A PLC is the key element behind today's industrial automation
But, what exactly is a PLC?
In this video I’ll walk you through the basics of what a PLC is and some of its practical
uses
and even go into some of the PLCs inner workings
Let’s start by learning what the commonly used acronym PLC stands for
PLC is short for programmable logic controller
In general, a PLC is an industrialized computer used to automate production equipment
and or processes
The PLCs hardware and software
allow it to monitor the status of incoming events
and control the outcomes
it also has the flexibility to reprogram its decision-making behaviors through software as
often as needed
Did you know you probably encounter PLC based controls every day without even realizing
it?
Aside from their primary use in automating factories and processes
PLCs are also used in simple applications such as car washes, elevators
and even amusement parks
more complex PLC applications include
water
and waste water treatment plants manufacturing assembly lines
machinery and bottling lines just to name a few
let's take a quick look at the PLCs history
the PLC is mostly recognized as being introduced by Bedford Associates in nineteen sixty
eight
originally called a modular digital controller or MODICON it became the product
of choice in meeting specifications
developed by general motors Hydra-matic Division Plan for replacing traditional relay
based machine control
it's use of ladder-logic programming was one of the main features that attracted it
to the factory personnel because it could be programmed the PLC allowed for quick changes
and reduction in wiring and troubleshooting time
this was a huge advantage over the older relay based control systems
making the same changes on relay based control systems meant equipment would have to be shut
down for long periods of time
to perform extensive rewiring
the PLC also has the advantage of taking up less panel space and consuming less energy
what do the individual words in PLC mean? It may make more sense to take the words in reverse
order
controller is the key word in programmable logic controller
the PLC monitors various conditions and based on these conditions determines an outcome
in other words
it has the ability to control devices wired to its output terminals based on the status of
the devices wired to its input terminals
Logic is how we treat the various conditions we monitor which determines our final
outcome
the decisions are based on logical rules that we teach the PLC by way of its programming
software
A simple example might be that we detect someone has opened a door
and it's nighttime
So a light is turned on so they can see where they are stepping
programmable software for the PLC is used to create these logical rules
because you can program the PLC to behave however you want
it can be adapted to virtually any application
the ability to program and reprogram the PLC makes it very versatile
and for this reason if future conditions change the PLC can be reprogrammed to meet these needs
we can envision how a PLC functions by examining the interaction of four internal areas
the primary area is the CPU
or central processing unit this is the brain or in other words where the decision-making
takes place
the second area is the memory
this is where the CPU stores the users control program, the I/O status
and data
the program is stored in non volatile memory so it is not lost in the event of power failure
the third area is communication
One or more communication ports are available to allow the users program to be loaded
into its memory from a PC or personal computer
And to also communicate with external devices including other PLCs
in order to exchange data the final area is the PLCs I/O the input and output signals to
real world devices
let's cover the I/O in more detail
Real world conditions are monitored by the PLCs inputs after decisions are made
by the users program
real world control devices is carried out by the PLCs outputs
here we see a systematic diagram example showing how inputs and outputs are wired to devices such
as push buttons, limit switches,
pilot lights, motor starters and solenoid valves
the PLCs I/O connections are often available with different input and output types such as DC and AC
voltage inputs
and also DC and AC relay type outputs
so
how are plcs programmed?
in most cases application software running on a personal computer is used to create the
PLCs decision-making logic
although other programming methods exist the most widely-used is still ladder logic
which is a carryover from relay based control systems
Ladder logic consist of contacts, coils and other functions arranged on rungs not
unlike the rungs on a ladder
In the simple example shown here a timer is activated by either an external device wired to input X1
or an
internal control relay labeled C1
notice the logic OR in the example
after a preset amount of time of ten seconds the timer signals completion by closing its
T1
contact and activating output Y1
which controls a real world device wired to the PLCs output terminals
END coil is shown in the last rung to tell the program to go back to the start
and check the rungs in order again
to better understand the internal workings of the PLC it helps to know how the ladder
logic is examined during execution
this is referred to as the program's scan
when the PLC is in its RUN mode
the CPU executes the users program with the first step being to read all the inputs
this is the same as taking a snapshot of the current status of the inputs
including all data registers
next routine house keeping is performed such as checking communications
from peripheral devices and updating register values
now the ladder logic is ready to be scanned the scan is performed left to right for
each instruction on the ladder logic rung
starting with the first rung and then each succeeding rung
until the END coil is encountered
the results for the programs logic conditions are determined computed and the outputs are updated
diagnostics are performed and if all is well the scan is repeated
the entire scan can take place in less than one to two milliseconds
the scan time is dependent on the size of the ladder logic program
the scan continues until the PLC is taken out of the RUN mode
Now that you have learned a bit more on what PLC is, how it works and what they can do for you why not
take a look at some of the other topics listed in the learn site
Until next time thank you for watching this tutorial from learn.automationdirect.com
have a great day