TRƯỜNG ĐẠI HỌC BÁCH KHOA HÀ NỘI
TRƯỜNG ĐIỆN – ĐIỆN TỬ
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ĐỒ ÁN I
Đề tài: Wireless power transfer system for wireless
gastrointestinal endoscopy device
Giảng viên hướng dẫn: PGS. Lê Minh Thùy
Sinh viên thực hiện: Trần Bùi Nam Sơn
MSSV: 20202767
Hà Nội, năm 2023
Table of Contents
INTRODUCTION............................................................................................................... 3
CHAPTER 1. OVERVIEW .................................................................................................. 4
I. Wireless power transfer (WPT): ............................................................................... 4
II. Wireless capsule endoscopy (WCE): ....................................................................... 5
II.1. Gastrointestinal........................................................................................................................ 5
II.2. Capsule endoscopy (CE):........................................................................................................... 6
II.3. Wireless power transfer (WPT) in WCE: ................................................................................... 7
CHAPTER 2. COIL DESIGN................................................................................................ 9
I. Transmitting coil (TC): .............................................................................................. 9
II. Receiving coil (RC) design ...................................................................................... 10
II.1. The Receiving subsystems: ..................................................................................................... 10
II.2. The receiving coil design: ....................................................................................................... 11
CHAPTER 3. COIL PARAMETERS CALCULATION ............................................................ 12
I. Prototype of WPT ................................................................................................... 12
I.1. Coupling coefficient:................................................................................................................ 12
I.2. Mutual inductance and power load: ...................................................................................... 12
I.3. Transferred voltage between the TC and the RC: ................................................................... 13
II. Stability of the WPT systems: ................................................................................ 14
II.1. Misalignment between the TC and the RC ............................................................................. 14
CONCLUSION ................................................................................................................ 16
REFERENCES .................................................................................................................. 17
2
INTRODUCTION
The gastrointestinal (GI) or digestive system extracts the essence of our diet and keeps
our body active, but nowadays an increasing number of people are being affected by GI
disorders. Early detection is important for effective prevention and treatment, wireless
capsule endoscopes (WCE) can serve this purpose of early detection screening. It can provide
comfortable, painless diagnosis to the patient and more importantly, it makes it possible for
a thorough diagnosis to be performed since the WCEs can travel to the parts where the typical
endoscopes fails to reach such as small bowel area.
However, to assure the quality of a diagnosis, the WCEs with specifications as image
resolution, frame rate and working time needed to be improved. And the first problem to be
mentioned is that these functions required more power, more than that of a battery with
approriate size planted in the endoscopy capsule can provide. In this regards, wireless power
transmission (WPT) system has been proposed to overcome this problem. This method can
offers flexibility for power adjustment that allows the transmission of the right amount of
power. And the WPT technology has been proven to be useful to power up healthcare
devices, especially in biomedical implants. However, WPT for WCE faces additional challenges
due to the unpredictable capsule orientation and motion and the need for compact size of
the RC and its power harvesting circuit. We need to find an appropriate solution which not
only can maintain a stable power suppy for the WCE system but also ensure the size of the
received part in the capsule to be small enough that it will not affect the detection process
and cause negative effect to patient’s health.
The unpredictable capsule orientation and motion leads to the misalignment between
the transmitting coils (TC) and receiving coils (RC), thus the power transmitted is unstable and
insufficient. To fix this problem, in this report, I propose a system of coils: a pair of Helmholtz
coil as TC placed outside of the body and the RC is designed as a three-dimensional orthogonal
coil with a ferrite core. The three coils perpendicular to each other enable the RC to generate
enough power at any orientation. By using this design, the problem of unstable power
transfer due to misalignment between the transmitting coil and receiving coil can be solved.
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CHAPTER 1. OVERVIEW
I. Wireless power transfer (WPT):
WPT is a technology that transmits power or electricity without any form of physical
connection between the load and the power source.
Figure 1. Working principle of WPT
WPT transmit power through the electromagnetic field. The transmitter produces a
period shifting electromagnetic field while the receiver collect power through
electromagnetic induction (present in the figure by the flux that going through the receiving
coil loop).
