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Int. J. Biomechatronics and Biomedical Robotics, Vol. 1, No. 2, 2010
Ionic polymer metallic composite as wearable impact sensor for sport science H.H. Chen, S.C. Fang and K.C. Aw* Mechanical Engineering, The University of Auckland, 20, Symonds Street, Auckland, New Zealand E-mail:
[email protected] E-mail:
[email protected] E-mail:
[email protected] *Corresponding author Abstract: Ionic polymer metallic composite (IPMC) is a type of ionic electroactive polymer that has electromechanical property that can be used as actuators or sensors. IPMC sensors offer several advantages over conventional sensors; such as flexibility, conformability and durability are useful for impact sensing. The aim of the paper is to demonstrate the ability of IPMC as an impact sensor to be used in sport science. The ability to measure impact force or energy is important in sport science as this information can be used to improve athletes’ performances. In this research, experiments were conducted for different impact energies and a mathematical model based on the impact sensor’s output voltage spike’s slope was developed. Testing using this model demonstrated that the accuracy of the measured impact energy is mostly within ±10% although an error of up to 25% was observed at low impact energies. Keywords: IPMC; impact sensor; sport science. Reference to this paper should be made as follows: Chen, H.H., Fang, S.C. and Aw, K.C. (2010) ‘Ionic polymer metallic composite as wearable impact sensor for sport science’, Int. J. Biomechatronics and Biomedical Robotics, Vol. 1, No. 2, pp.88–92. Biographical notes: H.H. Chen completed her BE (Hon) in Mechatronics Engineering from the University of Auckland in November 2009. S.C. Fang completed his BE (Hon) in Mechatronics Engineering from the University of Auckland in November 2009. K.C. Aw is a Senior Lecturer in the Department of Mechanical Engineering at The University of Auckland. He received his MSc in Advanced Manufacturing Systems from Brunel University and earned his PhD in Applied Physics. He has over a decade of industrial experiences at Intel, Altera and Navman before joining the university. He teaches undergraduate courses and supervises post-graduate research in mechatronics engineering. His current research interests are bio-mechatronics, micro-system and smart materials.
1
Introduction
In recent years, many athletes have resorted to sports science to improve their field performance. Some sports such as rugby, high jump and running can generate a substantial amount of impact force or energy onto the limbs. Therefore, being able to quantify and analyse this impact force or energy can help athletes to further improve their techniques, which can also lead to reduction in injuries. However, conventional impact sensors, i.e., strain gauges lack the robustness and mechanical flexibility for wearable sensors. In addition, strain gauges also need to be mounted on rigid surfaces which may not always be possible. The development of ‘smart materials’ such as ionic polymer metallic composite (IPMC) has the potential to overcome
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these problems. IPMC is largely still in its research stage. Many research studies have been conducted in implementing IPMC as actuators for artificial muscles and have shown promising results (Jung et al., 2003; Kim and Tadokoro, 2007; Reece, 2007). IPMC is a type of electro-active polymer (EAP) and belongs to a class called ionic EAP. A bending motion is generated with an applied voltage and a reverse effect occurs as such a deformation to the material will cause a voltage potential to form across its surfaces. The sensing behaviour of an IPMC is a result of the uneven distribution of charges with respect to the material’s neutral axis (Shahinpoor et al., 1998; Bonomo et al., 2003). By mechanically bending an IPMC, one side of the material contracts while the other side expands and spreads. This
Ionic polymer metallic composite as wearable impact sensor for sport science deformation creates a stress gradient onto the material that forces the ions to move towards the sparse side of the membrane (Shahinpoor et al., 1998; Bonomo et al., 2003; Punning et al., 2007). This movement of ions creates an unbalanced distribution of charges that can be read as a voltage potential through the electrodes. Since IPMC is essentially a piece of polymer, it has outstanding mechanical flexibility and is very conformal. In addition, it will not interfere with the movements of a body which makes it wearable. For example, an advantage that can be brought by IPMC impact sensors can be applied in footwear to measure the impact energy of a foot striking a hard surface. It can also be placed in the shoulder pads of a rugby player to measure the impact energy/force during a rugby scrum. A mathematical model has been developed for an IPMC strip used as a rotary joint sensor (van den Hurk et al., 2009) and this principle was adopted in this research. In theory, by fixing the bending angle, different impact energy would translate into different bending rates, hence outputting different voltage responses. Figure 1
2
shock absorber on the top and soft sponge at the bottom. The shock absorber will reduce bouncing and the material type can be change to cater for different application. The soft sponge on the other hand has to be carefully selected to allow for the intended range of impact energies to completely bend the IPMC by 8 mm. Different sponge materials will affect the bending speed and hence has to be changed for different energy range. Figure 2 shows an oscilloscope screen shot of the sensor’s output signal to an impact from a mass dropped onto it. Also shown in Figure 2 is the noise that is present in an unfiltered output signal from the sensor. Hence, the output signal from the sensor needs to be filtered before it can be used. Figure 2
A screen shot of a voltage response from an IPMC impact sensor with unfiltered noise (see online version for colours)
An impact sensor arrangement using IPMC
Experimental set-up
There are many researches that describe the effect of bending on IPMC voltage response (Biddis and Chau, 2006). Most studies have focused on the behaviour of an IPMC when bent in a cantilever fashion. An impact on an IPMC cantilever strip in an arrangement shown in Figure 1 produces a bend and generates a voltage spike in the millivolt range followed by a voltage recovery. The IPMC used is Nafion© based of 0.2 mm thick, 10 mm wide and 30 mm long. The IPMC’s sandwiching materials are the Figure 3
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The amplitude of the voltage spike is proportional to the bending speed of the IPMC cantilever strip. A simple experiment was conducted where a known mass was dropped at different known height allowing different impact energies to be measured. The output response from the IPMC for the corresponding impact energy will be amplified, acquired using a data acquisition card, digitally filtered and then computed using a computer as shown in Figure 3.
