WIRELESS POWER TRANSFER

By

Shu-Hui Cheng

David Chavez

ECE 445, SENIOR DESIGN PROJECT

FALL 2011

TA: Ryan May

7 December 2011

Project #16

i

ABSTRACT

This project describes an implementation of a wireless charger for USB consumer devices. The

smart charger is able to automatically sense the presence of a nearby electronic device and detect

its internal battery level. When the battery level of a USB device in proximity is dropped below a

certain preset threshold, the smart charger will be initiated through an ultrasound communication

system and start to charge the device. The charging process will be stopped automatically once

the battery is fully charged. The automated charging capability avoids excessive charging power

and makes the proposed charger eco-friendly. It is also a worry-free charge since users do not

have to plug in a charger.

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TABLE OF CONTENTS

WIRELESS POWER TRANSFER .............................................................................................. i

CHAPTER 1: INTRODUCTION ................................................................................................ 1

1.1 Purpose......................................................................................................................................... 1

1.2 Specifications ........................................................................................................................... 1

1.3 Subprojects .................................................................................................................................. 2

1.3.1 Battery Detection Unit ........................................................................................................ 2

1.3.2 Communication Unit........................................................................................................... 2

1.3.3 Antenna Coil Unit ............................................................................................................... 2

1.4 Background on Wireless Power Transfer Theory ................................................................... 3

1.5 Review on Previous Designs ....................................................................................................... 4

CHAPTER 2: DESIGN DESCRIPTIONS ................................................................................. 4

2.1 System Electrical Specifications ................................................................................................ 4

2.2 Initial Design Blocks ................................................................................................................... 5

2.3 Design Procedure ........................................................................................................................ 6

2.3.1 USB Battery Detection ........................................................................................................ 6

2.3.2 Communication ................................................................................................................... 8

2.4 Design Details .............................................................................................................................. 9

2.5 Design Calculations ................................................................................................................... 12

2.5.1 Antenna Coil Design ......................................................................................................... 12

2.5.2 Power Transmission Efficiency ........................................................................................ 14

2.5.3 System Efficiency .............................................................................................................. 14

CHAPTER 3: EXPERIMENTAL RESULTS .......................................................................... 15

3.1 Design Verification .................................................................................................................... 15

3.1.1 USB Battery Detection ...................................................................................................... 15

3.1.2 Communication Testing ................................................................................................... 16

3.1.3 Antenna Coil Testing Results ........................................................................................... 18

3.1.3.1 Waveform Measurements ............................................................................................. 20

3.1.3.2 Range Measurement ...................................................................................................... 22

3.1.3.3 Capacitance and Inductance ......................................................................................... 22

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CHAPTER 4: COST ANALYSIS ............................................................................................. 22

4.1 Cost Analysis ............................................................................................................................. 22

4.2 Part List ..................................................................................................................................... 23

CHAPTER 5: CONCLUSIONS ................................................................................................ 23

5.1 Accomplishments ...................................................................................................................... 23

5.2 Ethical Considerations .............................................................................................................. 24

5.3 Conclusions ................................................................................................................................ 24

5.4 Future work / Alternatives ....................................................................................................... 25

REFERENCE………………………………………………………………………………………………………………………………………26

APPENDIX A Block Diagrams

APPENDIX B Schematics

APPENDIX C Hall Effect Sensor

APPENDIX D Picture

1

CHAPTER 1: INTRODUCTION

The idea of wireless power transfer originated from the inconvenience of having too many wires

sharing a limited amount of power sockets. We believe that many people have the same

experience of lacking enough sockets for their electronic devices. Thus by creating a wireless

power transfer system, it would help clean up the clutter of wires around power sockets making

the space more tidy and organized.

1.1 Purpose

The purpose of this project is to produce a platform which can detect the battery level of an

electronic device, such as a cell phone, then be able to automatically charge the device when the

battery level of the device drops below a certain threshold. Our project will use resonant

induction charging which can charge multiple devices at the same time as long as they have the

same resonant frequency.

Benefits include:

Safe wireless power transfer

Compatibility with USB devices

Eliminates power outlet clutter

Intuitively self-charging

Features include:

Ability to charge multiple devices

Automatic detection of battery life and the need for charging

1.2 Specifications

The project was split up into battery detection, communication, and wireless power transfer. For

each indivudal module specification, refer to section 1.3.

