Comparison of Energy Harvesting Techniques for Wearable Activity

Monitoring Devices

Ana Cimpian

, Bastien Granier

, Gearóid Ó Laighin

and Maeve Duffy

¹Bioelectronics Research Cluster, Electrical & Electronic Engineering, National University of Ireland, Galway, Ireland

²Power Electronics Research Centre, Electrical & Electronic Engineering, National University of Ireland, Galway, Ireland

Keywords: Energy Harvesting, Electromagnetic, Piezoelectric, Self- sustained System, Activity Monitoring Devices.

Abstract: Piezoelectric and electromagnetic generation are the two most commonly used energy harvesting

techniques. The aim of this paper is to compare the two techniques in terms of their potential for powering a

wearable monitoring device. An electromagnetic generator and a piezoelectric system are proposed to

power an activity monitoring device located in the shoe. Results to date indicate that the electromagnetic

generator produces the best output power levels.

1 INTRODUCTION

Energy harvesting is a research field that has

become of more interest with the growth in

popularity of portable electronic devices and their

increased power requirements. Even though there

have been several studies comparing different

energy harvesting techniques (Mateu, 2005) and

(Paulo, 2010), very little has been done to compare

techniques for a given application.

This work focuses on harvesting energy from the

human body during normal everyday activities, for

powering wearable electronic applications. The aim

of this work is to propose a self- sustained activity-

monitoring device destined for every day monitoring

of elderly people. This will include comparing the

performance of two generators supplying power to a

pedometer system, where the generators are based

on the most commonly applied energy harvesting

techniques: electromagnetic and piezoelectric

A growing trend has been observed in the

development of activity monitoring devices: e.g. like

FitBit (Fitbit, US), Nike sensors (Nike, Inc., US) and

the Shimmer (Shimmer Research, Ireland). These

are most commonly used in monitoring sports

activities in order to increase performance.

Lately, such monitoring devices have also been

employed in health care, for monitoring the mobility

and activity levels of elderly people. Clearly, for

such devices to be worn or attached to the body, the

most important characteristics are small size,

usability and the ability to give real time information

with a reliable, low maintainence power source.

With a view to improving the rate of use by elderly

people, the possibility of harvesting energy

expended in activity to produce a self-sustained

pedometer is proposed, thereby removing the need

for users to recharge batteries.

2 ACTIVITY MONITORING

DEVICES

Typical power consumptions values for some of the

commonly applied components in activity

monitoring devices are compared in Table 1.

Table 1: Typical power consumption levels for devices

employed in activity monitoring.

Device Power consumption (mW)

Accelerometers 0.16

Low power processors 0.93- 1.4

Other sensors 0.16- 11.6

LEDs 100

Bluetooth

Sleep mode 2

During transmission 24

Considering that the power consumption of

individual circuit blocks varies significantly between

sleep and active modes, the design of a generator

that produces pulses of power was considered a

simpler and more achievable target. The system will

contain an nRF chip that will transmit a pulse of data

273

Cimpian A., Granier B., Ó Laighin G. and Duffy M..

Comparison of Energy Harvesting Techniques for Wearable Activity Monitoring Devices .

DOI: 10.5220/0004252102730277

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2013), pages 273-277

ISBN: 978-989-8565-34-1

 2013 SCITEPRESS (Science and Technology Publications, Lda.)

every time a step is taken, thus providing a self-

sustained pedometer. The selected chip for this

application is the nRF24AP2-1CH (Nordic

Semiconductor, Norway) and it operates on voltages

between 1.9V and 3.6V and power levels between

0.040 and 60 mW. Typical current consumption for

tasks performed by the device are displayed in Table

Table 2: Current requirements of the nRF chip.

Activity Current requirements

Sleep mode 500 nA

Broadcast 14-42 uA

Acknowledging a package 18- 52 uA

Active mode 17mA

3 WEARABLE GENERATOR

3.1 Electromagnetic Generator Design

Electromagnetic power generation is a widely used

technique in portable applications like those

presented by (Sodano, 2005) and (Arnold, 2007).

Although different materials and techniques have

been suggested, the main drawback of

electromagnetic generators is that they have

considerable dimensions.

The generator proposed in this paper is based on

on the sliding magnet principle described in (Carroll,

2011). Based on the theory that as long as the mass

of the magnet is considerably smaller than the mass

of the foot continuous movement of the magnet can

be sustained by external forces due the movement of

the foot, no additional or deliberate effort from the

user is needed.

The prototype generator was assigned a volume

of 50x15x15 mm

and disk shaped NdFeB magnets

with diameters of 10mm were determined to be

optimum for the generator cross sector. Applying

optimization rules, it was determined that the most

suitable generator prototype consists of 1 coil, with

an optimum coil length of 17.3 mm centred along

the 50 mm generator length. The copper wire used

has a diameter of 0.22 mm, this leading to the main

coil having 300 turns disposed in three layers in

order to fit the allocated space and resulting an

internal coil DC resistance of 6 Ω.

In order to maximise the power output of the

generator, but still maintain its size in the allocated

volume, two additional half coils have been

considered in the remaining generator length at

either end of the main coil. A prototype of the

electromagnetic generator is shown in Figure 1.

