Series Cascade Control Research Articles

Optimizing the series cascade control structure for nonminimum phase system regulation

AbstractThis work elucidates the control of integrating a nonminimum phase system via a series cascade scheme with fractional‐order P.I. (Proportional–Integral) plus D (Derivative) controller. The traditional Internal Model Control (IMC) is adopted for inner loop controller design. The feedback D controller is synthesized with the outer loop process model, showing the proposed work's universality. The outer loop controller is suggested in the IMC framework after the accountability of fractional‐filter and inverse response compensator. This combination is revealed to enhance performance without compromising robustness. The Riemann sheet principle is explored to compute the stability of the suggested controller. The sensitivity analysis has asserted the robustness. More importantly, the optimal value of controller settings is achieved via the Teaching Learning Based Optimization (TLBO) algorithm. This TLBO algorithm uses an objective function that minimizes Integral Square Error. Two illustrative problems are utilized to examine the recommended control structure's virtue.

Relocated internal model control based cascade control strategy for stable and unstable systems with delay

To control unstable and stable plants a new series cascade control structure (SCCS) is proposed. The proposed SCCS incorporates two or three controllers (setpoint tracking controller, secondary disturbance rejection controller and stabilising controller for only unstable plants). Internal model control scheme (IMCS) augmented with integration of square error (ISE) minimisation along with robustness considerations are applied to design the secondary loop disturbance rejection controller. In contrast, relocated IMCS augmented with ISE minimisation is applied for the outer loop or setpoint tracking controller. Routh Hurwitz stability criteria and maximum sensitivity considerations are used to design the proportional-derivative (PD) controller, which stabilises the unstable plant. It is important to note that the suggested SCCS only needs four controller parameters to be tuned, which is less than the recently reported techniques. Robust stability analysis of the proposed SCCS is performed in the presence of uncertainty. The simulation results depict that the suggested approach imparts pre-eminent improvement in closed-loop performance such as faster setpoint tracking and better disturbance rejection for nominal and perturbed plants.

An Analytical Design Method of PI and PID Controllers for Industrial Series Cascade Processes With Time Delay Under Robustness Constraints

In this paper, an analytical method is proposed to design Proportional-Integral (PI) and Proporional-Integral-Derivative (PID) controllers for two-stage industrial series cascade processes with transport delay under robustness constraints. The main rationale behind using series cascade control structure is that the disturbances in the inner loop are suppressed by the secondary controller before being transmitted to the outer loop. The presented design procedure consists of two steps: In the first step, the controller Cb(s) in the inner loop is designed, while in the second step the controller Ca(s) in the outer loop is obtained. The obtained controller is of PI or PID-type structure depending on the number of selected terms used in Maclaurin’s approximation of the transfer function of the high-order controller. By specifying the robustness constraints within design procedure one define values of adjustable parameters to achieve compromise between robustness and performance indicators. The result is efficient suppression of load disturbances, evaluated by the integral of the absolute error (IAE). The step reference response can be additionally reshaped by using Two Degree of Freedom – 2DoF control structure via suitable selection of set-point weighting factor b, 0≤b≤1, which acts on the control signal through displaced proportional action of the controller. The proposed design method is analyzed with simulations on wide class of typical representatives of industrial processes including stable, integral and unstable processes with time delay. A comparison with recent studies shows the effectiveness of the proposed tuning method for industrial cascade processes.

Modified optimal series cascade control for non-minimum phase system

Abstract This article contributes to handling the Non-Minimum Phase (NMP) system with time delay in the existence of uncertainty and disturbances. The series cascade control scheme is used to overcome such problems. The secondary loop architecture in a series cascade scheme is formulated in the Internal Model Control (IMC) framework. The tuning of fractional-filter via a delayed version of Bodes’ ideal transfer function approach of primary loop controller in a series cascade arrangement shows the novelty of this work. The primary loop controller is designed in the IMC framework after accountability of inverse response and dead-time compensator. Furthermore, Particle Swarm Optimization (PSO) is adapted to accomplish the optimal value of controller settings. The Riemann sheet principle is used to determine stability. The sensitivity investigation is performed to know the robustness of the offered controller. For the effectiveness of the suggested scheme, two case studies are revealed in this paper.

