· web viewthe tuning circuit is shown as fig 3.4.1. according to the theory, if the circuit...

The design of Transmission Impedance

Matching Network for Resonant Wireless

Power Transmission Physics Project

Abstract

AbstractAs wireless power transmission is a non-fixed, node to surface way of energy

transmission, it doesn’t have the many problems existing in the field of wired

power transmission. Currently, the technically advanced modes of wireless

power transmission are inductive transmission and resonant transmission. In

comparison with inductive transmission, resonant transmission has the

advantage in transmission distance. The essay features on series resonant

transmission. While the load on the receiving end is fixed, an L type impedance

matching network is added between the high frequency power source and the

transmitting coil to improve the efficiency of wireless power transmission. The

tests showed that tuning is vital in transmission, and that joint tuning has a

better effect than independent tuning. Once tuning is done in the case of critical

coupling or under coupling, the system will remain resonant even if the distance

between the transmitting and receiving coil is changed. In addition, a circuit

simulation model in accordance with the test results is built. Based on this

model, a single variable impedance matching network is designed and built. Test

results showed that within a certain distance, a relatively high efficiency of

wireless power transmission can be obtained using this simple impedance

matching network.

Keywords: resonant; wireless power transmission; impedance; circuit

simulation models.

I

Catalog

Catalog

Abstract............................................................................................................................................. I

Catalog...................................................................................................................................................... II

Part 1 Introduction..............................................................................................................................4

1.1 A brief Introduction on Wireless Power Transmission......................................4

1.2 Different Theories of Wireless Power Transmission and their Features...5

1.2.1 Far Field Wireless Power Transmission............................................................5

1.2.2 Near Field Wireless Power Transmission.........................................................5

1.3 The Objective of this Project..........................................................................................6

Part 2 Preparations and The Train of Thought of the Project..........................................7

2.1 Preparations of the Project.............................................................................................7

2.3.1. Impedance matching network........................................................................7

2.3.2. Experimental Equipment (Adjustable Capacitor).......................................7

2.3.3. Choice of the frequency of the power transmission.............................8

2.3.4. Choice of the structure of the power transmission...............................8

2.3.5. Choice of Load........................................................................................................8

2.3.6. Data and construction of the coil..................................................................8

2.2 The Train of Thought of the Project............................................................................8

Part 3 Establishment and Simulation of the Circuit Model.............................................10

3.1 The Establishment and Calculation of the Basic Model..................................10

3.2 The Measurement of Mutual Inductance...............................................................13

3.3 The Measurement of the Transmitting and Receiving coils’ Parameters15

3.4 Methods of Tuning...........................................................................................................17

3.5 The Basic Structure of the L Type Impedance Matching Network.............21

3.6 Application of the Impedance Matching Network on the Basic Model....24

3.7 The Building of the Advanced Model and the Impedance Matching

Network....................................................................................................................................... 27

3.8 The Building of the Ultimate Model and the Impedance Matching

Network....................................................................................................................................... 31

3.9 Optimization of the Impedance Matching Network..........................................34

II

Catalog

Part 4 Experimental Data Analysis and Summary..............................................................38

4.1 Analysis of Impedance Matching Test Results with the Same Receiving

Coil and Transmitting Coil...................................................................................................38

4.2 Analysis of Impedance Matching Test Results with Different Receiving

Coils and Transmitting Coils...............................................................................................40

4.3 Conclusion...........................................................................................................................43

4.4 Outlooks............................................................................................................................... 44

Part 5 Reference.................................................................................................................................45

Part 6 Acknowledgements.............................................................................................................46

III

Part 1 Introduction

Part 1 Introduction

1.1 A brief Introduction on Wireless Power Transmission

The human society has entered the era of electricity after the second

industrial revolution. After years of development, electricity has become an

essential part in people’s lives. From mobiles phones to cars, from planes to the

space stations in outer space, they all need electricity to operate.

Nowadays, the transmission of electricity is mainly based on node to node wired

transmission, this kind of transmission has a lot of problems such as friction, burning

in and so on. Besides, traditional wired power transmission can’t meet some special

needs such as in mines, under water or medical implants. Furthermore, a large

number of wires and sockets has brought great inconvenience in people’s lives.

