Abstract
Conditions are given under which a second-order system can be regularizedby output feedback. It is shown that under these conditions, proportionaland derivative output feedback controls can be constructed such thatthe closed-loop system is regular and has index at most one. This propertyensures the solvability of the resulting system of dynamic-algebraic equations.A reduced form is given that allows the system procedures used toestablish the theory are based only on orthogonal matrix decompositionsand can therefore be implemented in a numerically stable way.