Regarding the transmission distance, this technology is categorized into far-field and
near-field WPT. The difference among these methods is normal based on the transmission
distance and frequency.
 Near-field WPT:
 Limited in range
 Power transmitted by a magnetic field through inductive coupling between
seperate coils of wire
 Far-field WPT:
 Long range transmission (several kilometers)
 Power transfered in form of radio frequency, microwave or laser
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Particularly, implantable biomedical devices mainly focus on near-field coupling
power transmission, which can guarantee the acceptable efficiency and power level.
Near-field WPT is widely adopted to support implanted biomedical devices, while
inductive coupling is the most popular option with an overall high transmission efficiency.
In this report, we focus mainly on the coupling coils design.
Figure 2. Schematic of WPT structure
II. Wireless capsule endoscopy (WCE):
II.1. Gastrointestinal
The gastrointestinal (GI) system, also known as the digestive system, is a complex
network of organs and structures in the body responsible for the breakdown, digestion, and
absorption of food, as well as the elimination of waste products.
Figure 3. GI tract
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Gastrointestinal (GI) tract diseases have become a great threat to human. Most
diseases such as bleeding, ulcer and tumor can be cured or controlled in their early stages, or
they will deteriorate into cancer or some other fatal disease. Diagnosing these diseases in
their early stages is of great importance. Endoscopy, which allows clinicians to directly view
the GI tract, has become the standard method of diagnosis in clinic.
Figure 4. Upper GI endoscopy
Endoscopy is a test to look inside your body by a long, thin tube with a small camera
inside, called an endoscope. It provides visual examination of the GI tract for the specialists.
However, traditional invasive endoscopies cannot examine the whole GI tract, leaving
the small intestine as a dead zone. They are inconvenient and cause intense pain for patients.
Furthermore, they can increase the risk of intestinal perforation, breathing difficulty and
infection during the inspection time.
II.2. Capsule endoscopy (CE):
Consequently, in order to avoid the traditional endoscopy drawbacks, capsule
endoscopy has caught increasing attention from gastroenterologists. Compared with
traditional methods, it is reportedly painless and comfortable for patients, it can also reach
and examine the small intestine and areas which are difficult to reach by normal method.
Figure 5. Capsule endoscope
Capsule endoscopy is a medical procedure used to visualize and diagnose conditions
within the gastrointestinal (GI) tract, specifically the small intestine. This procedure involves
swallowing a small, pill-sized capsule equipped with a tiny camera and a light source. As the
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capsule travels through the digestive system, it captures images of the gastrointestinal tract,
allowing doctors to obtain a detailed view and have a more accurate diagnosis.
In WCE, the magnetic based actuation mechanism is one of the efficient control
methods. This method consists of a small permanent magnet embedded inside the capsule
which interacts with the magnetic field generated by a large external magnetic source
resulting in an actuation force. The main advantage of such magnetic-based control is that it
minimizes power demand at the WCE, because the power required at the WCE to achieve
actuation and locomotion are almost negligible.
The power requirement for WCE increases with the increase of the quality of the
captured image and the addition of the advanced features such as video frame rate, good
work rate and substantial working time.
Figure 6. Power requirement for different types of WCE
The most basic commercial wireless capsule (CWC) typically requires about 30 mW of
electric power, mainly to power up image sensor, light emitting diodes (LEDs), and radio
frequency transceiver.
II.3. Wireless power transfer (WPT) in WCE:
A method of providing power to the capsule endoscope without the need for physical
connections or batteries. Power is transmitted wirelessly to the capsule from an external
power source.
Wireless power transmission (WPT) system refers to a system that transfers electrical
power wirelessly from a transmitter to a receiver in the form of electromagnetic waves. In
the context of WCE, WPT is used to provide the necessary power to the capsule endoscope
as it travels through the gastrointestinal tract, capturing images and transmitting data. This
eliminates the need for the capsule to carry an onboard power source, such as batteries,
which could limit its size and functionality.
