Control systems multiple loop

Before proceeding we should emphasize that these control systems involve loops that are not separate but share either the single manipulated variable or the only measurement. In this respect the multiple-loop control systems of this chapter are generically different from those we will study in Chapters 23 and 24. 

Figure 8-62 depicts a hypothetical distributed control system. A number of different unit configurations are illustrated. This system consists of many commonly used DCS components, including multiplexers (MUXs), single/multiple-loop controllers, programmable logic controllers (PLCs), and smart devices. A typical system includes the following elements as well ... 

Block diagram reduction Control systems with multiple loops... 

A control system may have several feedback control loops. For example, with a ship autopilot, the rudder-angle control loop is termed the minor loop, whereas the heading control loop is referred to as the major loop. When analysing multiple loop systems, the minor loops are considered first, until the system is reduced to a single overall closed-loop transfer function. 

M i (input) and Jfr0 (output), and Gyb(s) is the product of all blocks in the whole loop—often termed the open-loop transfer function of the control system. It is possible to apply the same rule successively to simplify certain multiple loop control schemes (e.g. cascade control—Section 7.13). 

Figure 1.1.4 Scientific and nonscientific elements of the energy system. Multiple interfaces and control loops exist between the key elements. Science and technology are the enabling elements for all energy processes and additionally serve the important purpose of informing decision makers about necessary regulatory and behavioral boundary conditions.

We noted earlier in this chapter that many reactions in the chemical industries are exothermic and require heat removal. A simple way of meeting this objective is to design an adiabatic reactor. The reaction heat is then automatically exported with the hot exit stream. No control system is required, making this a preferred way of designing the process. However, adiabatic operation may not always be feasible. In plug-flow systems the exit temperature may be too hot due to a minimum inlet temperature and the adiabatic temperature rise. Systems with baekmixing suffer from other problems in that they face the awkward possibilities of multiplicity and open-loop instability. The net result is that we need external cooling on many industrial reactors. This also carries with it a control system to ensure that the correct amount of heat is removed at all times. 

Complex, multiple-loop, process control systems do not pose any new problems of analysis. The closed-loop characteristics of the smaller loops become the individual component characteristics of successively larger loops. 

The techniques for treating the synthesis of simple control systems are generally applicable to complex systems if the complexity arises from multiplicity of loops rather than intransigence of loop elements. 

In such cases control systems with multiple loops may arise. Typical examples of such configurations, that we will study in the present chapter, are the following ... 

Bode diagram, 330-31, 334-37 frequency response, 323-24 interacting capacities, 197-200 noninteracting capacities, 194-96 pulse transfer function, 619 Multiple-input multiple-output system, 20 discrete-time model, 586 discrete transfer function, 612 input-output model, 83-85, 163-68 linearization, 121-26 transfer-function matrix, 164, 166 Multiple loop control systems, 394-409 Multiplexer, 560, 564 Multivariable control systems, 461-62 alternative configurations, 467-84 decoupling of loops, 503-8 design questions, 461-62 interaction of loops, 487-94 selection of loops, 494-503 Multivariable process (see Multiple-input multiple-output system)... 

Chap. 20 Control Systems with Multiple Loops... 

Chapter 20. Chapter 6 of Shinskey s book [Ref. 3] is an excellent reference for multiple-loop control systems. It treats cascade, selective control loops, and adaptive systems. Besides the general treatment of each control configuration, it discusses the practical considerations guiding the selection and design of such control systems. In particular, it covers the following items which could attract the interest of the reader ... 

Mees, A.I. P.E. Rapp. 1978. Periodic metabolic systems Oscillations in multiple-loop negative feedback biochemical control networks. J. Math. Biol. 5 99-114. 

External field-mediated assembly techniques offer flexible and robust control of micro components in fluidic systems. Simultaneous use of multiple fields helps eliminate the disadvantages of individual methods and enables more complex assemblies. Most external field assemblies do not require closed-loop control however, methods like magnetic robots and manipulation of individual components to target locations with external fields require manual or closed-loop control systems. 

Identify the causes or conditions that lead to deviations. For example, low flow can be caused by the failure of the flow control loop. Events can be caused by a single failure or by multiple failures. Ensure that the identified causes are the minimum that will lead to the process deviation. The most common initiating causes are related to control system failures, which can happen multiple times over the life of the process. If the consequence is significant, safety systems are generally required to address identified process hazards. 

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