2

The Hall Effect sensor detects the battery level through current in the USB and when the current

is above a certain point, meaing low battery level, the sensor sends a signal to trigger the

microcontroller.The microcontroller then sends a signal to the transmitting ultrasound to start the

communication process. This is the battery detection part. The receiving ultrasound is fed with

40 KHz sine wave and the wave is rectified through a bridge rectifier and stablized through a

capacitor. This is the communication part. The stablized DC signal turns on the switch to start

the charging process. After the switch is on, a DC signal is fed into a voltage divider from an AC

to DC power supply to power the osicllator. The oscialltor outputs a 13.56MHz sinusodial wave

to the amplifier for greater power. When the ampfilied power is fed to the transmitting antenna

coil, the transmitting antenna coil induces magnetic field to the the receving antenna coil and the

power is received by the receiving antenna coil. This AC power is then rectified into DC voltage

and stablized through a voltage regulator and then charge the device through USB.

1.3 Subprojects

1.3.1 Battery Detection Unit

The Hall Effect sensor and the PIC microcontroller are charged through four 3V button cell

batteries and lowered through a 5V Voltage regulator. The clock oscillator is connected to the

PIC for sufficient clock pulse and is also powered by button battery.

1.3.2 Communication Unit

The transmitter is an ultrasonic piezoelectric transducer with a signal amplifier. When the battery

level is low, the microcontroller turns on the piezoelectric transducer to transmit an ultrasound

signal to turn on the charging dock. The ultrasound receiver is a piezoelectric microphone with a

signal amplifier which can convert an ultrasound signal into electrical signal and then amplifies

the signal.

1.3.3 Antenna Coil Unit

The loop antenna converts an electrical current into an electromagnetic field. In the near field

region, the magnetic field dominates and therefore electrical energy is transmitted wirelessly

through magnetic field coupling. The loop antenna is built with copper wire gauge 12 at

3

resonance frequency 13.56MHz. An electrical small loop is desired for our project so that it can

be fit into a cellphone form factor. A balun is connected to the loop antenna because the output

of the power amplifier is an unbalanced signal.

The coil/loop antenna is made of an inductor and a capacitor, therefore using the equation below

to calculate its resonance.

√

1.4 Background on Wireless Power Transfer Theory

The concept of wireless power transfer can be traced back to 1820 when Andre-Marie Ampere

developed his law which states that an electric current produces a magnetic field. Following the

work by Michael Faraday (1830), James C. Maxwell (1864) and Heinrich R. Hertz (1888),

Nikola Tesla experimentally demonstrated wireless power transfer in 1891 [1]. In Tesla’s

experiment, he designed a resonant circuit that is able to couple a high frequency current into

another resonant circuit of a similar structure. With his circuit, he was able to power wirelessly

(without any physical interconnecting conductor) a light bulb.

The theory behind wireless power transfer is already detailed in the Maxwell’s equations,

The last two curl equations state that a time-varying magnetic flux generates an electric field, and

a time-varying electric flux generates a magnetic field. Therefore, if a time-varying electric

current can be generates, the time-varying current will induces a time-varying magnetic field.

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This time-changing magnetic field can “somehow” be picked up and induce a time-varying

electric field, or an AC voltage across a receiving load. Tesla’s contribution lies on the design of

a circuit than can generate/receive a time-varying magnetic field in free-space. It shall be

emphasized that Tesla’s method is not based on the direct transfer of energy through the use of

propagating electromagnetic wave. Tesla’s method is actually a near-field method, whereas the

use of propagating electromagnetic wave (like transmission of microwave power through an

antenna) is a far-field method. The two methods differ by the transmission range as well as the

angular coverage of the system. Near-field method, though has a shorter range, the energy is

more confined than far-field method.

1.5 Review on Previous Designs

Although Tesla demonstrated wireless power transfer over a century ago, the subject was not

well investigated until researchers at Los-Alamos National Laboratory developed the first

passive radio-identification (RFID) tag [2]. The RFID tag is passive because the chip inside it is

powered by the signal that incidents on it. Later in 2007, as the research group at MIT

demonstrated the wireless powering of a light bulb, the research effort into wireless power

transfer got further boosted [3].

Indeed, in 2006, a senior design project group at Illinois has completed a project entitled

“Wireless Power Adapter for Rechargeable Devices” [4], which was almost a year ahead of the

MIT group. In the project, the group successfully demonstrated that a cell phone can be

wirelessly charged.

