1. University of Genoa
Ph.D. School in
Science and Technology for Information and Knowledge
Cycle XXII
February 2010
Vibration-Based Energy Scavenging
for Power Autonomous Wireless
Sensor Systems
by
Luigi Pinna
A dissertation submitted to the University of Genoa
for the degree of Doctor of Philosophy
Ph.D. Course in Nanotechnologies
Coordinator: Ph.D. Chiar. mo Prof. Ermanno Di Zitti
Advisor: Ph.D. Chiar. mo Prof. Maurizio Valle
Co-Advisor: Ph.D. Ravinder S. Dahiya
Co-Advisor: Ph.D. Ing. Gian Marco Bo
Settore Scientifico-Disciplinare [SSD]: ING-INF/01 Elettronica

2. To Family and Friends

3. Preface
Now, the name of this talk is ”There is Plenty of Room at the
Bottom” - not just - ”There is Room at the Bottom.” What I
have demonstrated is that there is room - that you can decrease
the size of things in a practical way. I now want to show that there
is plenty of room. I will not now discuss how we are going to do
it, but only what is possible in principle - in other words, what
is possible according to the laws of physics. I am not inventing
anti-gravity, which is possible someday only if the laws are not
what we think. I am telling you what could be done if the laws
are what we think; we are not doing it simply because we haven’t
yet gotten around to it.
Richard P. Feynman
This thesis describes the work carried out between January 2007 and Febru-
ary 2010, at the Canovatech-DIBE joint Lab, University Campus of Savona,
DIBE-University of Genoa, Genoa, Italy. I was the recipient of a doctoral
fellowship to work at the DIBE-Canova Tech joint Lab under the supervi-
sion of Prof. Maurizio Valle from DIBE, University of Genoa, and Ing. Gian
Marco Bo from Canova Tech, Srl, Padova, Italy.
This thesis is about energy scavenging or harvesting for power autonomous
wireless micro/nano sensor-based systems with focus on vibration to elec-
tricity conversion. The work presented in this thesis is primarily focused on
the design and implementation of a power unit for the power supply of a
vibration-based power autonomous wireless sensor system. The power unit
is composed by two basic blocks, which are the vibration-based energy har-
vester (scavenger) and an integrated power management circuit, which is
composed by an AC-DC converter that rectifies the AC voltage generated by
the vibration-based generator and a DC-DC converter or voltage regulator.
In particular, this thesis work is related to the design of the integrated power
i

4. management circuit powered by a Piezoelectric Bender Generator (PBG)
which converts ambient mechanical vibrations into electricity.
The thesis contains 8 chapters. Chapter 1 introduces the motivations
about why the need of replacing a battery as power source for the new gen-
eration of electronic wireless sensor systems, with MEMS or nano sensors
arrays, integrated on single silicon chip with ultra low power CMOS-based
electronics. An overview of the general energy harvesting state of art is also
discussed in the Chapter 1 in order to introduce the energy harvesting topic
and what could be the better renewable ambient energy source from which
harvesting the energy - opportunely conditioned by a transducer and a power
management circuit - to be used to power the wireless sensor system in place
of batteries. Then, by presenting the reference system for this work the
various steps, issues and objectives are also explained in the last section of
Chapter 1.
Chapter 2 focuses on vibration-based transducers giving the reader the
know how about a vibration-based generator general model and the state
of the art of the three basic vibration-based generators (i.e. electrostatic,
electromagnetic, piezoelectric). The study of the state of the art of vibration-
based generators (VBG) must first be addressed in order to identify what
VBG is suitable 1) to be used to convert ambient mechanical vibrations to
electricity and hence powering the embedded units of the reference system;
2) to be fabricated with MEMS technologies and integrated with CMOS-
based electronics. A comparison among the advantages and disadvantages
of vibration-based generators - discussed in the Chapter 2 - addressed us
to explore the possibility of using piezoelectric bender generator (PBG) as
harvester (scavenger) to convert mechanical vibrations to electricity.
A feasibility case study - reported and discussed in the Chapter 3 - has
the goal to explore and study the use of PBG as power source for wireless
sensor systems, in particular for a wireless tire pressure measurement system
embedded on the wheel of a car. The wheel of the car is a very extreme en-
vironment where mechanical vibrations and radial accelerations magnitude
can reach values three order of magnitude larger than those ones of com-
mon ambient vibrational sources. In regard to the integration of the wireless
sensor system, knowing if a VBG scaled at micro/nano sizes could still be
able to power a wireless sensor system and the knowledge of the limits of the
VBG is an important aspect that an electronic designer should know. By this
knowledge the electronic designer can design and optimize the electronic cir-
cuits in order to adapt them to the energy made available by the micro/nano
harvester. Chapter 3, therefore, attempts to explain these aspects with a
number of examples.
Another important aspect that must be addressed and that is also useful
ii

5. for an electronic designer is the availability of having an equivalent model of
the power source - which in this case is the piezoelectric generator - in SPICE.
If the electromechanical model of the scavenger can be in fact implemented
in SPICE, then, it is extremely convenient to analyze the complete system
(i.e. mechanical and electrical and electronic parts) in SPICE. Therefore,
Chapter 4 presents the development of the SPICE model of the piezoelectric
bender generator, based on an electromechanical model - suited to be mod-
eled in SPICE - from literature which takes into account both geometrical
and physical parameters of the generator itself.
Vibration-based generators produces AC voltages that need to be con-
verted, regulated and stabilized in DC before being used to power electronic
systems. A diode bridge rectifier is the common and simplest approach fol-
lowed in literature to realize the AC-DC conversion. However, the integration
of only passive devices as diodes on chip does not take advantage of the flex-
ibility of active devices usage along with optimized control circuits which
could give the possibility to manage in a smart way the power and voltage
generated by VBGs. Performance, efficiency and low-power consumption
can be improved, and this is an important aspect above all in the context
of energy scavenging applications, where the available energy may be poor.
However, a fully active solution for the bridge rectifier, might need a more
complicated control circuit to drive the active devices, which can increase
the complexity and power consumption of the system. Therefore, trade-
offs among simplicity, efficiency, flexibility and performance are necessaries.
Moreover, because of the PBG is able to generate high level output volt-
ages, power devices could be more suitable to be used for the bridge rectifier.
Therefore, a semi-active approach for the bridge rectifier with power passive
and active devices is presented and developed in the Chapter 5.
A DC-DC switching converter with its high efficiency - around the 90% -
is a basic unit of a power management system. It can be used to regulate and
adapt the diverse voltage levels needed to power the electronics belonging a
system and also can be used to optimize the power transfer towards the final
load. Moreover, because of they are realized with active devices which need
to be controlled suitable control algorithms can be implemented. The major
effort in the development of the power management circuit is therefore the
design of the driver circuit, which controls the switches of the converter.
In this work, the voltage regulated by a step-down buck converter is used
not only for being compared with a reference voltage in order to perform
the regulation, but, also is used to power the control circuits of the power
management system - i.e. the rectifier control circuit and the driver circuit.
The design issues addressed to make the voltage regulator self-powered are
discussed in the Chapter 5. Moreover, the simulation results of the reciprocal
iii

6. interaction between PBG and the scavenging system - SPICE model of the
PBG, the semi-active bridge and voltage regulator - is presented, in terms of
stress, strain rate, mechanical and electrical powers.
Chapters 6 and 7 concern the design, development and fabrication of a
prototype ASIC and test printed circuit board and the experimental valida-
tion test results. The developed ASIC comprises of the semi-active bridge
rectifier and the switching part of the buck converter which have been inte-
grated on chip. The control circuits and the LC filter of the step-down buck
converter have been inserted in the test PCB. This hybrid solution - ASIC
and PCB - has been adopted in order to have more flexibility in the design
and for the experimental tests, integrating the key components - rectifier
and switching part of the DC-DC converter - in the ASIC, while, the other
components - control circuits and LC filter - left outside the ASIC.
In order to validate the SPICE equivalent model of the PBG is necessary
to compare it with a realistic PBG. By performing experimental tests with the
prototype PBG and hence comparing the measured results with the simulated
ones, it would allow understanding what are the limits of the developed
SPICE model and hence, through a careful study and understanding of the
results obtained, optimizing the developed SPICE model in order to make
it closer to the realistic counterpart. The concluding chapter of this work
presents then the preliminary experimental tests realized with a fabricated
PBG prototype, either tested alone or connected to the ASIC and test board.
Finally, a critical evaluation of the work, through Chapter 1 to Chapter 7,
is done and presented in the conclusions section. Moreover, in the context of
the future trends, an analytical mathematical analysis of a vibration-based
hybrid generator, realized joining a PBG with an electromagnetic generator
is also presented.
iv

7. Acknowledgements
The path to a Ph.D. has been a long and winding one. The route was not
always well illuminated, but there have been many people who lighted up
the way by providing guidance, encouragement and support. It would be
belittling their efforts if I don’t express my gratitude to them.
I wish to start expressing my gratitude with my supervisors, not because
it is customary, but because they have been showing me the way all along.
Prof. Maurizio Valle has been a very patient supervisor of this ever expanding
project, which he strongly supported with inspiring enthusiasm, right from
beginning. His support, critical reviews and appreciation provided important
clues and guidelines.
I would like to thank Ing. Gian Marco Bo for his support, friendly advices,
generously sharing his expertise and pervasive knowledge in the field of power
management circuits and electronic engineering and I am sure this knowledge
will help me shape my professional career.
A really special thanks goes to Ravinder who has been extremely support-
ive, kept always an eye on the progress of my work and always was available
whenever I needed his advises and help. This last year of my Ph.D. would
have never been as it has been without his friendship, help and support.
Besides the support from supervisors, a thesis also needs the helping
hands of fellow researchers and colleagues that make the work more comfort-
able.
I would like to thank Alessandro and the Canova Tech company for fund-
ing and sponsoring the project related my Ph.D. on the development of a
power management system for energy scavenging applications.
My gratitude goes also to all other members of Canova Tech for their
technical support, generous hospitality and the pleasant time they gave me
during my stays in Padova.
I would like to thank Fabrizio from Canova Tech who has been generous
with his time in helping me understand circuit design issues and in helping
with instrumentation.
Among many colleagues I would like to thank Andrea Guerra who has
v

8. been a very great friend and flatmate during the first year of my Ph.D. here
in Savona. I would like to thank Sergio from Canova Tech and Leonardo,
colleague, friend and flatmate during these three years and Marco Antonio -
associated with MUSES lab at University Campus of Savona and also Can-
dice, Andrea and Lorenzo - all associated with microelectronics lab at DIBE,
University of Genova.
I am ever grateful to my Father, Mario, my Mather, Lucia, my Sister,
Rosy and her Husband, Mirko, who have given me their unequivocal support
throughout, for their constant love and confidence in me, as always, for which
my mere expression of thanks likewise does not suffice.
I am very grateful for my girlfriend and loved one, Sabina, for her love,
support, understanding and patience during the last two years of my Ph.D.
By any chance, if your name is not listed, rest assured that my gratitude
is not less than for those listed above.
Again, thank you all!
vi

