Robotic bats: Learning how bats fly

Dr. Neil Canter, Contributing Editor | TLT Tech Beat May 2017

A new process is unraveling aerodynamic secrets that one day could be transferred to aircraft design.

KEY CONCEPTS
• Bats use more than 40 joints to create a flexible, musculoskeletal system needed to support powered flight.
• A robotic bat known as Bat Bot (B2) has now demonstrated the ability to fly.
• The robotic bat is prepared with self-lubricating bearings. 

BATS HAVE LONG BEEN A FASCINATING SUBJECT both from the scientific and cultural standpoints. They are the only mammal that flies and are an important member of most ecosystems because they prey on insects.

The aerodynamics of how bats fly are of interest to researchers because bats exhibit a very sophisticated powered flight mechanism that involves wing flexibility in contrast to insects and birds. The latter two animals mainly have wings that are rigid structures. 

Dr. Alireza Ramezani, postdoctoral researcher at the Coordinated Science Laboratory at University of Illinois Urbana-Champaign in Urbana, Ill., says, “I have become impressed with how bats fly, particularly after seeing them move through the air in slow motion. Bats use more than 40 joints that interlock their bones and muscles to create a musculoskeletal system that can support flight. The action of this system creates the structural flexibility used by bats to use their wings to fly.”

The mechanism for bat flight has been determined through high-speed imaging. Ramezani says, “Bats fly by using a combination of downstrokes and upstrokes. In the downstroke, the wings drop leading to a buildup of aerodynamics producing lift and thrust. In the upstroke, the wings move up to position for the next downstroke. During this step, the surface area exposed to the air is minimized to reduce chances of losing aerodynamic lift.”

Efforts have been underway to develop drones based on the flying motion of bats and other animals. A previous TLT article described research done to develop drones based on the motion of bees (1). A micro-aerial vehicle known as a RoboBee was developed that has the ability to perch on objects through a process known as electrostatic adhesion. This enables the drone to fulfill its function yet minimizes energy use.”

Past efforts to develop robotic bats have not been successful mainly because the on-board computer actuation technology needed to control the drone has not been available. Ramezani says, “Miniature on-board computing, which required the use of data acquisition units that could convert analog data to digital data, was not available until recently. Without this technology, it is impossible to control the flight of any open-loop unstable object.”

A robotic bat with an on-board data acquisition system has now been developed that can perform autonomous flight.

PRINCIPAL COMPONENT ANALYSIS
Ramezani, in collaboration with Seth Hutchinson, professor of electrical and computer engineering at University of Illinois Urbana-Champaign, and Soon-Jo Chung, associate professor of aerospace and Bren Scholar at the Division of Engineering and Applied Science at the California Institute of Technology in Pasadena, Calif., has developed a robotic bat known as Bat Bot (B2) that contains soft, articulated wings similar to those displayed by bats (see Figure 1). The researchers used a technique known as principal component analysis (PCA) to simplify the wing motion of bats. 


Figure 1. Bat Bot (B2), a robotic bat, has been developed that imitates the flight of bats through the use of flexible wings. (Figure courtesy of University of Illinois Urbana-Champaign and the AAAS.)

Ramezani says, “PCA is used to break complex three-dimensional motions into simple motions that can be evaluated through mathematical computations. Bat joint movements are projected in the subspace of eigen modes, which isolates the various components of the wing conformation.”

PCA analysis is used as a building block to characterize the complexity of the motion of the bat’s wings. Ramezani says, “We used this technique to reduce the number of active and passive joints from more than 40 in the bat to nine (five active and four passive). The less important joints were not used.”

These nine joints were embedded in B2. 

One part of the bat’s anatomy that is aerodynamically significant is the tail and legs. Ramezani says, “The animal uses the legs to control the aerodynamic force, which will stabilizes the bat’s position in the air. The tail acts to control aerodynamic flow to ensure that the bat does not hit turbulence while flying. This helps the animal to avoid separation of air flow at the end of its body.”

The flexible joints present demanded that a stretchable fabric be articulated into the skeleton as B2’s skin. Ramezani says, “This means that we could not use conventional fabrics such as nylon because they are not stretchable. Instead, the skeleton of the robot is covered with an ultrathin (56 microns thick), silicon-based membrane.”

When asked if any lubricants are used to help B2 fly, Ramezani replied that self-lubricating bearings were placed in B2’s skeleton. 

Flight experiments were conducted in a large indoor space on the University of Illinois campus. A demonstration of the flights can be seen in the press release at the following link: http://engineering.illinois.edu/news/article/21209. 

Ramezani says, “Control channels were used to regulate asymmetric or asynchronous motion including the up and down movement of B2. This helps to keep B2 oriented at specific angles in the air so that it can maintain flight.”

Besides the curiosity to develop a robot that flies in a similar fashion to a bat, there are other motivations for the researchers. B2 is prepared with a flexible, soft material so that any collisions will not cause harm in a work environment. Ramezani says, “Based on this characteristic, we envision that B2 is suitable for use in construction zones to make sure that a specific building is being constructed the way it should based on the building information model.”

Future work will involve finding a way for B2 to perch. The value of developing flexible wing flight similar to a bat is that hopefully in the future this very efficient aerodynamic process can be translated into future wing designs for airplanes.

Additional information can be found in a recent article (2) or by contacting Ramezani at aramez@illinois.edu. 

REFERENCES
1. Canter, N. (2016), “RoboBees: Learning to perch,” TLT, 72 (8), pp. 8-9.
2. Ramezani, A., Chung, S. and Hutchinson, S. (2017), “A biomimetic robotic platform to study flight specializations of bats,” Science Robotics, 2 (3), eaal2505.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.