Eee3420 lecture05 rev2011

1Week © Vocational Training Council, Hong Kong.

│ Lecture 5 │

Flow-chart based process control design

EEC3420 Industrial ControlDepartment of Electrical Engineering

2© Vocational Training Council, Hong Kong.

EEE3420 Industrial Control

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Learning Objectives To design control process based on flow-chart technique



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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



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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



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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.



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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.



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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



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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



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• 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



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The general method for constructing flowcharts is:

1. Understand the process.2. Determine the major actions, these are drawn as

blocks.3. Determine the sequences of operations, these are

drawn with arrows.4. When the sequence may change use decision

blocks for branching.



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Two basic techniques that can be used:

• the first presented uses blocks of ladder logic code

• the second uses normal ladder logic



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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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• 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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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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• 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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The first part of the ladder logic required will reset the logic to an initial condition as shown



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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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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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The ladder logic for operation F3 shown opens the inlet valve and moves to operation F4



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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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The ladder logic for operation F5 shown is very similar



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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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Implementation of flowchart using 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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• each of the blocks in the flowchart are labeled

• the connecting arrows (transitions) in the diagram are also labeled



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• The first section of ladder logic shown indicates the transition logic

• This presents when the transitions between functions should occur



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• 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



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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



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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



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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



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Flow-chart based process control design

End of Lecture 5

RevisionTRiLOGI PLC simulator operation manualMitsubishi FX series programming manual

Eee3420 lecture05 rev2011

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