Water skating robots take off?

Abstract

An insect-inspired microbot that uses the motion of its wings to skate across the surface of water at high speed is presented in work from China aimed at creating microbots that can also fly. The water strider-inspired microbot's hydrophobic legs allow it to stand on the surface of the water thanks to surface tension. Bioinspired engineering seeks to create technology based on ideas taken from the amazing and varied biology we see around us everyday. Insects are a rich source of that inspiration, with their wide range of specialised abilities, including the ability to walk on water using surface tension displayed by gerridae such as the water strider. A group of researchers at Shanghai Jiao Tong University have been working on robots that can copy this feat, because, as team member Prof. Weiping Zhang explained, “Water striders can support themselves stably and move rapidly on the water. We feel that this is a very low energy consumption mechanism, especially in insect-scale.” In this issue of Electronics Letters, Prof. Zhang and his colleagues present a micro-bionic robot that can move quickly on the water, using flapping-wings and skating-legs, and is, they say, the lightest and one of the fastest water skating robots ever developed. Weighing just 165mg, their robot can skate across the surface of water at 151mm/s but Prof. Zhang tells us that these achievements are only a part of the significance of this work, “First, the study develops an efficient movement for aquatic robots, which can significantly improve the robot's working duration and cruising radius. Second, this study successfully combines the large thrust generation mechanism of flapping-wings flight and the low resistance movement form of surface tension skating to lay the technical foundation for further development of flight-skating amphibious robots.” This study was also the world's first attempt to combine these two forms of motion, and achieved a certain degree of movement performance. Previous studies have focused on actively driving a robot's legs to achieve water skating. The leg-driving approach produces motion through direct interaction with the water surface, using a rowing-like motion to push the robot forward. Unfortunately, at higher driving frequencies it is easy to break the water surface with this skating method, so it is difficult to increase the skating speed. The Shanghai team have avoided this problem by decoupling the drive mechanism from their robot's legs. Instead of driving the legs directly they use the flapping mechanism of the robot's flexible wings as the drive mechanism; producing aerodynamic force to move the robot across the water's surface. This increases stability and allows a higher skating speed, it also improves manoeuvrability. All of which can be controlled by adjusting the frequency of flapping motion. The motion of the robot's wings is produced by lead zirconate titanate (PZT) piezoelectric actuators; a method already used in flying robot designs. The wings themselves are made from polyester film with carbon fibre ‘veins’. The microbot stands on the water surface thanks to titanium alloy legs with a hydrophobic coating. The flapping wing laser displacement test equipment in the lab at Shanghai Jiao Tong University. Inset: The robot's polyester film wings incorporate carbon fibre 'veins'. These wings, rather than the legs, are used to drive the robot's water skating. The exact details of these tiny components in combination are key to the success of the work. “The major challenges we faced were the design of the wings and legs,” says Zhang. “Aerodynamic elastic deformation of flexible wings occurs when they flap at a high frequency, while large deflection elastic deformation of the titanium alloy legs treated with hydrophobic coating also happens when it undertakes a load during the movement of robot. We must ensure the stability of the forces produced by these two kinds of deformation, through the bionic design of veins, symmetrical distribution and horizontal assembly of wings, symmetrical PZT bimorph actuator design, periodic alternating signal and centre of gravity design.” The Shangai team are now working to further optimise the structural design of their microbot and develop their multi-DOF control technology; with the ultimate aim of producing an amphibious robot that can fly and skate with multiple degrees of freedom. The next step in the work is to further study the aerodynamic control mechanism and attempt to change the amplitude and direction of the aerodynamic forces produced in order to achieve transition between flight and water-skating. In terms of using such robots they foresee applications in aquatic detection, signal collection, military reconnaissance, marine meteorology and water pollution monitoring. “With such low-cost, low-power, fast-moving microbots as a platform, a mobile sensor network can be built up, effective distributed operations can be achieved as well.”