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Study on distribution of magnetic field interference in infrared seekers and shielding plans
Accepted Manuscript Study on Distribution of Magnetic Field Interference in Infrared Seekers and Shielding Plans Zhilei Ge, Yang Cheng, Mengjin Xu, Yan Huang, Suyun Liu, Yanni Wang PII: DOI: Reference:
S0263-2241(18)30898-4 https://doi.org/10.1016/j.measurement.2018.09.064 MEASUR 5922
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
20 May 2018 22 September 2018 24 September 2018
Please cite this article as: Z. Ge, Y. Cheng, M. Xu, Y. Huang, S. Liu, Y. Wang, Study on Distribution of Magnetic Field Interference in Infrared Seekers and Shielding Plans, Measurement (2018), doi: https://doi.org/10.1016/ j.measurement.2018.09.064
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Study on Distribution of Magnetic Field Interference in Infrared Seekers and Shielding Plans Zhilei Ge1, Yang Cheng, Mengjin Xu, Yan Huang, Suyun Liu and Yanni Wang School of Astronautics, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China
Abstract: Measuring the geomagnetic field accurately in real time is essential for the high-precision geomagnetic navigation for missiles. Infrared seekers contain a permanent magnet, which interferes with geomagnetic measurement terribly. The existing researches on modeling and shielding are generally on basis of mathematic methods for compensation. Here, we explore a physical shielding method to weaken the magnetic field and improve the accuracy of geomagnetic measurement. The Galerkin finite element method for scalar magnetic potential are first introduced, and distribution of interferential magnetic field in infrared seekers is analyzed, with an emphasis on various shielding schemes simulated using finite element software ANSYS. In addition, measured values are obtained via physical experiments accordingly. In comparison with measured values, good shielding effects have been achieved. The results are of guiding significance not only for improving the accuracy of magnetic measurement, but for presenting a magnetic shielding method based on finite element method.
Keywords：geomagnetic navigation; magnetic shielding; scalar magnetic potential; finite element; infrared seeker
1. Introduction The geomagnetic field is passive, stable and corresponding to geographic location. It can be used in navigation because of its good independence, low cost and all-weather use [1], which shows good application prospect in autonomous navigation of missiles, ships, vehicles and underwater vehicles. Currently, geomagnetic navigation has become a popular research subject. Geomagnetic navigation consists of 1
Corresponding author: ZhiLei Ge, School of Astronautics, Northwestern Polytechnical
University, Xi’an, Shaanxi 710072, China, Email: [email protected]
three parts: geomagnetic database preparation, geomagnetic data acquisition and geomagnetic matching [2]. In the acquisition of geomagnetic data, it is necessary to use the magnetometer to detect the magnetic field information in real time. However, as the magnetometer can be easily interfered with the vehicle and environmental error sources, there is still no better way to acquire accurate geomagnetic information, which mainly restricts the development and application of geomagnetic navigation technology. In geomagnetic navigation, geomagnetic measurement errors chiefly include instrumental error, variation of geomagnetic field and aircraft magnetic field [3], and the aircraft magnetic field influences on geomagnetic measurement mostly. The infrared seeker, as the terminal guidance equipment, is widely used in missiles. It has an important internal part called “seeker coordinator”. The seeker coordinator contains a permanent magnet which interferes with geomagnetic measurement badly when the missile is in geomagnetic navigation. As the interferential magnetic field is relatively large and it is hard to compensate it directly by a mathematic method [4-8], it is essential to establish the infrared seeker magnetic field model and explore a physical method to weaken the influence of interferential magnetic fields. Conventional models for aircraft magnetic interferential fields include Tolles-Lawson equation [9-10], and 16-order linear model for aeromagnetic compensation. These models classify the main sources of magnetic interference as permanent magnetic field, induced magnetic field and eddy-current magnetic field. Compensation equations with multiple parameters can be set up and parameters can be obtained through parameter estimation. However, the interferential magnetic model is just established for the whole carrier without analyzing the internal interferential magnetic field. In addition, using the magnet simulation method [11], we obtain the equivalent of the carrier with multiple magnetic simulation bodies (including magnetic dipoles, globe and rotational ellipsoid) and conduct mathematic calculation with basic electromagnetic knowledge. However, it is only an analytic method limited to combination of basic current and magnetic fields. It can only simulate simple objects and is not suitable for complex interference source models. In order to more accurately simulate and analyze the interferential field, and with the advance in computer performance, the finite element method is currently widely used to model interferential magnetic field. Zhang [12-14] and Li [15] used finite element software
ANSYS and Ansoft Maxwell to build the models of induced magnetic field and eddy-current magnetic field of submarines and guided missiles. In magnetic shielding, many scholars conducted theoretical derivation of the shielding effects of simple geometries based on the principle of electromagnetic field. Koroglu et al.[16] made computation, simulation and comparisons of shielding effects of cylindrical shields using the analytic method, the finite element method and the neural network method [17]. Besides, many scholars used the finite element method to study magnetic shielding. Paperno [18] and Cazacu [19-20] used finite element software respectively to analyze the shielding effect of multilayer shielding. In China, Zhai [21], Zhang [22], Shao [23] and Wu [24] conducted finite element simulation of magnetic shielding devices for specific designs. Currently, no literature makes reference to the magnetic shielding of infrared seekers. Therefore, the investigation of this paper can provide some reference for reduction of infrared seekers’ magnetic field in geomagnetic navigation. The paper uses finite element software ANSYS to simulate the distribution of interferential magnetic field produced by infrared seekers. The magnetic field is shielded by various shielding facilities of different shapes, thickness and layers. Then effects of the shielding schemes on reducing the disturbing magnetic field are analyzed. Furthermore, the physical experiments are carried out to verify the simulation results. As the infrared seeker is in a lock-out state during geomagnetic navigation and the coil does not work, no current magnetic field is produced. Therefore, this paper only studies the magnetostatic field produced by the infrared seeker in the lock-out state.
