Piezoelectric Force Response of Novel 2D Textile ... - IEEE Xplore

IEEE SENSORS JOURNAL, VOL. 13, NO. 12, DECEMBER 2013

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Piezoelectric Force Response of Novel 2D Textile Based PVDF Sensors Andrzej S. Krajewski, Member, IEEE, Kevin Magniez, Richard J. N. Helmer, and Viktoria Schrank

Abstract— This paper describes the development of 2D flexible sensors designed by integration of conductive fibrous electrodes and piezoelectric polyvinylidene fluoride (PVDF) fibers into a conventional plain woven polyester fabric. The piezoelectric properties and electrical response to the mechanical deformation of the sensors were tested using an electromechanical device built inhouse. Both the amplitude of movement and the frequency of the sensors were controlled using this device and the signal efficiency of these sensors was tested for maximum signal response to the sine frequencies between 80 and 1000 Hz. The electrical signal generated by the sensors was correlated to the fineness of the PVDF fibers used, the distance between the electrodes and the nature of the electrodes. Relationships between sensor output signal under load and the type of structure were thus established. Index Terms— Piezoelectric fiber, piezoelectric measurement, woven sensor.

I. I NTRODUCTION

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HE conversion of mechanical, thermal or chemical energy into electrical energy using piezoelectric materials has received great attention in the past few years. Recently, a considerable amount of research has been focused on the development of textile based piezoelectric materials which harbor potential in various applications such as energy harvesting, sensing and actuation. However the primary challenge in the design of these smart textiles is to find a suitable piezo-electric substrate material which can provide the desired functionality and which can be easily meshed into a hybrid textile structure. Much of the previous reported work in this field has shown that ceramic based piezoelectric devices perform better than their polymer based counterparts. However the brittleness of ceramic materials has hindered their implementation in textile. As a result there has been a shift to exploring polymer based piezoelectric technologies which offer more flexibility in design and processing. Manuscript received March 5, 2013; revised May 14, 2013; accepted July 16, 2013. Date of publication July 19, 2013; date of current version October 9, 2013. This work was supported in part by the Commonwealth Scientific and Industrial Research Organization, Materials Science and Engineering, and the Institute for Frontier Materials, Deakin University. The associate editor coordinating the review of this paper and approving it for publication was Prof. Sang-Seok Lee. A. Krajewski and R. J. N. Helmer are with the Materials Science and Engineering Division, Commonwealth Scientific and Industrial Research Organization, Geelong 3216, Australia (e-mail: [email protected]; [email protected]). K. Magniez is with Deakin University, Geelong 3216, Australia (e-mail: [email protected]). V. Schrank is with the Institute for Textile Technology, RWTH Aachen University, Aachen 52074, Germany (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2013.2274151

The surface area offered by piezoelectric fibres is greater than that offered by a film; therefore one would expect an improvement in piezoelectric performance when using piezoelectric fibres rather than a film in piezoelectric clothing. Most of the initial scientific work evolved around the coating of the textile fabric with a thin layer of piezo-resistive polymers such as polypyrrole (PPy, a π-electron conjugated conducting polymer) [1] or carbon-loaded rubber [2], or the insertion of piezoelectric films into textile fabric [3]. The long term performance of these systems after mechanical friction or repetitive washing is however questionable. Other researchers have focused on the development of piezoelectric fibrous substrates based on piezoelectric zirconate titanate (PZT) ceramic and poly (vinylidene fluoride) (PVDF) polymer. For instance, Guillot investigated the energy harvesting properties of woven fabrics containing PZT ceramic fibers coated with an acrylate oligomer [1]. Swallow et al. designed a series of microcomposite materials by embedding unidirectionally aligned PZT and PVDF piezoelectric fibers into a polymer matrix for energy harvesting in glove structures. The performance of the micro-composites with different fiber diameters and material thicknesses were investigated and showed some success. Other similar approaches using micro-composites have been reported [4], [5]. In a more novel publication, Laxminarayana et al. [6] demonstrated the direct and reverse conversion of mechanical energy into electrical energy using electrospun poly (vinylidene difluoride) PVDF/carbon nanotube nanofibrous membranes. However the preparation of the electrospun membranes is not efficient and requires the use of toxic chemicals. Recent developments in the processing of piezoelectric polymers such as PVDF have enabled the scientific community to progress further in this field. The work on PVDF fibers is very valuable for the scientific and industrial community since polymeric fibers can offer improved flexibility compared to their inorganic counterparts. The other advantage in producing flexible piezoelectric fibers is the ability to produce a large surface area in wearable technologies. The evolution of the piezo β crystal phase content in PVDF films [7], [8] and more recently on fibers [9] has been addressed in several papers focusing on the effect of both unidirectional stretching ratios and temperatures. Siores et al. [10] was the first to patent a continuous melt-spinning process for the development of piezoelectric PVDF fibers. The authors showed that by controlling the cold drawing temperature, drawing ratio and applied electric field, the piezoelectric properties of the fibers can be optimized. Their combination with conventional fibers and conductive electrodes into a weave textile has been described

