2012 International Conference on Biomedical Engineering and Biotechnology
The Design and Implementation of the Wireless Power Transmission System of Video
Capsule Endoscopy
Shi Yu
Yan Guozheng, Jia Zhiwei, Zhu Bingquan
Department of Instrument Science and Engineering
Shanghai Jiao Tong University
Shanghai 200240, People’s Republic of China
flowers3344@sjtu.edu.cn
Department of Instrument Science and Engineering
Shanghai Jiao Tong University
Shanghai 200240, People’s Republic of China
gzhyan@sjtu.edu.cn
transmission efficiency of wireless power supply system at
the frequency ranging from 100 KHz to 400 KHz. Powered
by a wireless power supply system, the VCE transmits
NTSC format video to the computer at the frame rate of 30
f/s with the image resolution of 320×240. Besides, the
working hour of the VCE is relatively limitless in the human
body.
Abstract—The video capsule endoscopy (VCE), as a new
technology, has been increasingly popular for diagnosis of
gastrointestinal (GI) tract diseases. Limited by the power supply,
however, most of capsule endoscopies can only work for 6 to 8
hours at a frame rate of 2 frames per second (f/s). Based on
electromagnetic induction, the wireless power transmission
(WPT) system can provide a power of 150mW at a frequency of
218 KHz for the VCE in the body. The transmitting module
adopts a solenoid pair structure with a uniform internal magnetic
field and the receiving module a three-dimensional coil structure
at a working frequency of 218 KHz. The result showed that the
VCE can work properly and transmit a 30fps NTSC video with
the resolution of 320×240. Such wireless power supply approach
provides power support for the further development of VCE
technology.
II.
Because of the wireless power supply based on
electromagnetic induction, the TC outside the body, as the
primary coil, can generate a changing magnetic field; and the
receiving coil (RC) inside the body, as the second coil, can
receive power. The two coils, together with the capacitance,
respectively form an LC resonant circuit, causing resonance
and coupling at the same frequency so as to transmit power.
The existing studies show that the transmitting and the
receiving coils, using the series resonant circuit, is of good
load characteristic and can transmit low-voltage and highcurrent power fit for driving the VCE [12].
As is shown in Figure 1, the transmitting system outside
the body comprises the controller, the power amplifier and
the power transmitting circuit. The oscillator signal at certain
frequency produced by the controller is received by the
power amplifier, which drives the LC resonant circuit and
thus generates an alternating magnetic field in the circuit.
Then the RC inside the body induces electric motive force
(EMF) in the alternating magnetic field and receives stable
Keywords - wireless power transmission; video capsule
endoscope (VCE); inductive coupling; Helmholtz coil
I.
INTRODUCTION
Since 2000 when the VCE was released by Given
Imaging Company in Israel, such medical device has been
playing its role in diagnosis of GI tract [1, 2]. Compared with
conventional endoscopies, the VCE can not only conduct
non-invasive diagnosis, but also obtain the image of the
whole GI tract. By far, companies such as Olympus,
IntroMedic and Jinshan Science and Technology (Group) etc,
have released their capsule endoscopies that can only work
for 6 to 8 hours at a frame rate of 2 f/s. However, such
devices fail to meet the need of diagnosis of the whole GI
tract in terms of time and effect [3].
All the capsule endoscopies mentioned above are batterypowered, yet cannot provide sufficient energy required by
the VCE system. However, the WPT system based on
electromagnetic induction can supply adequate energy for a
relatively long time, which will greatly increase the working
hour and image frame rate and offer power supply to the
wireless VCE robot, a next-generation controllable product
of capsule endoscopies [3-7].
This paper designs a double solenoid transmitting coil
(TC) which can generate a uniform alternating magnetic field
in its working field in order to transmit a unified power to the
VCE all over the place. And the receiving module is of threedimensional coil structure thereby calculating sufficient
energy regardless of body postures. The working frequency
is determined at 218 KHz through comparing the
978-0-7695-4706-0/12 $26.00 © 2012 IEEE
DOI 10.1109/iCBEB.2012.408
WIRELESS POWER TRANSMISSION SYSTEM
Figure 1. Sketch of a power transmission system.
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power for loading through rectifier and voltage regulator. In
addition, according to the law of electromagnetic coupling,
the power transmission functions most efficiently when the
LC circuits of the transmitting end and the receiving end
cause resonance at the same frequency.