As a result, wireless power transmission technology was developed. With this

technology, electricity can be transported without wires. Wireless power

transmission was first presented by American engineer Nicola Tesla in the end of the

nineteenth century. He lighted a phosphorescent light bulb without any wire

connected. It was an important exploration in the history of wireless power

transmission.

In recent years, wireless power transmission has experienced rapid

development both in technology innovation and application standards. Magnetic

induction wireless charging technology is a mature technology. It is the use of the

DC / AC power of the transmitter to convert the magnetic induction generated by

the driver coil to the coupling on the receiver coil, which forms AC current in the

secondary winding. In that way, DC voltage can be obtained after rectifying.[8],[9]

Magnetic induction wireless charging technology has established two application

standards: Wireless Power Consortium and Power Matters Alliance. Magnetic

resonance wireless charging technology, meanwhile, transfers energy by achieving

resonance in the LC circuits of the two coils. The process is done by adjusting the

electromagnetic wave frequency of the transmitting side to fit the resonant

frequency of the receiving side. The application standard for resonant wireless

charging technology is Alliance for Wireless Power (A4WP).

4

Part 1 Introduction

Thanks to the extensive research of wireless power transmission done by

domestic and foreign scholars, great progress has been made in the theory and

application of both near field and far field wireless power transmission.

1.2 Different Theories of Wireless Power Transmission and their

Features

1.2.1 Far Field Wireless Power Transmission

Far field wireless transmission includes radiation wireless power

transmission and laser wireless power transmission. Currently, radiation

wireless power transmission is poor in directivity and transmission

power. Laser wireless power transmission is excellent in transmission

power, but the current technology is not mature.

1.2.2 Near Field Wireless Power Transmission

Near field wireless power transmission includes magnetic induction

wireless charging technology and magnetic resonance wireless charging

technology. Their main difference is whether resonance occurs in the

system.

The mechanism of magnetic inductive wireless charging technology is similar to

that of a separable transformer. The iron core is replaced by the air gap,

resulting in the lack of directed channels to hinge the magnetic flux lines to

the receiving coil. Thus, this kind of power transmission with high power and

efficiency can only be obtained within a short transmission distance, usually

smaller or equal to the diameter of the transmitting coil. As the distance

increases, the transmission power and efficiency drops sharply.

Magnetic resonance wireless charging technology uses the mechanism of

resonance, so that a relatively high power and efficiency can be obtained at a

medium distance. Furthermore, its power transmission is not affected by the

non-magnetic obstacles. So, in a word, the resonance type has a longer

5

Part 1 Introduction

transmission distance and a higher transmission stability compared to that of

the induction type.

However, it is worth noting that because the magnetic resonance wireless

power transmission is based on the principle of resonance, if the two coils

does not meet the conditions of resonance in terms of capacitance and

inductance (i.e. offset), the transmission efficiency will be seriously affected.

To restore resonance to the system, it is needed to adjust the coil capacitance

and inductance, that is, to adjust the impedance of the system (i.e. tuning).

1.3 The Objective of this Project

This project will explore and design a set of simple two-coil series resonant

impedance matching network on the transmitting side, which can make the load

get the maximum transmission power in a certain distance, so as to improve the

transmission efficiency of wireless transmission.

6

Part 1 Introduction

Part 2 Preparations and The Train of Thought of the Project

2.1 Preparations of the Project

2.3.1. Impedance matching network：In a wireless power transmission system, the receiving end is equivalent to a

load in a high frequency circuit, whose impedance will change in accordance with

the change in the resistance of the load and the distance between the

transmitting and receiving end. Thus, an impedance matching system is needed

to match the receiving end with the transmitting end. [3]

In order not to consume extra power, the electronic components used in the

network can only be energy-storing components such as capacitance and

inductance. There are mainly three types of matching networks: L-type, T-type

and -π type. An L-type matching network is used, in which both C3 and C4 are

variables. (As shown in Fig 2.1.1)

2.3.2. Experimental Equipment (Adjustable Capacitor): The duplex adjustable

capacitor used in the project is a CBM-233P model. The three pins are marked O,

G and A, in which pin G is a common pin.

