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A New Omnidirectional Wireless Power Transmission Solution for The Wireless Endoscopic Micro-Ball
Xiaomeng Li#1, Tianjia Sun#, Guolin Li*, Xiang Xie#2, Yingke Gu# and Zhihua Wang#
#Institute of Microelectronics, *Department of Electronic Engineering, Tsinghua University Beijing, China
[email protected] [email protected]
Abstract—The wireless endoscopic Micro-Ball is used to capture images of gastrointestinal (GI) tracts and save them into Flash memories. The batteries will be exhausted after the Micro-Ball is excreted from human body. So a new wireless power transmission (WPT) solution is proposed to deliver energy from an image reader to the Micro-Ball to fetch the stored images. In this solution, considering that the posture of the Micro-Ball is uncertain when it is placed into the image reader and the volume of the Micro-Ball is too small to contain multiple receiving coils, we employ multiple emitting coils in the image reader and one single receiving coil inside the Micro-Ball. Additionally, an adaptive control mechanism is proposed to select the optimum emitting coil with the highest power efficiency to transmit wireless power to the Micro-Ball placed inside the image reader. As a result, the Micro-Ball can work well in the image reader under all postures with highest power efficiency. The experimental results prove that the power delivering efficiency is in the range of 32% to 36%.
I. INTRODUCTION The wireless endoscopic capsules containing one or two
cameras is designed for the examination of gastrointestinal (GI) tracts[1]. Due to the random movement of the wireless capsule in the digestive tracts, it may omit some critical spots during the checking procedure. In order to receive the image data transmitted from capsule in real time, a receiving antenna array has to be attached on the patients’ body. This limits the patients’ movement and makes them feel uncomfortable.
Thus, the former research[2] has presented the Micro-Ball Endoscopy System. Compared with the traditional capsule endoscopy, the Micro-Ball containing six cameras can reduce the blind area. Because there are six cameras, the power consumption of the Micro-Ball is much higher than that of the traditional capsule endoscopy. Instead of transmitting images outside the human body, the Micro-Ball stores images into Flash memories to save energy. After the Micro-Ball is excreted from human body, the batteries would be exhausted because of the high power consumption. Then place the Micro-ball into an image reader. There is a wireless power emitter in the image reader and a power receiver in the Micro-Ball. The emitter supplies wireless energy to the Micro-Ball, and the images stored in the Flash are read out. The above
workflow of the Micro-Ball Endoscopy System was described in reference [3] and is depicted in Figure 1.
Figure 1. The Workflow of the Micro-Ball Endoscopy System
Considering the random posture of the Micro-Ball when it is put into the image reader, an omnidirectional WPT solution is needed to meet this application’s requirements. Most of the former researches adopt multiple receiving coils[3][4][5][6]. However, the Micro-Ball has a diameter about 13mm~15mm and most of the spaces are occupied by six cameras and batteries. The left spaces are too limited to contain multiple receiving coils and related components.
This paper presents two new ideas to solve the above problems. One is a new WPT structure. Compared with the other WPT system which adopts a single emitting coil and multiple receiving coils, this new structure uses multiple emitting coils and only one receiving coil. This new structure not only guarantees that the transmitted wireless power is omnidirectional, but also makes the Micro-Ball small enough. The other idea proposed in this paper is an adaptive control mechanism. The mechanism chooses only one emitting coil with the highest power delivering efficiency to emit wireless power. The new structure and the adaptive control mechanism are two key factors to the success of this solution.
II. THE NEW STRUCTURE In order to transmit omnidirectional wireless power to the
Micro-Ball when it is placed into the image reader and save the space inside the Micro-Ball, this new structure adopts multiple emitting coils and a single receiving coil. As shown
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in Figure 2, the new structure contains five emitting coils on the image reader and a single receiving coil inside the Micro-Ball. The five emitting coils are fixed on the image reader along the direction of three orthogonal axes. The Coil 1 and Coil 3 are placed along the direction of the x-axis. The Coil 2 and Coil 4 are placed along the direction of the y-axis. For this reason, Coil 1 and Coil 3 are perpendicular to both Coil 2 and Coil 4. The Coil 5 is placed at the bottom and along the direction of z-axis. It is perpendicular to all the other four emitting coils. The single receiving coil is placed on one plane of the cube in the Micro-Ball[2].
