diagrama secuencial_gp1

6. Documentation of an electrohydraulic control system

Festo Didactic GmbH & Co. KG TP 601 139

GRAFCET is a specification language for the functional description of the sequential

part of control systems. GRAFCET in accordance with DIN EN 60848 enables the

graphic representation of the mode of operation of a control system irrespective of

the technology used. GRAFCET, i.e. its predecessor the function chart, is used in

numerous areas of automation for the planning and documentation of the

sequential part of control systems. GRAFCET can for instance be found in use in

power stations, process engineering applications or material flow systems.

In 2002, the German standard DIN 40719-6, function chart, was replaced by the

European standard DIN EN 60848, GRAFCET. The European origin of the standard is

recognisable by the name. GRAFCET is an abbreviation and originates from the

French:

GRAphe Fonctionnel de Commande Etape Transition.

GRAFCET is also internationally standardised through the IEC 60848 standard.

The advantages of the new DIN EN 60848 standard are:

The standard is valid throughout Europe.

The description of functions has been simplified .

The standard incorporates new functions. This includes the introduction of

hierarchical levels. Hierarchical levels are necessary for precisely defined

coarse/fine structures of control system behaviour, for modes of operation and

the EMERGENCY-STOP function of complex control systems.

6.3

GRAFCET

Replacement of function

chart by GRAFCET


140 Festo Didactic GmbH & Co. KG TP 601

A GRAFCET primarily describes two aspects of a control system in accordance with

defined rules:

The actions to be executed (commands),

The sequence of execution.

A GRAFCET also known as a GRAFCET chart is therefore divided into two parts.

The sequential or structural part describes the temporal sequence of the process,

whereby the process is structured into consecutive steps..

The sequential part does not describe what actions are to be executed individually.

These are contained in the action or function part. In the case of the example shown,

these are the blocks on the righthand side of the steps as well as the transition

conditions between the steps.

S1*1B2* *3B1* "Start condition"2B1 B4

3B2 "Position of downstream station"

3B1 "Position of magazine"



2B1 "Vacuum generated"

2B1 "Vacuum no longer generated"

1B2 "Part released"

1B1 "Part ejected"

1

3M1:=0

3M1:=1

3M1:=0

3M1:=1

2M1

2M2

1M1:=0

1M1:=1

3M2:=1

3M2:=0

3M2:=1

3M2:=0

2

4

7

9

5

8

6

3

P1

1B2* *3B1 "Initial position"2B1

"Initial position display"

"To downstream station"

"Eject part"

"To magazine"

"Generate vacuum"

"Release part"


"Deposit part"

"To magazine"

Function partStructural part

GRAFCET (function chart) for a distribution process

Structure of a GRAFCET

CarlosResaltadopregunta 4

geoResaltado4.1



The sequences are divided into steps. Each step is shown in the form of a box,

whereby a square is preferred to a rectangle. An alphanumerical designation must

be entered in the top centre of the step field.

A step is either active i.e. it is currently being executed or inactive.

2 93 8B

Example of steps

The status of a step can be interrogated or represented via its step variable. The

step variable is a boolean variable and either has the value 1 (step active) or the

value 0 (step inactive).

2

X2

Step 2 Step variable of step 2

Example of steps and step flags

Each chain of steps has one initial step. The initial step designates the initial

position of the control system. The control system is in this initial step immediately

after it is switched on. The initial step can be recognised by its double frame. The

example shows step 1 as an initial step.

1

Example of an initial step

Steps

Initial step



A transition is the connection from one step to the next. A transition is represented

by a line at a right angle to the connection between the two steps. However, if

required for reasons of clarity, it can also be shown by a horizontal line, which leads

to another step.

(4)

4

5

(5)

Example of a sequence structure consisting of steps and transitions

A transition can be given a transition name. This is to be entered on the left in

brackets order to prevent confusion.

