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Nathan Sharpes*, Dušan Vučkovićand Shashank Priya
Floor Tile Energy Harvester for Self-Powered
Wireless Occupancy Sensing
Abstract: We investigate a concept that can reduce the
overall power requirement of a smart building through
improvements in the real-time control of HVAC and indoor
lighting based on the building occupancy. The increased
number of embedded sensors necessary to realize the
smart building concept results in a complex wiring and
power structure. We demonstrate a floor tile energy har-
vester for creating a wireless and self-powered occupancy
sensor. This sensor termed as “Smart Tile Energy
Production Technology (STEP Tech)”can be used to con-
trol automation in smart buildings such as lighting and
climate control based upon the real-time building occu-
pancy mapping. The sensor comprises of piezoelectric
transducer, energy harvesting circuit and wireless commu-
nication. Modeling and optimization procedure for the
piezoelectric cymbal transducer is described within the
framework of tiles. The design and selection of a packaging
technique and construction of a durable floor tile enclosure
aimed at protecting the bulk piezoceramic is discussed
within the constraint that the deflection of the tile should
be minimal such that it is not readily perceivable by
humans, thus not disturbing their gait. Experimental
results demonstrate that the piezoelectric tile could provide
a promising solution for wireless occupancy sensing.
Keywords: energy harvesting, occupancy sensing, piezo-
electric, cymbal transducer, smart building, smart tile,
STEP tech
DOI 10.1515/ehs-2014-0009
Introduction
The increasing reliance of society on technological applica-
tions within the building environment has resulted in a
necessity to find methodologies for reducing overall
power consumption. In this decade, energy consumption
in the United States has risen to more than 93 quadrillion
Btu per year, with around 41% of this energy consumption
coming from buildings in the residential and commercial
sectors (EIA 2011). The concept of the smart building, or a
building that is aware of its occupancy and power con-
sumption, can help in reducing the overall power draw of
buildings (Nguyen and Aiello 2013). Specifically, significant
improvements can be made in the highest power consum-
ing building functions, such as HVAC and lighting, by
controlling them in real-time based upon building occu-
pancy information (Agarwal et al. 2010; Weng and Agarwal
2012). However, the increased number of sensors necessary
to realize the smart building concept can also lead to extra
power draw and complexity in wiring. Therefore, a self-
powered occupancy sensor (one not requiring grid or bat-
tery power) would be of great benefit and major step
toward realizing the smart building concept. Such a device
must then power itself by deriving the energy from its
environment. To fill this need, a floor tile energy harvester
is proposed for the purpose of creating a wireless and self-
powered occupancy sensor. This “Smart Tile Energy
Production Technology (STEP Tech)”will be used to auto-
mate and control various functions in smart buildings, such
as lighting and climate control, based upon the real-time
building occupancy mapping. In realizing such a device,
there are several challenges that need to be addressed: (i)
selection of the electromechanical transducer, (ii) modeling
and optimization of the electromechanical transducer, (iii)
developmentofapackagingtechnique and construction of
a durable floor tile enclosure that can not only protect the
transducer but also the harvesting circuitry, and (iv) inte-
gration of the tile harvester with the wireless controls. In
addition, there should be limited deflection of the tile to a
level on the order of the deflection of shoe soles, so that the
gait of the human occupants within the building is not
disturbed as they walk over the tile.
In order to not disturb the motion of occupants in the
building, the operation of a floor tile energy harvester
should most closely mimic that of a normal floor surface
in terms of appearance and mechanical stiffness. For this
reason, STEP Tech tiles are designed to be placed directly
below the top floor surface (e.g. carpet, laminate,
*Corresponding author: Nathan Sharpes, Center for Energy
Harvesting Materials and Systems, Virginia Tech, Blacksburg, VA,
24060, USA, E-mail: nsharpes@vt.edu
Dušan Vučković,DELTA/Technical University of Denmark, Hørsholm,
2970, Denmark, E-mail: d.vuckovic85@gmail.com
Shashank Priya, Center for Energy Harvesting Materials and
Systems, Virginia Tech, Blacksburg, VA, 24060, USA,
E-mail: spriya@vt.edu
Energy Harvesting and Systems 2015; aop
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decorative tile), as is illustrated in Figure 1(a), so that the
floor surface appears normal. STEP Tech is also designed
to fit the dimensions of standard size decorative tiles (12
in 12 in) and be as thin as possible, so that it can be
easily retrofitted into existing tile or other flooring alter-
natives. Additionally, STEP Tech is meant to be placed at
doorways or at boundaries between rooms and building
sections, so that when stepped upon, a signal is wire-
lessly transmitted to the centrally located smart building
receiver/controller, as depicted in Figure 1(b). In this
manner, real-time occupancy of individual rooms within
a building can be recorded and used, for example, to turn
the lights on/off (Figure 1(a)).
Mechanical stiffness of walking surfaces plays a key
role in controlling many aspects of the human gait cycle,
as we are very perceptive to even small changes in stiff-
ness (or cushioning) of walking surfaces (Hennig,
Valiant, and Liu 1996). Therefore, a floor tile energy
harvester must feel (i.e. have similar stiffness and deflec-
tion) like a normal floor, if it is to avoid the perception of
building occupants and not alter their normal gait pat-
terns (i.e. not affect their normal behavior). If an area of
the floor is perceived as different, individuals will avoid
(i.e. simply step over) those areas, and thus the data on
room occupancy will have inherent inaccuracy. While
terrestrial locomotion of humans is capable of many
different adaptations (Alexander 1991), a sudden change
in ground conditions (i.e. stepping on a “soft”tile) is not
desirable. This consideration is a hindrance to the energy
production of the tile, as available potential energy to be
harvested is in direct proportion to the tile deflection.
Though, when harvesting energy from humans, consid-
eration of human behavior is of paramount importance.
Previous efforts on floor tile energy harvesters
have been left largely to the commercial sector and
have relied on linear to rotation conversion mechanisms
to change the vertical deflection of the tile surface into
rotary motion for induction generators (Brezet et al. 2012;
Kemball-Cook and Tucker 2013; Paulides et al. 2009;
Seow, Chen, and Khairudin 2011). These studies have
reported vertical deflections of around 10 mm that is
dependent on the applied force (i.e. body weight). These
relatively large deflections have been generally accepted,
as the novelty and “greenness”of this technology has,
thus far, outweighed the encumbrance. Allowable floor
deflections are not only left to subjective human percep-
tion but also objectively defined by law (International
Code Council 2011). Section 1604.3.1 of the Virginia
Construction Code defines the maximum allowable
deflection of floor members subject to live (i.e. human,
animal) loads as l=360, where lis the length of the floor
member. By this rule, both the Pavegen (Seow, Chen, and
Khairudin 2011) (600 mm in length dimension ¼1.67 mm
allowable deflection) and Sustainable Dance Floor
(Paulides et al. 2009) (650 mm in length dimension ¼
1.8 mm allowable deflection) would not be allowed to be
installed in buildings in the state of Virginia, as they
exceed allowable deflection limits by several times.
