is.

  A sensor with high resolution inevitably has a small dynamic range. In order to operate the sensor without saturating, the DC component input to the sensor must be reduced. Therefore, in order to detect a small magnetic body, it is necessary to set the solid angle θ1 of the vector of the DC magnetic flux density 8 and the vector of the detection axis 2 of the magnetic sensor 1 to be orthogonal or nearly orthogonal.

  When the magnetic substance detector is installed, the above problem can be solved by measuring the direction of the DC magnetic flux density 8 using a measuring instrument and adjusting the solid angle θ1 with the detection axis 2 to be orthogonal. However, the direction of the DC magnetic flux density 8 varies depending on the installation environment, and is not a fixed direction. To adjust this, measuring instruments and the skill of the installer are required. In order to reduce the total cost including the installation cost, it is necessary to make it possible to use the high-resolution magnetic sensor 1 in a strong DC magnetic flux density environment even if it is installed without special measuring instruments or skills.

  FIG. 5 shows a schematic diagram of the diagnostic room 10 in which the MRI diagnostic apparatus is installed. The central axis 5 of the magnet 4 of the MRI apparatus is installed horizontally. There are a wall 11, a wall 12, a wall 13, and a wall 14 in the four directions of the diagnostic room 10, a ceiling 15 above, and a floor 16 below. In order to reduce the DC magnetic flux density B leaking to the outside of the diagnosis room, the width and depth of the diagnosis room is about 6 to 15 m. The width, depth, and shape vary depending on the installation environment.

  A door 17 is installed on one of the walls 11 to 14 of the examination room 10. The height of the door 17 is about 2 m and the width is about 1 m, and there is no significant difference depending on the installation environment. The door 17 is installed on any one of the walls 11 to 14. However, the wall on which the door 17 is installed is undefined, and the relative position with the magnet 4 varies depending on the installation environment. Although the actual height of the ceiling 15 is generally higher than the height of the door, the magnetic body detector according to the present invention has no problem if it detects a magnetic body that enters through the door 15. The height is about 2 m. The central axis 5 of the magnet 4 is installed horizontally, and this direction is hereinafter referred to as the x-axis. The distance between the central axis 5 and the floor 16 is about 1 m, and there is no significant difference depending on the installation environment. In FIG. 3, the shape of the examination room is rectangular when viewed from above, but the shape is not defined as long as the walls 11 to 14 are substantially vertical.

  FIG. 6 three-dimensionally shows a vector 8 of DC magnetic flux density B by a three-dimensional magnetic flux line generated by a magnet 4 whose central axis is installed horizontally. The vector 8 has various directions and scalar amounts in three dimensions depending on the space represented by the coordinates of x, y, and z. The scalar quantity of the magnetic flux density leaking outside the diagnostic room 10 is set to a value of 500 μT or less as a recommended standard for installation. Hereinafter, the calculation is performed using specific numerical values as examples, but numerical values such as the strength of the magnet and the size of the examination room are calculation examples, and are not restricted by these numerical values.

In order to simulate a magnet equivalent to 1.5T, a current of about 4.4 MA / m ² is passed through a magnet 4 having a diameter of 1 m and a length of 1 m to generate a magnetic field. The size of the examination room 10 to be 500 μT or less which is the recommended standard for the leakage DC magnetic flux density B is 9 m in the x-axis direction and 7.2 m in the y-axis direction. Hereinafter, the calculation is performed assuming that the door 17 is installed on the wall of the diagnostic room 10 having such a shape. The coordinate origin is the center of the magnet 4.

  FIG. 7 shows a vector 8 of the DC magnetic flux density B on the plane where z = 0. FIG. 8 shows the DC magnetic flux densities Bx, By, Bz, and Bt of the x, y, z direction components and the scalar quantity in the wall 11 shown in FIG. Similarly, FIG. 9 shows DC magnetic flux densities Bx, By, Bz, and Bt in the wall 12. Note that the DC magnetic flux densities of the walls 13 and 14 are obtained by inverting the signs of the walls 11 and 12 and have the same absolute value.

