The Micromechanical Flying Insect (MFI) project aims to create a 25 mm, 100 mg flapping micro air vehicle. A maximum of 1400 uN of lift force by a single transmission/wing of the MFI has been produced on a test stand with oversized actuators. To achieve takeoff on a lightweight composite airframe, this high thrust must be produced using at-scale actuators. Previous attempts at MFI takeoff have suffered from low flapping amplitude and hence low thrust, which can be seen as a lack of power delivered to the wing. Power density of the actuators and structure used to drive the wing is of critical importance, especially for flapping flight. In this work, a design framework using power density as a metric is formulated not only for the MFI but for millirobots in general. The power density of an MFI 10 mg piezoelectric bending actuator is directly measured for the first time with a custom dynamometer and found to be 467 W/kg at 275 Hz. It is mathematically shown that adopting an actuator geometry that places a uniform strain profile on the piezoelectric element rather than a bending geometry can provide 2.6 times the energy delivered for the same volume of piezoelectric material. A new thorax (actuator/transmission) design is introduced which couples a single, uniform strain flextensional actuator to 2 wings. A key concept of the thorax design is using carbon fiber side beams near their singularities to provide the necessary transmission ratio. Two 2 iterations of flextensional MFI designs are presented, finally obtaining 42 degrees of total wing stroke at 225 Hz. An important limitation of the structure is serial compliance in its transmission stages, which absorbs useful motion preventing proper power transmission from the actuator. Despite the low stroke amplitude stroke due to serial compliance, 37.5 mg of thrust was produced from the design using all at-scale components. Future improvements to the new MFI structure design are discussed, which through iteration can eventually yield an autonomous flying vehicle.





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