Since electromagnetic transducers require unaccepta-
bly large tile deflections, a piezoelectric transducer is
Figure 1 Illustration of (a) intended placement of individual STEP
Tech tiles under floor surface in doorways and (b) distribution of
STEP Tech tiles in a sample floor plan
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sought for this high-force, low-displacement application.
Many shoe-mounted piezoelectrics from literature rely on
forced bending to stress the piezoelectric materials
(Kymissis et al. 1998). This approach is acceptable for
shoes, which will last a couple of years; however, for a
tile, which must last some tens of years, this technique
will become less effective over time. Piezoelectrics may
also be stacked in a column and compressed in the d
33
direction (Abramovich et al. 2012). It is preferable to
use piezoelectrics in the d
33
mode, as it is typically
two to three times larger than the d
31
coefficient (APC
International 2013). However, for d
31
mode transducers,
mechanical amplifiers can be used to manipulate the
input forces, as exemplified in the cymbal transducer.
Instead of using the cymbal shape to increase displace-
ments as an actuator (Fernandez et al. 1998; Sun et al.
2005), it can be used to increase the force on the piezo-
electric materials as an energy harvester (Kim et al. 2004;
Kim, Priya, and Uchino 2006; Zhao, Yu, and Ling 2010). It
has been shown that rather than using a circular cymbal,
a rectangular cymbal could be used to take better advan-
tage of the crystal orientation in the piezoelectric material
(Luo et al. 2007).
The goal of this work is to demonstrate a self-pow-
ered architecture for autonomous control of the essential
building functions dependent upon the occupancy. In
this study, the transducer is designed based upon the
rectangular cymbal configuration and will be discussed
in the Modeling section with both Analytical and Finite
Element Analysis models.
Modeling
Prior studies on the cymbal transducer have mostly uti-
lized finite element simulation and experimental methods
(Fernandez et al. 1998; Kim et al. 2004; Kim, Priya, and
Uchino 2006; Luo et al. 2007; Sun et al. 2005; Zhao, Yu,
and Ling 2010) to predict the behavior of cymbal shape.
However, a simplified analytical model is lacking that
can be used on a regular basis for the design and perfor-
mance optimization. With an analytical model, one can
easily examine the performance of different geometries in
order to identify the optimum for a given set of boundary
conditions. After the optimum dimensions have been
identified, finite element analysis can be conducted to
verify the predictions of the analytical model. This dra-
matically reduces the quantity of experimentation
required to achieve the desired results. In this study, we
follow this thought process to arrive at the optimum STEP
Tech tile.
Analytical
We begin our analysis by first defining the schematic
representation of the cymbal transducer, shown in
Figure 2(a). The vertices of the angle bends in the end
caps are labeled A-F so we may identify the individual
segments of the cymbal by the lines connecting the two
points. Distributed load, P, is applied to section BC, and
section EF is considered to be resting on a rigid surface.
Following the assumptions of Fernandez et al. (1998) that
cymbal end cap bends behave as pin connected joints
and the end cap members are rigid, such that there is no
energy loss due to end cap bending, we add the conjec-
ture that the bulk piezoceramic between the end caps
behaves as a stiff spring. These assumptions are repre-
sented in Figure 2(b). Therefore, each cymbal section is
considered to be a simple truss, and the force distribution
in the structure is solved using the method of joints.
Figure 2(c) shows the sign of the force in each section,
with the cymbal end caps being in compression, and the
piezoceramic layer being in tension. By the method of
joints,
AB ¼CD ¼DE ¼AF ¼P
2 sin θ;½1
and
BC ¼EF ¼P
2cot θ:½2
Therefore,
AD ¼Pcot θ:½3
It is worth noting here that the cotangent of the angle θ
essentially acts as an amplification factor to the applied
force, P, as for any angle θless than 45°, the cot θis
greater than one.
With force exerted on the piezoelectric layer known, we
can examine the resulting deflections of the cymbal in both
the horizontal (x-axis) and vertical (y-axis) directions. The
deflection in the horizontal direction is caused by the length-
ening of the piezoelectric layer under tensile forces, and
following our assumptions for a member with initial length,
x,thedeflection,dx,asshowninFigure2(d),isequalto,
dx ¼Pcot θðÞx
EpA;½4
where Epis the Young’s modulus of the piezoelectric and
Ais the cross-sectional area of the piezoelectric layer,
which for a rectangular cymbal is the thickness of piezo-
electric, tp, multiplied by its depth, b. The elongation of
the piezoelectric materials in the horizontal x-direction is
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accompanied by a subsequent compression of the end caps
in the vertical y-direction. Examining one of the slanted
segments of the cymbal end cap, such as segment AB, we
see that it has some initial length, R, and rectilinear com-
ponents x0and y. From our assumption of rigid cymbal end
cap members, as deflection occurs, the length of segment
AB remains R; however, the rectilinear components are
given the addition of dx=2anddy=2 respectively, as is
presented in Figure 2(e). The deflections are halved in this
case since the cymbal structure is symmetric about the
center planes of the horizontal and vertical axes and we
are analyzing a single end cap segment. Finally, with this
information we solve for deflection in the y-direction by
applying the Pythagorean Theorem to the deflected seg-
ment in Figure 2(e), where we find,
dy ¼2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
R2x0þdx
2
2
sy
2
43
5:½5
Note that for this application, where the cymbal transdu-
cer is used as an energy harvester, the quantity, dy, will
always be negative.
After establishing eqs [1]–[5], it becomes evident that
the performance of the cymbal transducer is largely depen-
dent on the angle, θ, which is governed by the cymbal
height, h, and inner width, di, labeled in Figure 3(a).
Using the analytical model presented thus far, the plots in
Figure 3(c)-(h) are generated employing the values in Table
1, with Figure 3(b) offering a qualitative illustration of cym-
bal cap geometry over the parametric sweep of diand h.The
applied load, P, for these simulations is 80 N, which is
several times smaller than an average adult’sweight,but
was chosen because multiple cymbal transducers will be
used inside the floor tile, which will distribute the applied
load. The determination of the number of cymbals is
described in the later sections of this paper.
From Figure 3(d) it can be seen that the largest ampli-
fication factors (the cotangent of Figure 3(c)) are observed
where diand hbecome small. Naturally, as amplification
factor is increased, higher force is applied to the piezo-
electric layer, subsequently increasing the magnitude of
displacements in both x(Figure 3(f)) and y(Figure 3(e))
directions. However, the deflection in the y-direction,
which is what would be felt by the person stepping on the
tile, is still significantly small, thus meeting the tile design
requirements. The stress in the piezoelectric layer, plotted
in Figure 3(g), is a direct result of x-direction displacement
and follows the same profile as Figure 3(f). Certainly, the
higher the amplifications factor, the more energy can ulti-
mately be harvested per footstep on the tile. However, the
limiting factor is the stress in the piezoelectric layer, which
for the chosen material (APC 850) has a yield strength of
approximately 36 MPa (APC International 2013).
