Control systems with quasi-critical damping

The concept of critical damping is extended to linear systems of order greater than two, and is then termed quasi-critical damping. When applied to a simple class of feedback systems, this condition is shown to have various simple interpretations in terms of (i) the poles of the closed-loop transfer function, (ii) the coefficients of the characteristic equation, or (iii) the open-loop lags. For practical systems, one of the open-loop lags must predominate over the others, and a very straightforward design rule may then be applied. Quasi-critical damping thus constitutes an appropriate design criterion of stability, particularly in the early theoretical stages of system synthesis.