Manufacturing Automation for Assembly and Inspection MM453
Manufacturing Automation for Assembly and
Inspection
MM453
Dr. Nigel Kent
Rm S366
nigel.kent@dcu.ie
Dublin City University
Sem 1 2022
Dr. Nigel Kent Manufacturing Automation for Assembly and Inspection, MM453 1/13
Manufacturing Automation for Assembly and Inspection MM453
Lecture 4
Lecture 4
Dr. Nigel Kent Manufacturing Automation for Assembly and Inspection, MM453 2/13
Manufacturing Automation for Assembly and Inspection MM453
Lecture 4
Ladder Logic
Ladder Logic
• Most industrial control processes use programmable logic
controllers (PLCs) to control the mechanical components such
as actuators valves motors etc.
• Traditionally ladder logic diagrams are used to program PLCs.
Ladder logic can be used to implement logic gates and
memory blocks from the previous lecture.
• Although the IEC 61131-3 standard is recommended for
ladder diagram symbols, many vendors deviate slightly from
these symbols.
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Manufacturing Automation for Assembly and Inspection MM453
Lecture 4
Ladder Logic
Most Common Ladder Logic Symbols
Input contact such as a switch, relay, photoelec-
tric sensor etc., in other words anything that can
provide an on/off signal.
Output load such as a motor, valve, actuator,
solenoid, alarm etc.
Inverse input, i.e. if the input is A this uses A
(Not A).
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Manufacturing Automation for Assembly and Inspection MM453
Lecture 4
Ladder Logic
Ladder AND Ladder OR
L L
A B A
A.B A+B
Note the L can be considered as ’Live’ - this will be covered in
later slides.
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Manufacturing Automation for Assembly and Inspection MM453
Lecture 4
Ladder Logic
• The left most rail of the diagram may
be regarded as live and the right as
neutral. Therefore there must be a
load or output on every rung to avoid
a short circuit.
• Inputs can be repeated on as many
rungs as required.
• An output can be created only once.
• The on/off condition of an output may
be used as an input. The symbol used
will indicate the output.input nature of
the variable.
• Variables are only updated at the end
of a ’scan cycle’ i.e. all rungs are
processed and on the last run all
variables are updated.
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Manufacturing Automation for Assembly and Inspection MM453
Lecture 4
Ladder Logic
Start Logic
G P
When the go button G is pressed the output P is energised, often
to turn turned on to a machine. When G is released the power is
off.
The state of G depends on the mechanical action of the switch
used. It is typical for most industrial systems to use push buttons
for Start and Stop operations. Why might this be the case?
How can P be kept in the on state after G has been de-pressed?
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Manufacturing Automation for Assembly and Inspection MM453
Lecture 4
Ladder Logic
Latching
G P
Now the output P is used as an input latches itself on when it is
OR’d with G.
However the system now requires a stop function. Where should
the Stop switch be placed
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Manufacturing Automation for Assembly and Inspection MM453
Lecture 4
Ladder Logic
Latching
G S P
By using Not S the output P is interrupted when the stop button
S is pressed.
In this configuration the input switch to S is configured as
normally closed. Why might this be the case?
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Manufacturing Automation for Assembly and Inspection MM453
Lecture 4
Ladder Logic
Lock and Switch Example
L P P
L S
P ′ = L.P + S.L
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Manufacturing Automation for Assembly and Inspection MM453
Lecture 4
Ladder Logic
Conveyor Example
Consider a conveyor system which rejects overweight items.
Weight Sensor A
Trap Door
Y
If an item is overweight as detected by sensor A, the trap door Y
is opened immediately and stays open for 4 seconds.
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Manufacturing Automation for Assembly and Inspection MM453
Lecture 4
Ladder Logic
In this case an ’On Delay Timer’ or ’TON’ timer can be used.
Within standard PLC programs there are many different timers.
TON and an ’Off Delay Timer’, ’TOF’ being the most common.
TON
Input Output
Time 4s
Generic TON Siemens TON
In the case of the ’TON’ timer, the output goes high after a
predefined delay, in this case 4 seconds.
Once the input goes low, the output goes low and the timer is
reset.
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Manufacturing Automation for Assembly and Inspection MM453
Lecture 4
Ladder Logic
• Because of the nature of the timer,
the output from sensor A cannot
be used to switch on the timer as A B Y
A would go low after the item
passes the weigh station, causing Y
the timer to reset to zero.
Y TON B
• Therefore we need to use the
output Y , which will be switched Time 4s
on by A and latched on by itself.
• In order to switch the system off
we need an interrupt, not B, B is
created with the 4s delay timer
which is switched on with Y .
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