Enhancement in series cascade control for non-minimum phase system

Enhancement in series cascade control for non-minimum phase system

An enhancement in series cascade control for non-minimum phase system

Abstract This work reveals an Internal Model Control (IMC)-based series cascade control for the non-minimum phase and time delay process. The combination of a higher-order fractional IMC filter and inverse response compensator for designing the outer loop controller illustrates the uniqueness of this work. For the time delay term, a higher-order approximation is considered. The standard IMC-PID structure adopts for the inner loop controller design. While the higher-order fractional filter coupled with inverse response compensator takes for the design of the outer loop controller. The suggested scheme demonstrates enhanced exhibition for setpoint tracking and disturbance rejection. Moreover, the sensitivity analysis is also accomplished to determine the robustness of the closed-loop system under process parameter variations.

Optimal Fractional Order IMC-Based Series Cascade Control Strategy with Dead-Time Compensator for Unstable Processes

In industrial unstable processes, disturbance rejection is more challenging task than setpoint tracking. So, cascade control structure is widely used in many chemical processes to reject disturbances. In this work, an advanced dead-time compensator-based series cascade control structure (SCCS) is suggested for unstable processes. The suggested SCCS has three controllers (named as primary, secondary and stabilizing controllers). Both primary and secondary controllers are designed using fractional order-based internal model control (IMC) approach. The stabilizing proportional–derivative controller is designed using maximum sensitivity considerations and Routh–Hurwitz stability criteria. Optimal values of the closed-loop time constants and fractional orders of IMC filters are obtained using constrained artificial bee colony (ABC) algorithm. This ABC algorithm uses a multi-objective function involving minimization of integral of absolute error, integral of time weighted absolute error and integral of squared error. Simulation studies are conducted using some benchmark plant models used in literature for illustrating the advantages of the proposed strategy compared to the state of the art. Moreover, robust stability of the proposed design is analysed and quantitative performance measures are also computed.

Analytical design of fractional IMC filter – PID control strategy for performance enhancement of cascade control systems

The cascade control is widely used in chemical industries for disturbance attenuation as an alternative to a single feedback loop. This article presents an internal model control (IMC)-based series cascade control system for different time delay process models. The novelty of the work lies in the use of fractional order IMC filter of the first order and higher order for designing controller in the outer loop based on specific maximum sensitivity. The design also considers different approximations for the time delay term. The controller designed for the inner loop assumes the standard PID controller structure and the outer loop controller takes a fractional filter PID structure. The proposed method gives improved performance for set point tracking and disturbance rejection. Further, the proposed method gives robust performance for noise in the measured signal. Also, the robustness analysis is performed to know the insensitivity of the closed-loop system for process parameter variations. The fragility of the controller is investigated for perturbations in the controller parameters.

New tuning strategy for series cascade control structure

In the presence of disturbances associated with the manipulated variable, performance of a single loop control structure can be improved with the help of a series cascade control. A design approach for a series cascade control system is proposed in this paper. The secondary controller is tuned by finding the centroid of the convex stability region to quickly attenuate the disturbances arising in the inner loop before they influence the controlled variable, whereas the primary controller is tuned by magnitude optimum criterion to achieve improved servo response in the outer loop. Servo and regulatory responses for the nominal model, as well as for the perturbed model of the plant obtained by the proposed method are compared with some recently reported tuning strategies. Simulation results show that the proposed tuning strategy yields improved closed-loop performance.

Modified Cascade Controller Design for Unstable Processes With Large Dead Time

In this article, a simple controller design with the fractional order calculations applies to unstable cascade processes with a significant time delay. Series cascade control system consists of two loops in which the inner loop is designed using the IMC principle based on the synthesis method. The primary controller is designed with the FOPI-FOPD controller to ensure stable and satisfactory closed-loop performances for the unstable processes. For the primary controller, five tuning parameters are involved, which tuned using the centroid of the convex stability region method. Different examples are given to illustrate the superiority of the proposed design over some existing designs. Results obtained from simulation reveals that with the proposed control design, enhanced closed-loop control performances are obtained for nominal and perturbed conditions.

