Control of a Tail-Sitter VTOL UAV Based on Recurrent Neural Networks

TL;DR

This paper introduces a unified control approach for tail-sitter VTOL UAVs using recurrent neural networks, enabling smooth, real-time control across all flight modes with stability and robustness.

Contribution

The paper presents a novel RNN-based controller that unifies control of multiple flight modes and reduces computational complexity for tail-sitter UAVs.

Findings

RNN approximates the nonlinear solver with negligible errors

The controller operates at 50 Hz in real-time

Demonstrated stability and robustness through simulations and experiments

Abstract

Tail-sitter vertical takeoff and landing (VTOL) unmanned aerial vehicles (UAVs) have the capability of hovering and performing efficient level flight with compact mechanical structures. We present a unified controller design for such UAVs, based on recurrent neural networks. An advantage of this design method is that the various flight modes (i.e., hovering, transition and level flight) of a VTOL UAV are controlled in a unified manner, as opposed to treating them separately and in the runtime switching one from another. The proposed controller consists of an outer-loop position controller and an inner-loop attitude controller. The inner-loop controller is composed of a proportional attitude controller and a loop-shaping linear angular rate controller. For the outer-loop controller, we propose a nonlinear solver to compute the desired attitude and thrust, based on the UAV dynamics and an aerodynamic model, in addition to a cascaded PID controller for the position and velocity tracking. We employ a recurrent neural network (RNN) to approximate the behavior of the nonlinear solver, which suffers from high computational complexity. The proposed RNN has negligible approximation errors, and can be implemented in real-time (e.g., 50 Hz). Moreover, the RNN generates much smoother outputs than the nonlinear solver. We provide an analysis of the stability and robustness of the overall closed-loop system. Simulation and experiments are also presented to demonstrate the effectiveness of the proposed method.