8
Magnetic Field Uniformity Ȗ (%)
III.
WIRELESS POWER TRANSMITTING COILS
The VCE in the alimentary canal moves along with the
digestive movement. Therefore, the alternating magnetic
field generated by the TC must be uniform so that the VCE
can receive stable power. Neither Helmholtz coils nor
solenoid functions ideally despite their being commonly
used. Hence, the solenoid pair structure is proposed based on
the analysis of advantages and disadvantages of the former
two structures, as shown in Fig. 2.
The magnetic field uniformity is defined as:
7
6
5
4
3
2
20
25
30
35
40
Number of Turns
Figure 3. the magnetic uniformity of the TC with different turns
Helmholtz coils
pair of solenoid coils
solenoid coils
2.4
(1)
Magnetic Flux Density Coefficient B (mT/A)
§ B − B2 ·
γ = max ¨ 1
¸
© B0 ¹
Helmholtz coils
pair of solenoid coils
solenoid coils
9
B1 and B2 represent the magnetic density of any two
points in the working field. And B0 is the magnetic density
of the center part. As illustrated in formula (1), the more
uniform the magnetic field is, the less the magnetic field
uniformity tends to be. The uniformity of different TC
structures have been analyzed by introducing formula (1),.
The result shown in Figure 3 indicates that as the number of
turns increases, the magnetic field uniformity of the TC
decreases, i.e. the magnetic field becoming more uniform.
With the same number of turns, the magnetic field
uniformity of Helmholtz coils excels that of the double
solenoid pair while the single solenoid pair works worst.
In the mean time, the magnetic flux density coefficient of
the center of TC magnetic field leads a linear increase as the
number of turns goes up. With the same number of turns, the
magnetic flux density coefficient of the single and the double
solenoid coils are quite close in numerical value while that of
the Helmholtz coils is relatively smaller and the gap widens
as the number of turns increases.
Giving a comprehensive consideration to the magnetic
field uniformity as well as the transmitting power, we have
chosen the solenoid pair structure with 26 turns as the TC.
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
20
25
30
35
40
Number of Turns
Figure 4. the magnetic flux density of the TC with different turns
IV.
WIRELESS POWER RECEIVING SUBSYSTEM
The design of the RC is strictly restrained by the limited
given space in terms of structure and size. With a view to the
random postures of the VCE in the body, the threedimensional (3D) coil structure is chosen as the RC, which is
shown in Fig. 5.
Cartesian coordinate system is established to describe the
receiving coil in the magnetic field [4]. Moreover, the
orientation changing of the receiving coil can transfer by
pivoting around two coordinate axes, so the projected area of
the receiving coils could be calculated as:
Figure 5. structure of the 3D receiving coil
Figure 2. (a) solenoid coils, (b) Helmholtz coils, (c) solenoid pair coils
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V.
­ S1 = S0 ⋅ cosθ1 cosθ2
°
® S2 = S0 ⋅ sin θ1 cosθ 2
° S = S ⋅ sin θ
3
0
2
¯
The electromagnetic field generated by the transmitting
coil may damage the human body. Careful evaluation is
necessary to ensure that the generated electromagnetic field
is at a safe level. According to the International Commission
on Non-Ionizing Radiation Protection (ICNIRP 1997),
electromagnetic influences on biological tissue include
thermal effects and stimulant action when the frequency is
between 100 KHz and 10 MHz, the specific absorption rate
(SAR, W/kg) and current density (A/m2) are the main
indexes of the two effects. Generally, SAR is expressed by
SAR = ıE2/ȡ, and current density J is expressed by J = ıE,
where ı is the electric conductivity of biological tissue (s/m),
E is the root mean square of the electric filed (V/m), and ȡ is
the density of the biological tissue (kg/m3). Thus, SAR and J
should be considered along with the transmitting coil (e.g.,
shape and turns), similar to frequency and driven current [13].
Table 1 shows the basic restrictions defined in the
guidelines set by the ICNIRP for limiting exposure to timevarying electrical, magnetic and electromagnetic forces.
