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Combinatorial and Sequencing Control

Chapter 13

Gunnar Lindstedt

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Combinatorial vs. Sequential

• Combinatorial – no memory– Only depends on current signals

• Sequential – contains state(s)– Next state is a function of:

• Current inputs• Current state

Applicable for:Machine and/or Control system

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Example: Combinatorial Control

MachineControlsystem

Task:

Run the machine while the “Run” signal is given if both operator A and operator B are present.

Give “Fail”-signal if machine doesn’t respond.

OpApresOpBpres

RunFail

MRun

Running

MRUN = OpApres AND OPBpres AND Run

Fail = MRun AND NOT Running

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Example: Sequential Control

Conveyor

Controlsystem

Task:

Move the object from A to B when the signal “MoveA2B” becomes true.

Give a true signal on ”Moved” when the object has reached B.

MoveA2B

Moved A B

Bdetect

ConvRun

Idle Moving

Ready

MoveA2B

BdetectNOT MoveA2B

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Consequently….

• The controlled machine may contain states

• The controller may contain states

• The macro view of controller and machine contain states if any subsystem contain states

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Focus on the Controller• Most controllers are sequential• Is this a sequential controller? :

while (true) {while (NOT MoveA2B) /* do nothing */ ;ConvRun := true;while (NOT Bdetect) /* do nothing */ ;ConvRun := false;Moved := true;while (MoveA2B) /* do nothing */ ;Moved := false;

}

If yes – where is the state?

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Event Driven Systems –State Concept

Sequencing circuits

– one state at a time– state transfer– conditions for each

state transfer

1 2

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Sequential Control Tools

• State diagrams• Petri nets• Sequential Function Charts -

Grafcet• Switching theory• PLC languages IEC61131

3

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PLC programming

The standard IEC 61131-3 implies:• definition of sequential function chart

(SFC), and 4 language options– instruction list (IL)– function block (FBD)– ladder diagram (LD)– structured text (ST)

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Automation in the Good Old Days….

F

i

Ladder (LD) is based on this model

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Ladder Frameworkpower supply ground

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Ladder Symbols

a.

b.

c.

DIN US Electric

a.) Normally open (NO), makeb.) Normally closed (NC), breakc.) Output, coil

4

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Boolean Logic in LD

D

A B Y1

C Y2

+ 24V GNDY1 = AꞏBY2= C+D

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Set – Reset in LD(State implementation)

S R

Y

Y

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Reflections…

• A relay has only one coil, consequently we use only one coil symbol with the same name.

• An arbitrary number of make/break elements can be used in a ladder diagram.

• Symbols may be physical or logical in a PLC.

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An elementary PLC structure

000102030405060708091011121314151617

202122232425262728293031323334353637

phase 1data input

phase 2processing

phase 3signal output

processor

programmemory

datamemory

input signals

input register

output signals

output register

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State Encoding in LD

A B Cx y

A x C B

B

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z

Alternative State Transitions

A B Cx y

A x C B

B

D

DNote!y and z have to be mutually excluding

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Several Ways to Enter a State

A1 B Cx1 y

A1 x1 C B

B

A2x2

A2 x2

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Example: A mixing tank is used to produce process water with correct temperature (T1 true, T2 false). Supply water with approximately correct temperature is filled with V1 to level N2. If the temperature is too high or too low the water is reduced to N1 using dump valve V3. Then hot or cold water is added to level N2 using H or C.This is repeated until the temperature is correct. Then the water is delivered using V2 and a new batch of water should be produced.

V1

V2 V3

H C

N0

N1

N2T1T2

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The Classical Transfer Line

A B C Dfinished

product

raw

material

Starving - no input product

Blocking - can not deliver finished product

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Simple Cell

M1 M2

input buffer output buffer

robot

buffer

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Petri Net of the simple cell

idle

load M1

working

finished M1

finished

unload M1

idle

load M2

working

finished M2

finished

unload M2

machine 1 machine 2

putget

buffer robot

ii iii iv v vi vii viiiii

C1getunload M2unload M1load M1load M2putC2

iiii

iiiivv

vivii

viii

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The Manufacturing Cell

M1 M2 M3

car

in out

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Controlling the Cell

scheduler

machine 1 machine 2 machine 3 robot buffer

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1970s: á la carte programming

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1980s: industrial packages

Exempel: TDC3000 SCADA SATT line

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1990s: open protocolsLarm Övervakning

Styrning

Data- in- samling

Recepthantering

Nätverk

Rapport- generator

Presentation

Databas

Specialmodul

Special- modul

8

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The Software Problem• Software ~2 yrs• Hardware ~5 yrs• Application ~15 yrs

Requires:• Standardised

interfaces (ActiveX, ODBC, DDE, OPC,...

• Solutions have to survive a generation shift (planned obsolescence)

• Has to permit stepwise implementation

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Two different developments

• The process industry– Replace the analog PID controller with a

computer– The instrumentation people

• The manufacturing industry– Replace the electromechanical relays with a

computer– The electricians

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Sequential Control Tools

• State diagrams • Petri nets • Sequential Function Charts –

Grafcet (SFC)• Switching theory• PLC languages IEC61131 (LD )

