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Front. Inform. Technol. Electron. Eng.  2016, Vol. 17 Issue (1): 74-82    DOI: 10.1631/FITEE.1500114
Original article     
Improving the efficiency of magnetic coupling energy transfer by etching fractal patterns in the shielding metals*
Qing-feng LI1,2,?(),Shao-bo CHEN1,2,Wei-ming WANG1,2,Hong-wei HAO1,2,Lu-ming LI1,2,3,?()
1School of Aerospace, Tsinghua University, Beijing 100084, China
2National Engineering Laboratory for Neuromodulation, Beijing 100084, China
3Center of Epilepsy, Beijing Institute for Brain Disorders, Beijing 100084, China
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Abstract  

Thin metal sheets are often located in the coupling paths of magnetic coupling energy transfer (MCET) systems. Eddy currents in the metals reduce the energy transfer efficiency and can even present safety risks. This paper describes the use of etched fractal patterns in the metals to suppress the eddy currents and improve the efficiency. Simulation and experimental results show that this approach is very effective. The fractal patterns should satisfy three features, namely, breaking the metal edge, etching in the high-intensity magnetic field region, and etching through the metal in the thickness direction. Different fractal patterns lead to different results. By altering the eddy current distribution, the fractal pattern slots reduce the eddy current losses when the metals show resistance effects and suppress the induced magnetic field in the metals when the metals show inductance effects. Fractal pattern slots in multilayer high conductivity metals (e.g., Cu) reduce the induced magnetic field intensity significantly. Furthermore, transfer power, transfer efficiency, receiving efficiency, and eddy current losses all increase with the increase of the number of etched layers. These results can benefit MCET by efficient energy transfer and safe use in metal shielded equipment.

Key words： Fractal pattern      Metal-layer-shield      Eddy current      Magnetic coupling energy transfer     
Received: 09 April 2015      Published: 05 January 2016
CLC:  TN992  
Fund:  National Natural Science Foundation of China(No. 51125028);National Key Technology R & D Program of China(No. 2011BAI12B07)
Corresponding Authors: Qing-feng LI,Lu-ming LI     E-mail: lqf05@mails.tsinghua.edu.cn;lilm@tsinghua.edu.cn
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Qing-feng LI,Shao-bo CHEN,Wei-ming WANG,Hong-wei HAO,Lu-ming LI. Improving the efficiency of magnetic coupling energy transfer by etching fractal patterns in the shielding metals*. Front. Inform. Technol. Electron. Eng., 2016, 17(1): 74-82.

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http://www.zjujournals.com/xueshu/fitee/10.1631/FITEE.1500114     OR     http://www.zjujournals.com/xueshu/fitee/Y2016/V17/I1/74


Improving the efficiency of magnetic coupling energy transfer by etching fractal patterns in the shielding metals*

Thin metal sheets are often located in the coupling paths of magnetic coupling energy transfer (MCET) systems. Eddy currents in the metals reduce the energy transfer efficiency and can even present safety risks. This paper describes the use of etched fractal patterns in the metals to suppress the eddy currents and improve the efficiency. Simulation and experimental results show that this approach is very effective. The fractal patterns should satisfy three features, namely, breaking the metal edge, etching in the high-intensity magnetic field region, and etching through the metal in the thickness direction. Different fractal patterns lead to different results. By altering the eddy current distribution, the fractal pattern slots reduce the eddy current losses when the metals show resistance effects and suppress the induced magnetic field in the metals when the metals show inductance effects. Fractal pattern slots in multilayer high conductivity metals (e.g., Cu) reduce the induced magnetic field intensity significantly. Furthermore, transfer power, transfer efficiency, receiving efficiency, and eddy current losses all increase with the increase of the number of etched layers. These results can benefit MCET by efficient energy transfer and safe use in metal shielded equipment.