1
3
42
5
Emitting Coils
Micro-Ball
x
y
z
Image Reader
A Single Receiving Coil
Figure 2. A New Structure of The WPT Solution
To evaluate the performance of this new structure, analysis of the magnetic field is conducted. Due to the symmetric structure of the power emitting coils and there is only one working emitting coil at any time, the analysis can be simplified to analyze the distribution of the magnetic field generated by only one emitting coil. As shown in Figure 3(a), the relative orientation and position between the receiving coil and the emitting coils can be represented by two angles after the Micro-Ball is placed into the image reader. The angle α represents the angle between the plane of the receiving coil and the plane xoz. The angle β represents the angle between the plane of the receiving coil and the plane xoy.
(a)Definitions of Two Angles (b) Magnetic Strength Generated by
Coil1 at The Center of The Receiving Coil
Figure 3. Analysis of The Magnetic Field
Here, we analyze the magnetic field generated by the Coil 1 in Figure 2 and we only need to consider the range of α and β from negative 45 degrees to positive 45 degrees. Once any angle is out of this range, another emitting coil will work and generate the same magnetic field as the former emitting coil does. According to Biot-Savart Law, the magnetic strength can be determined by the two angles. Meanwhile, the magnetic strength is also proportional to the turns of the emitting coil (N) and the current (I) in the emitting coil. We get
B f α, β, N, I (1)
On condition that N=1 and I=1A, the relationship between the magnetic strength at the center of the receiving coil generated by one emitting coil and the two angles is shown in Figure 3(b). If N and I change, B will change in the same proportion.
From the above figure, we find that in the range we concern the magnetic strength does not change tremendously. The maximum B is 4.6×10-5T when |α| |β| 0°, while the minimum B is 3.9 × 10-5T when |α| |β| 45° . Similarly, other emitting coils can also generate the same magnetic field. Figure 4 indicates that the distribution of the magnetic strength at the center of the receiving coil inside the image reader varies periodically, and the range of variation is less than 15%. So it is reasonable to assume that the magnetic field is nearly uniform in the image reader when only one emitting coil works. Whatever the posture of the Micro-Ball is when it is put into the image reader, it can receive the wireless power with high efficiency all the time.
Figure 4. Distribution of Magnetic Field inside The Image Reader
If the structure adopts a single emitting coil and a single receiving coil, it is hard for the doctors to place the Micro-Ball with the best posture exactly in the actual working conditions. Furthermore, placing an external marker on the Micro-Ball to show the position of the receiving coil is also inappropriate. Due to the marker will block the line of sight of the cameras in the Micro-Ball, this method cannot be employed. Compared with the new structure presented in this work, the above methods for supplying wireless energy cannot meet the requirement of the Micro-Ball Endoscopy System. To sum up, this new structure is practical.
III. THE ADAPTIVE CONTROL MECHANISM An adaptive control mechanism is proposed in this section
for the new structure presented above. Under the control of this mechanism, only one emitting coil with the highest power delivering efficiency is activated to emit the wireless power.
A. Theoretical analysis of the adaptive control mechanism The principle of the adaptive control mechanism is that the
emitting coil couples with the receiving coil and the emitting coil side has an equivalent impedance of the receiving coil side. The greater the equivalent impedance is, the stronger the degree of coupling is, which corresponds to a higher power delivering efficiency. This mechanism detects the variation of the electric parameter caused by the variation of the equivalent impedance, and then selects the emitting coil with the highest power delivering efficiency. This new WPT solution can be
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modeled by Figure 5(a) when only one emitting coil is activated to work.