Figure 1: Electromagnetic generator prototype.

Open circuit AC measurements were undertaken

for a set of pre-established walking speeds. The

generator produces voltages between 0.5 V peak for

a walking speed of 2km/h (representing very slow

walking), and 2.6 V peak for 12 km/h (representing

running).

Through a set of AC measurements, it was

determined that the location for the generator

preferred for both performance and ease of wear, is

on the exterior side of the foot at the heel area. The

same walking speeds as in the case of open circuit

tests were applied with a series of load resistances

attached.

As expected AC measurements showed that the

generator performs best when the load resistance

matches the internal resistance of the main coil.

Unexpectedly, the average power decreased when

the subjects switched from walking fast to a running

state. The results of Figure 2 show that the optimal

speed is 8 km/h. At this point, the average power

graph shows that the values recorded are up to 4.5

mW average power, although the highest peak to

peak voltage was produced for a 10Ω load

resistance.

Figure 2: Peak- to- peak voltages and maximum output

power values at different walking speeds for different load

resistance for the electromagnetic generator.

BIODEVICES2013-InternationalConferenceonBiomedicalElectronicsandDevices

274

3.2 Piezoelectric Generator Design

Research in the field of piezoelectric energy

harvesting has been reviewed in papers like

(Sodano, 2005) and (Lefeuvre, 2006). Many authors

focus on the development of piezoelectric materials,

most of which are based on lead zirconate titanate,

also known as PZT (Anton, 2007). Based on a

comparison of generated power levels, Kymissis has

determined that different types of piezoelectric

material have different responses strongly connected

to the individual’s gait patterns (Kymissis, 1998).

The first aim of this research was to determine

the most suitable form of piezoelectric material for

producing a shoe in-sole generator, where the

highest forces available are due to the pressure of the

person’s weight acting under the sole of the foot.

Two different types of piezoelectric materials

have been taken into consideration: a PVDF film

having an area of 6x12 mm² from Measurements

Specialities (Measurement Specialities, US), and a

set of piezoceramic disks with diameters of 10 and

20 mm, and thicknesses of 1 and 2 mm from PI

(PhysikInstrumente GmbH & Co., Germany).

Table 3 compares the electrical performance of

the selected materials in terms of open-circuit

voltage as calculated, V

OCcal

, for an applied pressure

level of 400 kN/m

, (typical under the heel during

walking) (Perttunen, 2002). Measured results, V

OCm

are also presented for an estimated pressure of 94

kN/m

, and it is seen that there is generally good

agreement in the trends of calculated and measured

values.

Due to the low capacitance of the piezoelectric

elements, the capacitance of the probe (16pF/10MΩ)

affects the open-circuit measurement, and therefore

measured results are derived from an equivalent

circuit model of the piezoelectric elements, using

values of equivalent series capacitance, C

, and

resistance, R

, as listed in Table 3. Clearly

impedance values are much higher than the

electromagnetic coil resistance described in section

3.1.

Table 3: Comparison of test piezoelectric material

properties.

Materials C

(pF)

()

OCcal

(V) V

OCm

(V)

Polymer film 708

>1 M

34.0 na

PIC155 2x10 mm² 504

6.3 k

21.6 4.9

PIC255 2x10 mm² 608

5.2 k

20.0 4.7

PIC255 1x10 mm² 1210

2.8 k

10.0 2.59

PIC255 2x20 mm² 2430

1.3 k

20.0 2.87

In order to compare power levels achievable

within the footprint area of a shoe heel, equivalent

circuit models of each piezoelectric element were

combined in series and parallel and results of peak

AC power were produced for a range of load

resistances. Results are presented in Figure 3.

The maximum peak instantaneous power

predicted is 30 W, which compares with 4.5 mW

average power for the electromagnetic generator of

section 3.1. Clearly, the piezoelectric generator is

not competitive in this environment and work is on-

going to review the choice of piezoelectric materials,

and methods for compensating the high impedance

of piezoelectric source capacitance. Another issue

that needs to be addressed in terms of designing a

piezoelectric generator is compensating the high

impedance capacity as this will lead to increased

output power values.

Figure 3: Maximum output power of piezocermaic disks

for different load resistance and associations.

4 ELECTROMAGNETIC AC/DC

CIRCUIT PERFORMANCE

After establishing that the electromagnetic generator

is better suited for the in-shoe design the most

optimal parameters and design for the generator

were selected. The next step was designing the

AC/DC conversion circuit, in order to provide a DC

power source as required by most portable devices.

Literature has proposed a number of different

rectification circuits (Wang, 2009) and (Rao, 2011).