Each transition must have a transition condition. The transition condition is a logic

expression that can assume the value 1 (TRUE) or 0 (FALSE). The transition to the

next step is effected if the transition condition is fulfilled. The transition condition is

entered on the righthand side of a transition.

(Press up)Pushbutton pressed (S1)and press up (1B1)

7

8

(Press down) Press down (1B2)

(Press up) S1*1B1

7

8

(Press down) 1B2

Examples of transition conditions

A transition condition can be represented in text form using a boolean expression or

by means of graphic symbols.

In the standard DIN EN 60848 , examples of transition conditions are formulated

mainly in the form of boolean expressions.

Please note: The dot or asterisk used describes an AND operation and the plus

symbol an OR operation. Negations are represented by means of a line above the

variable name.

Transitions and transition

conditions

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A time-dependent transition condition is used if the initial step is to progress to the

next step after the expiry of a defined time The transition condition comprises the

time and status of the active step, both divided by an oblique slash.

5s/X9

9

10

Example of time-dependent execution of a step

In the example shown, X9 is the step variable of step 9, which represents the

boolean status of step 9.

The transition condition has the value TRUE for five seconds after step 9 is activated

and assumes the value FALSE immediately afterwards, thereby deactivating the

preceding step.

The period of activation of step 9 is therefore five seconds.

In order to create an error-free sequence structure, steps and transitions must

always alternate!

Each step is allocated one or several actions, which are executed when the step is

active.

An action is represented in the form of a square of optional width-to-height ratio.

The standard recommends using the same height for the action field and step

symbol.

Actions have different behaviour. The behaviour of an action is represented by

means of corresponding additions.

If several actions are allocated to one step, these can be graphically represented in

different ways. Please note: The order of representation does not represent a

temporal sequence!

Important

Actions

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Action 1

Action 1

Action 1 Action 1

Action 2

Action 2

Action 3

Action 3

Action 2

Action 2

Action 3

Action 3

77

77

7777

Examples for the representation of a step with several actions

Continually active actions means: The value 1, i.e. TRUE, is assigned to the specified

variable for as long as the corresponding step is active. Once the step is no longer

active, the value 0, i.e. FALSE, is assigned to the variable.

Designation in the action field can be effected in different ways. Text can be in the

form of a command or indicative form. It is also possible to directly indicate the

name of a variable.

3M2

Valve coil 3M2

Switch valve coil 3M2

4

4

4

Example of continuously active action

The actions described all have the same behaviour:

As long as step 4 is active, value 1 is assigned to variable 3M2.

Continuously active actions



The value 1 (TRUE) is assigned to the variable described in the action only if the

corresponding step is active and the assignment condition is fulfilled. If the

assignment condition is not fulfilled, the value 0 (FALSE) is assigned to the variable.

In the case of the example this means:

If step 3 is active and assignment condition B12 is fulfilled, then variable 1M2 is

assigned the value 1. In all other cases, the value of the variable 1M2 is 0.

1M23

B12

Example of a continuously active action with assignment condition

At the point when the corresponding step is activated, the value specified in the

action is assigned to the variable. The value of the variable remains stored until it is

overwritten by another action.

Since the assignment of the value is effected during the activation of a step, i.e.

when a rising signal edge is available, the action is identified by means of an upward

arrow.

4M1:=1

4M1:=0

9

14

C:=C+115

Examples of storing actions if step is activated

In the case of the example this means:

As soon as step 9 becomes active, the value 1 is assigned to valve coil 4M1. If step 9

is no longer active, variable 4M1 retains the value 1 until this value is overwritten by

another action. If step 14 becomes active, the value 0 is assigned to valve coil 4M1.

Variable 4M1 retains the value 0 until the value of the variable is overwritten by

another action. If step 15 is active, the value of the variable C will be increased

precisely once by 1.

Continuously active action

with assignment condition

Storing action if step is

activated



When the step is deactivated, the variable is assigned the value specified in the

action. The value for the variable remains stored until it is overwritten by another

action.

