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App
licat
ion
of F
orm
al M
etho
ds
for D
esig
ning
PLC
sA
Pet
ri N
et A
ppro
ach
Ali
Pou
yan
Emai
l: ap
ouya
n@ie
ee.o
rg
Pro
gram
mab
le L
ogic
Con
trolle
r -P
LC
A P
LC is
a s
peci
fical
ly d
esig
ned
digi
tal
cont
rol d
evic
e th
at a
ccep
ts in
put s
igna
ls
and
emits
out
put s
igna
ls b
ased
on
a co
ntro
l log
ic p
rogr
am.
PLC
s ar
e co
mm
only
use
d as
seq
uenc
e co
ntro
llers
in in
dust
rial a
utom
ated
sy
stem
s.
Cha
ract
eris
tics
of a
PLC
It ha
s a
prog
ram
mab
le m
emor
y to
sto
re
the
inst
ruct
ions
.Th
e st
ored
inst
ruct
ions
in th
e m
emor
y ar
e us
ed to
impl
emen
t sev
eral
func
tions
su
ch a
s: lo
gic,
seq
uenc
ing,
tim
ing,
co
untin
g, a
nd a
rithm
etic
func
tions
.Th
e va
rious
func
tions
are
use
d to
con
trol
mac
hine
s or
pro
cess
es.
PLC
-H
isto
ry
The
hist
ory
of P
rogr
amm
able
Log
ic
Con
trolle
rs (P
LCs)
goe
s ba
ck o
nly
as fa
r as
196
8to
the
Gen
eral
Mot
ors
Cor
pora
tion
(GM
). P
rior t
o 19
68, t
he in
dust
rial c
ontro
l fu
nctio
ns w
ere
perfo
rmed
by
cont
rol r
elay
s(C
Rs)
.
CR
s D
isad
vant
ages
They
are
cap
able
onl
y of
on/
off c
ontro
l, so
the
cont
rol s
chem
e fo
r com
plic
ated
syst
ems
is
quite
exp
ensi
ve.
They
are
qui
te b
ulky
.Th
ey a
re p
ower
-hun
gry,
so
high
pow
er
cons
umpt
ion
resu
lts in
hea
t gen
erat
ion.
Whe
n a
rela
y fa
ils, t
roub
lesh
ootin
gor
loca
ting
the
faile
d re
lay
is d
iffic
ult.
They
are
har
dwire
d, s
o an
y ch
ange
in th
e co
ntro
l pro
gram
requ
ires
rela
ys to
be
rew
ired.
GM
Crit
eria
for P
LCs
-196
8
Eas
ily p
rogr
amm
ed o
r rep
rogr
amm
ed
with
a m
inim
um o
f dow
ntim
e.E
asily
mai
ntai
ned.
Rug
ged
enou
gh to
ope
rate
in a
n in
dust
rial e
nviro
nmen
t.N
ot p
ower
-hun
gry.
Com
petit
ive
in c
ost.
Firs
t Gen
erat
ion
PLC
s
The
maj
or c
hara
cter
istic
of t
he fi
rst
gene
ratio
n w
as th
at th
ey d
id h
ave
prim
itive
se
lf-di
agno
stic
indi
cato
rsth
at a
ided
tro
uble
shoo
ting.
As
time
prog
ress
ed, P
LCs
acqu
ired
the
capa
city
to d
o ar
ithm
etic
, man
ipul
ate
data
, an
d co
mm
unic
ate
mor
e ef
ficie
ntly
with
the
prog
ram
mer
.
Toda
y’s
PLC
s
They
are
eco
nom
ical
, use
r-fri
endl
y de
vice
s th
at
have
ben
efite
d fro
m a
dvan
ces
in m
icro
proc
esso
r an
d m
emor
y te
chno
logi
es.
Toda
y’s
PLC
s ar
e ca
pabl
e of
per
form
ing
com
plic
ated
con
trol r
outin
es a
nd c
an
com
mun
icat
e w
ith o
ther
PLC
s an
d ho
st
com
pute
rs in
sop
hist
icat
ed c
ontro
l net
wor
ks.
