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PROYECTO FIN DE CARRERA

ENERGY SCAVENGING FOR

AUTOMOTIVE SENSORS USING

MICRO-ELECTRIC GENERATORS

AUTOR: ARTURO AGUILERA FERNÁNDEZ

MADRID, Junio 2009

UNIVERSIDAD PONTIFICIA COMILLAS

ESCUELA TÉCNICA SUPERIOR DE INGENIERÍA (ICAI)

INGENIERO INDUSTRIAL

Energy Scavenging for Automotive Sensors using Micro-Electric Generators

Executive Summary

PRODUCCIÓN DE ENERGÍA PARA SENSORES DEL

AUTOMOVIL USANDO GENERADORES MICRO-ELECTRICOS

Autor: Aguilera Fernández, Arturo.

Director: Anthony, Carl.

Entidad Colaboradora: University of Birmingham.

RESUMEN DEL PROYECTO

1. OBJETIVO DEL PROYECTO

- Investigar la generación energética a partir de fuentes cinéticas usando micro-

generadores.

- Emplear el reloj cinético como herramienta experimental para evaluar la

capacidad de generación energética.

2. MÉTODO EMPLEADO

- Determinación de la potencia requerida por los sistemas monitorizados de

control de presión de neumáticos (TPMS) y de las implicaciones de su

perfeccionamiento.

- Ingeniería inversa y experimentación del reloj cinético.

- Teorización del correspondiente generador rotacional electromagnético al

alimentar los sensores de presión inalámbricos.

- Modelado, simulación e implementación de los datos experimentales y teóricos

obtenidos.

- Miniaturización.

3. PRINCIPALES RESULTADOS

Energía renovable: Una mejora para TPMS.

El sistema TPMS directo resuelve los problemas de seguridad automovilística

causados por bajas presiones en los neumáticos. Importantes ventajas medio

ambientales y económicas se obtienen al sustituir la batería, que alimenta el

modulo de radio frecuencia del sensor con un mínimo de 2 mW, por un

dispositivo de generación y almacenamiento de energía libre de mantenimiento.

La fuente renovable más útil en el entorno del interior de un neumático es la


Executive Summary

generación cinética electromagnética. En particular, la generación rotacional

aprovecha al máximo la inercia rotacional de la rueda y vence las limitaciones de

los generadores comunes lineales debidas a las restricciones del desplazamiento

interno.

El reloj cinético: Un dispositivo de transformación y almacenamiento de energía

rotacional electromagnética.

Los resultados experimentales sobre el reloj cinético revelan la tecnología de

conversión electromagnética de energía rotacional. El dispositivo transforma la

rotación del péndulo a través de la amplificación y transmisión del movimiento a

un rotor magnético que genera tensión en una bobina. Niveles de potencia

razonables se generan así gracias al desplazamiento angular relativo entre la masa

y la estructura. Esta generación irregular consigue la autonomía del sistema

cuando se acompaña de una batería recargable. Su implementación para alimentar

TPMS obliga a localizar el dispositivo cerca del sensor orientándolo

paralelamente al plano de rotación de la rueda.

Funcionamiento de esta fuente de potencia autosuficiente para sensores de

presión inalámbricos.

Su funcionamiento general recae en un movimiento oscilatorio de la masa

caracterizado por grandes amplitudes y altas frecuencias. Este patrón de

generación se localiza para velocidades constantes del vehículo superiores a 15

km/h, dónde la aceleración centrifuga es más de 10 veces superior a la aceleración

gravitacional. El máximo nivel de potencia alcanzado abarca desde 2 mW hasta

una generación saturada constante de 3 mW.

Por debajo de una velocidad constante del vehículo de 5 km/h, dónde el campo

gravitatorio es mayor que el campo centrífugo, el método de generación se acerca

a un generador convencional. Este funcionamiento reposa en la conversión de una

rotación continua gracias al movimiento estacionario absoluto de la masa causado

por la resistencia vertical de la gravedad. La potencia máxima generada en este

caso no excede los 2 mW. Entre ambos métodos, un comportamiento caótico

genera a su vez insuficientes niveles de potencia para TPMS.

Las mayores amplitudes se obtienen en un movimiento resonante oscilatorio que

puede ser establecido calibrando los valores de los parámetros para esta aplicación


Executive Summary

particular. La supresión del método de generación continua introduciendo esta

última respuesta en todas las condiciones de funcionamiento vencería los bajos

niveles de generación a bajas velocidades ayudado por la adición de un sistema de

administración y almacenaje de energía.

Un sensor TPMS completamente autónomo a escala milimétrica.

Los resultados experimentales de miniaturización alcanzan una potencia máxima

de 4,4 mW. Futuras investigaciones deben enfocar la generación rotacional

electromagnética a escala milimétrica como una tecnología viable para la

producción de milivatios. El desarrollo de MEMS permitirá la introducción de

esta unidad de fuente infinita en el interior del propio sensor de presión.

4. CONCLUSIONES

- Actualmente, un rediseño del sistema de conversión electromagnética del reloj

cinético para ampliar su potencia permitirá indudablemente a los sistemas TPMS

trabajar con alta fiabilidad bajo todas las posibles condiciones de funcionamiento

durante toda la vida útil del vehículo.

- En un futuro, el diseño de un sistema MEMS a escala milimétrica alcanzará

seguramente la completa autonomía de los sensores TPMS aportando importantes

ventajas medio ambientales, económicas y de seguridad a nivel global.

5. SUMARIO

Enfocado a TPMS, este proyecto ha logrado utilizar la conversión de energía

mecánica del movimiento de una rueda para situar una unidad de fuente infinita

con el propio sensor. De ahí, la tecnología de micro-generación y almacenamiento

del reloj cinético ha sido resuelta basada en desarrollos teóricos y experimentales.

Consecuentemente, el conversor electromagnético de energía rotacional ha sido

modelado, simulado e implementado para alimentar la aplicación de TPMS.

Angulo relativo con respecto al tiempo para respectivamente ambos funcionamientos.

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Executive Summary

Finalmente, se propone un diseño en miniatura. Los resultados finales alientan

futuros logros a corto plazo utilizando la energía renovable a través de

generadores micro-eléctricos para la industria automovilística.

ENERGY SCAVENGING FOR AUTOMOTIVE SENSORS USING

MICRO-ELECTRIC GENERATORS

1. AIMS

- Investigate energy harvesting from kinetic sources using micro-generators.

- Use kinetic wristwatch as an experimental tool for assessing energy generation

capability.

2. OBJECTIVES

- Determine power requirements of Direct TPMS, and implications of its

improvement.

- Reverse engineering and experimentation of kinetic wristwatch.

- Theorise on rotational electromagnetic generator powering wireless pressure

sensors.

- Modelling, simulation and implementation of experimental and theoretical data

obtained.

- Attempt miniaturisation.

3. MAIN RESULTS

Energy harvesting: A TPMS improvement.

Direct TPMS resolves automobile safety problems caused by low pressure tyres.

Important environmental and economic advantages are obtained from the

substitution of the battery, which powers a minimum of 2 mW to the sensor RF,

by a free-maintenance energy harvesting and storage device. The most advisable

renewable source in tyre environment is kinetic electromagnetic generation. In

particular, rotational generation makes the most of wheel rotational inertia and

overcomes common linear harvester limitations due to internal displacement

restrictions.


Executive Summary

The kinetic watch: A rotational electromagnetic energy harvesting and storage

device.

Experimentation results of kinetic wristwatch reveal rotational electromagnetic

energy harvesting technology. The device damps its proof mass rotation by

amplifying and transmitting the motion to a magnetic rotor which generates

voltage into a coil. Reasonable power levels are then scavenged from the relative

angular movement between proof mass and frame. This irregular generation

achieves autonomy accompanied by a rechargeable battery. Its implementation to

power TPMS obliges to locate this device next to the sensor oriented parallel to

wheel rotational plane.

Operation of this self-renewable power source for wireless pressure sensors.

Its general operation relies on the oscillating motion of the proof mass

characterised by large amplitudes and high frequencies. This scavenging pattern

takes place for constant vehicle speeds above 15 km/h, where the centrifugal

acceleration is more than 10 times the gravitational acceleration. Maximum power

level achieved goes from 2 mW to a constant saturated generation of 3 mW.

Under a vehicle constant speed of 5 km/h, when the gravitational field is higher

than the centrifugal field, the harvesting method approaches a conventional

generator. This operation relies on scavenging continuous rotation due to the

absolute stationary motion of the proof mass caused by vertical opposition of

gravity. The maximum power generated in this case does not exceed 2 mW.

Between both methods, the chaotic motion generates as well insufficient power

levels for TPMS.

Relative angle position with respect to time for respectively both operations.

The largest amplitudes are obtained for oscillating resonant motion which can be

established redesigning parameter values for this particular application. The

elimination of continuous harvesting method, introducing that response at all

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Executive Summary

operating conditions will overcome poor generation at low speed, helped by the

addition of an energy management and storage system.

A millimetre-scaled complete autonomous TPMS sensor.

Experimental miniaturization results reach a maximum power of 4,4 mW. Future

researches have to focus on millimetre-scaled rotational electromagnetic

generation as a viable milliwatt powering technology. MEMS performance will

permit to introduce this infinite source unit into the pressure sensor itself.

4. CONCLUSIONS

- At present, a power scaled up design of kinetic watch inductive harvesting

system in its centimetre scale will definitely enable TPMS to work with high

reliability under all possible operating conditions during the vehicle entire life.

- A future MEMS design will surely achieve a millimetre-scaled complete

autonomous TPMS sensor which will contribute to important global

environmental, economic and safety advantages.

5. SUMMARY

Focusing on TPMS, this project has managed how to use mechanical energy

harvesting from wheel motion to place an infinite source unit with the sensor

itself. Thereby, kinetic wristwatch micro-generation and storage technology has

been solved based on theoretical developments and experimentation. Consequent

rotational electromagnetic energy harvester has been modelled, simulated and

implemented for powering TPMS application. Finally, a miniaturization design

has been approached. Final results encourage short term future achievements

using energy scavenging through micro-electric generators for the automotive

sensor industry.


29/04/2009

Energy Scavenging for

Automotive Sensors using

Micro-Electric Generators BEng Engineering Project

School of Mechanical Engineering

Arturo Aguilera Fernández

Supervisor: Dr. Carl Anthony


Acknowledgements

Acknowledgements

I would like to thank people who, in one way or another, have allowed this

engineering project to be completed. Thank you very much for your useful help

and support during current academic year.

First of all, many thanks to Dr. Carl Anthony who played a decisive role

throughout the development of this paper providing me with constant guidance

and assistance.

Secondly, special thanks to Mr. Alan Saywell for his collaboration during

the experimentation stage. Many thanks also to Dr. Mike Keeble for making

possible to back this work with high quality pictures. Likewise, I want to express

my gratitude to many other persons of the University of Birmingham who have

cooperated in experimental measurements lending me the instrumental equipment

needed.

Thirdly, my thanks go as well to David Cheneler for his collaboration

during the simulation stage, and to Imperial College of London for supplying a

PSpice energy harvesting simulator.

Finally, thanks to the technical support of UK Seiko for providing me with

really useful information.

Definitely, my most sincere gratitude goes to the University of

Birmingham for giving me the opportunity of living this research experience

abroad.


