PLC

What is Automation ?

          Automation is basically the delegation of human control function to technical equipment for...

Ø Increasing Productivity

Ø Increasing Quality

Ø Reducing Cost

Ø Increasing Safety in working conditions

What is PLC ?

PLC (Programmable logic controller)

           A programmable logic controller, PLC, or programmable controller is a digital computer used for automation of typically industrial electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many machines, in many industries.

           A programmable logic controller (PLe) is a specialized computer used to control machines
and processes. It uses a programmable memory to store instructions and execute specific functions that include on & off control. timing. counting. sequencing, arithmetic, and data handling.

          PLC’s are designed to operate near the equipment they are meant to control. This means that they function in hot, humid, dirty, noisy and dusty industrial environments. PLC’s can operate in 60 Deg C as well as 0 Deg C, with tolerable relative Humidities ranging from 0% to 95% non-condensing.

• Programming Languages in PLC's

          Standard IEC 61131-3 defines consistent set of programming languages for programmable controllers. It consists of four traditional languages and one higher level language. The following chart clears your idea.

Programming language specification chart

Ø Ladder Logic (LAD/LD) [Graphical]

          The ladder logic language, which was developed to simplify the task of programming PLC's are introduced. Ladder logic was originally a written method to document the design and construction of relay racks as used in manufacturing and process control. For example as shown below, 

Example of bidirectional movable arm

Ø Structured Text (ST) [Textual]

          Structured text is one of the five languages supported by the IEC 61131-3 standard, designed for programmable logic controllers (PLCs). Text based language for developing complex algorithms or manipulating ASCII. The variables and function calls are defined by the common elements so different languages within the IEC 61131-3 standard can be used in the same program.

Complex statements and nested instructions are supported:

• Iteration loops (REPEAT-UNTIL; WHILE-DO)
• Conditional execution (IF-THEN-ELSE; CASE)
• Functions (SQRT(), SIN())

For example as shown below...

Example of structured text programming

Ø Instruction List (IL) [Textual]

          Instruction List (IL) is one of the 5 languages supported by the IEC 61131-3 standard. It is designed for programmable logic controllers (PLCs). It is a low level language and resembles assembly. for example as shown below, 

Example of IL language

Ø Sequential Function Chart (SFC)

          It has elements to organize programs for sequential and parallel control processing. State-like language with operation with numerous, complex steps for machine sequence or batch process. For example as shown below, 

SFC Example

Ø Function Block Diagram (FBD) [Graphical]

          It helps to develop process loops that drive analog values for bigger process. The Function Block Diagram (FBD) is a graphical language for programmable logic controller design, that can describe the function between input variables and output variables. A function is described as a set of elementary blocks. Input and output variables are connected to blocks by connection lines.

Inputs and outputs of the blocks are wired together with connection lines, or links. Single lines may be used to connect two logical points of the diagram:

• An input variable and an input of a block
• An output of a block and an input of another block
• An output of a block and an output variable

For example as shown below...

FBD language example

• Various PLC's avilable in market

1) Allen Bradley micrologix 1200

Allen Bradley Micrologix 1200 setup configuration

2) Allen Brdley 1500

3) Allen bradley 1100

4) Allen bradley 1000

5) Siemens simantic S7-200

Siemens S7-200 setup configuration

6) Allen Bradley micrologix 1100

Allen Bradley Micrologix 1100 setup configuration

7) Allen Bradley micrologix 1200

Allen Bradley Micrologix 1200 Demo PLC

8) GE Fanuc PLC

GE Fanuc setup configuration

9) ABB PLC

ABB PLC setup

• Typical Wiring diagram of PLC

Wiring diagram of Allen Bradley Micrologix 1200 setup