design and

7/27/2019 Design And

1/8

Research article

Design and fabrication of magnetic couplings

in vacuum robotsPinkuan Liu, Yulin Wang and Jun Wu

State Key Laboratory of Mechanical System and Vibration, School of Mechanical Engineering,Shanghai Jiao Tong University, Shanghai, China

AbstractPurpose The purpose of this paper is to discuss the design and fabrication of magnetic couplings to use for vacuum robots. The permanent magneticcoupling (PMC) is appropriate for torque transmission in ultrahigh vacuum and highly clean environments. However, conventional structures of PMC arealways unsuitable to use for vacuum robots.Design/methodology/approach Two types of design scheme for radial magnetic couplings are introduced and compared. The major characteristicof the novel design scheme is that the inner part uses a nonmagnetic mantle to enclose the magnets and yoke, and the outer part uses two end closuresto position magnets. The locating groove on the end closure may be manufactured as T-shape or dovetail shape.

Findings The 3D finite element analysis simulation results and experimental studies have demonstrated that the proposed Design B had a lowercontamination rate and a higher transmission efficiency than the Design A.Research limitations/implications The limitation of the research to date is that issues of control, path-planning, and communication have not yetbeen addressed.Practical implications The proposed PMC is successfully applied in vacuum robots which uses combined direct drive techniques and magnetictransmit techniques.Originality/value These results suggest that the proposed PMC is suitable for using in vacuum robots.

Keywords Robotics, Magnetism, Power transmission systems

Paper type Research paper

1. IntroductionPermanent magnetic couplings (PMC) are device that

transmitting torque through magnetic force with no

mechanical contact from a primary drive to a secondary

follower. In particular, it is very appropriate for torque

transmission in ultrahigh vacuum, highly clean, hazardous or

corrosive environments, and has been widely used in the fields

of industriy and defense. The PMCs as critical component are

increasingly used in much of developing vacuum robot for

wafer handling related to the field of semiconductor. It is

desirable for transmitting torque or rotary motion from the

atmosphere to the vacuum environment. The wafer holder

arms of vacuum robot, which operate in the reaction

chamber, are magnetically coupled to a directly driving motor

outside of the vacuum area. For example, in ultrahigh vacuum,a coaxial drive mechanism is incorporated to the design of a

compact wafer-transfer robot, in which multiple sets of

magnetic couplings with different configurations are

simultaneously used to couple different axes, a non-magnetic

vacuum partition wall separates the inner and outer rotor to

maintain the high vacuum from atmosphere environment.

Synchronours-type couplings among different types of

PMC can transmit higher torque than others for identical

volum e of m agnetic m aterials. On the basis of the

configuration, these types of couplings can be further

categorized into radial, axial and mixed-type couplings. The

axial-type couplings usually generate considerable axial force,

for example attractive or repulsive force, therefore, they are

only suitable to transmit lower torque. However, they possess

an advantage of easy adjustment of the air gap. On the other

hand, the radial-type couplings can transmit higher torque

level by simply increasing the length of the two concentric

coupling members or increasing the amount of pole-pairs by

augmenting the diameter of couplings. The radial-type

couplings are desirable for application in wafer-transfer

robot. This type of PMC consists of outer and inner rotors,

the outer rotor has permanent magnets of changing polarity

on the inner side and the inner rotor has them on the outside.

The north and south poles of the rotors are opposite to eachThe current issue and full text archive of this journal is available atwww.emeraldinsight.com/0143-991X.htm

Industrial Robot: An International Journal

36/3 (2009) 230237

q Emerald Group Publishing Limited [ISSN 0143-991X]

[DOI 10.1108/01439910910950487]

This paper is an updated and revised version of the paper originallypresented at 2008 International Conference on Intelligent Robotics andApplications (2008 ICIRA), Wuhan, China, October 15-17, 2008. Thiswork was supported in part by the Science & Technology Commission ofShanghai Municipality under Grant 07PJ14051 and 071111008.

230


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other, the magnetic field is completely symmetric in non-

operative state. When the rotors are twisted over a torsion

angle, the magnetic field line are moved, hence the torque is

transmitted from primary shaft to follower one through the

air gap.