2. Galerkin finite element method for scalar magnetic potential As the study only involves soft magnetic materials, hard magnetic materials and magnetic conductive areas and there is no current existing, the magnetic field intensity is irrotational, which can be represented by scalar potential. Using scalar magnetic potential has less unknown variables than vector magnetic potential, which can reduce the computing time. Therefore, scalar magnetic potential is used for analysis in the paper. The Maxwell’s equations can be transformed into the following:
H 0 B 0
(1)
Where B and H represent the magnetic induction intensity and magnetic field intensity
of
the
calculation
domain
respectively
satisfying
the
equation
B H + M , in which is the permeability, and M is the magnetization intensity of ferromagnetic material. As H is irrotational, the scalar magnetic potential can be introduced:
H
(2)
Substituting B H + M into B 0 in Eq. (1), we get H + M 0 . After substituting Eq. (2) into it, we obtain: M
(3)
There is non-linear ferromagnetic medium in the calculation domain, permeability is the function of the position and magnetic induction intensity B . Therefore, Eq. (3) is a non-linear Poisson equation. With the boundary conditions, we can solve Eq. (3), and get the scalar magnetic potential , and arrive at the total magnetic field which is H gard . The Galerkin finite elment method is widely used for high accuracy and wide applicability. Therefore, it’s applied for scalar magnetic potential in this paper. The solution area is first subdivided into limited elements which form nodes. Then the potential function is expanded by means of the primary function and node place value to denote the discrete expression with approximate solution as follows: n
ˆ j N j
(4)
j 1
Where N j is the shape function. j is the unknown coefficient and n is the total number of nodes. The Galerkin method is considered one of the weighted residual methods, which focus on building a set of mutually linearly independent weighting function
Wi i 1, 2,
, n , where equation error (residual) with mean value is zero. Therefore,
the weighted residual method is used to solve Eq. (3) and get the residual equation,
where the weighted integral of the residual in the computational domain is zero:
W d W M d 0
i
i
(5)
From Green theorem, we get:
W d W n dS W d
(6)
W M d W M ndS W Md
(7)
i
i
S
i
S
i
i
i
Where S is the whole boundaries of the domain . Combining Eq. (6) and Eq. (7), we obtain:
W d W Md i
i
(8)
Take the primary function in the expansion Eq. (4) as the weighting function, namely:
Wi Ni
(9)
Substituting Eq. (9) and Eq. (4) into Eq. (8), we get:
n
j 1
Ni j N j d Ni Md
(10)
As j is the coefficient, we can place it outside the gradient symbol. According to the gradient algorithm, we get: n
j 1
j
Ni N j d Ni Md
(11)
So we can make it into: n
s j 1
j ij
Fi
i 1, 2,
n
Where sij Ni N j d , and Fi Ni Md .
So we can create a matrix expression:
(12)
s11 s 21 sn1
s1n 1 F1 s2 n 2 F2 snn n Fn
s12 s22 sn 2
(13)
Or S F
(14)
In this way, we can obtain the value of undetermined coefficient j . After backward substitution of the solved j in the expansion Eq. (4), we get an approximate solution.
3. Finite element model and simulation Finite element simulation requires building a 3-dimention model of infrared seekers and geomagnetic field, the diagram of which is shown in Fig. 1. In a state of mechanical lock, the seeker’s magnetic field is only produced by the permanent magnet. Therefore, simulation of the infrared seeker’s magnetic field is that of the permanent magnet. As shown in the figure, an inertial coordinate system OXYZ is established, where O is a fixed point in space; three axes of OX , OY and OZ are mutually perpendicular lines, forming a right-handed system; the plane XOZ lies in the horizontal plane; the axis OX points to the magnetic south, namely the direction of the north geographic pole. Therefore, magnetic vector Be lies in the plane XOY . Assume that the infrared seeker is placed horizontally and let the center of the permanent magnet be located in point O , and the head of the seeker point to the OX direction. Since the center axis of the permanent magnet coincides with that of the seeker under the condition of mechanical lock, we can determine the position and orientation of the permanent magnet in space. Finally, let the N pole of the permanent magnet point to axis OY , then we can determine their final position in space. In simulation, the shape of permanent magnet is expressed as a cylindrical body, whose sizes and orientations are shown in Fig. 1 Outside the permanent magnet, an inclined large cylindrical body with 6m in both diameter and height is used to simulate the air medium , where the axis is located in the plane OXYZ . The inclined angle I is the angle between the magnetic vector
and the horizontal plane. In this way, when we apply the scalar magnetic potential on the upside and downside of the cylindrical body, we will produce a magnetic field evenly distributed from the upside to the bottom side within the cylindrical body. Based on the values obtained from repeated measurement of the geomagnetic vector and the infrared seeker magnetic field in the local, we can determine the parameters of finite element simulation, in which I 40 and the intensity is around 54500nT. Namely we assume the scalar potential on the upper surface of the cylinder MAG 260 ; that of the lower surface MAG 0 ; the permanent magnet’s
permeability is rp 1.06 ; the coercive force is HCp 945000 A m and the air permeability ra 1.000038 .
6m
6m Be 54500nT
Y 1.1cm
North pole
3.1cm
4.3cm
O
X
Z
I 40 Fig. 1. Diagram of the finite element model for infrared seeker magnetic field.
Then the shielding device of the infrared seeker magnetic field is designed. Since the infrared seeker is located in the front of the missile, we can’t wrap it up to shield it. Neither can we put the shielding device around the permanent magnet or close to it, otherwise it will affect the infrared seeker. Thus, we can only place a device with high permeability in the rear of the infrared seeker to divert the magnetic field for the shielding effect. The shielding device needs to be as large as possible and as near the permanent magnet as possible. As the medium and long-range cruise missiles and air-to-air missiles have long ranges, the missile diameter is large accordingly. Suppose that the diameter is 30cm. we can fully utilize the diameter. Since the shielding device does not wrap up the magnetic field source, and the magnetic induction line still can
skirt it, so the separation can only have some effect in a position relatively near the device. If the shielding device is too small or far from the magnetic field source, the magnetic induction line will be easy to bypass it making the shielding performance poor. Therefore, the shielding device with structure shown in Fig. 2 is adopted. Its basal diameter is around 30cm. The cylinder has one end closed and one end open. Compared with a circular plate, this structure, which has the cylindrical part around it, can block more magnetic induction lines off. At the same time, Fig. 2 also shows the positions of the shielding device and the permanent magnet. The shielding device axis coincides with that of the inertial coordinate system OX . The opening points to the negative direction of axis OX . The bottom surface is 10cm from the central point O of the permanent magnet.
Y 10cm
X
O
Z Fig. 2. Permanent magnet and its shielding device.
