PLC Mitsubishi - Omron - Siemens - Allen Bradley
 What IS a PLC? The letters PLC, in electronic control and automation realms, are an abbreviation and stand for, Programmable Logic Controller "What’s one of those?" You say!
In the business sense, the abbreviation stands for Public Limited Company, but that's not what we're talking about here of course.The Programmable Logic Controller (PLC) little beastie is basically a computer. Even the smallest PLC has a microprocessor, that qualifies it as a computer. Yes a computer, but not in the generally accepted (home PC) form, they are what you would term as an industrial computer. Built to withstand very harsh environments. Find out what makes a PLC tick and how PLCs work. Their
appearance (of the smaller ones at least) is rather nondescript as most
look like a smallish box with rows of screws on top and bottom. This is
because they are manufactured to withstand harsh environments and be very reliable. They don't have a mouse, keyboard or even a screen, they don't even talk the same language as a PC. Of course ultimately all computers use binary when running a program, but I'm referring to how humans program them. Incidentally if your interest is of a level to want fault find or to write programs with them. Have a look at our PLC tutorial page or register yourself for our "PLC Programming From the Start" self teach course that's in the making with the form on the bottom of the page here In
the picture, above at the top, is a Mitsubishi FX1S-20MR, the 20 stands
for the amount of inputs and outputs, in this case 12 inputs and 8
outputs. The 'FX' is the (Mitsubishi) family of PLC that it belongs to
and the '1S' part is its model type. - BTW - Just for your information, the abbreviation PLC
and the term 'Programmable Logic Controller' are in fact registered
trademarks of the Allen-Bradley Company, although nowadays they are
widely used as a generic description. You could divert into PLC history here if you like to learn how they came to be. Otherwise carry on and find out more about them. - BTW - A
Programmable Logic Controller is a device that a user can program to
perform a series or sequence of events. These events are triggered by
stimuli (called inputs) received at the programmable logic controller
through delayed actions such as time delays or counted occurrences. Once
an event triggers, it actuates in the outside world by switching on or
off electronic control gear or the physical actuation of devices. A
Programmable Logic Controllers will continually loop through its user
defined program waiting for inputs and giving outputs at the specific
programmed times. As you would imagine in the world of computers they have their own language.
This language which is used to program the Programmable Logic
Controller can be used in three formats, ladder, instruction list and
logic symbol. More about this a bit later on.
Programmable Logic Controllers first came about as a replacement for
automatic control systems that used tens and hundreds (maybe even
thousands) of hard wired relays, motor driven cam timers and rotary
sequencers. More often then not, a single PLC can be
programmed to replace thousands of relays and timers. These Programmable
Logic Controllers were first befriended by the automotive manufacturing
industry, this enabled software revision to replace the laborious
re-wiring of control panels when a new production model was introduced. Many
of the earliest Programmable Logic Controllers expressed all decision
making logic in a program format called Ladder Logic, which from its
appearance was very similar to electrical schematic diagrams. This
of course was perfect for the electricians of the day, whom quite able
to follow and trace out circuit problems with electrical schematic
diagrams. So using ladder logic became second nature to
them allowing the electricians an relatively easy transition from hard
wired circuits to software driven circuits. This is the
reason this program notation was chosen, to reduce training time for the
existing technicians. Other early Programmable Logic Controllers used
an instruction list type form of programming, based on a stack-based
logic solver. Which was far most difficult to master.
So, what’s a Program? I’m glad you asked! A
program is a connected series of instructions written in a language that
the Programmable Logic Controller can understand. There are three forms
of program format for PLC’s these are Ladder, Instruction and SFC/STL.
