Piezoelectric polymer films as power converters for ...

Microelectronics Reliability 48 (2008) 897–901

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Piezoelectric polymer films as power converters for human powered electronics Ewa Klimiec a,*, Wiesław Zaraska a, Krzysztof Zaraska a, Kazimierz P. Ga˛siorski b, Tadeusz Sadowski b, Michał Pajda b a b

Institute of Electron Technology Cracow Division, ul. Zabłocie 39, 30-701 Kraków, Poland ´ ska 9, 30-418 Kraków, Poland Institute of Leather Industry Cracow Division, ul. Zakopian

a r t i c l e

i n f o

Article history: Received 27 November 2007 Received in revised form 20 March 2008 Available online 12 May 2008

a b s t r a c t Piezoelectric polymer film material allows for conversion of mechanical energy into electrical energy that can be used for supplying electronic devices. While this method does not allow obtaining large useful power, recent advances in electronic technology, in particular wide availability of submicron low-power CMOS processes, have made feasible the idea of using piezoelectric polymers as power converters for human powered electronics. This concept allows to overcome the necessity of using battery as a power source, which is one of the main obstacles to widespread adoption of wearable computing devices. Of particular interest is harvesting energy from walking, which can be achieved by using piezoelectric polymers. In this paper maximum power has been calculated that can be drawn from walking energy owing to application of the copolymer polyethylene–polypropylene (PE–PP) shoe insole. The amount of electric energy obtained from a PE–PP foil of a thickness of 11 lm for a single step of a duration of 1 s – that is equivalent to a frequency of 1 Hz – amounts to 340 nJ. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Some polymer materials exhibit piezoelectric properties, i.e. generation of surface electric charge when subjected to a mechanical stress, as well as a reciprocal effect, that is a change of dimensions when subjected to an electric field. The prerequisite for the piezoelectric properties of electroactive polymers (EAP) is their permanent polarization that results from stable arrangement of molecular dipoles or an uncompensated surface or volume electric charge. Piezoelectric film material allows for the conversion of mechanical energy applied to the material into electrical energy that can be used for supplying electronic devices. For the last several decades, the computer industry has been driven by Moore’s law. Although it has originally referred to transistor density in an integrated circuit, it has also been found applicable to other parameters of the computer system, such as memory size, CPU speed, RAM, and hard drive capacity. These advances have enabled the development of first portable, and later wearable, computing devices. However, a certain critical element of a wearable computing system does not scale according to Moore’s law; i.e. the power source. For the last several years, there has been no significant improvement in battery energy density. The inadequateness of battery power sources warrants research of alternative power sources, such as those harvesting energy from a human body movement, e.g. from walking [1–5]. This concept allows to overcome the necessity of using battery as a power source, * Corresponding author. Tel.: +48 126563144; fax: +48 126563626. E-mail address: [email protected] (E. Klimiec). 0026-2714/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2008.04.001

which is one of the main obstacles to widespread adoption of wearable computing devices [6,7]. Multiple approaches to harvesting energy from walking have already been presented [1,2]. Most notable ones are: use of electromechanical generators, in particular electromagnetic converters based on rare earth magnets, use of piezoceramics and piezopolymers. The possibility to recover some of the energy dissipated during walking or running owing to storing the charge produced by the piezoelectric materials embedded into shoes, as well as new devices powered in such a way have been investigated at the MIT Media Laboratory [1]. Kymissis et al. [2] examined three different devices built into a shoe and used for generating electrical power while walking: piezoceramic PZT strip, multilayer PVDF (polyvinylidinefluoride) foil stave and rotary magnetic generator. Improvement of the composition of organic piezoelectric materials and their production methods aimed at an increase in the generated electrical signals and foil durability have been subject of many publications. In this paper we focus on using new piezoelectric polymers, in particular PE–PP foil. Compared to the competing technologies, this method has several advantages, namely: low mass, lack of moving parts and flexibility. The chief disadvantage, compared to the other solutions, is low-power output; however recent advances in electronic technology, in particular wide availability of submicron low-power CMOS processes, have made feasible the idea of using EAPs as power converters for human powered electronics. The aim of this paper is to investigate feasibility of using a polypropylene–polyethylene copolymer PE–PP as a power source for human powered electronics. We focus on the application in form

