Wilamowski B.M., Irwin J.D. The Industrial Electronics Handbook. Second Edition: Industrial Communication Systems

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Quality of Service 19-13

2.and.3.will.start.to.drop,.but.WRED.will.also.continue.to.randomly.drop.packets.with.CoS.0.and.1..

 en

.re

sult

.is.th

.th

ere

.wi

.be.mo

.pa

ckets

.dr

opped

.wi

.Co

.0.an

.1.th

.wi

.Co

.2.an

.3.

19.4.2 High availability Solutions for the routers

As. mentioned. previously,. another. feature. for. the. “service. level”. solution. is. the. availability. of. the.

net

work.

.Non

.of.the.pre

vious

.QoS.sol

utions

.wil

.wor

.if.the.net

work

.dev

ices

.(sw

itches

.and

/or

.rou

ters)

whe

.mus

.be.imp

lemented

.are.not.wor

king.

. is.mea

.tha

.any.com

plete

.net

work

.QoS.sol

ution

.mus

tak

.in

.ac

count

.th

.re

dundancy

.fo

.at.le

ast,

.th

.mo

.se

nsible

.ro

uters

.in.th

.ne

twork.

Nowadays,

.thi

.is.imp

lemented

.alm

ost

.uni

versally

.by.usin

.the.Vir

tual

.Rou

ter

.Red

undancy

.Pro

tocol

(VR

RP).

. e.VRR

.[26

]

.obl

iges

.to.dup

licate

.rou

ters

.and.lin

.to.ens

ure

.con

tinuity

.of.ser

vice

.acr

oss

fai

lures.

.It.int

roduces

.the.con

cept

.of.“vi

rtual

.rou

ter”

.tha

.is.add

ressed

.by.IP.cli

ents

.req

uiring

.rou

ting

ser

vice.

. e.re

.ro

uting

.se

rvice

.is.pr

ovided

.by.ph

ysical

.ro

uters

.ru

nning

.th

.VR

RP.

 ere

.ar

.a.nu

mber

.of.de

scriptive

.te

rms

.in

troduced

.by.VR

RP:

•

. Virtual.router,.a.single.router.image.created.through.the.operation.of.one.or.more.routers.run-

ning

.VR

RP.

•

. Vir

tual

.rou

ter

.ID.or.VRI

.a.num

erical

.ide

ntication

.of.a.par

ticular

.vir

tual

.rou

ter.

.It.mus

.be.

uni

que

.on.a.ne

twork

.se

gment.

•

. Vir

tual

.rou

ter

.IP,.an.IP.add

ress

.ass

ociated

.wit

.a.VRI

.tha

.the.oth

.hos

.use.to.obt

ain

.net

work

ser

vice.

.It.is.ma

naged

.by.th

.VR

.in

stances

.be

longing

.to.a.VR

ID.

•

. Vir

tual

.MAC.add

ress,

.a.pre

dened

.MAC.add

ress

.use

.for.all.VRR

.act

ions

.(fo

.Ethe

rnet)

.ins

tead

of.th

.re

.ad

apter

.MA

.ad

dress(es).

.It.is.de

rived

.fr

.th

.VR

ID.

•

. Mas

ter,

.the.VRR

.ins

tance

.tha

.per

forms

.the.rou

ting

.fun

ction

.for.the.vir

tual

.rou

ter

.at.a.giv

tim

.On

.on

.ma

ster

.is.ac

tive

.at.a.ti

.fo

.a.gi

ven

.VR

ID.

•

. Bac

kup,

.oth

.VRR

.ins

tances

.for.a.VRI

.tha

.are.act

ive,

.but.not.in.the.mas

ter

.sta

te.

.Bac

kups

.are.

rea

.to.ta

.on.th

.ro

.of.th

.ma

ster

.if.th

.cu

rrent

.ma

ster

.fa

ils.

•

. Pri

ority,

.a.val

.assi

gned

.to.di

erent

.VRR

.ins

tances,

.as.a.way.to.det

ermine

.whi

.rou

ter

.wil

tak

.on.th

.ro

.of.th

.ma

ster

.if.th

.cu

rrent

.ma

ster

.fa

ils.

Figure

.19.

.sho

.a.simp

.VRR

.con

guration,

.wit

.two.rou

ters

.con

necting

.to.a.net

work

.clo

.and.

VRR

.pr

oviding

.a.re

silient

.ro

uting

.fu

nction

.fo

.th

.cl

ient

.ma

chines

.in.th

.lo

cal

.ar

.ne

twork.

Router

.rA.is.the.mas

ter

.of.vir

tual

.rou

ter

.VRI

.1.and.rou

ter

.rB.is.at.the.bac

kup

.sta

te.

.Whe

.a.VRR

ins

tance

.is.in.the.mas

ter

.sta

.for.a.VRI

.it.sen

.mul

ticast

.pac

kets

.to.the.reg

istered

.VRR

.mul

ticast

add

ress

.ad

vising

.ot

her

.VR

.in

stances

.th

.it.is.th

.ma

ster

.fo

.th

.VR

ID,

.wi

.a.tw

ofold

.ob

jective:

•

. If.a.VRR

.ins

tance

.wit

.a.hig

her

.pri

ority

.for.tha

.VRI

.is.sta

rted,

.the.new.VRR

.ins

tance

.can.

for

.an

.el

ection

.an

.ta

.on.th

.ma

ster

.ro

le.

•

. VRR

.ins

tances

.in.the.bac

kup

.sta

.for.the.VRI

.lis

ten

.for.the.mas

ter’s

.pac

kets;

.if.an.int

erval

ela

pses

.wit

hout

.a.pac

ket

.bei

.rec

eived,

.the.ins

tances

.in.the.bac

kup

.sta

.tak

.act

ion

.to.ele

.a.

new.ma

ster.

.Fi

gure

.19

.8,

.on

.of.th

.fo

llowing

.mu

.be.tr

.ab

out

.ro

uter

.rA

•

. It.ha

.a.hi

gher

.pr

iority

.th

.ro

uter

.rB

•

. If.bo

.ro

uters

.ha

.th

.sa

.pr

iority,

.it

.in

terface

.IP.ad

dress

.is.hi

gher

.th

.of.ro

uter

.rB

.rou

ter

.rA.fai

ls,

.ae

.a.sho

.int

erval

.rou

ter

.rB.wou

.not

ice

.tha

.no.mul

ticast

.pac

ket

.has.bee

rec

eived

.and.tra

nsitions

.to.the.mas

ter

.sta

te,

.tak

ing

.ove

.the.han

dling

.of.the.vir

tual

.IP.add

ress

.and.sen

ing

.it

.ow

.mu

lticast

.pa

ckets.

