Autostabilizing airframe articulation: Animal inspired air vehicle control

The sparse sensing and limited articulation that are characteristic of human-engineered robotic systems contrast dramatically with sensorimotor systems observed in nature. Animals are richly imbued with sensors, have many points of articulation and are heavily over-actuated. In fact, the compliant nature of the body (or Plant) of most animals requires constant control input to the muscles for postural maintenance. In this study, we show how flying insects use a compliant airframe to maintain flight stability via active articulation of the frame. We first derive the equations of motion for a model flying insect, inspired by the hawkmoth, a large fast flying and agile insect. By linearizing the equations of motion about a hovering equilibrium, we demonstrate that abdominal motions are sufficient to stabilize flight on a scale of 50ms. We then tested whether these insects use the abdomen for flight control by first measuring the open-loop transfer function between visual pitch rotations and abdominal movement in a tethered moth preparation. The measured transfer function was consistent with an abdominal control strategy. We then closed the loop and found that moths actively stabilize visual pitch rotations using abdominal motion as the only control input. The behavior was robust to variations in gain and to a variety of visual stimuli. These experiments establish airframe articulation as a plausible control mechanism for active flight.