Full-actuation of a dynamical system really provides great potential for system control, and yet this potential is seldom utilized or even recognized. In this paper, the direct parametric approach for fully-actuated second-order nonlinear systems recently proposed is generalized to the case of fullyactuated second-order nonlinear systems in descriptor forms. It is revealed that, with this proposed direct parametric approach, a fully-actuated descriptor system, no matter linear or nonlinear, can actually be turned, by a set of state proportional plus derivative feedback controllers, into a linear descriptor system with the dynamical part of the system being a constant linear one with a desire eigenstructure. A general complete parametric expression for all such controllers is established based on the solution to a type of fully-actuated second-order Sylvester matrix equations. What is more, in such a realization the approach also provides all the degrees of freedom which are represented by three parameter matrices and may be further utilized to improve the system performance.Index Terms-Fully-actuated second-order descriptor systems, direct parametric approach, eigenstructure, degrees of freedom, nonlinear systems.
I. INTRODUCTIONSecond-order systems capture the dynamic behavior of many natural phenomena, and have found applications in many fields ([1]-[7]). Among most reported results, control designs of such practical systems are carried out by using the first-order system framework. Note that most of these practical systems are time-varying and/or highly nonlinear, these reported approaches based on first-order system theory are generally impossible to provide a complete answer for the stability or the performance of the closed-loop system.Through some investigation, it is observed that the dynamical models for such applications are really originally nonlinear, and often appear in a matrix second-order form. What is more, most of the dynamical models of such practical systems are fully-actuated, that is, there are as many control variables as the number of state variables.Full-actuation is really a very good property of a system. It in fact provides great potential and allows the system