Table of Contents

Table of Contents

INTRODUCTION 1

1. Aims of the project .............................................................................................. 2

2. Objectives of the project ...................................................................................... 3

3. Methods of research ............................................................................................. 4

CHAPTER I: Introduction to TPMS 5

1. Tyre pressure monitoring system......................................................................... 7

2. Safety implications of the project ........................................................................ 9

3. Commercial implications of the project .............................................................. 9

4. Environmental implications of the project ........................................................ 10

5. Conclusion of chapter I ...................................................................................... 11

CHAPTER II: Literature Review of Energy harvesting 12

1. Energy harvesting .............................................................................................. 13

2. Linear electromagnetic micro-generator............................................................ 16

3. Energy storage ................................................................................................... 23

4. Conclusion of chapter II .................................................................................... 24

CHAPTER III: The Kinetic Watch 25

1. Taking Seiko kinetic watch apart ...................................................................... 26

2. Seiko AGS properties ........................................................................................ 27


Table of Contents

2.a. Oscillating weight ................................................................................. 28

2.b. Gear train .............................................................................................. 28

2.c. Generating rotor .................................................................................... 29

2.d. Generating coil...................................................................................... 31

2.e. Energy conversion interface ................................................................. 32

2.f. Step motor ............................................................................................. 38

3. Rotational electromagnetic micro-generator ..................................................... 41

3.a. Non resonant oscillating rotational generator ....................................... 41

3.b. Resonant oscillating rotational generator ............................................. 43

3.c. Continuous rotational generator ............................................................ 44

4. Conclusion of chapter III ................................................................................... 45

CHAPTER IV: Powering TPMS Sensors 46

1. Double pendulum............................................................................................... 47

2. Gravitational electromagnetic micro-generator ................................................. 49

3. Centrifugal electromagnetic micro-generator .................................................... 51

4. Conclusion of chapter IV ................................................................................... 59

CHAPTER V: Experimentation 60

1. Experimental starting ......................................................................................... 61

2. Experiment 1 ...................................................................................................... 63

3. Experiment 2 ...................................................................................................... 64

4. Conclusion of chapter V .................................................................................... 66

CHAPTER VI: Results 68


Table of Contents

1. Electromagnetic circuit ...................................................................................... 69

2. Oscillating operation .......................................................................................... 70

3. Continuous operation ......................................................................................... 75

4. Conclusion of chapter VI ................................................................................... 78

CHAPTER VII: Miniaturisation 79

1. Scaling considerations ....................................................................................... 80

2. Design proposal ................................................................................................. 81

3. Conclusion of chapter VII ................................................................................. 83

CONCLUSIONS 84

REFERENCES 87

APPENDIX


Table of Figures

Table of Figures

Figure 1: Tyre profile and wear on tread face. ........................................................ 6

Figure 2: Phaeton direct TPMS. .............................................................................. 7

Figure 3: Direct TPMS. ........................................................................................... 8

Figure 4: Phaeton pressure sensor package. ........................................................... 8

Figure 5: World automotive pressure sensors market. .......................................... 10

Figure 6: Energy harvesting and storage device. ................................................... 13

Figure 7: Main advantages and disadvantages of the three primary mechanical

energy converters. .................................................................................................. 14

Figure 8: Piezoelectric transducer. ........................................................................ 15

Figure 9: Electrostatic transducer. ......................................................................... 15

Figure 11: Lineal energy harvester. ....................................................................... 16

Figure 10: Faraday’s & Lenz’s laws. ..................................................................... 16

Figure 12: Vibrational harvester. ........................................................................... 17

Figure 13: Block diagram of a vibrational electromagnetic harvester. ................. 19

Figure 14: Damping effect. .................................................................................... 22

Figure 15: Power density of energy harvesting components ................................. 23

Figure 16: Pulsar kinetic watch. ............................................................................ 27

Figure 17: Electric circuit of Pulsar kinetic watch. ............................................... 28

Figure 18: Proof mass. ........................................................................................... 28

Figure 19: Gear train. ............................................................................................. 29

Figure 20: Rotor and stator. ................................................................................... 29

Figure 21: B-H curve of rare earth cobalt magnet. ................................................ 30

Figure 22: Coil block. ............................................................................................ 32

Figure 23: Rc measurement circuit. ....................................................................... 32

Figure 24: Battery. ................................................................................................. 33

Figure 25: Circuit block. ........................................................................................ 33

Figure 26: Simple model of a micro-generator...................................................... 34

Figure 27: Voltage output after signal through a full bridge diode rectifier. ........ 34


Table of Figures

Figure 28: Additional capacitor to produce a DC voltage output. ......................... 35

Figure 29: Quartz unit. ........................................................................................... 37

Figure 30: Step motor. ........................................................................................... 39

Figure 31: Step motor. ........................................................................................... 40

Figure 32: Rotational harvester. ............................................................................ 41

Figure 33: Double pendulum. ................................................................................ 47

Figure 34: Rotating pendulum. .............................................................................. 51

Figure 35: Rotating pendulum with one plane of oscillation. ............................... 54

Figure 36: Rotating pendulum from reference ij. .................................................. 56

Figure 37: Rotating pendulum with gear train. ...................................................... 58

Figure 38: Marine mammal package. ................................................................... 61

Figure 39: Experimental assembly. ....................................................................... 62

Figure 40: Experimental device. ............................................................................ 62

Figure 42: Open-circuit generated voltage. ........................................................... 63

Figure 41: Voltage measurement circuit................................................................ 63

Figure 43: Generation measurement circuit. ......................................................... 64

Figure 44: Intensity generated. .............................................................................. 65

Figure 45: Voltage generated. ............................................................................... 65

Figure 46: Power generation. ................................................................................. 66

Figure 47: Natural frequency wn and wheel speed Ω with respect to vehicle

velocity. ................................................................................................................. 71

Figure 48: Rotor frequency wr regarding v............................................................ 71

Figure 49: Mass acceleration ϴr regarding v. ....................................................... 72

Figure 50: Mass restoring torque Tc regarding v. .................................................. 72

Figure 51: rΩ2/g ratio regarding v. ........................................................................ 73

Figure 52: Mass relative angle ϴr(t) for v= 6 km/h. .............................................. 74

Figure 53: Mass relative displacement ϴr for v= 60 km/h. ................................... 75

Figure 54: Mass relative displacement ϴr for v= 2 km/h. ..................................... 76

Figure 55: Power generated under v = 5 km/h. ..................................................... 77

Figure 56: Stator winding pattern. ......................................................................... 81

Figure 57: Power regarding rotor speed. ............................................................... 82

Figure 58: Miniaturisation proposal. ..................................................................... 83

Figure 59: Autonomous TPMS. ............................................................................. 83


Table of Tables

Table of Tables

Table 1: Characteristics of rare earth cobalt. ......................................................... 30

Table 2: Magnetic circuit dimensions. .................................................................. 69

Table 3: Operational properties of the permanent magnet. ................................... 70

Table 4: Air gap results.......................................................................................... 70

Table 5: Parameter of wheel and oscillating weight. ............................................. 70


1

INTRODUCTION


Introduction 2

Introduction

These days, car manufacturers are at the forefront of developing

innovative sensing technologies due to high quantity of possible applications that

are present in the automotive industry. Moreover, vehicle environment represents

the most challenging conditions for micro-sensor systems. New sensor packages

are designed to obtain a competitive advantage or meet government regulations.

The subsequent customer acceptance involves trustworthiness and low cost.

Therefore, the output has to be stable during car lifetime, and the device has to be

small, as well as, easy to mount in its place. However, the current necessity of

wiring the sensor system back to vehicle power sources adds a significant higher

cost. Hence, this limitation has to be overcome by improving energy micro-

harvesting.

1. Aims of the project

This engineering project focuses specifically on tyre pressure monitoring

systems (TPMS). At the moment, their electricity supply is the vehicle

electrochemical battery or replaceable button cells. Therefore, this work is aimed

at using mechanical energy harvesting from wheel motion to place an infinite

source unit with the sensor itself. With last purpose, this project aims for solving

kinetic wristwatch micro-generation technology, and its electromagnetic energy

conversion and consequent storage. As a result, the design of a new autonomous

pressure sensor package will provide environmental, economic and technical

advantages.


Introduction 3

2. Objectives of the project

Particular objectives of this engineering project are the followings.

Firstly, a research about tyres pressure and TPMS has to be done to deduce

commercial and environmental implications of the project, as well as, to

understand power requirements of this sensoring system.

Secondly, a literature review concerning energy harvesting and consequent

energy storage has to be made to discriminate between different energy generation

options. Subsequently, a model of a common linear electromagnetic generator will

be done to depict usual kinetic harvesters and approach electromagnetic energy

conversion.

Thirdly, a reverse engineering of a kinetic wristwatch will have to be done

with the purpose of understanding the running of this particular human rotational

energy harvesting device. Hence, a Seiko AGS system watch will be taken apart,

and later, specific researches, measurements and calculations will be done to

assess properties of different components. Furthermore, this rotational harvester

will be portrayed depending on the source of excitation and compared with the

previous linear model.

Fourthly, the studied rotational harvester will be analysed and described

mounted in its wheel application.

Fifthly, Seiko wristwatch will have to be tested to obtain data of its

harvesting generation capacity through experimentation. Consequently, using all

previous data and information, calculations will be made to discuss if this

commercial device is able to deliver the power required by a TPMS sensor. In

addition, software implementations of the device performance could help this

discussion.

Finally, preceding results will be used to try to achieve a miniaturized

design of a rotational electro-magnetic micro-harvester. Final conclusions will

then be expounded.


Introduction 4

3. Methods of research

This project has required several methods of research to carry out the

work.

First of all, the introduction and literature review has been got from library

and online catalogue researches.

In addition, deduction of formulas to describe harvesting phenomena of

the kinetic wristwatch has been based on mechanical books and modern

periodicals, journals and university publications because of the topical subject

discussed in this paper. Those formulations are generally based on international

system of units otherwise it will be specifically pointed out. Miniaturization

design proposal is also based on up-to-date scientific and experimental

publications due to the nowadays lack of data, information and knowledge about

rotational electromagnetic micro/nano-generators.

Finally, measurements, models and experiments were carried out thanks to

cooperation and proposals of many personnel of the University of Birmingham,

who are mentioned in Acknowledgements, due to requirements of specific

knowledge, instrumentation and equipment.


5

CHAPTER I

Introduction to TPMS


Chapter I 6

Chapter I

Introduction to TPMS

Vehicle motion depends mainly on contact forces between tyre and road,

and consequently on tyre characteristics (Figure 1). Recommended tyre air

pressure, which is specified by the manufacturer, distributes the necessary load to

cause the correct amount of frictional force for enabling vehicle performance. As

studied, low pressure tyres induce poor handling, squealing, overheating,

premature tread wear, increasing self aligning torque, and steer problems.

Furthermore, low pressure increases braking distance, and traction is not

improved. In extreme cases, tread separation or even wheel rim detachment can

occur. So incorrectly inflated tyres give rise to safety, economic and

environmental problems.

Figure 1: Tyre profile and wear on tread face.

Moreover, in practice, a tyre can deflate up to half of its air pressure

without appearing it. Therefore, a system capable to manage low pressures

monitoring for alleviating those concerns would be hugely beneficial.


Chapter I 7

1. Tyre pressure monitoring system

A TPMS is an electronic system which monitories pressure of vehicle

tyres. The driver obtains the information in real time via different possible

displays. Manufacturers focus principally on direct TPMS, which use a pressure

sensor inside each tyre of the car, because of their higher level of advantageous

details. For instance, this kind of system can identify any combination of

simultaneous tyre under-inflations, and cancel pressure variations due to weather

or friction temperature effects. Hence, indirect TPMS, which measure the pressure

using parameters available outside the pneumatic tyre, are not discussed here.

As shown in Figure 2 and Figure 3, direct TPMS send collected data from

sensors located inside each tyre to a control unit for subsequently being processed

and sent to the instrument cluster. Hence, each sensor package contains a radio

Figure 2: Phaeton direct TPMS.


Chapter I 8

frequency module (RF) in order to overcome the wheel rotational boundary,

avoiding earlier complex rotating contact wiring. Consequently, each sensor has

to be powered by a battery, as depicted in Figure 4. And those batteries involve a

maintenance cost for the customer when they become exhausted. Furthermore,

pressure sensors could be damaged during battery replacement.

Figure 3: Direct TPMS.

The following technological challenge is then the extension of battery

power used essentially by the RF. Depending on the sampling rate, the supply

voltage of a pressure sensor package is typically from 1,8 V to 3,6 V, and its

power consumption is normally between 2 mW and 5 mW thanks to its sleep

state. Power management techniques permit batteries to operate longer. However

it is insufficient. The design of a new maintenance-free sensor package will

overcome direct TPMS drawbacks. Consequent safety, commercial and

environmental implications are presented next.

Figure 4: Phaeton pressure sensor package.


Chapter I 9

2. Safety implications of the project

In the United States, the National Highway Traffic Safety Administration

(NHTSA) announced that 533 deceases in road are linked to tyre problems in one

year; and if all vehicles would have had TPMS, 8400 injuries could have been

avoided and 120 fatalities would have been saved every year.

In Europe, the German DEKRA said that tyre irregularities are behind

41% of road injured accidents. The French road safety organization, Sécurité

Routière, made public that 9% of fatal accidents are caused by tyre under-

inflation.

Confirmed by statistics, tyre pressure condition is one of the most

important safety aspects of a vehicle, and therefore TPMS save lives. Hence, a

new generation of direct TPMS, which would not demand maintenance and would

be more reliable having a lower cost, will encourage car manufacturers and

customers to install this system in every vehicle as standard safety equipment.

3. Commercial implications of the project

In the United States, Clinton administration wrote the TREAD Act

because of the high number of deaths caused by accidents following a tyre tread

separation. After September 2007, all vehicles were required to install TPMS

which warn when the air pressure of a tyre decreases more than 25% of the

manufacturer recommendation. Frost & Sullivan divulged that $80,7 millions

were generated in US pressure sensor market in 2005. Thereafter, US revenues are

expected to increase at 30,7% until 2012, when they will be around $526,7

millions.

While in US direct TPMS development is based on safety legislation

reasons, Europe approaches TPMS from a more environmental point of view. A

new generation of direct TPMS would allow an international standardization and


Chapter I 10

high cost savings, contributing to the growth of this segment in Europe and Asia

Pacific markets. As a result, world pressure sensors demand forecast in the

automotive market is expected to be really significant, as recorded in Figure 5.

Figure 5: World automotive pressure sensors market.

4. Environmental implications of the project

On one hand, under-inflation influences tyres wear and fuel efficiency.

NHTSA publishes that tyres can lose air pressure between 20 kPa and 60 kPa

yearly. In addition, 40% of drivers over Europe and US check tyres less than one

time a year. Consequently, more than 40% of vehicle owners are driving with low

pressure tyres. Furthermore, the European Union estimates that a 2% increase of

fuel consumption and a 25% decrease of tyre life are caused by a 40 kPa tyre

deflation. As a result, tyre under-inflation generates 200 millions of prematurely

wear tyres, 20 million litters of unnecessary consumed fuel and 2 million tonnes

of CO2 throw into the environment just in Europe.

