286 1189-1-pb

5

Energy Harvesting from Passive Human Power

Faruk YildizSam Houston State University

E-mail : [email protected]

ABSTRACTSustaining the power resource for autonomous wire-less and portable electronic devices is an important issue. Ambient power sources, such as a replace-ment for batteries, can minimize the maintenance and the cost of operation by harvesting different forms of energy from the potential energy sources. Researchers continue to build high-energy-density batteries, but the amount of energy available in the batteries is not only finite but also low, limit-ing the lifetime of the system. Extended lifetime of electronic devices is very important and also has more advantages in systems with limited accessibil-ity. This research studies one form of ambient en-ergy sources: passive human power generated from a shoe/sneaker insole when a person is walking or running and its conversion and storage into usable electrical energy. Based on source characteristics, electrical-energy-harvesting, conversion, and stor-age circuits were designed, built, and tested for low-power electronic applications.

INDEX TERMSCircuit Analysis, Circuit Design, Energy Conserva-tion, Energy Conversion, Energy Storage, Piezoelec-tric Devices, Piezoelectric Materials

I. INTRODUCTIONEnergy harvesting is the conversion of ambient en-ergy into usable electrical energy. When compared to energy stored in common storage elements, such as batteries and capacitors, the environment repre-sents a relatively infinite source of available energy. Researchers have been working on many projects to generate electricity from human power, such as exploiting, cranking, shaking, squeezing, spinning, pushing, pumping, and pulling [1]. Several types of flashlights were powered with wind-up generators in the early 20th century [2]. Later versions of these devices, such as wind-up cell phone chargers and ra-

dios, became available in the market. The commer-cially available Freeplay’s [3] wind-up radios require 60 turns in one minute of cranking, which allows for the storage of 500 Joules of energy in a spring. The spring system drives a magnetic generator and ef-ficiently produces enough power for about an hour of radio play. Recently researchers have performed several studies in alternative energy sources that could pro-vide small amounts of electricity to low-power elec-tronic devices. These studies focused on investing and obtaining power from different energy sources such as vibration, light, sound, airflow, heat, waste mechanical energy, and temperature variations. The piezoelectric energy-harvesting method converts mechanical energy into electrical energy by straining a piezoelectric material [4] which causes charge separation across the device, producing an electric field and consequent voltage drop propor-tional to the stress applied. The oscillating system is typically a cantilever beam structure with a mass at the unattached end of the lever, since it provides higher strain for a given input force [5]. The volt-age produced varies with time and strain, effectively producing an irregular AC signal. The piezoelectric energy conversion produces relatively higher volt-age and power-density levels than an electromag-netic system. Moreover, piezoelectricity has the ability of elements, such as crystals, and some types of ceramics to generate an electric potential from a mechanical stress [6]. If the piezoelectric material is not short circuited, the applied mechanical stress induces a voltage across the material. The problem of how to get energy from a person’s foot to other places on the body has not been suitably solved. For a radio frequency identi-fication (RFID) tag or other wireless device worn on the shoe, the piezoelectric shoe insert offers a good solution. However, the application space for such devices is extremely limited, and, as mentioned ear-

6 journal of applied science & engineering technology 2011

lier, not very applicable to some of the low-powered devices such as wireless sensor networks. Active human power, which requires the user to perform a specific power- generating motion, is common and may be referred to separately as active human-pow-ered systems [7]. An example of energy harvesting using uni-morph piezoelectric structures was conducted by Thomas, Clark, and Clark [8]. This research was fo-cused on a unimorph-piezoelectricity circular plate, which is a piezoelectric (PZT) layer assembled on an aluminum substrate. The vibrations were driven from a variable-ambient- pressure source, such as a scuba tank or blood pressure monitor. The research-ers showed that by creating the proper electrode pat-tern on the piezoelectric element, (thermal regroup-ing), the electrode was able to produce an increase in available electrical energy. In the system, as the can-tilever beam vibrates, it experiences variable stresses along its length. Regrouping the electrodes target-ing specific vibration modes resulted in maximum charge collection. This type of design may offer the possibility for miniaturization and practicality to piezoelectric energy-harvesting technology. The piezoelectric active-fiber composites (AFCs) are made by Advanced Cerametrics Incor-porated (ACI) [9] from a uniquely-flexible ceramic fiber that is able to capture wasted ambient energy from mechanical vibration sources and convert it into electric energy. The piezoelectric composites’ fiber-spinning lines are capable of generating elec-tricity when exposed to an electric field. In piezo-electric fiber composite bimorph (PFCB) architec-ture, the fibers are suspended in an epoxy matrix and connected using inter-digitized electrodes to create an AFC. Advance Cerametrics Incorporated has demonstrated through testing that thin fibers with a dominant dimension, a length, and a very small cross-sectional area are capable of optimizing both the piezo and the reverse-piezo effects. An investigation into the improvement of an energy-harvesting-system performance and ef-ficiency using a PFCB is considered in this research. The PFCB characteristics and properties were inten-sively studied in order to build an efficient energy harvesting circuit for further study. The efficiency of the PFCB was measured by building an operation-al-difference, amplifier instrumentation test circuit and following an energy-harvesting and battery- charging circuit. Only one type of piezoelectric ele-

