In this work we present a method to analyze the robustness of stability of a time-delay system (TDS) with respect to the delays. This is done by computing the delays for which the system has a purely imaginary eigenvalue. These delays, called critical delays , generate potential points for a stability switch, i.e., the point where the system switches from a stable to unstable. To derive the method, we find a Lyapunov-type equation , equivalent to the characteristic equation of the TDS. Unlike the characteristic equation, the Lyapunov-type equation does not have any non-exponential terms if the eigenvalue is imaginary. This allows us to solve the Lyapunov-type equation by rewriting it to a quadratic eigenvalue problem for which there are efficient numerical methods. For the scalar case, we find a new explicit expression for the curves in the stability chart. The method is applied to previously solved examples as well as previously unsolved problems of larger dimension.
NV 20150504