2. PLC- History
Developed to replace relays in the late 1960s
PLC began in the 1970s, and has become the most
common choice for manufacturing controls common choice
for manufacturing controls.
The PLC was invented in response to the needs of the
American automotive manufacturing industry (primarily
General motors).
Costs dropped and became popular by 1980s
Now used in many industrial designs
3. PLC Background
The first Programmable Logic Controller (PLC) was developed
by a group of engineers at General Motors in 1968
The company was looking for an alternative to replace
complex relay control systems :
Must be programmable (and reprogrammable)
Designed to operate in an industrial environment
Must accept 120 V ac signals from pushbuttons, switches
Outputs must switch and continuously operate loads such
as motors and relays of two amps rating
Before the 1980’s, programmable controllers were called
PCs.
4. PLC Background
Originally hardwired arrays of relays were used to
control the operation of heavy machines that
contain motors and other high power devices.
PLCs were originally used to substitute the
switching relay networks used in industrial
applications, but now they can also be used
implement other tasks such as timing, delaying
counting, calculating, comparing and processing of
analog signals
5. P L C
A Programmable controller is a solid state user
programmable control system with functions to
control logic, sequencing, timing, arithmetic data
manipulation and counting capabilities
PLC basically operates by looking at its inputs and
depending upon their state, turninig ON/OFF its
output.
Used in many real world applications such as
machining, packing, material handling, automated
assembly etc..
6. PLC Functions
PLC performs the functions of conventional relays,
timers, counters.
Capable of storing instructions, such as
sequencing, timing, counting, arithmetic, data
manipulation, and communication, to control
industrial machines and processes.
8. Basic PLC Structure…
Hardware of the PLC are
only consist of several
components; such as Power
Supply Unit, CPU Unit,
Memory Unit and I/O Unit.
At the small PLC; Power
Supply, CPU, Memory and
I/O stay in the same Unit but
at the big PLC; Power
Supply, CPU, Memory and
I/O in the separate Unit.
9. Basic PLC Structure…
• PLC can be considered as a box full of hundreds or
thousands of separate relays, counters, timers and
data storage locations.
• These relays are simulated through bit locations in
registers
10. Basic PLC Structure…
Power Supply:
The Power Supply is used to give power to all the parts.
The most PLC works with power 24V DC or 220V AC. The big
PLC has the separate power supply and the small PLC has
power supply which is one. The Power Supply is also provided
by the battery back up, so that the program of memory user do
not lose.
Memory Unit:
The Memory Unit has function to save the data and
program will be used by PLC. This Memory is divisible into two
type that is ROM and RAM. ROM contains the data like facility of
the logic program, facility to edit the program, facility to monitor
the program, facility for the communications, etc.
11. Basic PLC Structure…
CPU Unit (Central Processing Unit):
The CPU is the brain of a PLC. The CPU is a 8 bit, 16 bit or
32 bit microcontroller. The CPU has function to handle
communications with the external apparatus,
interconnectivity between the parts of PLC, executes the
program, managements the memory, observing or
perceiving the input signal and gives the output signal. The
CPU is also provided by the indicator lamp as indicator of
the happening of mistake and damage.
12. Basic PLC Structure…
INPUT RELAYS-(contacts)These are connected to the outside
world. They physically exist and receive signals from switches,
sensors, etc. Typically they are not relays but rather they are
transistors.
INTERNAL UTILITY RELAYS-(contacts) These do not receive
signals from the outside world nor do they physically exist. They
are simulated relays and are what enables a PLC to eliminate
external relays. Some are always on while some are always off.
Some are on only once during power-on and are typically used
for initializing data that was stored.
COUNTERS-These again do not physically exist. They are
simulated counters and they can be programmed to count
pulses. Typically these counters can count up, down or both up
and down.
13. Basic PLC Structure…
TIMERS-These also do not physically exist. They come in many
varieties and increments. The most common type is an on-delay
type. Others include off-delay and both retentive and non-
retentive types. Increments vary from 1ms through 1s.
OUTPUT RELAYS-(coils)These are connected to the outside
world. They physically exist and send on/off signals to solenoids,
lights, etc. They can be transistors, relays, or triacs depending
upon the model chosen.
DATA STORAGE-Typically there are registers assigned to
simply store data. They are usually used as temporary storage
for math or data manipulation. They can also typically be used to
store data when power is removed from the PLC.
17. PLC Operation
The PLC program is executed as part of a repetitive
process referred to as a scan. We can think of this scan
cycle as consisting of 3 important steps:
• Check input status
• Execute program
• Update output status
The program is executed using the status of the inputs.
Once the program is completed, CPU performs internal
diagnostics and communication tasks. The scan cycle
ends by updating the outputs, then starts over.
18. PLC Operation…
Step 1: check i/p status
The PLC takes a look at each input to determine
if it is on or off. In other words, is the sensor
connected to the first input on? How about
the second input? How about the third? This goes on
and on through the entire program.
It records this data into its memory to be used during
the next step.
19. PLC Operation…
Step 2: Execute Program
The PLC executes the program one instruction at a
time, called the “Execute Program” stage. Since it
already knows which inputs are on/off from the
previous step it will be able to decide whether the
first output should be turned on based on the state
of the first input. It will store the execution results
for use later during the next step.
20. PLC Operation…
Step 3: Update status
Finally the PLC updates the status of the outputs. It
updates the outputs based on which inputs were
on during the first step and the results of executing
your program during the second step
After this PLC goes back to step 1 and repeat the
steps continuously.
21. PLC Programming- Ladder Logic
Ladder logic is one form of drawing electrical logic
schematics, and is a graphical language very
popular for programming Programmable Logic
Controllers.
Ladder logic was originally invented to describe
logic made from relays.
The name is based on the observation that
programs in this language resemble ladders, with
two vertical "rails" and a series of horizontal
"rungs" between them
22. PLC Programming- Ladder Logic..
There are many logic symbols available in Ladder
Logic - including timers, counters, math, and data
moves such that any logical condition or control loop
can be represented in ladder logic. Some are:
Normally Open Contact
Normally Open Coil
Normally Closed Contact
Normally Open Coil
Basic AND & OR Gates
Basic Timers & Counters
24. PLC Programming- Ladder Logic..
The symbols used in relay ladder logic consist of
a power rail to the left, a second power rail to the
right
Individual circuits that connect the left power rail
to
the right. The logic of each circuit (or rung) is solved
from left to right
25. PLC Programming- Ladder Logic..
The logic of the rung above is such:
If Input1 is ON (or true) - power (logic) completes
the circuit from the left rail to the right rail - and
Output1 turns ON (or true).
If Input1 is OFF (or false) - then the circuit is not
completed and logic does not flow to the right - and
Output 1 is OFF (or false).
26. PLC Programming- Ladder Logic..
Read real world
inputs
Evaluate ladder
Write real world
outputs