Shape memory alloy (SMA) actuator wires provide a natural analog for biological muscles. Additionally, SMA wires are lightweight and have an extremely high energy density, making them desirable for aerospace applications. Therefore, it is natural to consider implementing SMA actuator wires in a flapping flight platform based on the biology of an animal, such as a bat.
This work focuses on the design and fabrication of a bat with SMA-actuated elbow and shoulder joints configured to mimic the flapping pattern of a bat. The joints themselves are made from super-elastic SMA ribbons that can bend through large angles and constrain the rotation of each joint to a single degree of freedom. The design process involves careful consideration of the SMA wire placement and joint geometry such that the desired joint rotation is induced without exposing either the super-elastic joint material or actuator wires to excessive stresses. A simplified bending and force model is used to estimate the bending and stresses, and an optimization algorithm is employed to maximize bending without exceeding stress limits.
Fabrication involves carefully attaching the small, 75 μm diameter, actuator wires such that they can be guided through the natural curves of the bat’s body, and their pre-strain can be tuned after assembly. The heating power input waveforms are shaped by a custom-built power control devise for multi-functional SMA wires such that the flapping motion mimics that of the natural flyer. The prototype has been tested to 100,000 cycles and installed in an exhibit at the Raleigh Museum of Natural Science as an example of how technology often finds inspiration from nature.