STANFORD UNIVERSITY: Researchers at Stanford create new method for recording bird flight in 3D
Researchers
in the Lentink lab developed a new way to record wing shape during bird flight
in 3D. This high-resolution, high-speed, automated reconstruction method could
be applied to any studies of movement.
By Taylor Kubota
The wind
rushing between skyscrapers is a substantial hurdle for anyone interested in
operating small drones in urban areas. Yet, pigeons seem to have little trouble
maneuvering through turbulent city skies. With sights set on unlocking the
secrets of birds’ smooth sailing, researchers at Stanford University have
developed a new method for recording the shape of birds’ wings during flight.
“We’re
trying to figure out how birds are capable of flying so well in these complex,
turbulent environments and a lot of that comes from how they deform the shape
of their wings, left versus right, to adjust to gusts quickly,” said David
Lentink, an assistant professor of mechanical engineering.
Birds
morph their wings through an incredible range of shapes, but until now we’ve
known little about the angle, twist and asymmetries of each wing beat. After
seven years of development, the Lentink lab may have figured out how to more
closely observe birds’ morphing skills. It has created a new way of
automatically recording wing shape that works at high speeds and results in
high-definition 3D reconstructions. Details of their work are published in the
March 27 issue of the Journal of Experimental Biology.
Recording
animal movement
Current
techniques for recording animals in motion often rely on tracking markers
attached to the animal or features of the animal like stripes or spots, an
approach that can’t directly or automatically reconstruct an entire wing
surface at high resolution. Other methods, which use patterned light, are more
easily automated but are too slow to record bird flight.
The
Lentink lab has built on previous structured-light techniques, but its version
automatically resolves body shape changes at high speed and in high resolution.
“The
great thing about this system is it’s the first fully automated, high-speed
reconstruction of birds in the world,” said Marc Deetjen, a graduate student in
the Lentink lab and senior author of the paper.
The
group’s setup consists of a video camera synced with a projector that projects
two overlapping patterns of light. The first layer is a dense grid which, by
covering much of the surface of the bird, gives the researchers a high
resolution image. The second is a set of unequally spaced lines, like a bar
code, projected perpendicular to the first. The irregular second pattern
assures that no two areas of the light field look alike. When the bird flies
through these patterns, its body acts like a projector screen and the straight
lines of light deform based on the bird’s shape.
An
algorithm developed by Deetjen matches the deformed pattern on the bird that is
captured by the camera with the original projected pattern. It then produces a
detailed 3D reconstruction of how the bird moved through the light field.
Test
flight
To test
their technique, the researchers trained Gary, a 4-year-old parrotlet, to fly
from one perch to another, with the light grid projected onto the bird as it
took off. Gary’s light coloring allowed the camera to capture a clear light
pattern, like a near-white projector screen. For this paper, the group only
recorded the top surface of the bird, but multiple cameras could create a
full-body reconstruction in the future.
Graduate
student Marc E. Deetjen trains a parrotlet to fly between perches.
Graduate
student Marc E. Deetjen trains a parrotlet to fly between perches. (Image
credit: Kurt Hickman)
The
researchers intended this as a simple test of their system but ended up with an
insight so unexpected and intriguing, they thought it was a mistake. After
recording a portion of four of Gary’s downstrokes, they computed the bird’s
effective aerodynamic angle of attack – how much the wing flips backward – and
found it was consistently between 55 degrees and 75 degrees in the first
downstroke and between 45 degrees and 60 degrees in the second. Most airplanes
stall when the angle of attack reaches about 15 degrees because even this angle
can create drag so significant that the airflow becomes separated from the
wing, resulting in reduced lift. The researchers concluded that the bird is
actually supporting its body weight using drag oriented upward. In addition,
the lift it generates is rotated forward so it functions as thrust.
“They’re
actually able to generate more total force on lift-off,” said Deetjen. “That
enables them to not only push up and overcome gravity but to accelerate
forward.”
Details
like this could bring scientists closer to replicating the efficient and acute
takeoff of birds in small flying machines, like drones, which are a specialty
of the Lentink lab. For their next step, the researchers are planning to apply
this technique in a specialized bird wind tunnel to investigate the many
mysteries of bird flight in turbulence.
Although
the team tested the technique on bird flight, it could be applied to many forms
of movement. For example, it could show what happens to a car’s shape during a
simulated crash. Lentink said he’s also been talking with a scientist who
studies flying snakes in Borneo who might want to give the technique a try.
“This is
a technique that goes all the way from animal locomotion to direct applications
in engineering, where things deform fast,” said Lentink. “We only need to
create one frame and then we can reconstruct the shape in 3D. This technique,
in principle, does not have a speed limit.”
Andrew
A. Biewener is also co-author on this paper. Lentink is also a member of
Stanford Bio-X.
This
work was funded by the National Science Foundation and Micro Autonomous Systems
and Technology at the Army Research Laboratory.
Source: http://news.stanford.edu/2017/04/10/new-method-recording-bird-flight-3d/