.tim

.rou

ter

.rB.wai

.bef

ore

.mak

ing

.its.sta

.tra

nsition

.is.cal

led

.the.mas

ter

.dow

.int

erval

.and.is.

bas

.on.the.len

gth

.of.tim

.bet

ween

.mas

ter

.upd

ates

.(ad

vertisement

.int

erval)

.and.a.val

.cal

led

.ske

tim

.ca

lculated

.fr

.th

.pr

iority

.va

lue.

19-14 Industrial Communication Systems

When.the.problem.with.router.rA.is.resolved,.and.depending.on.its.conguration,.either.of.two.

situations.would.occur:

•

. If.Router.rA.is.congured.to.start.as.master,.it.will.force.an.election.immediately.by.sending.its.

rst.advertisement.as.master..Router.rB.will.receive.this.advertisement.and.transition.to.backup.

state.

•

. If.Router.rA.is.congured.to.start.as.backup,.it.will.transition.from.initialization.to.backup.state..

Nothing.will.happen.until.it.receives.an.advertisement.from.router.rB..Router.rB.has.a.lower.

priority.than.router.rA,.so.router.rA.will.start.an.election.by.commencing.its.transition.to.master.

state.and.sending.an.advertisement..When.it.receives.this.advertisement,.router.rB.will.transition.

to.backup.state.because.router.rA.has.higher.priority.

Although

.VRRP.is.based.on.Cisco’s.proprietary.Hot.Standby.Router.Protocol.(HSRP,.[26]).concepts,.

VRRP.is.actually.a.standardized.version.of.Cisco’s.HSRP.. ose.protocols,.while.similar.in.concept,.are.

not.compatible.. ere.are.a.great.number.of.implementations.besides.the.Cisco.devices.one.

references

. 1.. R..Zurawski,.ed.,.e Industrial Communication Technology Handbook,.CRC.Press,.Boca.Raton,.

FL,.2005.

. 2.. W..Stallings,.High Speed Networks and Internets. Performance and Quality of Service,.Prentice.Hall,.

Englewood.Clis,.NJ,.2002.

. 3.. J..García.Tomás,.J.L..Raya,.and.V..Rodrigo,.Alta velocidad y calidad de servicio en Redes IP,.Ed..

Ra-Ma,.Alfaomega,.Mexico,.2002.

VRID 1

Virtual IP 192.168.44.1

rA rB

Master

Backup

Router rB VRID 1

Priority: 20

Ethernet IP address:

192.168.44.3

LAN

Router rA VRID 1

Priority: 250

Ethernet IP address:

192.168.44.2

FIGURE 19.8 Simple.VRRP.example.

Quality of Service 19-15

. 4.. itSMF. International,. IT Service Management: An Introduction,. itSMF. Library,. Van. Haren.

Pub

lishing,

.Zal

tbommel,

.Hol

land,

.200

. 5.. R..An

derson,

.Se

curity Engineering,

.Jo

.Wi

ley

.&.So

ns,

.Ne

.Yo

rk,

.2001.

. 6.. RFC.219

.Site

Security Handbook,

.IET

.(In

ternet

.Eng

ineering

.Tas

.For

ce),

.Sep

tember

.199

ava

ilable

.at.ht

tp://www.ietf.org/,

.access.in.Ju

ly,

.2010.

. 7.. ISO/IEC. 27001,. Info

rmation

. tec

hnology—Security

. tec

hniques—Information

. sec

urity

. man

age-

ment

.system

s—Requirements,

.an.ISO.(Int

ernational

.Organiza

tion

.for.Sta

ndardization)

.sta

ndard,

Gene

va,

.Sw

itzerland,

.2005,.ava

ilable

.at.ht

tp://www.iso.org/,

.access.in.Ju

ly,

.2010.

. 8.. ISO

/IEC

.154

08,

.Inf

ormation

.tech

nology—Security

.tech

niques—Evaluation

.cri

teria

.for.IT.secu

rity—

Part

.1:.Intr

oduction

.and.gen

eral

.mod

el,

.an.ISO.(In

ternational

.Org

anization

.for.Sta

ndardization)

sta

ndard,

.Gen

eva,

.Swi

tzerland,

.200

.ava

ilable

.at.htt

p://www.iso.org/,

.acc

ess

.in.July

.201

. 9.. IEEE.1402,.Gui

de for Electric Power Substation Physical and Electronic Security,

. e.Ins

titute

.of.

Elec

trical

.an

.Ele

ctronics

.Eng

ineers,

.In

c.,

.Pi

scataway,

.NJ,.2000.

. 10.. Pro

cess

.Con

trol

.Sec

urity

.Req

uirements

.For

um,

.spo

nsorized

.by.Nat

ional

.Ins

titute

.of.Sta

ndards

.and.

Tec

hnologies,

.Gait

hersbug,

.MD,.avai

lable

.at.htt

p://www.isd.mel.nist.gov/projects/processcontrol/,

access.on.Ma

.2009.

. 11.. ANSI/ISA-TR99.00.01-2007,.Secu

rity Technologies for Industrial Automation and Control Systems,

an.ANSI/ISA.(Amer

ican

.Nat

ional

.of.Sta

ndards

.Ins

titute/International

.Societ

.of.Aut

omation)

.sta

dard,

. 2007,. Was

hington

. D.C.,. avai

lable

. at. htt

p://www.standardsportal.org/usa_en/sdo/isa.aspx,

access.in.July,.2010.

. 12.. R..Fro

om,

.B..Siva

subramanian,

.and.E..Frahim,.Bui

lding Cisco Multilayer Switched Networks,

.Cisco.

Pres

.In

dianapolis,

.IN,.2006.

. 13.. RFC.1633,.Int

egrated Services in the Internet Architecture: An Overview,

.IETF.(Int

ernet

.Engin

eering

Tas

.Fo

rce),

.Jun

.1994,.ava

ilable

.at.ht

tp://www.ietf.org/,

.access.in.Ju

ly,

.2010.