However, conventional PMC are always unsuitable for

vacuum robot because that the robot needs the PMC run in

vacuum environment with an extremely low-contaminationrate and high reliability. It is essential to present some more

feasible designs applied in vacuum condition with pressure

range from 1025 to 1027 Pa. Some considerations related

design and fabrication are involved to suit high vacuum and

ultrahigh vacuum application requirement.

Angular stiffness, synchronous rotary accuracy and air gap

are of key specifications for design of PMCs. First, angular

stiffness depends on the maximal coupling torque of the

PMC. If maximum coupling torque is exceeded, the power

transmission is interrupted. The synchronous rotary accuracy

is predominately related to quantity of the minimal torsion

angle. The air gap is determined by the thickness of the

vacuum barrier.

Various methods to analyze the transmitted torque of

magnetic couplings have been employed, mainly including

analytical methods (Yao et al., 1995) and finite element

analysis (FEA) methods (Wu et al., 1997). The 3D FEA has

been shown to be a powerful and effective tool for the analysis

and design of synchronous magnetic couplings, taking the

end-leakage effects into account (Wang et al., 2008).

In order to decrease the contamination rate and increase

the reliability, two types of design of radial magnetic couplings

were proposed in this paper. The first type denoted as Design

A has two configurations including first one used screw to fix

the magnets on the yokes and second one used mucilage to

adhesive them. The second type denoted as Design B mainly

include two characteristics: the inner rotor of PMC uses a

nonmagnetic mantle to enclose the magnet blocks and the

yoke which has several locating grooves; the outer rotor ofPMC has two end closures which have the T-shape groove or

the dovetail shape to position magnets. The design and

fabrication of two types of PMC are described and discussed.

The Design A was then compared with the Design B base on

their structural characteristics and fabrication. The

simulations on above two types of design through the 3D

FEA method were carried out. In order to verify the validity of

simulations, a five degrees of freedom (DOF) measuring

equipment for magnetic couplings was also built. The

maximal coupling torques were measured. Results from

FEA and testing were discussed and analyzed. In the end, the

PMC of the proposed Design B was successfully applied in

constructing a novel vacuum wafer transfer robot which used

the combined direct drive motor and magnetic transmit

technique.

2. Design and fabrication on the PMC

2.1 General description

Design scheme in this paper focuses on the typical cylindrical

couplings with iron yokes and magnets located separately

without jointing with each other. The alternate magnetized

poles were allocated circumferentially. The yokes with high

permeability short-circuited the inner- and outer-magnet

rings of the couplings to avoid magnetic leakage, thus

improving the efficiency of torque transmission. In these

designs, maximal coupling torque of the PMCs need be up to

75 Nm, the air gap is defined as width of 5 mm so that it can

allow sufficient thickness of the vacuum barrier to ensure its

high stiffness and strength. The magnet material which was

radially magnetized was of Nd-Fe-B with residual flux density

denoted as Br of 1.19 T, and coercive force as Hc of 835.8 kA/

m in the magnetized direction. In our designs, the sector-

shaped magnets with same arc angle are suit to merge thecylindrical curvature, so that the couplings are capable of

keeping uniformity of the air gap between the couplings and

vacuum barrier. Similarly, the ratio of pole width to pole pitch

denoted as a was set to be 0.72 (Hornreich and Shtrikman,

1978) in order to obtain higher efficiency of magnetic

material. Furthermore, yoke was made of steel ASTM 1045.

The bulk conductivity denoted as k of yoke is 2,000,000 S/m,

and the BH-curve of the relative permeability of yoke can be

seen in Figure 1. The structure of yoke with enough thickness

can carry the required flux without saturation. In these

designs, the thickness of magnets was assumed to be equal for

the inner rotor and the outer one of magnetic couplings.

2.2 Design scheme I

Two types of configuration for Design A were proposed. The

first type through socket head screws to connect a set of

magnets on the yoke as shown in Figure 2, of which

Figure 2(a) is the photograph of the prototype, and

Figure 2(b) is the schematic diagram of the design. From

the figure, we can see that 14 magnets with sector-shaped

were separately located on the inner and outer side at interval

of constant angle. The second type used mucilage to adhesive

them together, as shown in Figure 3, where Figure 3(a) is the

photograph of the prototype, and Figure 3(b) is the schematic

diagram of the design. The yoke of these two types both were

machined locating grooves to position each magnet on the

socket of yoke.

