Computing stability domains in the space of time delay and controller coefficients for FOPDT and SOPDT systems

In this paper, a method is proposed to compute the maximum allowable time delay for first-order plus dead time and second-order plus dead time systems, in order to maintain stability. Designing first order controllers for such systems to preserve stability for a longer time delay is the main aim of this paper. The procedure uses the properties of the phase diagram of the open-loop transfer functions of such systems. The effects of the variation of the controller coefficients on the maximum allowable time delay of the systems are investigated and the stability domains in the space of the uncertain delay and the controller coefficients are computed. The results can be used to design first-order robust controllers for first and second order processes containing uncertain delays.