topic 9 grafcet - · pdf file˘ ˇ grafcet & ladder diagram chapter 9 : grafcet and ladder...

��

GRAFCET & Ladder Diagram

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

step 0

step 1

step 2

step 3

step 4

step 5

output 1

output 2

output 3

input 1

input 2

input 3

input 4

input 5

input 6

output 4

Transitions

Directed Links

Actions

��


The GRAFCET (cont’d)

Block Diagram of a PLC-controlled automatic system :

CONTROLUNIT

OPERATIVEUNIT

COMMANDS

INFOMATION

OPERATORIN

STR

UC

TIO

NS

PIL

OT

LIG

HTS

PSA2 FP2-C1 XY64D2T XY64D2T

��

��

� � � � � � � �

� � � � � � � �

� � � � � � �

� � � ��

��


The GRAFCET (cont’d)

1

2

3

extend cylinder A

extend cylinder B

retract cylinder A

retract cylinder B

start

cylinder A extended

cylinder B extended

cylinder A retracted � cylinder B retracted

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

Initial Step (double box) Initiated by First Cycle Bit

Transitions

Transitions (AND operation)

Actions

Two Actions In One step

These are sensors !

��


Different parts of a GRAFCET

ACTIONS ACTIONS

ACTIONS ACTIONS

1

2

3 6

4

5

7

8

9

1

9 Labelled Connector : indicating where control has come from (source

Initial Step : defining the initial situation of the automated system.

Transition : between steps and with which logic conditions are associated.

Directed Link : connecting steps to transitions, and transitions to steps.

Step : a steady state with which command or actions may be associated.

Simultaneous Activation : indicated by transition followed by a double horizontal line.

Start of Sequence Selection : indicated by a single horizontal line followed by two or more transitions.

End of Sequence Selection : indicated by one or more transitions followed by a single Horizontal line.

Simultaneous De-activation : indicated a double horizontal line followed by transition.

Labelled Connector : indicating where control is going to (destination Connector).

��


Grafcet Design – Sequence How to do the sequence. 1. Represent the steps (step_1 and step_2) by internal

Relay bits R1 and R2. R1 and R2 will either be “1” (active) or “0” (inactive).

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

3. Do not worry about outputs at the moment. 4. The initial step should be activated by the PLC

internal scan cycle (the very first cycle). After 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 need to on to the avtive state (therefore latch It) using step_1. Step_1 latched. 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!) (But then there will be 2 active steps: step_1 and step_2.We want to have only one active step. Therfore, when step_2 becomes active, step_2 should de-activate (kill-off) step_1. Similarly, the step after step_2, that is step_1, should be used to de-activate (kill-off) 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 back).

1

2

START

STOP

1st_cycle_bit Step-1

Step-1

Step-2

1st_cycle_bit

Step-1

Step-1

Step-2

Start

Step-1

Step-2

Step-2

1st_cycle_bit

Step-1

Step-2

Step-1

Stop

Start

Step-2 Step-1

Occurs only once upon PLC power on

To kill Step 2 when Step 1 is active

�

�

��


Grafcet Design – Output

How to do the output: 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, And so on) we can determine the outputs (i.e. What happens at each step).

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

3. That means upon powering up of PLC (remember 1st_ cycle_ bit → 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. (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).

2

1

START

STOP

ON RED

ON GREEN 2

Step-1

Step-2 ON GREEN

ON RED

1

START

STOP

ON RED ON GREEN

ON BUZZER 2

Step-1 ON RED

ON GREEN

Step-2 ON BUZZER

Work the Outputs from right to left

�

��


OR

Other examples.

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.

1

START

STOP

ON RED ON GREEN

ON RED ON BUZZER

2 Step-1

Step-2

Step-1

Step-2 ON BUZZER

ON GREEN

ON RED

1

START

STOP

ON RED ON BUZZER

ON GREEN ON BUZZER

2

Step-1

Step-2

Step-1

Step-2 ON GREEN

ON RED

ON BUZZER

Outputs may be in any order

No Repeated Outputs !!

