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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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AbstractA simplified tuning guideline for internal model control (IMC) based modified Smith predictor technique is reported here for unstable lag dominated first-order processes with dead time (UFOPDT). Pole location in right half section of s-plane signifies the unstable behaviour of UFOPDT processes. Mostly, chemical processes like isothermal chemical reactor, bioreactor, dimerization reactor, fluid catalytic cracker etc. are found to be lag dominated and unstable along with considerable time delay. Smith predictor technique based control methodology is considered to be a well-accepted approach for such cases. However, conventional Smith predictor technique is not capable enough for controlling UFOPDT processes. Whereas modified Smith predictor is found to be quite competent in such cases as its design involves more than one controller. Modified Smith predictor structure is capable to provide desirable closed loop response during set point tracking along with the load recovery phases. To mitigate the tuning complexity of multiple controllers involved in modified Smith predictor designing, suggested IMC structure employs single tuning parameter λ i.e. closed loop time constant for all three controllers concerned. Noticeable performance enhancement is reported by the proposed scheme as no overshoot is observed during set point tracking. Moreover, smooth and efficient load rejection behaviour is also obtained. Supremacy of the proposed tuning is established through closed loop performance comparison with others’ reported modified Smith predictor based tuning relations for chemical reactor and bioreactor in terms of performance indices as well as stability margins.

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Improved disturbance rejection behaviour with modified Smith predictor is reported here for controlling integrating first-order plus time delay processes. Due to location of a pole at origin, process is said to be integrating in nature. In addition, due to presence of considerable dead time, it is very difficult to obtain the desired output from such processes using conventional control technique. In practice, a good number of chemical processes (e.g. distillation, evaporation, combustion, drying etc.) are integrating as well as delay dominating in nature. To ascertain desirable close-loop response for processes with large dead time, Smith predictor is a renowned methodology due to its simplicity and efficacy. But, this technique fails to perform satisfactorily for integrating processes with time delay. A good alternative can be considered as modified Smith predictor. This technique involves more than one controller for achieving desirable servo as well as regulatory responses. To avoid the tuning complexity of controllers, our proposed scheme involves comparatively less number of controllers with relatively simple tuning guide line. Distinct feature of the proposed tuning scheme is that process overshoot can be restricted within acceptable limit as well as improved load recovery can also be achieved. Efficacy of the proposed scheme is substantiated through performance assessment as well as stability study in comparison with well-known modified Smith predictor based tuning relations is also reported.