Surface Tension Driven Water Strider Robot using Circular Footpads Onur Ozcan * , Han Wang * , Jonathan D. Taylor, and Metin Sitti Carnegie Mellon University, Department of Mechanical Engineering, Pittsburgh, PA, USA Abstract—Water strider insects have attracted many re- searchers’ attention with their power efficient and agile water surface locomotion. This study proposes a new water strider insect inspired robot, called STRIDE II, which uses new circular footpads for high lift, stability, payload capability, and a new elliptical leg rotation mechanism for more efficient water surface propulsion. The lift, drag and propulsion forces and the energy efficiency of this robot are modeled and experiments are conducted to verify these models. A maximum lift capacity of 53 grams is achieved with a total of 12 footpads, each 4.2 cm in diameter for a robot weighing 21.75 grams. For this robot, a propulsion efficiency of 22.3% is measured. Maximum forward and turning speeds of the robot are measured as 71.5 mm/sec and 0.21 rad/sec, respectively. These water strider robots could be used in water surface monitoring, cleaning, and analysis in lakes, dams, rivers and sea. Index Terms— Biologically Inspired Robots, Mobile Robotics, Surface Tension, Miniature Robots. I. I NTRODUCTION Researchers have recently focused on the surface-tension- driven locomotion of water-walking arthropods such as water striders and fisher spiders [1], [2], [3], [4], [5], [6]. The theory behind their lift, propulsion and drag mechanisms have been revealed and enabled the development of vari- ous robotic counterparts of these water-walking arthropods. Being inspired by these insects, there have been several studies to design and manufacture bio-inspired legged robots to achieve power efficient, fast, silent, and stable legged locomotion on deep or very shallow water surfaces. Hu et al. [3] proposed a mechanical water strider powered by an elastic thread. Suhr et al. [7] developed a controllable water strider robot utilizing three piezoelectric unimorph actuators. Song et al. [8], [9] studied the numerical modeling of the supporting legs by respectively developing a rigid-leg model and a compliant-leg model, and built a non-tethered water strider robot with two miniature DC motors and a lithium polymer battery. Suzuki et al. [10] showed two water strider robots with hydrophobic microstructures on the surface of the supporting legs respectively driven by a vibration motor and a slider-crank mechanism. Shin et al. [11] developed a water jumping robot that is able to achieve a vertical jumping motion on the water surface with a latch mechanism driven by a shape memory alloy actuator. In this work, to achieve an efficient and fast legged propul- sion, a new improved water strider robot called STRIDE II using a DC motor actuated four-bar elliptical leg rotation mechanism for water propulsion is proposed. This robot has concentric circular footpads that are designed, analyzed, and * These authors contributed equally to this work. manufactured using laser cutting to generate more lift force per unit area and greater stability when compared to STRIDE [9]. Moreover, the drag force model of the supporting structure and the propulsion mechanism are investigated and explained in detail. Finally, the robustness and the payload capacity are improved with the new design while keeping the features like silent operation, little subsurface disturbance, and maneuvering capabilities in both deep and shallow water of the older version, STRIDE [9]. II. PROBLEM STATEMENT Water strider insect locomotion exemplifies robust and efficient water surface walking because of the lift force mechanism, low drag force on supporting legs, and the elliptical trajectory of the propelling legs. Therefore, these three features should be captured in the design of a water strider inspired robot. The lift force mechanism that a water strider insect dom- inantly uses is the surface tension force of water that is linearly proportional to the length of the supporting legs. Since the weight of the insect scales with its volume, if its size is smaller, the surface tension force is used as the lift force mechanism instead of buoyancy. To mimic the water strider insect, the robot should use surface tension as the dominant part of the lift force; therefore, the robot should have a relatively low weight and small size but long legs to support itself on water. The water strider robot should also have enough payload capacity to carry on-board electronics, power supply, actuators, and sensors for control, autonomous locomotion, and potential future applications like monitoring water quality. On the other hand, for a robot to have a high payload capacity using surface tension, the required leg lengths might be unrealistically long. Therefore, the supporting structures are designed as concentric circular footpads, which increase the total length subjected to lift force while keeping the total area of the supporting structures relatively small. The lift force mechanism and the results are explained in detail in section III. The drag forces that a water strider insect experiences are relatively low at the supporting legs, enabling them to move rapidly and efficiently on the water’s surface. This is due to the lift force mechanism of a water strider, which does not require the insect to break the water surface to stay afloat. Therefore, in order to claim that the designed robot is efficient for water surface locomotion, the drag force model for the robot should be established, which is explained in detail in section IV. 2010 IEEE International Conference on Robotics and Automation Anchorage Convention District May 3-8, 2010, Anchorage, Alaska, USA 978-1-4244-5040-4/10/$26.00 ©2010 IEEE 3799