On the other hand, 16 millions of yearly new manufactured cars have to

follow the TREAD Act in the United States. As a result, 65 millions of batteries

are thrown out into the environment annually just in US.


Chapter I 11

Definitely, a new technological introduction of battery-less direct TPMS

will overcome these environmental issues.

5. Conclusion of chapter I

Direct TPMS technology is limited by wireless pressure sensors powering.

Many TPMS advantages disappear when sensors have to be powered with an

external or replaceable source. In comparison, a new maintenance-free sensor

package design will involve important safety, economic and environmental

advantages. An approach to remove batteries from these low power sensor devices

could be power harvesting. Thereby, wheel kinetic energy could be converted into

usable electric energy. In conclusion, the objective is to conceive a small power

source placed into the sensor package which will enable direct TPMS to work

under all possible operating conditions during the vehicle entire life with low cost

and high reliability.


12

CHAPTER II

Literature Review of

Energy Harvesting


Chapter II 13

Chapter II

Literature Review of Energy Harvesting

Limited accessibility of TPMS pressure sensors requires them to turn into

a completely autonomous micro-device. Energy harvesting motivation is exactly

to overcome environmental issues of throw-away batteries. Therefore, a

possibility to achieve a self-powered package is extracting energy from a self-

renewing environmental source. That challenging energy has to be converted and

stored because of its intermittent properties. Thereby, a harvesting generator will

be replenishing the consumption of the RF. Consequently, it is essential to

approach different harvesting methods to recognise the most suitable

environmental source of pressure sensors application conditions.

Figure 6: Energy harvesting and storage device.

1. Energy harvesting

Energy harvesting or scavenging is the conversion of environmental

energy into electrical energy. In other words, it is the process of ambient energy

capture and storage (Figure 6). This power technology is then an endless source

with non environmental effects. As quantified, pressure sensors require low power

and an energy harvesting micro-system is capable to scavenge milliwatts required.

However possible power densities depend on the specific application and


Chapter II 14

generator design. An energy harvesting generator is based either on solar, thermal

or kinetic source (Figure 7).

Figure 7: Main advantages and disadvantages of the three primary

mechanical energy converters.

On one hand, a 100 mm2 photovoltaic cell scavenges 1 mW of power.

However, solar harvesting can just be taken into account if the sensor is hit by a

minimum of five hours of sunlight. On the other hand, thermoelectric devices

provide high reliability but low efficiency with temperature differences under

10ºC. Thus thermal harvesting has just to be considered in very hot applications

with a smooth surface. Both last sources generate enough power to supply a micro

pressure sensor; however conditions of the inside of a pneumatic tyre (no sun-

radiation and low thermal gradients) force this work to concentrate on kinetic

based harvesting, which is divided into three methods.


Chapter II 15

Figure 8: Piezoelectric transducer.

Kinetic energy harvesting converts the displacement of the transducer

device into electrical energy. Piezoelectric transducers produce a voltage drop

proportional to the piezoelectric material deformation or strain (Figure 8). And

electrostatic converters rely on the capacitance change of an initially charged

vibrational variable capacitor (Figure 9). However, properties of a wheel motion

demand this paper to focus on electromagnetic micro-generators because, even

though electromagnetic harvesting usually extracts the energy from vibration too,

it gives as well the possibility to scavenge kinetic energy from rotational motion.

Moreover, electromagnetic systems are more reliable working at large

accelerations. Electromagnetic micro-generation is then a promising method for

TPMS.

Figure 9: Electrostatic transducer.


Chapter II 16

2. Linear electromagnetic micro-generator

Electromagnetic energy harvesting converts mechanical energy to a

current in a conductor via an electromagnetic field. Based on Faraday’s law, the

variation of a magnetic flux within a conductive circuit induces an electric

voltage. By Lenz’s law, this voltage polarity creates a current whose magnetic

field is opposite to the magnetic flux variation, trying always to keep the total

magnetic flux constant, as illustrates Figure 10.

Consequently, the electric generation relies on a relative movement

between a conductor and a magnet (Figure 11). The following analysis of a usual

vibration based electromagnetic generator refers to papers [CHIN00], [BEEB08]

and [GILB08].

Figure 11: Lineal energy harvester.

Typically, the model of a lineal energy harvesting device takes the form of

a spring, mass and damper system, as illustrated in Figure 12. A magnet of mass

m hangs from the device case, where a coil is fixed, through a spring of stiffness

k. A viscous damper of coefficient ct represents the parasitic losses cm and the

Figure 10: Faraday’s & Lenz’s laws.


Chapter II 17

electrical energy extracted ce, since ct = cm + ce. y(t) is the position of the entire

device at time t, and z(t) is the relative position of the magnet referred to its

equilibrium position inside the device.

Figure 12: Vibrational harvester.

An input mechanical force fm(t) causes the vibration of the generator.

Hence, the magnet oscillates provoking its relative movement with regard to the

coil. The resulting variation of the magnetic flux linkage induces a voltage e(t)

and a current i(t) in the coil, getting the output power of the system. The

mechanical work is transformed into stored energy in the inductance L and into

heat in the resistance Rc when the coil is connected to a resistive load R.

Firstly, the magnet equation of motion is deduced from Newton’s second

law as

f t = mz t + cm z t + kz t (II. 1)

Hence the transfer function between the relative displacement z(t) and the total

force f(t) exerted on the magnet is

Z(s)

F(s)=

1

ms2 + cm s + k (II. 2)

Secondly, the induced voltage in the coil of N turns of side length l

moving at a velocity z t , which is supposed a sinusoidal of frequency ω, into a

magnetic flux of density B is

e t = NBlz t (II. 3)


Chapter II 18

Therefore voltage of the load R is

v t = i t R =e t

R + Rc + jωLR

v(t) =NBlRz t

R + Rc + jωL (II. 4)

And using Laplace transform, the output voltage generated v(t) is linked to the

relative displacement z(t) by a first order system

V(s)

Z(s)=

NBlRs

R + Rc + Ls (II. 5)

Thirdly, the current i(t) induced in the coil causes an opposite

electromechanical force fe(t) which is defined by Lorentz force law as

fe t = NBli t (II. 6)

Hence the concept of electromagnetic constant is defined as

ke =e(t)

z (t)=

fe(t)

i(t)= NBl (II. 7)

Furthermore, the total force exerted on the magnet can be expressed as

f t = fm t − fe t = fm t − NBli t

f(t) = fm t −NBl

Rv t (II. 8)

As a result of equations (II.2), (II.5) and (II.8), the block diagram of Figure

13 presents the transfer function of the system which relates the output voltage

v(t) with the input force fm(t) as (II. 9)

V(s)

Fm (s)=

1ms2 + cm s + k

NBlRsR + Rc + Ls

1 +1

ms2 + cm s + kNBlRs

R + Rc + LsNBl

R


Chapter II 19

Figure 13: Block diagram of a vibrational electromagnetic harvester.

After some simplification, the transfer function of the vibrational electromagnetic

generator becomes

V(s)

Fm (s)=

NBlRs

ms2 + cm s + k (R + Rc + Ls) + (NBl)2s (II. 10)

Knowing that the mechanical time constant is much higher than the electrical time

constant, the third order system can be simplifies again to a second order system.

V(s)

Fm (s)=

NBlRs

mRs2 + cm R + (NBl)2 s + kR (II. 11)

Therefore the simplified transfer function of the studied vibrational generator is

defined as

V(s)

Fm (s)=

NBlm s

s2 + 2ζωns + ωn2

(II. 12)

where ωn and ζ are respectively the spring natural frequency and the damping

factor expressed as

ωn = k

m

ζ =cm R + NBl 2

2R mk=

cm

2ωnm+

NBl 2

R2ωnm

ζ = ζm + ζe (II. 13)


Chapter II 20

knowing that ζm and ζe are respectively the mechanical and electrical damping

factors. Supposing that the conductor moves from the zero magnetic field to the

highest magnetic field B, the ideal electrical damping coefficient ce is

ce =(ke)2

R + Rc + jωL=

(NlB)2

R + Rc + jωL (II. 14)

Assuming an harmonic source of motion, the input displacement is a

sinusoidal y(t) = Y0 sin(ωt) whose maximum acceleration is y max = −Y0ω2.

The average input force becomes

Fm s = −mY s = −mY0

2ω2 (II. 15)

Hence, the corresponding output voltage is

V s =V s

Fm s Fm s

V s =

−Y0

2ω2NBls

s2 + 2ζωns + ωn2

(II. 16)

And knowing that s = jω,

V(jω) 2 =

Y02

2ω6 NBl 2

ωn2 − ω2 2 + 2ζωnω 2

=mY0

2 ωωn

3

ω3 (NBl)2

2ωnm

1 − ωωn

2

2

+ 2ζωωn

2

(II. 17)

Therefore, the average useful power generated by the linear electromagnetic

generator is

Pe = V 2

R=

mY02

ωωn

3

ω3ζe

1 − ωωn

2

2

+ 2ζωωn

2

(II. 18)


Chapter II 21

The peak of output power occurs at resonance when ω = ωn. The expressions

become

Pe max =mY0

2ωn3ζ

e

4ζ2

VPe max = 2Pe max R =Y0ωnNBl

2ζ (II. 19)

The electromagnetic generator has then to be designed matching its natural

frequency with the vibration present on the environment of application. In

addition, the output voltage could be higher by increasing the coil and the magnet

mass m; however those are always limited by the size of the device case which is

determined by its specific application. Furthermore, the maximum power is

generated for ζp = ζe, obtaining

Pe max =mY0

2ωn3

16ζe (II. 20)

This maximum value can be achieved adjusting ce = cp using the optimum load R

given by

R = Rc +(NlB)2

cm (II. 21)

The total power dissipated on the harvesting system is

P =mY0

2 ωωn

3

ω3ζ

1 − ωωn

2

2

+ 2ζωωn

2

(II. 22)

Its maximum takes place also when the vibration frequency ω equals the resonant

frequency ωn.

P =mY0

2ωn3

4ζ (II. 23)


Chapter II 22

Figure 14: Damping effect.

If the damping factor ζ increases, the power bandwidth increases as well, whereas

the peak effect decreases. Thus, a high damping factor should be used when the

source frequency changes, and on the contrary, the damping factor should be low

when the frequency of vibration is fixed. This reasoning is explained in Figure 14.

Finally, a solution for the relative displacement at steady state for the input

y(t) = Y0 sin(ωt) is

z t =Y0ω

2

km − ω2

2

+ ct

m ω 2

sin ωt + ϕ (II. 24)

where Φ is correspond to the phase angle as

ϕ = tan−1ctω

k − ω2m (II. 25)

The energy generated relies also on the frequency ω and amplitude Y0 related with

the mass displacement z; however the maximum displacement zmax is as well

limited by the size of the device.


Chapter II 23

In conclusion, optimum operation of linear electromagnetic harvesters

depends highly on frequency and requires a resonant oscillating design. Whereas a

non resonant harvesting can be significantly more efficacious in cases with a wide

range of low frequencies and high amplitudes, its power density is lower. Power

levels of these devices are limited essentially by the oscillating mass m, the

maximum internal displacement zmax and the frequency ω and amplitude Y0 of the

source motion. As a result, the power density decreases with the device size and

the maximum generated power scales as linear dimension raised to the power of

four.

3. Energy storage

Energy extracted by an electromagnetic generator from wheel kinetic

source is low and irregular. Therefore an intermittent charger is needed to store

the energy on a rechargeable battery of nickel metal hydride or lithium ion based

for subsequently powering the RF application of the pressure sensor via a

regulator circuit. Hence, battery charging efficiency and its power density is

crucial, as shows Figure 15.

Figure 15: Power density of energy harvesting components

compared to primary batteries.


Chapter II 24

As a result, wheel kinetic energy, which has normally been lost in the

environment, can now be harvested to power TPMS sensor packages extending

hugely their lifetime and overcoming primary battery disadvantages.

4. Conclusion of chapter II

Electromagnetic energy harvesting solution has been identified as the most

appropriate method for the particular environment of a tyre. An inductive micro-

generator and rechargeable battery system achieves TPMS sensors autonomy and

consequent independence from customer. However, power levels of common

kinetic energy harvesting devices are limited by internal displacement restrictions.

This limitation could be eliminated by damping instead the motion of a rotating

mass. Therefore, it is necessary to implement a rotational micro-generator using

the same previous principles to try to overcome power limitations of linear

harvesting and make the most of rotational kinetic energy of wheels. At the

moment, this technology is already used in some kinetic wristwatches.


25

CHAPTER III

The Kinetic Wristwatch


Chapter III 26

Chapter III

The Kinetic Wristwatch

In the mechanical field, the kinetic wristwatch has achieved to delete its

power maintenance employing human passive energy harvesting because of its

low power consumption. Thus the self-winding wristwatch is the precursor of

rotational power harvesting technology. The challenge is to do a reverse

engineering of the commercialized Seiko Automatic Generating System (AGS)

watch with the purpose of analysing the rotational micro-generator and

determining its power levels compared with previously detailed linear micro-

generator.

