miniproject_wireless power transmission

1. INTRODUCTION

We wish to present a small scale demonstration of wireless power transfer between two

coupled series LC tuned circuits, each consisting of a copper conductor loop acting as an

inductor and a capacitor. Both LC circuits are tuned to equal individual resonant

frequencies. One of them is a part of a 12 kHz frequency RC Phase shift oscillator

powered by 15 volts DC, while another is loaded. Brought in proximity, copper loops

share a small mutual inductance, essentially forming a transformer. In order to transmit

significant amount of power through this transformer, a very large amount of reactive

power needs to circulate in its primary. Receiver coil's leakage inductance is in turn

canceled out by another capacitor, allowing for the maximum power transfer to the load.

Experimenting with copper loop orientations, one can find positions of the receiver close

to transmitter where no power is received, as total magnetic flux crossing through the

receiver loop is zero. Hence this is a directional method of power transmission.

Figure.1 WPT model

Due to small size of the apparatus, very little power is actually radiated in far field, with

losses being mainly ohmic heating. Hence this method is also sometimes known as non-

radiative or near-field power transmission. This technology would be helpful for

recharging batteries of pacemakers. Further power cords can be avoided.

Department of EEE 1 GEC Thrissur

2. HISTORY

Nicola Tesla proposed theories of wireless power transmission in the late 1800s and early

1900s. One of his more spectacular displays involved remotely powering lights in the

ground at his Colorado Springs experiment station.

Figure.2 Nicola Tesla’s inductive coupling experiment

The idea behind the project was to create a small tabletop demonstrator of magnetically

coupled wireless power transfer, resembling a miniature version of the MIT witricity

device. The goal was to keep the circuit simple with easily obtainable parts, and to keep

voltage and power levels low so the device is safe for handling and doesn't require special

methods of cooling.

In the early 2000s, Professor Marin Soljacic and a team of physicists from the

Massachusetts Institute of Technology (MIT) used magnetic resonance coupling to enable

energy transfer over midrange distances. They transmitted power over a two-meter

distance, from the coil on the left to the coil on the right, where it powers a 60W light

bulb. They wrote a paper in 2006 and an article for the Journal of Science in 2007

describing their research. A lot of interest was created, and in October, 2007, they formed

the WiTricity Corporation to further develop the technology and commercialize it.


3. PRINCIPLE

R

V

C

0

L

Figure.3 RLC series circuit

Resonance is identified with engineering situations which involve energy storing

elements subjected to a forcing function of varying frequency. Specifically resonance

is the term used to describe the steady state operation of the circuit or system at that

frequency for which the resultant response is in time phase with the source function

despite of presence of energy storing elements.

In series resonance, there is a series arrangement of L and C along with resistance R.

The effective current flow caused by sinusoidal function is given by

I= VR+ j(ωL−1/ωC )

=VZ

A change in frequency means a change in the magnitude and phase angle of the

complex impedance. As ω increases, the reactance part of Z decreases, thus causing an

increase in current. As ω continues to increase, a point is reached when reactance is

zero. At this ω0,

ω0 L− 1ω0C

=0

ω02= 1

LC


ω0=1

√LC

The frequency ω0 is called resonant frequency of the circuit. At resonance the

impedance of the circuit is minimum and specifically it is equal to R. consequently,

when a series RLC circuit is at resonance, the current is maximum and is also in time

phase with the voltage.

Two circuits give two resonant frequencies whose separation depends on the value of

the mutual inductance M (the ratio of the voltage in the secondary to the rate of change

of primary current with time, and the unit is the henry). This has a reactance at the

operating frequency Xm = ωM). The mutual inductance coupling between primary and

secondary can be related to their self-inductance by means of the coupling constant k.

k= M

√L1 L2

Since k is defining the relationship between magnetic flux linkages in the circuit, it can

never be greater than 1. A value of k=1 means that all the flux produced by the primary

is linked with the secondary and vice versa. A value of k greater than 1 would mean

that more than all of the flux produced by the primary is linked with the secondary.

The coupling constant is independent of the number of turns in a coil. The number of

turns in a coil determines the magnetic field, which will be produced for a given

current. The coupling constant is concerned with how the lines of magnetic force

produced by one coil interact with another coil, and hence the coupling constant

between two air spaced coils depends only on their physical size and disposition in

space. Hence to obtain the best coupling between primary and secondary in an air-

cored transformer we can only change the size and spatial relationships of the coils.