Since the value is assigned when the step is deactivated, i.e. when the falling signal

edge applies, the action is designated by means of a downward arrow.

4M1:=012

Example of a storing action when step is deactivated

In the case of the example shown, this means:

If step 12 becomes inactive, the value 0 is assigned to variable 4M1. The assignment

is executed until variable 4M1 is overwritten in another action.

The variable described in the action is assigned the specified value only if the step is

active and if a rising edge occurs for the expression which represents the events.

The flag by the action symbolises that the action is not executed with a latching

function until an event occurs.

The arrow pointing upwards indicates that the action is executed when the event has

a rising edge.

Part_ok:=16

2B1

Example of a storing action if an event occurs

In the case of the example shown, this means:

If step 6 is active and the value of variable 2B1 changes from 0 to 1, then the action

represented is executed. The value 1 is assigned to the variable part _ok and is

assigned until the variable part _ok is overwritten by another action.

An action can also be executed as soon as an event is no longer true. The falling

edge of the event or assignment condition is represented by a downward arrow.

Storing action if step is

deactivated

Storing action during event



If an action is executed with time delay, then the continuously active action with

assignment conditon can be extended by a time period.

The active step and the time are specified as assignment condition. The assignment

condition is not fulfilled until the specified time has expired, when the variable

specified in the action receives the value 1 1.

4M127

2s/X27

0 2 4 6 8 10 12s

Step 27

4M1

2 s

Example of a time-delayed, continuously active action with time diagram

The following applies in the case of the example:

If step 27 is active (the status variable of step X27 has the value 1), the action

represented is executed when the specified time of 2 seconds has expired: The

value 1 is assigned to variable 4M1. The assignment is executed for as long as step

27 is active.

Please note: The time diagrams shown do not form part of the GRAFCET of a control

system. They are merely intended to explain and describe a time-delayed action.

Delayed, continuously active

action



A time-limited action is obtained via the negation of the time-delayed action

condition.

5M229

5s/X29

0 2 4 6 8 10 12s

Step 29

5M2

5 s

Example of a time-limited, continuously active action with time diagram

The following applies for the example:

If step 29 is active, the represented action is executed for the duration of 5 seconds.

If the associated step is active for less than 5 seconds, the action is also executed

for a correspondingly shorter period.


system. They are merely intended to describe and explain a time-limited action.

Equivalent representation:

An equivalent representation for a time-limited, continuously active action is

obtained via the time-dependent transition condition.

5M2

30

29

5s/X29

Example of a time-limited, continuously active action

Here again, the value TRUE is only assigned to the variable 5M2 for five seconds

after step 29 is activated.

Time-limited, continuously

active action



Instead of the active step, it is also possible to use a variable of your choice. In this

case, the time on the left is started via the rising edge of the specified variable. The

action is executed when the time has elapsed. A time indicated on the right is

started via the falling edge of the variable and extends the duration of the action.

The prerequisite for this is that the step remains active.

2M131

2s/B9/4s

0 2 4 6 8 10 12s

Step 31

B9

2M1

4 s

2 s

Example of a continuously active action with time-dependent assignment condition

The following applies in the case of the example:

If step 31 is active and the value of assignment condition B9 changes from 0 to 1,

then the time delay of 2 seconds starts. When the 2 seconds have elapsed, the

value 1 is assigned to the variable 2M1. If the value of assignment condition B9

changes from 1 to 0, then the variable 2M1 continues to hold the value 1 for 4

seconds.


system. They are merely intended to describe and explain an action with time-

dependent assignment condition.

Three basic forms of sequence structure can be created by combining the elements

of step and transition:

Linear sequence

Sequence divergence (alternative divergence)

Sequence splitting (parallel divergence)

Regardless of the form of sequence structure, steps and transitions must always

alternate. Sequence structures always evolve from top to bottom.

