Design, modeling, control, and implementation of bi-copter UAVs

Abstract

Unmanned aerial vehicles (UAVs) emerged as the premier platform for data collection tasks due to their superior mobility and mechanical simplicity. When performing those tasks, UAVs usually operate in confined spaces while carrying professional sensors that are precise but heavy. The bi-copter UAV is the optimal choice when there is a simultaneous demand for large payload capacity, long operating time, and size restriction.

The first problem this thesis focuses on is the designing, modeling, controlling, and implementation of compact yet efficient bi-copter UAV platforms. Through theoretical analysis of the efficiency and practicality of 12 common configurations, we show that bi-copter has 30% higher efficiency than quad-copters of the same width. Moreover, we demonstrate the feasibility of a servo-based tandem rotor bi-copter approach through an exemplary design, Gemini mini. Such a 10 inches bi-copter can carry a 380 g LiDAR and hover for 13 minutes (the equivalent quad-copters can barely operate for more than 10 minutes). Besides presenting the design process, we also demonstrate its flight performance under external disturbances and during a 40 cm narrow gap fly-through.

The second issue that we engage in is improving the UAV operation efficiency through aerial-ground locomotion. Since flying UAVs' primary power consumption is the thrust generated to counteract gravity, obtaining force support from the ground will save the majority of hovering power. We integrated a single passive wheel into the bi-copter, Gemini W. After adopting the ground locomotion approach, Gemini W saves up to 77% battery by adding merely 1% to the original UAV weight.

The third difficulty that we tackle is to further improve the agility of the bi-copter UAV by addressing the drawback brought by servo motors, which is for vectoring the motor thrust direction. Issues such as non-linearity, response lag, non-minimum phase, and backlash in servo motors and their contained gearboxes seriously restrict the bi-copter's performance. Moreover, when servo-based bi-copters have a large size or heavy payload capacity, the servo motors will become heavy, bulky, and hard to maintain. To resolve these challenges, we first adopt the swashplate approach, which failed to be validated by experiments due to safety concerns caused by the severe vibration of the rotating swashplate mechanism. Next, we innovatively embrace the swashplate-less cyclic pitch-varying technique instead. The swashplate-less mechanism decouples the vectorized thrust using sinusoidal motor thrust and three passive hinges. This novel method not only resolves the issues caused by servo motors, but also makes the system mechanically simpler, more reliable, and cost-effective. We build a swashplate-less based bi-copter, Gemini II, and achieve full attitude control with only two actuators. Lastly, we demonstrate the validity and flight performance of Gemini II by conducting tests, including a power consumption test, poking disturbance test, gust wind disturbance test, step response test, and agile trajectory tracking.

This thesis systematically exhibits a series of bi-copter designs, including detailed mechanical designs, avionics, motor tests, and each design's modeling, control, and implementation. Overall, our proposed bi-copter platforms possess multiple virtues in terms of compactness, efficiency, agility, and robustness.