CHAPTER 2: DESIGN DESCRIPTIONS

2.1 System Electrical Specifications

Input voltage AC 120 V @ 60 Hz

Output voltage DC 4.2 V

Output current 25 mA

Wireless transfer frequency 13.56 MHz

5

2.2 Initial Design Blocks

The system consists of three major components: battery indicator, transducer/receiver unit,

wireless power transfer unit. The battery indicator outputs a signal when the battery level of the

portable device to be charged reaches a certain threshold value. When the battery of the portable

device is below a certain threshold, the first LED of battery indicator will turn off and then the

edge detector will detect a falling edge. The SR latch holds the signal from the edge detector and

then turns on the switch. The signal from the switch is fed to a transducer that links to a receiver

in the charging dock. The transducer is an ultrasound transducer that emits an ultrasound signal.

After the receiver in the charging dock detects a signal, the signal is fed into a rectifier to convert

it from sine wave to a unipolar signal. The unipolar signal will feed into low pass filter to convert

it into DC voltage. To reduce ripples of the DC voltage, voltage regular is applied, results a flat

DC voltage which can turn on the switch. The switch turns on the power supply unit in the dock

and power is drawn from the AC wall outlet. The 60 Hz AC current is then converted to a higher

frequency that is suitable for wireless power transfer (for example, 13.56 MHz in the ISM band).

The up-converted AC current is then fed to the wireless power transfer unit. The wireless power

transfer unit is implemented by a pair of resonant loop antenna/coil, voltage divider, oscillator

and power amplifier. After the AC-to-DC converter transfers the AC power supply to DC

voltage, the 5 volts will then feed into voltage divider so that its output can reach to the operating

frequency of the oscillator. The following power amplifier will amplifies the output of the

oscillator. The signal is fed to the coil antenna through the balun. The loop antenna/coil is

brought to resonance by capacitive loading and using multiple windings. The received antenna

will pick up the magnetic field and transfer it to electrical signal and then the signal is rectified

into unipolar signal by bridge rectifier. The unipolar signal then is fed into voltage regular to

convert to DC voltage. The output of the voltage regulator is connected to the USB charging port

of the portable device to be charged. When the battery is fully charged, it will light up the last

LED in the battery indicator. This causes a rising edge and is then detected by edge detector.

This will release the latch to turn off the switch and then turn off the Ultrasonic transmitter

which will stop charging the platform. The initial and final block diagrams are displayed in

Appendix A.

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2.3 Design Procedure

2.3.1 USB Battery Detection

At the beginning, we designed a battery indicator for battery detection. However the

disadvantage part of this is that a wire needs to connect to the battery to detect the voltage drops.

Since the detection on USB port is always 5V, the only choice we have is to connect a wire to

the battery in the cell phone. After testing the current changse in different battery level, we

decided to use PIC micorcontroller with Hall Effect sensor to detect the current since the current

change is inversly proportional to the battery level.

Using a multimeter from the lab we calculated the current as the phone charged. The results are

shown below.

Table 1. Current, voltage, and power at different battery levels while charging

Battery Life (%) Current (A) Voltage (V) Power (W)

> 95 ≈0.05 5.1 ≈0.255

95 ≈0.3 5.1 ≈1.53

90 ≈0.35 5.1 ≈1.785

75 ≈0.65 5.05 ≈3.2825

70 ≈0.7 5.05 ≈3.535

0 ≈0.85 4.7 ≈3.995

The above values for current in Table 2.2.1 varied as it charged, thus only approximations. Also,

the battery life was determined through a phone application. Something that was noted during

these tests was that this battery was charging at about 5V, while our voltage regulator was set at

4.2V. This does not appear to be a problem. In this case the phone would charge at a slightly

lower rate or the current would increase through the USB cord.

Using a Hall Effect sensor to detect the magnetic field created by the current, we would be able

to detect the current of the USB line. The Hall Effect Sensor we chose was the ACS712 [5]

which operates between -5A and +5A outputting a voltage value between 1.5V to 3.5V. The

current and voltage was linearly related with 2.5V corresponding to 0A. However, sensing if the

battery needed charging through the current created another challenge. This challenge was that

the battery sensing would only be possible when the device is charging, thus when the device

7

finally stops charging, there would be no way of sensing the battery and sending out a signal that

it needs to be charged.

The solution to this was to implement a microcontroller that could pulse a 40kHz ultrasound

signal when the device needed to be charged and be able to stop pulsing when the device no

longer needed charging. When the microcontroller detected the device no longer needed

charging it would stop pulsing the 40kHz ultrasound for one minute, then begin pulsing the

signal again to continue charging. The microcontroller would then detect if the battery needed to

continue being charged or still fully charged and act accordingly as stated above. Due to time

constraints, we chose to implement the PIC16F887 [6] which are offered in the lab. Using the

flow chart from Figure 1, we programmed the microcontroller.

