A Design Study to Harvest Electrical Energy from
Walking and Running Motions
by
LC')
Cf)
Kelsey C. Seto
MI
Submitted to the Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
.) LL
U,
Bachelor of Science in Mechanical Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2015
@ Massachusetts Institute of Technology 2015. All rights reserved.
Signature redacted
A uthor ...................
Department of M1 anical Engineering
May 8, 2015
Signature redacted
Certified by..................
-7
Sang-Gook Kim
Professor of Mechanical Engineering
Thesis Supervisor
Signature redacted
A ccepted by .........................................................
Annette Hosoi
Professor of Mechanical Engineering
Undergraduate Officer
<
=
co
2
.................
a
A Design Study to Harvest Electrical Energy from Walking
and Running Motions
by
Kelsey C. Seto
Submitted to the Department of Mechanical Engineering
on May 8, 2015, in partial fulfillment of the
requirements for the degree of
Bachelor of Science in Mechanical Engineering
Abstract
This thesis studies two different methods of harvesting electrical energy from everyday
activities such as walking and running. It is a design study that aims to create a device
which can be attached or incorporated into a shoe, ideally a military boot, so that
soldiers can charge back-up batteries for their devices while out in the field. The goal
was to create a device that could achieve a peak energy harvesting power output on
the order of 0.1 Watts. The original concept for the device involved the use of macro
piezoelectric fiber harvesters which harness strain energy from the sole of the shoe as it
naturally bends and flexes throughout daily activity. Strain testing indicated the the
maximum peak power output that could be expected from these actuators was on the
order of 10- 4 W to 10- 3 W, and testing of the harvesters themselves yielded peak power
values on the order of 10
7
W to 10-6W. These low power values turned the design
study away from the use of piezoelectrics and a design incorporating a miniature air
turbine coupled with an electromagnetic generator was introduced. Initial testing
on this proof of concept device yielded peak power values on the order of 10- 4 W to
10- 3 W with much room for improvement. It was concluded that this sort of device
would be more effective for harvesting energy from the shoes, and future iterations
of the initial prototype were proposed.
Thesis Supervisor: Sang-Gook Kim
Title: Professor of Mechanical Engineering
3
4
Acknowledgments
I would first and foremost like to thank my thesis advisor Professor Sang-Gook Kim
for all of his support and guidance throughout the research and writing processes. He
has been extremely patient and understanding with me and has provided me with
insightful direction for both my research and my career ahead. If it had not been for
his teachings in my micro and nano engineering laboratory, I am not sure that I would
have found myself pursuing an education in the field in the first place. He is an inspiration to me and I am truly grateful to have had the opportunity to work with him.
I would also like to extend thanks to Ruize Xu (soon to be Dr. Ruize Xu), the
incredible PhD candidate who worked with me throughout the entire research process, answering all of my pestering questions and showing me the ropes of testing
piezoelectric harvesters. I could not have done this without him.
Finally, I would like to thank my friends and family for their continuous support
and encouragement.
5
6
Contents
Walking and Running Energy . . . . . . . . . . . . . . . . . . . . .
14
1.2
Past Energy Harvesting Studies . . . . . . . . . . . . . . . . . . . .
15
4
.
17
Piezoelectric Energy Harvesting
Strain Energy . . . . . . . . . . . . . . . . . . .
2.2
Piezoelectric Fibers . . . . . . . . . . . . . . . .
19
2.3
M ethod
. . . . . . . . . . . . . . . . . . . . . .
21
2.3.1
Quantifying Strain . . . . . . . . . . . .
21
2.3.2
Testing Piezoelectric Energy Harvesters .
22
Results and Discussion . . . . . . . . . . . . . .
23
Exploring New Methods of Harvesting Energy
35
.
.
.
.
.
.
2.1
2.4
3
.
1.1
. . . . . . . . . .
17
3.1
Apparatus and Procedure
. . . . . . . . . . . .
37
3.2
Results and Discussion . . . . . . . . . . . . . .
40
.
2
13
Introduction
.
1
43
Conclusion
A Tables
45
B Figures
49
7
8
List of Figures
2-1
Strain gauge attached to the sole of the shoe under the ball of the foot
2-2
d33 type PZT energy harvester secured inside the shoe under the ball
21
of the foot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
2-3
Shoe strain while walking
. . . . . . . . . . . . . . . . . . . . . . . .
25
2-4
Shoe strain while running
. . . . . . . . . . . . . . . . . . . . . . . .
26
2-5
Shoe strain while rocking back and forth . . . . . . . . . . . . . . . .
27
2-6
Strain power output while walking
. . . . . . . . . . . . . . . . . . .
28
2-7
Strain power output while running
. . . . . . . . . . . . . . . . . . .
29
2-8
Strain power output while rocking back and forth . . . . . . . . . . .
30
2-9
Measured power output from PZT energy harvester at the midpoint .
31
2-10 Measured power output from PZT energy harvester at the ball of the
.. . . . . . . . . . . . . . . . . . . . . . .
32
3-1
Optical disk drive motor [19] . . . . . . . . . . . . . . . . . . . . . . .
36
3-2
Salad spinner that converts translational to rotational motion [18] . .
36
3-3
Miniature motor used as a generator in this study [8] . . . . . . . . .
37
3-4
Standard dental handpiece [16]
. . . . . . . . . . . . . . . . . . . . .
38
3-5
Various turbine designs used in modern pressure-driven dental drills [1]
3-6
Dental drill air turbine [2]
. . . . . . . . . . . . . . . . . . . . . . . .
39
3-7
Blood pressure pump bulb [3] . . . . . . . . . . . . . . . . . . . . . .
39
B-1 Comparison of d33 and d31 MFC actuators [11] . . . . . . . . . . . .