  8 and 9, it can be seen that the scalar quantity Bt of the DC magnetic flux density has a large value of 200 to 500 μT. When the detection axis 2 of the magnetic sensor 1 is horizontal and installed in the x direction or the y direction, it can be seen that the DC magnetic flux density is applied up to 500 μT, although it is 0 μT depending on the location. Therefore, when the detection shaft 2 is installed horizontally, the dynamic range of the magnetic sensor 1 is required to be 500 μT or more in order to operate without saturation. A sensor with a large dynamic range inevitably has a low resolution. Therefore, even if a small magnetic body passes through the door, it cannot be detected.

  8 and 9 again, the DC magnetic flux density Bz in the z direction, that is, the vertical direction is 0 at any position. This is because the DC magnetic flux density B is not generated in the direction orthogonal to the central axis 5 of the magnet 4. Therefore, if the magnetic sensor 1 is arranged on the plane of z = 0 and the detection shaft 2 is installed in the vertical direction, the high-resolution magnetic sensor having a low dynamic range is saturated regardless of the installation position of the door 17. Never do. If the magnetic sensor has a low dynamic range but a high resolution, a small magnetic substance can be detected.

  However, in order to arrange the magnetic sensor 1 on the plane of z = 0, the central axis 5 of the magnet 4 must be measured and aligned with that position. In actual installation work, specialized knowledge is required to perform such work, which is not realistic. It is necessary to presuppose that the distance between the z = 0 plane and the magnetic sensor 1 does not become zero due to various factors.

  FIG. 10 shows a vector 5 of the DC magnetic flux density B on the plane where y = 0 by changing the viewpoint. The generated vector 8 of DC magnetic flux density B is exactly the same as in FIG. However, the distance between the ceiling 15 and the floor 16 is different.

  FIG. 11 shows the DC magnetic flux densities Bx, By, Bz, and Bt of the wall 11 at y = 0. FIG. 12 shows DC magnetic flux densities Bx, By, Bz, and Bt at x = 0 of the wall 12. As shown in FIG. 11, the maximum value of Bz is 150 μT even at the position of the ceiling 15 that is farthest from the central axis 5, that is, z = 1 m and the floor 16, that is, z = −1 m, and Bx, It can be seen that the value is smaller than the maximum value of By, 500 μT.

  When the detection shaft 2 of the magnetic sensor 1 is installed on the vertical shaft 3, the value of the DC magnetic flux density Bz applied to the detection shaft 2 is smaller than when the detection shaft 2 is installed in the horizontal direction. This is because the distance between the center axis 5 of the magnet and the ceiling 15 and the floor 16 is shorter than the distance between the walls 11 to 14. As the z coordinate moves away from z = 0, the value of the DC magnetic flux density Bz increases, but a value smaller than Bx or By is sufficient.

  The maximum tolerance of the mounting position of the magnetic sensor 1 is 0.25 m, and the DC magnetic flux density Bx, By, Bz, and Bt when the magnetic sensor 1 is mounted at the position of the wall 11 and the wall 12 at the position of z = 0.25 m. The results of calculating are shown in FIGS. It should be noted that the magnetic flux density at the position of z = −0.25 m only changes in sign and has the same absolute value.

  13 and 14 that the maximum value of the direct current magnetic flux density Bz in the vertical direction is 50 μT. Therefore, when the detection shaft 2 is installed in the vertical direction, the detection operation can be performed without being saturated even if the magnetic sensor 1 having a dynamic range of 1/10 is used. This means that if the detection shaft 2 is installed in the vertical direction, a small magnetic material can be detected without saturation even if the magnetic sensor 1 having a resolution 10 times higher is used if it is installed in the vertical direction. It is.

  Next, a change amount delta Bz of the magnetic flux density B in the z-axis direction when the magnetic body enters the examination room through the door is obtained. FIG. 15 shows a schematic diagram of the positions of the magnet and the door. Although the position of the door is indefinite, the calculation is performed when the door is installed at four positions as a representative example. The position of the door A is 18, the position of the door B is 19, the position of the door C is 20, and the position of the door D is 21. When the door is installed at a position symmetrical with respect to the x-axis and the y-axis, the absolute value is the same only by changing the sign of the change amount delta Bz. When a door is installed in the middle of the doors 18 to 21, the value of the variation delta Bz is within the range of values detected by those doors.

