Control System Design Concepts

Having learned a little about hardware and about several strategies used in control, we are now ready to talk about some basic concepts for designing a control system. At this point the discussion will be completely qualitative. In later chapters we will quantify most of the statements and recommendations made in this section. Our purpose here is to provide a broad overview of how to go about finding an effective control structure and designing an easily controlled process. 

A consideration of dynamics should be factored into the design of a plant at an early stage, preferably during pilot-plant design and operation. It is often easy and inexpensive in the early stages of a project to design a piece of process equipment so that it is easy to control. If the plant is designed with little or no consideration of dynamics, it may take an elaborate control system to try to make the most of a poor situation. 

For example, it is important to have large enough holdups in surge vessels, reflux drums, column bases, etc., to provide effective damping of disturbances (a much-used rule of thumb is 5 to 10 minutes). A sufficient excess of heat transfer area must be available in reboilers, condensers, cooling Jackets, etc., to be able to handle the dynamic changes and upsets during operation. The same is true of flow rates of manipulated variables. Measurements and sensors should be located so that they can be used for eflcctive control. 

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In this chapter we will study control equipment, controller performance, controller tuning, and general control-systems design concepts. Some of the questions that wc will explore are how do we decide what kind of control valve to use what type of sensor can be used and what are some of the pitfalls that you should be aware of that can give faulty signals what type of controller should we select for a given application and how do we tune the controller. 

The first set of case studies illustrates errors due to the inadequate design of the human-machine interface (HMI). The HMI is the boundary across which information is transmitted between the process and the plant worker. In the context of process control, the HMI may consist of analog displays such as chart records and dials, or modem video display unit (VDU) based control systems. Besides display elements, the HMI also includes controls such as buttons and switches, or devices such as trackballs in the case of computer controlled systems. The concept of the HMI can also be extended to include all means of conveying information to the worker, including the labeling of control equipment components and chemical containers. Further discussion regarding the HMI is provided in Chapter 2. This section contains examples of deficiencies in the display of process information, in various forms of labeling, and the use of inappropriate instrumentation scales. 

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... 

For a new plant, the problem of designing the control system can be quite difficult as a consequence of unit-to-unit interactions. Thus, understanding the potential sources of these interactions and finding ways in which they can be substantially mitigated are important to achieve effective plant operations. In this chapter, we introduce several key concepts in plantwide control Appendix H deals specifically with how to develop a control system design for a new plant. 

Frequency response concepts and techniques play an important role in control system design and analysis. In particular, they are very useful for 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). 

One control valve (or degree of freedom) must be used for each controlled variable. This relationship between controlled variables and degrees of freedom (or control valves or manipulated variables) is known as variable pairing and is an important concept in control system design. 

In electronics and communications, the drivers are the need for further miniaturization, higher performance, and new optical technologies that provide entirely new products. For example, in aircraft, control systems have progressed from mechanical hydraulic components to fly-bywire electronic systems to the new concept of fly-by-light optical systems. This progression has depended on the development of the appropriate materials to design the performance systems. 

First we will look briefly at some of the control hardware that is currently used in process control systems transmitters, control valves, controllers, etc. Then we will discuss the performance of conventional controllers and present empirical tuning techniques. Finally we will talk about some important design concepts and heuristics that are useful in specifying the structure of a control system for a process. 

To illustrate the concept, consider a single distillation column with distillate and bottoms products. To produce these products while using the minimum amount of energy, the compositions of both products should be controlled at their specifications. Figure 8.13u shows a dual composition control system. The disadvantages of this structure arc (1) two composition analyzers are required, (2) the instrumentation is more complex, and (3) there may be dynamic interaction problems since the two loops are interacting. This system may be difficult to design and to tune. 

There are no inherent linear limitations in feedforward control. Nonlinear feedforward controllers can be designed for nonlinear systems. The concepts are illustrated in Example 11.3. 

Also, there is serious question as to whether such a system would be capable of obtaining a RCRA Part B Permit to operate without formal air emission control systems. These open tank systems are designed to be crude but effective. When one begins to collect flammable and toxic gases over such open tanks and to allow access of fork lift trucks to deliver and retrieve hoppers of slag, the logic of the system falls apart very quickly and one returns to the reactor concept or other options. 

The upper limit of efficiency of the biophotolysis of water has been projected to be 3% for well-controlled systems. This limits the capital cost of useful systems to low cost materials and designs. But the concept of water biophotolysis to afford a continuous, renewable source of hydrogen is quite attractive and may one day lead to practical hydrogen-generating systems. 

As described above, volume-phase transitions in gels with immobilized enzymes are available for the biochemical creation of mechanical energies when coupled with enzymatic changes within the gel phase. In the design of such immobilized enzyme systems, the concept of controlling the phase transition threshold by... 

This citation talks to a core concept of computer systems validation development procedures and system specifications. Note that the firm was not cited for lack of validation testing of the software but for lack of design controls. Effective design controls would have included written design procedures. Adherence to these procedures would have... 

Throughout the design of a chemical plant, issues relating to safety, economics and environmental impact must be considered. By doing so, the risks associated with the plant can be minimised before actual construction. The same principle applies whatever the scale of the process. The field of process control (Chapter 8) considers all these issues and is, indeed, informed by the type of hazard analyses described in Chapter 10. The objectives of an effective control system are the safe and economic operation of a process plant within the constraints of environmental regulations, stakeholder requirements and what is physically possible. Processes require control in the first place because they are dynamic systems, so the concepts covered in the earlier chapters of this book are central to process control (i.e. control models are based on mass, energy and momentum balances derived with respect to time). Chapter 8 focuses on the key aspects of control systems. 

The activity of synthesis occurs throughout a design, from original process conception to construction and operation. Examples are 1) the synthesis of a control system for fixed plant configuration, 2) the synthesis of a start-up procedure for a new process, 3) the selection of materials of construction and 4) the layout of the plant to improve safety. All of these activities involve discrete decision making this aspect is a common denominator of all synthesis activities. 

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