In order to realize our assumption that a negligible
amount of energy is lost to bending of the cymbal end
caps, the end caps themselves must be as thin as possi-
ble, up to the point where they fail due to buckling, as
they are under compression. The cymbal end cap mem-
bers are treated as pinned-pinned plates under axial
compression, as per our assumptions. The critical load
for plate buckling is found using Euler’s formula,
Pcr ¼π2EI
L2
e
;½6
where Pcr represents the minimum load which can be
applied to a plate before causing buckling, Ethe Young’s
P
PP
Figure 2 Schematic representations of analytical model with cymbal end caps in black and piezoelectric material in orange, and
(a) diagram of cymbal with labeled points and applied force, (b) diagram with ideal model assumptions, (c) illustration of force sign in
the cymbal members, (d) member responsible for horizontal (x-axis) deflection and (e) member responsible for vertical ( y-axis) deflection
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Buckling Region
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Buckling Region
h
di
di
do
dc
(a)
h
(b)
Figure 3 Results of cymbal transducer analytical modeling with (a) notation of model parameters, (b) qualitative illustration of cymbal
geometry as function of parameters diand h. Plots (c)–(h) reveal the dependency of angle θ, amplification factor, magnitude of dy,dx,
stress in the piezoelectric layer and critical thickness (to buckling) of cymbal end caps, respectively. All responses are to a static 80 N load
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modulus of the material, Ithe moment of inertia and Lethe
effective length (a multiple of the physical length, deter-
mined by the boundary conditions). For our case, a load is
prescribed and we wish to find the necessary geometry to
support that load. We then re-write eq. [6] as,
P¼
π2Ec1
12 bt3
cmin
1ν2
c
a2:½7
where the effective length is now 1 ν2
c
a2, because we
are assuming pinned-pinned joints (Rees 2009). Here νcis
the Poisson’s ratio of the cymbal end cap, ais the physi-
cal length, bthe width and tcmin is the minimum end cap
material thickness necessary to prevent buckling due to
prescribed applied load, P. Next, eq. [7] is solved for the
minimum thickness,
tcmin ¼12P1ν2
c
a2
π2Ecb
1
3
:½8
The cymbal structure can experience buckling either on the
slanted side sections or along the top member, depending
on the relative values of diand h. Therefore eq. [8] makes
two distinct curves as atakes the value of dito examine
buckling in the top section and takes the value of Rto
examine buckling in the slanted side section. These two
curves are plotted in Figure 3(h), where the greater of the
two solutions is displayed for the varying geometry.
With this analysis, cymbal dimensions were chosen
such that the stress in the piezoelectric layer would be
approximately 75% of the yield strength, giving a Factor of
Safety of 1.33, while keeping y-axis deflection at a minimum.
The Factor of Safety is kept low to maximize the perfor-
mance, and because the tile will be protected from over-
loading, as is described in a later section. Other
considerations were to keep the height of the transducer
small, such that the overall height of the floor tile could
remain slim, making it easier to install in a floor, and keeping
the inner width of the cymbal (di) large enough, such that
thetopplateofthefloortilecouldhaveagoodsurfacetorest
upon. The chosen design point is highlighted on plots of
Figure 3 with a magenta marker and corresponds to di¼5
mm and h¼2 mm. For this geometry, the cymbal end caps
would need to be at least approximately 230 μmthick.In
keeping with the Factor of Safety of 1.33, the next closest
available material thickness(material soldin standard thick-
nesses) was chosen to give tc¼305 μm.
Finite element analysis
To check the validity of the analytical model and assump-
tions prior to fabrication, a finite element analysis of a
cymbal transducer of the dimensions chosen in the previous
section was conducted. The dimensions are detailed in
Figure 4(a), along with the mesh view overlaid on the cymbal
transducer assembly. The material properties for the steel
end caps and PZT piezoelectric layer were estimated using
the available material information and were identical to that
used in the analytical model, listed in Table 1. The interface
of the cymbal end caps and piezoelectric layer was taken to
be rigidly bonded. This analysis considers 32,115 tetrahedral
elements with an equivalent 80 N distributed load applied
across the top surface of the transducer and was conducted
using the Stress Analysis Environment of Autodesk Inventor
Professional 2013.
Examining Figure 4(b), where the coloring represents
the First Principal Stress, we see that the cymbal end caps
are largely in uniform compression within each segment,
with only a slight amount of tensile stress on the surface by
the obtuse angles due to the occurrence of some finite
bending in that region. Looking at the First Principal
Stress in just the piezoelectric layer, shown in Figure 4(c),
we find that this layer is in uniform tension, with the
exception of stress concentrations in the corners caused
by material necking as it meets the bonded boundary con-
dition. The dark colored regions on either end of the piezo-
electric layer are located where the end caps are bonded
and are in slight compression to support the vertical com-
ponent of the reaction load applied by the end caps.
Vertical (y-axis) displacement of the cymbal transducer is
shown in Figure 4(d) and horizontal (x-axis) displacement
is shown in Figure 4(e). Note the displacement in the hor-
izontal direction is axisymmetric and thus the displace-
ment, dx (elongation of the piezoelectric layer), is taken to
be twice the maximum value of the Figure 4(e).
A comparison of the results from the analytical and
finite element models is shown in Table 2. Here we find
good agreement between the two models, with the slight
Table 1 Model parameter values used for plots in Figure 3
Parameter Unit Value Description
PN 80.0 Applied load
d
c
mm 50.0 Overall width of cymbal
d
o
mm 40.0 Width of outer cymbal bends
d
i
mm 1.0–d
o
/2 Inner width of cymbal
hmm 1.0–d
o
/6 Height of cymbal cap
E
p
GPa 63.0 Young’s Modulus of piezoelectric (PZT)
t
p
μm 508.0 Thickness of piezoelectric
bmm 50.0 Overall length of cymbal
E
c
GPa 200.0 Young’s Modulus of end caps (Steel)
νc- 0.3 Poisson Ration of end cap (Steel)
t
c
mm 0.1–1.0 Thickness of end caps
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differences being attributed to the bending of the cymbal
end caps, which is neglected in the analytical model.
The bending predicted by the FEA gives additional ver-
tical displacement, dy, which subsequently takes away
from horizontal displacement, dx. This energy loss is
then reflected in the slightly reduced stress observed in
the piezoelectric layer. The analytical model was subse-
quently concluded to be sufficiently accurate and a
slightly conservative representation of the performance
of the cymbal transducers in this specific application.
