Stability of third-order control systems with time lag using Pontryagin´s method for zero determination

Pontryagin´s method for determining zeros is applied to establish the stability criteria for the 3rd-order retarded system characterised by the normalised transcendental equation H(s, es) = s(s2 + as + b)es + K = 0 Various cases are considered in detail. Stability charts based on the stability criteria established are also presented. Critical values of the system parameters obtained from these stability charts agree with the results obtained by the graphical method.