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Industrial Programmable Logic
Controllers (PLCs)
Eng. R. L. Nkumbwa
Copperbelt University
School of Technology
2010
Programmable Logic Controllers
(PLC’s)
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A programmable logic controller (PLC) is a
special purpose computer aimed at implementing
control solutions.
Historically PLC’s have been used mainly for onoff or logic type applications.
However, modern PLC’s have become
increasingly sophisticated and can now cover
quite complex control tasks.
Eng. R. L. Nkumbwa @ CBU
2010
Programmable Logic Controllers
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These notes emphasize the switching capability
of a PLC. However, the reader is urged to think
of the dynamics of the underlying system.
For example, we recently heard of a seemingly
straightforward application of a PLC to a
reservoir level control problem.
The algorithm had many features but in essence
conformed to the rule that if the level was too low
then the pumping rate should be
Eng. increased.
R. L. Nkumbwa @ CBU
2010
Programmable Logic Controllers
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This can be seen as an approximate form of integral
control.
However, pure integral control when applied to a tank
(which is also an integrator) will lead to self sustained
oscillations.
The reader is encouraged to verify this by drawing a root
locus plot for the closed loop poles.
The moral of this story is that when implementing a
controller in a PLC one needs to consider the combined
dynamics of the control law and system.
In this way a PLC is no different from any other form of
Eng. R. L. Nkumbwa @ CBU
control implementation platform.
2010
Industrial Robustness of PLC’s
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If you have ever had a personal computer lockup or reset in the middle of something important
you will appreciate that a similar failure in a large
industrial application would have severe
consequences.
A PLC is simply an industrial computer designed
to be durable enough to withstand the rigors of
industrial applications.
Eng. R. L. Nkumbwa @ CBU
2010
Industrial Robustness of PLC’s
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It is common in the mining industry to have
PLC’s on large mobile equipment to monitor and
control everything from lights to large electrical
drives.
The vibration and operating temperature of this
environment require the use of specialized
control equipment such as PLC’s.
Eng. R. L. Nkumbwa @ CBU
2010
Some Physical Tolerances of leading
PLC brands
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Eng. R. L. Nkumbwa @ CBU
2010
Typical Areas of Application
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For past 30 years PLC’s have been in the first
level of control because of their ability to run
uninterrupted for extended periods of time
(usually measured in months or even years).
The PLC layer in industrial applications is shown
in figure below
Eng. R. L. Nkumbwa @ CBU
2010
Control hierarchy of PLC’s
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Eng. R. L. Nkumbwa @ CBU
2010
Ladder Logic
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The remained of this chapter will look at Ladder
Logic, which is the most common programming
structure used in PLC’s.
Several examples of common Ladder Logic
programs will be demonstrated to show the
simple structure of this coding style.
Eng. R. L. Nkumbwa @ CBU
2010
Relay Ladder Logic
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Ladder logic is possibly the simplest
programming language.
The principle behind the language is illustrated
by a simple electrical relay. (Electrical relay logic
preceded PLC’s as initially all logic was
implemented in hard wired panels using actual
relays, timer, Counters etc.)
Eng. R. L. Nkumbwa @ CBU
2010
Simple Relay Circuit
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Eng. R. L. Nkumbwa @ CBU
2010
Circuit representation in Ladder Logic
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The above circuit is represented in Ladder
logic as shown in figure below (only the low
voltage circuit is used in ladder logic
diagrams):
Eng. R. L. Nkumbwa @ CBU
2010
Boolean Logic
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The above diagram can be expressed in Boolean
logic “Motor Coil = Switch”, ie if the Switch is on
the Motor coil is energized and Motor is
activated.
All ladder logic takes the form of that shown in
Figure above.
Thus it connects the positive and neutral rail
representing positive and negative power supply
to the relay.
Eng. R. L. Nkumbwa @ CBU
2010
Ladder Logic Instructions
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The three most common instructions in
ladder logic are:
Eng. R. L. Nkumbwa @ CBU
2010
PLC Switching
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The majority of simple PLC switching tasks can
be implemented using these simple instructions.
However there exists a set of 60 to 100 individual
instructions such as those above which form the
complete PLC language.
Other functions vary from simple timers and
counters to more sophisticated control functions
such as PID controllers and advanced arithmetic
functions.
Eng. R. L. Nkumbwa @ CBU
2010
Multi Input/Output Rungs
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Eng. R. L. Nkumbwa @ CBU
2010
Multi Input/Output Rungs
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Each of the inputs in series such as ‘S2’ and ‘S3’
are equivalent to the Boolean “AND”{.} statement
while a “Bridge” in a rung is seen as an “OR”{+}
statement.