Experimental methods
Manufacturing
The cymbal transducers deflect only a small portion of their
height (around 3%) when fully compressed. For this reason,
it is necessary to develop a manufacturing technique such
that the dimensions of each of several cymbal transducers in
the floor tile are most nearly the same. Therefore, a press-
brake manufacturing technique was adopted in order to
produce a consistent cymbal end cap. The press, shown on
the left, and the brake, shown on the right, in Figure 5(a),
were machined in a Tormach PCNC 1100 mill in a slightly
exaggerated profile to allow for spring-back in the steel end
caps after pressing. Flat pieces of steel shim were inserted
into the press brake and bent into the desired shape, as seen
in Figure 5(b). The bending pressure was applied by a
hydraulic press, shown in Figure 5(c), operating at approxi-
mately 20 kpsi. Piezoelectric elements were then packaged
(discussed in a later section) as shown in Figure 5(d). The
elements were arranged in five parallel 60 mm by 10 mm
pieces rather than one 50 mm by 50 mm piece due to material
availability. The assembly was then bonded together using
Loctite 120 HP epoxy, which was allowed to cure in a kiln at
65°C for no less than 12 hours. The completed assembly is
showninFigure5(e).
Testing procedure
In order to verify the performance of the cymbal transdu-
cers in the target application, it is necessary to have the
ability to impose a repeatable force input onto the sys-
tem. The force applied to the ground during walking
varies from person to person and even between steps
(a)
(b)
(c)
(d)
(e)
Figure 4 Results of finite element analysis of cymbal structure
subject to an equivalent 80 N distributed static load across top
surface using 32,115 tetrahedral elements with (a) model dimension
drawings and finite element mesh, (b) first principal stress of
assembly, (c) first principal stress in piezoelectric layer, (d) vertical
(y-axis) displacement and (e) horizontal (x-axis) displacement
Table 2 Comparison of Analytical Model and Finite Element
Analysis of the cymbal assembly of the chosen dimensions.
Analytical Model shows good agreement with FEA and discrepancies
are due to the small amount of bending in the cymbal caps
Parameter Unit Model FEA
dy μm 156.6 172.8
dx μm 17.52 15.16
Piezostress MPa 27.6 27.03
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made by the same person. Thus, we first seek to find a
mechanical means of applying force onto the cymbals so
that the same force profile may be applied across multi-
ple tests. This will allow for comparative analysis across a
range of cymbal variables. To establish the requirements
of the mechanical test stand, we first institute the force
profile that needs to be simulated. In prior work, Martin
and Marsh (1992) have provided the ground reaction force
for persons walking at their preferred pace, from which
the fourth-order polynomial curve fit in Figure 6(a) was
derived. The curve was obtained under the assumption
that the first peak (heel strike) occurs at 25% of the
contact time, the trough (when both feet are on the
ground) occurs at 50% of the contact time, and the
second peak (toe-off) occurs at 75% of the contact time.
The constraints were also set such that the first and
second peaks were of the same approximate amplitude,
which is true under steady-state walking, and the curve
tends to zero at 0% and 100% contact time. With these
constraints in place, the coefficients of the polynomial
were solved as a function of T, the total contact time (i.e.
period of the step), Fp, the peak force, and Fvthe force in
the valley of the curve as shown in Figure 6(a). The
values of T,Fp, and Fvwere chosen to be 0.63, 1.17,
and 0.7, based on the findings of Martin and Marsh
(1992). Note the ground reaction force (Vertical Force in
Figure 6(a)) is given as a multiple of body weight (BW).
To impose the desired force profile on the transdu-
cers, the test stand shown in Figure 7(a) was constructed.
The stand consists of a rigid aluminum frame, pneumatic
piston, control valves, and measurement sensors. High
pressure air (~110 psi building supply) was fed into the
system which was then regulated to the desired pressure
by either an electronic or manual pressure regulator. A
three-way solenoid valve (Parker 71315SN) then applies
the supply pressure to the piston or exhausts the piston
pressure to the open atmosphere. Pressure inside the
piston was monitored using a Wika model A-10 pressure
transducer. The force exerted on the cymbal transducer
0 0.2 0.4 0.6 0.8
0
20
40
60
80
100
Time (s)
Applied Load (N)
Theoretical
Experimental
Approximation
0 20 40 60 80 100
0
0.2
0.4
0.6
0.8
1
1.2
Vertical Force (BW)
% Contact Time
(a)
(b)
4+3+2+ + =0
=1
4265 − 320 = 1
3−256 +352
=1
264 − 116 =10+12
=0
Figure 6 (a) Fourth-order polynomial curve fit of vertical ground
reaction force data from Martin and Marsh (1992) and (b) typical
experimental step force approximation compared to theoretical
actual force profile (example for target 80 N load)
50 mm
Figure 5 Manufacturing process for production of cymbal transdu-
cers whereby (a) press (left) and brake (right) are machined and
used to bend (b) flat pieces of steel into shape when under pressure
from (c) a hydraulic press. (d) Piezoelectric elements are then pack-
aged and (e) the assembly is bonded together using epoxy
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was then inferred from the piston pressure using the
piston geometry. For this work, a Bimba double acting
piston of 2 in diameter was used.
In practice, the force profile of Figure 6(a) is difficult
to be accurately recreated using the described test rig.
The difficulty arises from the limitations of various com-
mercially available electronic pressure regulators to
regulate the pressure quickly enough or without unac-
ceptable amount of overshoot. After several trials, the
electronic pressure regulator was replaced with a manual
regulator, which was set at a pressure that results in the
desired force amplitude. The three-way solenoid valve
then creates a pseudo-square-wave of force, which was
taken to be an experimental approximation, and is shown
in Figure 6(b). For this approximation, inlet and exhaust
pneumatic piping size was chosen such that the rising
and falling slopes of the approximated force profile most
closely matches the theoretical gait force profile.
In Figure 7(a), force was applied to single transducers
using a hard rubber (~75 A Shore hardness) tip to evenly
distribute the load. Multiple cymbals were tested using
the test stand, as in Figure 7(b), by applying the load
through approximately rigid acrylic plates simulating a
floor tile surface. Measurements were taken using
National Instruments MyDAQ and NI9229 platforms.
Finally, the system was tested in a floor tile enclosure
under human footstep loads, as detailed in Figure 7(c).
PZT packaging
The brittle nature of lead zirconium titanate (PZT) can
cause the formation of cracks over time when operating
under cyclic loading conditions. Such cracks in the bulk
material can reduce performance due to a complex com-
bination of factors (Zhang and Gao 2004). This effect has
been shown in practice by the East Japan Railway
Company when testing their “Power-Generating Floor,”
where they expressed the need to package the piezoelec-
tric elements with a protective rubber structure (East
Japan Railway Company 2008). It has been shown that
PZT may be made substantially more compliant by creat-
ing a macrofiber composite (Wilkie et al. 2000). It is,
however, simpler and cheaper if some of the benefits
realized by a fiber composite could be realized using
bulk piezoelectric material. Therefore, in this study it
was examined if a bulk laminate composite can be used
to improve the performance of the cymbal transducer and
keep the cost of ceramic at the minimal level.