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E. Klimiec et al. / Microelectronics Reliability 48 (2008) 897–901

of a shoe insole, made of PE–PP. In particular, we are interested in determining these properties of the piezomaterial, which are useful in designing power converters, i.e. piezoelectric constants and internal resistance. The results of laboratory measurements of power generated by a PE–PP foil are compared with the signals generated by a shoe insole in the natural conditions, while walking. Finally, we discuss feasibility of using a PE–PP shoe insole for powering an electronic device. 2. Materials The investigated polymer material was prepared in the form of a foil 11 lm thick. This amorphous piezoelectric foil is a self-made new product based on polypropylene–polyethylene (PE–PP) copolymer. Relevant physical properties of the foil have been summarized in Table 1. The application of this foil enabled us to attain a voltage signal three times higher than that for PVDF foil. The test samples with a surface area of 10 cm2 (the surface of measuring electrode) were placed between metal capacitor electrodes and subjected to cyclic stress (5 Hz). The piezoelectric voltage and charge (measured values) and piezoelectric constants g33 and d33 (calculated values) for the PE–PP foil, investigated in the previous work [8], are presented in Figs. 1 and 2 as a function of mechanical stress. As defined by European Standard EN503242:2002, g33 and d33 are: piezoelectric voltage (stress) constant and piezoelectric charge (strain) constant, respectively, taken along the direction 33, i.e. the electric field and the mechanical stress are parallel to each other and to the polarization axis. As can be seen from these plots, the values of g33 and d33 depend

strongly on the stress applied to a sample. This dependence is stronger for small stress values. In a stress range of 0.3–10 N/ cm2, piezoelectric constants g33 and d33 range from 12,000 to 3000 mV m/N and from 150 to 28 pC/N, respectively. This foil exhibits high sensitivity and gives an immediate voltage response to the applied stress. The observed nonlinearity of piezoelectric constants, crucial in sensor applications, is of no importance to this application since we are not interested in precise force/voltage conversion, but in a total charge generated under stress. 3. Conversion of mechanical energy into electrical energy of PE– PP foil piezoelectric element

Table 1 Physical properties of the investigated PE–PP foil Property

Unit

Value

Thickness Specific weight Surface weight

lm g/cm3 g/m2

11 0.92 10.12

N/mm2 N/mm2

80 72

% % N/mm2 °C

60 65 133 Max 60

Tensile strength at break in direction Longitudinal Transverse Elongation at break in direction Longitudinal Transverse Elasticity modulus Operating temperature

Fig. 2. Piezoelectric charge (measured values) and the d33 value (calculated) as a function of mechanical stress applied to the test PE–PP 11 lm thick foil. Measuring electrode surface area 10 cm2.

Conversion of mechanical energy into electrical energy and maximum power attainable by the use of a PE–PP foil were investigated experimentally under laboratory conditions and under walking conditions. The piezoelectric elements were loaded with a resistance ranging from 2 to 165 MX and from 2 to 40 MX under laboratory conditions and under walking conditions, respectively. In both cases, the dependence of r.m.s. voltage value on the load resistance was determined and the power was calculated for respective resistances, from the following equation: PRMS ¼

U2RMS ; RL

ð1Þ

where PRMS is power (root-mean-square value), URMS is voltage (root-mean-square value), RL is load resistance. Then a power vs. load resistance graph was plotted. This way, a load resistance of the piezoelectric element was determined at which the system attains its maximum power. 3.1. Laboratory investigations – maximum power

Fig. 1. Piezoelectric voltage (measured values) and the g33 value (calculated) as a function of mechanical stress applied to the test PE–PP 11 lm thick foil. Measuring electrode surface area 10 cm2.

Laboratory investigations were carried out using a measurement rig setup shown in Fig. 3 and measurement circuit diagram is presented in Fig. 4. The laboratory made apparatus was applied for piezo tests. The voltage was measured and recorded by means of a LeCroy LT342 digital oscilloscope. The foil was placed between two metal electrodes 3 mm thick with a surface area of 10 cm2. The investigations were performed along the axis 33, i.e. the polarization of a piezoelectric element and the force acting on it were parallel to each other and perpendicular to the electrode surfaces. The foil was subjected to stresses of 5 N/cm2 at a frequency of 5 Hz. The dependence of r.m.s. voltage and power on load resistance is shown in Fig. 5. Generated power


899

Fig. 6. Schematic cross-section of the trainer insole with a piezoelectric element. 1 – PE–PP foil, 2 – electrodes, 3 – electrical leads, 4 – lower part of the insole, 5 – upper part of the insole made of synthetic material of a thickness of 4 mm.

