Predictive control of unstable time delay series cascade processes with measurement noise.

In this paper, generalized predictive control strategy is applied for unstable time delay series cascade processes. The control structure consists of a secondary loop with a secondary controller designed based on simple internal model control (IMC) method and the primary loop consists of a set-point achievement controller and is designed using synthesis method. Further, a generalized predictor consisting of two filters that deals with time delay, disturbance rejection and noise rejection capabilities is also designed in the primary loop. The filters are designed to ensure internal stability as well. For the predictor, only one tuning parameter is involved, which can be tuned in such a way that a trade-off is attained between disturbances attenuation and performance under uncertainties. In this work, various examples of cascade processes with unstable dynamics and time delay are considered for simulation. Results obtained from simulation reveals that with the suggested predictive control scheme, enhanced closed loop control performances are obtained for both nominal and perturbed conditions both in the presence and absence of measurement noise. The comparison is also verified quantitatively with the literature methods using integral absolute error (IAE) and total variation (TV) measures.

Optimal controller synthesis for second order time delay systems with at least one RHP pole.

An optimal H2 minimization framework is proposed in this paper for devising a controller of PID in nature, based on a refined IMC filter configuration. The tuning strategy is for controlling time delay system with at least one pole which falls on the right half of the s-plane. An underdamped model based filter is used in place of the unity damping ratio (critically damped) filter available in the literature to improve the reset action. The method has a single adjustable closed loop tuning parameter. Guidelines have been provided for choosing the pertinent tuning parameter based on the sensitivity function. Simulation work has been executed on diverse unstable models to support the advantages of the proposed scheme. The proposed controller yields improved performances over other recently reported tuning techniques in the literature. Experimental implementation is carried out on an inverted pendulum for demonstrating the practical applicability of the present method. The efficacy of the intended controller design is quantitatively analyzed using the time integral performance index.

Enhancing the performance of parallel cascade control using Smith predictor.

Parallel cascade controllers are used in chemical processing industries to improve the dynamic performance of a control system in the presence of disturbances. In the present work, a delay compensator has been incorporated in the primary loop of the parallel cascade control system. The secondary controller is designed using the internal model control (IMC) method. The primary controller is designed based on a direct synthesis method for the delay-free system. Design of controllers for slow (when the secondary loop dynamics is slow i.e. process contains poles sufficiently slower than the desired closed loop response) as well as fast dynamics (when the inner loop dynamics is fast i.e. process contains poles sufficiently faster than the desired closed loop response) of the secondary process is considered. The method provides robust control performances. Significant improvement in the closed loop performances are obtained with the delay compensator over that of a conventional parallel cascade control system. Several case studies are considered to show the advantage of the proposed method when compared to other recently reported methods.

Analytical design of modified Smith predictor in a two-degrees-of-freedom control scheme for second order unstable processes with time delay.

A modified form of Smith predictor is designed for controlling unstable second order plus time delay (USOPTD) processes, with/without a zero. USOPTD process transfer functions arise in modeling of many chemical and biological systems, such as isothermal continuous stirred tank reactors (CSTR) carrying out autocatalytic reactions, crystallizers and non-isothermal CSTRs. The modified Smith predictor scheme requires the design of three controllers. Synthesis method is used for the design of the three controllers, and analytical formulas are given for these controllers. The method has two tuning parameters, and guidelines are provided for selecting the tuning parameters. Robust disturbance rejection performances are achieved. Different examples are considered, and simulation studies are given, to show the effectiveness of the proposed method. A significant improvement is obtained, when compared with recently reported methods.

Set point weighted modified Smith predictor for integrating and double integrating processes with time delay.

A simple method of designing the controllers for a modified form of Smith predictor is proposed for integrating and double integrating processes with time delay. The modified Smith predictor has two controllers, namely, a set point tracking controller and a load disturbance rejection controller for obtaining good set point tracking and load disturbance rejection, respectively. The set point tracking controller is designed using the classical direct synthesis method based on the process model without considering the time delay. The disturbance rejection controller is considered as a proportional-derivative (PD) controller and is designed using optimal gain and phase margin approaches. Set point weighting is considered for reducing undesirable overshoots and settling times in the modified Smith predictor. Guidelines are provided for selection of the desired closed loop tuning parameter in the direct synthesis method and the set point weighting parameter. The method gives significant load disturbance rejection performances. Illustrative examples are considered to show the performances of the proposed method. A significant improvement in control performance is obtained when compared to recently reported methods.