Biologically inspired robotic fish for dense 3D data collection

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This article was authored by Michael Da Silva and Robin Hall.

Introduction

Life on earth has a tenuous link to each other. Over the years, due to climate change, this harmonious relationship has been tested. We have witnessed firsthand how climate change is rapidly changing the ocean ecosystem. 

One issue caused by climate change is rapid global temperature rise. When scientists look to make climate associations using temperature data, they generally use fixed temperature loggers attached to buoys or on the ocean floor. Unfortunately, this approach discounts the area between the ocean's surface and floor. Variable ocean conditions create microclimates, pockets of the ocean that are unaffected by general climate trends. Scientists have shown that most organisms experience climate change via these microclimates. Fish are greatly affected by this rapid increase in temperature as they can only lay eggs in a minimal range of temperatures. Microclimates are changing temperature with celerity. Hence, many species cannot adapt quickly enough to survive. At this rate, 60% of fish species could go extinct by 2100. 

Of course, fish are not the only organisms affected by the rapid increase in temperature. Coral in the Great Barrier Reef can only survive in a minimal temperature threshold, and as temperature increases, reefs are experiencing mass coral bleaching. AIMS, the Australian Institute for Marine Science, the government agency that monitors the Great Barrier Reef, utilizes divers pulled behind boats to record reef observations and collect data. Unfortunately, this has led to some casualties due to shark attacks. They have begun deploying large, almost seven feet in length, ocean gliders that can mitigate this risk. Unfortunately, these robots come with a hefty price tag of $125,000 to $500,000. They are also too large to navigate portions of the reef.

Biologically inspired robotic fish for collecting three-dimensional temperature data 

Our solution is building a tetherless, biologically inspired robotic fish. This fish can navigate the complex environment of the Great Barrier Reef. In addition, it takes dense three-dimensional temperature data throughout the water column. Moreover, we use a non-hazardous and affordable material for the fish's body. Our approach is in stark contrast to traditional autonomous underwater vehicles that utilize propellers that are noisy and incongruous to underwater life. We chose to mimic the motion of real fish to reduce the environmental impact of our robot and enable close observation of other real fish. 

We are, of course, not the first person to build a robot fish. In 1994, MIT produced the RoboTuna, a fully rigid fish robot, and since then, there have been many different iterations of fish robots. Some have been made of fully rigid materials like the RoboTuna and used motors that run the caudal tail (rear fin) actuation that powers the fish. However, this does not replicate the fluid motion achieved by real fish as they swim. A possible solution would be to use soft materials. Many of these designs utilize a silicone, pneumatically or hydraulically actuated tail. Unfortunately, these robots cannot operate in rough environments since any cuts or abrasions to the silicone could cause a leak in the system and lead to a total failure in the actuation of the tail. Other robots have combined the more durable rigid materials, actuated with cables, and then attached a soft silicone end that bends with the force of the water. 

We have improved this design by creating a 3D printed, cable-actuated wave springtail made from soft materials. The wave springtail gives the robot its biologically inspired shape, but it can bend like the silicone-based robots and real fish fluidly. The wave spring is entirely 3D printed from NinjaFlex, a flexible material that is affordable and easy to use. While this material is very affordable, it is also very durable, withstands harsh treatment, and runs for hundreds of thousands of cycles without any degradation to any of the robot's systems. 

The wave spring itself has a biologically inspired design. Reef fish are morphologically diverse but share a similar body shape which we emulate with a tapered oval design. The wave spring itself is composed of a mesh of diamond-shaped cells that can compress and bend. To restrict our robot to only lateral bending, we added supports down the dorsal and ventral edges of the wave spring. 

Attached to the wave spring are fins that have been laser cut from EPDM rubber. In biological undulatory motion, a traveling wave moves down the length of the fish, and due to the variable stiffness of the fish's body, this wave increases in both amplitude and phase shift. This motion improves the speed and mobility of the fish, and we attempt to replicate that variable stiffness in our robot. The rubber used to make the fins is half as stiff as the NinjaFlex wave springtail, and this decrease in stiffness allows the tails to maintain their shape but deformed more easily under pressure. 

To better understand the nature of this tail, we evaluated how different parameters affected the force output of the tail in the closed environment of a fish tank. We found that specific physical and motor parameters increased the tail's force output, thereby increasing the performance of the fish robot. 

To advance the state-of-the-art, we are using Macro Fiber Composites as actuators as an alternative to the servo motors. Incorporating this smart material would allow us to make the fish tremendously lighter while giving us the opportunity for a vivid representation of a fish's undulatory locomotion pattern. 

We are currently only at the beginning stages of this project. Robin has successfully created a robot that can swim like a fish, but so far, it cannot swim as fast, nor is it as maneuverable. We will look to improve the robot's performance to ensure that it can successfully navigate any ocean environment by investigating the impact of a multi-module tail that will allow the fish to swim more like an eel. Most importantly, we will add the sensors required to collect data like temperature, which is imperative to a better understanding of the oceans' rapidly changing microclimates and the effect on many species, including fish. 

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