In the case shown in Figure above the Boolean
expression for the Coil is:
Eng. R. L. Nkumbwa @ CBU
2010
Multi Output Rungs
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It is also possible to have multi output rungs
provided the OTE instructions are never placed
is series, they must always be in parallel and in
contact with the Neutral (right) rail, as shown in
figure below. (While multi output rungs are
necessary in some cases, it is often beneficial to
divide the rung into several smaller rungs for the
purpose of program clarity)
Eng. R. L. Nkumbwa @ CBU
2010
Multi Output Rungs
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Eng. R. L. Nkumbwa @ CBU
2010
Start-Stop Rung
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Any large rung can always be broken down into simple
Boolean expressions such as for the simple rungs shown
above.
Just as common electrical relays have auxiliary outputs,
which are used for feedback and indication, a similar
strategy is used in ladder logic.
To demonstrate this point a simple START/STOP rung is
examined.
Two Push Buttons (PB) are used for the ‘start’ and ‘stop’
inputs (Push buttons return a High or Low signal to the
PLC only for the duration during which the button is held
Eng. R. L. Nkumbwa
@ CBU
down), Consider the logic shown in Figure
below.
2010
Use of Auxiliary Output
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Eng. R. L. Nkumbwa @ CBU
2010
Push Button Example
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In this case the push buttons are “normally open” i.e. they
return a low signal in their normal position and a high
signal when pressed.
Initially assume the Coil output is off, and the Push
Buttons (PB) not activated so they are also off.
When the ‘Start PB’ is pressed the top path through the
rung is satisfied with start = 1and stop = 0 so the coil is
turned ON.
Note this also satisfies the bottom path through the rung
on the next scan since coil = 1 and stop = 0.
When the start button is released the top path through
the rung becomes false but the rung is still satisfied by
L. Nkumbwa
@ CBU PB
the bottom path, ie the coil remains Eng.
on R.
after
the start
2010
is released.
Push Button Example
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Pressing the ‘stop’ button (stop =1) breaks the
rung so the coil is turned OFF and remains off
after the stop button is released.
Eng. R. L. Nkumbwa @ CBU
2010
Other PLC Programming languages
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While Ladder logic is the most common form of
PLC language it is worth noting that other
languages do exist which where developed in
conjunction with Relay Ladder Logic.
Eng. R. L. Nkumbwa @ CBU
2010
Other Languages
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Blocks are built from small ladder logic subroutines and used
through the code as user defined ladder logic instructions, the
advantages of this approach is the reduction of repetitive ladder logic
code.
Sequential Function Chart (SFC) programming is similar to
programming by computer flow chart. In SFC the program advances
step by step through various blocks (where action happens such as a
motor is started).
Transition conditions determine when the program advances from
one block to another.
Both the action blocks and the transition conditions are created using
ladder diagrams.
Structured text, uses simple instructions common to medium level
Eng. R. L. Nkumbwa @ CBU
programming languages: If , While, Then etc.
2010
Other Languages
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Note: Some programming packages allow the
user to switch between Relay Ladder Logic and
Structured text representations of the code.
Eng. R. L. Nkumbwa @ CBU
2010
PLC Modules
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The modular nature of PLC components makes the
design, instillation and maintenance of the first level of
control much simpler.
Similar to the peripheral devices, which can be obtained
for a personal computer, there are literally thousands of
interface modules supported by PLC’s.
While these modules may be numerous they all have the
same function, obtaining or delivering control signals and
information between the measurement level and the
operator interface level.
The modular nature is possibly the greatest strength of
PLC’s and several common modules will be described
below.
Eng. R. L. Nkumbwa @ CBU
2010
Digital Inputs/Outputs (I/O)
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By far the most common industrial signals used in PLC’s
are simple digital control signals.
Referred to as Digital I/O (Digital input/output), they can
be measured in there thousands for substantial industrial
plants.
They are used to measure an amazing verity of events.
The most common digital signal encountered is a simple
switch, A switch is nothing more than an electrical switch
used to indicate some physical position.
Eng. R. L. Nkumbwa @ CBU
2010
Digital Inputs/Outputs (I/O)
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Digital I/O signals come in many forms from 12
Volt DC to 240 Volt AC, but the most common
industrial signals are 24Volt DC and 120V AC.
There are Digital I/O modules to measure all of
these signals, an extra variation is the number of
signals per module ranging from 8, 16 and 32
etc.