. 14.. RFC.247

.De

nition of the Dierentiated Services Field (DS eld in the IPv4 and IPv6 headers),

.IET

(In

ternet

.Eng

ineering

.Tas

.For

ce),

.Dec

ember

.199

.ava

ilable

.at.htt

p://www.ietf.org/,

.acc

ess

.in.Jul

.201

. 15.. RFC. 2475,. An A

rchitecture for Dierentiated Services,

. IETF. (Int

ernet

. Engin

eering

. Tas

. For

ce),

Decem

ber

.1998,.ava

ilable

.at.ht

tp://www.ietf.org/,

.access.in.Ju

ly,

.2010.

. 16.. RFC.2205,.Res

ource ReSerVation Protocol (RSVP),

.Versio

.1.Func

tional

.Spe

cication,

.IETF.(Int

ernet

Engin

eering

.Ta

.Fo

rce),

.Sep

tember

.1997,.ava

ilable

.at.ht

tp://www.ietf.org/,

.access.in.Ju

ly,

.2010.

. 17.. RFC.2210,.e U

se of RSVP with IETF Integrated Services,

.IETF.(Int

ernet

.Engin

eering

.Tas

.For

ce),

Sept

ember

.1997,.ava

ilable

.at.ht

tp://www.ietf.org/,

.access.in.Ju

ly,

.2010.

. 18.. RFC. 3936,. Pro

cedures for Modifying the Resource reSerVation Protocol (RSVP),

. IETF. (Int

ernet

Engin

eering

.Ta

.Fo

rce),

.Oc

tober

.2004,.ava

ilable

.at.ht

tp://www.ietf.org/,

.access.in.Ju

ly,

.2010.

. 19.. RFC.2748,.e C

OPS (Common Open Policy Service) Protocol,

.IETF.(Int

ernet

.Engin

eering

.Tas

For

ce),

.Ja

nuary

.2000,.ava

ilable

.at.ht

tp://www.ietf.org/,

.access.in.Ju

ly,

.2010.

. 21.. RFC.2597,.Ass

ured Forwarding PHB Group,

.IETF.(Int

ernet

.Engin

eering

.Tas

.For

ce),

.Jul

.1999,.

avai

lable

.at.ht

tp://www.ietf.org/,

.access.in.Ju

ly,

.2010.

. 22.. RFC.3246,.An E

xpedited Forwarding PHB (Per-Hop Behavior),

.IETF.(Int

ernet

.Engin

eering

.Tas

For

ce),

.Ma

rch

.2002,.ava

ilable

.at.ht

tp://www.ietf.org/,

.access.in.Ju

ly,

.2010.

. 23.. RFC.3550,.RTP:.A Tr

ansport Protocol for Real-Time Applications,

.IETF.(Int

ernet

.Engin

eering

.Tas

For

ce),

.Ju

.2003,.ava

ilable

.at.ht

tp://www.ietf.org/,

.access.in.Ju

ly,

.2010.

. 24.. RFC.3413,.Si

mple Network Management Protocol (SNMP) Applications,

.IETF.(In

ternet

.Eng

ineering

Tas

.Fo

rce),

.De

cember

.2002,.ava

ilable

.at.ht

tp://www.ietf.org/,

.access.in.Ju

ly,

.2010.

. 25.. RFC.2309,.Rec

ommendations on Queue Management and Congestion Avoidance in the Internet,

.IETF.

(Int

ernet

.Engin

eering

.Tas

.For

ce),

.Apr

.1998,.avai

lable

.at.htt

p://www.ietf.org/,

.access.in.Jul

.2010.

. 26.. RFC.3768,.Vir

tual Router Redundancy Protocol (VRRP),

.IETF.(Int

ernet

.Engin

eering

.Tas

.For

ce),

Apr

.2004,.ava

ilable

.at.ht

tp://www.ietf.org/,

.access.in.Ju

ly,

.2010.

20-1

20.1 Introduction

Industrial.control.applications.use.controllers,.sensors,.and.actuators.to.close.control.loops..Sensors.

and.actu

ators

.are.usua

lly

.dist

ributed

.topol

ogically

.thro

ugh

.the.cont

rolled

.plan

.As.ubiq

uitous

.embed

ded

.cont

rollers,

.sens

ors,

.and.actu

ators

.are.more.and.more.comm

on,

.whil

.inex

pensive

.and.reli

able

comm

unication

.syst

ems

.are.read

.for.use.in.the.indu

stry,

.many.of.the.cont

rol

.loop

.are.clos

.using.

indu

strial

.comm

unication

.syst

ems.

.A.larg

.part.of.the.indu

stry

.cont

rol

.loop

.are.disc

rete

.in.natu

re,

wher

.a.sequ

ence

.of.disc

rete

.acti

ons

.are.done.aer.a.sequ

ence

.of.sensi

ngs,

.e.g.,.aer.a.disp

lacement

.has.

been.comp

leted,

.a.motor.has.to.stop..From.the.cont

rol

.side,.a.disc

rete

.syst

.has.as.major.cons

traint,

the.syst

.resp

onse

.time

.or.the.maxi

mum

.time.that.can.be.acce

pted

.betw

een

.occu

rrence

.of.a.stim

ulus

and.the.reac

tion

.on.the.plan

.Prev

ious

.chapt

ers

.of.the.book.addr

ess

.how.the.indu

strial

.comm

unication

syst

ems

.att

ain

.the.goa

.of.sen

ding

.mes

sages

.kee

ping

.thi

.con

straint.

Continuous

.cont

rol

.loop

.die

.from.discret

.cont

rol

.loop

.in.that.the.cont

inuous

.sign

.meas

urement

is.used.by.the.cont

roller

.to.calc

ulate

.(usi

.die

rential

.equa

tions)

.the.cont

inuous

.sign

.to.be.send.to.the.

actu

ator.

. ose.cont

inuous

.sign

als

.can.be.prop

erly

.appr

oximated

.by.time

-discretized

.sequ

ences

.of.valu

that.main

tain

.the.dyna

mic

.char

acteristics

.of.the.samp

led

.syst

em.

.A.goal.when.conn

ecting

.thro

ugh

.a.com-

unication

.netwo

.sens

ors,

.cont

rollers,

.and.actu

ators

.in.a.cont

inuous-time

.cont

rol

.loop.is.to.main

tain

the.dyna

mic

.char

acteristics

.of.the.extr

acted

.data.from.the.sequ

ence

.of.mess

ages.