ment, PFCB, was available to test with a small con-stant shaker. The shaker functioned as an ambient vibration source (passive human power) and was used to vibrate the PFCB to produce electricity for the energy-harvesting circuit. A photograph of the PFCB with the inter-digitized electrodes to align the field (energy-harvesting circuit) with fibers is shown in Figure 1.

Figure 1. Basic specifications of the PFCB

The PFCB energy source is modeled as a steady AC power source for the circuit components of the energy-harvesting circuit. Since all the electronic components for the energy-harvesting and battery-charging circuit require DC voltage to operate, this AC source is then converted to a DC voltage source.

II. SYSTEM DESIGNFor the purpose of energy harvesting and storage, shoe or sneaker insoles are good sources of me-chanical stress, deformation, and vibration when a person is walking or moving his/her feet. With this method, waste-ambient mechanical energy was con-verted to electrical voltage through a unique ener-gy-harvesting circuit. An overall energy-harvesting model is shown in Figure 2 to explain implemen-tation steps and potential applications. In order to have the best efficiency and output power, the cir-cuit was designed and developed according to the ambient-source, characteristics, PZT ceramic- fiber composite and load constraints. The energy- har-vesting system is capable of capturing even minute amounts of stress and vibrations, then converting them to electric power sufficient to run low-power electronic systems.

Device size (mm): 130 x 10 x 1

Active elements: AFCB

Mode: d33

Full scale voltage range (V): ±400

Full scale power range (mW): ±120

EnErgy HArVEsting FroM PAssiVE HuMAn PoWEr 7

Figure 2. overall energy-harvesting model

Power CharacteristicsA piezoelectric energy source is most often mod-eled as an AC voltage source because of its AC pow-er characteristics and features. Piezoelectric fiber composite can be connected in series with the ca-pacitors and resistors to reduce or smooth a high-voltage input produced by PFCB. For test purposes, the tip of a mechanical pencil was used to flick the tip of the PFCB product in order to provide the ini-tial disturbance. The test equipment used included a multi-meter, shaker, and oscilloscope, connected to each other properly to obtain voltage readings from the PFCB. The first test for voltage output de-pended on time variation and was conducted with-out any mass placed on the tip of the PFCB. This was followed by a second test with variable masses that were placed on the tip of the PFCB to observe the output voltage levels. As the more mass that is added on the tip of the PFCB, the more time passes until vibration of the PFCB stops. The voltage from the PFCB increases depending on the mass and force applied to the tip of the PFCB. The plot in Figure 3 is the peak-to-peak volt-

age level, frequency, and corresponding time. AC signals and voltage outputs were similar to each other; however, the time required for the vibration to decay was observed to be longer when more mass was attached on the PFCB. All signal outputs, that occurred until the vibration decayed completely, could not be shown on the oscilloscope screen. However, it was observed that the time until vibra-tion stops is longer with more mass attached on the PFCB. The resonant frequency does not depend upon the pencil flicks. It depends upon the mass (including distributed mass and lumped-tip mass) and distrib-uted spring constant of the cantilevered beam. For a sinusoidal excitation, the most energy is transferred when excited at the resonant frequency. The decay depends upon the input resistance of the measur-ing device (electrical damping) and the mechanical damping from the material and from the air.

