H∞ control using compensators with access to the command signals

H^∞ design in which the compensator is assumed to have immediate access to the command signal is investigated. The authors analyze both linear and nonlinear systems and find that the advantage to this control architecture is great. In terms of the classical Doyle-Glover solution it is found that a solution exists if and only if the X Riccati equation has a positive semi-definite stabilizing solution. The Y Riccati equation and the constraints it introduces are eliminated. This obviously produces improved performance in the linear case, and in the nonlinear case it opens the possibility of finding solutions where finding solutions to the traditional nonlinear H^∞ control problem requires solving very hard equations. In practice one might desire more conservative solutions and produce them with hybrids between this and conventional H^∞ controllers. However, the first step is to work out the extreme case, which is done here