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57 3

Distributed control

sys t ems

Distributed control systems for industrial

automation have been available since the

mid-1970s, and originally programmable electronics

was used to realise the functions previously done

with relays or hard-wired electronics. Newer

systems use more efficient programming methods,

and greater emphasis is laid on user friendliness

and flexibility

by Thomas Pauly

The concept of 'instrumentation'

originates from the time when the

operator controlled the process

manually, using instruments

directly connected to the process.

From the beginning, automation

implied the closing of individual

loops with mechanical, hydraulic or

pneumatic controllers. During the

1950s and 1960s electronic process

controllers were developed, with

each controller realising a single

loop.

The front panel of such a con-

troller included displays for actual

and set-point values, and controls

for manually adjusting values.

Electronic controllers made it

easier to connect a computer for co-

ordinating several loops, and the

computer could also be used for

process optimisation. The instru-

mentation systems of the type with

which we are familiar today were

developed in the mid 1970s. These

systems were built up from process

stations handling multiple loops,

and operator workstations com-

municating via a data highway.

Operator workstations using colour

visual display units (VDUs) replaced

the ubiquitous front panel, and

more advanced functions were inte-

grated into the operator stations.

As process stations were gener-

ally very limited in the storage of

historical data and calculating abil-

IEE.1987

ity etc., such functions necessitated

the use of general-purpose comput-

ers connected via a data highway.

PLC system

Programmable logic controllers

(PLCs), introduced in the early

1970s, were developed as replace-

ments for relay systems and hence

were limited to Boolean functions;

simple calculations and monitoring

of analogue limit values etc. were

added later. Towards the end of the

1970s, communication facilities and

operator workstations were added

to PLC system s, similar to . instru-

mentation systems. Like these,

PLCs lacked functions for advanced

control and calculations. Again such

functions were provided by a separ-

ate computer connected to the data

highway.

Integration of

instrumentation and PLC

systems

The traditional way to integrate

instrumentation and PLC functions

is to interconnect a system of each

kind by means of an interface unit

(Fig.l). The instrumentation system

is usually on a higher level, since it

has more advanced operator and

calculation functions. This method

of integration has several disadvan-

tages,

besides the obvious one of

several different suppliers:

the operator must use different

screens for each function

alarms mus t be acknowledged in

both systems

different programming languages

and documentation methods

interlock signals are need ed

between the systems.

Certain suppliers offer both instru-

mentation and PLC systems; and

we can see a general effort to

integrate the different products by

allowing them to communicate via

the same highway. From the func-

tional viewpoint, however, the two

systems are still different, since

they have to be compatible with

earlier systems.

SC D systems

SCADA is an acronym for Super-

visory Control

And

Data Acquisition

and comes from telemetry applica-

tions,

where simple local units

acquire and transmit data to the

central processor. Such systems are

primarily intended for supervision

and manual control. In the SCADA

system, the central computer con-

tains the database describing the

entire process, and is kept updated

via slow communication links from

the remote units, imposing special

demands on the communication

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prod,

plan

optimisations

management

etc.

*' - *

:

' '

1 The traditional way of interconnecting instrumentation and PLC functions is via a specially programmed interface

unit or gatew ay

protocols to ensure an acceptable

transmission efficiency. Most

SCADA systems provide communi-

cation with instrumentation and

PLC systems via interface units con-

nected to the data highway, but

there is still the need for a higher-

level computer for the functions that

cannot be accomplished in the

instrumentation and PLC systems.

new system concept

A modern, integrated automation

system combines instrumentation

and PLC functions, and the super-

visory functions of the SCADA

system: ASEA Master is an exam-

ple of such a system. A system of

this kind is built around a local area

network (LAN) (Fig.2). The principle

of a data highway is that a certain

memory area is reflected from one

station to another. In the LAN it is a

matter of messages being transmit-

ted between functions in the com-

municating stations; a local area

network consequently offers more

scope when the system contains

many different functions.

The integrated system is divided

into process stations and operator

workstations. The database, which

may be considered as a description

of the process and its status, is fully

distributed across the process

stations the entire control func-

tion is also located in the process

station. Operator workstations con-

trol only the presentation of infor-

mation, and they must access the

process stations directly via the

LAN for the real-time data. The pro-

cess station can handle both logic

and regulatory control, thus avoid-

ing the disadvantages associated

with two different systems.