In general, the countersunk hole was cut on the magnet and

the threaded hole was machined on the iron for the type ofusing screws. However, the loss of torque will occur obviously,

because that the countersunk hole decreased the volume of

magnetic material. Besides, the screw would also change the

distribution of magnetic field. Furthermore, overusing the

screw may result in bringing containment and particles into

vacuum and clean environment, then changing the pressure of

vacuum environment.

Though it was the most convenient approach to adhesive

the magnet on the socket of the yoke with mucilage, the

properties such as strength, elasticity and ductility of

Figure 1 The BH-curve of the relative permeability of yoke

Fluxdensity(T)

Magnetic field strength (A/m)

0.0

0.5

1.0

1.5

2.0

2.5

50,000 50,000 1,00,000 1,50,000 2,00,000 2,50,000 3,00,000 3,50,0000


Pinkuan Liu, Yulin Wang and Jun Wu


Volume 36 Number 3 2009 230237

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the adhesive layer are of critical issue. Since the magnetic

coupling torque between inner and outer rotors was

permanently remained while running, the vary load was

exerted on the PMC, failure of strength will potentially occur

in the adhesive layer. On the other hand, it was possible that

the mucilage contaminated the vacuum environment.

Therefore, the above two types of configuration of the

PMCs were undesirable for using in constructing the vacuum

robot. It was essential to design some more reasonablestructures for the special application.

2.3 Design scheme B

Two novel types of configuration for PMC were proposed in

this design. In the improved configuration than Design A, the

inner rotor of PMC used a cover made of un-conducted

magnetic materials enclosing the magnets and yoke as shown

in Figure 4. Rim of the cover mate to the flange of the yoke,

then hold them together through screws. The yoke also had

locating grooves to position each magnet on the socket of

yoke, just was similar to the types of Design A. While using

the insulation shell served as a reliable seal to separate the two

coupling halves from two different medium environments, the

cover will ensure a lower outgas rate and atom contaminationrate than the Design A.

The outer rotor of PMC consists of upper- and lower-end

caps, hollow cylinder and magnets. Two types of end caps are

designed and fabricated. One of the end caps was machined

into T-shaped grooves by EDM, other one of that was cut into

dovetail groove. End caps with T-shaped or dovetail grooves

were separately mounted on upper and lower flange of the

hollow cylinder, and held tightly by screws. Grooves on

upper- and lower-end caps were aligned, and then formed the

locating grooves for retaining the magnet. Magnets were also

adhesive on the yoke by mucilage in order to increase the

fastener strength. The configuration of the PMC with

T-shaped and dovetail groove are, respectively, shown in

F igures 5 and 6 . T he above two figures depict the

configuration of PMC, structure of outer rotor and end cap

and shape of magnet in detail. The yoke was manufactured as

cylinder-shaped without locating grooves.

As we can see by comparing Figures 5 and 6, it was easier to

fabricate magnets and end caps into dovetail shape than

T-shape. However, it was more difficulty to mount the

magnets into the dovetail groove than into the T-shape groove

because of the tolerances of the positioning groove from

machining and assembly. These two types of PMC from

Design B both had a higher material utilization rate and a

lower contamination rate than Design A with screw to fix, and

had higher connection reliability than Design A with mucilage

Figure 2 Design of the PMC using screws to fix magnets on yoke: (a) photograph of the prototype; (b) schematic diagram of the design

Outer rotor

N

SS

SS

S

SS

S

S

N

N

N

N

N

N

N

N

Inner rotor

(a) (b)

Figure 3 Design of the PMC using mucilage to adhesive magnets onyoke: (a) photograph of the prototype; (b) schematic diagram of the

design

(a)

(b)

Outer rotor

Outer rotor

inner rotor

inner rotor

Figure 4 The inner rotor of configuration for PMC and the cover

inner rotor

cover





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4/8

to adhesive. Therefore, these two novel types of the PMC

were more suitable for using in constructing a vacuum robot.

The photographs of two prototypes for Design B are shown in

Figure 7.

