- The Salamandra robotica I and Salamandra robotica II model the system of neural networks that guides both swimming and walking movements in live salamanders.
- The Pleurobot takes bio-inspired engineering a step further by modeling salamander skeletal kinematics.
- Amphibious biobots can be used for a variety of applications, including environmental monitoring animal behavior studies, and search-and- rescue missions.
Undulating at the surface, the Salamandra robotica cruises through the water until it reaches the shore, then ambles forward through the sand with a lumbering gait on its four paddle-like, rotating legs. Unlike its amphibian muse, this robot salamander has no brain to dictate its motion, just a simple central pattern generator that controls its movement and a spinal cord that runs along its modular body. Built to test the limits of engineering through bio-inspired technology, the Salamandra robotica models the neural system that guides locomotion in live salamanders.
A team of researchers at Switzerland’s École Polytechnique Fédérale de Lausanne (EPFL) modeled the neural networks of a salamander’s spinal cord in order to better understand how animals that use both swimming and walking gaits move.
A salamander’s brain stimulates the animal’s neurobiological networks and creates ‘traveling’ or ‘standing’ waves that generate the movement of its body. ‘Traveling waves’ move along the body, from front to back, and are used to propel the salamander’s swimming body forward. ‘Standing waves’, on the other hand, involve fixed points along the salamander’s body, so that the animal moves its upper and lower body in opposite directions.
“When an animal walks, it actually does this gait in synchrony with the legs,” explains Dr. Alessandro Crespi, a researcher with the EPFL Biorobotics Laboratory. “This is very important, because if the synchronization between the body and the legs is [poor], then the movement is extremely ineffective. That’s actually a point that we tested with the robots, because you cannot really do this kind of testing in the animals. In robots, you can, for example, change the coordination between the legs and the body, and then you can figure out…the effect of changing the coordination on the actual speed of walking [for] the robot.”
Building the models
The researchers simulated locomotion through the biobots in order to study processes in completely controlled and simple subjects.
According to Dr. Crespi, “…it’s extremely difficult to simulate [aspects] like hydrodynamics and contact forces with the ground with friction. It’s actually much easier to create an embodiment like a robot that contains our model and uses the real environment compared to a computation of simulations.”
The models, Salamandra robotica I and Salamandra robotica II, don’t take into account the biological complexities of a real animal. Crespi explained that their team “made the hardware model as simple as possible” so that they could isolate and simulate solely the process of locomotion. For example, the differences in vertebra length and weight distribution of the body that scientists would find studying a live salamander were not factors that were included in the creation of the Salamandra robots.
Crespi explained, “The modeling was done in collaboration with neurobiologists from the University of Bordeaux in France, who are studying real salamanders. So we didn’t figure out the structure using the robots, but we used the robots to test that the structure we figured out was explaining the model…we wanted to verify if [the structure] can actually make our robots move in the real environment and not just a simulated model on a PC screen.”
The next level of the salamander models, the Pleurobot, more closely resembles the physical structure of a salamander. This model enables the researchers to study the animals’ skeletal kinematics, or how the structural elements of the body guide an animal’s motion capabilities. Using the Pleurobot model, the EPFL team could analyze how the salamander’s body structure aids its swimming and walking movements. Both the Pleurobot and the Salamandra robotica I and II were modeled by studying the movements of live salamanders under an X-ray machine.
“We [performed] a lot of manual tracking [of points along the body, shown in the video below] to extract all the [points of the] vertebrae from the cineradiography [X-ray recordings], so that we could see exactly how every vertebra is moving inside the animal while the animal is walking and swimming.”
Video courtesy of the Biorobotics Laboratory, EPFL
In order for the Salamandra robotica models to switch between walking and swimming gaits, the EPFL team programmed the robot’s central pattern generator. This algorithmic model controls the movements of the spinal cord to generate different gaits based on the level of stimulation sensed by the model’s leg oscillators. A lower stimulation level initiates the walking gait, whereas a higher stimulation generates a swimming gait. But since the robots do not have a brain to operate changes in gait, they must externally sense the presence of water. The researchers used an external water leakage sensor that senses if the biobot is partially or fully immersed in water and where the biobot is making contact with the water. A programmed algorithm in the central pattern generator makes the switch between gaits. The switch for the Pleurobot must be manually implemented with a remote controller.
Also, the Salamandra robotica models are waterproof, whereas the Pleurobot is not, so it wears a specially-designed neoprene casing that protects it while allowing it to swim freely.
Biobots for conservation
The Salamandra robotica models and the Pleurobot can all be adapted for use in conservation research and technology.
A variant of the EPFL team’s Pleurobot model, the Crocodile-bot, was created to mimic crocodilian movements. Crocodile-bot used the hardware structure of the Pleurobot to star in the BBC documentary series Spy in the Wild. The Pleurobot’s legs, unlike those of the Salamandra robotica models, move freely. Their lifelike movements were adapted for the Crocodile-bot. The hardware was built at the EPFL and was housed in an external envelope created to look like a real-life crocodile and built especially for the documentary.
Researchers from the EPFL, in collaboration with the BBC, developed crocodile and monitor lizard robots for the documentary series Spy in the Wild. Video courtesy of the Biorobotics Laboratory, EPFL
Yet another model—the Envirobot— will be used for environmental monitoring, such as to test pollution levels in lakes, during which it can swim freely, collect data, and return to the surface. The biobot is equipped with a GPS receiver and long-range communication capabilities. It was modeled closely after the Amphibot, a legless version of the Salamandra robotica I and II.
Such an application is fitting: salamanders themselves are highly sensitive to their environments, and the presence of species such as the southern torrent salamanders can signal a healthy ecosystem.