Inputs may be repeated

Outputs

�

��


1 UNPROCESSED

MATERIAL IN POSITION

LOWER STAMP

RAISE STAMP

LOWER DIE

EVACUATE PART

RAISE DIE

Material in position and cycle start

End of compression

Stamp in high position

Die in low position

Evacuation completed

Die in high position

2

6

5

4

3

GRAFCET of the Press Start PB Stamp

Die

( steps )

( input = sensor )

( outputs / actions )

Stamp

Stamp

Die

Die

Stamp

Stamp

Done manually by operator

��


SEQUENCE

Step-2

1st_cycle_bit

Die-High Step-6

Step-1 Step-2

Step-1

Step-1 Material Start

Step-2 Step-3

Step-2

Step-2 End-High

Step-3 Step-4

Step-3

Step-3 Stamp-High

Step-4 Step-5

Step-4

Step-4 Die-Low

Step-5 Step-6

Step-5

Step-6 Step-5 Evacuate

Step-6 Step-1

OUTPUT

Step-3

Step-4

Step-5

Step-6

Lower Stamp

Raise Stamp

Lower Die

Evacuate part

Raise Die

Ladder Diagram of the Press

��


Project 1 - Moving a Wagon

Left Side Right Side

Moving A Wagon

1

2 Move Wagon to the Right

3

Start PB pressed

Left Side reached

Right Side reached

Move Wagon to the Left

Start PB

Step-3

Step-1

Step-2

1st_cycle_bit

Left-side

Step-1

Step-3

Step-2

Step-1 Start

Step-2 Step-3

Step-2 Right-side

Step-3 Step-1

SEQUENCE OUTPUT

Step-2

Step-3

Move-right

Move-left

��


Convert GRAFCET �� Ladder Diagram

��

��

��

��

��

��

��

��

��

��

Sequence Output

Use Ext_A

Use A+

Use On Red

Use R1 or Step 1

��


1

2

3

Run & System initialised

Cylinder B extended

Cylinder A extended

On Green Light Extend Cylinder A

Step 4 / TIM001 / 3s . Drill_low

Drill_ high

Cylinder A retracted . Cylinder B retracted

Extend Cylinder B

Lower Drill

Raise Drill

Retract Cylinder A Retract Cylinder B

4

5

6

Drill-high

Drill-Low

Drill

Part / Material

Cylinder B Cylinder A

GRAFCET of the Drill

Cylinder A and Cylinder B hold the material (part to be drilled) tightly while the drilling machine drills the hole.

A+ A - B -

B+

System initialization : Retract Cylinder A Retract Cylinder B Off Green Light

sensors

sensors

��


Project 2

SEQUENCE

1 st _cycle_bit

Step - 1 Step - 2

Step - 6 Cyl.A ret Cyl.B ret

Step - 1 Run

Step - 2 Step - 3

Step - 2 Cyl.A ext

Step - 3 Step - 4

Step - 4 Tim001

Step - 5 Step - 6

Initialise ? d

Drill - Low

Step - 5 Drill - high

Step - 6 Step - 1

Step - 3 Cyl. B ext

Step - 4 Step - 5

Step - 1

Step - 2

Step - 3

Step - 4

Step - 5

Step - 6

E xt . C yl A

Step - 2

Step - 4

Step - 3

Step - 5

OUTPUT

Step - 6

Step - 4 TIM

001

3 s

RAISE - DRILL

R et . C yl A

R et . C yl B

E xt . C yl B

Ladder Diagram of the Drill

ON GREEN LIGHT

LOWER -DRILL

��


Grafcet Design Automatic Hand Dryer The system incorporates a diffuse sensor to detect the the presence of a pair of hands. When a pair of hands are placed just below the the Automatic Hand Dryer, heated air flow is turned on. When the person has completed drying his hands, the removal or absence of hands will be detected. The heated air will continue to flow for a further 3 seconds before the heated air flow is turned off.

1

2

Off Hand Dryer

3

Presence of Hands

Step 3 / Timer5 / 3s

Absence of Hands

On Hand Dryer

Automatic Hand Dryer

Fill in the blanks :

Inputs Outputs

��


Automatic Hand Dryer

Step - 1

Step - 2

DIF

Ste p - 0

Step - 0 1 st _cycle_bit

Step - 0 Step - 1

Step - 2 Tim 005

Step - 1 Step - 0

Step - 1

DIF

Step - 2

SEQUENCE

Step - 2

Step - 0

Step - 1

Step - 2

OUTPUT

Step - 2

OFF HAND DRYER

ON H AND DRYER

Presence

Absence

DIF (=ON) = Presence of hands

Timer5

3s

��


GRAFCET with multiple selections

A Grafcet is generally made up of several sequences (several series of steps that execute one after the other) and it is often necessary to exclusively select one of these sequences. Example:

