Geomagnetic interference shielding device and use method

Abstract

The invention discloses a geomagnetic interference shielding device and a use method, the device comprises a shielding shell, a shielding end cover, a PLC controller, a vector magnetic sensor and a magnetic field compensation device, the shielding shell is a hollow cuboid with an opening at one side, the shielding end cover is connected to the opening of the shielding shell to form a closed anti-interference shielding box body, and the PLC controller is connected with the vector magnetic sensor. An electric device fixing device is arranged in the box body and is used for fixing an electric device and shielding external geomagnetic interference; the PLC, the vector magnetic sensor and the magnetic field compensation device are respectively arranged on the outer wall of the shielding shell; the output end of the vector magnetic sensor is connected with the PLC, the output end of the PLC is connected with the magnetic field compensation device, and the PLC is used for controlling the magnetic field compensation device to generate a magnetic field, so that the geomagnetic field is shielded. According to the invention, the detected geomagnetic field in the environment can be offset, and the geomagnetic field is shielded while the signal receiving of the electric device is not influenced, so that the influence of the geomagnetic field on the work of the electric device is avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of interference shielding, and in particular to a geomagnetic interference shielding device and a use method thereof. Background Art

[0002] The geomagnetic field refers to the natural magnetic phenomenon inside the earth. The earth can be regarded as a magnetic dipole, with one pole located near the geographic North Pole and the other near the geographic South Pole. The imaginary straight line (magnetic axis) passing through these two magnetic poles is inclined at about 11.3 degrees to the earth's rotation axis. The earth's magnetic field extends tens of thousands of kilometers into space to form the earth's magnetic field. The earth's magnetic field has the function of shielding the earth from the charged particles carried by the solar wind. The earth's magnetic field is squeezed by the force of charged particles in the daytime area (the side facing the sun), and extends outward in the nighttime area (the side facing away from the sun).

[0003] The geomagnetic field includes two parts: the basic magnetic field and the variable magnetic field. The basic magnetic field is the main part of the geomagnetic field, originating from the interior of the solid earth, and is relatively stable, belonging to the static magnetic field. The variable magnetic field includes various short-term changes in the geomagnetic field, mainly originating from the outside of the solid earth, and is relatively weak. The earth's variable magnetic field can be divided into two types: quiet changes and disturbance changes.

[0004] The effect of the geomagnetic field on the compass is used for marching and navigation to determine direction. People can also look for mineral deposits based on the distribution characteristics of the geomagnetic field on the ground. Changes in the geomagnetic field can affect the propagation of radio waves. However, when the geomagnetic field is strongly disturbed by sunspot activity, long-distance communications will be seriously affected or even interrupted. Geomagnetic interference can affect the operation of electrical devices, affecting their normal operation and even causing damage to them. The existing method is usually to store electrical devices in a closed shielding box. This method is better for some electrical devices, but when some electrical devices need to receive external signals, sealing the electrical devices in this way will seriously affect their reception of signals.

[0005] Therefore, there is an urgent need for a geomagnetic interference shielding device that can shield the geomagnetic field without affecting the signals received by electrical devices. Summary of the invention

[0006] In order to solve the deficiencies in the prior art, the present invention provides a geomagnetic interference shielding device, which places electrical components in the closed anti-interference shielding box to shield external geomagnetic interference, thereby solving the problem that geomagnetic interference may affect the operation of electrical components and even cause damage to the electrical components.

[0007] The present invention adopts the following technical solution.

[0008] The first aspect of the present invention discloses a geomagnetic interference shielding device, comprising: a shielding shell, a shielding end cover, a PLC controller, a vector magnetic sensor and a magnetic field compensation device, wherein the shielding shell is a hollow rectangular parallelepiped with one side open, the shielding end cover is connected to the opening of the shielding shell to form a closed anti-interference shielding box body, and an electrical device fixing device is provided in the box body for fixing the electrical device and shielding external geomagnetic interference;

[0009] The PLC controller, the vector magnetic sensor and the magnetic field compensation device are respectively installed on the outer wall of the shielding shell; the output end of the vector magnetic sensor is connected to the PLC controller, and is used to transmit the result of the vector magnetic sensor detecting the external magnetic field to the PLC controller;

[0010] The output end of the PLC controller is connected to the magnetic field compensation device, which is used to control the magnetic field compensation device to generate a magnetic field to offset the geomagnetic field in the environment detected by the vector magnetic sensor, thereby shielding the geomagnetic field.

