A Novel Polypyrrole-based Sensor for Robotics Applications

A Novel Polypyrrole-based Sensor for Robotics Applications

10th IFAC Symposium on Robot Control International Federation of Automatic Control September 5-7, 2012. Dubrovnik, Croatia A Novel Polypyrrole-based ...

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10th IFAC Symposium on Robot Control International Federation of Automatic Control September 5-7, 2012. Dubrovnik, Croatia

A Novel Polypyrrole-based Sensor for Robotics Applications A. P. Tjahyono*. K.C. Aw,** J. Travas-Sejdic*** 

*Mechanical Engineering, The University of Auckland, New Zealand (e-mail: [email protected]) ** Mechanical Engineering, The University of Auckland, New Zealand (email: [email protected]) *** Polymer Electronic Research Centre, Chemistry Department The University of Auckland, New Zealand, (e-mail: [email protected]) Abstract: A novel large strain sensor that can be used for the control of air muscle and robotic rotary joint is shown here. This sensor is made from polypyrrole coated on a natural rubber substrate using vapour phase polymerisation technique and then oxidized with ferric chloride to adjust the conductivity. The operation of this sensor is based on the change of the resistance of polypyyrole when stretched or strained and hence measuring the contraction and extension of an air muscle or the angular displacement of a rotary joint. A gauge factor of 1.86 was achieved. Keywords: strain sensor, polypyrrole, air muscle, rotary joint. (Kiefer et al., 2007, 2008). This effect has been utilised to produce actuators such as artificial muscles (Kiefer et al., 2010; Chu et al. 2004; Spinks et al. 2004) and drug delivery systems (Sirivisoot et al. 2011; George et al., 2006). The piezoresistive effect works in reverse where physical changes in the polymer’s geometry generate changes in the electrical conductivity of the polymer. As such, PPy is highly suitable as strain sensors (Li et al, 2005 ; Xue et al., 2007 ).

1. INTRODUCTION Intrinsically conducting polymers are one of the emerging smart materials where polymers have the capability to conduct electricity. The existence of conducting polymers can be found in literatures as early as 1862 where polyaniline was prepared through anodic oxidation (Letheby et al. 1862) which was further studied and verified (Szarvasy, 1900). It was not until the major breakthrough in 1970s that conducting polymers were considered for their massive potentials, realised by the collaboration between three scientists; Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa. Their potentials are based on their high electrical conductivity merged with the excellent mechanical properties of polymers such as flexibility and lightweight. These properties are also adjustable by varying the fabrication parameters.

Conducting polymer films can only withstand limited strain before breaking and cannot perform well in measuring large strain (Murray et al., 1998). To overcome this problem, substrates were employed to provide the necessary support and the surface for conducting polymer film deposition. Generally, fabricating a strain sensor using this approach means that the mechanical properties are highly attributed to the substrate while the conducting polymers introduce the electrical conductivity. This practice is commonly found in the research field of smart textiles which are conductive fabrics produced by coating conducting polymers onto commercial fabrics such as nylon, polyester and Lycra (Bunsomsit et al., 2002; Molina et al., 2008; Wu et al., 2005). Although excellent results have been demonstrated with smart textiles using conducting polymers, the intended applications are mainly aimed at enhancing the usability of clothing beyond its current use as a protective layer. As a strain sensor for robotic applications, the substrate requires having some degree of rigidity and fabrics are not an ideal material due to its soft structure. Furthermore, repetitive strain can cause permanent elongation on individual fibres where the strain may not be distributed equally. This can lead to individual fibres having different mechanical properties that will affect the strain sensing performance. The proposed solution is to replace fabric with natural rubber (NR), which has good combination of rigidity and elasticity. Previous studies have succeeded in fabricating a strain sensor using

Among the conducting polymers, polypyrrole (PPy) has gained popularity in the research field due to its high conductivity, good stability and ease of synthesis either chemically or electrochemically. As a result, PPy has been widely studied as sensors, actuators and in other applications such as heat generation (Hakansson et al., 2004) and electromagnetic interference (EMI) shielding (Avlone et al., 2008). As a gas sensor, it can interact with various chemical vapours where the oxidation state of PPy changes according to the vapour it has been exposed to with high sensitivity and fast response (Inzelt et al., 2008 ; Wallace et al., 2009). This change in oxidation state can be observed through its conductivity change ie. exposure to NH3 reduces the conductivity of PPy as NH3 is a electron-donor that reverses the oxidation state of p-doped PPy. In addition, the application of an electrical field induces a redox reaction in the PPy that promote the movement of solvated ions in or out of the polymer structure changes the polymer’s geometry 978-3-902823-11-3/12/$20.00 © 2012 IFAC

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IFAC SYROCO 2012 September 5-7, 2012. Dubrovnik, Croatia

PPy and NR substrate where PPy powders are embedded into the structure of the rubber directly (Bunsomsit et al., 2002). Compared to the coating methods, that approach requires knowledge of manufacture rubber as well as access to the equipments to produce rubber with consistent mechanical properties.

