Decoupling Controller Design Based on Gain and Phase Margin Specifications for a Coupled Tank System Model

The objective of a multi-variable control involves maintaining various control variables at independent set points. The interactions present in the system affects more than one controlled variables because of the manipulated variable. Decouplers are designed to reduce the interactions in between the loops in to achieve a satisfactory responses when there is presence of non-minimum phase zeros,multiple time delays and large uncertainty. The dynamic and static decoupling are the two types of decoupling strategies. In this thesis, these control strategies are discussed. In practice, there exists certain process unmodelled dynamics. Hence, there is a necessity to examine the robust stability of a system to check whether the control system stability is ascertained in presence of these unmodelled dynamics. This thesis deals with designing a controller along with decoupler to achieve the desired performance of a TITO system. At first, a decoupler is being designed from the plant matrix. Then, a first order plus dead time model is obtained for each of the decoupled process on the basis of the frequency response fitting. After getting the FOPDT model a decentralized PI/PID controller for each reduced order decoupled model is designed to obtain desired gain and phase margins. The present technique is applied to a coupled tank system. The characteristics like non-minimum phase and non-linear characteristics make the control of coupled tank liquid level system, a standout amongst the most difficult benchmark control problems. The main objective of the coupled tank system is to maintain a desired level of liquid in the two tanks independent of each other when the water enters the tank and when the water flows out. The coupling impact here in this framework is a coupling switch that permits stream of water in the tank at higher level to a tank at lower level. Lastly, robust stability of the control system is analyzed in the presence of various process uncertainties like additive uncertainty and multiplicative uncertainties. The stability analysis is examined using the small gain theorem or the spectral radius criterion. The robust stability of the coupled tank system is also determined

An extended approach of inverted decoupling

This paper presents an extension of the inverted decoupling approach that allows for more flexibility in choosing the transfer functions of the decoupled apparent process. In addition, the expressions of the inverted decoupling are presented for general n × n processes, highlighting that the complexity of the decoupler elements is independent of the system size. The realizability conditions are stated in order to select a proper configuration, and the different possible cases for each configuration are shown. Comparisons with other works demonstrate the effectiveness of this methodology, through the use of several simulation examples and an experimental lab process

Centralized multivariable control by simplified decoupling

This paper presents a generalized formulation of simplified decoupling to n×n processes that allows for different configurations depending on the decoupler elements set to unity. To apply this decoupling method, the realizability conditions are stated. Then, from the previous decoupling in combination with a decentralized control, the formulation of a centralized control by simplified decoupling is developed. After reducing the controller, this last proposed method is modified to a multivariable PID control. From an implementation point of view, the windup problem is addressed for these methods, and an anti-windup scheme for multivariable PID controllers is proposed. Comparisons with other works demonstrate the effectiveness of these methodologies, through the use of several simulation examples and an experimental lab process

Centralized Inverted Decoupling Control

This paper presents a new methodology of multivariable centralized control based on the structure of inverted decoupling. The method is presented for general n×n processes, obtaining very simple general expressions for the controller elements with a complexity independent of the system size. The possible configurations and realizability conditions are stated. Then, the specification of performance requirements is carried out from simple open loop transfer functions for three common cases. As a particular case, it is shown that the resulting controller elements have PI structure or filtered derivative action plus a time delay when the process elements are given by first order plus time delay systems. Comparisons with other works demonstrate the effectiveness of this methodology through the use of several simulation examples and an experimental lab process

Multivariable PID control by decoupling

This paper presents a new methodology to design multivariable PID controllers based on decoupling control. The method is presented for general n×n processes. In the design procedure, an ideal decoupling control with integral action is designed to minimize interactions. It depends on the desired open loop processes that are specified according to realizability conditions and desired closed loop performance specifications. These realizability conditions are stated and three common cases to define the open loop processes are studied and proposed. Then, controller elements are approximated to PID structure. From a practical point of view, the windup problem is also considered and a new anti-windup scheme for multivariable PID controller is proposed. Comparisons with other works demonstrate the effectiveness of the methodology through the use of several simulation examples and an experimental lab process

A COMPARISON OF MULTI-LOOP PI/PID CONTROLLER DESIGN WITH REDUCTION AND WITHOUT REDUCTION TECHNIQUE

Proportional, Integral and Derivative (PID) controller are most widely used controller in chemical process industries because of their simplicity, robustness and successful practical application. Many methods have been proposed for design of Multi-loop PI/PID controller for Multi-Input Multi-Output (MIMO) process. In this paper we have compared two methods for two by two processes with time delays. One is model order reduction and other is without reduction. Performance index and robustness has been used as the criterion for comparison. Several commonly used simulation examples are included for demonstrating effectiveness of the proposed methods and the results obtained are comparatively same

Decoupling Control and Soft Sensor Design for an Experimental Platform

This chapter presents the design and implementation of a decoupling control strategy for an experimental platform and pilot plant, dedicated to the study of the fouling phenomena which occur in industrial tubes. Initially, a set of tests was done for the identification and validation of FOPDT models suitable to the four processes of the multivariable system: flow-voltage, flow-current, pressure-voltage, and pressure-current. After, the interaction between the inputs and outputs of the system was analyzed by the RGA and RNGA matrices. The static decoupling and decentralized PID controllers tuned by the Ziegler-Nichols and IMC methods were designed. Then, the set point tracking response was simulated and implemented using MATLAB and LabVIEW software, respectively. Finally, the concept of soft sensor was applied to monitor the output variables of the experimental platform, for a better performance of the decoupling control

Smith Predictor with Inverted Decoupling for Square Multivariable Time Delay Systems

Versión del autorThis paper presents a new methodology to design multivariable Smith predictor for n×n processes with multiple time delays based on the centralized inverted decoupling structure. The controller elements are calculated in order to achieve good reference tracking and decoupling response. Independently of the system size, very simple general expressions for the controller elements are obtained. The realizability conditions are provided and the particular case of processes with all of its elements as first order plus time delay systems is discussed in more detail. A diagonal filter is added to the proposed control structure in order to improve the disturbance rejection without modifying the nominal set-point response and to obtain a stable output prediction in unstable plants. The effectiveness of the method is illustrated through different simulation examples in comparison with other works

Inverted decoupling internal model control for square stable multivariable time delay systems

Accepted manuscriptThis paper presents a new tuning methodology of the main controller of an internal model control structure for n×n stable multivariable processes with multiple time delays based on the centralized inverted decoupling structure. Independently of the system size, very simple general expressions for the controller elements are obtained. The realizability conditions are provided and the specification of the closed-loop requirements is explained. A diagonal filter is added to the proposed control structure in order to improve the disturbance rejection without modifying the nominal set-point response. The effectiveness of the method is illustrated through different simulation examples in comparison with other works

Double-Loop Multi-Scale Control using Routh-Hurwitz Dimensionless Parameter Tuning for MIMO Processes

This paper presents a new approach to controlling MIMO processes by using the double-loop multi-scale control scheme in the decentralized control architecture. The decentralized PID control system has been used in process industry despite its several limitations due to process interactions, time-delays and right half plane poles. To overcome the performance limitation due to process interactions, decoupling controllers are often added to the decentralized PID control system. The proposed strategy based on the double-loop multi-scale control scheme has some advantages over the existing control strategies for MIMO processes. An advantage of the proposed scheme over the decentralized PID control with decoupling system is that, the proposed strategy has a fixed number of dimensionless tuning parameters that are easy to tune. For an n×n MIMO process, the proposed scheme requires the tuning of only 3 to 6 dimensionless parameters instead of the 3n original PID parameters