Topic 9 GRAFCET - Angelfire

24 Pages·2005·0.43 MB·English

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Chapter 9 : GRAFCET and Ladder Diagram

The GRAFCET is a tool for describing the specifications of the control unit of an
automated system.

The working of an automatic equipment may be described graphically by a group of:

• STEPS to which are associated certain ACTIONS
• TRANSITIONS which are conditions to be fulfilled
• DIRECTED LINKS linking steps and transitions
Transitions
step 0
input 1
Directed Links
step 1 output 1
input 2
step 2 output 2
input 3
Actions
step 3 output 3
input 4
step 4 output 4
input 5
step 5
input 6
GRAFCET & Ladder Diagram
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The GRAFCET (cont’d)

Block Diagram of a PLC-controlled automatic system :

OPERATOR

TS NS
H O
G TI
LI (cid:8) (cid:4) (cid:5)(cid:3) (cid:4) (cid:5)(cid:6)(cid:7) C (cid:1)(cid:2)(cid:3) (cid:4) (cid:5)(cid:6)(cid:7)
OT TRU
PIL NS
I

(cid:8) (cid:4) (cid:5)(cid:3) (cid:4) (cid:5)(cid:6)(cid:7)

COMMANDS

CONTROL OPERATIVE

UNIT UNIT
(cid:1)(cid:2)(cid:3) (cid:4) (cid:5)(cid:6)(cid:7)

INFOMATION
(cid:3) (cid:9) (cid:8) (cid:10) (cid:11) (cid:12)(cid:12) (cid:7)(cid:13)(cid:7)
(cid:3) (cid:17)(cid:10) (cid:7)
(cid:14) (cid:15) (cid:10) (cid:16) (cid:1)(cid:2)(cid:11) (cid:7)

PSA2 FP2-C1 XY64D2T XY64D2T

GRAFCET & Ladder Diagram
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The GRAFCET (cont’d)

Initial Step (double box)
Initiated by First Cycle Bit

0 WAIT step ( - doing nothing, in this case )

Transitions
start

1 extend cylinder A

cylinder A extended
Actions
2 extend cylinder B

cylinder B extended

retract cylinder A

3

retract cylinder B

(cid:1)
cylinder A retracted cylinder B retracted

Two Actions
Transitions In One step
(AND operation)
These are sensors !
GRAFCET & Ladder Diagram
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Different parts of a GRAFCET
Labelled Connector : indicating where
9 control has come from (source
Initial Step : defining the initial situation of
the automated system.
1
Transition : between steps and with which
logic conditions are associated.
Step : a steady state with
which command or actions 2
Directed Link : connecting steps to
may be associated.
transitions, and transitions to steps.
Simultaneous Activation :
indicated by transition followed
by a double horizontal line. 3 ACTIONS 6 ACTIONS
Start of Sequence Selection :
indicated by a single horizontal
line followed by two or more
transitions.
4 ACTIONS 7 ACTIONS
End of Sequence Selection :
indicated by one or more
transitions followed by a
single Horizontal line.
5 8
Simultaneous De-activation :
9
indicated a double horizontal line
followed by transition.
Labelled Connector :
1 indicating where control is going
to (destination Connector).
GRAFCET & Ladder Diagram
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Grafcet Design – Sequence

How to do the sequence.

1. Represent the steps (step_1 and step_2) by internal
Relay bits R1 and R2.
1
R1 and R2 will either be “1” (active) or “0” (inactive).

2. Only 1 step should be active at any one time. START

3. Do not worry about outputs at the moment.
2
4. The initial step should be activated by the PLC
internal scan cycle (the very first cycle). After
STOP
which it will be activated by step_2 and stop_PB
(during the return loop).

Step_1 activated by 1st cycle bit, but it will turn
off during the 2nd cycle onwards. Hence, the 1st_cycle_bit Step-1
need to on to the avtive state (therefore latch
It) using step_1.
Occurs only once
(cid:2) upon PLC power
on
Step_1 latched. 1st_cycle_bit

Step-1

Step-1
When step_1 is active and start_PB is pressed, step_2
will be active and need to be latched (because
start_PB will eventually be released!)
Step-1 Start

Step-2

Step-2

(But then there will be 2 active steps: step_1 and
1st_cycle_bit Step-1
step_2.We want to have only one active step. Therfore,
when step_2 becomes active, step_2 should de-activate
Step-1 Step-2
(kill-off) step_1.

