plc introduction

Introduction to PLC for
Industrial Automation
Prepared by
V.Kumaresan

Agenda
• Introduction to PLC Software and hardware
• Micro PLC hardware
• PLC Logical Rack and addressing
• PLC discrete and analog IO-s
• PLC IO connection- sinking and sourcing mode
• PLC expansion modules
• Introduction to building a sequencing programme
• Selection of PLC for Industrial automation
• Conclusion

Overall PLC based Automation System

Basic Elements of PLC controller

Introduction to PLC- Sensor and Actuator

Introduction to PLC- Digital Inputs

Introduction to PLC- analog input and Digital output

Introduction to PLC- Analog outputs

PLC – POWER SUPPLY
• Power supply
Many PLC controllers ( bigger ones )work either at 24 VDC or
220 VAC supplied by separate modules.
• Small and medium series PLC-s contain the supply module as
part of the controller . User has to determine how much current
is required .Different types of modules use different amounts of
electrical current.
• PLC controller is supplied with "pure“ and regulated supply
(e.g from a UPS) . This electrical supply is usually not used for
external inputs or outputs.
• Separate power supplies are provided for inputs or outputs
• Some of the smaller PLC controllers supply their inputs with
voltage from a small supply source already incorporated into a
PLC.

Introduction to PLC- Memory Size

PLC - Memory
• Memory
FLASH memory is used nowadays by a PLC for a process
control system. FLASH memory contents can be changed when
user program needs changing. Earlier EPROM memory were
used which had to be erased with UV lamp The use of FLASH
technology has greatly simplified reprogramming of the PLC-s
User memory is divided into blocks having special functions.
Some parts are used for storing input and output status as "1"
or as "0" in specific memory locations . Each input or output
has one corresponding bit in memory.
• Other parts of memory- or example, timer value, or counter
value-are used to store variable contents used in user program.

PLC Memory
Program is retained in case of power failure in
PROM
RAM loses the data with
Power loss

Introduction to PLC – Memory Organization

Introduction to PLC- Programming

PLC Programming – Statement List

PLC Programming – Functional Block Diagram

Introduction to PLC- Scan Cycle

Process any
Communications Requests
Perform the CPU Diagnostics
Scan Cycle
Writes to the outputs
Reads the inputs
Execute the Program
S7 200 Scan Cycle
Reading the inputs:
The S7–200 copies the state of the physical
inputs to the process–image input. register
Executing the control logic in the program:
The S7–200 executes the instructions of the
program and stores the values in the various
memory areas.
Processing any communications requests:
The S7–200 performs any tasks required for
communications.
Executing the CPU self–test diagnostics:
The S7–200 ensures that the firmware, the
program memory, and any expansion modules
are working properly.
Writing to the outputs:
The values stored in the process–image output
register are written to the physical outputs.
• The S7–200 continuously cycles through the control logic in your program,
reading and writing data.
• The S7–200 executes a series of tasks repetitively. This cyclical execution of
tasks is called the scan cycle.

Introduction to PLC- Programming
STEP 7

Programming a PLC controller
• PLC controller can be reprogrammed through a computer or
through manual programmers (consoles).
• This means that PLC controller can programmed through a PC
if one has the software needed for programming- e.g. STEP 7
Programming Software
• Once the system is ready, one may download the right
program into a PLC . It is also good to repeat this from time to
time to ensure that the program in a PLC has not changed
• This avoids hazardous situations in factory rooms (some
industries have established communication networks which
regularly check programs in PLC controllers to ensure
execution only of the right programs.

Programming a PLC by Programming terminal

Programming a PLC through A PC

Programming with PC based software

PLC - Programming
• Every program for a PLC controller possesses various useful
options such as:
• Forced switching on and off of the system inputs/outputs (I/O
lines), program follow up in real time as well as documenting a
diagram.
• This documenting is necessary to understand and define
failures and malfunctions. Programmer can add remarks,
names of input or output devices, and comments that can be
useful when finding errors, or with system maintenance.
• Adding comments and remarks enables any technician (and not
just a person who developed the system) to understand a
ladder diagram right away.

Common programming for Multiple PLC-s

PLC Hardware Specification- S7 -200 Micro

PLC Hardware Specification - S7 200 Famuly

PLC Hardware Specification - S7 200

Digital or Discrete IO addressing

PLC IO Status table in the Logical Rack

PLC Inputs
2.7 PLC controller inputs
Intelligence of an automated system depends largely on the ability of a PLC
controller to read signals from different types of sensors and input devices. Keys,
keyboards and by functional switches are a basis for man versus machine
relationship. On the other hand, in order to detect a working piece, view a
mechanism in motion, check pressure or fluid level you need specific automatic
devices such as proximity sensors, marginal switches, photoelectric sensors, level
sensors, etc. Thus, input signals can be logical (on/off) or analogue. Smaller PLC
controllers usually have only digital input lines while larger also accept analogue
inputs through special units attached to PLC controller. One of the most frequent
analogue signals are a current signal of 4 to 20 mA and milivolt voltage signal
generated by various sensors. Sensors are usually used as inputs for PLCs. You can
obtain sensors for different purposes. They can sense presence of some parts,
measure temperature, pressure, or some other physical dimension, etc. (ex.
inductive sensors can register metal objects).
Other devices also can serve as inputs to PLC controller. Intelligent devices such as
robots, video systems, etc. often are capable of sending signals to PLC controller
input modules (robot, for instance, can send a signal to PLC controller input as
information when it has finished moving an object from one place to the other.)

