Gemini II: Design, Modeling, and Control of a Compact Yet Efficient Servo-less Bi-copter.

Unmanned Aerial Vehicles (UAV) are widely used in the field for tasks that require 3D movements in space. The simultaneous demand for heavy payload capacity, long operating time, and size restriction pose a challenge to UAV design. To solve this conundrum, this paper proposes a novel bi-copter with only two actuators. Unlike tandem rotor bi-copters that utilize two servomotors to achieve yaw and roll control, our novel design, the Gemini II bi-copter, controls attitude by using cyclic flapping response in hinges that connect the blades. This passive cyclic pitch-varying mechanism makes the UAV no longer depend on two heavy and expensive servo motors or swashplate to vector the thrust. This change of propulsion system not only makes the UAV mechanically simpler, more reliable, and cost-effective, but also enhances the UAV performance by mitigating the issues of backlash, non-linearity, and non-minimum phase caused by servo motors. To the best of the authors' knowledge, the Gemini II, built entirely on off-the-shelf electronics, is the first servo-less bi-copter that can precisely control its 3D position and orientation with two actuators only. To demonstrate the flight performance and applications of the proposed novel UAV, we conduct path following experiments along with manual poking and wind disturbances tests.