Figure 1. MCU Flow Chart

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2.3.2 Communication

Another modification with respect to the old design is that we eliminated low pass filter and

voltage regulator in the charging platform. After implementing the circuit, we noticed that the

signal after the amplifier is stable enough to turn on the switch by adding a capacitor to reduce

the ripples. The ultrasonic signal was AC coupled, amplified, fully rectified, put through a low

pass filter, and finally a voltage regulator.

Due to the microcontroller being only able to output a 5V peak to peak square wave through its

PWM function, an AC coupling capacitor of 100uF was added in series to eliminate the 2.5VDC

offset at the receiving ultrasonic sensor. The ultrasonic sensors were then tested and found to

have more than a 16 inch reach at 5V peak to peak. Also, as the transmitter frequency varied

away from 40kHz the receivers ability to pick up the signal decreased dramatically. Through

these observations, it can be said that the ultrasonic transmitter/receiver was acting as a bandpass

filter centered around 40kHz. At the same time, the BS270 MOSFET [7] that was going to be

implemented as the switch for the charging dock to transmit power or not was found to need a

gate voltage of 7V or more to fully allow the charging dock to transmit power. These findings

resulted in the elimination of the low pass filter and the voltage regulator, for they were no

longer needed in the design.

Knowing that some noise would still be flowing into the ultrasonic sensor we decided to filter

out the low voltage signals by using a diode bridge to rectify the 40kHz ultrasound signal from

the ultrasound receiver. In this case we decided to go with the 1N5817 Schottky diode because of

its low 0.45V forward voltage compared to the typical 0.7V. Although diodes do exist that have

a lower voltage drop, the 0.45V voltage drop was not high enough to affect the range of the

ultrasonic sensors.

Now that the 40kHz signal was able to be read and properly filter out the low voltage noise from

the ultrasound receiver, the signal had to be amplified above 7V to be able to turn on the

MOSFET. To do this we chose to use the LMC 6482 [8] operational amplifier to boost the

signal. By placing an amplifier, it would amplify not only the signal we needed, but random

noise from the rectifier. By taking this into account, we chose to adjust the resistor values to a

point where the gain would be enough to drive the on signal to rail voltage, but not high enough

9

to let random noise power on the MOSFET to a relevant value. By amplifying the on signal to

rail voltage, we were no longer able to use the 5VDC from the wall converter to power on the op

amp. This resulted in having to power the op amp with three CR1616 [9] in series, which are 3V

button cell batteries.

2.4 Design Details

2.6.1 Battery Detection

The Battery Detection Module was designed so that we could know whether the battery needed

to be charged or not. This module uses an ACS712 Hall Effect sensor and a PIC16F887 as its

primary components. The Hall Effect current sensor had a linear relationship with the voltage

and therefore only needed to be powered and connected to the USB through pin 1 [14]. As for

the PIC16F887, it needed to be programmed following the flow chart seen in Figure 1.

By using the Hall Effect current sensor, we associated each current with a certain voltage value

which then needed to be read by the PIC. From the datasheet and the Hall Effect voltage input to

the PIC, the Analog to Digital Converter (ADC) needed to be set up. Since the Hall Effect sensor

was designed to operate between -5A to 5A the voltage output between the 0A to 1A range only

varied from 2.5V to about 2.7V. For this reason, the resolution of the ADC should not be large

and thus calculated with the following equation.

(2.6.1)

With the PIC needing 5V to power on, the reference voltage was set to that same value.

Although up to 16 bits of resolution can be used, we felt that the 10bits was sufficient for

operation at 4.8828mV/bit as calculated in equation (2.6.1). Using the resolution per bit, the

digital assigned value for each voltage could be determined with equation (2.6.2). Referring back

to Table 1 we approximated a turn on/off charging current to be around .5A which corresponded

to a voltage output of about 2.6V.

(2.6.2)

10

Reading the datasheet of the PIC16F887 closely, we decided to use a 20MHz clock oscillator

[15] to make the PIC more stable and accurate. This frequency was then set up to be divided by

32 so as to maintain the speed of the PIC within readable range.

Finally, the PWM signal needed to be set up to drive the 40kHz 5Vpk-pk ultrasound to the

ultrasonic transmitter. To do this it was necessary to set the Timer2 register value, which the

datasheet provided an equation. Plugging in the already known values in equation (2.6.3) the

equation was reduced to equation (2.6.4). Although using a prescalar of 4 or 16 would have also

given a relatively accurate 40kHz signal, prescalar 1 was more exact and therefore was used in

the PIC code. This resulted in a Timer2 value of 124 and a duty cycle of 124/2 = 62.

