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Production Systems Control - Prof. Luca FerrariniA.A. 2011-2012

Automation Engineering 2011 - 2012

Production Systems Control

Sequential Function ChartProf. Luca FerrariniEng. Giancarlo Mantovani

Politecnico di Milano – Dipartimento di Elettronica e Informazione

A.A. 2011-2012 Production Systems Control - Prof. Luca Ferrarini 2

► Introduction

► Basic elements

► Basic operators

► Evolution rules

► LD and SFC

Elementi Base Operatori Base LD ed SFCIntroduzione Regole di Evoluzione

Index


� History:

� SFC is a programming language defined in IEC61131 standard

� SFC was born from Grafcet, which is a representation language definedby AFCET (Association française pour la cybernétique économique et technique)

� SFC is its implementation as a Programming language for PLCs

Introduction

Basic elements Basic operators LD and SFCIntroduction Evolution rules


� In 1975 in France, si a commitee was formed to define a newformalism

� Result: GRAFCET, from “GRaph” + AFCET (Association française de cybernétique économique et technique)

� In 1988 standard IEC 848 (International ElectrotechnicalComission) is defined:

� «Preparation of function charts for control systems», with name«Sequential Function Chart», based on grafcet

� In 1993 SFC is included in IEC61131-3 standard on PLCprogramming

History



� SFC was born for the following reasons:

� Use formal models to represent control logics function for industrial automation

� Standardize representation formalisms

� Enrich automata with parallelism

� Simplify Petri nets in order to adapt them to industrial applications

� Be easily interpretable by control devices

Introduction




Basic elements

� Few and simple ideas

� Step: sequence execution state

� Transition: passage between two steps during program execution

� Action: every step is associated with one or more actions to be performed

� Logic conditions: enables transition trigger, so execution evolution (true = transition triggered, false = transition stopped)

� Oriented arc: identifies passages between steps through a transition which is enabled by a logic condition

A.A. 2011-2012 Production Systems Control - Prof. Luca Ferrarini


Basic elements - Step

7

� Step:

� ID (usually an integer sequential number)

� Can be active or inactive

� Graphical representation: square (with double border if starting step)

� Has associated actions to be performed when the step is “active” serie di “azioni. To every action a qualifier is associated, specifyingexecution time or duration (N, S, P, etc.)

2Turn

Motor on

Step

Identifier

Active step is marked

X

Qualifier:• P Activation• N Deactivation• S Set• R Reset• …

Action


� Qualifiers:

Elementi Base Operatori Base LD ed SFCIntroduzione Regole di Evoluzione

N Non-Store Action terminates when the step becomes inactive.

S Set (Stored)Actions continues even if the step becomes inactive and is terminated when the action is reset.

R Reset Terminates an action activated with qualifiers S, SD, SL or DS.

L Time LimitedAction is started when step becomes active and continues until the step becomes inactive or a certain time interval has passed.

D Time DelayedA timer is set when the step becomes active; if the step is still active and time is elapsed, the action is started and terminates when step goes inactive.

P PulseThe action is performed when the step goes active and is executed only once.

SDStored and time Delayed

The action is started with a time delay even if the step has gone inactive in the meanwhile and is performed until it is reset.

DSDelayed andStored

A timer begins counting when step becomes active. If the step is still active after timer completion, the action is started and carries on until is reset.

SLStored and time Limited

The action starts when the step becomes acrive and is carried on until it is not reset or a certain time is elapsed.

Basic elements - qualifiers




Basic elements – Transitions

� Transitions

� Identifier

� Enabled or disabled

� Can be triggered

� Associated to a logic condition (or “receptivity”)

� Graphical representation: horizontal segment

C1 AND C2

Transizione Logic condition (receptivity)


� Connection rules:

� A step can be directly connected only with transitions, not with steps; similarly, a transition can be directly connected with steps, not with other transitions.

