12 SFC programming i Training PLC Allen Bradley

6 – 10 December 2010QUALIFIER • Each action non-boolean and boolean uses a qualifier to determine when it starts and stops • The default qualifier is Non-Stored • The action starts w

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WHAT IS SFC ?

• Stand for: SEQUENTIAL FUNCTION CHART

• SFC is similar to a flow-chart of your process

• SFC defines the steps or states through which your system

progresses

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6 – 10 December 2010

SFC SCHEMA

Sequential Function Chart (SFC) Programming I 12

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WHEN TO USE SFC ?

Use SFC to:

• Organize the functional specification for your system

• Program and control your system as a series of steps and

transitions

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SFC OVERVIEW

SIMULTANEOUS BRANCH

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SFC OVERVIEW

SELECTION BRANCH

TEXT BOX

STOP

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BASIC ELEMENTS IN SFC

STEP

• Represent a major function of process

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TRANSITION

to go to the next step

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SELECTION BRANCH

takes the first true path

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SIMULTANEOUS BRANCH

executed at least once If the transition is false, the SFC repeats

the previous step

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LOOP BACK

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SEQUENCE

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STEP STRUCTURE

• Each step uses a tag to provide information about the step

• Access this information via the Monitor Tags tab of the Tags

window

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and last scan)

first and last scan

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STEP STRUCTURE

active during any of its executions

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STEP STRUCTURE

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ACTION TYPE

NON BOOLEAN

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ACTION TYPE

BOOLEAN

when the bit is on

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• Each action (non-boolean and boolean) uses a tag to provide

information about the action

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ACTION STRUCTURE

(milliseconds)

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QUALIFIER

• Each action (non-boolean and boolean) uses a qualifier to

determine when it starts and stops

• The default qualifier is Non-Stored

• The action starts when the step is activated and stops when

the step is deactivated

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QUALIFIER

Qualifier Meaning Function

stop when the step is deactivated

Edge)

Start when the step is activated AND execute only once

when the step is deactivated

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QUALIFIER

this action

Time Limited

Stay active until a Reset action turns off

this action or a specific time expires, even if the step is deactivated

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QUALIFIER

AND execute once when the step is deactivated

(Falling Edge)

Start when the step is deactivated AND execute only once

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Start a specific time after the step is activated, even if the step is deactivated before this time AND stay active until a

Reset action turns off this action

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QUALIFIER

Delayed

Start a specific time after the step is

activated and the step is still active AND

stop when the step is deactivated

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Start a specific time after the step is

activated and the step is still active AND stay active until a Reset action turns off

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TURN OFF DEVICE AT THE END OF

A STEP

When the SFC leaves a step, you have several options on how to

turn off devices that the step turned on

The Options are:

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DON’T SCAN

• Default option for handling the last scan of a step

• All data keeps its current values when the SFC leaves a step

• Requires additional instructions to clear any data that you

want to turn off at the end of a step

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TO TURN OFF A DEVICE AT THE

END OF A STEP:

Use a P0 Pulse (Falling Edge) action to clear the required data

Make sure that the P0 action or actions are last in the order of

actions for the step

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EXAMPLE

This action turns on the conveyor When conveyor_state

turns on, the conveyor turns on

Before the SFC leaves the step, the P0 action turns off the

conveyor On the last scan of the step, conveyor_state turns

off This turns off the conveyor

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PROGRAMMATIC RESET

• All actions execute according to their logic

• All data keeps its current values

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END OF A STEP:

• To your normal logic, add logic that clears the required data

Use the LS bit of the step or the Q bit of the action to condition

the execution of the logic

• Use a P0 Pulse (Falling Edge) action to clear the required data

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EXAMPLE: USING LS BIT

When the step is not on its last scan (conveyor_fwd.LS = 0),

this statement turns on conveyor_state When conveyor_state

On the last scan of the step (conveyor_fwd.LS =1), this

statement turns off conveyor_state When conveyor_state

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EXAMPLE: USING Q BIT

When the action is not on its last scan (conveyor_start.Q =1),

this statement turns on conveyor_state When conveyor_state

On the last scan of the action (conveyor_start.Q =0), this

statement turns off conveyor_state When conveyor_state

turns off, the conveyor turns off

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EXAMPLE: USING PO ACTION

Before the SFC leaves the step, the P0 action turns off

the conveyor On the last scan of the step,

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AUTOMATIC RESET

• Execute P and P0 actions according to their logic conditions

• Clear tags to the left of [:=] assignments

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END OF A STEP:

Control the state of the device with an assignment or instruction

such as:

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EXAMPLE:

When the SFC leaves the step, conveyor_state turns off

This turns off the conveyor

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Lets you stop the execution of an entire SFC or a path of a

simultaneous branch and wait to restart.

When an SFC reaches a stop element:

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STOP STRUCTURE

Property Function

active

Each stop uses a tag to provide the following information about

the stop element:

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DAY 3

NEXT TOPIC : SEQUENTIAL FUNCTION

CHART (SFC) PROGRAMMING II

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