Development and characterisation of elastomeric tape sensor fabrics for elbow angle measurement

Transcription

1 Indian Journal of.fibre & Textile Research Vol. 36, December 2011, pp Development and characterisation of elastomeric tape sensor fabrics for elbow angle measurement T Kannaian & R Neelaveni Department of Electrical and Electronics Engineering, PSG College of Technology, Coimbatore , India and G Thilagavathi a Department of Textile Technology, PSG College of Technology, Coimbatore , India This paper deals with the development of electro-active elastomeric fabric sensor using silver coated polyamide yarn at the centre of the tape along with polyester and rubber yarns and its electrical characterisation for the application towards elbow angle measurement. Box and Behnken 3-variable and 3-level experimental design has been used and 15 different samples are produced. The resistance change in the samples caused by linear extension as well as the angular movement of the elbow is measured. The variables for sensor design have been optimized using gauge factor ( r/ x) values as dependent variable and the results show that the variables such as number of conductive threads, picks per inch and rubber: polyester thread ratio influence the r/ x value negatively. Different sensor characteristics have also been analysed. The sample with 4 conductive threads, 20 picks/inch and 1:8 polyester : rubber ratio shows a higher gauge factor of and has proportional resistance change with length; this sample is therefore chosen to be good among all the 15 samples developed. It is also observed that this sample with higher sensitivity value can be used to measure elbow joint up to 90 0 angle. Keywords: Conductive yarn, Elastomeric sensor, Elbow angle, Goniometry, Resistance change, Silver coated yarn, Warp crimp 1 Introduction The diversification in the field of textiles is huge today and wearable electronics or electro-active textiles play more vital roles. The electro-active textiles can simply have an electronic circuit integrated within a fabric or the fabric itself can be made conductive with chemicals, conductive polymers or resins and metallic coating at fibre, yarn or fabric stage. The unique textile properties like large surface area, lightweight, and flexibility deploy them as sensors 1. The biomedical sensing includes heart rate, respiration of children, ECG, body posture and gesture movement measurement, strain sensor, and pressure sensor 2. Zhang et al. 3 developed a conductive knitted fabric from carbon fibre and steel. Contacting electrical resistance resulted from two overlapped yarns is attributed to be the key factor contributing to the resistance strain response. Gibbs and Asada 4 used an array of conductive fibre fixed over a spandex pant along with an elastic cord at top to continuously measure single-axis or multi-axis joint movement with a Kalman filter which requires a To whom all the correspondence should be addressed. thilagapsg@yahoo.co.in one time calibration. Xiaoyin et al. 5 coated a nano layer of polypyrrole on nylon and tested for strain with resistance. The sensor uses strain gauge principle and shows a change in resistance for 50% extension. Elastosil mixture along with a lycra fabric shows a piezoresistive property and can be used as a strain sensing fabric for hand posture and gesture measurement 6. A conductive polymer formed from pyrrole monomer, ferric chloride and 1, 5-naphthalenedisulfonic acid tetra hydrate was coated over nylon-lycra fabric which also shows a strain gauge principle 7. This paper discusses the design and development of an elastomeric fabric sensor working in strain gauge principle to measure the angle flexed by the elbow. 2 Materials and Methods 2.1 Materials The elastomeric tape sensor was produced with Shieldex HC Z, i.e. silver coated polyamide yarn of dtex, polyester yarn of 210 denier and rubber threads of 40 gauge in warp and polyester of 210 denier in weft. The silver coated polyamide yarns were supplied by M/s Statex Gmbh, Germany.