An experimental set-up
Sensor signal Mass impacting on the IPMC
Amplified signal Amplifier
DAQ
Digital data in a computer
Digital filter
Compute and analyse data
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Figure 4 shows the hardware used to conduct the impact test. The drop mass generates impact energy by transferring potential energy into kinetic energy. The amount of impact energy can varies according to the height of the drop mass. Figure 4
Impact test rig (see online version for colours)
Sliding guides
essential to quantify Ekinetic just before impact as this will be the actual energy converted into impact energy, Eimpact . Assume that after the drop-mass, mmass, impacted onto the shock absorber and the soft sponge (see Figure 1) of mass, msandwich, and the IPMC of mass mipmc; the three masses (mmass + msandwich + mipmc) become one body and travel at the same final velocity vfinal. The final velocity, vfinal will be the velocity bending the impact sensor. Since msandwich and mipmc are relatively very small compare to mmass then (mmass + msandwich + mipmc) ≈ mmass. The drop-mass kinetic energy, Ekinetic will be converted into impact energy, Eimpact , then
Drop mass
Eimpact = 1 mmass v 2final 2 Proximity sensors
Since the vfinal can be determined by the two proximity sensors (vfinal = d t ), then Eimpact can be computed with relative accuracy. Figure 5
A drop mass of 0.204 kg was used in the experiment. The mass had a drop range of 20 cm to provide sufficient varying impact energies for obtaining adequate data points. The mass had a rectangular tip with a slightly wider head than the IPMC strip to enable the strip to bend properly upon impact. The drop mass runs along two slider guides and is locked in place by a middle and bottom plate for stability. Upon impact, the mass will land on top of the free end tip of the IPMC strip, causing a bending motion, hence generating a voltage spike. Two proximity sensors were used in this project to estimate the final velocity of the drop mass just before impact. The sensors were placed one on top of another with a known distance, d, apart. Together with knowing the time, t taken to travel between the two sensors, the final velocity, vfinal of the drop mass before impact can be estimated. The final velocity, vfinal can be estimated with the simple equation of vfinal = d t . This velocity can also be assumed to be the bending velocity on the IPMC strip. The mathematical derivation relating the IPMC’s bending speed, vfinal to the impact energy, Eimpact of the drop mass is as follows: The gravitational potential energy, Epotential of the drop-mass, mmass at a height, h is defined as E potnetial = mmass gh
As the drop-mass is dropped from a height, it will be converted into kinetic energy, Ekinetic travelling at velocity, v, if there were no losses. E potential = Ekinetic =
1
2 mmass v
2
However, due to frictional loss along the sliding guides, not all Epotential will be converted into Ekinetic. Therefore, it is
3
An output voltage response showing the voltage spike and slope (see online version for colours)
Results and discussions
Analysing the voltage response from the IPMC when impacted by the drop-mass suggested that both the amplitude of the voltage spike (Vpeak ) and the slope of the voltage spike ( S = V peak Δt ) are proportional to the impact energy as shown in Figure 5. The plots of the impact energy versus the normalised amplitude of the voltage spike (Vpeak / V peak ( max ) ) and the impact energy versus the normalised slope of the voltage spike, (S/Smax) are shown in Figure 6 and 7 respectively. Figures 6 and 7 show that the impact energy is either related to the normalised amplitude of the voltage spike (V peak _ norm ) or to the normalised slope of voltage spike (Snorm). However, in this work Snorm will be used to quantify the impact energy as the curve fitting is slightly better. Using the mathematical equation obtained from Figure 7 ( Eimpact = 0.051e 1.4648 Snorm , where Eimpact = impact energy and Snorm = the normalised slope of the voltage spike), the impact energy, Eimpact of a mass impacting on the impact sensor can be measured from S. Table 1 summarises the calculated (theoretical) Eimpact , measured Eimpact by the IPMC and the percentage error of the measured Eimpact with respect to the calculated Eimpact .