[0011] Preferably, the shielding shell comprises an outer frame and a metal shielding plate connected to the outer frame wall, and the opening of the shielding shell is arranged on the side wall of the outer frame;

[0012] The opening is connected to the shielding end cover, which includes a rectangular frame, a metal shielding plate is arranged inside the rectangular frame, and the rectangular frame is connected to the opening of the outer frame through screws.

[0013] Preferably, the side of the shielding end cover is vertically connected to a bottom plate, the bottom plate can slide in the shielding shell, and a limit clamping plate is provided on the upper surface of the bottom plate for limiting the position of the electrical component.

[0014] Preferably, the electrical component fixing device includes a pressure plate assembly, which is located above the base plate and connected to the inner surface of the shielding shell for pressing the electrical component. The pressure plate assembly includes a flat plate and an inclined plate. The flat plate is parallel to the bottom surface of the shielding shell, and the inclined plate is connected to the flat plate at a set angle to prevent the electrical component from getting stuck.

[0015] Preferably, the pressure plate assembly also includes an elastic member, the upper end of the elastic member is connected to the inner surface of the shielding shell, and the lower end of the elastic member is connected to the flat plate; when the electrical device is inserted into the inner cavity of the shielding shell, the inclined plate and the flat plate are lifted up, and the elastic member is compressed, and under the elastic force of the elastic member, the inclined plate and the flat plate are pressed down to compress the electrical device.

[0016] Preferably, the magnetic field compensation device includes an outer shell, two parallel side plates are arranged inside the outer shell, namely a first side plate and a second side plate, a rotating rod is rotatably arranged between the two side plates, wherein a driving motor is installed on the side wall of the first side plate, and the driving motor is connected to one end of the rotating rod and drives the rotating rod to rotate.

[0017] Preferably, the rotating rod is a semicircular arc-shaped rod body, a rotating motor is installed on the arc-shaped convex side of the rotating rod through one end of a support shaft, and a compensation coil is provided at the other end of the support shaft.

[0018] Preferably, a power supply is further provided inside the outer shell, and the power supply is electrically connected to the compensation coil.

[0019] The second aspect of the present invention discloses a method for using a geomagnetic interference shielding device, based on the geomagnetic interference shielding device, comprising the following steps:

[0020] The shielding shell and the shielding end cover form a closed anti-interference shielding box body, and the electrical components are placed in the closed anti-interference shielding box body to shield the external geomagnetic interference;

[0021] When the electrical device needs to receive external signals, the shielding shell is unfolded, or the shielding end cover is removed from the shielding shell, and the magnetic field components of the earth's magnetic field in the environment at various angles of the outer wall of the shielding shell are detected by the vector magnetic sensor;

[0022] The detection results of the vector magnetic sensor are transmitted to the PLC controller, thereby controlling the magnetic field compensation device to produce a magnetic field of the same magnitude as the geomagnetic field but in the opposite direction, so as to offset the geomagnetic field in the detected environment.

[0023] Preferably, when the magnetic field compensation device is used, the PLC controller controls the output of the magnetic field compensation device based on the detection result of the vector magnetic sensor, and drives the rotating rod to rotate through the driving motor;

[0024] At the same time, the rotating motor drives the support shaft and the compensation coil to rotate, and the compensation coil is adjusted in multiple dimensions so that the directions of the magnetic field components at the angles of the compensation coil and the outer wall of the detected shielding shell are consistent;

[0025] The power supply output is controlled to adjust the size of the magnetic field generated by the compensation coil to offset the geomagnetic field in the environment.

