Energy Harvesting from 

Wood Floor Vibration Using a 

Piezoelectric Generator

Jiangming Kan

Robert J. Ross

Xiping Wang

Wenbin Li

United States Department of Agriculture

Forest

Service

Forest Products

Laboratory

Research Note

FPL–RN–0347

April

2017
Abstract

Vibration can occur in wood floor systems as a 

consequence of a variety of human activities, ranging from 

common daily movements associated with individuals 

living in homes to high-intensity activities associated with 

sporting events that are held in large sports arenas. For 

example, the potential for harvesting energy from a wooden 

floor system in public buildings during daily rush hours 

could be substantial. In this pilot study, an energy 

harvesting system was developed to investigate the 

feasibility of harvesting energy from a vibrating wooden 

floor system. The purpose of this publication is to provide a 

summary of the experimental technique we developed and 

to present data obtained from its use.

Keywords: energy harvesting, piezoelectric property, 

vibration, wood floor systems

Acknowledgments

This project was conducted under the cooperative research 

agreement between Beijing Forestry University, Beijing, 

China, and the USDA Forest Service, Forest Products 

Laboratory (FPL), Madison, WI. Financial support to 

Dr. Kan’s research visit to FPL was provided by the China 

Scholarship Council.

April 2017

Kan, J.; Ross, R.J.; Wang, X.; Li, Wenbin. 2017. Energy harvesting from 

wood floor vibration using a piezoelectric generator. Research Note FPL–

RN–0347. Madison, WI: U.S. Department of Agriculture, Forest Service, 

Forest Products Laboratory. 7 p.

A limited number of free copies of this publication are available to the 

public from the Forest Products Laboratory, One Gifford Pinchot Drive, 

Madison, WI 53726-2398. This publication is also available online at 

www.fpl.fs.fed.us. Laboratory publications are sent to hundreds of libraries 

in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the 

University of Wisconsin.

The use of trade or firm names in this publication is for reader information 

and does not imply endorsement by the United States Department of 

Agriculture (USDA) of any product or service.

Contents

Introduction........................................................................ 1

Fundamentals of Piezoelectric Material............................. 2

Materials and Methods....................................................... 3

Data Analysis..................................................................... 4

Results and Discussion....................................................... 4

Conclusion ......................................................................... 6

References.......................................................................... 7

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Introduction

The Beijing Forestry University and USDA Forest Service, 

Forest Products Laboratory (FPL) are investigating various 

methods and technologies surrounding energy usage in 

wood structures. Investigations on the use of various 

insulation methods and other energy saving techniques are 

ongoing. One of the areas proposed for investigation is the 

concept of capturing, or harvesting, mechanical energy 

generated by appliances and occupants as they live and 

work in wood structures. A significant amount of research 

has been conducted to investigate energy harvesting for a 

wide range of applications—from harvesting energy from 

the boots of soldiers to power their portable electronic field 

equipment to using harvesting technologies to provide 

power for in-service sensors for monitoring transportation 

structures.

Wood floor systems are widely used in residential and 

commercial buildings, as well as in large public facilities. 

Movement can occur in wood floor systems as a 

consequence of various human activities ranging from

walking to intense sports activities (Saidi and others 2006). 

For example, the walking pace of a human is generally 

between 0.9 and 16 steps per second (Braune and Fischer 

1987). For a 68-kg man walking at 2 steps per second, the 

available power that can be generated by the fall of his heel 

is 67 W (Starner 1996; Starner and Paradiso 2004). 

The use of piezoelectric generators to harvest energy from

the movement of humans has been extensively studied. 

Individual human motion, in general, can be characterized 

as being of relatively low frequency and large amplitude. 

The theoretical limits of piezoelectric energy harvesting on 

human applications have suggested that 1.27 W could be 

obtained from walking by using a piezoelectric generator 

(Gonzalez and others 2001). One of the earliest examples of 

a shoe-mounted generator incorporated a hydraulic system 

coupled to cylindrical lead zirconatetitanate (PZT) stacks. 

The hydraulic system amplifies the force on the 

piezoelectric stack while decreasing the stroke. A 1/17th

scale model was built and tested and was found to generate 

5.7 ± 2.2 mW kg-1 during walking, which suggested that 

6.2 W could be generated with a full-size generator on a 

75-kg subject (Antaki and others 1995).

Scientists have also developed a generator using 

polyvinylidine fluoride (PVDF) inserted in a sports training 

shoe in which the bending movement of the sole strained the 

PVDF stacks, producing a charge from the d31 mode 

(Kymissis and others 1998). At a foot fall frequency of 

0.9 Hz, this generator produced an average power of 

1.3 mW into a 250-kΩ load. Shenck and Paradiso (2001) 

tested a piezoelectric generator using a compressible 

dimorph that was located in the heel of a work boot. Under 

excitation of 0.9 Hz, the generator produced an average of 

8.4 mW into a 500-kΩ load.

