Bio-inspired robotic locomotion is a subcategory of bio-inspired design. It is about learning concepts from nature and applying them to the design of real-world engineered systems. More specifically, this field is about making robots that are inspired by biological systems, including Biomimicry. Biomimicry is copying from nature while bio-inspired design is learning from nature and making a mechanism that is simpler and more effective than the system observed in nature. Biomimicry has led to the development of a different branch of robotics called soft robotics. The biological systems have been optimized for specific tasks according to their habitat. However, they are multifunctional and are not designed for only one specific functionality. Bio-inspired robotics is about studying biological systems, and looking for the mechanisms that may solve a problem in the engineering field. The designer should then try to simplify and enhance that mechanism for the specific task of interest. Bio-inspired roboticists are usually interested in biosensors (e.g. eye), bioactuators (e.g. muscle), or biomaterials (e.g. spider silk). Most of the robots have some type of locomotion system. Thus, in this article different modes of animal locomotion and few examples of the corresponding bio-inspired robots are introduced.

Biolocomotion Biolocomotion or animal locomotion is usually categorized as below:

Locomotion on a surface Locomotion on a surface may include terrestrial locomotion and arboreal locomotion. We will specifically discuss about terrestrial locomotion in detail in the next section.

Locomotion in a fluid Locomotion in a blood stream or cell culture media swimming and flying. There are many swimming and flying robots designed and built by roboticists. Some of them use miniaturized motors or conventional MEMS actuators (such as piezoelectric, thermal, magnetic, etc),R. Fearing, S. Avadhanula, D. Campolo, M. Sitti, J. Jan, and R. Wood, "A micromechanical flying insect thorax," Neurotechnology for Biomimetic Robots, pp. 469–480, 2002.G. Dudek, M. Jenkin, C. Prahacs, A. Hogue, J. Sattar, P. Giguere, A. German, H. Liu, S. Saun- derson, A. Ripsman, et al., "A visually guided swimming robot," in IEEE/RSJ International Conference on Intelligent Robots and Systems, IROS, pp. 3604–3609, 2005.A. Alessi, A. Sudano, D. Accoto, E. Guglielmelli, "Development of an autonomous robotic fish," In Biomedical Robotics and Biomechatronics (BioRob), 2012 4th IEEE RAS & EMBS International Conference on (pp. 1032-1037). IEEE. while others use animal muscle cells as motors.

Behavioral classification (terrestrial locomotion)

There are many animal and insects moving on land with or without legs. We will discuss legged and limbless locomotion in this section as well as climbing and jumping. Anchoring the feet is fundamental to locomotion on land. The ability to increase traction is important for slip-free motion on surfaces such as smooth rock faces and ice, and is especially critical for moving uphill. Numerous biological mechanisms exist for providing purchase: claws rely upon friction-based mechanisms; gecko feet upon van der walls forces; and some insect feet upon fluid-mediated adhesive forces.R. M. Alexander, Principles of animal locomotion. Princeton University Press, 2003

Legged locomotion Legged robots may have one,M. H. Raibert, H. B. Brown, "Experiments in balance with a 2D one-legged hopping machine," ASME Journal of Dynamic Systems, Measurement, and Control, pp75-81, 1984.M. Ahmadi and M. Buehler, "Stable control of a simulated one-legged running robot with hip and leg compliance," IEEE Transactions on Robotics and Automation, vol. 13, no. 1, pp. 96– 104, 1997.P. Gregorio, M. Ahmadi, and M. Buehler, "Design, control, and energetics of an electrically actuated legged robot," IEEE Transactions on Systems, Man, and Cybernetics, Part B: Cybernetics, vol. 27, no. 4, pp. 626–634, 1997. two,R. Niiyama, A. Nagakubo, and Y. Kuniyoshi, "Mowgli: A bipedal jumping and landing robot with an artificial musculoskeletal system," in IEEE International Conference on Robotics and Automation, pp. 2546–2551, 2007. four,M. Raibert, K. Blankespoor, G. Nelson, R. Playter, et al., "Bigdog, the rough-terrain quadruped robot," in Proceedings of the 17th World Congress, pp. 10823–10825, 2008. six, or many legsS. Wakimoto, K. Suzumori, T. Kanda, et al., "A bio-mimetic amphibious soft cord robot," Transactions of the Japan Society of Mechanical Engineers Part C, vol. 18, no. 2, pp. 471–477, 2006. depending on the application. One of the main advantages of using legs instead of wheels is moving on uneven environment more effectively. Bipedal, quadrupedal, and hexapedal locomotion are among the most favorite types of legged locomotion in the field of bio-inspired robotics. Rhex, a Reliable Hexapedal robotU. Saranli, M. Buehler, and D. Koditschek, "Rhex: A simple and highly mobile hexapod robot," The International Journal of Robotics Research, vol. 20, no. 7, pp. 616–631, 2001. and CheetahY. Li, B. Li, J. Ruan, and X. Rong, "Research of mammal bionic quadruped robots: A review," in Robotics, IEEE Conference on Automation and Mechatronics, pp. 166–171, 2011. are the two fastest running robots so far. is another hexapedal robot inspired by cockroach locomotion that has been developed at Stanford University.J. Clark, J. Cham, S. Bailey, E. Froehlich, P. Nahata, M. Cutkosky, et al., "Biomimetic design and fabrication of a hexapedal running robot," in Robotics and Automation, 2001. Proceedings 2001 ICRA. IEEE International Conference on, vol. 4, pp. 3643–3649, 2001. This robot can run up to 15 body length per second and can achieve speeds of up to 2.3 m/s. The original version of this robot was pneumatically driven while the new generation uses a single electric motor for locomotion.S. Kim, J. Clark, and M. Cutkosky, "isprawl: Design and tuning for high-speed autonomous open-loop running," The International Journal of Robotics Research, vol. 25, no. 9, pp. 903– 912, 2006.