Communication

. netw

orks

. for. feed

back

. cont

rol

. are. in. incr

easing

. use.. A. netw

ork-based

. cont

rol

(NBC

)

.syst

.is.comp

osed

.of.dist

ributed

.node

.shar

ing

.feed

back

.cont

rol-related

.info

rmation

.throu

a.comm

unication

.chan

nel.

. e.basi

.con

guration

.is.give

.by.one.sens

.node.that.coll

ects

.proce

sens

.data

.one.cont

roller

.node.that.rece

ives

.the.sens

.data

.calc

ulates

.the.cont

rol

.acti

.and.send

Network-Based Control

20.1. Introduction.....................................................................................20-1

20.2

. Mutu

.Con

cepts

.in.Con

trol

.and.in.Com

munications.............20-2

20.3

. Arch

itecture

.of.Net

worked-Based

.Con

trol.................................20-2

Connection.Types

20.4. Network.Eects.in.Control.Performance....................................20-6

20.5

. Desig

.in.NBC.

................................................................................20-6

Design.Constraints.in.the.Network.Side. •. Design.Constraints.

in.the.Control.Side. •. Network.and.Control.Co-Design

20.6. Summary...........................................................................................20-7

References.....................................................................................................20-8

Josep M. Fuertes

Universitat Politècnica

de Cat

alunya

Mo-Yuen Chow

North Carolina

Stat

Uni

versity

Ricard Villà

Universitat Politècnica

de Cat

alunya

Rachana Gupta

North Carolina

Stat

Uni

versity

Jordi Ayza

Universitat Politècnica

de Cat

alunya

20-2 Industrial Communication Systems

it.to.an.actuator.node,.and.that.actuator.node.that.actuates.on.the.process..At.least.two.feedback.

control.nodes.are.needed.to.have.a.proper.NBC.system.(as.one.of.those.nodes.can.have.two.functions,.

i.e.,.a.sensor.and.controller.node.or.a.controller.and.actuator.node)..In.both.cases,.there.is.messaging.of.

sensor.to.controller.or.controller.to.actuator.nodes..In.general,.we.should.consider.that.the.network.is.

shared.with.other.nodes,.they.being.feedback.control.or.any.other.applications.. e.result.is.that.NBC.

systems.can.contain.a.large.number.of.interconnected.devices.(controllers,.actuators,.sensors).that.use.

the.network.for.interchanging.control.messages.

Feedback

.control.imposes.several.restrictions.on.the.communication.networks.. e.multiple.access.

schemes.of.common.industrial.buses.serialize.the.sending.of.the.messages.through.the.shared.media..

As.a.result,.communication.delays.are.created.in.the.feedback.control.loop..Delays.inside.a.control.loop.

tend.to.instabilize.the.controlled.plant..In.addition,.packets.may.be.lost.in.a.noisy.channel,.breaking.the.

control.loop.path.and.having.also.a.possible.bad.consequence.on.the.controlled.process.. is.chapter.

reviews.the.eects.on.the.control.side.of.the.communication.technology..It.also.talks.about.how.a.proper.

use.of.the.communication.channel.can.favor.the.dynamic.operation.of.the.control.loop.and.how.the.

control.law.is.to.be.adjusted.to.take.into.consideration.the.communication-imposed.constraints.

20.2 Mutual Concepts in Control and in Communications

In.a.networked.control.system.(NCS),.the.feedback.and.the.command.signals.use.a.communication.net-

work

.link..When.introducing.a.network.in.the.control.loop,.some.inconveniences.such.as.band-limited.

channels,.delays,.and.packet.dropouts.occur..Such.network.may.also.be.shared.by.other.applications,.

resulting.in.the.channel.being.inaccessible,.as.sharing.a.communication.channel.imposes.a.wait.time.

until.the.channel.is.accessible.again..From.the.point.of.view.of.control,.it.represents.an.additional.delay.

between.the.sensor.measurement.and.the.control.actuation,.resulting.in.a.potential.loss.of.control.per-

formance.

.Classical.discrete.controller.design.imposes.a.periodic.sampling.scheme,.which.also.imposes.

hard.real-time.operation.. e.classical.denition.of.hard.real-time.system.is.that.the.actions.or.tasks.

shall.be.executed.within.a.hard.deadline.or.the.result.is.no.longer.valid..In.a.controlled.system,.the.

restriction.is.more.severe,.as.the.sampling.and.actuation.is.required.to.be.with.(not.within).a.prescribed.

period..As.in.many.cases.strictly.periodic.operation.cannot.be.assured,.due.to.message.loss.or.commu-

nication

.incurred.delays,.several.schemes.can.be.used.

20.3 architecture of Networked-Based Control

When.a.traditional.feedback.control.system.is.closed.via.a.communication.channel.(such.as.a.network),.

which.maybe.shared.with.other.nodes.outside.the.control.system,.then.the.control.system.is.classied.as.

an.NCS.or.NBC.system..All.denitions.found.in.literature.for.NBC.have.one.key.feature.in.common.. is.

dening.feature.is.that.information.(reference.input,.plant.output,.control.input,.etc.).is.exchanged.among.

control.system.components.(sensor,.controller,.actuator,.etc.).using.a.shared network.(see.Figure.20.1).

Supervisor

Plant-1

Plant-2

Plant-m

Sensor-t

Network

Sensor-2

Sensor-1

Controller-n

Controller-2

Controller-1

FIGURE 20.1 Typical.NBC.architecture.

Network-Based Control 20-3

Thus,.the.basic.capabilities.of.any.NBC.are.information.acquisition.(sensors/users),.command.

(controllers/users),. communication,. and. network. and. control. (actuators).. In. broader. terms,. NCS.

research.and.application.is.categorized.into.two.parts.

. 1..Control of network:.Study.and.research.on.communications.and.networks.to.make.them.suitable.

for.real-time.NCS,.e.g.,.routing.control,.congestion.reduction,.ecient.data.communication,.and.

networking.protocol.

. 2.. Control over network:. is.deals.more.with.control.strategies.and.control.systems.design.over.

the.network.to.minimize.the.eect.of.adverse.network.parameters.on.NCS.performance.such.as.

network.delay.

.chapter.is.mainly.focused.on.“control.over.network.”

20.3.1 Connection types

 ere.are.two.major.types.of.control.systems.that.utilize.communication.networks.. ey.are.(1).shared-

network

.control.systems.and.(2).remote.control.systems.