[ ]

(2.6.3)

[ ] (2.6.4)

The final step in operating the battery detection involved powering the PIC and Hall Effect

sensor. In this case, 4 3V coin cell batteries were connected in series and fed through a KA7805

linear voltage regulator with a 0.33µF capacitor to ground at the input and a 0.1 µF capacitor to

ground at the output. This output was a steady 5V which was necessary to power on the PIC and

Hall Effect sensor.

2.6.2 Communication

The Communication Module was designed to turn the MOSFET on when a 40kHz ultrasound is

detected and off when the 40kHz signal is not transmitting and noise is present. Seeing how the

microcontroller pulsed a 5Vpk-pk 40kHz wave, a 100µF AC coupling capacitor was connected

in series in order to eliminate the 2.5Vdc offset. Due to the .9V forward voltage drop of the diode

bridge rectifier, the receiving ultrasound signal needed to be greater than .9Vpk-pk in order for a

signal to be read on the output of the bridge rectifier.

If the ultrasonic receiver read a signal that was larger than .9V it would then be amplified using

the LMC 6482 as a non-inverting signal amplifier with the following equation.

(

) (

) (

) (2.6.2.1)

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By setting R2 to 10kΩ and R1 to 51.1Ω, we were able to obtain such a high gain. By using this

gain, we were able to increase the range of the ultrasonic sensor. This high gain is able to detect

and amplify the signal to the necessary rail voltage of about 8V from the coin cell batteries as

soon as the voltage surpasses .93559V. In the video posted on the web page the range can be

seen to extend about a foot and a half, which far exceeds the power transmission range. When

the ultrasound transmitter was off, the noise that was seen at the amplifier edge appeared to be

about .1V, which according to equation (2.6.2.1) resulted in about 0.5mV noise signal.

3.6.3 Wireless transmission

After the receiver in the charging dock detects a signal, the signal is fed into a rectifier to convert

it from sine wave to a unipolar signal. The unipolar signal is then fed into an amplifier and a

capacitor which reduces ripples of the DC voltage. The DC voltage turn on the switch and the

switch turns on the power supply unit in the dock and power is drawn from the AC wall outlet.

The 60 Hz AC current is then converted to a higher frequency that is suitable for wireless power

transfer (for example, 13.56 MHz in the ISM band). The up-converted AC current is then fed to

the wireless power transfer unit.

Figure 2. Overview of the wireless power transfer system

As shown in Figure 2, the wireless power transfer unit consists of a pair of resonant loop

antenna, voltage divider, oscillator and power amplifier. After the AC-to-DC converter transfers

the AC power supply to DC voltage, the 5 volts will then feed into voltage divider so that its

Parallel LC resonant

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output 2.5V for the operating frequency of the oscillator.This oscillator converts the input signal

into a sine wave with desired frequency at 13.56MHz. The following power amplifier amplifies

the output of the oscillator and then feed to the coil antenna through the balun to balance the

signal. So that the signal can be transferred into the loop antenna is brought to resonance by

capacitive loading and using multiple windings. The loop antenna can convert an electrical

current into an electromagnetic field. In the near field region, the magnetic field dominates and

therefore electrical energy is transmitted wirelessly through magnetic field coupling. The loop

antenna is built with copper wire gauge 12. The received antenna will then pick up the magnetic

field and transfer it to electrical signal and then rectify into unipolar signal by bridge rectifier.

The unipolar signal then is fed into voltage regular to convert it into DC voltage. The output of

the voltage regulator will be around 5V to charge the battery. The diode there is to avoid current

going from battery to the wireless power transfer unit.

When the battery is fully charged, the Hall Effect sensor detects a small current. This causes a

trigger from the sensor to the PIC microcontroller. The PIC microcontroller then turns off the

Ultrasonic transmitter which will stop charging the platform.

2.5 Design Calculations

2.5.1 Antenna Coil Design

( ) ( )

L = inductance (µH)

r = mean radius of coil (cm)

N = number of turns

l = length in cm

N = 6 turns

r = 3.5cm

l = 4cm

L 2.42μH

13

C = capacitance

√

C 56.9pF

(a)

(b)

Figure 3. Fabricated antenna coil

Antenna coil

pair in the

system

14

The fabricated antenna coil is shown in Figure 3. Both the transmitting antenna and receiving

antenna are structured in the same way. Both antennas have the same number of turns.