The reason of choosing this kind of capacitance is that it has a large

adjustable range. The adjustable range of OG section is 5.5pF~60pF and the

adjustable range of AG section is 5.5pF~141pF. In other words, the biggest

7

Fig 2.1.1: L-type matching network

Part 1 Introduction

adjustable range of this capacitance is 11pF~201pF, which can be achieved by

connecting O point &A point. Besides, parallel connected constant capacitance

and switches are used when a bigger adjustable range is needed.

2.3.3. Choice of the frequency of the power transmission: Among the 3

existing power transmission standards(A4WP,PMA,WPC), PMA&WPC are

designed for electro-magnetic induction power transmission and only A4WP

standard is designed for resonance power transmission. Its standard frequency is

6.78MHz, so this project selects 6.78MHz as the frequency of the output

frequency of the high-voltage power source.

2.3.4. Choice of the structure of the power transmission：Resonance

power transmission has two main structures, the 4 coil structure and the 2 coil

structure. According to the reference[7], the efficiency of transmission of these

two structures are about the same at the same circumstance. As a result, this

project use the 2 coil structure to carry out the experiment.

2.3.5. Choice of Load:The project originally planed to use light bulbs of 12V,

2.4W as the load, but its resistance will increase greatly when the voltage rises.

Besides, the resistance of the light bulb is measured 95 when it’s working underΩ

normal voltage. As a result, this project uses a 95 resistance as the load.Ω

2.3.6. Data and construction of the coil：This project designed and

constructed 4 coils, each of it is constructed by fixing 2.5mm varnished wire onto

plastic boards using hot melt glue. Among them, coil 1 and 2 imitate the coils

mentioned in the paper High Power Resonant Wireless Power Transmission by

professor Y. Li. Coil 3 and 4are designed according to the formation of Flat-helix

coils and are quite different from coil 1&2 so that they can test whether the

designed impedance matching system can adapt to different coils.

2.2 The Train of Thought of the Project

8

Part 1 Introduction

The construction of the system is shown in Fig 2.2.2. There are four

adjustable components （ C1&C2 are two tuning capacitances, and C3&C4a are

two adjustable capacitances of the impedance matching network）so that the

system is quite complicated. In order to make the system simpler, the project

managed to reduce the number of adjustable components on the basis of the

stimulation model. Some components can be replaced by fixed components at

the cost of a slight decline in efficiency, so that the impedance matching network

can be much simpler and more feasible.

9

Fig 2.2.2: A simple diagram of the system

Fig 3.1.1：The basic model of the system

Part 3 Establishment and Simulation of the Circuit Model


3.1 The Establishment and Calculation of the Basic Model

The basic model of the system is shown as in fig 3.1.1.

The actual effect of the mutual inductance (Shown as M in Fig 3.1.1) is equivalent

to a controlled voltage source, as is shown in Fig 3.1.2.

10

Fig 3.1.2：Basic model which takes mutual inductance into account


According to the Law of Kirchhoff, the sum of the voltage around the closed loop

circuit equals to zero, thus the following equations can be established.

Where stands for the alternating voltage generated by the signal generator,

stands for the mutual inductance between the transmitting and receiving coil,

stands for the internal resistance of the signal generator, the value is 50 , Ω R1 and

R2 are the high frequency resistance of the transmitting and receiving coils. After

tuning is complete, the following equation should be obtained.

Therefore:

11

Part 1 Introduction

12


Plugging in (1) and (2)

By simplifying (6) and (7), we get

[5]

From equation (9), it can be seen that the

transmission module is equivalent to a simple

circuit consisting three resistances in series

(Fig 3.1.3). Where stands for the

impedance mapping of the receiving coil on

the transmitting side.

13

Fig 3.1.3： The simplified model of the transmission module


3.2 The Measurement of Mutual Inductance

To obtain the precise value of the mutual inductance, the circuit shown as

Fig3.2.1 is designed and built.

The signal from the signal source is a sine wave at a frequency of 6.78MHz.The

peak to peak voltage on the load and on the receiving coil

is obtained using

the Lvyang LDS31010F multifunctional oscilloscope. According to the two variables

obtained, the mutual inductance can be calculated using the following equation.