3. Analysis and experiment

3.1 3D FEA simulation

The 3D FEA module of Ansofts Maxwell 11 was employed

in this study to simulate the PMC. Physical model mainly

includes the outer rotor, inner rotor and air gap, therefore,

the finite element (FE) model is built according to this

physical model. The cross-section view of the PMC is shown

in Figure 7. In which the label H represents for the thickness

of magnet; the label R1 stands for the inner radius of inner

magnetic coupling; the label R2 is denoted as outer radius of

inner magnetic coupling; the label R3 stands for the inner

radius of outer magnetic coupling; the label R4 stands for the

outer radius of outer magnetic coupling; the label air_gap

stands for the length of air gap; and the label L stands for theaxial length of PMC. From Figure 8, we can see that vacuum

barrier, which is applied to separates the atmosphere and

vacuum environment, was not modeled in this FE model

because its permeability is close to air. Similarly, the thickness

of yokes was assumed to be uniformity along radial direction.

On the other hand, an area outside both upper and lower end

of the PMCs was included to model the end-leakage effect.

The width of the air layer considered in this model was set as

about three times the magnetic thickness in order to meet the

requirement of sufficient solving accuracy (Wu et al., 1997).

Since the magnetic coupling was periodically symmetrical

for each pole, the fractional FE models were established. The

master and slave boundaries were applied in this paper, which

can simplify model complexity. The magnetic field on theslave boundary was subjected to match the field on the master

boundary. The magnitude of the magnetic field on both

boundaries was equal. The fields on the two boundaries can

either point in the same direction, or in opposite directions, as

specified during the modeling process.

The FE models from the PMCs with four types of

configuration from the Designs A and B were, respectively,

shown in Figure 9(a)-(d). The parameters used in FE

simulation were listed in Table I. Four FE model were

Figure 5 Configuration of PMC using T-shape groove

PMC

T-shaped end cap

outer rotor

T-shaped magnet

Figure 6 Configuration of PMC using dovetail groove

PMC

outer rotor

dovetail end cap

dovetail magnet

Figure 7 The photograph of two prototypes of the PMC

outer rotor

inner rotor

PMC Dovetail shape

T shape

Figure 8 The cross-section view of PMC

R2 H

H

L

R1

R3

R4 air_gap





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analyzed in order to compute thecouplingtorques of thePMCs.

The first model was Design A with screws fixing, the second

model was also Design A using mucilage to adhesive only, and

the third model was Design B with the T-shape groove and the

last model from DesignB used thedovetailgroove. Thematerial

of the end caps for the last two models was aluminum. The

results of each model, which were maximal coupling torque of

PMC, were summarized in Table II in next subsection. The

maximal torque is yielded when the twisted angle is half-a-pole

pitch (Huang et al., 2001).

3.2 Experimental set-up

The experimental set-up in order to measure the static

coupling torques of PMCs was established as shown in

Figure 10. The Z-positioning stage 1 is vertically mounted on

the base 3, and the Z-stage driven and operated by manual, its

stroke can be up to 250 mm in z-direction. The X-Y

positioning stage 2 is stacked on the moving table of the

Z-stage 1, this stage 2 with precision leadscrew and linear

ball bearing can provide linear motion of 25 mm, respectively,

in x- and y-direction. The inner rotor of the PMC 5 is

connected coaxially to shaft of the torque sensor, 4 by mean

of mechanical coupler. The torque sensor is vertically

mounted on the angle bracket, which is fastened to the

moving table of the X-Y positioning stage 2 by mean of a

Figure 9 FE model of the PMCs: (a) FE model with screw fixing; (b) FE model using mucilage to adhesive; (c) FE model with the T-shape groove;(d) FE model with the dovetail groove

(a) (b)

(c) (d)

xz

z

z

y

y

y

y

z

Table I Parameters used in FE analysis

Design A Design B

Screws

fixing

Mucilage

fixing

T-shape

groove

Dovetail

groove

Air_gap (mm) 5

Br (T) 1.19

Hc (KA/m) 835.8

a 0.72k (S/m) 2,000,000

R2 (mm) 40 66 41 66

H (mm) 7 5 6 6

L (mm) 48 30 50 26

n 14 12 12 12





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screw connection. uz, ux positioning stage 6, which can rotate

range from 0 to 3608 about z-axis and from 0 to 58 about

x-axis, respectively. The outer rotor of the PMC 5 is fastened

to the chuck of the stage 6 by mean of a flange adapter. The

torque sensor 4 used in this experiment is the ORT-803 type,

and the controller of torque meter 7 is the TN-2 type of theOriental Technology Company. The A/D acquisition card is

the ADLINK PCI-9221, which is a 16-bit high-resolution

multi-function data acquisition (DAQ) card, with 16-channel

single terminal or eight-channel differential input capable of

up to 250kS/s sampling rate. The schematic diagram of the

working principle of the experiment set-up is shown in

Figure 11.

