Gain and phase margin controller tuning: FOPTD or IPDT model-based methods?

Control problems have been experienced with FOPTD-based PID tuning methods when the normalized dead-time of the process model is small. One proposed solution has been to treat the FOPTD system model, with small normalized dead-time, as an integral plus dead time (IPDT) process for controller tuning purposes. There has been very little work though on investigating the transition point, as a normalized dead-time value, when the designer should choose to use either FOPTD-based or IPDT-based PID control design methods. This contribution considers finding the transition point(s) for the gain and phase margin design method. For a range of values of normalized dead-time the performance of the IPDT-based PI controller will be compared with that of the FOPTD-based PI controller for both set-point response and load disturbance rejection.