Controller multiloop controllers

In this chapter we have considered control problems with multiple inputs (manipulated variables) and multiple outputs (controlled variables), with the main focus on using a set of single-loop controllers (multiloop control). Such MIMO control problems are more difficult than SISO control problems because of the presence of process interactions. Process interactions can produce undesirable control loop interactions for multiloop... 

Control Strategies for Multivariable Control Problems If a conventional multiloop control strategy performs poorly due to control loop interactions, a number of solutions are available ... 

Pairing of Controlled and Manipulated Variables A key decision in multiloop-control-system design is the pairing of manipu-... 

The MFC strategy is very different from conventional multiloop control strategies and thus initially unfamiliar to plant personnel. 

The component controllers used in the controller subsystem portion of the DCS can be of various types and include multiloop controllers, programmable logic controllers, personal computer controllers, singleloop controllers, and fieldbus controllers. The type of elec tronic con-troUer utihzed depends on the size and func tional characteristic of the process apphcation being controlled. See the earlier section on distributed control systems. 

Multiloop Controllers The multiloop controller is a DCS network device that uses a single 32-bit microprocessor to provide control functions to many process loops. The controller operates independent of the other devices on the DCS network andTcan support From 20 to 500 loops. Data acquisition capability for up to 1000 analog and discrete I/O channels or more can also be provided by this controller. 

The multiloop controller contains a variety of func tion blocks (for example, PID, totalizer, lead/lag compensator, ratio control, alarm, sequencer, and Boolean) that can be soft-wired together to form complex control strategies. The multiloop controller, as part of a DCS, communicates with other controllers and man/machine interface (MMI) devices also on the DCS network. 

Stand-alone systems. Multiloop controllers) or programmable logic controllers (PLC) typically used to control part of a process, and larger supervisory control and data acquisition (SCADA) systems/distributed control systems (DCS) used to control the process or service as a whole (e.g., bulk primary production plant, building management systems). These self-contained systems are a component of an automated manu-... 

But current versions of MPC have significant disadvantages in comparison with conventional multiloop control ... 

The optimal robust controller designed with one of the new synthesis techniques is generally not of a form that can be readily implemented. The main benefit of the new synthesis procedure is that it allows the designer to establish performance bounds that can be reached under ideal conditions. In practice, a decentralized (multiloop) control structure is preferred for ease of start-up, bumpless automatic to manual transfer, and fault tolerance in the event of actuator or sensor failures. Indeed, a practical design does not start with controller synthesis but with the selection of the variables that are to be manipulated and measured. It is well known that this choice can have more profound effects on the achievable control performance than the design of the controller itself. This was demonstrated in a distillation example [17, 18] in which a switch from reflux to distillate flow as the manipulated variable removes all robustness problems and makes the controller design trivial. 

In the multiloop controller strategy each manipulated variable controls one variable in a feedback proportional integral derivative (PID) control loop. Taking a single-feed, two-product distillation column with a total condenser and a reboiler as an example, a basic list of possible controlled variables includes the distillate and bottoms compositions, the liquid levels in the reflux accumulator and the column bottom, and the column pressure. The main manipulated variables are the reflux, distillate, and bottoms flow rates and the condenser and reboiler heat duties. 

Part V (Chapters 19 through 22) deals with the description, analysis, and design of more complex control systems, with one controlled output. In particular, Chapter 19 introduces the concept of feedback compensation with Smith s predictor, to cope with systems possessing large dead times or inverse response. Chapter 20 describes and analyzes a variety of multiloop control systems (with one controlled output) often encountered in chemical processes, such as cascade, selective, and split-range. Chapter 21 is devoted exclusively to the analysis and design of feedforward and ratio control systems, while Chapter 22 makes a rather descriptive presentation of adaptive and inferential control schemes why they are needed and how they can be used. 

The first troubleshooting step is to determine if the user I/O section is operating properly. This can be done by comparing the display functions with independent functions not a part of the system or loop being checked. If the I/O section is shared with several multiloop controllers, it can be checked with a known good controller. 

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