Step-2 Stop
Similarly, the step after step_2, that is step_1, should be
used to de-activate (kill-off) step-2.

Step-1 Start Step-2
Provision must be made to go back to step_1 again when
Step_2 is active and stop_PB is pressed.(This is to loop
Step-2 Step-1
back).
To kill Step 2

when Step 1
is active (cid:2)

GRAFCET & Ladder Diagram
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Grafcet Design – Output

How to do the output:

1 ON RED
1. Now that we have got the sequence, (sequence
Ensures that step_1 and start_PB goes to step_2,
And step_2 and stop_PB goes back to step_1, START
And so on) we can determine the outputs (i.e.
What happens at each step).
22 ON GREEN

2. We can decide to turn on red_light in step_1
STOP
And turn on green_light in step_2 (if that is
What is required).

Step-1 ON RED
3. That means upon powering up of PLC (remember
1st_ cycle_ bit fi hence the initial step : step_1),
the red light turns. When start_PB is pressed,
green_light turns on. When stop_PB is pressed,
red light turns on, and so on. Step-2 ON GREEN
(Remember : no change in sequence, only the output).

OR

We can decide to turn on the buzzer in step_2 and turn on both the lights in step_1.
So that when start_PB is pressed, the red_light and green_light will turn on.
(Remember : still no change in sequence, only the output).
(cid:2)

Work the

Outputs from

right to left

Step-1 ON RED
ON RED
1
ON GREEN

ON GREEN
START

Step-2 ON BUZZER

2 ON BUZZER

STOP
GRAFCET & Ladder Diagram
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OR

Other examples.

Step-1 ON GREEN

ON RED
1
ON GREEN
ON BUZZER
Step-2
START

Step-1 ON RED
2 ON RED
ON BUZZER
Step-2

STOP

Outputs may be
in any order
Inputs may
be repeated

Outputs

Step-1 ON RED

ON RED
1 ON BUZZER Step-2 ON GREEN

START
Step-1 ON BUZZER

2 ON GREEN Step-2
ON BUZZER
(cid:2)

STOP

No Repeated

Outputs !!

Note : Beware of repeated output in the ladder diagram program.

Even though the GRAFCET contains two instances of ON BUZZER
The OUTPUT ladder diagram should contain only one instance of ON BUZZER.
It is wise therefore to state the output first and see which steps need this output.
That is work the output part of the ladder diagram from right to left.
GRAFCET & Ladder Diagram
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GRAFCET of the Press

Start PB
Stamp

Done manually by operator
UNPROCESSED
MATERIAL
1
IN POSITION Stamp
Die

Material in position and cycle start

( steps )

LOWER STAMP
2

Stamp

End of compression ( i n p ut = sensor )

RAISE STAMP
3

Stamp

Stamp in high position

LOWER DIE
4
Die
( outputs / actions )

Die in low position

EVACUATE PART
5

Stamp

Evacuation completed

RAISE DIE
6
Die

Die in high position

GRAFCET & Ladder Diagram
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SEQUENCE Ladder Diagram of the Press

1st_cycle_bit
Step-1
Step-6 Die-High
Step-1 Step-2
Step-1 Material Start Step-2
Step-2 Step-3
Step-2 End-High Step-3
Step-3 Step-4
Step-3 Stamp-High Step-4
Step-4 Step-5
Step-4 Die-Low Step-5
Step-5 Step-6
Step-5 Evacuate Step-6
Step-6 Step-1
OUTPUT
Step-2
Lower Stamp
Step-3
Raise Stamp
Step-4
Lower Die
Step-5
Evacuate part
Step-6
Raise Die
GRAFCET & Ladder Diagram
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Proj ect 1 - Moving a Wagon

Left Side Right Side

Start PB

1

Start PB pressed
Moving A Wagon

2 Move Wagon to the Right

Right Side

reached

Move Wagon to the Left
3

Left Side
reached

OUTPUT

SEQUENCE

1st_cycl e_bit Step- 1 Step-2 Move-right

Step-1 Step-2
Step-3 Move-left

Step-3 Left-side

Step-1 Start Step-2

Step-2 Step-3

Step-2 Right-side Step-3

Step-3 Step-1

GRAFCET & Ladder Diagram