Figure 3.the voltage decay of the open-circuit PFCB

The PFCB was carefully clamped on the shaker with plastic bumpers to avoid damaging the part during its vibrations. The wiring between all modules was done carefully to allow for read-ing the voltage outputs from the oscilloscope and multi-meter displays. The PFCB layer and material properties were not known to predict the frequency rate accurately, so the value had to be determined experimentally on the test fixture. In order to allow the calculation of the cur-rent output, wires from the PFCB electrodes were connected to the oscilloscope probes through a 1 kΩ resistor. The current outputs could not be mea-sured by a multi-meter and were observed from the oscilloscope screen when the PFCB was vibrated by the shaker at 60Hz. It was not possible to plot the current and power outputs due to a lack of the proper data-acquisition system during the research. However,


very-low-current outputs were still produced that might be harvested with a proper energy harvesting circuit. From the vibration test results, it was de-termined that at the variable frequency, the power generated from the PFCB is sufficient for low-pow-er electronic devices. The obtained values would be enough to build an energy harvesting circuit to charge a small-scale storage device such as a bat-tery, capacitor, or super capacitor, albeit slowly.

III. ENERGY HARVESTING CIRCUIT DESIGNAfter testing the power output and the working characteristics of the PFCB at different vibration levels and attached masses, the researcher built the energy-harvesting circuit used to charge the bat-teries under low-current levels. The mechanical-to-electrical energy conversion is usually managed by the energy-harvesting circuits including convention-al buck-boost converters, bridge rectifiers, and bat-tery-charging circuits [10,11,12,13]. The energy har-vesting circuit was designed, developed, and built according to the ambient source and piezoelectric fiber composites’ low-current constraints in order to produce efficient power output. The energy-harvesting and battery charg-ing circuit design was built with typical compo-nents that could decrease high-input voltages and increase low-input currents from the PFCB to pro-vide sufficient charge currents to the batteries. The circuit was designed to start charging when the bat-tery voltage drops beyond a nominal value, and it stops charging when voltage is reached at the bat-tery nominal voltage. The Linear Technology SPICE simulator (LTSPICE) simulation interface that shows the over-all circuit is depicted in Figure 4; it represents the system circuit modules that are simulated together to test the output power level of the circuit [14]. All the necessary simulations were conducted us-ing SwitcherCADTM Spice III, because of the Linear

Technology based DC-DC buck-boost converter and battery-charging circuit components [15]. Initially, a full-wave bridge rectifier was placed between the PFCB and the operational- am-plifier-instrumentation circuit that converts AC signals to DC signals. A full-wave bridge rectifier is very efficient, converting positive and negative cy-cles from the PFCB and supplying DC voltage to the battery through the battery- charging part of the en-ergy-harvesting circuit. Since the current produced from PFCB was low, an intermediate-operational-amplifier (op-amp) circuit was used to test various current levels that was generated by PFCB in differ-ent decaying times. [16]. This instrumentation cir-cuit consisted of operational amplifiers, resistors, and intermediate/storage capacitors to implement the circuit at ±15V. A buck-boost converter and battery- charg-ing circuit is the last part of the simulation inter-face before the storage unit. The op-amp part of the energy-harvesting circuit consisted of three single operational amplifiers that are configured as differ-ence amplifiers. This implies that the voltage differential be-tween the two branches is the output of the circuit. This op-amp-instrumentation-circuit design (Figure 5) helped to observe voltage outputs from the PFCB and the capacity changes of the capacitors when the PFCB was being vibrated. Capacitors C1 and C11 were charged de-pending on the voltage generated by the PFCB, which was observed by the oscilloscope through the operational-amplifier-instrumentation circuit.

Figure 4. Energy-harvesting circuit simulation interface


Figure 5. operational amplifier instrumentation circuit

The general operational amplifier in Figure 5 was used to observe the charging phase of the C11 intermediate-storage capacitor. The 5V IC (ini-tial voltage) was supplied across capacitors in both circuits. In circuit A, the voltage across the capacitor with the 10MEG impedance, representing a flux digi-tal multi-meter, was measured. The voltage across capacitor C1 dropped almost 2V when the circuit was simulated at the same input voltage. However, the voltage across capacitor C11 in the operational-amplifier circuit stayed at constant volt-age other than negligible voltage drops. The voltage level across both capacitors, C1 and C11, was simu-lated and is plotted in Figure 6 in order to compare voltage drops across the capacitors. When the PFCB was placed on the constant shaker, it started generating voltages by charging the capacitors. The operational- amplifier circuit kept the initial voltage level constant to allow for an accurate reading of the voltage levels of the in-termediate-storage capacitor. In circuit A, the volt-age readings would not be accurate because of the

voltage drops across the capacitor when measuring the voltage with a digital multi-meter. However, the capacity readings across the capacitor C11 would be accurate, since the operational amplifier keeps the initial voltage-level constant.