9. Contents
Preface i
Acknowledgements v
1 Introduction 1
1.1 Motivation: wireless micro/nano sensor-based electronic system 1
1.2 Wireless power autonomous micro/nano sensor-based system . 5
1.2.1 Renewable ambient energy sources . . . . . . . . . . . 6
1.2.2 Vibrational sources: a powerful option to batteries . . . 7
1.3 Thesis objectives: proposed harvesting system architecture . . 9
2 Vibration-based energy harvesting 16
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2 General model of a vibration-based generator . . . . . . . . . 16
2.3 State-of-the-art of vibration-based generators . . . . . . . . . . 21
2.3.1 Electrostatic generators . . . . . . . . . . . . . . . . . 22
2.3.2 Electromagnetic generators . . . . . . . . . . . . . . . . 24
2.3.3 Piezoelectric bender generators . . . . . . . . . . . . . 27
2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3 Case Study: Piezoelectric Bender Generator for Wireless
Tire Pressure Measurement Systems 36
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.2 Wireless smart sensors energy/power requirements estimation 37
3.3 Experimental results . . . . . . . . . . . . . . . . . . . . . . . 40
3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4 SPICE model of Piezoelectric Bender Generator 47
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
vii

10. 4.2.1 Analytical model . . . . . . . . . . . . . . . . . . . . . 48
4.2.2 Mechanical side and electrical block equations . . . . . 50
4.2.3 Piezoelectric coupling equations . . . . . . . . . . . . . 51
4.2.4 PBG model transfer function with resistive load . . . . 51
4.3 SPICE implementation . . . . . . . . . . . . . . . . . . . . . . 52
4.4 Simulation results . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5 Design and analysis of a Vibration-Based Energy Scavenging
System 58
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5.2 Vibration-Based Energy Scavenging Circuit . . . . . . . . . . 59
5.2.1 Semi-Active Bridge Rectifier . . . . . . . . . . . . . . . 59
5.2.2 Voltage regulator . . . . . . . . . . . . . . . . . . . . . 61
5.2.3 Transient and steady-state . . . . . . . . . . . . . . . . 65
5.3 SPICE simulation results . . . . . . . . . . . . . . . . . . . . . 70
5.3.1 Semi-Active Bridge rectifier simulation results . . . . . 71
5.3.2 Voltage regulator simulation results . . . . . . . . . . . 72
5.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
6 Test chip design 81
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
6.2 ESD protections . . . . . . . . . . . . . . . . . . . . . . . . . . 83
6.2.1 LV input pad protection . . . . . . . . . . . . . . . . . 83
6.2.2 HV IO protections to protect the HV switches . . . . . 87
6.3 Layout of the test chip . . . . . . . . . . . . . . . . . . . . . . 89
7 Test board design and experimental validation 91
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
7.1.1 Test board voltage level shifter . . . . . . . . . . . . . 93
7.1.2 Test board driver . . . . . . . . . . . . . . . . . . . . . 94
7.2 Test board experimental validation results . . . . . . . . . . . 95
7.2.1 Efficiency of the test board voltage regulator . . . . . . 96
7.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
8 Conclusions and future trends 103
8.1 Preliminary experimental results . . . . . . . . . . . . . . . . . 108
8.2 A proposal for a hybrid vibration-based generator . . . . . . . 114
8.3 Electromechanical analytical model of a vibration-based piezo-
electric and electromagnetic generator . . . . . . . . . . . . . . 114
viii

11. 8.3.1 PBEMG model with resistive load: coil connected in
parallel with the PBG output . . . . . . . . . . . . . . 118
8.4 Combining piezoelectric and electromagnetic SPICE models . 121
8.4.1 SPICE modeling of the mechanical and electrical sides 122
A Publications 124
B SPICE Netlist of the Piezoelectric Bender Generator 126
C AMIS I3T50u devices overview 127
C.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
C.2 n-type VDMOS transistor: VFNDM50 . . . . . . . . . . . . . 128
C.3 p-type VDMOS transistor: LFPDM50 . . . . . . . . . . . . . 129
C.4 High Voltage diode: FID50U . . . . . . . . . . . . . . . . . . . 129
D Semi-Active Bridge rectifier dimensioning 131
Bibliography 135
ix

12. List of Figures
1.1 block diagram of a battery-supplied wireless sensor system. . . 2
1.2 (a) Top view and cross-sectional diagrams of nanosensors ar-
ray with silicon CMOS circuitry [Xu et al., 2004]. (b) Micro-
graphs of the test chip with an array of nanowires and red-out
circuit and the UWB transmitter chip [Narayanan, 2004]. . . . 4
1.3 Comparison of powers generated from vibrations, solar, and
various battery chemistries with respect to the life time of the
power source expressed in years [Roundy et al., 2004]. . . . . . 9
1.4 Block diagram of a wireless power autonomous sensor system. 10
1.5 Block diagram of the wireless power autonomous sensor sys-
tem with semi-active bridge and DC-DC switching voltage reg-
ulator blocks. . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.6 Block diagram of the power management blocks realized as
ASIC and PCB. . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1 General model of the linear mass-spring-damper system rep-
resenting the resonant inertial generator (a). Free-body dia-
grams at an arbitrary instant including effects of the absolute
motion of the frame, y(t) (b). . . . . . . . . . . . . . . . . . . 18
2.2 Electrostatic generators structures: in-plane overlap converter
(left), in-plane gap closing converter (center), out-of-plane gap
closing converter (right) . . . . . . . . . . . . . . . . . . . . . 23
2.3 Electromagnetic generators: (a) MEMS-based by Beeby et al.,
Wang et al., respectively; (b) Millimeter scale by Torah et al.,
by Glynne et al., respectively . . . . . . . . . . . . . . . . . . 24
2.4 Illustration of the two modes of piezoelectric conversion from
input mechanical stress (denoted as σ1). In the figure the
strain is denoted as S1. . . . . . . . . . . . . . . . . . . . . . . 27
2.5 Vibration-based piezoelectric generators: (a) Micromachined
cantilever by Marzencki et al., Choi et al. and Fang et al.,
respectively; (b) Millimeter scale by Roundy et al. and Leland
et al., respectively. . . . . . . . . . . . . . . . . . . . . . . . . 29
x

13. 2.6 Piezoelectric nanogenerator based on Zinc Oxide nanowire ar-
rays by Wang, Z. L. et al.. . . . . . . . . . . . . . . . . . . . . 30
3.1 (a) Centripetal force progress during the roto-translational
motion of an automobile tire, and its action onto Piezoelectric
Bender Generator. (b) Schematic of the energy harvesting cir-
cuit. (c) Set up of the experiment with the test board mounted
on the outer rim of the wheel of an automobile. . . . . . . . . 40
3.2 Measured Voltage and computed energy curves for different
car speeds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.3 Measured Voltage and computed energy curves for different
PBG thicknesses. . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.4 Measured Voltage and computed energy curves for different
distances of PBG from the wheel center. . . . . . . . . . . . . 42
3.5 Power flow from PBG to sensor and radio block in case of
not considering and considering the minimum supply voltage
required by the sensor and radio block to operate. . . . . . . 44
4.1 (a) Piezoelectric bender generator with a proof mass placed on
the free end of the bender. (b) Piezoelectric bender generator
wired for series and parallel operation mode. . . . . . . . . . . 49
4.2 (Bimorph electromechanical circuit model. . . . . . . . . . . . 49
4.3 SPICE schematic subcircuit of the Piezoelectric Bender Gen-
erator model. . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.4 Comparison among the MATLAB and SPICE (a) and the
Roundy simulated and experimental measured (b) powers and
voltages versus load resistance in case of bimorph wired for the
parallel operation mode. . . . . . . . . . . . . . . . . . . . . . 55
4.5 SPICE and MATLAB powers (a) and voltages (b) versus load
resistance in case of bimorph wired for the series operation
mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
5.1 Block diagram of vibration based energy scavenging system
with PBG (excited by a vibration source), the voltage regula-
tor system and control circuit (supplied by the output of the
DC-DC voltage regulator). . . . . . . . . . . . . . . . . . . . . 59
5.2 Schematic of the semi-active bridge rectifier with the ZCC
control circuit inside the dashed rectangle. . . . . . . . . . . . 60
5.3 Schematic of the voltage regulator circuit with the semi-active
bridge rectifier and the SPICE model of the PBG. . . . . . . . 62
5.4 Internal schematic view of the monostable circuit. . . . . . . . 63
xi

14. 5.5 Internal schematic view of the flip flop D master slave. . . . . 63
5.6 Internal schematic view of the SPICE model of a one-pole
comparator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.7 Voltage regulator control circuit cycle timing diagram in case
of Iout,PBG < (IV dd,Driver + ILoad). . . . . . . . . . . . . . . . . 67
5.8 Voltage regulator control circuit cycle timing diagram in case
of Iout,PBG ≥ (IV dd,Driver + ILoad). . . . . . . . . . . . . . . . . 70
5.9 (a) Simulated rectifier average input power (left) and load
power (right) of the semi-active and diode bridge rectifiers
vs. load resistance. (b) Simulated rms load voltage (left) and
efficiency (right) of the semi-active bridge and diode bridge
rectifiers vs. load resistance. . . . . . . . . . . . . . . . . . . . 72
5.10 Simulated mechanical input power for the 3.3 V, 1.8 V and
1.4 V regulated voltages versus load resistance (a). Simulated
mechanical input power, PBG output power and load Power
at various load resistances in case of the 3.3 V regulation (b). . 73
5.11 Efficiency (a) and simulated PBG output current curves (b)
for the 3.3 V, 1.8 V and 1.4 V regulated voltages versus the
load resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . 74
5.12 Simulated strain rate curves for the 3.3 V, 1.8 V and 1.4 V
regulated voltages versus the load resistance (a). Simulated
PBG output voltage curves for 3.3 V, 1.8 V, 1.4 V versus load
resistance (b). . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
5.13 Simulated stress curves for the 3.3 V, 1.8 V and 1.4 V regulated
voltages versus the load resistance (a). Comparison among the
three different regulated voltage curves versus load resistance
(b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
5.14 Simulated load power for 3.3 V, 1.8 V, 1.4 V versus load re-
sistance (c). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
6.1 SPICE schematic of the test chip core. . . . . . . . . . . . . . 82
6.2 Test chip connection diagram and top view of the Dual-In Line
Package pin out of the test chip. . . . . . . . . . . . . . . . . 82
6.3 Test Chip core SPICE symbol with ESD protections blocks
diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
6.4 LV input protection scheme based on local ESD clamp protec-
tions and the path follow by the ESD event from the input pad
to VSS or VDD, and viceversa. In the figure are also shown
the AMIS I3T50u standard cells for the ESD1, ESD2, ESD3
and ESD4 blocks. . . . . . . . . . . . . . . . . . . . . . . . . . 85
xii