Most digital modules have electrical protection
usually in the form of optocoupling to prevent
damage to the PLC from standard
Eng. R. electrical
L. Nkumbwa @ CBU
2010
faults.
Analogue Inputs/Outputs (I/O)
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Analogue signals enter and leave the PLC in
voltage and current form, The resolution of most
analogue signals is between 11 and 16 bits.
While voltage is commonly used in practice it can
be sensitive to noise (induced voltages in
electrical equipment rooms can be substantial)
This makes current control the superior choice.
The most common analogue signals appear in
the form of a 4-20mA current signal,
i.e. @ CBU
Eng. R. L. Nkumbwa
2010
Analogue Inputs/Outputs (I/O)
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A signal of 4.0mA equates to zero,
A signal of 20mA equates to 2048 for an 11 bit input,
The reason for the 4.0mA starting point is a failsafe feature, if the instrument fails or the signal
cable is damaged the current falls to zero and
the PLC can alarm the operating system of the
failure immediately.
This fail-safe feature is not possible for voltage
signals which pass through zero
volts.
Eng. R. L. Nkumbwa @ CBU
2010
Other interface modules
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Digital I/O and Analogue I/O signals comprise the
vast majority of PLC signals.
However, not all devices can be controlled by
these simple signals and it is often required that
PLC’s communicate to foreign instruments using
many different communications protocols,
Again in most instances there is simply a module
predesigned to make this interface possible.
Some examples include RS232, RS485, DH485,
DH+, Modbus, Ethernet etc. Eng. R. L. Nkumbwa @ CBU
2010
Memory
(Addressing , Internal Registers)
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The memory in PLC’s needs to be predefined by
the programmer, similar to defining variables in
standard programming.
The memory is divided into data files, each data
file has a unique number and a character prefix
which refers to the type of data.
For example, in the Allen Bradley PLC-5
system, we have;
Eng. R. L. Nkumbwa @ CBU
2010
Allen Bradley PLC-5 System
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Eng. R. L. Nkumbwa @ CBU
2010
Allen Bradley PLC-5 System
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The ASCII character (or characters) denotes the data
area type. Other types of defined words include T-timers,
C-counters and S-status registers etc.
Note that the Output and Input numbers are dropped
from file 0 and 1 respectively, this is because these files
are not programmable.
Status file 2 (S2:##) is also fixed within Allen Bradley
PLC’s. (Status bits include such information as arithmetic
overflow warnings, communication status and processor
time and dates etc)
Eng. R. L. Nkumbwa @ CBU
2010
Allen Bradley PLC-5 System
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The internal data areas of Integers and Floating
point addresses simply point to a linear memory
region.
However the Input and Output address actually
correspond to a physical address. For example
I:12/03 has the format, I:{Rack address}{slot
number}/{Input number}.
Physically this is the 3rd input on the 2nd slot in
the 1st Rack.
Eng. R. L. Nkumbwa @ CBU
This input is show below in figure,
2010
Physical I/O Addressing
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Eng. R. L. Nkumbwa @ CBU
2010
Physical I/O Addressing
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Note: PLC’s usually start counting from zero and
use the octal counting system. ie an eight slot
rack will have slots numbered from 0 to 7.
This is the case for racks, slots and input
numbers.
So the first possible input would be Rack 0, slot 1
input 0 (Slot 0 in Rack 0 is not available for
inputs since it is always reserved for the Central
Processing Unit, CPU)
Eng. R. L. Nkumbwa @ CBU
2010
Addressing Format
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The addressing format has been demonstrated
here because in writing a PLC program the
address is used by the PLC in the ladder logic.
For example, if in the Start /Stop logic of figure
(6) the Start PB = I:13/04 and Stop PB = I:13/05
and the output Coil = O:12/03 then the ladder
logic would look more like figure below
Eng. R. L. Nkumbwa @ CBU
2010
Start/Stop Addressing Example
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Eng. R. L. Nkumbwa @ CBU
2010
PLC Programming Architecture
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A PLC program can be divided into many small
subroutines. In this respect PLC’s replace function and
procedures with subroutines.
Each subroutine is executed from top to bottom in a
predetermined order.
Once all the subroutines have been scanned it simply
begins again with the first subroutine.
The nature of subroutines makes a modular
programming structure possible, It is always beneficial to
group segments of code which serve similar purposes.
Eng. R. L. Nkumbwa @ CBU
2010
PLC Programming Architecture
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For example a program may be divided in the
following way:
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Subroutine U:20 – Pumps sequencing logic file.
Subroutine U:21 – Pump No.1 alarming and drive file.