Fig. 3. Piezoelectric foil measurement rig setup, 1 – piezo test apparatus, 2 – test sample, 3 – load resistors, 4 – force meter, 5 – oscilloscope.

parts of the shoe insole along the edges. Electric diagram of the measurement system was similar to the one used for the laboratory investigations, the difference consisted in the replacement of the piezoelectric tester by a shoe insole. A schematic cross-section of the insole is shown in Fig. 6. The mass of a walking man’s body was 68 kg. The electrode surface area amounted to 100 cm2. The stresses within the piezoelectric element take place during walking not only along the axis 33 (compression) but also along the axes 31 and 32 (tension) as well as 35 and 36 (torsion). The recorded voltage is the resultant of all stresses across the piezoelectric element. Its plot recorded with an oscilloscope at a load of 20 MX is shown in Fig. 7. The investigated material, similarly to other piezoelectric materials, generates a bipolar output voltage when subjected to stresses acting in only one direction (unipolar).

Fig. 4. Measurement circuit diagram, Es – e.m.f. of the source of electrical energy, Zs – internal impedance of the source, RL – load resistance, A – high impedance amplifier, DO – oscilloscope.

Fig. 7. A plot of the voltage across the PE–PP piezoelectric element placed inside a trainer. x-axis: 1 div = 1 s, y-axis: 1 div = 2 V.

Fig. 5. The dependence of r.m.s. voltage (measured values) and of r.m.s. power (calculated values) on load resistance of the electrical energy source at stresses of 5 N/cm2.

was calculated for particular points on the curve of voltage. In calculations, input impedance of the amplifier – that amounts to 500 MX and is connected in parallel with RL – has been taken into account. The conclusion follows from the presented data that the maximum power at the stresses 5 N/cm2 has been obtained at a load resistance of 50 MX (this means that the source has such internal resistance). For an r.m.s. voltage of 750 mV, the maximum power amounts to 11.25 nW. 3.2. Investigation under walking conditions – maximum power The investigations under natural conditions were conducted recording the voltage generated across the PE–PP foil during walking. The foil was a component of an insole placed inside typical trainer shoes. Application of the piezoelectric films was performed in a simple and durable way by gluing to the upper and bottom

Fig. 8. The dependence of r.m.s. voltage (measured values) and power (calculated values) on load resistance of the voltage source, during walking.

900


Fig. 9. A plot of the power across the PE–PP piezoelectric element placed inside a trainer. x-axis: 1 div = 1 s, y-axis: 1 div = 0.5 lW.

Duration of a single step amounted to 1 s, which is equivalent to a frequency of 1 Hz. The dependence of r.m.s. voltage and of power on load resistance is presented in Fig. 8. Maximum power is obtained at a load of 20 MX (thus, one may conclude that the internal impedance of the source amounts to 20 MX). For a PP–PE foil of a thickness of 11 lm placed inside the trainers, the r.m.s. power value amounts to 340 nW. Duration of a single step amounts to 1 s, thus, the electrical energy obtained for a single step is 340 nJ, for a single layer of the PE–PP foil. For all the 500 measured voltage values shown in Fig. 7, instantaneous power has been calculated, and shown in Fig. 9. The maximum value of instantaneous power was about 1.45 lW. 4. Discussion The results of the investigations on the maximum power that can be generated in the PE–PE foil under laboratory conditions and under natural conditions are collected in Table 2. The results of the laboratory investigations allow us to predict only very approximately the behaviour of the piezoelectric foil under natural conditions. The major reason of it is that in the laboratory investigations, the stresses were applied mainly in the direction 33, while under natural conditions, the stresses act on the foil in all directions. Also, internal resistance of the foil changes. Nevertheless, the laboratory investigations and the study under natural conditions, jointly, allow for a wider view on the issue of the application of piezoelectric foils as power generators. The amount of electrical energy obtained from a single layer of the PE–PP foil of a thickness of 11 lm for a single step of a duration of 1 s – that is equivalent to a frequency of 1 Hz – amounts to 340 nJ. In order to obtain higher power values from a piezoelectric element made of the PE–PE foil, the studies are conducted on joining foil layers. The feasibility of using investigated PE–PP shoe insole for powering a microelectronic device is related to recent development in the field of low powered electronics. For example, a commercially available MSP 430 family microcontroller supplied at 3 V and clocked at 1 MHz consumes about 900 lW (supply current 300 lA); however, when clocked at 4 kHz, the supply current drops to 1.8 lA (4.8 lW). The device can also be switched into a standby