20.3.1.1

Shared-Network Control Systems

Using

.shared-network.resources.to.transfer.measurements,.from.sensors.to.controllers.and.control.signals.

from.controllers.to.actuators.can.greatly.reduce.the.complexity.of.connections.. is.method,.as.shown.in.

Figure.20.2,.is.systematic.and.structured,.provides.more.exibility.in.installation,.and.eases.maintenance.

and.troubleshooting..Furthermore,.networks.enable.communication.among.control.loops.. is.feature.is.

extremely.useful.when.a.control.loop.exchanges.information.with.other.control.loops.to.perform.more.

sophisticated.controls,.such.as.fault.accommodation.and.control..Similar.structures.for.NBC.have.been.

applied.to.automobiles.and.industrial.plants.

20.3.1.2

remote Control Systems

.the.other.hand,.a.remote.control.system.can.be.thought.of.as.a.system.controlled.by.a.controller.

located.far.away.from.it.. is.is.sometimes.referred.to.as.teleoperation.control..Remote.data.acquisition.

systems.and.remote.monitoring.systems.can.also.be.included.in.this.class.of.systems.

 ere

.are.two.general.approaches.to.design.an.NBC.system.. e.rst.approach.is.to.have.several.sub-

systems

.form.a.hierarchical.structure,.in.which.each.of.the.subsystems.contains.a.sensor,.an.actuator,.

and.a.controller.by.itself,.as.depicted.in.Figure.20.3.. ese.system.components.are.attached.to.the.same.

control.plant..In.this.case,.a.subsystem.controller.receives.a.set.point.from.the.central.controller.C

.. e.

subsystem.then.tries.to.satisfy.this.set.point.by.itself.. e.sensor.data.or.status.signal.is.transmitted.back.

via.a.network.to.the.central.controller.

Network

: Controller

: Sensor,

: Actuator,

FIGURE 20.2 Shared-network.connections.

20-4 Industrial Communication Systems

 e.second.approach.of.network.control.is.the.direct.structure,.as.shown.in.Figure.20.4.. is.struc-

ture

.has.sensors,.actuators,.and.a.controller.of.a.control.loop.connected.directly.to.a.network..Both.the.

hierarchical.and.direct.structures.have.their.own.pros.and.cons..Many.NCS.include.features.from.both.

structures,.to.benet.from.their.advantages,.thus.dening.the.hybrid.NCSs.

.details.can.be.found.in.[1,2].

20.3.1.2.1 Communication Networks

.communication.network.is.the.backbone.of.the.NBC..Reliability,.security,.ease.of.use,.and.availability.

are.the.main.issues.while.choosing.the.communication.type..A.communication.network.giving.support.

to.an.NBC.should.have.a.deterministic.end-to-end.message.response.as.a.principal.property.. is.is.the.

only.way.to.guarantee.the.real-time.response.required.by.the.control.algorithm.

.ARPANET.developed.by.Advanced.Research.Projects.Agency.Network.of.the.U.S..Department.

of.Defense.in.1969.was.the.world’s.rst.operational.packet.switching.network.and.the.predecessor.of.the.

global.Internet..Later.came.eldbus.(around.since.1988)—an.industrial.network.system.for.real-time.

distributed.control..Fieldbus.is.a.generic.term,.which.described.a.modern.industrial.digital.communica-

tions

.network.intended.to.replace.the.existing.4–20.mA.analog.signal.standard.. is.network.is.a.digi-

tal,

.bidirectional,.multi-drop,.serial-bus.used.to.link.isolated.eld.devices,.especially.in.the.automated.

manufacturing.environment..Probus.(process.eldbus).is.a.standard.for.eldbus.communication.in.

automation.technology.and.was.rst.promoted.in.1989.by.BMBF.(German.department.of.education.and.

research).[3].

.basic.medium.control.access.scheme.is.the.time.division.multiple.access,.where.a.fraction.of.the.

channel.is.given.to.all.the.nodes.. ere.are.several.medium.access.control.schemes.used.by.industrial.

networks.. e.two.more.common.are.based.on.a.token-passing.scheme.and.a.random.access.scheme..

 e.token-passing.scheme.uses.a.portion.of.the.network.bandwidth.for.transmitting.an.authorizing.

Control signal

Sensor measurement

Network

FIGURE 20.4 Data.transfers.of.direct.structure.

Set point

Network

Status or sensor

measurement

FIGURE 20.3 Data.transfers.of.hierarchical.structure.

Network-Based Control 20-5

message.(token).that.gives.rights.to.its.receiving.node.to.transmit.one.or.more.queued.messages.. e.

toke

.is.usu

ally

.sen

.to.an.ord

ered

.and.cyc

led

.lis

.of.nod

es.

. e.res

ult

.is.tha

.the.net

work

.giv

.an.

ord

ered

.acc

ess

.to.all.the.nod

.and.eac

.nod

.can.kno

.in.adv

ance

.the.max

imum

.del

.a.giv

.mes

sage

wil

.in

cur

.be

fore

.ar

riving

.to.it

.de

stination.

.ran

dom

.acc

ess

.sche

.all

ows

.any.nod

.to.tra

nsmit

.when

ever

.it.nee

.to.sen

.a.queu

.mes

sage

pro

vided

.not.any.othe

.nod

.is.sen

ding

.a.mes

sage.

.If.ther

.is.a.mes

sage

.bei

.sen

.thr

ough

.the.net

work,

the.nod

.wai

.til

.the.end.of.thi

.mes

sage

.and.then.it.att

empts

.to.sen

.its.queu

.mes

sage

.ae

.an.

act

ive

.wai

.wher

.a.com

petition

.bet

ween

.the.wai

ting

.nod

.wil

.giv

.the.mes

sage

.acc

ess

.to.onl

.one.of.

them

. e.res

ult

.is.tha

.a.ran

dom

.acc

ess

.sche

.is.gen

erally

.non

deterministic.

.top.of.the.acc

ess

.sch

eme

.is.the.end

-to-end

.com

munication

.rel

ationship

.bet

ween

.nod

es.

. e.mos

com

mon

.re

lationships

.ar

.th

.ma

ster–slave,

.pr

oducer–consumer,

.an

.cl

ient–server.

•

. Mas

ter–Slave:

. e.net

work

.has.mas

ter

.nod

.and.sla

.nod

es.

.Sla

.nod

.onl

.rea

.to.mas

ter

nod

.com

mands.