In order to analyze the effect of devices with different thickness, shape and layers on shielding the infrared seeker magnetic field, devices with different shielding structures in terms of height, thickness and layers are designed for the shielding simulation in the paper. Its different parameters are shown in Table 1. Table 1. Parameters of magnetic field shielding device. No.
Thickness
mm
Number of layers
cm
Height
1
0.4
1
10
2
0.4
2
10
3
1
1
10
4
2.5
1
10
5
3.5
1
10
6
2.5
1
5
7
2.5
1
15
As shown in Table 1, if two of the three parameters are kept unchanged, the influence of the other parameter on the shielding effect can be analyzed. For example, comparisons of 1, 3, 4, 5 make it possible to analyze the influence of thickness on the shielding effect. Comparisons of 1, 2, 3, 4 make it possible to analyze the influence of the number of layers on the shielding effect. Comparisons of 4, 6, 7 make it possible to analyze the influence of the height on the shielding effect. Shielding devices requires materials with high permeability, such as pure iron, silicon steel, permalloy, iron-aluminum alloys, amorphous alloys and nanocrystalline magnetically soft alloy. Pure iron, permalloy, amorphous alloys and nanocrystalline magnetically soft alloy are ideal materials with relatively large initial permeability. As permalloy contains precious metal, it is relatively expensive. In addition, many amorphous alloys are limited by its critical dimensions and the production technology, so they are not widely applied in industry. Since pure iron has relatively high permeability and coercivity, this paper uses the pure iron DT4C with the highest magnetism in electricity as the material for shielding simulation and testing, in which the B-H curve is shown in Fig. 3.
Fig. 3. B-H curve of pure iron DT4C.
It can been seen from Fig. 4, the magnetic field distributions of the permanent magnet in geomagnetic field are simulated using software ANSYS, both when there is no shielding device, and when the shielding device with thickness of 2.5cm and height of 10cm.
Fig. 4. Nephograms for magnetic field distribution without shielding device(left side) and with shielding device(right side).
Fig. 4 shows that there are relatively strong magnetic fields around the permanent magnet. When the distance is increased, the magnetic induction intensity decreases. When some certain distance is reached, it will gradually approach the value of the geomagnetic field. When there is a shielding device near the permanent magnet, the magnetic induction intensity at the back of the permanent magnet will decrease markedly, and the spot near to the shielding device has relatively weak intensity. When the distance increases, it will gradually recover the value of the geomagnetic field. We can take the magnetic induction intensity of the magnetic field at points -30cm, -40cm, -50cm, -60cm, -70cm, -80cm, -90cm, -100cm, -110cm, -120cm and -130cm away respectively, which are used as the simulation and measurement references.
4. Magnetic shielding experiment of Infrared Seeker This paper conducts verification experiments of the magnetic field produced by the infrared seeker under conditions of no-shielding and multiple shielding devices. The measurement diagram is shown in Fig. 5.
y
o I
Be
z
Y North pole
x
Z
O
X
120 110 100 90 80 70 60 50 40 30 20 10 0 (cm) Fig. 5. Diagram for magnetic shielding experiment of Infrared Seeker.
The measurement will be carried out on a south-north geomagnetic line on the horizontal plane. The center of the infrared seeker’s permanent magnet is located at 0cm from the northernmost part, with the height equal to the missile diameter of 15cm; the pole N points to the positive direction of the axis Y ; the bottom of the shielding device is placed at the back of the infrared seeker and 10cm away from the center of the permanent magnet. The raw material used for making the shielding device is sheet material of pure iron DT4C, with 0.4mm, 1mm, 2.5mm and 3.5mm in thickness respectively. Its carbon content is not more than 0.025%; the coercivity is 48A/m; the largest relative permeability is 162338. After laser cutting, bending, and argon arc welding, it is processed into the desired shape. Finally, after annealing in the vacuum furnace to eliminate the effect of residual stress on magnetic property, the machining of the shielding device is completed. For the measurement, the magnetometer used is the FVM-400 vector magnetic flux gate manufactured by American firm MEDA. Its resolution ratio can reach 1nT in a 100000nT field. It can measure magnetic field in three directions: the x , y and
z directions. The oxyz system in Fig. 5 is the magnetic field measurement coordinate system, where the origin is located at the center of probe of the magnetometer. Its three coordinate axes are parallel to the three axis of the OXYZ system. Moving the probes can record the three coordinate values of the magnetic field on the infrared seeker’s central axis at points 30cm, 40cm, 50cm, 60cm, 70cm, 80cm, 90cm, 100cm, 110cm, 120cm and 130cm from the permanent magnet’s center. After repeating measurements of the magnetic induction intensity at different test positions with the same shielding device, we can get many measured values. After averaging measured values, we can obtain the total magnetic induction intensity, which is used as the final measured value of that test position. See Fig. 6 for the actual measurement and
shielding devices.
Fig. 6. The measurement and shielding devices.
5. Results and discussion 5.1 Simulation and measurement Fig. 7 and Fig. 8 show measured geomagnetic values, simulation values and measured values of magnetic induction intensity in different test positions both with shielding devices and without shielding devices. The first subplot of the picture shows the total magnetic induction intensity; the second subplot shows the value of the x-component; the third part shows the value of the y-component; the forth part shows the value of the z-component.
Fig. 7. Magnetic induction intensity without shielding device.
It’s seen from Fig. 7 that, under the condition of no shielding device, the magnetic field produced by infrared seeker is very large when the distance is very
close, and then decreases as the distance grows. In addition, the simulated values of the total magnetic induction intensity are nearly the same as the measured values. The maximum error between the simulated magnetic induction intensity and the measured value is 0.18 μT when the distance increases. Fig. 8 shows the curve of measured geomagnetic values, simulated values and measured values of magnetic induction intensity with the distance when the thickness of the shielding device is 1mm. It’s seen that, when the shielding device is placed behind the infrared seeker, there is a relatively large decrease in the total magnetic induction intensity and the y-component. If the magnetic detector in the geomagnetic navigation is within 90cm from the permanent magnet, the interferential magnetic field will be pretty high and it is difficult to be eliminated by mathematic method. Besides, the maximum error between the simulated magnetic induction intensity and the measured value is 0.247 μT with the distance changing.
Fig. 8. Magnetic induction intensity with shielding device.
