Beyond biomimicry: Scientists find better-than-nature run style for six-legged robots

Researchers have found a running style for six-legged robots that significantly improves on the traditional nature-inspired method of movement.

The research, conducted by scientists at the École Polytechnique Fédérale de Lausanne (EPFL) and the University of Lausanne (UNIL) in Switzerland, found that as long as the robots are not equipped with insect-like adhesive pads, it is faster for them to move with only two legs on the ground at any given time.

Robotics has in the past few years made heavy use of biomimicry – the practice of mimicking natural systems – resulting in six-legged robots being designed to move like insects. In nature, insects use what is known as a tripod gait, where they have three legs on the ground at a time, so it had been assumed that this was the most efficient way for similarly legged robots to move.

However, by undertaking a series of computer simulations, tests on robots and experiments on Drosophila melanogaster – better known as the common fruit fly – the scientists found that the two-legged approach, which they have dubbed the bipod gait, results in faster and more efficient movement.

The core goal of the research, which is published today in the journal Nature Communications, was to confirm whether the long-held assumption that a tripod gait was best was indeed correct.

“We wanted to determine why insects use a tripod gait and identify whether it is, indeed, the fastest way for six-legged animals and robots to walk,” said Pavan Ramdya, study co-lead and corresponding author.

Initially, this involved the use of a simulated insect model based on the common fruit fly and an algorithm designed to mimic different evolutionary stages. This algorithm simulated different potential gaits to create a shortlist of those that it deemed to be the fastest.

This, however, shed light on why insects have a tripod gait – and why it may not be the best option for robots. The simulations showed that the traditional tripod gait works in combination with the adhesive pad found on the ends of insects’ legs to make climbing over vertical surfaces such as rocks easier and quicker.

Robots, however, are typically designed to walk along flat surfaces, and so the benefits of such a gait are lost.

“Our findings support the idea that insects use a tripod gait to most effectively walk on surfaces in three dimensions, and because their legs have adhesive properties. This confirms a long-standing biological hypothesis,” said Ramdya. “Ground robots should therefore break free from only using the tripod gait”.

Study co-lead authors Robin Thandiackal (left) and Pavan Ramdya with the six-legged robot used in the research. Images courtesy of EPFL/Alain Herzog

To for always corroborate the simulation’s findings, the researchers built a six-legged robot that could move either with a bipod or tripod gait, and which quickly confirmed the research by being faster when moving with just two legs on the ground at once.

However, they went further by confirming that the adhesive pads were in fact playing a role in the insect’s tripod movement.

They did this by equipping the fruit flies with tiny polymer boots that would cover the adhesive pads, and so remove their role in the way the insects moved. The flies’ responses confirms their theory: they began moving with a bipod-like gate rather than their conventional tripod-style movement.

“This result shows that, unlike most robots, animals can adapt to find new ways of walking under new circumstances,” said study co-lead author Robin Thandiackal.

As bizarre as the research sounds, it provides valuable new insights both for roboticists and biologists, and could lead to a new standard in the way that six legged robots are designed to move.

“There is a natural dialogue between robotics and biology: Many robot designers are inspired by nature and biologists can use robots to better understand the behavior of animal species,” added Thandiackal. “We believe that our work represents an important contribution to the study of animal and robotic locomotion.”

Transparent hydrogel robot ensnares fish with giant claw

Engineers at MIT have built transparent, gel-based robots that move when water is pumped in and out of them. The robots are able to perform a variety of tasks, including kicking a ball underwater and catching and releasing a live fish.

Made entirely from hydrogel, a tough, rubbery, nearly transparent material that’s composed mostly of water, the robots consist of an assemblage of hollow hydrogel structures. These structures are precisely designed and connected to rubbery tubes that, when researchers pump water through them, inflate the structures in orientations that enable the robots to curl up or stretch out.

“Hydrogels are soft, wet, biocompatible, and can form more friendly interfaces with human organs,” said group leader Xuanhe Zhao, associate professor of mechanical engineering and civil and environmental engineering at MIT.

“We are actively collaborating with medical groups to translate this system into soft manipulators such as hydrogel ‘hands,’ which could potentially apply more gentle manipulations to tissues and organs in surgical operations.”

The robots were developed in part to mimic glass eels, focusing on these small creatures’ combination of force, speed and camouflaging transparency. In order to replicate these elements, researchers used 3D printing and laser cutting techniques to print their hydrogel recipes into robotic structures and other hollow units, which they bonded to small, rubbery tubes that are connected to external pumps.

The robots’ endurance was tested across a variety of designs, with the team finding that the hydrogel structures were able to withstand repeated use of up to 1,000 cycles without rupturing or tearing. More interestingly, however, they were able to achieve their glass eel mimicry as, placed underwater against coloured backgrounds, each design was found to make the robot almost entirely camouflaged.

Moreover, unlike other materials commonly used in soft robotics, the robots’ acoustic and optical properties were found to be near equal to that of water. To demonstrate these properties, the team built a hand-like robotic gripper that was able to open and close when water was pumped through. Placed in a tank with a goldfish, the gripper was strong and fast enough to catch the fish as it swam past.

“[The robot] is almost transparent, very hard to see,” Zhao said. “When you release the fish, it’s quite happy because [the robot] is soft and doesn’t damage the fish. Imagine a hard robotic hand would probably squash the fish.”

Images and video courtesy of Melanie Gonick/MIT

The demonstration proves the capability of the robots to perform pressure-related tasks at speed and opens up the possibility of developing the soft robotics into new fields. Moving forward, the team plans to identify and tailor the robots to specific applications, such as surgery.

“We want to pinpoint a realistic application and optimize the material to achieve something impactful,” graduate student Hyunwoo Yuk said.

“To our best knowledge, this is the first demonstration of hydrogel pressure-based acutuation. We are now tossing this concept out as an open question, to say, ‘Let’s play with this. ‘”