Observer-less Output Feedback Global Tracking Control of Lossless Lagrangian Systems

TL;DR

This paper presents a novel observer-less output feedback control method that achieves global tracking stability for lossless Lagrangian systems using only position measurements, solving longstanding open problems in the field.

Contribution

It introduces a globally stable, observer-free control approach for Lagrangian systems, including those with higher relative degree and actuator dynamics, using simple, Lipschitz continuous controllers.

Findings

Achieves uniform global asymptotic stability with simple controllers.

Extends to systems with higher relative degree and actuator dynamics.

Solves open problems in observer-less output feedback control.

Abstract

We obviate the use of observers for the purpose of output feedback tracking control of Lagrangian systems and solve some long-standing yet well-documented open problems. As often implemented in control practice, we replace unavailable derivatives with approximate differentiation. Our contribution consists in establishing uniform global asymptotic stability in closed-loop, for Lagrangian systems without dissipative forces (friction) using only position feedback. Firstly, for fully-actuated relative-degree-two systems, the controller is reminiscent of passivity-based controllers for robot manipulators and consists in a linear dynamic system together with a globally-Lipschitz control law. Establishing a global uniform result, all the more with such a simple controller, is particularly valuable relatively to the literature of output-feedback control of systems with non-globally-Lipschitz nonlinearities in the unmeasured variables. This first contribution solves a long-standing open problem and, as a matter of fact, recasted in a general context this result is at the edge of what is achievable -see [24]. Then, we show that our control approach may be applied to a more general problem, that of tracking control of Lagrangian systems augmented by a chain of integrators (with relative degree greater than two). As a corollary, we solve the global-tracking position-feedback control problem for flexible-joint robots but also for systems coupled with output-feedback linearizable actuator dynamics. Finally, we discuss remaining open problems of fairly general interest in the realm of analysis and design of robust nonlinear systems.