It is possible to perform advanced

calculations in the process station

which require a special computing

station in traditional systems. Joint

control of different parts of a pro-

cess can be programmed in the pro-

cess stations, irrespective of

whether the process parts are con-

trolled by the same or different

stations, as the process stations uti-

lise the LAN to exchange informa-

tion. A minicomputer may be con-

nected to the local area network,

generally via a gateway, to handle

more long-term information, and to

perform production control and opti-

misation.

As the communication system is

isolated from the application soft-

ware by the LAN, it is possible to

expand or modify the existing com-

munication network, and even to

upgrade it by adding new com-

munication systems (e.g. MAP)

without affecting application soft-

ware. In addition, the system can

be expanded step by step from sim-

ple machine control to complete

automation of an entire factory by

the subsequent addition of further

nodes on the LAN.

The system utilises symbolic

addressing of the distributed data-

bases,

such that measuring points,

controllers etc. are identified by

names. The system itself translates

the names into physical addresses

(i.e.

station and database

addresses) and the user is not

involved with cross-reference

tables.

Suppliers of traditional instrumen-

tation or PLC systems are faced

with the difficulty of introducing

more advanced processing because

of the need for compatibility with

earlier systems. Fig.3 illustrates one

way of solving this conflict. The old

system continues to be the actual

control system, possibly supple-

mented by a local operator work-

station. The new system communi-

cates with the old via a special

access station which includes a pro-

cess database that is cyclically

updated from the process stations.

All differences between the old and

the new system are reconciled here,

e.g. the translation of physical

addresses into symbolic ones, but

the user is obliged to build the

cross-referencing database into the

access station.

Process stations:

The following

functions are typical of those

included in a process station within

an integrated automation system:

logic and sequentia l control

closed-loop control, including

self-tuning adaptive control

calculations and process optimi-

sation

alarm/event recording; logging of

measured values

man-machine communication in

the form of traditional panels and

VDUs.

The application programming is

done in a high-level language using

function elements in a graphic rep-

resentation, with automatic docu-

mentation.

Function block language: Th e

application software for one such

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57 5

typical system is programmed with

a function block language called

ASEA MasterPiece Language

(AMPL). A special feature of this

language is that each function is

treated as a black box that is, an

element, with inputs and outputs.

Such an element may have a simple

function; e.g. a logic AND function,

or a complex one, namely a com-

plete PID controller. The inputs and

outputs of an element may be con-

nected to the inputs and outputs of

other elements, or to the process

I/O; this simple connection activity

constitutes the 'programming' and

is done graphically using a pro-

gramming terminal. The resulting

AMPL program may be automatic-

ally documented in graphical form.

In addition to function elements,

the language includes a number of

structure elements allowing the

application program to be sub-

divided into convenient modules, to

ease commissioning and fault find-

ing. The following example illus-

trates how such a high-level lan-

guage can achieve a simple, power-

ful solution to a problem, where the

solution in ladder diagram would be

considerably more complex. A num-

ber of change-over switches are to

be used for manual operation, but

only one operation at a time may be

performed. The program has to

ensure that manual operation is

only possible when the signal

'manual' is active. In addition, it

has to prevent more than one opera-

tion being performed simultane-

ously. Fig. 4 shows th e solution. The

signal 'manual' is the execution

condition for the entire module.

Because the inverse of this signal is

connected to the reset input of the

module, none of the module outputs

will be active when 'manual' is

inactive.

The different change-over

switches are connected to a conver-

sion element which converts one of

16 lines to the equivalent integer. If

more than one input to this element

is active, the element generates a

fault signal. The integer passes to a

demultiplexing element which con-

verts it back to one of 16 lines, and

the fault signal is connected to the

reset input of this element. Thus,

when more than one of the inputs

to the first element is active, all the

outputs of the other element will be

held at zero.

Closed loop control:

AMPL also

has very powerful closed-loop con-

trol functions. The PID controller

element PIDCON, for example,

includes:

several control modes, and tun-

able parameters

cascade inputs

tracking functions for smooth

transition between control modes

direct manua l control of the out-

put signal

2 A modern integrated automation system combines instrumentation PLC

and SCADA functions. Nodes of the system are connected via a LAN

detectio n of limit values and

handling of events

powerful presentation and dia-

logue at the operator workstation.