� Graphical conventions:

� Arc with right angles

� Arc directed downwards: without arrow

� Arc directed upwards: with arrow

1

2

3

Start

Tank target level reached

ActivatePump P1

Switch-offPump P1

Basic elements – Oriented arc


Pump is off


� In SFC it is possible to represent two basic features fordiscrete events systems:

� Choice (OR operators)

� Parallelism (AND operators)

� Graphical operators characterized by a “start” ( divergence )and an “end” ( convergence ) correspond to these two features

Basic operators



� Choice start

� Two transitions after a single step

NOTE: mutual exclusion is necessary in order to have a deterministic behavior of the program

3

4 5

C1 NOT(C1)


Basic operators – Or divergence


� Choice end

� A step with two transitions before

9 10

11

C1 C2

Basic operators – Or convergence



� Parallelism start

� Simultaneous activation of more steps

14

15

13

14


Basic operators – AND divergence


� Parallelism end

� Simultaneous deactivation of two or more steps (syncronization)

15

22

20 21

Basic operators – AND convergence



� A transition is enabled (by the associated logic condition) if allthe steps before it are active

� A transition can be triggered if is enabled and its receptivityhas logical value true (the logic condition is verified)

16

22

20 21

Enabled

Evolution rules



� Rule 1 – if a transition can be triggered, it is actually triggered :all steps before the transition are deactivated, while all steps after itare activated

� Rule 2 – all triggerable transitions in a certain time instant aretriggered at the same time

17

Evolution rules



Transition enabled

� Transition triggering

18

17

15

16

Evolution rules – AND divergence




Transition passed

19

Elementi Base Operatori Base LD ed SFCIntroduzione

17

15

16


Evolution rules – AND divergence



20

22

20 21


Evolution rules – AND convergence

Transition enabled



Transition passed

21

22

20 21


Evolution rules – AND convergence



� C1 true, C2 false

3

4 5

C1 C2

22


Evolution rules – OR divergence



� C1 true, C2 false

3

4 5

C1 C2

23





� C1 true, C2 true

3

4 5

C1 C2

24





� C1 true, C2 true

3

4 5

C1 C2

25





� C1 true, C2 true → Ambiguous situtation which can be solved with mutual exclusion or transition priority (interpretation depends on the PLC..)

3

4 5

C1 C2 AND NOT(C1)

26




� Note

� By applying evolution rules it can happen that a step (31) should be activated and deactivated at the same time

� Conventionally, this step stays active (if it has an associated action which is being performed, this action is not stopped and started again)

� But this situation should be used with caution…

27

30

31

32

C10

C11

C10 and C11both true


Evolution rules


� Note

� By applying evolution rules it can happen that a step (31) should be activated and deactivated at the same time

� Conventionally, this step stays active (if it has an associated action which is being performed, this action is not stopped and started again)

� But this situation should be used with caution…

28

30

31

32

C10

C11

C10 and C11both true


Evolution rules


SFC LD

PROs CONs PROs CONs

• Easier control logicsdesign

• Allows functionaldesign of projectspecifications

• The project has evena documentationfunction

• Easier maintainanceof already developedfunctions

• Even if it was bornmore than 25 yearsago and is present in IEC industrial standard, is notsupported by all PLCs

• Supported by allPLCs

• Really low levellanguage

• Difficult to read and so to maintain

• Not suitable forcomplex projects


LD vs SFC


SFC LD

PROs CONs PROs CONs

• Easier control logicsdesign

• Allows functionaldesign of projectspecifications

• The project has evena documentationfunction

• Easier maintainanceof already developedfunctions

• Even if it was bornmore than 25 yearsago and is present in IEC industrial standard, is notsupported by all PLCs

• Supported by allPLCs

• Really low levellanguage

• Difficult to read and so to maintain

• Not suitable forcomplex projects


LD vs SFC


T3

2 3

4

5 6

T1T2

1

T4

Cond1Cond2

Cond3Cond4

31

� Consider the generic step [4] of an SFC…


Boolean equivalent equations


� State activation equation:� X4 = X1 • Cond1 + X2 • X3 • Cond2

� State permanence equation :� X4 = X4 • NOT (Cond3 + X5 • X6 • Cond4)

� Output equation (depends on qualifier):� N: Action4 = X4� ...