15. 6.5 Test chip core with 40V HV IO ESD protection strategy both
for the LFPDM50 in HS configuration (right) and for the
VFNDM50 in LS configuration (left) schematic diagrams. . . . 88
6.6 Layout of the test chip with ESD protections (a). SEM picture
of the test chip (b). . . . . . . . . . . . . . . . . . . . . . . . 90
7.1 Schematic of the circuit comprising of the Power Amplifier
and transformer to step up the AC voltage magnitude of the
Function Generator till 36 Vpeak (a). Voltage level shifter
circuit (b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
7.2 Schematic internal view of the driver circuit. . . . . . . . . . . 94
7.3 Test board schematic circuit set for the start up verification. . 96
7.4 Test board schematic circuit set for the control switches rec-
tifier verification. . . . . . . . . . . . . . . . . . . . . . . . . . 96
7.5 (left) Rectified voltage measured at the output VO1 (top wave
form) and single voltage wave form measured at the input AC1
(bottom wave form). (right) Rectified voltage measured at the
output VO1 (top wave form) and pulse wave form applied at
the gate G1 of the chip (bottom wave form). . . . . . . . . . . 97
7.6 Measured load voltage (a) and output power (b) with respect
to the resistive load and for different input voltage values in
input to the test chip. . . . . . . . . . . . . . . . . . . . . . . 98
7.7 Set up to estimate the effective input power. . . . . . . . . . . 99
7.8 Measured input power (a) and efficiency (b) with respect to
the resistive load and for different input voltage values in input
to the test chip. . . . . . . . . . . . . . . . . . . . . . . . . . . 100
7.9 Test board circuit and set up of the experimental tests. . . . . 101
8.1 The fabricated prototypes - PBG1 and PBG2 - made of PSI-
5A4E with a steel proof mass attached to the free end, and
clamped to a steel support. . . . . . . . . . . . . . . . . . . . . 110
8.2 The prototype generator (PBG1) mounted on the vibrometer
(Tira TV50018) used to perform the experimental tests. . . . . 110
8.3 Schematic diagram of the test set up for the open circuit
voltage (left) and resistive load voltage measurements (right)
across the output of the PBG prototype. . . . . . . . . . . . . 111
8.4 Measured open circuit voltages (peak values) at various fre-
quencies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
8.5 Measured open circuit voltages (peak values) and powers vs.
load resistance at 32 Hz (resonance frequency of the PBG) . . 112
xiii

16. 8.6 Measured PBG output voltage and rectified voltage with re-
spect to time (a). Voltage regulator measured open circuit
voltage and for a 10 kΩ resistive load (b). . . . . . . . . . . . 113
8.7 Model of a linear vibration-based electromagnetic generator. . 116
8.8 Equivalent electromechanical Piezoelectric Bender ElectroMag-
netic Generator model. . . . . . . . . . . . . . . . . . . . . . . 119
8.9 Equivalent electromechanical Piezoelectric Bender ElectroMag-
netic Generator model, in case of parallel connection of the coil
output with the PBG output. . . . . . . . . . . . . . . . . . . 119
8.10 Equivalent SPICE model circuit of the Piezoelectric Bender
ElectroMagnetic Generator. . . . . . . . . . . . . . . . . . . . 122
C.1 Symbol and cross-section of the VFNDM50 and LFPDM50
VDMOS transistors and cross-section of the FID50U diode,
respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
xiv

17. List of Tables
1.1 Comparison of various power sources for power au-
tonomous sensor systems. . . . . . . . . . . . . . . . . . . 8
2.1 Summary of vibration-based electrostatic genera-
tors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.2 Summary of vibration-based electromagnetic gen-
erators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.3 Summary of vibration-based piezoelectric bender
generators . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1 ATA6285/6 parameters. . . . . . . . . . . . . . . . . . . . 38
3.2 Summary of the energy/power consumption estima-
tions to perform a sensor sampling and transmis-
sion for power autonomous wireless sensors. . . . . 39
6.1 Summary of the ESD protection cells specifications. 84
6.2 NPOR electrical parameters. . . . . . . . . . . . . . . . 86
6.3 HIPOR electrical parameters. . . . . . . . . . . . . . . 89
C.1 Summary of the AMIS I3T50-U devices used. . . . . . 128
C.2 VFNDM50 device parameters. . . . . . . . . . . . . . . . 128
C.3 LFPDM50 device parameters. . . . . . . . . . . . . . . . 129
C.4 FID50U device characteristics. . . . . . . . . . . . . . . 130
D.1 Results of the parametric analysis in case of Typ-
ical (Typ), Worst Case Speed (WCS), Worst Case
Power (WCP), conditions operation. . . . . . . . . . . 132
D.2 Results of the parametric analysis in case of Typi-
cal (Typ), Minimum (Min) and Maximum (Max) con-
ditions operation obtained by varying the multi-
plier of the diode. . . . . . . . . . . . . . . . . . . . . . . 133
xv

18. Chapter 1
Introduction
1.1 Motivation: wireless micro/nano sensor-
based electronic system
The demand for completely self-powered integrated electronic systems for a
number of applications, such as implantable devices [Platt et al., 2005], real-
time monitoring of the health of infrastructures, environmental monitoring,
medical/health care systems, has resulted in an increased research activity
for energy harvesting devices and systems, micro and nano electromechani-
cal systems (MEMS/NEMS), nanosensors, ultra low power electronics. Such
systems comprising of on-chip sensors or actuators, power management elec-
tronics, elaboration unit or also DSPs, and suitable RF circuitry, require their
own power supply which in most cases is the conventional electrochemical
battery, as shown in Fig. 1.1. In the recent years the research community has
focused its effort on 1) increasing the energy/power efficiency of electronic
systems, or stated in other terms, in the effort of reducing as much as pos-
sible the current and power consumption of each embedded basic electronic
component of a wireless sensor system, in the optic to enhance the lifetime of
the system; 2) scaling as much as possible the dimensions of sensors and elec-
tronic units to make the wireless system, ubiquitous, pervasive, non-invasive;
3) integrating all the units composing the system on single silicon chip.
In these last years the research activity has addressed at the development
of power management techniques both hardware and software, in order to
reduce the power consumption of the electronic units of a wireless sensor
system, and hence, increasing the life time of the wireless sensor battery.
The hardware approach has addressed at lowering as much as possible the
energy consumption of electronic devices, by developing techniques like Dy-
namic Voltage Scaling (DVS) [Pering et al., 1998], optimized wake-up pro-
1

19. Figure 1.1: block diagram of a battery-supplied wireless sensor system.
cedures [¨Ostmark et al., 2006], and so forth. Data converters, whose FOM
(Figure Of Merit) of recent ADCs is better than 50fJ/conversion; micro-
controllers, microprocessors or DSPs, which are reaching the 10µW/MMAC
(Watts per millions of multiply-accumulate operations per second) accord-
ing to Gene’s law1
; transceiver or RF units, which are reaching the level of
3nJ per received-transmitted bit. This trend and effort has the aim to build
up an energy/power autonomous system [Belleville et al., 2009]. The soft-
ware approach has addressed at developing optimized algorithms to make
running on the CPU unit of a wireless sensor system, allowing managing in
a smart way the low useful power made available by batteries and, at the
same time, increasing the lifetime of batteries and of wireless sensor system
[Barboni and Valle, 2008].
Even if, both hardware and software approaches are important for limit-
ing the power consumed by electronic systems, however, they do not solve the
problem of the battery which always needs to be replaced. Task this, which
can become very tedious, expensive and risky especially in cases like im-
plantable devices. Besides, in case of ecologically-sensitive places, like lakes,
rivers [Harnett, 2008], forests, the use of wireless sensor systems, powered
by batteries, could be impractical not only for the issue due to the battery
replacement for hundreds or thousands of wireless sensor systems spread ev-
erywhere to realize a network in the environment to be monitored, but also
1
Gen’s Law states that power dissipation will be reduced at roughly the same rate that
performance increases. Performance is suppose to double every 18 months, so according
to Gene’s Law, power dissipation will decrease by half every 18 months. Of course, this
presents a problem because now leakage power is approaching the same level as active
power dissipation.
2

20. because they could be physically embedded in the environment itself. Fur-
ther, batteries contain toxic chemicals and need to be quickly retrieved, after
the batteries discharge.
At nano and micro scale sizes, powering nano and microelectromechanical
systems (MEMS) by means of batteries could become unpractical. Moreover,
because of the size of the battery, compromising also the big effort of scaling
the electronic devices. In fact, the size of a battery is the most limiting factor
for reducing the wireless sensor system sizes. As example, the Crossbow mica
mote is powered by two AA size batteries that occupy 90% of the entire device
volume. Even if the battery technology is improved in the last years, batteries
are the largest and most expensive component of a wireless sensor system, and
furthermore, the most limiting factor for the life time of the wireless sensor
itself because of the limited lifespan of the battery. Therefore, wireless sensor
systems need other powering techniques and renewable energy/power sources
from ambient for replacing batteries and hence really making ubiquitous and
self-sustaining the wireless sensor systems.
MEMS-based devices (sensors and actuators) have been integrated with
success in a lot of different electronic applications, to build up integrated
microsystems. One of the key advantages of MEMS-based devices is that
they can be integrated with state-of-the-art silicon microelectronics on a very
large scale (e.g. micromachined accelerometers for automobile crash tests and
detection [Ferraresi and Pozzi, 2009], microsystems integrating chemical/gas
sensors on a single chip with CMOS-based readout electronic to build up
electronic noses [Yang et al., 2010], and so forth). MEMS systems can then
sense a wide variety of different phenomena at low cost and efficiently, and
they can be integrated and used with success in Wireless Sensor Networks
and practically in a wide range of industrial sectors: automotive, telecom-
munications, aerospace, data storage and biotechnology.
Nanotechnology is a field of research, which is rapidly growing and evolv-
ing and it is offering to researchers a way for manipulating and controlling
the matter at an extremely small scale size, even to the level of molecules
and atoms. Nanoscale engineering (by the use of top-down2
and bottom-
up3
approaches) can offer both control and manipulative ability over indi-
2
The top-down approach often uses the traditional workshop or microfabrication meth-
ods where externally-controlled tools are used to cut, mill, and shape materials into the
desired shape and order. Micropatterning techniques, such as photolithography and inkjet
printing belong to this category.
3
The bottom-up approach uses the chemical properties of single molecules to cause
single-molecule components to self-organize or self-assemble into some useful conformation,
or rely on positional assembly. These approach utilizes the concepts of molecular self-
assembly and/or molecular recognition.
3

21. (a)
(b)
Figure 1.2: (a) Top view and cross-sectional diagrams of nanosensors array
with silicon CMOS circuitry [Xu et al., 2004]. (b) Micrographs of the test
chip with an array of nanowires and red-out circuit and the UWB transmitter
chip [Narayanan, 2004].
vidual atoms and molecules, determining physical, chemical and even bio-
logical material properties. The sensitivity that nano scale sensors can ob-
tain can therefore be much greater than the sensitivity of microsensors and
other MEMS-based sensors. In addition, due to the incredibly tiny dimen-
sions of nanosensors, a large number of low-power-consuming nanosensors
with diverse functionalized properties can be array-connected and integrated
on a single silicon chip along with CMOS-based electronics, as shown in
Fig. 1.2 [Xu et al., 2004]. Already developed nanosensor devices including
NEMS-based cantilever sensors [Li et al., 2007], which are sensitive to ultra-
small masses and forces, nanowires [Fan et al., 2008] and carbon nanotubes
[Li et al., 2004] used as chemical, pressure, humidity, accelerometers, biosen-
4