DC buck-boost converter and battery charging circuit

DC-DC converters efficiently step-up (boost), step-down (buck), or invert DC voltages without the neces-sity of transformers. In these structures, switching capacitors are usually utilized to reduce or increase physical size requirements. DC-DC converters assist in product-size reduction for portable electronic devices where increased efficiency and regulation of input power are necessary for optional require-ments. Taking the above features of the buck-boost converters into consideration, a linear-technology based LT1512 integrated circuit (DC-DC buck-boost SEPIC constant current/voltage battery charger) was used to regulate the high-output voltage that was produced from the PFCB to charge small-scale bat-teries for test purposes [17]. A LT1512 battery-charging circuit was added to the energy-harvesting circuit, considering its characteristics, based upon an application data sheet. Since buck-boost converters are very sensitive, proper design, in conjunction with supporting com-ponents and physical layout, is necessary to avoid electrical noise generation and instability. The con-siderations, including LTSPICE modeling, converter selection, circuitry building, debugging, and power-output improvements, were followed step-by-step to obtain adequate energy-harvesting circuitry. This circuit would maximize the power flow from the piezoelectric device and was implemented in coordination with a full-wave bridge rectifier, in-termediate-storage capacitor, and voltage-sensitive switching circuit. It was observed that, when using the energy-harvesting circuit, over twice the amount of energy was transferred to the battery compared to direct charging alone. However, if the power-harvesting medium produced less than 2.7V, power flow into the battery was reduced due to losses in the additional circuit components and the threshold characteristics of the LT1512. For the purpose of storing energy in the intermediate storage unit, a capacitor was placed before the voltage-sensitive circuit and buck-boost converter. The voltage-sensitive circuit consists of di-Figure 6. Voltage levels across intermediate storage

capacitors


odes, MOSFET switches, and resistors to transfer the energy from the intermediate capacitor to the battery through the DC-DC buck-boost converter [18]. The MOSFET switches and zener diodes on the voltage-sensitive circuit sense the voltage in the intermediate capacitor and transfer the energy when the capacitor reaches specific voltage levels. The volt-age level in the intermediate capacitor is controlled by the zener diodes until the capacitor is discharged by transferring its energy to the battery. Depending on the zener diode values, the stored energy in the capacitor is transferred to the storage unit through DC-DC buck-boost converter and battery-charging circuit (The switch should be between 5V-15V). Due to known high-discharge rates of the ca-pacitors, the zener-diode-voltage values of 12V and 6.2V were chosen (which are small values for the pur-pose of energy harvesting from PFCB) to avoid loos-ing stored energy in the intermediate capacitor. One of the biggest benefits of the intermediate capacitor and voltage-sensitive switching circuit is to increase the amount of transferred energy from the PFCB. Cir-cuit loss is reduced throughout the energy-harvesting circuit caused by the electronic components. The circuit shown in Figure 7 was designed has four phases to represent the overall energy- har-vesting circuit modules. The first module is a mechanical-to- elec-trical energy conversion module and functions the same as PFCB producing AC power. The second module has rectification (the conversion of AC volt-age to DC voltage) and an energy-storage unit (inter-mediate capacitor). The third module is a voltage-switching circuit that senses the voltage level of the

intermediate capacitor and transfers it to the battery through a DC-DC buck-boost converter and battery-charging circuit. The fourth module is the model of a buck-boost converter and battery-charging circuit representing the exact characteristics of the LT1512 SEPIC-constant-current/voltage integrated circuit. The voltage level after the intermediate ca-pacitor and voltage-sensitive switch simulation is plotted in Figure 8. V

IN (the capacitor voltage level)

reaches only 15V, and starts discharging by trans-ferring voltage to the battery. When (V

IN) starts

decreasing, (VOUT

) increases until 15V with the ca-pacitor voltage at 15V. Both (V

IN) and (V

OUT) start de-

creasing by transferring energy to the battery. (VOUT

) reaches zero voltage while transferring its energy to the battery; simultaneously, the (V

IN) value de-

creases in order to reach 15V again. The charge and discharge steps are repeated while PFCB produces electricity from vibrations.