As
time
prog
ress
ed, P
LCs
acqu
ired
the
capa
city
to
do
arith
met
ic, m
anip
ulat
e da
ta, a
nd
com
mun
icat
e m
ore
effic
ient
ly w
ith th
e pr
ogra
mm
er.
Toda
y’s
PLC
s -c
ontin
ued
It is
wid
ely
belie
ved
that
one
of t
he a
reas
w
here
sys
tem
s en
gine
erin
g m
etho
dsst
ill ne
ed a
con
sist
ent d
evel
opm
enti
s th
at o
f lo
gic
cont
rolle
rs.
Spe
cific
atio
n fo
rmal
ism
and
desi
gn
crite
ria a
re s
till f
ar fr
om b
asic
ana
lytic
requ
irem
ents
, in
this
are
a.
Sta
tem
ent o
f the
Pro
blem
The
prob
lem
of c
ontro
l lea
ds to
the
desi
gn
of s
eque
nce
cont
rol s
oftw
are
for
auto
mat
ed d
iscr
ete
even
t dyn
amic
sy
stem
s(D
ED
S).
Seq
uenc
e co
ntro
l is
mai
nly
asyn
chro
nous
, w
ith c
oncu
rrent
con
trolt
o m
aint
ain
sync
hron
isat
ion
and
excl
ude
conf
lictin
g m
achi
ne a
ctio
ns.
Sta
tem
ent o
f the
Pro
blem
The
exis
ting
(mod
ellin
g) la
ngua
ges
are
fund
amen
tally
pro
cedu
ral l
angu
ages
and
it is
stil
l diff
icul
t to
unde
rsta
ndth
e pr
ogra
m’s
be
havi
our w
hen
prog
ram
siz
ebe
com
es
larg
e.Th
ey d
o no
t pro
vide
form
al v
erifi
catio
n fo
r th
e de
sign
ed c
ontro
l sof
twar
e.
PLC
Sta
ndar
d La
ngua
ges
-IE
C
IL: I
nstru
ctio
n Li
stS
T: S
truct
ured
Tex
tFB
D: F
unct
ion
Blo
ck D
iagr
amS
FC: S
eque
ntia
l Fun
ctio
n C
hart
LLD
: Lad
der L
ogic
Dia
gram
sN
one
of th
e ab
ove
lang
uage
s ar
e de
sign
m
etho
dolo
gies
, nor
are
they
wid
ely
acce
pted
or t
este
d, e
xcep
t LLD
.
LLD
s
Eve
n if
LLD
sas
sum
e a
rele
vant
im
porta
nce
in th
e in
dust
rial m
arke
t, th
ere
is n
ot y
et a
sta
ndar
d in
tegr
ated
tool
, whi
ch
is s
uffic
ient
ly s
impl
e to
use
, pow
erfu
l, ve
rsat
ile, u
nam
bigu
ous,
and
with
whi
ch it
is
pos
sibl
e to
car
ry o
ut p
roof
s on
form
al
corre
ctne
ssbe
side
sth
e cl
assi
cal
valid
atio
n th
roug
hsi
mul
atio
n.
Bui
ldin
g B
lock
s of
Log
ic C
ontro
llers
Logi
cal O
RLo
gica
l And
Con
curre
ncy
Syn
chro
niza
tion
Logi
cal “
OR
”
Logi
cal “
AN
D”
Com
bina
tiona
l Log
ic F
unct
ions
Pro
pose
d M
etho
d
We
have
pro
pose
d a
conc
eptu
al a
nd
prac
tical
met
hodo
logy
to d
esig
n co
ntro
l so
ftwar
e sy
stem
s (L
ogic
con
trolle
rs) w
ith
Pet
ri N
etth
eory
.
Pro
pose
d M
etho
d
In p
artic
ular
, the
pro
pose
d sp
ecifi
catio
n fo
rmal
ism
is b
ased
on
a sp
ecia
l cla
ss o
f P
etri
nets
, tai
lore
d to
mod
el th
e co
ntro
l flo
w in
dis
cret
e sy
stem
s.
