MURPHY AND SITTI: WAALBOT: AN AGILE SMALL-SCALE WALL-CLIMBING ROBOT UTILIZING DRY ELASTOMER ADHESIVES 337 to travel along curved paths and around tight corners. IR communication is used to control the robot. The prototype was not able to climb on inverted surfaces because of its high mass, but was able to climb on smooth acrylic surfaces with slopes up to 110 ◦ and make floor to wall transitions. Climbing reliability of the prototype was limited because of the inability of the robot to maintain synchronization between the two legs. The sensors synchronize the legs only three times per revolution. Since the motors and drive trains differ slightly in friction, they run at different speeds. While the sensors are sufficient for foot positioning for safe turning, they do not provide synchrony throughout forward steps. When the feet are stepping even slightly asynchronously the assumptions made for the robot analysis no longer apply. In this case, the front feet are not necessarily preloaded correctly and the robot often falls. Future versions of the <strong>Waalbot</strong> will utilize absolute encoders to track and synchronize the legs. One of the major disadvantages of this robot design is that there is very little redundancy in case of adhesion failure. Most of the time, there are only two feet attached to the surface. Inverted climbing was not possible with the prototype due to the mass of the robot. As the adhesives used on the feet of the robot gathered dust and other contaminants, their performance degraded quickly. Therefore, these elastomer adhesives are not suitable for dirty outdoor environments, walking across indoor floors, or for long term tasks. Furthermore, if improper foot placement occurs, the possibility of adhesion failure during a plane transition is a dangerous flaw for a climbing robot. Sensing and planning for plane transitions is an important future challenge. Future work includes implementing the synthetic dry adhesives in place of the conventional adhesives when the technology is mature. Further miniaturization of the robots is required to improve performance due to increased area-to-mass ratio. Decreasing the robot mass will increase payload capacity, allowing for other sensors or tools, and lower power consumption for longer operation time. The force model will be improved by including the ankle joint as well as nonuniform adhesion pressure distribution across the surface of the footpads. Initial results from tests with a new smaller scale prototype, which addresses many of the issues discussed, show that these challenges can be overcome and suggest that reliable climbing with this design is possible. ACKNOWLEDGMENT The authors would like to thank W. Tso and M. Tanzini for their robot design, fabrication, and testing assistance, and all the members of the <strong>NanoRobotics</strong> <strong>Lab</strong>oratory at <strong>Carnegie</strong> <strong>Mellon</strong> <strong>University</strong> for their help and discussions. REFERENCES [1] W. Yan, L. Shuliang, X. Dianguo, Z. Yanzheng, S. Hao, and G. Xueshan, “Development and application of wall-climbing robots,” in Proc. IEEE Int. Conf. Robot. Autom., 1999, pp. 1207–1212. [2] S. Hirose, A. Nagakubo, and R. Toyama, “Machine that can walk and climb on floors, walls and ceilings,” in Proc. Int. Conf. Adv. Robot., 1991, vol. 1, pp. 753–758. [3] S. Ryu, J. Park, S. Ryew, and H. Choi, “Self-contained wall-climbing robot with closed link mechanism,” in Proc. Int. Conf. Intell. Robots Syst., 2001, vol. 2, pp. 839–844. [4] L. Briones, P. Bustamante, and M. Serna, “Wall-climbing robot for inspection in nuclear power plants,” in Proc. Int. Conf. Robot. Autom., 1994, pp. 1409–1414. [5] R. Pack, J. L. Christopher, and K. Kawamura, “A rubbertuator-based structure-climbing inspection robot,” in Proc. IEEE Int. Conf. Robot. Autom., Albuquerque, NM, 1997, vol. 3, pp. 1869–1874. [6] B. Luk, A. Collie, V. Piefort, and G. Virk, “Robug iii: A tele-operated climbing and walking robot,” in Proc. UKACC Int. Conf. Control, 1996, vol. 1, pp. 347–352. [7] T. Yano, T. Suwa, M. Murakami, and T. Yamamoto, “Development of a semi self-contained wall climbing robot with scanning type suction cups,” in Proc. Int. Conf. Intell. Robots Syst., 1997, vol. 2, pp. 900–905. [8] J. Grieco, M. Prieto, M. Armada, and P. Gonzalez de Santos, “A six-legged climbing robot for high payloads,” in Proc. Int. Conf. Control Appl., 1998, pp. 446–450. [9] S. Kim, A. T. Asbeck, M. R. Cutkosky, and W. R. Provancher, “Spinybot II: Climbing hard walls with compliant microspines,” in Proc. Int. Conf. Adv. Robot., 2005, pp. 601–606. [10] T. Bretl, S. Rock, and J.