(a) The Emitting Coil Couples with The Receiving Coil
(b) Emitting Coil in No-Load State (C) Emitting Coil in Loaded State
Figure 5. The Schematic Circuit of The Adaptive Control Mechanism
As depicted in Figure 5(a), the left circuit connecting to a voltage source represents the emitter while the right circuit represents the receiver. The source Vi has an internal resistance Rs. L1 is the inductance of the emitting coil, and C1 is the tuning capacitor connected to the emitting coil. The circuit works at a selected resonant frequency by tuning the capacitor C1. R1 is the parasitic resistance of the emitting coil. In a similar way, L2, C2 and R2 are the inductance, tuning capacitor and parasitic resistance of the receiving coil. RL represents the load connected to the receiving coil. M is the mutual inductance between the two coils. The mutual inductance M can be represented by M k L L (2)
The parameter k is the coupling coefficient.
According to Figure 5(a), the model of the WPT solution can be expressed by Equation (2). V0 R R j ωL 1
ωC jωMjωM RL R j ωL 1ωC II
(3)
Since the circuit works at the selected resonant frequency, the impedance of the emitting coil and the receiving coil turns into a resistance. An detailed deduction was proposed to prove that the power delivering efficiency relates to the coupling coefficient k[7]. The efficiency can be expressed by Equation (4).
η Q QQ Q (4)
The parameter Q1 and Q2 are quality factors of the emitting and receiving coil, which are determined by the size of the coil and the working frequency. If the working frequency and the size of the coil are determined, Q is a constant. When the Micro-Ball is not placed into the image reader, it can be
supposed that the WPT solution is in the no-load state. The parameter k in Equation (2) is zero and we get a Vab as shown in Figure 5(b). When the Micro-Ball is put into the image reader, the working state of the WPT solution changes and we get a changed value of Vab as shown in Figure 5(c). We mark this loaded Vab as V . The calculation of the increasing rate of Vab can be expressed as followed according to Equation (2) and Equation (3). ∆V % V VV L L RSR RS R RL R L L (5)
The value of the coupling coefficient k is very small under normal conditions and we always get k << 0.1. Therefore, we come to a conclusion that k ω L L RS R RL R (6)
and ∆V % C · η
η，C L L RR Q Q RS R RL R (7)
According to Equation (7), the power delivering efficiency is positive related to the ∆V %. It means that the emitting coil with a highest delivering efficiency will show a prominent change of Vab when the Micro-Ball is placed into the image reader. In the following experiment, this conclusion is verified. As shown in Figure 6, the efficiency is from one emitting coil to the receiving coil. When the efficiency reaches 36%, the measured value of ∆V % is 56.7% and the theoretical value is 56.2%. When the efficiency is 32%, the measured value is 46% and the theoretical value is 47%. From Figure 6 we find that the measured value is matching the theoretical Equation (7). This will lead a design to implement the adaptive control mechanism as shown in the following.
Figure 6. Relation between Efficiency And Increasing Rate of Vab
B. Implementation of the adaptive control mechanism A circuit is designed to implement the adaptive control
mechanism as shown in Figure 7. According to the analysis above, to select the emitting coil with the highest power delivering efficiency to emit wireless power, this circuit detects the greatest ∆V % among the five emitting coils.
Figure 7. Implementation of The Adaptive Control Mechenism
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This circuit contains an AC source, an envelope detection circuit, an A/D convertor, a multiplexer and a digital controller. The envelope detection circuit is used to collect Vab and the A/D converter is used to process the signal of Vab collected by the envelope detection circuit in order to meet the requirements of the digital controller. The digital controller is used to send a control signal to the multiplexer, and controls the different switches on or off. When this WPT solution is in no-load state as shown in Figure 5(b), i.e., before the Micro-Ball is placed into the image reader, the digital controller uses a polling mode to collect five Vab of each emitting coil. When the Micro-Ball is placed into the image reader as shown in Figure 5(c), the digital controller gets five loaded V in the similar way. After calculating five ∆V % , the digital controller selects one coil with the greatest ∆V % to emit the wireless power.