2.5.2 Power Transmission Efficiency

Efficiency =Power at the receive loop antenna

Power at the transmit loop antenna

= 6.8 %

2.5.3 System Efficiency

ystem efficiency Power delivered to battery

Power drawn from wall AC supply

= 6.6 %

2.6 Schematics

Wireless power transfer unit:

Wireless power receive unit:

15

CHAPTER 3: EXPERIMENTAL RESULTS

3.1 Design Verification

3.1.1 USB Battery Detection

Since we were unable to draw enough current in the USB for the Hall Effect Sensor to be

tested, using dc power supply current was varied from 0A to 1A while testing the voltage.

This graph is shown in Appendix C.1 and is similar to what is seen in the datasheet [1].

Since the microcontroller could not be fed a voltage from the Hall Effect sensor, we

simulated a high current signal (ie., above 2.6V) and checked the output of pin 17 to verify

the 40kHz 5Vpk-pk signal as shown in Figure 3.1.1.1. The voltage was then decreased and

increased to simulate a low current, full battery, followed by a need for charge. The PIC then

took about one minute before pulsing the 40kHz signal again. This test along with a range

test for the ultrasound can be seen on the course website that simulates the current dropping

after being high for a while, then waiting one minute before checking if the signal needs

charging again.

When initially designed, four 3V coin cell batteries were used to power the PIC. Looking at

the datasheet of these CR1616 batteries, the capacity for these batteries is only 55mAh. By

leaving them plugged in and operating for too long they had accidentally run low and

deemed the communication part of the demo inoperable.

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Figure 3.1.1.1 PWM signal from PIC16F887

3.1.2 Communication Testing

This part of the project was easy to test by breaking it up into a series of checkpoints and

verifying the correct signal. To start, we set up the ultrasonic sensors approximately 4 inches

apart and powered on the PIC to output the 40kHz wave. In Figure 3.1.2.1 the PWM signal

from the PIC is shown with the output of the ultrasonic receiver. While the 40kHz signal was

still on the output after the bridge rectifier showed that the frequency had doubled to about

79.7kHz and the voltage amplitude of the signal had dropped about .9V to about .547V as

shown in Figure 3.1.2.2. With this received signal from the bridge rectifier it was then

amplified to the positive rail voltage of the batteries powering the op amp which is around

7V and shown in A.5. When the signal was cut off and no signal was being read, then the

output voltage was insignificant as shown in Figure 3.1.2.3.

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Figure 3.1.2.1 40kHz square wave from PWM with 40kHz output from the Ultrasonic Receiver

Figure 3.1.2.2 Bridge Rectifier Signal from Ultrasonic Receiver

The output of the rectifier after the signal fed from Ultrasound. The two ultrasound sends 40kHz

for communication and the maximum range for two ultrasound to communicate successfully is

about 30cm.

18

Figure 3.1.2.3 Amplified 40kHz signal when in range

The output of amplifier after the signal fed from the rectifier. From this graph , the ripples can be

reduced by adding a capacitor and then is able to turn on the switch.

3.1.3 Antenna Coil Testing Results

3.1.3.1 Testing for Two Coil Antenna

0

5

10

15

20

25

30

35

40

0 2 4 6 8

Distance(cm)

Vo

ltag

e(V

)

19

The graph on the left shows its correspondence to the theoretical graph since voltage has a linear

relationship with magnetic field.

3.1.3.2 Resonance Frequency Testing

This graph is a measurement of peak-to-peak voltage at receive coil as the frequency changes

and it confirms the coil resonating at 13.5 MHz.

0

50

100

150

200

250

300

350

12.5 13 13.5 14 14.5 15 15.5

Frequency(MHz)

Vo

ltag

e (m

V)

20

3.1.4 Waveform Measurements

Results and Graphs

Oscillator output: 1.77 Vpp @13.48 MHz

1st

amplifier output: 4.44 Vpp

2nd

amplifier output: 12.74 Vpp

The gain from the 1st

amplifier output to the

2nd

amplifier output is

9.2 dB which is slighltly

off form the typical gian

of this amplifier 13dB.

This is due to the

impedance mismatching.

This is measured by

using oscillascope

connecting to the output

o f the 2nd

amplifier and

the ground of the circuit.

The output of the

Oscillator is not a

perfect sine wave since

we need to add a filter

to filter the noise out.