1. Taking Seiko kinetic watch apart

The first experimental approach to Seiko AGS technology involves taking

apart a Pulsar kinetic watch whose model is PAR087X1 and Cal. YT57. All

experimental works of this project will be using this device. The watch has then

been dismantled to understand how it works following precisely instructions of

catalogue [SEIK08]. The assembling instruction used and the resulting

chronological pictures, where the background white segments measure 1 cm, are

presented in Appendix 1. Consequently, the running mechanism and particularly

the rotational harvesting system of the watch were revealed. Therefore, the main

characteristics of Seiko kinetic wristwatch are described below.


Chapter III 27

2. Seiko AGS properties

The studied wristwatch utilizes the motion of the arm to freely rotate a

semi-circular weight mounted off its center of mass on a ball bearing spindle.

Thus the oscillating proof mass winds the harvesting mechanism. A high ratio

transmission gear train attached to a generating permanent magnet rotor amplifies

the rotational movement. The high spinning speed of the rotor transforms the

inertial rotation into magnetic charges and induces an electric voltage and current

into the coil by means of a ferromagnetic stator circuit. Then the sinusoidal

generated power is rectified and stored in the energy storage unit. Subsequently,

the electrical energy required to run the quartz based hands system is supplied.

Figure 16 and Figure 17 show a comprehensive view of this wearable device.

Theoretically, the wristwatch generates on average 5 μW when it is worn, and

1 mW when is forcibly shaken. Each specific part is detailed next.

Figure 16: Pulsar kinetic watch.


Chapter III 28

Figure 17: Electric circuit of Pulsar kinetic watch.

2.a. Oscillating weight

Parameters of the oscillating weight (Figure 18) were determined using a

digital scales and a calliper gauge. As a result, its radius and mass values are

respectively Rp = 13,5 mm and m = 4,8 g. Measurements were done 5 times and

the average of them was consider as the final reading. This method has been

carried out for all experimental measurements and tests of the project.

2.b. Gear train

The angular velocity of the generating rotor ωr is related to the relative

angular velocity of the oscillating weight ϴr by a gear ratio n of the transmission

train depicted in Figure 19. Knowing that the number of tooth of each gear is

Z1 = 76, Z2 = Z4 = 7 and Z3 = 61, the transmission ratio is defined by

Figure 18: Proof mass.


Chapter III 29

ωr = nϴr n =

Z1

Z2

Z3

Z4=

Z1

Z4=

76

7≈ 95 (III. 1)

2.c. Generating rotor

The generating rotor is a permanent magnet made of rare earth cobalt,

whose characteristics are shown in Table 1. First of all, its dimensions were

measured. Hence, its diameter and thickness are respectively dr = 2,6 mm and

lr = 0,4 mm. And its speed in an ordinary running is in the range of 10 000 rpm to

100 000 rpm. The permanent magnet rotates relatively to the stator, and therefore

the magnetic environment of the coil changes inducing a voltage in it according to

Faraday’s law. The following explanation of this section refers to publication

[NASA79].

Figure 20: Rotor and stator.

Figure 19: Gear train.


Chapter III 30

The excitation performance of the magnet and its operating properties rely

on the magnetic circuit installation (Figure 20). The type and size of permanent

magnet is established depending on magnetic requirements, mechanical design

and cost.

Table 1: Characteristics of rare earth cobalt.

The operating point of the magnet chosen is determined with the second

quadrant of the specific B-H curve for achieving a particular flux density in the air

gap. As shown in Figure 21, those graphs also draw curves of permeance ratio

Bm/Hm and energy product BmHm. The best energetic efficiency of a magnet takes

place when its operating conditions coincide with its maximum energy product,

which quantifies the magnetic energy that the permanent magnet supplies. In

addition, following equations are used to dimension the magnet and design the

magnetic circuit. CGS system of units have been used in this section for

simplification, since μ0 = 1 and Hg = Bg.

Figure 21: B-H curve of rare earth cobalt magnet.


Chapter III 31

Designating Hm and Hg respectively for the magnetic field intensity of the

magnet and air gap (in oersted), lm and lg for respective lengths (in cm) and Vf for

the reluctance drop in the ferromagnetic circuit (in gilbert), Ampere’s law sets out

Hm lm = Hg lg + Vf (III. 2)

Furthermore, the cross sectional area of the magnet Am is related with the flux

density in the gap Bg by

Bm Am = BgAgK (III. 3)

where Bm is the flux density in the magnet (in gauss) and Ag the cross sectional

area of the air gap (in cm2). The leakage factor K quantifies the flux lost between

the side of the magnet and the beginning of the magnetic circuit. It is determined

by experimental formulas obtained for usual circuit configurations. In the case the

magnet is situated right next the air gap, the leakage factor is deduce from (III. 4)

K = 1 + 0,67pm

lg

Ag 1,7

0,335lm

0,335lm + lg+

lg

lm

where pm is the perimeter of the magnet cross section. From equations (III.2) and

(III.3) and neglecting Vf, the volume of the magnet is obtained with

Am lm =Bg

2Ag lgK

Bm Hm (III. 5)

Likewise, the permeance ratio expression is attained.

Bm

Hm=

Ag lm K

Am lg (III. 6)

2.d. Generating coil

The generating coil block (Figure 22), which measured side length is

l = 2 mm, is modelled as a voltage source, a resistor and an inductor in series.

Two experiments were carried out to determine the values of the equivalent

electric components. With this purpose, small cables had to be soldered to the


Chapter III 32

terminals of the coil aided by a microscope. The result is shown in Appendix 2 as

well as next experimental electric assemblies.

On one hand, the first experiment consisted of the electrical assembly

illustrated in Figure 23. A voltmeter U and an ammeter A measure respectively

the voltage and current across the coil which are supplied by an intensity source I.

The resistor R1 = 99 kΩ is a protection against a possible high voltage across the

vulnerable micro-wire of the coil. The final readings were V = 33,6 mV and

I = 0,1 mA. Therefore the coil resistance is obtained.

Rc =V

I= 336 Ω (III. 7)

On the other hand, the coil was connected to a precision component

analyser that estimated a coil impedance of L = 191,4 mH.

2.e. Energy conversion interface

The electricity generated in the coil is rectified and stored in a titanium

lithium ion rechargeable battery (Figure 24) whose operating voltage range goes

Figure 23: Rc measurement circuit.

Figure 22: Coil block.


Chapter III 33

from 0,45 V to 2,2 V. This storage unit is able to supply around 6 months of

energy from full charge to stoppage.

Subsequently, the circuit block (Figure 25) is in charge of the control of

voltage and amperage. Using quartz oscillations, it produces a precise electric

signal that is converted into a rotational motion by micro step motor. Finally a

gear train transmits this motion to move the hands and indicate the time. Hence,

the watch consumption is less than 1 μA with 1,55 V supplied from a battery.

Figure 25: Circuit block.

Figure 24: Battery.


Chapter III 34

Rectification

In its very simplest form, an electromagnetic generator is modelled as an

AC voltage source as shown in Figure 26. However, this output is not useful for

most electronic applications. The generator is first connected to a full bridge

rectifier, which consists of four standard diodes connected in such a way that the

voltage reaching the load is always positive, as shown in the graph in Figure 27.

Figure 26: Simple model of a micro-generator.

Figure 27: Voltage output after signal through a full bridge diode rectifier.


Chapter III 35

In the ideal-diode model, the device acts as a perfect conductor with no

voltage drop in the forward direction and acts as an open circuit in the reverse

direction. For a real diode, the output voltage is less than the input voltage due to

a drop across the diode, typically 0.7 V for silicon diodes at room temperature.

In order to provide a relatively stable voltage for electronics, a capacitor is

added to the output terminals of the bridge rectifier (Figure 28). If it is small

enough, the capacitor is charged up to the first peak of the voltage input. The

relationship between the current and voltage in a capacitor can be given by

i = Cdv(t)

dt

So the current is related to the change in voltage and the storage capacity of a

capacitor.

Once the input voltage drops below the voltage stored in the capacitor, the

capacitor slowly discharges until the next peak of the input. As a general rule, the

size of the capacitor required to smooth the voltage is

C =iT

2vr

where i is the average load current, T is the period of the bridge input voltage, and

vr is the peak-to-peak ripple voltage.

Figure 28: Additional capacitor to produce a DC voltage output.


Chapter III 36

The size of the capacitor is typically sized to supply DC to the load.

However, because the current that can be delivered from the generator is very

small, the charge must first be built up on a capacitor or stored in a rechargeable

battery before it can be used.

Current portable electronic devices have different low power or sleep

modes to save energy during times of inactivity. The management of these modes

is very important in relation with an energy harvesting strategy, allowing to refill

the energy reservoir of the system during these periods of low activity. This

means that generally, a discontinuous operation use model is mandatory for the

energy harvesting approach.

Quartz unit

The amplitude of oscillation of a quartz resonator is of the order of a

thousandth of a millimetre. In addition the frequency of oscillation is normally

greater than 10 000 Hz. In our case of study, the oscillator oscillates at a highly

stable rate of 32 768 times per second. This is because the frequency is a function

of the elastic properties of quartz and the size of the crystal used, the frequency

decreasing as the size increases. The maximum size of available good-quality

crystals limits the lower frequency that can be obtained to the value quoted. It is

obvious therefore that mechanical methods cannot be used to detect or maintain

the vibrations of quartz. However, in addition to other useful properties, quartz is

piezoelectric, which enables these functions to be performed electronically.

The direct piezoelectric effect is the generation of electric charge on the

surface of some crystalline materials when they are strained mechanically. The

inverse piezoelectric effect takes place when a crystal is strained as a result of

applying to it an electric field. Piezoelectric materials are not uncommon, but

quartz combines the effect with good chemical and mechanical stability, and with

very low internal frictional losses, and it is therefore ideally suited for use as an

oscillator.

An important fact about both the direct and inverse effects is that they are

linear. This means that the effect is proportional to the cause: in the direct effect,


Chapter III 37

the magnitude of the charge generated is proportional to the strain; in the inverse

effect, the strain is proportional to the field.

If a piece of quartz is set into vibration it is undergoing a mechanical strain

which is varying sinusoidally at the frequency of vibration. As a result of the

direct piezoelectric effect, electric charge is generated at the crystal surfaces, also

varying sinusoidally at the same frequency. If two metal electrodes are deposited

on the surfaces, the charges induce a voltage between them which is proportional

to the charge. The voltages can be detected by electronic means. Vibrations of

quartz can therefore be detected by means of the direct effect.

The inverse piezoelectric effect affords a means of maintaining the crystal

in oscillation. Two metal electrodes are deposited on the crystal surfaces. A

voltage applied between these sets up a field in the crystal, deforming it. If the

voltage between the electrodes varies at the frequency of oscillation of the crystal,

and if the position of the electrodes is chosen in such a way that the deformation

set up by the field is of the same form as that in the vibration, then energy is fed

into the oscillations to overcome frictional loss.

Figure 29: Quartz unit.

The basic electronically maintained quartz crystal controlled oscillator is

shown diagrammatically in Figure 29. A piece of quartz crystal with a natural

resonant frequency at the required oscillation frequency has two pairs of metal

electrodes deposited on its surfaces. The direct piezoelectric effect induces


Chapter III 38

voltages between one pair which are connected to the input of an electronic

amplifier of gain A. The output voltage from the amplifier is fed to the second

pair of electrodes, and maintains the oscillations by the inverse piezoelectric

effect. The gain A of the amplifier is independent of frequency, and the gain β of

the quartz crystal, which is actually considerably less than unity, exhibits a sharp

resonance peak. So the product Aβ exhibits a similar peak and exceeds unity only

over a very narrow frequency range. The use of more sophisticated electronics

makes it possible to dispense with one pair of electrodes.

The quartz controlled oscillator is usually spoken of as an electronic

oscillator. It is perhaps as well to point out that it is really still a mechanical

oscillator, depending on the vibrations of a small piece of quartz, and is merely

electronically maintained.

2.f. Step motor

The step motor converts the electrical signal in to a precise rotational

motion that is transmitted to the hands through the gear train. Current

consumption of this tiny motors is 0,8 μA with a resistance between 1,7 kΩ and

2,1 kΩ.

The frequency divider accepts the signal generated by the quartz oscillator

and reduces its frequency to about 1 Hz to drive the display. It consists essentially

of a long chain of fairly simple circuits called bistables, each of which reduces the

frequency by a factor of five.

The main advance in this part of the watch has been the steady reduction in

its power consumption, which allows the use of higher quartz frequencies and

gives longer battery life. The introduction of a form of integrated circuit

construction called CMOJ; gave the most dramatic improvements here.

Focusing on the analogue display, the method of driving the seconds hand

is important. It is always by means of a small electric motor driven by the output

of the frequency division chain.


Chapter III 39

The stepping motor is driven by pulses of current fed to coils on the stator.

At each pulse, the rotor steps forward through a set angle, the value of which is a

matter of design. A typical design is sketched in Figure 30. The rotor is a

magnetised disc, which has north and south magnetic poles alternately round its

periphery, three of each. It rotates about an axis through its centre, and it is placed

between the poles of the stator, which is energised by a coil wound on it.