.Mas

ter

.nod

.sha

.the.cha

nnel

.acc

ess

.by.usin

.a.rou

nd-robin

.(i.

e.,

.toke

.pas

ing)

.sc

heme.

.Pr

obus

.is.an.ex

ample

.of.th

.sc

heme.

•

. Pro

ducer–Consumer:

. e.net

work

.has.nod

.tha

.are.pro

ducers,

.con

sumers,

.or.bot

.A.pro

ducer

nod

.whe

.it.has.a.mes

sage

.to.sen

.loo

.for.the.rs

.opp

ortunity

.to.sen

.it.in.a.bro

adcast

.way

Con

sumer

.nod

.pic

.up.thi

.mes

sage

.con

tents

.as.soo

.as.the

.nee

.it..As.an.exa

mple,

.Con

trol

Are

.Net

work

.(CA

.use

.thi

.par

adigm.

.CAN.is.one.of.the.acc

epted

.el

dbus

.sta

ndards—a

ser

ial,

.asy

nchronous,

.mul

ti-master

.com

munication

.pro

tocol

.desi

gned

.for.app

lications

.nee

ding

hig

h-level

.da

.in

tegrity

.an

.da

.ra

tes

.of.up.to.1.Mb

ps.

•

. Cli

ent–Server:

.In.thi

.cas

.the.net

work

.has.cli

ent

.nod

.and.ser

ver

.nod

es.

. e.cli

ent

.nod

.sen

req

uests

.to.the.ser

ver

.nod

.for.ser

vices.

.Bas

.on.the.bes

.eo

rt,

.the.ser

ver

.pro

vides

.thi

.ser

vice

and.not

ies

.the.com

pletion.

. is.sch

eme

.is.not.nec

essarily

.det

erministic

.as.the.ser

ver

.nod

.tak

as.mu

.ti

.as.it.ne

eds.

.res

ult

.of.app

lication

.of.tho

.acc

ess

.and.rel

ationship

.sch

emes

.is.that.a.giv

.ind

ustrial

.net

work

.may.

par

tially

.use.a.cli

ent–server

.par

adigm

.and.a.pro

ducer–consumer

.or.mast

er–slave

.in.con

junction

.wit

.any.

of.the.acc

ess

.sch

emes.

.How.the.rel

ationship

.is.to.be.ope

rated

.dep

ends

.on.the.tra

nsaction

.to.be.don

.For.

exa

mple,

.in.an.ind

ustrial

.app

lication,

.dur

ing

.the.com

missioning

.phas

.a.cli

ent

.app

lication

.ins

talls

.an.

aut

omation

.pro

gram

.int

.a.Pro

grammable

.Log

.Con

troller

.(PL

C),

.whi

.act

.as.a.ser

ver,

.whi

.dur

ing

the.PLC.cyc

lic

.ope

ration,

.it.con

nects

.to.sen

sors

.and.act

uators

.bas

.on.a.mast

er–slave

.sch

eme.

 ere

.is.a.wid

.var

iety

.of.con

curring

.el

dbus

.sta

ndards

.and

.the

refore,

.man

.tim

.int

eroperability

bec

omes

.an.iss

ue.

.Ano

ther

.com

munications

.net

work

.use

.in.NCS

—Ethernet—has

.evo

lved

.into.the.

mos

.wid

ely

.imp

lemented

.phy

sical

.and.lin

.lay

.pro

tocol

.toda

.mai

nly

.bec

ause

.of.the.low.cos

.of.the.

net

work

.com

ponents

.and.the

.bac

kward

.com

patibility

.wit

.the.exi

sting

.Ethe

rnet

.inf

rastructure.

.Now.

we.hav

.fas

.Ethe

rnet

.(1

0–100

.Mbp

.and.Gig

abit

.Ethe

rnet

.(1

000

.Mbp

.[5]

.Rec

ently,

.sw

itched

.Ethe

rnet

bec

ame

.a.ver

.pro

mising

.alt

ernative

.for.rea

l-time

.ind

ustrial

.app

lication

.bec

ause

.it.ele

minates

.the.col

li-

sion

.in.tr

aditional

.Eth

ernet

.[4

Overall,

.the.cho

ice

.of.net

work

.dep

ends

.upo

.the.desi

red

.app

lication.

.Tod

ay,

.wit

.the.hel

.of.tec

nologies

.lik

.GPS

.ele

ctronics

.atl

.(Go

ogle

.map

s),

.we.are.loo

king

.at.mul

ti-agent

.tra

c

.con

trol

.in.urb

are

.wit

.ec

ient

.veh

icle

.com

munication

.[7]

.Mil

itary,

.sur

gical,

.and.oth

.eme

rgency

.med

ical

.app

li-

cations

.can.use.ded

icated

.opti

cal

.net

works

.to.ens

ure

.fas

t-speed

.and.rel

iable

.dat

.com

munication.

. e.

Int

ernet

.is.the.mos

.sui

table

.and.ine

xpensive

.cho

ice

.for.man

.app

lications,

.whe

.the.pla

.and.the.con

troller

.ar

.fa

.aw

.fr

.ea

.ot

her

.[6

20.3.1.2.2 Sampling

ssaging

in NB

Sampling

.is.the.act

ivity

.of.mea

suring

.the.fee

dback

.con

trol

.var

iable

.to.be.sen

.to.the.con

troller.

. e.cyc

lic

con

trol

.con

ceptual

.sch

eme

.fol

lows

.the.ord

er:

.sam

pling,

.con

trol

.law.com

putation,

.ord

.to.act

uator.

. e.

mos

.usu

.req

uirement

.for.thi

.seq

uence

.is.tha

.it.has.to.be.exe

cuted

.bas

.on.a.xe

.clo

.cyc

.tim

Gen

eral

.con

trol

.law.mod

els

.ass

ume

.tha

.the.tim

.spe

.bet

ween

.suc

cessive

.sam

ples

.is.a.con

stant,

.cal

led

20-6 Industrial Communication Systems

the.sampling.period.T,.while.the.time.spent.between.the.sampling.and.the.actuation.instants.is.negli-

gible

.or.cons

tant

.and.know

.but.the.samp

ling

.and.actu

ation

.mess

ages

.take.some.time

.whic

.depe

nds

on.the.acce

.sche

.and.the.rela

tionship

.used

.and.the.cont

rol

.law.requ

ires

.some.comp

utation

.time

so.the.abov

.assu

mption

.does.not.hold

. is.eec

.beco

mes

.more.impo

rtant

.as.the.netw

ork

.tra

.load.

incr

eases.