[0026] The beneficial effect of the present invention is that, compared with the prior art,

[0027] The present invention forms a closed anti-interference shielding box body by arranging a shielding shell and a shielding end cover, and places the electrical device in the closed anti-interference shielding box body to shield external geomagnetic interference; when the electrical device needs to receive external signals, the present invention unfolds the shielding shell, or removes the shielding end cover from the shielding shell, and uses a vector magnetic sensor to detect the magnetic field components of the geomagnetic field in the environment at various outer wall angles of the shielding shell, and transmits the detection result of the vector magnetic sensor to a PLC controller, thereby regulating the size of the magnetic field generated by the magnetic field compensation device, and the magnetic field is opposite to the direction of the detected magnetic field, and is used to offset the detected geomagnetic field in the environment. In this way, the geomagnetic field can be shielded without affecting the reception of signals by the electrical device, thereby avoiding the influence of the geomagnetic field on the operation of the electrical device. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the structure of the present invention;

[0029] Figure 2 It is a schematic diagram of the separate structure of the outer frame and the metal shielding plate of the present invention;

[0030] Figure 3 It is a schematic diagram of the separate structure of the shielding shell, the shielding end cover and the magnetic field compensation device of the present invention;

[0031] Figure 4 It is a schematic diagram of the internal structure of the magnetic field compensation device of the present invention;

[0032] In the figure: 1. shielding shell; 11. outer frame; 12. metal shielding plate; 2. shielding end cover; 3. PLC controller; 4. vector magnetic sensor; 5. magnetic field compensation device; 51. outer shell; 52.1. first side plate; 52.1. second side plate; 53. rotating rod; 54. driving motor; 55. rotating motor; 56. supporting shaft; 57. compensation coil; 58. power supply; 6. limit card plate; 7. flat plate; 8. inclined plate; 9. elastic member; 10. bottom plate. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The embodiments described in this application are only part of the embodiments of the present invention, not all of them. Based on the spirit of the present invention, other embodiments obtained by ordinary technicians in this field without creative work are all within the scope of protection of the present invention.

[0034] In the description of the present invention, it is necessary to understand that the terms "upper", "lower", "front", "back", "left", "right", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship are based on the orientation or position relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0035] like Figure 1 As shown, embodiment 1 of the present invention provides a geomagnetic interference shielding device, including a shielding shell 1, a shielding end cover 2, a PLC controller 3, a vector magnetic sensor 4 and a magnetic field compensation device 5;

[0036] The shielding shell 1 is a hollow cuboid with one side open. The shielding end cover 2 is connected to the opening of the shielding shell 1 to form a closed anti-interference shielding box body. An electrical component fixing device is provided in the box body to fix the electrical components and shield external geomagnetic interference.

[0037] The PLC controller 3, the vector magnetic sensor 4 and the magnetic field compensation device 5 are respectively installed on the outer wall of the shielding shell 1; the output end of the vector magnetic sensor 4 is connected to the PLC controller 3, and is used to transmit the result of the vector magnetic sensor 4 detecting the external magnetic field to the PLC controller 3;

[0038] The output end of the PLC controller 3 is connected to the magnetic field compensation device 5 to control the magnetic field compensation device 5 to generate a magnetic field to offset the geomagnetic field in the environment detected by the vector magnetic sensor 4, thereby shielding the geomagnetic field.

[0039] In a preferred but non-limiting embodiment of the present invention, Figure 2 As shown, the shielding shell 1 includes an outer frame 11 and metal shielding plates 12 connected to the front and rear, upper and lower, and right side walls of the outer frame 11 . The opening of the shielding shell 1 is set on the left side wall of the outer frame 11 .

[0040] The shielding end cover 2 includes a rectangular frame, and a metal shielding plate 12 is also arranged inside the rectangular frame. The rectangular frame is connected to the left opening of the outer frame 11 by screws.

[0041] The shielding shell 1 and the shielding end cover 2 are composed of an outer frame 11, a rectangular frame, a metal shielding plate 12 and other structures. Due to the material limitation of the metal shielding plate 12, the external magnetic field is shielded and will not affect the electrical components inside. The material of the metal shielding plate 12 is aluminum, copper and the like.

[0042] In a preferred but non-limiting embodiment of the present invention, Figure 3As shown, a bottom plate 10 is provided at the opening of the shielding end cover 2 facing the shielding shell 1, and the bottom plate 10 can slide in the shielding shell 1. A limiting clamping plate 6 is provided at one end of the upper surface of the bottom plate 10 away from the shielding end cover 2, and the limiting clamping plate 6 is used to clamp the electrical components installed on the inner side of the shielding shell 1.

