Relative stability and robust stabilityAclosed-loop feedback system is either stable or not stable. We say that a feedback controlsystem that displays a bounded response to a bounded input is stable. Note that this meansthat the closed-loop feedback system must have a bounded output to every bounded input.This is known as absolute stability. We might wonder about marginally stable closed-loopsystems wherein the response remains bounded, but does not decay with time. Typicallyourdesign specifications require the closed-loop feedback system tracking error response,represented in the frequency domain as E(s)=Y(s)-R(s), to decay to zero-not just remainbounded-and this is the focus of this discussion. Obviously, it is not practical to test theresponse to every bounded input; however, a necessary and sufficient condition for the closed-loop feedback system to be stable is that all the poles of the closed-loop transfer function lie inthe left-half s-plane-this is a primary require