Considering

.an.NBC.sche

.(Fig

ure

.20.2

.in.whic

.the.cont

rol

.loop

.consi

.of.plan

.G,.and.con-

rollers

.C,.whic

.exch

ange

.thro

ugh

.mess

ages

.the.samp

led

.vari

ables

.y(kT).and.the.cont

rol

.vari

ables

c(kT),.wher

.k.is.the.numbe

.of.the.samp

le.

.As.soon.as.the.mess

ages

.are.sent.thro

ugh

.the.netw

ork,

.they.

expe

riment

.a.dela

.so.the.cont

roller

.rece

ives

.a.dela

yed

.valu

.of.the.samp

led

.signa

.y(kT + t

),.while.

the.actu

ator

.rece

ives

.a.dela

yed

.valu

.of.the.cont

rol

.signa

.c(kT + t

),.where.t

.and.t

.are.the.time.delay.

betw

een

.sens

.to.cont

roller

.and.cont

roller

.to.actu

ator,

.resp

ectively.

.In.orde

.to.ensu

.syst

.perf

or-

mance,

.com

munication-induced

.tim

.del

ays

.hav

.to.be.con

sidered

.at.the.con

troller

.desi

.sta

ge.

20.4 Network Effects in Control Performance

 e.main.control.performance.parameters.in.a.controlled.system.are.stability.and.the.system.error..It.is.

clea

.that.stab

ility

.must.be.ensu

red

.in.all.cont

rolled

.syst

ems.

. e.syst

.erro

.meas

ured

.on.a.time.win-

ow,

.show

.the.abil

ity

.of.a.closed

-loop

.syst

.to.recov

.aer.a.pert

urbation.

.In.gene

ral,

.we.can.stat

.that.

the.smal

ler

.the.syst

.erro

.the.bett

.the.perf

ormance.

.perf

ormance

.of.an.NBC.syst

.depe

nds

.on.the.cont

rol

.stra

tegy,

.the.mess

age

.sche

duling

.algo

rithm,

.and.the.reli

ability

.of.the.comm

unication

.netw

ork.

.We.shal

.cons

ider

.the.case.of.limi

ted

comm

unication

.reso

urces

.(sen

sors,

.cont

rollers,

.and.actu

ators),

.whic

.shar

.and.comp

ete

.with.othe

node

.for.acce

ssing

.the.netw

ork.

. e.mess

age

.acce

.sche

.and.the.comm

unication

.rela

tionship

.are.

used.as.sche

duling

.stra

tegies

.to.assig

.netw

ork

.acce

.to.node

. e.resu

lting

.sche

duling

.will.caus

.a.

shor

tening

.or.inc

reasing

.of.the.mes

sage

.lat

encies,

.thu

.in

uencing

.sys

tem

.per

formance.

.dela

.in.the.cont

rol

.loop.redu

ces

.the.syst

.perf

ormance

.[8]..As.larg

.is.the.dela

.wors

.is.the.per-

ormance.

.If.the.dela

.is.know

.it.can.be.incl

uded

.into.the.cont

rol

.desig

.as.a.para

meter

.and.its.eec

can.be.mini

mized.

.But.in.a.netw

orked

.cont

rol,

.the.netw

ork-induced

.dela

.usua

lly

.vari

.from.actu

tion

.to.actu

ation

.as.the.mess

ages

.are.sent.only.aer.comp

eting

.for.and.gain

ing

.acce

.to.the.netw

ork.

 e.resu

.is.that.the.actu

.dela

.vari

.from.cycl

.to.cycl

.whic

.not.only.degr

ades

.perf

ormance

.but.

can.driv

.the.syst

.to.inst

ability.

.In.part

icular,

.the.sens

.to.cont

roller

.dela

.can.be.time

-stamped,

.so.

the.cont

roller

.can.know.exac

tly

.the.time.inst

ant

.of.the.samp

led

.inst

ance,

.and.from.ther

.calc

ulate

.the.

actu

ation

.valu

.But.the.mess

age

.from.the.cont

roller

.to.the.actu

ator

.can.sue

.also.unpr

edictable

.dela

not.bein

.possi

ble

.to.intr

oduce

.it.into.the.cont

rol

.calc

ulation.

.One.possi

ble

.way.to.over

come

.this.situ-

tion.and.obtaining.predictability.is.using.the.one-shot.model.[9].where.the.actuation.is.synchronized.

at.the.end.of.the.per

iod.

Another

.impo

rtant

.eec

.in.the.NCS.occu

.when.a.mess

age

.is.lost.(the

.can.be.seve

ral

.caus

.for.

losin

.a.mess

age).

.It.is.clea

.that.losin

.samp

ling

.or.cont

rol

.mess

ages

.may.also.degr

ade

.the.perf

ormance

of.the.cont

rolled

.syst

em.

.From.the.cont

rol

.side,.ther

.are.die

rent

.stra

tegies

.for.redu

cing

.the.eec

.of.

lost.mess

ages,

.whil

.from.the.comm

unication

.syst

ems

.side,.some.kind.of.redu

ndancy

.can.be.used.in.

high

-integrity

.syst

ems.

.One.of.the.used.meth

ods

.is.incl

uding

.some.kind.of.obse

rver

.or.pred

ictor

.that.

calc

ulates

.or.pre

dicts

.the.los

.val

.[10

20.5 Design in NBC

Designing.an.NBC.system.requires.a.careful.work.within.the.areas.of.industrial.communications,.with.

refe

rence

.to.the.netw

ork

.used.and.the.comm

unication

.rela

tionship;

.the.cont

rol

.syst

.desig

.area

with.speci

.emph

asis

.on.incl

uding

.the.indu

ced

.netw

ork

.dela

.into.the.desi

gned

.cont

rol

.law;.and.the.real

time

.comp

utation

.area

.as.all.the.cont

rol

.task

.toge

ther

.with.othe

.task

.that.poss

ibly

.use.the.netw

ork,

.are.

to.be.depl

oyed

.in.comp

uting

.plat

forms

.that.shal

.ensu

.thei

.cont

inued

.execu

tion.

