Multiple-loop control

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 ... 

Ulerich, N. H., and Powers, G. J., On-Line hazard aversion and fault detection Multiple loop control example. AIChE Fall National Meeting, New York, 1987. 

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). 

Real Time Hazard Aversion and Fault Detection Multiple Loop Control Example... 

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. 

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)... 

Multiple loop control (cascade, selective, split range)... 

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 ... 

Figure H.8 Proposed decentralized (multiple-loop) control system specification for the full reactor/flash unit plant.

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. 

This loop is, however, affected by the availability of the reactant oxygen, which in surplus destroys the precursor VPO. Further, oxygen is positively needed to activate and re-oxidize the VxOy sites but leads also to more water formation that in turn hydrothermally deactivates the active mass. Likewise, water is needed to separate, via hydrolysis, the vanadium phosphate into VxOy and mobile phosphate. The multiplicity of the feedback loops is at first puzzling but explains the apparent stable steady state that can be reached with a catalyst undergoing so many chemical and microstructural transformations the multiplicity of controls prevents one single factor becoming dominant and thus potentially destabilizing the whole process. 

The results obtained in the closed-loop control are summarised in Fig. 1 where five time varying curves are presented. The first one is the controlled variable (< B), the second is the input or control variable (A / F) he third is the feed concentration change ( ao), the fourth is the performance index (or objective function) and the last one is the computational time required to get the solution. It should be noted that it happens to the system under consideration to exhibit a steady-state input multiplicity and thus several solutions. This point is well discussed in [12],... 

Another fundamental state variable that can be regulated during the cycle is cavity pressure. Closed loop control of cavity pressure could automatically compensate for variations in melt viscosity and injection pressure to achieve a consistent process and consistency of molded products. Adaptive control methods have been developed to track the cavity pressure profile at one location in the mold. In these earlier works, cavity pressure control was handicapped by the absence of actuators for distributed pressure control, as conventional molding machines are equipped with only one actuator (the screw), which prevents the simultaneous cavity pressure control at multiple points in the mold. This problem has been solved with the development of dynamic melt flow regulators that allow control of the flow and pressure of the polymer melt at multiple points in the mold.[ °l... 

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. 

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 ... 

Chap. 20 Control Systems with Multiple Loops... 

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