Singularity-free path following control for miniature unmanned helicopters

Abstract

A singularity-free nonlinear controller is presented for the miniature unmanned helicopter to follow a reference path described by implicit expressions. Based on the time-scale separation principle, the controller is designed with hierarchical inner-outer loop structure. The outer-loop position controller is constructed with hyperbolic tangent function, and temporary-path generation method is developed to keep the control matrix invertible and obviate large control energy. The desired command attitude can be derived from position controller without singularity by choosing appropriate controller parameters. The inner-loop attitude controller is designed with the unit-quaternion attitude representation and backstepping technique to achieve attitude tracking. Numerical simulation is provided to verify the theoretical results.