Household devices produce relatively small magnetic fields. For this reason, chargers

hold devices at the distance necessary to induce a current, which can only happen if the


coils are close together. A larger, stronger field could induce current from farther

away, but the process would be extremely inefficient. Since a magnetic field spreads in

all directions, making a larger one would waste a lot of energy.

Induction can take place a little differently if the electromagnetic fields around the

coils resonate at the same frequency. The theory uses a curved coil of wire as an

inductor. A capacitance plate, which can hold a charge, attaches to each end of the coil.

As electricity travels through this coil, the coil begins to resonate. Its resonant

frequency is a product of the inductance of the coil and the capacitance of the plates.

As long as both coils are out of range of one another, nothing will happen, since the

fields around the coils aren't strong enough to affect much around them. Similarly, if

the two coils resonate at different frequencies, nothing will happen. But if two

resonating coils with the same frequency get within a few meters of each other,

streams of energy move from the transmitting coil to the receiving coil. According to

the theory, one coil can even send electricity to several receiving coils, as long as they

all resonate at the same frequency. The researchers have named this non-radiative

energy transfer since it involves stationary fields around the coils rather than fields that

spread in all directions.

3.1 RESONANT INDUCTIVE COUPLING

Resonant inductive coupling or electro-dynamic induction is the near field wireless

transmission of electrical energy between two coils that are tuned to resonate at the

same frequency. The equipment is sometimes called resonant or resonance

transformer. While many transformers employ resonance, this type has a high Q and is

often air cored to avoid 'iron' losses. The two coils may exist as a single piece of

equipment or comprise two separate pieces of equipment.

Resonant transfer works by making a coil ring with an oscillating current. This

generates an oscillating magnetic field. Because the coil is highly resonant any energy

placed in the coil dies away relatively slowly over many cycles; but if a second coil is

brought near it, the coil can pick up most of the energy before it is lost, even if it is

some distance away. The fields used are predominately non-radiative, near field


(sometimes called evanescent waves), as all hardware is kept well within the 1/4

wavelength distance they radiate little energy from the transmitter to infinity.

One of the applications of the resonant transformer is for the CCFL inverter. Another

application of the resonant transformer is to couple between stages of a super

heterodyne receiver, where the selectivity of the receiver is provided by tuned

transformers in the intermediate-frequency amplifiers. Resonant transformers such as

the Tesla coil can generate very high voltages with or without arcing, and are able to

provide much higher current than electrostatic high-voltage generation machines such

as the Van de Graff generator. Resonant energy transfer is the operating principle

behind proposed short range wireless electricity systems such as Witricity and systems

that have already been deployed, such as passive RFID tags and contactless smart

cards.


4. TWO LEVEL WIRELESS POWER TRANSMISSION

Two level wireless power transmissions incorporate ideas of witricity and smart home. At

first level, power was transmitted wirelessly from source point to destination point. At

second level, appliance was powered by the output and the control and selection of

appliance was performed by RF transmitter-receiver circuitry.

Figure.4 Block diagram of transmission side

At the first level, AC input was given to a step down transformer of suitable rating. The

voltage stepped down to a suitable value, and was converted to DC via suitable ADC

converters like bridge rectifiers. The DC thus obtained was used to power the oscillator

circuit, which generates an alternating waveform of desired higher frequency. It was

connected to the transmitter coil which is coupled inductively with the receiver coil.

Figure.5 Block diagram of receiver side

The RF module (Tx/Rx) was used for acting as a wireless remote, which could be used to

drive an output from a distant place. RF module, as the name suggests, uses radio

frequency to send signals. These signals are transmitted at a particular frequency and a


baud rate. A receiver can receive these signals only if it is configured for that frequency. A

four channel encoder/decoder pair has also been used in this system. The input signals, at

the transmitter side, are taken through four switches while the outputs are monitored on a

set of four LEDs corresponding to each input switch. The circuit can be used for designing

Remote Appliance Control system. The outputs from the receiver can drive

corresponding relays connected to any household appliance. From the receiving coil,

appliance was connected to decoder via relay. The decoder was responsible to select the

appliance according to the signal from the switch.

Figure.6 Block diagram of RF Transmitter side

The switch board has several switches corresponding to each appliance. Pressing of a

switch selects a distinct pin of the encoder. This encoder gave respective output to the RF

transmitter. It was coupled with an RF receiver which selected respective decoder line,

turning the respective relay ON. A particular appliance was enabled and it was supported

by the wirelessly transmitted power.

The principle of witricity along with inductive coupling was responsible for power

transmission in the first phase. In the second phase, RF receiver-transmitter circuitry

performed the task of controlling and selection of devices. Thus, there was a two level

wireless control of a device.


