Passive Stability for Cooperative Transport of a Cable-Suspended Payload by Two Aerial Robots

Abstract

With plenty of potential applications in various industries such as agriculture, construction, emergency response and transportation. Aerial robots, in particular, quadrotors have attracted tremendous attention in payload transporting tasks. In order to overcome the limitation of the payload carrying capacity of a single robot, cooperative transporting of a cable-suspended payload by multiple aerial robots provides a possible solution. Recently, there have been studies on the dynamical behavior and realization of the aerial transportation task by multiple aerial robots. However, previous accomplishments are mostly limited to indoor environments because accurate and reliable position feedback from a motion capture system is required. The stringent requirements cannot be met with feedback provided by GPS in outdoor environment. There have also been attempts to realize the cooperative transport task in outdoor settings, but substantial peripheral hardware, such as a companion computer and a camera, is required for localization and communication between robots.

In this thesis, we address the cooperative transport problem with two robots in the leader-follower configuration. We seek to achieve this by installing mechanical air stabilisers on both the payload and the follower robot. The passive approach replaces the algorithm-based strategy with a mechanical solution. Considering the aerodynamics damping forces of the added dampers, the system can be stabilised at the equilibrium point with little modification to the regular flight controller on the follower robot. To achieve this, we firstly study the dynamical model formed by two robots and a cable-suspended payload via applying a Lagrangian-Euler formulation. In the model, the effects of both air drag from the propellers and damping forces on the air dampers are taken into consideration. The state dynamics are linearised for stability analysis and the results show that the system can be stabilised by adding those mechanical dampers. The degree of the stability is quantified through an examination of root locus and damping ratio.

In order to demonstrate the proposed passive approach in real flight, we first performed experiments to identify the damping coefficients introduced in the modelling. Following this, to validate the feasibility of the method, we carried out a cooperative transport experiment with two robots by employing the passive stabilizers. The results show that the system consisting of two quadrotors (68 g and 73 g) and a payload (30 g) can be stabilised by the mechanical dampers when travelling at 1 meter per second. The results show promise of the passive approach in the collective transport via aerial robots, as well as the potential of the strategy in outdoor environments.

Date of Award	4 Aug 2020
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Pakpong CHIRARATTANANON (Supervisor)