Fig. 1 Simulation model
Fig. 2 Three etching schemes for the metal sheet (a) Four 0.3 mm deep slots begin at the disk center and end at a distance L from the center; (b) Three 0.3 mm deep slots begin at the disk center and end at a distance L from the center with a fourth slot ending at the disk boundary; (c) The slots all extend to the edge with varying slot depth h. All the slots have a fixed 0.3 mm width
Fig. 3 Simulation results for the patterns in Fig. 2(a) Results of PT and η for the pattern in Fig. 2a; (b) Results of PE and PT/PTR for the pattern in Fig. 2a; (c) Results of PT and η for the pattern in Fig. 2b; (d) Results of PE and PT/PTR for the pattern in Fig. 2b; (e) Results of PT and η for the pattern in Fig. 2c; (f) Results of PE and PT/PTR for the pattern in Fig. 2c
Fig. 4 Slot fractal patterns (a) Tree fractal; (b) Helical fractal; (c) Cross fractal; (d) Hilbert fractal with order two; (e) Hilbert fractal with order three; (f) H-shaped fractal with order one; (g) H-shaped fractal with order two; (h) Snow fractal. The arrows in the metal sheet represent the flow direction of the eddy currents
Scheme PE(%) PT(%) η(%) PT/PTR(%) Slots length(mm) Suppression efficiency(%/mm)
Fig. 4a 10.1 98.7 92.8 88.0 245.66 0.366
Fig. 4b 23.9 96.6 84.1 74.7 280.88 0.271
Fig. 4c 13.3 98.2 90.7 84.3 140.00 0.619
Fig. 4d 11.5 98.4 91.8 86.7 247.50 0.357
Fig. 4e 4.0 99.6 97.1 95.2 489.25 0.196
Fig. 4f 16.6 97.5 88.5 81.9 125.00 0.667
Fig. 4g 6.5 99.1 95.3 92.8 292.50 0.320
Fig. 4h 3.5 99.6 97.4 96.4 635.00 0.152
No slot 100.0 81.8 51.1 38.6 Null Null
No metal Null 100.0 100.0 100.0 Null Null
Table 1 Results for different slot fractal patterns in 4
Fig. 5 Frequency responses with the slot fractal pattern in Fig. 4g: (a) PT and η; (b) PE and PT/PTR
Fig. 6 Model for suppressing eddy currents in three metal layers
Scheme PE(%) PT (%) η(%) PT/PTR (%)
No copper 100.0 100.0 100.0 100.0
No slot 88.1 34.2 44.3 50.0
Etching one layer 99.9 46.0 53.7 56.3
Etching two layers 103.7 94.0 94.0 93.8
Table 2 Eddy current suppression in three metal layers
Fig. 7 Experimental platform
Fig. 8 The coils and the metal samples(a) Transmitting coil; (b) Receiving coil, PCB copper disk without slots, PCB copper slotted disk without breaking the edge, and PCB copper slotted disk to the edge (from left to right and from top to bottom); (c) Titanium board
Fig. 9 Four experimental designs for evaluation of slots’key pat-tern features(a) No metal is presented between the transmitting coil and the receiving coil; (b) A PCB copper layer without slots is present between the coils; (c) A PCB copper layer slotted without breaking the edge is present between the coils; (d) A PCB copper layer with slots reaching the edge is present between the coils
Scheme PE(%) PT (%) η(%) PT/PTR (%)
Fig. 9a 0 100.0 100.0 100.0
Fig. 9b 100.0 64.7 20.6 8.7
Fig. 9c 79.0 75.2 27.9 12.4
Fig. 9d 6.7 95.8 86.2 72.0
Table 3 Results for the four conditions in Fig. 9
Fig. 10 Suppressing eddy current effects of slots in multiple metal layers (a) A titanium board is present between the transmitting coil and the receiving coil; (b) Two additional PCB copper layers are present above the receiving coil; (c) Two additional PCB copper layers with one layer slotted are present above the receiving coil; (d) Two additional PCB copper layers with both layers slotted are present above the receiving coil
Fig. 11 Comparisons of simulation and experimental results in Fig. 10 (a) Comparison of PT; (b) Comparison of η; (c) Comparison of PE; (d) Comparison of PT/PTR
Fig. 12 Eddy current distribution shapes in Fig. 2 (a) Eddy current distribution in Fig. 2a with L=20 mm; (b) Eddy current distribution in Fig. 2b with L=20 mm; (c) Eddy current distribution in Fig. 2c with h=0.2 mm. The arrows in the figures represent the eddy currents directions in the metal sheet
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