Robotic tuna uses variable-stiffness tail for more efficient swimming
Given that they're such naturally proficient swimmers, the physical structure of fish is increasingly being copied in the design of underwater robots. Scientists have now discovered that by adjusting the stiffness of their tails, those bots can swim much more efficiently.
In real fish, the tail muscles can be stiffened up for optimum high-speed sprinting, or loosened off for better low-speed cruising and maneuverability. Fish-inspired robots, however, have to compromise – their tails are set to one stiffness which isn't ideal in all situations.
"Having one tail stiffness is like having one gear ratio on a bike," says the University of Virginia's Prof. Dan Quinn. "You’d only be efficient at one speed. It would be like biking through San Francisco with a fixed-gear bike; you’d be exhausted after just a few blocks."
Unfortunately, it's very difficult to determine when and if fish do actually change their tail stiffness. Working with postdoctoral researcher Qiang Zhong, Quinn turned to fluid dynamics and biomechanics to derive a theoretical model. In a nutshell, the model stated that tail stiffness should increase with swimming speed squared.
In order to put their theory to a real-world test, the scientists built a robotic tuna known as AutoTuna. Based on the tail-stiffness model, the device utilizes a programmable tendon to automatically vary the stiffness of its tail as it swims in a lab-based water channel. Remarkably, it can swim over a wider range of speeds than an otherwise-identical fixed-tail-stiffness robot, while using almost half as much energy.
The researchers are now investigating how the technology could be applied to robots based on other types of swimming animals.
"Stiffness-tuning mechanisms like ours can be miniaturized pretty easily, so they could support robots of various sizes and shapes," Quinn tells us. "The harder part is to figure out how stiff the robot should be at various swimming frequencies and speeds. We used a physical model and water channel tests to develop a control law for our robot to use as it tuned its tail stiffness automatically. That model would need to be recalibrated if you made the robot much bigger (e.g. a dolphin-like robot) or switched to a different swimming type (e.g. a stingray-like robot), but that’s totally doable."
A paper on the research was recently published in the journal Science Robotics.