Figure 30: Step motor.

When the stator coil is not energised, the stator is not magnetised. In this

situation the rotor takes up one of the two positions shown in Figure 30.a and

Figure 30.b. To see that this is so, displace the rotor slightly as in Figure 30.c and

Figure 30.d. Now the south pole marked S' and the north pole N' are attracted

back to the pole-pieces and the forces of attraction turn the rotor as shown. When

the rotor reaches the positions of Figure 30.a and Figure 30.b, the force on all the

poles is radial and there is no further turning effect. These two positions are

therefore stable equilibrium positions. Work has to be done to move the rotor

away from these positions, for example to change from one to the other.


Chapter III 40

Now suppose the rotor to be in the stable position shown in Figure 31.a

and let a current be sent through the energising coil so that the left-hand stator

pole becomes a north pole. Then this north pole will repel the rotor poles N', N".

If the system were perfectly symmetric, the turning effect of the various forces

would exactly cancel. But the air gap between stator and rotor is not quite

uniform. Therefore the repulsion of N' which is nearer the stator pole, is stronger

than that of N". The rotor therefore starts to turn clockwise. As it does so pole S

moves closer to the stator north pole and is attracted to it. Rotation continues until

the situation in Figure 31.b is achieved, in which the stator north pole is adjacent

to two of the three south poles in the rotor, and the stator south pole is adjacent to

two of the three rotor north poles. The rotor has moved through 60° and is now in

the second stable position, so that if the energising current is removed, it remains

stationary.

A current pulse in the opposite direction will move the rotor through

another 60° to the next stable position. The current drive to the motor has

therefore to consist of pulses of opposite polarity at each of which the rotor turns

1/6 revolution. If the current pulses are separated by 1 second, a ten-to-one

reduction gear gives the correct stepping speed for a seconds hand.

Figure 31: Step motor.


Chapter III 41

3. Rotational electromagnetic micro-generator

In a first approach, Seiko AGS watch is clearly a rotational

electromagnetic energy harvesting and storage device, and it demonstrates that

rotational kinetic motion can be directly used to scavenge power. Free rotation of

the proof mass achieves satisfactorily to eliminate preceding linear displacement

constraints. Moreover, the shape of the mass permits the device to take advantage

of rotational and also linear excitations. Resonant operation is not a requirement,

because excitations in wristwatch application have normally large amplitudes in

comparison with the device size. Following principles of linear harvesting, this

section models and analyses rotational kinetic energy harvesting based on

previous watch explanations depending on different sources of motion. The

development of this entire section is based on paper [YEAT07].

3.a. Non resonant oscillating rotational generator

The rotational energy harvesting device is simplified taking the form of

just a semi-circular mass and damper system, as illustrated in Figure 32. The

angular velocity of the frame Ω (t) and the angular velocity of the proof mass θ (t)

are coupled by an electromagnetic transducer with a damping coefficient D. The

consequent damping torque is then proportional to the relative rotational velocity

between both parts, being expressed as

TD = D Ω (t) − θ (t) (III. 8)

Figure 32: Rotational harvester.


Chapter III 42

Hence, the linear differential equation of motion can be directly raised.

Iθ t = D Ω t − θ t (III. 9)

where I = mRp2/4 is the moment of inertia of the semi-circular mass about the

axis of rotation, with Rp the radius of the proof mass m. Furthermore, the

electrical power generated by the rotational harvester is obtained as

Pe = TD Ω t − θ t = D Ω t − θ t 2

Pe =I2

Dθ (t)2 (III. 10)

Assuming a rotational harmonic excitation, the input and output displacements of

the system are respectively Ω t = Ω0 sin(ωt) and ϴ t = ϴ0 sin ωt + ϕ ,

whose maximum angular acceleration is θ = −θω2. In the same manner as in the

linear model, application of Laplace to equation (III.9) gives respectively the

amplitude and phase functions of the rotational system.

ϴ0

Ω0=

D

D2 + ω2I2 ϕ = cos−1

ϴ0

Ω0 (III. 11)

Thus the average generated power can be rewritten as

Pe =I2ϴ0

2ω4

2D=

IΩ02ω3

2

DωI

1 + DωI

2 (III. 12)

Hence when D = ωI, the maximum power extracted by a non resonant rotational

electromagnetic harvester is

Pe max =mRp

2Ω02ω3

16 (III. 13)

Furthermore, the optimum operating point takes place when ϕ = π / 4 and

ϴ0 = Ω0/ 2 = ϴr0, with ϴr0 the amplitude of the relative displacement

ϴr(t) = ϴ(t) - Ω(t).


Chapter III 43

In this first case, displacement constraint of the power level is overcome

since it is possible to scavenge mass displacements with large amplitudes of

multiple cycles. Hence low mechanical resistance is of vital importance. However

as proved by the optimum operating conditions, this is considered in practice a

rare operating case because the source of excitation have to present even larger

amplitudes. Therefore, resonant operation is required to increase effectiveness.

3.b. Resonant oscillating rotational generator

Resonant condition is required to take advantage of large amplitudes

caused by non internal displacement limitations. Thus a spring k has to be added

to the modelled system. Its applied torque is

Tk = k Ω(t) − θ(t) (III. 14)

And the new equation of motion is then given by

Iθ t = D Ω t − θ t + k Ω t − θ t (III. 15)

As in the linear model, the damping coefficient is divided into the electrical

conversion De and the parasitic losses Dm, since D = De + Dm. Repeating the

analytical process, the generated power obtained for resonant rotational generation

is

Pe =I2Ω0

2ω4

2De

De2

De + Dm 2 (III. 16)

Furthermore, the maximum power is obtained as well when De = Dm.

Pe max =I2Ω0

2ω4

8Dm=

m2Rp4Ω0

2ω4

32Dm (III. 17)

Finally, the relative internal displacement at resonance is

ϴr0 =IΩ0ω

D (III. 18)


Chapter III 44

Hence larger amplitudes are achieved. Then the generated power of

resonant rotational harvesting increases but the frequency dependence, which was

characteristic of linear harvesting, appears. Comparing with linear harvesting,

rotational harvesting takes better advantage of the excitation if amplitudes are

lower than the dimensions of the device. This method gives then the possibility of

high power densities. However, it requires improvements on large angular ranged

springs and low parasitic losses.

3.c. Continuous rotational generator

Assuming now a rotational continuous excitation, the angular velocity of

the frame is taken as constant. Considering in addition the gravitational torque Tg

acting against the damping torque TD, the equation of motion becomes non linear.

Iθ t = D Ω − θ t − mgRg cos ϴ t (III. 19)

where g is the acceleration of gravity and Rg = 4R/3π is the distance from the

center of mass of the semi-circular proof mass to the rotational axis. A solution

can be obtained if the proof mass is considered immobile since θ (t) = θ (t) = 0.

D =mgRg cos ϴ(t)

Ω (III. 20)

Therefore, when θ(t) = 0, the power generated by a gravitational continuous

rotational harvester with a static mass is

Pe = D Ω − θ (t) 2

= mgRgΩ (III. 21)

In a similar way of a conventional generator, gravity force orients proof

mass downwards while the frame is forced to rotate. In this case, the device

presents a high dependence on orientation.


Chapter III 45

4. Conclusion of chapter III

After detailing rotational electromagnetic energy harvesting methods, it

can be concluded that Seiko AGS technology overcomes linear harvesting

constraints and it permits to improve power levels, scavenging oscillating or

continuous rotational sources of excitation. However, in TPMS application, the

studied harvesting device will be mounted outside the wheel axle of rotation.

Therefore, the performance of the rotational harvester need to be implemented in

its operating place because of the possible appearance of other influence forces.


46

CHAPTER IV

Powering TPMS Sensors


Chapter IV 47

Chapter IV

Powering TPMS Sensors

The studied Seiko rotational harvester scavenges power from the relative

displacement between its frame and proof mass for oscillating and continuous

excitations. The objective of the project is to power wireless sensors of TPMS.

Therefore the harvesting system has to be situated inside the tyre near the sensor.

In this concrete application, the harvesting device is subjected to a continuous off-

center rotation with high acceleration peaks which is determined by the wheel

motion. Dynamics and motion of the studied rotational harvesting device have

consequently to be analysed and implemented when operating in its application

place in a vehicle wheel.

1. Double pendulum

When the rotational harvester is operating in its application, the centre of

rotation of its proof mass is distanced a distance r from the centre of rotation of

the frame. Thus the path of the device is circular. At first glance, this axis

misalignment is approached by the mechanical problem of a double pendulum.

The nomenclature used is specified in Figure 33.

Figure 33: Double pendulum.


Chapter IV 48

The mass position is expressed as

x = r sin Ω + l sin(θ)

y = −r cos Ω − l cos(θ) (IV. 1)

The kinetic energy and the potential energy of the problem are respectively

E =1

2mv2 =

1

2m r2Ω 2 + l2ϴ 2 + 2rl Ω ϴ cos Ω − ϴ (IV. 2)

U = mgy = −mgr cos Ω − mgl cos(θ) (IV. 3)

Hence the Lagrangian L = E - U of this dynamical system results in (IV. 4)

L =1

2mr2Ω 2 +

1

2ml2ϴ 2 + mrl Ω ϴ cos Ω − ϴ + mgr cos Ω + mgl cos(θ)

The Euler-Lagrange differential equation for the angle θ is defined as

d

dt ∂L

∂ϴ −

∂L

∂ϴ= 0 (IV. 5)

and its terms are given by

∂L

∂ϴ = ml2ϴ + mrlΩ cos Ω − ϴ (IV. 6)

d

dt ∂L

∂ϴ = ml2ϴ + mrlΩ cos Ω − ϴ − mrlΩ sin Ω − ϴ Ω − ϴ (IV. 7)

∂L

∂ϴ= mrl Ω ϴ sin Ω − ϴ − mgl sin(θ) (IV. 8)

As a result, the equation of motion for the double pendulum through the angle θ is

lθ + rΩ cos Ω − θ − rΩ 2 sin Ω − θ + g sin(θ) = 0 (IV. 9)

In the same manner, terms of Euler-Lagrange differential equation for the

angle Ω are

∂L

∂Ω = mr2Ω + mrlϴ cos Ω − ϴ (IV. 10)

d

dt ∂L

∂Ω = mr2Ω + mrlϴ cos Ω − ϴ − mrlϴ sin Ω − ϴ Ω − ϴ (IV. 11)


Chapter IV 49

∂L

∂Ω= −mrl Ω ϴ sin Ω − ϴ − mgr sin(θ) (IV. 12)

Therefore the second equation of motion that describes the motion of the mass m

is

rΩ + lθ cos Ω − θ + lθ 2 sin Ω − θ + g sin(Ω) = 0 (IV. 13)

From equations (IV.9) and (IV.13), it is learned that the double pendulum

dynamical system is non linear and chaotic. Therefore it is impossible to obtain an

analytical solution to the problem.

As in previous on-axis configuration, the proof mass is always subjected

to the gravitational force. However, the off-axis situation introduces the

centrifugal force. This rotational force increases rapidly with the angular velocity

of the frame Ω . As a result, the mechanical problem can be solved from two

perspectives of simplification depending on the ratio between gravitational force

and centrifugal force exerted on the mass. On one hand, at low rotational speed Ω

or/and small offset dimension r, just the gravitational force can be considered to

influence the mechanical system. On the other hand, at high rotational speed

or/and high offset distance, the gravitational force is neglected and just the

centrifugal force is considered part of the dynamic problem. Both operating

solutions are analysed next.

2. Gravitational electromagnetic micro-generator

This section characterise the behaviour of the studied rotational energy

harvesting device as a gravitational electromagnetic generator. Power is

scavenged thanks to the relative position between frame and mass due to the work

done by the gravitational force trying to keep the proof mass oriented downwards

(Figure 33). Results of this study refer to paper [TOH_08].

In this situation, the rotational centrifugal force is neglected due to a very

low velocity of excitation. Thus the device is operating as a continuous rotational


Chapter IV 50

generator. At a constant excitation speed Ω , the mass stabilizes at a certain angle

ϴ where the gravity torque Tg matches the damping magnetic torque TD, as

demonstrates equation (III.19). Therefore, the maximum power generated is given

by equation (III.21). Furthermore, knowing that keΩ is the electromotive force

where ke is the electromagnetic constant, the output power generated is given by

Pe = keΩ

2

2(R + Rc) (IV. 14)

The optimal power generated to the external load R is obtained when this last

equals the coil resistance Rc. When R = Rc,

Pe max = keΩ

2

4Rc (IV. 15)

Total power of the harvesting device is then twice the power generated to the

external load. Thus from equations (III.21) and (IV.15), the maximum velocity of

rotation before the proof mass m flips over is

Ω max =2mgRgRc

ke2 (IV. 16)

Knowing that the electromagnetic torque is kei, this condition can be also

expressed by a maximum current on the coil,

imax =mgRg

ke (IV. 17)

The rotational harvester will continue to scavenge energy above this limit but the

power level obtained cannot be determined by this first case of study.

