UNCERTAINTY COMPENSATION FOR A FLEXIBLE LINK MANIPULATOR USING NONLINEAR H-INFINITY CONTROL

In a flexible-link manipulator, in general the effect of some paramete rs such as payload, friction amplitude and damping coefficients cannot be exactly measured. One possibility is to consider the above as para meters with uncertainty. In this paper, constant as well as L-2-bounde d deviations of parameters from their nominal values are considered as uncertainties. These uncertainties make it difficult for a linear con troller to achieve desired closed-loop performance. To remedy this pro blem, a nonlinear dynamical model of a flexible-link manipulator which has a constant input vector field (g in (x) over dot = f(x) + g(x)u) is obtained. Based on recent results in nonlinear robust regulation wi th an H-infinity constraint a nonlinear controller is designed for the flexible-link manipulator. The contribution of this paper is in demon strating that the nonlinear controller has a larger domain of attracti on than the linearized controller. In fact, for the single-link flexib le manipulator considered in this paper, the linear H-infinity control ler results in instability for step changes in the desired output of g reater than 3.6 rad, whereas the nonlinear H-infinity controller yield s desired step changes of 2 pi rad. Simulation results demonstrating t he advantages and superiority of the nonlinear H-infinity controller o ver the linear H-infinity controller are presented.