49
B-2 Schematic of the dental handpiece [5] . . . . . . . . . . . . . . . . . .
50
foot
.
. . . . . . ....
9
38
10
Measured Strain Testing Results . . . . . . . . . . .
24
2.2
Power predictions for PZT energy harvesters . . . .
.
25
2.3
Measured power output from PZT energy harvester
29
.
2.1
.
List of Tables
. . . . .
45
A.2 Outputs of pre-existing walking energy harvesters [21]
. . . . .
46
A.3 Strain Gauge Specifications [12] . . . . . . . . . . . .
. . . . .
46
A.4 M FC Specifications [11]
. . . . .
47
.
.
.
A.1 Estimation of available walking energy [21] . . . . . .
.
. . . . . . . . . . . . . . . .
11
12
Chapter 1
Introduction
The world already harvests energy from water, wind, steam, and other natural sources,
yet one natural source that has remained rather untapped throughout the alternative energy boom is human power. The average moderately active 20 year-old man
burns about 2,800 kilocalories or about 12,000 kiloJoules per day and the average
moderately active 20 year-old woman burns about 2,200 kilocalories or about 9,000
kiloJoules per day1 [14]. The majority of that energy is used for internal functions
such as respiration, but a significant amount of energy is also used for mobility: predominantly walking and running. This kinetic energy of walking and running motions
is absorbed by the environment but has the potential to be collected and converted
into useful electrical energy for charging batteries. This thesis aims to evaluate two
different methods of harnessing this energy: firstly using piezoelectric energy harvesters to collect strain potential energy and secondly using a mechanical setup to
directly convert kinetic energy.
Chapter two of this thesis describes in detail the available strain energy stored in
shoe soles throughout the course of walking and running motions and the potential
for this energy to be harvested using piezoelectric energy harvesters.
Chapter three steps away from the discussion of strain energy and proposes a new
method of harvesting energy from walking and running motions by directly converting
'The average man is assumed to be 5 feet 10 inches tall and weigh 154 pounds and the average
woman is assumed to be 5 feet 4 inches tall and weigh 126 pounds
13
kinetic energy to electrical energy through the use of an air pocket and an air turbine.
Initial results of this mechanism are discussed and future iterations are proposed.
Chapter four describes future applications for this energy harvesting technology
and proposes further research and development of the mechanism required to create
a functional and practical prototype.
1.1
Walking and Running Energy
To clearly define the energy harvesting goals of this study, it is necessary to first
define and quantify the amount of energy available for harvesting from walking and
running motions. Several research studies have been carried out in the past to serve
this purpose; however, the numbers vary significantly, ranging from 5W to 324W of
output power 2 . The following equation is used by Thad Starner to estimate a walking
power output of 67W [17]:
68kg -
9.8m
2
2steps
. 0.05m
se
67W
(1.1)
This equation assumes that a subject weighing 68kg takes a step twice a second and
lifts his or her foot 5cm off of the ground with every step. However, considering that
the shoe that the subject is wearing is only depressed about 1cm with every step,
it can be estimated that the shoes stores significantly less than the 67W of power
predicted above [21]. Predicting that the maximum energy return out of the shoe is
about 50%, the maximum power output from the shoe can be estimated retaining the
same assumed subject weight and walking frequency:
9.8m
2steps
68kg - s 2 -0.Olm -sec
50%
sec
sec
6.7W
2 See Table A. 1 in the appendices of this thesis for more detailed results
14
(1.2)
wh
-
. -,
."Aa- 1--
"'.
Increasing the frequency from 2 steps per second to 5 steps per second to represent
running motions yields:
68kg -
9.8m
2
sec2
. 0.01m -
5steps
sec
-50%
16.75W
(1.3)
On the basis of these calculations, the goal energy harvesting power output for this
study was set at 1% of this available power: about 0.1W.
1.2
Past Energy Harvesting Studies
Different devices have been developed with varying levels of success in attempts to
harvest energy from walking and running motions.
These devices, documented in
Table A.2 of the appendices, report power outputs ranging from 0.25mW to 650mW
average power and 1.4mW to 1W peak power. One study of particular interest was
carried out in the MIT Media Lab and involved the development of two piezoelectric
harvesters fitted for shoes [10]. The first of these devices was a polyvinylidinefluoride
(PVDF) stave placed in the insole of the shoe underneath the ball of the foot, and the
other was a lead zirconate titanate (PZT) unimorph placed in the insole underneath
the heel. The stave consisted of 16 layers of piezoelectric material to maximize energy
collection. It harvested a peak power of 18mW from the strain energy produced by
the natural bending of the shoe sole over and area of 65 cm 2 . This yields about
277V'. The PZT unimorph yielded a peak power of 80 mW from the impact of the
heel strike over an area of 49 cm 2 . This yields about 1.63'W; however, the study
determined that piezoelectric energy harvesters should not be implemented directly
under the heel as the high impact damages the devices.
The Media Lab also developed magnetic rotary generators that were mechanically
driven by the impact of the heel strike [10]. These generators yielded power outputs
roughly 2 orders of magnitude larger than the piezoelectric devices; however, they
were found to be very cumbersome and difficult to discretely integrate into the shoes.
The pros and cons of these three devices were taken into consideration when
15
studying and developing energy harvesters throughout this thesis.
16
Chapter 2
Piezoelectric Energy Harvesting
As piezoelectric devices are being developed, they are becoming progressively more
useful for energy harvesting. This can be partially attributed to the development of
piezoelectric mechanisms which can operate at low frequencies and low accelerations.
As demonstrated in Ruize Xu's work towards developing an optimized doubly clamped
beam resonator for coupling with piezoelectric harvesters, piezoelectrics have the
potential to harvest energy from vibrations resonating at ambient frequencies [20].