Finally, adding the term representing the damping generation, the

complete equations of motion describing the Seiko rotational harvesting device on

its application are (IV. 18)

lθ + rΩ cos Ω − θ − rΩ 2 sin Ω − θ + g sin(θ) +ke

2(θ −Ω )

2ml2Rc= 0

rΩ + lθ cos Ω − θ + lθ 2 sin Ω − θ + g sin(Ω) +ke

2(θ −Ω )

2ml2Rc= 0


Chapter IV 51

3. Centrifugal electromagnetic micro-generator

From another point of view, the studied rotational harvester can be

modelled as well as a centrifugal electromagnetic generator. High velocities of

excitation justify neglecting the gravitational force when compared with the

centrifugal force. Following dynamic analysis are based on publications

[GENT05] and [CONR05].

A first approximation models the current dynamic problem as a rotating

pendulum just subjected to a centrifugal force Fc considering constant the rotation

of the frame Ω . In other word, the mechanical system turns into a freedom

pendulum of length l attached to the outside r of a rotating disc. Nomenclature

used is defined in Figure 34, where ϴr is now the relative angle between

pendulum and frame.

Figure 34: Rotating pendulum.


Chapter IV 52

The position of the mass m located on the point P is

OP =

r cos Ω t + l cos(ϕ) cos(Ω t + ϴr)

r sin Ω t +l cos(ϕ) sin(Ω t + ϴr)

l sin ϕ

(IV. 19)

The velocity of point P is then obtained by differentiation, (IV. 20).

VP =

−rΩ sin Ω t −l ϕ sin ϕ cos(Ω t + ϴr) − l (Ω + ϴr )cos(ϕ) sin(Ω t + ϴr)

r Ω cos Ω t −l ϕ sin ϕ sin(Ω t + ϴr) + l (Ω + ϴr )cos(ϕ) cos(Ω t + ϴr)

l ϕ cos ϕ

Hence the kinetic energy of the mass m is achieved as E =1

2m VP

2

(IV. 21).

E =1

2m r2Ω 2 + l2ϕ 2 + l2(Ω + ϴr

)2cos2 ϕ

− 2rlΩ ϕ sin ϕ sin ϴr

+ 2rlΩ (Ω + ϴr )cos(ϕ) cos(ϴr)

The first equation of motion related with the plane of rotation of the disc xy is

determined by Euler-Lagrange neglecting the potential energy.

d

dt ∂E

∂ϴr −

∂E

∂ϴr= 0 (IV. 22)

whose terms are given by

∂E

∂ϴr = m l2 Ω + ϴr

cos2 ϕ + rlΩ cos(ϕ) cos(ϴr) (IV. 23)

d

dt ∂E

∂ϴ = m l2ϴr

cos2 ϕ

− 2l2ϕ (Ω + ϴr )cos(ϕ) sin ϕ

− rlΩ ϕ sin ϕ cos(ϴr)

− rlΩ ϴr cos(ϕ) sin ϴr (IV. 24)

∂E

∂ϴ= m −rlΩ ϕ sin ϕ cos(ϕ) − rlΩ Ω + ϴr

cos(ϕ) sin ϴr (IV. 25)


Chapter IV 53

Doing the same Lagrange calculations for the second equation of motion related

with the plane xz perpendicular to the disc, the dynamic equations describing the

motion of the rotating pendulum are (IV. 26)

lϴr cos2 ϕ − 2l2ϕ (Ω + ϴr

)cos(ϕ) sin ϕ + rΩ 2 cos(ϕ) sin ϴr = 0

lϕ + l Ω + ϴr

2cos(ϕ) sin ϕ + r Ω 2 sin(ϕ) cos ϴr = 0

This solution is clearly non linear. However, assuming the condition of small

oscillations, the equations of motion can be linearized.

lϴr + rϴrΩ

2 = 0

lϕ + r + l ϕΩ 2 = 0 (IV. 27)

These dynamical equations correspond respectively with the motion of a

pendulum of length l subjected to a constant acceleration force of rΩ 2 and

(r + l)Ω 2. Hence, natural frequencies of the pendulum referred to the rotating

frame in the plane of rotation xy and its perpendicular plane xz are

ωϴr=

rΩ 2

l= Ω

r

l

ωϕ = (r + l)Ω 2

l= Ω 1 +

r

l (IV. 28)

On one hand, it can be deduced that frequency ωϕ is always larger than the

angular velocity of excitation. On the other hand, two possible situations take

place in the plane xy. If l < r, frequency ωϴr is also larger than the excitation

velocity; whereas if l > r, the frequency of oscillation ωϴr is lower than the

excitation speed. Thus, frequencies of both planes tend to match in the case the

pendulum length l is really small in comparison with the disc radius r. Whereas in

a very long pendulum condition, plane xy do not present any oscillation and plane

xz tend to match the excitation velocity, in other words, this means that the mass

displacement describes a circle inclined with respect to the spin axis z.


Chapter IV 54

Assuming condition of small oscillations, the component of the centrifugal

force Fc perpendicular to segment CP in the plane xz (Figure 34), which acts as a

restoring force and generates the oscillating motion, is given by

Fc sin(ϕ) ≈ m(r + l)ωϕ2ϕ (IV. 29)

Comparatively, the respective restoring component of the centrifugal force in the

plane xy is smaller as

Fc sin(ϴr − α) ≈ m r + l ωϴr

2 ϴr − α (IV. 30)

Figure 35: Rotating pendulum with one plane of oscillation.

Moreover, Seiko rotational harvester is forced to oscillate in just a plane.

Thus, if the rotating pendulum already described is constrained to oscillate in a

plane which makes an angle ψ with the axis of rotation z, the angles describing the

movement of P become

ϴr = ϴp sin ψ

ϕ = ϴp cos ψ (IV. 31)

where θp is the oscillating amplitude of the pendulum in its plane, as illustrates

Figure 35.


Chapter IV 55

Substituting these functions in equation (IV.21), the kinetic energy of the mass

becomes

E =1

2m r + l 2Ω 2 + l2ϴp

2

− lϴp2Ω 2 r + l cos2 ψ

+ 2l(r + l)Ω ϴp sin ψ (IV. 32)

Repeating the same analytic process, the linearized equation of motion is

lϴp + ϴpΩ

2 r + l cos2 ψ = 0 (IV. 33)

And the respective natural frequency obtained is

ωn = Ω r

l+ cos2 ψ (IV. 34)

The device is then operating as an oscillating rotational generator because the

natural frequency ωn equals zero when the excitation speed vanishes and the

restoring force is just caused by the centrifugal field. Hence, its maximum power

generated is given by equation (III.13).

Furthermore, the harvesting device can be improved at high excitation

speeds as a resonant oscillating rotational generator attaching a spring of stiffness

k between the mass and frame, which adds a restoring force toward the radial

direction. Introducing the potential energy of the spring U =1

2kϴp

2 on the

dynamic system and repeating the analytical demonstration, the linearized

equation of resonant motion is

ml2ϴp + mlΩ 2 r + l cos2 ψ + k ϴp = 0 (IV. 35)

And the natural frequency is then

ωn = k

ml2+ Ω 2

r

l+ cos2 ψ (IV. 36)

Therefore the device improves its maximum power generated given by equation

(III.17).


Chapter IV 56

According to previous gravitational harvesting method, Seiko rotational

harvester is definitely restricted to be oriented in plane xy (ψ=90º) with the

purpose of generating notably more power at very low excitation speeds.

Moreover, the device will take significantly more advantage of the inertial

trajectory in its real application where peaks of acceleration exist, and therefore

the maximum generated power will greatly increase. Consequently, the rotational

harvester has definitely to be embedded in the same plane of the rotational

excitation at a radius r in the interior of the tyre. This specific operation is

analysed in detail next.

The current dynamical situation is now viewed from a relative reference ij

attached to the frame rotation, as illustrates Figure 36. From this point of view, the

frame of the device is stationary. The centrifugal force is depicted as a radial

Figure 36: Rotating pendulum from reference ij.


Chapter IV 57

conservative potential field whose source is the center O of the wheel. And the

gravitational force, which rotates at velocity Ω with regard to reference ij, adds a

sinusoidal excitation of amplitude g and frequency Ω to the mass m in a given

position. This section takes advantage of the fact that at a certain excitation

velocity the centrifugal acceleration is large enough to take efficient advantage of

the rotational kinetic energy neglecting comparatively small disturbances caused

by the internal work done by the gravitational acceleration.

From Figure 36, the restoring torque caused by the centrifugal field is

expressed as

Tc = −l sin θr − α Fc (IV. 37)

And the value of the centrifugal force is

Fc = mΩ r cos(α) + l cos(θr − α) (IV. 38)

From equations (IV.37), (IV.38) and the geometrical expression

r sin(α) = l sin θr − α (IV. 39)

the oscillating restoring torque becomes

Tc = −mrlΩ 2 sin(θr) (IV. 40)

In this optimized operating configuration, the natural frequency of the

pendulum is given by ωn = Ω r/l from equation (IV.34) with ψ=90º. In this case

of application, the radius r of the wheel rim is much larger than the pendulum

length l. Therefore the natural frequency of oscillation is always going to be

proportional and higher than the frequency of excitation. This can then lead to

small displacement amplitudes and consequently to insufficient power levels.

However, the actual semi-circular proof mass has to be included as a

distributed pendulum on this dynamical model. Thus the natural frequency of the

distributed pendulum in a centrifugal field becomes

ωn = m(rΩ 2)Rg

I= Ω

mrRg

I (IV. 41)


Chapter IV 58

Therefore, the distributed rotational harvester can be installed to operate in

resonant conditions if the distance between the center of gravity and the rotational

harvesting axis O satisfies

Rg =I

mr (IV. 42)

The natural frequency equals then the frequency of excitation at all operating

conditions. As a result, the distributed proof mass permits to achieve larger

angular displacements.

Finally, an electromagnetic generator system is used to scavenge the

energy at a point of transmission T through a gear train of rotational inertia IT, as

shows Figure 37. Thus the velocity of the generating rotor ωr is related with the

velocity of the distributed pendulum ϴr by the transmission ratio n (equation

III.1). Therefore, the kinetic energy of the entire system is given by

E =1

2I2 +

1

2IT

2 =1

2 I + ITn2 ϴr

2 (IV. 43)

Figure 37: Rotating pendulum with gear train.


Chapter IV 59

where the equivalent rotational inertia is defined as

I′ = I + ITn2 (IV. 44)

The natural frequency of the system becomes then

ωn = Ω mrRg

I′ (IV. 45)

And the condition to achieve resonance at every excitation frequency is

I′ = mrRg (IV. 46)

Consequently, this complete dynamic implementation of the studied rotational

harvester during operation solves motion constraints of previous configurations. It

is then proved that the device can be designed for achieving power levels required

by a specific application.

4. Conclusion of chapter IV

It can be concluded from the previous dynamic analysis that Seiko

rotational electromagnetic harvesting system can be design to scavenge reasonable

power levels for TPMS application. The geared distributed pendulum has to be

oriented in the vertical plane of wheel rotation. Thus the main operating method

scavenges the energy from oscillating motion caused by the centrifugal field. The

maximum energy generation demands a resonant response to constant excitation

speeds for achieving the largest amplitudes of the internal magnetic rotor motion.

At very low speeds, the system scavenges the energy from the opposition of the

gravitational field.


60

CHAPTER V

Experimentation


Chapter V 61

Chapter V

Experimentation

It has been analytically demonstrated that studied Seiko harvesting

technology can be redesigned to power other electronic packages. This is

confirmed by a recent innovative design based on watch AGS components which

supplies power to sensors mounted on marine mammals, scavenging the motion of

the animal. Since the sensor package is larger, as shows Figure 38, the levels of

power were scaled up between 5 mW and 10 mW. This chapter aims to test Seiko

wristwatch to obtain experimental data of its power potential.

1. Experimental starting

Seiko wristwatch testing called for being able to spin the mass spindle at a

specific controlled angular speed. With this purpose, an assembly was devised to

replace the mechanical function of the proof mass.

Figure 38: Marine mammal package.


Chapter V 62

Firstly, a screw of diameter 0,8 mm was retouched to replace the one

holding the proof mass to the ball bearing and to create a point of joining for the

assemblage. A second piece was manufactured from a small aluminium block

with the objective of holding the screw, and allowing to spin the center of rotation

of the device. The assemblage of both parts was made by strong glue for metals

with high strain resistance. Drawings of both pieces are attached in Appendix 3.

Tolerances were determined considering that just dimensions concerning the

assemblage join were critical. At the end, the final assembly, which is shown in

perspective in Figure 39, was installed on device, and the result is photographed

in Figure 40.

A first experimental approach was made to check the assembly. Hence the

experimental device was turned by hand connected to a voltmeter U, as draw in

Figure 41. Photographs of this electrical assembly and following experiments are

included in Appendix 4. As a result, the running of the experimental device was

confirmed, and the maximum voltage obtained by hand was Vmax = 1,1 V.

Figure 40: Experimental device.

Figure 39: Experimental assembly.


Chapter V 63

2. Experiment 1

Subsequently, the experimental device was installed in a small lathe with

the purpose of spinning its handle piece at a constant known velocity.

Furthermore, the same previous circuit of Figure 41 was assembled again. The

experiment was about reading rms voltage measurements at steady state as

velocity of excitation was increased gradually. The rotational speed of the lathe

was precisely determined using a laser digital tachometer (Appendix 4).