Although many of the systems proposed have

shown good results, a number of them require

additional powering for active switches. As it is

desired that the proposed generator to be a self-

sustained system and that the generator voltage pulse

needs only to be rectified but not necessarily

regulated, the AC/DC conversion circuitry needed is

considered to be a more simplistic application of

basic conversion principles. Full investigations were

conducted for full-wave rectifier and half-wave

rectifiers and voltage doubler circuits. A set of

ComparisonofEnergyHarvestingTechniquesforWearableActivityMonitoringDevices

275

simulations were performed with the aid of pSpice

simulation software (Cadence Design Systems, UK)

for each type of rectifier with several load

resistances and capacitors values, and the results are

shown in Figure 4. The source used in each

simulation is represented by the generator open-

circuit waveform and a 6Ω resistor representing coil

resistance. In order to maximise the output power,

the PMEG 2010EH diodes were used as they present

lower forward voltage (Caroll, 2005).



Figure 4: Simulations results of maximum output power

for rectification circuits depending on load resistances for

3.5mF capacitor.

Due to the overall reduced voltage drop across

the diodes in the full- wave rectifier circuit, the

doubler circuit performed the best. The maximum

output power was achieved for 250 Ω, with a peak

instantaneous power value of 3 mW. Due to the fact

that the rectification was performed for a pulsed type

of power a capacitance value of 3.5mF was used, as

it provided increased values for the output voltage.

Figure 5 presents the input and the rectified

waveforms for the optimum load resistance.

Figure 5: Typical waveform generated by the prototype

during walking and the rectified waveform with a doubler

circuit for the optimum load resistance (250Ω).

5 CONCLUSIONS

Two different energy harvesting techniques have

been compared in order to prove their suitability to

power an activity monitoring device. The

electromagnetic generator has proved to be superior

to the piezoelectric system. With the given space of

50x15x15 mm

the generator produced power levels

of 4.5 mW AC and 0.8 mW for pulsed DC, almost

sufficient to meet the requirements of the nRF chip.

Work is on-going to investigate the effect of the

additional half coils over the output power.

However, the optimum speed being around 7-8 km/h

might make this type of energy harvesting system

more suitable for more active people. For the

piezoelectric system, the selection of materials

available for this work has not given the desired

outcome, with the piezoceramic disks giving low

output powers and being subjective to damage due

to repeated stress caused by the constant pressure

applied during walking. Although piezoelectric

components are capable of providing high output

voltage, the output power is limited by the loading

of the capacitive impedance.

REFERENCES

L. Mateu, F. Moll, and U. Polit, “Review of Energy

Harvesting Techniques and Applications for

Microelectronics,” Energy.

J. Paulo and P. D. Gaspar, “Review and Future Trend of

Energy Harvesting Methods for Portable Medical

Devices,” Engineering, vol. II, 2010.

H. A. Sodano, D. J. Inman, and G. Park, “Comparison of

Piezoelectric Energy Harvesting Devices for

Recharging Batteries,” Power, vol. 16, no. 10, pp.

799-807, 2005.

Arnold, D.P.; ‘Review of microscale magnetic power

generation’, IEEE Transactions on Magnetics, Vol.

43, No. 11, November 2007, pp. 3940 – 3951

Carroll. D, Duffy. M, Modelling, design, and testing of an

electromagnetic power generator optimized for

integration into shoes, Journal Article 2011, 8

September 9, 2011, Proceedings of the Institution of

Mechanical Engineers, Part I: Journal of Systems and

Control Engineering, Volume 226 issue 2, pages 256-

270

E. Lefeuvre, Badel, C. Richard, L. Petit, and D. Guyomar,

“A comparison between several vibration-powered

piezoelectric generators for standalone systems,”

Sensors and Actuators A: Physical, vol. 126, no. 2, pp.

405-416, Feb. 2006.

S. R. Anton and H. Sodano, “A review of power

harvesting using piezoelectric materials (2003–2006),”

Smart Materials and Structures, vol. 16, no. 3, pp.

R1–R21, Jun. 2007.

J. Kymissis, C. Kendall, J. Paradiso, and N. Gershenfeld,

“Parasitic power harvesting in shoes,” Digest of

Papers. Second International Symposium on Wearable

Computers (Cat. No.98EX215), pp. 132–139.

J. Perttunen, "Foot loading in normal and pathological

ime

5.0s 5.5s 6.0s 6.5s 7.0s 7.5s 8.0s 8.5s 9.0s 9.5s 10.0s

V(0,R2:1) V(R1:1,V1:-)

2.0V

1.0V

2.0V

BIODEVICES2013-InternationalConferenceonBiomedicalElectronicsandDevices

276

walking", PhD thesis, University of Jyvaskyla, 2002.

Y. Rao, David P. Arnold, “An Input-Powered Active

AC/DC Converter with Zero Standby Power for

Energy Harvesting Applications”, Energy Conversion

Congress and Exposition (ECCE), 2010 IEEE, 12-16

Sept., pp 4441 – 4446

G. Wang, C. Luo, H. Hofmann, L. Rome, ”Power

Electronic Circuitry for Energy Harvesting Backpack”,

Energy Conversion Congress and Exposition, ECCE

2009, 20-24 Sept. 2009, pp 3544 – 3549

D. Carroll, M. Duffy, “Demonstration of Wearable Power

Generator”, 11th European Conference on Power

Electronics and Applications, EPE’05 Dresden,

Germany, paper no. 884, CD ROM, September 2005.

ComparisonofEnergyHarvestingTechniquesforWearableActivityMonitoringDevices

277