According to the theory, the value of mutual inductance should decline rapidly

as the distance between transmitting and receiving coil increases. The test results (fig

3.2.2) coincides with the theory.

14

Fig 3.2.1：The circuit for the measurement of mutual inductance

Fig 3.2.2 The variation trend of mutual inductance vs the distance between transmitting and receiving coil


15


3.3 The Measurement of the Transmitting and Receiving coils’

Parameters

The equivalent circuit model of the coil is shown as in Fig

3.3.1, the coil has an inherent resonant frequency

. Through connecting a capacitance in parallel

between the main coil, the resonant frequency can

be changed to . Using the vector network analyzer to measure the

two different frequencies, the inductance and the distributed

capacitance can be calculated with the following equation.

After simplifying, we get

Wire

Diameter（mm

）Turns

Inner

Diameter（cm

）

Distance

between

turns(cm)

Coil 1

2.5

8 10 1

Coil 2 8 10 1

Coil 3 5 19 0.6

Coil 4 10 5 0.6

In the experiment, the capacitance

used is 330pF±10%. From the

experiment, we get that and . Using equation (13) and

16

Fig 3.3.1 ：Equivalent circuit model of the coil


(14), we get that the inductance of coil 1 and 2 are both 12 , while their

distributed capacitance are both 10.4pF±10%. The inductance of coil 3 is 13

, its distributed capacitance is 4.79pF±10%. The inductance of coil 4 is

10 , its distributed capacitance is 4.36pF±10%.

The high frequency resistance of the coils is difficult to measure with the

current instruments, so it is calculated using a formula.

For a cylindrical copper conductor with radius , length , conductivity ,its

DC resistance is equal to the following formula.

[5]

The skin depth can also be presented.

[5]

Using formula (15) and (16), the expression for high frequency resistance R

is

[5]

Through calculations we can get that the high frequency resistance is 0.35Ω

for coil 1 and 2, 0.28 for coil 3, and 0.27 for coil 4.Ω Ω

17


3.4 Methods of Tuning

Because the inherent resonant frequencies of coil 1 and 2 are not 6.78MHz,

and that the resonant power can only reach a maximum when the circuit is

resonant, series resonant capacitors are needed in the coil part of the circuit, so

as to make series circuit resonate at a frequency of 6.78MHz.

At the beginning of the experiment, the method of independent tuning is

used. The tuning circuit is shown as Fig 3.4.1.

According to the theory, if the circuit is resonant, the voltage across the 50Ω

resistor should be half of the open circuit voltage. In the experiment, the peak to

peak voltage output of Vs is 10V, and the open circuit voltage is 9.8V, so the

system should be resonant when the voltage across the resistor is equal to 4.9V.

However, in actual situations, there are unavoidable deviations in tuning. If the

deviation for both coils are rather high, the system cannot work under a resonant

condition. When using independent tuning, it is found that after tuning, the

voltage on the load can be further promoted after adjusting the tuning capacitor,

which proves that resonance in the system can hardly be obtained using this

method. In order to achieve better resonance in the system, the original tuning

circuit is improved

18

Fig 3.4.1: The original tuning circuit


The improved tuning circuit is shown as Fig 3.4.1

Using this method of joint tuning, the system reaches resonance when the

voltage across the load RL reaches its peak value. After tuning, a vector network

analyzer replaces the signal generator in the circuit to measure the resonant

frequency of the coil. The target frequency is 6.78MHz and the actual frequency

is 6.75MHz. The difference is 0.4%(Fig 3.4.2). It can be seen that this testing

method has tuning deviation, but is far lower than that of the former method and

is easier to operate.

19

Fig 3.4.1 Improved Tuning Circuit

Fig 3.4.2: Using the vector network analyzer to analyze the frequency of the coils*(Target Frequency 6.78MHz, Actual Frequency 6.75MHz)


As is mentioned in 2.1, the ranges of the duplex adjustable capacitor used in

the project are 5.5pF~60pF for OG and 5.5pF~141pF for AG. Thus it is necessary

to determine which range to use in order to get the minimum step size. Using

equation (9) and the parameter of the main coils measured in 3.3, a simulation of

the change in the voltage of the load versus the changes in the tuning capacitors

C1 and C2 is made. The result is shown in Fig 3.4.3.

