G.A. Rathy, Sr Lecturer/Electrical, NITTTR, Chennai

Relay is an electrically operated device that mechanically switches electric circuits.

It consists of a coil and an internal switch.

.

One relay coil may be used to actuate more than one set of contacts.

Those contacts may be normally-open, normally-closed, or any combination of the two.

Allow a relatively small electric signal to switch a relatively large electric signal,

Relays also offer electrical isolation between coil and contact circuits. This means that the control and power circuit(s) are electrically insulated from one another.

One circuit may be DC and the other AC

PLC•PLC is a solid state equipment to perform

logical,decision making for industrial control

applications.

• Used for Counting, Calculating, Comparing

and Processing of analog signals.

• Mainly designed to replace the relay logic.

• It is similar to a PC to operate in the industrial environment

• It is equipped with inputs, outputs and a control programming language

Wires

Real elements software

Process changes – changeable

Advantages of PLC over relay control panel

• Small

• Inexpensive

• Low power consumption

• Ease of expandability

Advantages of PLC

PLC BASED CONTROL PANEL

Definition of PLC

PLC - solid state device

- event driven device (event taking place in the field will result in a output)

Parts of PLC

•The processor

•The memory

•The power supply

• I/O’s

Parts of PLC

CPU

Processor

8K

• RAM

- Provides an excellent means for easily creating and altering a program

• ROM

- Information found in ROM is placed by the manufacturer for the internal use and the operation of PLC

• EEPROM

- It provides permanent storage of the program. Easily changed using Standard programming device.

Memory

Data Table

House Keeping

User Program

Timers, Counters, Inputs, Outputs

User program

(For processor)

Memory Map of PLC

Power Supply

 

Two types of power supplies.

These are 24 VDC and 120/240 VAC.

I/Os

InputsLimit switchPush buttonSensorsSelectorSwitches

MotorMotor startersSolenoidsIndicator device

Digital or Analog

Digital or Analog

PLC consists of input modules, a central processing unit and output modules

An input accepts a variety of digital or analog signals from various field devices(sensors) and converts them into a logical signal that can be used by the CPU

The CPU makes decision and executes control instructions based on the program instructions in memory.

Output modules convert the control instructions from the CPU in to digital or analog signal that can be used to control field devices

PLC uses three types of outputs: 

• Relay• Transistor• Triac

THE INPUT/OUTPUT SYSTEM

1. Types of input/output devices2. Input interfaces3. Output interfaces4. I/O power distribution wiring

• Temperature

• Light

• Speed

• Pressure

• Small motors, Valves

Analog or Digital Input Modules

INPUT INTERFACING

Communication

The PLC can be programmed using PC or hand held Programmer. The communication between the PLC and the PC is carried out using the RS 232 C Serial port. The operating mode can be controlled from a computer running the programming software.

Two Modes of Operation

• Stop Mode

• Run Mode

Programming and Downloading- STOP OR OFFLINE

Verifying the program –RUN OR ONLINE

will execute the application program residing in the memory

Power onPower up

Perform diagnostic checks Fault

Scan Logic

Scan Inputs

Scan Outputs

Communication

Power fail

Fault

Set outputs to safe state

Wait for fault to clear

Power up

Power Failure

Turn Outputs off

Prepare for power up

Stop

Programming

• Ladder

• Statement List

• Functional Block diagram

• Understood by workers

• Fast

• Graphic display and shows how power flows

Ladder

PLC Scan  In RUN mode, the PLC executes the user program. It repeats the program continuously. Execution of the program once is called PLC Scan time   

The PLC scan consists of

 

        Input processing

        Program processing

        Output processing

I/P Image

Memory

in

Processor

Program

processing

O/P Image

memory in

Processor

Thank you

Memory System The memory system consists of four units• File sections• Files• Words• Bits

 

Files 0 and 1 executive software of the Micro Logix 1000.

Keeping track of what is happening while the PLC is operating.

Data about the processor, including type, configuration, and passwords access

• File 2 holds the main ladder program that is entered into the PLC’s memory.

• This ladder program controls the machine or process.

• File 3 stores an error fault routine

• It is executed when a recoverable, or fixable, fault occurs in the PLC’s control program.

• When this routine is executed, the Micro Logix corrects the problem to get the system up and running again.

• File 4 stores the high-speed counter interrupt program

• It is executed when a high-speed counter instruction causes an interruption in the control program.

• File 5 contains the selectable timed interrupt program

• Used to interrupt the normal program scan so that a subroutine can be executed immediately.

Files 6–15 store the subroutine ladder programs that are called by the main ladder control program.

This area can store up to ten subroutines

Data File Section

File 0 is the output file.

It stores data about the status of each output device connected to the output terminals.

Each of the PLC’s outputs is mapped to an address bit in this file.

File 0 is also known as the output image table. The output file contains one word.

File 1 is the input file.

It stores data about the status of each input device.

The input file is also known as the input image table.

inputs is mapped to a specific bit in the input file.

The input file contains two words to account for all of the possible input devices that can be connected

File 2 is the status file. It stores information about how the PLC is operating and how it is set up.

File 3 is the binary, or bit file.

It stores data about the status of internal coils and contacts. The binary file contains 32 words.

Because the binary file stores data about internal instructions, its bits do not map real field devices, as the bits in the input and output files do.

File 4 is the timer file.

It contains data about the timers used in the control program.

It includes data about each timer’s status, preset value, and accumulated value.

The Micro Logix can use up to 40 timers in its control program, and it dedicates three words for each timer.

The timer file has 120 words, three for each of its 40 timers.

File 5 is the counter file.

It stores data about the 32 counters available in the Micro Logix 1000.

It has three control words for each counter: the counter’s status, preset value, and accumulated value.

Each piece of counter data is stored in its own word.

The counter file has 96 words, three words for each of the 32 counters.

File 6 is the control file.

It stores information used by specialized PLC instructions, like shift and sequencer instructions.

This file can hold data for 16 instructions. It uses three words for each instruction.

Therefore, this file can contain up to 48 words.

File 7 is the integer file.

This file stores miscellaneous numerical data, such as constant and variable data, binary codes, and mask patterns.

The integer file uses 105 words to store all of this information.

I : 0 / 0

• The file label• The word label• The bit label