Figure 8. Voltage input and output simulation of voltage-sensitive switch

The DC-DC converter and battery-charging circuit design, a part of the energy harvesting circuit simulation interface (Figure 9) is employed to han-dle the decrease or increase of voltage levels and adjust it according to the battery specifications. The

Figure 7. intermediate-voltage-sensitive switch with hysteresis


voltage output of the circuit can be easily modified by using different resistance values if a different battery is integrated to the system.

Figure 9. Energy-harvesting and charging circuit

Circuit SimulationThe simulation of the important circuit components through the LT1512-SEPIC-battery- charging circuit (including input voltage, battery- charging voltage, and current) are depicted in Figure 10. All three im-portant aforementioned parameters of the energy-harvesting and battery- charging circuit were sim-ulated together to examine the consistency of the voltage/current levels on the circuit-design-simula-tion interface.

Figure 10. Battery-charging values simulation

The input voltage (VIN

) simulation plot was generated by the vibrations through the PFCB while being shaken. This voltage level was measured after rectification of the AC voltage signal that came from the PFCB unit as a DC voltage and served as the in-put for the buck-boost converter and the voltage-regulator circuit. Since the maximum input voltage of an LT1512- integrated circuit is 30V

MAX, a zener

diode was placed between VIN

and the ground of the LT1512 in order to avoid damaging the internal chip components of the LT1512. The input voltage (V

IN) and regulated voltage (V

OUT) are compared in

order to check the input and output voltage differ-ences after regulation. The input voltage levels that were larger or less than (3.6V) were regulated by the LT1512 buck-boost converter in order to charge the

battery at the nominal voltage level, which is 3.6V at 60mAh for the test battery. The battery-charging current (IR

OUT) and

battery-charging voltage (VOUT

) simulation plots are depicted to indicate battery-charging values. Both voltage and current levels were supplied at a steady state for proper battery charging, IROUT =5mA, which is a standard charging current for the bat-tery, and VOUT =3.6V nominal charging voltage. The charging current that was generated by the PFCB was less than 1mA but was increased to 5mA by the intermediate capacitors. The intermediate capacitors were charged to the minimum charging threshold of the battery and then released to the battery terminals by discharging themselves, allow-ing the capacitors to accept charge voltages from the PFCB again. However, the current level could not increase to charge the battery at the quick-charge phase because of the low current produced by the PFCB. The specific voltage and current levels that are specified in the simulation plot can charge at 3.6V at 60mAh for a fully discharged battery in ap-proximately 27 hours with constant vibrations.

Building the CircuitConsidering the variable output voltages from the PFCB, an energy-harvesting circuit was designed to charge small-scale NICD/NIMH batteries at the con-stant-charging phase. The printed circuit board for the energy-harvesting circuit was designed and built as small as possible to fit even the smallest places for power generation, including the battery soldered on the circuit. However, for test purposes, the in-strumentation circuit on the prototyping board and the energy-harvesting circuit were placed near the measuring equipment with the PFCB assembled to

Figure 11. Energy-harvesting, conversion, and charging circuit


allow for the reading of output values on the oscillo-scope display. The energy-harvesting circuit, which is soldered on the printed circuit board, is shown in Figure 11. A protective box should be designed and built to protect the circuit components and the bat-tery from bending and experiencing deformations from the vibration sources.

Storage Unit TestsOne problem often encountered when using pow-er-harvesting systems is that the power produced by the piezoelectric material is often not sufficient to power most electronics. Therefore, methods are needed to accumulate energy in an intermediate storage device so that it may be used as a power source. A capacitor is typically used to accumulate the energy. However, capacitors have characteristics that are not ideal for many practical applications such as limited capacity and high leakage rates. For the purpose of intermediate storage units, typical capacitors were used in the energy-harvesting cir-cuit without causing any critical issues. A group of capacitors were connected in parallel with the re-sistors in order to smooth the delivered voltage, making the output voltage easily read by the multi-meter. According to the approximate displacement and frequency levels, stored energy in the capaci-tors was calculated. The current levels for battery-charging pur-poses were calculated according to the input voltage levels. If the input voltage is increased, the output current would automatically increase by decreas-ing the battery-charging time. All calculations were done according to the energy stored in 13 seconds