Also, piezoelectric crystals embedded in the heel of a shoe 

have been demonstrated by the Electric Shoe Company 

(Drake 2001). This generator could recharge a mobile phone 

after 5 days of walking. Yoon and others (2005) used a 

simply supported curved piezoelectric unimorph, similar to 

a piezoelectric bimorph, to verify the analytical model of the 

curved PZT unimorph. They found that it is more effective 

to increase the width of the unimorph rather than the length. 

The height at the center and the thickness of the substrate 

material should be maximized within the capacity of the 

manufacturing process and the available compressing force. 

In an analytical study on PVDF inserts, Mateu and Moll 

(2005) compared homogeneous (two layers of PVDF 

bonded together) and heterogeneous beams (PVDF 

bimorph) with different boundary conditions in both 

rectangular and triangular shapes. The overall conclusion 

was that the best PVDF structure was a simply supported 

asymmetric bimorph beam under a distributed load, with a 

large ratio of substrate to PVDF thickness being preferable.

Energy Harvesting from Wood Floor 

Vibration Using a Piezoelectric Generator

Jiangming Kan, Professor

School of Technology, Beijing Forestry University, Beijing, China

Robert J. Ross, Supervisory Research General Engineer

Xiping Wang, Research Forest Products Technologist

Forest Products Laboratory, Madison, Wisconsin

Wenbin Li, Professor

School of Technology, Beijing Forestry University, Beijing, China
Research Note FPL–RN–0347

2

Almost all energy generators introduced in previous 

research were used on wearable computing electronic 

systems or implanted sensor nodes for a specific purpose. 

No information has been reported on the use of piezoelectric 

generators in wood floor systems to extract energy from 

floor vibration.

The objective of this pilot study was to investigate the 

feasibility of using a piezoelectric generator to harvest 

vibration energy from a wood floor system.

Fundamentals of Piezoelectric 

Material

Piezoelectric materials have been used for many years to 

convert mechanical energy into electrical energy. The 

piezoelectric effect was discovered by Jacques and Pierre 

Curie in 1880 (APC International Ltd. 2002). They found 

that if certain crystals were subjected to mechanical strain, 

they became electrically polarized, and the degree of 

polarization was proportional to the applied strain. This is 

called direct piezoelectric effect. Conversely, these materials 

deform when exposed to an electric field. This is called 

converse piezoelectric effect. The direct piezoelectric effect 

is responsible for the material’s ability to function as a 

sensor or generator. The converse piezoelectric effect is 

accountable for its ability to function as an actuator. A 

material is deemed piezoelectric when it has this ability to 

transform mechanical strain energy into electrical energy 

and likewise transform electrical energy to mechanical 

strain energy.

The mechanical and electrical behavior of a piezoelectric 

material can be modeled by two linear constitutive 

equations (Riemer and Shapiro 2011). These equations 

contain two mechanical and two electrical variables:

{D} [d]{T} [ ]{E} T = + ε (1)

{S} [S ]{T} [d] {E} E t = + (2)

where {D} is the three-dimensional electric displacement 

vector; {T} is the stress vector; [ɛT] is the dielectric 

permittivity matrix at constant mechanical stress; [sE] is the 

6-by-6 matrix of compliance coefficients at constant electric 

field strength; {S} is the six-dimensional strain vector; [d] is 

the 3-by-6 matrix of the piezoelectric constant; and {E} is 

the electric field vector. The superscript t denotes

transposition of a matrix. Equation (1) is for direct 

piezoelectric effect, and Equation (2) is for converse 

piezoelectric effect:

When the material is deformed or stressed, an electric 

voltage can be recovered along any surface of the material 

(via electrodes). Therefore, piezoelectric properties must 

contain a sign convention to facilitate this ability to recover 

electric potential in three directions. The piezoelectric 

material can be generalized for two mode operations. The 

first is the stack configuration that operates in the 33 mode,

and the second is the bender that operates in the 13 mode. 

The sign convention assumes that the poling direction is 

always in direction 3.

In the 33 mode as shown in Figure 1, the electric voltage is 

recovered in direction 3 and the material is strained in the 

poling direction, or direction 3. In the 13 mode as shown in 

Figure 2, the electric voltage is measured in direction 3 and 

the material is strained in direction 1, or perpendicular to the 

poling direction. These two modes of operation are 

particularly important when defining the piezoelectric 

constants.