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as the ideal way in making smart piezoelectric materials which can be used for many smart wearable applications. This is because in woven structures, the piezoelectric fibers (acting as charge generator) can inter-connect with the conductive electrodes in a number of ways. However, careful choices of design and conductive charge carrier have to be made for optimum performance. In this work, we designed a number of 2D flexible sensors by integration of various types of conductive fibrous electrodes and piezoelectric PVDF fibers into a plain woven polyester fabric. The aim of this research was to comparatively and consistently test the piezoelectric properties of these sensors. The testing of piezo materials is typically achieved using cantilever oscillators, four point bending or compression tests. Nonetheless, textile based piezo sensors are more flexible and their testing requires adapted methodologies. Herein, we describe the design and principle of an electromechanical device suitable for testing the piezoelectric properties of flexible textiles. The results produced in this work captured the magnitude of signal response of 2D flexible PVDF fiber based sensors to the various frequencies of the stimulus signal that was required to deform the fabric. The piezoelectric properties are correlated to the fineness of the PVDF fibers used, the distance between the electrodes and the nature of the electrodes. Relationships between sensor output signal under load and the type of structure have been established.

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II. M ATERIALS AND M ETHODS A. Preparation and Structure of the Flexible Sensors 1) Preparation of the Piezoelectric Fibers: Piezo polymer polyvinylidene fluoride (PVDF) was converted into bulk continuous fiber of 20 single filaments using a single screw Busschaert Fiber extruder. PVDF was supplied by Arkema Group (grade Kynar 710). The extruder was heated from 240 to 280 degrees, and the speed of the single screw was set to 20 rpm. The polymer flow rate varied between 50 cubic centimeters and 80 cubic centimeters per minute. By adjusting the speeds of the cold and heated rolls on the extruder, the fibers were stretched by 75% of their original length. This process produced PVDF single filaments of 37 and 50 microns in diameter, respectively. The piezoelectric β crystal phase in these filaments accounted for approximately 60% content (not shown here). 2) Design and Preparation of Flexible Woven Sensors: The PVDF fibers were used to produce flexible woven textile sensors. The design and preparation of woven sample is an important step in order to achieve meaningful data. The samples were prepared under the same weaving conditions (Fig. 1) in order to have consistent tension and mechanical properties (in terms of material flexibility and thickness). The PVDF fibers and conductive fibrous electrodes were integrated into plain polyester weave in order to produce a two-dimensional (2D) flexible piezoelectric woven sensor. The woven structure contains non-conductive nylon spacer yarn, separating the conductive yarn from one another in order to avoid shorting. The overall structure is shown in the following Fig. 1. The conductive fibrous electrodes were

(c) Fig. 1. Process manufacturing of 2D flexible PVDF fiber based sensors. (a) Weaving process. (b) Plain weave fragment of the sensing area. (c) Fabric layout.

connected to a copper strip using conductive epoxy glue. Each electrode was connected to a metallic snap-button via insulated electrical wire. The snap-button performed as a physical connector between the sensor and the measuring instrument. In the first set of the experiments, we looked at the effect of the PVDF fiber diameter on the efficiency of the sensors. Silver coated nylon yarns (containing 20 filaments of 50 microns) were used as the electrodes. In the second set of experiments, the piezoelectric performances of the sensor were investigated as influenced by the distance between the electrodes. Sensors were constructed using PVDF fibers having fiber diameters of 50 microns. Two silver coated nylon yarns (containing 20 filaments of 50 microns) were used as the electrodes. The distance between

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TABLE I T YPE OF C ONDUCTIVE E LECTRODES

Fig. 3.