To minimize the onset of brittle fracture or cracking
in the PZT, it should be laminated with a more ductile
material. Additionally, the laminate should have a higher
Young’s modulus than PZT in order to adequately trans-
fer strain across any cracks which do form, but should be
kept sufficiently thin so as to take a minimal amount of
strain energy away from the PZT layer. It was thus
decided to laminate the bulk PZT pieces with brass,
because of its high ductility, electrical conductivity, and
Young’s modulus approximately 40% greater than PZT.
To examine the influence of the laminate brass layer,
three cases illustrated in Figure 8(a) were considered,
including a baseline case of unlamented (plain) PZT, a
25 μm brass laminate case, and a 50 μm brass laminate
case. The brass was laminated at the top and bottom
surfaces of the PZT using All-Spec CW2400 conductive
Test Platform With Cymbal
High Pressure
Air Supply
Regulated to
desired
pressure
Pressurize or
depressurize
piston
Pressure
inside
piston
cylinder
Piston
Geometry
Force
Applied
Piezo
Voltage
Computer
Controller
Pressure Sensor
Solenoid Valve
Test Platform
Pneumatic Piston
Regulator
Air Supply
Bread Board Connections
and Storage Setup
National Instruments
MyDAQ and NI9229
LabVIEW System
Control and Measurement
Rigid Acrylic Plates
Cymbal Transducers
Tile Enclosure
Human Step
Figure 7 Experimental methods, including (a) custom test stand,
illustration of flow of information and testing of single transducer,
(b) multiple transducer testing using custom test stand and simu-
lated floor tile top, and (c) testing using human step inputs
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epoxy, bonded in a kiln at 80°C for no less than 2 hours.
For these tests, smaller scale test specimens were used to
minimize the material waste during ultimate breaking
strength tests. The test specimens were of similar geome-
try to the dimensions shown in the drawing of Figure
4(a), however only 10 mm in depth. The test specimens
were then tested individually using the pneumatic test
stand, as shown in Figure 7(a), in terms of energy output
and ultimate breaking strength. Energy output was deter-
mined by recording the voltage in a storage capacitor
when the test specimen was subjected to a force impulse
of 20 N load for 0.63 seconds. The storage capacitor was a
10 μF electrolytic capacitor and connected to the output
from the test cymbal through a full-bridge diode (BAV21)
rectifier. Energy produced was then inferred by measur-
ing the voltage across the capacitor, by the relationship,
1=2CV2, where Cis the capacitance and Vis the voltage of
the capacitor respectively. Ultimate breaking strength
was determined by slowly increasing the load on the
test cymbal until the moment of failure. Results are
given as a percent increase or decrease over the unpack-
aged case, as it is a comparative analysis, and to avoid
confusion with the reported performance of the full size
cymbals.
As shown in Figure 8(b), the addition of the brass
laminate causes a dramatic increase in breaking strength,
as well as an increase in energy output per simulated step
input. The increase in energy production is postulated to
result from a more even stress distribution in the PZT
caused by the addition of the laminate. The increase in
breaking strength is more intuitively caused by the addi-
tional cross-sectional area and stiffness and subsequent
reductioninstressperload.Weseethatwhilebothlami-
nate cases offer improvements in both categories, the great-
est improvement, which we define as percent improvement
in energy output per simulated step multiplied by percent
improvement in ultimate breaking strength, is produced by
the 25 μm laminate. Therefore, this packaging scheme was
used for subsequent implementation in the tile.
Optimal number of transducers
We now describe the selection of optimum number of cym-
bal transducers that should be employed within the tile
enclosure. The number of cymbal transducers determines
the stress each one of them will experience andsubsequently
the charge produced by the PZT layer. The larger the number
of transducers used, themore potential there is for energy to
be harvested before the cymbals become overloaded.
However, the more transducers which are used, the lower
the sensitivity of the tile, or the amount of energy produced
per load (i.e. person’s weight). Consequently, there exists an
optimum number of transducers to be employed for max-
imum energy harvesting from a target weight. Figure 9 illus-
trates this concept by showing a qualitative comparison of
(a)
(b)
PZT
Brass
PZT
Brass
PZT
PZT - Plain
PZT - 25 μm Brass
PZT - 50 μm Brass
-+82%
+28%
-
+96%
+151%
Energy Output/Step Breaking Strength
Figure 8 (a) Illustration of three packaging techniques investigated
including plain PZT, PZT with 25 μm brass laminate, and PZT with 50
μm brass laminate. (b) Results of packaging test, comparing perfor-
mance in terms of ultimate breaking strength and energy output per
simulated step input
Energy / Step
Weight
Weight Optimized Cymbal Performance
Optimized for Average NFL Player (115 kg - 14 Cymbals)
Optimized for Average Adult (82 kg - 8 Cymbals)
Optimized for Average Child (32 kg - 4 Cymbals)
Figure 9 Qualitative illustration energy production per step as a
function of weight for several cases of optimal number of cymbals
used for target weights of a child (8 year old) (Fryar, Gu, and Ogden
2012), average adult (Fryar, Gu, and Ogden 2012), and average NFL
player (Merron 2004) as examples
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energy producedper step on the tile, as a function of weight
for an optimum number of transducers for the weight of an
average child (8 year old) (Fryar, Gu, and Ogden 2012),
average adult (Fryar, Gu, and Ogden 2012), and average
NFL player (Merron 2004), as examples.
TheflatareasofthecurvesinFigure9representcondi-
tion where the transducers would become overloaded and
experience failure if not for the design of the tile enclosure
(discussed in the Tile Enclosure Design section).We see that
a tile optimized for a child can make much more energy per
step at low weights, but has a much lower maximum possi-
ble energy production compared to a tile optimized for an
adult or NFL player. We do note, however, that after a certain
minimum weight, the floor tile will produce a constant
amount of energy per step, regardless of the load. This type
of performance is quite desirable as the function of tile (i.-
e. sending a signal when stepped upon) can be decoupled
from a person’sweightorgaitstyle.Therefore,wedeter-
mined the optimal number of transducers such that the
maximum possible energy harvested per step is just suffi-
cient to power the signal transmission circuitry. For any
person above a certain minimum weight, the tile will func-
tion in a similar manner. For reasons discussed in the Circuit
Design section, this minimum weight was found to be
around 36kg and an optimal number of cymbal transducers
was found to be five. In this application, a minimum neces-
sary number of transducers were used. However, if the tile
was built to harvest as much energy as possible, the number
of cymbal transducers would be chosen to correlate with the
expected average weight of occupants that would step on the
tiles most regularly.