Pro
pose
d M
etho
d
The
mai
n ob
ject
ive
of th
e pr
opos
ed
met
hodo
logy
is to
redu
ce th
e de
sign
effo
rt du
e to
the
com
plex
ity o
f con
trol p
robl
ems
and
to p
rovi
de d
esig
n cr
iteria
that
fit t
he
fund
amen
tal e
ngin
eerin
g ap
proa
ch to
co
mpl
exity
: the
incr
emen
tal d
esig
n.
Pro
pose
d M
etho
d
The
met
hodo
logy
is b
ased
on
conc
epts
co
min
g fro
m a
num
ber o
f pro
ven
theo
rem
s.
Pet
ri N
ets:
A S
naps
hot
In 1
962
Car
l Ada
m P
etri,
a G
erm
an
mat
hem
atic
ian,
intro
duce
d P
etri
nets
as
a m
athe
mat
ical
tool
for d
escr
ibin
g re
latio
ns
betw
een
cond
ition
s an
d ev
ents
.N
ow P
etri
net t
heor
y is
an
impo
rtant
fiel
d in
theo
retic
al c
ompu
ter s
cien
ce a
s a
high
le
vel f
orm
al s
peci
ficat
ion
lang
uage
.
Pet
ri N
ets:
A S
naps
hot
Pet
ri ne
ts c
an b
e sh
own
grap
hica
lly u
sing
a
3-el
emen
t set
of s
ymbo
ls:
a pl
ace:
repr
esen
ted
by c
ircle
a tra
nsiti
on: r
epre
sent
ed b
y a
bar o
r a b
oxan
arc
: rep
rese
nted
as
a di
rect
ed a
rc
conn
ectin
g ei
ther
a p
lace
to a
tran
sitio
n or
a
trans
ition
to a
pla
ce
Pet
ri N
ets:
A S
naps
hot
A P
etri
net s
truct
ure,
N, i
s de
fined
as
a fo
ur-
tupl
e, N
= (
P, T
, V, F
)whe
reP
∪P
= {
p1,p
2,...
, pn
} is
a fin
ite s
et o
f pla
ces,
n ≥
0,T
= { t
1,t2
,...,
tm }
is a
fini
te s
et o
f tra
nsiti
ons,
m ≥
0, (T
∪P
form
the
node
s of
N),
V ⊆
{ (P×T
)∪(T×P
)} is
a s
et o
f dire
cted
arc
s
(or a
flow
rela
tion)
, and
F : V
→ℵ
is a
mul
tiplic
ity fu
nctio
n,
ℵ=
{0,1
,2,…
}, P
∩T=
∅an
d T≠
∅.
Bui
ldin
g B
lock
s of
the
Seq
uenc
e C
ontro
l
Bui
ldin
g B
lock
s of
the
Seq
uenc
e C
ontro
l
LLD
of a
Log
ic E
quat
ion
Y =
((X
0+X
1+X
2).(X
4.X5)
) + X
3
( Y
)
X0
X1
X2
X3
X4
X5
(b
)
Pet
ri N
et M
odel
of a
Log
ic E
quat
ion
Dis
adva
ntag
es
LLD
-bas
ed c
ontro
llers
bec
ome
cum
bers
ome
both
to d
esig
n an
d de
bug
whe
n ap
plie
d to
co
mpl
ex s
eque
ntia
l tas
ks in
dis
cret
e-ev
ent
man
ufac
turin
g sy
stem
s.
Whe
n a
prob
lem
is d
etec
ted
in th
e sy
stem
, ca
use-
traci
ng a
nd lo
catin
g be
com
es e
xtre
mel
y di
fficu
lt.
LLD
s do
n’t p
rovi
de a
ny to
ols
for a
naly
sis
and
perfo
rman
ce c
hara
cter
istic
s of
the
syst
em o
ther
th
an b
asic
sim
ulat
ion
for v
erifi
catio
n of
pro
gram
s.
