Insights from tree-climbing geckos’ hard landings lead to better and more controlled landings in robotic aerial vehicles.
Agile, flying robots already play an important role in a number of sectors and applications, including data collection, search and rescue, crop monitoring and forest fire management. However, even the most advanced drones have limited ability to land on precarious terrain and difficult perches, such as the side of a building, tree or pole.
“Fast landing on vertical surfaces is one of the biggest challenges in aerial robotics,” explained Ardian Yousufi, Max Planck Group Leader at MPI for Intelligent Systems and the Swiss Federal Laboratory for Materials Science Technology. “Emulating this maneuver would expand their application space, such as in a debris field after an earthquake, or to assist firefighters, among other search and rescue scenarios.”
Current robots rely on rotors or ailerons to slow down and reorient themselves before landing, Jusufi says. “Landing on a wall requires the integration of multiple sensor streams to control aerodynamic forces to bring the robot into the desired upslope body orientation for a special landing maneuver,” he explained. “The process of integrating multiple sensor streams is computationally expensive, resulting in slow response times to environmental perturbations.”
On a field trip to Singapore’s rainforest, Jusufi encounters the Asian flat-tailed gecko, known not only for its unparalleled climbing abilities, but also for its ability to glide between trees and land on vertical surfaces.
“I was surprised to notice that these lizards bump headfirst into the tree trunk and throw themselves backwards, head over heels at extreme angles from the vertical surface, to land,” Jussoufy said. “They collide with the tree at the astonishing speed of 22 km/h.”
These lizards rely on their torsos and tails to dissipate the kinetic energy built up during their glide, holding their landings by pressing their tails against their bodies and preventing them from falling over their heels. “I saw the potential of this mechanism in creating multimodal robots capable of landing in similar settings,” Jussoufy said.
In a recent study published in Advanced intelligent systems, Jusufi and his group therefore developed a soft-bodied prototype based on the gecko’s size, shape and weight, and which uses what he called a “fall arrest response”. As with the gecko, the robot’s tail was critical to ensure a safe landing, along with the stiffness of the torso.
“The compliant torso allows the robot to dissipate significant amounts of kinetic energy upon impact,” explained Chelapurath, the lead author on this study. “After impact, the flexible torso allows the robot’s hind limbs to grip the surface, and the rigid tail reduces bounce.”
As the tail is pressed against the wall, it provides torque countermeasures and prevents the robot from tipping forward and falling upside down. “In this spirit, morphing structures and adaptive bracing are increasingly enabling unprecedented robot locomotion with simplified control provided by biomimetic materials and systemic relationships,” Jussouffi said.
Surprisingly, for the emergency landing to work properly, the team determined a full length tail was needed – a half tail wouldn’t do. “This is particularly interesting because it supports the idea that these lizards potentially evolved to have tails that were the right length for their body’s capacity for locomotion,” said Pranav Khandelwal, one of the study’s authors.
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, demonstrating the versatility of this bio-inspired landing mechanism.
“The fall-arrest response of geckos when crash landing on a wall highlights the importance of compliance in back and tail structures to provide resilience to uncertainty in unstructured natural terrain,” commented Robert Wood, a professor at Harvard University who was not involved in the the survey. “And more generally, the work of Jusufi and his lab highlights the utility of using bio-inspired robots to investigate questions in biology in unprecedented ways.”
This study provides new insights into the requirements for hard landings and how they can be used to increase stability and simplify controlled landings in aerial vehicles.
Max Planck’s group believes there is potential to extend the landing resistance by further fine-tuning the robot’s material properties and testing it on different challenging surfaces in different environments to expand 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
Image rendering credit: Ardian Jusufi Lab