A bio-inspired approach to underwater legged robotics

Oceans have a critical importance for Earth's natural and social ecosystems and there is increasing demand for marine and underwater operations for commercial and scientific goals. However, the vastity and the extreme environmental conditions of the Oceans limit the workspace and efficacy of human actions. Robots have the potential to overcome this limitation and research in the field of underwater robotics is thriving. Despite its importance in applications, interaction with the seabed is among the open challenges of traditional underwater robotics which is mostly based on vehicles that swim by means of thrusters. In this work, a bio-inspired approach to underwater legged robotics is proposed to overcome this issue and enhance exploration and operations on the seabed. First, the general principles of the bio-inspired approach followed are described with a particular focus on their application to mobile robotics and more specifically to legged locomotion. Then, such principles are applied to a series of three robotic prototypes of growing complexity to study different aspects of underwater legged locomotion and progressively develop a robot for which the bio-inspired approach brings quantitative and qualitative benefits to be harnessed in field applications. In particular, in underwater legged locomotion the interaction with the fluid is a key aspect, that is experimentally studied through a monopedal robot with a springy leg and a deformable body. Here the contribution of shape dependent forces can be modulated by changing body size and underwater weight to affect stability and performance of hopping locomotion. Results are analysed through the use of a bio-inspired model that accounts for the contributions of underwater environment and legs' compliance. The same model is used as a reference in the design and control of a four legged vehicle, SILVER. In this case, the hopping behavior of the quadruped is predicted with good precision by the single-legged model which, in turn, can be used to tune the timing of the leg actions without any sensory feedback. Other locomotion strategies such as crawling and walking are implemented on SILVER to demonstrate the versatility granted by legs also underwater. The concept of embodying the dynamics of the fundamental single legged model in the construction of a multi-legged vehicle and using it as the basic control layer, culminates in the design and development of a hexapod robot with segmented legs, SILVER2. The novel design of the robot allows an even greater locomotion versatility at the cost of a further level of abstraction to match the linear leg of the bio-inspired model and previous prototypes with the segmented one. Data collected during several sea trials is used to demonstrate the unique capabilities granted by the bio-inspired approach presented, which complement the ones of traditional underwater robots. Finally, SILVER2 is used in field applications that can benefit from the bio-inspired approach to underwater legged robotics presented. Particular attention is devoted to the issue of sunken plastic litter. With this regard, SILVER2 is used to sample sediment to assess the presence of micro-plastics, and to collect macro-plastic with a soft robotic arm.