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An all-proportional-derivative (PD) control-based modified Smith predictor design is reported here for second-order delay-dominated integrating processes. The proposed control structure is realized with two PD controllers along with a first-order filter towards achieving the desired closed-loop response. To eliminate the tuning complexity, reported internal model control (IMC) scheme suggests a single tuning parameter λ (i.e. closed-loop time constant) to tune both the PD controllers along with the filter present in the modified Smith predictor designing. The forward path PD controller parameters are obtained as per the IMC tuning guideline suggested for servo tracking, whereas the feedback path PD controller is realized based on Routh stability analysis with a goal towards improved regulatory responses. The firstorder filter present in the feedback path helps to ensure robust closed-loop performance. Considerable performance enhancement is observed during set point tracking by the proposed scheme where no overshoot is observed even with smaller rise time. In addition, smooth and reasonably quick load rejection behaviour is also found during a regulatory response. Superiority of the proposed scheme is also substantiated in comparison with others' reported dead-time compensating techniques in terms of closed-loop performance indices as well as stability margins.

An improved auto-tuning scheme is proposed for Ziegler-Nichols (ZN) tuned PID controllers (ZNPIDs), which usually provide excessively large overshoots, not tolerable in most of the situations, for high-order and nonlinear processes. To overcome this limitation ZNPIDs are upgraded by some easily interpretable heuristic rules through an online gain modifying factor defined on the instantaneous process states. This study is an extension of our earlier work [Mudi RK., Dey C. Lee TT. An improved auto-tuning scheme for PI controllers. ISA Trans 2008; 47: 45-52] to ZNPIDs, thereby making the scheme suitable for a wide range of processes and more generalized too. The proposed augmented ZNPID (AZNPID) is tested on various high-order linear and nonlinear dead-time processes with improved performance over ZNPID, refined ZNPID (RZNPID), and other schemes reported in the literature. Stability issues are addressed for linear processes. Robust performance of AZNPID is observed while changing its tunable parameters as well as the process dead-time. The proposed scheme is also implemented on a real time servo-based position control system.

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