It can be seen from Fig 3.4.3 that the capacitance of C1 and C2 are both below

60pF when the system is resonant, so the OG range should be chosen as its step

size is smaller. From the sectional graph of Fig 3.4.3(Fig 3.4.4), we can also see

that C1 has a greater effect in case of tuning. Thus C1 should be adjusted before

C2 while tuning, it would be easier to reach the resonant point.

Once tuning is done in the case of critical coupling or under coupling, the

system will remain resonant even if the distance between the transmitting and

receiving coil is changed. Therefore, C1 and C2 can remain fixed after tuning is

done, reducing the number of variable components from four to only two.

20

Fig 3.4.3: The change in the voltage of the load vs the changes in the tuning capacitors C1 and C2

Fig 3.4.4: The influence that C1 and C2 poses on tuning


21


3.5 The Basic Structure of the L Type Impedance Matching

Network

The basic structure of the L type impedance matching network is shown as Fig

3.5.1.

根据阻抗公式，The impedance is for a capacitance, for an inductance, and for

a resistance. In order to get the maximum power on the load, the sum of the

impedance of the network and the load should be equal to 50 . Thus theΩ

following equation can be derived.

Whereas refers to the mapping of the load on the receiving end .

The expression of is , which is derived from equation (9).

By simplifying equation (15), we get

22

Fig 3.5.1 The model of the L type impedance network


If the equation is to hold true, the real and complex parts on the two sides of

the equation must be equal to each other respectively. Thus we get equation

(17) and (18).

From equation (17), equation (19) can be derived.

Plug in equation (18).

From the expression of C3 (equation (19)), it can be inferred that the system

is only effective when . When , the “L reverse” network can be

adopted, as shown in Fig 3.5.2.

In this situation, all other parameters remain the same as the L type, only the

expression of C3 is changed into equation (21). 23

Fig 3.5.2: The model of the L reverse type impedance network


24

Fig 3.6.1: The model of the transmission side


3.6 Application of the Impedance Matching Network on the

Basic Model

Because the impedance matching network has a more obvious effect with a

longer distance, the impedance network at 30cm is first simulated and built. The

model of the transmission end is shown in Fig 3.6.1.

According to equation (19), . As and that is

positive, it can be derived that . After plugging in all of the variables, we

get that . In order not to change inductors in the following experiments,

the value of L is chosen at . By plugging in this value, . The

formula for the power on the load is shown as follows.

Suppose that the impedance of branch ① is , the impedance of branch ② is

25


, and the sum of the impedance of the two branches is , the following

equations can be derived.

The model of the receiving end is shown as Fig 3.6.3.

The formulas for the power and voltage across the load are shown as follows,

26

Fig 3.6.3：The model of the impedance matching network on the transmission side


whereas stands for the sum of the power in the circuit.

The resonant mode used in the circuit is series resonance, also called voltage

resonance. Therefore, if the peak to peak voltage output is 10V, the voltage across

the tuning capacitor can be as high as 500V, which is far above the 100V withstand

voltage of the capacitor. For safety reasons, the peak to peak voltage output is

lowered to 2V, so that the voltage across the tuning capacitor can be lowered to

100V, so as to prevent the breakdown of the capacitor.

The simulations based on the formulas above indicates that the maximum

voltage across the load is 0.8V. However, in the experiment, the maximum voltage

reached is only 0.45V, which is a 78% difference from the simulated value. This

proves that this model is far from the actual situation.

27

Fig 3.7.4 The advanced model of the transmission system (without the impedance matching system)


3.7 The Building of the Advanced Model and the Impedance

Matching Network

After re-analyzing the previous model, one possible reason for the deviation is

found. That is the previous model neglected the distributed capacitance of the main

coil. The advanced model is built with the distributed capacitance taken into account,

as is shown in Fig 3.7.4.

Suppose that the impedance of the receiving end is , it can be derived that

28


From equation (9), the impedance of the transmission end of the basic model can be

expressed using the following equation.

It can be seen that the distributed capacitance does have an effect on the

calculation of impedance.

When calculating the impedance of

the transmission end, the mutual

inductance between the coils (noted as

M) needs to be taken account of. The

model of the transmission end is shown

as Fig 3.7.5.