  As an example of the magnetic material, a 10 mm square and 100 mm long iron rod is imitated by a driver that is a general tool. The case where this magnetic body 22 approached from the doors 18-21 (doors AD) is calculated. Since the magnetic body 22 is rod-shaped, it has a direction. FIG. 16 shows the amount of change Delta Bz of the magnetic flux density when entering perpendicularly to the door. Similarly, FIG. 17 and FIG. 18 show the amount of change Delta Bz in the magnetic flux density when entering parallel to the door and when entering vertically. The magnetic body 22 is assumed to pass through the center of the doors 18 to 21. The state where the magnetic body 22 starts moving from a position 1 m away from the door, passes through the door, and moves to the position of 0.5 m inside the examination room 10 is calculated. The distance between the magnetic body 22 and the door is displayed as a relative position, and the distance at the time of passing through the door is 0.

  As shown in FIGS. 16 to 18, when the magnetic body 22 enters through the door, a change in the magnetic flux density in the vertical direction occurs. When the detection axis 2 of the magnetic sensor 1 is made to coincide with the vertical axis 3, the change amount delta Bs of the scalar amount = the change amount delta Bz of the magnetic flux density in the vertical direction. The passage of the magnetic body 22 is detected by determining the value of the change amount delta Bs of the scalar amount.

  16-18, even if the same magnetic body 22 is used, it can be seen that the value of the amount of change delta Bs of the scalar amount differs depending on the position of the door to enter and the orientation of the magnetic body 22. For example, as shown in FIG. 17, when the magnetic body 22 is parallel to the door and the door 21 (door D) is passed, the difference in the amount of change Delta Bs of the scalar amount is a large value of 750 nT. However, as shown in FIG. 18, the difference in the amount of change Delta Bs of the scalar amount when the same magnetic body 22 is made vertical and passed through the door 20 (door C) is only 1.5 nT. Such a small scalar change amount delta Bs cannot be detected even by the high-resolution magnetic sensor 1.

  Therefore, as shown in FIG. 19, not only one side of the door but also a plurality of magnetic sensors are provided on both sides of the door and in the vertical direction. A magnetic flux density vector at the position of the sensor 1 when there is no magnetic body 7 is indicated by 23. The scalar quantity of the vector 24 of the DC magnetic flux density in the direction of the detection axis 2 applied by 23 is Bs24. Similarly, a vector of DC magnetic flux density at the position of the sensor 25 is indicated by 27. The scalar quantity of the vector 28 of the DC magnetic flux density in the direction of the detection axis 26 applied by 27 is Bs28.

  When the magnetic body 7 enters this space, the scalar quantity and direction of the DC magnetic flux density change. When the position 23 of the magnetic sensor 1 changes to a DC magnetic flux density vector 29, the DC magnetic flux density in the direction of the detection axis 2 changes to a vector 30, and the scalar quantity changes from Bs24 to Bs30. However, depending on conditions, the amount of change from Bs24 to Bs30 may be small. If the amount of change from Bs24 to Bs30 is smaller than the resolution, this cannot be detected.

  Similar to the magnetic sensor 1, the scalar amount and direction of the DC magnetic flux density also change at the position of the magnetic sensor 25. When the position 27 of the magnetic sensor 25 changes to a DC magnetic flux density vector 31, the DC magnetic flux density in the direction of the detection axis 26 changes to a vector 32, and the scalar quantity changes from Bs28 to Bs32. However, at the position of the magnetic sensor 25, the magnetic body 7 enters at a different angle from the magnetic sensor 1. Therefore, the change amount of Bs28 and Bs32 is different from the change amount of Bs24 to Bs30, and can be detected.

  Even if one magnetic sensor does not detect the magnetic body 7, there is no problem as a detection function of the entire apparatus as long as another magnetic sensor detects it.