This is tested by using

oscillascope connecting

to the out put of the

oscillator and ground of

the circuit. The gain

from the oscillator to

the 1st amplifier is 8db

which is slightly off

from the typical gain

13dB of this amplifier

since the impedance

matching is saller than

the required impedance

50 ohm

21

Balun output (fed to transmit coil): 33.4 Vpp

Receive coil output: 7.94 Vpp @13.48 MHz

Bridge rectifier output:

The balun converts the

unbalance signal to a

balance and differential

signal. Therefore

theoutput of the balun

seems t ampliy the input

signal from the 2nd

amplifier. Due to the

small resistance causing

the impedance

mismatching, the power

decreases significantly

as the distance increase

and the grea loss of

power makes the receive

coil receiving around

8Vpp.

The bridge rectifier

rectifies the AC

signal from the

antenna coil to a DC

signal and there is a

loss during this

rectifing process.

Aprroximately each

diode in recitfier

absorbs 1V from the

input signal.

22

3.1.5 Range Measurement

Distance between Two Coil (cm) DC Voltage to the USB (V)

Closest ~0.1 4.17

1 2.88

2 1.26

3.1.6 Capacitance and Inductance

The measurement of the inductance of the antenna coil is around 0.8uH.

The measurement of the capacitance of the antenna coil is

CHAPTER 4: COST ANALYSIS 4.1 Cost Analysis

Labor:

people

Working Hours

23

4.2 Part List

Description Part Price ($) Quantity Total ($)

Charging ANNEX

Battery Detection

Ultrasonic Transmitter 400ST160 7.4 1 7.4

PIC PIC16F887 2.8 1 2.8

5V regulator KA7805 0.65 1 0.65

20MHz oscillator FOX1100 1.1 1 1.1

Coin Cell Batteries FOX1100 0.78 4 3.12

USB Charger

Copper Wire 1ft. 4 1 4

Surface mount RF Schottky Diodes HSMS 2828 1.55 1 1.55

Voltage Regulator MIC5209-4.2YS 2.41 1 2.41

Charging DOCK

0

Communication System

0

Ultrasonic Receiver 400SR160 7.4 1 7.4

Low Pass Filter and Full wave Rectifier LMC6482 2.21 1 2.21

Schottky Diodes 1N5817 0.074 4 0.296

MOSFET BS270 0.0566 1 0.0566

Charging System 0

AC-to-DC Power Supply VOF-15-5 17.46 1 17.46

Voltage Divider LM2681 1.02 1 1.02

Oscillator ASE2-13.500HZ-ET 2.4 1 2.4

Power Amplifier BBA-322-A 17.09 1 17.09

Balun XFA-0201-1WH 2.2 1 2.2

Copper Wire 1ft. 4 1 4

Power Amplifier

568-6212-1-N

599-1026-1-N 24.81 1 24.81

Total

101.9726

Grand total: $25101.9726

CHAPTER 5: CONCLUSIONS

5.1 Accomplishments

The communication unit with ultrasound works in 30cm apart and the battery detection through

current can be adjusted for needed current in the electronic devices. The Wireless transmission

unit can transfer power from annex to platform and light up an LED.

24

5.2 Ethical Considerations

• With this responsibility, not only will we reassure that the final product will meet its

expectations, we will assure it is safe for use and put warning labels on items deemed unsafe

if tampered with

• Through thorough testing of each component, we will guarantee that our performance claims

are accurate

• Ideally this project will further open the door to exploration in wireless power transfer.

5.3 Conclusions

Through the testing of the USB ports, we were able to fix the battery detection problem in the

beginning. We decided to implement the Hall Effect sensor and the PIC microcontroller for

detection so that the circuit is able to detect the battery level through USB port which is a more

convenient way to do it. The battery detection module and the communication unit are able to

work as we designed.

The wireless transmission unit is not able to function as our design that we expect to see the unit

can charge a battery. However, the tranmission does tranfer power the the charging annex that an

LED can be lighted up. The low power transfer efficiency is due to the low current ouput at the

voltage regulator. A good way to increase the current is to add a current amplifier after the

voltage regulator so that the power is sufficient to charge the deive since the output voltage is

great enough in this case (around 4.1V). In order to charge more different devices, a good

impedance matching is recommended. We can construct an impedance matching circuit by

inserting discrete L and C elements between the balun and the output of the final stage amplifier

to achieve 50 ohms impedance.

Overall, the circuit is able to light up an LED and automatical battery detection. We could have

completed the project as we expected if our shipping of the components did not take 3 weeks to

receive all of them.

