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Level 3 - Programmable logic controllers (PLC's)
Session 1 - History and an introduction to PLC's
Tutor name.......................................
Course.............................................
Student name................................................
Session 1 complete (signed by tutor)...................... date...................
PLC’s are now used widely in industry. They are everywhere.
So why use them?
A bit of history
Before the 1950’s all assembly and production lines were heavily
dependent on manual labour – people.
Problems
Quality of work varies
Get tired
Go on strike
Toiletry needs
Sickness
Dangerous, H+S issues
Need good environment to work in
Very boring, repetitive
Labour is expensive
In about the 1950’s in an effort to reduce the human impact and
problems Numerical Control (NC) systems were used.
This consisted of using hundreds of relays to make logic circuits that
would control systems.
Single relay
All this……..
….…to control this
Problems
Relays are very bulky
Consume a lot of power
Unreliable – mechanical switches
Expensive
Not flexible in terms of reprogramming the system
So now we are in the 1960’s
Transistors were now becoming widely used which reduced the bulk,
power consumption and reliability problems.
However there was still the problem of lack of control flexibility.
The USA makes a commitment to go to the moon.
Saturn V launch vehicle
(LEM)
Lunar excursion module
www.youtube.com/watch?v=FzCsDVfPQqk
There was a need for lightweight and compact control systems to
enable complex control of the propulsion and guidance/navigation
systems. The Apollo missions really pushed things on.
To get 1KG into space takes 68KG of fuel
Transistors still too heavy and power hungry.
To combat this there was a need for a way of logically controlling a
system that was:Compact
Light
Low power consumption
Reprogrammable
This was one of those moments in history that changed
absolutely everything!
The microprocessor
It consists of thousands (or millions) of components (mainly
transistors) integrated onto a single piece of silicon. Together with
memory it has formidable computing and control abilities
Its beauty is its re-programmability.
One big problem with the microprocessor is that it’s a pig to program
and requires a high level of expertise.
It is also not very good at being connected up to things like sensors
and output devices (motors etc). It needs a lot of electronics to
Interface it to the outside world it is controlling.
So in 1969 General Motors in the USA came up with the first
Programmable Logic Controller. It’s pretty much the same as the ones
we have today.
 It’s easy to program
 It’s easy to connect up to the system you are trying to
control.
What’s in a PLC?
Lots of things, but let’s keep it simple.
PLC
Program
Memory
Programming
computer
Input
Interface
24V Power
supply
Communication
Interface
ROM
Memory
RAM-data
Memory
Microprocessor
Input
Interface
Ouput
Interface
System
Sensors
System
Output
devices
Microprocessor
 The microprocessor controls all activities within the PLC.
 Looks at the inputs
 Sets the output’s at the correct levels
 Communicates with the programming computer to download programs and monitor
the state of the inputs and outputs
Input and output interface
 The PLC needs to be able to communicate with the outside world. This is done using
input and output interfaces.
 It ensures that the input sensors can talk to the PLC and the PLC can talk to the
output devices or actuators.
 It protects the delicate internals of the PLC from the outside world.
Program memory
 This is where the PLC stores the programs that you have written.
 Obviously the size of the memory must be sufficient to hold the largest program
envisaged. Usually measured in Kbytes or Mbytes but measured in rungs for PLC’s.
 Program memory is permanent once the program has been downloaded until you
want to change it. The program will still be there if you switch off the PLC.
 It’s actually EEPROM (Electrically Erasable Programmable Read Only Memory)
ROM (read only memory)
 This holds the operating system program (like Android or Windows)
 It is permanent memory and not lost when the PLC is switched off.
RAM (random access memory)
 This is temporary memory where information such as data from input sensors,
outputs, timers, counters and other program memory elements are stored.
 Also used when mathematical calculations or logic operations are carried out.
 When the power is switched off the memory is erased and all data is lost.
Communication interface
 This allows the PLC to communicate with the programming computer.
 It can talk to the programming computer to allow programs to be loaded into
program memory.
 When the PLC program is running and controlling the system it allows the
programming computer to show the state of all the inputs and outputs and any
other features being used (counters and timers etc) via the monitor program. This is
because the PLC can tell the programming computer what the status is of the above
via the communication link.
Power supply
 For most PLC's 24V DC supply.
 It powers the PLC and in small systems the input devices and output devices.
 A separate external regulated power supply is needed.
 Some run off 230V mains voltage, the PLC converting the 230V AC into 24V DC
internally.
System Model
System
Sensors
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PLC
System
Output
Devices
To control a system the PLC needs information about what is happening in the
system at any moment in time.
To do this it needs sensors placed around the system giving the correct information.
Depending upon what that information is it will either turn on or turn off its output
devices which alter how the system functions.
You are a system and you do this all the time, all day, every day, till you die.
Think about walking down the street.
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You use visual sensors to tell you where you are and what is happening.
You use balance sensors to enable you to stand up straight and not fall over when
walking on a slope say.
Touch sensors give feedback as to exact bodily position and muscle activity.
You listen for traffic and other people around you.
The information streaming in allows your brain to control all your muscles (output
devices) to navigate a safe route from A to B.
Session 1 knowledge check
Answer all questions then get your tutor to check them.
1) Give three problems with labour based production lines
1. ..........................................................................................................................
2. ..........................................................................................................................
3. ..........................................................................................................................
2) What device enabled the first numerical controlled
machines to be developed?
..........................................................................................................................
3) What is the name of the electronic device that enabled
men to land on the moon?
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4) What is the name of the company that developed the
first PLC?
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5) Below is a block diagram of the internal structure of a
PLC. Label all the blocks.
PLC
6) Match the memory types to their purpose by drawing a
line. State whether the information stored is temporary or
permanent.
ROM
data storage (temporary/permanent)
RAM
application program storage (temporary/permanent)
Program memory
operating system (temporary/permanent)
7) What is the supply voltage most PLC's operate at?
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8) Give two reasons a PLC needs a communication interface.
1. ......................................................................................................
2. ......................................................................................................
9) Any control system knows what is happening in the
system by the use of ....................... It can control the
behaviour of the system by altering the condition of
...................... devices.