  As described above, by making the detection axis 2 of the magnetic sensor 1 coincide with the vertical axis 3, it is possible to use the high-resolution magnetic sensor 1 with a low dynamic range regardless of the position of the door. However, as shown in FIG. 20, when the magnetic sensor 1 is attached to the housing 33, the detection shaft 2 and the housing shaft 34 of the housing 33 do not always coincide. Similarly, when the casing 33 is attached to the wall 35, the casing axis 34 and the vertical axis 3 do not always coincide with each other. It is impossible to make the detection axis 2 completely coincide with the vertical axis 3 due to errors such as various tolerances.

Therefore, to be the solid angle of the detection axis 2 and the vertical axis 3 provided in advance a certain tolerance theta _D. However, theta when _D is too large, the DC magnetic flux density Bs generated in the detection axis 2 of the magnetic sensor 1 is increased, it is impossible to operate accidentally saturated. If the DC magnetic flux density Bs in the detection axis direction is set to 30% or less of the DC magnetic flux density B, the solid angle θ _D between the detection axis 2 and the vertical axis 3 of the magnetic sensor 1 is reduced to 15 ° or less from the above equation (1). It ’s fine. Within this range, it becomes difficult to saturate.

  The above description is applied when the central axis 5 of the magnet 4 is installed in the horizontal direction. However, in some open type MRI diagnostic apparatuses, the central axis 5 of the magnet 4 is installed in the vertical direction. In such an MRI diagnostic apparatus, the DC magnetic flux density B in the vertical direction is large and the DC magnetic flux density B in the horizontal direction is small. Therefore, when the detection shaft 2 of the magnetic sensor 1 is installed in the vertical direction, it cannot be saturated and operate.

In an environment where the central axis 5 of the magnet 4 is installed in the vertical direction, the detection axis 2 of the magnetic sensor 1 is installed horizontally. Similarly, since there is a mounting tolerance, the solid angle θ _D between the detection axis 2 and the horizontal axis 6 of the magnetic sensor 1 may be 15 ° or less. Within this range, it becomes difficult to saturate.

  The direction of the detection axis 2 of the magnetic sensor 1 differs depending on the installation direction of the central axis 5 of the magnet 4. Therefore, it is impossible to apply to two types of MRI diagnostic apparatuses with one magnetic substance detector. However, since two types of MRI diagnostic apparatuses are not used at the same place, there is no particular problem.

  There is no problem if the direction of the detection axis 2 of the magnetic sensor 1 is 15 ° or less, but there is no problem even if a function for adjusting the angle is provided in consideration of special environmental conditions.

  Next, the noise resistance performance is examined. There are various types of magnetic sensors. Here, a sensor that converts the magnetic flux density B into an electric quantity is assumed. The output of the sensor is an electric quantity having a correlation with the input magnetic flux density B, and is an analog value. There is no problem whether the magnetic flux density B detected by the magnetic sensor 1 is direct current or alternating current.

  As a general method for determining the presence or absence of an object, there is a method using a comparison circuit (comparator) that determines an analog value of a sensor output with a certain threshold value. In addition, as shown in FIG. 21, as a general method, the analog value of the sensor output is converted into a digital value using a conversion circuit 37, and compared with a threshold value using an operation comparison circuit 38. There is also a method of making an announcement using 36. Notification means include light and sound. Some general conversion circuits 37 have a function of selecting a plurality of inputs, and the outputs of the plurality of magnetic sensors 1 can be processed by a single conversion circuit 37. The power supply for each sensor is supplied by a power supply circuit 39. In FIG. 21, a case where four magnetic sensors are provided will be described as one embodiment. Of course, the number of magnetic sensors is not limited to four.

  As shown in FIG. 21, in the magnetic body detector as in the present invention, a plurality of magnetic sensors 41 to 44 must be provided on the left and right sides of the door 40. On the other hand, a comprehensive determination of the presence or absence of a magnetic material must be performed by one determination unit 45. However, the position of the determination unit 45 is not necessarily the upper part of the door 40, and may be either the left or right of the door 40, or may be a position completely different from the door 40. Since the size of a general door is about 2 m in height and about 1 m in width, the creepage distance between the determination unit 45 and the sensors 41 to 44 installed in the upper center of the door 40 needs to be 1.5 m or more. When the determination unit 45 is installed on either the left or right side of the door, the distance to one magnetic sensor is shortened, but the creepage distance to the opposite magnetic sensor is at least 3 m. Whatever means is used, the total cable length is 3 m or more.