It’s found in Fig. 7 and Fig. 8 that, when the distance is 40-110cm from the permanent magnet, the errors between the total magnetic induction intensity and the geomagnetic field are far lower than when there is no shielding device at the same distance, and with the increase of the distance, the geomagnetic errors decrease; when the distance is above 120cm, with or without shielding device, the magnetic field in the infrared seeker is basically the same with the geomagnetic field. Therefore, the shielding device can improve the accuracy of geomagnetic measurement. In addition, the simulated magnetic induction intensity values follow the measured values with the maximum error less than 0.25 μT , which demonstrates the validity and accuracy of
the infinite element method for magnetic shielding. 5.2 Comparison of shielding effects In this paper, the shielding effects of different shielding schemes are compared. The control variable method is used to analyze the change of the three variables on the shielding effect where two of the three variables—the thickness, the number of layers and the height of the shielding device remain unchanged while the other changes.
Fig. 9. Magnetic field measurement of the shielding devices with different thickness.
Fig. 9 shows the measured values of magnetic field under the condition of the shielding devices with different thickness, which illustrate that shielding devices can weaken the infrared seeker magnetic field markedly. It can been seen from Fig. 9 that the effect of 2.5mm-thick shielding device is better than that of 3.5mm-thick device, while the values are not much different in the y direction. Therefore, it’s found that the thicker the shielding device is, the better the shielding performance will be in terms of the total magnetic induction intensity and the component. But when the device is too thick, it will affect the shielding effect instead. In practice, there is no need for the shielding devices with high thickness. The result shows that shielding devices with thickness of 2.5 achieve the better effect.
Fig. 10. Magnetic field measurement of the shielding devices with different layers.
To study the influence of the number of layers on the shielding effect, Fig. 10 shows that, when the distance is larger than 40cm, shielding devices with two layers and thickness of 0.4mm have better effects than those with a single layer and the same thickness in terms of the total magnetic induction intensity and the x-component and y-component. Compared with shielding devices with thickness of 1mm, the total magnetic induction intensity still has a better effect, even though the effects of the x-component and y-component are similar. Compared with shielding devices with thickness of 2.5mm, those with two layers and thickness of 0.4mm have slightly better effects in terms of the total magnetic induction intensity. Thus, two-layered shielding devices have better shielding performance. As two-layered shielding devices have smaller total thickness and light weight, they are more suitable for weapons such as missiles.
Fig. 11. Magnetic field measurement of the shielding devices with different height.
To analyze the influence of the height on the shielding effect, Fig. 11 indicates that, under the design scheme, the height of the shielding device has a relatively large impact on the shielding effect. In general, the higher the shielding device is, the larger the decrease of the total magnetic induction intensity and the y-component in a near position will become. However, in a spot above 40cm, the increase of height is not consistent with increased shielding effect. Comparing shielding device with height of 5cm with that with height of 10cm, it is found that 10cm-high device have better shielding results than the 5cm-high device in terms of both the total magnetic induction intensity and the y-component. Therefore, higher shielding devices adopted for this paper are reasonable. Comparing shielding device with height of 10cm with that with height of 15cm, it is found that the 10cm-high device more approaches the geomagnetic field in terms of the total magnetic induction intensity and the x-component and y-components. In addition, the x-component of 15cm-high shielding devices has large errors occurring at a distance of 40-70cm. Therefore, higher shielding devices can’t increase the shielding effect, but will produce errors in the direction of the magnetic field, so it is not appropriate to use much higher shielding devices. The result shows that shielding device with thickness of 2.5 cm and height of 10cm has the better shielding performance. To sum up, shielding devices designed by this paper use pure iron DT4C as the material. The structure with thickness of 2.5cm and height of 10cm has a better shielding effect. In a position above 40cm, the largest error between its magnetic induction intensity and the geomagnetic field is only 0.592 μT , which can provide a relatively large scope for installation of the magnetometer. When factors such as weight and price are taken into consideration, structure with an appropriate height, smaller thickness and two layers can be used, such as two-layered shielding devices with thickness of 0.4mm and height of 10cm, because it can produce a better shielding effect while reducing the cost and the burden of the missile. If a 0.4mm-thick and two-layered devices is used, the magnetometer can be installed at a distance larger than 70cm. But when the distance reaches 120cm, the magnetic induction intensity with no-shielding case is basically the same as that with the shielding case, both approaching the geomagnetic field after weakening. Therefore, when the magnetometer is installed at a distance larger than 120cm, it will have a relatively accurate measurement, without the need for a shielding device.
In the meantime, to verify the shielding effects, we have made comparisons with mathematic methods presented in Ref[13] and Ref[14], which both use the algorithms for the mathematic compensation for the geomagnetic field measurement when the interferential magnetic field exists in geomagnetic navigation. Table 2．Comparison of Shielding effects with mathematic methods. Physical shielding
Differential evolution algorithm in Ref[13]
New method in Ref[14]
Error before shielding 7.993
Nearly 10
Nearly 2
compensation（ μT ）
0.592
0.055
0.03
Shielding efficiency
92.59%
99.55%
98.5%
or compensation（ μT ） Error after shielding or
It can be seen from Table 2 that, the method proposed in the paper can shield nearly 9% of the interferential magnetic field when the distance is close to infrared seekers, although the shielding effect in the physical method is slightly lower than that in the mathematic methods. In fact, infrared seekers in missiles can’t be fully wrapped, which indicates that the interferential magnetic field can’t be completely shielded by shielding devices. Both the physical shielding method proposed in this paper and the mathematic compensation methods can be combined to improve the accuracy for geomagnetic navigation, which is worthy researching in the future.
6. Conclusions In this paper, the interferential magnetic field in infrared seekers based on the scalar magnetic potential method has been analyzed with ANSYS, and various shielding schemes are designed to weaken the magnetic field in order to improve the accuracy of geomagnetic measurement. Comparative studies are carried out on the shielding effect of shielding devices with different thickness, number of layers and height through physical experiments. It’s found that devices with thickness of 2.5mm and height of 10cm have the best shielding effectiveness. The probe of the magnetometer is best installed more than 40cm from the permanent magnet. The larger the distance is, the smaller the interference from the infrared seeker magnetic field will be. When the distance is larger than 120cm, it is not necessary to install a shielding device. The proposed method in the paper is of guiding significance not
only for improving the magnetic measurement accuracy, but for presenting a magnetic shielding method based on finite element method.
Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 61374209).