Figure 1—The 33 mode of the piezoelectric effect

described in an orthogonal system. P is the direction of 

polarization.

Figure 2—The 13 mode of the piezoelectric effect

described in an orthogonal system. P is the direction of 

polarization.
Energy Harvesting from Wood Floor Vibration Using a Piezoelectric Generator

3

The piezoelectric charge constant α is defined as the electric 

polarization generated in a material per unit mechanical 

stress applied to it. Alternatively, it is the mechanical strain 

experienced by the material per unit electric field applied to 

it. For example, α31 is the induced polarization in direction 3 

per unit stress applied in direction 1. Also, it is the 

mechanical strain induced in the material in direction 1 per 

unit electric field applied in direction 3.

The piezoelectric voltage constant g is defined as the 

electric field generated in a material per unit mechanical 

stress applied to it. Alternatively, it is the mechanical strain 

experienced by the material per unit electric displacement 

applied to it. For example, g31 is the induced electric field in 

direction 3 per unit stress applied in direction 1. Also, it is 

the mechanical strain induced in the material in direction 1 

per unit electric displacement applied in direction 3.

Another important constant for piezoelectric material is the 

coupling factor keff, which is a measure of the effectiveness 

with which electrical energy is converted into mechanical 

energy and vice versa. At frequencies well below the 

resonant frequency of the piezoelectric body, keff is given by

energy input

2 energy converted keff = (3)

This expression holds for both electro-mechanical and 

mechano-electrical conversions. As with other piezoelectric 

constants, coupling factors carry subscripts. k33, for instance, 

is the coupling factor for longitudinal vibrations of a very 

long, slender rod (in theory infinitely long, in practice with a 

length/diameter ratio >10) under the influence of a 

longitudinal electrical field. k31 is the coupling factor for 

longitudinal vibrations of long rod under the influence of a 

transverse electric field. The real efficiency is the ratio of 

the converted useful energy to the energy taken up by the 

transducer. A tuned and well-adjusted transducer working in 

its resonance region could be more than 90% efficient. If it 

is outside its resonance region, however, its efficiency could 

be very low.

Piezoelectric materials are widely available in many forms 

including single crystal (quartz), piezoceramic (PZT), thin 

film (sputtered zinc oxide), screen-printable thick films 

based on piezoceramic powders (Baudry 1987; White and 

Turner 1997), barium titanate (BaTiO3), and polymeric 

materials such as PVDF. The piezoelectric constants of 

common materials are given in Table 1.

Materials and Methods

Wood Floor System

The wood floor system used in this pilot study was 

constructed of nine 40- by 225-mm Douglas-fir joists 

spaced 380 mm on center, with a span of 3.65 m (Fig. 3). 

The floor deck was 20-mm-thick plywood panels, which 

were fastened to the joists with drywall screws. The floor 

system was rigidly supported on both ends by portable 

concrete barriers (Fig. 3).

Energy Harvesting System

Figure 4 shows the energy harvesting system used in this 

study. The experimental setup consisted of a beam-type 

piezoelectric generator, an energy harvesting electrical

interface, a digitizer (NI-USB-5132, National Instruments, 

Austin, TX), and a personal computer with signal-recording 

software developed by the authors. The conversion chain 

starts with a mechanical energy source: floor vibration 

resulting from human motions. The vibration energy is 

converted into electrical energy via a piezoelectric generator 

installed in the floor. The produced electricity is thereafter 

regulated by electrical interface before being stored in a 

storage system or powering the load (electrical device).

Piezoelectric Generator

A bending type piezoelectric generator (D220-A4-503YB, 

PiezoSystems Inc., Woburn, MA) was selected for this 

Table 1—Constants of common piezoelectric materials

Property PZT-5Ha PZT-5A BaTiO3

b PVDFc

d33 (10-12 CN-1) 593 374 149 –33

d31 (10-12 CN-1) –274 –171 78 23

g33 (10-3 VmN-1) 19.7 24.8 14.1 330

g31 (10-3 VmN-1) –9.1 –11.4 5 216

k33 0.75 0.71 0.48 0.15

k31 0.39 0.31 0.21 0.12

Relative permittivity 

(ε/ε0)

3400 1700 1700 12

Curie temperature (°C) 195 365 120 110

a

PZT, cylindrical lead zirconatetitanate.

b

Barium titanate.

c

PVDF, polyvinylidine fluoride.

Figure 3—The laboratory-constructed wood floor system 

used for energy harvesting experiments.
Research Note FPL–RN–0347

4

experiment and mounted underneath the floor deck. The 

piezoelectric generator was constructed of PZT material 

deposited on both sides of a brass substrate (Fig. 5).