Xu's bi-stable nonlinear resonance-based energy harvester is designed to generate
electric power from vibrations ranging from 200-300Hz and excitation accelerations of
0.5g. This is a vast improvement from the previous resonator design which generated
power from vibrations close to 1.3kHz and excitation accelerations of about 5g [20].
These strides to lower operating frequencies make piezoelectric devices more practical
for real-life applications such as harnessing power from walking and running motions.
2.1
Strain Energy
The Wheatstone Bridge circuit is a mechanism used to determine the resistance
change in a connected strain gauge.
When the sole of the shoe bends, the strain
changes and the coupled strain gauge, which is a variable resistor in the Wheatstone Bridge circuit, experiences a change in resistance which consequently alters the
voltage output of the system.
17
This change in voltage output Vmeas can be used in combination with the constant
of proportionality for the bridge Kmn, the gauge factor of the strain gauge F, and the
initial, unstrained resistance of the strain gauge RO to determine strain of the bent
shoe sole E.
S
-Vmeas
Km - Fg- Ro
(2.1)
Strain values can then be converted to energy density p, energy U, and power P
using the specifications of the strain gauges used. Assuming that all of the bending
and straining of the shoe sole occurs in the elastic regime of the stress strain curve,
the energy density can be determined by calculating the area underneath the linear
portion of the stress strain curve using the Young's Modulus E of the strain gauge.
12
-EE
2
P
(2.2)
Multiplying the energy density p by the volume V of the strain gauge yields the total
amount of strain potential energy stored in the strain gauge.
Umech
1
=
2
EE2 . V
(2.3)
Taking the derivative of the strain energy stored in the gauge with respect to time
yields the power output of the system.
1
-( IEVE2)
d
Pmech
dt2
(2.4)
The peak and average power defines the system's potential to charge batteries and
produce a useful output for the design purposes of this study.
18
M
11,111
lllng ip
|
|
V||
IIIIi
IIlrl11|
11|
if111111,
2.2
Piezoelectric Fibers
Piezoelectric fibers sandwiched between conductive elastic substructures induce a
charge when the material is strained. When these fibers are lined up side by side, the
straining of the arrangement creates a difference in electrical potential, generating a
small electric field and inducing a current flow in the device. This flow of electrical
energy can be harvested from the device and has the potential to charge batteries for
use in other applications. The amount of strain energy that these fibers can harvest
is determined by the electromechanical coupling coefficient K2 . This constant is
specific to the piezoelectric material and mechanism involved and represents the ratio
between the amount of electrical energy can be stored in the piezoelectric fibers and
the amount of mechanical strain energy applied to the fibers or vice versa. The larger
the K2 value, the more effective the piezoelectric energy harvester.
The ratio of the amount of electrical energy that can be extracted from these
harvesters to the amount of electrical energy that can be stored in the harvester is
dependant on the efficiency of the specific harvester itself. This efficiency is typically
in the range of 10 - 15%.
Based on the amount of available mechanical energy calculated from strain testing
Umech,
the maximum amount of stored electrical energy Ueec in the piezoelectric
energy harvester can be calculated:
Uelec
Umech
K
2
(2.5)
The amount of stored electrical energy can then be used to predict the amount of
energy that can be expected to be extracted from the energy harvester by applying
the efficiency of the particular device:
E
= Efficiency -Ue1ec
-
(2.6)
And, consequently, because power is simply the derivative of energy, the following
19
relationships also apply:
Peiec
Pmech- K 2
(2.7)
Put
Ef f iciency - Peiec
(2.8)
This power output Pnt is representative of the potential for a piezoelectric energy
harvester of the same size as the strain gauge to harness energy and charge devices.
The energy harvesters do not necessarily need to be the same size as the strain gauges
which are very small compared to the surface area of the shoe sole. To more accurately
predict the power output of the piezoelectric device, it is more useful to express this
power as a power per unit area P
. Multiplying this value by the actual area of the
piezoelectric energy harvester will yield the expected power output for that specific
device Ppzt:
PZt = Pot -Areat
Areagauge
(2.9)
Experimentally, Pmea, can be determined by connecting the piezoelectric energy
harvester to a known external resistance R and measuring the electrical potential
drop Vmeas across that resistor while straining the harvester. Electrical power Pmeas
is defined as a function of this change in electrical potential across the resistor and
the current I through the resistor. The current flow I through the resistor is simply
defined as the drop in electrical potential Vmeas across the resistor divided by the
resistance R of the external resistor, so the power can be defined as such:
Pmeas = Vmeas *I
V
R
V2
Pmeas =
(2.10)
meas
R
(2.12)
This power Pmeas is representative of the actual potential for energy harvester to
charge power sources for other useful applications.
20
2.3
2.3.1
Method
Quantifying Strain
To determine the total potential strain energy that could be harvested from the
bending of a shoe sole, simple strain tests were carried out on different points along
the sole. A women's size 5 rubber-soled tennis shoe was used throughout all of the
experiments. This particular shoe was chosen for it's relatively thin, flat, flexible sole.
Axial strain gauges were used for all strain measurements and were attached using
a cyanoacrylate adhesive. The two gauges were positioned so that their axes were
aligned from the heel to the toe of the shoe.
Figure 2-1: Strain gauge attached to the sole of the shoe under the ball of the foot
As seen in Figure 2-1, The first strain gauge was placed beneath the ball of the
foot, where the shoe bends most while the wearer is walking or running and where
the most strain energy is expected to be stored and released. The second was placed
on the sole halfway between the ball of the foot and the arch of the foot. The wires
connected to each of the strain gauges were also secured to the sole and to the body
of the shoe to prevent unwanted movement and resulting noise in the signal. All tests
21
were carried out on a flat, indoor surface.
