The crash-landing robot takes inspiration from the gecko

Insights from the hard landings of tree-climbing geckos lead to better and more controlled perching in robotic aerial vehicles.

Agile, flying robots are already playing an important role in many fields and applications, including data collection, search and rescue, crop monitoring, and forest fire management. However, even state-of-the-art drones have limited ability to land on uncertain terrain, such as near buildings, trees or poles.

“Fast landing on vertical surfaces is one of the biggest challenges in aerial robotics,” said Ordian Jusoufy, MPI for Intelligent Systems and Max Planck Group Leader at the Swiss Federal Labs for Materials Science Technology. “Imitating this move will expand their application space, such as in post-earthquake rubble areas, or to assist fire-fighters among other search and rescue scenarios.”

Current robots rely on rotors or ailerons to slow down and realign themselves before landing, says Jusufi. “Landing on the wall requires integrating multiple sensor streams to control the aerodynamic forces and bring the robot into the desired pitch-up body orientation for a dedicated perching maneuver,” he explained. “The process of integrating multiple sensor streams is computationally expensive, resulting in slow response times to disturbances from the environment.”

It was during a field trip to Singapore’s rainforest that Jusufi found the Asian flat-tailed gecko, famous not only for its unique climbing ability, but for its ability to glide between trees and ground on vertical surfaces.

“I was amazed to see that these lizards hit the trunk of a tree head first and then pitch backwards head-over-heels at extreme angles from a vertical surface to land,” Jusufi said. “They hit a tree at 22 kilometers per hour.”

These lizards rely on their trunks and tails to dissipate the kinetic energy accumulated during their gliding, by pressing their tails against the trunk to cushion their landings and prevent them from falling head over heels. “I see the potential of this mechanism to create multi-modal robots capable of perching in similar settings,” Jusufi said.

In a recently published study Advanced Intelligent Systems, Jusufi and his group therefore developed a soft-body prototype based on the size, shape and weight of the gecko, and what he calls a “fall arrest response”. Like the gecko, the robot’s tail was important to enable safe landing, as well as the rigidity of the torso.

“A compliant torso allows the robot to dissipate a large amount of kinetic energy upon impact,” said Chellapurath, lead author of the study. “After impact, the bent torso allows the robot’s hind limbs to engage with the surface, and the stiff tail reduces rebounding.”

By pressing the tail against the wall, this provides a counter torque and prevents the robot from spinning head first and falling head over heels. “In this spirit, morphing structures and adaptive stiffness will increasingly enable unprecedented robotic ambulation with simple control provided by biomimetic materials and system relationships,” Jusufi said.

Pressing the tail against the wall provides counter torque and prevents the robot from turning head first. Illustration by Melanie Ackerman

Surprisingly, for the crash landing to work properly, the team determined a full-length tail was needed – a half tail would not do. “This is particularly interesting because it supports the idea that the body of these lizards probably evolved to have a tail length appropriate for locomotion,” said Pranab Khandelwal, one of the authors of the study.

The scientists also tested different approach angles and impact speeds to account for different approach trajectories to simulate real-world scenarios. The “fall arrest response” worked well even when the approach angle and speed changed, showing the versatility of this bio-inspired perching mechanism.

“The fall arrest response of geckos crash landing on walls highlights the importance of compliance in back and tail structures to ensure robustness to uncertainty in unstructured natural terrain,” commented Harvard University professor Robert Wood, who was not involved in the study. “And more broadly, the work of Jusufi and his lab highlights the utility of using bioinspired robots to explore questions in biology in unprecedented ways.”

This study provides new insights into the requirements of hard landings and how they can be used to increase stability and simplify controlled perching on aircraft.

Max Planck’s group realizes that there is potential to extend the landing robustness by further fine-tuning the robot’s physical properties and by testing the robot’s capabilities on various challenging surfaces in different environments.

Reference: Ardian Jusufi, et al., Morphologically Adaptive Crash Landing on a Wall: Soft-Bodied Models of Gliding Geckos with Varying Material Stiffnesses, Advanced Intelligent Systems (2022). DOI: 10.1002/aisy.202200120

Feature Image Credit: Ardian Jusufi Laboratory

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