5. CONSTRUCTION

5.1 TRANSMITTER SIDE

Single phase AC input-230V/50Hz- was sent to a 230V/15V step down transformer, from

where it was sent to a bridge rectifier with filter. The DC thus obtained was used to power

an oscillator. The output was given to the transmitter coil, which is coupled with the

receiving coil.

Figure.7 Elaborate block diagram of transmitter side

In our prototype, we have used an RC phase shift oscillator designed to operate at desired

frequency of 12 kHz. Further, we have used an inverting amplifier operating at gain of 4.5.

The output of the inverting amplifier is fed to the transmitter coil.

5.2 RECEIVER SIDE

The inductively transferred power was then connected to four appliances. Here 4 LEDs are

connected to denote 4 appliances. They are connected to an HT12D decoder via relay. The

output of the decoder is fed to an RF receiver. The RF module, as the name suggests,

operates at Radio Frequency. The corresponding frequency range varies between 30 kHz

& 300 GHz. In this RF system, the digital data is represented as variations in the amplitude

of carrier wave. This kind of modulation is known as Amplitude Shift Keying (ASK).


Figure.8 Elaborate block diagram of receiver side

The RF transmitter was connected to switches via HT12E encoder. When one of the

switches were ON, the respective line in the HT12E encoder is activated, which is passed

to the RF transmitter.

Figure.9 Elaborate block diagram of RF transmitter side

The 434 MHz RF transmitter sent a signal to the RF receiver, from where it went to the

HT12D decoder. The decoder selected the appropriate relay and activates that line. The

respective appliance was activated and it was supported by the wirelessly transmitted

power.


6. CIRCUIT DIAGRAM OF WIRELESS POWER TRANSMISSION


7. WORKING

7.1 TRANSMITTER SIDE

7.1.1 RC Phase Shift Oscillator

The RC phase shift oscillator consists of an opamp as the amplifying stage with three RC

cascading networks as the feedback network. The feedback network provides a fraction of

the output voltage back to the input of the amplifier. The opamp is in the inverting mode.

Therefore, any signal which appears in the inverting terminal is shifted 180o at the output.

An additional 180o required for the oscillations as per the Barkhausen criterion is provided

by the cascaded RC network. Thus the total phase around loop becomes 360o.

7.1.2 Design:

Frequency of the oscillation,

f 0=1

2 π √6 RC

Let C= 0.01µF; R = 510Ω;

fo ≈ 12 kHz

Gain,

A=R f

R1

=29

Let R1 = 1.2kΩ;

Rf ≈ 33 kΩ


Figure.10 Circuit diagram of transmitter side

7.1.3 Inverting Amplifier

This is one of the most popular opamp circuits. The polarity of the input voltage gets

inverted at the output. If a sine wave is fed to the input of this amplifier, the output will be

an amplified sine wave with 180o phase shift. Rf is the feedback resistance and R1 is the

input resistance. Inverting amplifier can be used as a scale changer because by varying

either Rf or R1, the amplitude of the output can be varied.

7.1.4 Design:

Gain,

A=−R f

R1

R1 = 2.2 kΩ; Rf = 10 kΩ;

A= 4.54


7.2 RECEIVER SIDE

7.2.1 RF Receiver-Transmitter

Transmission through RF is better than IR (infrared) because of many reasons. Firstly,

signals through RF can travel through larger distances making it suitable for long range

applications. Also, while IR mostly operates in line-of-sight mode, RF signals can travel

even when there is an obstruction between transmitter & receiver. Next, RF transmission is

more strong and reliable than IR transmission.

Figure.11 Circuit diagram of receiver side depicting RF Tx/Rx

RF communication uses a specific frequency unlike IR signals which are affected by other

IR emitting sources. This RF module comprises of an RF Transmitter and an RF Receiver.

The transmitter/receiver (Tx/Rx) pair operates at a frequency of 434 MHz. An RF

transmitter receives serial data and transmits it wirelessly through RF through its antenna

connected at pin4. The transmission occurs at the rate of 1Kbps - 10Kbps.The transmitted

data is received by an RF receiver operating at the same frequency as that of the

transmitter. The RF module is often used along with a pair of encoder/decoder. The


encoder is used for encoding parallel data for transmission feed while reception is decoded

by a decoder.

