Research Project Report

30.580 Research Projects – Project Report Pocket Power

Rajasekaran Ganeshkumar 1

Project Title: Pocket Power

(A low cost, light-weight, tiny and robust ‘last call’ charging system)

Name of the student: Rajasekaran Ganeshkumar

(1000799)

Project Supervisor: Dr. Erik Wilhelm

Objective:

New products drive business. To remain competitive, there is need for continuous search for new

methods to evolve products. In this project, our objective is to design and develop a low cost pocket

power that can charge your mobile phone or handheld devices. Smartphones are, for the most part

really nice. However the battery is a big issue. It’s inevitable, at some point during a busy day travel,

your phone is running out of battery particularly when you need it. This project aims to solve that

problem by coming up with an energy-harvesting device that can charge your phone for one last call

on the go. To address this problem, we use reverse engineering and redesign methodology. We start

by formulating the customer needs, followed by reverse engineering, creating a functional model to

meet this goal. The concepts formulated are used to ideate new design and develop a tiny, lighter in

weight, ergonomic product that meets the design specification intact.

Reverse Engineering:

Reverse engineering is the process of discovering the technological principles of a device, object or

system through analysis of its structure, function and operation. It often involves disassembling

something (a mechanical device, electronic component) and analyzing its components and workings

in detail to support creation of a new device that does the same thing without using the original or

simply duplicating. We follow a systematic methodology proposed by Kris L Wood et.al [1] that

helps us to understand the product evolution in better way and how to execute the planned redesign.

Several tasks are done to execute an effective redesign of a commercial product and we follow them

for the Pocket power as described in steps below.

JITENDRA KUMAR BEHERA



Step 1: Investigation and Hypothesis

Investigation is process of exploring the working principle of the device. Helps us understand the

overall product function though we have very little knowledge of the internal components of the

product. A black box model is created after knowing the inputs and outputs flow of the pocket

power, helps us to have an unbiased perception of the possible product evolution.

Fig 1. Black Box Model

Step 2: Customer feedback and need analysis

Based on the black box model, we have managed to look for already available solution that uses

hand crank to charge your mobile phone for last call/sms in an emergency. Customer needs are

gathered for the product, which are used to describe the product design specification in a complete

way. For the purpose of our methodology, the task for getting feedback from the users is done by

circulating questionnaires over the web for an appropriate sample space of twenty. With the feedback

forms information is interpreted as below

Fig 2. Customer feedback analysis

!!!!!!!!!!!!!!!!! !0" 5" 10" 15" 20"

Yes"

No"

If"there"is"a"handy"self6powered"keychain"based"mobile"charger"at"a"price"around"

10$,"will"you"give"a"try?"

Keychain)based&mini&Energy'Harvester

!to#charge#hand%held%

DC#Power#output 5V#/#1#A

USB$Output LED$Indications

No#of#rotations

Mechanical"energy"(linear"or"rotational"

action)

Total&time&of&input



!

Step 3: Functional Prediction

We were able to analyze the data from the customer feedback and begin the development of the

product by predicting the functions or tasks required. It includes forming a structure or flow diagram

explaining the mechanism of the product. A flow is a physical phenomenon intrinsic to a product

operation or sub-function. For example in our product is converting mechanical energy to electrical

energy.

Fig 3. Predicted functional flow of the product

Step 4: Teardown and Experimentation

We managed to find few already available solutions to the above addressed issue and analyze its

operation by disassembling them. It is studies to know whey these products are unsuccessful.

a) Hand-crank generator

The mechanics of hand-powered generators are not too complicated. They work like other generators

except your arm rather than other sources provides the initial energy.



Figure 4. Hand crank generator / Dissassembly

b) Battery backup

Figure 5. Battery Backup plugged into Smartphone

They are hand carry external batteries that can charge your smartphone by just plugging them.

