Dcs Controller

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ISA is the international society for measurement and control ISA is the international society for measurement and control

Volume EMC 02.01

Distributed ControllerHardware and Software Basics

Samuel M. Herb, Author

Controller Hardware Structures Controller Software Structures Controller Redundancy

Connections to the Controller

Taken from the book: Understanding Distributed Processor Systems for Control


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Notice

The information presented in this publication is for the general education of the reader. Because neither theauthors nor the publisher have any control over the use of the information by the reader, both the authors andthe publisher disclaim any and all liability of any kind arising out of such use. The reader is expected toexercise sound professional judgment in using any of the information presented in a particular application.

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Copyright 2000 Instrument Society of America.

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Editor s Introduction

This mini-book is available both in downloadable form, as part of the ISA Encyclopedia of Measurement and Control, and bound in a print format.

Mini-books are small, unified volumes, from 25 to 100 pages long, drawn from the ISA catalog of reference and technical books. ISA makes mini-books available to readers who need narrowly focusedinformation on particular subjects rather than a broad-ranging text that provides an overview of the entire

subject. Each provides the most recent version of the material in some cases including revisions that havenot yet been incorporated in the larger parent volume. Each has been re-indexed and renumbered so it canbe used independently of the parent volume. Other mini-books on related subjects are available.

The material in this mini-book was drawn from the following ISA titles:

Understanding Distributed Processor Systems for Control , by Samuel M. Herb, Chapters 4-7.Order Number: 1-55617-645-7

To order: Internet: www.isa.orgPhone: 919/549-8411Fax: 919/549-8288Email: [email protected]


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Chapter 4. Controller Hardware Structures.............................................. 1Traditional Process Controllers ...........................................................................1

Architectures of Controllers ................................................................................2

Chapter 5. Controller Software Structures................................................ 7Programming .......................................................................................................7Organizing Execution Time for Control Action ..................................................8Software Structure Advances.............................................................................11

Programming vs. Configuration ........................................................................11Function Blocks.................................................................................................12Object-Oriented.................................................................................................13Connecting the Blocks ......................................................................................14Changing Control Strategy ................................................................................16

Chapter 6. Controller Redundancy.......................................................... 21Single Loop Integrity Myth.............................................................................21Redundant Mainframe Computers ....................................................................22Microprocessor-Based Shared-Loop Redundancy...........................................22Microprocessor-Based Redundancy in Single Card Controller Racks ............24

Windows NT as Controller ................................................................................26Power Interruptions ..........................................................................................27

Chapter 7. Connections to the Controller .............................................. 29Input/Output Subnetworks to Field Devices ....................................................29Digital Conversion of Inputs and Outputs ........................................................30Remote Input/Output Connections ..................................................................32Influence of Fieldbus.........................................................................................35Input/Output Subnetworks to Humans ............................................................36

X Windows ........................................................................................................38 X-Window Architecture ....................................................................................40

Incorporating X Windows in a Control System.................................................40Customer Response...........................................................................................41

Index............................................................................................................ 45

Table of Contents


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4444Here we go with that word understanding again. To understand the microprocessor-based controller needs a little understanding of the traditional analog controllers from

which it evolved. Different vendors took different approaches to their respectivedesigns based upon how they viewed the role their analog counterparts.

Traditional Process ControllersRecall that programmable logic controllers (PLCs) used in factory automation began asreplacements for banks of relays, with no operator interface other than start/stop but-tons to initiate action and lamps to follow the progress of an operation and notify theoperator of its completion. The origins of process control were quite different.

Early process controllers were physically one with the operator faceplate. They included not just a process variable (PV) indicator on a calibrated scale but also a setpoint (SP) indicator on that scale and a control signal output indicator. On some instru-ments, the later was not an indication of the controller output, but rather the actualposition of the end element (valve, motor drive unit, etc.) coming from a feedback sig-nal. This was all part of a process loop (Figure 4-1): process to sensor, sometimesthrough a transmitter, to report process conditions to the controller, to drive the final(end) element, which adjusted the process.

Figure 4-1. Simple Process Loop Using a Traditional Single Loop Controller

Process Variable (PV)

Output

Set point (SP)

Process

Analog Loop

Controller Hardware Structures


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2 Chapter 4

In the distributed controller file or rack the card cage became thelocation of several loops that shared a processor (Figure 4-2). Likebefore, there were still hard wires to sensors and final elements, but

within this controller rack there were no longer direct wires and tradi-

tional connections. What starts to be quite unique about a microproces-sor-based controller compared to the traditional controller is theopportunity for significant interconnection of the analog and discrete.They both had to be converted to the digital domain. Now alarmed

values in one loop could trigger actions in another. There was even the not-so-obviousfact that an endless number of wires can be connected to a digital point in the sys-tem with no voltage/current loading. In the past, this last idea placed a severe restric-tion on one s ability to develop many kinds of control strategies . This new freedom from hardware and hard wiring is permitting many control strategies, that were merely hypothetical in the past to become practical and inexpensive. Hence they offeredanother opportunity to introduce productivity where it had been not possible in thepast a new way to see!

