Dynamics, Control, and Estimation for Aerial Robots Tethered by Cables or Bars

TL;DR

This paper thoroughly analyzes the nonlinear dynamics of tethered aerial robots, establishing their differential flatness, controllability, and observability, and proposes robust nonlinear control and state estimation methods tested under various challenging conditions.

Contribution

It introduces a comprehensive characterization of tethered aerial robots, including differential flatness, and develops globally convergent nonlinear controllers and observers.

Findings

System is differentially flat with respect to specific output pairs.

Proposed observer and controllers are almost globally convergent.

Robustness confirmed under nonlinear effects, noise, and actuator deviations.

Abstract

We consider the problem of controlling an aerial robot connected to the ground by a passive cable or a passive rigid link. We provide a thorough characterization of this nonlinear dynamical robotic system in terms of fundamental properties such as differential flatness, controllability, and observability. We prove that the robotic system is differentially flat with respect to two output pairs: elevation of the link and attitude of the vehicle; elevation of the link and longitudinal link force (e.g., cable tension, or bar compression). We show the design of an almost globally convergent nonlinear observer of the full state that resorts only to an onboard accelerometer and a gyroscope. We also design two almost globally convergent nonlinear controllers to track any sufficiently smooth time-varying trajectory of the two output pairs. Finally we numerically test the robustness of the proposed method in several far-from-nominal conditions: nonlinear cross-coupling effects, parameter deviations, measurements noise and non ideal actuators.