Output feedback stability and passivity in nonstationary and nonlinear systems

Recent publications have shown that under some conditions linear time-invariant systems become strictly positive real with constant feedback. To expand the applicability of this result to nonstationary and nonlinear systems, this paper first reviews a few pole-zero dynamics definitions in nonstationary systems and relates them to stability and passivity of the systems. The paper then shows the sufficient conditions that allow a system to become stable and strictly passive via static (constant or nonstationary) output feedback. Applications in robotics and adaptive control are also presented.