Eee3420 lecture06 rev2011

EEC3420 Industrial Control
Department of Electrical Engineering
│ Lecture 6 │
SFC based Process Control Design
© Vocational Training Council, Hong Kong. Week 1

EEE3420 Industrial Control
© Vocational Training Council, Hong Kong. Week
2
Learning Objectives
 Know the background of Sequential Function Chart
(SFC)
 Understand the operation of SFC
 Process design using SFC

Introduction to Sequential Function
Chart
• a special high-level language to describe control
sequences in graphical schedules
• at the late 70s the first function chart program
Grafcet was developed in France
• the base for the definition of the international
standard IEC 848 (“Preparation of function charts for
control systems”)
• used to structure the internal organization in a
control program
• written in a language that is defined to perform
sequential control functions
3

Chart
SFC describes the control sequences with
predefined rules for:
• Controls that have to be executed and in
which order they shall be done.
• Execution details for each instruction
4

Chart
The SFC can be divided into two parts, the
“sequence“ part and the “object” or “control”
part.
In the “sequence” part the order between the
control steps is described and in the “object” or
control” part is the internal actions that shall be
executed.
5

Chart
According to IEC 611131-3 (1998-11-18) page
86 the SFC elements give a division of the
control program in a number of steps and
transitions connected to each other by directed
links.
To every step there is one or several actions
and to each transition there is a condition
connected.
6

Chart
Step:
The program behavior in a step follows a
number of rules defined by the associated
actions that is connected to the step. The step
can be either active or inactive. At any given
moment, its active steps, the internal and the
output variable values define the state of the
control program.
7

Chart
Step:
Graphically a block that contains a step-name
represents the steps. A vertical line attached to
the top of the step represents the directed link
to the step. A vertical line connected
graphically represents the link from the step to
the bottom of the step
8

Chart
Step:
The step that is “active” is the step that is
currently executed. To indicate if a step is
active or inactive, there is a step flag. The step
flag is represented by a Boolean, the value of
the step flag is one if the step is active and
zero if the step is inactive.
9

Chart
Step:
The time that is spent in a step is saved as the
variable “step elapsed time” it keeps it value
when a step is inactivated. The value on “step
elapsed time” is reset when a step is activated.
10

Chart
Step:
The control program must have an initial state,
in this state the internal and output variables
have their initial values and the control
program stand in its initial step. The initial step
is the step that is initially active and there shall
be exactly one initial step. The initial step is
represented graphically by a step with double
lines for boarder.
11

Chart
Step:
The number of steps per SFC and the
accuracy for the “step elapsed time” is
dependent on the implementation.
12

Chart
Transitions:
There are transitions between every step
Thanks to the transition the program can pass
from one or more preceding steps to one or
more successor steps. When the program
passes a transition the successor step(s)
becomes active and the preceding step
becomes inactive. The transition is made along
the vertical directed link.
13

Chart
Transitions:
To each transition there are associated steps,
which is called transition conditions. The
transition condition shall result in an evolution
of a simple Boolean expression. Sometimes
the user wants the transition condition to
always be true, and then the symbol 1 or the
keyword true shall represent it.
14

Chart
Actions:
Every action is associated with a step. The
step can have none or several actions
associated. If there is no associated action to
the step, it will be considered as a WAIT
function.
15

Chart
Actions:
The WAIT function is a function that is waiting
for the successor transition to be true. An
action can be described in several ways, for
example with a ladder-diagram, logical circuits
or with Boolean expressions.
16

Chart
Action blocks:
This is a graphical element for the combination
of a Boolean variable with one of the action
qualifiers to produce an enabling condition. The
action block contributes with a kind of Boolean
indicator variable; it can be set by a specific
action to indicate its completion, time-out, error
conditions, etc.
17

Chart
Action blocks:
The graphical concatenated action blocks can
have multiple indicator variables, but just one
common Boolean input variable, it shall act
simultaneous for all the concatenated blocks.
18

Chart
Action qualifier:
Each step/action association shall have an
associated action qualifier. The action qualifier
can have the following values according to IEC
61131-3 (1998-11-18) page 97.
19

Chart
Action qualifier:
20

21
Representation of a sequential
process by SFC
Basic
component
symbols used in
the SFC
(IEC 6113-3)

Basic structures of SFC
The SFC syntax can handle much more than
just an iterative execution of the same control
instructions.
The initial step, step(s) and transitions can be
connected in several ways, which makes it
possible to describe many complicated
functions.
22

23
Simple sequence,
this is just a step
followed by a
transition or a
transition followed
by a step

24
Alternative parallel
sequences consist of
two or more transition
succeeding a step, so
that the execution can
take alternative ways
depending on external
conditions.