Subroutine U:22 – Pump No.2 alarming and drive file.
Subroutine U;23 – Pump No.3 alarming and drive file.
Eng. R. L. Nkumbwa @ CBU
2010
PLC Programming Architecture
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Here subroutine 20 controls the sequencing of
21,22 and 23 while the drive files are virtually
identical for each pump by starting and stopping
the pump while reporting alarms and the healthy
status to file 20.
Fault finding problems in the code is greatly
simplified when all the code for a faulty device is
listed in a single subroutine.
Eng. R. L. Nkumbwa @ CBU
2010
PLC Programming Architecture
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The development of a plant wide PLC standard is an
essential step in automating a large control system.
The benefits are twofold, if the programming standard is
well understood many code developers can work on the
code producing a uniform control system.
The second advantage is that for anyone not involved in
the code production only needs to learn one standard to
have a good understanding of the program structure of all
the plants PLC’s.
Eng. R. L. Nkumbwa @ CBU
2010
Commercially Available PLC’s
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The next slides give details of some commercial
available PLC’s.
The purpose of this is to show the variety of
PLC’s available on the market.
Eng. R. L. Nkumbwa @ CBU
2010
Allen-Bradley PLC’s
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Below are AB Plcs
Note that AB is part of the Rockwell
Automation, a global leading Automation
giant.
Eng. R. L. Nkumbwa @ CBU
2010
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Eng. R. L. Nkumbwa @ CBU
2010
Siemens Simatic PLC’s
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Below are Siemens Simantic PLCs
Eng. R. L. Nkumbwa @ CBU
2010
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Eng. R. L. Nkumbwa @ CBU
2010
Modicon TSX PLC’s
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See Modicon TSX PLC below
Eng. R. L. Nkumbwa @ CBU
2010
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Eng. R. L. Nkumbwa @ CBU
2010
Other PLC Terminology
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On-line: Refers to the program code currently in the PLC memory.
Off-line: Refers to the copy of the program code stored on the
programming computer.
Up-Load: Refers to the act of copying the current On-line code from
the PLC to the programming computer . (Note the previous code on
the programming computer is lost during this transfer if a copy is not
made)
Down-Load: Refers to the act of copying the code from the
programming computer into the PLC’s memory. (note the previous
PLC code is lost during this transfer).
Note: It is important to realize that changing the On-line program will
not change the Off-line program on the programming computer.
Invariably at the end of any edit there must follow an Up-load or
R. L. Nkumbwa
CBU
Down-Load to make the Online and Off-lineEng.
programs
the @
same.
2010
Other PLC Terminology
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Run Mode: The PLC actively scanning the code and
driving outputs.
Program Mode: The PLC is not being scanned (All
outputs disabled)
Remote Mode: programming computer can change
the PLC between Run and Program.
Note: There is a physical key on the front of each
PLC processor module, which allows the PLC to be
switched between Run, Remote and Program Mode.
Eng. R. L. Nkumbwa @ CBU
2010
Programming Procedure
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PLC’s are very versatile in the respect that
programming changes can be made on-line
while the processor is in Run Mode.
This is equivalent to making changes in a
compiled ‘C’ program while it is being executed.
To introduce programming edits into the existing
code while in Run Mode the following procedure
is followed.
Eng. R. L. Nkumbwa @ CBU
2010
Programming Procedure
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Insert edit rungs into the On-line PLC code (as many changes as
required). These edits are effectively invisible to the processor during
this stage.
Once all edit rungs are entered switch the processor to TEST, during
TEST mode any new inserted code is allowed to be scanned along
with existing code, if any code is marked for deletion this code is
prevented from being scanned at this stage.
Once you are confident that the edit rungs are correct Assemble the
code which permanently activates new code and permanently
removes any code marked for deletion.
Eng. R. L. Nkumbwa @ CBU
2010
Programming Example:
Simple start/stop ladder logic.
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Eng. R. L. Nkumbwa @ CBU
2010
Programming Example:
Simple start/stop ladder logic.
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This is as it would appear in a windows
based programming package.
Text in the Yellow and Green box’s is the
Address Description and Address Symbol
respectively.
The symbols often correspond to SCADA
(System Control and Data Acquisition)
software tags.
Eng. R. L. Nkumbwa @ CBU
2010
Alternative Start/Stop using
Latch and Unlatch instructions.
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Eng. R. L. Nkumbwa @ CBU
2010
References
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Garry Johnston (2010),University of Newcastle,
Australia
Eng. R. L. Nkumbwa @ CBU
2010
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Eng. R. L. Nkumbwa @ CBU
2010