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This work is aimed at developing a low cost, large strain sensor using NR substrate with PPy coating. The strain will cause a change in the electrical resistance. Hence, it can be used to measure the extension or contraction of an artificial muscle. It can also be used to measure the angular displacement of a rotary joint such as the finger joints of a humanoid. NR was chosen as the substrate due to its excellent resilience and elasticity. Commercial NR strips were purchased and used to produce the strain sensor. PPy as thin film is coated onto the NR substrate by means of vapour phase polymerisation (VPP) technique that provides a good adhesion between the two components of the strain sensing element. Here this sensor will use the acronym NR/PPy sensor.

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Fig. 1. Test profile (linear strain versus time).

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The NR substrate used to prepare the samples was 1.5 mm thick strips (from NZ Rubber and Foam) cut to 50 mm x 2 mm pieces. Deposition of PPy film onto the NR surface was carried out using vapour phase polymerisation (VPP) technique by using PPy vapour under vacuum.

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Fig. 2. The actual resistance profile of the sensor showing an upward drift as the test is repeated.

The optimised fabrication process requires a 40 s of oxygen plasma treatment at 200 W of the natural rubber then followed by dipping in a 0.5 M of FeCl before vapour phase polymerization with a 0.1 M pyrrole monomer solution over 2 hours while the rubber is being stretched throughout the deposition process.

However, a normalised, ΔR/RO plot as in Fig. 3, allows this NR/PPy sensor to be used to determine the relative linear strain caused by extension/contraction if the two working extremes can be established. The ΔR/RO of each strain value was averaged to determine the mathematical relationship between the averaged ΔR/RO and strain (Fig 3b). From this plot, the gauge factor is calculated to be approximately 1.86 with a mathematical relationship in Equation 1.

2.2 Linear Strain Test

ΔR/RO = 1.86 x strain

All testing were conducted at room temperature (22C to 27C) and the humidity can varied between 50% and 70%. An NR/PPy sensor fabricated using the established optimised parameters was subjected to 800 test cycles to evaluate their repeatability and stability over time. The test was conducted over 40 days with a 20 cycles/day and up to 20% of the original length for each cycle. The test profile is shown in Fig. 1.Fig. 2 shows the electrical resistance profile of a NR/PPy sensor when the linear strain profile of Fig. 1 was applied. The shape of the electrical resistance versus time profile remains similar, although there is an upwards shift over the time. This upward shift of the electrical resistance means that the absolute electrical resistance value varies over time and cannot be used to represent the absolute strain.

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2.3 Angular Strain Test This NR/PPy sensor can also be used to measure the bending angle of a rotary joint such as a robotic finger joint. The test profile and the electrical resistance profile for the sensor are shown in Fig. 4(a) and Fig. 4(b) respectively. As with the linear motion, the same upward drift is also evident in this test and hence only the normalised electrical resistance, ΔR/RO, can be used to determine the relative angular position. The ΔR/RO for each strain value from the 10 cycles (limited to a maximum angular displacement of 90) was averaged and the non-linear relationship as in Fig. 5. From Fig. 5, the maximum hysteresis is approximately 10 at the centre region of the plot that translates to 11.1% of hysteresis.

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Fig. 5. The relationship of ΔR/RO versus rotary angle.

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3. SENSOR AND PID CONTROL

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The capability of the NR/PPy sensor to be used as a strain sensor that measures and controls the bending of a finger joint on a hand exoskeleton that is actuated by an air muscle is demonstrated here. The pneumatic pressure of the air muscle is controlled via a solenoid valve using a proportional-integral-derivative (PID) controller with a desired input signal to contract the air muscle and then return it back to the starting position. The resistance is read from the NR/PPy sensor and the ΔR/RO is computed, and then input into the PID controller. This is the feedback to the PID controller. The amplified output from the PID controller will control a solenoid valve (SMC ITV0030-3BS) that will regulate the pneumatic pressure to control the extension of the air muscle. A laser rangefinder (L-GAGE LG10A65PU) was mounted onto the testing rig as a secondary sensor, independent from the PID controller and solely used to determine the actual bending of the finger joint. The readings

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Fig. 4. (a) The rotary strain test profile and (b) the resistance profile showing upward drift for a rotary motion.