-C. Latombe, “Motion planning for a three-limbed climbing robot in vertical natural terrain,” in Proc. Int. Conf. Robot. Autom., 2003, pp. 2947–2953. [11] R. Altendorfer, N. Moore, H. Komsuoglu, M. Buehler, H. B. B. Jr., D. Mc- Mordie, U. Saranli, R. Full, and D. Koditschek, “Rhex: A biologically inspired hexapod runner,” Auton. Robots, vol. 11, no. 3, pp. 207–213, 2001. [12] R. Quinn, D. Kingsley, J. Offi, and R. Ritzmann, “Improved mobility through abstracted biological principles,” in Proc. IEEE/RSJ Int. Conf. Intell. Robots Syst., 2002, pp. 2652–2657. [13] J. M. Morrey, B. Lambrecht, A. D. Horchler, R. E. Ritzmann, and R. D. Quinn, “Highly mobile and robust small quadruped robots,” in Proc. Int. Conf. Intell. Robots Syst., 2003, vol. 1, pp. 82–87. [14] K. Daltorio, A. D. Horchler, S. Gorb, R. Ritzmann, and R. Quinn, “A small wall-walking robot with compliant, adhesive feet,” in Proc. Int. Conf. Intell. Robots Syst., 2005, pp. 4018–4023. [15] K. Autumn, Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing, and R. J. Full, “Adhesive force of a single gecko foot-hair,” Nature, vol. 405, pp. 681–685, 2000. [16] K. Autumn, M. Sitti, Y. A. Liang, A. M. Peattie, W. R. Hansen, S. Sponberg, T. W. Kenny, R. Fearing, J. N. Israelachvili, and R. J. Full, “Evidence for van der waals adhesion in gecko setae,” Proc. Nat. Academy Sci., vol. 99, pp. 12 252–12 256, 2002. [17] M. Sitti and R. Fearing, “Synthetic gecko foot-hair micro/nanostructures as dry adhesives,” J. Adhesion Sci. Technol., vol. 17, no. 5, pp. 1055–74, May 2003. [18] A. K. Geim, S. V. Dubonos, I. V. Grigorieva, K. S. Novoselov, A. A. Zhukov, and S. Y. Shapoval, “Microfabricated adhesive mimicking gecko foot-hair,” Nature Mater., vol. 2, pp. 461–463, Jun. 1, 2003. [19] N. J. Glassmaker, A. Jagota, C.-Y. Hui, and J. Kim, “Design of biomimetic fibrillar interfaces: 1. making contact,” J. R. Soc., Interface, vol. 1, no. 1, pp. 23–33, Nov. 2004. [20] C. Menon, M. Murphy, and M. Sitti, “Gecko inspired surface climbing robots,” in Proc. IEEE Int. Conf. Robot. Biomimetics, 2004, pp. 431–436. [21] B. Aksak, M. Murphy, and M. Sitti, “Adhesion of biologically inspired vertical and angled polymer microfiber arrays,” Langmuir, vol. 23, no. 6, pp. 3322–3332, 2007. Michael P. Murphy (M’07) received the B.Sc. degree in mechanical engineering from Yale <strong>University</strong>, New Haven, CT, in 2003, and the M.Sc. degree in mechanical engineering from <strong>Carnegie</strong> <strong>Mellon</strong> <strong>University</strong>, Pittsburgh, PA, in 2005, where he is currently working toward the Ph.D. degree. His current research interests include mobile robot systems, biologically inspired miniature robots, and micro/nanoscale smart materials. Mr. Murphy received the Best Biomimetics Paper Award at the 2004 IEEE International Conference on Robotics and Biomimetics.
338 IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 12, NO. 3, JUNE 2007 Metin Sitti (S’94-M’00) received the B.Sc. and M.Sc. degrees in electrical and electronic engineering from Bogazici <strong>University</strong>, Istanbul, Turkey, in 1992 and 1994, respectively, and the Ph.D. degree in electrical engineering from the <strong>University</strong> of Tokyo, Tokyo, Japan, in 1999. He was a Research Engineer in the Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) Robotics Department, TUBITAK Marmara Research Center, Kocaeli, Turkey, during 1994–1996. He was a Research Scientist and a Lecturer in the Department of Electrical Engineering and Computer Sciences, <strong>University</strong> of California, Berkeley, during 1999–2002. He is currently an Assistant Professor in the Mechanical Engineering Department, <strong>Carnegie</strong> <strong>Mellon</strong> <strong>University</strong> (CMU), Pittsburgh, PA, and the Robotics Institute, CMU. His current research interests include micro/nanoscale robotic dynamics, systems, and control, biologically inspired miniature robots, and micro/nanoscale manipulation and manufacturing systems. Dr. Sitti received a National Science Foundation CAREER Award and the CMU Struminger Award in 2005. He was elected a Distinguished Lecturer for the IEEE Robotics and Automation Society for 2006–2008. He received the Best Paper Award at the IEEE/RSJ International Conference on Intelligent Robots and Systems in 1998, the Best Video Award at the IEEE Robotics and Automation Conference in 2002, and the Best Biomimetics Paper Award at the IEEE Robotics and Biomimetics Conference in 2004.
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