Excitation

Two excitation sources were used to generate the floor 

movement. One excitation source consisted of a small 

electric motor whose rotational frequency was continuously 

adjustable up to 30 Hz. This excitation source was used to 

simulate walking. An inflatable ball dribbled continuously 

was also used, to simulate what a sports floor might be 

subjected to.

Electrical Interface

Two electrical interfaces were used in the experiments. One 

was an EH 300 energy harvesting module (Advanced Linear 

Devices, Inc., Sunnyvale, CA) with two 3300 pF parallelconnected capacitors for low power intermittent duty and 

long storage time applications. The other is a universal 

energy harvesting evaluation kit (CBC-EVAL-09, Cymbet 

Corporation, Elk River, MN) with an energy processor 

(EnerChip EP CBNC915-ACA, Cymbet Corporation) and a 

solid state battery module (EnerChip CBC51100 100 μAh, 

Cymbet Corporation). Under each excitation condition, the 

two electrical interfaces were used respectively to regulate 

and store the energy harvested from the vibration. The effect 

of the electrical interfaces on the output power of the energy 

harvesting system was evaluated based on the experimental 

results.

Experimental Procedures

Energy harvesting experiments were conducted with four 

different combinations of electrical interfaces and excitation 

sources: (1) EH 300 energy harvesting module with motor 

excitation; (2) EH 300 energy harvesting module with 

bouncing ball excitation; (3) CBC-EVAL-09 universal 

energy harvesting evaluation kit with motor excitation; and 

(4) CBC-EVAL-09 universal energy harvesting evaluation 

kit with bouncing ball excitation.

In the cases of motor excitation, the wood floor was excited 

by an electric motor with the rotational frequency that 

produced the maximum output voltage of piezoelectric. In 

the cases of bouncing ball excitation, the wood floor was 

excited by repeated impact from a basketball when a person 

constantly bounced the ball while walking on the floor. The 

maximal output voltage of the beam-type piezoelectric 

generator and the voltage of the capacitor or battery of the 

electrical interface were recorded once every 30 s by the 

digitizer and recorded by the signal-recording software.

Data Analysis

To model the electrical behavior of a vibrating piezoelectric 

generator, we consider the following: (1) the generator is 

excited around its resonance frequency; and (2) the 

displacement of the vibration movement remains in the 

linear range. Then, the structure with piezoelectric elements 

can be modeled by a mass + piezoelectric element + 

spring + damper (Roundy and Wright 2004). The resonant 

frequency is conversely proportional to the squared root of 

mass M. Adding the proof mass can decrease its resonant 

frequency to about 10 Hz, which is in the main-energy 

frequency range. Therefore, in this study, the piezoelectric 

generator was excited around its natural frequency. In this 

case, the harvested power varied with different electrical 

load and reached a maximum harvested power for an 

optimal load (Minazara and others 2007).

The energy stored in the capacitors harvested from the wood 

floor vibration was calculated as

( ) 2

1

2

2 2

1 ∆E = C U −U (4)

where U1 represents the initial voltage across the capacitors, 

U2 is the voltage across the capacitors after being charged, C

is the capacitance, and ΔE represents the energy stored in 

the capacitors.

The average power (P) from the piezoelectric generator was 

calculated by dividing the stored energy by charging time

(Tc).

The average charging current Ic was determined by

( )

c

2 1 c T

U U C I − × = (5)

Results and Discussion

The vibration levels observed were different for different 

excitation methods. The maximal vibration amplitude was 

observed when the bouncing ball was used to generate 

Figure 4—The energy harvesting system using a 

piezoelectric generator applied to a wood floor structure.

Figure 5—Beam-type piezoelectric generator.
Energy Harvesting from Wood Floor Vibration Using a Piezoelectric Generator

5

vibrations on the floor. The measured vibration levels for 

walking simulated by motor excitation were relatively low.

Figure 6a shows the frequency spectrum of the vibration 

signal obtained from the floor deck, and Figure 6b shows 

the frequency spectrum of the vibration signal obtained 

from the floor joist. The vibration energy of the floor deck 

was concentrated in the frequency range of 0–130 Hz, 

whereas the vibration energy of the floor joist was 

concentrated in the frequency range of 0–420 Hz. The peaks 

corresponded to the main frequencies of the wood floor 

system and human motions. The peaks observed were less 

than 420 Hz, regardless of the speed of the bouncing ball 

and the weight of the person walking on the wood floor. The 

interesting frequency band to harvest energy, where 90% of 

the energy of the vibrations induced by human motions was 

concentrated, was approximately between 0 and 420 Hz 

(Figs. 6 and 7). It is therefore essential to select and tune a 

piezoelectric generator that resonates in the frequency range 

of 0 to 420 Hz, where 90% of vibration energy is available.