Complimentary Sensitivity Function based Novel Cascade Control Structure for Automatic Generation Control

This paper introduces a series cascade control structure for automatic generation control. The control structure consists of two loops such as Primary loop and auxiliary loop (secondary loop). The secondary loop controller is designed using internal model control (IMC) approach. The primary loop controller is a PID controller which is tuned using desired complimentary sensitivity function. The beauty of the control structure is that it effectively nullifies the disturbances entering to the secondary loop as well as primary loop. The efficacy of the proposed controller is shown by comparing the simulation results with the existing methods in the literature.

Control performance assessment based on sensor fusion techniques

Control performance assessment techniques are widely studied and many performance assessment indices have been proposed. In this paper, a control performance assessment technique for multi-loop control systems is presented based on the decision fusion strategy. Since decisions based on individual indices can lead to erroneous results, decision fusion of different indices can improve the assessment accuracy, especially in multi-loop control systems in the presence of loop interactions. Performance assessment indices are individually evaluated and decisions based on these indices are fused. The results of simulation and practical implementation on series cascade control structures illustrate the effectiveness of the proposed algorithm.

A simple method of tuning series cascade controllers for unstable systems

A simple method is proposed to design P/PI controllers for a series cascade control system for unstable first order plus time-delay (FOPTD) systems. In this paper, the controller design for unstable FOPTD systems cascaded in series with stable/unstable FOPTD systems is considered. The proposed method is based on equating the coefficients of corresponding powers of s and s 2 in the numerator to α 1 and α 2 times those of the denominator of the closed-loop transfer function for a servo problem. The open loop system consists of an unstable FOPTD system cascaded in series with a stable/unstable FOPTD system. Only two tuning parameters (α 1 and α 2) are required for the design of controllers. The closed-loop performances are evaluated for both the servo and regulatory problems and the performances are found to be better than that of the well established synthesis method. The robustness for uncertainty in the model parameters is studied and compared with that of the controllers designed by the synthesis method.

Simulation analysis of Series Cascade control Structure and antireset windup technique for a jacketed CSTR

Simulation analysis of Series Cascade control Structure and antireset windup technique for a jacketed CSTR

Enhanced cascade control for a class of integrating processes with time delay

Unlike self-regulating processes, cascade control strategies for control of integrating processes with time delay are limited. A novel series cascade control structure to enhance the closed loop performance is proposed for integrating and time delay processes. The proposed controller structure has only two controllers and a setpoint filter. The inner loop controller is designed based on IMC approach and the primary setpoint filter is based on optimal performance index. The primary load disturbance rejection controller, a PID controller in series with a lead/lag compensator, is designed on the basis of the desired closed-loop complementary sensitivity function. The robustness analysis is carried out using Kharitonov's theorem. Simulation results demonstrate the efficacy of the proposed method by showing satisfactory nominal and robust performances.

Diagnosis of Cascade Control Loop Status Using Performance Analysis Based Approach

On the basis of the achievable performance of the control outputs, fault diagnosis of two cascade control systems, including series cascade control (SCC) and parallel cascade control (PCC), is developed. Without any prior knowledge of complicated operating processes and/or external inputs perturbing the operating system, the accurate fault identification can be achieved by a series of the statistical hypothesis procedures applied to the currently measured data. To isolate possible faults, the output variances of the primary and secondary loops are separated into cascade-invariant (CI) and cascade-dependent (CD) terms, respectively, by the Diophantine decompositions. After a sequence of the hypotheses tests is performed on the CI and the CD terms of current control and the achievable performance conditions, the hierarchical diagnosis trees for the primary and secondary outputs are established, respectively, to explore possible faults. The final faults can be inferred by merging both diagnosis trees. The st...