February 22, 2013
Secrets Of Bat Flight Revealed Through Robotic Bat Wing
[ Watch the Video: Researchers Build A Robotic Bat Wing ]
April Flowers for redOrbit.com - Your Universe OnlineSearching for the secrets of bat flight, researchers have built a robotic bat wing which mimics the wing shape and motion of the lesser dog-faced fruit bat.
The wing is designed to flap when attached to a force transducer - which records the aerodynamic forces generated by the moving wind - in a wind tunnel. Researchers can then evaluate the energy required to execute wing movements by measuring the power output of the three servo motors that control the robot´s seven movable joints.
By producing enough thrust to overcome drag and enough lift to carry the weight of the model species, the robot wing has shown in testing that it can match the basic flight parameters of bats.
The work was done in labs of Brown professors Kenneth Breuer and Sharon Swartz, who are the senior authors on the paper. Breuer, an engineer, and Swartz, a biologist, have studied bat flight and anatomy for years.
Since bats don't take requests and cannot fly while connected to instruments, a robotic wing was the next best thing.
“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.”
Because they can control each of the kinematic parameters - or movement capabilities - individually, the model does exactly what the researchers want it to do.
“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.”
Future research papers will detail experimental results from the robot, but this first paper includes some preliminary results from a few case studies.
The aerodynamic effects of wing folding were the subject of one experiment. During the upstroke, bats and some birds fold their wings back. A prior study at Brown showed that this was for energy conservation, but how folding effected aerodynamic forces wasn´t clear. The robot wing shows that folding is all about lift.
Positive lift is generated by the downstroke in a flapping animal. Some of that lift, however, is undone by the upstroke, which generates negative lift. The research team ran trials with and without wing folding, which showed that folding the wing on the upstroke dramatically decreases that negative lift, increasing net lift by 50 percent. Such data not only gives insight into the mechanics of bat flight, but it could also aid in the design of small flapping aircraft as well.
Complex structures, bat wings span most of the length of the bat's body from shoulder to foot. Two arm bones and five finger-like digits support and move the wing. Super elastic skin, able to stretch up to 400 percent without tearing, covers the wing. The robot's plastic bones were carefully fabricated on a 3D-printer to match proportions of a real bat. According to the researchers, the joints are actuated by servo motors that pull on tendon-like cables, which in turn pull on the joints.
The robot doesn't match the complexity of a real bat's wing, which has 25 joints and 34 degrees of freedom, as it isn't feasible to create an exact simulation given today's technology. Bahlman says this wouldn´t be desirable anyway. Because of its simplicity, the model distills bat flapping down to five fundamental parameters: 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.
Just building the robot taught the engineers many new things. “We learned a lot about how bats work from trying to duplicate them and having things go wrong,” he said.
For example, during testing the tongue and groove joint used for the robot's elbow broke multiple times because the forces on the wing would spread open the groove. This would eventually break it open. Eventually, Bahlman wrapped steel cable around the joint to keep it intact, similar to the way ligaments hold joints together in real animals.
The weakness of the robot's elbow might tell us a lot about the musculature of elbows in real bats. For example, bats have a large set of muscles at the elbow that are not positioned to flex the joint. The same muscles in humans help us to turn our palms up or down, which bats cannot do. This made that muscle set rather mysterious. Working with the robot suggests these muscles may be adapted to resist bending in a direction that would break the joint open.
More lessons were garnered from the wing membrane, which often tore at the leading edge. Bahlman reinforced that spot with elastic threads, which ended up looking a lot like the tendon and muscle that reinforce leading edges in bats, underscoring how important those structures are.
Bahlman has a lot of plans for the operational model.
“The next step is to start playing with the materials,” he said. “We´d like to try different wing materials, different amounts of flexibility on the bones, looking to see if there are beneficial tradeoffs in these material properties.”
The findings of this study and a description of the robot were recently published in the journal Bioinspiration and Biomimetics.