Bats are incredibly agile and maneuverable flyers, but they are also uncooperative research subjects. Since bats are unlikely to follow directions from humans, University researchers developed a robotic bat wing to study how changing different parameters, such as wing motion and wind speed, affect the bat’s ability to fly.
The researchers described the robotic bat wing in a study in the journal Bioinspiration and Biomimetics last month. It is not the first robotic wing — others mimicking bird and insect flight already exist, said Joseph Bahlman GS, who led the project and designed and constructed the robotic bat wing as part of his dissertation. But the new wing is unique because of its ability to actively fold and expand just like a real bat wing, Bahlman said.
The work was conducted in the labs of Kenneth Breuer ’82 P’14 P’16, professor of engineering, and Sharon Swartz ’84, professor of ecology and evolutionary biology and engineering, who served as senior authors on the paper.
Understanding bat flight may hold the key to building bat-sized flapping aircrafts, also known as micro aerial vehicles, said Soon-Jo Chung, an assistant professor of aerospace engineering at the University of Illinois at Urbana-Champaign who studies bio-inspired flight. Chung was not involved in the study.
“The mainstream model for bio-inspired flight is insects,” Chung said. But “it is difficult to put anything meaningful in a small, insect-scale airplane because you have to miniaturize everything.”
Bat-sized aircrafts would be big enough to carry small equipment, such as cameras, and could access areas that are too difficult or too dangerous for humans to reach, he said.
Building a bat wing
The robotic wing was inspired by the lesser dog-faced fruit bat, a medium-sized bat that the researchers have already studied extensively, Bahlman said. When fully extended, the robotic wing is 20 cm long. “We really wanted to make sure that we could produce something that was life-sized and moved at life-like speeds,” he said.
Bahlman spent two and a half years constructing this specific model. Like human hands, bat wings have many joints. He designed the robot’s skeleton to include seven of the joints most important for mimicking the motion of bat flight, using cables to act as the tendons that operate these joints in a live bat. The cables connect to three motors.
“The actuation system is similar to and inspired by real muscular systems since these cables connect a motor to a joint, the same way a tendon connects muscle to a joint,” Bahlman said.
Bahlman used a 3-D printer to create the plastic skeleton and covered it with a thin rubber sheet for the wing’s membrane. The membrane was particularly challenging to build because the researchers needed a material that was lightweight, thin and stretchy.
To collect data, the researchers suspended the bat wing inside a wind tunnel using force transducers, which measure the aerodynamic forces produced by the bat wing, including lift, the force generated to counteract gravity, and thrust, the force generated to move forward. The researchers also measured the power output of the motors to determine the energy required for the wing to flap, Bahlman said.
Bahlman, Breuer and Swartz are currently working on another paper on the results of their experiments with the bat wing.
Groundbreaking interdisciplinary research
Throughout the development process, the robotic wing broke frequently. But those breaks turned out to be blessings in disguise.
“Learning why it breaks gives us a lot of feedback on the mechanical stresses real animals have,” Breuer said.
The breaks also revealed how bats naturally cope with the stresses of flying. For example, to better understand why the robot’s elbow joint broke open, the researchers examined the structure of a real bat’s elbow joint and realized that the robot’s joint needed ligaments as reinforcement, Bahlman said.
“I like how this project in particular was a great example of how the biology informs the engineering, but the engineering also informs the biology,” Bahlman said.
But interdisciplinary research also has its challenges. Breuer described scientific disciplines as having different “cultures.” When Swartz and Breuer began their collaboration over 10 years ago, they had to overcome disciplinary differences including jargon, definitions of good experiments and ways of framing questions, Breuer said.
“I think both of us would say that the work we’ve done together has been some of the most rewarding work of our scientific careers,” Swartz said.
Bahlman said he finds interdisciplinary research both challenging and rewarding.
“The benefit is that you learn both worlds. The challenge is that you have to learn both worlds,” Bahlman said. Swartz and Breuer’s research group was selected by the National Science Foundation to present its work at the American Association for the Advancement of Science’s meeting in Boston last month.
“One of the things that the Foundation likes to do is support work that breaks new ground, and some of that most exciting new work is at disciplinary interfaces,” Swartz said.
The group spent three days sharing their work with a wide audience, ranging from children to anthropologists and astrophysicists.
“I’ve never been more proud of my research group than I was over those three days,” Swartz said.
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