Robotic fish enable better understanding of the mechanics of underwater flow

These lateral line sensing fish robots represent a joint effort of experts in fish biology (University of Bath, UK), underwater robotics (Tallinn University of Technology, Estonia), mechanical engineering (Riga Technical University, Latvia), signal analysis and flow perception (Verona University, Italy) and of sensor technology (Italian Institute of Technology). All fish have an organ capable of sensing in this way, but so far this has had no technological counterpart on man-made underwater vehicles. However, in an article published in 'Proceedings of the Royal Society A', FILOSE team members describe a robotic fish that is controlled with the help of lateral line sensors. Over the last four years, the FILOSE collaboration has investigated fish lateral line sensing and locomotion with the aims of understanding how fish detect and exploit flow features, and of developing efficient underwater robots based on biological principles. Though flow is a highly volatile state of matter, it can be measured and characterised based on salient features that do not change much such as flow direction or turbulence intensity, for example. These salient features can then be described as a "flowscape" – a flow landscape that helps fish and robots to orient themselves, navigate and control their movements. "So far, flow in robotics is treated as a disturbance that drives the robots away from their planned course", says Prof. Maarja Kruusmaa, the scientific coordinator of the FILOSE project. "We have shown that flow is also a source of information that can be exploited to better control the vehicle. Also, flow can be a source of energy if we can understand the flow dynamics and interact with eddies and currents in a clever way". Experiments with flow sensing and actuation by FILOSE have demonstrated that a fish robot can save energy by finding energetically favourable regions in the flow where the currents are weaker or by interacting with eddies so that they help to push the robot forward. The robots are also able to detect flow direction and swim upstream or hold station in the flow while compensating for the downstream drift by measuring the flow speed. A FILOSE robot hovering in the wake of an object in the flow was demonstrated as a way of reducing energy consumption. "It is similar to reducing your effort in the tailwind of another cyclist or reducing fuel consumption by driving behind a truck", says Prof. Kruusmaa. Several prototype artificial lateral lines and robot actuators were developed in FILOSE to experimentally investigate different aspects of sensing and locomotion in fluids, such as how to use compliant materials to swim in turbulence; how to build robots that are mechanically simple but still behave like fish, how to interpret flow features and use them for controlling the vehicles, and how to measure robot's own motion from the flow signals. The FILOSE project has contributed to understanding of the "fish-centric" viewpoint of the aquatic environment. "Robotic experiments have also helped us to understand fish behaviour," says FILOSE collaborator Prof. William Megill, who led the University of Bath's contribution to the project. "By recording flow sensor data from a robotic fish head which we've programmed to move like a real fish in similar flow conditions, we are able to understand what fish can perceive."