First-of-its-kind inflatable drone perches like a bird for search and rescue

A robotics team at ASU developed a first-of-its-kind quadrotor drone with both an inflatable frame and a novel gripper for collision resilience.
Sade Agard
A first-of-its kind quadrotor drone with an inflatable frame and grasper.
A first-of-its kind quadrotor drone with an inflatable frame and grasper.

Arizona State University 

  • The fragility of current drone designs makes them susceptible to damage, thus limiting their use in search and rescue missions. 
  •  A robotics team has designed and tested a first-of-its-kind 'soft' quadrotor drone and grasper for collision resilience.
  • The work could also boost drone efficacy in forest fire monitoring, military reconnaissance, and even planetary exploration. 

After catastrophic events, such as the 7.8 magnitude earthquakes that shocked Turkey and Syria on February 6, 2023, search and rescue efforts are in a race against time. 

It is crucial for emergency response teams to promptly locate any gaps or openings in the debris of a building where survivors could potentially be trapped and to do so before potential hazards such as natural gas leaks, mains water flooding, or unstable concrete slabs further endanger survivors.

High-tech tools such as thermal imaging equipment and sensitive listening devices can be employed to detect signs of life. Small aerial drones can also be deployed to explore inaccessible areas. Still, the fragility of current designs makes them susceptible to damage, thus limiting their use. 

Now, a robotics team from Arizona State University has designed and tested a first-of-its-kind quadrotor drone with an inflatable frame. Uniquely, its stiffness is tunable, or adjustable, to absorb shocks from unexpected collisions and recover from unplanned taps and thumps.

To gain a deeper understanding, Interesting Engineering (IE) interviewed Professor Wenlong Zhang, one of the research paper's corresponding authors.

First-of-its-kind soft drone with an avian-inspired grasper 

The focus must shift away from avoiding environmental contact, stated Zhang. He argues that for drones to carry out diverse jobs, they must be able to physically interact with their surroundings. 

Additionally, a soft body delivers the material compliance needed for dynamic maneuvers like perching on unsteady objects, as well as absorbing impact forces to provide collision robustness.

To further explain, perching can be seen as a form of controlled collision. When birds land and settle on tree branches or other structures, they are essentially colliding with them, bending their legs to absorb their momentum. Their flexible joints and soft tissues absorb the force of impact. while their feet have a passive tendon-locking mechanism that allows them to grip uneven surfaces without exerting extra muscle energy to stay put.

Even tinier insects like flies make use of both collision and perching techniques, and their compliant bodies aid in reducing the force of impact.

"[We designed] a 'soft' drone with the chassis being inflatable beams," Zhang told IE. In the team's paper, this is referred to as SoBAR—a soft-bodied aerial robot.

"By controlling the amount of air in the actuator, we can tune the stiffness of the chassis to achieve collision resilience," he added.

"In addition, we integrated an inflatable beam with 'bistable' material (sheet metal, think about a tape measure) to design a grasper for the drone to perch on objects without consuming energy." 

Bistable refers to the grasper's capacity to exist in two resting states that do not require power: open and closed. Upon landing, it responds by rapidly snapping shut and firmly attaching to objects with diverse shapes and dimensions.

"[Our drone] can perch on pretty much anything. Also, the bistable material means it doesn't need an actuator to provide power to hold its perch. It just closes and stays like that without consuming any energy," Zhang clarified in a previous statement

According to the paper, the new drone's grasper carries out this 'snap-through buckling' function by absorbing the impact energy and converting it into a closed-form grip. Better yet, they claim it can adapt to a range of sizes and shapes within approximately four milliseconds (ms).

"Then, when needed, the gripper can be pneumatically retracted, and the drone can just take off," Zhang added. 

Making the SoBAR-perching drone

"The enabling technology is pneumatic-driven textile actuators. Our team has been working on actuators made of fabrics over the past few years," he told IE.

Pneumatic-driven textile actuators are soft robotic devices that use pressurized air to produce motion in textile-based materials. They are made by integrating textile materials, such as fabric or fibers, with pneumatic systems that can inflate or deflate the material to achieve movement.

First-of-its-kind inflatable drone perches like a bird for search and rescue
Fabrication of the SoBAR-perching drone with its components

Creating the new drone's grasper first involves aligning the heat-sealed actuator with the woven fabric after laser cutting. The woven fabric sheets are then stitched together along the edges using a superimposed seam. The heat-sealed actuator is then inserted to create the soft frame.

Following this, the pneumatic connector and 3D-printed motor mounts are added and aligned with the inflated soft-bodied frame. "Finally, mount the propellers and motor pairs, electronics, and the perching mechanism," explained the researchers in their paper. 

"These actuators are low-cost, lightweight, and easy to manufacture and customize. We have applied these textile actuators on wearable assistive devices for healthcare applications (e.g., rehabilitation and power augmentation)," said Zhang.

The research team claims to be the first (to the best of their knowledge) to create a multirotor aerial robot that exclusively employs an inflatable fabric-based soft body to regulate its rigidity and assimilate impacts. 

They propose their study promotes a new category of aerial robot structures that utilize superior-strength inflatable fabrics to absorb collision and perching impact forces competently.

Moreover, the researchers believe they have introduced a new class of lightweight grippers for aerial robots that depend on a combination of high-strength inflatable fabrics and a bistable mechanism. 

'There is definitely a lot more to be done'

"One challenge is the more complex aerodynamics of the drone since the frame is soft and flexible," revealed Zhang. "We have developed new models and flight controllers for SoBAR, but there is definitely a lot more to be done to make the drone highly robust and efficient."

For instance, the team highlighted a way to maximize the soft drone's energy efficiency by optimizing its design for improved stiffness at various internal pressures. 

They reasoned this can be achieved by varying its cross-section design, including internal soft truss structures, and/or developing a rigid-soft hybrid frame.

Similarly, the HFB grasper, although observed to be adaptable to many irregular shapes, it is mounted against a flat plate, which slightly limits its bending range. The team anticipates redesigning the grasper to bend and curl more effectively. 

They also describe how the motor mounts on the arms of the SoBAR had to be readjusted during tests due to minor slippage. One way to address this would be to add antislip fabric at the interface between the motor mount and the fabric frame.

"Besides that, we are also working on integrating onboard sensors and planning algorithms so that the drone can fly autonomously with minimal human supervision," added Zhang. 

High-impact drones for serving the military

"This new class of drones presented in this work can be applied in many scenarios where it is challenging to avoid collisions, such as disaster relief and reconnaissance," Zhang said to IE

First-of-its-kind inflatable drone perches like a bird for search and rescue
Concept image of drone employed for military operations.

"Since the drone can perch on objects without consuming energy, it holds the potential to extend the battery life of small unmanned aerial vehicles and enable long-term environment, traffic, and infrastructure monitoring," he added.

From this perspective, he reasoned, pretty much anyone who currently uses drones in their job may benefit from this work. 

The next action plan is to prove the capability of SoBAR for dynamic autonomous perching in the real world by having it visually detect suitable perches.

The team aims to investigate aggressive trajectory planning and sturdy control strategies that would enable SoBAR to approach objects at various angles and heights while navigating a complex, obstacle-filled environment. 

They asserted that such future efforts have the potential to facilitate battery recharging techniques for extended outdoor surveillance as well as search and data collection missions.

Zhang believes such dynamic environmental interaction can boost the efficacy of drones in search and rescue missions, forest fire monitoring, military reconnaissance, and even planetary exploration.

"There are so many functionalities possible with conformable, reconfigurable soft aerial robots, so we hope our work here leads to even more novel, bio-inspired designs," Zhang concluded.

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