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Highlights
1）The interferential magnetic field can be shielded by the physical method. 2）92.6% of the interferential magnetic field near infrared seekers is shielded. 3）Devices with thickness of 2.5mm and height of 10cm are the best for shielding. 4）The magnetometer is best installed over 40cm from the permanent magnet.
S0263-2241(18)30898-4 https://doi.org/10.1016/j.measurement.2018.09.064 MEASUR 5922
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
20 May 2018 22 September 2018 24 September 2018
Please cite this article as: Z. Ge, Y. Cheng, M. Xu, Y. Huang, S. Liu, Y. Wang, Study on Distribution of Magnetic Field Interference in Infrared Seekers and Shielding Plans, Measurement (2018), doi: https://doi.org/10.1016/ j.measurement.2018.09.064
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Study on Distribution of Magnetic Field Interference in Infrared Seekers and Shielding Plans Zhilei Ge1, Yang Cheng, Mengjin Xu, Yan Huang, Suyun Liu and Yanni Wang School of Astronautics, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China
Abstract: Measuring the geomagnetic field accurately in real time is essential for the high-precision geomagnetic navigation for missiles. Infrared seekers contain a permanent magnet, which interferes with geomagnetic measurement terribly. The existing researches on modeling and shielding are generally on basis of mathematic methods for compensation. Here, we explore a physical shielding method to weaken the magnetic field and improve the accuracy of geomagnetic measurement. The Galerkin finite element method for scalar magnetic potential are first introduced, and distribution of interferential magnetic field in infrared seekers is analyzed, with an emphasis on various shielding schemes simulated using finite element software ANSYS. In addition, measured values are obtained via physical experiments accordingly. In comparison with measured values, good shielding effects have been achieved. The results are of guiding significance not only for improving the accuracy of magnetic measurement, but for presenting a magnetic shielding method based on finite element method.
Keywords：geomagnetic navigation; magnetic shielding; scalar magnetic potential; finite element; infrared seeker
1. Introduction The geomagnetic field is passive, stable and corresponding to geographic location. It can be used in navigation because of its good independence, low cost and all-weather use [1], which shows good application prospect in autonomous navigation of missiles, ships, vehicles and underwater vehicles. Currently, geomagnetic navigation has become a popular research subject. Geomagnetic navigation consists of 1
Corresponding author: ZhiLei Ge, School of Astronautics, Northwestern Polytechnical
University, Xi’an, Shaanxi 710072, China, Email: [email protected]
three parts: geomagnetic database preparation, geomagnetic data acquisition and geomagnetic matching [2]. In the acquisition of geomagnetic data, it is necessary to use the magnetometer to detect the magnetic field information in real time. However, as the magnetometer can be easily interfered with the vehicle and environmental error sources, there is still no better way to acquire accurate geomagnetic information, which mainly restricts the development and application of geomagnetic navigation technology. In geomagnetic navigation, geomagnetic measurement errors chiefly include instrumental error, variation of geomagnetic field and aircraft magnetic field [3], and the aircraft magnetic field influences on geomagnetic measurement mostly. The infrared seeker, as the terminal guidance equipment, is widely used in missiles. It has an important internal part called “seeker coordinator”. The seeker coordinator contains a permanent magnet which interferes with geomagnetic measurement badly when the missile is in geomagnetic navigation. As the interferential magnetic field is relatively large and it is hard to compensate it directly by a mathematic method [4-8], it is essential to establish the infrared seeker magnetic field model and explore a physical method to weaken the influence of interferential magnetic fields. Conventional models for aircraft magnetic interferential fields include Tolles-Lawson equation [9-10], and 16-order linear model for aeromagnetic compensation. These models classify the main sources of magnetic interference as permanent magnetic field, induced magnetic field and eddy-current magnetic field. Compensation equations with multiple parameters can be set up and parameters can be obtained through parameter estimation. However, the interferential magnetic model is just established for the whole carrier without analyzing the internal interferential magnetic field. In addition, using the magnet simulation method [11], we obtain the equivalent of the carrier with multiple magnetic simulation bodies (including magnetic dipoles, globe and rotational ellipsoid) and conduct mathematic calculation with basic electromagnetic knowledge. However, it is only an analytic method limited to combination of basic current and magnetic fields. It can only simulate simple objects and is not suitable for complex interference source models. In order to more accurately simulate and analyze the interferential field, and with the advance in computer performance, the finite element method is currently widely used to model interferential magnetic field. Zhang [12-14] and Li [15] used finite element software
ANSYS and Ansoft Maxwell to build the models of induced magnetic field and eddy-current magnetic field of submarines and guided missiles. In magnetic shielding, many scholars conducted theoretical derivation of the shielding effects of simple geometries based on the principle of electromagnetic field. Koroglu et al.[16] made computation, simulation and comparisons of shielding effects of cylindrical shields using the analytic method, the finite element method and the neural network method [17]. Besides, many scholars used the finite element method to study magnetic shielding. Paperno [18] and Cazacu [19-20] used finite element software respectively to analyze the shielding effect of multilayer shielding. In China, Zhai [21], Zhang [22], Shao [23] and Wu [24] conducted finite element simulation of magnetic shielding devices for specific designs. Currently, no literature makes reference to the magnetic shielding of infrared seekers. Therefore, the investigation of this paper can provide some reference for reduction of infrared seekers’ magnetic field in geomagnetic navigation. The paper uses finite element software ANSYS to simulate the distribution of interferential magnetic field produced by infrared seekers. The magnetic field is shielded by various shielding facilities of different shapes, thickness and layers. Then effects of the shielding schemes on reducing the disturbing magnetic field are analyzed. Furthermore, the physical experiments are carried out to verify the simulation results. As the infrared seeker is in a lock-out state during geomagnetic navigation and the coil does not work, no current magnetic field is produced. Therefore, this paper only studies the magnetostatic field produced by the infrared seeker in the lock-out state.