5

8 6 9

10 7

11

Action A Action C

Action G Action D

Action E

x y z

a c

g

b

d

The above GRAFCET consists of a switch which allow to select one out of three possible sequences depend on the transition conditions x, y and z. The different transitions corresponding with their conditions x, y and z may be enable simultaneously and they could be cleared simultaneously if the transition conditions x, y and z were true at the same time. In order to avoid this from happening, the transition conditions must be mutually exclusive. It is also possible to introduce the priorities among the different sequences.

k

��


GRAFCET with multiple selections

12

13 14

a.b

The transition conditions a.b and a.b are mutually exclusive. If a and b are both present, the transition from step 12 will not be cleared.

a.b

12

13 14

a

The transition 12-13 has higher priority than 12-14: the transition 12-13 will be cleared if both a and b are true at the same time.

a.b

f.e

Action E

15

14

13

12

Action G

Action H

Action J

g

h

j

f.e

Jump from step 12 to step 15 if condition f.e is true.

k

Action K

19

18

17

16

Action L

Action M

Action P

l

n.m

p

n.m

Repeat 17-18 if condition n.m is not obtained and n.m is true.

��


Step-6 A

Step-11 E

Step-8 C

Step-10 D

OUTPUT

Step-5

Step-6

x

Step-7

Step-6

Step-5

Step-9

z

Step-10

Step-9

Step-9

Step-10

b

Step-11

Step-10

Step-5

Step-8

y

Step-11

Step-8

Step-6

Step-7

a

Step-11

Step-7

Step-4

Step-5

?

Step-6

Step-5

Step-7

Step-8

g

c

Step-11

Step-10

Step-11

d

Step-12

Step-8 Step-9

SEQUENCE

Step-7 G

��


Label Addr Label Addr Head Tail

X1 X2

TOSS COIN GREEN RED

Y1 Y2 Y3

1

2

TOSS COIN

ON GREEN

ON RED 3

TAIL

HEAD

STEP - 2 - 3/ Timer7 / 3s

GRAFCET : COIN TOSSER

TOSS COIN 1 st _cycle_bit

Step - 1 Step - 2

Step - 1

Step - 1

Step - 2

Head

Step - 1

Step - 2

Step - 1

Step - 3

Tail

Step - 1

Step - 3

Step - 2

Step -

Input Output

3

Timer7

Step - 1

Step - 3 ON RED

Step - 2 ON GREEN

Step - 2 Timer7

3s Step - 3

OUTPUT SEQUENCE

Step - 3

�� IO Listing or IO Address Assignment Table

��


SOLUTION : Sequence & Output

FAIL-PUSHER-EXT

EXTEND FAIL-PUSHER

EXTEND PASS-PUSHER

LOAD NEXT COMPONENT

START

PASS

PASS-PUSHER-EXT

FAIL

MATERIAL (IN POSITION)

1

2

4 3

5

GRAFCET: COMPONENT INSPECTION

��


(c) Design a GRAFCET for the following system in Fig 1 (initial step = step 1):

Detection of clasps on cardboard box: The system incorporates a through-beam (TB) optical sensor, an inductive sensor (IND�) and a counter. The presence of a new clasp increments the counter (INCR_CTR). The counter serves only to show the number of clasps on each box that passes by. No other action need to be taken by the system based on the number of clasps detected. The counter is reset (RESET_CTR) for every new box that arrives.

(inputs) (outputs) PLC input-output diagram

Fig. 1 : Detection of clasps on cardboard box

PLC INCR_CTR RESET_CTR

TB

IND�

Counter

��


Solution :

��


(d) Design a GRAFCET for the following system in Fig Q3d (initial step = step 1):

Automatic gauge level monitoring: The system incorporates two through-beam (TB) optical sensors to monitor the level of the fluid in the stationary glass gauge as shown in Fig Q3d. When the level falls below the lower limit, a valve situated on top of the gauge is turned on automatically to add more fluid. When the fluid level reaches above the upper limit, the valve is turned on for a further 10 seconds before it is turned off.

Note : The control of filling level through glass gauge makes use of the laws of refraction. The fluid, being the optically denser medium, breaks the light beam. Without the fluid the sensor is activated.

(inputs) (outputs) PLC input-output diagram

Automatic On/Off Valve Emitters Detectors

Fig. Q3d : Automatic Gauge Level Monitoring

PLC On Valve Off Valve

TB_Top

TB_Bottom

��

��

TB_Bottom

TB_Top

��


Solution :

topic 9 grafcet - · pdf file˘ ˇ grafcet & ladder diagram chapter 9 : grafcet and ladder...

Documents