1

FINAL YEAR PROJECT PROGRESS REPORT

BY RONAN DUNNE B.E. ELECTRONIC ENGINEERING

Electromagnetic shielding techniques for inductive powering applications

Supervisor: Dr. Maeve Duffy

12/01/09

2

Table of Contents

Table of Contents......................................................................................................ii

1. Introduction........................................................................................................ 1

1.1 - Background/ History.................................................................................. 1

1.2 - Inductive Power Transfer........................................................................... 1

1.3 - Transmitter and receiver coils.................................................................... 2

1.4 - Applications.............................................................................................. 3

1.4 - Electromagnetic Shielding......................................................................... 5

2. Proposals for tackling project.............................................................................. 7

2.1 - Software.................................................................................................... 7

3. Progress to date.................................................................................................. 8

3.1 - Research........................................................................................................ 8

3.2 - Programming and simulation......................................................................... 8

3.3 - Transmitter coil........................................................................................... 10

3.4 - Receiver coil................................................................................................ 10

4. Task list and project plan.................................................................................. 11

4.1 - January........................................................................................................ 11

4.2 - February......................................................................................................11

4.2 - March.......................................................................................................... 12

5. References ..............................................................12

3

1. Introduction:

1.1 Background/History

Inductive power transfer is an age old concept. It was originally taught up by a

physicist and engineer named Nikola Telsa. Telsa invented the Telsa coil in 1891. The

Telsa coil was developed to transmit electrical energy without wires. It is basically an

air cored resonant transformer. The Telsa coil along with many of Telsas other

ingenious inventions have paved the way for future technological advances in the area

of inductive power transfer.

1.2 Inductive Power Transfer

Inductive power transfer is the wireless transfer of electrical power from a source to

an object requiring power. It works by using inductive coupling. Inductive coupling

involves the use of magnetic fields to stimulate the movement of current through a

wire. A current could also be induced in the wire by placing a second coil of wire

within the magnetic field created by the first one. Thus, if the wire is formed into a

coil, the magnetic field it produces is amplified by several degrees, making the

magnetic field a lot bigger than if the electric wire was straight.

The advantages to using inductive power transfer is that you do not have the problem

of hazardous, inconvenient cables and wires.

Fig1.0 Hazardous/ incontinent cables

4

1.3 Transmitter and receiver coils

In order to transfer power wirelessly one must use transmitter and receiver

coils(primary and secondary coils). The transmitter coil is connected to a power

source, this will produce a magnetic field. The strength of this magnetic field depends

on the distance it is from the coil, the strength decrease the further away you are from

the coil. In order for a current to be induced a receiver coil must be added. This must

be placed inside the region containing the magnetic field created by the transmitter

coil. The transmitter and receiver coils are never connected. Mutual Inductance is a

process which allows the energy to be transferred using electromagnetic coupling. For

this process to occur the receiver coil must be relatively close to the transmitter coil.

Fig 1.1 Transmitter and receiver coils

5

1.4 Applications

There are many applications present today which use inductive power transfer

technology. One simple example is the electric toothbrush.

An electric toothbrush and its base contain two coils, a primary and a secondary. The

primary coil is located in the base. When the base is plugged in current is supplied to

this coil and it produces a magnetic field. The actual toothbrush itself contains the

secondary coil which is connected to the battery. When the toothbrush is attached to

its base, the magnetic field induces a current in the secondary coil which recharges the

battery.

Fig1.3 Electric toothbrush

6

Another application which uses inductive power transfer is a charging platform for

charging electronic devices. The charging platform works in the same way as the

electric toothbrush. The charging platform contains inbuilt primary coils which induce

a current in the secondary coils in the mobile devices when they are brought close to

the platform.

Fig 1.4 Charging platform

In these sort of applications the transmitter and receiver coils must be close together

as the magnetic fields they produce are relatively small. In order to for these

applications to work from a greater distance the magnetic field would have to be

much stronger and larger. However there is a problem with using a large magnetic

field, because magnetic fields spread in all directions, a large one would prove very

inefficient and would result in a waste of energy. LC Resonant circuit could be used

to help transmit energy from a greater distance. Resonance circuits respond

selectively to signals of a given frequency while discriminating against signals of

different frequencies. It consists of an inductor and a capacitor, when connected

together, an electric current can alternate between them at the circuits resonant

frequency.

7

Fig 1.4 LC resonant circuit

If the coils in an application are within a certain distance of each other (determined by

the magnetic field produced) and they both have the same resonant frequency then the

current can tunnel from the transmitter coil to the receiver coil. But if the coils go out

of range or if they have different resonant frequencies then they will not transfer

power.

