Eee3420 lecture05 rev2011

EEC3420 Industrial Control
Department of Electrical Engineering
│ Lecture 5 │
Flow-chart based process control
design
© Vocational Training Council, Hong Kong. Week 1

EEE3420 Industrial Control
© Vocational Training Council, Hong Kong. Week
2
Learning Objectives
 To design control process based on flow-chart technique

3
Flow-chart
• in flowchart, blocks are connected using
arrows to indicate the sequence of the steps
• different blocks imply different types of
program actions
• programs always need a start block, but PLC
programs rarely stop so the stop block is
rarely used
• other important blocks include operations and
decisions

4
Flowchart
• flowchart is ideal for a
process that has
sequential process
steps
• steps will be
executed in a simple
order that may
change as the result
of some simple
decisions

A simple flowchart with automatical
entering to the first step
• ARROW paths are
represented by T1:500 and
T2:501
• PROCESS boxes are
represented by F1:1000 and
F2:1001
• arrow paths are implemented
by means of the “Transition
Logic” ladder
• process boxes are realized by
the “Function Logic” ladder.
5

A simple flowchart with automatical
entering to the first step (without KEEP function)
• ARROW paths are
represented by T1:500 and
T2:501
• PROCESS boxes are
represented by F1:1000 and
F2:1001
• arrow paths are implemented
by means of the “Transition
Logic” ladder
• process boxes are realized by
the “Function Logic” ladder.
6

7
The DECISION box
• there are two output
transitions in a decision box
• each output transition is
accompanied by a Boolean
condition
• When the input 01 is 0, the
output 1000 will be off and
1001 on, and when the input
01 is 1, the output 1000 will
be on and 1001 off

8
Representation of a sequential
process by flowchart
• a sequential process of a
Tank Filler for controlling
a large water tank
• when start button is
pushed the tank will start
to fill, and the flow out will
be stopped when full, or
the stop button is pushed
the outlet will open up, and
the flow in will be stopped

9
• execution starts at the top
• first operation is to open
the outlet valve and close
the inlet valve
• next, a single decision
block is used to wait for a
button to be pushed

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• when the button is pushed
the yes branch is followed,
the inlet valve is opened,
and the outlet valve is
closed
• the flow chart goes into a
loop that uses two decision
blocks to wait until the tank
is full, or the stop button is
pushed

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• if either case occurs the
inlet valve is closed and the
outlet valve is opened
• system then goes back to
wait for the start button to
be pushed again
• when the controller is on
the program should always
be running, so only a start
block is needed

The general method for constructing flowcharts is:
1. Understand the process.
2. Determine the major actions, these are drawn as
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blocks.
3. Determine the sequences of operations, these are
drawn with arrows.
4. When the sequence may change use decision
blocks for branching.

Two basic techniques that can be used:
• the first presented uses blocks of
ladder logic code
• the second uses normal ladder
logic
13

Implementation of flowchart using
block logic
• name each block in
the flowchart as
shown
• each of the
numbered steps will
then be converted to
ladder logic
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block logic
• each block in the
flowchart will be
converted to a block
of ladder logic
• to do this we will use
the Interlock relay
(or the MCR relay)
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block logic
If the first ILock line is
true then the ladder
logic on the following
lines will be scanned
as normal to the
subsequent ILoff line
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block logic
• if the first line is
false the lines to
the subsequent
ILoff block will all
be forced off
• if a normal output
is used inside an
ILock block, it may
be forced off
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block logic
The first part of the ladder logic required will reset the logic
to an initial condition as shown
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block logic
This logic turns on the outlet valve and turns off the inlet valve
It then turns off operation F1, and turns on the next operation
F2
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block logic
The ladder logic for operation F2 shown is simple, and when
the start button is pushed, it will turn off F2 and turn on F3
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block logic
The ladder logic for operation F3 shown opens the inlet
valve and moves to operation F4
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block logic
The ladder logic for operation F4 shown turns off F4, and
if the tank is full it turns on F6, otherwise F5 is turned on
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block logic
The ladder logic for operation F5 shown is very similar
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block logic
The ladder logic for operation F6 shown turns the outlet valve
on and turns off the inlet valve. It then ends operation F6 and
returns to operation F2
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sequence bits
In general there is a preference for methods that do
not use Ilock (or MCR) statements or latches
The flowchart used in the previous example can be
implemented without these instructions using the
following method
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sequence bits
• each of the blocks in
the flowchart are
labeled
• the connecting arrows
(transitions) in the
diagram are also
labeled
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sequence bits
• The first section of
ladder logic shown
indicates the transition
logic
• This presents when
the transitions
between functions
should occur
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sequence bits
• The logic shown will
keep a function on, or
switch to the next
function
• F1 will be turned on by
T1 and once function
F1 is on it will keep
itself on, unless T2
occurs shutting it off
28

sequence bits
This shows the
output functions of
the water tank
system, from the
flowchart it can be
seen that the outlet
valve is open in
function blocks F1,
F2 and F6
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High Level Process Representation
using Flowchart
With the aid of high level instructions in PLCs, the
flowchart may now be used to present a process at a
relatively high level perspective
30

Cart control system
The following example shows a flowchart
representation of a control process of cart movement
and how to implement the process using the high
level instructions offered by the Mitsubishi FX PLC
31

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Cart control system
The limit switches LS1 to LS8 detect the position of
the cart and the push buttons SB1 to SB8 are the call
buttons to initiate the calling of the cart.
Upon receiving the cart call signal, the control system
determines whether the call is coming from the high
position or from the lower position and then sends
corresponding signal to drive the cart to the
destination position

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Cart control system
During the movement of the cart, no further call signal
will be handled, and when the cart arrives the
destination, the cart will stay there for 30s and after
that the system resumes its cart call service

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Cart control system
Based on the process specification,
the flowchart shown below is
constructed

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Cart control system
IO and flags assignment table
Limit switch inputs Push button inputs Others
X000 LS1 X010 SB1 Y000 motor brake
X001 LS2 X011 SB2 Y001 motor forward
X002 LS3 X012 SB3 Y002 motor reverse
X003 LS4 X013 SB4 Y003 call in-service light
X004 LS5 X014 SB5 D100 cart stop position
X005 LS6 X015 SB6 D110 cart call position
X006 LS7 X016 SB7 M101 cart call inhibit
X007 LS8 X017 SB8 M102 system started
X021 system start button
X022 system stop button
M000 intermediate relay

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Cart control system
When first start, reset the content of
D100 & D110 to 0. The MOVP
instruction will only be executed
when there is a rising edge present

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Cart control system
The content of D100 is
filled with the stop
position of the cart

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Cart control system
The Cart Call Inhibit coil
is used to control an
output indicator light
Y003 to show that the
cart call is in service.
The content of D110 is
filled with the position of
cart call

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Cart control system
The CMP instruction is used to
determine whether the cart call is
coming from a higher position or
from a lower position. When the
value of D110 (cart call position)
is less than that of D100 (cart
stop position) , then M000 will be
energized and in turn switches
Y002 (motor reverse) on

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Summary
 Flowcharts are suited to processes with single flow
of execution
 Flowcharts are suited to processes with clear
sequence of operation
 Flowchart may be implemented by using block logic
 Flowchart may also be implemented using
sequence bits
 Flowchart can be used to represent process from a
high level perspective and the details of the
process is implemented by the high level
instructions of the PLC

Flow-chart based process control design
41
End of Lecture 5
 Revision
TRiLOGI PLC simulator operation manual
Mitsubishi FX series programming manual

Eee3420 lecture05 rev2011

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