When the analog output signals of the magnetic sensors 41 to 44 are connected to the conversion circuit 37 using long cables 46 to 47, noise _NL is superimposed on the cables 46 to 47. Since the magnetic flux density change delta B due to the intrusion of the magnetic material is a small value, the detection output change delta Vo of the magnetic sensors 41 to 44 is also a small value. When the external noise _NE is superimposed on the small detection output change delta Vo and the noise _NL is added, whether the change delta Vi of the input signal of the conversion circuit 37 is changed by the delta Vo or not is changed by _NL . It is impossible to determine whether it has been performed, and malfunction occurs. Accordingly, correct operation cannot be performed unless the noise N _L is reduced.

The size of the noise _NL varies depending on various factors, but the largest factor regarding the device itself is the length of the cables 46 to 47. If the length of cable 46-47 is long, inevitably the inductance _{L L} is increased, the greater the mutual inductance _{M L} between external noise _{N E.} The larger the mutual inductance M _L is the noise N _L to be superimposed to the cable becomes large, causing a malfunction. In order to prevent this, it is necessary to shorten the lengths of the cables 46 to 47. However, it is impossible to shorten the physical distance between the magnetic sensors 41 to 44 and the determination unit 45.

Therefore, as shown in FIG. 22, conversion circuits 48 to 51 are provided in the immediate vicinity of the magnetic sensors 41 to 44, respectively. The distance of the conversion circuit 48 to 51 and the magnetic sensors 41 to 44 is extremely short, the mutual inductance _{M A} of the inductance _{L A} and external noise _{N E} of the connecting cable is reduced. Thus the noise N _A due to external noise N _E superimposed on converting circuit 48-51 is reduced, it becomes possible to reduce malfunction. Since the cables 46 to 47 are long, the noise _NL is largely superimposed as in FIG. 21. However, since the amount is a digitized amount, the influence of the noise _NL can be reduced to 0 if the transmission quality is ensured. It is.

  Another factor that causes noise to be superimposed is the power source. As shown in FIG. 21, when power is supplied from one power supply circuit through cables 46 to 47, the cable is long, and thus the resistance component R is included in the power supply line. When the current consumption fluctuates due to load fluctuation, the power supply voltage VcL supplied to the magnetic sensors 41 to 44 fluctuates due to the resistance component R. As a result, since the power supply voltage of the magnetic sensors 41 to 44 varies, the detection output Vo varies depending on the magnetic flux density.

  In order to prevent this, as shown in FIG. 22, power supply circuits 52 to 55 are provided in the immediate vicinity of the magnetic sensors 41 to 44, respectively. Although VcL varies due to load variation, the power supply voltage VcA supplied to the magnetic sensors 41 to 44 does not vary.

  As shown in FIG. 22, providing a plurality of conversion circuits 48 to 51 and power supply circuits 52 to 55 is disadvantageous in terms of cost. However, it is necessary to ensure performance.

  Even if noise countermeasures as described above are taken, the noise does not become zero. Noise always intrudes due to various factors. The magnitude of the noise varies greatly depending on the installation environment. When the threshold value for determining the presence / absence of detection is fixed, there is no problem in an environment where noise is small, but there is a case where malfunction occurs in an environment where noise is large.

  Therefore, a threshold value input device 56 that can easily set the threshold value from the outside is provided, and a threshold value that matches the installation environment is set. As a specific threshold input device 56, a variable resistor or the like is assumed.