22. sors [Jang, 2009] all deployed in array configurations, can allow a sensor
system having high surface to volume ratio, which can improve detection
sensitivity and response time due to more reaction area per volume and re-
duced diffusion time. Examples, of integration of nanosensors array with
CMOS-based electronics using the technique called dielectrophoretic (DEP)
assembly and the design of a low-power transmitter for wireless communi-
cations can be found in [Narayanan, 2004], as shown in Fig. 1.2. Other
interesting work can be found in [Fan et al., 2008], where Scientists at the
U.S. Department of Energy’s Lawrence Berkeley National Laboratory and
the University of California at Berkeley have created the world’s first all-
integrated sensor circuit based on nanowire arrays, combining light sensors
and electronics made of different crystalline materials.
Next future step will be the integration of biosensors [Jang, 2009] and
nano-scale devices (e.g. nanosensors) [Xu et al., 2004] into low-cost, ultra-
low power wireless sensor systems. Besides, nanosensors, ultra low-power
electronics (i.e. front-end electronic, ADC, DSP, and RF unites, power man-
agement circuitry), and ambient energy scavengers can be integrated on a
tiny single silicon-based chip to build up the next generation of ubiquitous
power autonomous wireless sensor systems.
1.2 Wireless power autonomous micro/nano
sensor-based system
A big effort in the research activity has therefore been done for reducing size,
improving sensors sensitivity with micro and nano sensor devices, improv-
ing efficiency and reducing the power consumption of all electronic blocks
composing a wireless sensor system. However, the vast reduction of size and
power consumption of CMOS-based circuitry has led to a large research ef-
fort based in the context of the vision of ubiquitous networks of wireless
sensor systems, and also highlight the issue of finding suitable power supply
sources in place of batteries. Moreover the new grand challenge faced by
circuits and systems communities is to design green electronic devices and
systems that consume less energy, thus lead to the reduction of global CO2
emission (e.g. 2010 International Conference on Green Circuits and Sys-
tems). Therefore, one of the major challenge for the developing of a power
autonomous wireless sensor system is to realize a power unit composed by
1) a suitable device as alternative to batteries that can be miniaturized us-
ing microfabrication technologies; it should be silicon-compatible in order
to be integrated with CMOS-based electronics; and above all it should be
5

23. able to harvest the whole energy needed for the power supply of the wire-
less sensor system from a renewable power source present in the ambient
where the system itself operates - this eliminate the need to have a local
energy reservoir which contains a limited energy and the entire lifetime of
the wireless sensor system; 2) designing and developing a low power/current
consuming and integrated power management circuit, which is efficient and
optimized to manage the environmental energy in order to make the sys-
tem - already communication-autonomous thanks to the wireless technology
- self-sustaining and ambient-aware.
1.2.1 Renewable ambient energy sources
Renewable power sources like light, thermal, radio frequency, sonic waves
and kinetic energy present within the sensor environment can be used to
generate electrical energy for the power supply of low power and ultra-low
power electronic devices. Many transducers exist to harvest the ambient
energy and converting it into electrical one and among them the most known
are the solar cells.
Solar cells can offer energy densities of about 100 mW/cm2
, when they are
enlightened by direct sunlight and of about 100 µW/cm2
, when enlightened
by artificial light [Paradiso and Starner, 2005]. However, due to the not so
high efficiency conversion of solar cells (i.e. about 10-24%), in case of outdoor
environment, only 15 mW/cm2
of power density can be truly harvested, and
in case of indoor environment the harvested power density decreases at only
10 µW/cm2
. Besides, solar cells are limited in dim ambient light conditions
and obviously unsuitable in embedded applications where no light may be
present.
Thermo Life Energy, Corp., developed and designed some little and
compact Low Power Thermoelectric Generators (LPTG), which allow (using
some thermopile couples which exploit the Seebeck effect) converting gradient
temperature to electrical energy. When both heat couple plates are thermally
connected with a heat source and a heat sink, heat flows through thermopiles
and is converted directly into electricity. With gradient temperatures of only
5 ◦
C, these LPTGs can supply powers of 30 µW, and up to 135 µW if the
gradient temperature goes up to 10 ◦
C.
Other energy harvesting approach, involves broadcasting RF energy to
power remote devices (e.g. electronic ID tags, smart cards). This solution
anyway is limited by the distance between the device to be powered and
the RF energy source. Using the simple expression of the power received
by an anisotropic antenna, which neglects reflections and interferences, here
6

24. reported
Pr =
P0λ2
4πR2
(1.1)
where P0 is the transmitted power, λ is the wavelength of the signal, R is
the distance between transmitter and receiver, the power received by the
electronic device to be powered [Smith, 1998], if it is assumed a distance be-
tween transmitter and receiver of 5 meters, a transmitted power of 1 W at
the frequency of 2.5 GHz, is about 50 µW. In an indoor environment, how-
ever, a more likely figure is 1/R4
, rather than 1/R2
. Passive radio-frequency
identification (RFID) systems derive their energy inductively, capacitively,
or radiatively from the tag reader. RFID tags generally consume between 1
and 100 µW, but the RF energy source should be very close to the tag in
order to succeed to power it.
In [Dietterich, 2009] an ultra-low power temperature sensor node has been
developed to harvest radio frequency energy broadcasted by a base station
(standard electric-powered) placed on the center of a wireless sensor network
radio frequency-covered area of 30 m of diameter. The developed sensor node
prototype includes a temperature sensor, an RF energy harvesting circuit, a
binary frequency shift keying (BFSK) receiver and transmitter, which share
the same antenna. The ultra-low power temperature sensor node succeeded
to measure a range of temperatures from -10 to 40 ◦
C, with an accuracy of
±0.5 ◦
C, consuming only 1 nJ per measurement.
Other studies have been made [Paradiso and Starner, 2004], to under-
stand how much energy is possible to extract from human body (e.g. by
gradient temperatures, breathing, blood flow, etc.) and from human activ-
ities (e.g. walking, keyboard typing, etc.), thus, using this energy to power
portable and wearable electronic devices [Paradiso and Starner, 2005]. The
most energy rich and most easily exploitable human activity energy source,
according to what stated in [Paradiso and Starner, 2004], occurs at the foot
during the heel strike. Some back-to-back unimorphs piezoelectric shoe in-
serts embedded in an insole have been able to generate an average power of
about 8.4 mW while a person was walking [Shenck and Paradiso, 2001].
1.2.2 Vibrational sources: a powerful option to batter-
ies
Among various sources the ambient vibrational one is the most promising
source from which harvesting or scavenging energy to be converted to electric-
ity, and powering low power electronic devices, such as, small electronic com-
ponents, wireless sensors, wireless implantable biosensors. Table 1.1 shows a
7

25. Table 1.1: Comparison of various power sources for power au-
tonomous sensor systems.
Source
Source Physical Harvested Power
Characteristic Efficiency (Range)
Solar
Office 0.1 mW/cm2 10-24% 10 µW/cm2
Outdoor 100 mW/cm2 15 mW/cm2
Vibrations
max power is
source and
device
dependent
1-10 m/s2 0.65 µW/cm2
10-500 Hz 375 µW/cm2
8.1 mW/cm2
Thermal Energy
Human 20 mW/cm2 0.1% 25 µW/cm2
Industry 100 mW/cm2 3% 1-10 mW/cm2
RF (EM Energy)
GSM 900MHz 0.3-0.03 µW/cm2 50% 0.1 µW/cm2
1800 Hz 0.1-0.01 µW/cm2
comparison among the various power sources described in the previous sec-
tion with the vibrational one. The amount of energy that can be obtained
depends basically on the quantity and form of vibrational energy present
in the environment, the efficiency, size and kind of the transducer and also
on the power management electronics. Furthermore, as it can be noted in
Fig. 1.3 where is shown a comparison among vibration, solar and different
technology battery powers with respect to the life time of the power source
[Roundy et al., 2004], if the projected lifetime is more than a few years, and
sufficient light energy is not available in the environment in which the device
should operate, mechanical vibration conversion is the most practical alter-
native to batteries and light. The use of vibrational energy available in the
ambient (such as household goods like microwave oven, refrigerators, washing
machines, industrial plant equipments, automobiles, buildings, bridges, and
so forth) where the electronic device operates, would allow the replacement
or at least the minimization of the requirement of external power sources or
batteries.
Environmental energy harvesting (scavenging) techniques, therefore, might
alleviate, if not solve altogether, the problem of battery-supplied electronic
systems, by developing meso-scale (order of several centimeters), micro or
nano-scale (order of micrometers and nanometers) energy harvesting (scav-
8

26. Figure 1.3: Comparison of powers generated from vibrations, solar, and
various battery chemistries with respect to the life time of the power source
expressed in years [Roundy et al., 2004].
enging) devices that can ”scavenge” the normally wasted energy from envi-
ronment and convert it into usable form, or gathering it into storage devices,
like capacitors or batteries, and hence, powering the electronics embedded
on wireless sensor systems. Vibrational sources are the most suitable candi-
date as possible power source for the wireless power-autonomous micro/nano
sensor-based system, and therefore in this thesis work they have been used
as the renewable ambient energy source.
1.3 Thesis objectives: proposed harvesting
system architecture
The block diagram shown in Fig. 1.4, of a possible newly architecture for
wireless power-autonomous micro/nano sensor-based system will be the ref-
erence of this thesis work. The system comprises of an array of micro/nano
sensors; a suitable interface to allow connecting the sensors array to the signal
conditioning circuitry (which performs amplification of the low level sensor
signals from sensors, adjust the output signal swing, sampling, converting
the signal from analog to digital format); an ultra low power elaboration
unit; an ultra low power RF module comprising of a transceiver; a harvester
(scavenger) connected to an optimized power management circuit to realize
the small, smart, efficient and low power/current consuming power unit.
The work presented in this thesis is primarily focused on the design and
implementation of the power unit for the power supply of the wireless sen-
sor system shown in Fig. 1.4. The power unit is composed by two basic
blocks, which are the vibration-based energy harvester (scavenger) and an
9