Limbless locomotion Terrain involving topography over a range of length scales can be challenging for most organisms and biomimetic robots. Such terrain are easily passed over by limbless organisms such as snakes. Several animals and insects including worms, snails, caterpillars, and snakes are capable of limbless locomotion. A review of snake-like robots is presented by Hirose et al.S. Hirose, P. Cave, and C. Goulden, Biologically inspired robots: snake- like locomotors and manipulators, vol. 64. Oxford University Press Oxford, UK, 1993 These robots can be categorized as robots with passive or active wheels, robots with active treads, and undulating robots using vertical waves or linear expansions. Most snake-like robots use wheels, which are high in friction when moving side to side but low in friction when rolling forward (and can be prevented from rolling backward). The majority of snake-like robots use either lateral undulation or rectilinear locomotion and have difficulty climbing vertically. Choset has recently developed a modular robot that can mimic several snake gaits, but it cannot perform concertina motion.R. Hatton and H. Choset, "Generating gaits for snake robots: annealed chain fitting and keyframe wave extraction," Autonomous Robots, vol. 28, no. 3, pp. 271–281, 2010. Researchers at Georgia Tech have recently developed two snake-like robots called Scalybot. The focus of these robots is on the role of snake ventral scales on adjusting the frictional properties in different directions. These robots can actively control their scales to modify their frictional properties and move on a variety of surfaces efficiently.H. Marvi, G. Meyers, G. Russell, D. Hu, "Scalybot: a Snake-inspired Robot with Active Frictional Anisotropy," ASME Dynamic Systems and Control Conference, Arlington, VA, 2011. Researchers at CMU have developed both scaled and conventional actuated snake-like robots.

Climbing Climbing is an especially difficult task because mistakes made by the climber could cause the climber to lose its grip and fall. Most robots have been built around a single functionality observed in their biological counterparts. Geckobots typically use van der waals forces that work only on smooth surfaces.O. Unver, A. Uneri, A. Aydemir, and M. Sitti, "Geckobot: a gecko inspired climbing robot using elastomer adhesives," in International Conference on Robotics and Automation, pp. 2329–2335, 2006. Being inspired from geckos, scientists from Stanford university have artificially created the adhesive property of a gecko. Similar to seta in a gecko's leg, millions of microfibers were placed and attached to a spring. The tip of the microfiber will be sharp and pointed in usual circumstances, but upon actuation, the movement of a spring will create a stress which bends these microfibers and increase their contact area to the surface of a glass or wall. Using the same technology, gecko grippers were invented by NASA scientists for different applications in space. Stickybots use directional dry adhesives that works best on smooth surfaces.S. Kim, M. Spenko, S. Trujillo, B. Heyneman, D. Santos, and M. Cutkosky, "Smooth vertical surface climbing with directional adhesion," IEEE Transactions on Robotics, vol. 24, no. 1, pp. 65–74, 2008.S. Kim, M. Spenko, S. Trujillo, B. Heyneman, V. Mattoli, and M. Cutkosky, "Whole body adhesion: hierarchical, directional and distributed control of adhesive forces for a climbing robot," in IEEE International Conference on Robotics and Automation, pp. 1268–1273, 2007.D. Santos, B. Heyneman, S. Kim, N. Esparza, and M. Cutkosky, "Gecko-inspired climbing behaviors on vertical and overhanging surfaces," in IEEE International Conference on Robotics and Automation, pp. 1125–1131, 2008.A. Asbeck, S. Dastoor, A. Parness, L. Fullerton, N. Esparza, D. Soto, B. Heyneman, and M. Cutkosky, "Climbing rough vertical surfaces with hierarchical directional adhesion," in IEEE International Conference on Robotics and Automation, pp. 2675–2680, 2009.S. Trujillo, B. Heyneman, and M. Cutkosky, "Constrained convergent gait regulation for a climbing robot," in IEEE International Conference on Robotics and Automation, pp. 5243–5249, 2010. The SpinybotA. Asbeck, S. Kim, M. Cutkosky, W. Provancher, M. Lanzetta, "Scaling hard vertical surfaces with compliant microspine arrays," The International Journal of Robotics Research, Vol.25, No. 12, pp. 1165-1179, 2006. and RiSEM. Spenko, G. Haynes, J. Saunders, M. Cutkosky, A. Rizzi, D. Koditschek, et al., "Biologically inspired climbing with a hexapedal robot," Journal of Field Robotics, vol. 25, no. 4-5, pp. 223– 242, 2008. robots are among the insect-like robots that use spines instead. Legged climbing robots have several limitations. They cannot handle large obstacles since they are not flexible and they require a wide space for moving. They usually cannot climb both smooth and rough surfaces or handle vertical to horizontal transitions as well.

Jumping