� Problem: they are not very readable and maintainable anditerpretation rules are missing…


Boolean equivalent equations


� The formal representation of evolution (or interpretation ) rulesof an SFC program is called evolution algorithm (orinterpretation algorithm)

� In literature there are many alternatives for evolutionalgorithms, because the same program can be interpreted indifferent ways…

33


SFC interpretation


1

2

3

A

B

Let’s make an example:� What if A and B are both verified?


SFC interpretation


1

2

3

A

B

Let’s make an example:� What if A and B are both verified?� Step 2 would be activated and…


SFC interpretation


1

2

3

A

B

� But “immediately” cannot be obtained with realdevices and in any case needs to be interpreted..


SFC interpretation

Let’s make an example:� What if A and B are both verified?� Step 2 would be activated and...� …”immediately” after it, step 3


� Evolution algorithms without stability search

� Algorithmic representation of evolution rules such as in presence of transitions sequences with logic condition equal to true, they are triggered in different cycles, not in the same one

� All the outputs associated to intermediate steps are assigned (even if activation token stays in the step for an arbitrarily small time)

� Four steps for each cycle:1. Input scanning2. Triggerable transitions definition3. New state definition (due to transition trigger)4. Output writing

37


SFC evolution algorithms


Steps initialization

Input scanning

Output writing

Input scanning

Triggerable transitions definition

New state definition

Output writing



� Evolution algorithmwithout stability

search


Steps initialization

Input scanning

Impulsive actions execution

Input scanning

Triggerable transitions definition

New state definition

Output writing

Varied condition

Impulsive actions execution

� Evolution algorithmwith stability search




� Ladder Diagram implementation of an SFC schema withoutstability search

� A memory bit is associated to each step (representing if thestep is active or not)

� A memory bit is associated to each transition (meaning that atransition is triggerable in a certain state)

� Program divided in 4 sections:1. Initialization2. Actions executions section3. Transitions evaluation secton4. Steps update section

40


SFC to LD translation – without stability search


� Executed once: initializes to 1 initial steps:

41

W1LBL

W1

Initial_step_1

Initial_step_k

LBL

L

L

L


SFC to LD translation – [1] Initialization

JMP


� For every action we need to perform these translation steps

� For every action with N qualifier , it is necessary to include a rung withinside all the steps performing that action, put under an OR condition.

� If the action is under other conditions (example: TIMER ), it isnecessary to include that condition in the rung (AND condition).

� For actions with set or reset qualifiers (S, R), Latch or Unlatch coils areneeded.

� For impulsive actions it is necessary to use positive/negative edgedetectors

42


SFC to LD translation – [2] Actions


Step i Action A

Step j Tk

Step k

Step m

Step n

Action B

L


SFC to LD translation – [2] Actions

Perform Action A (N qualifier ) if you are in step i, k or in step j and Tk=1 (timer elapsed) (D/L qualifiers )

Start performing Action B (S qualifier ) when step m or n become active (until Action B is reset)


� The bit associated to the transition is high if the transitio n istriggerable

� A rung is defined for each transition

44

Step i Step j Receptivity Transition A


SFC to LD translation - [3] Transitions

=

Receptivity

j

A

i


� In correspondence of a triggerable transition:� 1 is written for steps after the transition (steps going active)� 0 is written for steps before the transition (steps going inactive)

45

Step i

Step j

Step k

Step m

L

L

U

U

Transition A


SFC to LD translation – [4] Steps update


� Timing variables are made with non retentive timers which st artcounting when steps become active (D/L qualifiers )

� They can be put in a separated section

46

Step i

Tk5000


SFC to LD translation – Timing variables


� T_tr bit means that “at least one transition has been triggered in this cycle during conditions evaluations”

� T_tr is activated during conditions evaluation if a transition is triggered (so the overall state has changed)

� IF T_tr==1 THEN JMP to Input scanning

� T_tr is unlatched at the beginning

47

T_tr

U

Step i

Step j

Step k

Step m

T_tr

L

L

L

U

U

Transition A


SFC to LD translation – Stability search algorithm

Input_Scanning

JMP

T_tr

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