The coaxiality between the inner and the outer rotor of the

PMC can be set by the X-Y positioning stage; the coupling

length between the inner and the outer rotor in axial direction

can be adjusted by the Z-positioning stage; the slope angle of

the outer rotor can be regulated by the ux positioning stage;

while the torsional angle is implemented by the uz positioning

stage. The maximal torque can be measured when the

torsional angle was half-a-pole pitch.The torque value measured is output as frequency signal

range from 5 to 15 kHz by mean of the torque sensor, and

then it was translated to the analog voltage signal range from

25 to 5 V by the torque sensor amplifier. Finally, the 16-bit

high-resolution DAQ card was used to convert the analog

voltage positional to digital signals range from 0 to 100 Nm.

3.3 Results and discussion

The testing results from four types of PMC are summarized

and compared with the results from FE simulation in Table II,

and the following conclusions can be drawn:

.

The error between these two methods is less than3 percent, which shows that the 3D FEA has a high

accuracy.. The method of using screws to fix will decrease the

maximal torque of PMC obviously. The countersunk hole

on the magnet made the volume of magnet block

decrease about 5 percent in the example of this paper,

and the screw also changed the distribution of magnetic

field. The result showed that the maximal torque

decreased about 15 percent compared with other models.

In order to discuss the influence of the deviation length

between the inner and the outer rotor in axial direction, and

the slope angle of the outer rotor, the prototype for T-shape

groove of PMC from Design B was taken as an experiment

example. In Figure 12, the deviation length was set as fourcases, which is 0, 5, 15 and 25mm. When the deviation

length changed less than 5 mm in this case, its influence is

small. However, when the deviation length changed more

than 10 mm in this case, the maximal torque and the stiffness

of PMC decreased greatly. Furthermore, the slope angle of

the outer rotor has little influence on the transmitted torque of

PMC when the slope angle changed from 0 to 28 (Figure 13).

4. Application for constructing vacuum robots

The shafting of vacuum robot was designed which combined

used the rotary direct drive technique and the magnetic force

transmission technique. With a direct drive motor system, it is

no need for mechanical linkage (gearbox, belt, etc.) between

the payload and the motor. Thus, it does not exit the

backlash, clearance, friction, or elasticity problems. This

makes for a much stiffer and more easily controlled system.

Furthermore, the direct drive solution reduces the number of

components, simplifies assembly and thus reduces the overall

cost of the system. All this increase the dynamic performance

and precision of the system, and help to create a cleaner robot

with lower particulate contamination. The partial cross-

section view of a drive section of the 2 DOF vacuum robot

can be seen in Figure 14.

In this design, the rotational and radial position of the arm

was dependant upon the motion of the two rotary direct drive

Figure 10 Experimental set-up: (1) Z-positioning stage; (2) X-Ypositioning stage; (3) base; (4) torque sensor; (5) PMC; (6) uz, uxpositioning stage; (7) controller of torque sensor

1

2

3

4

5

6

7

Figure 11 Schematic diagram of experimental design

AEMC

A/D

Acquisition

Card

Torque

Sensor

Amplifier

xx

z

z

y

x

xy

z

Table II The results comparison of different models

Screws

fixing

Mucilage

fixing

T-shape

groove

fixing

Dovetail

groove

fixing

FEA (Nm) 61.4 71.2 70.7 72.2

Experiment (Nm) 60.9 72.3 72.6 70.9

Error (%) 0.8 1.5 2.6 1.8





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system 1, 2, which were arranged co-axially. Each drive

system was functionally identical and mechanically similar,

and each contained three elements: the drive motor, the

rotary position encoder, and the drive shaft. The drive motor

and the drive shaft both had a large, open, inner diameter.