Not all programming tools can work with all programming formats. Generally
hand held programming panels only work with instruction format while
most graphic programming tools work with both instruction and ladder
format. Specialist programming software will also allow SFC style
programming but that’s for another time. We will only be concerning ourselves with Ladder Logic
programming here, because it's the most widespread in use today,
probably because it's the easiest to grasp and get into the quickest. Now,
there's one big difference between a PLC and a PC type computer; as
mentioned above, they only have one program to run. Unlike the PC,
which is capable of running several programs at once within the Windows
framework. Any of these could one or many many more of the different
programs that could be installed on the PC. Why? In one word, speed. A
PLC will be designed to run its one program at a very fast speed, only
branching out from within the main bit when an event happens. Events
that happen in real time. This gives our little PLC beastie the ability
to respond very quickly to any of the events under its control via an
input. Its response would then be carried out via an
output. For example controlling a machines production running at 30,000
units an hour! Such as an offset web printing press churning out
newspapers or book pages. Ladder Logic, (the PLC
programming language) is very closely associated to relay logic. In
relay logic there are both contacts and coils that can be loaded and
driven in different configurations. As there are in ladder logic, but a
lot more configurations are possible. However the basic principal
remains the same. The program is written to switch the
desired outputs for a given set of inputs energized. The 'hello world'
program equivalent for a PLC would be a light bulb and a switch (see
below). The switch is the input and the bulb would be controlled by the
output. So, when the switch (input) is on, the bulb (output) is on.

A coil (relay logic terminology) drives outputs of the PLC (a ‘Y’
device, e.g. Y01) or drives internal coils (‘M’ device) timers, counters
or flags. Each coil has associated contacts. These contacts are
available in both normally open (NO) and normally closed (NC) configurations. The term normally refers to the status of the contacts when the coil is not
energized. Using a relay analogy, when the coil is off, a NO contact
would have no current flow, that is, a load being supplied through a NO
contact would not operate. However, a NC contact would allow current to
flow, hence the connected load would be active. Activating
the coil reverses the contact status, that is, the current would flow
in a NO contact and a NC contact would then inhibit the flow. Physical
inputs to the PLC (X devices) have no programmable coil. These devices
may only be used in a contact format, again with NO and NC types
available. Because of the close relay logic association,
ladder logic programs can be read as current flowing from the left
vertical line to the right vertical line. This current must pass through
the input (switch) configuration in order to switch the output coil Y0
on. Therefore in the example below, switching X0 on and X1
being off would causes the output Y0 to also to switch on. However, if
X1 were to switch on while X0 was on, the output coil would then switch
off.

This is a very basic example of course, as they are very capable of
automating a complete warehouse or running very complex machines on
their own. Then, as you would imagine, the program it would be
running would have many twists and turns to respond to the 10's and
quite possibly even 100's of inputs and outputs. These inputs in
conjunction with the program would be dictating the on and off pattern
of the outputs at any given time. Here are just a few examples of PLC programming applications that have been successfully completed and are in use today. - Manufacturing Industry
- Lead acid battery plant, complete manufacturing system - Extruder factory, silo feeding control system - Travel Industry
- Escalator operation, monitored safety control system - Lift operation, monitored safety control system - Aerospace
- Water tank quenching system - Printing Industry
- Offset web press print register control system - Multistage screen washing system - Food Industry
- Filling machine control system - Main factory feed water pump duty changeover system - Textile Industry
- Industrial batch washing machine control system - Closed loop textile shrinkage system - Hospitals
- Coal fired boiler fan change-over system - Film Industry
- Servo axis controlled camera positioning system - Corrugating
- Main corrugation machine control system - BOBST platten press drive and control system - Plastics Industry
- Extruder factory, silo feeding control system - Injection moulding control system - Agriculture
- Glasshouse heating, ventilation & watering system - Foundry
- Overhead transportation system from casting process to shotblasting machine - Leisure
- Roller coaster ride and effects control system - Greyhound track 'Rabbit' drive system
As I said these are just a few examples of plc programming applications, there are many many
more in use today. In point of fact, there are far too many to list
here, the PLC is today's unseen hero controlling a massive range of
equipment. Manufacturers such as Mitsubishi, Allen Bradley, Omron and Siemens have been around for a long time and produce very high quality
equipment through years of development (they should be paying me for
advertising). It is quite probable that your machines have one of these
makes controlling it. So spare a thought for the
Programmable Logic Controller as you journey through this site, it does
do a lot of work and it doesn't get out much!
Register for Programmable Logic Controller programming training HERE Return to Technical page Return to Machine Shop Projects page Return to Home Page
PLEASE feel free to use this button and donate - You Are Secure! |