Tile enclosure design
The main functions of the tile enclosure are to provide a flat
and rigid surface to transfer force to the cymbal transducers,
protect the cymbals from overstraining, and protect the cym-
bals and circuitry components from the environment. To
ensure a consistently flat and sufficiently rigid surface, it
was chosen to construct the tile enclosure from Delrin
®
,a
thermoplastic which exhibits high stiffness, dimensional
stability, low thermal expansion, and resistance to water
and other chemicals. Channels were cut (CNC Mill) into the
top and bottom plates, into which the supports are fitted. The
support strips were bonded only to the bottom plate, using
Loctite 120 HP epoxy, and were sized such that they do not
come into contact with the top plate when the tile is not
loaded and the top plate rests on the cymbal transducers. In
Figure 10(a), we can see that when the tile is loaded, the
cymbal transducers are compressed, although before the
piezoelectric layer is strained to the point of failure, the top
plate comes in contact with the supports, preventing further
deflection. In this way, the cymbal transducers are protected
from failure due to high loads. In addition, after a certain
minimum load (i.e. person’s weight), energy production is
held relatively constant, as the deflection of the cymbal
transducers is mechanically limited. This leads to a more
(a) (b)
Cymbal
Transducers
Circuitry
Superfluous wire to run to
DAQ equipment when
testing
Cymbal
Foot
Bottom Plate Top Plate
Piezo
Supports
Unloaded State Loaded State
To D AQ
(c)
(d)
*All dimensions in mm
Figure 10 (a) Illustration of tile enclosure parts in unloaded and loaded states, (b) fabricated enclosure (with top plate removed) used when
testing with human step inputs, (c) final fabricated enclosure (with top plate removed) with all components internal to the tile,
(d) dimensioned drawing of floor tile enclosure, and (e) fully assembled STEP Tech floor tile with size 9 (US) shoe for scale
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consistent operation (i.e. occupancy sensing and signal
transmission), as a known amount of energy is delivered to
the circuitry.
When developing the cymbal transducers and circui-
try with human step loading, as shown in Figure 7(c), the
output from the cymbal transducers was run outside the
tile enclosure to the data acquisition (DAQ) modules, as
shown in Figure 10(b), through small holes in the enclo-
sure, which were later filled. Once the system was verified
to function as anticipated, the circuitry was wired into the
enclosure and superfluous wire which was used to reach
the DAQ equipment in Figure 10(b) was wound in the extra
space within the tile as shown in Figure 10(c). A dimen-
sioned drawing of the tile enclosure is shown in Figure 10
(d) with the fully assembled STEP Tech floor tile shown in
Figure 10(e), along with a size 9 (US) shoe for scale.
Circuit design
With the mechanics of the tile established, circuitry was
designed to fulfil the target application of detection of build-
ing occupancy. The detection of human presence was
achieved by transmitting a wireless signal using the energy
generated by a person stepping on the tile. In this way, each
time a signal was received from the tile, it would indicate that
a person has stepped on it. In order to achieve this function-
ality, the design was examined into four stages: (i) determin-
ing the optimal electrical connection strategy for the
multiple cymbals, in order to maximize the collected energy;
(ii) determining a rectification method; (iii) sizing the storage
capacitor; and (iv) utilizing the collected energy through an
energy management section in order to power a wireless
transmitter. Each of these stages will be addressed in more
detail in the following sub-sections.
Electrical connection strategy
While the cymbal transducers are loaded mechanically in
parallel, their electrical connection strategy must be
decided in order to realize the most efficient operation
of the circuitry. When a 71 kg person stepped on the tile,
it was seen that the output open circuit voltage of the
cymbals is in range of 20 V to 30 V. Therefore, it can be
expected that the maximum power point (MPP) will be
one half of the open circuit voltage. If we consider con-
necting the cymbals in series, this would yield the max-
imum power point voltage of approximately 75 V in the
case of a tile with 5 cymbals. This was derived under the
assumption that all cymbals were the same, and that
their maximum power point voltage was 15 V for each
cymbal. In this case, the energy management stage
would be faced with the task of converting the 75 V
stored on the capacitor to 3 V, which is the voltage
typically used by sensors and wireless modules. After
searching for high efficiency DC/DC converters with the
required input voltage rating, it was concluded that the
conversion from 75 V to 3 V would be very inefficient.
Furthermore, integrated solutions for low power switch-
ing DC/DC converters capable of performing the task are
not easily available. Therefore, connecting the cymbals in
series was not an option. On the other hand, connecting
the cymbals in parallel would result in approximately 15
V maximum power point. At this voltage level, it was
possible to use off-the-shelf converters, while providing
high efficiency in energy transfer from the buffer capaci-
tor to the rest of the circuitry. It was therefore decided to
connect the cymbals electrically in parallel.
Rectification of the AC signal
In the literature, several different approaches have been
demonstrated for piezoelectric output rectification (Beeby
and White 2010; Erturk and Inman 2011; Guyomar et al.
2005). These circuits have been primarily designed to
operate with stable periodic oscillations that would gen-
erate a predictable output of the harvester. However, the
output of the cymbals when stepped on is an irregularly
shaped single pulse. Furthermore, the amount of avail-
able energy after one step is in order of one millijoule.
Hence, the rectification circuit selected has to be very
efficient in order to not consume more energy than the
gain of using the selected rectifier circuit. Furthermore,
the storage capacitor can be considered empty before
each step occurs, as an indeterminate amount of time
will pass between the subsequent foot falls on the tile.
Advanced rectifier designs typically require stable power
for the control circuitry. All these limitations made the
use of the advanced rectification a very challenging task.
Therefore, a standard diode bridge was selected.
Selection of a storage capacitor
As with many energy harvesting applications, in this
specific tile application there is not sufficient instanta-
neous energy produced to power the payload device
(wireless signal transmitter in this case) continuously.
Therefore, an intermittent storage capacitor is necessary
to accumulate energy until signal transmission can be
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powered. Here, there is no need for accumulating energy
between transmissions, principally because occupancy
has already been counted, and also because energy is
only collected when the tile is being stepped upon.
Therefore, the capacitor buffer size was selected based
on the value that would produce the most optimal energy
output from a single step. Empirically, the value of the
buffer capacitor was selected to be a 10 µF ceramic X7R
capacitor rated for 25 V, as it stores the most energy per
step, demonstrated in Figure 11. Note that in Figure 11
one simulated step input was signified by two rises in
capacitor voltage, where one rise is caused by force being
applied and the second rise by the force being removed.
The regions of decreasing voltage in-between rises are
caused by the high leakage current of the Schottky diodes
used in the full bridge rectifier above 15 V. Schottky
diodes are thus not used in the final circuit design.
Energy management
Wireless signal transmission was performed using the
ez460-RF2500 platform by Texas Instruments (Texas
Instruments 2009). The energy required for starting up the
transmitter and performing a wireless transmission, based
on the documentation provided by Texas Instruments, was
approximately 100 µJ (Morales and Shivers 2007). The
amount of energy harvested during the stepping onto the
tile, by an approximately 71 kg person, was borderline
sufficient to perform the transmission. This, however, indi-
cates that a lighter person wouldn’t generate sufficient
energy, by stepping onto the tile, to perform a transmission.
