Water Strider Inspired At-scale Water Jumping Robot

Abstract

Water striders shows remarkable movement on the water surface. They can walk and slide freely, and sometimes jump and leap out of the water surfaces. These extreme locomotion inspires the robotic researchers to break the mobility limitation of the robots by developing the robot that is capable of walking and jumping on water. To build the robotic water striders, the hydrodynamics acting on the driving legs with the water surface is analyzed theoretically to abstract the principles and criteria for the robot design. We found that the curvature force of the water surface is the largest and dominant hydrodynamic force in jumping on water of the water striders. The design criteria is established for the robot to obtain the maximum curvature forces from the water surface and not to break the water surface which causes reducing the reaction force and the momentum transfer to the robotic water striders significantly. The criteria is that the reaction force per submerged leg length should be below the maximum water surface curvature force generated by the surface tension of water. The dimensions of the water striders is around of 2~3 cm in body length, 3~5 cm in leg length and weight of 50mg. It is much smaller and lighter than the conventional robotic systems and machines. The robotic water striders weigh about 1/6 of a single 3mm nut which is basic element of conventional machines. The smart composite microstructures (SCM) process is employed to build the small scale robotic structures. The precision laser machining and laminating process with fiber reinforced composite materials enable to fabricate millimeter scale articulated robotic structures with minimum friction loss in working. The shape memory alloy (SMA) actuator is embedded on the robot as an artificial muscle of the robot. To satisfy the design criteria with maximum momentum transfer, the flea inspired catapult mechanism, called the torque reversal catapult, is employed to design the controllable jumping mechanism. The unique force profile of the torque reversal catapult mechanism enables to maximize the momentum transfer with the low reaction force. Moreover, the robot structure is simplified dramatically by applying the passive triggering components. Finally, the robotic water strider prototypes are built with 2 cm in body length, 3~5cm in leg length and around 55 mg in weight. The robots can be designed with various design parameters to change the jumping power of the robots to meet the design criteria. The results show that the robotic water strider jumps and leaps out of the water surface smoothly with the takeoff velocity of 1.5 m/s. If the robotic water strider satisfies the design criteria, the takeoff velocity on water and ground are almost same because the water surface generates enough forces that can endure the driving force of the legs. Water-ground velocity ratio of the experimental results show how efficient the robot can jump on water compared with jumping on the ground. When the reaction force per submerged length exceed the maximum curvature force of the water surface, the takeoff velocity on water decrease rapidly compared with the jumping on the ground. By developing the robotic water strider, the robot itself would be used as an environment surveillance robots. In addition, the enabling technologies expand the scale limitation in the wide spectrum of the robotics.