For near-field application where the distance is relatively short, inductive coupling
technique is preferred. A WPT system based on induction coupling mainly consists of two
subsystems: the power transmitter and the power receiver. In this report, we consider:
 Power transmitter: power transmission section comprises of
 An oscillator: generate excitaiton signal
 A power amplifier circuit: amplify the excitation signal into high amplitude
electric current
 Transmitting coil (TC): produce a strong alternating magnetic field
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 Power receiver: receiving section comprises of
 Receiving coil (RC): A 3-coil set wound around a ferrite core, the magnetic field
induced by the TC interact with this RC and induces an AC current in the RC
 A rectifier: convert AC current to DC current
 A regulating circuit: smoothen and stabilize the DC current
 All components compacted inside the capsule
Figure 7. Illustration of WPT system in WCE
The TC and the RC resonate at the same frequency, at resonance point, all reactive
power in the coils is terminated ,thus raise the transfer efficiency.
For pratical use of WPT system, it must fulfill the following standard requirements:
i. Firstly, the system must be able to deliver stable and sufficient power to
ensure the smooth working of the WCE
ii. Secondly, the system must meet the requirements of safety issues (such as
the level of the electromagnetic field generated from the TC must not be
harmful to human,…)
iii. Finally, the receiving coil (RC) and its regulation circuit should be small
enough to be planted in the capsule.
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CHAPTER 2. COIL DESIGN
I. Transmitting coil (TC):
The transmitting coil induces magnetic field when the TC driving signal is applied (TC
driving signal comes from a power source and is amplified by a power amplifier circuit (PA)
before coming to the TC).
Figure 8. Sketch of the power transmission system
The intensity of the generated magnetic field depends on the magnitude of the driving
TC signals.
The uniformity of the generated magnetic field depends totally on the type and the
design structure of the TC. And one of the most suitable methods which have been introduced
for generating uniform magnetic fields is using the Helmholtz coils system.
Figure 9. Helmholtz coil (PA = Power Amplifier)
With Helmhotlz coil, the uniformity of the magnetic field is better in the inner region.
In this set up way, the Helmholtz coil configuration allows good confinement of the magnetic
field in the patient’s body, and hence may reduce the risk of unnecessary exposure.
Design considerations of the Helmholtz coil used in this system are coil dimensions,
uniformity level and the number of turns of the coil. In order to hold the human body and
provide sufficient work space, the diameter of the coil should be greater than the transverse
dimension of the body.
For any coil type, the main parameters detemining the quality of the TC are the
diameter, number of turns and wire gauge. Smaller diameter of TC improves efficiency
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because it allows a better coupling between the TC and the RC but may increase
electromagnetic exposure on human’s body. Even though larger TC can fix this problem, it will
require larger driving energy.
In this report, we suggest design the TC as a pair of Helmholtz coils, which consists of
two identical circular magnetic coils that are placed symmetrically.
Figure 10. Example of pratical Helmholtz TC coils model
The TC positions are fixed, the distance between the TC and RC coils is about 50-150 mm. The
longer the distance, the weaker the transmitting electromagnetic field becomes, so the
electric power generated to RC is very limited.
II. Receiving coil (RC) design
II.1. The Receiving subsystems:
The receiving subsystem consists of a receiving coils wound around a ferrite core, a
rectifier and a regulate circuit.
Figure 11. Schematic of the receiving subsystem
Working principle: Time varying magnetic field perpendicular to the cross sectional
area of the receiver coil includes an AC current in it and then AC potential is generated at the
terminals of the receiver coil.
Factors affecting the received power includes:
i. The RC dimensions
ii. The relative alignment between TC and RC
iii. The ferrite core used
iv. The matching between RC and load impedance
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v. The quality factor of TC and RC
vi. Rectifier circuit after the RC
II.2. The receiving coil design:
The RC is designed as a three-dimensional orthogonal coil with a ferrite core. The three
coils perpendicular to each other enable the RC to generate enough power at any orientation.
 The three coils: are made of a AWG33-enameled wire reeled around a cube core and
the three reeled coils are approximately round in shape.
 The ferrite cube core: can improve the transmission efficiency significantly and it is
made of Mn-Zn material with high initial permeability and low loss factor. Commonly
used ferrite cores are 3F4 (µ = 1000) [71], R5K (µ =5000) [62] or R10K (µ = 10000). The
advantages of ferrite core depends in their initial permeability µ
Figure 12. Practical image of three-dimensional RC
We choose the diameter to be about the diameter of the battery typically employed
in WCE, which is around 8mm. The induced voltage at the receiver relies on the maximum
possible number of turns.
