Fractional-Order PID Controllers Design for Inverse Response Processes Control

In the last decades the use of fractional calculus in control applications has risen especially in the process modelling and the control algorithm design and controller tuning which are indispensable stages in the design of a control system. Several works have proved the effectiveness of the use of fractional order proportional-integral-derivative controllers, to satisfy performance considerations in the set-point tracking and in the load disturbance rejection, as well as for achieving robustness levels. Most of the studies have focused the tuning of FOPID controllers based on stable dynamics well characterized by first and second order plus dead time models as well as integrating Dynamics. However, when the process presents an inverse response there are not so many Works that explain how design the closed-loop system based on performance and robustness considerations and using a FOPID controller. In this work we present a methodology to compare the performance provided by the use of PID and FOPID when a robustness constraint is considered for inverse response dynamics in the control of an isothermal continuous stirred tank reactor (CSTR) and a process model with a transfer function which has large values for the normalized dead time and for the ratio between the zero time constant and the dominant time constant.