Solutions Advantages Disadvantages

Hand Crank Low cost ($10-$20)

No batteries, available in small form factor and weighs less

Unreliable power output Not ergonomic

Battery backup Reliable power

Sturdy and ergonomic designs

Heavy (as if carrying one more smartphone)

Expensive (>$40) Energy and Power limited!

Table 1. Comparison between the solutions available



Step 5: Forming Engineering Specifications

The last phase of the reverse engineering entails the forming of specifications, benchmarking and

choosing the product systems that is to be evolved. This helps us in redesign process. Good energy

system requires a better understanding of where and how the energy is used. Review on the power

consumption of the smartphone entities and overall system is studied using energy profiler.

Table 1. Power required by different entities in a mobile phone [2]

Technology/Entity Action Power [mW] Energy [J]

CPU Usage Boot (50%) 400

Display 30% intensity 65

Voice 2G call 700

Total 1165 360

To know particularly the power and energy consumed by a smartphone to charge its battery from

zero percent to full charge is measured using Energy consumption meters [3]. As we know there are

many players in the smartphone market, we consider two namely iPhone 5 and Galaxy S3 as they are

told to be best ones in the segment. The table below shows how much energy does these

smartphones and time taken to charge the battery to full.

Table 3. Energy consumed by a Smartphone [3]

Smartphone Measured Energy Consumed

0% to 100%

Time taken Calculated energy consumption

0% to 3% Samsung S3 12.2 Watt-hours 2 hrs 15 min (6W) 0.40 Watt-hours

iPhone 5 9.5 Watt-hours 1 hrs 50 min (5.5W) 0.34 Watt-hours

Assuming the usage time around 5 min, total energy required by the smartphone for one last call in

emergency is around 0.1 Watt-hours (360 J).

Design Specification:

From the reverse engineering methodology and the steps followed, we derive the final design

specifications. Weight of the product is considered most important parameter to be taken care as the

customer feedback and need analysis implies the factor that device weight and cost influence them to

buy the charger. From the above engineering specifications, we come to know the average power

needed to charge the smartphone is 5W. So our final design specifications are tabled below and using

which we ideate designs.



Table 4. Design Specifications

Parameters Specifications

Size 3-4 cm

Weight 25 g

Output Voltage 5 V

Output current 1 A

Power 5 W

Total cost <10$

Re-design:

The underlying principal of electrical power generation defines the efficiency of generation and type

of required human action. Designing power-harvesting devices therefore has to be considered from

the user interaction perspective because different interaction activities require quite difference power

generators. The following designs are some solutions as we consider human power generation as a

user interface design problem.

a) Design 1

The design was inspired from widely available hand crank flashlights. That uses the to and fro

motion of the lever converted to rotational motion using the gears that in-turn converted to Electrical

energy using a dc motor. We could be able to generate the required power but the size and weight of

the final device becomes larger and out of the considerations.

Fig 6. Design 1 Fig 7. Design 2



b) Design 2

This ideation came from analogy of the lawn mover or motor started we used earlier days. The

working mechanism is to pull a thread and release it, where the thread is connected to a spring that

stores the enough energy and when the thread is released, it uses the stored energy to rotate the shaft

of the motor thus generating electricity. The power generated depends on the thread length, which is

a major constraint on developing this device. Also the electricity generation is using the motor

where the size and weight matters. These constraints make the above two proposed designs un-

realizable.

Though the energy generation using DC motors gives enough power needed to charge the

smartphone, we have few concerns using them, as it weighs more and bigger in size. Thus we

studied other energy harvesting mechanisms such as piezoelectric, electromagnetic and electret.

Piezoelectric mechanism is not potentially viable to generate power around 5W, whereas in

electromagnetic, we need to use permanent magnets that can disturb other electronic devices around.

c) Proposed design: Electret based Energy Harvester

Electrets: (formed of “elektr-” from electricity and “-et” from magnet) Dielectric materials that

hold quasi-permanent electric charge. One way to think of an electret is as a capacitor that always

carries a charge and that you never have to recharge. Recently the electret principle was utilized to

convert human finger tapping inot power to actuate an array of flashing LEDs [4].