7.2.2 HT12D Decoder

HT12D is a decoder integrated circuit that belongs to 212 series of decoders. This series of

decoders are mainly used for remote control system applications, like burglar alarm, car

door controller, security system etc. It is mainly provided to interface RF and infrared

circuits. They are paired with 212series of encoders. The chosen pair of encoder/decoder

should have same number of addresses and data format. In simple terms, HT12D converts

the serial input into parallel outputs. It decodes the serial addresses and data received by,

say, an RF receiver, into parallel data and sends them to output data pins. The serial input

data is compared with the local addresses three times continuously. The input data code is

decoded when no error or unmatched codes are found. A valid transmission in indicated by

a high signal at VT pin. HT12D is capable of decoding 12 bits, of which 8 are address bits

and 4 are data bits. The data on 4 bit latch type output pins remain unchanged until new is

received.

Figure.12 HT12D decoder


7.2.3 HT12E Encoder

HT12E is an encoder integrated circuit of 212 series of encoders. They are paired with

212 series of decoders for use in remote control system applications. It is mainly used in

interfacing RF and infrared circuits. The chosen pair of encoder/decoder should have same

number of addresses and data format. Simply put, HT12E converts the parallel inputs into

serial output. It encodes the 12 bit parallel data into serial for transmission through an RF

transmitter. These 12 bits are divided into 8 address bits and 4 data bits.

Figure.13 HT12E Encoder

HT12E has a transmission enable pin which is active low. When a trigger signal is

received on TE pin, the programmed addresses/data are transmitted together with the

header bits via an RF or an infrared transmission medium. HT12E begins a 4-word

transmission cycle upon receipt of a transmission enable. This cycle is repeated as long as

TE is kept low. As soon as TE returns to high, the encoder output completes its final cycle

and then stops.


7.2.4 Microswitch

Figure.14 Micro switch

Micro switch is used for getting 5 volt (VDD) while in normal condition and ground

condition while in pressed position. Resistors are connected for limiting current.

7.2.5 RF module (433 MHz)

This radio frequency (RF) transmission system employs Amplitude Shift Keying (ASK)

with transmitter/receiver (Tx/Rx) pair operating at 434 MHz. The transmitter module takes

serial input and transmits these signals through RF. The transmitted signals are received by

the receiver module placed away from the source of transmission.

The system allows one way communication between two nodes, namely, transmission and

reception. The RF module has been used in conjunction with a set of four channel

encoder/decoder ICs. Here HT12E & HT12D have been used as encoder and decoder

respectively. The encoder converts the parallel inputs (from the remote switches) into

serial set of signals. These signals are serially transferred through RF to the reception

point. The decoder is used after the RF receiver to decode the serial format and retrieve the

original signals as outputs. These outputs can be observed on corresponding LEDs.

Encoder IC (HT12E) receives parallel data in the form of address bits and control bits. The

control signals from remote switches along with 8 address bits constitute a set of 12

parallel signals. The encoder HT12E encodes these parallel signals into serial bits.

Transmission is enabled by providing ground to pin14 which is active low. The control

signals are given at pins 10-13 of HT12E. The serial data is fed to the RF transmitter

through pin17 of HT12E.


Figure.15 RF Tx/Rx

Transmitter, upon receiving serial data from encoder IC (HT12E), transmits it wirelessly

to the RF receiver. The receiver, upon receiving these signals, sends them to the decoder

IC (HT12D) through pin2. The serial data is received at the data pin (DIN, pin14) of

HT12D. The decoder then retrieves the original parallel format from the received serial

data.

When no signal is received at data pin of HT12D, it remains in standby mode and

consumes very less current (less than 1µA) for a voltage of 5V. When signal is received by

receiver, it is given to DIN pin (pin14) of HT12D. On reception of signal, oscillator of

HT12D gets activated. IC HT12D then decodes the serial data and checks the address bits

three times. If these bits match with the local address pins (pins 1-8) of HT12D, then it

puts the data bits on its data pins (pins 10-13) and makes the VT pin high. An LED is

connected to VT pin (pin17) of the decoder. This LED works as an indicator to indicate a

valid transmission. The corresponding output is thus generated at the data pins of decoder

IC.

A signal is sent by lowering any or all the pins 10-13 of HT12E and corresponding signal

is received at receiver’s end (at HT12D). Address bits are configured by using the by using

the first 8 pins of both encoder and decoder ICs. To send a particular signal, address bits

must be same at encoder and decoder ICs. By configuring the address bits properly, a

single RF transmitter can also be used to control different RF receivers of same frequency.


To summarize, on each transmission, 12 bits of data is transmitted consisting of 8 address

bits and 4 data bits. The signal is received at receiver’s end which is then fed into decoder

IC. If address bits get matched, decoder converts it into parallel data and the corresponding

data bits get lowered which could be then used to drive the LEDs. The outputs from this

system can either be used in negative logic or NOT gates can be incorporated at data pins.