Consequently, the open-circuit voltage of the device was obtained with respect to

constant excitation speed Ω in rpm. Resulting curve is illustrated in Figure 42.

Figure 42: Open-circuit generated voltage.

0

5

10

15

20

25

30

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Vo

ltag

e (

V)

Speed (rpm)

Figure 41: Voltage measurement circuit.


Chapter V 64

As observed in the graph, it can be concluded that the output voltage and the

angular velocity of the source are linearly related by

V = 0,01482π

60Ω = 0,00155Ω (V. 1)

3. Experiment 2

This second experiment aims for characteristic operational curves of Seiko

generator with respect to constant excitation speed. The lathe was then used again

as source of excitation. As demonstrated, the maximum power generation occurs

when the coil resistance Rc matches with the external load R. Thus the circuit of

Figure 43 was assembled using R2 = 327 Ω , an ammeter A and a voltmeter U

(Appendix 4).

Figure 43: Generation measurement circuit.

Along the experiment, rms current and rms voltage measurements were

read at a steady state as the lathe spindle speed was increased gradually. As a

result, curves of intensity and voltage generated by the device with respect to the

excitation speed were obtained, and drawn respectively in Figure 44 and Figure

45. Consequently, the maximum average power delivered by Seiko device was

directly deduced, since P = VI. This last graph is shown in Figure 46.


Chapter V 65

Figure 44: Intensity generated.

Figure 45: Voltage generated.

0

0,5

1

1,5

2

2,5

3

3,5

0 200 400 600 800 1000 1200 1400 1600

Inte

nsit

y (

mA

)

Speed (rpm)

0

0,2

0,4

0,6

0,8

1

1,2

0 200 400 600 800 1000 1200 1400 1600

Vo

ltag

e (

V)

Speed (rpm)


Chapter V 66

Figure 46: Power generation.

It can be observed that at high frequencies of excitation the device

generating behaviour becomes constant. Voltage and reactance of the harvesting

generator are proportional to the frequency of excitation. And their values are

given respectively by equation (V.1) and X = LΩ . Therefore the output

alternating current is expressed as

I =V

Z=

0,00155Ω

Rc + R 2 + LΩ 2

(V. 2)

Thus the generating saturation observed appears because the resistance term can

be neglected in comparison with the much higher inductance term at high speeds

of excitation. Hence, V and Z are proportional to the excitation speed at high

frequencies, and consequently its quotient I is constant.

4. Conclusion of chapter V

It has been proved that Seiko rotational harvester achieves a maximum

power around 3 mW. This power level is enough to feed a common TPMS sensor

0

0,5

1

1,5

2

2,5

3

3,5

0 200 400 600 800 1000 1200 1400 1600

Po

wer

(mW

)

Speed (rpm)


Chapter V 67

package and permit its RF transmissions. However, it has to be determined if the

speed levels required to obtain that generation are reached while the harvester

operates in its application. A numerical discussion has then to be done using

previous experimental curves, dynamic formulations and scavenging methods

described about Seiko rotational electromagnetic generator.


68

CHAPTER VI

Results


Chapter VI 69

Chapter VI

Results

This chapter presents results obtained from calculations done based on

rotational energy harvesting theory and experimental information which have

been set out along this paper. Next numerical discussion quantifies then the

operation of Seiko rotational harvester when mounted for TPMS application.

1. Electromagnetic circuit

The magnet shape is simplified to a square of side dr and thickness lr, and

the radial distance of the air gap between the rotor and the magnetic circuit is

estimated around eg = 0,1 mm. Thus knowing that Am = drlr, pm = 2lm + 2lr and

lg = 2eg, dimensional parameters of the permanent magnet and air gap are

presented in Table 2.

dr (cm) lr (cm) lm (cm) Am (cm2) pm (cm) lg (cm) Ag (cm

2)

0,26 0,04 0,26 0,0104 0,6 0,02 0,0104

Table 2: Magnetic circuit dimensions.

It is assumed that the permanent magnet, whose properties are shown in

Table 1, is a samarium cobalt rare earth magnet (SmCo) whose grade is YX18T. It

is considered that this magnet operates at maximum energy product conditions

BmHm. Its point of operation can then be situated in the demagnetization curve of

Figure 21. Consequently, its permeance ratio BmHm, flux density Bm and flux

intensity Hm are graphically deduced. The resulted operating properties of the

rotor are resumed below in Table 3.


Chapter VI 70

BmHm (G.Oe) Bm/Hm Bm (G) Hm (G)

1,80E+07 1 4243 4243

Table 3: Operational properties of the permanent magnet.

Furthermore, the leakage factor is obtained from equation (III.4). And finally the

flux density on the air gap Bg is calculated from equation (III.3). Those final

results are shown in Table 4.

K Bg (G) Bg (T) B (T)

2,13 1993 0,1993 0,1993

Table 4: Air gap results.

Neglecting reluctance in the ferromagnetic circuit Vf = 0, it can be

concluded that the magnetic field B going through the coil, with the permanent

magnet YX18T designed for a maximum harvesting generation, is equal to the

value of flux density on the air gap Bg (Table 4).

2. Oscillating operation

Firstly, common vehicle dimensions have been chosen for the external

radius of a wheel rmax and also its rim radius r, where the energy harvesting device

is placed. Those parameters are presented together with dynamic properties of the

proof mass in Table 5.

r (m) rmax (m) m (kg) Rp (m) Rg (m) I (kg.m2)

0,2032 0,205 0,0048 0,0135 0,00573 2,19E-07

Table 5: Parameter of wheel and oscillating weight.

Moreover, the vehicle speed v and the corresponding wheel speed Ω are related

by

Ω =v

3,6rmax (VI. 1)


Chapter VI 71

Hence, the acceleration ϴr and restoring torque Tc of the proof mass for a certain

relative angle ϴr can be calculated respectively from equations (IV.27) and

(IV.40). Furthermore, the natural frequency of the mass wn, and consequently the

rotor frequency wr are deduced from equations (IV.41) and (III.1). Those

calculations have been done for ϴr = 30º until v = 120 km/h, and results are

shown in Figure 47, Figure 48, Figure 49 and Figure 50 below.

Figure 47: Natural frequency wn and wheel speed Ω with respect to vehicle velocity.

Figure 48: Rotor frequency wr regarding v.

0

1000

2000

3000

4000

5000

6000

7000

8000

0 20 40 60 80 100 120

Fre

qu

en

cy (

rpm

)

v (km/h)

Natural frequency Wheel velocity

0

100000

200000

300000

400000

500000

600000

700000

800000

0 20 40 60 80 100 120

Fre

qu

en

cy o

f ro

tor

(rp

m)

v (km/h)


Chapter VI 72

Figure 49: Mass acceleration 𝚹𝐫 regarding v.

Figure 50: Mass restoring torque Tc regarding v.

Furthermore, the ratio of the centrifugal acceleration rΩ 2 and the

gravitational acceleration g = 9,8 m/s2 can be inferred with respect to the vehicle

speed. As confirms Figure 51, the hypothesis of neglecting the gravitational force

in comparison with the centrifugal field is perfectly justified. At 5 km/h both

accelerations are equal. At 10 km/h the centrifugal acceleration is already 4 times

higher than the gravity, and at 20 km/h the quotient increases to 15. Therefore,

0

100000

200000

300000

400000

500000

600000

0 20 40 60 80 100 120

Ac

ce

lera

tio

n o

f ϴ

r (r

ad

/s2)

v (km/h)

0

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0 20 40 60 80 100 120

Res

tori

ng

to

rqu

e (

Nm

)

v (km/h)


Chapter VI 73

apart from very low speeds of the vehicle, Seiko rotational generator operates

definitely as an oscillating energy harvesting device.

Figure 51: 𝐫Ω 𝟐/g ratio regarding v.

Secondly, a Matlab model of the device operation in TPMS application has

been programmed with the purpose of obtaining the oscillating amplitude of the

proof mass from non linear equations of motion (IV.18) when Ω = 0. The

electromagnetic constant ke used has been estimated around 0,04 Vs/rad from

equation (IV.15). Consequently, the number of turns N of the coil results 100

from equation (II.7). Simulations of this model make a distinction again between

the stationary motion of the weight at very low speeds of the wheel and its more

common oscillating displacement.

Three different behaviours of the weight are then observed. At very low

speeds the harvesting method changes because of the mass stationary motion, and

therefore this particular case will be detailed on next section. From v = 5 km/h

(Ω = 65 rpm), where gravity and centrifugal force coincide, until v = 16 km/h

(Ω = 207 rpm), where centrifugal field is just one order of magnitude higher, a

zone of motion transition between stationary and sinusoidal large oscillations

takes place. In this particular zone, the mass response to a constant excitation

0

100

200

300

400

500

600

0 20 40 60 80 100 120

Ra

tio

ac

en

t./g

v (km/h)


Chapter VI 74

speed progresses from random oscillations with small amplitudes, as the example

of Figure 52, to sinusoidal oscillations with progressively less dampening effect.

Figure 52: Mass relative angle ϴr(t) for v= 6 km/h.

Above those speeds of excitation, the mass motion follows large oscillating

amplitudes with high natural frequencies, as for instance the case of Figure 53.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-250

-200

-150

-100

-50

0

50

Time (s)

Rela

tive a

ngle

(deg)



Chapter VI 75

Figure 53: Mass relative displacement ϴr for v= 60 km/h.

Moreover, Figure 46 demonstrates that for wheel velocities Ω higher than

160 rpm (v = 12 km/h), Seiko generator produces more than 2 mW. And above

600 rpm (v = 47 km/h), its generation stabilizes around 3 mW. As a result, it is

demonstrated that Seiko harvester is surely able to power a TPMS sensor at a

constant vehicle speed higher than 15 km/h, scavenging large oscillations of the

proof mass.

3. Continuous operation

At very low vehicle speed under 5 km/h the gravitational acceleration is

higher than the centrifugal force, and therefore the proof mass tends to be

stationary. The maximum velocity before flip-over is confirmed by equation

(IV.16). Hence the relative angle of the weight increases infinitely, as

demonstrated the model in example of Figure 54.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08-180

-160

-140

-120

-100

-80

-60

-40

-20

0

Time (s)

Rela

tive a

ngle

(deg)



Chapter VI 76

Figure 54: Mass relative displacement ϴr for v= 2 km/h.

Thus the Seiko generator operates in this zone as a continuous energy harvesting

device. The power generated on this situation is then calculated from equation

(III.21). Results, which are presented in Figure 55, show that the power level

along this behaviour is always under 2 mW. As a result, it is demonstrated that at

very low speeds, in which the mass oscillating motion do not occurs, the power

generation is not enough to feed directly a TPMS sensor package.

0 0.5 1 1.5 2 2.5-50

0

50

100

150

200

250

300

350

Time (s)

Rela

tive a

ngle

(deg)



Chapter VI 77

Figure 55: Power generated under v = 5km/h.

All estimations of this project have been done considering constant the

wheel velocity Ω because of the appearance of the non linear chaotic behaviour.

In real application, the introduction of very high accelerations of the wheel mostly

at speeds lower than 15 km/h will increase the achieved power level. Hence,

generation difficulties of the zone of transition between 5 km/h and 15 km/h will

be certainly overcome. Furthermore, the addition of an energy processor and

storage interface will make available for use the energy harvested under 2 mW at

very low vehicle speeds. Finally, it has to be specified that an initial acceleration

perturbation is essential to provoke the required oscillating motion at high speeds.

If accelerations were not considered, the relative angle ϴr of proof mass would

stay nil along cycles at very high speeds, and no power would be generated. In

other words, the mass would always maintain a radial orientation, rotating at the

same speed of the wheel. Therefore, wheel perturbations and accelerations are

fundamental for scavenging suitable power levels from this environment.

Finally, the theoretical power levels discussed in this section have been

confirmed by a PSpice model built with ICES software. The simulations have

been carried out with the characteristics parameters defined for Seiko device.

Appendix 5 present the model utilized.

0

0,0002

0,0004

0,0006

0,0008

0,001

0,0012

0,0014

0,0016

0,0018

0,002

0 1 2 3 4 5

Po

we

r g

en

era

ted

(W

)

v (km/h)


Chapter VI 78

4. Conclusion of chapter VI

It can be concluded that Seiko current wristwatch is prepared for

scavenging enough power for TPMS sensors above 15 km/h. A device redesign

will be able to overcome generation problems at low speeds trying to eliminate the

gravitational harvesting method introducing oscillating resonance at all operating

conditions. As in all harvesting devices, a storage system will manage and make

the most of zones of poor harvesting power. Moreover, optimizing as much as

possible the power needed by a TPMS wireless sensor system through power

management techniques is the first approach before improving a compact design.


79

CHAPTER VII

Miniaturization


Chapter VII 80

Chapter VII

Miniaturization

In the centimetre scale, the experimented wristwatch system can deliver

milliwatts required by sensor application through rotational motion conversion. A

micro-electro-mechanical system (MEMS) is the integration of mechanics and

electronics on a common silicon substrate. Thus electromechanical devices are

produces by a micro-fabrication technology that specifically adds structural layers

or etches away parts of a silicon wafer. Therefore, if miniaturization of Seiko

rotational electromagnetic device could be achieved, it would be possible to

develop a complete autonomous TPMS sensing system.