The hollow design allowed for tubing, fixtures, rotary unions

and wiring to travel up through the center of the shaft and the

motor. The rotation of the arm was driven by the drive shaft

6, while the radial motion was driven by the shaft 7. The drive

shaft 6 runs through the hollow shaft 7 and the direct drive

system 2.The two concentric hollow drive shafts 6 and 7 connected

the two outer parts of the PMC 3, 4, respectively, and the

outer parts of PMC transmited torque to the inner parts by

magnetic force through the insulation shell 5, which separated

the atmospheric region and the vacuum region. The drive and

control mechanisms for the arm were completely outside the

insulation shell. The structure of the insulation shell was

designed as a novel stepped type. It was helpful to

simultaneously use two sets of magnetic couplings with

different dimension to couple different axes, and the cross-

coupling of multiple sets of PMC will decrease greatly

because of obviously different demension. In order to insure

the dem and of ultrahigh vacuum and highly cleanenvironment and a higher reliability for the vacuum robot,

the proposed Design B of T-shape groove and dovetail shape

groove of PMC were used. Finially, the robot arm rotated and

extended through the rotation of the two concentric hollow

drive shafts 8 and 9 which attached to the inner parts of PMC.

5. Conclusion

Based on the analysis, simulations and measurement of all

above PMC structure types, the conclusion can be obtained

that the proposed Design B both had a lower contamination

rate than the Design A; and they also had a higher

transmission efficiency than the screws fixing type of Design

A, and had higher connection reliability than the mucilage

fixing type of Design A. The maximal torque decreased about

15 percent for the screws fixing type compared with other

types.

Besides, the influences of the deviation length and the slope

angle between the inner and the outer rotor were discussed.

The results showed that when the deviation length changed

less than 5 mm in this case, its influence is small. However,

when the deviation length changed more than 10 mm, the

maximal torque and the stiffness of PMC decreased greatly.

The slope angle of the outer rotor has little influence on the

transmitted torque of PMC when the slope angle changed

from 0 to 28.

Figure 12 The influence of deviation length

80

70

60

50

40

30

20Torque(Nm

)

10

0

10

2 0 2 4 6

Torsion angle ()

8 10 12 14

0 mm

5 mm

15 mm

25 mm

Figure 13 The influence of slope angle

80

70

60

50

40

30

20Torque(Nm)

10

0

10

0 2 4 6

Torsion angle ()

8 10 12 14

2 degree

0 degree

Figure 14 The partial cross-section view of a drive section of the 2 DOFvacuum robot: (1), (2) rotary direct drive system; (3), (4) outer parts ofthe PMC; (5) insulation shell; (6)-(9) concentric hollow drive shaft

8

9

5

4

3

2

1

6

7





236


8/8

Furthermore, the shafting of vacuum robot was introduced,

and the proposed Design B of PMC was verified to be feasible

and can be applied in the vacuum robot successfully.

References

Hornreich, R. and Shtrikman, S. (1978), Optimal design of

synchronous torque couplers, IEEE Transactions on

Magnetics, Vol. 14, pp. 800-2.

Huang, S.M., Chen, W.L., Yau, C.H. and Sung, C.K. (2001),

Effects of misalignment on the transmission characteristics

of magnetic couplings, Proceedings of the Institution of

Mecha nical Engi neers, Part C: Jour nal of Mechan ical

Engineering Science, Vol. 215 No. 2, pp. 227-35.

Wang, Y.L., Liu, P.K. and Wu, J. (2008), Near-optimal

design and 3D finite element analysis of multiple sets of

radial magnetic couplings, IEEE Transactions on Magnetics,

Vol. 44 No. 12, pp. 4747-53.

Wu, W., Lovatt, H.C. and Dunlop, B. (1997), Analysis and

design optimisation of magnetic couplings using 3D finite

element modelling, IEEE Transactions on Magnetics,

Vol. 33, pp. 4083-94.Yao, Y.D., Chiou, G.J., Huang, D.-R. and Wang, S.-J. (1995),

Theoretical computations for the torque of magnetic

coupling, IEEE Transactions on Magnetics, Vol. 31,pp. 1881-4.

Corresponding author

Pinkuan Liu can be contacted at: [email protected]





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