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Fig. 6. A PID controller fedback with NR/PPy sensor to control the rotation of a finger joint to (a) 40 and (b) 50. 592

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from the NR/PPy sensor will be compared with the actual joint rotation (by laser rangefinder) that is being controlled, to gauge the overall performance of the NR/PPy sensor for use in the PID controller. From Fig. 6, the actual rotation of the finger joint (40 and 50) and the calculated angular rotation from the NR/PPy sensor are very close to the desired angle. There are some overshoot in the response and this can be reduced further by fine-tuning the controller. When this test was ran repeatedly, the maximum angular error is approximately +/-5 and this is value coincides with the maximum hysteresis of 10

Kiefer, R., Bowmaker, G.A., Kilmartin, P.A. and TravasSejdic, J. (2010). Effect of polymerisation potential on the actuation of free standing poly-3,4ethylenedioxythiophene films in a propylene carbonate electrolyte, Electrochimica Acta, volume (55), 681-688. Letheby, H. (1862). On the production of a blue substance by the electrolysis of sulphate of aniline, Journal of Chemical Society, volume (15), 161-163. Li, Y., Cheng, X.Y., Leung, M.Y., Tsang, J., Tao, X.M. and Yuen, M.C.W. (2005). A flexible strain sensor from polypyrrole-coated fabrics, Synthetic Metals, volume (151), 89-94. Murray, P., Spinks, G.M., Wallace, G.G. and Burford, R.P. (1998). Electrochemical induced ductile-brittle transistion in tosylate-doped (pTS) polypyrrole, Synthetic Metals, volume (97), 117-121. Molina, J., del Río, A.I., Bonastre, J. and Cases, F. (2008). Chemical and electrochemical polymerisation of pyrrole on polyester textiles in presence of phosphotungstic acid, European Polymer Journal, volume (44), 2087-2098. Sirivisoot, S., Pareta, R. and Webster, T. J. (2011). Electrically controlled drug release from nanostructured polypyrrole coated on titanium, Nanotechnology, volume (22), 085101. Spinks, G. M., Zhou, D., Truong, V. and Wallace, G. G. (2004). Enhanced control and stability of polypyrrole electromechanical actuator, Synthetic Metals, volume (140), 273-280. Szarvasy, E. (1900). Electrolytic preparation of indulines dyes, Journal of the Chemical Society volume (77), 207212. Wallace, G.G., Spinks, G.M., Kane-Maguire, L.A.P. and Teasdale, P.R. (2009). Conductive electroactive polymer: intelligent polymer systems, 3rd edition, CRC Press. Wu, J., Zhou, D., Too, C.O. and Wallace, G.G. (2005). Conductive polymer coated lycra, Synthetic Metals, volume (155), 698-701. Xue, P., Tao, X. M. and Tsang, H. Y. (2007). In situ SEM studies on strain sensing mechanisms of PPy-coated electrically conducting fabrics, Applied Surface Science, volume (253), 3387-3392.

4. CONCLUSIONS In this paper, the development of a large strain sensor made from natural rubber coated with PPy is presented. The deposition process of PPy onto the rubber surface was undertaken through VPP of pyrrole. Although the linear gauge factor for the sensor is only 1.86, we have demonstrated it is capable to be used in a hand exoskeleton where size and weight is very important. The downside of this sensor is that they show some hysteresis. Nevertheless, we have successfully demonstrated the application of the NR/PPy sensors with reasonable accuracy (+/-5) in conjunction with a PID controller to control the turning angle of a finger joint in an exoskeleton actuated by an air muscle. REFERENCES Avlone, J., Florio, L., Henn, A. R., Lau, R., Ouyang, M. and Sparavigna, A. (2008). Electromagnetic shielding with Polypyrrole-coated fabrics, Journal of Thermoplastic Composite Materials, volume (20), 241-254. Bunsomsit, K., Magaraphan, R., O’Rear, E.A. and Grady, B.P. (2002). Polypyrrole-Coated Natural Rubber Latex by Admicellar Polymerisation, Colloid and Polymer Science, volume (280), 509-516. Chu, S.Y., Kilmartin, P.A., Cooney, R.P and Travas-Sejdic, J. (2009). Effects of applied stress and long-term stability on PPy(CF3SO3) linear actuator, Synthetic Metals, volume (159), 2286-2288. George, P.M., LaVan, D.A., Burdick, J.A., Chen, C.-Y., Liang, E. and Langer, R. (2006). Electrically Controlled Drug Delivery from Biotin-Doped Conductive Polypyrrole, Advanced Materials, volume (18), 577-581. Hakansson, E., Kaynak, A., Lin, T., Nahavandi, S., Jones, T. and Hu, E. (2004). Characterization of conducting polymer coated synthetic fabrics for heat generation, Synthetic Metals, volume (144), 21-28 Inzelt, G. (2008). Conducting polymers: a new era in electrochemistry, Springer, Berlin Heidelberg. Kiefer, R., Chu,, S.Y., Kilmartin, P.A., Bowmaker, G.A., Cooney R.P. and Travas-Sejdic, J. (2007). Mixed-ion linear actuation behaviour of polypyrrole, Electrochimica Acta volume (52), 2386-2391. Kiefer, R., Bowmaker, G.A., Cooney, R.P., Kilmartin, P.A. and Travas-Sejdic, J. (2008). Cation driven actuation for free standing PEDOT films prepared from propylene carbonate electrolytes containing TBACF 3SO3, Electrochimica Acta, volume (53), 2592-2599.

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