Implanted biomedical devices using inductive power transfer are also being

developed. The transmitter and receiver coils in these devices are much further apart

resulting in low inductive coupling levels. One example of a biomedical system which

is being developed is a system which is designed to help stroke victims who suffer

from a walking disability known as foot drop. Foot drop means that the patient is

unable to lift the front part of the leg due to paralysis or weakness of the foot which

can cause the foot to be dragged forward on the ground. One answer to this problem is

to provide stimulation using electrodes to the damaged nerves using Functional

Electrical Stimulation (FES). The system being designed consists of an externally

worn transmitter, which is inductively coupled to an implanted receiver unit. It works

in a similar way to the applications mentioned previously. Inductive coupling is used

to transfer pulses from the transmitter to the implanted receiver circuit. These pulses

are applied to the relevant nerve endings via electrodes. In order to improve the

energy transfer process, resonant circuits are implemented in both the transmitter and

receiver circuits.

Fig 1.5 Transmitter circuit for foot drop application

8

1.5 Electromagnetic Shielding

Electromagnetic shielding is the process of limiting the penetration of electromagnetic

fields into a space, by blocking them with a barrier made of conductive material. In an

inductive charging platform it is required to supply electromagnetic shielding to the

bottom of the platform.

Fig 1.6 Charging platform

This is done to avoid any loss in electromagnetic flux which may escape through the

bottom. If the platform was not shielded and it was placed on a metallic desk it could

result in current being induced in the desk due to the flux which is created by the

charging platform. This could lead to undesirable energy transfer and heat effects in

the metallic desk. The electromagnetic shield used in the charging platform consists of

two layers. The first layer is a thin layer of soft magnetic material and the second

layer is a thin layer of conductive material. The soft magnetic material used is ferrite

4F1. The conductive material used is copper. Shielding effectiveness is defined as the

ratio between the field strength at a given distance from the source without the shield

introduced and the field strength with the field introduced.

In order to analyze the influence of the double-layer electromagnetic shield one must

determine the inductance and the impedance of the shielded planar spiral windings.

One must also calculate the thickness of the shielding materials. Simulation and

measurement must be done to determine the conductivity, permeability and thickness

of several different shielding plates. In order to investigate the different shielding

techniques one must carry out several case studies. These include testing the shielding

effectiveness when we use a dielectric material and a conductive material (copper) as

our double layer substrate and another study is of when we use a magnetic material

9

(ferrite) and copper sheet as the layers of our substrate.

Fig 1.6 Schematic view of the planar winding with shielding

Fig 1.7 Proposed cross-sectional structure of a PCB transformer shielded with ferrite plates

and copper sheets.

10

2. Proposals for tackling project

The objective of this project is to investigate different Electromagnetic shielding

techniques for inductive powering applications. This is done by applying modelling

techniques to compare the performance of different shielding layers for both

applications and to develop effective shielding solutions. Methods to be applied

include analytic and FEA modelling. The two software applications needed are

Matlab and Ansoft. I must also design a demonstrator inductive powering circuit

which will light an LED. This can be achieved by constructing and testing a circuit

containing transmitter and receiver coils. A resonant circuit can be added to the

circuits to improve the energy transfer.

I must also investigate the performance of different magnetic materials used in

shielding. This is done by defining the spiral coil and shield patterns for PCB

fabrication and by producing Gerber files for PCB manufacturing

2.1 Software

Mathlab:

Matlab is a high-level language and interactive environment that enables you to

perform computationally intensive tasks faster than with traditional programming

languages such as C, C++, and Fortran.

I need matlab to solve a complicated numerical integration formula. I used this

formula to calculate the Mutual inductance.

Ansoft, Maxwell SV: Maxwell 2D Student Version (SV), is used for analyzing electromagnetic fields in

cross-sections of structures. Maxwell SV uses finite element analysis (FEA) to solve

two-dimensional (2D) electromagnetic problems.

To analyze a problem, you need to specify the appropriate geometry, material

properties, and excitations for a device or system of devices. The Maxwell software

then does the following:

Automatically creates the required finite element mesh.

Calculates the desired electric or magnetic field solution and special quantities of

interest, such as force, torque, inductance, capacitance, or power loss. You can select

any of the following solution types: Electrostatic, Magnetostatic, Electrostatic, Eddy

11

Current, DC Conduction, AC Conduction, Eddy Axial.

Allows you to analyze, manipulate, and display field solutions.

3. Progress to Date

3.1 Research

Extensive research has been carried out on topic of inductive power transfer and its

applications.

The main areas covered include:

transmitter and receiver circuits.