Input interface
2.8 Input adjustment interface
Adjustment interface also called an interface is placed between input lines and a CPU
unit. The purpose of adjustment interface to protect a CPU from disproportionate
signals from an outside world. Input adjustment module turns a level of real logic to
a level that suits CPU unit (ex. input from a sensor which works on 24 VDC must be
converted to a signal of 5 VDC in order for a CPU to be able to process it). This is
typically done through opto-isolation, and this function you can view in the following
picture.
Opto-isolation means that there is no electrical connection between external world
and CPU unit. They are "optically" separated, or in other words, signal is transmitted
through light. The way this works is simple. External device brings a signal which
turns LED on, whose light in turn incites photo transistor which in turn starts
conducting, and a CPU sees this as logic zero (supply between collector and
transmitter falls under 1V). When input signal stops LED diode turns off, transistor
stops conducting, collector voltage increases, and CPU receives logic 1 as
information.

Typical AC input Module with wiring

PLC Output interface
c2.10 Output adjustment interface
Output interface is similar to input interface. CPU brings a signal to LED diode and
turns it on. Light incites a photo transistor which begins to conduct electricity, and
thus the voltage between collector and emmiter falls to 0.7V , and a device attached
to this output sees this as a logic zero. Inversely it means that a signal at the output
exists and is interpreted as logic one. Photo transistor is not directly connected to a
PLC controller output. Between photo transistor and an output usually there is a
relay or a stronger transistor capable of interrupting stronger signals.

Wiring of an Output module – Discrete output

Wiring to a motor through interposing relay

Common ratings of Discrete IO modules

Connection for an analog input module – single
ended

Sinking-Sourcing Concept
• PLC has input and output lines through which it is connected
to a system it directs. Input can be keys, switches, sensors
while outputs are led to different devices from simple
signalization lights to complex communication modules.
• This is a very important part of the story about PLC
controllers because it directly influences what can be
connected and how it can be connected to controller inputs or
outputs.
• Two terms most frequently mentioned when discussing
connections to inputs or outputs are "sinking" and "sourcing".

Sinking-Sourcing Concept
• The most brief definition of these two concepts would be:
• SINKING = Common GND line (-)
SOURCING = Common VCC line (+)
First thing that catches one's eye are "+" and "-" supply, DC
supply. Inputs and outputs which are either sinking or
sourcing can conduct electricity only in one direction, so they
are only supplied with direct current.
• Each input or output has its own return line, so 5 inputs would
need 10 screw terminals on PLC controller housing. Instead,
we use a system of connecting several inputs to one return
line as in the following picture. These common lines are
usually marked "COMM" on the PLC controller housing.

Sinking and Sourcing Inputs
• Input lines
Sensor outputs can be different depending on a sensor itself
and also on a particular application. Following pictures display
some examples of sensor outputs and their connection with a
PLC controller.
• Sensor output actually marks the size of a signal given by a
sensor at its output when this sensor is active. In one case this
is +V (supply voltage, usually 12 or 24V) and in other case a
GND (0V).
• Another thing worth mentioning is that sinking-sourcing and
sourcing - sinking pairing is always used, and not sourcing-
sourcing or sinking-sinking pairing.

Sinking and Sourcing outputs
• Output lines
PLC controller output lines usually can be:
-transistors in PNP connection
-transistors in NPN connection
-relays
The following two pictures display a realistic way how a PLC
manages external devices. It ought to be noted that a main
difference between these two pictures is a position of "output
load device". By "output load device" we mean some relay,
signalization light or similar.

Sinking and sourcing type inputs

Sinking and sourcing type outputs

PLC I/O expansion
2.11 Extension lines
Every PLC controller has a limited number of input/output lines. If needed this
number can be increased through certain additional modules by system extension
through extension lines. Each module can contain extension both of input and output
lines. Also, extension modules can have inputs and outputs of a different nature from
those on the PLC controller (ex. in case relay outputs are on a controller, transistor
outputs can be on an extension module).

