Variable-Pitch-Propeller Mechanism Design, and Development of Heliquad for Mid-flight Flipping and Fault-Tolerant-Control

TL;DR

The paper tackles the limitations of fixed-pitch quadcopters by introducing a Variable-Pitch-Propeller (VPP) mechanism on a quadcopter designated as Heliquad. It derives the mechanism's input-output relation $\gamma=f(\xi)$ via loop-closure constraints, analyzes kinematic singularities, and derives the minimum actuator torque $\tau_s$ required for pitch changes. A unified non-switching cascaded attitude-rate controller, augmented with neural-network-based control allocation, handles nonlinear aerodynamics and actuator faults to achieve full attitude control, including yaw-rate, on three actuators. Experimental results demonstrate mid-flight flipping, three-actuator flight, and safe recovery after actuator failure, with neural-network outputs remaining bounded throughout.

Abstract

This paper presents the design of Variable-Pitch-Propeller mechanism and its application on a quadcopter called Heliquad to demonstrate its unique capabilities. The input-output relationship is estimated for a generic mechanism. Various singularities and actuator sizing requirements are also analyzed. The mechanism is manufactured, and the validated input-output relationship is implemented in the controller of Heliquad. Heliquad is controlled by a unified non-switching cascaded attitude-rate controller, followed by a unique Neural-Network-based reconfigurable control allocation to approximate nonlinear relationship between the control input and actuator command. The Heliquad prototype's mid-flight flip experiment validates the controller's tracking performance in upright as well as inverted conditions. The prototype is then flown in upright condition with only three of its working actuators. To the best of the authors' knowledge, the cambered airfoil propeller-equipped Heliquad prototype demonstrates full-attitude control, including yaw-rate, on three working actuators for the first time in the literature. Finally, the utility of this novel capability is demonstrated by safe recovery and precise landing post-mid-flight actuator failure crisis. Overall, the controller tracks the references well for all the experiments, and the output of the NN-based control allocation remains bounded throughout.

Figures (18)

• Figure 1: Heliquad's Variable-Pitch-Propeller actuator can make it fly with full-attitude control despite the complete failure of an actuator. It can also perform mid-flight flips. The videos are included in the supplementary material.
• Figure 2: (a) The mechanism schematic used for analysis. $r_0$ is the crank length. (b) Actual VPP actuator mounted on Heliquad. The servo-motor is mounted behind the slot. The actuator has two blades. Another link similar to Link 4 is behind it.
• Figure 3: Frames of reference. $\{E\}$ is the inertial and $\{B\}$ is the body frame of refernce. Subscript of $\Omega$ represents actuator number.
• Figure 4: Control loop schematic. Fault Detection and Isolation (FDI) method is not discussed in this paper.
• Figure 5: Input-Output relationship for the VPP mechanism. All the readings were taken on the same setup