Control of the Robotic Butterfly by Abdominal and Wings Motions

In this work, a robotic butterfly with new control method is created inspired by the flight dynamics of butterflies; the dynamic of the robot and the flow field generated are also scrutinized. The butterflies fly with significant body motion in nature. Their abdomen rotates periodically during flight, and lead to the body angle and wing kinematics change concurrently. Previous studies also suggest that the body motion and flight trajectories are closely integrated. These evidences show that it is highly possible that butterfly able to control their flight modes via abdomen movements, which motivates us to create a butterfly robot with controllable abdominal motion. The body of our robot contains two parts - thorax and abdomen. The joint between them can fold as the real butterfly. The wing span and the weight of the robot are around 50 cm and 330 g, respectively. Two four–linker mechanisms were adopted to achieve flapping and abdominal motion, and were driven by a motor operated at 67 rpm. The mechanism allows the wing and abdomen to move synchronized. We recorded the motion of this robot with a camera by hinging it at the thorax, and then analyzed the interaction among the motions of thorax, wings and abdomen. The flow field generated by the robot were also analyzed with particle image velocimetry (PIV) technique. The results indicate that the abdominal motion largely affects the phase of thorax angle. For the test of the robot without the abdominal motion, the thorax and wing motion are in phase. The robot generates neither lift nor thrust with this motion since the down- and up-stroke jet are generated oppositely. In contrast, when the robot with abdominal motion, the phase of thorax angle is delayed, which is similar to that observed from the real butterflies. The flow field indicates that the directions of jet-flow generated in down- and up- stroke significantly twisted downward and backward, respectively, and are beneficial for robot to generate lift and thrust forces. Our current robot cannot generate enough lift to stay aloft, however, this work reveals the importance of the abdominal motion in the flight control of butterfly, which might be an alternative strategy to control MAVs in the future.