27. Figure 1.4: Block diagram of a wireless power autonomous sensor system.
integrated power management circuit, which is composed by an AC-DC con-
verter which rectifies the alternative signal delivered by the vibration-based
generator; a DC-DC converter or voltage regulator which adapts the level of
voltage to the storage element characteristics (e.g. battery) in the case of
energy harvesting systems, or (and) it adapts the voltage to the requirements
of the electronics units embedded in the wireless sensor system, in the case
of energy scavenging (harvesting) applications which require (not require)
a continuous operation. In fact, energy harvesting techniques aim to col-
lect ambient energy to help power systems, possibly storing energy when it
is not required (e.g. buffer batteries, capacitors, springs, supercapacitors)
[Ottman et al., 2002]. Energy scavenging techniques aim, on the contrary,
to scavenge the energy from the ambient to power electronics systems for
continuous operation [Metzger et al., 2007]. Therefore, different power man-
agement circuit approaches are used for energy harvesting and scavenging
systems according to the application, the amount of environmental energy
that can be converted into electricity, the kind and efficiency of the envi-
ronmental energy transducer. However, hybrid solutions where a storage
element, which could be a battery as well as a capacitor or supercapacitor,
can be adopted.
The study of the state-of-art of vibration-based generators (VBG) - i.e.
electrostatic, electromagnetic and piezoelectric - must first be addressed in
order to identify the suitable VBG to be used to convert ambient mechanical
10

28. vibrations to electricity and hence powering the embedded units of the sys-
tem in Fig. 1.4. A comparison among the advantages and disadvantages of
vibration-based generators - which will be discussed in the Chapter 2 - ad-
dressed us to explore the possibility of using piezoelectric bender generator
(PBG) as harvester (scavenger) to convert mechanical vibrations to elec-
tricity. Therefore, the focus of this thesis work is on the optimal design of
integrated power management electronics for a piezoelectric generator drove
by vibrations.
A feasibility case study - which will be reported and discussed in the
Chapter 3 - has the goal to explore and study the use of PBG has power
source for wireless sensor systems, in particular for a wireless tire pressure
measurement system embedded on the wheel of a car. The wheel of the
car is a very extreme environment where mechanical vibrations and radial
accelerations magnitude can reach values three order of magnitude larger
than those ones of common ambient vibrational sources. In fact, it is con-
sidered - according to Cantatore et al. - the only one environment which
could excite a vibration-based generator scaled to micro sizes so that it can
generate enough power density for the power supply of a state-of-art wire-
less sensor microsystem with on-board radio and sufficient processing power
[Cantatore and Ouwerkerk, 2006]. In regard to the integration of the system
depicted in Fig. 1.4, knowing if a VBG scaled at micro/nano sizes could
still be able to power a wireless sensor system and their limits is an im-
portant aspect that an electronic designer should know. By this knowledge
the electronic designer can design and optimize the electronic circuits in or-
der to adapt them to the energy made available by the micro/nano harvester.
Another important aspect that must be addressed and that is also use-
ful for an electronic designer is the development of a realistic model of the
power source, which in this case is the vibration-based generator. The
characteristic equations of the general model of a vibration-based genera-
tor [Williams and Yates, 1996] - as will be introduced and discussed in the
Chapter 2 - can give to the designer the knowledge of a roughly estimation
of the maximum power that a VBG can produce. Moreover, those character-
istic equations do not take into account neither the geometrical and physical
parameters of the generator, nor the kind of the vibration-based genera-
tor. The power output can be roughly estimated given only the magnitude
and frequency of input vibrations, the overall size (and therefore mass) of
the device, and knowledge of the mechanical and induced electrical damp-
ing ratios. Moreover, the general model is well suited for electromagnetic
generators rather than for piezoelectric generators. The electromechanical
11

29. model of the PBG developed by Roundy et al. [Roundy et al., 2004] can
give the designer a more accurate estimation of the power and voltage that
the generator can produce, because based on an analytical mathematical ap-
proach which takes into account both geometrical and physical parameters
of the generator itself. Anyway, for an electronic designer it is useful to
have available a SPICE model of the power source above all in the context
of the integration of the electronics on chip and the possibility to simulate
the overall system - SPICE model of source and integrated electronic - with
SPICE. The power sources which SPICE makes available (i.e. AC voltage
independent sources) are not suited to represent a vibration-based genera-
tor, in particular a PBG. Moreover, a SPICE model of the VBG can give
the designer the possibility of studying the reciprocal interaction among the
mechanical and electrical parameters, that the use of the only power sources
available in SPICE cannot offer. Therefore, based on the electromechanical
model of the PBG reported in [Roundy et al., 2004], which is suited to be
implemented in SPICE the first objective of this thesis work to be pursued
will be the implementation in SPICE of the electromechanical model of the
PBG - which will be discussed in the Chapter 4.
Vibration-based generator produces AC voltages that need to be con-
verted, regulated and stabilized in DC before being used to power electronic
systems. A diode bridge rectifier is the common and simplest approach fol-
lowed in literature to perform the AC-DC conversion. However, integrating
only passive devices as diodes on chip does not take advantage of the flexi-
bility of the use of active devices with optimized control circuit which could
give the possibility to manage in a smart way the power, and voltage gener-
ated by VBGs. Performance, efficiency and low-power consumption can be
improved, and this is an important aspect above all in the context of energy
scavenging applications, where the available energy may be poor. However,
a fully active solution for the bridge rectifier, might need a more complicated
control circuit to drive the active devices, which can increase the complex-
ity and power consumption of the system. Therefore, a compromise among
simplicity, efficiency, flexibility and performance is necessary. In this work a
semi-active approach for the bridge rectifier is proposed. The design of the
semi-active bridge rectifier is the second objective of this work and it will be
discussed in Chapter 5.
The voltage regulator is a basic unit of the power management system. It
can be used to regulate and adapt the diverse voltage levels needed to power
the electronics belonging a system and also for optimizing the power transfer
toward the final load, which could be a battery [Ottman et al., 2002] or the
12

30. Figure 1.5: Block diagram of the wireless power autonomous sensor system
with semi-active bridge and DC-DC switching voltage regulator blocks.
units embedded in the wireless sensor system shown in Fig. 1.4. A Low
Drop Out (LDO) linear voltage regulator can be used [Leland et al., 2006]
in a store-and-release power management strategy to allow or not allow the
power transfer from a storage element to the load. This energy harvesting
strategy is adopted when the power generated by the harvester is not enough
to power the circuit for continuous operation. The power management circuit
should then store the energy generated by the harvester into a capacitor or
battery, till sufficient energy is stored and hence it can be released to power
the electronic circuit. When the energy level decreases to a minimum value,
the power management stops the energy flow toward the load and starts
again to store the energy by charging the capacitor. The use of LDO linear
voltage regulators, however, has the drawback of the low conversion efficiency
- around the 40% - being dependent on the ratio between the output voltage
and input voltage.
DC-DC switching converter having high conversion efficiency - around
the 90% - can be used to perform more efficient power management strate-
gies for the energy harvesting from vibrational sources. Suitable adaptive
control algorithms can be implemented by using DSP [Ottman et al., 2002]
or built by using suitable circuits which implement the algorithm relations
[Ottman et al., 2003] in order to control the switches of the DC-DC switch-
ing converter by varying the duty-cycle to optimize the power stored by the
battery. Maximum power transfer from energy harvester to the load can
therefore be performed by means of smart control switch strategies.
13

31. Figure 1.6: Block diagram of the power management blocks realized as
ASIC and PCB.
In the case which both vibrational source and VBG can allow the con-
tinuous operation of the electronic system to be powered, e.g. a wireless
sensor, energy scavenging systems are better suited, as aforementioned, and
DC-DC switching converter can be used to perform two tasks which are the
maximum power transfer toward the load and/or regulation of the voltage
generated by the scavenger at the useful level in order to adapt it to the
right level needed by the circuit to be powered. In this last case, if differ-
ent circuit blocks need different voltage levels, hence, more DC-DC switching
regulators can be present in the power management block. DC-DC switching
converters are well suited to be used for energy harvesting and scavenging ap-
plications, thanks to their high efficiency and flexibility which allow them to
be controlled by different switching control strategies. Therefore, the DC-DC
switching converter as voltage regulator for energy scavenging applications
will be used also in this work. Because of a PBG generates high level voltages,
the use of a step-down DC-DC switching converter - i.e. a buck converter -
is needed.
The major effort in the development of the power management circuit
is the design of the driver circuit, which controls the switches of the con-
verter. In this work, the voltage regulated by the buck converter is used
not only for being compared with a reference voltage in order to perform
the regulation, but, also is used to power the control circuits of the power
management system - i.e. the rectifier control circuit and the driver circuit.
14

32. The block diagram shown in Fig. 1.5 illustrates the proposed architecture
for the power management system powered by the PBG. Therefore, the third
objective of this thesis work is the design of an integrated, power/current-
aware and self-sustaining voltage regulator. The issues addressed to make
the voltage regulator self-powered will be discussed in the Chapter 5, where
a SPICE analysis and simulation results of the whole system comprising of
the SPICE model of the PBG, the semi-active bridge and voltage regulator
will be presented.
The last part of this work concerns the design, development, fabrication
of a prototype ASIC, a test printed circuit board and a PBG prototype.
Fig. 1.6 shows the diagram block with the parts of the power management
circuit which have been integrated and those ones realized with discrete com-
ponents on printed circuit board. As it can be seen, only the rectifier and the
switching part of the step-down buck converter have been integrated. The
control circuits and the LC filter of the buck have been inserted in the test
PCB. This hybrid solution - ASIC and PCB - has been adopted in order to
have more flexibility in the design and for the experimental tests, integrating
the key components - rectifier and switching part of the DC-DC converter -
in the ASIC, while, the other components - control circuits and LC filter -
left outside the ASIC. Chapters 6, and 7 will present the design of the ASIC
test chip and test board, and the experimental validation test results.
In order to validate the SPICE equivalent model of the PBG is necessary
to compare it with a realistic PBG. By performing experimental tests with
the prototype PBG and hence comparing the measured results with the sim-
ulated ones, would allow understanding what are the limits of the developed
SPICE model and hence, through the study and understanding of the results
obtained, optimizing the developed SPICE model in order to make it closer
to the realistic counterpart. The concluding chapter of this work presents
the preliminary experimental tests realized with a fabricated PBG prototype,
either tested alone or connected to the ASIC and test board.
Moreover, in the context of the future trends, an analytical mathematical
analysis of a vibration-based hybrid generator, realized joining a PBG with
an electromagnetic generator is also presented.
15

33. Chapter 2
Vibration-based energy
harvesting
2.1 Introduction
The subject of this chapter are vibration-based energy generators, which
convert kinetic energy in the form of mechanical vibration present in the
environment where the generators are used, into electricity. Vibrations are
typically converted into electricity by means of electrostatic, electromagnetic
or piezoelectric transducers. The amount of energy/power that can be gen-
erated by these transducers depends fundamentally upon the quantity of
vibrations available in the application environment, the efficiency of both
the transducer and power management electronic.
Following sections illustrate the general model used to describe vibration-
based generators, based on a one-degree-of-freedom mass-spring-damper sys-
tem, and the state of art of the three kind of vibration-based generators
reported in the literature. A summary of the three vibration-based genera-
tors performances is given at the end of the chapter.
2.2 General model of a vibration-based gen-
erator
In the literature vibration-to-electricity converters are described like a one-
degree-of-freedom second order spring-mass-damper system connected to an
inertial frame, which acts as fixed reference forced by a vibration source.
Fig. 2.1 shows the equivalent system model in which there is an inertial
mass m, a spring of stiffness k and a damper of damping coefficients ce and
16