In order to address this issue transmission was delayed
until the person steps off the tile. A similar amount of
energy is generated as a person steps off the tile as when
stepping on. Therefore, as is addressed in the Optimal
Number of Transducers section, the minimum weight of a
person necessary to power signal transmission is around 36
kg. Based on the measurements, when the stepping off is
completed, the voltage reached values around 15 V. As a
result, it was necessary to convert the voltage on the storage
capacitor to the range of 1.8 V to 3.6 V, which is required for
stable operation of the radio and MCU on the ez430-RF2500
board. The conversion is done by using a switching DC/DC
regulator. Such a regulator is found in the LTC3129-1 nano-
watt buck-boost DC/DC (Linear Technology Corporation
2013). This DC/DC converter was selected first because the
circuit is rated for the voltage range expected on the buffer
capacitor, and second it has analog control, which can be
used for selecting a threshold voltage at which the conver-
ter should activate. By applying a voltage level above 1.22 V
on the analog control pin, the converter would activate.
Therefore, by providing the voltage from the input using a
simple resistor divider it was possible to set the activation
voltage at a desired input storage capacitor voltage level.
However, this circuit has a narrow input hysteresis, so it is
necessary to extend it in order to allow the output to use as
much energy as possible before the DC/DC converter turns
off. The hysteresis extension is implemented by connecting
a diode and a capacitor to the RUN input, which controls
the converter’s operation, as shown in Figure 12(a).
The circuit rectifies alternating voltage input from the
cymbal transducers during a step. As the voltage on the
input capacitor, C8, rises, the voltage on the capacitor
connected to the RUN pin, C7, also rises, at a rate defined
by the voltage divider and the diode voltage drop. When
the voltage on capacitor C7 reaches the threshold voltage
of the converter, 1.22 V, the converter starts operation. As
soon as the converter starts operating, the voltage on
capacitor C8 will drop, as the harvester is not capable
of supplying sufficient power to prevent this, as the foot-
step on the tile is completed. With the drop of voltage on
C8, if it weren’t for the diode D5, the voltage on the RUN
pin would reflect the voltage change on the input, lead-
ing to disabling the converter once the voltage on this pin
reaches 1.11 V. However, as the diode is preventing the
discharge of the capacitor C7, the LTC3129-1 continues
operation, draining the input capacitor until it reaches
1.9 V, where the LTC3129-1 enters under-voltage condition
1.5 2 2.5 3
0
5
10
15
20
25
Time (s)
Capacitor Voltage (V)
1
μ
F
2.2
μ
F
10
μ
F
47
μ
F
410N load
2.57 µJ/N
1.18 µJ/N
0.76 µJ/N
0.23 µJ/N
Figure 11 Stored voltage for five cymbal transducers in parallel
under 410 N simulated step for varying storage capacitor values.
Values of maximum energy stored per step per cymbal per load are
noted for each capacitance value
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and stops powering the output. This process is qualita-
tively illustrated in Figure 12(b), with measured voltages
on the input and output sides of the DC/DC converter
under simulated stepping loads shown in Figure 12(c).
Introduction of the diode D5 and the capacitor C7 to
the circuitry has the effect of realizing a low pass filter on
the RUN pin. This in turn introduces a delay in response
to the changing input. The value of the resistance divider
is in megaohm range, in order to reduce the losses during
charging. Therefore, the time constant of the RC network
would be in the second range, depending on the values
of the resistors and capacitor used. This delay could be
potentially used to the advantage of the circuit. Let
VTHR-DC/DC be the voltage level required for the input
storage capacitor to reach in order to provide sufficient
energy to perform a wireless transmission. This voltage
level will be reached, as explained earlier, during the
stepping off from the tile. If a person’s weight is the
same as the minimum weight required for the system to
operate, after the person has stepped off, the buffer
would reach the required voltage VTHR-DC/DC after the
delay introduced by the RC circuit. However, if a person
is heavier, the voltage on the input buffer capacitor
would continue to rise as the circuit is not being activated
because of the said delay. In this way, more energy will
be accumulated in the storage capacitor before the circuit
is activated, hence providing additional energy to the
circuit, increasing robustness.
Discussion
STEP Tech was tested in a “real world”setting at the
Change the World Science & Engineering Careers Fair at
Dulles Town Center in Dulles, Virginia. The event was
attended by several hundred individuals, and the STEP
Tech tile was stepped upon many times, by persons
weighing an estimated range of 25–130 kg. The demon-
stration showed the ability of the STEP Tech tile to har-
vest enough energy from a single step to transmit a
wireless signal to a lamp which turned on when tile is
stepped upon, and turned off when the tile was stepped
upon again. A sample demonstration is shown in several
frames in Figure 13. From this demonstration it was
(a)
Voltage
Step On
Step Off
C1
D2 D3
C8
C7 Vout
Signal Transmission
9 9.5 10 10.5 11 11.5 12
0
1
2
3
4
5
6
7
Time (s)
Voltage (V)
Vin
Vout
Step On
Step Off
Signal Transmission
(b) (c)
Figure 12 (a) STEP Tech circuitry schematic, with (b) qualitative illustration of circuit function, and (c) measured voltage response of circuit to
simulated footstep forcing input to the tile
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conceptualized how a distributed network of tiles could
control not only smart building lighting but also climate
control and other systems, based on real-time occupancy
measurements. Additionally, the individuals stepping on
the tile unanimously stated that they could not perceive
any vertical deflection of the tile’s top plate while stepping
upon it. This response indicates that if the tile were
embedded into a floor, it would be unperceivable to build-
ing occupants and thus not alter their normal gait patterns.
The vertical deflection of the tile is unperceivable
by humans because it is mechanically limited to appro-
ximately 0.5 mm. By Section 1604.3.1 of the Virginia
Construction Code, STEP Tech tiles would be allowed to
deflect 0.85 mm (International Code Council 2011).
Therefore, the installation of STEP Tech tiles would be
allowed in buildings in the state of Virginia, unlike other
commercially available floor tile energy harvesters, which
report an order of magnitude greater deflection (Paulides
et al. 2009; Seow, Chen, and Khairudin 2011). In addition,
the level of deflection exhibited by STEP Tech tiles is
not readily perceivable by persons stepping on the tile.
Subsequently, STEP Tech will not disturb the gait of
smart building occupants when imbedded under the floor
surface (e.g. carpet, laminate, decretive tile), and thus not
cause them to deviate from their normal route (i.e. avoid
“soft”tiles). This then allows for accurate real-time occu-
pancy mapping and occupancy driven smart building sys-
tem control, which will lead to substantial energy savings
(Agarwal et al. 2010; Nguyen and Aiello 2013; Weng and
Agarwal 2012). Most importantly, these savings are realized
without using sensors which consume power themselves,
as STEP Tech derives its energy from human motion.
