Feedback control design techniques

MacFarlane, A.G.J. and Kouvaritakis, B. (1977) A design technique for linear multivariable feedback systems. International Journal of Control, 25, pp. 837-874. 

In regard dynamics and control scopes, the contributions address analysis of open and closed-loop systems, fault detection and the dynamical behavior of controlled processes. Concerning control design, the contributors have exploited fuzzy and neuro-fuzzy techniques for control design and fault detection. Moreover, robust approaches to dynamical output feedback from geometric control are also included. In addition, the contributors have also enclosed results concerning the dynamics of controlled processes, such as the study of homoclinic orbits in controlled CSTR and the experimental evidence of how feedback interconnection in a recycling bioreactor can induce unpredictable (possibly chaotic) oscillations. 

The use of feedback-control techniques to modulate combustion processes in propulsion systems has recently received extensive attention [1-3]. Most of the previous studies involved direct implementation of existing control methods designed for mechanical devices, with very limited effort devoted to the treatment of model and parametric uncertainties commonly associated with practical combustion problems. It is well established that the intrinsic coupling between flow oscillations and transient combustion responses prohibits detailed and precise modeling of the various phenomena in a combustion chamber, and, as such, the model may not accommodate all the essential processes involved due to the physical assumptions and mathematical approximations employed. The present effort attempts to develop a robust feedback controller for suppressing combustion instabilities in propulsion systems. Special attention is given to the treatment of model uncertainties. Various issues related to plant... 

Combustion instability that leads to performance deterioration and excessive mechanical loads, which could result in reduced life and premature failure, is an important issue with modern gas turbine engines and ramjet and scramjet combustors. Various techniques of passive and active control to reduce combustion instabilities and improve performance are addressed. Since extensive, promising research is being carried out to develop sensors and actuators, these techniques can be used in practical combustors in the near future. The topics covered in Section 3 provide the required chemical, kinetic, and fluid dynamic understanding to help the designer who is involved in active feedback control for combustion systems. 

The state estimation technique can also be incorporated into the design of optimal batch polymerization control system. For example, a batch reaction time is divided into several control intervals, and the optimal control trajectory is updated online using the molecular weight estimates generated by a model/state state estimator. Of course, if batch reaction time is short, such feedback control of polymer properties would be practically difficult to implement. Nevertheless, the online stochastic estimation techniques and the model predictive control techniques offer promising new directions for the improved control of batch polymerization reactors. 

Design of Feedback Control Systems Using Frequency Response Techniques 344... 

Part IV (Chapters 13 through 18) covers the analysis and design of feedback control systems, which represent the control schemes encountered most often in a chemical plant. Emphasis has been placed on understanding the effects which various feedback controllers have on the response of controlled processes, and on the selection of the most appropriate among them. The subject of controller tuning has been deemphasized, and as a consequence, the traditional root-locus techniques and frequency response tuning methods have been scaled down. 

In Chapters 17 and 18 we will study a new technique which is often used to design feedback controllers. Quite different from everything we have seen so far, it is called frequency response analysis. 

Latency Latency is the time interval during which receipt of new information cannot change an output even though it arrives prior to the output. While latency time can be reduced by using various types of design techniques, it cannot be eliminated completely. Controllers need to be informed about the arrival of feedback affecting previously issued commands and, if possible, provided with the ability to undo or to mitigate the effects of the now unwanted command. 

The most important and challenging problems in active and passive stmctural control systems are the formulation and solution of optimal control and nonlinear constrained optimization needed to develop appropriate closed loop feedback control algorithms and the optimal placement, which is the central focus of this book. State-of-the-art techniques for optimal design of passive and active control systems are described in detail in various chapters written by researchers aroimd the world. I welcome this new book for offering a very good overview of the current developments in the field. 

Models can often be used to guide the designed experiments that need to be run on the commercial process as part of the qualification effort. Out of this activity come the aims and limits of the process variables, which either directly or indirectly are under closed-loop feedback control and some of which are then monitored by statistical process control techniques (Ref 2). 

Having dealt with the stability of closed-loop systems, we can consider our next topic, the design of feedback control systems. This important subject is considered in Chapters 12 and 14. A number of prominent control system design and tuning techniques are based on stability criteria. 

In previous chapters, Laplace transform techniques were used to calculate transient responses from transfer functions. This chapter focuses on an alternative way to analyze dynamic systems by using frequency response analysis. Frequency response concepts and techniques play an important role in stability analysis, control system design, and robustness analysis. Historically, frequency response techniques provided the conceptual framework for early control theory and important applications in the field of communications (MacFarlane, 1979). We introduce a simplified procedure to calculate the frequency response characteristics from the transfer function of any linear process. Two concepts, the Bode and Nyquist stability criteria, are generally applicable for feedback control systems and stability analysis. Next we introduce two useful metrics for relative stability, namely gain and phase margins. These metrics indicate how close to instability a control system is. A related issue is robustness, which addresses the sensitivity of... 

Frequency response techniques are powerful tools for the design and analysis of feedback control systems. The frequency response characteristics of a process, its amplitude ratio AR and phase angle, characterize the dynamic behavior of the process and can be plotted as functions of frequency in Bode diagrams. The Bode stability criterion provides exact stability results for a... 

In this chapter, we consider the design and analysis of feedforward control systems. We begin with an overview of feedforward control. Then ratio control, a special type of feedforward control, is introduced. Next, design techniques for feedforward controllers are developed based on either steady-state or dynamic models. Then alternative configurations for combined feedforward-feedback control systems are considered. This chapter concludes with a section on tuning feedforward controllers. 

In this section, the Direct Synthesis (DS) method presented in Section 12.2 is extended to the design of digital controllers. We begin with special cases that lead to a PID controller, and then show how other types of digital feedback controllers can be derived using the Direct Synthesis technique. Both Gc and G must be expressed as discrete-time in the closed-loop transfer function (17-59). In Direct Synthesis, the designer specifies the desired closed-loop transfer function YlYg d- The controller that yields the desired performance is obtained from (17-59)... 

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