Direct drive of flapping wings under resonance with instantaneous wing trajectory control

In this study, we present a motor-driven flapping-wing actuator, designed to operate at its resonant frequency using a torsion spring. The wing is driven by a DC motor directly through gear transmission. Linear torsion springs mounted on the load shaft creates restoring torque when the wing is displaced from its mid-stroke position. The actuator dynamics is obtained using system identification. The flapping motion of the wing is achieved by closed-loop motor control, for example, tracking a sinusoidal wave with the frequency tuned to match the resonant frequency of the system. PID and LQR controllers are applied for instantaneous wing kinematics tracking: A PID controller is able to precisely track the trajectory with relatively large control input; on the other hand, a linear quadratic regulator (LQR) achieves large flapping amplitude with small input effort. We show that the mechanism is able to track sinusoidal motions with different amplitude, bias and frequencies with relatively large range by changing the springs, therefore generating roll and pitch torques that can be used for flight control. Then a Hopf oscillator based central pattern generator is also shown to be an alternative trajectory to track. The proposed wing actuation mechanism provides an at-scale wing testing platform for flapping wing micro aerial vehicles.