2. Galerkin finite element method for scalar magnetic potential As the study only involves soft magnetic materials, hard magnetic materials and magnetic conductive areas and there is no current existing, the magnetic field intensity is irrotational, which can be represented by scalar potential. Using scalar magnetic potential has less unknown variables than vector magnetic potential, which can reduce the computing time. Therefore, scalar magnetic potential is used for analysis in the paper. The Maxwell’s equations can be transformed into the following:
H 0 B 0
(1)
Where B and H represent the magnetic induction intensity and magnetic field intensity
of
the
calculation
domain
respectively
satisfying
the
equation
B H + M , in which is the permeability, and M is the magnetization intensity of ferromagnetic material. As H is irrotational, the scalar magnetic potential can be introduced:
H
(2)
Substituting B H + M into B 0 in Eq. (1), we get H + M 0 . After substituting Eq. (2) into it, we obtain: M
(3)
There is non-linear ferromagnetic medium in the calculation domain, permeability is the function of the position and magnetic induction intensity B . Therefore, Eq. (3) is a non-linear Poisson equation. With the boundary conditions, we can solve Eq. (3), and get the scalar magnetic potential , and arrive at the total magnetic field which is H gard . The Galerkin finite elment method is widely used for high accuracy and wide applicability. Therefore, it’s applied for scalar magnetic potential in this paper. The solution area is first subdivided into limited elements which form nodes. Then the potential function is expanded by means of the primary function and node place value to denote the discrete expression with approximate solution as follows: n
ˆ j N j
(4)
j 1
Where N j is the shape function. j is the unknown coefficient and n is the total number of nodes. The Galerkin method is considered one of the weighted residual methods, which focus on building a set of mutually linearly independent weighting function
Wi i 1, 2,
, n , where equation error (residual) with mean value is zero. Therefore,
the weighted residual method is used to solve Eq. (3) and get the residual equation,
where the weighted integral of the residual in the computational domain is zero:
W d W M d 0
i
i
(5)
From Green theorem, we get:
W d W n dS W d
(6)
W M d W M ndS W Md
(7)
i
i
S
i
S
i
i
i
Where S is the whole boundaries of the domain . Combining Eq. (6) and Eq. (7), we obtain:
W d W Md i
i
(8)
Take the primary function in the expansion Eq. (4) as the weighting function, namely:
Wi Ni
(9)
Substituting Eq. (9) and Eq. (4) into Eq. (8), we get:
n
j 1
Ni j N j d Ni Md
(10)
As j is the coefficient, we can place it outside the gradient symbol. According to the gradient algorithm, we get: n
j 1
j
Ni N j d Ni Md
(11)
So we can make it into: n
s j 1
j ij
Fi
i 1, 2,
n
Where sij Ni N j d , and Fi Ni Md .
So we can create a matrix expression:
(12)
s11 s 21 sn1
s1n 1 F1 s2 n 2 F2 snn n Fn
s12 s22 sn 2
(13)
Or S F
(14)
In this way, we can obtain the value of undetermined coefficient j . After backward substitution of the solved j in the expansion Eq. (4), we get an approximate solution.
3. Finite element model and simulation Finite element simulation requires building a 3-dimention model of infrared seekers and geomagnetic field, the diagram of which is shown in Fig. 1. In a state of mechanical lock, the seeker’s magnetic field is only produced by the permanent magnet. Therefore, simulation of the infrared seeker’s magnetic field is that of the permanent magnet. As shown in the figure, an inertial coordinate system OXYZ is established, where O is a fixed point in space; three axes of OX , OY and OZ are mutually perpendicular lines, forming a right-handed system; the plane XOZ lies in the horizontal plane; the axis OX points to the magnetic south, namely the direction of the north geographic pole. Therefore, magnetic vector Be lies in the plane XOY . Assume that the infrared seeker is placed horizontally and let the center of the permanent magnet be located in point O , and the head of the seeker point to the OX direction. Since the center axis of the permanent magnet coincides with that of the seeker under the condition of mechanical lock, we can determine the position and orientation of the permanent magnet in space. Finally, let the N pole of the permanent magnet point to axis OY , then we can determine their final position in space. In simulation, the shape of permanent magnet is expressed as a cylindrical body, whose sizes and orientations are shown in Fig. 1 Outside the permanent magnet, an inclined large cylindrical body with 6m in both diameter and height is used to simulate the air medium , where the axis is located in the plane OXYZ . The inclined angle I is the angle between the magnetic vector
and the horizontal plane. In this way, when we apply the scalar magnetic potential on the upside and downside of the cylindrical body, we will produce a magnetic field evenly distributed from the upside to the bottom side within the cylindrical body. Based on the values obtained from repeated measurement of the geomagnetic vector and the infrared seeker magnetic field in the local, we can determine the parameters of finite element simulation, in which I 40 and the intensity is around 54500nT. Namely we assume the scalar potential on the upper surface of the cylinder MAG 260 ; that of the lower surface MAG 0 ; the permanent magnet’s
permeability is rp 1.06 ; the coercive force is HCp 945000 A m and the air permeability ra 1.000038 .
6m
6m Be 54500nT
Y 1.1cm
North pole
3.1cm
4.3cm
O
X
Z
I 40 Fig. 1. Diagram of the finite element model for infrared seeker magnetic field.
Then the shielding device of the infrared seeker magnetic field is designed. Since the infrared seeker is located in the front of the missile, we can’t wrap it up to shield it. Neither can we put the shielding device around the permanent magnet or close to it, otherwise it will affect the infrared seeker. Thus, we can only place a device with high permeability in the rear of the infrared seeker to divert the magnetic field for the shielding effect. The shielding device needs to be as large as possible and as near the permanent magnet as possible. As the medium and long-range cruise missiles and air-to-air missiles have long ranges, the missile diameter is large accordingly. Suppose that the diameter is 30cm. we can fully utilize the diameter. Since the shielding device does not wrap up the magnetic field source, and the magnetic induction line still can
skirt it, so the separation can only have some effect in a position relatively near the device. If the shielding device is too small or far from the magnetic field source, the magnetic induction line will be easy to bypass it making the shielding performance poor. Therefore, the shielding device with structure shown in Fig. 2 is adopted. Its basal diameter is around 30cm. The cylinder has one end closed and one end open. Compared with a circular plate, this structure, which has the cylindrical part around it, can block more magnetic induction lines off. At the same time, Fig. 2 also shows the positions of the shielding device and the permanent magnet. The shielding device axis coincides with that of the inertial coordinate system OX . The opening points to the negative direction of axis OX . The bottom surface is 10cm from the central point O of the permanent magnet.
Y 10cm
X
O
Z Fig. 2. Permanent magnet and its shielding device.
In order to analyze the effect of devices with different thickness, shape and layers on shielding the infrared seeker magnetic field, devices with different shielding structures in terms of height, thickness and layers are designed for the shielding simulation in the paper. Its different parameters are shown in Table 1. Table 1. Parameters of magnetic field shielding device. No.
