A robotic trout not only swims silently through the water at speeds similar to its natural counterpart, it can also harvest enough energy to power sensors. The work of researchers in the United States could enable robots to infiltrate schools of fish in order to study them, monitor marine pollution, or even act as marine spies.

Swimming robots are nothing new, but they often require bulky and loud motors with complex mechanics. Newer robotic fish have been made from soft, intelligent materials, but they can swim slowly or depend on changes in temperature.

A picture that shows a small tank filled with water from above and in it two of the robot trout

Alper Erturk of the Georgia Institute of Technology believes that piezoelectric materials are the answer to overcoming these limitations. His group made the first piezoelectric robotic fish without an appendage in 2013. Now, Erturk’s team has built on this work by making a robot the size and shape of a rainbow trout and reaching speeds comparable to reality.

Piezoelectric materials are made up of flexible fibers that expand and contract like muscles in response to electricity. But they can also generate electricity if the fibers are set into vibration by an external force such as wind or flowing water. The material has already been used to build flying and land-based robots – but this is the first underwater prototype.

Erturk’s robot fish has a silicone tail fin sandwiched between two layers of piezoelectric material. This material contains lead zirconate titanate fibers separated between layers of flexible epoxy resin. This is overlaid with waterproof electrodes embedded in a polyimide film.

A photo of the robot fish with its black rubber body half-open in the middle to reveal the electronic components inside.  Below is a drawing of the same fish with a transparent outer shell to show the internal components

The tail fin is removable and makes up a third of the robot’s 30.5 cm length – similar to that of an adult trout. A hollow body and nose, 3D printed from polylactic acid, hold electronics, batteries and energy-collecting components. A bluetooth receiver makes it possible to control the vibrations of the fin via a phone app.

Experiments in the water showed that the robot could achieve a thrust comparable to that of a rainbow trout and reached a swimming speed of almost one body length per second. When the robot is tethered, water turbulence can cause the tail fin to vibrate, allowing it to gain energy – but not enough to propel its own movement.

“It is feasible to supply it with electricity yourself, but it would take a long time to generate the flow energy and store it in a battery,” says Erturk. “It would take many hours of harvest time to swim for minutes.” However, the recovered energy could be used for wireless sensors on board to monitor underwater environments. By using piezoelectric actuators, the concept could be scaled to robots up to millimeter-sized.

‘The [researchers] show exciting results with a piezo-based actuator that is applied to a robot fish, “says Robert Katzschmann, who develops soft robots at the Swiss Federal Institute of Technology in Zurich.” I would be interested to know whether the robot can also generate energy when it is used unbound in running water The piezo-based actuator design enables reversible energy conversions and could also enable simultaneous energy harvesting and noise attenuation in industrial environments. ‘

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