1. Scaling considerations

Dimensional factors of the harvesting device have to be discussed to

determine if the adoption of miniaturization design, whose cost is supported by

the commercial implication of the project, is accurate. Some parameters have to

be considered to keep the power generated at the level required by the application

as the device scale decreases. Even so, it do not exist any conclusive theory or

experimental studies about miniaturized rotational generators under a diameter

lower than 5 mm. Therefore, issues as dynamics of high rotation, winding

resistance or magnetism of permanent magnet are unexplored at those

microscopic levels.

It is defined that A is a characteristic length of the rotational harvesting

device. On one hand, permanent magnet magnetization Bm and consequently the

electromagnetic constant ke scale as A2. On the other hand, coil resistance Rc


Chapter VII 81

scales as 1/A. As a result, the power generated at a constant rotation scales rapidly

as A5. These scaling considerations have been confirmed with the PSpice model.

Since the magnet size has to remain constant as the device decreases to

maintain power levels, it is feasible to expect an optimization of the generator in

the millimetre scale providing a significant reduction of volume and mass.

Consequently, the objective of the design is to minimize the magnetic

degradation. Furthermore, the second challenge is the fabrication of high

performance miniaturised windings capable to make the most of the magnetic

field received.

2. Design proposal

Recent experiments about high speed permanent magnet generators make

an important progress in this subject. The experimental work that is going to be

summarized in this section is presented in paper [HERR08]. A three phase stator

winding pattern of four poles and six turn per pole has been developed

maximizing the amount of copper of the given volume under a magnet of diameter

2 mm. This flat coil technology is illustrated in Figure 56.

Figure 56: Stator winding pattern.

Its linear open-circuit voltage curve shows that a voltage of amplitude 6,3 mVrms,

is achieved when the coil is excited by a 2 mm SmCo rotor spinning at


Chapter VII 82

72 000 rpm. And the maximum open-circuit voltage obtained achieves 120 mVrms.

Furthermore, the maximum single phase generated power is 2,2 mW at

392 000 rpm across a resistive load of 1,8 Ω, which corresponds with a three

phase power of 6,6 mW, as is illustrated in the experimental graph of Figure 57. It

can be then deduced that a direct consequence of miniaturization is the

requirement of higher rotational speed of excitation, because the installation of a

multiplier gear train become impossible.

Figure 57: Power regarding rotor speed.

Moreover, paper [TOH_08] suggests an interesting millimetre-scaled

design of a rotational electromagnetic harvester, which is depicted in Figure 58.

Combining this new winding technology with the proposed structure, the

experimental output power will be doubled, because two stator coils could be

installed. Therefore, this harvesting device would be sufficiently capable to

deliver the power needed by TPMS sensors.

In this design, high rotational speeds require a strong mechanical structure

and low-loss stable bearing capable to maintain the exact air gap under the

dynamic stress exerted. Micro-ball bearing technology is developed accordingly

in the experimental purpose of paper [GHAL08]. The main problem is how to

obtain high rotational speeds demanded from the rotor with a miniaturized mass.

And therefore future experiments have to be done to investigate miniaturised

rotational dynamics.


Chapter VII 83

Figure 58: Miniaturisation proposal.

3. Conclusion of chapter VII

In conclusion, current promising results indicate that a miniaturized

rotational electromagnetic generator is a reasonable and feasible approach for

generating milliwatts in a millimetre scale. Future experimentation will surely

overcome present ignorance and limitations about rotational micro-harvesting, and

consequently achieve the design of a new autonomous TPMS sensor package

(Figure 59) that will provide eagerly awaited environmental, safety and economic

advantages.

Figure 59: Autonomous TPMS.


84

CHAPTER VIII

Conclusions


Chapter VIII 85

Chapter VIII

Conclusions

Low pressure tyres cause important safety concerns, and Direct TPMS is

hugely beneficial alleviating those issues. This system will be highly improved

developing a reliable energy harvesting device that will make the wireless sensor

package truly autonomous, eliminating completely the need for battery changes.

This objective involves significant commercial and environmental advantages.

Rotational electromagnetic conversion is the most appropriate energy

harvesting method for powering this application from wheel motion. In particular,

kinetic wristwatch technology eliminates maintenance through rotational

inductive micro-generation and later storage in a rechargeable battery. Power is

scavenged from the relative displacement between the proof mass and the frame.

And the generation level relies on either a continuous or an oscillating motion of

the weight.

Powering TPMS sensors, that harvesting device has to be oriented with the

plane of rotation of the wheel. Generally, its operation is based on scavenging the

kinetic energy from high frequency oscillations caused by the centrifugal field,

which dominates tremendously facing gravity insignificant perturbations. At very

low speeds, its operation depends on scavenging the energy from the stationary

motion of the weight caused by predominant vertical opposition of the

gravitational field. Complying with this pattern, the device generates between

2 mW and 3 mW above a vehicle speed of 15 km/h. Under this limit, the

generation falls off into unsatisfactory power levels for TPMS radio frequency

modules.

The largest amplitudes of the internal permanent magnet and consequent

maximum output power are achieved for oscillating resonant motion of the


Chapter VIII 86

weight. Therefore a device redesign will overcome poor generation at low speeds

deleting the gravitational harvesting method through the introduction of that

response at all operating constant conditions. The addition of an energy

management and storage system will definitely enable wireless pressure sensors to

accomplish their transmissions under all possible operating conditions during the

vehicle entire life with low cost and high reliability, thanks to rotational

electromagnetic energy harvesting.

Subsequent miniaturisation proposal is able to achieve a maximum single-

phase power of 4,4 mW. Whereas it is not conceived how to achieve the high

inertial excitations required by the rotor, experimental results encourage future

researches to focus on millimetre-scaled rotational electromagnetic generation as a

viable milliwatt powering technology. Consequently, future MEMS performance

will surely permit to introduce this infinite source unit into the pressure sensor

itself achieving an autonomous package that will contribute to important

environmental, economic and safety advantages.


87

REFERENCES


References 88

References

[WEST94] Automotive sensors, M H Westbrook & J D Turner, IOP, 1994.

[CHOW95] Automobile Electronics, Eric Chowanietz, BH, 1995.

[FREN89] Tyre Technology, Tom French, Adam Hilger, 1989.

[GILL92] Fundamentals of Vehicle Dynamics, Thomas D. Gillespie, Society of

Automotive Engineers, 1992.

[HEIN02] Advanced Vehicle Technology, HeinzHeisler, BH, 2002.

[NHTS00] Federal Motor Vehicle Safety Standards; Tire Pressure Monitoring

Systems; Controls and Displays, DOT, NHTSA, 49 CFR Part 571, Docket No.

NHTSA 2000-8572, RIN 2127-AI33.

[DIXO91] Tires, Suspension and Handling, 2nd

edition, John C. Dixon, SAE,

1991.

[OHBA92] Intelligent Sensor Technology, Ryoji Ohba, WILEY, 1992.

[BERN93] Maintenance-Free Batteries, D. Berndt, RSP, 1993.

[RIST94] Sensor technology and devices, Ljubisa Ristic, Artech House, 1994.

[NHTS01] Tire Safety Information; Proposed Rule, DOT, NHTSA, 49 CFR Parts

657, 571, 574 and 575, Docket No. NHTSA-01-11157, RIN 2127-AI32.

[VENK07] MEMS Pressure Sensors in the Automotive Industry, Prashanth

Venkatesh, Frost & Sullivan, 2007.

[REAB97] Batteries for automotive use, P.Reabeck & J.G. Smith, RSP, 1997.

[TORR05] Harvesting ambient energy will make embedded devices autonomous,

Erick O. Torres & Gabriel A. Rincon-Mora, EE Times, 2005.

[GOET05] Photovoltaic Solar Energy Generation, A. Goetzberger & V.U.

Hoffmann, Springer, 2005.

[MERR82] Sunlight to Electricity, 2nd

edition, Josepha Merrigan, MIT Press,

1982.

[ROWE94] Handbook of Thermoelectrics, D.M. Rowe, CRC, 1994.

[HARM67] Thermoelectric and Thermomagnetic Effects and Applications,

Harman & Honig, McGraw-Hill, 1967.


References 89

[GAUT02] Piezoelectric Sensorics, G.Gautschi, Springer, 2002.

[CROS87] Electrostatics Principles, Problems and Applicacions, J.A. Cross,

Adam Hilger, 1987.

[SHEN87] Applied Electromagnetism, 2nd

edition, Liang Chi Shen & Jin Au

Kong, PWS, 1987.

[KRAU89] Electromechanical Motion Devices, Paul C. Krause & Oleg

Wasynczuk, McGraw-Hill, 1989.

[BISW__] Design of a MEMPS using a Microgenerator, Nipun Biswas, Nikhit

Nair, Rosa Ciprian & Anindita Bhattacharya.

[CHIN00] PCB Integrated Micro-generator for Wireless Systems, Neil N.H.

Ching, Gordon M.H. Chan, Wen J. Li, Hiu Yung Wong & Philip H.W. Leong,

The Chinese University if Hong Kong, 2000.

[BEEB08] Kinetic Energy Harvesting, S.P. Beeby, R.N. Torah & M.J. Tidor,

University of Southampton, 2008.

[GILB08] Comparison of Energy Harvesting Systems for Wireless Sensor

Networks, James M. Gilbert & Farooq Balouchi, University of Hull, 2008.

[JENS84] Fundamentals of Energy Storage, Johanness Jensen & Bent Sorensen,

Wiley-Iterscience, 1984. (Chapter 7: Electrochemical Energy Conversion and

Storage p. 141-205.)

[GABA83] Lithium Batteries, Jean-Paul Gabano, Academic Press, 1983.

[VULL08] Energy Harvesting or Micropower Generation for Wireless

Autonomous Sensors, Ruud Vullers, Holst Center/IMEC-NL, 2008.

[RAWL93] The Science of Clocks & Watches, 3rd

edition, A.L. Rawlings, British

Horological Institute, 1993.

[SEIK08] Parts Catalogue & Technical Guide, Cal. 5M62A, 5M63A, UK Seiko,

2008.

[WRIG06] Energy Scavenging/Harvesting, Paul Wright, ITRIS, 2006.

[NASA79] Electromechanics and Electric Machines, S.A. Nasar & L.E.

Unnewehr, John Wiley & Sons, 1979.

[WEAV82] Electrical and Electronic Clocks and Watches, J.D. Weaver, Newnes

Technical Books, 1982.

[YEAT07] Energy harvesting from motion using rotating and gyroscopic proof

masses, E.M. Yeatman, Imperial College London, 2007.


References 90

[TOH_08] A continuously rotating energy harvester with maximum power point

tracking, Tzern T. Toh, Paul D. Mitcheson, Andrew S. Holmes & Eric M

Yeatman, Imperial College, 2008.

[GENT05] Dynamics of rotating systems, Giancarlo Genta, Springer, 2005.

[CONR05] Development of an Inertial Generator for Embedded Applications in

Rotating Environments, Stephen D. Conrad, University of Cincinnati,

Massachusetts Institute of Technology 2005.

[PARA05] Energy Harvesting for Mobile Computing, Joe Paradiso, Responsive

Environments Group, MIT Media Lab, 2005.

[MOOR88] The Modern Bicyble Dynamo, Guy S.M. Moore, Hatfield Polytechnic,

1988.

[COME86] Microprocessor-Based System Design, David J. Comer, HRW, 1986.

[RAO_08] Imperial College Energy-Harvesting Simulator (ICES), Kondala Rao,

Paul D. Mitcheson & Tim C. Green, Imperial College London, 2008.

[ARMS05] Power Management for Energy Harvesting Wireless Sensors, S.W.

Arms, C.P. Townsend, D.L. Churchill, J.H. Galbreath & S.W. Mundell,

MicroStrain, 2005.

[BROD82] The Physics of Microfabrication, Ivor Brodie & Julius J. Muray,

Plenum, 1982.

[HERR08] Ultraminiaturized High-Speed Permanent-Magnet Generators for

Milliwatt-Level Power Generation, Florian Herrault, Chang-Hyeon Ji & Mark G.

Allen, IEEE, 2008.

[GHAL08] Design, Fabrication, and Characterization of a Rotary Micromotor

Supported on Microball Bearings, Nima Ghalichechian, Alireza Modafe, Mustafa

Ilker Beyaz, & Reza Ghodssi, IEEE, 2008.


APPENDIX


Appendix 1

Appendix 1

Taking Seiko Kinetic Watch Apart


Appendix 1


Appendix 2

Appendix 2

Generating Coil Experimentation

Measurement of resistance.

Measurement of impedance.

Soldered joints.


Appendix 3

Appendix 3

Assembly Drawings


Appendix 4

Appendix 4

Watch Experimentation

Experimental assembly checking circuit. Digital tachometer.

Measurement of open-circuit voltage at a constant speed.

Measurement of current/voltage generated to an external load at a constant speed.


Appendix 5

Appendix 5

ICES Model


Appendix 6

Appendix 6

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