On the basis of the data provided by the Visible Human
Project of the National Library of Medicine, we establish a
simulation model and adopt a finite integration technique to
calculate current density and SAR. As the parameters of the
transmitting coil vary, the corresponding indexes change,
and these indexes are compared with the restrictions in
evaluating the safety of the transmitting setup. When the
transmitting coil is driven, the attributed current density and
(2)
Where S0 is the enclosed area of each coils, S1, S2 , S3 are
the projected area of coil1, coil2 and coil3, θ1 is the angle
pivoting around X axis, θ 2 is the angle pivoting around Y
axis. When the three dimensions are serial connected, the
equivalent projected area S is:
S = S1 + S2 + S3 = ( cosθ1 cosθ2 + sinθ1 cosθ2 + sinθ2 ) ⋅ S0
(3)
And the corresponding efficiency is:
η=
ω 2 B12 μr2 RL
( R2 + RL )
2
R1
( cosθ1 cos θ 2 + sin θ1 cosθ 2 + sin θ2 )2 ⋅ S 2
0
(3)
The average received power is:
1
P=
4π 2
³θ
ε 2 (t )
RL
(4)
dt
HUMAN TISSUE SAFETY
The frequency of wireless power supply system has
exerted a great impact on the transmission efficiency and
received power [10]. We have thus compared the average
received power and transmission efficiency at the frequency
ranging from 100 KHz to 400 KHz and the result is
illustrated in Fig. 6.
Fig. 6: By rectifying, connecting in series and voltagestabilizing, current in the RC is output to the 121ȍ resistance,
which is converted from power.
The result shows that the average received power and
transmission efficiency gets highest at the frequency of 218
KHz in the above range. Hence, the 3D coil structure is
chosen for the power supply of the VCE.
TABLE I.
BASIC RESTRICTIONS OF THE ICNIRP.
Localized SAR(W Kg-1)
Frequency
range
(MHz)
Current density
for head and
trunk (A/m2)
Head and trunk
Limbs
0.1-10
f/100
10
20
4.0
160
3.5
140
3.0
120
2.5
100
2.0
80
1.5
60
Transmission Efficiency (%)
Average Received Power (mW)
Average Received Power (mW)
Transmission Efficiency (%)
1.0
40
0.5
100
150
200
250
300
350
400
Frequency (KHz)
Figure 7. (a) Distribution map of induced current density: x = 0. (b) y = 0.
(c) z = 0. (d) Distribution map of SAR value: x = 0. (e). y = 0. (f) z = 0.
Figure 6. average received power and transmission efficiency on the load
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SAR in the body can be calculated as shown in Fig. 7. Using
these results, we can easily determine the maximum value of
the current density and SAR for comparison with the
restrictions of the ICNIRP.
VI.
TEST OF THE VCE
The TC is wound with 180 turns of Litz wire, the
framework of which is made of an ABS cylinder with a
diameter of 400mm. It can generate a ĭ400mm×200mm
uniform alternating magnetic field. And the RC with an
external diameter of ĭ10mm×10.5mm is made of a
6.7mm×6.7mm×7.6mm magnetic core wound with 10 turns
of enameled copper wire. It is connected with the rectifier
and voltage regulator and integrated into the VCE with a size
of ĭ11mm×29mm.
Test result shows that the VCE works well in the TC with
an emission current of 1.3A, a power of 8.2W and a
frequency of 218 KHz and the video signal sent by the image
acquisition system is vivid and fluent, with the image
resolution of 320×240, which is shown in Fig. 8 and Fig.9.
Figure 9. the inner image of isolated intestinal
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VII. CONCLUSION
The device in this experiment adopts solenoid pair
structure as the TC and a three-dimensional coil structure as
the RC, providing a power of over 150mW for the VCE at a
frequency of 218 KHz. The VCE transmits NTSC format
video to the computer at the frame rate of 30 f/s with the
image resolution of 320×240. Due to the wireless power
supply system, the VCE can work properly in the required
time at such a high frame rate and resolution.
[5]
[6]
[7]
ACKNOWLEDGMENT
[8]
This work has been supported by the National Natural
Science Foundation of China (NSFC) (No. 31170968), the
National High Technology Research and Development
Program of China (863) (No. 2008AA04Z201), Pre-research
Programme on Manned Spaceflight (PR China, No. 010203)
and Science and Technology Commission of Shanghai
Municipality (PR China, No. 09DZ1907400). Our special
thanks goes to Mr. Cheng-geng Wu for his help with
manufacturing the device.
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Figure 8. overview of VCE system
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