Time-dependent assignment

condition

Sequence structures



In the case of alternative divergence, two or several transitions follow a step. The

sub-sequence is activated and executed and its transition condition is the first to be

fulfilled. Since is it possible to select precisely one sub-sequence in the case of

alternative divergence, the transition conditions must be mutually exclusive.

S1*S4*1B1*2B1*3B1 S1* *1B1*2B1*3B1S4

1B2 2B2

1

1M1:=1 2M1:=1

3M1:=1

2A 2B

3

Example of an alternative divergence

In the case of parallel divergence, the fulfilment of a transition condition leads to the

simultaneous activation of several sub-sequences. The evolution of these sub-

sequences takes place simultaneously, but independently of one another. The

convergence of the sub-chains is synchronised. Only when all parallel sub-

sequences are executed completely, can a transition to the step below the double

line take place in this example to step 6.

4B2 5B2

4M1

3M1:=0

5M1

3M1:=1

4A

6

4B

3

5A 5B

3B2

4B1*5B1

Example of a parallel divergence

Alternative divergence

Parallel divergence



Sequences are generally executed cyclically, i.e. they represent a loop. In order to

represent a loop structure, a line must evolve from the bottom up. As this direction

is the opposite of the usual top-down direction of a sequence, it must be denoted by

an arrow.

1

9

Example of a feedback loop in a sequence structure

If a directed link in a GRAFCET has to be interrupted because the GRAFCET is

complex or extends over several pages, then the identifier of the target step and the

number of the page on which it appears must be specified.

9

Step 10Page 2

Example of a breakpoint in a sequence structure

Feedback loop and skips



The sequence of the distribution station from the Modular Production System MPS

of Festo Didactic can be described by means of the basic elements of GRAFCET.

S1*1B2* *3B1* "Start condition"2B1 B4





2B1 "Vacuum generated"

2B1 "Vacuum no longer generated"

1B2 "Part released"

1B1 "Part ejected"

1

3M1:=0

3M1:=1

3M1:=0

3M1:=1

2M1

2M2

1M1:=0

1M1:=1

3M2:=1

3M2:=0

3M2:=1

3M2:=0

2

4

7

9

5

8

6

3

P1

1B2* *3B1 "Initial position"2B1

"Initial position display"


"Eject part"

"To magazine"

"Generate vacuum"

"Release part"


"Deposit part"

"To magazine"

Function partStructural part

GRAFCET (function chart) for a distribution process

Explanations for the purpose of clarification can be added in the form of comments

at any point of a GRAFCET. Comments are represented in inverted commas.

Example for a GRAFCET

Comments



The standard takes into consideration new elements to describe control systems.

These include the introduction of hierarchical levels, which are necessary for the

precise definition of coarse/fine structures of a control system, for modes of

operation and for the EMERGENCY-STOP function in complex control systems .

If different hierarchical levels are used, then a GRAFCET is split up into several parts.

These parts are known as SUB-GRAFCETs and are given a name, prefixed by a G.

The main elements of the structure are:

Forcing commands

Inclusive steps

Macrosteps

A master GRAFCET controls the subordinate GRAFCETs with so-called forcing

commands. A forcing command is linked to a step and represented in the form of a

rectangle with double lines. The steps are shown in a curly bracket and are forced.

G9{100}

G1{ }

G2{INIT}

G4{ }

5

9

7

12

Examples of forcing commands

There are four types of forcing commands. These are explained with the help of

examples.

Forcing a sub-GRAFCET into a specific situation:

If step 5 becomes active, step 100 is activated in sub-GRAFCET 9; all other steps

of G9 are deactivated. In a structure with parallel divergence, several steps can

be forced. The notation in this case is as follows: G9{100, 200, 300}. Sub-

GRAFCET G9 does not require an initialising step.

Forcing a sub-GRAFCET into the current situation (freeze command):

If step 9 becomes active, the sub-GRAFCET G1 is frozen in the current position for

as long as step 9 is active.