Ionic polymer metallic composite as wearable impact sensor for sport science Figure 6
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The impact energy versus the normalised peak voltage (see online version for colours)
Impact Energy (J)
1
0.1 1.8601x
y = 0.0358e 2 R = 0.8767
0.01 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Normalized peak voltage (Vpeak/Vpeak(max))
The impact energy versus the normalised slope of the voltage spike (see online version for colours) 1
Impact energy (J)
Figure 7
0.1 1.4648x
y = 0.051e 2 R = 0.891
0.01 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Normalized voltage slope (S/Smax)
0.8
0.9
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H.H. Chen et al.
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Summary of the calculated and measured impact energy
Calculated energy (J)
Eimpact =
mmass v 2final 2
Measured energy (J) by the IPMC Eimpact = 0.051e1.4648 Snorm
0.0425
0.0531
24.93
0.0606
0.0539
–11.08
0.0773
0.0701
–9.32
0.0884
0.0921
4.18
0.0989
0.0923
–6.67
0.1125
0.1107
–1.64
0.1372
0.1419
3.41
0.1438
0.1417
–1.44
0.1513
0.1631
7.84
0.1808
0.1891
4.59
The result illustrates that the IPMC is able to provide a fairly accurate measurement of the Eimpact using Snorm. The accuracy of the measured energy is approximately within ±10% with the exception at low impact energies. To be able to use the IPMC as an impact sensor in the field, further work has to be carried out to improve its reliability. Nevertheless, an IPMC can be used to measure impact energy aimed for sport science as wearable sensor. However, it is important that the IPMC is calibrated with at least two known impact energies (a high and a low impact energies) before it can be used for proper measurements as the environmental condition of the IPMC affects the absolute voltage value but not the trend.
4
Percentage error (%)
Conclusions
It has been demonstrated that an IPMC can be used to measure the impact energy. Experiments have shown that two quantities, i.e., IPMC’s voltage spike (V peak ) and slope of the voltage spike (S) can be used to quantify the impact energy. However, the normalised S (Snorm) is used due to a slightly better curve fitting of the experimental data. In this work, the IPMC as an impact sensor has demonstrated that it is capable of measuring the impact energy to within ±10% with the exception at low impact energy.
References Biddiss, E. and Chau, T. (2006) ‘Electroactive polymeric sensors in hand prostheses: bending response of an ionic polymer metal composite’, Med. Eng. Phys., Vol. 28, pp.568–578. Bonomo, C., Del Negro, C., Fortuna, L. and Graziani, S. (2003) ‘Characterization of IPMC strip sensorial properties: preliminary results’, Proceedings of the International Symposium on Circuits and Systems, 25th–28th May, pp.816–819.
Jung, K., Nam, J. and Choi, H. (2003) ‘Investigations on actuation characteristics of IPMC artificial muscle actuator’, Sens. Actuators, Vol. 107, pp.183–192. Kim K. J. and Tadokoro, S. (2007) Electroactive Polymers for Robotics Applications: Artificial Muscles and Sensors, Springer-Verlag London Ltd., London, pp.37–49. Punning, A., Kruusmaa, M. and Aabloo, A. (2007) ‘Surface resistance experiments with IPMC sensors and actuators’, Sens. Actuators, Vol. 133, pp.200–209. Reece P. L. (2007) Progress in Smart Materials and Structures, Nova Science, New York, pp.92–110. Shahinpoor, M., Bar-Cohen, Y., Xue, T., Simpson, J.O. and Smith, J. (1998) ‘Ionic polymer-metal composites (IPMC) as biomimetic sensors and actuators’, Smart Mater. Struct., Vol. 9, pp.R15–R30. van den Hurk, A., Chew, X.J., Aw, K.C. and Xie, S.Q. (2009) ‘A rotary joint sensor using ionic polymer metallic composite’, Int. Conf. on Smart Materials and Nanotechnology in Engineering, 8th–11th July, Weihai, China.