Fig. 2. Concept of the electromechanical device used for testing of 2D flexible PVDF fiber based sensors.

electrodes varied from one to two spacers, using either PVDF or polyester yarns. In the third part of the experiments, various types of electrodes (Table I) were used and the efficiency of the corresponding sensors was measured. Sensors were constructed using the PVDF fibers having fiber diameters of 50 microns. B. Design of the Electromechanical Device and Testing of the Sensors 1) Design and Signal Processing: Fig. 2 shows a conceptual representation of the electromechanical device. Similar apparatus have been reported for energy harvesting purposes [11]. A 100 VA audio speaker, for which the amplitude of movement and the frequency can be easily controlled, was used as a deformation driver. Three wires were glued on the membrane top surface of the speaker, their top ends being soldered together. A rare earth magnet (of approximately 3 mm in diameter and 2 mm thickness) sourced from Alpha Magnetics Pty Ltd was embedded in a non-conductive casing and glued on top of the wires. The corresponding rare earth magnet with the same diameter and thickness was positioned on the opposite side of the fabric. The top magnet was covered by a thin layer of resin in order to prevent shorting the electrodes built into the sensor. The piezoelectric fabric to be tested was inserted between two acrylic plates (10 mm thick). One end of the fabric was clamped at one end of the plate

Signal processing system – block diagram.

whilst the other end was put in tension using a weight of 100 grams (preventing any eventual buckling of the fabric during movement of the speaker up or down). A hole of 25 mm in diameter (corresponding to the sensing area) was cut out of the acrylic plate. In order to shield the internal electronic circuitry from any external noise sources, the device was placed in an earthed galvanized steel metal container. The block diagram of the signal processing system is shown in Fig. 3. The electromagnetic device is driven by the sine wave function generator. The speaker was energized by the 40 VA universal amplifier, Module M034. Oscillations of the speaker were controlled at various frequencies and amplitudes using a XR2206 function generator. The signal from the piezoelectric fabric sensor was fed into differential amplifier. The input resistance of this circuit is 4 M. The total amplification of the signal conditioning circuit together with the buffer is 1. The signal from the buffer was fed into a National Instrument USB-6210 card connected to a PC. The software is designed to collect the data from all sensors using Lab View National Instrument software package. A digital noise filtering was performed using the National Instrument software. 2) Operation and Testing: The device was designed to relatively compare the piezo-electric properties of the 2D textile sensors. Both the variation in voltage signal provided to the mechanical movement generator and the piezo-electric signal generated by the PVDF sensor were monitored and recorded by the National Instrument Data Acquisition card simultaneously. The peak-to-peak values of the signals generated by the sensor as well as signals provided to the mechanical movement generator were calculated and averaged for the time of test duration. The average voltage Ux is calculated according to n 0 abs(Uxi ) (equation 1) where n is the number of Ux = n samples acquired during the test and Uxi is the peak-topeak voltage sample acquired by the NI data acquisition card (10 kHz sampling frequency, 200000 collected samples). Each sensor was tested for maximum voltage response to the sine frequencies by sweeping the vibration frequency between 80 Hz and 200 Hz. The samples of sensing responses in terms of voltage of the sensors subjected to the sine wave shaped stimulus at frequencies of 80 Hz, 120 Hz and 200 Hz are shown in Fig. 4. The stimulus sine wave amplitude was set to ±0.4 V. Amplitudes higher than ±0.4 V caused the magnets used

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Fig. 6. Response to the sine wave shaped stimulus for 37 μm and 50 μm PVDF fibre diameters.

Fig. 4. Samples of sensing response of 2D flexible PVDF fiber based sensors to sine wave shaped stimulus.

Fig. 7. Impact of the distance between the electrodes on the sensitivity of the PVDF based sensor.