8. CHALLENGES

Some of the biggest challenges for WiTricity in developing the technology were the

miniaturizing of the components, which would increase their manufacturability and

make them small enough to integrate into electric devices, and having enough

communications and control so that the system could operate in the real world.

The original experiments that showed that 60 watts could be created over a two meter

distance featured coils that were two feet in diameter, and a lot of signal generators and

amplifiers. There was an obvious need to miniaturize the components. Moreover,

electric devices have metal content and other electrical components.

The witricity devices produced so far could only operate at short distances. They do

not give enough energy to power large vehicles and equipment. Also the cost of

supplying it to the public is very high. The concept is still under production and

improvement.


9. ADVANTAGES

Power cords can be avoided. This enables a shift from traditional wiring systems to

modern wireless systems.

Fine tuning is possible using ferrite rods by impedance matching.

Even when non-metallic objects come in between, no distortion is caused.

The device can easily be designed to operate at any frequency from few Hertz to

MHz range. Scalable Design Enables Solutions from mill watts to Kilowatts.

Transmission through RF is better than IR. Signals through RF can travel through

larger distances making it suitable for long range applications. IR mostly operates

in line-of-sight mode; RF signals can travel even when there is an obstruction

between transmitter & receiver. RF transmission is more strong and reliable than

IR transmission.

It is portable. It can be set up at any location at suitable clearances.

The circuit is simple with easily obtainable parts.

Voltage and power levels are low. The device is safe for handling. It is safe for

people and animals.

It does not require special methods of cooling.

In some applications, they can even replace batteries.


10.APPLICATIONS

Industrial, military, household robots

Portable personal electronics

Electric vehicles

Less dependence on batteries

Implanted medical devices- pacemakers

Power supply for MEMS or nano robots

Sensors with difficult access, as in golf courts

Electrically heated clothes

Room warming


11. FUTURE SCOPE

Modern science has now made it possible to use electricity without having to plug

in any wires. This concept is called witricity which seems to have a bright future in

providing wireless electricity.

In future, with witricity, where there will be no need of power cables and batteries.

The city just has to be covered with witricity hot spots wherein you can use your

electric gadget battery and wire free making it more convenient to carry around and

much lighter. With witricity, there will be no need of charging batteries, or buying

new batteries for your electrical gadgets.

Just as beneficial witricity may be, there are some contraindications to the concept,

with debates if it is risky living next to power lines and having a low power

witricity network running in the home.

However despite these contraindications, witricity has a bright future with the

many advantages it provides in terms of weight, convenience and portability of

electrical appliances.


12. CONCLUSION

As wireless power transmission is in the developmental stage, lots of work is still to be

done in improving it as the device used for their research disclosed that witricity power

applications operate at only 40% efficiency. The potential applications of witricity are

expected to materialize in the new future, of say a few years’ time, after the necessary

modifications are made to them.

The future witricity power applications permit us to use wireless energy, without having to

replace or recharge batteries. There will be no need of getting rid of these batteries either

or of remembering to recharge batteries periodically. In addition to this, with witricity,

there is no need of plugging in any wires and plugs and thus face a mess of wires.


13. INFERENCE

At the end of this mini project, we came to know about many applications and limitations

of several electronic equipments namely opamps, relays, RF module etc.

We realized that amplification at high frequencies were less efficient unless for special

purpose opamps, as their gain-bandwidth product is restricted to around 4MHz in ordinary

opamps. We familiarized with RF module (Tx/Rx at 434 MHz) and preferred it to IR,

keeping in mind, the wide clearance possible in case of radio frequency operation.

We tried several coils-solenoidal (air and iron core), circular and cylindrical- and chose a

convenient model for the sake of convenience of accurate experimental result, as our

operating voltage was low. We found that maximum power transfer takes place at resonant

frequency of the RLC series circuit. Therefore we designed our oscillator in such a way

that the frequency of oscillation matched with the resonant frequency.

Several experiments were conducted with laboratory equipments for accurate results and

measurement.


REFERENCES

[1] ‘Electrical Engineering Fundamentals’, Vincent Del Toro

[2] ‘RF based wireless remote’, STrobotix, Chandigarh labs & Chawla Radios.

[3] ‘Understanding Relays’, Kevin R. Sullivan

[4] Datasheets, Fairchild semiconductors; www.fairchildsemi.com

[5] www.mit.edu/~soljacic

[6] www.engineersgarage.com

[7] www.witricity.com

[8] www.kidela.com

[9] www.witricitypower.com