Hierarchical GRAFCETs

Forcing commands



Forcing a sub-GRAFCET into a vacant situation:

If step 12 becomes active, the sub-GRAFCET G4 is set to the vacant position; no

steps are activated. This means that all steps are de-activated by G4.

Forcing a sub-GRAFCET into the initial position:

If step 7 becomes active, sub-GRAFCET G2 is initialised, whereby only those

steps identified as initialising steps are activated. All other steps of G2 are

deactivated.

The GRAFCET describing the control behaviour of the MPS distribution station is

divided into three sub-GRAFCETs.

G1: Sub-GRAFCET of the modes of operation (upper hierarchical level)

G10: Sub-GRAFCET of automatic operation (lower hierarchical level)

G100: Sub-GRAFCET of manual/reset operation (lower hierarchical level)

Forcing commands only apply to the upper hierarchical level.

G10 applies to the lower hierarchical level, the sub-GRAFCET for automatic

operation, and G100, the sub-GRAFCET for manual/reset operation.

EMERGENCY STOP*S_Manual

1

EMERGENCY STOP*Reset_OK*S_Automatic

3

G10{ }

G100{INIT}

G10{INIT}

G100{ }

2 " /Reset"Manual

"Automatic"

EMERGENCY STOP


G1: Sub-GRAFCET of modes of operation (upper hierarchical level)

Higher-order sub-GRAFCET for the MPS distribution station

Example of application using

forcing commands



All three GRAFCETs are started simultaneously.

G1, like any other GRAFCET, starts in the initialising step 1, where it executes two

forcing commands:

G10, the automatic operation, is force-deactivated for as long as G1 is in step 1.

G100, the reset mode, is forced into its initialising step. G100 executes its

initialising step for as long as step 1 of G1 is active.

Once the EMERGENCY-STOP is released and manual operation is selected, G1

progresses to step 2, where a forcing command is output:

G10, automatic operation, is forced into its initialising step, where it remains for

as long as step 2 of G1 is active.

The forcing command in step 2 of G1 no longer applies for sub-GRAFCET G100. G100

is therefore no longer dependent on a forcing command. Normal operation of G100

is thus released. The station is reset.

G1 advances to step 3, if the successful execution of the reset sequence is signalled

via the variable Reset_OK, the EMERGENCY-STOP is not actuated simultaneously

and automatic operation is selected. G1 jumps back to initialising step 1, if

EMERGENCY/STOP is actuated in step 2.

A forcing command is output again in step 3 of G1:

G100, the reset sequence, is force-deactivated. None of the steps of G100 are

executed. G100 remains deactivated for as long as step 3 is active.

The forcing command in step 3 of G1 no longer applies for sub-GRAFCET G10. G10 is

therefore no longer dependent on a forcing command . Normal operation of G10 is

thus released.



EMERGENCY STOP 2B1 B4*S_Automatic*S1*1B2* *3B1* EMERGENCY STOP*S_Manual*S4

3B2 1B2

3B1 3B2

3B2

3B1

2B1 2B1

2B1

1B2 1B2* *3B12B1

1B1 1s/X102

10 100

3M1:=0 1M1:=0

3M1:=1 3M1:=0

3M1:=0 Reset_OK:=1

3M1:=1

2M1 2M2

2M2

1M1:=0 3M1:=1 3M2:=0

1M1:=1 2M1

3M2:=1

Reset_OK:=0

3M2:=0 3M2:=1

3M2:=1

3M2:=0

11 101

13 103

16 106

18

14 104

17

15 105

12 102

P1 P2

1B2* *3B12B1 Flashing cycle

G10: Sub-GRAFCET of automatic mode (lower hierarchical level)

G100: Sub-GRAFCET of manual/reset mode (lower hierarchical level)

Subordinate sub_GRAFCETs for the MPS distribution station

One option for the structuring of a GRAFCET is the use of inclusive steps. The

inclusive step is identified by a drawn-in octagon.