Network-Based Control 20-7

20.5.1 Design Constraints in the Network Side

 e.main.design.constraints.in.the.network.side.are.the.network.bandwidth.assigned.to.the.controlled.

syst

em,

.the.resu

lting

.samp

ling

.to.actu

ation

.dela

.and.the.prob

ability

.of.mess

age

.drop

. e.assig

ned

band

width

.is.used.to.inte

rchange

.mess

ages

.betw

een

.the.sens

.node

.the.cont

rol

.node

.and.the.actu

ator

node

.On.a.peri

odic

.cont

rol

.mode

.this.band

width

.has.to.prov

ide

.enou

.room.for.at.leas

.one.samp

ling

mess

age

.and.one.cont

rol

.mess

age

.in.each.cont

rol

.peri

od.

. e.netw

ork

.mess

age

.sche

duler

.has.to.resp

ect

all.of.the.cont

rol

.loop

.timi

.cons

traints,

.each.one.with.its.own.samp

ling

.peri

od.

.Depe

nding

.on.the.

type.of.acce

.sche

.and.comm

unication

.rela

tionship,

.the.assig

nment

.of.loop.band

width

.can.beco

cruc

ial

.in.the.res

ulting

.net

work

.ind

uced

.del

ay.

20.5.2 Design Constraints in the Control Side

In.the.control.side,.the.constraints.are.mainly.given.by.the.dynamic.response.of.the.controlled.system..

 e.dyna

mic

.resp

onse,

.also.know

.as.the.cont

rol

.syst

.band

width,

.can.be.expr

essed

.as.the.rang

.of.

freq

uencies

.the.cont

rol

.syst

.is.able.to.comp

ensate

.and.is.dire

ctly

.rela

ted

.to.the.char

acteristic

.poly

no-

mial

.of.the.clos

ed-loop

.syst

em.

. is.cont

rol

.band

width,

.toget

her

.with.the.samp

ling

.theo

rem,

.expr

esses

the.rang

.of.samp

ling

.peri

ods

.that.can.be.chos

.for.the.disc

retized

.cont

rol

.law.. e.appl

ication

.of.the.

samp

ling

.theo

rem

.to.a.disc

rete

.cont

rol

.syst

.allo

.to.foll

.one.of.the.acce

pted

.rule.of.thumb.that.

stat

.that.the.samp

ling

.freq

uency

.shou

.be.4–20.time

.the.syst

em’s

.cont

rol

.band

width.

. e.resu

.is.

that.ther

.is.some.exi

bility

.in.sele

cting

.the.cont

rol

.peri

.T..Samp

ling

.fast.(sma

.T).usua

lly

.allo

bett

.perf

ormance,

.but.requ

ires

.the.use.of.more.comm

unication

.band

width.

.Slow.samp

ling

.(larg

.T).

degr

ades

. the. cont

rol

. perf

ormance,

.but.redu

ces

. the.netw

ork

.usag

.A. trad

e-o

. must.be.consi

dered

betw

een

.the.com

mon

.res

ource

.usa

.(ne

twork

.ban

dwidth)

.and.the.con

trol

.per

formance.

20.5.3 Network and Control Co-Design

From.the.above.constraints,.it.naturally.appears.the.consideration.of.co-design.of.the.network.and.the.

cont

rol.

. e.assig

nment

.of.the.mess

age

.sche

duling

.and.the.cont

rol

.law.desig

.is.done.in.para

llel

.as.

each.one.of.the.indi

cators

.depe

nds

.on.the.othe

rs.

. e.cont

rol

.law.can.incl

ude

.some.esti

mations

.of.the.

samp

ling

.to.act

uation

.del

ays,

.the.con

trol

.per

iod

.can.be.cho

sen

.dep

ending

.on.the.net

work

.con

ditions.

.addi

tion,

.othe

.consi

derations

.and.para

digms

.can.be.appl

ied.

. e.prec

eding

.trig

gering

.sche

for.the.cont

rol

.loop.exec

ution

.is.base

.on.a.cons

tant

.time.base

.it.is.time.trig

gered.

. is.sche

.has.

been.prov

.to.be.quit

.eci

ent

.for.cont

inuous

.signa

ls,

.so.the.resu

.is.pred

ictable

.and.dete

rministic,

but.dela

.are.larg

.An.alte

rnative

.is.to.use.an.even

t-trigger

.para

digm.

.In.this.case,.the.cont

rol

.mess

ages

are.trig

gered

.by.the.occu

rrence

.of.even

ts,

.e.g.,.when.a.pert

urbation

.prod

uces

.a.cont

rol

.erro

.[11]

. is.

sche

.is.quit

.eci

ent

.for.disc

rete

.signa

.as.netw

ork

.band

width

.is.used.when.it.is.nece

ssary,

.but.

redu

ced

.when.the.cont

rolled

.syst

.is.stat

ionary.

. e.draw

back

.of.this.sche

.is.that

.alth

ough

.dela

are.low.in.aver

age,

.it.is.neit

her

.pred

ictable

.nor.dete

rministic.

.Guar

anteeing

.the.stab

ility

.and.perf

or-

mance

.of.tho

.eve

nt-driven

.sys

tems

.req

uires

.res

earch

.and.app

lied

.eo

.[12

,13].

20.6 Summary

NBC.systems. are.control.and. communication. systems. where. messages. are. used. to.close. the. loops.

betw

een

.sens

ors,

.cont

roller,

.and.actu

ator

.node

.thro

ugh

.a.shar

ed,

.band

-limited,

.digi

tal

.comm

unication

netw

ork.

.Desig

ning

.this.kind.of.syst

.invo

lves

.the.inte

gration

.of.the.disc

iplines

.of.digi

tal

.comm

uni-

cations,

.cont

rol

.desig

.and.real

-time

.comp

uting.

.In.this.chapt

er,

.we.have.show

.the.arch

itectures

.for.

NBC.syst

ems,

.the.conc

epts

.asso

ciated

.with.the.netw

ork

.side.seen.from.the.cont

rol

.side,.the.conc

epts

rela

ted

.to.the.cont

rol

.side.seen.from.the.netw

ork

.side,.and.the.prin

cipal

.para

digms

.and.cons

traints

.for.

desig

ning

.an.NBC.syst

em.

. is.NBC.chapt

.aims.to.prov

ide

.a.desc

ription

.not.only.of.the.fund

amental

aspe

cts

.in.thi

.pra

ctical

.are

.but.als

.whe

.som

.of.the.pri

ncipal

.res

earch

.dir

ections

.lie