Inductive Power transfer applications

Resonant circuits

Electromagnetic shielding and the different electromagnetic shielding techniques.

Software (Matlab, Ansoft)

All of which are mention earlier in detail.

3.2 Programming and Simulation Programmed analytic formulas for electromagnetic fields around planar windings in air

Matlab programming: I used Matlab to solve a complicated formula which was used to find the mutual inductance between two planar windings. In order to solve this formula I first had to do extensive research on how to program in Matlab. The main areas I needed to cover were the basics of matlab, while loops and numerical integration.

Ansoft, Maxwell SV: Maxwell was used to simulate the inductance. This was done by drawing a model of the planar winding using the same dimensions used in the matlab program. The main steps involved determining the inductance were i) drawing the model, ii)setting up the materials which were to be used iii) setup the boundaries iv) setup executive parameters, then by clicking the solve button you are provided with a inductance matrix. Once this was done I used the post processor to analyse the solution. The post processor allowed me to plot the lines of magnetic flux and create magnetic field plots which shows the magnetic field throughout the region.

12

Fig 1.8 Magnetic field around the windings

I got good agreement between the two software application for the value of mutual

inductance.

13

3.3 Transmitter coil

I built a wire-wound version of the transmitter coil investigated for the inductive

charging platform . The inductance and resistance was then measured.

Fig 1.9 Measured inductance and resistance

3.4 Receiver coil

I also built a receiver coil. This consists of a coil of very thin wire with 222 turns.

This involved wrapping the very thin wire carefully around piece of a filter of a pen.

This was a long process and required patience and concentration in order to make sure

the number of turns was counted correctly.

14

4. Task List and Project Plan

4.1 January

Design a demonstrator inductive powering circuit for an LED Test a receiver coil similar to that being investigated for biomedical

applications: determine the range of open-circuit voltage levels possible at different locations around the transmitter coil.

Investigate (by circuit analysis and testing) the level of improvement in open-circuit voltage provided by including resonant capacitors on (i) the receiver side and (ii) receiver and transmitter sides

Determine the conditions required in the system to provide sufficient current on the receiver side for lighting an LED; e.g. larger transmitter coil, higher frequency, more transmitter / receiver turns, need for magnetic core in receiver coil

4.2 February

Continue to investigate the effect of different shielding techniques using analytic and FEA models

Magnetic vs. conductive shield vs. combined magnetic and conductive shield.

Compare field levels at reference points around the device including shielded areas and receiver coil locations. Compare with the air-coil case.

Predict the effect of different shield solutions on power transfer between transmitter and receiver circuits

Investigate the performance of different magnetic materials in shielding Define spiral coil and shield patterns for PCB fabrication; allow for the

investigation of different copper thicknesses and shield patterns. Produce Gerber files for PCB manufacture

Perform preliminary tests on wire transmitter coil; use ferrite

substrates, copper plates, etc. for shielding. Test for the cases of unloaded and loaded receiver coils (LED demonstrator).

Investigate the effect of material parameters (permeability and conductivity) for a range of magnetic alloys in analytic & FEA models (data will be provided by supervisor)

Investigate the effect of magnetic / conductor layer thickness for a given operating frequency

Compare the time taken for analytic and FEA models to provide solutions

4.3 March

15

Develop analytic models for predicting magnetic field levels and transmitter coil inductance for different shield structures

Account for multi-layer transmitter coils Account for patterned conductor and magnetic layers Investigate the effect of current flowing in a receiver circuit (spiral coil

located close to the transmitter) Verify models with tests performed on different PCB structures

Health and safety issues.

Power levels will be very low during testing; therefore there should not be any cause for risk to the student.

5. References

Websites

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/serres.html

http://en.wikipedia.org/wiki/RF_shielding

http://www.mathworks.com/products/matlab/

http://www.ansoft.com/products/em/maxwell/

http://www.tech-faq.com/wireless-power-induction.shtml

Papers:

Evaluation of the Shielding Effects on Printed-Circuit-Board Transformers Using Ferrite Plates and Copper Sheets by S. C. Tang, Member, IEEE, S. Y. (Ron) Hui, Senior Member, IEEE, and Henry Shu-hung Chung, Member, IEEE, November 2002 Extended Theory on the Inductance Calculation of Planar Spiral Windings Including the Effect of Double-Layer Electromagnetic Shield by Y. P. Su, Student Member, IEEE, Xun Liu, Member, IEEE, and S. Y. (Ron) Hui, Fellow, IEEE, July 2008 Inductive Powering for Biomedical Applications, Chevalerias O, OReilly S, Alderman J, September 2004