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OR u2

u1 y 1> u2

u1 yu u y1 2 0 0 00 1 11 0 11 1 1

y = u + u 1 2

&u2

u1 yAND u2

u1 yu u y1 2 0 0 00 1 01 0 01 1 1

y = u u 1 2

NOR u2

u1 y 1> u2

u1 yu u y1 2 0 0 10 1 01 0 01 1 0

y = u + u 1 2

&u2

u1 yNAND u2

u1 yu u y1 2 0 0 10 1 11 0 11 1 0

y = u u 1 2

1u yNOT u y u y0 11 0

y = u

=1u2

u1 yXOR u2

u1 yu u y1 2 0 0 00 1 11 0 11 1 0

&u y

SLock

u y y = u u 1 2 S

DIN/IEC UStruthtable

booleanexpression

ElementaryBooleanexpressions

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De Morgans theorem

zyxzyx

zyxzyx

)(

)(

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Three NOR = AND

u2

u1 y 1>

0u1

1>

0u2

1>

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u1

y1

y2

y3

y4

u2

u3

u4

1

& & & & & & & &

1

1

1

1

1

1

1

>

>

>

>

input bufferwith inversion

AND matrix

OR matrix

Structure ofPLD, PROM

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PLC programming

The standard IEC 61131-3 implies:• definition of sequential function chart

(SFC), and 4 language options– instruction list (IL)– function block (FBD)– ladder diagram (LD) – structured text (ST)

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From ladder to PLC codeInstruction List (IL)

u1 u2

u2

y1

y1 y2

y3

u3ld u1or y1and u2and u3out Y1out Y2ldi u2out Y3

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From ladder to PLC

u1 u2

u3 u4

y1 ld u1and u2ld u3and u4orbout Y1

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From Boolean to PLC

&

&&

& 1>

u1

y1

u2

u3

u4

u5

u6

ld u1and u2ld u3and u4ld u5and u6orbanbout Y1

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PLC programming

The standard IEC 61131-3 implies:• definition of sequential function chart

(SFC), and 4 language options– instruction list (IL) – function block (FBD)– ladder diagram (LD) – structured text (ST)

11

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Function Block Diagram (FBD)

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PLC programming

The standard IEC 61131-3 implies:• definition of sequential function chart

(SFC), and 4 language options– instruction list (IL) – function block (FBD) – ladder diagram (LD) – structured text (ST)

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Structured Text (ST)

Strongly resembles Pascal

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PLC programming

The standard IEC 61131-3 implies:• definition of sequential function chart

(SFC), and 4 language options– instruction list (IL) – function block (FBD) – ladder diagram (LD) – structured text (ST)

12

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Controlling a simple Batch Tank

1

2

3

4

5

tank empty

discharge valve closedpump on

pump offheater on

heater offwait time=”time out”

open discharge valve

empty * start

full

temp

wait time

empty

Function Comment

This expression is true if the sensor for the low levelindicates empty=1 and a start command is given

Start filling operation

The level has reached the upper limit value

End of filling operation and start of heating time

The desired temperature has been reached

The waiting time is “time out”

The waiting time is over

The tank is empty

Initial stateFilling state

Heating state

Wait state

Emptying state

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The Sequential Function Chart

1

2

3

4

5

tank empty

discharge valve closedpump on

pump offheater on

heater offwait time=”time out”

open discharge valve

empty * start

full

temp

wait time

empty

Function Comment

This expression is true if the sensor for the low levelindicates empty=1 and a start command is given

Start filling operation

The level has reached the upper limit value

End of filling operation and start of heating time

The desired temperature has been reached

The waiting time is “time out”

The waiting time is over

The tank is empty

The Sequencepart

The Actions

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The finite state concept

• The process is represented by 5 states• Only 1 state at a time!• A transition signal marks the change from

one state to another one• A condition for transition (from a sensor, a

timer, an operator)• In each state there is an action

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Grafcet

• Developed in France 1977• Graf + AFCET (Association Francaise

pour la Cybernetique Economique et Technique)

• French standard 1982• IEC standard 1988 - IEC 848

(=SFC, Sequential Function Chart)• Essential part of IEC 61131-3

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Alternating Parallel Branches

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Grafcet for a Drill(Alternating parallel branches)

drill in positionup

set acknowledge time=time_1

show drill at work piece alarm

set start rotationset drill to work pieceset max time

21

22

23

24 30

drill at work piece * max timedrill at work piece * NOT max time

drill ready * NOT time_1 * work piece

drill start

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Simultaneous Parallel Branches

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FromSFC

to ladder

Simul-taneousparallel

branchess4

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FromSFC

to ladder

Alternatingparallel

branches

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PLC open

• PLC open is an organisation to support the standard 61131-3

• Manufacturer and product independent• Members of the PLC open group are

expected to deliver products that follow the 61131-3 standard

http://www.plcopen.org/

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PLC Open (2)• Open systems can be replaced with

software or hardware from a third party• Some companies call products open

incorrectly• The IEC 61131 standard encourages

interchangeable systems• Open architecture controllers replace a

PLC with a standard computer

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Batch Operation

• Batch operation software is standardised in the ISA S-88 standard (ISA)

• S-88 defines models and terminology

www.isa.org/isa88

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Structure of a Batch Operation

Process cellProcess unitProcess enhet

Process cell

Process unit

Process cell

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Important Concepts

• Process cell• Process unit• Batch• General recipe• Control recipe• Phase

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Batch – from the start• The customer has a

manufacturing procedure (recipe)

• How to structure the software ?

Add 10 l of waterAdd 5 l of ethanolMix 3 min, 300 rpmAdd 2 l of saltMix 2 min, 1000 rpmHeat until 45o CEmpty after 5 min

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Batch (2)

Identify a sequence of operations in the unit

1

2

3

4

5

6

7

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Batch (3)

• Identify phases and parameters for each unit– Raw material (sort, volume) – Mixing (time, speed) – Temperature control (setpoint value)– Empty (time)

• Write the executing of the recipe

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US US US

Nedbrytning

Rening

Torkning

Batch (4) - several units

• Identify the equipment need• Write recipe• Start the production

Break down

Drying

Cleaning