To harvest the maximum energy, the piezoelectric generator 

should be tuned to the resonant mode of the floor structure.

Table 2 shows the average power harvested from the wood 

floor vibration by the piezoelectric generator with an EH 

300 energy harvesting module in different excitation modes 

and the average charging current of the different electric 

interfaces by different excitation mode. The relationships 

between the charging time and the voltage across the energy 

storage (capacitors or batteries) with different electric 

interfaces in different excitation mode are shown in 

Figure 8.

The experimental results indicated that the maximum output 

voltage with motor excitation was lower than that with the 

bouncing ball excitation. One possible reason is that the 

amplitude of the wood floor vibration excited by the 

bouncing ball was larger than that excited by a motor, even 

though the frequency of the bouncing ball was far lower 

than that of the motor.

The average output power of the energy harvesting system 

depends on the excitation mode. The average power 

harvested through motor excitation was greater than that 

harvested through a bouncing ball, although the maximum 

output voltage by motor excitation was lower than that by 

bouncing ball. That is because the average output power 

depends not only on the maximum output voltage but also 

Figure 6—Single-sided amplitude spectrum of vibration 

signals captured from the wood floor system (a) in deck, 

(b) in joist.

Figure 7—Cumulative sum of the acceleration squared

(a) in deck, (b) in joist.
Research Note FPL–RN–0347

6

on the frequency of the floor excitation. The frequency of 

the bouncing ball was far lower than that of the motor, thus 

resulting in less average output power by the bouncing ball

than that by motor excitation.

The charging current showed a similar relationship with the 

excitation mode for the same reason. Figure 8 shows the 

relationship between the voltage across the energy storage 

and charging time. The slope of the curves indicates the 

charging rate of the capacitors or battery. The experimental 

results indicated that the piezoelectric generator produced 

more power from the vibration of the wood floor induced by 

the motor than that by the bouncing ball.

The ultimate output power from an energy harvesting 

system also depends on the electric interface used. There

could be an issue on the impedance matching between the 

output impedance of piezoelectric generator and the input 

impedance of the electrical interface. The advanced energy 

management can improve the efficiency of energy 

harvesting as the maximum power point tracking (MPPT) 

algorithms do in the CBC-EVAL-09 universal energy 

harvesting evaluation kit. The energy harvesting system that 

scavenges wood floor vibration energy induced by human 

motion should include an advanced energy management 

subsystem to adapt the excitation mode in different 

application sites.

The maximum power harvested from wood floor vibration 

by a single piezoelectric generator is not enough to power 

common electric devices. This was expected because the 

vibration energy in a specific location generated by a single 

excitation source is very limited. Another factor was that the 

efficiency of the piezoelectric generator was relatively low 

because the piezoelectric material used was not the best one 

on the market. Further research is needed to focus on 

efficiency improvement of the piezoelectric generator and 

the energy congregation of multigenerators.

Conclusions

In this preliminary study, we investigated the feasibility of 

harvesting vibration energy from the wood floor vibration 

induced by human motions. From analysis of the 

experimental results, we concluded the following:

1. The main frequency of the wood floor vibration varies 

in different excitation modes, but the dominant 

frequency range of wood floor vibration was between 0 

and 420 Hz. In an energy harvesting system, the 

piezoelectric generator should be tuned to the resonant 

mode in this range, allowing the maximum amount of 

energy to be harvested.

2. The output power from the piezoelectric generator was

related to the frequency and amplitude of the excitation. 

Higher frequency and larger amplitude of the excitation 

can produce more power for the same piezoelectric 

generator.

3. The energy harvesting system can scavenge energy 

from the wood floor vibration and store it in the 

capacitors or batteries. Future research should focus on 

examining the feasibility to power an electronic system 

by using multiple piezoelectric generators and multiple 

excitation sources in in-situ wood floor structures.

Table 2—Performance of the energy harvesting systems tested

Excitation method

Energy 

harvesting 

system

Capacitance

(μF)

Charging 

time

(s)

Max. output 

voltage

(V)

Energy 

stored

(J)

Average 

power

(mW)

Average 

charging 

current

(mA)

Motor excitation EH300 6600 330 14.575 0.148 0.447 0.102

Bouncing ball EH300 6600 930 16.303 0.304 0.327 0.046

Figure 8—Voltages across the energy storage in relation to 

charging time.
Energy Harvesting from Wood Floor Vibration Using a Piezoelectric Generator

7

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