Table 2 Power generated in the PP–PE element under laboratory and walking conditions Conditions

Surface area (cm2)

Stress (N/cm2)

Frequency (1/s)

Load resistance (MX)

Power (nW)

laboratory Natural (walking)

10 100

5 7 (mean value)

5 1

50 20

11 340

(sleep mode), where the supply current is as low as 0.1 lA (which corresponds to 0.3 lW power dissipation) [9]. Such excellent performance can also be exhibited by analog devices; for example, a commercially available MAX4132 operational amplifier has a quiescent current as low as 1 lA [10]. We have demonstrated that it is possible to obtain 0.3 lW of useful power from a single layer of PE–PP copolymer. If multiple piezoelectric layers are used, the harvested power can be increased tenfold, to about 3 lW. This is enough to power a microcontroller, provided that the clock frequency is kept low and/or an extensive use of sleep mode is made. For example, we can envisage an MSP430 microcontroller operating at 4 kHz, which sleeps for 90% of time. That theoretically gives us up to 400 instructions per second, at the power consumption of 0.75 lW (at 3 V). This is enough to realize a simple demonstrator application, e.g. a step counter. The problem, however, is that the amplitude of the generated voltage is unsuitable for supplying a low-power electronic device. Such devices are designed to operate from a DC voltage of 1.8 to 3.3 V, whereas the foil generates AC voltage of about 10Vp–p (peak-topeak) (see Fig. 7). Therefore, a power converter circuit needs to be placed between the generator and the powered device (load), in order to convert the output voltage of the foil into a voltage suitable for powering a low-power CMOS device. An example of such circuit has been given in [2]. We are currently working on design of a power converter suitable for the investigated material. So far, we have successfully designed and simulated a power converter which can operate from a piezoelectric foil with the output voltage of 20Vp–p, and the internal resistance of 2 MX. The results of preliminary studies indicate that such a generator can be created by appropriately connecting multiple PE–PP layers. Thus, we believe that it is possible to use a PE–PP copolymer as power source for a useful microelectronic device, provided that: (1) a multilayer PE–PP material, with the output power of 3 lW under walking conditions is made, and (2) a power converter with at least 25% efficiency can be constructed. Our future work is expected to focus on these two problems, optimizing the design of the insole and constructing a simple demonstrator application. 5. Conclusion Our results show that at present it is possible to use a PE–PP piezoelectric polymer foil placed in a typical insole of trainer shoes as a micropower generator, producing during normal walking for a single layer about 0.3 lW. Application of multilayer foil in future works should increase the generated power by one order of magnitude. Acknowledgements The authors wish to express their gratitude to Mr. Andrzej Cichocki and Mr. Wiesław Prochwicz, M.Sc. Eng. for their assistance with electric measurements and editing of this paper. The investigated material was fabricated in the framework of an EU FP6 Project No. 507378 ‘‘CEC-MADE-SHOE – Custom, Environment, and Comfort made shoe”. This work was funded by Polish Ministry of Science and Higher Education in range of the Research Project No. N507 4634 33. References [1] Fletcher R. Force transduction materials for human–technology interfaces. IBM Syst J 1996;35:630–8. [2] Kymissis J, Kendall C, Paradiso J, Gershenfeld N. Parasitic power harvesting in shoes. In: 2nd int symp on wearable computing, 1998. p. 132–9. [3] Starner T, Paradiso JA. Human generated power for mobile electronics. In: Piguet C, editor. Low power electronics design. CRS Press; 2004. Available from: .

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