Thickness
mm
Number of layers
cm
Height
1
0.4
1
10
2
0.4
2
10
3
1
1
10
4
2.5
1
10
5
3.5
1
10
6
2.5
1
5
7
2.5
1
15
As shown in Table 1, if two of the three parameters are kept unchanged, the influence of the other parameter on the shielding effect can be analyzed. For example, comparisons of 1, 3, 4, 5 make it possible to analyze the influence of thickness on the shielding effect. Comparisons of 1, 2, 3, 4 make it possible to analyze the influence of the number of layers on the shielding effect. Comparisons of 4, 6, 7 make it possible to analyze the influence of the height on the shielding effect. Shielding devices requires materials with high permeability, such as pure iron, silicon steel, permalloy, iron-aluminum alloys, amorphous alloys and nanocrystalline magnetically soft alloy. Pure iron, permalloy, amorphous alloys and nanocrystalline magnetically soft alloy are ideal materials with relatively large initial permeability. As permalloy contains precious metal, it is relatively expensive. In addition, many amorphous alloys are limited by its critical dimensions and the production technology, so they are not widely applied in industry. Since pure iron has relatively high permeability and coercivity, this paper uses the pure iron DT4C with the highest magnetism in electricity as the material for shielding simulation and testing, in which the B-H curve is shown in Fig. 3.
Fig. 3. B-H curve of pure iron DT4C.
It can been seen from Fig. 4, the magnetic field distributions of the permanent magnet in geomagnetic field are simulated using software ANSYS, both when there is no shielding device, and when the shielding device with thickness of 2.5cm and height of 10cm.
Fig. 4. Nephograms for magnetic field distribution without shielding device(left side) and with shielding device(right side).
Fig. 4 shows that there are relatively strong magnetic fields around the permanent magnet. When the distance is increased, the magnetic induction intensity decreases. When some certain distance is reached, it will gradually approach the value of the geomagnetic field. When there is a shielding device near the permanent magnet, the magnetic induction intensity at the back of the permanent magnet will decrease markedly, and the spot near to the shielding device has relatively weak intensity. When the distance increases, it will gradually recover the value of the geomagnetic field. We can take the magnetic induction intensity of the magnetic field at points -30cm, -40cm, -50cm, -60cm, -70cm, -80cm, -90cm, -100cm, -110cm, -120cm and -130cm away respectively, which are used as the simulation and measurement references.
4. Magnetic shielding experiment of Infrared Seeker This paper conducts verification experiments of the magnetic field produced by the infrared seeker under conditions of no-shielding and multiple shielding devices. The measurement diagram is shown in Fig. 5.
y
o I
Be
z
Y North pole
x
Z
O
X
120 110 100 90 80 70 60 50 40 30 20 10 0 (cm) Fig. 5. Diagram for magnetic shielding experiment of Infrared Seeker.
The measurement will be carried out on a south-north geomagnetic line on the horizontal plane. The center of the infrared seeker’s permanent magnet is located at 0cm from the northernmost part, with the height equal to the missile diameter of 15cm; the pole N points to the positive direction of the axis Y ; the bottom of the shielding device is placed at the back of the infrared seeker and 10cm away from the center of the permanent magnet. The raw material used for making the shielding device is sheet material of pure iron DT4C, with 0.4mm, 1mm, 2.5mm and 3.5mm in thickness respectively. Its carbon content is not more than 0.025%; the coercivity is 48A/m; the largest relative permeability is 162338. After laser cutting, bending, and argon arc welding, it is processed into the desired shape. Finally, after annealing in the vacuum furnace to eliminate the effect of residual stress on magnetic property, the machining of the shielding device is completed. For the measurement, the magnetometer used is the FVM-400 vector magnetic flux gate manufactured by American firm MEDA. Its resolution ratio can reach 1nT in a 100000nT field. It can measure magnetic field in three directions: the x , y and
z directions. The oxyz system in Fig. 5 is the magnetic field measurement coordinate system, where the origin is located at the center of probe of the magnetometer. Its three coordinate axes are parallel to the three axis of the OXYZ system. Moving the probes can record the three coordinate values of the magnetic field on the infrared seeker’s central axis at points 30cm, 40cm, 50cm, 60cm, 70cm, 80cm, 90cm, 100cm, 110cm, 120cm and 130cm from the permanent magnet’s center. After repeating measurements of the magnetic induction intensity at different test positions with the same shielding device, we can get many measured values. After averaging measured values, we can obtain the total magnetic induction intensity, which is used as the final measured value of that test position. See Fig. 6 for the actual measurement and
shielding devices.
Fig. 6. The measurement and shielding devices.
5. Results and discussion 5.1 Simulation and measurement Fig. 7 and Fig. 8 show measured geomagnetic values, simulation values and measured values of magnetic induction intensity in different test positions both with shielding devices and without shielding devices. The first subplot of the picture shows the total magnetic induction intensity; the second subplot shows the value of the x-component; the third part shows the value of the y-component; the forth part shows the value of the z-component.
Fig. 7. Magnetic induction intensity without shielding device.
It’s seen from Fig. 7 that, under the condition of no shielding device, the magnetic field produced by infrared seeker is very large when the distance is very
close, and then decreases as the distance grows. In addition, the simulated values of the total magnetic induction intensity are nearly the same as the measured values. The maximum error between the simulated magnetic induction intensity and the measured value is 0.18 μT when the distance increases. Fig. 8 shows the curve of measured geomagnetic values, simulated values and measured values of magnetic induction intensity with the distance when the thickness of the shielding device is 1mm. It’s seen that, when the shielding device is placed behind the infrared seeker, there is a relatively large decrease in the total magnetic induction intensity and the y-component. If the magnetic detector in the geomagnetic navigation is within 90cm from the permanent magnet, the interferential magnetic field will be pretty high and it is difficult to be eliminated by mathematic method. Besides, the maximum error between the simulated magnetic induction intensity and the measured value is 0.247 μT with the distance changing.
Fig. 8. Magnetic induction intensity with shielding device.