Each strain gauge was individually wired to a wheatstone bridge circuit which
was calibrated specifically to each of the strain gauges to compensate for any slight
manufacturing variations. The wheatstone bridge was then fed into an amplifier, and
the resulting output voltage signal was read and analyzed.
The placement of these strain gauges enabled the general mapping of strain energy
across the sole of the shoe. This allowed for a better understanding of where to place
the energy harvesters to optimize the energy output for the specifications of the
particular harvesters used.
Each of the strain gauges at the different locations along the shoe sole was subject
to three different tests: walking, running in place, and rocking back and forth from
heel to toe. Multiple actions were tested to produce a spectrum of energy data to
understand the different motions that the piezoelectric energy harvesters would be
able to handle at each location along the sole.
2.3.2
Testing Piezoelectric Energy Harvesters
A smart-materials Macro fiber composite (MFC) d33 effect actuator/sensor 1 was used
throughout testing as a piezoelectric energy harvester. To eliminate some inconsistencies, the harvester was first flexed by hand to break it in before testing began.
The physical difference between the two types of MFCs is explained by Figure B-1 in
the appendices of this thesis.
To avoid damaging the PZT fibers, the piezoelectric energy harvesters were attached to the inside of the shoe sole as opposed the exterior sole where the shoe makes
direct contact with the ground. The harvesters were wired into a closed loop in series
with a resistor. Various resistors were used over the course of the trials to compare
outputs. The voltage drop across this resistor was measured and used in conjunction
with the resistance value of the resistor used to calculate the energy collected and
'Important specifications for this particular device and a visualization of the difference between
d31 and d33 actuators/sensors can be found in Table A.4 and Figure B-1 respectively in the appendices of this thesis.
22
Figure 2-2: d33 type PZT energy harvester secured inside the shoe under the ball of
the foot
power produced throughout the tested motions. As before, the following motions
were tested with the piezoelectric energy harvesters in place: walking, running in
place, and rocking back and forth from heel to toe.
2.4
Results and Discussion
As was expected, the sole of the shoe was subject to significantly more straining at the
ball of the foot than at the midpoint between the arch of the foot and the ball of the
foot. Although not entirely apparent in the relaxed walking motions, measuring the
shoe sole strain during running and rocking motions made it clear that more strain
energy was being stored towards the ball of the foot where the shoe typically bends
most.
Due to the difference in the position of the strain gauges, numerical comparisons
were made between the peak-to-peak amplitude of the strain AEma, in the shoe sole
over the course of one step (or analytically speaking one cycle) as opposed to the
23
absolute strain c to account for the fact that the gauges experienced different resting
strain once the shoe conformed to the wearer's foot.
Position
Midpoint
Motion
' Emax
Pmech,max
walking
0.296%
0.367mW
running
0.197%
0.302mW
rocking
0.292%
0.577mW
walking
0.282%
0.498mW
running
0.365%
2.28mW
rocking
0.626%
4.57mW
Ball of
Foot
Table 2.1: Measured Strain Testing Results
As seen in Figure 2-3, the walking motions produced strain amplitudes and profiles
that were very comparable between the two points on the shoe sole. There was only
a 0.014% strain difference between the respective
Zemax
values at the midpoint and
at the ball of the foot. This indicates that the strain was relatively constant in the
middle area of the shoe while the subject was walking.
However, as the activity
became more rigorous, this difference increased. As seen in Figure 2-4 and Figure 2-5
and as quantified in Table 2.1, the difference in peak-to-peak strain amplitude between
the two points on the shoe increased significantly to 0.168% strain and 0.334% strain
as the subject carried out running and rocking motions respectively. While the ball
of the foot experienced a change of 0.344% strain in peak-to-peak strain amplitude
between walking and rocking motions, the midpoint did not experience such large
amounts of fluctuation. The change in peak-to-peak strain amplitude between the
same motions at the midpoint was a minuscule -0.004%.
From this data we can determine that, as predicted, the ball of the foot of the
shoe houses significantly more strain potential energy over the course of various active
motions than does the midpoint between the ball of the shoe and arch. Analyzing the
corresponding strain power produced throughout the various motions further indicates
24
0.6
Ball of Foot
---
0.5
- Midpoint
0.4
0.3
-
-
0.2
(I)
0.1
0
-0.1
-0.2'
I
I
1
0
I
2
3
Time [s]
i
I
4
5
6
Figure 2-3: Shoe strain while walking
that this location also has a higher potential energy harvesting power output.
Position
Midpoint
Motion
Pout,max/Area
Pzt,max
walking
94.6 P
370.8 [uW
running
78.0
305.76 pW
rocking
148.8 'W
583.3 [LW
walking
128.5
503.7 puW
running
587.8 t'
2.30 mW
rocking
1178.6 P
4.62 mW
m
P
Ball of
CM2
Foot
Table 2.2: Power predictions for PZT energy harvesters
Converting the strain energy versus time data for each of the trials yields the strain
power output at each of the two points of interest for the three different motions. The
maximum strain power output
Pmech,max
listed in Table 2.1 at the midpoint between
the ball of the foot and the arch was rather consistent across all three motions.
25
........
......
....
0.6
Ball of Foot
-
Mid nint
8
9
-
0.5
- --
0.40.30
1'3 E
0.120.1
1,
%%.,~.,*I
-0.2
0
1
2
3
4
1,
5
Time [s]
7
6
10
Figure 2-4: Shoe strain while running
Using these values, Equations 2.7 and 2.8 were used in combination with the area of
the strain gauge to determine the maximum power output per unit area that would
be extracted by the PZT energy harvester at this point on the shoe. Documented
above in Table 2.2, this maximum harvestable power per area Pout,max/cm2 ranged
from about 80O
to 150w. Subsequently, using the active area of the MFC energy
harvester used in this study, it was predicted that the maximum power output for
the harvesters Pzt,max would range from about 300pLW to 600pW.
