1 A Novel Water Running Robot Inspired by Basilisk Lizards Steven Floyd, Terence Keegan, John Palmisano, and Metin Sitti NanoRobotics Laboratory, Department of Mechanical Engineering, Carnegie Mellon University, PA 15213, USA Abstract— This paper introduces a novel robot which can run on the surface of water in a manner similar to basilisk lizards. Previous studies on the lizards themselves have characterized their method of propulsion and their means of staying afloat. By slapping and stroking their feet into the water, the lizard effects a momentum transfer which provides both forward thrust and lift. The design of a biomimetic robot utilizing similar principles is discussed, modeled, and prototyped. Functionally, the robot uses a pair of identical four bar mechanisms, with a 180 o phase shift to achieve bipedal locomotion on the water’s surface. Computational and experimental results are presented and reviewed with the focus being a maximization of the lift to power ratio. After optimization, two legged models can experimentally provide 12-15 g/W of lift while four legged models can provide 50 g/W of lift. This work opens the door for bipedal and quadrupedal robots to become ambulatory over both land and water, and represents a first step toward studies in amphibious stride patterns; step motions equally conducive to propulsion on water and land. Index Terms— Biomimetics, legged robots, basilisk lizard, walk- ing on water. I. I NTRODUCTION Small, lightweight animals have a large variety of floatation mechanisms open to them. There are spiders and insects which float using surface tension, and propel themselves using menis- cus in the water and marangoni flows. Larger animals have fewer options. Lizards, aquatic birds, and marine mammals, with their larger bulk and higher mass, utilize buoyancy, viscous drag and momentum transfer [1]. The basilisk lizard (Basiliscus sp.) is capable of running across the surface of water at approximately 1.5 m/s, and a stepping rate of 5-10 Hz (per leg). Four factors influence the lizard’s ability to stay afloat: a) body mass, b) characteristic length, c) running speed, and d) shape of the foot. All of these variables are inter-related, and the morphological relations to the lizard’s water running have been characterized in [2]–[4], [6], [7]. Biomimetic robots are those machines which emulate some aspect of a living system. In this case, the ability to run over water is what our robot attempts to duplicate. This robot em- ploys momentum transfer for both lift and propulsion, instead of surface tension, which other water walking robots employ [8], [9]. The goal is not to copy nature, but to understand the principles of operation, and use or improve on them for use in our own creations. The knowledge gained by this work will help expand the limits of legged robot locomotion. A legged robot capable of walking across land and water quite literally has the entire world open to it. Further work in this field can lead to completely amphibious bipedal or quadrupedal motion. Appli- cations include exploration and search and rescue in partially flooded or marsh-like environments, and of remote controlled toy models which can run anywhere. This work can also help increase the understanding of the basilisk lizard and its ability to walk on both land and water. In this paper, we use the work of others to develop an understanding of basilisk water running. We then adapt this knowledge to a general four bar mechanism interacting with water to create a computer model with real, predictive value. We first emulate, and then optimize the stepping path of a basilisk lizard. To establish our model’s validity, we built several prototypes, and measured their ability to lift weight out of the water. Lastly, we found ways to improve on nature, and provide our devices with more lifting ability with lower power expenditure. II. LIZARD WATER RUNNING A basilisk’s water running stride can be roughly divided into four phases: slap, stroke, recovery up and recovery down [4]. The forces experienced by the leg and foot are different in each phase, and have differing effects on the lizard’s ability to stay afloat. These phases are shown in Fig. 1. Surface tension effects on the ability to run on the water’s surface are negligible. A. Slap Phase During each step on the water, an initial slap at the interface pushes up on the basilisk’s foot. For younger, lighter lizards, this slap force can provide all of the lift necessary to stay afloat. The lizard’s ability to generate excess lift during the slap phase of the stroke declines as the mass of the lizard increases [3]. The slap phase begins when the foot first contacts the water. Foot motion is primarily downward, and the magnitude of the upward force is much greater than anywhere else in the step. From [3], the maximum slap impulse (I max slap ) is a function of the effective radius of the foot (r eff ) and the peak velocity during the slap (u peak ): I max slap = 4 3 r 3 eff u peak (1) Proceedings of the IEEE/RSJ Intelligent Robotic Systems Conference, Beijing, China, Oct. 2006.