The sensors currently used in the buildings are mainly
motion detectors which emit radiation into a space and
detect occupancy by measuring what is reflected back to
the device. Infrared sensors can also be used to detect
motion through a plane. Since the premise of operation of
these sensors requires that they emit energy, their power
consumption is in the range of hundreds of milliwatts to
several watts, depending on the device range and coverage
area (Leviton Manufacturing Co. 2011). Since these devices
are constantly scanning their environment, this power con-
sumption ispersistent 24 hours aday, regardless of building
occupancy. Certainly, if a room containing several hundred
watts of lighting can be controlled with a one watt sensor,
the advantages are obvious. However, the elimination of
passive energy consumption of traditional occupancy sen-
sors, offered by an energy harvesting solution like STEP
Figure 13 Lighting control demonstration whereby (a) a STEP Tech tile is placed some distance away from a lamp and when (b) the tile is
stepped upon, enough energy is collected to (c) transmit a signal to the lamp to turn on. When the tile is stepped upon once again (e), the
same process occurs only this time (f) turning the lamp off
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Tech, is favorable. Naturally, the receiver for the control unit
has some power consumption, but since it only needs to
listen for a signal (rather than broadcast and receive), the
control unit operates nearly exclusively in stand-by mode,
consuming only some microwatts of power.
In addition, the control of building function through
traditional occupancy sensors has proven inadequate,
requiring sophisticated data analysis and redundant sen-
sor measurements to achieve a higher certainty of occu-
pancy detection (Dodier et al. 2006). Consequently,
occupancy measurements are often left under-sampled,
and rely on sensor timeouts to decide when to turn lights
off, as anyone who has ever had to get up and wave their
arms around to turn the lights back on in a motion
detector driven lighting controlled room has experienced.
STEP Tech offers the additional information about the
number of people in a given area, as a signal is sent
each time there is a footfall onto the tile, unlike tradi-
tional motion detectors which only see there is some level
of motion in an area. Furthermore, STEP Tech tile could
be arranged to capture successive footfalls, giving infor-
mation about the direction of occupant motion.
Since the Pb(Zr,Ti)O
3
composition was discovered in
the mid 1950s, many studies on developing high perfor-
mance piezoelectric materials have been conducted. The
commercialized piezoelectric ceramic compositions with
high d
33
value of 1,000 pC/N or compositions with high
g
33
value of 40 10
–3
VmN
-1
can be found. However,
compositions having high d
33
usually show low g
33
value
while high g
33
compositions possess low d
33
. Thus, the
achievement of high d·gcoefficient from single piezo-
electric composition is challenging. Recently, a very
different strategy for overcoming the fundamental limita-
tions imposed by electrodynamics and thermodynamics
has been proposed via designing a textured microstruc-
ture, which can result in large magnitude of d·gcoeffi-
cient (Yan, Wang, and Priya 2012). The reason for
realizing high d·gcoefficient in textured ceramics is: (1)
<001> texturing (grain orientation along the <001> crys-
tallographic direction) of piezoelectric ceramic produces
engineered domain state similar to that of <001> single
crystals, thus, resulting in high dvalues; (2) the existence
of low εtemplates produces a composite microstructure
and suppresses the dielectric constant (ε) of piezoelectric
ceramic. Thus, utilizing textured ceramics within the
cymbal transducer would result in some improvement of
the output power response.
Apart from the piezoelectric materials, the power
density of STEP Tech could be improved by further exam-
ination of the cymbal transducer. In this study, the end
caps of the cymbal were made as thin as possible to
minimize energy loss in bending the end caps and max-
imize the stress delivered to the piezoelectric layer.
Cymbal dimensions, particularly the bend angle, were
then chosen to fulfill a prerequisite Factor of Safety.
Since the amplification factor of the cymbal transducer
follows the cotangent of the bend angle, further decreas-
ing the bend is predicted to increase performance.
However, it is difficult to produce a cymbal end cap
with angles smaller than those described in this work.
This is because the bends become less sharp with
decreasing angle, as plastic deformation in the pressing
process is not readily induced, because of the smaller
applied strain. Subsequently, buckling of the end caps
becomes more spontaneous, due to the imperfect small
angle bends. It is postulated that the end caps could be
made slightly thicker, allowing for smaller angles to be
created while still guarding against buckling. Therefore,
while some performance is lost by using thicker end caps,
the performance gain for using smaller bend angles
would still yield an overall increase in performance.
Outside the engineering challenges, occupancy sen-
sors in general face challenges with acceptance, as it is
difficult to accurately predict cost savings (Von Neida,
Manicria, and Tweed 2001). It is well established that cost
savings are to be had by implementing occupancy sen-
sors; however, as savings are ultimately directed by
building occupancy and human tendency, they are inher-
ently highly variant. In addition, there is also the con-
sideration that frequent switching of lighting on/off can
decrease light fixture lifetime, due to more rapid thermal
cycling. In the case of fluorescent lights, it has been
simulated that the calendar life of lighting (i.e. time
between when lighting is replaced/required mainte-
nance) may be decreased by frequent switching; how-
ever, cost savings will nonetheless still be realized
(Maniccia et al. 2001). The greatest energy (and subse-
quently cost) savings are ultimately determined by what
is done with the building occupancy information. STEP
Tech offers a more reliable method of measuring occu-
pancy without having any passive power consumption,
as well as, offering savings in simplified installation and
wiring costs when compared to traditional means of
occupancy sensing.
Summary
In this study, a self-powered wireless occupancy sensor
has been designed in order to take a major step toward
practicalizing the smart building concept. Our device
takes the form of a floor tile, which powers the wireless
16 N. Sharpes et al.: Floor Tile Energy Harvester
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signal transmission of occupancy information by deriving
energy from human gait when stepped upon. This Smart
Tile Energy Production Technology (STEP Tech) has
demonstrated the ability to control automated process
in smart buildings, in this case lighting, and can be easily
extended to climate control and many other electroni-
cally controlled building systems. In realizing STEP
Tech, several major challenges were addressed, including
the modeling and optimization of a rectangular cymbal
piezoelectric transducer, the development of a packaging
technique and construction of a durable floor tile enclo-
sure aimed at protecting and prolonging the useable life
of the piezoceramic, as well as, the design of energy
harvesting circuitry for optimal energy conversion and
wireless signal transmission, and finally experimentation
and demonstration to prove “real-world”functionality.
These accomplishments were made while also limiting
the deflection of the tile to an unperceivable level, so
the gait of the building occupants is not disturbed as
they walk about, preserving the integrity of the measured
occupancy.
Acknowledgments: The authors would also like to
acknowledge the contributions of: Jeffery Cotter
(Undergraduate researcher at Virginia Tech, Blacksburg,
VA) and Rob Culbertson (US Army Veteran) (Teacher at
Thomas Jefferson High School for Science & Technology,
Alexandria, VA).
Research funding: This research was supported through
the National Science Foundation through CEHMS and
Fundamental Research Program. Piezoelectric materials
were donated by American Piezo Ceramics Inc.
(Mackeyville, PA).
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