(as reported by tests). The graph in Figure 12 com-pares voltage and current levels and average power output in 13 seconds. When a resistive load is relatively large, the power output from the PFCB does not produce sig-nificantly more power. The results of using a larg-er capacitor to smooth the voltage output suggest that the size of the smoothing capacitor affects the amount of power that can be delivered to a resis-tive load (battery). This result is attributed to the non-ideal behavior of the capacitor, which leads to internal losses. Following construction of the ener-gy-harvesting circuit, NICD- and NIMH-type batter-ies were charged to determine the battery charging time that could be effectively observed for each battery, with constant frequency. After testing the voltage levels of the PFCB using the capacitors, the PFCB was then tested with the batteries to observe battery-charging efficiency. For this purpose, a per-manent magnet shaker was used to induce vibra-tions; three rechargeable batteries were used for the experiment (Fig. 13). A PFCB consisting of two active-fiber composites (bimorph) was clamped to a thin piece of metal on the constant shaker for the energy-harvesting experiment. The constant vibra-tions from the shaker were applied to the PFCB at 60Hz. The voltage measurements from the batteries were taken every hour, and it appeared that the in-crease was minor. The reason for the slow charging was the very low current produced from the PFCB and the losses across the energy-harvesting and bat-tery-charging circuits. Because of the low-charging current, the test battery was not able to be charged at the specified standard charging time. However, this charging experiment was conducted only with a single PFCB, which is not recommended for charg-ing batteries. In some applications, more than three PFCBs are connected in parallel to increase the cur-rent levels and efficiency of the energy harvesting system. The number of PFCBs would be increased to charge the batteries at a specified time frame to avoid voltage drops across the batteries while pow-ering the electronic application. The batteries used in the experiment are listed in Table I with the basic specifications that are needed as charging param-eters. In order to charge batteries, the PFCB inter-digitized electrodes were connected to the battery terminals through the energy harvesting circuit. This experimental test showed that batter-ies were being charged with constant current/volt-

Figure 12. Energy stored in 13 seconds


age in longer time frames than the specific time frame in the battery datasheets. The last column in Table I shows the time required to charge the batter-ies with one PFCB. If more PFCBs are used for the en-ergy harvesting, charging time would be decreased considerably.

IV. SNEAKER SOLE EXPERIMENTA sneaker-insole was considered as a possible am-bient energy source to generate electricity through a PFCB that could be used to charge low-scale re-chargeable batteries. The batteries are expected to power low-power electronic applications such as a radio, MP3 player, mobile phone, and GPS unit. A typ-ical MP3 player was chosen as an electronic device to make the power estimation between generated- and consumed- power levels. Table 1 demonstrates that the PFCB was able to produce enough voltage with low current as an input power to charge a small- scale rechargeable battery, depending on the time a person walks or runs. The analytical estimation of the bat-tery-charging time and walking-time relationship was calculated for the fully discharged batteries. Howev-er, in the case of powering the electronic application, the batteries were placed in the system fully charged. It is essential to determine if the gained and stored power compensates for the consumption of the elec-tronic device while it is operating. If the produced power compensates for the daily consumptions and the leakages of the electronic device, a sneaker insole as an ambient energy source is a feasible source for the electronic application. For experiment purposes, the sneaker sole was cut carefully to place the PFCB in the correct po-sition for maximum efficiency when a person walks or runs. The base where the PFCB was placed was su-per glued with a piece of thin wood in order to place the PFCB properly on a smooth surface. Following the proper placement of the PFCB, the insole of the sneaker was again covered with the piece taken from the sneaker sole to avoid any damages when a per-

son starts walking. Several pennies were attached to the tip of the PFCB to increase the vibration time while the PFCB was being shaken by the foot-steps. The power output of the PFCB was directly propor-tional to the force and mass applied on the PFCB to induce vibrations. However, the output power of the energy-harvesting circuit was a constant voltage and current to avoid damaging the battery (because of the unregulated voltages produced by the PFCB). Therefore, the amount of energy generated by hu-man power through the PFCB was determined by human power that was available to shake the piezo-electric material. The stronger the force and mass applied, the more electrical power was generated from the PFCB while the person walked or ran. As a result of vibrations, the fibers in the frame of the shoe sole generated electricity with high potential and low current. The electricity was conducted and stored in a battery or capacitor in the circuit placed into a special compartment of the shoe insole. The photograph of the redesigned sneaker insole with the assembled PFCB is shown in Figure 13.

Figure 13. redesigned sneaker insole with PFCB

The wires coming from the PFCB were ex-tended by wires to the energy-harvesting-circuit in-put. In order to avoid voltage drops due to long wires, the PFCB and energy-harvesting circuit were placed in close proximity. The efficient point of this experi-

TypeNom.