Architectures of ControllersThese electronic devices called controllers frequently had special function cards in theinitial stages of their design. There would be a special card devoted specifically toinputs and outputs and a special card to retain all of the algorithms (or functionsblocks) that were used in this system. Other cards acted as a calculator, stored the data-base, digitally communicated over the data highway, communicated to external devicesremotely connected to that controller, and so on.

Two very general controller architectures emerged and by the mid-1990s populated themajority of DCSs still operating by the tens of thousands worldwide. Both types influ-

Figure 4-2. Several Loops Share the Same Digital Controller

Typical Distributed Control Loops

(Digital Communications)Data Highway Control

Room

Field Signals (Analog or Digital)


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Controller Hardware Structures 3

enced the approach used in the newest of designs and are worth understanding, espe-cially if your company owns a few. Each version has distributed processing, but they distributed it in different ways.

In what I call the shared-function version, all the loops coming into that particular controller shared several microprocessor cards (Figure 4-3). Microprocessors were functionally distributed into I/O processing, control processing, communication pro-cessing, and so on. In this particular structure, the same card sets were used for all ver-sions of control in most cases. As a result, all the controllers were alike from a hardware

viewpoint and relatively easy to maintain and expand in their own way. They could berecognized by their having such cards as an output card (or a hold card, perhaps calleda hold station), an input conditioning card, a database card, an algorithm card, and anexternal communication card. These cards were usually accompanied by the other required cards, such as a power supply card and a data highway card. The earlier con-trollers made by EMC, Fischer and Porter, Foxboro Microspec, Honeywell TDC2000,Leeds & Northrup s MAX 1, Moore Products MYCRO and Powell System s Micontended to be of this type.

The major advantage of the shared function controller is that all the control-lers have the same hardware/software sets, making ordering, installing, changingorders, training, maintaining, and storing of spare parts very simple.

The other generalized construction was to have individual microprocessor cards for each loop or loop set (Figure 4-4). Some cards were for loop control, different ones

were used for logic control. The functions of output, input, signal conditioning, and soon occurred on each card by the same processor. There were also separate cards for

Figure 4-3. Physical Controller Structure: Shared-Function Controller Rack

All control loops share several cards Same sets for all Function processing is distributed:

! I/O ! Control ! Data highway ! Communication

OutputInput Conditioning

Data Base Algorithm (on some)

Communication (External)Data Highway (Internal)


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4 Chapter 4

programming beyond the vendor s standard set of algorithms. Frequently, thesecards are called multifunction, multipurpose, or similar names. Different combina-tions of cards were unique in each controller rack depending on the customer s order.Each controller rack would also have a highway card and a power supply card, some-

times another card for diagnostics, and still another card for a bus driver to connectthese cards together. These systems in those days tended to be Bailey, Fisher Controls,Foxboro I A, Rosemont, Taylor, and Toshiba.

The major advantage of the individual loop controller approach is that the lossof a processor only impacts one loop or small cluster of loops.

Later designs began to combine these ideas in ways not so easy to classify, taking advan-

tage of ever increasing processor power and memory capacities. Emerging from both of these earlier designs is a more current design in which all the functions are embedded ona single card, or module (Figure 4-5). Most architectures being sold in the mid-1990s use

what are called multifunction controllers rather than loop controllers, logic controllers,and separate application controllers. This approach also provides the advantages of thesingle hardware/software set. This is the form that soft control within PCs is taking.

The advantages of having multiple loops residing within the same card makes it evenmore possible to create some very powerful multiloop control strategies. This capabil-ity defeats the purpose of single loop integrity, which is near impossible to achieve

with any kind of interlocking control strategy. The only protection for today s controlstrategies is redundant controllers, which are now more practical and reasonably priced than before. (I personally have always called single loop integrity a marketingmyth that was based upon a battle between two vendors in the late 1970s.)

Figure 4-4. Physical Controller Structure: Individual Loop Controller Rack

Individual cards for each control loop Different sets per order Select card for each needed function: ! Loop ! Logic ! Data Acquisition ! Multifunction

Highway

Bus DriverDiagnostics (some)

Each Card Has one P for Algorithm, Data Base, etc.


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Controller Hardware Structures 5

Expect to see wider use of multiple configuration languages within the same module.The International Electrotechnical Commission s standard IEC 61131-3 defines fivesuch languages, each of which has been popular with various PLC vendors. Four of these languages are function blocks for continuous control, ladder logic for discreterequirements, sequential function charts for sequencing and batch control, and struc-

tured text for complex calculations. (The fifth is like assembly language, not very usefulas a user-friendly language for process configuration). One process vendor has beenusing multiple configuration languages within the same module since 1992, and offersthe ability to blend the configuring of all the languages in the same graphic technique.More process vendors are beginning to employ one or more of the IEC languages intheir control configuration.

Figure 4-5. Single Control Module with Multiple Languages (and Capabilities)Within the Same Module

Continuous, discrete, sequential control Four blended languages (IEC 1131-3)

FI 134:=FT 130 (PT 450)/(TT 673)

Function Blocks

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