A ball shaped keychain design is proposed which incorporates the electret layers. The radius of the

ball is 3cm and the cross section of the proposed design is given here.

Figure 8. Layered structure of the proposed energy harvesting device



Electret material used is TEFLON or PTFE (50 micron thick) as it has the capability to store the

static electric charges for longer period compared to other materials.

Principle of Operation:

The operation of the Electret Generators relies on

1. The movement of the two conductive sheets (silver polyester) relative to each other and the

electret (PTFE).

2. Energy is created when a user moves electrodes. As the relative positions of the sheets

change, the distribution of the induced charges, the electric field, and the total capacitance

between the conductors change, resulting in an electric potential difference between the

conductors.

Figure 9. Electret Energy Harvesting: principle of operation

In our application, the field source is the semi-permanent charge on the surface of the thin and

flexible sheet of PTFE. When the sheet is brought near to conductive objects like sheets of silver, the

charge on the PTFE attracts the free charges of opposite polarity. These charges accumulate on the

surface of the Teflon sheet as shown in figure, because PTFE has very low electron affinity than

rubber. When the conductive sheets are moved relative to each other, then field source are converted

into electrical energy that can do work.

Tapping or Pressing on the top of the keychain are simple, intuitive gestures that everyone is

familiar with everyday experiences and have numerous same kind of applications. The generator

utilizes the layered structure described previously, when the top electrode is pressed, it touches the

electret (PTFE) creating a voltage spike. As the electrode is flexible, it recoils to its original position

when the user’s hand is retracted.

The electrical energy generated is used to charge a capacitor and only released to the load when a

certain amount of energy is reached. This results in significantly increased power density to charge

the smartphone.



Summary:

We have proposed and presented a key chain based charger; an electret based energy harvesting

device that generates electrical energy from user interaction (pressing) to charge the smartphone for

one last call. The device can be assembled using the thin film materials like PTFE and silver

polyester that are readily available. These materials are light, flexible and inexpensive, which make

the final device a light, tiny and robust ‘last call’ charger.

The proposed design does have certain limitations. Firstly, implanting the initial charges to the

Teflon material is difficult. As the Teflon material is highly resistive, the current output from this

design is very low. Thus we need to procure specific ultra-capacitors that can store and release the

energy or look for other options like energy store-and-release devices already available; making the

device bigger in size.

Future Works:

To proceed is to procure the above-mentioned materials and prototype the design. The measurements

and experiments from the prototype help us to understand the operation better. The characteristics of

the device will tell us about the harvesting circuit requirements. We believe, the AC voltage spikes

from the tapping generator should be regulated using rectifiers and converted to DC signal, which is

then fed to a capacitor.

Acknowledgements:

I would like to thank Dr. Erik Wilhelm for his support and help on this project. I would like to thank

Dr. Hong Yee Low for the guidance throughout the course and SUTD for the funding and

encouragement.

References:

[1] Kevin N Otto, Kristin L Wood, “Product Evolution: A reverse engineering and redesign

methodology”, Research in Engineering Design 1998

[2] Perrucci GP, “Survey on Energy Consumption Entities on the Smartphone platform”, Nokia

Research Centre, 2006

[3] Aron Carroll, “An analysis of Power Consumption in a Smartphone”, 2007

[4] Mustafa EK et.al., “Paper Generators: Harvesting Energy from Touching, Rubbing and Sliding”,

UIST’13 - Disney Research 2013

[6] Awad Saad, “New electret charging technique for energy harvesting” SPIE Newsroom 2012



[7] http://blog.opower.com/2012/09/how-much-does-it-cost-to-charge-an-iphone-5-a-

thought-provokingly-modest-0-41year/

[8] Customers need analysis questionnaire form. https://docs.google.com/forms/d/

1ZLIpDhE85OBZ6idVdKT7GzaBBx5Bed3D52cILweT5No/viewform

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