25

5.4 Future work / Alternatives

For the future work, there are many ways to improve the wireless power transmission. To reduce

the size of the coil, we can make a multilayer coil which can be made planar for easy integration

with device platform. We can also load the antenna coil with ferrite to concentrate the magnetic

field so that the transmission range can be increased. For maximum power tranfer, a 50

impedance matching is needed. We can construct an impedance matching circuit by inserting

discrete L and C elements between the balun and the output of the final stage amplifier. To

improve the transmission range, we can insert more RF power amplifiers and current amplifier

after voltage regulator. For battery detection improvement, we can try to detect from the output

battery level of cell phone’s operating system.

26

References

[1]. Nikola Tesla, US patent No. 454,622, “System of Electric Lighting.”, 1891.

[2]. http://www.transcore.com/pdf/AIM%20shrouds_of_time.pdf

[3]. http://web.mit.edu/newsoffice/2007/techtalk51-30.pdf

[4]. J. ukkar and P. H. Hirschboeck, “Wireless Power Adapter for Rechargeable Devices”,

Senior Design Project Report, 2006.

[5]. Allegro Micro ystems, Inc., “Fully Integrated, Hall Effect-Based Linear Current Sensor IC

with 2.1 kVRMS Isolation and a Low-Resistance Current Conductor”, [Online Document],

October 2011 [cited 5 December 2011], Available HTTP:

http://www.allegromicro.com/Products/Current-Sensor-ICs/Zero-To-Fifty-Amp-Integrated-

Conductor-Sensor-ICs/~/media/Files/Datasheets/ACS712-Datasheet.ashx

[6]. Microchip, “PIC 16F887”, [Online Document], October 2011 [cited 5 December 2011],

Available HTTP:

[7]. Agilent Technologies, “H M -282x urface Mount RF chottky Barrier Diodes”, [Online

Document], May 2009 [cited 26 October 2011], Available HTTP:

http://www.avagotech.com/docs/AV02-1320EN

[8]. MICREL, “MIC5209 500mA Low-Noise LDO Regulator”, [Online Document], August

2000 [cited 26 October 2011], Available HTTP:

http://www.datasheetcatalog.org/datasheet/Micrel/mXsvxvq.pdf

[9]. CUI INC, “ eries: VOF-15 Description: AC-DC Power upply”, [Online Document],

September 2011 [cited 26 October 2011], Available HTTP: http://products.cui.com/CUI_VOF-

15-5_Datasheet.pdf?fileID=5125

[10]. National emiconductor, “LM2681 witched Capacitor Voltage Converter”, [Online

Document], January 2003 [cited 26 October 2011], Available HTTP:

http://www.national.com/ds/LM/LM2681.pdf

[11]. ABRACON CORPORATION, “2.5Vdc CMOS Compatible SMD Crystal Clock

Oscillator”, [Online Document], June 2011 [cited 26 October 2011], Available HTTP:

http://www.abracon.com/Oscillators/ASE2series.pdf

[12]. LINX TECHNOLOGIE , “BBA eries RF Amplifier Data Guide”, [Online Document],

January 2003 [cited 26 October 2011], Available HTTP:

27

http://www.linxtechnologies.com/resources/data-guides/bba-xxx-a.pdf

[13]. RFMD, “XFA-0201-1WH 1:1 MT Transformer”, [Online Document], [cited 26 October

2011], Available HTTP: http://www.rfmd.com/CS/Documents/XFA-0201-1WHDS.pdf

[14]. Access Communications PTY LTD, “U B Reference”, [Online Document], July 2007

[cited 5 December 2011], Available HTTP: http://www.accesscomms.com.au/reference/usb.htm

[15]. FOX Electronics, “TTL Clock Oscillator F1100E”, [Online Document], 1998 [cited 5

December 2011], Available HTTP: http://www.brookdale.com/Fox/f1100e.pdf

1

APPENDIX A: BLOCK DIAGRAMS

Figures A.1 and A.2 show the transition between our initial design and our final design.

Figure A.1 Initial block diagram from Design Review

2

Figure A.2 Final block diagram

3

APPENDIX B: SCHEMATICS

B.1 Charging Platform Final Schematic

4

B.2 Charging Annex Final Schematic

5

APPENDIX C Battery Testing and Communication plots and pictures

Figure C.1 Hall Effect Sensor Output Voltage vs. Input Current

2.45

2.5

2.55

2.6

2.65

2.7

2.75

0 0.2 0.4 0.6 0.8 1 1.2

Vo

ltag

e (

V)

Current (A)

6

APPENDIX D Pictures

Picture D.1 The entire circuit with 3 major parts: battery detection unit, communication unit and

wireless transfer unit