In nature, when a bird, insect, or bat takes flight, it looks effortless and simple. Understanding how these animals generate lift and move through the air proves to be much more complex. Assistant Professor David Lentink leads a group of graduate researchers from Stanford University who are using ATI’s Force/Torque Sensors to unravel this mystery. The team has invented a so called “aerodynamic force platform” for measuring the aerodynamic forces during free flight experiments with well-trained birds. The experiments are performed in vivo, which is Latin for “within the living” and indicates that the experiment parallels naturally-occurring behavior. Remarkably, the new method is “no-touch” and allows the bird to freely fly around without being burdened by instrumentation in any way, making it exceptionally animal-friendly.
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Flight research encompasses the study of wing structure, wing movements, and their interactions with the air. Of particular interest are the forces animals generate during flight to stay aloft and perform various maneuvers. Formerly, these forces had to be measured indirectly based on theoretical models and measured flow fields, or by tethering the animals to a sensing system. However, the tethers cannot be used for higher order animals such as birds, especially because it would disrupt their well-being and natural flying behavior. Scientifically, a tethered force measurement approach fails to depict what really happens as animals fly, resulting in misleading and inaccurate modeling of flight behavior. Lentink’s team investigated three commonly used models and found some disappointing results. “All three performed worse than we hoped for, and did not predict the lift reliably, which shows that work is needed to improve the models,” reports Stanford graduate student Diana Chin.
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One of the resident flight experts at Stanford is a Pacific Parrotlet named Obi. As with all of the other birds in the Lentink Lab, Obi was trained using positive reinforcement only--to fly on cue. In previous experiments, Obi flew through a laser sheet saturated with small mist particles while wearing custom-fit laser safety goggles. The goggles were designed by former graduate student Eric Gutierrez and used to keep Obi safe during the flight test. Gutierrez was able to reconstruct the flow field around Obi’s wings by recording the motion of the particles, in response to the bird’s wingbeat, with high-speed cameras. He combined these flow field measurements with commonly used aerodynamic models to derive instantaneous force estimates. These estimates were then compared to direct measurements made during a separate set of parrotlet flights inside the Aerodynamic Force Platform. The comparisons revealed the poor predictive ability of the popular models, further highlighting the value of direct force measurement methods.
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Because so little is known about the aerodynamic forces generated during flight, the Stanford research team faced a daunting task: they needed a way to capture small, yet dynamically changing, lift forces without the use of tethers. It was imperative to create a more natural environment where they could train an animal to fly on cue, and observe the forces generated by its wingbeats. These objectives became the catalyst for the Aerodynamic Force Platform and ultimately led to the development of the new in vivo experiments, which directly measured the lift generated by a flying animal for the first time.
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The Aerodynamic Force Platform (AFP) is an enclosed flight chamber that features ATI’s Nano43 Force/Torque Sensors mounted to the top and bottom. During flights in the AFP, wingbeats create pressure changes in the air which are translated by ATI’s Nano43 Sensors into aerodynamic force measurements. Chin indicates, “The sensitivity and precision of the Nano43 sensors were ideal for measuring the small forces generated by the birds in the Aerodynamic Force Platform, and the rapid sampling rate was necessary to resolve forces within wingbeats.” The ATI F/T Sensors provide force feedback from six different axes, and record changes as they occur, which in this case is at very high speeds. The outputs from these experiments offer a more comprehensive look at dynamic pressure changes during flight, something that was not possible previously.
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Stanford’s integration of the ATI Force/Torque Sensors with the Aerodynamic Force Platform harvests more accurate data from each flight test. The AFP is a huge breakthrough for the team, and for flight research; pairing this new technology with in vivo experiments provides a deeper look into animal flight mechanics. Lentink, Chin, Gutierrez, and the rest of the team have drawn more informed conclusions about bird flight that will advance the technology of flying robots. With these techniques, the team will develop biomimetic robots that can aid in search and rescue missions, perform surveillance in hazardous conditions, or even deliver medical supplies.
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Click here for more information on Stanford's flight experiments.
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Click here for more information on our Force/Torque Sensors.
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