[0043] Further preferably, a pressure plate assembly is provided on the inner upper side of the shielding shell 1 , the pressure plate assembly is located on the upper side of the bottom plate 10 , and the pressure plate assembly is used to press the electrical components.

[0044] The pressure plate assembly includes a flat plate 7 and an inclined plate 8 disposed at an end of the flat plate 7 facing the opening of the shielding shell 1 , and the inclined plate 8 is connected to the flat plate 7 at a certain angle.

[0045] The upper surface of the pressure plate assembly is connected with an elastic member 9, the upper end of the elastic member 9 is connected to the upper surface of the inner cavity of the shielding shell 1, and the lower end of the elastic member 9 is connected to the flat plate 7;

[0046] In actual use, the electrical component is placed on the upper surface of the base plate 10. According to the shape of the electrical component, the limit card plate 6 is set to a shape that matches the electrical component, so that the electrical component can be fixedly connected to the limit card plate 6, thereby fixing the electrical component and preventing the electrical component from moving.

[0047] The electrical device is inserted into the inner cavity of the shielding shell 1 as the bottom plate 10 is inserted into the inner cavity of the shielding shell 1. After the electrical device is inserted into the inner cavity of the shielding shell 1, the inclined plate 8 and the flat plate 7 are lifted up, and the elastic member 9 is compressed. Under the elastic force of the elastic member 9, the inclined plate 8 and the flat plate 7 are pressed down to press the electrical device tightly and stabilize it. Since the inclined plate 8 is set to be inclined, when the electrical device is inserted, the inclined plate 8 contacts the electrical device first, and since the inclined plate 8 is inclined, the electrical device is prevented from being stuck.

[0048] In a preferred but non-limiting embodiment of the present invention, Figure 4 As shown, the magnetic field compensation device 5 includes an outer shell 51, and two parallel side plates are arranged inside the outer shell 51, which are a first side plate 52.1 and a second side plate 52.2. A rotating rod 53 is rotatably arranged between the two side plates, and a driving motor 54 is installed on the side wall of one of the first side plates 52.1. The driving motor 54 is connected to the rotating rod 53 and drives the rotating rod 53 to rotate.

[0049] The rotating rod 53 is a semicircular rod body, and a rotating motor 55 is installed on the arc convex side of the rotating rod 53 through one end of a support shaft 56, and a compensation coil 57 is provided at the other end of the support shaft 56. A power supply 58 is provided inside the outer shell 51, and the power supply 58 is electrically connected to the compensation coil 57.

[0050] Both ends of the rotating rod 53 are provided with short shafts, and the output shaft of the driving motor 54 is connected to one of the short shafts.

[0051] Further preferably, the outer shell 51 is made of wood, plastic or glass, and the wood, plastic or glass material does not affect the magnetic field generated by the compensation coil 57 being emitted from the outer shell 51 .

[0052] It is worth noting that the above material is only one material selection of the embodiment. In engineering practice, those skilled in the art may implement the present invention using any material that does not affect the magnetic field emitted by the compensation coil, which falls within the scope of the present invention.

[0053] It is worth noting that magnetic sensors are devices that detect corresponding physical quantities by converting changes in the magnetic properties of sensitive components caused by external factors such as magnetic fields, currents, stress strains, temperature, and light into electrical signals.

[0054] When an electric current passes through a wire, it generates a magnetic field. The size and direction of this magnetic field is determined by the size and direction of the current. This is because the current itself is generated by the movement of charged particles (such as electrons) in a conductor. As these charged particles move, they generate an electric and magnetic field, and the size and direction of these fields depend on the direction and speed of the charged particles. In a wire, because the charged particles are moving in close proximity, the magnetic fields they generate interact with each other to create an overall magnetic field.

[0055] Therefore, by changing the current and power supply direction of the power supply 58 to the compensation coil 57, the direction and magnitude of the magnetic field generated by the compensation coil 57 can be changed.