According to the computation

methods of equation (25), it can be

derived that the impedance of the

29

Fig 3.7.5: The advanced model of the transmission end


transmission end , whereas .

According to the impedance matching network based on the advanced model

(shown in Fig 3.7.6), , and .

Similar to the former model, let the impedance of branch ① be , the

impedance of branch ② be , the sum of the impedance of the two branches be

. The following formulas can be derived.

30

Fig 3.7.6: The model of the impedance matching network based on the advanced model


The model of the impedance of the receiving end is shown as Fig 3.7.7.

According to formula (25),

31

Fig 3.7.7: The model of the impedance matching system based on the advanced model进阶模型的阻抗匹配接收端模型


According to the simulation based on formula (29) and (30), the maximum

voltage across the load is equal to 900mV. However, in the experiment, the

voltage across the load can only reach 600mV, which is a 33% deviation from the

theoretical value. This means that the advanced model still does not match the

actual situation and needs further modification.

32


3.8 The Building of the Ultimate Model and the Impedance

Matching Network

According to the advanced model, the choice of inductor has little or no effect

根 on transmission efficiency. In fact, when working at high frequencies,

every inductor and capacitor has loss resistance within it, which is

proportional to the impedance of the component. The loss resistance can be

neglected when the mapping is rather big, but when the distance reaches

30cm, becomes almost equal to the loss resistance. If the loss resistance is

still neglected in the model, there will unavoidably be deviations. The ultimate

model with the loss resistance taken into consideration is shown as Fig 3.8.1.

can be derived using formula (25).

33

Fig 3.8.1The ultimate model for the system


can be derived using formula (27)

Thus the expression of is

The expression of the voltage across the load is shown as formula (32) and (33).

Whereas real refers to taking the real part of the variable.

34


Take , The variation in the voltage of the load is shown as Fig 3.8.2.

Though adjusting the impedance matching network, the maximum voltage

reached is about 0.6V, which is only a 5% deviation from the simulated . This

proves that the ultimate model coincides with the actual situation. According to the

ultimate model, the loss resistance , whereas Q is the quality factor. Suppose

the quality factor are the same for different kinds of color ring inductors, the only

way to lower the loss resistance is by decreasing the impedance. As mentioned

earlier in 3.6, and that the smallest available inductor is 1 , so the final

choice of is 1 .

35

Fig 3.8.2： The variation in the peak to peak voltage of the load versus the adjustment of the impedance matching network

Fig 3.9.2: Section view of Fig 3.9.1


3.9 Optimization of the Impedance Matching Network

The graph of the variation of the peak to peak voltage across the load versus the

variation of the capacitance is shown as Fig 3.9.1, based on simulations.

According to the section view (Fig 3.9.2), it can be inferred that C4 doesn’t have a

big influence on the voltage once it is bigger than 1000pF, while C3 has a greater

36

Fig 3.9.1: the variation of the peak to peak voltage across the load versus the variation of the capacitance (distance=30cm)


impact on the voltage.

If the conclusion can be extended to the full distance of 10cm-30cm, then C4 can

be fixed to a specific value, only remaining C3 adjustable. In this way, the matching

process can be greatly simplified without affecting the transmission efficiency. The

section view of the variation of the peak to peak voltage across the load versus the

variation of the capacitance (distance=10 to 25cm) are shown in Fig 3.9.3 to Fig 3.9.6.

37

Fig 3.9.3: The variation in the peak to peak voltage across the load versus the adjustment in C3 and C4 (Distance=25cm)



Through the above simulation, the former hypothesis can be proved, that in the range of 10cm ~ 30cm, the changes in C4 after it exceeds 1000pF has no obvious effect on the voltage. C3, meanwhile, has a greater impact on the voltage. Thus C4 can be changed into a fixed capacitor so as to simplify the adjustments. According to the simulation, C4 needs to be greater than 1200pF, so

38




in further experiments, the value of C4 is fixed at 1200pF. The voltage across the load with C4 adjustable and the voltage across the load with C4 fixed at 1200pF are compared using the simulation methods. The table is shown as follows.