It is a schematic explanatory drawing which shows one structural aspect of the magnetic body detector concerning this invention. It is a schematic explanatory drawing which shows the other structure aspect of the magnetic body detector concerning this invention. It is a schematic explanatory drawing which shows the magnetic flux density at the time of installing a magnetic sensor in a magnetic field. It is a schematic explanatory drawing which shows the magnetic flux density at the time of installing a magnetic sensor and a magnetic body in a magnetic field. It is a schematic explanatory drawing which shows the diagnostic room where the MRI diagnostic apparatus is installed. It is the schematic explanatory drawing which showed the vector of the magnetic flux density which a magnet generate | occur | produces three-dimensionally. It is the schematic explanatory drawing which showed the vector of the magnetic flux density on the plane of z = 0. It is a graph which shows the magnitude | size of the magnetic flux density in one wall. It is a graph which shows the magnitude | size of the magnetic flux density in one wall. It is the schematic explanatory drawing which showed the vector of the magnetic flux density on the plane of y = 0. It is a graph which shows the magnitude | size of the magnetic flux density in one wall. It is a graph which shows the magnitude | size of the magnetic flux density in one wall. It is a graph which shows the magnitude | size of the magnetic flux density in one wall. It is a graph which shows the magnitude | size of the magnetic flux density in one wall. It is the schematic explanatory drawing which showed the relationship between a magnet, a door, and the rod-shaped magnetic body which penetrates. It is a graph which shows the change of the magnetic flux density which generate | occur | produces with the rod-shaped magnetic body which penetrate | invades in a perpendicular direction to a door. It is a graph which shows the change of the magnetic flux density which generate | occur | produces with the rod-shaped magnetic body which penetrates into a door in the parallel direction. It is a graph which shows the change of the magnetic flux density which generate | occur | produces with the rod-shaped magnetic body which penetrate | invades in the perpendicular direction. It is a schematic explanatory drawing of the magnetic flux density at the time of providing multiple magnetic sensors. It is a schematic explanatory drawing which shows the positional relationship of a magnetic sensor, a housing | casing, and a wall. It is a block diagram of a magnetic substance detector provided with one conversion circuit and one power supply circuit. It is a block diagram of a magnetic substance detector provided with a plurality of conversion circuits and power supply circuits.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Magnetic sensor 2 Detection axis 3 Vertical axis 4 Magnet 5 Center axis 6 Horizontal axis 7 Magnetic body 8 DC magnetic flux density 9 DC magnetic flux density 10 Examination room 11 Wall 12 Wall 13 Wall 14 Wall 15 Ceiling 16 Floor 17 Door 18 Door A
19 Door B
20 Door C
21 Door D
22 Magnetic body 23 DC magnetic flux density 24 DC magnetic flux density 25 Magnetic sensor 26 Detection shaft 27 DC magnetic flux density 28 DC magnetic flux density 29 DC magnetic flux density 30 DC magnetic flux density 31 DC magnetic flux density 32 DC magnetic flux density 33 Housing 34 Housing shaft 35 36 Notification Function 37 Conversion Circuit 38 Operation Comparison Circuit 39 Power Supply Circuit 40 Door 41 Magnetic Sensor 42 Magnetic Sensor 43 Magnetic Sensor 44 Magnetic Sensor 45 Determination Unit 46 Cable 47 Cable 48 Conversion Circuit 49 Conversion Circuit 50 Conversion Circuit 51 Conversion Circuit 52 Power Supply Circuit 53 Power circuit 54 Power circuit 55 Power circuit 56 Threshold input device

Claims (5)

  In a magnetic substance detector having a magnetic sensor for detecting a magnetic flux density, a threshold value, a comparison function for comparing the output of the magnetic sensor with the threshold value, and a function for notifying a result of the comparison function, the detection of the magnetic sensor A magnetic substance detector characterized in that a solid angle between an axis and a vertical axis or a horizontal axis is within 15 °.   The magnetic body detector according to claim 1, wherein the magnetic body detector includes a plurality of the magnetic sensors and each magnetic sensor is installed at a different position.   In the magnetic substance detector, a conversion circuit for converting the analog output amount of the magnetic sensor into a digital amount is installed in the immediate vicinity of each magnetic sensor, an arithmetic circuit for calculating the digital amount of the conversion circuit, and an operation of the arithmetic circuit The magnetic body detector according to claim 2, further comprising a function for comparing a result with the threshold value.   4. The magnetic substance detector according to claim 2, wherein the number of power supply circuits is the same as that of the magnetic sensors, and the power supply circuits are installed in the immediate vicinity of each magnetic sensor and the conversion circuit. Magnetic body detector.   The magnetic body detector according to any one of claims 1 to 4, wherein the magnetic body detector has a function of making the threshold variable.