VAmpacity

mAh

ChargeQuickCharge Charge

timehI

mATim

hI

mATh

NIMH 1.2 80 8 15 47

NICD 3.6 60 6 14 20 7 36

NIMH 3.6 60 6 14 20 7 41

table 1. rechargeable Battery specifications


ment demonstrated that if two sneakers (each with one PFCB attached) are worn, the output from the vi-brations of both PFCBs never stops, even if a person walks slowly. Once the first PFCB starts vibrating, it takes at least one second until the vibrations stop. In this time period, the other PFCB starts shaking when the person takes another step. In this manner, battery- charging is decreased to almost half of the standard charging time as specified in the previous sections. Since the current level produced through the energy-harvesting circuit for battery charging has been calculated, the next comparison estimated whether the system could produce enough power for a typical MP3 player. The walking time also was calculated to compute how much walking is needed to compensate the energy consumption of the elec-tronic application (Figure 14).

Figure 14. Block diagram of comparison estimation for energy harvesting

Overall energy used/produced relationship estimations were conducted considering (I

1), overall

current consumption, and (I2), current gained from the

PFCB in the sneaker insole, through the human power.

I1 (LOSS/24HRS)

= [(IBATTERY_LEAKAGE

) + (I

HARVEST_ LEAKAGE) + (I

MP3_ LEAKAGE) + (I

MP3)] (1)

where I1

(LOSS/24HRS) is the current loss per 24 hours,

and IMP3

is the current consumption of the MP3.

Equation 1 calculates the overall current consumption of the MP3 player including the to-tal leakages on stand-by mode, energy- harvesting circuit, and batteries. According to Equation 1, it is

also necessary to determine the power consumption of the MP3 player (P

MP3) separately per usage, before

a calculation of the overall energy consumption. It is assumed that a person walks/runs about 1 hour in 24 hours. The power characteristics of a small, low-power MP3 player were provided by one of the MP3 manufacturer’s data sheets. The specifications of the MP3 player were used to calculate the power consumption for one hour of use (P

MP3). According

to the specifications, the power usage of the MP3 player in one hour is calculated as 72mW.

IMP3

= 60mA;

VMP3

= 1.2V; and PMP3

= I * V = 60mA * 1.2V =

72mWh.

There are certain components in the sys-tem that is always at stand-by to sense the wake-up signals. These components drain some quiescent currents from batteries while they are on standby (including the MP3 player, the energy-harvesting cir-cuit, and the battery that keeps the device up and running). The total leakages and quiescent current for the system components during the playing of the MP3 player were calculated using Equation 1. Power consumption of the MP3 player (P

MP3) was

found separately and included in overall current consumption in 24 hours as calculated below.

I1 (LOSS/24HRS)

= [(IBATTERY_LEAKAGE

) + (IHARVEST_ LEAKAGE

) + (IMP3_

LEAKAGE) + (I

MP3)]; so

I1 (LOSS/24HRS)

= [(270μA) + (984μA) + (13mA)] + [(60mA)] =74mAh.

The value for I1 is converted to the power unit in or-

der to compare the power gain and the power loss. Then

P1 (LOSS/24HRS)

= 0.074A * 1.2V = 0.088Wh (88mW).

The total energy drained from the batter-ies was estimated at 0.088W for one day. This value is not exact energy consumption; it may change according to how often a person changes the pa-rameters of the MP3 player while using the device. Since the total energy loss (P

1) was estimated, the

next step was to calculate the energy gain from the energy-harvesting system. This equation enables


calculation of the total gained current from human power through the PFCB that is assembled in the sneaker insole during one day (24 hours), the bat-tery charging time assumed.

Thus:

I2 (GAIN/24HRS)

= IG * T; (2)

where I2 (GAIN/24HRS)

is the total current recovered/stored per 24 hours, IG is the current gathered while a person walks for one hour, and T is the time per-son walked/run (in hour). For this application, the gained current from the PFCB (I

2) should be more than or equal to the

overall current loss (I1) in 24 hours (I

1≤I

2). Otherwise,

the MP3 player would be operating inconsistently due to insufficient current. The total energy gained from the system would depend on the time a person walks/runs during a 24-hour period:

I2 (GAIN/24HRS)

= (0.005A * 2hrs) = 0.01A.