IV. EXPERIMENTS To verify the validity of our analysis, prototypes of the
image reader and the Micro-Ball were fabricated. Figure 8(a) describes the prototype of the image reader. It is comprised of five plastic square boards, whose edge length is 20mm. The five emitting coils are all with a diameter of 11mm and they are placed as shown in Figure 2. Figure 8(b) depicts the prototype of the Micro-Ball with a diameter of 14mm. The cube has an edge length of 8mm and the receiving coil on one plane of the cube has a diameter of 7mm. An energy recovery circuit with a red LED is connected to the receiving coil.
(a) Image Reader (b) Micro-Ball
Figure 8. Prototype of The WPT Solution
According to Equation (4), a high value of Q1Q2 corresponds to a high efficiency. The method of optimizing the receiving and emitting coils is to search an optimal value of Q1Q2. At the frequency of 3.17MHz, the value of Q1Q2 has the largest value. Figure 9 depicts the prototype of this WPT solution works in practice. A signal generator outputs a sinusoidal signal with a frequency of 3.17MHz. Tuning capacitors were connected to the receiving and emitting coils in order to tune the frequency resonate at 3.17MHz. The receiving coil’s tuning capacitor is inside the Micro-Ball.
Figure 9. The WPT Solution Works in Practice
Agilent 8753SE S-Parameter Network Analyzer is used here to measure S11 and S21. As shown in Figure 10, the best efficiency reaches 36% at 3.17MHz. This situation happens when αand βin Figure 3(a) are all zero. The worst case appears when |α| |β| 45°. At this moment the efficiency drops to 32%. This result is exactly matching the distribution of the magnetic field depicted in Figure 4.
(a) S11 of Emitting Coil (b) S21 from Emitting Coil to Micro-Ball
Figure 10. S11 And S21 Measured by S-Parameter Network Analyzer
The proposed WPT structure and the adaptive control mechanism ensure that whatever the posture of the Micro-Ball in the image reader is, the emitting coil with the highest power delivering efficiency can be selected to work. Experiments indicate that 150mW can be successfully delivered, which can meet the requirement of the Micro-Ball.
V. CONCLUSION This paper presents a new wireless power transmission
solution for the Micro-Ball endoscopy system. In this solution, a new structure and an adaptive control mechanism are proposed. The new structure ensures that the omnidirectional wireless power can be transmitted to the Micro-Ball inside the image reader. The adaptive control mechanism can select the emitting coil with the highest power delivering efficiency to emit the wireless power. The experiments show that the power delivering efficiency can be guaranteed between 32% and 36%.
REFERENCES [1] Gavriel Iddan, Gavriel Meron, Arkady Glukhovsky, et al. “Wireless
capsule endoscopy”,Nature,2000,pp.417-418A.B. [2] Y. Gu, X. Xie, Z. Wang, et al. “A new Globularity Capsule Endoscopy
System with Multi-Camera”, IEEE Biomedical Circuits and System Conference, pp. 289-292, Nov. 2009
[3] An asymmetric resonant coupling wireless power transmission link for Micro-Ball Endoscopy. Sun, Tianjia; Xie, Xiang; Li, Guolin, et al. Conf Proc IEEE EMBC, vol.1, pp. 6531-6534, 2010
[4] Lenaerts, B.; Puers, R. “An omnidirectional transcutaneous power link for capsule endoscopy”, International Workshop on Wearable and Implantable Body Sensor Networks, pp.4, 2006
[5] Lenaerts B, Puers R. “An inductive power link for a wireless endoscopy”. Biosensors and Bioelectronics, Vol.22, No.7, pp.1390-1395, Feb 15 2007
[6] Ryu M, Kim JD, Chin HU, et al. “Three-dimensional power receiver for in vivo robotic capsules”, Medical & Biological & Engineering & Computing, Vol.45, No.10, pp.997-1002, Oct 2007
[7] Kumar, A.; Mirabbasi, S.; Mu C.; “Resonance-based wireless power delivery for implantable devices”, IEEE Biomedical Circuits and Systems Conference Page(s): 25 – 28, 2009
AC Source
Micro-Ball
20mm
11mm
Tuning Capacitor
-9.2db
7mm
8mm
Tuning Capacitor
-4.5db
14mm
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