Feb.24, 2013
Bats are spectacular fliers. Their wings span most of the length of a bat's body, from shoulder to foot and supported and moved by two arm bones and five finger-like digits. Over those bones is a super-elastic skin that can stretch up to 400 percent without tearing. For years researchers are studying the machinations of bats wing. The strong, flapping flight of bats offers great possibilities for the design of small aircraft.
Researchers at Brown University have developed a robotic bat wing for better understanding dynamics of flapping flight in real bats.
The robot, which mimics the wing shape and motion of the lesser dog-faced fruit bat, is designed to flap while attached to a force transducer in a wind tunnel. As the lifelike wing flaps, the force transducer records the aerodynamic forces generated by the moving wing. By measuring the power output of the three servo motors that control the robot's seven movable joints, researchers can evaluate the energy required to execute wing movements.
Testing showed the robot can match the basic flight parameters of bats, producing enough thrust to overcome drag and enough lift to carry the weight of the model species.
(A robotic bat wing lets researchers measure forces, joint movements, and flight parameters — and learn more about how the real thing operates in nature. Credit: Breuer and Swartz labs/Brown University)
This robot bat wing can generates data that could never be collected directly from bats. "We can't ask a bat to flap at a frequency of eight hertz then raise it to nine hertz so we can see what difference that makes," said Joseph Bahlman, a graduate student at Brown who led the project. "They don't really cooperate that way."
With this robot model, researchers can control each of its movement capabilities — kinematic parameters — individually. One interesting discovery is, for example, that bats fold their wings back during the upstroke for lifting and not for saving energy.
"We can answer questions like, 'Does increasing wing beat frequency improve lift and what's the energetic cost of doing that?'" Bahlman said. "We can directly measure the relationship between these kinematic parameters, aerodynamic forces, and energetics."
No surprise this eight-inch robot mimics is fabricated on a 3D printer. A 3D printer created plastic bones and the skin is made of a silicone elastomer. The joints are actuated by servo motors that pull on tendon-like cables, which in turn pull on the joints.
Though robot is just a simple version of a real bat's wing, which has 25 joints and 34 degrees of freedom, the model is still very useful for studying the functionality of bat's wing, such as flapping frequency, flapping amplitude, the angle of the flap relative to the ground, the amount of time used for the downstroke, and the extent to which the wings can fold back.
During testing, for example, the tongue and groove joint used for the robot's elbow broke repeatedly. The forces on the wing would spread open the groove, and eventually break it open. Bahlman eventually wrapped steel cable around the joint to keep it intact, similar to the way ligaments hold joints together in real animals.
The wing membrane provided more lessons. It often tore at the leading edge, prompting Bahlman to reinforce that spot with elastic threads. The fix ended up looking a lot like the tendon and muscle that reinforce leading edges in bats, underscoring how important those structures are.
A paper describing the robot and presenting results from preliminary experiments is published in the journal Bioinspiration and Biomimetics. The work was done in labs of Brown professors Kenneth Breuer and Sharon Swartz. Breuer, an engineer, and Swartz, a biologist, have studied bat flight and anatomy for years.
( Brown professors Kenneth Breuer and Sharon Swartz)
Data like that will not only give new insights into the mechanics of bat flight, it could aid the design of small flapping aircraft. The research was funded by the U.S. Air Force Office of Scientific Research and the National Science Foundation.
Source: Brown University
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