Crash-sitting robots take inspiration from geckos

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

Agile flying robots are already playing an important role in a number of sectors and applications, including data collection, search and rescue, crop surveillance and forest fire management. However, even the latest technology drones have a limited ability to land on unsafe and difficult terrain, such as on the side of a building, tree or pole.

“Rapid landing on vertical surfaces is one of the biggest challenges in aerial robotics,” explained Ardian Jusufi, Head of the MPI Max Planck Group for Intelligent Systems and the Swiss Federal Laboratories for Materials Science Technology. “Emulating this maneuver would expand their scope of application, such as in a rubble field after an earthquake, or to assist firefighters, among other search and rescue scenarios.”

Current robots rely on rotors or rails to slow and reorient themselves before landing, Yusufi says. “Landing on a wall requires the integration of multiple sensor streams to control aerodynamic forces to bring the robot into the desired body orientation for a dedicated landing maneuver,” he explained. “The process of integrating multiple sensor streams is computationally expensive, leading to slow response times to environmental perturbations.”

It was during a field trip in the rainforests of Singapore that Yusuf came across the Asian flat-tailed gecko, known not only for its unparalleled climbing skills, but also for its ability to glide between trees and surface soil vertical.

“I was surprised to notice that these lizards hit the tree trunk headfirst and turn backwards, upside down at extreme angles from the vertical surface, in order to descend,” Yusufi said. “They hit the tree at an astonishing speed of 22 km/h.”

These lizards rely on their torso and tail to dissipate the kinetic energy accumulated during their glide, cushioning their landing by pressing their tail against the trunk and preventing them from falling head over heels. “I saw the potential of this mechanism in creating multimodal robots capable of landing in similar environments,” said Yusufi.

In a recent study published in Advanced intelligent systemsTherefore, Yusuf and his group developed a soft-bodied prototype based on the gacko’s size, shape and weight, and which uses what he called a “falling stop response”. As with the gecko, the tail of the robot was critical to enable a safe landing, along with stiffening the torso.

“A compliant torso allows the robot to dissipate significant amounts of kinetic energy during impact,” explained Chellapurath, lead author on the study. “After impact, the bent torso allows the robot’s hind limbs to engage with the surface, and the rigid tail reduces recoil.”

As the tail presses against the wall, it provides a counter torque and prevents the robot from rotating head first and falling head over heels. “In this spirit, morphing structures and adaptive stiffening enable more and more unprecedented robotic ambulances with simplified control provided by biomimetic materials and systems relationships,” said Yusufi.

The tail is pressed against the wall providing a counter torque and preventing the robot from rotating head first. Illustrations by Melanie Eckermann

Surprisingly, for crash landing to work properly, the team determined a full tail was needed – a half tail wouldn’t work. “This is particularly interesting because it supports the idea that these lizards potentially evolved to have tails that are the right length for their body’s locomotion capacity,” said Pranav Khandelwal, one of the study’s authors.

The scientists also tested different approach angles and impact speeds to calculate different approach trajectories in order to simulate real-world scenarios. “Fall arrest response” worked well even when approach angle and speed changed, demonstrating the versatility of this bio-inspired landing mechanism.

“The fall arrest response of geckos crashing into a wall highlights the importance of compliance in back and tail structures to provide stability to uncertainty in unstructured natural terrain,” commented Robert Wood, a professor at Harvard University, who was not included in the study. “And more broadly, Yusuf and his lab’s work highlights the utility of using bio-inspired 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 landings in aircraft.

The Max Planck group thinks it has the potential to extend the landing’s durability by further fine-tuning the robot’s material qualities and testing it on a variety of challenging surfaces in different environments to push the robot’s capabilities.

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 Lab

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