Distance (cm)

The voltage

across the load

with C4

adjustable (V)

The voltage

across the load

with C4 fixed at

1200pF (V)

Difference Percentage

10 1.2662 1.2377 2.251%15 1.1976 1.1976 0.000%20 0.9431 0.9425 0.064%25 0.8529 0.8502 0.317%30 0.6554 0.647 1.282%

As can be seen from the table, the percentage difference of the fixed and

adjustable voltage is always below 2.5%, within the allowable range. This proves that

this impedance matching network construction method is theoretically feasible. At

this point, the variable in the system remains only the impedance matching capacitor

C3, which makes the whole system simple and efficient.

39

Part 4 Experimental Data Analysis and Summary


4.1 Analysis of Impedance Matching Test Results with the

Same Receiving Coil and Transmitting Coil

After setting up the test device according to the model diagram in 3.8, adjust

C3 to the maximum voltage on the load. Use the Tektronix TDS1012 oscilloscope

to read the voltage across the load. The data table is as follows.

The peak to peak voltage across the load in condition of no match and transmission

match under different distances

10cm 15cm 20cm 25cm 30cm

No match voltage (V) 1.02 1.28 0.95 0.6 0.43

Tx match voltage(V) 1.25 1.3 1.15 0.82 0.65

After getting the maximum voltage across the load, a vector network

analyzer replaces the signal generator in the circuit to measure the impedance of

the system. (Shown in Fig 4.1.1)

The impedance of the system at this point is 52.5-7.7j , which is only a 5%Ω

deviation from the target value 50 , falling inside the Ω 10% allowing range. The

result can be interpreted as the matching of the system is complete, which proves

the effectiveness of the impedance matching system.

40

Fig 4.1.1: Using the vector network analyzer to measure the impedance of the system(The left picture indicates how the circuit is connected, the right picture indicates the result)


Let the Q of the capacitance be , and the Q for the inductance be 33. The

value of the mutual inductance M is listed in 3.3. The simulation is done using the

same method as noted in 3.9. The graph of the transmission efficiency versus the

distance is shown as Fig 4.1.2. The definition of efficiency in this project is the

power on the load divided by the maximum output power of the power source. In

the figure, the experimental values are also shown in order to test the validity of

the model.

From the graph, it can be concluded that the simulated value are close to the

measured value, with the biggest difference being only 8%. This proves the

validity of the ultimate model.

41

Fig 4.1.2: The graph of transmission efficiency versus the distance


4.2 Analysis of Impedance Matching Test Results with

Different Receiving Coils and Transmitting Coils

According to the testing methods in 3.3, the parameters of coil 3 and coil 4

are measured using the vector network analyzer. For coil 3,

. For coil 4, .

Replace the receiving circuit with coil 3 and coil 4 respectively. Adjust C3 to

the maximum voltage on the load. Use the Tektronix TDS1012 oscilloscope to

read the voltage across the load. The data table is as follows.

Coil 3:

The peak to peak voltage across the load in condition of no match and transmission

match under different distances (Coil 1 Transmits, Coil 3 Receives)




The graph of transmission efficiency versus the distance is shown as Fig 4.2.1：

42Fig 4.2.1: The graph of transmission efficiency versus the distance


Coil 4：The peak to peak voltage across the load in condition of no match and transmission

match under different distances (Coil 1 Transmits, Coil 4 Receives)




The graph of transmission efficiency versus the distance is shown as Fig 4.2.2：

In the two sets of data, the biggest diversion happened at 15cm of coil 4, the

difference is 24.2%. Considering that 15cm is the peak of transmission efficiency,

it is possible for the simulation to divert from the actual situation. Meanwhile, the

simulation coincides with the actual situation in terms of the variation trend of

the transmission efficiency versus the distance, so such deviation noted above

can be considered normal.

It can be concluded from the three sets of data above, that the impedance

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Fig 4.2.2: The graph of transmission efficiency versus the distance


matching effect is not obvious at 10cm ~ 15cm. At a point, the voltage with

impedance matching can even be smaller than no matching. This is because of

the loss resistance inside capacitors and inductors. At 15~30cm, however, the

impedance matching network can effectively improve the load voltage. The effect

is most obvious at 30cm. The maximum voltage increase is about 66.7%, which is

a 178% increase in terms of transmission power.