The maximum strain power output
Pmech,max
of the three different motions at
the ball of the foot had a much larger range as predicted by the increasing strain in
the shoe at that location as the activity became more rigorous. Unlike the
Pmech,max
values at the midpoint, the maximum strain power output spanned almost a full order
of magnitude at the ball of the foot throughout the three different motions. These
Pmech,max
values correspond with maximum harvestable power per unit area values
Pout,max/cm2 ranging from about 130"
to 1200'W
and maximum power output
Ppzt,max ranging from about 0.5mW to 5mW, a significantly larger range and higher
output than the harvestable power output at the midpoint. Comparisons between
26
0.6
Ball of Foot
Miinint
-
0.5
0.40.3
-
0.2
0
lI'
-02
Ii
10.
-012 1I
0
I
1
2
3
4
5
Time [s]
6
7
8
9
10
Figure 2-5: Shoe strain while rocking back and forth
the two cases can be seen in Figures 2-6, 2-7 and 2-8.
At 12001, a shoe sole surface area of 83.3cm 2 would be required to reach the goal
power output of 0.1W. Considering that this high power to area ratio. only occurs at
a very small region of the shoe underneath the ball of the foot, it is unreasonable to
predict that 0.1W is an achievable power output for these energy harvesters.
If the system were able to extract the maximum power from this study 4.62mW
constantly, it would take 216.5 hours or roughly 9 days to charge a single AAA battery.
This would be a somewhat acceptable time frame which could be decreased by using
multiple energy harvesters or harvesters with larger surface areas; however, this power
value is the absolute peak power that was measured during this study, not the average
power. This means that, assuming the user is walking, running, or rocking at some
rate close to 1-2Hz, this power output would only be achieved for an instant every
half to full second. In order to achieve an average power even close to this value to
harvest any significant amount of energy in a reasonable amount of time, the user
would need to be straining the sole of his or her shoe many orders of a magnitude more
frequently than humanly possible. Though the operating frequencies of piezoelectric
27
5
x
10
Ball of Foot
Midpoint
-
----
4
3
2
0
0~
0
0-w-
-1
-2
-3
-4
0
1
2
3
Time [s]
4
5
6
Figure 2-6: Strain power output while walking
energy harvesters are dropping as new developments arise (as exemplified by Ruize
Xu's work [20]), much progress still needs to be made before piezoelectric harvesters
can efficiently harvest energy from extremely low frequency walking and running
motions.
Although the data from the strain testing did not prove to be very promising for
the effective use of piezoelectric energy harvesters in shoes, MFC actuators were still
briefly tested on the shoes to confirm the low outputs.
28
:100-3
5
Ball of Foot
- Midpoint
---
4
3
2
1
U)
0
0
-
0~
-1
-2
-3
A
0
1
2
3
4
5
Time [s]
6
7
9
8
10
Figure 2-7: Strain power output while running
Position
Midpoint
Motion
Pmeas,max
walking
62.8 nW
running
477.5 nW
rocking
58.5 nW
walking
172.1 nW
running
1.146 pW
rocking
85.2 nW
Ball of
Foot
Table 2.3: Measured power output from PZT energy harvester
The comparison of the MFC maximum power output values
Pmeas,max
in Table 2.3
with the maximum power output values Put,max predicted in Table 2.2 shows clear
evidence that the energy harvesters are actually much less efficient than the strain
testing had predicted. This makes sense because the predictions were calculated as
absolute maximum possible peak power with the most optimistic efficiency assump29
5
x 10
----
4-
Ball of Foot
Midpoint
3
21
00A
-2-3-4
0
1
2
3
4
5
Time [s]
6
7
8
9
10
Figure 2-8: Strain power output while rocking back and forth
tions; however, these efficiency assumptions do not explain why the actual power
output values are roughly three degrees of magnitude smaller than predicted. Most
of the power output values predicted through strain testing trials are on the the order
of 10' or 10' Watts while the actual values are on the order of 10' or 10-6 Watts.
It is also peculiar that in both cases, when the MFC actuator was placed at the
ball of the foot and when it was placed at the midpoint between the ball of the foot
and the arch, the power output was significantly larger during the running motion
that during either of the other two motions. This data is visualized in Figures 2-9
and 2-10. The strain data predicted that maximum power output would be detected
during the rocking motion as opposed to the running motion; however, the data
collected from the MFC actuator itself indicated that the rocking motion was the
least powerful of the three.
These unexpected discrepancies between the predicted and measured values of the
maximum MFC power output can be potentially attributed to multiple factors:
1. Surface placement of the original strain gauges
30
0.35
walking
running
rocking
0.3-
-
0.25
-
00.2
C3) 0.15-
0L
-
0.1
-
0.05
0
0
-
0.5
1
1.5
-
2
2.5
Time[s]
3
-13.5
4
4.5
5
Figure 2-9: Measured power output from PZT energy harvester at the midpoint
2. MFC alignment and position
3. Coupling between the MFC actuator and the shoe
4. Strain gradient in the shoe sole
Due to the fact that the strain gauges used to test the strain energy and power in
the shoe sole were adhered to the surface of the sole as opposed to being inset into
the material, it is possible that the original strain measurements might have been
affected by the direct contact force on the gauges. If this were the case, the strain
and strain power output measurements would be higher than they should be.