Dis
adva
ntag
es
In th
e co
ntex
t of a
gile
man
ufac
turin
g sy
stem
s, in
whi
ch c
ontro
l seq
uenc
es n
eed
to b
e re
gula
rly m
odifi
ed, t
o m
eet t
he
dyna
mic
ally
cha
ngin
g re
quire
men
ts o
f the
ou
ter e
nviro
nmen
t, LL
D-b
ased
seq
uenc
e co
ntro
llers
are
not
read
ily a
dapt
able
to th
e ch
ange
s ne
eded
to fu
lfil t
he s
yste
m
spec
ifica
tion
chan
ge.
Pet
ri N
et M
odel
vs.
LLD
Mod
el
Bot
h th
eore
tical
ly a
nd p
ract
ical
ly, a
form
al
(PN
) mod
el o
f a c
ircui
t tha
t can
be
anal
ysed
and
ver
ified
is a
lway
s pr
efer
red
to a
LLD
sta
tic g
raph
.Th
e dy
nam
ics
of a
sys
tem
can
be
stud
ied
thro
ugh
the
trans
ition
firin
gs a
nd
dist
ribut
ion
of to
kens
in a
PN
mod
el.
Cha
ract
eriti
csof
the
Pro
pose
d A
ppro
ach
Ther
e is
no
need
to c
onst
ruct
sep
arat
e m
odel
s (m
odul
es) o
f diff
eren
t con
trol
ta
sks
and
mer
ge th
em to
bui
ld u
p an
ex
pand
ed s
yste
m. T
he s
yste
m is
ex
pand
ed b
y kn
ittin
g th
e co
ntro
l tas
ks
to th
e ba
sic
proc
ess
in a
str
uctu
ral
rule
-bas
ed in
crem
enta
l fas
hion
whi
lepr
eser
ving
the
desi
red
prop
ertie
s.
Cha
ract
eris
tics
of th
e P
ropo
sed
App
roac
h
The
logi
c co
ntro
ller c
an b
e de
sign
ed a
t an
y le
velo
f abs
trac
tion.
Fur
ther
mor
e, a
de
taile
d ex
pand
ed s
yste
m c
an b
e re
duce
dto
a s
yste
m w
ith h
ighe
r lev
el
of a
bstr
actio
nac
cord
ing
to c
erta
in
rule
s.
Cha
ract
eris
tics
of th
e P
ropo
sed
App
roac
h
It is
a ru
le-b
ased
app
roac
h w
hich
can
be
impl
emen
ted
(cas
t to
code
) as
a C
AD
tool
for l
ogic
con
trol
ler d
esig
n
Exa
mpl
e
Con
side
r a m
anuf
actu
ring
cell
cons
istin
g of
a re
volv
ing
benc
h w
ith th
ree
wor
ksta
tions
: loa
ding
, dril
ling,
and
qua
lity
cont
rol a
nd u
nloa
ding
. The
revo
lvin
g be
nch
is ro
tate
d to
put
the
piec
es u
nder
co
rrect
mac
hine
s. T
hree
pro
cess
es c
an
prog
ress
con
curre
ntly
, ie.
, loa
ding
, dril
ling,
an
d qu
ality
con
trol.
Exa
mpl
e -1
o p1t1
t3p4
t4p5
t5p6
p2t2
p3t6
p7t7
Basic
task
(pro
cess
)-drill
ing
The
drilli
ng ta
sk m
odel
led
by a
Pet
ri N
et
Exa
mpl
e -2
o p1t1
t3p4
t4p5
t5p6
p2t2
p3t6
p7t7
p8t8
p9t9
p10
Inte
grat
ing
the
load
ing
task
to th
e sy
stem
Exa
mpl
e -3
o p1
t1t3
p4
t4p
5t5
p6
p2
t2p
3t6
p7
t7
p8
t8p
9t9
p1
0
p1
1
t10
p1
2
t11
p1
3
t12
p1
4
t13
p1
5
t14
p1
6t1
5p
17
t16
ext
ern
al
en
ab
ling
si
gn
al
The
final
mod
el a
fter i
nteg
ratin
g qu
ality
con
trol t
ask