25
Simultaneous parallel
sequences, are made
up of two or more steps
placed parallel after a
transition. The parallel
steps can be
simultaneously active.

Implementation of SFQ by basic
ladder building block
26
Simple
Sequence

Simple Sequence :
The Boolean equations for X2 and X3 are:
X2 = ( X1•a + X2 )•/X3
X3 = ( X2 •b + X3 ) •/X4
Here X2 is an active step. When X2 is active, the actions
Y1 and Y2 are asserted. When X2 is inactive, the
execution of Y1 and Y2 will stop.
27

28
Alternative
parallel
sequence

Alternative parallel sequence :
The Boolean equations for X2, X3, X4 and X5 are:
X2 = ( X1•a + X2 ) • /X3 • /X4
X3 = ( X2•b + X3 )•/X5 ‘alternate parallel branch
X4 = ( X2•c + X4 )•/X5
X5 = ( X3•d + X4•e + X5 )•/X6
There are two conditions entering step X5.
29

30
Simultaneous
parallel
sequence

Simultaneous parallel sequence :
The Boolean equations for X2, X3, X4, X5, X6, X7 and
X8 are:
X2 = ( X1 • a + X2 ) • /X3
X3 = ( X2 • b + X3 ) • /X4
X5 = ( X2 • b + X5 ) • /X6
X6 = ( X5 • d + X6 ) • /X7
X4 = ( X3 • c + X4 ) • /X8
X7 = ( X6 • e + X7 ) • /X8
X8 = ( X4 • X7 • f + X8 ) • /X9
31

32
Branching
sequence

Branching sequence :
X2 = ( X1 • a + X2 ) • /X3 • /X5
X3 = ( X2 • b + X3 ) • /X4
X4 = ( X3 • c + X4 ) • /X5
X5 = ( X2 • e + X4 • d + X5 ) • /X6
There are two branches for step X2, it enters the step
X3 if condition b is asserted and it enters the step X5
if condition e is asserted.
33

34
Repeating
sequence

Repeating sequence :
X2 = ( X1 • a + X4 • e + X2 ) • /X3
X3 = ( X2 • b + X3 ) • /X4
X4 = ( X3 • c + X4 ) • /X5 • /X2
X5 = ( X4 • d + X5 ) • /X6
35

36
Initial step

Initial step :
To enter into Step 0, either a “First Scan” input signal or
nand logic of all the rest of the steps can be used. So the
Boolean equation for Step 0 is:
Step0 = ( FirstScan + Step0 ) • /Step1
or if the nand logic of all the rest of the steps is used, then
Step0 = ( /Step1 • /Step2 • /Step3 + Step0 ) • /Step1
37

38
Inserting
blank step

Inserting blank step :
The looping in the SFC on the left cannot be implemented
as it is impossible for Setp2 to serve as the setup point and
the exit point for Step1 at the same time. So a blank step
Step3 is inserted to implement the required looping.
39

Inserting blank step :
The Boolean equation for Step1, Step2 and Step3 are as
follows:
Step1 = ( Step3 • 1 + Step1 ) • /Step2
Step2 = ( Step1 • LS1 + Step2 ) • /Step3
Step3 = ( Step2 • LS2 + Step3 ) • /Step1
40

Example of process control by using
SFC
Design the logic to move a lift between 3 floors, and
the control functions as follows:
– The lift has for each floor one button which, if
pressed, causes the lift to visit (i.e. move to and
stop at) that floor.
– Each floor has a button to request an up-lift or a
down-lift. They are cancelled when a lift visits the
floor.
– A lift without requests should remain in its final
destination and await further requests.
41

SFC
42
There are six cases:
1. Consider first at
the 3rd floor called
by the 1st floor.

SFC
43
2. Next consider the
case of the lift at
the 1st floor called
by the 3rd floor

SFC
44
3. Then consider
the case of the lift
at the 3rd floor
called by the 2nd
floor

SFC
45
4. Then consider
at the 1st floor
called by the 2nd
floor.

SFC
46
5. Then consider
at the 2nd floor
called by the 1st
floor.

SFC
47
6. Finally consider
the case of the
Lift at the 2nd floor
called by the 3rd
floor.

48
Summary
 SFCs are suited to processes with
single/parallel flow of execution
 SFCs are suited to processes with clear
sequence of operation
 SFC may be implemented by using block
logic
 SFC may also be implemented using
sequence bits

SFC based Process Control Design
49
End of Lecture 6
 Revision
The IEC 61131-3 Programming Language
Specification

Eee3420 lecture06 rev2011

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