To preserve the integrity of the actuators, the MFCs were not placed on the bottom
sole of the shoe as the strain gauges were during the strain testing studies. Instead,
they were attached to the interior sole of the shoe underneath the foot. This interior
sole was made from a material different from the exterior sole, and the difference in
the material's elastic properties likely had an impact on the power output. The fact
that the MFC was closer to the foot than the strain gauges also likely contributes to
31
0.9
----
0.8
walking
-
running
rocking
-
0.7
0.61if
20.5-
.I
if
0.4 -
ipi
III !iiJ~III_
0.10-
hII
I
4I
2.
..
03
osJ~~
ItriI ~
2. It
foIt
foott
the discrepancy in the order of magnitude.
The MFC actuator was secured to the inner sole of the shoe with tape instead of
the epoxy typically used to adhere these actuators to surfaces. This choice was made
to preserve the shoe; however, the tape did not secure the actuator at all points along
the surface area as the epoxy would do. This likely allowed the MFC to somewhat
separate from the sole surface while being strained, resulting in a low power output
reading.
The strain gauges used to predict the maximum power output had a significantly
smaller active area than the MFC actuators (see Tables A.3 and A.4 in the appendices of this thesis for specifications). The predicted maximum power output values
Ppzt,max listed in Table 2.2 were calculated based on the assumption that the expected
power output per unit area
PoUn ,max /Area
was constant across the region of interest.
Realistically, this Pout,max/Area is a function of location on the shoe sole, so the ma-
jority of the area covered by the MFC actuator likely had a lower power output per
32
unit area than the value calculated during strain testing.
Error aside, the MFC actuators were not efficient enough to perform up to the
standards necessary to achieve the desired power output for this design study.
33
34
Chapter 3
Exploring New Methods of
Harvesting Energy
In order to use piezoelectric energy harvesters to charge any sort of battery in a
reasonable amount of time, the devices would need to be several orders of magnitude
more efficient than they are currently. For the scope of this project, it was decided
that piezoelectric energy harvesters would not be the best choice of device to employ
in shoes for harvesting energy from running and walking motions.
The idea of using a miniature electromagnetic generator within the shoe to convert
kinetic mechanical energy into electrical energy was proposed, and the initial design
consisted of a ratchet mechanism which would convert the transverse impact of the
shoe's heel striking the ground into rotational motion of a rotor which would drive the
electromagnetic generator. The core components of this mechanism were inspired by
the idea of backdriving something like an optical disk drive motor, pictured in Figure
3-1, with a miniature version of the mechanism found is most salad spinners, pictured in Figure 3-2. If some mechanism were robust enough to withstand the impact
of the heel strike, then this concept could be considered plausible; however, before
considering sturdiness and product reliability, both comfort and design integration
were brought forth as concerns with this design.
Most shoes, especially shoes designed for performance and activity, have springlike soles for comfort. They allow the users to sink into their steps to keep from
35
Figure 3-1: Optical disk drive motor
[19]
Figure 3-2: Salad spinner that converts
translational to rotational motion [18]
subjecting the their feet and subsequently their joints to large and often detrimental
impulses. Similar to airbags, but on a much less extreme scale, shoe soles lengthen
the time of impact of the foot on a surface to decrease the rate at which the person's
momentum changes with each footfall. This cushioning shoe sole property provides
comfort.
The ratchet mechanism derived from the salad spinner would not be easily altered
to fit this sort of model to provide comfort to the user; however, the air pockets that
shoe companies already incorporate into their designs already suffice to dampen the
foot fall. Considering this method of dissipating energy which has already been proven
to provide ample comfort to the user, the design of the proposed energy harvester
evolved and the ratchet to electromagnetic generator coupling was replaced by a
coupling between a miniature air turbine and an electromagnetic generator. A proof
of concept model was developed with the purpose of estimating an order of magnitude
of the potential power output of the system when subjected to walking and running
motions.
Similar to the calculations used in Chapter 2 to determine the amount of power
collected but the piezoelectric energy harvester, power was calculated using the measured voltage output of the back-driven motor Vmea,
of the motor Ri:
36
and then know internal resistance
P Vmeas - I
(3.1)
Vmeas
(3.2)
Ri
V2
P =me"a
Ri
3.1
(3.3)
Apparatus and Procedure
Parts for the prototype were sourced from common pre-existing mechanisms. The
three main sourced items were:
1. Miniature Motor
2. Pressure-Driven Dental Drill
3. Blood Pressure Cuff
Figure 3-3: Miniature motor used as a generator in this study [8]
The miniature motor, pictured in Figure 3-3, was selected for its size and potential
to be easily integrated into a small attachment for the shoe. Motors of this scale are
commonly coupled with off-balanced masses to produce vibrations in devices such as
cell phones. This small-scale motor was chosen to be back driven as a generator for
the system.
37
Figure 3-4: Standard dental handpiece [16]
Figure 3-5: Various turbine designs used in modern pressure-driven dental drills [1]
The dental drill, pictured in Figure 3-4, is a tool widely used by dentists to drill
holes in teeth for cavity removal and other procedures. The tool is capable of generating large amounts of torque for this purpose and consists of a miniature air turbine
that is driven by compressed air of pressures between 30-40psi and rotates at noload frequencies up to 400,000rpm. The device works by taking in high pressure air
through valves entering the base of the handle, forcing that air through a turbine at
the head of the device where the air turns a rotor and attached dental drill bit, and
then passing the air back through a valve at the base of the handle. A schematic of
the mechanism itself within the housing can be found in the appendices of this thesis
in Figure B-2.
The turbine within the drill was the part of interest for this study as it has already
been optimized for widely used medical equipment. This means that energy losses to
friction in the piece have already been minimized, and the parameters of the rotor
itself have been optimized to provide significant amounts of torque. Evident in Figure
3-5, various geometries of rotors can be found within the dental handpiece market
38
Figure 3-6: Dental drill air turbine [2]
Figure 3-7: Blood pressure pump bulb [3]
and are used to operate at different pressures and output various ranges of torques,
but for cost purposes we used a very simple handpiece turbine pictured in Figure 3-6.
