| Research on flapping-wing micro-aerial vehicles (MAV) has grown steadily in the past decade, aiming to address unique challenges in morphological construction, power requirements, force production, and control strategy. In particular, vehicles inspired by hummingbird or insect flight have been remarkably successful in generation of adequate lift force for levitation and vertical acceleration; however, finding effective methods for motion control still remains an open problem. Here, to address this problem we introduce and analyze a bio-inspired approach that guarantees stable and agile flight maneuvers. Analysis of flight muscles in insects/hummingbirds suggests that mechanical impedance of the wing joint could be the key to flight control mechanisms employed by these creatures. Through developing a quasi-steady-state aerodynamic model for insect flight, we were able to show that presence of a similar structure - e.g. a torsional spring - at the base of each wing can result in its passive rotation during a stroke cycle. It has been observed that manipulating the stiffness of this structure affects lift production via limiting the extent of wing rotation. Furthermore, changing the equilibrium point of the passive element introduces asymmetries in the drag profile which can generate considerable amounts of mean thrust while avoiding significant disturbances on lift production. Flight simulations using a control strategy based on these observations - a.k.a. the Mechanical Impedance Manipulation (MIM) technique - demonstrate a high degree of maneuverability and agile flight capabilities. Correlation analysis suggests that by limiting the changes in mechanical impedance properties to frequencies well below that of stroke, the coupling between lift and thrust production is reduced even further, enabling us to design and employ simple controllers without degrading flight performance. Unlike conventional control approaches that often rely on manipulation of stroke profile, our bio-inspired method is able to operate with constant stroke magnitudes and flapping frequencies. This feature combined with low bandwidth requirements for mechanical impedance manipulation result in very low power requirements, a feature that is of great importance in development of practical flapping-wing MAVs. |
