Modified parallel cascade control strategy for stable, unstable and integrating processes

This manuscript presents a modified parallel cascade control structure (PCCS) for a class of stable, unstable and integrating process models with time delay. The proposed PCCS consists of three controllers. Internal Model Control (IMC) approach is used to design the disturbance rejection controller in the secondary loop. Parameters of the proportional-integral (PI) controller which is used for setpoint tracking is obtained by equating the first and second derivatives of desired and actual closed loop transfer functions at the origin of s-plane. Routh Hurwitz stability criterion is used to design the proportional-derivative (PD) controller which stabilizes the unstable/integrating primary process model. An analytical expression is proposed for computing the desired closed loop time constant of the primary loop in terms of plant model parameters so as to achieve an user-defined maximum sensitivity. Based on extensive simulation studies, a suitable value for the secondary closed loop time constant is also recommended. This is an advantage of the present work over the reported parallel cascade control schemes where authors provide a suitable range of values for the closed loop time constants. The proposed tuning strategy requires tuning of four/six controller parameters for stable/unstable and integrating process models which is less compared to the reported strategies. Simulation results illustrate that the proposed method yields significant improvement in closed loop performance compared to some of the recently reported tuning strategies for both nominal and perturbed process models.