Finally the blood pressure cuff was acquired solely for the use of the hand pump
bulb, pictured in Figure 3-7 and the tubing. Once again, this is a product that has
already been iterated and optimized for medical devices. The rubber bulb is easy
enough to compress by hand, and adjustment of the flow valve at the base of the bulb
allows for the application of a maximum of about 30psi via hand pumping. There is
a potential to increase this pressure value when considering that the air bulb will be
subject to the impact of a footfall as opposed to a hand squeeze. The bulb is also
equipped with an air inlet which allows air to refill the pocket once the bulb has been
released. This inlet closes when an outside force is applied to the bulb to maintain
pressure and to force air through the valve and connected tubing.
To assemble these three core components, the dental drill was first disassembled.
The connector piece that typically joins the handpiece to the compressed air source
39
was removed and the drill's flow valves were exposed. The tubing from the blood
pressure cuff was then attached to the drill's air intake valve and the other end of
the tubing was attached to the pump bulb. The air outlet valve of the dental drill
was left untouched so air flowing out of the turbine could flow back out into the
environment.
The back of the head of the drill was then removed, exposing one
end of turbine within. The spindle of the miniature motor was inserted into the hole
running through the axis of the turbine's rotor. For the sake of preserving the part for
future configurations and iterations, the initial tests were run with out permanently
adhering any of the components.
With the components in place, the pump bulb was subjected to footfalls ranging
from a typical walking step to jumping up and down. The flow valve at the base of
the pump bulb was set to be as closed as possible to allow for maximum pressure
output.
3.2
Results and Discussion
From initial testing, this setup generated power outputs on the order of 10-4 to 10-
Watts. Although formal tests have not been carried out, these ballpark measurements
are enough to predict significantly higher power outputs than those generated by the
MFC actuators. The use of the electromagnetic generator not only allows for the
direct conversion of the kinetic energy of the footfall to potential energy, but it has
the potential to generate a much higher average power output as the inertia of the
rotor inside of the generator will keep the rotor spinning for a period of time in
between steps. This slower response is much more ideal for the low frequency inputs
of walking and running motions while the MFC actuators are more ideal for very high
frequency inputs.
Knowing that this setup is a plausible method for obtaining reasonable electrical
power outputs for harvesting and storing energy, there are several improvements that
should be imposed upon the system to maximize these power outputs:
1. Improve Couplings
40
2. Decrease the length of tubing
3. Optimize pressure bulb
Neither of the couplings between the motor and turbine and the turbine and the
tubing were ideal for minimizing energy losses in the apparatus. The motor was not
rigidly adhered to the turbine as it was very difficult to align the central axes of the
two components. The flexibility in the coupling was necessary to keep the system from
jamming and preventing the generator from operating properly. In future iterations,
the turbine and generator rotors should be combined into a single piece to ensure
alignment and to prevent energy losses to friction in an extra coupling.
This apparatus would also benefit from the limiting of the distance across which
the air must travel between the pump and the turbine. Energy loss in fluid flow is a
function of this length, so attaching the pump closer to the turbine will increase the
power output-of the system while also shrinking the device itself.
The pressure bulb, as is, has been optimized for hand pumping; however, footfalls
apply much more force to the pump than hand squeezing. Experimenting with the
size, material, flow valve size and air inlet size would benefit the apparatus. Optimizing these parameters would produce a higher air pressure out of the flow valve and
into the air turbine, ultimately generating a higher power output.
41
42
" ..........
I
Chapter 4
Conclusion
After carrying out strain testing on the sole of a shoe, it was determined that, even in
the most optimal situation imaginable, one in which the energy harvester is perfectly
coupled to the outer sole of the shoe at the point of maximum straining under the
ball of the foot, an MFC actuator might be able to harness a peak output power
on the order of 10-Watts. When actually tested, the MFCs yielded a maximum
peak output power three orders of magnitude smaller than that. Combined with
that fact that these actuators would need to be sensing this peak power at very high
frequencies as opposed to the 1Hz - 5Hz of walking or running, it can be concluded
that piezoelectric energy harvesters are not the ideal choice for harnessing energy to
charge batteries on the go.
The newly proposed device, which involved an air pocket pump coupled to a
miniature air turbine and subsequently coupled to a miniature electromagnetic generator, produced much more promising results. This apparatus built from sourced
parts for testing purposes yielded peak power outputs on the order of 10- 3 W, the
absolute maximum power that was expected from the piezoelectric energy harvesters
in the most ideal conditions, in a non-optimal, proof-of-concept state. It was also
much more well suited for low frequency inputs closer to the walking or running frequencies as the inertia of the turbine and coupled generator rotor induced a much
slower response.
In further iterations of the turbine generator device, the sourced parts should be
43
eliminated and replaced by custom pieces to allow for the optimization of parameters.
The generator rotor and the turbine should be combined into one piece to ensure a
sound coupling between the two elements, and the tubing attached between the pump
and the turbine inlet valve should be shorted and coupled in a more permanent way
to minimize energy losses. These alterations, combined with the optimization of the
pump for foot falls and heel strikes should increase the device efficiency and hopefully
increase the power output by at least one order of magnitude.