Fig. 5. Sensitivity of the of 2D flexible PVDF fiber based sensors to the sine wave shaped stimulus.

for clamping the sensor to the membrane of the speaker to dislodge causing false reading of the sensor’s generated response. The maximum signal response for all tested sensors was within the range of 128 Hz ± 2 Hz. The sensing signal sensitivity under frequencies of 80 Hz, 120 Hz and 200 Hz is shown in Fig. 5. There was a rapid decline in sensitivity of the sensor when the stimulus frequency was set to below 80 Hz. At the 40 Hz the sensitivity was around 60 mV. However the signal was very noisy and therefore unreliable. III. R ESULTS AND D ISCUSSION The 2D piezoelectric sensors were prepared with two polyester yarn spacers between the silver coated nylon electrodes in the weft direction and the PVDF fibers in the warp direction (Fig. 1). The changes in sensor voltage response to vibrations when using PVDF fibre diameters of 37 and 50 microns are depicted in Fig. 6.

It can be noted that the overall voltage response of the sensor constructed using this particular configuration is relatively low. In addition, the diameter of the PVDF filament did not seem to have a significant effect on the response of the sensor. From our perspective however the handling of the thicker 50 micron PVDF fibers is easier and it leads to fewer fiber breakages during the weaving process. We then tried to understand the effect of the distance between electrodes. The number of spacer yarn was reduced to one and consequently as it can be seen from Fig. 7 that the voltage response of the sensor to vibrations was significantly improved. The difference in voltage level may also be associated with fiber compaction during the weaving process. The length of the sensing area in the warp direction was found to be 33 and 41 mm using PVDF spacer yarn (single or double, respectively) whilst 29 and 43 mm was measured with polyester (single or double yarn, respectively). Therefore, it is clear that the distance between the electrodes, which is influenced by both the type of spacer and the fiber compaction, have a significant effect on the overall voltage signal generated by the piezo-sensor. In the last set of experiments, the sensors were constructed using various types of electrodes. The fabrics were woven

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IV. C ONCLUSION

Fig. 8. Impact of the electrode material in the sensitivity of the PVDF based sensor.

in such a way that the distance between the electrodes was kept the same for all tested sensors (in order to minimize any effect of the distance on the signal generated by the sensor). The results displayed in Fig. 8 show that the best voltage response to sine wave shaped vibrations was achieved when using the silver coated copper (SCC) electrodes. This result can be attributed to the large area of contact offered by these electrodes (a single electrode consist of 20 individual filaments of 40 microns in diameter each). On the other hand, the SCN (silver coated nylon) electrodes, for which the area of contact was larger than any other electrodes (a single SCN electrode consists of 20 individual filaments of 50 microns each in diameter) did not perform overly well. During the weaving process, the weft electrode most probably underwent small surface damages due to friction with the warp. This would have induced a non-continuous silver surface coating which in turn would reduce the electrical conductivity of the electrode (considering that its nylon core is non-conductive). The sensor constructed using either the steel or titanium electrodes performed reasonably well (second best). This could be a result of tight fit achieved during the weaving with the electrodes. However these electrodes (which consisted of a single wire of 10 and 20 microns in diameter, respectively see Table I) were quite delicate to work with. Therefore, coupling of multiple wires into a thicker and stronger yarn would facilitate their weaving, and would also in turn increase the contact surface area. Bundling wires would also decrease overall resistance of the electrode which may also have an impact on the generated signal. The worst signal response was noted when using the aluminum electrodes. We believe that the electrical discharge which occurred during testing might have induced some oxidation at the contact point. Metallic aluminum is very reactive with atmospheric oxygen, and a thin passivation layer of aluminum oxide (electrical insulator) can form very rapidly upon discharge on any exposed surface. Also because the fabric is periodically stretched during testing, expansion and contraction of the more ductile aluminum might have caused some significant changes in electrical connectivity to the PVDF fibre.