It’s found in Fig. 7 and Fig. 8 that, when the distance is 40-110cm from the permanent magnet, the errors between the total magnetic induction intensity and the geomagnetic field are far lower than when there is no shielding device at the same distance, and with the increase of the distance, the geomagnetic errors decrease; when the distance is above 120cm, with or without shielding device, the magnetic field in the infrared seeker is basically the same with the geomagnetic field. Therefore, the shielding device can improve the accuracy of geomagnetic measurement. In addition, the simulated magnetic induction intensity values follow the measured values with the maximum error less than 0.25 μT , which demonstrates the validity and accuracy of
the infinite element method for magnetic shielding. 5.2 Comparison of shielding effects In this paper, the shielding effects of different shielding schemes are compared. The control variable method is used to analyze the change of the three variables on the shielding effect where two of the three variables—the thickness, the number of layers and the height of the shielding device remain unchanged while the other changes.
Fig. 9. Magnetic field measurement of the shielding devices with different thickness.
Fig. 9 shows the measured values of magnetic field under the condition of the shielding devices with different thickness, which illustrate that shielding devices can weaken the infrared seeker magnetic field markedly. It can been seen from Fig. 9 that the effect of 2.5mm-thick shielding device is better than that of 3.5mm-thick device, while the values are not much different in the y direction. Therefore, it’s found that the thicker the shielding device is, the better the shielding performance will be in terms of the total magnetic induction intensity and the component. But when the device is too thick, it will affect the shielding effect instead. In practice, there is no need for the shielding devices with high thickness. The result shows that shielding devices with thickness of 2.5 achieve the better effect.
Fig. 10. Magnetic field measurement of the shielding devices with different layers.
To study the influence of the number of layers on the shielding effect, Fig. 10 shows that, when the distance is larger than 40cm, shielding devices with two layers and thickness of 0.4mm have better effects than those with a single layer and the same thickness in terms of the total magnetic induction intensity and the x-component and y-component. Compared with shielding devices with thickness of 1mm, the total magnetic induction intensity still has a better effect, even though the effects of the x-component and y-component are similar. Compared with shielding devices with thickness of 2.5mm, those with two layers and thickness of 0.4mm have slightly better effects in terms of the total magnetic induction intensity. Thus, two-layered shielding devices have better shielding performance. As two-layered shielding devices have smaller total thickness and light weight, they are more suitable for weapons such as missiles.
Fig. 11. Magnetic field measurement of the shielding devices with different height.
To analyze the influence of the height on the shielding effect, Fig. 11 indicates that, under the design scheme, the height of the shielding device has a relatively large impact on the shielding effect. In general, the higher the shielding device is, the larger the decrease of the total magnetic induction intensity and the y-component in a near position will become. However, in a spot above 40cm, the increase of height is not consistent with increased shielding effect. Comparing shielding device with height of 5cm with that with height of 10cm, it is found that 10cm-high device have better shielding results than the 5cm-high device in terms of both the total magnetic induction intensity and the y-component. Therefore, higher shielding devices adopted for this paper are reasonable. Comparing shielding device with height of 10cm with that with height of 15cm, it is found that the 10cm-high device more approaches the geomagnetic field in terms of the total magnetic induction intensity and the x-component and y-components. In addition, the x-component of 15cm-high shielding devices has large errors occurring at a distance of 40-70cm. Therefore, higher shielding devices can’t increase the shielding effect, but will produce errors in the direction of the magnetic field, so it is not appropriate to use much higher shielding devices. The result shows that shielding device with thickness of 2.5 cm and height of 10cm has the better shielding performance. To sum up, shielding devices designed by this paper use pure iron DT4C as the material. The structure with thickness of 2.5cm and height of 10cm has a better shielding effect. In a position above 40cm, the largest error between its magnetic induction intensity and the geomagnetic field is only 0.592 μT , which can provide a relatively large scope for installation of the magnetometer. When factors such as weight and price are taken into consideration, structure with an appropriate height, smaller thickness and two layers can be used, such as two-layered shielding devices with thickness of 0.4mm and height of 10cm, because it can produce a better shielding effect while reducing the cost and the burden of the missile. If a 0.4mm-thick and two-layered devices is used, the magnetometer can be installed at a distance larger than 70cm. But when the distance reaches 120cm, the magnetic induction intensity with no-shielding case is basically the same as that with the shielding case, both approaching the geomagnetic field after weakening. Therefore, when the magnetometer is installed at a distance larger than 120cm, it will have a relatively accurate measurement, without the need for a shielding device.
In the meantime, to verify the shielding effects, we have made comparisons with mathematic methods presented in Ref[13] and Ref[14], which both use the algorithms for the mathematic compensation for the geomagnetic field measurement when the interferential magnetic field exists in geomagnetic navigation. Table 2．Comparison of Shielding effects with mathematic methods. Physical shielding
Differential evolution algorithm in Ref[13]
New method in Ref[14]
Error before shielding 7.993
Nearly 10
Nearly 2
compensation（ μT ）
0.592
0.055
0.03
Shielding efficiency
92.59%
99.55%
98.5%
or compensation（ μT ） Error after shielding or
It can be seen from Table 2 that, the method proposed in the paper can shield nearly 9% of the interferential magnetic field when the distance is close to infrared seekers, although the shielding effect in the physical method is slightly lower than that in the mathematic methods. In fact, infrared seekers in missiles can’t be fully wrapped, which indicates that the interferential magnetic field can’t be completely shielded by shielding devices. Both the physical shielding method proposed in this paper and the mathematic compensation methods can be combined to improve the accuracy for geomagnetic navigation, which is worthy researching in the future.
6. Conclusions In this paper, the interferential magnetic field in infrared seekers based on the scalar magnetic potential method has been analyzed with ANSYS, and various shielding schemes are designed to weaken the magnetic field in order to improve the accuracy of geomagnetic measurement. Comparative studies are carried out on the shielding effect of shielding devices with different thickness, number of layers and height through physical experiments. It’s found that devices with thickness of 2.5mm and height of 10cm have the best shielding effectiveness. The probe of the magnetometer is best installed more than 40cm from the permanent magnet. The larger the distance is, the smaller the interference from the infrared seeker magnetic field will be. When the distance is larger than 120cm, it is not necessary to install a shielding device. The proposed method in the paper is of guiding significance not
only for improving the magnetic measurement accuracy, but for presenting a magnetic shielding method based on finite element method.
Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 61374209).
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Highlights
1）The interferential magnetic field can be shielded by the physical method. 2）92.6% of the interferential magnetic field near infrared seekers is shielded. 3）Devices with thickness of 2.5mm and height of 10cm are the best for shielding. 4）The magnetometer is best installed over 40cm from the permanent magnet.