44
Appendix A
Tables
Power Output
Weight
Walking Pace
5 W
52kg
2 steps
see
Method of Estimation
40-ply
triangular
PVDF plate
with a center metal shim deflected 5cm [17]
8.4 W
68kg
2 steps
sec
Mechanical spring and a rotary
generator conversion, with 67W
available power [17]
14 W
70kg
1 step
sec
130% of the weight exerted with
1cm heel displacement [15]
16.4 W
70kg
1
step
sec
Based on the heel and toe force
and maximum 1cm allowable displacement [4]
67 W
68kg
2 steps
sec
Assume
the
fall
of
the heel
through 5cm [17]
324 W
68kg
2 steps
see
Power consumed during walking
[13]
Table A.1: Estimation of available walking energy [21]
45
Power Output
300-650mW
Device Description
Cylindrical PZT stack with hyrdaulic amplifiers [4]
20mW peak power
2mm flexible plastic substrate
1mW average power
sandwiched in the middle of 16
layers of PVDF sheets [10]
80mW peak power
Unimorph strip of spring steel
2mW average power
bonded to piezoceramic material
[10]
1W peak power
0.25mW average power
800mW
AC
electromagnetic
generator
with lever and flywheel [10]
Electroactive polymers or dielectric elastomers membrane [9]
1.4mW peak power
27.5mW peak power
Triboelectric nano-generator [7]
Piezoelectric bimorphs coupled
with amplification mechanism [6]
Table A.2: Outputs of pre-existing walking energy harvesters [21]
Table A.3: Strain Gauge Specifications [12]
Manufacturer
Micro-Measurements
Model
CEA-06-240UZ-120
Resistance
120Q t 0.3%
Gage Factor Q 24 degrees C
2.140 +0.5%
Active Length
7.25mm
Active Width
3.82mm
Active Height
0.1mm
46
Table A.4: MFC Specifications [11]
Manufacturer
Smart Material
Model
M 2814 P1 (d33)
Active Length
28mm
Active Width
14mm
k33 Coupling Coefficient
0.69
47
48
Appendix B
Figures
MFC P1 Type (d 33 effect)
epoxy
Elongator
" powerful actuator
* sensitive sensor
Contractor
* Low Impedance sensor
" energy generator
MFC P2 Type (d 3 1 effect)
metal layer - surface
metal layer - IDE
metal layer - IDE
I
I
-
-
PZT
epoxy
+
+
+
+
- PZT
Figure B-1: Comparison of d33 and d31 MFC actuators [11]
49
U.S. Patent
May 11, 1976
of 2
Sheet I
,-~--
3,955,284
-,
228
4'
/
Je
12
12
.
111/01/
?"G4.-jo
16
,Z G 6.
/542
52
57
H4
36
-
I J8
--36
54
/G. 7
Figure B-2: Schematic of the dental handpiece
50
.
.50
[5]
Bibliography
[1] American Dental Accesories. http://www.amerdental.com/blog/tech-tips-22/.
http://www.amerdental.com/turbines[2] American Dental Accessories.
handpieces-and-dental-parts/turbines?limit=all.
http://www.allheart.com/prestige-medical-replacement-bulb-and[3] AllHeart.
valve-combo/p/pm80bva/.
[4] J[ames] F. Antaki, G[ina] E. Bertocci, E[lizabeth] C. Green, A[hmed] Nadeem,
T[homas] Rintoul, R[obert] L. Kormos, and B[artley] P. Griffith. A gait-powered
autologous battery charging system for artificial organs. ASAIO Journal, 1995.
[5] J.E. Balson. Disposable dental drill assembly, May 11 1976. US Patent 3,955,284.
[6] M[ingjing] Cai and L[onghan] Xie. Increased piezoelectric energy harvesting from human footstep motion by using an amplitication mechanism.
Center for Nutrition Policy and Promotion, 2014.
[7] J[un] Chen, L[ih-Juann] Chen, T[e-Chien] Hou, Z[hong Lin] Wang, Y[a] Yang,
and H[ulin] Zhang. Triboelectric nanogenerator built inside shoe insole for harvesting walking energy. Elsevier Journal, 2013.
[8] Ebay. http://www.ebay.com/itm/load-3-7v-85ma-39500rpm-coreless-motor-size7mm-dia-16-5mm-l-lmm-shaft-dia-/190881446150.
[9] V[iktoria] Greanya, C[ynthia] Daniell, and C[olleen] Nehl. Energy harvesting
technology development at darpa. Materials Research Society Journal, 2011.
and
Paradiso,
J[oseph]
Kendall,
C[lyde]
Kymissis,
[10] J[ohn]
shoes.
in
harvesting
power
Parasitic
Gershenfeld.
N[eil]
Second IEEE International Conference on Wearable Computing
Journal,
1998.
[11] Smart Material. http://www.smart-material.com/media/datasheet/.
http://www.vishaypg.com/micro-measurements/stress[12] Micro-Measurements.
analysis-strain-gages/all-linear-patterns/.
[13] D[udley] J. Morton. Human locomotion and body form, a study of gravity and
man. Baltimore, the Williams and Wilkins Co. Journal, 1952.
51
[14] United States Department of Agriculture. Estimated calorie needs per day by
age, gender, and physical activity level. Applied Physics Letters Journal, 2014.
[15] J[oseph] A. Paradiso and T[had] Starner. Energy scavenging for mobile and
wireless electronics. Pervasive Computing IEEE Journal, 2005.
[16] Dental Office
handpieces.
Products.
https://www.dentalofficeproducts.com/dental-
[17] T[had] Starner. Human-powered wearable computing. IBM Systems Journal,
1996.
[18] Target. http://www.target.com/p/mini-salad-spinner/-/a-516266.
[19] TT-Store. http://www.tt-store.com/images//ps2/.
[20]
Nuize
Xu. The design of low-frequency, low-g piezoelectric micro energy harvesters. Master's thesis, Massachusetts Institute of Technology, 2012.
[21] R[uize] Xu, S[ang-Gook] Kim, and K[elsey] Seto. Human body mountable energy
harvesters. SUTD-MIT InternationalDesign Center, 2014.
......
...
.."opIlrol
.
52