The value for I2 is converted to the power unit in

order to make comparison between the power gain and the power loss. Then

P2 (GAIN/24HRS)

= 0.01A * 1.2V = 0.012Wh (12mW).

P1 and P

2 were calculated and converted to the en-

ergy value in order to make a comparison ratio if energy gain is greater than energy loss in order to balance the system power.

(3)

where EG is the overall energy gain, EOUTPUT

is energy loss through the MP3 player, and E

INPUT is the energy

gain from PFCB;

energy loss ratio.

As estimated above, the energy gain is 7.3 times smaller than the overall energy consumption of the MP3 player in a day. The harvested and stored energy is not sufficient to run an MP3 player for one hour with one-hour of daily walking. The energy loss/gain graph depending on time given in Figure15 to permit a visual comparison of these variables.

Figure15. Energy gain/loss depends on the time a person walks/runs

Figure 15 shows that the gained power is too low to play an MP3 player for one hour with one hour of walking. To improve the energy gain, more than one PFCB should be placed into the sneaker in-sole to increase the generated power to balance the power consumption of the MP3 player. Another so-lution to make the harvesting circuit efficient would be improving or redesigning the circuit to increase the current flow to the battery to decrease the bat-tery charging time. Also, if the time a person walks increases (more than one hour), the battery charging time would be decreased depending on how long a person walks in a day.

V. CONCLUSIONThe advances made from this research build the framework for further experimentation with the tools necessary to use the PFCB effectively in nu-merous applications. The sensing capabilities of the PFCB were investigated, and shown in an ener-gy-harvesting system through an experiment and battery-charging circuit. The energy-harvesting circuit can be improved to increase current levels from the PFCB while decreasing voltage levels for battery-charging purposes. The increase of current during the vibration of PFCB would decrease the battery- charging time by supplying more energy to the electronic device. The special device should be in the proper position so that the maximum amount of vibration can be created for energy-harvesting purposes. As the energy yield increases and wear-able electronic devices become more efficient, foot-powered energy scavenging systems can drive more components of wearable computers, reducing the


need for batteries, or enabling them to be charged while the energy is being discharged. These sys-tems provide for a host of applications in situations where power resources are inaccessible, such as during hiking expeditions or military missions. As applications of energy harvesting from human pow-er increase, so should ease of use. The piezoelectric application conditioning electronics to harvest en-ergy from sneaker insoles could be commercialized as separate components to be modularly linked to electronics embedded in the heel via a weatherproof connector. As shoe/sneaker insoles become worn, they could be replaced by the consumer. With ap-propriate adaptations, sneaker/shoe mounted en-ergy harvesting systems are likely to power a wide range of low power electronic devices.

REFERENCES

[1] T. Starner and J.A., Paradiso, Human-generated power for mobile electronics,”. Low-Power Electronics Design, C. Piguet, Ed. CRC Press, New York, NY, chp 45, pp. 1–35, 2004.

[2] “Self-contained generating and lighting unit,” U.S. Patent and Trademark Office, patent 1,184,056, 1916.

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[6] D.A. Skoog, J.F. Holler, and S.R. Crouch, Prin-ciples of instrumental Analysis, 6th ed., Flor-ence, KY: Cengage Learning, Brooks/ Cole, 2006.

[7] S. J. Roundy, Energy Scavenging for Wireless Sensor Nodes with a Focus on Vibration to Electricity Conversion.” PhD dissertation, The University of California, Berkeley, 2003.

[8] J.J. Thomas, W. W. Clark, “Harvesting Energy from Piezoelectric Material,” IEEE, University of Pittsburgh: IEEE CS, vol. 4, no.1, pp. 69-71 1536-1268, 2005.

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[10] “DC-DC Converter Basics,”2008. [Online]. Available: http://www.powerdesigners.com/InfoWeb/design_center/articles/DC-DC/con-verter.shtm [Accessed February 15, 2010]

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FARUK YILDIz attended Taraz State University, Ka-zakhstan, and graduated with a BS Computer Sci-ence degree. He received his Master’s Degree from City University of New York with a Computer Sci-ence and completed his Doctoral degree from the University of Northern Iowa in Industrial Technol-ogy. He is a faculty member in the Industrial Tech-nology Program at Sam Houston State University. His research interests include energy harvesting, conversion, charging circuits, and ambient energy sources. He is a current member of IEEE, ATMAE, ASEE, EPT communities.

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