2 KANNAIAN et al.: DEVELOPMENT & CHARACTERIZATION OF ELASTOMERIC TAPE SENSOR FABRICS Methods Elastomeric Tape Sensor Production Elastomeric tapes were woven in M/s Sakthi Tapes, Tirupur. Twill (3/1) was the woven structure and 4-8 conductive yarns were woven in middle of the tape. The elastomeric tape sensor fabric design was optimized with Box and Behnken 3-variable and 3-level experimental design. Three independent variables used were number of conductive threads (X 1 ), picks per inch (loom state) (X 2 ) and polyester: rubber thread ratio (X 3 ) in three different levels (Table 1). The dependent variable was the sensitivity factor or gauge factor, i.e. ratio of change in resistance for a small change in length. Response surface equation was formed with the help of least square regression method using SYSTAT statistical package. The coded levels of Box and Behnken 3-variable 3-level experimental design are shown in Table 2 and the sample specifications of 15 experiments are shown in Table Resistance Measurement The elastomeric tape sensors were extended with the help of fabric extension meter from an initial length of 15cm. They were extended up to 30 % (4.5 cm) with an interval of 0.5cm. The resistance change (linear resistance) thus caused due to the extension was noted down using Agilent 34401A 6 ½ digit multimeter. The sensitivity factor was calculated by the slope of the curve for resistance with extension, as shown below: Sensitivity factor (Gauge factor) = r/ x... (1) where r is the change in resistance for a change in distance x, for which the curve was linear. The developed sensors were also charecterised through multiple elongation-contraction cycles using Instron tensile tester Elbow Angle Measurement The elastomeric tapes were attached to an elbow sleeve using a velcro (Fig. 1). The effect of the resistance change with the angular movement of the elbow was measured by moving the hand with a sensor integrated sleeve over a planar surface marked with various angles (Fig. 2). 3 Results and Discussion 3.1 Electrical Resistivity for Linear Extension All the 15 samples were extended upto 25-30% extension and the changes in resistance with extension Table 1 Coded levels of variables as per Box and Behnken 3-variable 3-level experiment Expt number Coded level of variables X 1 X 2 X X 1 Number of conductive threads, X 2 Picks/inch (loom state), and X 3 Polyester : rubber thread ratio. Variable Table 2 Actual values corresponding to coded levels of elastomeric fabric Level in Box-Behnken design Low ( 1) Middle (0) High (+1) Conductive thread no. (X 1 ) Picks/inch (loom state) (X 2 ) Polyester : rubber thread ratio (X 3 ) 1:6 1:8 1:10 Table 3 Sample specification of 15 experiments as per Box and Behnken design Expt No. Conductive thread no. (X 1 ) Picks/inch (loom state) (X 2 ) Polyester :rubber thread ratio (X 3 ) : : : : : : : : : : : : : : :8

3 438 INDIAN J. FIBRE TEXT. RES., DECEMBER 2011 is shown in Fig. 3. The samples are found to have a change in resistance from 2.66% to 8.66% for an extension of 20% and then attain saturation. The resistance change with extension was plotted and the gauge factor for the samples is shown in Table 4. Sample 1 shows a higher linear change in resistance with extension and higher gauge factor value of 0.583ohms/cm. The change in resistance is found to be more linear and significant up to 10% extension, which can be attributed to the change in geometrical structure. There is a sharp change in the contact area Fig. 1 Elbow sleeve attached with the sensor between the loops of the conductive yarns and hence the resistance curves for the samples show more steady change within the 10% extension. There is no resistance change in elastomeric tape beyond 20% extension. The initial resistance change for 15cm original length in all the 15 samples ranges from 4.5 ohms/cm to 10.5 ohms/cm. The samples were also tested on Instron with an initial length of 15 cm for 40 % extension, i.e. up to 21 cm. The sample 1 which is tested for hysteresis curve for 10 cycles is shown in Fig. 4a. Sample shows a consistent increase with load applied and the curves merge as one, indicating that it has a good recovery property with a less growth factor. The tensile loading of the sample shows that it requires gf for an extension of 40% from original length with a CV% of 2.3%. It is observed from Fig. 4b that the sample shows a constant and proportional change in the force Fig. 2 Measurement of resistance with varying elbow flexion angles Fig. 3 Resistance vs extension of samples

4 KANNAIAN et al.: DEVELOPMENT & CHARACTERIZATION OF ELASTOMERIC TAPE SENSOR FABRICS 439 required with the extension. It requires a small force for extension of same length compared with other samples. 3.2 Optimization of Variables for Sensor Fabric Design The following response surface equation is obtained with gauge factor ( r/ x) value (Y) as Sample No. Table 4 Gauge factor of samples Gauge factor r/ x dependent variable and X 1, X 2, and X 3 as independent variables with the help of SYSTAT software package: Y= X X X X X X 1 X X 2 X X 1 X 3 R 2 =0.978 The negative coefficients of all the variables such as number of conductive threads, picks/inch and polyester: rubber ratio in a response surface equation indicate that the increase in their values decreases the r/ x value and the higher R 2 value of indicates their significant relation. The variable polyester: rubber ratio (X 3 ) influences strongly the gauge factor. Contour plots are also plotted using the SYSTAT software at 0, +1, -1 levels of dependant variables. In the present case, for discussion the contour plots are taken at 0 levels for the response. At this level, the effect of two variables is studied. The major observation from all the contour graphs (Fig. 5) is that the higher sensitivity is achieved at lower levels of the variables selected. Fig. 4 Load-extension curves of Sample 1 for (a) 10 load cycles, and (b) different tensile loading