The results of the experiment shows that this impedance matching network

can suit coils of different parameters.

44


4.3 Conclusion

(1) Joint tuning has a better effect than independent tuning. Once tuning is

done in the case of critical coupling or under coupling, the system will remain

resonant even if the distance between the transmitting and receiving coil is

changed.

(2) In the establishment of the simulation model, factors including mutual

inductance, distributed capacitance and loss resistance need to be taken into

consideration. The simulation results based on the ultimate model and the test

results of the maximum voltage obtained on the load with impedance matching is

basically the same, which proves the validity of the ultimate model.

(3) According to the ultimate model, an adjustable impedance matching

system is designed with only one variable. Within a certain distance, a relatively

high efficiency of wireless power transmission can be obtained using this simple

impedance matching network

(4) This impedance matching network can match the coils of different

parameters within a certain range.

45


4.4 Outlooks

There are still many shortcomings in the project. Afterwards, the project can

advance in these following aspects.

1. Choose the inductance and capacitance with high quality index (Q). The inductors

and capacitors chosen in this project have rather low quality indexes, thus the

loss resistance consumed much power and lowered the efficiency of wireless

power transmission. Changing the inductance and capacitance into those with

higher quality index can promoted the transmission efficiency effectively, and

therefore promote the effect of impedance matching.

2. Establish an automatic control circuit by adding a directional coupler, and a

logarithmic amplifier in the transmission circuit. The directional coupler can

couple a portion of the incident signal and reflected signal from the

transmitting loop. Input the two coupling signals to the logarithmic amplifier.

The output amplitude value voltage and the phase value voltage of the

logarithmic amplifier can reflect the matching situation on the transmitting

end..

46

Part 5 Reference

Part 5 Reference[1] Xiao B. Research on Transmission Efficiency of Wireless Transmission [D].

Chengdu, University of Electronic Science and Technology of China

[2] Imura Takehiro,Uchida Toshiyuki,Hori Yoichi.Experimental Analysis of High

Efficiecy Power Transfer Using Resonance of Magnetic Antennas for the Near Field-

Geometry and Fundamental Characteristics [C].IEEE Japan Industry Applications

Society Conference,2008:539-542.

[3] Beh TeckChuan,Kato Masaki,Imura Takehiro. Basic Study on Improving Efficiecy

of Wireless Power Transfer via Magnetic Resonance Coupling Based on Impedance

Matching[C]. IEEE International Symposium on Industrial Electronics 2010,

2010:2011-2016.

[4] Li Y. Research on High Power Resonance Wireless Power Transmission Method

and Experiment [D], Hebei Technology University, 2012

[5] Reinhold Ludwig, Pavel Bretchko. RF Circuit Design Theory and Applications[M],

Publishing House of Electronic Industry

[6] Калантаров,П.Л. ,Цейтлин,Л.А., Inductance calculation manual[M], China

Machine Press, 1992

[7] Zhang Xian,Yang Qingxin,Chen Haiyan. A Novel Two-port Network Model and

Experimental Validation for Resonantly Coulping Power Transmission System[C].

The Sixth International Conference on Electromagnetic Field Problems and

Applications,2012.

47

Part 5 Reference

[8] Chunbo Zhu, Kai Liu, Chunlai Yu, Rui Ma, Hexiao Chen, Simulation and

Experimental Analysis on Wireless Energy Transfer Based on Magnetic

Resonance[C].IEEE Vehicle Power and Propulsion Conference(VPPC), Sept.3-5, 2008.

48

Part 5 Reference

Part 6 AcknowledgementsThis project is completed under the guidance of my instructor. In the scheming

session of the project, he gave me many suggestion as to how the project should be

carried out. He is very concerned about our research process, and strive to create the

best conditions for us throughout the project. He also taught us the fundamentals of

circuit design so that we can carry on with the design of the impedance matching

system.

I would also like to thank the scholars involved in this paper, this paper cited the

works of several scholars, which have helped me a lot in my research process.

Without their hard work, it would be very difficult for me to complete this project.

Finally, I would like pay thanks to my family, it is for their endless love that I can

successfully complete the project.

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· web viewthe tuning circuit is shown as fig 3.4.1. according to the theory, if the circuit...

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