[0056] Embodiment 2 of the present invention provides a method for using a geomagnetic interference shielding device, and the specific steps are as follows:

[0057] Step 1, the shielding shell 1 and the shielding end cover 2 can form a closed anti-interference shielding box body, and the electrical components are placed in the closed anti-interference shielding box body to shield external geomagnetic interference;

[0058] Step 2, when the electrical device needs to receive an external signal, the shielding shell 1 is unfolded, or the shielding end cover 2 is removed from the shielding shell 1, and the magnetic field components of the geomagnetic field in the environment at various angles of the outer wall of the shielding shell 1 are detected by the vector magnetic sensor 4;

[0059] Step 3, transmitting the detection result of the vector magnetic sensor 4 to the PLC controller 3, so as to control the magnetic field of corresponding magnitude produced by the magnetic field compensation device 5.

[0060] It is worth noting that the magnetic field is opposite to the direction of the detected magnetic field and is used to offset the detected geomagnetic field in the environment. This can shield the geomagnetic field without affecting the reception of signals by the electrical devices, thereby preventing the geomagnetic field from affecting the operation of the electrical devices.

[0061] When the magnetic field compensation device 5 needs to be used, the PLC controller 3 controls the output of the magnetic field compensation device 5 based on the detection result of the vector magnetic sensor 4, and drives the rotating rod 53 to rotate through the driving motor 54;

[0062] At the same time, the rotating motor 55 drives the support shaft 56 and the compensation coil 57 to rotate, so that the compensation coil 57 can be adjusted in multiple dimensions, so that the compensation coil 57 can be consistent with the direction of the magnetic field component at each outer wall angle of the detected shielding shell;

[0063] The power supply 58 outputs control to adjust the size of the magnetic field generated by the compensation coil 57 so as to offset the earth's magnetic field in the environment.

[0064] The beneficial effect of the present invention is that, compared with the prior art,

[0065] The present invention forms a closed anti-interference shielding box body by arranging a shielding shell and a shielding end cover, and places the electrical device in the closed anti-interference shielding box body to shield external geomagnetic interference; when the electrical device needs to receive external signals, the present invention unfolds the shielding shell, or removes the shielding end cover from the shielding shell, and uses a vector magnetic sensor to detect the magnetic field components of the geomagnetic field in the environment at various outer wall angles of the shielding shell, and transmits the detection result of the vector magnetic sensor to a PLC controller, thereby regulating the size of the magnetic field generated by the magnetic field compensation device, and the magnetic field is opposite to the direction of the detected magnetic field, and is used to offset the detected geomagnetic field in the environment. In this way, the geomagnetic field can be shielded without affecting the reception of signals by the electrical device, thereby avoiding the influence of the geomagnetic field on the operation of the electrical device.

[0066] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A geomagnetic interference shielding device, comprising: A shielding shell (1), a shielding end cover (2), a PLC controller (3), a vector magnetic sensor (4) and a magnetic field compensation device (5), characterized in that: The shielding shell (1) is a hollow rectangular parallelepiped with one side open, and the shielding end cover (2) is connected to the opening of the shielding shell (1) to form a closed anti-interference shielding box body, and an electrical component fixing device is provided in the box body for fixing the electrical components and shielding external geomagnetic interference; The PLC controller (3), the vector magnetic sensor (4) and the magnetic field compensation device (5) are respectively mounted on the outer wall of the shielding shell (1); the output end of the vector magnetic sensor (4) is connected to the PLC controller (3) and is used to transmit the result of the vector magnetic sensor (4) detecting the external magnetic field to the PLC controller (3); The output end of the PLC controller (3) is connected to the magnetic field compensation device (5) and is used to control the magnetic field compensation device (5) to generate a magnetic field to offset the geomagnetic field in the environment detected by the vector magnetic sensor (4), thereby shielding the geomagnetic field.

2. A geomagnetic interference shielding device according to claim 1, characterized in that: The shielding shell (1) comprises an outer frame (11) and a metal shielding plate (12) connected to the wall of the outer frame (11); the opening of the shielding shell (1) is arranged on the side wall of the outer frame (11); The opening is connected to the shielding end cover (2), the shielding end cover (2) comprises a rectangular frame, a metal shielding plate is arranged inside the rectangular frame, and the rectangular frame is connected to the opening of the outer frame (11) by means of screws.

