A group of researchers from the Shenyang Institute of Automation (SIA) of the Chinese Academy of Sciences has presented a soft robotic fish driven by an electrohydraulic system. The development is capable of reaching high speed and performing complex three-dimensional maneuvers. The results were published on October 5 in the journal IEEE/ASME Transactions on Mechatronics.
Soft robotic fish are seen as a promising tool for underwater environmental monitoring and resource exploration. However, traditional designs often sacrifice speed for maneuverability, especially when performing complex movements in three dimensions. The new work offers a solution to this long-standing contradiction.
The engineers drew inspiration from the way real fish coordinate muscles and skeleton, and from the way the swim bladder regulates buoyancy. They combined rigid and flexible elements into a single propulsion system and added active buoyancy control. As a result, they managed to combine a fast thrust with flexible spatial movement.
The key element is an electrohydraulic actuator with two degrees of freedom. This means the device can move in two independent directions. Local rigid constraints reduce unwanted deformation during actuation. Two drive units work alternately, imitating the contractions of antagonist muscles, and create high-frequency oscillations of the tail.
In experiments, the robotic fish showed a maximum speed of 2.13 body lengths per second - about 26 cm/s (10 inches per second). The minimum cost of transport (a measure of energy expenditure for moving a unit mass a unit distance) was 9.4 J/(kg*m). According to the authors, this is a new speed record among soft robotic fish using body and caudal fin undulatory propulsion (BCF propulsion).
To control orientation, the researchers created a biomimetic swim bladder that operates through a liquid-to-gas transition. By regulating local buoyancy, the system creates a pitching moment - a rotational force that tilts the robot forward or backward - and allows the position to be changed from -20 deg to 58 deg.
The combined operation of the electrohydraulic propulsor and dynamic posture adjustment gives the robotic fish the ability to perform horizontal cruising, turns, S-shaped swimming, ascent, descent and spiral ascent. Thus, the device goes beyond purely horizontal movement and performs complex three-dimensional maneuvers.
The authors emphasize that the combination of a rigid-flexible electrohydraulic actuator with active buoyancy control makes it possible to preserve structural compliance and achieve high speed and maneuverability in 3D. This is a potential approach to creating highly efficient underwater robots. At the same time, this is a laboratory prototype, not a finished commercial product.