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CHAPTER 3. COIL PARAMETERS CALCULATION
I. Prototype of WPT
I.1. Coupling coefficient:
 The amount of power received in the WCE systems depends on the coupling degree
between the TC and the RC, which is identified by the coupling coefficient k.
 k is defined as followed:
𝑀
k=
≤ 1
𝐿
√𝐿𝑟 𝑡
where: M is the mutual inductance
𝐿𝑟 and 𝐿𝑡 are the self inductance of the two coils respectively and can be easily
be measured by the inductance meter
 Coupling cofficient k depends on the geometry of coils, their relative spacing and the
medium surrouding them.
I.2. Mutual inductance and power load:
Figure 13. Circuit model of the WPT system
Where:



𝐿𝑡 , 𝐿𝑟 and 𝑅𝑡 , 𝑅𝑟 are the inductance and impedance of the two coils respectively.
𝐶𝑡 and 𝐶𝑅 are the tuning capacitors which can cause resonance in the system and
therefore remove the reactive power in coils, improve the coupling rate of the two
coils.
𝑅𝐿 is the load resistance, 𝑉𝑡 is the transmitting voltage and f is the working frequency.
 The mutual inductance M of the TC and RC with a single turn can be obtained using
the following equation:
𝑀𝑟𝑡 =
𝜇0 𝑅𝑟 2𝜋 2𝜋 sin 𝜃𝑟 sin 𝜃𝑡 + cos 𝛼 cos 𝜃𝑟 cos 𝜃𝑡
∫ ∫
𝑑𝜃𝑟 𝑑𝜃𝑡
4𝜋 0 0
𝑑𝑟𝑡
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Where
 µ0 is the air magnetic permeability of the core
 𝑑𝑟𝑡 is the distance between the TC and the RC
𝑑𝑟𝑡 = √(𝑅 cos 𝜃𝑡 − 𝑟 cos 𝜃𝑟 )2 + (𝑅 sin 𝜃𝑡 − 𝑟0 − 𝑟 sin 𝜃𝑟 cos 𝛼)2 + (𝑑0 + 𝑟 sin 𝜃𝑟 sin 𝛼)2
 Then the mutual inductance M between the TC with 𝑁𝑡 turns and RC with 𝑁𝑟 turns is:
M = 𝑁𝑡 𝑁𝑟 𝑀𝑟𝑡
Then, by finding M and measure Lr and Lt we can calculate the coupling coefficient k.
 The working power of the load, which is the received power of the transmission
system, can be expressed as:
The above formula and figure indicate that matching between the tuning capacitance
𝐶𝑡 and 𝐶𝑟 and the load resistance 𝑅𝐿 can maximize the receiving power 𝑃𝑟 .
I.3. Transferred voltage between the TC and the RC:
Figure 14. Schemactic of the power receiving subsystems
 When the receiving subsystems is placed in the alternating magnetic field, the induced
voltages V1, V2 and V3 will be produced in the three coils.
 Only the electric power with the maximum coupling voltage will be rectified to supply
the WCE working, and the other coils will not offer power. When the orientation of
the WCE changes, another coil with the maximum coupling voltage will offer power.
 The electric power V being used is:
V = max(V1, V2, V3)
 The minimum voltage that the RC can receive is:
𝑉𝑚𝑖𝑛 =
2√3
3
𝜋𝑓𝐵𝑁𝑆
Where:
 𝑓 is the changing frequency of the magnetic field, also known as the
frequency of the driving signal.
 B is the magnetic flux density
 N is the number of turns
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 S is the area if the coils
 In any orientation the three-dimension RC can receive electric power more than
voltage value 𝑉𝑚𝑖𝑛 , and the received inductive power can produce stable energy after
being rectified and regulated.
II. Stability of the WPT systems:
II.1. Misalignment between the TC and the RC
Ideally, the cross-sectional area of the coil must be perpendicular to the direction of
the magnetic field induced from the TC to ensure maximum power transfer efficiency.
