Dynamic Modeling and Control of A Flexible Four-Rotor UAV

This paper presents a novel design for mini rotor-craft Unmanned Aerial Vehicles (UAV) that are intended to be used in tight spaces. In this design, the UAV is allowed to flex when subject to external forces that occur during collisions. A four link mechanisms with revolute flexure joints is used as a support frame, which allows both flexibility and controllability. The paper presents the design of such a flexible UAV, denoted as ParaFlex, its modeling, and a closed-loop PI controller for stabilization at hover. Through simulation study, the robustness of the ParaFlex is demonstrated in the face of an impact. The goal of this work is to propose and evaluate a new design of four-rotor UAV that will make it robust to collisions which is common in cluttered and tight spaces. UAVs are well suited for applications such as surveillance, search and rescue, remote monitoring, target tracking, and aerial photography. The future road map for UAV calls for robust navigation in urban setting. Robust to external forces, indoor ingress, precision hover, perching are some of the key challenges put forth in the UAV vision 1 . Vertical takeoff and landing (VTOL) UAVs have the basic advantage of the ability to hover and land in tight spaces. Many applications demand UAVs to navigate in tight spaces such as ducts lines, collapsed buildings, and hazardous environments. UAVs are fragile (they easily become unstable) and hence they often need a safe trajectory to navigate successfully without collision. This limits the speed and performance of the mission. While they can be somewhat protected by adding shrouds or cages around the UAV, the problem still persists. Feedback control based solutions are possible but may prove to be either expensive or difficult to implement. This is because collision generally involves large magnitude impulsive force and the corrective actions have to be generated at high bandwidth. Even if the control loop runs at high bandwidth, slow actuators may limit the performance. The main contribution reported in this paper is the novel design that introduces flexibility (passive or active suspension) to the four-rotor UAV shown in figure 1. The basic reason for introducing flexibility is to absorb collision forces and allow the controller sufficient time to react. There can be many different ways to introduce flexibility. We present one particular design called ParaFlex that is very similar to Quadrotor but has skewable frame.