34. cm, named electrically induced damping coefficient and mechanical damp-
ing coefficient, respectively. The spring and the damper are both connected
between the inertial frame and the inertial mass. In Fig. 2.1, y(t) is the
absolute motion of the frame and that one of the proof mass is denoted as
x(t) = y(t) + z(t), where z(t) represents the displacement of the mass with
respect to its rest position relatively to the frame. The inertial frame is
useful for transmitting vibrations to the suspended inertial mass with the
result of producing the relative displacement z(t) between the inertial mass
and the frame. The relative displacement can then be used to generate en-
ergy by causing work to be done against the damping force. The nature of
this damping force realized by an electric or magnetic field, or by strain-
ing a piezoelectric material defines the type of vibration-based generator
(VBG) [Mitcheson et al., 2004]. Besides, such a system possesses a natural
resonant frequency, therefore vibration-based generators can be designed in
order to match the fundamental frequency of the environmental vibration
source. Matching the two frequencies allows to magnify the environmental
vibration amplitude by the quality factor of the resonant system.
With respect to the general model depicted in Fig. 2.1 the transducer
is described by the damper, because the conversion mechanism damps the
mass. The expression for the electrically induced damping coefficient, ce is
different for each kind of VBG [Beeby et al., 2006b, Mitcheson et al., 2004].
The inertial frame (who acts as a fixed reference) is excited by a sinusoidal
vibration source modeled as ¨y(t) = A sin(ωt − φ). Through the frame vibra-
tions are transmitted to the suspended inertial mass, producing the relative
displacement z(t) between them. This relative displacement is sinusoidal in
amplitude, so that it can drive a suitable transducer to generate electrical
energy [Williams and Yates, 1996].
Consider the mass-spring-damper system as shown in Fig. 2.1. Appli-
cation of the Newton’s law to the free-body diagram of Fig. 2.1 (b) yields
mg − k (x + ∆st − y) − cT ( ˙x − ˙y) = m¨x (2.1)
or
m¨x + cT ˙x + kx = cT ˙y + ky (2.2)
where cT = ce + cm. In (2.1) and (2.2) the static deflection of the spring
denoted as ∆st = (mg)/k cancels the gravity term mg because of the static-
equilibrium position condition of the system [Kelly, 2000]. Define
z(t) = x(t) − y(t) (2.3)
as the displacement of the inertial mass relative to the displacement of the
17

35. (a) (b)
Figure 2.1: General model of the linear mass-spring-damper system repre-
senting the resonant inertial generator (a). Free-body diagrams at an arbi-
trary instant including effects of the absolute motion of the frame, y(t) (b).
frame. Equation (2.1) is rewritten using z as the dependent variable
m¨z + cT ˙z + kz = −m¨y. (2.4)
Dividing equation (2.4) by m yields
¨z + 2ζT ωn ˙z + ω2
nz = −¨y (2.5)
where cT /m has been replaced by 2ζT ωn, with ζT defined as the equivalent
damping ratio and ωn = k/m defined as the natural frequency of the
system. If the absolute motion of the frame is given by a single-frequency
harmonic of the form
y(t) = Y sin ωt (2.6)
equation (2.5) becomes
¨z + 2ζT ωn ˙z + ω2
nz = ω2
Y sin ωt (2.7)
where ω can be defined as the fundamental frequency of the vibration source,
and (ω2
Y ) is the acceleration amplitude, denoted as A, of the vibrational
source. The standard steady-state solution of (2.7) for the inertial mass
displacement is given by
z(jω) =
ω
ωn
2
1 − ω
ωn
2
+ j2ζT
ω
ωn
Y sin (ωt − φ) . (2.8)
18

36. The square of the module of (2.8) is given by
|z|2
=
ω
ωn
4
1 − ω
ωn
2 2
+ 2ζT
ω
ωn
2 Y 2
(2.9)
and the phase φ is given by
φ = tan





2ζT
ω
ωn
1 − ω
ωn
2





. (2.10)
The mechanical power converted into electrical is equal to the power removed
from the mechanical system by the electrically induced damping coefficient,
ce. The electrically induced force is then, Fe = ce ˙z, and the related power is
defined as
Pe =
v
0
Fedv = ce
v
0
vdv (2.11)
where v = ˙z, then, the expression for the power becomes
Pe =
1
2
ce ˙z2
= ζeωnm ˙z2
(2.12)
where ce = 2ζeωnm. Knowing that ˙z = jωz and that | ˙z| = ω|z|, substituting
this last one relation into (2.12) and considering equation (2.9), it yields
[Williams and Yates, 1996]
Pe =
mζeY 2
ω2
ωn
ω
ωn
4
1 − ω
ωn
2 2
+ 2ζT
ω
ωn
2 (2.13)
where ζT = (ζp + ζm + ζe) is the total damping ratio. The term ζp represent
the parasitic damping caused by undesirable effects such as air resistance.
If the fundamental mode of the vibrational source matches the resonance
frequency, ω = ωn, of the system, the maximum power can be generated by
the system and it is given by
Pe,max =
mζeY 2
ω3
n
4ζ2
T
(2.14)
19

37. or in terms of the acceleration amplitude of the vibrational source
Pe,max =
mζeA2
4ωnζ2
T
. (2.15)
Equation (2.15) shows that the system must be designed so that its resonance
frequency (or natural frequency of the system) matches the lowest fundamen-
tal frequency of the vibrational source, due to the fact that the maximum
power at resonance is inversely proportional to frequency.
Equations (2.14) and (2.15) are steady-state solutions and are valid only
at resonance. From (2.13) it can be noted that the damping factor controls
the selectivity of the device, i.e. for applications where the vibration fre-
quencies are well known and concentrated around one point, a low damping
factor would give a more peaked response and power. According to (2.14)
or (2.15), if the damping factor reduces up to zero it is possible to obtain an
infinite generated power at resonance.
However, the damping factor cannot be equal to zero and so the electri-
cally induced power that can be generated has a finite value. Reducing the
damping factor, however, increases the displacement of the mass - z(t) - up
to a maximum limit value at resonance. At resonance, equation (2.9) gives
then, the maximum displacement of the mass
zmax =
Y
2ζT
(2.16)
which should be taken into account when designing vibration-based generator
(VBG) if the mass can move out the frame - limited in size and geometry
- where the generator itself is inserted. The maximum power that can be
generated by a VBG in terms of maximum displacement, is therefore
Pe,max = mζT ω3
nz2
max. (2.17)
As it has been said in the introduction, the characteristic equations of the
general model of a vibration-based generator [Williams and Yates, 1996] re-
ported above can give to the designer the knowledge of a roughly estimation
of the maximum power that a VBG can produce. The characteristic equa-
tions of the general model as illustrated do not take into account neither
the geometrical and physical parameters of the generator, nor the kind of
the vibration-based generator. The power output can be roughly estimated
given only the magnitude and frequency of input vibrations, the overall size
(and therefore mass) of the device, and knowledge of the mechanical and in-
duced electrical damping ratios, by using equations (2.13), (2.14), (2.15) and
20

38. (2.17). Moreover, the general model is well suited for electromagnetic genera-
tors rather than for piezoelectric generators [Williams and Yates, 1996]. The
general model, moreover, is not suited to be used as reference model from
which developed a SPICE model for a VBG. Then, Chapter 4 will introduce
a better model, based on an analytical electromechanical model, which de-
scribes better the behavior of a real VBG, it introduces better expressions
for the estimation of the power which can be generated by the VBG, but,
also, being based on an electromechanical model - described with equivalent
electrical elements of those ones of the general model - can be used as refer-
ence model from which deriving a SPICE equivalent version. The following
sections give the reader an overview of the state-of-the-art of vibration-based
generators, in order to understand what VBG is better suited to be integrated
with CMOS-based electronics and microfabricated. .
2.3 State-of-the-art of vibration-based gener-
ators
Transduction mechanisms used to convert vibrations into electricity can ex-
ploit the mechanical strain or the relative displacement, depending on the
kind of transducer. Anyway, the characteristic shared by vibration-based
generator is that they generate an AC output voltage, which needs to be
prior converted into DC before to be used to power electronic devices. The
basic and common vibration-based generators usually reported on literature
and used to perform the mechanical to electrical conversion are piezoelectric,
electrostatic and electromagnetic.
Piezoelectric materials subjected to mechanical strain induced by me-
chanical stress applied by vibrational source become electrically polarized.
The degree of polarization is proportional to the applied strain. The piezo-
electric material subjected to strain exhibits a creation of a negative charge
on the compressed piezoelectric surface and a creation of a positive one in
the tensed piezoelectric surface, resulting in a negative and positive voltages,
respectively, across the piezoelectric material. Therefore, if two electrodes
are sputtered onto the top and bottom surface of the piezoelectric material
a differential open circuit voltage can be obtained across the piezoelectric
material.
Electromagnetic vibration-based generators exploit as transduction mech-
anism the relative displacement in terms of velocity. They can consist of a
fixed coil and a mobile permanent magnet, or vice versa. The relative mo-
tion between the permanent magnet and the coil induces the creation of an
21

39. electromotive force into the coil proportional to the velocity of the moving
part according to the Lorentz’s Force and Faraday’s Law of induction. If a
resistive load is connected to the coil output terminals, a current starts to
flow into the coil.
Electrostatic vibration-based generators are practically a variable capaci-
tor, consisting of two conductors (one fixed and the other one movable) with
usually the air as dielectric. In this case is the relative motion between con-
ductors to allow converting vibrations in electricity and the generated voltage
is a function of the relative position between conductors.
Following sections give the reader a summary of the state-of-the-art of
the three kinds of VBGs.
2.3.1 Electrostatic generators
A vibration-based electrostatic generator is simply a variable capacitor, whose
plates are electrically separated from each other by a dielectric that can be
air, insulator or vacuum. In order to allow the beginning of the harvesting
process the capacitance needs to be previously charged by a priming volt-
age. The work done against the electrostatic force between the two plates of
the capacitor, made moving relatively from each other by vibrations provides
the harvested energy. Electrostatic generators are principally fabricated with
MEMS techniques. There are three basic types of MEMS vibration-based
electrostatic generators, which are the in-plane overlap converter, in-plane
gap-closing converter and the out-of-plane gap-closing converter (see Fig.
2.2) [Roundy et al., 2002]. The operating principle of the electrostatic gen-
erator bases its working on the variation of the electrostatic force between the
two plates. The electrostatic force variation depends on the structure of the
electrostatic generator and if the charge (i.e. charge constrained electrostatic
generator) or the voltage (i.e. voltage constrained electrostatic generator)
between the plates is held constant.
In-plane overlap converters can be operated only charge constrained, be-
cause the electrostatic force varies with inverse proportionality with respect
to the square of the displacement of the mass if the charge is held constant and
the voltage made varying. On the contrary, if the in-plane overlap converter
is operated voltage constrained the electrostatic force remains constant.
In-plane gap closing converters can be operated either in charge and volt-
age constrained. If operated charge constrained the electrostatic force varies
proportionally with the mass displacement, while, if operated voltage con-
strained the electrostatic force varies with inverse proportionality with re-
spect to the square of the mass displacement.
22