5 440 INDIAN J. FIBRE TEXT. RES., DECEMBER 2011 It is observed from Fig. 5a that a higher sensitivity is obtained for the less number of conductive threads and at lower picks/inch. The sensitivity decreases as the number of conductive threads and picks/inch increase. This may be because of residual strain (strain introduced during weaving) present in the conductive yarn, which is high at higher picks/inch. As shown in Fig. 5b, the sensitivity decreases as the polyester: rubber ratio and number of conductive threads increase. This may be because at lower percentage of rubber threads, the sample shows better crimp and hence good extensibility. It is also observed from Fig. 5c that the sensitivity decreases as the polyester: rubber ratio and picks/inch increase. 3.3 Electrical Resistivity with Varying Elbow Flexion Angles Figures 1 and 2 show the elastomeric sensor fabric fixed on human hand and various positions of elbow respectively. The resistance changes for various angles of movement of elbow are shown in Fig. 6. The initial resistance at 0 degree angle for the 15 samples is ranging from 4.5 ohms/cm to 10.5ohms/cm. Sample 1 with 4 conductive threads, 20 picks/inch and 1:8 polyester: rubber ratio shows good linearity and hence this can be used for measuring angular movements. The samples tend to have a good repeatability property for angle resistance change. There is a steady increase in resistance up to 90 flexion of the Fig. 5 Contour plots of (a) slope vs. no. of conductive threads & picks/inch, (b) slope vs no. of conductive threads & polyester : rubber ratio, and (c ) slope vs. picks/inch & polyester : rubber ratio Fig. 6 Resistance vs elbow flexion angle for Box-Behnken design samples

6 KANNAIAN et al.: DEVELOPMENT & CHARACTERIZATION OF ELASTOMERIC TAPE SENSOR FABRICS 441 Fig. 7 Loop structures (a) without extension, (b) at 5% extension, (c ) at 10% extension, (d) at 20% extension, and (e) at maximum extension elbow after that the change in resistance is not significant. 3.4 Effect of Extension on Geometrical Structure in Tape The tape structures are woven in a tensioned state which when relaxed makes the warp yarn, except the rubber yarns, to shrink and form a loop like structure. The warp crimp of the conductive yarn used is found to be ~ 180%, which varies according to change in the experimental design. Because of the compact woven structures, the loops of conductive yarn thus formed touch each other (Fig. 7 a). When the tapes extend, the contact area between the loops gradually decreases (Figs 7 a d), and hence there is an increase in the path of the current flow. This increase in the path of the current flow increases the resistance of the conductive yarn. Thus, there is a change in the resistance when extended. The resistance change is arrested after 90 or 20% extension, because the loops were fully out of contact by then (Fig. 7e). 4 Conclusion Elastomeric electroactive tape sensor fabrics have been developed using silver coated polyamide yarn along with polyester and rubber threads. The process variables for sensor design are optimized using Box and Behnken experimental design with gauge factor as dependent variable. The samples have been tested for change in resistance with linear extension and angular movements for specific elbow angle measurement application. It is observed that the resistance changes up to 20% extension. Out of the 15 samples studied, sample with 4 conductive yarns, 20 picks/inch and 1:8 polyester: rubber ratio shows a good gauge factor ( r/ x) value of 0.583ohms/cm, which is a good sensitivity value for fabric and hence is chosen as the best sensor for goniometry applications. On attachement of this sample to an elbow sleeve for joint angle measurement, it is found that up to 90 joint movement can be measured with repeatability. The variable polyester : rubber ratio (X 3 ) influences strongly the r/ x values. References 1 Tushar Ghosh & Anuj Dhawan, Electronic textiles and their potential, Indian J Fibre Text Res, 31 (2006) Anand S C, Kennedy J F, Miraftab M & Rajendran S, Medical Textile and Biomaterials for Healthcare (Woodhead Publishing Limited, Cambridge), 2006, Hui Zhang, Xiaoming Tao, Tongxi Yu & Shanyuan Wang, Conductive knitted fabric as large-strain gauge under high temperature, Sensors Actuators, 126 (2006) 129.

7 442 INDIAN J. FIBRE TEXT. RES., DECEMBER Gibbs P & Asada H H, Wearable conductive fiber sensor arrays for measuring multi-axis joint motion, Proceedings, 26th Annual International Conference of the IEEE, Xiaoyin Cheng E, Yang Li, Xiaoming Tao, Pu Xue, Xiaoxiang Cheng, Hing Yee C W M, Tsang J & Mei Yi Leung, Polypyrrole-coated fabric strain sensor with high sensitivity and good stability, Proceedings, 1st IEEE International Conference on Nano/Micro Engineered and Molecular Systems, Federico Lorussi, Enzo Pasquale Scilingo, Mario Tesconi, Alessandro Tognetti & Danilo De Rossi, Strain sensing fabric for hand posture and gesture monitoring, IEEE Transac Information Technol Biomed, 9 (2005) 3. 7 Bridget J Munroa, Toni E Campbell, Gordon G Wallace & Julie R Steele, The intelligent knee sleeve: A wearable biofeedback device, Sensors Actuators, 131 (2008) 541.