Transcript Design Review Second Semester

Wireless Power Transfer
Via Inductive Coupling
SENIOR DESIGN GROUP 1615
RYAN ANDREWS, MICHAEL DONOHUE, WEICHEN ZHANG
Objective
Statement:
Design, build and optimize a Wireless Power Transfer (WPT) system
utilizing inductive coupling. Explore power modulation to a receiver
under changing conditions such as distance and pitch that a free
floating sensor may experience. Model efficiency in air and water to
determine the validity of inductive coupling in water as a means of
power transfer.
Approach:

Use a closed loop microcontroller system.

Voltage across load will provide feedback to source.

Frequency and voltage will be modulated to alter Power.
Transmitter
Transmitter Overview:

Arduino Due generates high frequency sin wave using on board DAC from 0 to 3.3V.

High pass filter removes DC component, sin wave now +/- 1.65V.

Buffer amp isolates sub circuits.

Low pass filter removes high frequency components of signal.

Series LC circuit, containing primary coil transmits AC to receiver.

Frequency of signal from Arduino controllers power transmitted.
Arduino Due

32bit ARM core processor

CPU clock speed 84MHz

Runs at 3.3V

Sin Wave generation of up to 1MHz at
0 to 3.3V

Device to device communication via


SPI

I2C

UART
2 on board DAC

Write Rate of 1.74MHz
High Pass Filter

Filters out DC component
of sin wave.

Cutoff Frequency = 482Hz
Reconstruction \ Low Pass Filter

Converts impulse of DAC
into smooth wave form.

Cutoff Frequency = 200kHz
Graphs Courtesy of http://sim.okawa-denshi.jp/
OpAmp Buffer
Use:

Isolates high pass, low pass filter
and primary LC sub circuits.

Filter will not influence resonant
frequency of coil.

Protects microcontroller from high
current generated in primary LC.
Specifications:

LM7171

Low cost - $2.81

Attenuation at 220MHz allowing
high frequency signals to pass
without attenuation.
Primary Coil

Litz wire having 30 strands of 0.1 mm
diameter.

Shielding directs field lines outwards
and protects the rest of the electronics
from field.

Strong coupling up to 7cm.
D = Diameter of Larger Coil
Graph Courtesy of Wireless Power Consortium
Resonance and Voltage Magnification
1
𝐿𝐶
= 2𝜋𝑓 = 𝑟𝑒𝑠𝑜𝑛𝑎𝑛𝑡 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦

𝜔𝑜 =

Strong fields build up over multiple
oscillations.

More power in oscillating field then being fed
in by the supply.

At resonance in a series RLC the impedances
of the C and L components are at a
minimum.

As the frequency moves away from
resonance power stored in the field lowers.

The voltage across the L and C elements are
180 degrees out of phase. Are equal and
opposite.
Simulated Voltage Across Primary Coil vs
Frequency
Q Factor (Magnification Factor)

Measure of rate of loss in relation
to energy stored in the resonator.

Ratio of reactive voltage to
supply voltage

High Q is desired in all WPT systems.

A higher q factor is directly related
to a larger bandwidth at
resonance.

High Q > 100

𝑄=𝑅

Calculated Q = 188
1
𝐿
𝐶
=
𝜔𝑜 𝐿
𝑅
where R is series Resistance
Coupling Coefficient k
A measure of the percentage of  𝑘 =
power from the EM field of the
primary coil is induced in the
secondary.
 𝑀=
 Ranges from 0 to 1




K>.5 is considered strongly
coupled
Inversely proportional to distance.
Decreases when out of coils are
out of alignment.

𝐿=
𝜙
𝐼
𝑀
𝐿1 𝐿2

𝐿 = 𝑠𝑒𝑙𝑓 𝑖𝑛𝑑𝑢𝑐𝑡𝑎𝑛𝑐𝑒 (𝐻)

𝑀 = 𝑚𝑢𝑡𝑢𝑎𝑙 𝑖𝑛𝑑𝑢𝑐𝑡𝑎𝑛𝑐𝑒 (𝐻)
𝜇𝑜 𝜋𝑛1 𝑛2 𝑎2 𝑏2

𝑎+𝑏 2 +𝑑2 ( 𝑎−𝑏 2 +𝑑2 )
2
2
𝑎 = 𝑟𝑎𝑑𝑖𝑢𝑠 𝑜𝑓 𝑝𝑟𝑖𝑚𝑎𝑟𝑦 𝑐𝑜𝑖𝑙 (𝑚)

𝑏 = 𝑟𝑎𝑑𝑖𝑢𝑠 𝑜𝑓 𝑠𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑐𝑜𝑖𝑙 (𝑚)

𝑑 = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 (𝑚)

𝑙 = 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑠𝑜𝑙𝑖𝑛𝑜𝑖𝑑 (𝑚)
= 𝜇𝑜 𝑁 𝜋𝑟 l
Power at Resonance Series LC

𝑃 = 𝑉𝐼𝑐𝑜𝑠(Ө)

At resonance theta is zero
producing max real power at the
coil.

Calculated resonant freq 125kHz

Tested resonance ≈ 130kHz

Near resonance Voltage can
reach ≈ 40V across inductor.

Eq 2 = Max efficiency

Eq 3 = Voltage Gain between coils
Receiver
Receiver Overview:

Secondary LC with same topography as transmitter to resonate at same frequency.

Full bridge rectifier plus smoothing capacitor for AC to DC conversion.

IC Chip DF04M used for bridge rectifier.

10uF smoothing cap chosen experientially. No visible ripple present.

Voltage Divider to scale down from 40Vmax to 5Vmax for ADC conversion.

Buffer protects microcontroller from high input current.

Microcontroller takes ADC conversion.
Micro-controller Interaction:

Performs ADC conversion every 1ms.

Target Voltage Specified in Advance

Transmits voltage to Arduino Due via SPI
communication.

Arduino steps down frequency by 500Hz
if Voltage is too large.

Steps up voltage by 500Hz if too low.

Start frequency selected to be 10KHz off
resonance

Max frequency at resonance.

Voltage Delivered kept constant under
changing distance / pitch.
Underwater Application
In the water
In the air
Speed of Sound at T=20° C
1484 m/s
343 m/s
Relative Permittivity
80 F/m
1 F/m
Relative Permeability
1 H/m
1.257 x 10 ^- 6 H/m
Conductivity
10^-3 S/m
3x 10 ^-15 to 8x 10^ -15 S/m

Predicted efficiency in salt water is approximately 20% as efficient as air.

Based on independent research from the WiTricity Corporation.

A device could be charged with no external ports using an internal coil.
Allowing for underwater devices to be fully water proof.

Device would have to be kept within transmission distance to avoid large loss.

Best to mechanically hold the device stationary.
Alternate Design Considerations

Larger coil for larger transmission distance.

Gain applied at transmitter buffers for greater power delivery.

Stepping down DAC amplitude rather than frequency.

Another Atmega328p was considered as a signal generator until the
DAC sample rate was determined to be too slow to produce proper
frequencies.