PLC with Local and Remote IO Racks

Typical PLC architecture for the Bar mill

Start_PB
M_Starter
M_StarterE_Stop
Output
Motor
Start / Stop Switch
Input
Motor Starter
The basic operation of the S7-200 is very
simple:
The S7-200 reads the status of the inputs.
The program that is stored in the S7-200 uses
these inputs to evaluate the control logic. As
the program runs, the S7-200 updates the
data.
The S7-200 writes the data to the outputs.
The S7-200 PLC continuously cycles through the control logic in your
program, reading and writing data.
The S7-200 Relates Your Program to the Physical Inputs and Outputs
Operation of PLC

Sequencing logic- Conventional and by PLC

Sequencing control Operation by PLC

Selection of PLC for an Industry
• Typical mill Layout

S7-300 and S7-400 PLC-s for
Larger systems

– The parallel I/O-s and the parallel I/O racks are those
located in the main PLC panel and are connected to
the Main PLC rack through extension of the back plane
through appropriate parallel cables. They have to be in
the vicinity of the CPU.
– Normally all the time critical functions are located in
the Parallel racks so that the I/O read and write time
will be saved.
– All non- time critical inputs and outputs could be
remote I/O-s and they can be located in remote
locations , i.e. remote from the central PLC CPU panel
( normally in an air-conditioned control room ) , in the
mill area itself ( in the separate I/O boxes under normal
mill ambience ).

– These I/O-s can be located very near to the equipment in the plant
and hence large cable expenses could be saved. Considering a
mill with couple of thousand I/O-s the saving could be huge.
– The I/O boxes will be connected to the main CPU rack through
medium speed serial communication links; a single set twisted or
some special serial communication cable.
– The disadvantage of serial I/O-s are that the information exchange
with the CPU is through serial communication and hence can take
couple of hundred milliseconds.
– Normally in a mill the non- time critical I/O-s form the bulk of the
I/O-s

• The inputs and outputs form bulk of the PLC hardware.
• The PLC CPU type is selected based on number of inputs and
outputs it has to handle and also the type control functions (
whether normal instructions or high end and faster
computations )
• Also the number of CPU-s depending on the mill and segregation
of the functionalities. In the example for the for the steel plant 5 –
numbers of CPU-s are selected based on the following
segregation of the mill functions :
1. Furnace and roughing mill ( Trio ) PLC based on a typical 90-30 CPU
2. The stands PLC for bulk sequencing functions by 90-30 CPU
3. Main mill PLC for time critical mill functions by 90-70 CPU
4. Cooling Bed ( finishing Area I ) PLC by 90-30 CPUand
5. Coiler ( Finishing Mill Area II ) PLC by 90-30 CPU
• For each area ( 1,2,4, 5 ) there will be 1500 I/O –s predominantly
digital I/O-s for sequencing functions

• The main mill PLC will be handling time critical mill functions like
– Cascade referencing scheme ;
– automatic loop control
– minimum tension controls
– Cut to length start / stop shear
– Cut length optimization
• Here the number of I/O-s will be not many but they will be
predominantly analog types and providing mill referencing and such
functions.
• Since they are time critical ( in the range of 5 to 30 m secs ) the I/O-s
will be mostly in the parallel racks near to the CPU.
• Apart from that the PLC instructions have to be with the least
service time and hard ware interrupts should also be accepted by
the PLC.

• The selection of I/O-s have to be done with lot
of care
• The main advantage of PLC is the possibility
of able to define the number of I/O-s upfront
to a great accuracy once the process
requirements are given in the form of the motor
and component list mentioned earlier.
• There are systematic methods of defining the
number of inputs and outputs for a function
• E.g. for a non-reversing motor starter the
number of required number of inputs are 5
and outputs are 3. Similarly for a reversible
motor starter the number of inputs will be 7 and
number of outputs will be 5.
• With this the number of I/O-s required for the
various MCC feeders can be estimated.

• Similarly for other functions.
• Hence the PLC hardware can be procured assuming an
additional 10 % I/O-s
• All inputs and outputs are available as input images
and output images in the PLC memory. These images
can be used any number of times with in the program
unlike the earlier hardware logic wherein the each and
every contact in the ladder represents separate physical
contact.
• Hence with the relay based circuits unless the total
functional details are available and the ladder diagram is
fully prepared the number of relays to be used and the
number of contacts can not be estimated.
• This is one of the most important advantages of PLC.

• There are different types of input and output cards based on
– Operating voltage
– Potential free or non-potential free contacts
– Time critical and non-time critical inputs
– Analog or digital
– Special inputs like temperature signals and such other physical
parameters
– Isolated or non- isolated
– Source type and sink type
– Number of inputs like 8 or 16 or 32 in a card
– Number of outputs 8, 16 or 32 in a card etc.
– Availability of spare inputs and outputs and replacement during
operation
• The selection is made based on the application, wiring requirements,
requirement of isolation , hot replacement of cards requirement and
of course costs.

Conclusion
• The PLC nowadays serve as a powerful tool in the
industries by which all the electrical and allied
equipment are interfaced .This enables total
monitoring of the plant centrally.
• The selection of PLC for an application has to be done
based on various standard definitions and standards
which make the execution, software writing and testing ,
commissioning / startups and modifications quite fast
and simple.
• To ensure greater productivity of the Mills , more and
more of PLC-s are hence getting incorporated
Thank you!

plc introduction

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plc introduction