40. Figure 2.2: Electrostatic generators structures: in-plane overlap converter
(left), in-plane gap closing converter (center), out-of-plane gap closing con-
verter (right)
Out-of-plane gap closing converters can be operated only voltage con-
strained because the electrostatic force varies with inverse proportionality
with respect to the mass displacement, while, it remains constant if operated
charge constrained.
Among the three structures the optimization of the design parameters of
the in-plane gap closing converter could allow obtaining a generated power
density higher than the other two structures and equal to about 116 µW/cm3
at the frequency of 120 Hz and acceleration amplitude of 2.25 ms−2
. The
drawback is that it needs a priming voltage of 5 V. Out-of-plane gap clos-
ing converter is the following second structure for which, with an optimized
design, a high power density can be obtained [Roundy et al., 2002].
Miao et al. have designed an electrostatic vibration-based generator
named Coulomb Force Parametric Generator (CFPG) whose principal char-
acteristic is to be non-resonant. Therefore, it can operate over a wide range
of excitation frequencies and amplitudes. According to the operating fre-
quency and amplitude it can generate voltages up to 220 V. The dimensions
of this device are about 11x11x0.4 mm3
with a proof mass of 0.12 g. The
authors have predicted a generated power of 80 µW at 30 Hz and 10 ms−2
of acceleration amplitude [Miao et al., 2006].
Ryoichi et al. have designed an electrostatic generator named Honeycomb-
type variable capacitor (HVC) that harnesses heart ventricular motion with
the aim of driving a cardiac pacemaker permanently in place of the common
battery. This electrostatic generator has generated a power of 36 µW, which
has been enough to drive a cardiac pacemaker in order to perform a con-
tinuous electric generation and cardiac pacing for more than 2 hours in the
animal experimental test performed with canine heart [Ryoichi et al., 2002].
23

41. (a)
(b)
Figure 2.3: Electromagnetic generators: (a) MEMS-based by Beeby et al.,
Wang et al., respectively; (b) Millimeter scale by Torah et al., by Glynne et
al., respectively
2.3.2 Electromagnetic generators
Electromagnetic vibration-based generators bases their operation on relative
motion between a coil and a permanent magnet. The best design consists of
to keep the coil fixed and to have a mobile permanent magnet, mounted on
the free tip of a cantilever beam, so that it can be used as an inertial mass.
Designs with mobile coil and fixed permanent magnet has been proposed in
the literature, but they do not allow generating high output power values.
For example, Fig. 2.3 shows the laterally vibrating silicon microgenerator re-
ported in [Beeby et al., 2006a] which exhibits a power density of 1.8nW/mm3
- in a volume of about 68 mm3
- and generates only 122 nW of power at the
resonance frequency of 9.5 kHz into a 110 Ω load and acceleration amplitude
24

42. of 3.5 m/s2
. The laterally vibrating silicon microgenerator had the wound
copper coil placed into the moving proof mass connected to the wafer bulk
by a supporting cantilever beam. Therefore, coils move in the plane of the
wafer relative to the fixed permanent magnets. Adhesively bonding the wires
along the cantilever, increases the mechanical damping and then it decreases
the generated power.
The second microgenerator design proposed always by Beeby et al., de-
veloped to overcome the problems raised with the lateral design (above all
designed to avoid electrical connections to moving parts) has been realized
attaching four magnets mounted in turn onto a tungsten proof mass at the
free end of a steel cantilever beam, clamped at the other end as shown in Fig.
2.3. The magnets in this case move relative to the coil, that is fixed. The
total volume of the device was 60 mm3
and the generated power delivered
to a resistive load of 100 Ω at the frequency of 350 Hz, and acceleration am-
plitude of 3 m/s2
was about 2.85 µW [Beeby et al., 2006a]. The optimized
design of this generator, consisting of the optimization of the magnet size for
the same cantilever structure of the prior design, produced an output voltage
of 87 mVrms across a resistive load of 9 MΩ from 0.6 m/s2
vibrations at the
frequency of 60 Hz, and a generated power of about 17.8 µW across a resis-
tive load of 150 Ω. This optimization had the aim to increase the generated
output voltage that however it still remained too low, even if the generated
power is increased of almost an order of magnitude, with respect to the prior
design [Torah et al., 2006].
A vibration-based linear electromagnetic micro-generator suitable to power
wearable body sensor nodes, consisting of a stator coil and a flexible stack
translator of alternately magnets and spacer, succeeded to generate an output
power of 2-25 µW, depending on the generator position on the human body.
A two-stage procedure has been conducted by authors in order to optimize
the generator design. The optimization has consisted of firstly, the opti-
mization of the geometric parameters of stator and translator for maximum
electromagnetic force capability, by magnetostatic finite element simulations.
Secondly, the optimization of mechanical resonance frequency and load re-
sistance in order to maximize the output power, by using lumped-parameter
simulations and measured acceleration data from human walking motion.
The designed linear electromagnetic micro-generator has a volume of 0.25 cm3
when worn on the body during walking [von B¨uren and Tr¨oster, 2007].
Wang et al. presents a micro electromagnetic energy harvester which can
convert low level vibration energy to electrical power. The microgenerator,
as shown in Fig. 2.3 consists of an electroplated copper planar spring, a per-
manent magnet and a copper planar coil with high aspect ratio. Mechanical
simulation have shown that the natural frequency of the magnet spring sys-
25

43. tem is 94.5 Hz. Electromagnetic simulations have shown that the line width
and the turns of the coil can influence the induced voltage. The optimized
vibration-based electromagnetic generator can generate 0.7 µW of maximum
output power with output voltage of 42.6 mVpp at resonance and input accel-
eration amplitude of 4.94 m/s2
(i.e low level environmental vibration source).
The not-optimized prototype, fabricated using MEMS micromachining tech-
nology have shown from experimental tests to be able to generate induced
voltage of 18 mVpp and output power of 0.61 µW for 14.9 m/s2
(i.e. not low
level environmental vibration source) external acceleration amplitude in cor-
respondence of its resonant frequency of 55 Hz . The measured output power
generated by the prototype has shown to have a value close to the value of
the simulated power of the design model. However, the strength of the input
vibration (i.e. 14.9 m/s2
) used to excite the prototype has resulted much
higher than that one (i.e. 4.94 m/s2
) used to excite the optimized model.
The input vibration source used to excite the prototype does not belong to
common low level vibration amplitude presents in the environment. Excit-
ing the prototype with input acceleration and input vibration amplitudes -
which have been used in simulation - the output voltage and power that the
prototype can generate reduce to very small values. Therefore the fabricated
micro electromagnetic generator proposed by Wang et al. has resulted to be
not suitable for harvesting low level vibration energy [Wang et al., 2009].
Serre et al. have designed a vibration-based electromagnetic inertial mi-
crogenerator for energy scavenging applications, named Velocity Damped
Resonant Generator (VDRG), consisting of a fixed micromachined coil and
a movable permanent magnet (that operates as an inertial mass) mounted
on a resonant structure (Kapton membrane). This generator in a volume
in the range of 0.6-0.7 cm3
if optimized could generate a power of 280 µW
[Serre et al., 2007].
Glynne-Jones et al. have designed and fabricated a cantilever-based elec-
tromagnetic generator, with overall volume of 3.15 cm3
, as shown in Fig. 2.3,
based on a moving coil between four magnets capable of generating useful
level of power of average value of 157 µW when mounted on the engine block
of a car with peak of power of 3.9 mW [Glynne-Jones et al., 2004].
Beeby et al. have also reported about a macro vibration-based elec-
tromagnetic generator - the Perpetuum PMG7 generator - designed to res-
onate at 50 Hz, able to produce an AC output power of about 3 mW if
made vibrated by a vibration acceleration amplitude of 0.5 m/s2
. The
mass of the generator was 85 g and with a overall volume of 41.3 cm3
[Beeby et al., 2006a].
26

44. Figure 2.4: Illustration of the two modes of piezoelectric conversion from
input mechanical stress (denoted as σ1). In the figure the strain is denoted
as S1.
2.3.3 Piezoelectric bender generators
Vibration-based generators which exploit the piezoelectric effect in order to
convert the mechanical energy from vibrations into electrical energy, usually
are formed as layer-based cantilevers. The number of layers define the kind of
piezoelectric generator - usually unimorph structures composed by a piezoce-
ramic layer attached to a metal layer or bimorph structures where two piezo-
ceramic layers are attached to a central metal layer. A proof mass attached
on to the free end of the bimorph complete the vibration-based piezoelectric
generator, usually called Piezoelectric Bender Generator (PBG). According
to how the electrodes are placed onto the surface of the piezoelectric mate-
rial, as shown in Fig. 2.4, the cantilever beam transducer can operate either
in the 31 mode (also called transverse mode) - electric field perpendicular to
the stress/strain direction - or in the 33 mode (also called thickness mode)
- electric field parallel to the stress/strain direction. In the 31 mode the
electrodes are deposited on the top and bottom surface of the piezoelectric
material , while, in the 33 mode the electrodes are interdigitated. The ad-
vantage of the 33 mode with respect to the 31 mode is that with the 33
mode it can be generated a voltage 20 times than that one generated by the
PBG in the 31 mode. At microscale sizes interdigited PBGs are preferred
because the advantages of the 33 mode over the 31 mode in order to increase
the generated voltage and power. Scaling down the dimensions of the PBG,
the power that it can be generated decreases, while the resonance frequency
of the PBG increases. Vibrational sources as reported in the introduction
have a frequency spectrum range of 10-500 Hz. Scaling the PBG at micro
scale sizes increases the resonance frequency of the PBG system, then, it be-
27