However, this is not always guaranteed in pratical situation. In the pratical experiment, the
position of the TC are fixed whereas the RC’s position and orientation changes constantly, the
mismatching of the moving RC and TC will reduce the induced electric power.
To increase the electric power in the RC, the only way is to improve the power
transmission efficiency, if we increase the electric power of the TC to generate more magnetic
field, it may be harmful to human body.
Three-dimensional RC where three coils are set perpendicular to each other can
minimize this problem and each coil has its own diameter.
Figure 15. Relative alignment between the TC and the RC
Where:
 𝑑0 is axial misalignment
 𝑟0 is lateral misalignment
  is pitch misalignment
The TC and the RC are simplified as a single coil respectively in figure 15.Their relative
spacing in the coordinate system in the coordinate system is represented by the axial
misalignment 𝑑0 , lateral misalignment 𝑟0 and pitch misalignment .
The stability of 𝑑0 and 𝑟0 depends on the TC design.
The stability of  depends on the RC design.
The parameter functions of the TC and the RC can be expressed as below:
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R and r are radius of the TC and the RC respectively, 𝜃𝑡 and 𝜃𝑟 are parameters of the fucntions.
II.2. Comparision between different coil types:
Figure 16. Overview of WPT stability
The Helmholtz coil system have better stability (maximum 83%) in comparision to the
solenoid based systems in terms of 𝒅𝟎 and 𝒓𝟎 . But in terms of , the stability of a Helmholtz
coil based system was poor due to unequal inductance of the RC set. More accurate RC set
design can improve the stability which occurs by the variation of .
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CONCLUSION
The development of WPT in this area is still immature that there are many aspects that
require improvements and further investigation to make the WPT system become practical
in this area. Among the main problems we can highlight the non-uniformity of magnetic fields
within the WCE operating region; the dissimilarity of electrical parameters at the receiver
coils; and misalignment between receiver and the transmitter coils. All these contribute to
fluctuation of the received power and affect system efficiency, therefore they must be further
investigated to improve the feasibility of WPT.
The proposed design of coupling coils in wireless power transmission system of wireless
capsule endoscopy has theoretically shown improvement in solving the problem of unstable
power transferred due to misalignment between the transmitting coil and the receiving coil.
The three-dimension receiver coil has shown good flexibility at receiving power regardless of
the position and orientation relative to the transmitter coil, thus guarantees to meet the
power requirement of the system. However in practical experiment, there might occurs some
errors such as the collide between the magnetic fields of the three receiving coils. Thus this
system still need more experiment and research in pratical conditions. In the future, the
material, size and shape of the ferrite core is to be considered and researched.
After researching this subject, I have learned the basic informations (definition, theory,
working principe, classification) of wireless power transfer system. I have also learned the
application of wireless power transfer in implanted medical area, in particular is WPT for
gastrointestinal endoscopy. Through that I have learned how to choose appropriate coil
design and calculate basic parameters. By doing this project, I have improved some helpful
life skills such as research skill, information systhesis skill and presentation skill.
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REFERENCES
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doi:https://doi.org/10.1016/j.proche.2009.07.119.
2. Carta, R., Thoné, J. and Puers, R. (2009). A 3D Ferrite Coil Receiver for Wireless
Power Supply of Endoscopic Capsules. Procedia Chemistry, 1(1), pp.477–480.
doi:https://doi.org/10.1016/j.proche.2009.07.119.
3. Hui, S.Y.R., Zhong, W. and Lee, C.K. (2014). A Critical Review of Recent Progress
in Mid-Range Wireless Power Transfer. IEEE Transactions on Power Electronics,
[online] 29(9), pp.4500–4511. doi:https://doi.org/10.1109/TPEL.2013.2249670.
4. A video wireless capsule endoscopy system powered wirelessly: design, analysis and
experiment Guobing Pan1, Wenhui Xin2, Guozheng Yan2 and Jiaoliao Chen1
5. Zhou, Y., Liu, C. and Huang, Y. (2020). Wireless Power Transfer for Implanted
Medical Application: A Review. Energies, 13(11), p.2837.
doi:https://doi.org/10.3390/en13112837.
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