This work showed that it is possible to design flexible textile-based sensor using unidirectional drawn piezoelectric PVDF fibers. The purposely built device which was used to examine the signal level response generated by the sensors to the sine wave shaped vibrations provided meaningful results, indicating some relationship between the output signal under load and the configuration of the sensor. Changes in the level of response to the mechanical deformation of the fabric were noted depending on the type of conductive electrodes used and depending on the distance between the electrodes. When the distance is reduced to one the level of signal generated by the sensor was significantly improved, but one has to bear in mind the electrical and shorting issues that can be associated with short distances. Both the large area of contact offered by the silver coated copper yarn (SCC) and their low resistance had a positive impact on the performance of the sensor. Further work will be carried out by varying the type of weave and the type of conductive electrodes in an attempt to optimize the piezoelectric output for sensing applications. ACKNOWLEDGMENT We gratefully acknowledge Mr. M. Neuenhofer from the Textile Institute RWTH Aachen Germany for all the help they provided during the duration of this project. We also gratefully acknowledge CSIRO Materials Science and Engineering weaving and workshop staff without whom, this device would be just another idea. R EFERENCES [1] S. Dong, Z. Sun, and Z. Lu, “Chloride chemical sensor based on an organic conducting polypyrrole polymer,” Analyst, vol. 113, no. 10, pp. 1525–1528, 1988. [2] D. De Rossi, F. Lorussi, A. Mazzoldi, P. Orsini, and E. P. Scilingo, “Monitoring body kinematics and gesture through sensing fabrics,” in Proc. 1st Annu. Conf. Microtechnol. Med. Biol., 2000, pp. 587–592. [3] J. Edmison, M. Jones, Z. Nakad, and T. Martin, “Using piezoelectric materials for wearable electronic textiles,” in Proc. 6th Int. Symp. Wearable Comput., 2002, pp. 41–48. [4] X. Chen, S. Xu, Y. Shiyou, and S. Nan, “Nanogenerator for mechanical energy harvesting using PZT nanofibers,” Nano Lett., vol. 10, no. 6, pp. 2133–2137, 2010. [5] C. E. Seeley, E. Delgado, J. Kunzman, and D. Bellamy, “Miniature piezo composite bimorph actuator for elevated temperature operation,” in Proc. ASME Conf., 2007, pp. 405–415. [6] K. Laxminarayana and N. Jalili, “Functional nanotube-based textiles: Pathway to next generation fabrics with enhanced sensing capabilities,” Textile Res. J., vol. 75, no. 9, pp. 670–680, 2005. [7] P. Sajkiewicz, A. Wasiak, and Z. Gocłowski, “Phase transitions during stretching of poly(vinylidene fluoride),” Eur. Polymer J., vol. 35, no. 3, pp. 423–429, 1999. [8] A. Salimi and A. A. Yousefi, “Analysis method: FTIR studies of β-phase crystal formation in stretched PVDF films,” Polymer Test., vol. 22, no. 6, pp. 699–704, 2003. [9] C.-H. Du, B.-K. Zhu, and Y.-Y. Xu, “Effects of stretching on crystalline phase structure and morphology of hard elastic PVDF fibers,” J. Appl. Polymer Sci., vol. 104, no. 4, pp. 2254–2259, 2007. [10] E. Siores, R. L. Hadimani, and D. Vatansever, “Piezoelectric polymer element and production method and apparatus therefor,” U.S. Patent 101 539 9.7, 2010. [11] (2010). Smart Materials Corporation Webpage http://www.smartmaterial.com/EH-product-main.html

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Andrzej S. Krajewski (M’10) received the M.Sc. degree in electronic engineering from the Department of Electric and Electronics, University of Mining and Metallurgy, Krakow, Poland, in 1979, and the Ph.D. degree in instrumentation in 2007 after working on improving nonwoven web quality with a new web scanning device. He is currently a Research Project Scientist with expertise in the area of flexible electronic and electronic textiles. His current research interests include the development of novel textile impact sensors and other smart textile devices.

Kevin Magniez received the M.Eng. degree in textile engineering from ENSAIT, Roubaix, France, the leading French school of textile engineering in 2001, and the Ph.D. degree in materials science in 2006 after working on the structure-property relationships in polymer blends and nanocomposites. He is currently a Researcher with extensive expertise in the area of meltprocessing and nanocomposites. His current research interests include the development of novel composite materials including self-healing, toughened and nanocomposite systems as well as smart textile materials.


Richard J. N. Helmer is actively involved in developing and applying wearable technologies for sports, health, and defense applications that combine smart materials with ICT infrastructure. His current research interests include fiber based sensors and energy devices. He has published over ten patent filings, a number of which have proceeded to grant and license, and over 50 scientific publications. He led the Advancing Human Performance Research Theme, CSIRO, from July 2008 to June 2009 and is a member of the Australian Institute of Sport and CSIRO Research Steering Committee, whilst leading multidisciplinary projects and fostering new research activity across Australia.

Viktoria Schrank, photograph and biography not available at the time of publication.