In process-control systems, compen-

sation of simple nonlinearities in

actuator signals, such as squaring,

square root, logarithms etc., is com-

monly required; complex nonlinear-

ities require comprehensive calcula-

tions to obtain a polynomial approx-

imation of the desired function. The

function-generator element (FUNG)

easily solves this problem in AMPL

by providing a function of one or

two input signals with the aid of a

break-point table. The desired func-

tion is approximated to a partly lin-

ear function, and the break points

are entered in tabular form. Up to

255 points can be specified, and the

function element carries out linear

interpolation between break points.

FUNG may also be used to compen-

sate for nonlinearities in the process

dynamics; the values of the control

parameters are set in accordance

with the measured value from the

process so-called gain schedul-

ing.

As a further example of a complex

function that is available as a single

element, the Self-Tuning Adaptive

Regulator (STAR element) can be

used for the troublesome control

tasks.

This controller continually

adjusts to changing process dynam-

ics,

which may be due to the age-

ing of the process equipment for

instance. The STAR element incor-

porates dead-time compensation

3 The differences between the old and new system are resolved in an access

station whose database must be built by the user

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ELECTRONICS & POWER SEPTEMBER 1987

4 This simple but powerful AMPL solution to problem wouldbeconsiderably m ore complex using forexample

ladder logic

and feedforward,

so

that

the con-

troller cantake account of process

disturbances without knowing

exactly what form

the

influence

takes;

i.e. the controller tea che s

itself.

Operator workstations:

The opera-

tor workstation represents the 'eyes

and hands' needed

to

monitor

and

control

a

process. The following

are

the basic requirements in anopera-

tor workstation using colour VDUs:

user configured proce ss displays,

standard list displays, trend

curves and reports

efficient, simp le, operator dia-

logue for control of plant

clear alarm/event presentation

on-line construction of displays.

Itis important that VDU information

is structured

in

such

a

way that the

operator canquickly find a particu-

lar display. This may be done in

several ways;

e.g.

with menus,

direct selection keys or by arranging

the displays in a hierarchy. In the

ASEA Master System,

for

example,

the operator uses menus and direct

selection keys;

in

addition, the user

can define

the

display hierarchy

online. In this way, the operator can

create links from

a

single process

display to up to ten other displays.

Manual control: Manual control

means that

the

operator

can

influ-

ence

the

automatic process control,

either by altering process conditions

(parameters,

se t

points

etc. or by

changing

to the

manual mode

and

thus overriding automatic control by

the system. Where several operator

stations are required,

it is

important

that

a

selection mechanism exists

to prevent several operators actuat-

ing an object simultaneously, and

to

ensure that the status of the object

(operating mode, any blocking etc.)

is always available

at all

work-

stations.

Also, it is

equally important that

the operator is not stressed by

unnecessary alarms

due to a

sequence of faults, or to faults in

process parts that are shut downor

being started up.

In

ASEA Master,

alarms aredetected in theprocess

station, andmay be blocked either

manually from the operator stations

or automatically by AMPL programs

in the process stations.

Expansion capability: The

inte-

grated system must allow theuser

to introduce automation

on a

small

scale, and then enlarge step by step

to give complete automation. It is

common

to

start

by

automating

subprocesses

or

individual

machines, and then tocombinethe

different systems

via a

communica-

tion link and add joint control func-

tions and a central control room.A

gradual expansion must

be

possible

without existing stations having

to

be modified in anyrespect, apart

from

the

additions t hat have

to be

madeforthe connection of, and the

exchange

of

information with,

the

new stations.

A new ASEA Master station con-

nected to MasterBus is automatic-

ally included in the bus communica-

tion. Any symbolic addresses referr-

ing to the new station are auto-

matically resolved after

the

connec-

tion.

The

user need only supple-

ment existing stations with the

application software needed

to

interact with

the new

station;

e.g.

new displaysor AMPL programsfor

joint control.

evelopment trends

Industrial control systems will

continue to bedeveloped to meet

market requirements and toexploit

the opportunities offered by modern

microprocessors. Key words inthis

development are integration, pro-

gramming with functional

lan-

guages, globally available data-

bases, andopenness to interfacing

with other systems and communica-

tion standards. Increasing import-

ance will

be

attached

to the

life-

cycle costs

of a

control system,

where the costs of programming,

installation, commissioning, main-

tenance

and

future modifications

and expansions are taken into

account.

Thomas Pauly is with ASEA Automation

AB,

Vasteras, Sweden