45. comes difficult to design it to match the fundamental mode of the vibrational
source.
In [Leland et al., 2006] is reported about a millimeter scale PBG of bi-
morph dimensions 31.5x12.7x0.51 mm3
by Piezo Systems, Inc., with a tung-
sten proof mass of 52 g attached onto the free end, which has been excited by
vibrations generated by continuous traffic on a staircase. This PBG, shown
in Fig. 2.5 produced an instantaneous power of 30 µW in the time interval of
816 ms (time required by the sensor and radio hardware to turn on, initialize,
and transmitting the measure), after 50 minutes of continuous traffic during
which sufficient energy has been stored onto a capacitor allowing powering a
temperature sensor and radio - a MICA2dot by Crossbow, Tech - to transmit
the measured temperature.
Choi et al., have designed a MEMS PBG device (see Fig. 2.5) based on
thin film PZT/SiNx with proof mass placed onto the free end of bimorph
dimensions 170x60x1 µm3
and total volume, comprising of proof mass, of
2.2e-3 mm3
. the Pt/Ti interdigitated electrodes are patterned onto the top
piezoelectric in order to employ the d33 mode. The MEMS PBG is able
to generate 1 µW of continuous electrical power to a 5.2 MΩ resistive load
(in condition of impedance matching) at 2.4 V DC and at the resonance
frequency of 13.9 kHz [Choi et al., 2006].
Fang et al. have reported of a MEMS PBG - as shown in Fig. 2.5 -
with a proof mass that allows transferring 2.16 µW of power to a 21.4 kΩ of
resistive load in condition of impedance matching, at a resonance frequency
of 608 Hz. A first prototype of this MEMS PBG resulted in about 0.89 V AC
peak to peak output voltage to overcome germanium diode rectifier toward
energy storage. This MEMS PBG operates with the 31 mode with can-
tilever dimensions of 2000x600x13.64 µm3
and that ones of the proof mass
600x600x500 µm3
. The total volume was 0.615 mm3
. It is a composite can-
tilever made up of an upper piezoelectric thick film, sandwiched between a
pair of metal (Pt/Ti) electrodes, and with a lower non-piezoelectric element
[Fang et al., 2006]. The same authors reported in [Liu et al., 2008b] about a
design and fabrication of an array of piezoelectric bender generators based
on thick-film piezoceramic materials connected in series. The PBGs array
are fabricated with micromachining techniques on bulk silicon. The charac-
teristic of the designed array of series-connected PBGs is that each PBG has
a different width and length (same thickness) in order to cover a frequency
bandwidth in the range of 200-400 Hz. The design of a vibration-based
generator is strictly dependent on the knowledge of the vibrational source
fundamental frequency mode of the application where the VBG will operate.
The VBG should be in fact designed in such way that its natural frequency
matches the fundamental frequency mode in order to generate the maximum
28

46. (a)
(b)
Figure 2.5: Vibration-based piezoelectric generators: (a) Micromachined
cantilever by Marzencki et al., Choi et al. and Fang et al., respectively; (b)
Millimeter scale by Roundy et al. and Leland et al., respectively.
power. The knowledge of this fundamental mode could be sometime not prior
known or the frequency could change or be random. The prototype gener-
ator designed and fabricate by Liu et al. hence wanted to have a measured
performance of 3.98 µW effective electrical power and 3.93 V DC output
voltage to resistive load. This device is promising to support networks of
ultra-low-power, peer-to-peer, wireless nodes.
Marzencki et al. have proposed a MEMS PBG with a seismic mass (see
Fig. 2.5) with dimensions 800x1200x525 µm3
comprising of the seismic mass
(the thickness of the cantilever is equal to 5 µm). The resonance frequency
of the MEMS PBG is equal to 1.3 kHz. The reported experimental results
stated the possibility of exploiting very low amplitude signals produced by
the PBG in order to charge a storage capacitor. It is shown that power of
2 µW at 1.6 V can be obtained from this MEMS PBG at 4g (g = 9.8 ms−2
)
excitation [Marzencki et al., 2007].
At macro and micro scale gravitation force plays the major role for driv-
29

47. Figure 2.6: Piezoelectric nanogenerator based on Zinc Oxide nanowire ar-
rays by Wang, Z. L. et al..
ing the oscillation induced by vibrations of the inertial mass attached onto
the free end of the cantilever beam. However, scaling the PBG at nano
sizes the gravitation not influences the motion of the mass anymore. There-
fore, other approaches need to be found. Piezoelectric bender generators
at nano scale have been investigated by Liu et al. [Liu et al., 2008a] (see
Fig. 2.6). The nano generator made of nanowires of ZnO with diameters
of ∼ 100 nm and lengths of 5 µm made growing on polymer substrate with
an effective area of 6 mm2
, this piezoelectric nano bender generator can pro-
duce a continuous current (of about 500 nA) and voltage (of about 10 mV).
Besides, considering the effective area it results in an effective current den-
sity of 8.31 µA/cm2
and a power density of 83 nW/cm2
. From the same
authors in [Wang et al., 2008] it is estimated that if every nanowire spreaded
in an effective area of 1 cm2
were involved in generating electricity the output
power could reach 10 µW/cm2
, which with the newly 3D integration tech-
nology1
could be also improved. Fig. 2.6 shows also the package fabricated
to contain the nanowires using the 3D integration technology with the zigzag
1
The 3D integration technology consists of stacking integrated circuits, heterogeneous
devices and technologies and connecting them vertically.
30

48. top electrode drove by an external ultrasonic wave or mechanical vibrations.
Through the relative deflection/displacement between the nanowires and the
zig zag electrode mechanical energy can be converted into electricity.
2.4 Summary
Vibration-based electrostatic generators have the advantages to be easily in-
tegrated in silicon-based microsystems, because they can be realized with
MEMS technology. Besides, miniaturization is facilitate because of the gen-
erated energy density increases by reducing the gap between conductors of
the capacitor. On the contrary, the generated energy density decreases when
decreasing the surface area. Further, high transduction damping at low fre-
quency can be achieved only incorporating small capacitor gaps and high
voltages, rising the risk of capacitor electrodes shorting and ’stiction’ when
wafer-scale implemented. However, electrostatic generators - being variable
capacitor - need to be prior polarized by a priming voltage, that could be very
high (> 100 V). Other drawbacks regard the high output impedance (order
to GΩ), generated low output current and very high level output voltages
(order of hundreds of volts).
The voltage (and so the power) that it can be generated by a vibration-
based electromagnetic transducer depends of the length and thickness of the
wire of the coil. The greater the length of the coil (and so the number of
turns) is, the greater the voltage and the power which can be generated
are. The problem is that the greater the number of turns, the greater the
self-inductance of the coil. A high self-inductance needs a long conduction
period to reach the value of current corresponding to optimal damping, and
this can lead to high resistive losses. Furthermore, adding more conductor
material to the coil 1) increases the area or the length over which flux must
be supported in the air gap between the magnetic materials and 2) requires a
larger volume of permanent magnet [Mitcheson et al., 2007]. The request of a
large length of the coil to achieve high voltages and the wafer-scale integration
is quite difficult to achieve because of the poor properties of planar magnets
and the limited number of turns which can be achieved. Electromagnetic
generators can offer a wide variety of configurations and can be realized
with various type of materials. They can generate high output current and
power. However, the major drawback of the electromagnetic generators is
that even if the power that they can generate can be comparable with the
power generated by piezoelectric generators, as will be illustrated in the next
section, the voltage values they can generate are lower than 1 V. Therefore,
the voltage generated by vibration-based electromagnetic generators needs to
31

49. be prior increased in order to be suitable to power practical wireless sensor
applications. Anyway, charge pumps, or voltage multipliers can be suitable
for increasing the low voltage generated by electromagnetic generators, and
besides, they operate also as rectifiers.
Piezoelectric generators offer the simplest approach for converting vibra-
tions into voltage by exploiting the piezoelectric effect which allows obtaining
voltage by straining the material directly. Piezoelectric bender generators are
capable of producing relatively high output voltages, powers and power den-
sities. However, because of the high output impedance - greater than 100 kΩ
- they are able to produce relatively small currents and in order to pro-
duce voltage and power the materials with which PBG are made require to
be directly strained, limiting performances and lifetime because of subject to
wear. The design of a PBG does not require complex geometries and it is the
simplest Vibration-Based Generator to fabricate. Several existing processes
for piezoelectric material deposition exist and can be exploited to fabricate
micromachined PBG to be integrated with silicon technology and then with
CMOS-based circuits. Moreover, PBG can be well miniaturized by using
high quality piezoelectric thin layers, whose properties are recently improved
and made close to that ones of bulk materials [Ledermann et al., 2003] in
order to allow the piezoelectric device working efficiently. It has been de-
cided therefore to explore the possibility of building the energy scavenging
integrated system employing the Piezoelectric bender generators as power
source because the most suitable among vibration-based generators to be
integrated with CMOS-based circuits.
32

50. Table2.1:Summaryofvibration-basedelectrostaticgenerators
Reference
PowerVoltageFAVolume
Characteristics
(µW)(V)(Hz)(m/s2)(mm3)
[Roundyetal.,2002]11N.A.1202.251000
In-planeoverlapconverter;
primingvoltageof5Vis
required
[Roundyetal.,2002]50(@dielectricgap0.25µm)N.A.1202.251000
In-planegapclosingconverter;
primingvoltageof5Visre-
quired
[Roundyetal.,2002]
20(@0.001atmospheres)N.A.1202.251000
Out-of-planegapclosingcon-
verter,Powerintheuseful
rangeforuseinsomeapplica-
tions;primingvoltageof5Vis
required
1e-3(@1atmospheres)N.A.1202.251000
Powertoolowtobeusedfor
anyapplication
[Miaoetal.,2006]80220301050
Coulomb-ForceParametric
Generator;primingvoltageof
100Visrequired
[Ryoichietal.,2002]362.46N.A.>10,000
Honeycomb-typevariableca-
pacitor;energyharversting
generatorforharnessingheart
ventricularmotionforcardiac
pacemaker;primingvoltageof
45Visrequired
33

51. Table2.2:Summaryofvibration-basedelectromagneticgenerators
Reference
PowerVoltageFAVolume
Characteristics
(µW)(V)(Hz)(m/s2)(mm3)
[Beebyetal.,2006a]0.1223.7e-3(@110Ω)9.5e33.568
Mobilecoil,fixedmagnets,mi-
cromachined
[vonB¨urenandTr¨oster,2007]2-25>1N.A.N.A.250
Lineargenerator,Mobilestack
magnets(translator),fixedcoil
(stator),body-worn
[Wangetal.,2009]0.742.6e-394.54.94
Micromachined,electro-plated
copperplanarspringwithper-
manentmagnet(mobile),cop-
perplanarcoil(fixed)
[Beebyetal.,2006a]2.8517e-3(@100Ω)350360
Cantilever-based,not-
optimized
[Torahetal.,2006]17.852e-3(@150Ω)600.6N.A.Cantilever-based,optimized
[Glynne-Jonesetal.,2004]157-3.9e3<1106N.A.3150
Cantilever-based,fourmoving
magnets,fixedcoil
[Beebyetal.,2006a]3e3N.A.500.541300
Commercialdevice-Per-
petuumPMG7
34

52. Table2.3:Summaryofvibration-basedpiezoelectricbendergenerators
Reference
PowerVoltageFAVolume
Characteristics
(µW)(V)(Hz)(m/s2)(mm3)
[Wangetal.,2008]12.4(@5.4MΩ)13.9e3N.A.2.2e-3
NanogeneratorbasedonZinc
Oxidenanowires
[Choietal.,2006]12.4(@5.4MΩ)13.9e3N.A.2.2e-3
Micromachined;Interdigitated
electrodes;exploitd33mode
[Fangetal.,2006]2.160.89AC(@21.4kΩ)608N.A.0.615
Micromachined;exploitd31
mode
[Marzenckietal.,2007]21.61.3e339.20.504
Micromachined;exploitd31
mode
[Roundyetal.,2004]210>101202.51000
PZT-5H/Brass;Tungsten
mass;exploitd31mode
[Roundyetal.,2003]375>101202.51000
PZT-5H/Brass;Tungsten
mass;exploitd31mode
35