On May 19, 2022, the leading international journal Science Robotics published an artile, titled “Aerial-aquatic robots capable of crossing the air-water boundary and hitchhiking on surfaces”, reporting the latest progress in aerial-aquatic hitchhiking robots made by Prof. Wen Li’s group from the School of Mechanical Engineering and Automation of Beihang University.
Robots are used to work in highly unstructured environments, such as multiterrain observation, multimedia operation and multi-environment exploration, so there is a wide range of potential needs for robots with the ability to perform rapid cross-medium motion (enlarging monitoring area) and efficient transient perching (extending working time). Compared with the traditional aerial robots, cross-medium hitchhiking robots can work for long periods of time and move objects cross the air-water boundary, which is of great importance in exploring fundamental scientific questions and developing high-performance cross-domain vehicles with potential uses. There are two primary challenges to implementing a robot with the ability to perform aerial-aquatic hitchhiking: 1) it requires a powerful, reversible, adaptable and robust adhesive device that functions both in air and underwater, allowing the robot to hitchhike on various surfaces and generate enough tangential force to resist the impact of high-speed incoming flow; 2) a cable-free robot that can perform a seamless aerial-aquatic transition to carry adhesion devices and working equipment.
By observing the biological remora disc under laboratory conditions, the research group found that a living remora fish can attach to a porous surface with only part of its disc. They revealed the morphological features of the biological remora disc’s redundant adhesion for long periods of time, including independent lamellar compartments and redundant, hydrostatically enhanced adhesion ( Fig. 1). Multiple bionic control studies revealed that the internal symmetrically arranged lamellas of the disc can form independent lamellar compartments for partial adhesion, and the external soft disc lip can form a whole adhesion to achieve redundant adhesion. When the hydrostatic expansion of the lamellae chamber further rotates the lamellae, a large number of spinules on the tip of the lamella can interlock with the adhesion surface to enhance friction to overcome the shear force, but the spinules do not destroy the adhesion of the independent lamellar compartment. While the conventional suction devices are sensitive to leakage and external impact, the redundant and adaptive adhesion of the remora disc in water and air increases the frictional stress and adhesion time by 44% and 458%, respectively. The discovery of this new mechanism is of great significance to realize the long-term "hitchhiking" of bionic robots on various surfaces.
Fig. 1. Morphological features, bionic structure and redundant adhesion of the remora disc
To address the challenge of the seamless aerial-aquatic transition, the robot needs to perform untethered, rapid, and consecutive transition between the air and water and move stably in the two media. The researchers designed simple, low-cost, and foldable morphing propellers, which can self-fold underwater and self-unfold in the air. The different shapes of the foldable propeller in the two media reduce the working speed range of the propeller in the two media and shorten the transitional time of the working speed. In addition, the transitional time of the robot was reduced by 61.1% compared with the commercial propeller, which significantly improves the speed of the robot crossing the water/air interface. On this basis, combined with the biomimetic remora disc and the highly mobile quadrotor robot, the research group developed an aerial-aquatic biomimetic robot ( Fig. 2). The self-adaptive folding propeller can realize a stable, continuous and rapid water-to-air transition (0.35s), taking 2.9 s per transition on average in consecutive air-water transitions.
Fig.2. Cross-medium properties and field applications of the aerial-aquatic hitchhiking robot
The aerial-aquatichitchhiking robot can also resist large external longitudinal and tangential forces, thus enabling the robot to rest on a stationary surface or “hitchhike” on a moving host to extend its working time and enlarge its monitoring area. The robot’s hitchhiking state saves power by a factor of almost 50 times and 19 times compared with remaining in a hovering state. In the wild, the robot can cross medium in the ocean and canyon streams, and can adhere to the moving ship bottoms and slippery rock surfaces for stable observation missions, respectively (see Fig. 2). In addition, such robotic forms may be promising for several open-environment applications, including long-term air and water observations, cross-medium operations, submerged structure inspections, marine life surveys, and iceberg detections ( Fig. 3).
Fig. 3. A depiction of the mission profile and design of an aerial-aquatic hitchhiking robot
In this research, Li Lei, a 2018 doctoral candidate from the School of Mechanical Engineering and Automation of Beihang University, is the first author. Wang Siqi, Zhang Yiyuan, and Song Shanyuan are the co-first authors, and Prof. Wen Li is the only corresponding author. This work was supported by the National Natural Science Foundation of China, the National Key R&D Program of China, etc.
Link to the article: https://www.science.org/doi/10.1126/scirobotics.abm6695
Link to download the article: https://www.science.org/stoken/author-tokens/ST-498/full
Reported by Song Wanheng
Reviewed by Zhang Zhigang
Edited by Jia Aiping
Translated by Liang Xiaochun