3. A geomagnetic interference shielding device according to claim 2, characterized in that: The side of the shielding end cover (2) is vertically connected to a bottom plate (10), the bottom plate (10) can slide in the shielding shell (1), and a limit card plate (6) is provided on the upper surface of the bottom plate (10) for limiting the position of the electrical component.

4. A geomagnetic interference shielding device according to claim 1 or 3, characterized in that: The electrical component fixing device comprises a pressure plate assembly, which is located above the bottom plate (10) and connected to the inner surface of the shielding shell (1) for pressing the electrical component. The pressure plate assembly comprises a flat plate (7) and an inclined plate (8). The flat plate (7) is parallel to the bottom surface of the shielding shell (1), and the inclined plate (8) is connected to the flat plate (7) at a set angle to prevent the electrical component from getting stuck.

5. A geomagnetic interference shielding device according to claim 4, characterized in that: The pressure plate assembly also includes an elastic member (9), the upper end of which is connected to the inner surface of the shielding shell (1), and the lower end of which is connected to the flat plate (7); when the electrical device is inserted into the inner cavity of the shielding shell (1), the inclined plate (8) and the flat plate (7) are lifted up, and the elastic member (9) is compressed. Under the elastic force of the elastic member (9), the inclined plate (8) and the flat plate (7) are pressed down to compress the electrical device.

6. The geomagnetic interference shielding device according to claim 1, characterized in that: The magnetic field compensation device (5) comprises an outer shell (51), wherein two parallel side plates are arranged inside the outer shell (51), namely a first side plate (52.1) and a second side plate (52.2), and a rotating rod (53) is rotatably arranged between the two side plates, wherein a driving motor (54) is mounted on the side wall of the first side plate (52.1), and the driving motor (54) is connected to one end of the rotating rod (53) and drives the rotating rod (53) to rotate.

7. A geomagnetic interference shielding device according to claim 6, characterized in that: The rotating rod (53) is a semicircular arc-shaped rod body. A rotating motor (55) is installed on the arc-shaped convex side of the rotating rod (53) through one end of a support shaft (56), and a compensation coil (57) is provided at the other end of the support shaft (56).

8. The geomagnetic interference shielding device according to claim 6, characterized in that: A power supply (58) is also provided inside the outer shell (51), and the power supply (58) is electrically connected to the compensation coil (57).

9. A method for using a geomagnetic interference shielding device, based on the geomagnetic interference shielding device according to any one of claims 1 to 8, characterized in that: The following steps are involved: The shielding shell (1) and the shielding end cover (2) form a closed anti-interference shielding box body, and the electrical device is placed in the closed anti-interference shielding box body to shield external geomagnetic interference; When the electrical device needs to receive an external signal, the shielding shell (1) is unfolded, or the shielding end cover (2) is removed from the shielding shell (1), and the magnetic field components of the geomagnetic field in the environment at various angles of the outer wall of the shielding shell (1) are detected by the vector magnetic sensor (4); The detection result of the vector magnetic sensor (4) is transmitted to the PLC controller (3), thereby controlling the magnetic field compensation device (5) to produce a magnetic field with a magnitude corresponding to the earth's magnetic field but opposite in direction, so as to offset the earth's magnetic field in the detected environment.

10. The method for using the geomagnetic interference shielding device according to claim 9, characterized in that: When the magnetic field compensation device (5) is used, the PLC controller (3) controls the output of the magnetic field compensation device (5) based on the detection result of the vector magnetic sensor (4), and drives the rotating rod (53) to rotate through the driving motor (54); At the same time, the rotating motor (55) drives the support shaft (56) and the compensation coil (57) to rotate, and the compensation coil (57) is adjusted in multiple dimensions so that the directions of the magnetic field components at the angles of the compensation coil (57) and the outer wall of the detected shielding shell are consistent; The output of the power supply (58) is controlled to adjust the size of the magnetic field generated by the compensation coil (57) so as to offset the earth's magnetic field in the environment.

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