12

Example of an inclusive step

The diagram means that step 12 contains further steps.

Inclusive steps



An inclusive step is incorporated into a GRAFCET in the same way as a normal step.

The steps contained are represented in a separate sub-GRAFCET and framed. The

step number of the inclusive step is entered at the top edge of the sub-GRAFCET

frame and the step name at the bottom edge of the frame. The step invoked first is

marked with an *. The steps contained are only executed for as long as the higher-

order inclusive step is active.

The diagram below represents the GRAFCET for the MPS distribution station using

inclusive steps.


1

Reset_OK*S_Automatic

"Manual/Reset"

"Automatic"

EMERGENCY STOP


2

3

Master GRAFCET with inclusive steps for the MPS distribution station

The steps contained in step 2 are represented within frame 2 (reset).

The steps contained in step 3 are represented in frame 3 (loop). This sub-

GRAFCET again contains an inclusive step 8.

The steps contained in step 8 are represented in frame 8 (sequence).



S1*1B2* *3B1*2B1 B4

X17*3B1

7 P1

1B2* *3B12B1

"Cycle running"8

*

3

Loop

S4

1B2

3B2

2B1

1B2* *3B12B1

1s/X102

100

1M1:=0

3M1:=0

Reset_OK:=1

2M2

3M1:=1 3M2:=0

2M1

Reset_OK:=0

3M2:=1

101

*

103

106

104

105

102

P2

2

Reset

Flashing cycle

Sub-GRAFCETs with contained steps for the MPS distribution station



3B2

1B1

2B1

1B2

3B2

2B1

3B1

10

1M1:=1

2M1

3M1:=1

2M2

1M1:=0

3M1:=0

3M1:=1

3M2:=1

3M2:=0

3M2:=1

3M2:=0

11

*

13

16

14

15

17

12

3M1:=0

8

Sequence

Sub-GRAFCETs with contained steps for the MPS distribution station

Macrosteps are particularly suitable for the coarse/fine structuring of a control. A

macrostep incorporates a substructure of a GRAFCET, thereby making the GRAFCET

more transparent. A macrostep does not create any different hierarchies.

A macrostep is identified by means of two horizontal double lines on the step

symbol. The step name starts with M as a prefix.

M4

Example of a macrostep

Macrostep M4 represents a sub-GRAFCET.

Macrosteps



A macrostep cannot be quit until the GRAFCET structure contained therein is

completely executed.

The rules regarding the identification of macrosteps are shown by means of an

example.

The example shows a GRAFCET using two macrosteps M2 and M4. During expansion,

the name of the first step is the same as that of the macrostep, but with the prefix E.

The name of the last step is also the same as that of the macrostep, but with the

prefix S. The steps in between can be given any name.

1

M2

M4

S1*1B2* *3B1*2B1 B4

S_Manual

S_Automatic

S_Automatic

3 P1

1B2* *3B12B1

"Manual/Reset"

"Automatic"

Example of a GRAFCET using macrosteps

If the GRAFCET is executed with the macrosteps, step 3 can only be activated if step

S2 in the expansion is active and the variable S_Automatic has the value 1 as a

transition condition.



S4 3B2

1B2 1B1

3B2 2B1

2B1 1B2

2B1

1B2* *3B12B1 3B2

1s/X22 3B1

E2 E4

1M1:=0 1M1:=1

3M1:=0 2M1

3M1:=1

Reset_OK:=1 2M2

2M2 1M1:=0

3M1:=1 3M1:=0

3M1:=1

3M2:=1

3M2:=0

2M1

Reset_OK:=0 3M2:=13M1:=0

3M2:=1

3M2:=0

3M2:=0

3M2:=1

21 41

23 43

S2 46

24 44

25 45

S4

22 42

P2

Flashing cycle

Macrostep M2"Reset"

Macrostep M4"Automatic"

Expansion representation of macrosteps M2 and M4

diagrama secuencial_gp1

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